Plaster mold casting

Dimensional tolerances cannot be achieved capably and Increase difficulty/cost substantially Consider another or secondary process 

8CC Plaster mold casting process capability chart. 

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Large and small shapes may be sUp cast from both plastic and nonplastic mixes by the usual techniques. Precise shapes, such as glass feeder parts, are made in this way as well as large flux blocks. The process requkes the formulation of a sUp of suitably stable character to be poured into a plaster mold to be dewatered. After it solidifies, the mold is removed and dried further before firing. 

Forming-Die Alloys. The tonnage of slab zinc used in this appHcation is small. The use of zinc alloy dies started in the aircraft industry during World War II (119). Zinc-based alloys cast in sand and plaster molds continue to be used for short-mn dies for steel and aluminum stampings in the automotive and aircraft industries (120). Considerable cost savings are realized with these low melting zinc-based alloys which are easy to poHsh, machine, weld, and remelt. 

Students will carve and assemble a plaster mold for casting. 

Plaster of paris casting, which gives better surface finishes, dimensional accuracy, finer detail, and a more solid structure than sand castings, but it is more expensive. The plaster mold is made by mixing plaster of paris with water, then pouring it around the pattern and allowing it to partially set. The pattern is then removed. Separate parts of the mold assembly are baked separately to complete setting and to drive off moisture. 

Roy SK (1968) Slip Casting in Plaster Molds. In Proc WW/ME Divs, Am Ceram Soc, Bedford Springs, Pa. Sept. 

Lehman L (1958) Plaster Molds and the Influence of their Properties on the Formation of a Slip Casting. Berichte der DKG 35 273-277... 

Plaster molds from wooden patterns, up to 40 castings depending on mold... 

The surface of the model is covered with a uniform layer of clay or plasticine. The thickness of the spacing layer determines the thickness of the final mold. The next step is to build up a rigid layer on top of the spacer. This support is either poured (plaster or casting resin) or built up in layers by spreading the material (polyester, polyurethane or epoxy resin reinforced with fiberglass matting). Once the support has set, it is separated from the spacer, which is then discarded. The support, which must contain air-escape and feeding holes, is now fitted over the model. The silicone rubber is poured into the hollow space between the support and the model and allowed to cure. The... 

Sprayed-metal mold n. A mold made by spraying molten metal onto a master form to obtain at shell of desired thickness, which may subsequently be backed with plaster, cement, casting resin, etc. Such molds are used most commonly in sheetforming processes. 

Of course you should be warned that a plaster mold will not last as long as one made from epoxy resin. Plaster tends to be brittle around the edges of the mold cavity. And chunks can easily chip away when removing finished parts. You will be lucky if you get more than 1 or 2 shots out of a plaster mold, but the cost of the plaster is so minimal tliat it is really the only way to learn how to produce molds using the casting method. Save the aluminum filled epoxy for those molds you plan to use a lot. 

Molds must be of a solid material able to withstand the force exerted on them by atmospheric pressure as well as the temporary heat of the plastic when it comes in contact with the mold. They can be constructed of many materials some of which include, wood, fiberglass, plaster, epoxy, cast aluminum, machined aluminum, etc. 

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