1.3.2 Permanent Mold Casting
• The basic process is,
1. A metal mold is made in two halves.
2. The mold is then coated with a refractory coating, or sometimes graphite is used instead. This acts as a thermal barrier, and as a parting agent.
3. Cores are then added as required.
4. The mold halves are mated and preheated to about 300-400°F.
5. Low melting point molten metal is poured into the dies.
6. Water channels, or heatsink fins are used to cool the mold quickly.
7. The mold is opened, and ejector pins are used to force the part out of the mold - this leaves small circular depressions on the surface of the part.
8. the sprue is removed, and the stub is ground off.
• The mold cavity is typically coated with a refractory coating to reduce heat damage, and ease part removal after casting. The materials also help control the cooling rate of the casting. Typical materials include,
- sodium silicate and clay
- sprayed graphite
• Molds are machined, including the cavity and gates. Typical mold materials include,
- cast iron and alloyed cast irons
- steel
- bronze
- graphite
- refractory metal alloys
• Typical core materials include,
- oil-bonded sand
- resin-bonded sand
- plaster
- graphite
- gray iron - most common
- low-carbon steel
- hot work die steel
• Low melting point metals can be cast
- aluminum
- zinc
- magnesium alloys
- brass
- cast iron
• Movable sections can be used to allow removal of cast parts.
• Can be used for thousands of parts before mold is replaced or repaired.
• Part sizes are from a few ounces to a hundred pounds.
• Typical applications are,
- pistons/cylinders/rods
- gears
- kitchenware
• Advantages,
- the mold can be chilled to speed cooling
- good surface finish
- good dimensional accuracy
- only one mold is required
• Disadvantages,
- limited numbers of alloys can be used
- complex shapes cannot be cast
- mold production is time consuming and costly
- mold sizes are limited
1.3.2.1 - Slush Casting
• Permanent mold casting can be used to produce hollow parts without using cores.
• In this process the mold is filled as normal, and solidification begins at the outer surface and moves inwards. After a short period of time the mold can be turned over, and the molten metal inside will run out. This leaves a thin shell in the mold.
1.3.2.2 - Pressure Casting
• In this process the normal permanent mold process is used, except instead of pouring molten metal, it is forced into the die under a moderate pressure or pulled in using vacuum). This pressure is maintained until the part has solidified.
• The constant pressure allows for filling of the mold as it shrinks.
1.3.2.3 - Die Casting
• The basic process is,
1. two permanent mold halves of a die (mounted in a press) are brought together.
2. the molten metal is injected through a runner and gate with pressures up to 100 ksi - 2000-5000 psi is common.
3. air escapes into overflow wells, and out vents, and metal fills the molds
4. the mold is chilled, and the injected metal freezes
5. the mold is separated, and knockout pins eject the part
6. the parts are cut off the runners and sprues
• Used for low melting point (non-ferrous) metals such as,
- zinc
- aluminum
- magnesium
- copper
- lead
- tin
• Can produce complex shapes at mass production rates.
• Metal dies,
- must withstand high pressures
- die life is shortened by extreme temperature fluctuations
- dies often made with carbon or special alloys
- multiple cavities can be used in the die
• Applications,
- automotive parts
- appliances
- office machines
- bathroom fixtures
- outboard motors
- toys
- clocks
- tools
• Die casting machines can use,
- hot chambers with a plunger - a reservoir of molten metal is used to directly feed the machine.
- a cold chamber - metal is ladled into the machine for each shot.
• Hot chamber machines are,
- good for low temperature zinc alloys (approx. 400°C)
- faster than cold chamber machines
- cycle times must be short to minimize metal contamination
- metal starts in a heated cylinder
- a piston forces metal into the die
- the piston retracts, and draws metal in
• Cold chamber machines,
- casts high melting point metals (>600°C)
- high pressures used
- metal is heated in a separate crucible
- metal is ladled into a cold chamber
- the metal is rapidly forced into the mold before it cools
• All die casting processes require a large press to hold mold halves together during a cycle.
• Advantages,
- intricate parts possible
- short cycles
- inserts feasible
- cycles less than 1 minute
- minimum finishing operations
- thin sections, high tolerances, good surface finish
• Disadvantages,
- metal die is costly
- porous parts
- not suited to large parts
- long setup times
- $5000-200,000 for machine
- metal melting point temperature must be lower than die
1.3.3 Centrifugal Casting
• The basic process is,
1. a mold is set up and rotated along a vertical (rpm is reasonable), or horizontal (200-1000 rpm is reasonable) axis.
2. The mold is coated with a refractory coating.
3. While rotating molten metal is poured in.
4. The metal that is poured in will then distribute itself over the rotating wall.
5. During cooling lower density impurities will tend to rise towards the center of rotation.
6. After the part has solidified, it is removed and finished.
• There are three variants on this process,
true centrifugal casting - long molds are rotated about a horizontal axis. This can be used to make long axial parts such as seamless pipes.
semicentrifugal casting - parts with a wide radial parts. parts such as wheels with spokes can be made with this technique.
centrifuging - the molds are placed a distance from the center of rotation. Thus when the poured metal reaches the molds there is a high pressure available to completely fill the cavities. The distance from the axis of rotation can be increased to change the properties
• Centrifugal and semicentrifugal casting used for axisymmetric parts (internally).
• Parts from 6” to 5’ in diameter can be made, but typical diameters are 10’ to 30’.
• Long tubes can be made that could not normally be rolled.
• Typical metals cast are,
- steel
- nickel alloys
- copper
- aluminum
• Typical applications are,
- train wheels
- jewelry
- seamless pressure tubes/pipes
• Advantages,
- good uniform metal properties
- no sprues/gates to remove
- the outside of the casting is at the required dimensions
- lower material usage
- no parting lines
- low scrap rates
• Disadvantages,
- extra equipment needed to spin mold
- the inner metal of the part contains impurities
1.3.5 Single Crystal Casting
• The process is effectively,
1. Prepare a mold so that one end is a heated oven, and the other end chilled. The part should be oriented so that the cooling happens over the longest distance.
2. Cast metal into the mold
3. Solidification will begin at the chill plate. These dendrites will grow towards the heated end of the part as long dendritic crystals. The part is slowly pulled out of the oven, past the chill plate.
4. Remove the solidified part.
• Parts made of a single crystal can have creep and thermal shock resistance properties.
• There are two variants to this technique,
directionally solidified - in this case the dendrites grow from the chill plate towards the other end.
single crystal - a helical constriction is used so that instead of parallel dendrites, only a single crystal is formed in the blade.