Secondary Operations
Chapter 9
Professor Joe Greene
CSU, CHICO
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MFGT 144
Chapter
9
Topics
Need for Secondary Operations
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• Assembly Operations
– Ultrasonic welding; Hot-gas welding
– Induction bonding; Spin (Friction) welding; Adhesive bonding
• Machining Operations
– Drilling and tapping; Reaming; Turning and milling
• Automatic Shape Cutting
– Water jet; Laser cutting
• Surface Finishes and Decorating Procedures
– Surface Prep: Flame, plasma process, acid etch
• Applied Finishes
– Painting;Electroplating; Vacuum Metallizing; Hot stamping
– Pad printing and screen printing
– Molded-in-color and symbols
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Need for Secondary Operations
• Secondary operations
– Any operation to a molded part that occurs after the part
is made.
• Painting, trimming, drilling, fasteners, assembly
– Should be minimized through injection molding design
– Will generally be more expensive than molded-in
features.
• When to consider secondary operations
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When volumes are small
When tooling costs are excessive
When time to build mold jeopardizes sales schedules
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When a labor is available from other sources in company
Assembly Operations
• Ultrasonic welding (Figure 9-1)
– Uses high frequency mechanical vibrations (20 to 40 kHz
per second transmitted through thermoplastic parts.
– Vibrations generate friction between the plastic parts
which leads to melting of the plastic.
– The two plastic parts melt and fuse together as bond.
– Can be used for staking, surface vibration welding, spot
welding, and inserting metal inserts.
• Thermoplastic materials can be welded
– Amorphous materials are easy to ultrasonically weld.
– Crystalline materials require greater amounts of energy
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and are much more sensitive to joint design and fixturing
Ultrasonic Welding
• Parameter Effects
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Materials: crystalline versus amorphous
Melt temperature
Melt index and viscosity
Material stiffness
Chemical makeup of plastic: Some dissimilar amorphous
plastics can be welded.
• Energy Directors (Fig 9-2)
– Purpose- direct energy from the horn of machine to the
desired point of welding.
– Focuses the ultrasonic energy to the point and causes the
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material to melt
Ultrasonic Welding
• Ease of welding
– Table IX-2 and IX-2 for Amorphous and Crystalline
– Function of joint design, part geometry, energy
requirements, amplitude, and fixturing.
– Based on near field welding, welding joint within 0.25
inches of horn contact surface
– Frequency is usually 20 kHz versus 40 kHz (20% of jobs)
– Vibration welding is lower frequency: 250 to 300 Hz
• automotive bumpers, or materials that are damaged by high Hz
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Ultrasonic Welding
• Variables that Influence Ultrasonic Welding
– Polymer structure
• Amorphous- molecules are random arrangement.
– Efficiently transmit ultrasonic vibrations and can be welded under a
wide range of force-amplitude combinations
• Crystalline- molecules are are spring-like in solid state.
– Internally absorb a percentage of the high-frequency mechanical
vibrations of the ultrasonic generator reducing efficiency of transmitting
to joint interface.
– Requires a higher amplitude
– Melt temperature
• Higher melt temperature more energy required.
– Stiffness (modulus of elasticity)
• Higher stiffness the better the transmission of the ultrasonic
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energy to the joint interface.
Ultrasonic Welding
• Variables that Influence Ultrasonic Welding
– Moisture content
• Hygroscopic materials- nylon, ABS, PC, Psulfone, PET, PBT
• Higher moisture content the lower the bonding efficiency
• Moisture turns into steam during welding step and creates
porosity in part and degrade resin at the joint interface.
• Molded parts should be dried prior to welding.
– Flow rates or viscosity
• Rate at which material flows when it becomes molten.
• Different materials should have similar viscosities or melt index
– Mold release agents
• Added to increase release of part from mold.
• Higher mold release the lower the bond strength.
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Ultrasonic Welding
• Variables that Influence Ultrasonic Welding
– Plasticizers
• High temperature boiling liquids or low temperature melting
plastics added to increase flexibility and elongation
• Higher plasticiser amount results in lower bond strength.
• Plasticizers interfere with a resin’s ability to transmit vibrations.
• Plasticizers swell polymer like a sponge.
