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Manufacturing Overview: Introduction to Processes & Systems
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INTRODUCTION AND OVERVIEW OF MANUFACTURING 1. What is Manufacturing? 2. Materials in Manufacturing 3. Manufacturing Processes 4. Production Systems 5. Organization of the Book University of Limerick Con Sheahan Advanced Manufacturing DM4038 Manufacturing is Important Technologically Economically Historically University of Limerick Con Sheahan Advanced Manufacturing DM4038 1 Manufacturing - Technologically Important • Technology - the application of science to provide society and its members with those things that are needed or desired Technology provides the products that help our society and its members live better What do these products have in common? – They are all manufactured Manufacturing is the essential factor that makes technology possible University of Limerick Con Sheahan Advanced Manufacturing DM4038 Manufacturing - Economically Important U.S. economy: •Manufacturing is one way by which nations create material wealth University of Limerick Sector % of GNP Manufacturing 20% Agriculture, minerals, etc. 5% Construction & utilities 5% Service sector – retail, transportation, banking, communication, education, and government 70% Con Sheahan Advanced Manufacturing DM4038 2 Manufacturing - Historically Important • Throughout history, human cultures that were better at making things were more successful Making better tools meant better crafts & weapons – Better crafts allowed people to live better – Better weapons allowed them to conquer other cultures in times of conflict To a significant degree, the history of civilization is the history of humans' ability to make things University of Limerick Con Sheahan Advanced Manufacturing DM4038 What is Manufacturing? • The word manufacture is derived from two Latin words manus (hand) and factus (make); the combination means “made by hand” “Made by hand” accurately described the fabrication methods that were used when the English word “manufacture” was first coined around 1567 A.D. Most modern manufacturing operations are accomplished by mechanized and automated equipment that is supervised by human workers University of Limerick Con Sheahan Advanced Manufacturing DM4038 3 Manufacturing - Technologically •Application of physical and chemical processes to alter the geometry, properties, and/or appearance of a starting material to make parts or products Manufacturing also includes assembly Almost always carried out as a sequence of operations •Figure 1.1 (a) •Manufacturing •as a technical •process University of Limerick Con Sheahan Advanced Manufacturing DM4038 Manufacturing - Economically •Transformation of materials into items of greater value by means of one or more processing and/or assembly operations Manufacturing adds value to the material by changing its shape or properties, or by combining it with other materials •Figure 1.1 (b) •Manufacturing •as an economic •process University of Limerick Con Sheahan Advanced Manufacturing DM4038 4 Manufacturing Industries • University of Limerick Industry consists of enterprises and organizations that produce or supply goods and services Industries can be classified as: 1. Primary industries - those that cultivate and exploit natural resources, e.g., farming, mining, forestry, fishing. 2. Secondary industries - take the outputs of primary industries and convert them into consumer and capital goods manufacturing is the principal activity 3. Tertiary industries - service sector Con Sheahan Advanced Manufacturing DM4038 Manufacturing Industries - continued Secondary industries include manufacturing, construction, and electric power generation Manufacturing includes several industries whose products are not covered in this book; e.g., apparel, beverages, chemicals, and food processing For our purposes, manufacturing means production of hardware – Nuts and bolts, forgings, cars, airplanes, digital computers, plastic parts, and ceramic products University of Limerick Con Sheahan Advanced Manufacturing DM4038 5 Production Quantity Q • The quantity of products Q made by a factory has an important influence on the way its people, facilities, and procedures are organized Annual production quantities can be classified into three ranges: • Production range Annual Quantity Q • Low production 1 to 100 units • Medium production 100 to 10,000 units • High production 10,000 to millions of University of Limerick Con Sheahan Advanced Manufacturing DM4038 Product Variety P • Product variety P refers to different product types or models produced in the plant Different products have different features – They are intended for different markets – Some have more parts than others The number of different product types made each year in a factory can be counted When the number of product types made in the factory is high, this indicates high product variety University of Limerick Con Sheahan Advanced Manufacturing DM4038 6 P versus Q in Factory Operations • Figure 1.2 P-Q Relationship University of Limerick Con Sheahan Advanced Manufacturing DM4038 More About Product Variety • Although P is a quantitative parameter, it is much less exact than Q because details on how much the designs differ is not captured simply by the number of different designs Soft product variety - small differences between products, e.g., between car models made on the same production line, with many common parts among models Hard product variety - products differ substantially, e.g., between a small car and a large truck, with few common parts (if any) University of Limerick Con Sheahan Advanced Manufacturing DM4038 7 Manufacturing Capability • University of Limerick A manufacturing plant consists of processes and systems (and people, of course) designed to transform a certain limited range of materials into products of increased value The three building blocks - materials, processes, and systems - are the subject of modern manufacturing Manufacturing capability includes: 1. Technological processing capability 2. Physical product limitations 3. Production capacity Con Sheahan Advanced Manufacturing DM4038 1. Technological Processing Capability • The available set of manufacturing processes in the plant (or company) Certain manufacturing processes are suited to certain materials – By specializing in certain processes, the plant is also specializing in certain materials Includes not only the physical processes, but also the expertise of the plant personnel Examples: – A machine shop cannot roll steel – A steel mill cannot build cars University of Limerick Con Sheahan Advanced Manufacturing DM4038 8 2. Physical Product Limitations • Given a plant with a certain set of processes, there are size and weight limitations on the parts or products that can be made in the plant Product size and weight affect: – Production equipment – Material handling equipment Production, material handling equipment, and plant size must be planned for products that lie within a certain size and weight range University of Limerick Con Sheahan Advanced Manufacturing DM4038 3. Production Capacity • Defined as the maximum quantity that a plant can produce in a given time period (e.g., month or year) under assumed operating conditions Operating conditions refer to number of shifts per week, hours per shift, direct labor manning levels in the plant, and so on Usually measured in terms of output units, such as tons of steel or number of cars produced by the plant Also called plant capacity University of Limerick Con Sheahan Advanced Manufacturing DM4038 9 Materials in Manufacturing • Most engineering materials can be classified into one of three basic categories: 1. Metals 2. Ceramics 3. Polymers Their chemistries are different Their mechanical and physical properties are dissimilar These differences affect the manufacturing processes that can be used to produce products from them Con Sheahan University of Limerick Advanced Manufacturing DM4038 In Addition: Composites •Nonhomogeneous mixtures of the other three basic types rather than a unique category •Figure 1.3 Venn diagram of three basic material types •plus composites University of Limerick Con Sheahan Advanced Manufacturing DM4038 10 1. Metals • Usually alloys, which are composed of two or more elements, at least one of which is metallic Two basic groups: 1. Ferrous metals - based on iron, comprises about 75% of metal tonnage in the world: Steel = Fe-C alloy (0.02 to 2.11% C) Cast iron = Fe-C alloy (2% to 4% C) 2. Nonferrous metals - all other metallic elements and their alloys: aluminum, copper, magnesium, nickel, silver, tin, titanium, etc. University of Limerick Con Sheahan Advanced Manufacturing DM4038 2. Ceramics • University of Limerick Compounds containing metallic (or semimetallic) and nonmetallic elements. Typical nonmetallic elements are oxygen, nitrogen, and carbon For processing, ceramics divide into: 1. Crystalline ceramics – includes: Traditional ceramics, such as clay (hydrous aluminum silicates) Modern ceramics, such as alumina (Al2O3) 2. Glasses – mostly based on silica (SiO2) Con Sheahan Advanced Manufacturing DM4038 11 3. Polymers • University of Limerick Compound formed of repeating structural units called mers, whose atoms share electrons to form very large molecules Three categories: 1. Thermoplastic polymers - can be subjected to multiple heating and cooling cycles without altering molecular structure 2. Thermosetting polymers - molecules chemically transform (cure) into a rigid structure – cannot be reheated 3. Elastomers - shows significant elastic behavior Con Sheahan Advanced Manufacturing DM4038 4. Composites • Material consisting of two or more phases that are processed separately and then bonded together to achieve properties superior to its constituents Phase - homogeneous mass of material, such as grains of identical unit cell structure in a solid metal Usual structure consists of particles or fibers of one phase mixed in a second phase called the matrix Properties depend on components, physical shapes of components, and the way they are combined to form the final material University of Limerick Con Sheahan Advanced Manufacturing DM4038 12 Manufacturing Processes • 1. Two basic types: Processing operations - transform a work material from one state of completion to a more advanced state – Operations that change the geometry, properties, or appearance of the starting material 2. Assembly operations - join two or more components to create a new entity University of Limerick Con Sheahan Advanced Manufacturing DM4038 Figure 1.4 Classification of manufacturing processes University of Limerick Con Sheahan Advanced Manufacturing DM4038 13 Processing Operations • Alters a material’s shape, physical properties, or appearance in order to add value Three categories of processing operations: 1. Shaping operations - alter the geometry of the starting work material 2. Property-enhancing operations improve physical properties without changing shape 3. Surface processing operations - to clean, treat, coat, or deposit material on exterior surface of the work University of Limerick Con Sheahan Advanced Manufacturing DM4038 Shaping Processes – Four Categories 1. 2. 3. 4. University of Limerick Solidification processes - starting material is a heated liquid or semifluid Particulate processing - starting material consists of powders Deformation processes - starting material is a ductile solid (commonly metal) Material removal processes - starting material is a ductile or brittle solid Con Sheahan Advanced Manufacturing DM4038 14 Solidification Processes • Starting material is heated sufficiently to transform it into a liquid or highly plastic state Examples: metal casting, plastic molding • University of Limerick Con Sheahan Advanced Manufacturing DM4038 Particulate Processing • Starting materials are powders of metals or ceramics Usually involves pressing and sintering, in which powders are first compressed and then heated to bond the individual particles University of Limerick Con Sheahan Advanced Manufacturing DM4038 15 Deformation Processes • Starting workpart is shaped by application of forces that exceed the yield strength of the material Examples: (a) forging, (b) extrusion University of Limerick Con Sheahan Advanced Manufacturing DM4038 Material Removal Processes • Excess material removed from the starting piece so what remains is the desired geometry Examples: machining such as turning, drilling, and milling; also grinding and nontraditional processes University of Limerick Con Sheahan Advanced Manufacturing DM4038 16 Waste in Shaping Processes • Desirable to minimize waste in part shaping Material removal processes are wasteful in unit operations, simply by the way they work Most casting, molding, and particulate processing operations use very little waste material Terminology for minimum waste processes: – Net shape processes - when most of the starting material is used and no subsequent machining is required – Near net shape processes - when minimum amount of machining is required Con Sheahan University of Limerick Advanced ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Manufacturing DM4038 Property-Enhancing Processes • Performed to improve mechanical or physical properties of work material Part shape is not altered, except unintentionally – Example: unintentional warping of a heat treated part Examples: – Heat treatment of metals and glasses – Sintering of powdered metals and ceramics University of Limerick Con Sheahan Advanced Manufacturing DM4038 17 Surface Processing Operations University of Limerick Cleaning - chemical and mechanical processes to remove dirt, oil, and other contaminants from the surface Surface treatments - mechanical working such as sand blasting, and physical processes like diffusion Coating and thin film deposition - coating exterior surface of the workpart Con Sheahan Advanced Manufacturing DM4038 Assembly Operations • University of Limerick Two or more separate parts are joined to form a new entity Types of assembly operations: 1. Joining processes – create a permanent joint Welding, brazing, soldering, and adhesive bonding 2. Mechanical assembly – fastening by mechanical methods Threaded fasteners (screws, bolts and nuts); press fitting, expansion fits Con Sheahan Advanced Manufacturing DM4038 18 Production Systems • People, equipment, and procedures used for the combination of materials and processes, constitute a firm's manufacturing operations A manufacturing firm must have systems and procedures to efficiently accomplish its type of production Two categories of production systems: – Production facilities – Manufacturing support systems Both categories include people (people make the systems work) University of Limerick Con Sheahan Advanced Manufacturing DM4038 Production Facilities • The factory, production equipment, and material handling systems Production facilities "touch" the product Includes the way the equipment is arranged in the factory - the plant layout Equipment usually organized into logical groupings, called manufacturing systems – Examples: Automated production line Machine cell consisting of an industrial robot and two machine tools University of Limerick Con Sheahan Advanced Manufacturing DM4038 19 Facilities versus Product Quantities • University of Limerick A company designs its manufacturing systems and organizes its factories to serve the particular mission of each plant Certain types of production facilities are recognized as the most appropriate for a given type of manufacturing: 1. Low production – 1 to 100 units/year 2. Medium production – 100 to 10,000 3. High production – 10,000 to >1,000,000 Different facilities are required for each of the three quantity ranges Con Sheahan Advanced Manufacturing DM4038 Low Production • Job shop is the term used for this type of production facility A job shop makes low quantities of specialized and customized products – Products are typically complex, e.g., space capsules, prototype aircraft, special machinery Equipment in a job shop is general purpose Labor force is highly skilled Job shop designed for maximum flexibility University of Limerick Con Sheahan Advanced Manufacturing DM4038 20 Medium Production • Two different types of facility, depending on product variety: Batch production – Suited to hard product variety – Setups required between batches Cellular manufacturing – Suited to soft product variety – Worker cells organized to process parts without setups between different part styles University of Limerick Con Sheahan Advanced Manufacturing DM4038 High Production University of Limerick Often referred to as mass production – High demand for product – Manufacturing system dedicated to the production of that product Two categories of mass production: 1. Quantity production 2. Flow line production Con Sheahan Advanced Manufacturing DM4038 21 Quantity Production • Mass production of single parts on single machine or small numbers of machines Typically involves standard machines equipped with special tooling Equipment is dedicated full-time to the production of one part or product type Typical layouts used in quantity production are process layout and cellular layout University of Limerick Con Sheahan Advanced Manufacturing DM4038 Flow Line Production • Multiple machines or workstations arranged in sequence, e.g., production lines Product is complex – Requires multiple processing and/or assembly operations Work units are physically moved through the sequence to complete the product Workstations and equipment are designed specifically for the product to maximize efficiency University of Limerick Con Sheahan Advanced Manufacturing DM4038 22 Manufacturing Support Systems • A company must organize itself to design the processes and equipment, plan and control production, and satisfy product quality requirements Accomplished by manufacturing support systems - people and procedures by which a company manages its production operations Typical departments: 1. Manufacturing engineering 2. Production planning and control 3. Quality control University of Limerick Con Sheahan Advanced Manufacturing DM4038 Overview of Major Topics • Figure 1.10 Overview of production system and major topics in Fundamentals of Modern Manufacturing. University of Limerick Con Sheahan Advanced Manufacturing DM4038 23 A spectacular scene in steelmaking is charging of a basic oxygen furnace, in which molten pig iron produced in a blast furnace is poured into the BOF. Temperatures are around 1650°C (3000 ° F). University of Limerick Con Sheahan Advanced Manufacturing DM4038 A machining cell consisting of two horizontal machining centers supplied by an in-line pallet shuttle (photo courtesy of Cincinnati Milacron). University of Limerick Con Sheahan Advanced Manufacturing DM4038 24 A robotic arm performs unloading and loading operation in a turning center using a dual gripper (photo courtesy of Cincinnati Milacron). University of Limerick Con Sheahan Advanced Manufacturing DM4038 Metal chips fly in a high speed turning operation performed on a computer numerical control turning center (photo courtesy of Cincinnati Milacron). University of Limerick Con Sheahan Advanced Manufacturing DM4038 25 A batch of silicon wafers enters a furnace heated to 1000°C (1800°F) during fabrication of integrated circuits under clean room conditions (photo courtesy of Intel Corporation). University of Limerick Con Sheahan Advanced Manufacturing DM4038 Two welders perform arc welding on a large steel pipe section (photo courtesy of Lincoln Electric Company). University of Limerick Con Sheahan Advanced Manufacturing DM4038 26 Automated dispensing of adhesive onto component parts prior to assembly (photo courtesy of EFD, Inc.). University of Limerick Con Sheahan Advanced Manufacturing DM4038 Assembly workers on an engine assembly line (photo courtesy of Ford Motor Company). University of Limerick Con Sheahan Advanced Manufacturing DM4038 27 Assembly operations on the Boeing 777 (photo courtesy of Boeing Commercial Airplane Co.). University of Limerick Con Sheahan Advanced Manufacturing DM4038 Die Design for Injecting Moulding & Pressure Die Casting DM4038 University of Limerick Con Sheahan Advanced Manufacturing DM4038 28 What is Injection Moulding? • Polymer is heated to a highly plastic state and forced to flow under high pressure into a mould cavity where it solidifies and the moulding is then removed from cavity • Produces discrete components almost always to net shape • More injection moulding machines used for plastic processing than any other equipment • Typical cycle time ∼10 to 30 sec, but cycles of one minute or more are not uncommon • Mould may contain multiple cavities, so multiple mouldings are produced each cycle • E.g. Bottle tops and outdoor furniture University of Limerick Con Sheahan Advanced Manufacturing DM4038 Injection Moulded Parts • Complex and intricate shapes are possible • Shape limitations: – Capability to fabricate a mould whose cavity is the same geometry as part – Shape must allow for part removal from mould • Part size from ∼ 50 g up to ∼ 25 kg, e.g., refrigerator doors, automobile bumpers • Injection moulding is economical only for large production quantities due to high cost of mould (usually P20 steel) University of Limerick Con Sheahan Advanced Manufacturing DM4038 29 Injection Moulding University of Limerick Con Sheahan Advanced Manufacturing DM4038 What is Pressure Die Casting? University of Limerick Con Sheahan Advanced Manufacturing DM4038 30 What is Pressure Die Casting? • Molten metal forced at high pressure into a mould, which is the inverse of the desired shape • Non ferrous metals normally used • Fine section and thin detail can be achieved • Mould is made by a toolmaker from steel, and precisionmachined to form the features of the desired part University of Limerick Con Sheahan Advanced Manufacturing DM4038 Advantages and Limitations • Advantages of die casting: –Economical for large production quantities –Good accuracy and surface finish –Thin sections are possible –Rapid cooling provides small grain size and good strength to casting • Disadvantages: –Generally limited to metals with low melting points –Part geometry must allow removal from die University of Limerick Con Sheahan Advanced Manufacturing DM4038 31 Injection Moulding Process University of Limerick Con Sheahan Advanced Manufacturing DM4038 Injection Moulding Machine Two principal components: 1. Injection unit – Melts and delivers polymer melt – Operates much like an extruder 2. Clamping unit – Opens and closes mould each injection cycle University of Limerick Con Sheahan Advanced Manufacturing DM4038 32 Injection Moulding Machine A large (3000 ton capacity) injection moulding machine (Photo courtesy of Cincinnati Milacron). University of Limerick Con Sheahan Advanced Manufacturing DM4038 Injection Moulding University of Limerick Con Sheahan Advanced Manufacturing DM4038 33 Injection Moulding – History • John Wesley Hyatt developed first injection moulding machine in 1872 to manufacture celluloid billiard balls University of Limerick Con Sheahan Advanced Manufacturing DM4038 University of Limerick Con Sheahan Advanced Manufacturing DM4038 34 How does it work? 1. Mould Closes Con Sheahan University of Limerick Advanced Manufacturing DM4038 How does it work? 2. Barrel contacts mould University of Limerick Con Sheahan Advanced Manufacturing DM4038 35 How does it work? 3. Material Injected University of Limerick Con Sheahan Advanced Manufacturing DM4038 How does it work? 4. Pressure maintained during cooling University of Limerick Con Sheahan Advanced Manufacturing DM4038 36 How does it work? 5. Barrel and screw move back, cooling continues. More material bought to heaters University of Limerick Con Sheahan Advanced Manufacturing DM4038 How does it work? 6. Mould opens, part attached to moving section and ejected. Part removed through gravity or mechanical system University of Limerick Con Sheahan Advanced Manufacturing DM4038 37 Injection Moulding University of Limerick Con Sheahan Advanced Manufacturing DM4038 Mould Timing and Terminology • Dry-cycle - Total Time required for the clamp to close and open (sum of mould opening and closing time). • Opening Time - Time it takes to open mould and usually eject part • Closing Time - Time it takes to close mould • Mould Protection - Halts moulding operation if a foreign material (dirt, flash, debris) is detected in mould cavity • Mould Open - Time the mould is actually open • Mould Closed - Time the mould is closed and usually includes » Injection Time, Hold Time, Cooling Time • Ejection Time - Time is takes to eject parts and is a part of Mould Open Time University of Limerick Con Sheahan Advanced Manufacturing DM4038 38 Injection Time • Injection Time and the Machine – Faster a machine can inject the faster the injection time. – All machines are rated in the number of cm3 per second • Injection Time and Mould Design – Pressure drop in the runner system from nozzle to the gate in the cavity » Smaller the pressure drop, the faster the mould will fill. » Larger runners yield smaller pressure drop, but take longer to cool. » In hot runners, valve gates can provide large passages • Number of cavities – More cavities, the longer it takes to fill • Product Shape – Long flow lengths cause longer injection time • L/t Ratio – Ratio of flow length to thickness. University of Limerick Con Sheahan Advanced Manufacturing DM4038 Time? University of Limerick Con Sheahan Advanced Manufacturing DM4038 39 Machine Configuration Power Unit Plasticizing / Injection Unit Mould Unit University of Limerick Con Sheahan Advanced Manufacturing DM4038 Injection Pressure on Material University of Limerick Con Sheahan Advanced Manufacturing DM4038 40 Injection Unit Aim: Liquefy the material and inject into mould. Hopper (Manual / Automatic) Barrel University of Limerick Generally holds more than two cycles (shots) worth of material – 50% rule Heaters Con Sheahan Advanced Manufacturing DM4038 Injection Unit (General Overview) • Heat sensitivity of material determines shot capacity (50%). • 20% rule for low heat sensitive material • 80% rule for high heat sensitive material (as material will degrade in melted condition) • E.g. Shot required = 60g for heat sensitive material. Therefore, 80% = 60g; 100%=75g. • Capacity of unit is rated in terms of weight of polystyrene it holds • Injection pressure – pressure used to perform filling of mould University of Limerick Con Sheahan Advanced Manufacturing DM4038 41 Q3(b)(i) 2005 80% rule: Ans = 75g 20% rule: Ans = 300g University of Limerick Con Sheahan Advanced Manufacturing DM4038 Heating Cylinder (Barrel) Three or more heater bands at each heater location Each zone is individually controlled (thermocouple monitors temperature, which is fed back to control unit) Barrel Outer sleeve: low quality steel University of Limerick Heaters Con Sheahan Inner sleeve: high quality tool steel Advanced Manufacturing DM4038 42 Hopper Contains approximately 2hrs worth of material Contains a ‘drawer magnet’ on it’s base to capture any loose steel particles from previous processing University of Limerick Con Sheahan Advanced Manufacturing DM4038 Injection (Metering) Screw University of Limerick Con Sheahan Advanced Manufacturing DM4038 43 Injection (Metering) Screw Aim: To auger material from hopper to heating area Also: Mixes and homogenises material Generates friction heating Three zones – Feed, Melt and Meter Feed Zone 50% University of Limerick Melt 25% Con Sheahan Min clearance = 0.08mm Meter 25% Advanced Manufacturing DM4038 Check Ring (Valve) – Non-return Mechanism The check ring is used to prevent the plastic from being blown back during the injection phase of the molding cycle. It is similar to a non return valve. University of Limerick Con Sheahan Advanced Manufacturing DM4038 44 Typical Specification Injection screw φ = 40mm Injection time = 1sec Screw Power = 6kW Screw speed = 195rpm Max screw stroke = 76mm Main motor = 15kW Heating zones = 3 (6kw) Max shot weight = 80g Toggle clamping, (80 tonnes) Platen size = 305*270mm 3 Platens, ground surface finish 4 tie-bars, High tensile steel Actuators: Barrel, Injection screw, Clamping Heat exchanger for moulds and oil Air supply University of Limerick Polymer compressed approx. 24:1 pressure Barrel temperature ~Barrel 70-200MPa ~ 200-400° °C Con Sheahan Advanced Manufacturing DM4038 Injection Moulding Pressures • Pressure depends on viscosity, flow rate and temperature of plastic and temperature of mould • Pressure to use: compromise between minimising residual stress and maximising output • Three stages, Initial, holding and Back Pressures University of Limerick Con Sheahan Advanced Manufacturing DM4038 45 Injection Moulding Operations • Cycle Time University of Limerick • Injection Pressure Con Sheahan Advanced Manufacturing DM4038 Injection Pressure Graphs University of Limerick Con Sheahan Advanced Manufacturing DM4038 46 Injection Moulding Thermal Process • Temperature History in part University of Limerick Con Sheahan Advanced Manufacturing DM4038 Injection Moulding Process • Fill time – Faster filling rate = shorter fill time – Volume of part divided by volumetric flow rate – Note: Pressure is a function of flow rate. Faster flow rate = higher pressures, except at very slow fill which causes larger core and smaller flow channel and then higher pressures. University of Limerick Con Sheahan Advanced Manufacturing DM4038 47 Developing Injection Pressure Injection pressure – used to perform filling of mould –Main system: ~14MPa –At nozzle: ~200MPa University of Limerick Con Sheahan Advanced Manufacturing DM4038 Holding Pressure Used to ensure material is packed into cavity ~40MPa University of Limerick Con Sheahan Advanced Manufacturing DM4038 48 Applying Back Pressure Screw begins to turn at end of moulding cycle. Pressure of molten polymer in front of screw forces screw backwards Max. 4MPa University of Limerick Con Sheahan Advanced Manufacturing DM4038 University of Limerick Con Sheahan Advanced Manufacturing DM4038 49 Pressure Die Casting Process University of Limerick Con Sheahan Advanced Manufacturing DM4038 Pressure Die Casting Pressures up to 200MPa University of Limerick Con Sheahan Advanced Manufacturing DM4038 50 University of Limerick Con Sheahan Advanced Manufacturing DM4038 Die Casting Machines • Designed to hold and accurately close two mould halves and keep them closed while liquid metal is forced into cavity • Two main types: 1.Hot-chamber machine 2.Cold-chamber machine University of Limerick Con Sheahan Advanced Manufacturing DM4038 51 Hot-Chamber Die Casting • Metal is melted in a container, and a piston injects liquid metal under high pressure into the die – High production rates - 500 parts per hour not uncommon – Applications limited to low melting-point metals that do not chemically attack plunger and other mechanical components – Casting metals: zinc, tin, lead, and magnesium University of Limerick Con Sheahan Advanced Manufacturing DM4038 Hot-Chamber Die Casting Cycle in hot-chamber casting: (1) with die closed and plunger withdrawn, molten metal flows into the chamber University of Limerick Con Sheahan Advanced Manufacturing DM4038 52 Hot-Chamber Die Casting Up to 15 cycles per minute Cycle in hot-chamber casting: (2) plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification. University of Limerick Con Sheahan Advanced Manufacturing DM4038 Cold-Chamber Die Casting Machine • Molten metal is poured into unheated chamber from external melting container, and a piston injects metal under high pressure into die cavity • High production but not usually as fast as hot-chamber machines because of pouring step • Casting metals: aluminum, brass, and magnesium alloys • Advantages of hot-chamber process favors its use on low melting-point alloys (zinc, tin, lead) University of Limerick Con Sheahan Advanced Manufacturing DM4038 53 Cold-Chamber Die Casting Cycle in cold-chamber casting: (1) with die closed and ram withdrawn, molten metal is poured into the chamber University of Limerick Con Sheahan Advanced Manufacturing DM4038 Cold-Chamber Die Casting Cycle in cold-chamber casting: (2) ram forces metal to flow into die, maintaining pressure during cooling and solidification. University of Limerick Con Sheahan Advanced Manufacturing DM4038 54 Die Design (Introduction) University of Limerick Con Sheahan Advanced Manufacturing DM4038 Mould diagram showing key elements University of Limerick Con Sheahan Advanced Manufacturing DM4038 55 University of Limerick Con Sheahan Advanced Manufacturing DM4038 University of Limerick Con Sheahan Advanced Manufacturing DM4038 56 Clamping Mechanism University of Limerick Con Sheahan Advanced Manufacturing DM4038 Clamping Mechanism • • • • • • • Holds two halves of mould in proper alignment and keeps mould closed during injection Opens and closes mould at appropriate times in moulding cycle Clamp unit rated by maximum clamp force a machine can produce (e.g. 150 tons clamping force) Clamping force = Injection Pressure x Total Cavity Projected Area Required force dependant on polymer / metal used Too little force results in molten material escaping through the sides of the mould Too much force can damage the mould / moulding machine University of Limerick Con Sheahan Advanced Manufacturing DM4038 57 Clamp Unit • Hydraulic – (Limited to maximum load – can damage system if accidentally overloaded) • Mechanical (Toggle) – Unlikely to fail with minor overloading – Linkages must be replaced regularly – Can only be run at one specified loading. University of Limerick Con Sheahan Advanced Manufacturing DM4038 University of Limerick Con Sheahan Advanced Manufacturing DM4038 58 Injection Moulding University of Limerick Con Sheahan Advanced Manufacturing DM4038 Q3(a) 2004 University of Limerick Con Sheahan Advanced Manufacturing DM4038 59 Flow System • Consists of: – Sprue Bushing, Runner, Gate, Mould Cavity. University of Limerick Con Sheahan Advanced Manufacturing DM4038 Sprue Bushing Runner Gate Mould Cavity To avoid sink marks as polymer solidifies, Mould solidifies first, then gate, runner and finally sprue. University of Limerick Con Sheahan Advanced Manufacturing DM4038 60 A B University of Limerick Con Sheahan Advanced Manufacturing DM4038 Sprue Bushing • Insert to get polymer into centre of mould cavity • Central core remains liquid until runner solid • Must not be larger than needed • Can be replaced and repaired easily • Demould easily • Design considerations – Tapered bore 2-4° – Bushing Sphere = Nozzle φ + 1mm – Minor φ = Nozzle φ + 1mm Con Sheahan University of Limerick – Major φ = Runner φ + 1.5 mm Advanced Manufacturing DM4038 61 University of Limerick Con Sheahan Advanced Manufacturing DM4038 Standard tolerance: University of Limerick Con Sheahan 0.000 R−+0.010 Advanced Manufacturing DM4038 62 Cold Slug Well • Receives material that has chilled at the front of the nozzle during cooling and ejection phase • Also provides a positive means whereby the sprue can be pulled from the sprue bushing for ejection purposes University of Limerick Con Sheahan Advanced Manufacturing DM4038 Runners University of Limerick Con Sheahan Advanced Manufacturing DM4038 63 Injection Mould Design – Runner Functions • Cavity filling with minimum of knit lines • Few restrictions as possible • Convey melt rapidly into cavity in the shortest way with minimum loss of heat and pressure • Material must enter cavity / cavities at all gates at the same time, under equal pressure and temperature • Central core must remain liquid until Gate and Cavity solidify. Polymer in contact with mould walls solidifies first leaving central core molten • The surface / volume ratio should be kept as small as possible to save material Con Sheahan University of Limerick Advanced Manufacturing DM4038 • Ease of demoulding Standard Runner – Design Considerations • Size (dependant on cavity) – CSA of runner must be large enough to permit melt to pass through and fill the impression before runner solidifies – Be large enough to allow application of packing pressure (shrinkage compensation) – Be a minimum to reduce material usage • Pros and cons of large runners – While large runners facilitate the flow of material at relatively low pressure requirements, they require longer cooling times, more material and resulting scrap, and increased clamping forces • Pros and cons of small runners – Designing the smallest adequate runner system will maximize efficiency in material usage and energy consumption in molding. However, runner size reduction is constrained by the molding machine's injection pressure Con Sheahan University of Limerick Advanced Manufacturing DM4038 capability. 