Casting & Welding Engineering(IE 203)

Second Year,

Industrial Engineering Dept.,

Faculty of Engineering,

Fayoum University

Dr. Ahmed Salah Abou Taleb

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Casting Processes

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Metal Casting Processes

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Casting Processes

Sand Casting

Shell Casting

Investment Casting

Gravity Die Casting

Pressure Die Casting

Centrifugal Casting

Vacuum Casting

Plaster & Ceramics Casting

Permanent Casting

Composite Casting

Envelop Pattern Casting

PolystreneCasting

Metal Casting Processes

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Expendable Casting

Sand Casting

Shell Casting

Investment Casting

Vacuum Casting

Plaster & Ceramics Casting

Composite Casting

Envelop Pattern Casting

Polystrene

Casting

Permanent Casting

Gravity Die Casting

Pressure Die Casting

Centrifugal Casting

Permanent Casting

Selection of Casting Process

• Type of metal.

• Number of castings required.

• Shape & size of castings.

• Dimensional accuracy required.

• Casting finish required.

• Economics.

• The number of finishing operations-the processes that provide the “closest dimensions, best surface finish, & the most intricate detail” usually require the smallest number of finishing operations.

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Expendable Casting

Processes

Sand Casting

Shell Casting

Investment Casting

Vacuum Casting

Plaster & Ceramics Casting

Composite Casting

Envelop Pattern Casting

PolystreneCasting

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The investment-casting process, also called the lost-waxprocess, was first used during the period 4000-3500 B.C. The pattern is made of wax or a plastic. The pattern is made by injecting molten wax or plastic into a metal die in the shape of the object.

All metals can be casted in investment process.

Applications: Jewellery, surgical instruments, vanes and blades of gas turbine, and bolts and triggers for fire arms.

Investment Casting (Lost Wax Casting)

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(a) Wax pattern(injection molding)

(b) Multiple patternsassembled to wax sprue

(c) Shell built immerse into ceramic slurry immerse into fine sand(few layers)

(d) dry ceramic melt out the waxfire ceramic (burn wax)

(e) Pour molten metal (gravity) cool, solidify

[Hollow casting: pouring excess metal before solidification

(f) Break ceramic shell (vibration or water blasting)

(g) Cut off parts(high-speed friction saw) finishing (polish) 8

Investment Casting (Lost Wax Casting)

Pros:

– Capability to cast parts with great complexity and intricacy.

– Close dimensional control ( 0.076 m tolerance).

– Good surface finish.

– Wax can be recovered and reuse.

– Additional machining normally not required.

Cons:

– Normally used for smaller parts.

– Relatively expensive.9

Investment Casting (Lost Wax Casting)

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Shell Casting (Dump Box)

The shell casting process, also called the Dump box process, is the process in which dry and fine sand mixed with resin is allowed to come into contact with heated metallic pattern, so that a thin and strong shell of mould is formed around the pattern. The shell is removed from pattern. After that the shell is placed into the flask with necessary back up materials and the molten metal is poured into the mould.

All metals can be casted in investment process.

Applications: Cylinders and cylinders heads for IC engines, small crank shafts, and automobile transmission parts, and brake arm.

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Shell Casting (Dump Box)

Steps 1: A metal pattern is heated and placed over a box containing sand mixed with thermosetting resin

Step 2: Box is inverted so that sand and resin fall onto the hot pattern, causing a layer of the mixture to partially cure on the surface to form a hard shell

Step 3: Box is repositioned so loose uncured particles drop away

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Shell Casting (Dump Box)

Step 4: Sand shell is heated in oven for several minutes to complete curing

Step 5: shell mold is stripped from pattern

Step 6: Two halves of the shell mold are assembled, supported by sand or metal shot in a box, and pouring is accomplished

Step 7: Finished casting with sprue removed

Pros:

– Smoother surface finish than sand casting.

– Surface finish of 2.5 m can be obtained.

– Good dimensional accuracy 0.25 mm on small to medium size parts.

– No further machining is needed.

– Capability for automation lowers the cost for larger quantities.