– Flame retardants
• Inhibits ignition or modifies burning chacteristics
• Generally inorganic oxides or halogenated organics
– Aluminum, antimony, boron, chlorine, bromine, sulfer, nitrogen,
– Typically, 1% to 2%
– not weldable
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• Higher flame retardant amounts results in lower bond strength
Ultrasonic Welding
• Variables that Influence Ultrasonic Welding
– Regrind
• Regrind is added to reduce cost of part
• Regind reduces melt temperature and reduces bond strength
– Colorants
• Generally do not inhibit weld strength unless greater that 5%
– Resin grade
• Different resin grades can have different melt temperatures and
molecular weights.
• Different weld grades are weldable if the two resins have
similar molecular weights and the melt temperatures should be
within 40°F of each other.
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Ultrasonic Welding
• Variables that Influence Ultrasonic Welding
– Fillers
• Added to reduce the price of the polymer and increase (Slightly
stiffness) and reduce CLTE
– talc, calcium carbonate, kaolin, organic fillers, silica, micas, etc.
• Enhance some plastics ability to transmit ultrasonic energy by
imparting higher stiffness. (For up to 35% filler)
• Are very abrasive and can cause excessive wear on surface.
• Require use of hardened steel or carbide-coated titanium horns
– Reinforcements
• Added to increase strength and stiffness and reduce CLTE
– glass fiber, carbon fiber, aramid fiber.
• Enhance the weldability of resin
– Short fibers result in better welds
– Long fibers clump at gate and reduce weldability
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Hot-gas Welding
• Similar to metal welding
– Welding rod composed of same material being welded is
placed along a beveled joint area.
– Heat is applied to the area by hot gas (air or nitrogen)
– Hot plastic melts the plastic and welding rod
– PVC (rigid) is most common material hot-gas welded
– Figure 9-3
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Induction (Electromatic) Bonding
• Figure 9-4 (Time required = less than 10 seconds)
– Process consists of activating an electrodynamic field to
excite a conductive bonding agent (metal wire strands)
– Heat is absorbed by the plastic components that surround
the bonding agent, causing the plastic to melt.
– Melted plastics fuse together and to the bonding agent.
– Slight pressure is applied during welding.
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Spin (Friction) Welding
• Figure 9-5 (Time required = less than 2 seconds)
– Process consists of one part spinning at speeds of 100 to
1000 RPM located near second part.
– The spinning produces friction & heat when parts touch.
– Slight pressure is applied during welding.
– Can produce weld with drill press or lathe.
– Can be used with most hard plastics.
– Requires part to be cylindrical.
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Adhesive
Bonding
Figure 9-6 (Time required = 10 seconds to minutes)
– Process consists of one adding a thermoset material
– The thermoset material (urethane and epoxy polymers)
– Ashland Chemical PLIOGRIP (Modified urethane)
• Acrylics, Phenolic resins, Structural Adhesive, Welding
Adhesives, Roofing Adhesives, Wood Bonding Adhesives
• Structural Adhesives
– Solventless PLIOGRIP®, AROWELDTM and AROGRIP® structural
adhesives bond reinforced thermosetting composites, thermoplastics,
metals and other substrates
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Machining Operations
• Drilling and Tapping Thermoplastics
– Carbide drills are most suitable
– Carbide tipped or diamond-tipped drills for mirror finish
– Flutes should be highly polished and drill cutting surfaces
should be chrome plated or nitrided to reduce wear
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Details of drill dimensions are shown in Fig 9-7
drill land, L, should be 1/16 in or less
Helix angle should be 30° to 40°
Point angle should be 60° to 90°
Drill feed should be approximately 0.0005 per revolution of
drill bit
• Drill speeds should range from 5000 rpm to 1000 rpm
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• Thermosets are more abrasive and require special bit
Machining Operations
• Reaming Thermoplastics and Thermosets
– Reamers should be fluted for best surface finish
– Reamer feeds and speeds should approximate those of
drilling operations.
– Water soluble coolants should be used to reduce heat
generation by friction.
• Turning and Milling
– Lath and mill cutters should be tugsten carbide or
diamond-tipped with negative back rake and front
clearance (Figure 9-8)
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Automated Shape Cutting
• Water Jet Cutting
– Most popular automated cutting process in industry
– Employs a force of a thin stream of water pressure (20 kpsi to
50kpsi) to create a powerful cutting point.