64 Standard Runner – Design Considerations • Layout: Runner systems should be balanced • Length (dependant on no of parts being produced) University of Limerick Con Sheahan Scra p – Circular – Parabolic – Trapezoid – Rectangular Cost of manufactu re • CSA shape Heat Loss – Dependant on diameter (determined from experience), CSA – Should be kept to a minimum to reduce pressure losses – Dependant on viscosity (low viscosity ⇒ long thin runner) Advanced Manufacturing DM4038 Factors Affecting Design of Runners – Moulding • Geometry • Volume • Wall thickness • Quality requirements: – Dimensional – Optical – Mechanical University of Limerick Con Sheahan Advanced Manufacturing DM4038 65 Factors Affecting Design of Runners – Moulding Material • Viscosity • Chemical composition • Freezing time • Softening time • Softening temperature • Softening range • Sensitivity to heat • Shrinkage University of Limerick Con Sheahan Advanced Manufacturing DM4038 Factors Affecting Design of Runners – Moulding Machine • Type of clamping • Injection pressure • Injection rate University of Limerick Con Sheahan Advanced Manufacturing DM4038 66 Factors Affecting Design of Runners – Injection mould • Automatic demoulding • Manual demoulding • Temperature of runner system University of Limerick Con Sheahan Advanced Manufacturing DM4038 Runner Guidelines • Common runners – Full-round runner – Trapezoidal runner – Modified trapezoidal runner (a combination of round and trapezoidal runner) – Half-round runner – Rectangular runner University of Limerick Con Sheahan Advanced Manufacturing DM4038 67 Runner Design – Circular Cross Section • Advantages – Smallest surface area relative to cross section – Slowest cooling rate – Low heat and frictional losses – Centre or channel freezes last therefore effective holding pressure • Disadvantages – Machining into both mould halves is difficult and expensive University of Limerick Con Sheahan Advanced Manufacturing DM4038 Runner Design – Parabolic Cross Section • Advantages – Best approximation of circular cross section – Simpler machining in one mould half only (usually moveable side for ejection purposes) • Disadvantages – More heat losses and scrap compared to circular University of Limerick Con Sheahan Advanced Manufacturing DM4038 Note: Most favourable 68 Runner Design – Trapezoidal Cross Section • Alternative to parabolic • Disadvantages – More heat losses and scrap compared to parabolic Unfavourable runner designs: University of Limerick Con Sheahan Advanced Manufacturing DM4038 Round vs Trapezoidal Runner Molecular distortion results in stresses developing Non-circular section results in unequal pressure Minimises distortion and hence residual stress in mould cavity University of Limerick Con Sheahan Advanced Manufacturing DM4038 69 Small CSA • Allows gate to solidify quickly • Allows simple degating • Small witness marks remain • Better control of filling multi-impression moulds • Packing to compensate for shrinkages is minimised University of Limerick Con Sheahan Advanced Manufacturing DM4038 University of Limerick Con Sheahan Advanced Manufacturing DM4038 70 Injection Mould Design – Runners The following factors affect runner design • Part Volume • Wall thickness (s) • Plastic material • Length of flow path • Resistance to flow • Surface \ Volume ratio D = Smax +1.5mm University of Limerick Con Sheahan • Heat losses • Friction losses • Cooling time • Amount of scrap • Manufacturing cost • Mould type Advanced Manufacturing DM4038 Graph of polymer mass as a function of runner diameter for different section thickness for injection moulding University of Limerick Con Sheahan Advanced Manufacturing DM4038 71 Graph of polymer mass as a function of runner diameter for different section thickness for injection moulding University of Limerick Con Sheahan Advanced Manufacturing DM4038 Graph of runner length versus corrected length for injection moulding University of Limerick Con Sheahan Advanced Manufacturing DM4038 72 Procedure for Runner Design (φ φ) Calculation • Determine G (weight of part) and s (wall thickness of part) • Find D` (runner diameter for material considered) • Determine L (length of runner to one cavity) • Find FL (length correction factor) from diagram • Correct runner diameter (D)=D`.FL University of Limerick Con Sheahan Advanced Manufacturing DM4038 Example • Part material – Polystyrene • Part weight – 200g • Part wall thickness – 2mm • Runner length – 100mm • Calculate runner diameter University of Limerick Con Sheahan Advanced Manufacturing DM4038 73 Part material – Polystyrene Part weight – 200g Part wall thickness – 2mm Runner length – 100mm University of Limerick Con Sheahan Part material – Polystyrene Part weight – 200g Part wall thickness – 2mm Runner length – 100mm Calculate runner diameter Advanced Manufacturing DM4038 Calculate runner diameter D`=4.5mm University of Limerick Con Sheahan Advanced Manufacturing DM4038 74 Part material – Polystyrene Part weight – 200g Part wall thickness – 2mm Runner length – 100mm Calculate runner diameter Correct runner diameter, D=D`.FL = 4.5(1.13) = 5.085mm D`=4.5mm FL = 1.13 University of Limerick Con Sheahan Advanced Manufacturing DM4038 Q4b(i) 2005 University of Limerick Con Sheahan Advanced Manufacturing DM4038 75 Runner System • Several types of runners – single part runner – multiple part runner » symmetrical runner » non-symmetrical runner – runner-less designs with hot manifolds University of Limerick Con Sheahan Advanced Manufacturing DM4038 Runner Design – Circular layout • Advantages: – Equal flow lengths to all cavities – Easy demoulding especially of parts requiring unscrewing device • Disadvantages – Only limited number of cavities accommodated University of Limerick Con Sheahan Advanced Manufacturing DM4038 76 Runner Design – Layout in Series • Advantages: – Space for more cavities than circular • Disadvantages – Unequal flow lengths – Uniform filling only possible with corrected channel diameters (e.g. by computer programs) University of Limerick Con Sheahan Advanced Manufacturing DM4038 Runner Design – Symmetrical Layout • Advantages: – Equal flow lengths to all cavities without gate correction • Disadvantages – Large runner volume – High scrap rates – Rapid cooling of melt » Remedy: Hot manifold or insulated runner University of Limerick Con Sheahan Advanced Manufacturing DM4038 77 Naturally Balanced Runner Systems University of Limerick Con Sheahan Advanced Manufacturing DM4038 Variations of naturally balanced runners in a single cavity mould. The result of the imbalance may cause problems with concentricity and/or core deflection. University of Limerick Con Sheahan Advanced Manufacturing DM4038 78 University of Limerick Con Sheahan Advanced Manufacturing DM4038 Meltflipper Technology University of Limerick Con Sheahan Advanced Manufacturing DM4038 79 8-cavity mould short shot from a 1944 Manual Plunger IMM. The right side of the mould contains MeltFlipper technology and the left side does not (Material = TPE). 1944 Model 1, Van Dorn manual molding machine University of Limerick Con Sheahan Advanced Manufacturing DM4038 Meltflipper Insert The Meltflipper insert rotates the melt, sending equal amounts of hot and cool material to each cavity. Photo courtesy of Beaumont Technologies Inc. University of Limerick Con Sheahan Advanced Manufacturing DM4038 80 Intra-cavity imbalance in 4-cavity automotive connector causing dimensional variations from cavity-tocavity in critical dimension features. The result will be a kitty-corner effect where every other cavity is dimensionally the same as indicated by Flow 1 and Flow 2 parts. University of Limerick Con Sheahan The warp is often mis-diagnosed as a cooling deficiency toward the centre of the mould. Advanced Manufacturing DM4038 Injecting Moulding Gate Design DM4038 University of Limerick Con Sheahan Advanced Manufacturing DM4038 81 University of Limerick Con Sheahan Advanced Manufacturing DM4038 – http://www.paralleldesign.com/moldability_101/ University of Limerick Con Sheahan Advanced Manufacturing DM4038 82 Functions of Gate • Connects runner to cavity • Controls flow of polymer (All cavities fill at same rate) • Additional heating of polymer due to mechanical work • Allows part to be easily separated from runner University of Limerick Con Sheahan Advanced Manufacturing DM4038 Fountain Effect Flow • Hot resin flow from the middle of the flow channel to the walls and cools University of Limerick Con Sheahan Advanced Manufacturing DM4038 83 Factors Affecting Gate Design • Moulded part – Geometry, Wall thickness, Direction of mechanical loading, Quality demands • Moulding material – Viscosity, shrinkage temperature, flow characteristics, fillers, • Generalities – Distortion, knit lines, Ease of de-moulding, ease of separation from moulded part, costs University of Limerick Con Sheahan Advanced Manufacturing DM4038 Knit Lines or Weld Lines • Occur where there are holes or hollow features in a mould and when multiple gates are used. • The primary cause of knit lines is the way the plastic flow re-joins after it goes around a metal core in the mold University of Limerick Con Sheahan Advanced Manufacturing DM4038 84 Gate Location Runner Gate Runner Gate Cavity Cavity Concentric gate Eccentric gate • Concentric gate prevents cold skin entering mould causing “Blush Marks”, and promotes jetting. • Blush marks – When cold polymer enters mould cavity and does not adhere to existing mould material. An abrupt transition between runner and gate traps cold polymer. • Eccentric position reduces machining to one mold and aligned to wall restricts jetting Con Sheahan University of Limerick Advanced Manufacturing DM4038 Gate Location Gate Runner Gate Runner Cavity Cavity Jetting and reforming Even filling (but may get blush marks) • Jetting occurs when mould fills along mid-side of cavity, better to fill along edge (eccentric). – Eccentric easier to machine on one side of cavity. • Directional properties influence polymer chains orientated along direction of gate giving higher tensile strength and crack resistance University of Limerick Con Sheahan Advanced Manufacturing DM4038 85 Gate Location University of Limerick Con Sheahan Advanced Manufacturing DM4038 Gate Location • Position the gate away from load-bearing areas. • Position the gate away from the thin section areas, or regions of sudden thickness change to avoid hesitation and sink marks University of Limerick Con Sheahan Advanced Manufacturing DM4038 86 Q4(b)(i) 2004 University of Limerick Con Sheahan Advanced Manufacturing DM4038 Jetting Jetting occurs when polymer melt is pushed at a high velocity through restrictive areas, such as the nozzle, runner, or gate, into open, thicker areas, without contacting with the mold wall. The buckled, snake-like jetting stream causes contact points to form between the folds of melt in the jet, creating small-scale "welds" University of Limerick Con Sheahan Advanced Manufacturing DM4038 87 Jetting Remedies Direct the melt against a metal surface. Use an overlap gate or a submarine gate as shown in the Figure below. University of Limerick Con Sheahan Advanced Manufacturing DM4038 Gate Design Guidelines • Polymer flows from thick to thin sections • Merging flow has equal flow lengths • Locate gate on hidden surface • Locate where it will not trap air • Locate where jetting avoided. • Note problem of sink marks on strengthening ribs when positioning gates • Take distortion into account University of Limerick Con Sheahan Advanced Manufacturing DM4038 88 Optimum gate size depends on: • Optimum gate size depends on: – Flow characteristics of the material – Wall section of moulding – Volume of material injected into impression – Temperature of melt – Temperature of mould • Gate thickness: – Is usually two-thirds the part thickness. – Controls packing time • Chose a larger gate for better appearance, low residual stress and better dimensional stability University of Limerick Con Sheahan Advanced Manufacturing DM4038 Runner Designs – 3 types 1. 2. 3. Runners that remain with the moulding and have to be cut off afterwards Runners that are automatically separated from the moulding and ejected separately Runners that are automatically separated from the moulding, but remain in the mould University of Limerick Con Sheahan Advanced Manufacturing DM4038 89 Gating Systems Type 1 1. Sprue 2. Edge 3. Disk 4. Ring Type 2 Type 3 1. Tunnel 1. Pinpoint (with (submarine) reversed sprue) 2. Pinpoint (in 3-plate 2. Runnerless mould) 3. Runner for stack moulds 4. Insulated runner 5. Hot manifold Con Sheahan University of Limerick Advanced Manufacturing DM4038 Gate Type – Sprue • Applications: – For temperature sensitive and high-viscous materials, high quality parts and those with heavy sections • Advantages – Results in high quality and exact dimensions • Disadvantages – Post operation for sprue removal – Visible gate mark University of Limerick Con Sheahan Advanced Manufacturing DM4038 90 Gate Type – Sprue Sprue always placed at thickest section of part University of Limerick • Small rectangular opening at channel • Low cost gate. Con Sheahan end of runner Advanced Manufacturing DM4038 Gate Type – Edge (Fan) • Applications: – For parts with large areas such as plates and strips and when reinforcements can’t flow though edge gate • Advantages – No knit lines – High quality and exact dimensions • Disadvantages – Post operation for gate removal • The cost of removing the part from the runner will affect part pricing and the runner itself may sometimes become a source of wasted material. University of Limerick Con Sheahan Advanced Manufacturing DM4038 91 Gate Type – Edge (Fan) Extended distributor channel Con Sheahan University of Limerick Advanced Manufacturing DM4038 Gate Type – Disk • Applications: – For axially symmetrical parts with core mounted at one side only • Advantages – No knit lines – No reduction in strength • Disadvantages – Post operation for gate removal University of Limerick Con Sheahan Advanced Manufacturing DM4038 92 Gate Type – Ring • Applications: – For sleeve-like parts with core mounted at both sides (hollow cylinder parts) • Advantages – Uniform wall thickness around circumference (minimise weld lines) • Disadvantages – Slight knit line – Post operation for gate removal • Covers the entire top of the cylindrical part so the resin flow is downward into the walls of the part University of Limerick Con Sheahan Advanced Manufacturing DM4038 Gate Type – Ring University of Limerick Con Sheahan Advanced Manufacturing DM4038 93 Gate Type – Tunnel • Applications: – Primarily for smaller parts in multicavity moulds and for elastic materials • Advantages – Automatic ejection gate removal at part • Disadvantages – For simpler parts only because of high pressure loss – Creates high shear. University of Limerick Con Sheahan Advanced Manufacturing DM4038 Gate Type – Tunnel University of Limerick Con Sheahan Advanced Manufacturing DM4038 94 Gate Type – Pinpoint (three plate mould) • Applications: – For multi-cavity moulds and centre gating • Advantages – Automatic gate removal • Disadvantages – Large amounts of scrap – High mould costs University of Limerick Con Sheahan Advanced Manufacturing DM4038 Gate Type – Pinpoint (with reversed sprue) • Applications: – For parts with automatic gate removal • Advantages – No post operation • Disadvantages – Preferably for thermally stable materials (PE, PS) – Limited use for others University of Limerick Con Sheahan Advanced Manufacturing DM4038 95 Three Plate Mould 1= Left plate 2= floating plate 3= stationary mold half A= undercut in core B= gate C= undercut D= runner E= drop F= parting line 1 G= parting line 2 University of Limerick Con Sheahan Advanced Manufacturing DM4038 University of Limerick Con Sheahan Advanced Manufacturing DM4038 96 Hot Runner System • The ideal injection molding system delivers molded parts of uniform density, and free from all runners, flash, and gate stubs. • To achieve this, a hot runner system, in contrast to a cold runner system, is employed. The material in the hot runners is maintained in a molten state and is not ejected with the molded part. Hot runner systems are also referred to as hot-manifold systems, or runnerless molding. FIGURE 1. Hot runner system types: (a) the insulated hot runner, (b) the internally heated hot-runner system, and (c) the externally heated hot-runner system University of Limerick Con Sheahan Advanced Manufacturing DM4038 Gate Type – Runnerless • Applications: – For thin walled parts and rapid sequence of cycles • Advantages – No loss of material for runner system • Disadvantages – Pronounced marks on part from nozzle Pinpoint gate Cold mould ⇒ low cycle time required University of Limerick Con Sheahan Advanced Manufacturing DM4038 97 Gate Type – Stack Moulds • Applications: – Flat and light weight parts – in multi cavity moulds • Advantages – Better utilization of machines plasticizing rate • Disadvantages – Large amount of scrap from voluminous runner system – Higher mould costs University of Limerick Con Sheahan Advanced Manufacturing DM4038 Gate Type – Insulated Runner Moulds • Applications: – For materials with large softening and melt temperature range and rapid sequence of cycles • Advantages – Automatic gate separation – Material loss from runner shutdown only after • Disadvantages – Danger of cold material getting into cavity after interruption University of Limerick Con Sheahan Advanced Manufacturing DM4038 98 Q4(a) 2002 University of Limerick Con Sheahan Advanced Manufacturing DM4038 Injecting Moulding Cavity Layout, Shrinkage, Distortion DM 4038 University of Limerick Con Sheahan Advanced Manufacturing DM4038 99 Cavities • The number of cavities depends on: – Available production time – Product quantity required – Machine shot size – Plasticizing capacities – Shape and size of moldings – Mold costs University of Limerick Con Sheahan Advanced Manufacturing DM4038 Mould Cavity Layout • Mould cavity forms the product while cooling • Cooling takes up to 80% of cycle time (~10secs) • All cavities must fill at same time or mouldings will have different properties – runners must be same length (see runner layout) • Mould cavities must not be too close, as cooling time may increase, injection pressures and ejector pins. • Mould forces must act through centre of platen • If one cavity is used then may need to use a threeplate mould University of Limerick Con Sheahan Advanced Manufacturing DM4038 100 No. of Cavities Technically Feasible Cavitity Layout Consideration of Part Dimensions Superposition of Clamping Area Input: n = 1 Input: Part Dimensions Input: Clamping Area Input: n = 2 Input: Part Dimensions University of Limerick Con Sheahan Input: Clamping Area Advanced Manufacturing DM4038 No. of Cavities Technically Feasible Cavitity Layout Consideration of Part Dimensions Superposition of Clamping Area Input: n = 3 Input: Part Dimensions Input: Clamping Area Mould and clamping unit are loaded unevenly if the cavities are loaded eccentrically University of Limerick Con Sheahan Advanced Manufacturing DM4038 101 Three Plate Mould Offsets sprue allows cavity to be centrally located Scrap content can be high in these designs. They are only used where cavity is too large to allow use of a dummy cavity Con Sheahan University of Limerick Advanced Manufacturing DM4038 Determine Centre of Area of Cavities Xm = ∑ ( Ai X i ) ∑ ( Ai ) ; Ym = Example: ∑ ( AY i i) ∑ ( Ai ) 60x60mm φ80mm 80mm Figure 3 80mm Y 80mm 60mm X University of Limerick Con Sheahan Advanced Manufacturing DM4038 102 Q4(b)(ii) 2003 Ans: X= 93.38mm Y=113.38mm Ans: Mould and clamping unit are loaded unevenly if the cavities are loaded eccentrically. This can cause wear and damage to the mould surface, the clamping mechanism and cause flash to form around the moulded part. University of Limerick Con Sheahan Advanced Manufacturing DM4038 Cavity Layout • Circular – Provides equal flow length , balances forces but has limited number of cavities • In series – Unequal flow lengths. More cavities possible than circular • Symmetrical – Provides equal flow lengths but large scrap volume. Only suitable for hot runner system • Difficult to have same conditions in all cavities • Cavity number (n) – related to number of parts required. Generally, one cavity per 10,000 parts required n= University of Limerick N 10,000 Con Sheahan Quantity required Advanced Manufacturing DM4038 103 Cycle time Cavity Layout Cavity number related to delivery date: n = N *t T Productio n time Cavity number related to clamping force: Clampin g Force n= Cross sectional area of part University of Limerick F A* P * f Factor of Safety Injection Pressure Con Sheahan Advanced Manufacturing DM4038 Cavity Layout Cavity number related to minimum shot capacity: 0.2Vs n= Vp Part + Runner mass Machine shot capacity Cavity number related to maximum shot capacity: n= University of Limerick 0.8Vs Vp Con Sheahan Advanced Manufacturing DM4038 104 Cavity Layout Cavity number related to plasticizing rate: n= University of Limerick t * Rp Plasticizing rate Vp Con Sheahan Advanced Manufacturing DM4038 Question 1 The injection-moulded component shown in the Figure below is required in batch quantities of 100,000 units. The process conditions are: Mass of part and = 10g Mass of runner system = 2g Maximum clamping force = 100tonnes Maximum injection pressure = 200MPa Factor of safety for clamping force = 20% Maximum shot capacity = 50g Calculate: Number of cavities based on clamping force Minimum number of cavities based on shot capacity Maximum number of cavities based on shot capacity Number of cavities required to minimise production time University of Limerick Con Sheahan Advanced Manufacturing DM4038 105 University of Limerick Con Sheahan Advanced Manufacturing DM4038 University of Limerick Con Sheahan Advanced Manufacturing DM4038 106 Q4(b)(ii)2003 University of Limerick Con Sheahan Advanced Manufacturing DM4038 Q4(b)(ii)2003 University of Limerick Con Sheahan Advanced Manufacturing DM4038 107 Shrinkage • While cooling from melting temperature to room temperatures, polymers shrink by about 1.5%. • 95% of this occurs in the mould while the remainder occurs a matter of weeks after manufacture • Gate position affects shrinkage as most polymers shrink across the direction of flow. • The addition of fillers reduces shrinkage in all directions • Shrinkage can affect assembly after manufacture University of Limerick Con Sheahan Advanced Manufacturing DM4038 Shrinkage (Sink Marks) University of Limerick Con Sheahan Advanced Manufacturing DM4038 108 Shrinkage – Definition • The difference between an arbitrary dimension in the cavity and the corresponding dimension in the moulding with reference to the cavity dimension l −l S = c m .100% lc University of Limerick Con Sheahan Advanced Manufacturing DM4038 Demoulding shrinkage Processing shrinkage Post shrinkage University of Limerick Con Sheahan Advanced Manufacturing DM4038 109 DFM for Injection Molding • Effects of shrinkage – Parts are designed with shrinkage included early in the design and before tool build » Shrink rates for common materials Material • Acetal • Acrylic • ABS • Nylon • PC • PE • PP • PS • PVC rigid • PVC flexible University of Limerick Max Shrinkage 2.5% 0.8% 0.8% 1.5% 0.7% 5.0% 2.5% 0.6% 0.5% 5.0% Con Sheahan Advanced Manufacturing DM4038 Distortion • Different cooling rates will cause residual stress and distortion (anisotropic shrinkage) – E.g.. A disk component will form a saucer shape if the centre cools first – Hot section in mould – Hot section in Part » Sink marks/voids. – Section thickness not constant » Use strengthening ribs » Offset sections University of Limerick Con Sheahan Advanced Manufacturing DM4038 110 Distortion – Disk Example University of Limerick Con Sheahan Advanced Manufacturing DM4038 Distortion University of Limerick Con Sheahan Advanced Manufacturing DM4038 111 Shrinkage / Distortion University of Limerick Con Sheahan Advanced Manufacturing DM4038 Q3(b) 2002 Q3(c) 2002 University of Limerick Con Sheahan Advanced Manufacturing DM4038 112 University of Limerick Con Sheahan Advanced Manufacturing DM4038 Boosting structural integrity with ribs • Problem: Structure must be stiff enough to withstand expected loads. Increasing thickness to achieve this is self-defeating, since it will: – Increase part weight and cost proportional to the increase in thickness. – Increase moulding cycle time – Increase the probability of sink marks. • However, well-designed ribs can overcome these disadvantages with only a marginal increase in part weight. • Typical uses for ribs – Covers, cabinets and body components with long, wide surfaces that must have good appearance with low weight – Rollers and guides for paper handling, where the surface must be cylindrical – Gear bodies, where the design calls for wide bearing surfaces on the center shaft and on the gear teeth – Frames and supports University of Limerick Con Sheahan Advanced Manufacturing DM4038 113 Ribs Design Rules • Keep part thickness as thin and uniform as possible – Will shorten cycle time, improve dimensional stability and eliminate surface defects – For greater stiffness, reduce spacing between ribs, enabling addition of more ribs • Rib geometry – Thickness, height and draft angle are related: » Excessive thickness produces sinks on opposing surface » Small thickness and large draft will lead to unacceptable filling – Ribs should be tapered (drafted) at one degree per side » Use smaller draft if steel in mould is carefully polished » Draft will increase rib thickness from the tip to the root, by ~0.175 mm /cm of rib height, for each degree of draft angle » The maximum recommended rib thickness at the root, is 0.8 times the thickness of the base to which it is attached » The typical root thickness ranges from 0.5 to 0.8 times the base thickness University of Limerick Con Sheahan Advanced Manufacturing DM4038 Recommended Rib Design Parameters. • See Figure 1 for recommended design parameters. University of Limerick Con Sheahan Advanced Manufacturing DM4038 114 Ribs Design Rules • Location of ribs, bosses, and gussets – Ribs aligned in the direction of the mold opening are the least expensive design option to tool. – Ribs are used in a design to increase the bending stiffness of a part without adding thickness. Ribs increase the moment of inertia, which increases the bending stiffness – Gussets can be used to support bosses that are away from the walls. The same design rules that apply for ribs also apply for gussets. University of Limerick Con Sheahan Advanced Manufacturing DM4038 Ribs Design Rules A boss should not be placed next to a parallel wall; instead, offset the boss and use gussets to strengthen it. Gussets can be used to support bosses that are away from the walls. The same design rules that apply for ribs also apply for gussets. University of Limerick Con Sheahan Advanced Manufacturing DM4038 115 Ribs Design Rules Alternative configurations As shown in Figure below, ribs can take the shape of corrugations. The advantage is that the wall thickness will be uniform and the draft angle can be placed on the opposite side of the mold, thereby avoiding the problem of the thinning rib tip. Honeycomb ribbing attached to a flat surface provides excellent resistance to bending in all directions. A hexagonal array of interconnected ribs will be more effective than a square array, with the same volume of material in the ribs. University of Limerick Con Sheahan Advanced Manufacturing DM4038 Q4(b)(ii) 2002 University of Limerick Con Sheahan Advanced Manufacturing DM4038 116 Mould Venting University of Limerick Con Sheahan Advanced Manufacturing DM4038 Mould Venting University of Limerick Con Sheahan Advanced Manufacturing DM4038 117 Injection Mould Design • Before an Injection Mold Design can be started the following criteria must be known, to enable a proper design. • Part Size; Function; Material type; Production required; Acceptable witness marks for parting line, method of ejection, and gate. • Machine Type; Tonnage; Platen Shot size; Locating Coolant Connector Type and Size Nozzle Spherical Radius, and • Processing Cycle time Possible types of Mouldbases University of Limerick Con Sheahan Ring 'O' hole size; size; Dimension Capacity No. of Cavities per No. of moulds to be ordered Study mold Advanced Manufacturing DM4038 A GENERAL APPROACH MAY FOLLOW THE FOLLOWING STEPS: INJECTION MOLD DESIGN REQUIRES SYNTHESIS OF MANY REQUIREMENTS AT THE SAME TIME • Start at part and work out • Determine gate location. CAE (Flow Analysis) is a superior tool to determine gate location • Determine an appropriate Parting Line Correct parting line location will help ensure proper ejection Also will ensure the part 'Sticks' to the ejection (core) side of the mold • Determine Runner Layout that suits no. of cavities and incorporate Cold Slug Wells • The next three items need to be 'Juggled": Ejection; Coolant lines; Cavity steel sizes. • Determine Cavity steel sizes (you should have an idea of cooling layout, at this point). Will coolant holes be needed in the cavity steels? • Determine Ejection method and location of ejector contact area • Determine Cooling Line location as per coolant line placement rules • Select a possible Moldbase (may change later) that suits the design requirements (Ejection travel, coolant connectors counterbored?, general size. • Determine the number and size of Support Pillars • Select a Sprue Bushing, and Locating Ring University of Limerick Con Sheahan Advanced Manufacturing DM4038 118 Steering wheel prototype mould trial University of Limerick Con Sheahan Advanced Manufacturing DM4038 University of Limerick Con Sheahan Advanced Manufacturing DM4038 119 Injecting Moulding Polymer Defects DM4038 University of Limerick Con Sheahan Advanced Manufacturing DM4038 Sinking and Voiding Possible cause • Unsuitable grade of material Suggested cure Ensure that gating is into thickest section of moulding Increase gate size Eliminate thick sections University of Limerick Con Sheahan Advanced Manufacturing DM4038 120 Sinking and Voiding Possible cause • Incorrect cooling arrangements Suggested cure Increase cooling time Lower mould temperature (sinking) Raise mould temperature (voiding) Con Sheahan University of Limerick Advanced Manufacturing DM4038 Sinking and Voiding Possible cause • Moulding conditions causing excessive localised shrinkage Suggested cure Increase feed setting Increase pressure hold-on time Increase injection pressure Lower temperatures Check that machine capacity is adequate University of Limerick Con Sheahan Advanced Manufacturing DM4038 121 Flashing Possible cause • Shut off faces of mould mismatched Suggested cure Reface and realign where necessary Ensure no foreign material on mould faces Check tool for deformation University of Limerick Con Sheahan Advanced Manufacturing DM4038 Flashing Possible cause • Mould opening up under pressure Suggested cure Check that projected area of moulding is within limits of machine locking capacity Increase clamp pressure if possible Decrease injection pressure Enlarge gates to enable filling pressure to be reduced Balance gates and runners where applicable Reduce injection speed University of Limerick Con Sheahan Advanced Manufacturing DM4038 122 Short Mouldings Possible cause • Inadequate shot capacity Suggested cure Increase injection pressure Increase mold temperature Increase injection speed University of Limerick Con Sheahan Advanced Manufacturing DM4038 Short Mouldings Possible cause • Material freezing off before complete mould filling Suggested cure Increase