DEBRE TABOR UNIVERSITY
FACULTY OF TECHNOLOGY
DEPARTMENT OF MECHANICAL ENGINEERING
Manufacturing Engineering-I (MEng3181)
Pre-requisites: Engineering Materials I and II
Prepared by: Berhe Syum (MSc)
DEBRE TABOR, ETHIOPIA
SEPTEMBER, 2019
P.O. BOX: 272
CHAPTER THREE
FUNDAMENTAL OF CASTING PROCESS
Casting is the process in which the molten metal (of which the component is to be made) is poured in a
mould (cavity) and allowed to solidify.
➢ The term casting is commonly used for metals and the term moulding is used for plastics.
➢ Simple and complicated shapes can be made from any metal that can be melted.
➢ The process can be applied on metals and plastics.
Metal casting process begins by:
❖ creating a mould, which is the ‘reverse’ shape of the part we need. The mould is made from a
refractory material, for example, sand.
❖ The metal is heated in an oven until it melts,
❖ the molten metal is poured into the mould cavity.
❖ The liquid takes the shape of cavity, which is the shape of the part.
❖ It is cooled until it solidifies.
❖ Finally, the solidified metal part is removed from the mould
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Advantages & Disadvantages of Casting
Advantages of casting
➢Can create complex part geometries
➢Can create both external and internal shapes
➢Some casting processes are net shape; others are near net shape
➢Cast material is isotropic. It has the same physical and mechanical properties along any direction.
➢It is economical, with very little wastage: the extra metal in each casting is remelted and re-used
➢Can produce a wide variety of sized parts:
Large parts: engine blocks, cylinder heads, railway wheels, pipes……etc.
Small parts: dental crowns, jewelers, gears, brake components.
Limitations of Casting
❖Limitations on mechanical properties
❖Poor dimensional accuracy and surface finish for some processes (sand casting)
❖Safety hazards to workers due to hot molten metals
❖Porosity (empty spaces within the metal ‐ reduces the strength of metal
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Categories of Casting Processes
1. Expendable mould processes:
✓uses a mould which is destroyed to remove casting:
✓Mould materials: sand, plaster, and similar materials, plus binders
2. Permanent mould processes:
❖uses a mould which can be used over and over:
❖Made of metal (or ceramic refractory material)
Advantages of Casting over Other Shaping Processes
• It is the cheapest and most direct way of producing a shape with certain desired mechanical
properties.
• Best suited where components are desired in low quantities as high cost of mechanical
working processes like rolling, forging, extrusion, etc.
• Intricate shapes having internal openings and complex sectional variations can be produced
• Heavy equipment like machine beds ships propellers, etc. can be cast easily in the required
size.
• High degree of reproducibility is possible.
• Can also be made in wide range of dimensional tolerances and surface finish.
• For certain materials like plastics; casting is exclusively selected as the production technique.
• Complicated parts of cam be cast as two or three pieces and then joined by welding.
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Summarizes different types of castings, their advantages, disadvantages and
examples
Process
Sand
Shell mould
Expendable
Pattern
Plaster Mould
Advantages
Wide range of metals, sizes, shapes,
low cost
Better accuracy, finish, higher
production rate
Wide range of metals, sizes, shapes
Disadvantages
Examples
Poor finish, wide tolerance Engine blocks, cylinder heads
Limited part size
Connecting rods, gear housings
Patterns have low strength Cylinder heads, brake
components
Complex shapes, good surface finish Non-ferrous metals, low
Prototypes of mechanical parts
production rate
Ceramic Mould Complex shapes, high accuracy, good Small sizes
Impellers, injection mould
finish
tooling
Investment
Complex shapes, excellent finish
Small parts, expensive
Jewelry
Permanent
Good finish, low porosity, high
Costly mould, simpler
Gears, gear housings
Mould
production rate
shapes only
Die
Excellent dimensional accuracy,
Costly dies, small parts,
Precision gears, camera bodies,
high production rate
non-ferrous metals
car wheels
Centrifugal
Large cylindrical parts
Expensive, limited shapes Pipes, boilers, flywheels
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, good quality
Foundry processes
Foundry processes consist of making moulds, preparing and melting the metal,
pouring liquid metal in to the moulds, cleaning the castings, and reclaiming the sand
for reuse. Molds may be made of metal, plasters, ceramics and other refractive
substances.