Cons:

– More expensive metal pattern, especially for small batch.

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Shell Casting (Dump Box)

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Polystyrene Casting (Lost Foam)

• Uses a mold of sand packed around a polystyrene foam pattern which vaporizes when molten metal is poured into mold

• Polystyrene foam pattern includes sprue, risers, gating system, and internal cores (if needed)

• Mold does not have to be opened into cope and drag sections.

• Other names: lost-foam process, lost pattern process, evaporative-foam process, and full-mold process

• Applications: Automobile engine,

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Step 1: Polystyrene foam pattern is coated with refractory compound

Step 2: Foam pattern is placed in mold box, and sand is compacted around the pattern

Step 3: Molten metal is poured into the portion of the pattern that forms the pouring cup and sprue

As the metal enters the mold, the polystyrene foam is vaporized ahead of the advancing liquid, thus filling the mold cavity

Polystyrene Casting (Lost Foam)

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Polystyrene Casting (Lost Foam)

• Pros:

– Pattern need not be removed from the mold

– No cope /drag is needed, all features are built into the pattern.

– Possibility for automated production.

• Cons:

– A new pattern is needed for every casting.

– Economic justification of the process is highly dependent on cost of producing patterns

Plaster and Ceramic Casting• Similar to sand casting in terms of process except

the mold is made from plaster of Paris (Gypsum).

• Plaster mold is for lower temperature alloys while ceramic mold is for higher temperature alloys.

Pros:

– Good surface finish and dimensional control.

– Capability to make thin cross sections.

Cons:

– Curing takes too long to render it unsuitable for volume production.

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Plaster and Ceramic CastingPlaster:

• Similar to sand casting in terms of process except the mold is made from plaster of Paris (Gypsum).

• In mold-making, plaster and water mixture is poured over plastic or metal pattern and allowed to set

– Wood patterns not generally used due to extended contact with water

• Plaster mixture readily flows around pattern, capturing its fine details and good surface finish

• Plaster mold is for lower temperature alloys

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Plaster and Ceramic CastingPros:

– Good surface finish and dimensional control.

– Capability to make thin cross sections.

Cons:

– Curing takes too long to render it unsuitable for volume production.

– Mold must be baked to remove moisture

• Moisture can cause problems in casting

– Mold strength is lost if over-baked

– Plaster molds cannot stand high temperatures

• Limited to lower melting point alloys

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Plaster and Ceramic Casting

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Ceramics:

• Similar to plaster except that mold is made of refractory ceramic material that can withstand higher temperatures than plaster.

• Can be used to cast steels, cast irons, and other high temperature alloys

• Advantages (good accuracy and finish) also similar

Sequence of operations in making a ceramic mold.

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Plaster and Ceramic Casting

Vacuum Casting

Step 2: Special flask with vents and filled

with sand is placed over the pattern.

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Step 1: Thin pre-heated plastic sheet sucks onto

the vacuum vented pattern surface.

Step 3: Another plastic sheet covers the sand

and vacuum is drawn.

Step 5: Cope and drag is assembled to

form the complete mold. The plastic

sheet is burnt away when in contact with

the molten metal.

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Vacuum Casting

Step 4: Vacuum on the pattern is

released to free the sand mold.

Pros:

– Sand can be recovered unlike shell mold.

– No chemical binder, and therefore no special treatment for the sand.

– No water mixed with the sand and therefore no moisture related problems

Cons:

– Relative slow

– Not readily adaptable to mechanization.

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Vacuum Casting

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Permanent Casting

Gravity Die Casting

Pressure Die Casting

Centrifugal Casting

Permanent Casting

Permanent casting

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• Uses a metal mold constructed of two sections designed for easy, precise opening and closing

• Molds used for casting lower melting point alloys are commonly made of steel or cast iron

• Molds used for casting steel must be made of refractory material, due to the very high pouring temperatures.

• Spray with lubricant (graphite, silica) to improve flow, increase life.

• Aluminum, magnesium, copper alloys and cast iron.