melt temperature Increase gate and runner sizes University of Limerick Con Sheahan Advanced Manufacturing DM4038 123 Short Mouldings Possible cause • Flow-back between screw and barrel Suggested cure Increase melt temperature Increase mold temperature Increase injection speed University of Limerick Con Sheahan Advanced Manufacturing DM4038 Crosspiece incompletely formed; scanning electron micrograph, University of Limerick Con Sheahan Advanced Manufacturing DM4038 124 Detail from previous University of Limerick Con Sheahan Advanced Manufacturing DM4038 Ejection Difficulties Possible cause • Insufficient cooling Suggested cure Increase cooling time Lower mould temperature Lower material temperature University of Limerick Con Sheahan Advanced Manufacturing DM4038 125 Ejection Difficulties Possible cause • Insufficient taper Suggested cure Increase as required Possible cause • Excess pressure in cavity Suggested cure Decrease injection pressure Decrease packing pressure and hold-on time University of Limerick Con Sheahan Advanced Manufacturing DM4038 Ejection Difficulties Possible cause • Poor surface finish on mould Suggested cure Clean and Polish mold Possible cause • Area / position of ejectors unsatisfactory Suggested cure Add larger or additional ejectors University of Limerick Con Sheahan Advanced Manufacturing DM4038 126 Ejection Difficulties Possible cause • Cores out of alignment Suggested cure Improve core alignment and / or alter gate position University of Limerick Con Sheahan Advanced Manufacturing DM4038 Air Entrapments Air bubble University of Limerick Con Sheahan Advanced Manufacturing DM4038 127 Air Entrapments Possible cause • During filling air is entrapped due to shape of moulding and if obstructed in the peripheral region near the surface, causes bubbles. Suggested cure Reduce screw decompression or decompress at reduced rate. (Especially if air entrapment is near gate) Optimize geometry with mould flow calculation Check design and condition of mould vents. University of Limerick Con Sheahan Advanced Manufacturing DM4038 Warping and Bowing Possible cause • Differential shrinkage caused by: a) High residual stress from flow conditions Suggested cure Decrease injection pressure Optimise pressure hold-on time Increase injection speed Raise material or mould temperature Use higher MFI grade Relocate gates University of Limerick Con Sheahan Advanced Manufacturing DM4038 128 Warping and Bowing Possible cause • Differential shrinkage caused by: b) High residual stress from unbalanced cooling Suggested cure Arrange water connections to give opposing temperature gradient to that of material e.g. connect water inlets to gate area and outlets from edges Rearrange cooling circuit layout to give correct gradient University of Limerick Con Sheahan Advanced Manufacturing DM4038 Warping and Bowing Possible cause • Differential shrinkage caused by: c) Wall thickness variations Suggested cure Redesign to give even wall thickness throughout If thickness variations are due to core deflection, improve core location and/or alter gate position University of Limerick Con Sheahan Advanced Manufacturing DM4038 129 Warping and Bowing Possible cause • Differential shrinkage caused by: d) Inadequate cooling Suggested cure Increase cooling time Lower mould temperature Lower material temperature University of Limerick Con Sheahan Advanced Manufacturing DM4038 Warping and Bowing Possible cause • Unsatisfactory ejection Suggested cure Check ejection system and correct as necessary Check manual handling University of Limerick Con Sheahan Advanced Manufacturing DM4038 130 Weld Line University of Limerick Con Sheahan Advanced Manufacturing DM4038 Poor Weld/Knit Lines Possible cause • Converging streams of materials not welding properly at interface Suggested cure Raise the temperature of the mold or molten plastic Increase the injection speed Move weld lines into more acceptable areas such as vented zones Use higher MFI grade where possible University of Limerick Con Sheahan Advanced Manufacturing DM4038 131 Poor Weld Lines Possible cause • Foreign material at interface of converging streams Suggested cure Ensure that pigments are well dispersed Possible contamination with other plastics in feed or barrel, moisture and oil. University of Limerick Con Sheahan Advanced Manufacturing DM4038 Scorch/Burn marks due to poor mould venting at the end of the flow path University of Limerick Con Sheahan Advanced Manufacturing DM4038 132 Burn Marks Possible cause • Air trapped in mould cavity Suggested cure Reduce injection speeds Optimize gas venting and degassing Reduce mold and melt temperatures Ensure correct gate location Check that core deflection is not resulting in air traps – if so correct and improve core location University of Limerick Con Sheahan Advanced Manufacturing DM4038 Delamination due to temperature differences University of Limerick Con Sheahan Advanced Manufacturing DM4038 133 Delamination Possible cause • Incompatible materials blended together Suggested cure Check for contamination e.g. from other plastics in hopper or cylinder Pre-dry the plastic properly before molding Purge the machine University of Limerick Con Sheahan Advanced Manufacturing DM4038 Delamination Possible cause • High shear stresses result in the formation of layers Suggested cure Raise material temperature Raise mould temperature Lower injection speed University of Limerick Con Sheahan Advanced Manufacturing DM4038 134 Streaks or Specks Colour streaks Moisture streaks University of Limerick Con Sheahan Advanced Manufacturing DM4038 Streaks or Specks Possible cause • Moisture content too high; (water vapour produced during melting) Suggested cure Use material adequately pre-dried to a residual moisture content of < 0.1 % Increase back pressure University of Limerick Con Sheahan Advanced Manufacturing DM4038 135 Streaks or Specks Possible cause • Poor pigment or master-batch dispersion • ‘Dead spots’ in plasticizing unit or hot runner system • Contamination Suggested cure Improve mixing procedure Increase screw backpressure Increase screw speed Increase temperature gradient in cylinder, with hopper end temperature lower Avoid dead spots Insure good cleaning of the plasticizing unit University of Limerick Con Sheahan Advanced Manufacturing DM4038 Black Streaks Black specks and black streaks are caused by overheated (degraded, burned) material or by contamination of the resin. University of Limerick Con Sheahan Advanced Manufacturing DM4038 136 Black Speckling \ Streaking Possible cause: • Processing using a screw which is too deeply flighted in feed section (air intake) • ‘Dead spots’ in the plasticizing unit or hot runner system • Defective non-return valve • Screw decompression is too great or too fast Suggested cures Raise temperature in feed section so that melting occurs earlier; use more suitable screw Check plasticizing unit and hot runners for impeded flow and correct Replace defective non-return valve Shorten path for screw decompression or decompress at a reduced rate University of Limerick Con Sheahan Advanced Manufacturing DM4038 Colour streaks due to deposits of material due to “dead spots” in a hot runner system University of Limerick Con Sheahan Advanced Manufacturing DM4038 137 Shrinkage Differs from Planned % Possible cause: If shrinkage is too low ... Suggested cure Raise mould temperature Raise material temperature if gate is unrestricted and low pressure can be used Decrease injection pressure Decrease pressure 'hold-on‘ time University of Limerick Con Sheahan Advanced Manufacturing DM4038 Shrinkage Differs from Planned % Possible cause: If shrinkage is too high ... Suggested cure Lower mould temperature Lower material temperature if gate is unrestricted Raise material temperature if gate is restricted Increase injection pressure Increase pressure 'hold-on' time Increase gate size University of Limerick Con Sheahan Advanced Manufacturing DM4038 138 Shrinkage Differs from Planned % Possible cause: Excessive difference between shrinkage in line with and across direction of flow Suggested cure Decrease injection pressure and raise material temperature Ensure correct mould temperature and cooling arrangements Increase number of gates University of Limerick Con Sheahan Advanced Manufacturing DM4038 Blisters Blisters can be defined as raised defects on the surface of a molded part caused by trapped gases in the part that could not escape before the surface began to ``skin'' during the molding process. University of Limerick Con Sheahan Advanced Manufacturing DM4038 139 Polymer Defects Blistering caused by: • Machine – back pressure too low • Mould – temperature too low • Material – too coarse Suggested cure: Optimize back pressure Increase mould temperature Use smaller particle sizes Con Sheahan University of Limerick Advanced Manufacturing DM4038 Polymer Defects • Blushing • Crazing / Cracking Cracking Blushing University of Limerick Con Sheahan Advanced Manufacturing DM4038 140 Polymer Defects • Blushing caused by: Machine – injection speed too fast Mould – temperature too low Material – Excessive moisture • Crazing / Cracking Machine – moulded in stresses Mould – insufficient draft or polish Con Sheahan University of Limerick Advanced Manufacturing DM4038 Polymer Defects • Flow lines • Jetting Flow-line University of Limerick Jetting Con Sheahan Advanced Manufacturing DM4038 141 Polymer Defects • Flow lines Machine – inadequate injection pressure Mould – temperature too low / cold skin Material – poor flow rate • Jetting Machine – excessive injection speed Mould – improper gate design location University of Limerick Con Sheahan Advanced Manufacturing DM4038 Jetting due to unsuitable choice of gate location and design University of Limerick Con Sheahan Advanced Manufacturing DM4038 142 Q4(b)(ii) 2002 University of Limerick Con Sheahan Advanced Manufacturing DM4038 Q3(c) 2003 University of Limerick Con Sheahan Advanced Manufacturing DM4038 143 Electrical Discharge Machining (EDM) University of Limerick Con Sheahan Advanced Manufacturing DM4038 1 Introduction Sometimes it is referred to as spark machining, spark eroding, burning, die sinking or wire erosion Its a manufacturing process whereby a desired shape is obtained using electrical discharges (sparks). Material is removed from the workpiece by a series of rapidly recurring current discharges between two electrodes, separated by a dielectric liquid and subject to an electric voltage. One of the electrodes – ‘tool-electrode’ or ‘tool’ or ‘electrode’. Other electrode - workpiece-electrode or ‘workpiece’. As distance between the two electrodes is reduced, the current intensity becomes greater than the strength of the dielectric (at least in some points) causing it to break. University of Limerick Con Sheahan Advanced Manufacturing DM4038 2 1 History This allows current to flow between the two electrodes. This phenomenon is the same as the breakdown of a capacitor. As a result, material is removed from both the electrodes. Once the current flow stops, new liquid dielectric is usually conveyed into the electrode zone enabling the solid particles (debris) to be carried away. Adding new liquid dielectric in the electrode volume is commonly referred to as flushing. Also, after a current flow, a difference of potential between the two electrodes is restored to what it was before the breakdown, so that a new liquid dielectric breakdown can occur. University of Limerick Con Sheahan Advanced Manufacturing DM4038 3 History In 1770, English Physicist Joseph Priestley studied the erosive effect of electrical discharges. Furthering Priestley's research, the EDM process was invented by two Russian scientists, Dr. B.R. Lazarenko and Dr. N.I. Lazarenko in 1943. In their efforts to exploit the destructive effects of an electrical discharge, they developed a controlled process for machining of metals. Their initial process used a spark machining process, named after the succession of sparks (electrical discharges) that took place between two electrical conductors immersed in a dielectric fluid. The discharge generator effect used by this machine, known as the Lazarenko Circuit, was used for many years in the construction of generators for electrical discharge. University of Limerick Con Sheahan Advanced Manufacturing DM4038 4 2 History New researchers entered the field and contributed many fundamental characteristics of the machining method we know today. In 1952, the manufacturer Charmilles created the first machine using the spark machining process and was presented for the first time at the European Machine Tool Exhibition in 1955. In 1969, Agie launched the world's first numerically controlled wire-cut EDM machine. Seibu developed the first CNC wire EDM machine in 1972 and the first system was manufactured in Japan. Recently, the machining speed has gone up by 20 times. This has decreased machining costs by at least 30 percent and improved the surface finish by a factor of 1.5 University of Limerick Con Sheahan Advanced Manufacturing DM4038 5 General Aspects of EDM EDM is a machining method primarily used for hard metals or those that would be very difficult to machine with traditional techniques. EDM typically works with materials that are electrically conductive, although methods for machining insulating ceramics with EDM have been proposed. EDM can cut intricate contours or cavities in hardened steel without the need for heat treatment to soften and re-harden them. This method can be used with any other metal or metal alloy such as titanium, hastelloy, kovar, and inconel. Also, applications of this process to shape polycrystalline diamond tools have been reported. University of Limerick Con Sheahan Advanced Manufacturing DM4038 6 3 EDM - System University of Limerick Con Sheahan Advanced Manufacturing DM4038 7 EDM - Components University of Limerick Con Sheahan Advanced Manufacturing DM4038 8 4 EDM - Components The main components in EDM: Electric power supply Dielectric medium Work piece & tool Servo control unit. The work piece and tool are electrically connected to a DC power supply. The current density in the discharge of the channel is of the order of 10000 A/cm2 and power density is nearly 500 MW/cm2. A gap, known as SPARK GAP in the range, from 0.005 mm to 0.05 mm is maintained between the work piece and the tool. Dielectric slurry is forced through this gap at a pressure of 2 kgf/cm2 or lesser. University of Limerick Con Sheahan Advanced Manufacturing DM4038 9 EDM – Working Principle It is a process of metal removal based on the principle of material removal by an interrupted electric spark discharge between the electrode tool and the work piece. In EDM, a potential difference is applied between the tool and workpiece. Essential - Both tool and work material are to be conductors. The tool and work material are immersed in a dielectric medium. Generally kerosene or deionised water is used as the dielectric medium. A gap is maintained between the tool and the workpiece. Depending upon the applied potential difference (50 to 450 V) and the gap between the tool and workpiece, an electric field would be established. Generally the tool is connected to the negative terminal (cathode) of the generator and the workpiece is connected to positive terminal (anode). University of Limerick Con Sheahan Advanced Manufacturing DM4038 10 5 EDM – Working Principle As the electric field is established between the tool and the job, the free electrons on the tool are subjected to electrostatic forces. If the bonding energy of the electrons is less, electrons would be emitted from the tool. Such emission of electrons are called or termed as ‘cold emission’. The “cold emitted” electrons are then accelerated towards the job through the dielectric medium. As they gain velocity and energy, and start moving towards the job, there would be collisions between the electrons and dielectric molecules. Such collision may result in ionization of the dielectric molecule. Ionization depends on the ionization energy of the dielectric molecule and the energy of the electron. University of Limerick Con Sheahan Advanced Manufacturing DM4038 11 EDM – Working Principle As the electrons get accelerated, more positive ions and electrons would get generated due to collisions. This cyclic process would increase the concentration of electrons and ions in the dielectric medium between the tool and the job at the spark gap. The concentration would be so high that the matter existing in that channel could be characterised as “plasma”. The electrical resistance of such plasma channel would be very less. Thus all of a sudden, a large number of electrons will flow from tool to job and ions from job to tool. This is called avalanche motion of electrons. Such movement of electrons and ions can be visually seen as a spark. Thus the electrical energy is dissipated as the thermal energy of the spark. University of Limerick Con Sheahan Advanced Manufacturing DM4038 12 6 EDM – Working Principle The high speed electrons then impinge on the job and ions on the tool. The kinetic energy of the electrons and ions on impact with the surface of the job and tool respectively would be converted into thermal energy or heat flux. Such intense localized heat flux leads to extreme instantaneous confined rise in temperature which would be in excess of 10,000oC. Such localized extreme rise in temperature leads to material removal. Material removal occurs due to instant vaporization of the material as well as due to melting. The molten metal is not removed completely but only partially. University of Limerick Con Sheahan Advanced Manufacturing DM4038 13 EDM – Working Principle Upon withdrawal of potential difference, plasma channel collapses. This ultimately creates compression shock waves on both the electrode surface. Particularly at high spots on work piece surface, which are closest to the tool. This evacuates molten material and forms a crater around the site of the spark. The whole sequence of operation occurs within a few microseconds. University of Limerick Con Sheahan Advanced Manufacturing DM4038 14 7 EDM – Schematic University of Limerick Con Sheahan Advanced Manufacturing DM4038 15 EDM – Working Principle Thus to summarise, the material removal in EDM mainly occurs due to formation of shock waves as the plasma channel collapse owing to discontinuation of applied potential difference Generally the workpiece is made positive and the tool negative. Hence, the electrons strike the job leading to crater formation due to high temperature and melting and material removal. Similarly, the positive ions impinge on the tool leading to tool wear. In EDM, the generator is used to apply voltage pulses between the tool and job. A constant voltage is not applied. Only sparking is desired rather than arcing. Arcing leads to localized material removal at a particular point whereas sparks get distributed all over the tool surface leading to uniform material removal. University of Limerick Con Sheahan Advanced Manufacturing DM4038 16 8 EDM – Working Principle University of Limerick Con Sheahan Advanced Manufacturing DM4038 17 EDM – Power & Control Circuits Two broad categories of generators (power supplies) are in use on EDM. Commercially available: RC circuits based and transistor controlled pulses. In the first category, the main parameters to choose from at setup time are the resistance(s) of the resistor(s) and the capacitance(s) of the capacitor(s). In an ideal condition, these quantities would affect the maximum current delivered in a discharge. Current delivery in a discharge is associated with the charge accumulated on the capacitors at a certain moment. Little control is expected over the time of discharge, which is likely to depend on the actual spark-gap conditions. Advantage: RC circuit generator can allow the use of short discharge time more easily than the pulse-controlled generator. University of Limerick Con Sheahan Advanced Manufacturing DM4038 18 9 EDM – Power & Control Circuits Also, the open circuit voltage (i.e. voltage between electrodes when dielectric is not broken) can be identified as steady state voltage of the RC circuit. In generators based on transistor control, the user is usually able to deliver a train of voltage pulses to the electrodes. Each pulse can be controlled in shape, for instance, quasi-rectangular. In particular, the time between two consecutive pulses and the duration of each pulse can be set. The amplitude of each pulse constitutes the open circuit voltage. Thus, maximum duration of discharge is equal to duration of a voltage pulse. Maximum current during a discharge that the generator delivers can also be controlled. Con Sheahan University of Limerick Advanced Manufacturing DM4038 19 EDM – Power & Control Circuits Details of generators and control systems on EDMs are not always easily available to their user. This is a barrier to describing the technological parameters of EDM process. Moreover, the parameters affecting the phenomena occurring between tool and electrode are also related to the motion controller of the electrodes. A framework to define and measure the electrical parameters during an EDM operation directly on inter-electrode volume with an oscilloscope external to the machine has been recently proposed by Ferri et al. This would enable the user to estimate directly the electrical parameter that affect their operations without relying upon machine manufacturer's claims. When machining different materials in the same setup conditions, the actual electrical parameters are significantly different. University of Limerick Con Sheahan Advanced Manufacturing DM4038 20 10 EDM – Power & Control Circuits When using RC generators, the voltage pulses, shown in Fig. are responsible for material removal. A series of voltage pulses (Fig.) of magnitude about 20 to 120 V and frequency on the order of 5 kHz is applied between the two electrodes. University of Limerick Con Sheahan Advanced Manufacturing DM4038 21 EDM – Power & Control Circuits University of Limerick Con Sheahan Advanced Manufacturing DM4038 22 11 EDM – Power & Control Circuits University of Limerick Con Sheahan Advanced Manufacturing DM4038 23 EDM – Power & Control Circuits University of Limerick Con Sheahan Advanced Manufacturing DM4038 24 12 EDM – Electrode Material Electrode material should be such that it would not undergo much tool wear when it is impinged by positive ions. Thus the localised temperature rise has to be less by properly choosing its properties or even when temperature increases, there would be less melting. Further, the tool should be easily workable as intricate shaped geometric features are machined in EDM. Thus the basic characteristics of electrode materials are: High electrical conductivity – electrons are cold emitted more easily and there is less bulk electrical heating High thermal conductivity – for the same heat load, the local temperature rise would be less due to faster heat conducted to the bulk of the tool and thus less tool wear. University of Limerick Con Sheahan Advanced Manufacturing DM4038 25 EDM – Electrode Material Higher density – for less tool wear and thus less dimensional loss or inaccuracy of tool High melting point – high melting point leads to less tool wear due to less tool material melting for the same heat load Easy manufacturability Cost – cheap The followings are the different electrode materials which are used commonly in the industry: Graphite Electrolytic oxygen free copper Tellurium copper – 99% Cu + 0.5% tellurium Brass University of Limerick Con Sheahan Advanced Manufacturing DM4038 26 13 EDM – Electrode Material Graphite (most common) - has fair wear characteristics, easily machinable. Small flush holes can be drilled into graphite electrodes. Copper has good EDM wear and better conductivity. It is generally used for better finishes in the range of Ra = 0.5 μm. Copper tungsten and silver tungsten are used for making deep slots under poor flushing conditions especially in tungsten carbides. It offers high machining rates as well as low electrode wear. Copper graphite is good for cross-sectional electrodes. It has better electrical conductivity than graphite while the corner wear is higher. Brass ensures stable sparking conditions and is normally used for specialized applications such as drilling of small holes where the high electrode wear is acceptable. University of Limerick Con Sheahan Advanced Manufacturing DM4038 27 EDM – Electrode Movement In addition to the servo-controlled feed, the tool electrode may have an additional rotary or orbiting motion. Electrode rotation helps to solve the flushing difficulty encountered when machining small holes with EDM. In addition to the increase in cutting speed, the quality of the hole produced is superior to that obtained using a stationary electrode. Electrode orbiting produces cavities having the shape of the electrode. The size of the electrode and the radius of the orbit (2.54 mm maximum) determine the size of the cavities. Electrode orbiting improves flushing by creating a pumping effect of the dielectric liquid through the gap. University of Limerick Con Sheahan Advanced Manufacturing DM4038 28 14 EDM – Electrode Wear University of Limerick Con Sheahan Advanced Manufacturing DM4038 29 EDM – Electrode Wear The melting point is the most important factor in determining the tool wear. Electrode wear ratios are expressed as end wear, side wear, corner wear, and volume wear. “No wear EDM” - when the electrode-to-workpiece wear ratio is 1 % or less. Electrode wear depends on a number of factors associated with the EDM, like voltage, current, electrode material, and polarity. The change in shape of the tool electrode due to the electrode wear causes defects in the workpiece shape. Electrode wear has even more pronounced effects when it comes to micromachining applications. The corner wear ratio depends on the type of electrode. The low melting point of aluminum is associated with the highest wear ratio. University of Limerick Con Sheahan Advanced Manufacturing DM4038 30 15 EDM – Electrode Wear University of Limerick Con Sheahan Advanced Manufacturing DM4038 31 EDM – Electrode Wear Graphite has shown a low tendency to wear and has the possibility of being molded or machined into complicated electrode shapes. The wear rate of the electrode tool material (Wt) and the wear ratio (Rw) are given by Kalpakjian (1997). University of Limerick Con Sheahan Advanced Manufacturing DM4038 32 16 EDM – Dielectric In EDM, material removal mainly occurs due to thermal evaporation and melting. As thermal processing is required to be carried out in absence of oxygen so that the process can be controlled and oxidation avoided. Oxidation often leads to poor surface conductivity (electrical) of the workpiece hindering further machining. Hence, dielectric fluid should provide an oxygen free machining environment. Further it should have enough strong dielectric resistance so that it does not breakdown electrically too easily. But at the same time, it should ionize when electrons collide with its molecule. Moreover, during sparking it should be thermally resistant as well. Generally kerosene and deionised water is used as dielectric fluid in EDM. University of Limerick Con Sheahan Advanced Manufacturing DM4038 33 EDM – Dielectric Tap water cannot be used as it ionises too early and thus breakdown due to presence of salts as impurities occur. Dielectric medium is generally flushed around the spark zone. It is also applied through the tool to achieve efficient removal of molten material. Three important functions of a dielectric medium in EDM: 1. Insulates the gap between the tool and work, thus preventing a spark to form until the gap voltage are correct. 2. Cools the electrode, workpiece and solidifies the molten metal particles. 3. Flushes the metal particles out of the working gap to maintain ideal cutting conditions, increase metal removal rate. It must be filtered and circulated at constant pressure. University of Limerick Con Sheahan Advanced Manufacturing DM4038 34 17 EDM – Dielectric The main requirements of the EDM dielectric fluids are adequate viscosity, high flash point, good oxidation stability, minimum odor, low cost, and good electrical discharge efficiency. For most EDM operations kerosene is used with certain additives that prevent gas bubbles and de-odoring. Silicon fluids and a mixture of these fluids with petroleum oils have given excellent results. Other dielectric fluids with a varying degree of success include aqueous solutions of ethylene glycol, water in emulsions, and distilled water. University of Limerick Con Sheahan Advanced Manufacturing DM4038 35 EDM – Flushing One of the important factors in a successful EDM operation is the removal of debris (chips) from the working gap. Flushing these particles out of the working gap is very important, to prevent them from forming bridges that cause short circuits. EDMs have a built-in power adaptive control system that increases the pulse spacing as soon as this happens and reduces or shuts off the power supply. Flushing – process of introducing clean filtered dielectric fluid into spark gap. If flushing is applied incorrectly, it can result in erratic cutting and poor machining conditions. Flushing of dielectric plays a major role in the maintenance of stable machining and the achievement of close tolerance and high surface quality. Inadequate flushing can result in arcing, decreased electrode life, and increased production time. University of Limerick Con Sheahan Advanced Manufacturing DM4038 36 18 EDM – Flushing Four methods: 1. Normal flow 2. Reverse flow 3. Jet flushing 4. Immersion flushing Con Sheahan University of Limerick Advanced Manufacturing DM4038 37 EDM – Flushing Normal flow (Majority) Dielectric is introduced, under pressure, through one or more passages in the tool and is forced to flow through the gap between tool and work. Flushing holes are generally placed in areas where the cuts are deepest. Normal flow is sometimes undesirable because it produces a tapered opening in the workpiece. Reverse flow Particularly useful in machining deep cavity dies, where the taper produced using the normal flow mode can be reduced. The gap is submerged in filtered dielectric, and instead of pressure being applied at the source a vacuum is used. With clean fluid flowing between the workpiece and the tool, there is no side sparking and, therefore, no taper is produced. University of Limerick Con Sheahan Advanced Manufacturing DM4038 38 19 EDM – Flushing Jet flushing In many instances, the desired machining can be achieved by using a spray or jet of fluid directed against the machining gap. Machining time is always longer with jet flushing than with the normal and reverse flow modes. Immersion flushing For many shallow cuts or perforations of thin sections, simple immersion of the discharge gap is sufficient. Cooling and debris removal can be enhanced during immersion cutting by providing relative motion between the tool and workpiece. Vibration or cycle interruption comprises periodic reciprocation of the tool relative to the workpiece to effect a pumping action of the dielectric. University of Limerick Con Sheahan Advanced Manufacturing DM4038 39 EDM – Flushing Synchronized, pulsed flushing is also available on some machines. With this method, flushing occurs only during the non-machining time as the electrode is retracted slightly to enlarge the gap. Increased electrode life has been reported with this system. Innovative techniques such as ultrasonic vibrations coupled with mechanical pulse EDM, jet flushing with sweeping nozzles, and electrode pulsing are investigated by Masuzawa (1990). University of Limerick Con Sheahan Advanced Manufacturing DM4038 40 20 EDM – Flushing For proper flushing conditions, Metals Handbook (1989) recommends: 1. Flushing through the tool is more preferred than side flushing. 2. Many small flushing holes are better than a few large ones. 3. Steady dielectric flow on the entire workpiece-electrode interface is desirable. 4. Dead spots created by pressure flushing, from opposite sides of the workpiece, should be avoided. 5. A vent hole should be provided for any upwardly concave part of the tool-electrode to prevent accumulation of explosive gases. 