➢Foundries may be classified on the basis of nature of work undertaken as:
Jobbing Foundry:
❖ Cater to the needs of a wide variety of customers.
❖Hence, size of the castings made and the compositions melted may range widely
from one day to the next.
Captive Foundries:
❖ located inside a manufacturing plant and cast specific components for their
own manufacturing plant.
❖Therefore, they have a single customer only.
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Classification of Foundry
According to the type of metal cast, foundries are classified as:
❖Ferrous Foundries,
❖Non-Ferrous Foundries,
❖Aluminum Foundries,
❖Grey Iron Foundries And
❖Brass Foundries.
Based on the degree of mechanization or automation employed, foundry may be
classified as:
➢Small Foundries with Manual Work,
➢Semi-Mechanized and
➢Mechanized or Automated Foundries.
Foundries can also be classified as:
✓Green sand foundries.
✓Shell molding foundries.
✓Investment casting.
✓Die casting (low pressure).
✓Pressure die casting
✓Gravity dies casting and Centrifugal casting.
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Phases of Founding
A. Process Selection:
❖There are a number of casting methods, from the usual method that is green sand molding to
investment casting techniques.
❖Each method is best suited for certain materials and specific components, though a given
component can be made by more than one method of casting.
❖The most important factors to be considered while selecting a casting process or molding
method are: Size of casting, Numbers of castings required, Minimum section thickness,
Dimensional accuracy, Surface finish & Composition of the cast alloy, and Properties and
structure of the casting.
❖ For example, if a smooth surface and uniform wall thickness is required, die casting is
preferable where as if the size is too large and surface finish is not mandatory, we use sand
casting.
B. Design and specification:
❖After process selection, the next step will be designing of the material to be cast with the
required dimensional and geometrical specifications. As much as possible, the design should be
light in weight, compact in size, cheap in cost and can perform its duty effectively.
❖The design must consider casting design and mould design, i.e. gating and riser design. While
preparing the drawing of the casting, the foundry engineer may provide the following
information: Parting line, Gate and riser location, Draft allowances, Machining allowances,
Casting tolerances, Cores and Locating points to be used in machining.
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Cont…
C. Determination of casting techniques:
➢The casting techniques are determined by considering the factors like,
✓Cost,
✓Strength consideration,
✓Dimensional accuracy,
✓Surface finish,
✓Machinability and
✓ Pressure tightness.
D. Molding Material Preparation
➢As we have discussed so far, in the mould the molten metal is poured and allowed
to solidify.
➢After the solidification the cast metal is withdrawn.
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Flow Diagram of Casting Production
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Reading Assignment
Patterns
➢Single piece pattern
➢Split pattern
➢Loose piece pattern
➢Gated pattern
➢Match plate patterns
➢Special patterns
Pattern allowance
✓Shrinkage allowance
✓Draft allowance
✓Machine allowance
✓Distortion allowance
✓Shaking or rapping allowance
Core and Core Making
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Sand Casting
❖ Is a method involving pouring a molten metal into sand mould.
❖Sand casting uses natural or synthetic sand (lake sand) which is mostly refractory
material called silica (SiO2).
❖Larger sized moulds use green sand (mixture of sand, clay and some water).
❖Sand can be re-used, and excess metal poured is cut-off and re-used also.
❖Refractoriness is related to the ability of sand to withstand high temperatures
without breaking down or fusing.
❖Backing sand is the old, repeatedly used mould sand, black in color due to
addition of coal dust and burning.
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Sand Casting
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Cont..
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Cont….
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Sand Casting Processes
1. Preparing a mould cavity of the desired shape with proper allowances
(inclination, shrinkage, and machining).
2. Melting the metal with acceptable quality and temp.
3. Pouring the metal into the cavity and providing means for the escape of air or
gases.
4. Solidification process must be properly designed and controlled to avoid
defects.
5. Mould removal.
6. Finishing, cleaning and inspection operations.
7. Heat treatment of casting is sometimes required to improve metallurgical
properties
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Types of Sand Mould
1. Green-sand moulds:
❖Are those sand moulds, in which moisture is present in the sand at the time of
pouring the molten metal.
❖Green sand moulds mixture of sand, clay, and water; “Green" means mould
contains moisture at time of pouring.
