Investment casting is the method of forming a mold for the lost wax casting process.
The first step in investment casting is to add the sprues (or gates) to the wax model. These will allow the molten metal to flow into the mold by gravity or centrifugal force, and for air to flow out. In very large molds, there may be seperate vents to prevent air pockets from causing an incomplete casting.
The wax model is mounted in a metal container. This may be anywhere from a soup can with a special base attached to a 55 gallon barrel ... or larger. Usually, the wax model is mounted so that the sprues support it from the bottom of the can. The investment material is mixed with water and poured carefully around the wax model. To avoid air pockets which will trap excess metal, several different methods may be used:
- De-bubblizing: The surface of the wax model is sprayed with alcohol or another substance to break up the surface tension of the water in the investment. This is more to ensure a smooth surface than to prevent large bubbles.
- Vacuum investing: After pouring in the investment, the container is placed under a bell jar which is evacuated using a vacuum pump. This causes any air bubbles in the solution to expand until they rise to the surface and are removed. The investment expands substantially during vacuum investing, so the container should be taller than the level of the investment material.
- Vibration: The container is vibrated (carefully, to avoid breaking the sprues loose) to shake loose air bubbles. On the vacuum investing machines made for small jewelry and dental castings, the bell jar is mounted on a spring-loaded platform that makes it easy to shake it a bit by hand.
Once the investment has cured and dried fully, it is ready for the burnout and casting. The mold is placed in a burnout oven and heated until the wax burns away. The casting is done with the mold still hot, to avoid failure due to thermal shock.
See also: Lost wax bronze casting process, treeing, wax injection, spin casting.