6. A flush box is useful if there is a hole in the cavity. University of Limerick Con Sheahan Advanced Manufacturing DM4038 41 EDM – Process Parameters The waveform is characterized by the: The open circuit voltage – Vo The working voltage – Vw The maximum current – Io The pulse on time – the duration for which the voltage pulse is applied ton The pulse off time – toff The gap between the workpiece and the tool – spark gap - δ The polarity – straight polarity – tool (-ve) The dielectric medium External flushing through the spark gap. University of Limerick Con Sheahan Advanced Manufacturing DM4038 42 21 EDM – Process Parameters The process parameters - mainly related to the waveform characteristics. University of Limerick Con Sheahan Advanced Manufacturing DM4038 43 EDM – Types – Sinker EDM Sinker EDM, also called cavity type EDM or volume EDM. Consists of an electrode and workpiece submerged in an insulating liquid such as oil or other dielectric fluids. The electrode and workpiece are connected to a suitable power supply. The power supply generates an electrical potential between the two parts. As the electrode approaches the workpiece, dielectric breakdown occurs in the fluid, forming a plasma channel, and a small spark jumps. These sparks happen in huge numbers at seemingly random locations. As the base metal is eroded, and the spark gap subsequently increased, the electrode is lowered automatically so that the process can continue. Several hundred thousand sparks occur per second, with the actual duty cycle carefully controlled by the setup parameters. These controlling cycles are sometimes known as "on time" and "off time“. University of Limerick Con Sheahan Advanced Manufacturing DM4038 44 22 EDM – Types – Sinker EDM The on time setting determines the length or duration of the spark. Hence, a longer on time produces a deeper cavity for that spark and all subsequent sparks for that cycle. This creates rougher finish on the workpiece. The reverse is true for a shorter on time. Off time is the period of time that one spark is replaced by another. A longer off time, for example, allows the flushing of dielectric fluid through a nozzle to clean out the eroded debris, thereby avoiding a short circuit. These settings can be maintained in micro seconds. The typical part geometry is a complex 3D shape, often with small or odd shaped angles. University of Limerick Con Sheahan Advanced Manufacturing DM4038 45 EDM – Types – Wire EDM (WEDM) Also known as wire-cut EDM and wire cutting. A thin single-strand metal wire (usually brass) is fed through the workpiece submerged in a tank of dielectric fluid (typically deionized water). Used to cut plates as thick as 300 mm and to make punches, tools, and dies from hard metals that are difficult to machine with other methods. Uses water as its dielectric fluid; its resistivity and other electrical properties are controlled with filters and de-ionizer units. The water flushes the cut debris away from the cutting zone. Flushing is an important factor in determining the maximum feed rate for a given material thickness. Commonly used when low residual stresses are desired, because it does not require high cutting forces for material removal. University of Limerick Con Sheahan Advanced Manufacturing DM4038 46 23 EDM – Material Removal Rate Con Sheahan University of Limerick Advanced Manufacturing DM4038 47 EDM – Material Removal Rate In EDM, the metal is removed from both workpiece and tool electrode. MRR depends not only on the workpiece material but on the material of the tool electrode and the machining variables such as pulse conditions, electrode polarity, and the machining medium. In this regard a material of low melting point has a high metal removal rate and hence a rougher surface. Typical removal rates range from 0.1 to 400 mm3 /min. MRR or volumetric removal rate (VRR), in mm3/min, was described by Kalpakjian (1997): where I - EDM current (A) Tw - Melting point of the workpiece (°C). University of Limerick Con Sheahan Advanced Manufacturing DM4038 48 24 EDM – Material Removal Rate Effect of pulse current (energy) on MRR & surface roughness. University of Limerick Con Sheahan Advanced Manufacturing DM4038 49 EDM – Material Removal Rate Effect of pulse on-time (energy) on MRR & surface roughness. University of Limerick Con Sheahan Advanced Manufacturing DM4038 50 25 EDM – Surface Integrity Surface consists of a multitude of overlapping craters that are formed by the action of microsecond-duration spark discharges. Crater size depends on physical and mechanical properties of the material composition of the machining medium discharge energy and duration. Integral effect of thousands of discharges per second leads to machining with a specified accuracy and surface finish. Depth of craters - the peak to valley (maximum) of surface roughness Rt. Maximum depth of damaged layer can be taken as 2.5 times of roughness Ra. According to Delpreti (1977) and Motoki and Lee (1968), the maximum peak to valley height, Rt, was considered to be 10 times Ra. University of Limerick Con Sheahan Advanced Manufacturing DM4038 51 EDM – Surface Integrity Average roughness can be expressed in terms of pulse current ip (A) and pulse duration tp (μs) by Surface roughness increases linearly with an increase in MRR. Jeswani (1978) - Graphite electrodes produce rougher surfaces than metal ones. Kuneida and Furuoya (1991) claimed that the introduction of oxygen into discharge gap provides extra power by the reaction of oxygen. This in turn increased workpiece melting and created greater expulsive forces that increased MRR and surface roughness. Choice of correct dielectric flow has a significant effect in reducing surface roughness by 50 %, increasing the machining rate, and lowering the thermal effects in the workpiece surface. Dielectrics having low viscosity are recommended for smooth surfaces. University of Limerick Con Sheahan Advanced Manufacturing DM4038 52 26 EDM – Surface Integrity Metallurgical changes occur in the surface – Temperature 8000 to 12,000°C. Additionally, a thin recast layer of 1 μm to 25 μm – depending on power used. Delpretti (1977) and Levy and Maggi (1990) claimed that the heat-affected zone (HAZ) adjacent to the resolidified layer reaches 25 μm. Some annealing can be expected in a zone just below the machined surface. Not all the workpiece melted by discharge is expelled into the dielectric. Remaining melted material is quickly chilled, primarily by heat conduction into the bulk of the workpiece, resulting in an exceedingly hard surface. Depth of annealed layer is proportional to power used. It ranges from 50 μm for finish cutting to ~ 200 μm for high MRR. Annealing is usually about two points of hardness below the parent metal for finish cutting. University of Limerick Con Sheahan Advanced Manufacturing DM4038 53 EDM – Surface Integrity In roughing cuts, the annealing effect is ~ five points of hardness below the parent metal. Electrodes that produce more stable machining can reduce the annealing effect. A finish cut removes the annealed material left by the previous rough cut. The altered surface layer significantly lowers the fatigue strength of alloys. It consists of a recast layer with or without microcracks, some of which may extend into the base metal, plus metallurgical alterations such as rehardened and tempered layers, heat-affected zones, and inter-granular precipitates. During EDM roughing, the layer showing microstructural changes, including a melted and resolidified layer, is less than 0.127 mm deep. During EDM finishing, it is less than 0.075 mm. Post-treatment to restore the fatigue strength is recommended to follow EDM of critical or highly stressed surfaces. University of Limerick Con Sheahan Advanced Manufacturing DM4038 54 27 EDM – Surface Integrity There are several effective processes that accomplish restoration or even enhancement of the fatigue properties. These methods include Removal of the altered layers by low-stress grinding or chemical machining Addition of a metallurgical-type coating Re heat-treatment Application of shot peening. University of Limerick Con Sheahan Advanced Manufacturing DM4038 55 EDM – Characteristics Can be used to machine any work material if it is electrically conductive. MRR depends on thermal properties (job) rather than its strength, hardness etc. The volume of the material removed per spark discharge is typically in the range of (1/1,000,000) to (1/10,000) mm3. In EDM, geometry of tool - positive impression of hole or geometric feature. Tool wear once again depends on the thermal properties of tool material. Local temperature rise is rather high, but there is not enough heat diffusion (very small pulse on time) and thus HAZ is limited to 2 – 4 μm. Rapid heating and cooling leads to surface hardening which may be desirable in some applications. Tolerance value of + 0.05 mm could be easily achieved by EDM. Best surface finish that can be economically achieved on steel is 0.40 m. University of Limerick Con Sheahan Advanced Manufacturing DM4038 56 28 Applications Drilling of micro-holes, thread cutting, helical profile milling, rotary forming, and curved hole drilling. Delicate work piece like copper parts can be produced by EDM. Can be applied to all electrically conducting metals and alloys irrespective of their melting points, hardness, toughness, or brittleness. Other applications: deep, small-dia holes using tungsten wire as tool, narrow slots, cooling holes in super alloy turbine blades, and various intricate shapes. EDM can be economically employed for extremely hardened work piece. Since there is no mechanical stress present (no physical contact), fragile and slender work places can be machined without distortion. Hard and corrosion resistant surfaces, essentially needed for die making, can be developed. University of Limerick Con Sheahan Advanced Manufacturing DM4038 57 Applications – EDM Drilling Uses a tubular tool electrode where the dielectric is flushed. When solid rods are used; dielectric is fed to the machining zone by either suction or injection through pre-drilled holes. Irregular, tapered, curved, as well as inclined holes can be produced by EDM. Creating cooling channels in turbine blades made of hard alloys is a typical application of EDM drilling. Use of NC system enabled large numbers of holes to be accurately located. University of Limerick Con Sheahan Advanced Manufacturing DM4038 58 29 Applications – EDM Sawing An EDM variation - Employs either a special steel band or disc. Cuts at a rate that is twice that of the conventional abrasive sawing method. Cutting of billets and bars - has a smaller kerf & free from burrs. Fine finish of 6.3 to 10 μm with a recast layer of 0.025 to 0.130 mm University of Limerick Con Sheahan Advanced Manufacturing DM4038 59 Applications - Machining of spheres Shichun and coworkers (1995) used simple tubular electrodes in EDM machining of spheres, to a dimensional accuracy of ±1 μm and Ra < 0.1 μm. Rotary EDM is used for machining of spherical shapes in conducting ceramics using the tool and workpiece arrangement as shown below. University of Limerick Con Sheahan Advanced Manufacturing DM4038 60 30 Applications - Machining of dies & molds EDM milling uses standard cylindrical electrodes. Simple-shaped electrode (Fig. 1) is rotated at high speeds and follows specified paths in the workpiece like the conventional end mills. Very useful and makes EDM very versatile like mechanical milling process. Solves the problem of manufacturing accurate and complex-shaped electrodes for die sinking (Fig. 2) of three-dimensional cavities. (Fig. 1) University of Limerick (Fig. 2) Con Sheahan Advanced Manufacturing DM4038 61 Applications - Machining of dies & molds EDM milling enhances dielectric flushing due to high-speed electrode rotation. Electrode wear can be optimized due to its rotational and contouring motions. Main limitation in EDM milling - Complex shapes with sharp corners cannot be machined because of the rotating tool electrode. EDM milling replaces conventional die making that requires variety of machines such as milling, wire cutting, and EDM die sinking machines. University of Limerick Con Sheahan Advanced Manufacturing DM4038 62 31 Applications – Wire EDM Special form of EDM - uses a continuously moving conductive wire electrode. Material removal occurs as a result of spark erosion as the wire electrode is fed, from a fresh wire spool, through the workpiece. Horizontal movement of the worktable (CNC) determines the path of the cut. Application - Machining of superhard materials like polycrystalline diamond (PCD) and cubic boron nitride (CBN) blanks, and other composites. Carbon fiber composites are widely used in aerospace, nuclear, automobile, and chemical industries, but their conventional machining is difficult. Kozak et al. (1995) used wire EDM for accurately shaping these materials, without distortion or burrs. Recently used for machining insulating ceramics by Tani et al. (2004). University of Limerick Con Sheahan Advanced Manufacturing DM4038 63 Applications – Wire EDM University of Limerick Con Sheahan Advanced Manufacturing DM4038 64 32 Wire EDM Introduction o A thin wire of brass, tungsten, or copper is used as an electrode. o Deionized water is used as the dielectric. The process is similar to standard EDM University of Limerick Con Sheahan Advanced Manufacturing DM4038 65 Wire EDM Introduction o Slowly cuts groove in shape of wire. o Wire is consumed and is slowly fed. o This process is much faster than electrode EDM. University of Limerick Con Sheahan Advanced Manufacturing DM4038 66 33 Wire EDM Characteristic of W-EDM University of Limerick Con Sheahan Advanced Manufacturing DM4038 67 Wire EDM Schematic illustration of the wire EDM process. As much as 50 hours of machining can be performed with one reel of wire, which is then discarded. University of Limerick Con Sheahan Advanced Manufacturing DM4038 68 34 Machine speed Wire EDM o Machine speed (mm2/min) = machine speed feed (mm/min) * workpiece thickness (mm) o Higher currents, and lower rest times increase the speed of this process. University of Limerick Con Sheahan Advanced Manufacturing DM4038 69 Wire EDM - Mechanism of material removal - melting and evaporation aided by cavitation - Medium - dielectric fluid - Tool materials - Cu, Brass, Cu-W alloy, Ag-W alloy, graphite - Material/tool wear = 0.1 to 10 - Gap = 10 to 125 micro m - maximum mrr = 5*103 mm3/min - Specific power consumption 1.8 W/mm3/min - Critical parameters - voltage, capacitance, spark gap, dielectric circulation, melting temperature - Materials application - all conducting metals and alloys - Shape application - blind complex cavities, micro holes for nozzles, through cutting of non-circular holes, narrow slots University of Limerick Con Sheahan Advanced Manufacturing DM4038 70 35 Wire EDM o The wire moves through the work piece like a band saw, removing material by electrical discharge o Dielectric fluid is applied to the work area o The wire is generally used only once; it is inexpensive University of Limerick Con Sheahan Advanced Manufacturing DM4038 71 Wire EDM Example of a wire EDM machine University of Limerick Con Sheahan Advanced Manufacturing DM4038 72 36 Wire EDM Example of a wire EDM machine University of Limerick Con Sheahan Advanced Manufacturing DM4038 73 Wire EDM Example of a wire used for an EDM machine This wire has been used; the wave pattern was formed during take-up University of Limerick Con Sheahan Advanced Manufacturing DM4038 74 37 Wire EDM Example of cores removed from a part using wire EDM to create the cavity in a high-pressure nozzle Holes were drilled in the interiors so that the wire could be strung through University of Limerick Con Sheahan Advanced Manufacturing DM4038 75 Wire EDM o A variation of EDM is wire EDM , or electrical-discharge wire cutting. In this process, which is similar to contour cutting with a band saw , a slowly moving wire travels along a prescribed path, cutting the work piece, with the discharge sparks acting like cutting teeth. o Wire EDM involves the use of a continuously moving conductive wire as the tool electrode. The tensioned wire of copper, brass, tungsten, or molybdenum is used only once, traveling from a take-off spool to a take-up spool while being “guided” to produce a straight narrow kerf in plates up to 100 mm thick. The wire diameter ranges from 0.05 to 0.25 mm. with positioning accuracy up to +0.005 mm. in machines with CNC or tracer control. The dielectric is usually deionized water because of its low viscosity. o This process is widely used for the manufacture of punches, dies, and stripper plates, with modern machines capable of routinely cutting die relief, intricate openings, tight radius contours, and corners. University of Limerick Con Sheahan Advanced Manufacturing DM4038 76 38 Wire EDM o This process is used to cut plates as thick as 300 mm, and for making punches, tools, and dies from hard metals, It can also cut intricate components for the electronics industry. o The wire is usually made of brass, copper, or tungsten; zinc- or brass-coated and multi-coated wires are also used. The wire diameter is typically about 0.30 for roughing cuts and 0.20 mm for finishing cuts. o The wire should have sufficient tensile strength and fracture toughness, as well as high electrical conductivity and capacity to flush away the debris produced during cutting. University of Limerick Con Sheahan Advanced Manufacturing DM4038 77 Wire EDM o The wire is generally used only once, as it is relatively inexpensive. It travels at a constant velocity in the range of 0.15 to 9 mm/mm (6 to 360 in/mm), and a constant gap (kerf) is maintained during the cut. o The cutting speed is generally given in terms of the cross-sectional area cut per unit time. o Typical examples are: 18,000 mm (28 in. for 50-mm (2-in.) thick D2 tool steel, and 45,000 mm (70 in. for 150-mm (6-in.) thick aluminum. These removal rates indicate a linear cutting speed of 18,000/50 = 360 mm/hr = 6 mm/mm, and 45,000/150 = 300 mm/hr = 5 mm/mm, respectively. o The trend in the use of dielectric fluids is toward clear, low-viscosity fluids. University of Limerick Con Sheahan Advanced Manufacturing DM4038 78 39 Wire EDM o Modern wire EDM machines (multi-axis EDM wire cutting machining centers) are equipped with the following features: 1. Computer controls for controlling the cutting path of the wire 2. Multi heads for cutting two parts at the same time, 3. Features such as controls for preventing wire breakage 4. Automatic self-threading features in case of wire breakage 5. Programmed machining strategies to optimise the operation. University of Limerick Con Sheahan Advanced Manufacturing DM4038 79 Wire EDM o Two-axis computer-controlled machines can produce cylindrical shapes in a manner similar to a turning operation or cylindrical grinding. o Many modern wire EDM machines allow the control of the feed and take-up ends of the wire to traverse independently in two principal directions, so tapered parts can be made. o Depending on size, capability, and quality, the cost of wire EDM machines is in the range of $150,000 to $300,000. University of Limerick Con Sheahan Advanced Manufacturing DM4038 80 40 Wire EDM Cutting a thick plate with wire EDM. University of Limerick Con Sheahan Advanced Manufacturing DM4038 81 Wire EDM o Also known as wire-cut EDM and wire cutting. o A thin single-strand metal wire (usually brass) is fed through the workpiece submerged in a tank of dielectric fluid (typically deionized water). o Used to cut plates as thick as 300 mm and to make punches, tools, and dies from hard metals that are difficult to machine with other methods. o Uses water as its dielectric fluid; its resistivity and other electrical properties are controlled with filters and de-ionizer units. o The water flushes the cut debris away from the cutting zone. o Flushing is an important factor in determining the maximum feed rate for a given material thickness. o Commonly used when low residual stresses are desired, because it does not require high cutting forces for material removal. University of Limerick Con Sheahan Advanced Manufacturing DM4038 82 41 Wire EDM o Special form of EDM - uses a continuously moving conductive wire electrode. o Material removal occurs as a result of spark erosion as the wire electrode is fed, from a fresh wire spool, through the work piece. o Horizontal movement of the worktable (CNC) determines the path of the cut. o Application - Machining of super hard materials like polycrystalline diamond (PCD) and cubic boron nitride (CBN) blanks, and other composites. o Carbon fiber composites are widely used in aerospace, nuclear, automobile, and chemical industries, but their conventional machining is difficult. Kozak et al. (1995) used wire EDM for accurately shaping these materials, without distortion or burrs. o Recently used for machining insulating ceramics by Tani et al. (2004). University of Limerick Con Sheahan Advanced Manufacturing DM4038 83 Wire EDM University of Limerick Con Sheahan Advanced Manufacturing DM4038 84 42 Wire EDM Typical EDM-WC products. University of Limerick Con Sheahan Advanced Manufacturing DM4038 85 Wire EDM o Utilises a traveling wire that is advance within arcing distance of the workpiece (0.001 in.) o Removes material by rapid, controlled, repetitive spark o Uses dielectric fluid to flush removed particles, control discharge, and cool wire and workpiece o Is performed on electrically conductive workpieces o Can produce complex twodimensional shapes University of Limerick Con Sheahan Advanced Manufacturing DM4038 86 43 Wire EDM o Numerically controlled wire EDM has revolutionised die making, particularly for plastic molders. Wire EDM is now common in tool-and-die shops. Shape accuracy in EDM-WC in a working environment with temperature variations of about 3°C is about 4 µm. If temperature control is within ± 1°C, the obtainable accuracy is closer to 1 µm. o No burrs are generated and since no cutting forces are present, wire EDM is ideal for delicate parts. o No tooling is required, so delivery times are short. Pieces over 16 in thick can be machined. Tools and parts are machined after heat treatment, so dimensional accuracy is held and not affected by heat treat distortion. University of Limerick Con Sheahan Advanced Manufacturing DM4038 87 Wire EDM The vertical, horizontal and slanted cutting with the µ-EDM-WC tool has successfully fabricated complex features and parts. An example is the impressive Chinese pagoda (1.25 mm × 1.75 mm) shown here where vertical and horizontal µ-EDM-WC cuts are illustrated University of Limerick Con Sheahan Advanced Manufacturing DM4038 88 44 Wire EDM o Texturing is applied to steel sheets during the final stages of cold rolling. o Shot blasting (SB) is an inexpensive method of texturing. o Limitations of SB include its lack of control and consistency of texturing, and the need for protection of other parts of the equipment holding the roll. o EDT, is a variation of EDM and proved to be the most popular. o Texturing is achieved by producing electrical sparks across the gap between roll (work piece) and a tool electrode, in the presence of dielectric (paraffin). o Each spark creates a small crater by the discharge of its energy in a local melting and vaporization of the roll material. o By selecting the appropriate process variables such as pulse current, on and off time, electrode polarity, dielectric type, and the roll rotational speed, a surface texture with a high degree of accuracy and consistency can be produced. University of Limerick Con Sheahan Advanced Manufacturing DM4038 89 Wire EDM Limitations o High specific energy consumption (about 50 times that in conventional machining); o When forced circulation of dielectric is not possible, removal rate is quite low; o Surface tends to be rough for larger removal rates; o Not applicable to non-conducting materials. University of Limerick Con Sheahan Advanced Manufacturing DM4038 90 45 Wire EDM • Main components: electrical conductive w/piece, continuously moving wire, dielectric fluid • Working operations • Terms: overcut, kerf, etc • Advantages, disadvantages University of Limerick Con Sheahan Advanced Manufacturing DM4038 91 Applications – EDM of Insulators A sheet metal mesh is placed over the ceramic material. Spark discharges between the negative tool electrode and the metal mesh. These sparks are transmitted through the metal mesh to its interface with the ceramic surface, which is then eroded. University of Limerick Con Sheahan Advanced Manufacturing DM4038 92 46 Applications – Texturing Texturing is applied to steel sheets during the final stages of cold rolling. Shot blasting (SB) is an inexpensive method of texturing. Limitations of SB include its lack of control and consistency of texturing, and the need for protection of other parts of the equipment holding the roll. EDT, is a variation of EDM and proved to be the most popular. Texturing is achieved by producing electrical sparks across the gap between roll (workpiece) and a tool electrode, in the presence of dielectric (paraffin). Each spark creates a small crater by the discharge of its energy in a local melting and vaporization of the roll material. By selecting the appropriate process variables such as pulse current, on and off time, electrode polarity, dielectric type, and the roll rotational speed, a surface texture with a high degree of accuracy and consistency can be produced. University of Limerick Con Sheahan Advanced Manufacturing DM4038 93 Advantages Some of the advantages of EDM include machining of: Complex shapes that would otherwise be difficult to produce with conventional cutting tools. Extremely hard material to very close tolerances. Very small work pieces where conventional cutting tools may damage the part from excess cutting tool pressure. There is no direct contact between tool and work piece. Therefore delicate sections and weak materials can be machined without any distortion. A good surface finish can be obtained. University of Limerick Con Sheahan Advanced Manufacturing DM4038 94 47 Disadvantages Some of the disadvantages of EDM include: The slow rate of material removal. For economic production, the surface finish specified should not be too fine. The additional time and cost used for creating electrodes for ram/sinker EDM. Reproducing sharp corners on the workpiece is difficult due to electrode wear. Specific power consumption is very high. Power consumption is high. "Overcut" is formed. Excessive tool wear occurs during machining. Electrically non-conductive materials can be machined only with specific set-up of the process University of Limerick Con Sheahan Advanced Manufacturing DM4038 95 Analytics Based Manufacturing on Electro-Discharge Machining (EDM) Defect Prevention and Condition Based Maintenance Seth McDonald, GE Aviation, April 2018 Electro-Discharge Machining in Aviation Film cooling is a method of cooling parts in hot sections of the engine High temperatures can be withstood by passing cool air through small holes in the surface of a part Cooling holes can be created using Electro-Discharge Machining (EDM) University of Limerick Con Sheahan Advanced Manufacturing DM4038 96 48 What is Electro-Discharge Machining? Electrode Wire Voltage is passed between two electrodes resulting in an electric discharge or spark which removes material Spark Dielectric Fluid Blade Outer Wall Blade Cavity Ideal means of precise and accurate hole drilling in exotic metal alloys University of Limerick Inner Wall Con Sheahan Advanced Manufacturing DM4038 97 EDM Potential Defect Modes Under Drill Over Drill Voltage Irregularity Machine Stoppage Defect modes drive rework, scrap, reduced cycle time University of Limerick Con Sheahan Advanced Manufacturing DM4038 98 49 On-Machine Monitoring & Controlling Voltage Breakthrough Detection System (BDS) Time Other Parameters Controller Image credit: Makino Con Sheahan University of Limerick Advanced Manufacturing DM4038 99 Velocity Signal Monitoring Cavity Cavity Wall Electrode Feed Velocity Outer Wall Drill Time Outcomes Eliminate Over Drills University of Limerick Con Sheahan Eliminate Under Drills Advanced Manufacturing DM4038 100 50 Voltage Variance Signal Monitoring Hole Drill Average Variance Maintenance Event Machine Down Voltage Variance Critical Shift Time Signal characterized for voltage shift and provides advanced alerting University of Limerick Con Sheahan Advanced Manufacturing DM4038 101 Off Machine Condition Base Maintenance Voltage Time Other Parameters Breakthrough Detection System (BDS) Condition Based Maintenance System Controller Outcomes Increased Uptime Image credit: Makino Voltage Driven Defects March 11, 2024 University of Limerick Con Sheahan Advanced Manufacturing DM4038 102 51 EDM Machine Optimization Drill Time High Machine variance Machine A Machine B Machine C Time Efforts underway to optimize machine efficiency University of Limerick Con Sheahan Advanced Manufacturing DM4038 103 Outcomes and Benefits 20% Increased Uptime 50% Voltage Driven Defects Eliminated Over Drills Eliminated Under Drills 10% Cycle Time Reduction Presents $125M Opportunity GE-to-GE University of Limerick Con Sheahan Advanced Manufacturing DM4038 104 52 SHEET METALWORKING 1. Cutting Operations 2. Bending Operations 3. Drawing 4. Other Sheet Metal Forming Operations 5. Dies and Presses for Sheet Metal Processes 6. Sheet Metal Operations Not Performed on Presses 7. Bending of Tube Stock University of Limerick Con Sheahan Advanced Manufacturing MF4038 1 Sheet Metalworking Defined Cutting and forming operations performed on relatively thin sheets of metal • Thickness of sheet metal = 0.4 mm (1/64 in) to 6 mm (1/4 in) • Thickness of plate stock > 6 mm • Operations usually performed as cold working University of Limerick Con Sheahan Advanced Manufacturing MF4038 2 1 Sheet and Plate Metal Products • Sheet and plate metal parts for consumer and industrial products such as – Automobiles and trucks – Airplanes – Railway cars and locomotives – Farm and construction equipment – Small and large appliances – Office furniture – Computers and office equipment University of Limerick Con Sheahan Advanced Manufacturing MF4038 3 Advantages of Sheet Metal Parts • High strength • Good dimensional accuracy • Good surface finish • Relatively low cost • Economical mass production for large quantities University of Limerick Con Sheahan Advanced Manufacturing MF4038 4 2 Sheet Metalworking Terminology • • • University of Limerick Punch-and-die - tooling to perform cutting, bending, and drawing Stamping press - machine tool that performs most sheet metal operations Stampings - sheet metal products Con Sheahan Advanced Manufacturing MF4038 5 Basic Types of Sheet Metal Processes 1. Cutting – Shearing action to separate large sheets – Blanking to cut part perimeters out of sheet metal – Punching to make holes in sheet metal 2. Bending – Straining sheet around a straight axis 3. Drawing – Forming of sheet into convex or concave shapes University of Limerick Con Sheahan Advanced Manufacturing MF4038 6 3 Sheet Metal Cutting Figure 20.1 Shearing of sheet metal between two cutting edges: (1) just before the punch contacts work; (2) punch begins to push into work, causing plastic deformation; University of Limerick Con Sheahan Advanced Manufacturing MF4038 7 Sheet Metal Cutting Figure 20.1 Shearing of sheet metal between two cutting edges: (3) punch compresses and penetrates into work causing a smooth cut surface; (4) fracture is initiated at the opposing cutting edges which separates the sheet. University of Limerick Con Sheahan Advanced Manufacturing MF4038 8 4 Shearing, Blanking, and Punching Three principal operations in pressworking that cut sheet metal: • Shearing • Blanking • Punching University of Limerick Con Sheahan Advanced Manufacturing MF4038 9 Shearing Sheet metal cutting operation along a straight line between two cutting edges • Typically used to cut large sheets Figure 20.3 Shearing operation: (a) side view of the shearing operation; (b) front view of power shears equipped with inclined upper cutting blade. University of Limerick Con Sheahan Advanced Manufacturing MF4038 10 5 Blanking and Punching Blanking - sheet metal cutting to separate piece (called a blank) from surrounding stock Punching - similar to blanking except cut piece is scrap, called a slug Figure 20.4 (a) Blanking and (b) punching. University of Limerick Con Sheahan Advanced Manufacturing MF4038 11 Clearance in Sheet Metal Cutting Distance between punch cutting edge and die cutting edge • Typical values range between 4% and 8% of stock thickness – If too small, fracture lines pass each other, causing double burnishing and larger force – If too large, metal is pinched between cutting edges and excessive burr results University of Limerick Con Sheahan Advanced Manufacturing MF4038 12 6 Clearance in Sheet Metal Cutting • Recommended clearance is calculated by: c = at where c = clearance; a = allowance; and t = stock thickness • Clearance allowance a is determined according to type of metal University of Limerick Con Sheahan Advanced Manufacturing MF4038 13 Sheet Metal Groups Allowances Metal group a 1100S and 5052S aluminum alloys, all tempers 0.045 2024ST and 6061ST aluminum alloys; brass, soft cold rolled steel, soft stainless steel 0.060 Cold rolled steel, half hard; stainless steel, half hard and full hard 0.075 University of Limerick Con Sheahan Advanced Manufacturing MF4038 14 7 Punch and Die Sizes • For a round blank of diameter Db: – Blanking punch diameter = Db - 2c – Blanking die diameter = Db where c = clearance • For a round hole of diameter Dh: – Hole punch diameter = Dh – Hole die diameter = Dh + 2c where c = clearance Con Sheahan University of Limerick Advanced Manufacturing MF4038 15 Punch and Die Sizes Figure 20.6 Die size determines blank size Db; punch size determines hole size Dh.; c = clearance University of Limerick Con Sheahan Advanced Manufacturing MF4038 16 8 Angular Clearance Purpose: allows slug or blank to drop through die • Typical values: 0.25 to 1.5 on each side Figure 20.7 Angular clearance. University of Limerick Con Sheahan Advanced Manufacturing MF4038 17 Cutting Forces Important for determining press size (tonnage) F=StL where S = shear strength of metal; t = stock thickness, and L = length of cut edge University of Limerick Con Sheahan Advanced Manufacturing MF4038 18 9 Question 1 A 150mm diameter disc is to be blanked from a strip of 2.3mm, half-hard cold rolled steel whose shear strength is = 260MPA. The clearance allowance for the material is a = 0.075. Determine (a) the appropriate punch and die diameters, and (b) blanking force University of Limerick Con Sheahan Advanced Manufacturing MF4038 19 Sheet Metal Bending Straining sheetmetal around a straight axis to take a permanent bend Figure 20.11 (a) Bending of sheet metal University of Limerick Con Sheahan Advanced Manufacturing MF4038 20 10 Sheet Metal Bending Metal on inside of neutral plane is compressed, while metal on outside of neutral plane is stretched Figure 20.11 (b) both compression and tensile elongation of the metal occur in bending. University of Limerick Con Sheahan Advanced Manufacturing MF4038 21 Types of Sheet Metal Bending • V-bending - performed with a V-shaped die and punch • Edge bending - performed with a wiping die and punch University of Limerick Con Sheahan Advanced Manufacturing MF4038 22 11 V-Bending • For low production • Performed on a press brake • V-dies are simple and inexpensive • Angles vary from very obtuse to very acute Figure 20.12 (a) V-bending; University of Limerick Con Sheahan Advanced Manufacturing MF4038 23 Edge Bending • For high production • Pressure pad required • Dies are more complicated and costly Figure 20.12 (b) edge bending. University of Limerick Con Sheahan Advanced Manufacturing MF4038 24 12 Stretching during Bending • If bend radius is small relative to stock thickness, metal tends to stretch during bending • Important