❖The grains are held together by moist clay.
❖They are the least expensive moulds.
❖They are used for casting small, medium and large moulds.
❖They do not require backing operations and they are less time consuming.
❖However, they are not very strong, exposed for sand erosion during pouring of
molten metal’s and may cause casting defects.
❖Moreover, they cannot be used for long time.
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2. Dry-sand mould
❖are made of sands that do not contain moisture to develop strength.
❖Binding materials are added and the sands are baked at a temperature of 1503000c for increasing the strength.
❖It uses organic binders rather than clay and mould is baked to improve
strength.
❖They are used for steel castings of small and medium size operation.
❖These moulds can be used for more than one casting, resist the erosion during
metal pouring and do not expose for casting defects due to moisture.
❖But the molding material is expensive and extra operation, equipment, space
and labor are required.
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3. Skin-dried mould
❖Drying mould cavity surface of a green-sand mould to a depth of 10 to 25 mm,
using torches or heating lamps.
❖In these moulds, the upper layer which remain around the pattern is harder than
the body so as to prevent sand erosion during pouring of molten metal.
❖The harder layer is obtained by spraying linseed oil, molasses or water on the
surface of the green sand and heated; and by mixing a binder with the portion of
the green sand;
❖Requires less equipment, less time, and less space than dry sand mould.
❖It is cheaper than dry sand, used for longer time and stronger than green sand.
❖However, it is not as strong as dry sand moulds.
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4. Loam moulds:
❖are used for large castings such as large cylinders, paper making rollers and bells.
❖Loam mortar a mixture of molding sand (50%) and clay (50%) mixed together to form a
stiff mud.
❖A porous brick work is cemented together with loam mortar.
❖The inner surface of the brick structure is faced with loam and may be swept by a swept
for a cylindrical or bell shaped casting.
❖These moulds are very strong and they have the advantages over dry sand moulds.
❖However, they take long time to prepare the mould and the mould material cannot be
used again.
5. Metal moulds:
❖are moulds made of metals.
❖They have a very long life, do not erode during pouring, can be saved for a long time,
have high production rate and have smooth surface and accurate shape.
❖However, the mould is costly and takes longer time to make.
6. Special moulds: include moulds which are made of plastics, cement, paper or rubber.
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Buoyancy in Sand Casting Operation
➢During pouring, buoyancy of the molten metal tends to displace the core. Core
displacement can cause casting to be defective.
➢Force tending to lift core = weight of displaced liquid less the weight of core itself
Fb = Wm - Wc
Where, Fb = buoyancy force;
Wm = weight of molten metal displaced; and
Wc = weight of core
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Chapter Four
Specail Casting Process
1. Polystyrene Casting/ lost foam process /Expandeble- Mold Casting
➢The pattern used in this process is made from polystyrene (this is the light, white
packaging material which is used to pack electronics inside the boxes).
➢Polystyrene foam is 95% air bubbles, and the material itself evaporates when the
liquid metal is poured on it.
➢May includes sprue, risers, gating system, and internal cores (if needed).
➢The mold does not have cope and drag sections tags.
➢Uses a mould of sand packed around a polystyrene foam pattern which vaporizes
when molten metal is poured into mould.
➢The process is useful since it is very cheap, and yields good surface finish and
complex geometry.
➢There are no runners, risers, gating or parting lines – thus the design process is
simplified.
➢The process is used to manufacture crank-shafts for engines, aluminum engine
blocks, manifolds etc.
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Polystyrene Casting /Expandable Molding
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1.
2.
3.
4.
5.
Steps for Polystyrene
Pattern of polystyrene is coated with refractory compound.
Foam pattern is placed in mould box.
Sand is compacted around the pattern.
Molten metal is poured into the portion of the pattern that forms the pouring cup and
sprue.Then, the polystyrene foam is vaporized ahead of the advancing liquid, thus allowing
the resulting mould cavity to be filled.
By cooling and solidification, the part is removed by breaking the shell.
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2. Shell-mould casting
❖Shell-mould casting Yields better surface quality and tolerances. The process is
described as follows:
1. The 2-piece pattern is made of metal (e.g. aluminum or steel), it is heated to
between 175°C-370°C, and coated with a lubricant, e.g. silicone spray.
