There are four main types of permanent mold casting: gravity, slush, low-pressure, and vacuum.
Gravity process The gravity process begins by preheating the mold to . to ease the flow and reduce thermal damage to the casting. The mold cavity is then coated with a
refractory material or a
mold wash, which prevents the casting from sticking to the mold and prolongs the mold life. Any sand or metal
cores are then installed and the mold is clamped shut.
Molten metal is then poured into the mold. Soon after solidification the mold is opened and the casting removed to reduce chances of
hot tears. The process is then started all over again, but preheating is not required because the heat from the previous casting is adequate and the refractory coating should last several castings. Because this process is usually carried out on large production run work-pieces automated equipment is used to coat the mold, pour the metal, and remove the casting. The method was developed by
William Britain in 1893 for the production of lead
toy soldiers. It uses less material than solid casting, and results in a lighter and less expensive product. Hollow cast figures generally have a small hole where the excess liquid is poured out. Similarly, a process called
slush molding is used in automotive dashboard manufacture, for soft-panel interiors with artificial leather, where a free-flowing (behaves like a liquid) powder plastic compound, either PVC or TPU, is poured into a hot, hollow mold and a viscous skin forms. Excess slush is then drained off, the mold is cooled, and the molded product is stripped out.
Low-pressure Low-pressure permanent mold (
LPPM) casting uses a gas at low pressure, usually between 3 and 15 psi (20 to 100 kPa) to push the molten metal into the mold cavity. The pressure is applied to the top of the pool of liquid, which forces the molten metal up a refractory pouring tube and finally into the bottom of the mold. The pouring tube extends to the bottom of the ladle so that the material being pushed into the mold is exceptionally clean. No risers are required because the applied pressure forces molten metal in to compensate for shrinkage. Yields are usually greater than 85% because there is no riser and any metal in the pouring tube just falls back into the ladle for reuse. The vast majority of LPPM casting are from aluminum and magnesium, but some are copper alloys. Advantages include very little turbulence when filling the mold because of the constant pressure, which minimizes
gas porosity and
dross formation. Mechanical properties are about 5% better than gravity permanent mold castings. The disadvantage is that cycles times are longer than gravity permanent mold castings.
Vacuum Vacuum permanent mold casting retains all of the advantages of LPPM casting, plus the dissolved gases in the molten metal are minimized and molten metal cleanliness is even better. The process can handle thin-walled profiles and gives an excellent
surface finish. Mechanical properties are usually 10 to 15% better than gravity permanent mold castings. The process is limited in weight to . ==Advantages and disadvantages==