Joel McKinney

Assistant Professor of Graphic Design/Multimedia/Sculpture

(507) 859-2421,

Foundry Process: Investment Molds

Page Two

This is a hollow wax casting from a latex mold.
Sprues and vents, also made of wax, are added. These will allow the molten metal into the mold and let the air and gasses inside escape. The physics behind spruing and venting are just basic hydraulic theories and are beyond the scope of this presentation. Think liquid. It flows down, tends not to fill air pockets that are high spots unless the air is vented up and out. Fluid levels tend to equalize so all of the vents must open at the same height that your pouring cup opens.
The sprued wax is often coated with one layer of dental investment to preserve fine surface detail and to avoid surface bubbles that end up as small metal warts. A fine investment can also me made from equal parts molding plaster and silica flour. Mix the dry powders thouroughly (wearing a particle mask) before adding to water.
Flasks are built to form the investment mold. The flasks shown consist of a layer of 30# tarpaper rolled up and duct taped securely (the plaster is heavy and runny), with a layer of chickenwire rolled loosely inside. The tarpaper contains the plaster until it has set, the wire re-enforces the plaster during burn-out, handling, and pouring.
Usually, the sprued, once-coated wax will be set, cup down, on the floor. Note the small amount of plaster that has been poured on the floor of the flask to seal the base to the plastic underneath. After the thin layer of plaster has set, the rest of the flask is carefully filled with a standard investment mixture of plaster, sand or grog, and luto (old, ground up molds). Filling is usually a two person job with one person pouring from a bucket while the other steers the stream with a hand and protects the model. Bubbles should be strenuously avoided.
In this picture there are fully filled flasks on the left and on the right, and a mold with the tarpaper portion of the flask removed in the center. The paper is removed before placing the wet flasks into the burn-out kiln.
The molds spend three days in the burn-out kiln, two at 400f, one at 1000f (but never raised more than 200 degrees an hour). When they are ready the furnace is prepared, and started. Here the furnace is shown with a cold charge, ready to light.
While the metal is melting the molds are removed from the kiln and lined up, ready to pour. Aluminum foil is placed over the holes where the vent and cup wax was. This helps prevent accidental inclusions of sand, etc. from falling, irretrievably, into the molds.

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