Casting Processes and their Advantages/Disadvantages

Slides 61-113

Expendable-mold Processes

Economic disadvantage because a new mold is needed for every cast

• shell molding

• expanded-polystyrene process

• investment casting

• plaster-mold and ceramic-mold casting

Permanent Mold Processes

Mold is removed and reused several times

• Die Casting

• Hot Chamber Die Casting

• Cold-chamber die casting

• Squeeze Casting

• Semisolid Metal Casting

• Centrifugal Casting

Shell Molding

Casting process in which the mold is a thin shell of sand held together by a thermosetting resin

Final sand mold prepared for casting

Advantages of Shell Molding

• Smoother cavity surface permits easier flow of molten metal and better surface finish

• Good dimensional accuracy

• Mold collapsibility minimizes cracks in casting

• Can be mechanized for mass production

Disadvantages of Shell molding

• More expensive metal pattern

• difficult to justify for small quantities

Steps in Shell Molding

1. Metal pattern is heated and placed over a box of sand mixed with TS resin

2. Box is inverted causing sand to fall and cure on the heated pattern

3. Box is flipped back so uncured sand falls off the pattern

4. Sand shell is heated in an oven to finish curing, and then it is stripped from the pattern

5. two halves of the shell are assembled and supported by sand in a box, pouring commences

Expanded Polystyrene Processes

uses a mold of sand packed around a polystyrene foam pattern, the foam vaporizes when molten metal is poured

Does not need to be opened into cope and drag sections

A.K.A. lost-foam process, lost pattern process, evaporative-foam process, and full-mold process

Steps in Polystyrene Process

1. polystyrene foam pattern is coated with refractory compound

2. Foam pattern is placed in a mold box and sand is placed around it

3. Molten metal is poured into the part of the mold that has the pouring cup and sprue

4. As the metal enters the mold the polystyrene foam is vaporized

Advantages of expanded polystyrene process

• Pattern does not need to be removed from the mold

• simplifies and expedites mold making because two halves are not required

Disadvantages of expanded polystyrene processes

• New pattern needed for every casting

• Economic justification for the process is dependent on the price of producing patterns

Applications of expanded polystyrene

• mass production of automobile engines

• Automated and integrated manufacturing systems can be used to mold the patterns and then feed them to a casting operation

aluminum engine head from expanded polystyrene process

Investment Casting (Lost Wax Process)

Wax pattern is coated with a refractory material to make the mold, wax is then melted away prior to pouring metal

a high precision casting process that can produce castings with high accuracy and intricate details

Steps in Investment Casting

1. Wax pattern produced

2. several patterns are attached to a sprue to create a pattern tree

3. Pattern tree coated in refractory material

4. full mold is made by sufficiently coating the pattern tree in refractory material to make it rigid

5. Mold is inverted and heated to melt the wax and let it drip out of the mold

6. Mold is preheated and then filled with molten metal

7. Mold is broken away from finished casting and parts are separated from the sprue

One-piece compressor stator made by investment casting

Advantages of Investment Casting

• Parts of great complexity can be made

• Close dimensional control and surface finish

• wax can be recovered for reuse

• Net shape process - additional machining not usually required

Disadvantages of Investment Casting

• Many processing steps

• Relatively expensive process

Plaster mold casting

Similar to sand casting but the mold is made of plaster of paris

Plaster mold casting steps

1. plaster and water mixture poured over plastic or metal pattern and allowed to set

2. Wood is not typically used due to extended exposure to water

3. Plaster mixture flows around the pattern freely to capture all of the details

advantages of plaster mold casting

• good accuracy and surface finish

• capable of making thin cross sections

disadvantages of plaster mold casting

• mold must be baked to remove moisture because moisture can cause problems in casting

• mold strength is lost if over baked

• plaster molds cannot withstand high temps, limited to lower melting point alloys

Ceramic Mold casting

similar to plaster but the mold is made of refractory ceramic material that can withstand higher temps than plaster