– Pierces plastic or composite material cleanly.
– Dust and chips are non-existent
– Used for flat sheet stock mostly
but can with the use of three and
five axis machines cut complex parts
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Automated Shape Cutting
• Laser Cutting
– Used when a fine polished finish on plastic edge is
required, such as on the edges of an acrylic sign.
– Laser cuts by focusing its concentrated beam at the exact
point of the cut, which causes the plastics to melt,
vaporize, and solidify, thus producing a smooth finish.
– Advantages
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Straight, burr-free cuts
Narrow cutting-width
Oxidation-free cuts,
Smooth profile
High cutting speeds
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Surface Finishes and Decorating Procedure
• Preparation of Surface
– Products that require postmold painting or decorating need clean
surface to ensure proper adhesion to paint or bond.
– Flame treatment
• Most common method of preparing polyolefins and acetals
• These materials are slippery in nature and resistant to paints
• Flame treatment consists of passing the molded product through a flame
– Causes the surface to oxidize and making it receptive to paints
– Surface is oxidized without charring surface.
– Corona Discharge
• Surface oxidation of plastic is achieved by passing the plastic over an
insulated metal cylinder beneath a high voltage conductor.
• An electric discharge strikes the surface of the plastic causing plastic to
oxidize
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Surface Finishes and Decorating Procedure
• Preparation of Surface
– Plasma Process
• Low pressure air is directed through an electrical discharge and expand into
a vacuum chamber containing the plastic.
• Nitrogen and oxygen gases are partially disassociated radicals in air react
with the surface
– Acid Etch
• Some plastics, e.g., PC and ABS, need additional surface preparation
• Acid wash attacks surface of the plastic and creates microscopic craters of
exposed resin.
• Craters will physically capture the decorative coating and lock it to the
plastic surface.
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Surface Finishes and Decorating Procedure
• Applied Finishes
– Painting
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Applied with brushing, spraying, rolling, or dipping
Manually, mechanically, or robotically
Most are sprayed with standard spray process (Fig 9-9)
Need proper surface prep, primer, oven Temp
– Plating (Electroplating)
• Requires plastic to be made conductive
• Apply conductive base metal to plastic surface.
• Metallic plating is used for decorative or functional
– plumbing fixtures, jewelry, circuit board traces, EMI shields, corrosion resistant
surfaces
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Surface Finishes and Decorating Procedure
• Applied Finishes
– Vacuum Metallizing (Deposition) Figure 9-10
• Plastic is coated with lacquer base coat.
• Then placed on a rack inside a vacuum chamber along with small clips of
the metal to be deposited,
• The metal clips are heated to the point of vaporizing
• Depoited on all line-of sight surfaces due to vacuum
• Gives bright metallic finish
• Less expensive than electroplating
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Surface Finishes and Decorating Procedure
• Applied Finishes
– Hot Stamping (Fig 9-11)
• Three methods of hot stamping
– Roll-on decorating: (Fig 9-12)
» Ideal for applying rolls or preprinted heat transfers to part surfaces
» Silicone rubber roller applies heat and pressure to release the print medium
onto the plastic substrate.
– Peripheral marking (Figure 9-13)
» Ideal for periphery of cylindrical or slightly conical parts
» Plastic product is rolled under a flat stamping die to release the print
medium onto plastic substrate
– Vertical Stamping (Fig 9-14)
» Ideal for small areas of flat or slightly crowned products
» Silicone rubber die is mounted to the heated head of a vertical machine and
positioned directly over the part to be decorated
» Rubber die contains raised graphics to be stamped and is heated
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» Rubber die is powered and pushes the foil against the plastic
Surface Finishes and Decorating Procedure
• Applied Finishes
– Pad Printing (Fig 9-15)
• Done like printing paper on a press
– A pad of rubber is inked with the image that is pressed onto a steel or nylon
plate on which the image is etched with ink screened into that image.
– Ink pad is brought to the plastic surface and pressed.
– Screen Printing (Fig 9-16)
• Ink or paint is forced through a mesh of a plastic or a metal
screen by pulling a squeegee across a screen that is placed
against the surface of the plastic.
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