2. Each heated half-pattern is covered with a mixture of sand and a thermoset
resin/epoxy binder. The binder glues a layer of sand to the pattern, forming a
shell. The process may be repeated to get a thicker shell.
3. The assembly is baked to cure it.
4. The patterns are removed, and the two half-shells joined together to form the
mould; metal is poured into the mould. When the metal solidifies, the shell is
broken to get the part.
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Shell-mould casting
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Steps of shell molding casting process
1. A heated pattern is placed over a
dump box containing a sand and
resin mixture, fig. (a)
2. The box is inverted and a shell
partially cures around the pattern
fig. (b)
3. The box is righted, the top is
removed, and placed in an oven
to further cure the shell fig.(c)
4. The shell is stripped from the
pattern fig. (d)
5. Matched shells are then joined
and supported in a flask ready for
pouring. fig. (e)
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3. Investment Casting (Lost Wax Process)
➢Has been used since ancient times to make jewelers.
➢It is also used to make other small (few grams, though it can be used for parts up to a few
kilograms).
➢It is a precision casting process - capable of castings of high accuracy and intricate detail.
➢ Ceramic slurry is applied around a disposable pattern, usually wax, and allowed to harden
to form a disposable casting mould.
➢disposable means pattern is destroyed during its removal from the mould and that the
mould is destroyed to recover the casting.
➢There are two distinct processes for making investment casting moulds:
➢The ceramic shell process: has become the predominant technique for engineering
applications, displacing the solid investment process.
➢The solid investment (solid mould) process: used to produce dental and jewelers castings
and has only a small role in engineering applications, mostly for nonferrous alloys.
An advantage: wax can carry very fine details, so it gives good dimensional tolerances &
excellent surface finish
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Steps in investment casting
1) Wax patterns are produced.
2) Several patterns are assembled to a sprue to
form a pattern tree.
3) The pattern tree is coated with a thin layer of
refractory material.
4) The full mould is formed by covering the
coated tree with sufficient refractory material
to make it rigid.
5) The mould is held in an inverted position and
heated to melt the wax and permit it to drip out
of the cavity.
6) The mould is preheated to a high temperature,
which ensures that all contaminants are
eliminated from the mould; it also permits the
liquid metal to flow more easily into the
detailed cavity; the molten metal is poured; it
solidifies.
7) The mould is broken away from the finished
casting parts are separated from the sprue.
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4. Die casting
❖Die casting is a very commonly used type of permanent mould casting process.
❖It is used for producing many components of home appliances (e.g. rice
cookers, stoves, fans, washing and drying machines, fridges), motors, toys and
hand-tools.
❖Good Surface finish and tolerance of die cast parts
❖There is almost no post-processing required.
❖Die casting moulds are expensive, and require significant lead time to fabricate.
❖Moulds in these casting operation are called dies; hence the name die casting Use
of high pressure to force metal into die cavity is what distinguishes this from other
permanent mould processes
❖There are two common types of die casting:
➢Hot-Chamber Die Casting and
➢Cold-Chamber Die Casting.
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A. Hot-Chamber Die Casting
❖Metal is melted in a container/ pressure chamber, and a piston
injects liquid metal under high pressure into the die.
❖High production rates - 500 parts per hr not.
❖Applications limited to low Mpt metals that do not chemically
attack plunger & do not alloy with the die material, steel;
❖Used for Casting metals: Zn, Zn alloys, Sn, Pb, and Mg
The basic cycle of operation is as follows:
1) die is closed and gooseneck cylinder is filled with molten metal;
2)
plunger pushes molten metal through gooseneck passage and
nozzle and into the die cavity; metal is held under pressure until
it solidifies;
3) die opens and cores, if any, are retracted; casting stays in ejector
die; plunger returns, pulling molten metal back through nozzle
and gooseneck;
4) ejector pins push casting out of ejector die. As plunger uncovers
inlet hole, molten metal refills gooseneck cylinder.
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B. 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.
❖Useful for casting high Mpt metals : Al, and Cu (and its
alloys), Brass, and Mn alloys
❖The operating cycle is:
1. Die is closed and molten metal is ladled into the cold
chamber cylinder;
2. Plunger/Ram pushes molten metal into die cavity; the
metal is held under high pressure until it solidifies;
3. Die opens and plunger follows to push the solidified
slug from the cylinder, if there are cores, they are
retracted away;
4. Ejector pins push casting off ejector die and plunger
returns to original position.