to estimate amount of stretching, so final part length = specified dimension • Problem: to determine the length of neutral axis of the part before bending University of Limerick Con Sheahan Advanced Manufacturing MF4038 25 Bend Allowance Formula Ab = 2π α ( R + K bat ) 360 where Ab = bend allowance; = bend angle; R= bend radius; t = stock thickness; and Kba is factor to estimate stretching • If R < 2t, Kba = 0.33 • If R 2t, Kba = 0.50 University of Limerick Con Sheahan Advanced Manufacturing MF4038 26 13 Springback Increase in included angle of bent part relative to included angle of forming tool after tool is removed • Reason for springback: – When bending pressure is removed, elastic energy remains in bent part, causing it to recover partially toward its original shape Con Sheahan University of Limerick Advanced Manufacturing MF4038 27 Springback Figure 20.13 Springback in bending is seen as a decrease in bend angle and an increase in bend radius: (1) during bending, the work is forced to take radius Rb and included angle b' of the bending tool, (2) after punch is removed, the work springs back to radius R and angle ‘. University of Limerick Con Sheahan Advanced Manufacturing MF4038 28 14 Bending Force Maximum bending force estimated as follows: F K bf TSwt 2 D where F = bending force; TS = tensile strength of sheet metal; w = part width in direction of bend axis; and t = stock thickness. For V- bending, Kbf = 1.33; for edge bending, Kbf = 0.33 University of Limerick Con Sheahan Advanced Manufacturing MF4038 29 Die Opening Dimension Figure 20.14 Die opening dimension D: (a) V-die, (b) wiping die. University of Limerick Con Sheahan Advanced Manufacturing MF4038 30 15 Question 2 University of Limerick Con Sheahan Advanced Manufacturing MF4038 31 Drawing Sheet metal forming to make cup-shaped, box-shaped, or other complex-curved, hollow-shaped parts • Sheet metal blank is positioned over die cavity and then punch pushes metal into opening • Products: beverage cans, ammunition shells, automobile body panels and sinks • Also known as deep drawing (to distinguish it from wire and bar drawing) University of Limerick Con Sheahan Advanced Manufacturing MF4038 32 16 Drawing Figure 20.19 (a) Drawing of cup-shaped part: (1) before punch contacts work, (2) near end of stroke; (b) workpart: (1) starting blank, (2) drawn part. University of Limerick Con Sheahan Advanced Manufacturing MF4038 33 Clearance in Drawing • Sides of punch and die separated by a clearance c given by: c = 1.1 t where t = stock thickness • In other words, clearance is about 10% greater than stock thickness University of Limerick Con Sheahan Advanced Manufacturing MF4038 34 17 Tests of Drawing Feasibility • Drawing ratio • Reduction • Thickness-to-diameter ratio University of Limerick Con Sheahan Advanced Manufacturing MF4038 35 Drawing Ratio DR Most easily defined for cylindrical shape: DR Db Dp where Db = blank diameter; and Dp = punch diameter • Indicates severity of a given drawing operation – Upper limit: DR 2.0 University of Limerick Con Sheahan Advanced Manufacturing MF4038 36 18 Reduction r • Defined for cylindrical shape: r Db Dp Db where Db = blank diameter; and Dp = punch diameter Value of r should be less than 0.50 University of Limerick Con Sheahan Advanced Manufacturing MF4038 37 Thickness-to-Diameter Ratio t/Db Thickness of starting blank divided by blank diameter • Desirable for t/Db ratio to be greater than 1% • As t/Db decreases, tendency for wrinkling increases University of Limerick Con Sheahan Advanced Manufacturing MF4038 38 19 Blank Size Determination • For final dimensions of drawn shape to be correct, starting blank diameter Db must be right • Solve for Db by setting starting sheet metal blank volume = final product volume • To facilitate calculation, assume negligible thinning of part wall University of Limerick Con Sheahan Advanced Manufacturing MF4038 39 Shapes other than Cylindrical Cups • Square or rectangular boxes (as in sinks), • Stepped cups • Cones • Cups with spherical rather than flat bases • Irregular curved forms (as in automobile body panels) • Each of these shapes presents its own unique technical problems in drawing University of Limerick Con Sheahan Advanced Manufacturing MF4038 40 20 Other Sheet Metal Forming on Presses Other sheet metal forming operations performed conventional presses • Operations performed with metal tooling • Operations performed with flexible rubber tooling University of Limerick Con Sheahan on Advanced Manufacturing MF4038 41 Ironing • Makes wall thickness of cylindrical cup more uniform Figure 20.25 Ironing to achieve more uniform wall thickness in a drawn cup: (1) start of process; (2) during process. Note thinning and elongation of walls. University of Limerick Con Sheahan Advanced Manufacturing MF4038 42 21 Embossing Creates indentations in sheet, such as raised (or indented) lettering or strengthening ribs Figure 20.26 Embossing: (a) cross-section of punch and die configuration during pressing; (b) finished part with embossed ribs. University of Limerick Con Sheahan Advanced Manufacturing MF4038 43 Guerin Process Figure 20.28 Guerin process: (1) before and (2) after. Symbols v and F indicate motion and applied force respectively. University of Limerick Con Sheahan Advanced Manufacturing MF4038 44 22 Advantages of Guerin Process • Low tooling cost • Form block can be made of wood, plastic, or other materials that are easy to shape • Rubber pad can be used with different form blocks • Process attractive in small quantity production University of Limerick Con Sheahan Advanced Manufacturing MF4038 45 Dies for Sheet Metal Processes Most pressworking operations performed with conventional punch-and-die tooling • Custom-designed for particular part • The term stamping die sometimes used for high production dies University of Limerick Con Sheahan Advanced Manufacturing MF4038 46 23 Punch and Die Components Figure 20.30 Components of a punch and die for a blanking operation. University of Limerick Con Sheahan Advanced Manufacturing MF4038 47 Progressive Die Figure 20.31 (a) Progressive die; (b) associated strip development • Progressive die performs two or more operations on a sheet metal at 2 or more station on a single stroke. High production rates, high cost. University of Limerick Con Sheahan Advanced Manufacturing MF4038 48 24 Stamping Press Figure 20.32 Components of a typical mechanical drive stamping press University of Limerick Con Sheahan Advanced Manufacturing MF4038 49 Types of Stamping Press Frame • Gap frame – Configuration of the letter C and often referred to as a C-frame • Straight-sided frame – Box-like construction for higher tonnage University of Limerick Con Sheahan Advanced Manufacturing MF4038 50 25 Figure 20.33 Gap frame press for sheet metalworking (ohoto courtesy of E. W. Bliss Co.); capacity = 1350 kN (150 tons) University of Limerick Con Sheahan Advanced Manufacturing MF4038 Con Sheahan Advanced Manufacturing MF4038 51 Figure 20.34 Press brake (photo courtesy of Niagara Machine & Tool Works); bed width = 9.15 m (30 ft) and capacity = 11,200 kN (1250 tons). University of Limerick 52 26 Figure 20.35 Sheet metal parts produced on a turret press, showing variety of hole shapes possible (photo courtesy of Strippet Inc.). University of Limerick Con Sheahan Advanced Manufacturing MF4038 53 Figure 20.36 Computer numerical control turret press (photo courtesy of Strippet, Inc.). University of Limerick Con Sheahan Advanced Manufacturing MF4038 54 27 Figure 20.37 Straight-sided frame press (photo courtesy of Greenerd Press & Machine Company, Inc.). University of Limerick Con Sheahan Advanced Manufacturing MF4038 55 Power and Drive Systems • Hydraulic presses - use a large piston and cylinder to drive the ram – Longer ram stroke than mechanical types – Suited to deep drawing – Slower than mechanical drives • Mechanical presses – convert rotation of motor to linear motion of ram – High forces at bottom of stroke – Suited to blanking and punching University of Limerick Con Sheahan Advanced Manufacturing MF4038 56 28 Operations Not Performed on Presses • Stretch forming • Roll bending and forming • Spinning • High-energy-rate forming processes. University of Limerick Con Sheahan Advanced Manufacturing MF4038 57 Stretch Forming Sheet metal is stretched and simultaneously bent to achieve shape change Figure 20.39 Stretch forming: (1) start of process; (2) form die is pressed into the work with force Fdie, causing it to be stretched and bent over the form. F = stretching force. University of Limerick Con Sheahan Advanced Manufacturing MF4038 58 29 Force Required in Stretch Forming F LtYf where F = stretching force; L = length of sheet in direction perpendicular to stretching; t = instantaneous stock thickness; and Yf = flow stress of work metal • Die force Fdie can be determined by balancing vertical force components University of Limerick Con Sheahan Advanced Manufacturing MF4038 59 Roll Bending Large metal sheets and plates are formed into curved sections using rolls Figure 20.40 Roll bending. University of Limerick Con Sheahan Advanced Manufacturing MF4038 60 30 Roll Forming Continuous bending process in which opposing rolls produce long sections of formed shapes from coil or strip stock Figure 20.41 Roll forming of a continuous channel section: (1) straight rolls, (2) partial form, (3) final form. University of Limerick Con Sheahan Advanced Manufacturing MF4038 61 Spinning Metal forming process in which an axially symmetric part is gradually shaped over a rotating mandrel using a rounded tool or roller • Three types: 1. Conventional spinning 2. Shear spinning 3. Tube spinning University of Limerick Con Sheahan Advanced Manufacturing MF4038 62 31 Conventional Spinning Figure 20.42 Conventional spinning: (1) setup at start of process; (2) during spinning; and (3) completion of process. University of Limerick Con Sheahan Advanced Manufacturing MF4038 63 High-Energy-Rate Forming (HERF) Processes to form metals using large amounts of energy over a very short time • HERF processes include: – Explosive forming – Electrohydraulic forming – Electromagnetic forming University of Limerick Con Sheahan Advanced Manufacturing MF4038 64 32 Explosive Forming Use of explosive charge to form sheet (or plate) metal into a die cavity • Explosive charge causes a shock wave whose energy is transmitted to force part into cavity • Applications: large parts, typical of aerospace industry University of Limerick Con Sheahan Advanced Manufacturing MF4038 65 Explosive Forming Figure 20.45 Explosive forming: (1) setup, (2) explosive is detonated, and (3) shock wave forms part and plume escapes water surface. University of Limerick Con Sheahan Advanced Manufacturing MF4038 66 33 Electromagnetic Forming Sheet metal is deformed by mechanical force of an electromagnetic field induced in the workpart by an energized coil • Presently the most widely used HERF process • Applications: tubular parts • Also called magnetic pulse forming University of Limerick Con Sheahan Advanced Manufacturing MF4038 67 Electromagnetic Forming Figure 20.47 Electromagnetic forming: (1) setup in which coil is inserted into tubular workpart surrounded by die; (2) formed part. University of Limerick Con Sheahan Advanced Manufacturing MF4038 68 34 Electrical Discharge Machining (EDM) University of Limerick Con Sheahan Advanced Manufacturing DM4038 1 Introduction Sometimes it is referred to as spark machining, spark eroding, burning, die sinking or wire erosion Its a manufacturing process whereby a desired shape is obtained using electrical discharges (sparks). Material is removed from the workpiece by a series of rapidly recurring current discharges between two electrodes, separated by a dielectric liquid and subject to an electric voltage. One of the electrodes – ‘tool-electrode’ or ‘tool’ or ‘electrode’. Other electrode - workpiece-electrode or ‘workpiece’. As distance between the two electrodes is reduced, the current intensity becomes greater than the strength of the dielectric (at least in some points) causing it to break. University of Limerick Con Sheahan Advanced Manufacturing DM4038 2 1 History This allows current to flow between the two electrodes. This phenomenon is the same as the breakdown of a capacitor. As a result, material is removed from both the electrodes. Once the current flow stops, new liquid dielectric is usually conveyed into the electrode zone enabling the solid particles (debris) to be carried away. Adding new liquid dielectric in the electrode volume is commonly referred to as flushing. Also, after a current flow, a difference of potential between the two electrodes is restored to what it was before the breakdown, so that a new liquid dielectric breakdown can occur. University of Limerick Con Sheahan Advanced Manufacturing DM4038 3 History In 1770, English Physicist Joseph Priestley studied the erosive effect of electrical discharges. Furthering Priestley's research, the EDM process was invented by two Russian scientists, Dr. B.R. Lazarenko and Dr. N.I. Lazarenko in 1943. In their efforts to exploit the destructive effects of an electrical discharge, they developed a controlled process for machining of metals. Their initial process used a spark machining process, named after the succession of sparks (electrical discharges) that took place between two electrical conductors immersed in a dielectric fluid. The discharge generator effect used by this machine, known as the Lazarenko Circuit, was used for many years in the construction of generators for electrical discharge. University of Limerick Con Sheahan Advanced Manufacturing DM4038 4 2 History New researchers entered the field and contributed many fundamental characteristics of the machining method we know today. In 1952, the manufacturer Charmilles created the first machine using the spark machining process and was presented for the first time at the European Machine Tool Exhibition in 1955. In 1969, Agie launched the world's first numerically controlled wire-cut EDM machine. Seibu developed the first CNC wire EDM machine in 1972 and the first system was manufactured in Japan. Recently, the machining speed has gone up by 20 times. This has decreased machining costs by at least 30 percent and improved the surface finish by a factor of 1.5 University of Limerick Con Sheahan Advanced Manufacturing DM4038 5 General Aspects of EDM EDM is a machining method primarily used for hard metals or those that would be very difficult to machine with traditional techniques. EDM typically works with materials that are electrically conductive, although methods for machining insulating ceramics with EDM have been proposed. EDM can cut intricate contours or cavities in hardened steel without the need for heat treatment to soften and re-harden them. This method can be used with any other metal or metal alloy such as titanium, hastelloy, kovar, and inconel. Also, applications of this process to shape polycrystalline diamond tools have been reported. University of Limerick Con Sheahan Advanced Manufacturing DM4038 6 3 EDM - System University of Limerick Con Sheahan Advanced Manufacturing DM4038 7 EDM - Components University of Limerick Con Sheahan Advanced Manufacturing DM4038 8 4 EDM - Components The main components in EDM: Electric power supply Dielectric medium Work piece & tool Servo control unit. The work piece and tool are electrically connected to a DC power supply. The current density in the discharge of the channel is of the order of 10000 A/cm2 and power density is nearly 500 MW/cm2. A gap, known as SPARK GAP in the range, from 0.005 mm to 0.05 mm is maintained between the work piece and the tool. Dielectric slurry is forced through this gap at a pressure of 2 kgf/cm2 or lesser. University of Limerick Con Sheahan Advanced Manufacturing DM4038 9 EDM – Working Principle It is a process of metal removal based on the principle of material removal by an interrupted electric spark discharge between the electrode tool and the work piece. In EDM, a potential difference is applied between the tool and workpiece. Essential - Both tool and work material are to be conductors. The tool and work material are immersed in a dielectric medium. Generally kerosene or deionised water is used as the dielectric medium. A gap is maintained between the tool and the workpiece. Depending upon the applied potential difference (50 to 450 V) and the gap between the tool and workpiece, an electric field would be established. Generally the tool is connected to the negative terminal (cathode) of the generator and the workpiece is connected to positive terminal (anode). University of Limerick Con Sheahan Advanced Manufacturing DM4038 10 5 EDM – Working Principle As the electric field is established between the tool and the job, the free electrons on the tool are subjected to electrostatic forces. If the bonding energy of the electrons is less, electrons would be emitted from the tool. Such emission of electrons are called or termed as ‘cold emission’. The “cold emitted” electrons are then accelerated towards the job through the dielectric medium. As they gain velocity and energy, and start moving towards the job, there would be collisions between the electrons and dielectric molecules. Such collision may result in ionization of the dielectric molecule. Ionization depends on the ionization energy of the dielectric molecule and the energy of the electron. University of Limerick Con Sheahan Advanced Manufacturing DM4038 11 EDM – Working Principle As the electrons get accelerated, more positive ions and electrons would get generated due to collisions. This cyclic process would increase the concentration of electrons and ions in the dielectric medium between the tool and the job at the spark gap. The concentration would be so high that the matter existing in that channel could be characterised as “plasma”. The electrical resistance of such plasma channel would be very less. Thus all of a sudden, a large number of electrons will flow from tool to job and ions from job to tool. This is called avalanche motion of electrons. Such movement of electrons and ions can be visually seen as a spark. Thus the electrical energy is dissipated as the thermal energy of the spark. University of Limerick Con Sheahan Advanced Manufacturing DM4038 12 6 EDM – Working Principle The high speed electrons then impinge on the job and ions on the tool. The kinetic energy of the electrons and ions on impact with the surface of the job and tool respectively would be converted into thermal energy or heat flux. Such intense localized heat flux leads to extreme instantaneous confined rise in temperature which would be in excess of 10,000oC. Such localized extreme rise in temperature leads to material removal. Material removal occurs due to instant vaporization of the material as well as due to melting. The molten metal is not removed completely but only partially. University of Limerick Con Sheahan Advanced Manufacturing DM4038 13 EDM – Working Principle Upon withdrawal of potential difference, plasma channel collapses. This ultimately creates compression shock waves on both the electrode surface. Particularly at high spots on work piece surface, which are closest to the tool. This evacuates molten material and forms a crater around the site of the spark. The whole sequence of operation occurs within a few microseconds. University of Limerick Con Sheahan Advanced Manufacturing DM4038 14 7 EDM – Schematic University of Limerick Con Sheahan Advanced Manufacturing DM4038 15 EDM – Working Principle Thus to summarise, the material removal in EDM mainly occurs due to formation of shock waves as the plasma channel collapse owing to discontinuation of applied potential difference Generally the workpiece is made positive and the tool negative. Hence, the electrons strike the job leading to crater formation due to high temperature and melting and material removal. Similarly, the positive ions impinge on the tool leading to tool wear. In EDM, the generator is used to apply voltage pulses between the tool and job. A constant voltage is not applied. Only sparking is desired rather than arcing. Arcing leads to localized material removal at a particular point whereas sparks get distributed all over the tool surface leading to uniform material removal. University of Limerick Con Sheahan Advanced Manufacturing DM4038 16 8 EDM – Working Principle University of Limerick Con Sheahan Advanced Manufacturing DM4038 17 EDM – Power & Control Circuits Two broad categories of generators (power supplies) are in use on EDM. Commercially available: RC circuits based and transistor controlled pulses. In the first category, the main parameters to choose from at setup time are the resistance(s) of the resistor(s) and the capacitance(s) of the capacitor(s). In an ideal condition, these quantities would affect the maximum current delivered in a discharge. Current delivery in a discharge is associated with the charge accumulated on the capacitors at a certain moment. Little control is expected over the time of discharge, which is likely to depend on the actual spark-gap conditions. Advantage: RC circuit generator can allow the use of short discharge time more easily than the pulse-controlled generator. University of Limerick Con Sheahan Advanced Manufacturing DM4038 18 9 EDM – Power & Control Circuits Also, the open circuit voltage (i.e. voltage between electrodes when dielectric is not broken) can be identified as steady state voltage of the RC circuit. In generators based on transistor control, the user is usually able to deliver a train of voltage pulses to the electrodes. Each pulse can be controlled in shape, for instance, quasi-rectangular. In particular, the time between two consecutive pulses and the duration of each pulse can be set. The amplitude of each pulse constitutes the open circuit voltage. Thus, maximum duration of discharge is equal to duration of a voltage pulse. Maximum current during a discharge that the generator delivers can also be controlled. Con Sheahan University of Limerick Advanced Manufacturing DM4038 19 EDM – Power & Control Circuits Details of generators and control systems on EDMs are not always easily available to their user. This is a barrier to describing the technological parameters of EDM process. Moreover, the parameters affecting the phenomena occurring between tool and electrode are also related to the motion controller of the electrodes. A framework to define and measure the electrical parameters during an EDM operation directly on inter-electrode volume with an oscilloscope external to the machine has been recently proposed by Ferri et al. This would enable the user to estimate directly the electrical parameter that affect their operations without relying upon machine manufacturer's claims. When machining different materials in the same setup conditions, the actual electrical parameters are significantly different. University of Limerick Con Sheahan Advanced Manufacturing DM4038 20 10 EDM – Power & Control Circuits When using RC generators, the voltage pulses, shown in Fig. are responsible for material removal. A series of voltage pulses (Fig.) of magnitude about 20 to 120 V and frequency on the order of 5 kHz is applied between the two electrodes. University of Limerick Con Sheahan Advanced Manufacturing DM4038 21 EDM – Power & Control Circuits University of Limerick Con Sheahan Advanced Manufacturing DM4038 22 11 EDM – Power & Control Circuits University of Limerick Con Sheahan Advanced Manufacturing DM4038 23 EDM – Power & Control Circuits University of Limerick Con Sheahan Advanced Manufacturing DM4038 24 12 EDM – Electrode Material Electrode material should be such that it would not undergo much tool wear when it is impinged by positive ions. Thus the localised temperature rise has to be less by properly choosing its properties or even when temperature increases, there would be less melting. Further, the tool should be easily workable as intricate shaped geometric features are machined in EDM. Thus the basic characteristics of electrode materials are: High electrical conductivity – electrons are cold emitted more easily and there is less bulk electrical heating High thermal conductivity – for the same heat load, the local temperature rise would be less due to faster heat conducted to the bulk of the tool and thus less tool wear. University of Limerick Con Sheahan Advanced Manufacturing DM4038 25 EDM – Electrode Material Higher density – for less tool wear and thus less dimensional loss or inaccuracy of tool High melting point – high melting point leads to less tool wear due to less tool material melting for the same heat load Easy manufacturability Cost – cheap The followings are the different electrode materials which are used commonly in the industry: Graphite Electrolytic oxygen free copper Tellurium copper – 99% Cu + 0.5% tellurium Brass University of Limerick Con Sheahan Advanced Manufacturing DM4038 26 13 EDM – Electrode Material Graphite (most common) - has fair wear characteristics, easily machinable. Small flush holes can be drilled into graphite electrodes. Copper has good EDM wear and better conductivity. It is generally used for better finishes in the range of Ra = 0.5 μm. Copper tungsten and silver tungsten are used for making deep slots under poor flushing conditions especially in tungsten carbides. It offers high machining rates as well as low electrode wear. Copper graphite is good for cross-sectional electrodes. It has better electrical conductivity than graphite while the corner wear is higher. Brass ensures stable sparking conditions and is normally used for specialized applications such as drilling of small holes where the high electrode wear is acceptable. University of Limerick Con Sheahan Advanced Manufacturing DM4038 27 EDM – Electrode Movement In addition to the servo-controlled feed, the tool electrode may have an additional rotary or orbiting motion. Electrode rotation helps to solve the flushing difficulty encountered when machining small holes with EDM. In addition to the increase in cutting speed, the quality of the hole produced is superior to that obtained using a stationary electrode. Electrode orbiting produces cavities having the shape of the electrode. The size of the electrode and the radius of the orbit (2.54 mm maximum) determine the size of the cavities. Electrode orbiting improves flushing by creating a pumping effect of the dielectric liquid through the gap. University of Limerick Con Sheahan Advanced Manufacturing DM4038 28 14 EDM – Electrode Wear University of Limerick Con Sheahan Advanced Manufacturing DM4038 29 EDM – Electrode Wear The melting point is the most important factor in determining the tool wear. Electrode wear ratios are expressed as end wear, side wear, corner wear, and volume wear. “No wear EDM” - when the electrode-to-workpiece wear ratio is 1 % or less. Electrode wear depends on a number of factors associated with the EDM, like voltage, current, electrode material, and polarity. The change in shape of the tool electrode due to the electrode wear causes defects in the workpiece shape. Electrode wear has even more pronounced effects when it comes to micromachining applications. The corner wear ratio depends on the type of electrode. The low melting point of aluminum is associated with the highest wear ratio. University of Limerick Con Sheahan Advanced Manufacturing DM4038 30 15 EDM – Electrode Wear University of Limerick Con Sheahan Advanced Manufacturing DM4038 31 EDM – Electrode Wear Graphite has shown a low tendency to wear and has the possibility of being molded or machined into complicated electrode shapes. The wear rate of the electrode tool material (Wt) and the wear ratio (Rw) are given by Kalpakjian (1997). University of Limerick Con Sheahan Advanced Manufacturing DM4038 32 16 EDM – Dielectric In EDM, material removal mainly occurs due to thermal evaporation and melting. As thermal processing is required to be carried out in absence of oxygen so that the process can be controlled and oxidation avoided. Oxidation often leads to poor surface conductivity (electrical) of the workpiece hindering further machining. Hence, dielectric fluid should provide an oxygen free machining environment. Further it should have enough strong dielectric resistance so that it does not breakdown electrically too easily. But at the same time, it should ionize when electrons collide with its molecule. Moreover, during sparking it should be thermally resistant as well. Generally kerosene and deionised water is used as dielectric fluid in EDM. University of Limerick Con Sheahan Advanced Manufacturing DM4038 33 EDM – Dielectric Tap water cannot be used as it ionises too early and thus breakdown due to presence of salts as impurities occur. Dielectric medium is generally flushed around the spark zone. It is also applied through the tool to achieve efficient removal of molten material. Three important functions of a dielectric medium in EDM: 1. Insulates the gap between the tool and work, thus preventing a spark to form until the gap voltage are correct. 2. Cools the electrode, workpiece and solidifies the molten metal particles. 3. Flushes the metal particles out of the working gap to maintain ideal cutting conditions, increase metal removal rate. It must be filtered and circulated at constant pressure. University of Limerick Con Sheahan Advanced Manufacturing DM4038 34 17 EDM – Dielectric The main requirements of the EDM dielectric fluids are adequate viscosity, high flash point, good oxidation stability, minimum odor, low cost, and good electrical discharge efficiency. For most EDM operations kerosene is used with certain additives that prevent gas bubbles and de-odoring. Silicon fluids and a mixture of these fluids with petroleum oils have given excellent results. Other dielectric fluids with a varying degree of success include aqueous solutions of ethylene glycol, water in emulsions, and distilled water. University of Limerick Con Sheahan Advanced Manufacturing DM4038 35 EDM – Flushing One of the important factors in a successful EDM operation is the removal of debris (chips) from the working gap. Flushing these particles out of the working gap is very important, to prevent them from forming bridges that cause short circuits. EDMs have a built-in power adaptive control system that increases the pulse spacing as soon as this happens and reduces or shuts off the power supply. Flushing – process of introducing clean filtered dielectric fluid into spark gap. If flushing is applied incorrectly, it can result in erratic cutting and poor machining conditions. Flushing of dielectric plays a major role in the maintenance of stable machining and the achievement of close tolerance and high surface quality. Inadequate flushing can result in arcing, decreased electrode life, and increased production time. University of Limerick Con Sheahan Advanced Manufacturing DM4038 36 18 EDM – Flushing Four methods: 1. Normal flow 2. Reverse flow 3. Jet flushing 4. Immersion flushing Con Sheahan University of Limerick Advanced Manufacturing DM4038 37 EDM – Flushing Normal flow (Majority) Dielectric is introduced, under pressure, through one or more passages in the tool and is forced to flow through the gap between tool and work. Flushing holes are generally placed in areas where the cuts are deepest. Normal flow is sometimes undesirable because it produces a tapered opening in the workpiece. Reverse flow Particularly useful in machining deep cavity dies, where the taper produced using the normal flow mode can be reduced. The gap is submerged in filtered dielectric, and instead of pressure being applied at the source a vacuum is used. With clean fluid flowing between the workpiece and the tool, there is no side sparking and, therefore, no taper is produced. University of Limerick Con Sheahan Advanced Manufacturing DM4038 38 19 EDM – Flushing Jet flushing In many instances, the desired machining can be achieved by using a spray or jet of fluid directed against the machining gap. Machining time is always longer with jet flushing than with the normal and reverse flow modes. Immersion flushing For many shallow cuts or perforations of thin sections, simple immersion of the discharge gap is sufficient. Cooling and debris removal can be enhanced during immersion cutting by providing relative motion between the tool and workpiece. Vibration or cycle interruption comprises periodic reciprocation of the tool relative to the workpiece to effect a pumping action of the dielectric. University of Limerick Con Sheahan Advanced Manufacturing DM4038 39 EDM – Flushing Synchronized, pulsed flushing is also available on some machines. With this method, flushing occurs only during the non-machining time as the electrode is retracted slightly to enlarge the gap. Increased electrode life has been reported with this system. Innovative techniques such as ultrasonic vibrations coupled with mechanical pulse EDM, jet flushing with sweeping nozzles, and electrode pulsing are investigated by Masuzawa (1990). University of Limerick Con Sheahan Advanced Manufacturing DM4038 40 20 EDM – Flushing For proper flushing conditions, Metals Handbook (1989) recommends: 1. Flushing through the tool is more preferred than side flushing. 2. Many small flushing holes are better than a few large ones. 3. Steady dielectric flow on the entire workpiece-electrode interface is desirable. 4. Dead spots created by pressure flushing, from opposite sides of the workpiece, should be avoided. 5. A vent hole should be provided for any upwardly concave part of the tool-electrode to prevent accumulation of explosive gases. 