Vacuum molding, also called the V-process, was developed in Japan around 1970. It uses a sand mold held together by vacuum pressure rather than by a chemical binder. Accordingly, the term vacuum in this process refers to the making of the mold rather than the casting operation itself.
Because no binders are used, the sand is readily recovered in vacuum molding.
Also, the sand does not require extensive mechanical reconditioning normally done when binders are used in the molding sand. Since no water is mixed with the sand, moisture related defects are absent from the product.
Disadvantages of the V-process are that it is relatively slow and not readily adaptable to mechanization.
Steps in vacuum molding
1. A thin sheet of preheated plastic is drawn over a match-plate or cope-and-drag pattern by vacuum—the pattern has small vent holes to facilitate vacuum forming
2. A specially designed flask is placed over the pattern plate and filled with sand, and a sprue and pouring cup are formed in the sand
3. Another thin plastic sheet is placed over the flask, and a vacuum is drawn that causes the sand grains to be held together, forming a rigid mold
4. The vacuum on the mold pattern is released to permit the pattern to be stripped from the mold
5. This mold is assembled with its matching half to form the cope and drag, and with vacuum maintained on both halves, pouring is accomplished.
The plastic sheet quickly burns away on contacting the molten metal. After solidification, nearly all of the sand can be recovered for reuse.