5. Pressure is maintained during solidification, then mould
is opened and part is removed.
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5. Centrifugal Casting
➢Is a method of casting parts having axial
symmetry.
➢The method involves pouring molten metal
into a cylindrical mold spinning about its
axis of symmetry.
➢the mould is rotated at high speed so
centrifugal force distributes molten metal to
outer regions of dies cavity
➢The mold is kept rotating till the metal has
solidified.
➢mold material: steels, Cast irons, Graphite.
The 3-types of centrifugal casting are;
❖True Centrifugal Casting,
❖Semi-centrifugal Casting &
❖ Centrifuge Casting.
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Cont…
A. True Centrifugal Casting
➢Molten metal is poured into rotating mould/horz. or vert./ to produce a tubular part.
➢No cores used
➢Parts: pipes, tubes, bushings, and rings.
➢Outside shape of casting can be round, octagonal, hexagonal, etc, but inside shape is
(theoretically) perfectly round, due to radially symmetric forces.
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B. Semi-centrifugal Casting
❖Centrifugal force is used to produce solid castings rather than tubular parts
❖Used to cast large size Axi-symmetrical objects.
❖Mold is placed Hrz. & rotates about Vrt. Axis.
❖Moulds are designed with risers at center to supply feed metal
❖Core is inserted at the center.
❖Density of metal in final casting is greater in outer sections than at center of rotation
Eg. wheels and pulleys, Gear Blanks
C. Centrifuge Casting
✓Mould is designed with part cavities located away from axis of
rotation, so that molten metal poured into mould is distributed to these
cavities by centrifugal force
✓Used for smaller parts
✓Radial symmetry of part is not required as in other centrifugal casting
methods.
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Furnaces for Casting Processes
❖Furnaces most commonly used in foundries: Cupolas, Direct fuel-fired furnaces,
Crucible furnaces, Electric-arc furnaces & Induction furnaces.
1. Cupolas
➢Vertical cylindrical furnace equipped with tapping spout near base
➢Used only for cast irons.
➢The "charge," consisting of iron, coke, flux, and possible alloying elements.
2. Direct Fuel-Fired Furnaces
✓Small open-hearth in which charge is heated by natural gas fuel burners located
on side of furnace.
✓Furnace roof assists heating action by reflecting flame down against charge.
✓At bottom of hearth is a tap hole to release molten metal.
✓Generally used for nonferrous metals such as copper-base alloys and aluminum.
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3. Crucible Furnaces
➢Metal is melted without
direct contact with burning
fuel mixture.
➢Sometimes called indirect
fuel-fired furnaces
• Used for nonferrous metals
such as bronze, brass, and
alloys of Zn and Al.
• Three types used in
foundries: (a) lift-out type,
(b) stationary, (c) tilting
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4. Electric-Arc Furnaces
➢Charge is melted by heat
generated from an electric arc.
➢High power consumption, but
electric-arc furnaces can be
designed for high melting
capacity.
➢Used primarily for melting
steel.
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5. Induction Furnaces
❖Uses alternating current passing through
a coil to develop magnetic field in metal.
❖Induced current causes rapid heating and
melting.
❖Electromagnetic force field also causes
mixing action in liquid metal.
❖Since metal does not contact heating
elements, the environment can be closely
controlled, which results in molten
metal’s of high quality and purity.
❖Melting steel, cast iron, and aluminum
alloys are common applications in
foundry work.
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Casting Quality and Casting Defect
There are numerous opportunities for things to go wrong in a casting operation,
resulting in quality defects in the product. The defects can be classified as follows:
❖Surface Defects: Blow, Scar, Blister, Drop, Scab, Penetration, Buckle
❖Internal Defects: Blow holes, Porosity, Pin holes, Inclusions, Dross
❖Visible Defects: Wash, Rat tail, Swell, Mis run, Cold shut, Hot tear, Shrinkage/Shift
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Surface Defects
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Internal Defects
➢Created mainly due to trapped gases and dirty metal.
➢Gases get trapped due to hard ramming or improper venting. These defects also
occur when excessive moisture or excessive gas forming materials are used for.
mould making.
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Visible defect
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