6. A flush box is useful if there is a hole in the cavity. University of Limerick Con Sheahan Advanced Manufacturing DM4038 41 EDM – Process Parameters The waveform is characterized by the: The open circuit voltage – Vo The working voltage – Vw The maximum current – Io The pulse on time – the duration for which the voltage pulse is applied ton The pulse off time – toff The gap between the workpiece and the tool – spark gap - δ The polarity – straight polarity – tool (-ve) The dielectric medium External flushing through the spark gap. University of Limerick Con Sheahan Advanced Manufacturing DM4038 42 21 EDM – Process Parameters The process parameters - mainly related to the waveform characteristics. University of Limerick Con Sheahan Advanced Manufacturing DM4038 43 EDM – Types – Sinker EDM Sinker EDM, also called cavity type EDM or volume EDM. Consists of an electrode and workpiece submerged in an insulating liquid such as oil or other dielectric fluids. The electrode and workpiece are connected to a suitable power supply. The power supply generates an electrical potential between the two parts. As the electrode approaches the workpiece, dielectric breakdown occurs in the fluid, forming a plasma channel, and a small spark jumps. These sparks happen in huge numbers at seemingly random locations. As the base metal is eroded, and the spark gap subsequently increased, the electrode is lowered automatically so that the process can continue. Several hundred thousand sparks occur per second, with the actual duty cycle carefully controlled by the setup parameters. These controlling cycles are sometimes known as "on time" and "off time“. University of Limerick Con Sheahan Advanced Manufacturing DM4038 44 22 EDM – Types – Sinker EDM The on time setting determines the length or duration of the spark. Hence, a longer on time produces a deeper cavity for that spark and all subsequent sparks for that cycle. This creates rougher finish on the workpiece. The reverse is true for a shorter on time. Off time is the period of time that one spark is replaced by another. A longer off time, for example, allows the flushing of dielectric fluid through a nozzle to clean out the eroded debris, thereby avoiding a short circuit. These settings can be maintained in micro seconds. The typical part geometry is a complex 3D shape, often with small or odd shaped angles. University of Limerick Con Sheahan Advanced Manufacturing DM4038 45 EDM – Types – Wire EDM (WEDM) Also known as wire-cut EDM and wire cutting. A thin single-strand metal wire (usually brass) is fed through the workpiece submerged in a tank of dielectric fluid (typically deionized water). Used to cut plates as thick as 300 mm and to make punches, tools, and dies from hard metals that are difficult to machine with other methods. Uses water as its dielectric fluid; its resistivity and other electrical properties are controlled with filters and de-ionizer units. The water flushes the cut debris away from the cutting zone. Flushing is an important factor in determining the maximum feed rate for a given material thickness. Commonly used when low residual stresses are desired, because it does not require high cutting forces for material removal. University of Limerick Con Sheahan Advanced Manufacturing DM4038 46 23 EDM – Material Removal Rate Con Sheahan University of Limerick Advanced Manufacturing DM4038 47 EDM – Material Removal Rate In EDM, the metal is removed from both workpiece and tool electrode. MRR depends not only on the workpiece material but on the material of the tool electrode and the machining variables such as pulse conditions, electrode polarity, and the machining medium. In this regard a material of low melting point has a high metal removal rate and hence a rougher surface. Typical removal rates range from 0.1 to 400 mm3 /min. MRR or volumetric removal rate (VRR), in mm3/min, was described by Kalpakjian (1997): where I - EDM current (A) Tw - Melting point of the workpiece (°C). University of Limerick Con Sheahan Advanced Manufacturing DM4038 48 24 EDM – Material Removal Rate Effect of pulse current (energy) on MRR & surface roughness. University of Limerick Con Sheahan Advanced Manufacturing DM4038 49 EDM – Material Removal Rate Effect of pulse on-time (energy) on MRR & surface roughness. University of Limerick Con Sheahan Advanced Manufacturing DM4038 50 25 EDM – Surface Integrity Surface consists of a multitude of overlapping craters that are formed by the action of microsecond-duration spark discharges. Crater size depends on physical and mechanical properties of the material composition of the machining medium discharge energy and duration. Integral effect of thousands of discharges per second leads to machining with a specified accuracy and surface finish. Depth of craters - the peak to valley (maximum) of surface roughness Rt. Maximum depth of damaged layer can be taken as 2.5 times of roughness Ra. According to Delpreti (1977) and Motoki and Lee (1968), the maximum peak to valley height, Rt, was considered to be 10 times Ra. University of Limerick Con Sheahan Advanced Manufacturing DM4038 51 EDM – Surface Integrity Average roughness can be expressed in terms of pulse current ip (A) and pulse duration tp (μs) by Surface roughness increases linearly with an increase in MRR. Jeswani (1978) - Graphite electrodes produce rougher surfaces than metal ones. Kuneida and Furuoya (1991) claimed that the introduction of oxygen into discharge gap provides extra power by the reaction of oxygen. This in turn increased workpiece melting and created greater expulsive forces that increased MRR and surface roughness. Choice of correct dielectric flow has a significant effect in reducing surface roughness by 50 %, increasing the machining rate, and lowering the thermal effects in the workpiece surface. Dielectrics having low viscosity are recommended for smooth surfaces. University of Limerick Con Sheahan Advanced Manufacturing DM4038 52 26 EDM – Surface Integrity Metallurgical changes occur in the surface – Temperature 8000 to 12,000°C. Additionally, a thin recast layer of 1 μm to 25 μm – depending on power used. Delpretti (1977) and Levy and Maggi (1990) claimed that the heat-affected zone (HAZ) adjacent to the resolidified layer reaches 25 μm. Some annealing can be expected in a zone just below the machined surface. Not all the workpiece melted by discharge is expelled into the dielectric. Remaining melted material is quickly chilled, primarily by heat conduction into the bulk of the workpiece, resulting in an exceedingly hard surface. Depth of annealed layer is proportional to power used. It ranges from 50 μm for finish cutting to ~ 200 μm for high MRR. Annealing is usually about two points of hardness below the parent metal for finish cutting. University of Limerick Con Sheahan Advanced Manufacturing DM4038 53 EDM – Surface Integrity In roughing cuts, the annealing effect is ~ five points of hardness below the parent metal. Electrodes that produce more stable machining can reduce the annealing effect. A finish cut removes the annealed material left by the previous rough cut. The altered surface layer significantly lowers the fatigue strength of alloys. It consists of a recast layer with or without microcracks, some of which may extend into the base metal, plus metallurgical alterations such as rehardened and tempered layers, heat-affected zones, and inter-granular precipitates. During EDM roughing, the layer showing microstructural changes, including a melted and resolidified layer, is less than 0.127 mm deep. During EDM finishing, it is less than 0.075 mm. Post-treatment to restore the fatigue strength is recommended to follow EDM of critical or highly stressed surfaces. University of Limerick Con Sheahan Advanced Manufacturing DM4038 54 27 EDM – Surface Integrity There are several effective processes that accomplish restoration or even enhancement of the fatigue properties. These methods include Removal of the altered layers by low-stress grinding or chemical machining Addition of a metallurgical-type coating Re heat-treatment Application of shot peening. University of Limerick Con Sheahan Advanced Manufacturing DM4038 55 EDM – Characteristics Can be used to machine any work material if it is electrically conductive. MRR depends on thermal properties (job) rather than its strength, hardness etc. The volume of the material removed per spark discharge is typically in the range of (1/1,000,000) to (1/10,000) mm3. In EDM, geometry of tool - positive impression of hole or geometric feature. Tool wear once again depends on the thermal properties of tool material. Local temperature rise is rather high, but there is not enough heat diffusion (very small pulse on time) and thus HAZ is limited to 2 – 4 μm. Rapid heating and cooling leads to surface hardening which may be desirable in some applications. Tolerance value of + 0.05 mm could be easily achieved by EDM. Best surface finish that can be economically achieved on steel is 0.40 m. University of Limerick Con Sheahan Advanced Manufacturing DM4038 56 28 Applications Drilling of micro-holes, thread cutting, helical profile milling, rotary forming, and curved hole drilling. Delicate work piece like copper parts can be produced by EDM. Can be applied to all electrically conducting metals and alloys irrespective of their melting points, hardness, toughness, or brittleness. Other applications: deep, small-dia holes using tungsten wire as tool, narrow slots, cooling holes in super alloy turbine blades, and various intricate shapes. EDM can be economically employed for extremely hardened work piece. Since there is no mechanical stress present (no physical contact), fragile and slender work places can be machined without distortion. Hard and corrosion resistant surfaces, essentially needed for die making, can be developed. University of Limerick Con Sheahan Advanced Manufacturing DM4038 57 Applications – EDM Drilling Uses a tubular tool electrode where the dielectric is flushed. When solid rods are used; dielectric is fed to the machining zone by either suction or injection through pre-drilled holes. Irregular, tapered, curved, as well as inclined holes can be produced by EDM. Creating cooling channels in turbine blades made of hard alloys is a typical application of EDM drilling. Use of NC system enabled large numbers of holes to be accurately located. University of Limerick Con Sheahan Advanced Manufacturing DM4038 58 29 Applications – EDM Sawing An EDM variation - Employs either a special steel band or disc. Cuts at a rate that is twice that of the conventional abrasive sawing method. Cutting of billets and bars - has a smaller kerf & free from burrs. Fine finish of 6.3 to 10 μm with a recast layer of 0.025 to 0.130 mm University of Limerick Con Sheahan Advanced Manufacturing DM4038 59 Applications - Machining of spheres Shichun and coworkers (1995) used simple tubular electrodes in EDM machining of spheres, to a dimensional accuracy of ±1 μm and Ra < 0.1 μm. Rotary EDM is used for machining of spherical shapes in conducting ceramics using the tool and workpiece arrangement as shown below. University of Limerick Con Sheahan Advanced Manufacturing DM4038 60 30 Applications - Machining of dies & molds EDM milling uses standard cylindrical electrodes. Simple-shaped electrode (Fig. 1) is rotated at high speeds and follows specified paths in the workpiece like the conventional end mills. Very useful and makes EDM very versatile like mechanical milling process. Solves the problem of manufacturing accurate and complex-shaped electrodes for die sinking (Fig. 2) of three-dimensional cavities. (Fig. 1) University of Limerick (Fig. 2) Con Sheahan Advanced Manufacturing DM4038 61 Applications - Machining of dies & molds EDM milling enhances dielectric flushing due to high-speed electrode rotation. Electrode wear can be optimized due to its rotational and contouring motions. Main limitation in EDM milling - Complex shapes with sharp corners cannot be machined because of the rotating tool electrode. EDM milling replaces conventional die making that requires variety of machines such as milling, wire cutting, and EDM die sinking machines. University of Limerick Con Sheahan Advanced Manufacturing DM4038 62 31 Applications – Wire EDM Special form of EDM - uses a continuously moving conductive wire electrode. Material removal occurs as a result of spark erosion as the wire electrode is fed, from a fresh wire spool, through the workpiece. Horizontal movement of the worktable (CNC) determines the path of the cut. Application - Machining of superhard materials like polycrystalline diamond (PCD) and cubic boron nitride (CBN) blanks, and other composites. Carbon fiber composites are widely used in aerospace, nuclear, automobile, and chemical industries, but their conventional machining is difficult. Kozak et al. (1995) used wire EDM for accurately shaping these materials, without distortion or burrs. Recently used for machining insulating ceramics by Tani et al. (2004). University of Limerick Con Sheahan Advanced Manufacturing DM4038 63 Applications – Wire EDM University of Limerick Con Sheahan Advanced Manufacturing DM4038 64 32 Wire EDM Introduction o A thin wire of brass, tungsten, or copper is used as an electrode. o Deionized water is used as the dielectric. The process is similar to standard EDM University of Limerick Con Sheahan Advanced Manufacturing DM4038 65 Wire EDM Introduction o Slowly cuts groove in shape of wire. o Wire is consumed and is slowly fed. o This process is much faster than electrode EDM. University of Limerick Con Sheahan Advanced Manufacturing DM4038 66 33 Wire EDM Characteristic of W-EDM University of Limerick Con Sheahan Advanced Manufacturing DM4038 67 Wire EDM Schematic illustration of the wire EDM process. As much as 50 hours of machining can be performed with one reel of wire, which is then discarded. University of Limerick Con Sheahan Advanced Manufacturing DM4038 68 34 Machine speed Wire EDM o Machine speed (mm2/min) = machine speed feed (mm/min) * workpiece thickness (mm) o Higher currents, and lower rest times increase the speed of this process. University of Limerick Con Sheahan Advanced Manufacturing DM4038 69 Wire EDM - Mechanism of material removal - melting and evaporation aided by cavitation - Medium - dielectric fluid - Tool materials - Cu, Brass, Cu-W alloy, Ag-W alloy, graphite - Material/tool wear = 0.1 to 10 - Gap = 10 to 125 micro m - maximum mrr = 5*103 mm3/min - Specific power consumption 1.8 W/mm3/min - Critical parameters - voltage, capacitance, spark gap, dielectric circulation, melting temperature - Materials application - all conducting metals and alloys - Shape application - blind complex cavities, micro holes for nozzles, through cutting of non-circular holes, narrow slots University of Limerick Con Sheahan Advanced Manufacturing DM4038 70 35 Wire EDM o The wire moves through the work piece like a band saw, removing material by electrical discharge o Dielectric fluid is applied to the work area o The wire is generally used only once; it is inexpensive University of Limerick Con Sheahan Advanced Manufacturing DM4038 71 Wire EDM Example of a wire EDM machine University of Limerick Con Sheahan Advanced Manufacturing DM4038 72 36 Wire EDM Example of a wire EDM machine University of Limerick Con Sheahan Advanced Manufacturing DM4038 73 Wire EDM Example of a wire used for an EDM machine This wire has been used; the wave pattern was formed during take-up University of Limerick Con Sheahan Advanced Manufacturing DM4038 74 37 Wire EDM Example of cores removed from a part using wire EDM to create the cavity in a high-pressure nozzle Holes were drilled in the interiors so that the wire could be strung through University of Limerick Con Sheahan Advanced Manufacturing DM4038 75 Wire EDM o A variation of EDM is wire EDM , or electrical-discharge wire cutting. In this process, which is similar to contour cutting with a band saw , a slowly moving wire travels along a prescribed path, cutting the work piece, with the discharge sparks acting like cutting teeth. o Wire EDM involves the use of a continuously moving conductive wire as the tool electrode. The tensioned wire of copper, brass, tungsten, or molybdenum is used only once, traveling from a take-off spool to a take-up spool while being “guided” to produce a straight narrow kerf in plates up to 100 mm thick. The wire diameter ranges from 0.05 to 0.25 mm. with positioning accuracy up to +0.005 mm. in machines with CNC or tracer control. The dielectric is usually deionized water because of its low viscosity. o This process is widely used for the manufacture of punches, dies, and stripper plates, with modern machines capable of routinely cutting die relief, intricate openings, tight radius contours, and corners. University of Limerick Con Sheahan Advanced Manufacturing DM4038 76 38 Wire EDM o This process is used to cut plates as thick as 300 mm, and for making punches, tools, and dies from hard metals, It can also cut intricate components for the electronics industry. o The wire is usually made of brass, copper, or tungsten; zinc- or brass-coated and multi-coated wires are also used. The wire diameter is typically about 0.30 for roughing cuts and 0.20 mm for finishing cuts. o The wire should have sufficient tensile strength and fracture toughness, as well as high electrical conductivity and capacity to flush away the debris produced during cutting. University of Limerick Con Sheahan Advanced Manufacturing DM4038 77 Wire EDM o The wire is generally used only once, as it is relatively inexpensive. It travels at a constant velocity in the range of 0.15 to 9 mm/mm (6 to 360 in/mm), and a constant gap (kerf) is maintained during the cut. o The cutting speed is generally given in terms of the cross-sectional area cut per unit time. o Typical examples are: 18,000 mm (28 in. for 50-mm (2-in.) thick D2 tool steel, and 45,000 mm (70 in. for 150-mm (6-in.) thick aluminum. These removal rates indicate a linear cutting speed of 18,000/50 = 360 mm/hr = 6 mm/mm, and 45,000/150 = 300 mm/hr = 5 mm/mm, respectively. o The trend in the use of dielectric fluids is toward clear, low-viscosity fluids. University of Limerick Con Sheahan Advanced Manufacturing DM4038 78 39 Wire EDM o Modern wire EDM machines (multi-axis EDM wire cutting machining centers) are equipped with the following features: 1. Computer controls for controlling the cutting path of the wire 2. Multi heads for cutting two parts at the same time, 3. Features such as controls for preventing wire breakage 4. Automatic self-threading features in case of wire breakage 5. Programmed machining strategies to optimise the operation. University of Limerick Con Sheahan Advanced Manufacturing DM4038 79 Wire EDM o Two-axis computer-controlled machines can produce cylindrical shapes in a manner similar to a turning operation or cylindrical grinding. o Many modern wire EDM machines allow the control of the feed and take-up ends of the wire to traverse independently in two principal directions, so tapered parts can be made. o Depending on size, capability, and quality, the cost of wire EDM machines is in the range of $150,000 to $300,000. University of Limerick Con Sheahan Advanced Manufacturing DM4038 80 40 Wire EDM Cutting a thick plate with wire EDM. University of Limerick Con Sheahan Advanced Manufacturing DM4038 81 Wire EDM o Also known as wire-cut EDM and wire cutting. o A thin single-strand metal wire (usually brass) is fed through the workpiece submerged in a tank of dielectric fluid (typically deionized water). o Used to cut plates as thick as 300 mm and to make punches, tools, and dies from hard metals that are difficult to machine with other methods. o Uses water as its dielectric fluid; its resistivity and other electrical properties are controlled with filters and de-ionizer units. o The water flushes the cut debris away from the cutting zone. o Flushing is an important factor in determining the maximum feed rate for a given material thickness. o Commonly used when low residual stresses are desired, because it does not require high cutting forces for material removal. University of Limerick Con Sheahan Advanced Manufacturing DM4038 82 41 Wire EDM o Special form of EDM - uses a continuously moving conductive wire electrode. o Material removal occurs as a result of spark erosion as the wire electrode is fed, from a fresh wire spool, through the work piece. o Horizontal movement of the worktable (CNC) determines the path of the cut. o Application - Machining of super hard materials like polycrystalline diamond (PCD) and cubic boron nitride (CBN) blanks, and other composites. o Carbon fiber composites are widely used in aerospace, nuclear, automobile, and chemical industries, but their conventional machining is difficult. Kozak et al. (1995) used wire EDM for accurately shaping these materials, without distortion or burrs. o Recently used for machining insulating ceramics by Tani et al. (2004). University of Limerick Con Sheahan Advanced Manufacturing DM4038 83 Wire EDM University of Limerick Con Sheahan Advanced Manufacturing DM4038 84 42 Wire EDM Typical EDM-WC products. University of Limerick Con Sheahan Advanced Manufacturing DM4038 85 Wire EDM o Utilises a traveling wire that is advance within arcing distance of the workpiece (0.001 in.) o Removes material by rapid, controlled, repetitive spark o Uses dielectric fluid to flush removed particles, control discharge, and cool wire and workpiece o Is performed on electrically conductive workpieces o Can produce complex twodimensional shapes University of Limerick Con Sheahan Advanced Manufacturing DM4038 86 43 Wire EDM o Numerically controlled wire EDM has revolutionised die making, particularly for plastic molders. Wire EDM is now common in tool-and-die shops. Shape accuracy in EDM-WC in a working environment with temperature variations of about 3°C is about 4 µm. If temperature control is within ± 1°C, the obtainable accuracy is closer to 1 µm. o No burrs are generated and since no cutting forces are present, wire EDM is ideal for delicate parts. o No tooling is required, so delivery times are short. Pieces over 16 in thick can be machined. Tools and parts are machined after heat treatment, so dimensional accuracy is held and not affected by heat treat distortion. University of Limerick Con Sheahan Advanced Manufacturing DM4038 87 Wire EDM The vertical, horizontal and slanted cutting with the µ-EDM-WC tool has successfully fabricated complex features and parts. An example is the impressive Chinese pagoda (1.25 mm × 1.75 mm) shown here where vertical and horizontal µ-EDM-WC cuts are illustrated University of Limerick Con Sheahan Advanced Manufacturing DM4038 88 44 Wire EDM o Texturing is applied to steel sheets during the final stages of cold rolling. o Shot blasting (SB) is an inexpensive method of texturing. o Limitations of SB include its lack of control and consistency of texturing, and the need for protection of other parts of the equipment holding the roll. o EDT, is a variation of EDM and proved to be the most popular. o Texturing is achieved by producing electrical sparks across the gap between roll (work piece) and a tool electrode, in the presence of dielectric (paraffin). o Each spark creates a small crater by the discharge of its energy in a local melting and vaporization of the roll material. o By selecting the appropriate process variables such as pulse current, on and off time, electrode polarity, dielectric type, and the roll rotational speed, a surface texture with a high degree of accuracy and consistency can be produced. University of Limerick Con Sheahan Advanced Manufacturing DM4038 89 Wire EDM Limitations o High specific energy consumption (about 50 times that in conventional machining); o When forced circulation of dielectric is not possible, removal rate is quite low; o Surface tends to be rough for larger removal rates; o Not applicable to non-conducting materials. University of Limerick Con Sheahan Advanced Manufacturing DM4038 90 45 Wire EDM • Main components: electrical conductive w/piece, continuously moving wire, dielectric fluid • Working operations • Terms: overcut, kerf, etc • Advantages, disadvantages University of Limerick Con Sheahan Advanced Manufacturing DM4038 91 Applications – EDM of Insulators A sheet metal mesh is placed over the ceramic material. Spark discharges between the negative tool electrode and the metal mesh. These sparks are transmitted through the metal mesh to its interface with the ceramic surface, which is then eroded. University of Limerick Con Sheahan Advanced Manufacturing DM4038 92 46 Applications – Texturing Texturing is applied to steel sheets during the final stages of cold rolling. Shot blasting (SB) is an inexpensive method of texturing. Limitations of SB include its lack of control and consistency of texturing, and the need for protection of other parts of the equipment holding the roll. EDT, is a variation of EDM and proved to be the most popular. Texturing is achieved by producing electrical sparks across the gap between roll (workpiece) and a tool electrode, in the presence of dielectric (paraffin). Each spark creates a small crater by the discharge of its energy in a local melting and vaporization of the roll material. By selecting the appropriate process variables such as pulse current, on and off time, electrode polarity, dielectric type, and the roll rotational speed, a surface texture with a high degree of accuracy and consistency can be produced. University of Limerick Con Sheahan Advanced Manufacturing DM4038 93 Advantages Some of the advantages of EDM include machining of: Complex shapes that would otherwise be difficult to produce with conventional cutting tools. Extremely hard material to very close tolerances. Very small work pieces where conventional cutting tools may damage the part from excess cutting tool pressure. There is no direct contact between tool and work piece. Therefore delicate sections and weak materials can be machined without any distortion. A good surface finish can be obtained. University of Limerick Con Sheahan Advanced Manufacturing DM4038 94 47 Disadvantages Some of the disadvantages of EDM include: The slow rate of material removal. For economic production, the surface finish specified should not be too fine. The additional time and cost used for creating electrodes for ram/sinker EDM. Reproducing sharp corners on the workpiece is difficult due to electrode wear. Specific power consumption is very high. Power consumption is high. "Overcut" is formed. Excessive tool wear occurs during machining. Electrically non-conductive materials can be machined only with specific set-up of the process University of Limerick Con Sheahan Advanced Manufacturing DM4038 95 Analytics Based Manufacturing on Electro-Discharge Machining (EDM) Defect Prevention and Condition Based Maintenance Seth McDonald, GE Aviation, April 2018 Electro-Discharge Machining in Aviation Film cooling is a method of cooling parts in hot sections of the engine High temperatures can be withstood by passing cool air through small holes in the surface of a part Cooling holes can be created using Electro-Discharge Machining (EDM) University of Limerick Con Sheahan Advanced Manufacturing DM4038 96 48 What is Electro-Discharge Machining? Electrode Wire Voltage is passed between two electrodes resulting in an electric discharge or spark which removes material Spark Dielectric Fluid Blade Outer Wall Blade Cavity Ideal means of precise and accurate hole drilling in exotic metal alloys University of Limerick Inner Wall Con Sheahan Advanced Manufacturing DM4038 97 EDM Potential Defect Modes Under Drill Over Drill Voltage Irregularity Machine Stoppage Defect modes drive rework, scrap, reduced cycle time University of Limerick Con Sheahan Advanced Manufacturing DM4038 98 49 On-Machine Monitoring & Controlling Voltage Breakthrough Detection System (BDS) Time Other Parameters Controller Image credit: Makino Con Sheahan University of Limerick Advanced Manufacturing DM4038 99 Velocity Signal Monitoring Cavity Cavity Wall Electrode Feed Velocity Outer Wall Drill Time Outcomes Eliminate Over Drills University of Limerick Con Sheahan Eliminate Under Drills Advanced Manufacturing DM4038 100 50 Voltage Variance Signal Monitoring Hole Drill Average Variance Maintenance Event Machine Down Voltage Variance Critical Shift Time Signal characterized for voltage shift and provides advanced alerting University of Limerick Con Sheahan Advanced Manufacturing DM4038 101 Off Machine Condition Base Maintenance Voltage Time Other Parameters Breakthrough Detection System (BDS) Condition Based Maintenance System Controller Outcomes Increased Uptime Image credit: Makino Voltage Driven Defects March 11, 2024 University of Limerick Con Sheahan Advanced Manufacturing DM4038 102 51 EDM Machine Optimization Drill Time High Machine variance Machine A Machine B Machine C Time Efforts underway to optimize machine efficiency University of Limerick Con Sheahan Advanced Manufacturing DM4038 103 Outcomes and Benefits 20% Increased Uptime 50% Voltage Driven Defects Eliminated Over Drills Eliminated Under Drills 10% Cycle Time Reduction Presents $125M Opportunity GE-to-GE University of Limerick Con Sheahan Advanced Manufacturing DM4038 104 52 BULK DEFORMATION PROCESSES IN METALWORKING 1. Rolling 2. Other Deformation Processes Related to Rolling 3. Forging 4. Other Deformation Processes Related to Forging 5. Extrusion 6. Wire and Bar Drawing University of Limerick Con Sheahan Advanced Manufacturing MF4038 1 Bulk Deformation Metal forming operations which cause significant shape change by deforming metal parts whose initial form is bulk rather than sheet • Starting forms: – Cylindrical bars and billets, – Rectangular billets and slabs, and similar shapes • These processes stress metal sufficiently to cause plastic flow into desired shape • Performed as cold, warm, and hot working operations University of Limerick Con Sheahan Advanced Manufacturing MF4038 2 1 Importance of Bulk Deformation • In hot working, significant shape change can be accomplished • In cold working, strength is increased during shape change • Little or no waste - some operations are near net shape or net shape processes – The parts require little or no subsequent machining University of Limerick Con Sheahan Advanced Manufacturing MF4038 3 Four Basic Bulk Deformation Processes 1. Rolling – slab or plate is squeezed between opposing rolls 2. Forging – work is squeezed and shaped between opposing dies 3. Extrusion – work is squeezed through a die opening, thereby taking the shape of the opening 4. Wire and bar drawing – diameter of wire or bar is reduced by pulling it through a die opening University of Limerick Con Sheahan Advanced Manufacturing MF4038 4 2 Rolling Deformation process in which work thickness is reduced by compressive forces exerted by two opposing rolls The rolling process (specifically, flat rolling). University of Limerick Con Sheahan Advanced Manufacturing MF4038 5 The Rolls Rotating rolls perform two main functions: • Pull the work into the gap between them by friction between workpart and rolls • Simultaneously squeeze the work to reduce its cross section University of Limerick Con Sheahan Advanced Manufacturing MF4038 6 3 Types of Rolling • Based on workpiece geometry : – Flat rolling - used to reduce thickness of a rectangular cross section – Shape rolling - square cross section is formed into a shape such as an I-beam • Based on work temperature : – Hot Rolling – most common due to the large amount of deformation required – Cold rolling – produces finished sheet and plate stock University of Limerick Con Sheahan Advanced Manufacturing MF4038 7 Rolled Products Made of Steel Some of the steel products made in a rolling mill. University of Limerick Con Sheahan Advanced Manufacturing MF4038 8 4 Diagram of Flat Rolling Side view of flat rolling, indicating before and after thicknesses, work velocities, angle of contact with rolls, and other features. University of Limerick Con Sheahan Advanced Manufacturing MF4038 9 Flat Rolling Terminology Draft = amount of thickness reduction d t o t f where d = draft; to = starting thickness; and tf = final thickness University of Limerick Con Sheahan Advanced Manufacturing MF4038 10 5 Flat Rolling Terminology Reduction = draft expressed as a fraction of starting stock thickness: r d to where r = reduction University of Limerick Con Sheahan Advanced Manufacturing MF4038 11 Shape Rolling Work is deformed into a contoured cross section rather than flat (rectangular) • Accomplished by passing work through rolls that have the reverse of desired shape • Products include: – Construction shapes such as I-beams, L-beams, and U-channels – Rails for railroad tracks – Round and square bars and rods University of Limerick Con Sheahan Advanced Manufacturing MF4038 12 6 A rolling mill for hot flat rolling. The steel plate is seen as the glowing strip in lower left corner (photo courtesy of Bethlehem Steel). University of Limerick Con Sheahan Advanced Manufacturing MF4038 13 Rolling Mills • Equipment is massive and expensive • Rolling mill configurations: – Two-high – two opposing rolls – Three-high – work passes through rolls in both directions – Four-high – backing rolls support smaller work rolls – Cluster mill – multiple backing rolls on smaller rolls – Tandem rolling mill – sequence of two-high mills University of Limerick Con Sheahan Advanced Manufacturing MF4038 14 7 Two-High Rolling Mill Figure 19.5 Various configurations of rolling mills: (a) 2-high rolling mill. University of Limerick Con Sheahan Advanced Manufacturing MF4038 15 Three-High Rolling Mill Various configurations of rolling mills: (b) 3-high rolling mill. University of Limerick Con Sheahan Advanced Manufacturing MF4038 16 8 Four-High Rolling Mill Figure 19.5 Various configurations of rolling mills: (c) four-high rolling mill. University of Limerick Con Sheahan Advanced Manufacturing MF4038 17 Cluster Mill Multiple backing rolls allow even smaller roll diameters Various configurations of rolling mills: (d) cluster mill University of Limerick Con Sheahan Advanced Manufacturing MF4038 18 9 Tandem Rolling Mill A series of rolling stands in sequence Figure 19.5 Various configurations of rolling mills: (e) tandem rolling mill. University of Limerick Con Sheahan Advanced Manufacturing MF4038 19 Thread Rolling Bulk deformation process used to form threads on cylindrical parts by rolling them between two dies • Important commercial process for mass producing bolts and screws • Performed by cold working in thread rolling machines • Advantages over thread cutting (machining): – Higher production rates – Better material