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
- 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
- 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
- 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
- The vacuum on the mold pattern is released to permit the pattern to be stripped from the mold
- 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.
Original equipment manufacturers (OEMs) utilize heavy gauge vacuum formed components for production quantities in the range of 250–3000 units per year. Vacuum-formed components can be used in place of complex fabricated sheet metal, fiberglass, or plastic injection molding. Typical industry examples include kiosks and automated teller machines, enclosures for medical imaging and diagnostic equipment such as magnetic resonance imaging (MRI) machines, engine covers in a truck cab, and railcar interior trim and seat components. Vacuum formers are also often used by hobbyists, for applications such as RC cars and masks.
There are some problems encountered in the vacuum forming process. Absorbed moisture can expand, forming bubbles within the plastic’s inner layers. This significantly weakens the plastic. However, this can be solved by drying the plastic for an extended period at high but sub-melting temperature. Webs can form around the mold, which is due to overheating the plastic and so must be carefully monitored. Webbing can also occur when a mold is too large or parts of the mold are too close together. Finally, objects that are formed often stick to the mold, which is remedied by using a draft angle of three degrees or more in the mold.