Applications and advantages of ceramic molding

Used to cast steels, irons, and other high temp alloys

Similar applications to plaster molding but with different metals

Has good dimensional accuracy and surface finish

Basic Permanent mold process

metal mold constructed of two sections for easy and precise opening and closing

molds for lower temps made of cast iron or steel

molds used for casting steel MUST be made out of refractory material due to high pouring temps

Steps in permanent mold process

1. mold is pre-heated and coated for lubrication and heat dissipation

2. cores (if used) are inserted and mold is closed

3. molten metal is poured and solidifies

4. mold is opened and the casting removed

Advantages of permanent mold casting

• good dimensional control and surface finish

• rapid solidification from metal mold causes finer grain structure making the casting stronger

Limitations of permanent mold casting

• generally limited to metals with lower melting points

• cannot handle geometries as complex as the ones sand casting covers

• high cost of mold

Applications for permanent mold casting

• because of high mold cost it is best suited to mass production, and can be automated like that

• typical parts : automotive pistons, pump bodies, and castings for aircraft/missiles

• common metals : aluminum, magnesium, copper based alloys, cast iron

• CANNOT be used for steels because of high pouring temps

Die Casting

molten metal is injected into mold cavity under high pressure

Pressure is maintained during solidification and then part is removed

molds = dies

Die casting machines

Designed to hold mold together while metal is forced into mold cavity

1. Hot Chamber machine

2. Cold Chamber machine

Hot Chamber Die Casting Machine

metal is melted in a container and a piston injects the metal under high pressure into the die

• High production rates (500/hr)

• limited to low melting point metals that do not attack plunger or other components

• casting metals: zinc, tin, lead, magnesium

Cold Chamber Die Casting Machine

molten metal is poured into an unheated chamber from an external container, piston injects metal under high pressure

• high production, but lower than HC because of pouring step

• casting metals: aluminum, brass, and magnesium alloys

HC die casting cycle

1. die closed and plunger withdrawn molten metal flows into the chamber

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

3. plunger is withdrawn, die is opened and casting ejected

CC Die Casting Cycle

1. with die closed and ram withdrawn molten metal is poured into the chamber

2. ram forces metal to flow into die maintaining pressure during cooling and solidification

3. ram is withdrawn die is opened and casting removed

truck cab floor die casting

Die Casting advantages

• economical for large production quantities

• good accuracy and surface finish

• thin sections feasible

• rapid cooling = small grain sizes and stronger casting

Die casting disadvantages

• generally limited to metals with low melting point

• part geometry has to allow removing the die

Squeeze Casting

Combo of casting and forging in which molten metal is poured into a pre-heated lower die and the upper die is closed to create the mold cavity AFTER solidification begins

Semisolid Casting

Family of net shape and near net shape processes performed on metal alloys between liquidous and solidus temps

Mix of molten and solid metals during casting making it mushy

to flow properly the mixture needs solid metal globules (stir the mixture to prevent dendrite formation)

Semisolid casting advantages

• complex part geometries

• thin part walls possible

• close tolerances

• zero or low porosity, high strength of casting

Centrifugal Casting

family of processes where the mold is rotated at high speed so centrifugal force distributes molten metal to outer regions of die cavity

• true centrifugal casting

• semicentrifugal casting

• centrifuge casting

True centrifugal casting

molten metal is poured into rotating mold to make tubular part

parts: pipes, tubes, bushings, and rings

outside shape can be different but inside is ideally perfectly round

Semicentrifugal casting

centrifugal force is used to create solid casting instead of tube

use risers in the center and density of metal in final casting is greater in outer sections than at the center of rotation

Often used on parts where the center is machined away (eliminates the part where quality is worst)

Semicentrifugal casting

Centrifuge Casting

mold is created with part cavities located away from the axis of rotation

molten metal is distributed to those cavities via centrifugal forces

used for small parts, does not require radial symmetry

Centrifuge casting