utilization – Stronger threads and better fatigue resistance due to work hardening University of Limerick Con Sheahan Advanced Manufacturing MF4038 20 10 Thread Rolling Figure 19.6 Thread rolling with flat dies: (1) start of cycle, and (2) end of cycle. University of Limerick Con Sheahan Advanced Manufacturing MF4038 21 Ring Rolling Deformation process in which a thick-walled ring of smaller diameter is rolled into a thin-walled ring of larger diameter • As thick-walled ring is compressed, deformed metal elongates, causing diameter of ring to be enlarged • Hot working process for large rings and cold working process for smaller rings • Applications: ball and roller bearing races, steel tires for railroad wheels, and rings for pipes, pressure vessels, and rotating machinery • Advantages: material savings, ideal grain orientation, strengthening through cold working University of Limerick Con Sheahan Advanced Manufacturing MF4038 22 11 Ring Rolling Ring rolling used to reduce the wall thickness and increase the diameter of a ring: (1) start, and (2) completion of process. University of Limerick Con Sheahan Advanced Manufacturing MF4038 23 Forging Deformation process in which work is compressed between two dies • Oldest of the metal forming operations, dating from about 5000 B C • Components: engine crankshafts, connecting rods, gears, aircraft structural components, jet engine turbine parts • Also, basic metals industries use forging to establish basic form of large parts that are subsequently machined to final shape and size University of Limerick Con Sheahan Advanced Manufacturing MF4038 24 12 Classification of Forging Operations • Cold vs. hot forging: –Hot or warm forging – most common, due to the significant deformation and the need to reduce strength and increase ductility of work metal –Cold forging – advantage: increased strength that results from strain hardening • Impact vs. press forging: –Forge hammer - applies an impact load –Forge press - applies gradual pressure University of Limerick Con Sheahan Advanced Manufacturing MF4038 25 Types of Forging Dies • Open-die forging - work is compressed between two flat dies, allowing metal to flow laterally with minimum constraint • Impression-die forging - die contains cavity or impression that is imparted to workpart – Metal flow is constrained so that flash is created • Flashless forging - workpart is completely constrained in die – No excess flash is created University of Limerick Con Sheahan Advanced Manufacturing MF4038 26 13 Open-Die Forging Three types of forging: (a) open-die forging. University of Limerick Con Sheahan Advanced Manufacturing MF4038 27 Impression-Die Forging Three types of forging: (b) impression-die forging. University of Limerick Con Sheahan Advanced Manufacturing MF4038 28 14 Flashless Forging Three types of forging (c) flashless forging. University of Limerick Con Sheahan Advanced Manufacturing MF4038 29 Open-Die Forging Compression of workpart between two flat dies • Similar to compression test when workpart has cylindrical cross section and is compressed along its axis – Deformation operation reduces height and increases diameter of work – Common names include upsetting or upset forging University of Limerick Con Sheahan Advanced Manufacturing MF4038 30 15 Open-Die Forging with No Friction If no friction occurs between work and die surfaces, then homogeneous deformation occurs, so that radial flow is uniform throughout workpart height and true strain is given by: h ln o h where ho= starting height; and h = height at some point during compression • At h = final value hf, true strain is maximum value University of Limerick Con Sheahan Advanced Manufacturing MF4038 31 Open-Die Forging with No Friction Homogeneous deformation of a cylindrical workpart under ideal conditions in an open-die forging operation: (1) start of process with workpiece at its original length and diameter, (2) partial compression, and (3) final size. University of Limerick Con Sheahan Advanced Manufacturing MF4038 32 16 Open-Die Forging with Friction • Friction between work and die surfaces constrains lateral flow of work, resulting in barreling effect • In hot open-die forging, effect is even more pronounced due to heat transfer at and near die surfaces, which cools the metal and increases its resistance to deformation University of Limerick Con Sheahan Advanced Manufacturing MF4038 33 Open-Die Forging with Friction Actual deformation of a cylindrical workpart in open-die forging, showing pronounced barreling: (1) start of process, (2) partial deformation, and (3) final shape. University of Limerick Con Sheahan Advanced Manufacturing MF4038 34 17 Impression-Die Forging Compression of workpart by dies with inverse of desired part shape • Flash is formed by metal that flows beyond die cavity into small gap between die plates • Flash must be later trimmed, but it serves an important function during compression: – As flash forms, friction resists continued metal flow into gap, constraining material to fill die cavity – In hot forging, metal flow is further restricted by cooling against die plates University of Limerick Con Sheahan Advanced Manufacturing MF4038 35 Impression-Die Forging Figure 19.14 Sequence in impression-die forging: (1) just prior to initial contact with raw workpiece, (2) partial compression, and (3) final die closure, causing flash to form in gap between die plates. University of Limerick Con Sheahan Advanced Manufacturing MF4038 36 18 Impression-Die Forging Practice • Several forming steps often required, with separate die cavities for each step – Beginning steps redistribute metal for more uniform deformation and desired metallurgical structure in subsequent steps – Final steps bring the part to final geometry • Impression-die forging is often performed manually by skilled operator under adverse conditions University of Limerick Con Sheahan Advanced Manufacturing MF4038 37 Advantages and Limitations • Advantages of impression-die forging compared to machining from solid stock: – Higher production rates – Less waste of metal – Greater strength – Favorable grain orientation in the metal • Limitations: – Not capable of close tolerances – Machining often required to achieve accuracies and features needed University of Limerick Con Sheahan Advanced Manufacturing MF4038 38 19 Flashless Forging Compression of work in punch and die tooling whose cavity does not allow for flash • Starting workpart volume must equal die cavity volume within very close tolerance • Process control more demanding than impression-die forging • Best suited to part geometries that are simple and symmetrical • Often classified as a precision forging process University of Limerick Con Sheahan Advanced Manufacturing MF4038 39 Flashless Forging Flashless forging: (1) just before initial contact with workpiece, (2) partial compression, and (3) final punch and die closure. University of Limerick Con Sheahan Advanced Manufacturing MF4038 40 20 Forging Hammers (Drop Hammers) Apply impact load against workpart • Two types: – Gravity drop hammers - impact energy from falling weight of a heavy ram – Power drop hammers - accelerate the ram by pressurized air or steam • Disadvantage: impact energy transmitted through anvil into floor of building • Commonly used for impression-die forging University of Limerick Con Sheahan Advanced Manufacturing MF4038 41 Drop forging hammer, fed by conveyor and heating units at the right of the scene (photo courtesy of Chambersburg Engineering Company). University of Limerick Con Sheahan Advanced Manufacturing MF4038 42 21 Drop Hammer Details Figure 19.20 Diagram showing details of a drop hammer for impression-die forging. University of Limerick Con Sheahan Advanced Manufacturing MF4038 43 Forging Presses • Apply gradual pressure to accomplish compression operation • Types: – Mechanical press - converts rotation of drive motor into linear motion of ram – Hydraulic press - hydraulic piston actuates ram – Screw press - screw mechanism drives ram University of Limerick Con Sheahan Advanced Manufacturing MF4038 44 22 Upsetting and Heading Forging process used to form heads on nails, bolts, and similar hardware products • More parts produced by upsetting than any other forging operation • Performed cold, warm, or hot on machines called headers or formers • Wire or bar stock is fed into machine, end is headed, then piece is cut to length • For bolts and screws, thread rolling is then used to form threads University of Limerick Con Sheahan Advanced Manufacturing MF4038 45 Upset Forging An upset forging operation to form a head on a bolt or similar hardware item The cycle consists of: (1) wire stock is fed to the stop, (2) gripping dies close on the stock and the stop is retracted, (3) punch moves forward, (4) bottoms to form the head. University of Limerick Con Sheahan Advanced Manufacturing MF4038 46 23 Heading (Upset Forging) Examples of heading (upset forging) operations: (a) heading a nail using open dies, (b) round head formed by punch, (c) and (d) two common head styles for screws formed by die, (e) carriage bolt head formed by punch and die. University of Limerick Con Sheahan Advanced Manufacturing MF4038 47 Swaging Accomplished by rotating dies that hammer a workpiece radially inward to taper it as the piece is fed into the dies • Used to reduce diameter of tube or solid rod stock • Mandrel sometimes required to control shape and size of internal diameter of tubular parts University of Limerick Con Sheahan Advanced Manufacturing MF4038 48 24 Swaging Figure 19.24 Swaging process to reduce solid rod stock; the dies rotate as they hammer the work In radial forging, the workpiece rotates while the dies remain in a fixed orientation as they hammer the work. University of Limerick Con Sheahan Advanced Manufacturing MF4038 49 Trimming Cutting operation to remove flash from workpart in impression-die forging • Usually done while work is still hot, so a separate trimming press is included at the forging station • Trimming can also be done by alternative methods, such as grinding or sawing University of Limerick Con Sheahan Advanced Manufacturing MF4038 50 25 Trimming After Impression-Die Forging Figure 19.29 Trimming operation (shearing process) to remove the flash after impression-die forging. University of Limerick Con Sheahan Advanced Manufacturing MF4038 51 Extrusion Compression forming process in which work metal is forced to flow through a die opening to produce a desired cross-sectional shape • Process is similar to squeezing toothpaste out of a toothpaste tube • In general, extrusion is used to produce long parts of uniform cross sections • Two basic types: – Direct extrusion – Indirect extrusion University of Limerick Con Sheahan Advanced Manufacturing MF4038 52 26 Direct Extrusion Figure 19.30 Direct extrusion. University of Limerick Con Sheahan Advanced Manufacturing MF4038 53 Comments on Direct Extrusion • Also called forward extrusion • As ram approaches die opening, a small portion of billet remains that cannot be forced through die opening • This extra portion, called the butt, must be separated from extrudate by cutting it just beyond the die exit • Starting billet cross section usually round • Final shape of extrudate is determined by die opening University of Limerick Con Sheahan Advanced Manufacturing MF4038 54 27 Hollow and Semi-Hollow Shapes Figure 19.31 (a) Direct extrusion to produce a hollow or semi-hollow cross sections; (b) hollow and (c) semi-hollow cross sections. University of Limerick Con Sheahan Advanced Manufacturing MF4038 55 Indirect Extrusion Figure 19.32 Indirect extrusion to produce (a) a solid cross section and (b) a hollow cross section. University of Limerick Con Sheahan Advanced Manufacturing MF4038 56 28 Comments on Indirect Extrusion • Also called backward extrusion and reverse extrusion • Limitations of indirect extrusion are imposed by – Lower rigidity of hollow ram – Difficulty in supporting extruded product as it exits die University of Limerick Con Sheahan Advanced Manufacturing MF4038 57 Advantages of Extrusion • Variety of shapes possible, especially in hot extrusion – Limitation: part cross section must be uniform throughout length • Grain structure and strength enhanced in cold and warm extrusion • Close tolerances possible, especially in cold extrusion • In some operations, little or no waste of material University of Limerick Con Sheahan Advanced Manufacturing MF4038 58 29 Hot vs. Cold Extrusion • Hot extrusion - prior heating of billet to above its recrystallization temperature – Reduces strength and increases ductility of the metal, permitting more size reductions and more complex shapes • Cold extrusion - generally used to produce discrete parts – The term impact extrusion is used to indicate high speed cold extrusion University of Limerick Con Sheahan Advanced Manufacturing MF4038 59 Extrusion Ratio Also called the reduction ratio, it is defined as Ao Af where rx = extrusion ratio; Ao = cross-sectional area of the starting billet; and Af = final cross-sectional area of the extruded section • Applies to both direct and indirect extrusion rx University of Limerick Con Sheahan Advanced Manufacturing MF4038 60 30 Extrusion Die Features Figure 19.35 (a) Definition of die angle in direct extrusion; (b) effect of die angle on ram force. University of Limerick Con Sheahan Advanced Manufacturing MF4038 61 Comments on Die Angle • Low die angle - surface area is large, which increases friction at die-billet interface – Higher friction results in larger ram force • Large die angle - more turbulence in metal flow during reduction – Turbulence increases ram force required • Optimum angle depends on work material, billet temperature, and lubrication University of Limerick Con Sheahan Advanced Manufacturing MF4038 62 31 Orifice Shape of Extrusion Die • Simplest cross section shape is circular die orifice • Shape of die orifice affects ram pressure • As cross section becomes more complex, higher pressure and greater force are required • Effect of cross-sectional shape on pressure can be assessed by means the die shape factor Kx University of Limerick Con Sheahan Advanced Manufacturing MF4038 63 Complex Cross Section Figure 19.36 A complex extruded cross section for a heat sink (photo courtesy of Aluminum Company of America) University of Limerick Con Sheahan Advanced Manufacturing MF4038 64 32 Extrusion Presses • Either horizontal or vertical – Horizontal more common • Extrusion presses - usually hydraulically driven, which is especially suited to semi-continuous direct extrusion of long sections • Mechanical drives - often used for cold extrusion of individual parts University of Limerick Con Sheahan Advanced Manufacturing MF4038 65 Wire and Bar Drawing Cross-section of a bar, rod, or wire is reduced by pulling it through a die opening • Similar to extrusion except work is pulled through die in drawing (it is pushed through in extrusion) • Although drawing applies tensile stress, compression also plays a significant role since metal is squeezed as it passes through die opening University of Limerick Con Sheahan Advanced Manufacturing MF4038 66 33 Wire and Bar Drawing Figure 19.40 Drawing of bar, rod, or wire. University of Limerick Con Sheahan Advanced Manufacturing MF4038 67 Area Reduction in Drawing Change in size of work is usually given by area reduction: A Af r o Ao where r = area reduction in drawing; Ao = original area of work; and Ar = final work University of Limerick Con Sheahan Advanced Manufacturing MF4038 68 34 Wire Drawing vs. Bar Drawing • Difference between bar drawing and wire drawing is stock size – Bar drawing - large diameter bar and rod stock – Wire drawing - small diameter stock - wire sizes down to 0.03 mm (0.001 in.) are possible • Although the mechanics are the same, the methods, equipment, and even terminology are different University of Limerick Con Sheahan Advanced Manufacturing MF4038 69 Drawing Practice and Products • Drawing practice: – Usually performed as cold working – Most frequently used for round cross sections • Products: – Wire: electrical wire; wire stock for fences, coat hangers, and shopping carts – Rod stock for nails, screws, rivets, and springs – Bar stock: metal bars for machining, forging, and other processes University of Limerick Con Sheahan Advanced Manufacturing MF4038 70 35 Bar Drawing • Accomplished as a single-draft operation - the stock is pulled through one die opening • Beginning stock has large diameter and is a straight cylinder • Requires a batch type operation University of Limerick Con Sheahan Advanced Manufacturing MF4038 71 Bar Drawing Bench Figure 19.41 Hydraulically operated draw bench for drawing metal bars. University of Limerick Con Sheahan Advanced Manufacturing MF4038 72 36 Wire Drawing • Continuous drawing machines consisting of multiple draw dies (typically 4 to 12) separated by accumulating drums – Each drum (capstan) provides proper force to draw wire stock through upstream die – Each die provides a small reduction, so desired total reduction is achieved by the series – Annealing sometimes required between dies to relieve work hardening University of Limerick Con Sheahan Advanced Manufacturing MF4038 73 Continuous Wire Drawing Figure 19.42 Continuous drawing of wire. University of Limerick Con Sheahan Advanced Manufacturing MF4038 74 37 Features of a Draw Die • Entry region - funnels lubricant into the die to prevent scoring of work and die • Approach - cone-shaped region where drawing occurs • Bearing surface - determines final stock size • Back relief - exit zone - provided with a back relief angle (half-angle) of about 30 • Die materials: tool steels or cemented carbides University of Limerick Con Sheahan Advanced Manufacturing MF4038 75 Draw Die Details Figure 19.43 Draw die for drawing of round rod or wire. University of Limerick Con Sheahan Advanced Manufacturing MF4038 76 38 Preparation of Work for Drawing • Annealing – to increase ductility of stock • Cleaning - to prevent damage to work surface and draw die • Pointing – to reduce diameter of starting end to allow insertion through draw die University of Limerick Con Sheahan Advanced Manufacturing MF4038 77 39 MF4038 Advanced Manufacturing PROCESSING OF INTEGRATED CIRCUITS 1. Overview of IC Processing 2. Silicon Processing 3. Lithography 4. Layer Processes Use in IC Fabrication 5. Integrating the Fabrication Steps 6. IC Packaging 7. Yields in IC Processing University of Limerick Con Sheahan Advanced Manufacturing MF4038 1 Integrated Circuit (IC) A collection of electronic devices such as transistors, diodes, and resistors that have been fabricated and electrically intraconnected onto a small flat chip of semiconductor material Silicon (Si) - most widely used semiconductor material for ICs Less common: germanium (Ge) and gallium arsenide (GaAs) Since circuits are fabricated into one solid piece of material, the term solid state electronics is used for IC technology University of Limerick Con Sheahan Advanced Manufacturing MF4038 2 1 Levels of Integration in Microelectronics Integration level year Small scale integration (SSI) 1959 Medium scale integration (MSI) 1960s Large scale integration (LSI) Very large scale integration (VLSI) Ultra large scale integration (ULSI) Giga scale integration University of Limerick Con Sheahan Number devices Approx. 10 - 50 50 - 103 103 - 104 104 - 106 106 - 108 109 - 1010 1970s 1980s 1990s 2000s Advanced Manufacturing MF4038 3 Overview of IC Technology An integrated circuit consists of hundreds, thousands, or millions of microscopic electronic devices that have been fabricated and electrically intraconnected on the surface of a silicon chip A chip is a square or rectangular flat plate that is about 0.5 mm (0.020 in) thick and typically 5 to 25 mm (0.2 to 1.0 in) on a side Each electronic device on the chip surface consists of separate layers and regions with different electrical properties combined to perform a particular electronic function University of Limerick Con Sheahan Advanced Manufacturing MF4038 4 2 IC Transistor Figure 35.1 Cross section of a transistor in an integrated circuit. Approximate size of the device is shown; feature sizes within the device can be less than 40 nm with current technology. *2016 device feature sizes down to 10 nm University of Limerick Con Sheahan Advanced Manufacturing MF4038 5 Integrated Circuit Highly magnified photograph of an Intel Pentium microprocessor (photo courtesy of Intel Corporation). University of Limerick Con Sheahan Advanced Manufacturing MF4038 6 3 iPhone 4S Circuit Board University of Limerick Con Sheahan Advanced Manufacturing MF4038 7 Processing Sequence for Silicon ICs 1. Silicon processing - sand is reduced to very pure silicon and then shaped into wafers 2. IC fabrication - processing steps that add, alter, and remove thin layers in selected regions to form electronic devices Lithography is used to define the regions to be processed on wafer surface 3. IC packaging - wafer is tested, cut into individual chips, and the chips are encapsulated in an appropriate package University of Limerick Con Sheahan Advanced Manufacturing MF4038 8 4 Sequence in IC Processing Figure 35.3 Sequence of processing steps in the production of integrated circuits: (1) pure silicon is formed from the molten state into an ingot and then sliced into wafers; (2) fabrication of integrated circuits on the wafer surface; and (3) wafer is cut into chips and packaged. University of Limerick Con Sheahan Advanced Manufacturing MF4038 9 Clean Rooms Much of the processing of ICs must be carried out in a clean room Ambiance of a clean room is more like a hospital operating room than a production factory Cleanliness is dictated by the microscopic feature sizes in an IC, the scale of which continues to decrease each year University of Limerick Con Sheahan Advanced Manufacturing MF4038 10 5 Trends in IC Feature Size Trend in device feature size in IC fabrication; also shown is the size of common airborne particles that can contaminate the processing environment. University of Limerick Con Sheahan Advanced Manufacturing MF4038 11 Clean Room Classification A number (in increments of ten) used to indicate the quantity of particles of size 0.5 m or greater in one cubic foot of air Class 100 clean room must maintain a count of particles of size 0.5 m or greater at less than 100/ft3 Class 10 clean room must maintain a count of particles of size 0.5 m or greater at less than 10/ft3 A clean room is air conditioned to 21C (70F) and 45% relative humidity University of Limerick Con Sheahan Advanced Manufacturing MF4038 12 6 Silicon Processing University of Limerick Microelectronic chips are fabricated on a substrate of semiconductor material Silicon is the leading semiconductor material today More than 95% of all semiconductor devices produced in the world Preparation of silicon substrate can be divided into three steps: 1. Production of electronic grade silicon 2. Crystal growing 3. Shaping of Si into wafers Con Sheahan Advanced Manufacturing MF4038 13 Electronic Grade Silicon Silicon is one of the most abundant materials in the earth's crust, occurring naturally as silica (e.g., sand) and silicates (e.g., clay) Principal raw material for silicon is quartzite, which is very pure SiO2 Electronic grade silicon (EGS) is polycrystalline silicon of ultra high purity So pure that impurities are measured in parts per billion (ppb) University of Limerick Con Sheahan Advanced Manufacturing MF4038 14 7 Crystal Growing The silicon substrate for microelectronic chips must be made of a single crystal whose unit cell is oriented in a certain direction Silicon used in semiconductor device fabrication must be of ultra high purity Substrate wafers must be cut in a direction that achieves the desired planar orientation Most widely used crystal growing method is the Czochralski process, in which a single crystal boule is pulled from a pool of molten silicon University of Limerick Con Sheahan Advanced Manufacturing MF4038 15 Czochralski Process Czochralski process for growing single crystal ingots of silicon: (a) initial setup prior to start of crystal pulling, and (b) during crystal pulling to form the boule. University of Limerick Con Sheahan Advanced Manufacturing MF4038 16 8 Shaping of Silicon into Wafers • Processing steps to reduce the boule into thin, disc-shaped wafers 1. Ingot (boule) preparation 2. Wafer slicing 3. Wafer preparation University of Limerick Con Sheahan Advanced Manufacturing MF4038 17 Preparation of the Boule The ends of the boule are cut off Cylindrical grinding is used to shape the boule into a more perfect cylinder One or more flats are ground along the length of the boule, whose functions, after the boule is cut into wafers, are: Identification Orientation of ICs relative to crystal structure Mechanical location during processing University of Limerick Con Sheahan Advanced Manufacturing MF4038 18 9 Grinding Operations on Boule Grinding operations used in shaping the silicon ingot: (a) a form of cylindrical grinding provides diameter and roundness control, and (b) a flat ground on the cylinder. University of Limerick Con Sheahan Advanced Manufacturing MF4038 19 Wafer Slicing • Cutting edge is a very thin ring-shaped saw blade with diamond grit on internal diameter • ID used for slicing rather than the OD for better control over flatness, thickness, parallelism, and surface characteristics of the wafer • Wafers are cut 0.5-0.7 mm (0.020-0.028 in.) thick, greater thicknesses for larger wafer diameters – To minimize kerf loss, blades are very thin: thickness ~ 0.33 mm (0.013 in) University of Limerick Con Sheahan Advanced Manufacturing MF4038 20 10 Wafer Slicing Wafer slicing using a diamond abrasive cut-off saw. University of Limerick Con Sheahan Advanced Manufacturing MF4038 21 Wafer Preparation • Wafer rims are rounded by contour-grinding wheel to reduce chipping during handling • Wafers are chemically etched to remove surface damage from slicing • A flat polishing operation is performed to provide surfaces of high smoothness for photolithography processes to follow • Finally, wafer is chemically cleaned to remove residues and organic films University of Limerick Con Sheahan Advanced Manufacturing MF4038 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e 22 11 Furnace at over 1000C (1800F) glows red as it awaits silicon wafers for baking. University of Limerick Con Sheahan Advanced Manufacturing MF4038 23 Lithography An IC consists of many microscopic regions on the wafer surface that make up the devices and intraconnections as specified in the circuit design In the planar process, regions are fabricated by steps that add, alter, or remove layers in selected areas of the wafer surface Each layer is determined by a geometric pattern representing circuit design information that is transferred to the wafer surface by lithography University of Limerick Con Sheahan Advanced Manufacturing MF4038 24 12 Lithographic Technologies • Several lithographic technologies are used in semiconductor processing: – Photolithography – Electron lithography – X-ray lithography – Ion lithography • The differences are in type of radiation used to transfer the mask pattern to the wafer surface University of Limerick Con Sheahan Advanced Manufacturing MF4038 25 Photolithography Uses light radiation to expose a coating of photoresist on the surface of the wafer Common light source in wafer processing is ultraviolet light, due to its short wavelength A mask containing the required geometric pattern for each layer separates the light source from the wafer, so that only the portions of the photoresist not blocked by the mask are exposed University of Limerick Con Sheahan Advanced Manufacturing MF4038 26 13 Photolithography Mask Flat plate of transparent glass onto which a thin film of an opaque substance has been deposited in certain areas to form the desired pattern • Thickness of glass plate is around 2 mm (0.080 in), while deposited film is only a few m thick - in some film materials, less than 1 m • The mask itself is fabricated by lithography, the pattern being based on circuit design data, usually in the form of digital output from the CAD system used by circuit designer University of Limerick Con Sheahan Advanced Manufacturing MF4038 27 Photoresist Organic polymer that is sensitive to light radiation in a certain wavelength range Sensitivity causes either increase or decrease in solubility of the polymer to certain chemicals Typical practice in semiconductor processing is to use photoresists sensitive to UV light UV light has a short wavelength compared to visible light, permitting sharper imaging of circuit details on wafer surface Also permits fabrication areas in plant to be illuminated at low light levels outside UV band University of Limerick Con Sheahan Advanced Manufacturing MF4038 28 14 Contact Printing Mask is pressed against resist coating during exposure Advantage: high resolution of the pattern onto wafer surface Disadvantage: physical contact with wafers gradually wears out mask Figure 35.10 Photolithography exposure techniques: (a) contact printing. University of Limerick Con Sheahan Advanced Manufacturing MF4038 29 Proximity Printing • Mask is separated from the resist coating by a distance of 10-25 m (0.0004-0.001 in) • Eliminates mask wear, but image resolution is slightly reduced Photolithography exposure techniques: (b) proximity printing. University of Limerick Con Sheahan Advanced Manufacturing MF4038 30 15 Projection Printing • High-quality lenses project image through mask onto wafer • Preferred technique because non-contact (thus, no mask wear), and optical projection can obtain high resolution Photolithography exposure techniques: (c) projection printing. University of Limerick Con Sheahan Advanced Manufacturing MF4038 31 Photolithography Processing Sequence Surface of the silicon wafer has been oxidized to form a thin film of SiO2 It is desired to remove the SiO2 film in certain regions as defined by mask pattern Sequence for a negative resist proceeds as follows: University of Limerick Con Sheahan Advanced Manufacturing MF4038 32 16 Photolithography Processing Sequence 1. The wafer is properly cleaned to promote wetting and adhesion of resist Photolithography process applied to a silicon wafer: (1) prepare surface. University of Limerick Con Sheahan Advanced Manufacturing MF4038 33 Photolithography Processing Sequence 2. A metered amount of liquid resist is fed onto center of wafer and wafer is spun to spread liquid and achieve uniform coating thickness apply photoresist University of Limerick Con Sheahan Advanced Manufacturing MF4038 34 17 Photolithography Processing Sequence 3. Soft bake - purpose is to remove solvents, promote adhesion, and harden resist • Temperature 90C (190F) for 10-20 minutes (3) soft-bake University of Limerick Con Sheahan Advanced Manufacturing MF4038 35 Photolithography Processing Sequence 4. Pattern mask is aligned relative to wafer and resist is exposed through mask (4) align mask and expose University of Limerick Con Sheahan Advanced Manufacturing MF4038 36 18 Photolithography Processing Sequence 5. Exposed wafer is immersed in developing solution, or solution is sprayed onto surface For negative resist, unexposed areas are dissolved, thus leaving SiO2 surface uncovered in these areas (5) develop resist University of Limerick Con Sheahan Advanced Manufacturing MF4038 37 Photolithography Processing Sequence 6. Hard bake to expel volatiles remaining from developing solution and increases adhesion of resist especially at newly created edges of resist film (6) hard-bake University of Limerick Con Sheahan Advanced Manufacturing MF4038 38 19 Photolithography Processing Sequence 7. Etching removes SiO2 layer at selected regions where resist has been removed (7) etch University of Limerick Con Sheahan Advanced Manufacturing MF4038 39 Photolithography Processing Sequence 8. Resist coating remaining on surface is removed Stripping is accomplished using either liquid chemicals or plasma etching Figure 35.11 (8) strip resist University of Limerick Con Sheahan Advanced Manufacturing MF4038 40 20 Other Lithography Techniques • As feature size in integrated circuits continues to decrease and UV photolithography becomes increasingly inadequate, other lithography techniques that offer higher resolution are growing in importance: – Extreme ultraviolet (EUV) lithography – Electron beam lithography – X-ray lithography – Ion lithography University of Limerick Con Sheahan Advanced Manufacturing MF4038 41 Layer Processes Used in IC Fabrication Steps to fabricate ICs on a silicon wafer consist of chemical and physical processes that add, alter, or remove regions that have been defined by photolithography Regions are insulating, semiconducting, and conducting areas that form the devices and their intraconnections in the IC Layers are fabricated one at a time, each layer requiring a separate mask, until all of the details of the electronic devices and conducting paths have been fabricated onto the wafer surface University of Limerick Con Sheahan Advanced Manufacturing MF4038 42 21 Layering Processes in IC Fabrication • Thermal oxidation – adds SiO2 layer on Si substrate • Chemical vapor deposition - adds various layers • Diffusion and ion implantation - alter chemistry of an existing layer or substrate • Metallization processes - add metal layers for electrical conduction • Etching processes - remove portions of layers to achieve desired IC details University of Limerick Con Sheahan Advanced Manufacturing MF4038 43 Thermal Oxidation of Silicon Exposure of silicon wafer surface to an oxidizing atmosphere at elevated temperature to form layer of silicon dioxide Oxygen or steam atmospheres are used, with the following reactions, respectively: Si + O2 SiO2 or Si + 2H2O SiO2 + 2H2 University of Limerick Con Sheahan Advanced Manufacturing MF4038 44 22 Functions of Silicon Dioxide (SiO2) SiO2 is an insulator, compared to Si which is a semiconductor • Used as a mask to prevent diffusion or ion implantation of dopants into silicon • Can be used to isolate devices in circuit • Provides electrical insulation between levels in multi-level