• CORE (hollow parts); metal core can be extracted from the part and sand core must be destroyed to remove

• Applications: Automotive piston, pump bodies.

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Permanent casting

Step 1: Mold is preheated and coated for lubrication and heat dissipation

Step 2: Cores (if any are used) are inserted and mold is closed

Step 3: Molten metal is poured into the mold, where it solidifies

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Permanent casting• Pros:

– Good dimensional control and surface finish

– Rapid solidification caused by metal mold results in a finer grain structure, so castings are stronger

• Cons:

– Generally limited to metals of lower melting point

– Simpler part geometries compared to sand casting because of need to open the mold

– High cost of mold

Pressure Die Casting

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• A permanent mold casting process in which molten metal is injected into mold cavity under high pressure.

• Pressure is maintained during solidification, then mold is opened and part is removed

• Molds in this casting operation are called dies.

• Use of high pressure to force metal into die cavity is what distinguishes this from other permanent processes.

• Two main types: 1. Hot chamber machine 2- Cold chamber machine

Pressure Die CastingHot-chamber machines :

• Metal molten in container attached to machine.

• Typical injection pressures are 7 to 35 MPa.

• The piston is subjected to the melting temperature of the metal and thus the process is often used for low melting point metals such as zinc, tin, lead or magnesium alloys.

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Pressure Die CastingHot-chamber machines :

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Step1: die closed and plunger withdrawn, molten metal flows into the chamber

Step 2: plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification.

Step 3: Plunger is withdrawn, die is opened, and casting is ejected

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Pressure Die CastingHot-chamber machines :

Pressure Die CastingCold-chamber machines

• Molten metal is poured into an unheated chamber from an external container.

• Typical injection pressures are 14 to 140 MPa.

• Often used for high melting point metal such as aluminum, brass, and magnesium alloys.

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Pressure Die CastingCold-chamber machines

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Step 1: die closed and ram withdrawn, molten metal is poured into the chamber

Step 2: Ram forces metal to flow into die, maintaining pressure during cooling and solidification

Step 3: Ram is withdrawn, die is opened, and part is ejected

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Pressure Die CastingCold-chamber machines

• Mold made of tool steel or mold steel.

• Tungsten and molybdenum (good refractory qualities) used to die cast steel and cast iron.

• Ejector pins required to remove part from die when it opens.

• Lubricants must be sprayed onto cavity surfaces to prevent sticking.

• Flash formation is common.

• Application: components for rice cookers, stoves, fans, washing drying machines, fridges, motors, toys, hand-tools, and car wheels.

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Pressure Die Casting

Pros:

– High production rates are possible.

– Economical for large quantities.

– Close tolerances are possible ( 0.076 mm).

– Good surface finish.

– Thin sections are possible (down to 0.5 mm).

– Rapid cooling, fine grain, high strength.

Cons:

– melting point of metals are low.

– shape restriction.

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Pressure Die Casting

Centrifugal Casting• A permanent mold made of metal or ceramic is rotated at high

speed (300 to 3000 rpm). The molten metal is then poured into the

mold cavity and due to centrifugal action the molten metal

conform to the cavity provided in the mould.

• Castings are known for their higher densities in the outer most

regions.

• The process gives good surface finish

• Three types are introduced:

1- True centrifugal casting 2- Semi centrifugal casting

3- Centrifugal casting

• Applications: pipes, bushings, gears, flywheels etc.

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Centrifugal CastingTrue centrifugal casting:

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• Molten metal is poured into rotating mold to produce a tubular part.

• In some operations, mold rotation commences after pouring rather than before.

• 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.

Centrifugal CastingSemi centrifugal casting:

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• Centrifugal force is used to produce solid castings rather than tubular parts

• Molds use risers at center to supply feed metal

• Density of metal in final casting is greater in outer sections than at center of rotation

• Often used on parts in which center of casting is machined away, thus eliminating the portion where quality is lowest

– Examples: wheels and pulleys

Centrifugal CastingCentrifugal casting:

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• Mold is designed with part cavities located away from axis of rotation, so molten metal poured into mold 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