45.3 MULTIPLE USE MOLD TECHNIQUES
45.3.1 Vacuum Casting
3. The head is lowered into molten metal in an induction furnace so that the lower face of the mold is submerged.
This process is relatively inexpensive and can be automated.
Thin walls, down to 0.02" are possible.
The process can be used effectively with reactive metals.
45.3.2 Permanent Mold Casting
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.
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.
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,
Molds are machined, including the cavity and gates. Typical mold materials include,
Typical core materials include,
Low melting point metals can be cast
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.
45.3.2.1 - 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.
45.3.2.2 - 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.
45.3.2.3 - Die Casting
2. the molten metal is injected through a runner and gate with pressures up to 100 ksi - 2000-5000 psi is common.
Used for low melting point (non-ferrous) metals such as,
Can produce complex shapes at mass production rates.
All die casting processes require a large press to hold mold halves together during a cycle.
45.3.3 Centrifugal Casting
1. a mold is set up and rotated along a vertical (rpm is reasonable), or horizontal (200-1000 rpm is reasonable) axis.
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.
45.3.4 Casting/Forming Combinations
These processes basically casting molten metal, but the use mechanical force to reshape.
45.3.4.1 - Squeeze Casting
3. Pressure is applied to the punch and die while the part solidifies. This pressure is lower than normally required for forging.
This method overcomes problems with feeding the die, and produces near net, highly detailed parts.
45.3.4.2 - Semisolid Metal Forming
This can produce better metal qualities in net shape parts requiring no finishing operations.
45.3.5 Single Crystal Casting
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.
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.
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.