metallization systems University of Limerick Con Sheahan Advanced Manufacturing MF4038 45 Thermal Oxidation Figure 35.12 Growth of SiO2 film on a silicon substrate by thermal oxidation, showing changes in thickness that occur: (1) before oxidation and (2) after thermal oxidation. University of Limerick Con Sheahan Advanced Manufacturing MF4038 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e 46 23 Alternative Process for Adding SiO2 When a silicon dioxide film must be applied to surfaces other than silicon, then direct thermal oxidation does not work An alternative process must be used, such as chemical vapor deposition University of Limerick Con Sheahan Advanced Manufacturing MF4038 47 Introduction of Impurities into Silicon University of Limerick IC technology relies on the ability to alter the electrical properties of silicon by introducing impurities into selected regions of the surface Doping - adding impurities into silicon surface Common doping elements are boron (B) which forms electron acceptor regions; and phosphorous (P), arsenic (As), and antimony (Sb), which form electron donor regions Techniques for doping silicon: 1. Thermal diffusion 2. Ion implantation Con Sheahan Advanced Manufacturing MF4038 48 24 Thermal Diffusion Process in which atoms migrate from regions of high concentration into regions of lower concentration • In semiconductor processing, diffusion is carried out to dope the silicon substrate with controlled amounts of a desired impurity • Two steps in thermal diffusion: 1. Predeposition - dopant is deposited onto wafer surface 2. Drive-in - heat treatment in which dopant is redistributed to obtain the desired depth and concentration profile University of Limerick Con Sheahan Advanced Manufacturing MF4038 49 Ion Implantation Vaporized ions of impurity element are accelerated by an electric field and directed at silicon substrate Atoms penetrate into surface, losing energy and finally stopping at some depth in crystal structure determined by mass of ion and acceleration voltage Advantages: Can be accomplished at room temperature Provides exact doping density University of Limerick Con Sheahan Advanced Manufacturing MF4038 50 25 Metallization Combines various thin film deposition technologies with photolithography to form very fine patterns of conductive material Functions of conductive materials on wafer surface: Form certain components (e.g., gates) of IC devices Provide intraconnecting conduction paths between devices on chip Connect the chip to external circuits University of Limerick Con Sheahan Advanced Manufacturing MF4038 51 Metallization Materials Aluminum - most widely used metallization material Favored for device intraconnections and connections to external circuitry Other materials: polysilicon (Si); gold (Au); refractory metals (e.g., W, Mo); silicides (e.g., WSi2, MoSi2, TaSi2); and nitrides (e.g., TaN, TiN, and ZrN) Applications such as gates and contacts University of Limerick Con Sheahan Advanced Manufacturing MF4038 52 26 Metallization Processes • Physical vapor deposition – PVD metallization processes include vacuum evaporation and sputtering • Chemical vapor deposition – CVD deposited materials include tungsten, molybdenum, and most silicides used in semiconductor metallization • Electroplating - occasionally used to increase thickness of thin films University of Limerick Con Sheahan Advanced Manufacturing MF4038 53 Etching Certain steps in IC manufacturing require material removal from surface, accomplished by etching away unwanted material Usually done selectively, by masking surface areas that are to be protected and leaving other areas exposed Two categories of etching process: 1. Wet chemical etching 2. Dry plasma etching University of Limerick Con Sheahan Advanced Manufacturing MF4038 54 27 Wet Chemical Etching Use of an aqueous solution, usually an acid, to etch away a target material Etchant is selected to chemically attack the specific material to be removed and not the protective layer Process variables: immersion time, etchant concentration, and temperature In its simplest form, etching is accomplished by immersing the masked wafers in etchant for a specified time and then immediately rinsing to stop the etching University of Limerick Con Sheahan Advanced Manufacturing MF4038 55 Chemical Etching Chemical etching reaction is isotropic, causing an undercut below protective mask Mask pattern (resist) must be sized to compensate for this effect Profile of a properly etched layer. University of Limerick Con Sheahan Advanced Manufacturing MF4038 56 28 Dry Plasma Etching Uses an ionized gas to etch a target material Ionized gas is created by introducing an appropriate gas mixture into vacuum chamber and RF electrical energy is used to ionize a portion of the gas to create a plasma The high energy plasma reacts with the target surface, vaporizing material to remove it University of Limerick Con Sheahan Advanced Manufacturing MF4038 57 Process Integration in IC Fabrication • An n-channel metal oxide semiconductor (NMOS) logic device will be used to illustrate processing sequence • Sequence for NMOS ICs is less complex than for CMOS or bipolar technologies, although processes for these IC categories are similar • Starting substrate is a lightly doped p-type silicon wafer, which will form the base of n-channel transistor University of Limerick Con Sheahan Advanced Manufacturing MF4038 58 29 IC Fabrication Sequence 1. A layer of Si3N4 is deposited by CVD onto Si substrate using photolithography to define the regions – the layer will serve as a mask for thermal oxidation in step (2) IC fabrication sequence: (1) Si3N4 mask is deposited by CVD on Si substrate University of Limerick Con Sheahan Advanced Manufacturing MF4038 59 IC Fabrication Sequence 2. SiO2 is grown in exposed regions of surface by thermal oxidation SiO2 regions are insulating and will isolate this device from other devices (2) SiO2 is grown by thermal oxidation in unmasked regions University of Limerick Con Sheahan Advanced Manufacturing MF4038 60 30 IC Fabrication Sequence 3. The Si3N4 mask is stripped by etching (3) the Si3N4 mask is stripped University of Limerick Con Sheahan Advanced Manufacturing MF4038 61 IC Fabrication Sequence 4. Another thermal oxidation is done to add a thin gate oxide layer to previously uncoated surfaces and to increase thickness of previous SiO2 layer (4) a thin layer of SiO2 is grown by thermal oxidation University of Limerick Con Sheahan Advanced Manufacturing MF4038 62 31 IC Fabrication Sequence 5. Polysilicon is deposited by CVD onto surface and then doped n-type using ion implantation (5) Polysilicon is deposited by CVD and doped n+ using ion implantation University of Limerick Con Sheahan Advanced Manufacturing MF4038 63 IC Fabrication Sequence 6. The polysilicon is selectively etched using photolithography to form gate electrode of transistor (6) the poly-Si is selectively etched using photolithography to define the gate electrode. University of Limerick Con Sheahan Advanced Manufacturing MF4038 64 32 IC Fabrication Sequence 7. Source and drain regions (n+) are formed by ion implantation of arsenic (As) into substrate, selecting an implantation energy level that penetrates the thin SiO2 layer but not the polysilicon gate or the thicker SiO2 isolation layer (7) source and drain regions are formed by doping n+ in the substrate University of Limerick Con Sheahan Advanced Manufacturing MF4038 65 IC Fabrication Sequence 8. Phosphosilicate glass (P-glass) is deposited onto the surface by CVD to protect the circuitry beneath (8) P-glass is deposited onto the surface for protection. University of Limerick Con Sheahan Advanced Manufacturing MF4038 66 33 IC Packaging The final series of operations to transform the wafer into individual chips, ready to connect to external circuits and prepared to withstand the harsh environment of the world outside the clean room Accomplished after all of the processing steps on the wafer have been completed University of Limerick Con Sheahan Advanced Manufacturing MF4038 67 Design Issues in IC Packaging Electrical connections to external circuits Materials to encase chip and protect it from the environment Humidity and corrosion Temperature Vibration and mechanical shock Heat dissipation Performance, reliability, and service life Cost University of Limerick Con Sheahan Advanced Manufacturing MF4038 68 34 Manufacturing Issues in IC Packaging • Chip separation - cutting wafer into individual chips • Connecting it to the package • Encapsulating the chip • Circuit testing University of Limerick Con Sheahan Advanced Manufacturing MF4038 69 IC Package Design Factors related to the design of an integrated circuit package: Number of input/output terminals required for an IC of a given size Materials used in IC packages Package styles University of Limerick Con Sheahan Advanced Manufacturing MF4038 70 35 Input/Output (I/O) Terminals Basic problem is to connect many internal circuits on the chip to I/O terminals so that the appropriate electrical signals can be communicated to the outside world As the number of devices in the IC increases, the required number of I/O terminals also increases The problem is aggravated by IC trends: Decreases in device size Increases in number of devices in IC University of Limerick Con Sheahan Advanced Manufacturing MF4038 71 IC Package Materials Ceramic (Al2O3) Advantages: hermetic sealing of IC chip and highly complex packages can be produced Disadvantage: poor dimensional control due to shrinkage during firing Plastic (epoxies, polyimides, and silicones) Not hermetically sealed, but cost is lower Generally used for mass produced ICs, where very high reliability is not required University of Limerick Con Sheahan Advanced Manufacturing MF4038 72 36 Two Basic Types of IC Package For assembling IC package to a printed circuit boards (PCB): 1. Through-hole mounting, also called pin-in-hole (PIH) technology – IC package and other components have leads inserted through holes in PCB and soldered on underside 2. Surface mount technology (SMT) – Components are attached to surface of board (in some cases, both top and bottom surfaces) University of Limerick Con Sheahan Advanced Manufacturing MF4038 73 Two Basic Types of IC Package Types of component lead attachment on a printed circuit board: (a) through-hole, and several styles of surface mount technology: (b) butt lead, (c) "J" lead, and (d) gull-wing. University of Limerick Con Sheahan Advanced Manufacturing MF4038 74 37 Major IC Package Styles • Dual in-line package (DIP) • Square package • Pin grid array • Some of these are available in both through-hole and surface mount styles, while others are designed for only one mounting method University of Limerick Con Sheahan Advanced Manufacturing MF4038 75 Dual In-Line Package (DIP) DIP is a very common form of IC package, available in both through-hole and surface mount configurations Dual in-line package with 16 terminals, shown here in through-hole configuration. University of Limerick Con Sheahan Advanced Manufacturing MF4038 76 38 Square Package Leads are arranged around periphery so that number of terminals on a side is nio/4 Square leaded chip carrier (LCC) for surface mounting with gull wing leads. University of Limerick Con Sheahan Advanced Manufacturing MF4038 77 Pin Grid Array (PGA) Two dimensional array of pin terminals on underside of a square chip enclosure Square matrix of pins maximizes number of leads on a package Ideally, entire bottom surface of package is fully occupied by pins, so pin count in each direction is square root of nio However, center area of package has no pins because this region contains IC chip University of Limerick Con Sheahan Advanced Manufacturing MF4038 78 39 Processing Steps in IC Packaging Wafer testing Chip separation Die bonding Wire bonding Package sealing Final testing University of Limerick Con Sheahan Advanced Manufacturing MF4038 79 Wafer Testing Testing (called multiprobe) is accomplished by computer-controlled equipment using needle probes that match connecting pads on the chip surface • Many of these tests are performed while ICs are still on wafer - before separation • When probes contact pads, tests are carried out to indicate short circuits and other faults, followed by a functional test • Chips that fail are marked with an ink dot – These defects will not be packaged University of Limerick Con Sheahan Advanced Manufacturing MF4038 80 40 Chip Separation Wafer is cut into individual chips using a thin diamond-impregnated saw blade • The wafer is attached to a piece of adhesive tape mounted in a frame – Adhesive tape holds individual chips in place during and after sawing – The frame is a convenience in subsequent handling of the chips • Chips with ink dots are now discarded University of Limerick Con Sheahan Advanced Manufacturing MF4038 81 Die Bonding • • Automated handling systems pick separated chips from tape frame and place them for die bonding Various techniques are used to bond the chip to the packaging substrate, including: 1. Eutectic die bonding – for ceramic packages 2. Epoxy die bonding – for plastic packages University of Limerick Con Sheahan Advanced Manufacturing MF4038 82 41 Wire Connections After die is bonded to package, electrical connections are made between contact pads on chip surface and package lead frame using small diameter wires Typical wire connection between chip contact pad and lead. University of Limerick Con Sheahan Advanced Manufacturing MF4038 83 Packaging of IC Chip Packaging of an integrated circuit chip: (a) cutaway view showing the chip attached to a lead frame and (b) encapsulated in a plastic enclosure. University of Limerick Con Sheahan Advanced Manufacturing MF4038 84 42 Final Testing Upon completion of packaging sequence, each IC must undergo a final test Purpose of test: Determine which units, if any, have been damaged during packaging Measure performance characteristics of each device University of Limerick Con Sheahan Advanced Manufacturing MF4038 85 Lecture – Electronic Packaging Electronics Packaging Is the physical means by which the components in a system are electrically interconnected and interfaced to external devices. It includes the mechanical structure that holds and protects the circuitry. A well designed electronics packages serves the following functions 1) Power distribution and signal interconnection 2) Structural support 3) Circuit protection from physical & chemical hazards 4) Dissipation of heat generated by the circuits 5) Minimum delays in signal transmission. University of Limerick Con Sheahan Advanced Manufacturing DM4038 86 43 Lecture – Electronic Packaging For complex systems the electronics package is organised into levels that comprise a “packaging hierarchy” University of Limerick Con Sheahan Advanced Manufacturing DM4038 87 Lecture – Electronic Packaging Level o = interconnections on the semi-conductor chip Level 1 = the packaged chip Level 2= is the printed circuit board assembly – pin-in-hole and SMT technology Level 3 = PCBs assembled to the chassis Level 4 – wiring and cabling inside the cabinet that contains the electronic system. University of Limerick Con Sheahan Advanced Manufacturing DM4038 88 44 Lecture – Printed Circuit Boards A PCB consists of one or more thin sheets of insulating material, with thin copper lines on one or both surfaces that interconnect the components attached to the board. In boards consisting of one or more layers, copper conducting paths are interleaved between the layers. PCBs are vital and widely used – because 1) They provide a structural platform for the components 2) A board with correctly routed interconnections can be mass produced consistently with out the variability associated with hand wiring 3) Nearly all of the soldering connection can be complete in a one-step automated process 4) An assembled PCB gives reliable performances 5) PCB can be dropped out for repair or replacement – without need to scrap the machine University of Limerick Con Sheahan Advanced Manufacturing DM4038 89 Lecture – Printed Circuit Boards A PCB is a laminated flat panel of insulating material designed to provide interconnection between electronic components attached to it. The conducting paths are made of copper and are called tracks Other copper areas called lands are available on the board surface for attaching and electrically connecting components Insulation materials in PCBs are usually polymer composites reinforces with glass fabrics or paper. Polymers include epoxy (widely used) and polyamide. E-glass is the usual fibre in glass-reinforcing fabrics, especially in epoxy PCBs. The usual thickness of the substrate layer is 0.8-3.2mm, and copper foil thickness or 0.04mm. University of Limerick Con Sheahan Advanced Manufacturing DM4038 90 45 Lecture – Printed Circuit Boards The materials forming the PCB structure must be •electrically insulating, •strong and rigid, •resistant to warpage, •dimensionally stable, •heat resistant •flame retardant. There are 3 principle types of PCB Single sided – where copper-foil is only on one side Double sided – where the copper foil is on both sides Multi-layer board –consisting of alternating layers of conducting foil and insulation. Multi-layered boards are used for complex circuit assemblies in which a large number of components must be interconnected with many track routings. University of Limerick Con Sheahan Advanced Manufacturing DM4038 91 Lecture – Printed Circuit Boards The materials forming the PCB structure must be •electrically insulating, •strong and rigid, •resistant to warpage, •dimensionally stable, •heat resistant •flame retardant. There are 3 principle types of PCB Single sided – where copper-foil is only on one side Double sided – where the copper foil is on both sides Multi-layer board –consisting of alternating layers of conducting foil and insulation. Multi-layered boards are used for complex circuit assemblies in which a large number of components must be interconnected with many track routings. University of Limerick Con Sheahan Advanced Manufacturing DM4038 92 46 Lecture – Printed Circuit Boards Copper Foil Insulating Substrate Single Sided Copper Foil Insulating Substrate Double Sided Copper Foil Copper Foil Insulating Substrate Multi-Layered Copper Foil Insulating Substrate Copper Foil University of Limerick Con Sheahan Advanced Manufacturing DM4038 93 Lecture – Printed Circuit Boards Production of the starting boards consists of pressing multiple sheets of woven glass that have been impregnated with partially cured epoxy. The number of sheets used in the starting sandwich determines the thickness of the final board. Copper foil is placed on one or both sides of the epoxy-glass laminated stack, depending on single, double or multi. Pressing is accomplished between tow steam-heated platens of a hydraulic press. The combination of the heat and pressure compacts and cures the epoxy glass layers to bond and harden the laminates into a single-piece board. The board is then cooled and trimmed to remove excess epoxy that has been squeezed out around the edges. PCBs are usually produced in large standard widths to match board handling systems on wave soldering, SMT equipment etc. University of Limerick Con Sheahan Advanced Manufacturing DM4038 94 47 Lecture – Printed Circuit Boards Glass Layer University of Limerick Glass Layer Con Sheahan Advanced Manufacturing DM4038 95 Lecture – Printed Circuit Boards The PCB fabricator uses a variety of processes to produce a finished PCB. Cleaning, shearing, drilling, Pattern imaging, etching and plating. Hole Drilling – tooling holes but also functional circuit holes. Insertion holes – for inserting component leads in plated through holes. Via holes which are later copper-plated and used as conduction paths from one side to the other. Fastening holes for connectors – not plated Circuit Pattern Imaging Pattern is transferred to the PCB using Photolithography, in which a light sensitive resist material is exposed through a mask to transfer the circuit pattern. University of Limerick Con Sheahan Advanced Manufacturing DM4038 96 48 Lecture – Printed Circuit Boards Dry film resists are commonly used in PCB fabrication. They consist of three layers a film of photosensitive polymer, sandwiched between a polyester support sheet on one side and a removable plastic cover sheet on the other. The cover sheet prevents the photosensitive material from sticking during storage and handling. To apply the cover sheer is removed and the resist film is placed on the copper surface. Hot rollers are used to press and smooth the resist onto the surface Alignment of the masks relative to the board relies on use of registration holes that are aligned with the tooling holds on the board. The interconnecting pattern is laid on top of the resist, the resist is the developed with involves removal of the unexposed regions of the negative resist from the surface. University of Limerick Con Sheahan Advanced Manufacturing DM4038 97 Lecture – Printed Circuit Boards After resist development certain areas of the copper surface remain covered by resist while other areas are now unprotected. The covered areas corresponded to the circuit tracks and lands (interconnecting pattern) The uncovered areas represent areas for removal Etching removes the copper cladding in the unprotected regions from the board by means of a chemical etchant. Etching is the step which transforms the solid copper film into the interconnecting pattern of the circuit Etching is done in an etching chamber in which the etchant is sprayed onto the surface of the PCB. Various etchants can be used e.g. ammonium hydroxide (NH4OH), cupric chloride (CuCl2) and ferric chloride (FeCl3) Parameters such as temperature, concentration and duration must be closely controlled to avoid under or over etching. After etching the PCB must be washed and rinsed. University of Limerick Con Sheahan Advanced Manufacturing DM4038 98 49 Lecture – Printed Circuit Boards A photographic plate carries the specified pattern The plate is placed above the board and exposed to light Photosensitive Layer Copper Substrat e Phase 2 Phase 1 Phase 3 The light turns the photosensitive solution opaque where the copper is required. The laminate is then Exposed to a chemical protectant and then placed in etching solution Effluents from PCB manufacture PCB Manufacture Air Emissions Tin Sulphuric The result of the above process is a PCB with excess copper removed leaving solely the copper which is needed to from a circuit Copper Sulphates Vinyl polymer Nitric Phosphoric Ammonia Phase 4 University of Limerick Con Sheahan Advanced Manufacturing DM4038 99 Lecture – Printed Circuit Boards – Through Hole Pin-in-hole follows the usual steps, component insertion, soldering, cleaning, testing and rework. Component insertion is accomplished by automatic insertion machines Some manual placement may be required for difficult – non standard components. Insertion involves, forming the leads, inserting the leads and cropping at the other side of the board. University of Limerick Con Sheahan Advanced Manufacturing DM4038 100 50 Lecture – Printed Circuit Boards – Through Hole Soldering can take place by hand or using a wave soldering machine. Wave soldering is where a PCB containing inserted components is moved by conveyor over a standing wave of molten solder. The position of the conveyor is such that only the underside of the board with components leads projecting through the holes is in contact with the solder. Capillary action and upward force of the wave cause the liquid solder to flow into the clearance between the leads and the hole to ensure a good solder joint. To aid soldering flux may be sprayed onto the under side of the PCB prior to soldering to clean the leads and help with solder activation. University of Limerick Con Sheahan Advanced Manufacturing DM4038 101 Lecture – Printed Circuit Boards – Through Hole Soldering can take place by hand or using a wave soldering machine. Purpose of Flux - Main role is to promote the flow of solder - Removes light oxidation Evaporate the solvents from the flux - Activate the flux - Prevent thermal cracking of components - Reduces thermal shock - Allows proper solder flow onto the board University of Limerick Con Sheahan Advanced Manufacturing DM4038 102 51 Lecture – Printed Circuit Boards – Through Hole University of Limerick Con Sheahan Advanced Manufacturing DM4038 103 Lecture – Printed Circuit Boards – Through Hole Conveyor Angle The assembly is conveyed, usually up a 4° to 12° slope, until its bottom surface contacts the crest of the solder wave, where the pads, protruding leads, plated holes, and bottom side surface mounted components are soldered. University of Limerick Con Sheahan Advanced Manufacturing DM4038 104 52 Lecture – Printed Circuit Boards – Surface Mount Surface mount technology uses an assembly method in which component leads are soldered directly on to the surface of the PCB rather than running through the board Eliminating the need for leads inserted into through holes – gives several advantages. Smaller components cant be made Packing densities can be increases Components can be mounted on both sides of the PCB Smaller PCBs can be used for the same system Drilling of multiple holes in the PCB is eliminated.. Typical areas of the board surface taken by SMT components range between 20and 60% compared to through hole components University of Limerick Con Sheahan Advanced Manufacturing DM4038 105 Lecture – Printed Circuit Boards – Surface Mount A solder paste is a suspension of solder powers in a flux binder. It has three functions 1 it is the solder – typically 80-90% of paste volume is solder. 2) It holds the flux 3) It is the adhesive that secures the components to the surface of the board. Leaded solder Typical leaded solder constitutes a mix of Tin and lead normally 63% tin and 37% lead Leaded solder will become molten at 183 degree C. Lead free solder Typical leaded solder constitutes a mix of Tin, silver and copper or Tin/silver. 96.5% Tin, 3% silver and 0.5% copper. Lead free solder will become molten at 217 degree C. University of Limerick Con Sheahan Advanced Manufacturing DM4038 106 53 Printed Circuit Boards – Surface Mount Soldering Process Solder Land Lead of Component Reflow Gaps in Stencil Allows solder to be placed on the PCB University of Limerick Solder Joint Stencil Con Sheahan Advanced Manufacturing DM4038 107 Printed Circuit Boards – Surface Mount Soldering Process University of Limerick Con Sheahan Advanced Manufacturing DM4038 108 54 Printed Circuit Boards – Surface Mount Placement Small components Screen Printer Board Loader Placement Large components For double sided sequence is repeated Wave Soldering AOI Reflow Electrical Testing University of Limerick Con Sheahan Advanced Manufacturing DM4038 109 Printed Circuit Boards – Surface Mount Screen Printing Paste can be applied directly using a spatula Using a rehometric pump Or a loaded cartridge Printing set-up such as stencil thickness, pressure, snap-off height influence the paste deposits. Typically when using a stencil contact printing is used. Increasing pressure can influence the life of the stencil – too much pressure may tear the stencil Typically the squeegee and stencil are made from stainless steel, stencil can vary from 5 thou to 8 thou thickness. Thicker stencil = more paste deposited. University of Limerick Con Sheahan Advanced Manufacturing DM4038 110 55 Printed Circuit Boards – Surface Mount Manual application Rheometric Solder Pump University of Limerick Con Sheahan Advanced Manufacturing DM4038 111 Printed Circuit Boards – Surface Mount Placement Rate: 20,000 cph 9,000 cph (C&P Head) 1,800 (P&P Head) Placement Accuracy: +68microm 3 Sigma (C&P Head) 38micro&nbsp;3 Sigma (P&P head) University of Limerick Con Sheahan Manual application Advanced Manufacturing DM4038 112 56 Printed Circuit Boards – Surface Mount Components held in reels When the reel runs out or preferably before it runs out an operator splices a second reel on – giving continuous supply of components Protective tape is rolled back and the placement nozzle drops down to pick the component from the reel. Vacuum is applied, and the component is pick and then placed on the board. University of Limerick Con Sheahan Advanced Manufacturing DM4038 113 Components held in reels Printed Circuit Boards – Surface Mount When the reel runs out or preferably before it runs out an operator splices a second reel on – giving continuous supply of components Protective tape is rolled back and the placement nozzle drops down to pick the component from the reel. Vacuum is applied, and the component is pick and then placed on the board. The placement machine holds the coordinates of the PCB on file, using reference geometry from known zero point the various components are placed. Each component has an identifier, e.g. component 1 may be a capacitor and located in X 125.3 Y 325.7 from the (0,0) University of Limerick Con Sheahan Advanced Manufacturing DM4038 114 57 Printed Circuit Boards – Surface Mount Inspection of the placed component prior to reflow may occur to check for position, accuracy, solder quality etc. PCB is then transferred to the reflow oven. Dependant on the PCB, solder type, component packing density the engineer sets the various zones in the oven to a profile University of Limerick Con Sheahan Advanced Manufacturing DM4038 115 Printed Circuit Boards – Surface Mount There are a number of zones in the over, these control the temperature of the PCB in the oven. The PCB must be preheated, to ensure components don’t crack or tombstone and to activate flux. The solder is held over its melting point. This allows the solder to wet to the leads of the components. The PCB is then cooled in a controlled process to avoid thermal shock. Nitrogen can be introduced to help soldering – this is more costly University of Limerick Con Sheahan Advanced Manufacturing DM4038 116 58 Typical lead-free solder Reflow Profile. Pre-heating, soaking, Reflow Zone and cooldown University of Limerick Con Sheahan Printed Circuit Boards – Surface Mount Advanced Manufacturing DM4038 117 Printed Circuit Boards – Surface Mount Typical leaded solder Reflow Profile. Pre-heating, soaking, Reflow Zone and cooldown – take note of the different temperatures University of Limerick Con Sheahan Advanced Manufacturing DM4038 118 59 Printed Circuit Boards – Surface Mount Inspection Automated Optimal inspection (AOI) typically takes place after reflow to ensure the PCB is fully and correctly populated. Due to the complex nature of the process and large number of process variables, there can be a number of defects attributed to the manufacturing process. These include Insufficient Paste This is where there is insufficient solder to form a mechanical or electrical connection to the PCB. This can be caused by a blockage in the stencil, forgetfulness on behalf of the operator, dry paste and others. The corrective action is typically to use a stencil wipe (automated every 5 PCBs) to prevent stencil from becoming blocked. Blockage University of Limerick Solder in position Con Sheahan Advanced Manufacturing DM4038 119 Printed Circuit Boards – Surface Mount Solder Balls Where excess solder can group together and form balls on the surface of the PCB, because they are not adhered to a component they may become dislodge easily and cause short circuits in the circuit. Caused by incorrect cleaning of PCBs, too much solder, wiping of PCB Solved by following correct operating procedures, wash protocols, adjusting squeegee pressure University of Limerick Con Sheahan Advanced Manufacturing DM4038 120 60 Printed Circuit Boards – Surface Mount Solder bridging Where solder forms a bridge between leads of a component there by short circuiting the component Caused by too much solder, incorrect squeegee pressure, poor solder masking, stencil too thick, Printing environment too warm (paste less viscous) Solution to ensure a regulated temperature environment, ensure quality incoming PCBs, examine stencil thickness and ensure its correct. University of Limerick Con Sheahan Advanced Manufacturing DM4038 121 Printed Circuit Boards – Surface Mount Tombstoning Components Caused by minute differences in wetting force from one side of a component to the other. When there is a sufficient imbalance in torque, relative to the mass of the component, the component is tipped upright (tombstoned), consigning the product to either scrap or rework. Caused by misalignment of paste, component terminations, pad geometries (one pad bigger than other) Corrective action – check paste is correct position – may need to jog the stencil, design in correct pad geometries and perform solderability testing of components to check for soldering. University of Limerick Con Sheahan Advanced Manufacturing DM4038 122 61 Printed Circuit Boards – Surface Mount Not soldered Where paste is present but hasn’t adhered to the component. Results in an incomplete joint in image below, solder should flow up and around the toe of the component however the solder is repelled away from the toe and remains on the land. Cause could be poor component solerability, incorrect plating finish, incorrect reflow profile – not melting solder properly Check components, check profile, check paste is in correct position, maybe try thicker stencil University of Limerick Con Sheahan Advanced Manufacturing DM4038 123 Printed Circuit Boards – Surface Mount Flipped Component Typically the component should sit as per Figure 1 – with the larger surface area on the pad. However sometimes it may be placed on its edge – this is a flipped component (Figure 2) Figure 2 Plan view Figure 1 Plan view Caused by the shutter speed of the placement device, not opening sufficiently and so dislodging or rotating the component on the nozzle, also possible component packaging may have too much room so component is presented for placement this orientation Action would be to check feeders, adjust shutter springs and check packaging University of Limerick Con Sheahan Advanced Manufacturing DM4038 124 62
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