U.S. Gypsum
Mold Making

Models

The model should be checked for any flaws. Patching or repairing should be done, first, with clay or plaster. Whenever copies are made, some details are lost; therefore make certain the model has desired detail on the cast. When the model is perfect, it can be fastened to any smooth flat surface such as glass, hardboard or plywood. If the model is a porous material such as wood, plaster, concrete, etc., it should be sealed with thinned orange shellac and thoroughly dried. If the model is made of non-porous material such as metal, glass or glazed ceramics, etc., a sealer coat may not be needed.

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Molds can be made of a variety of materials. All molds, however, are grouped into two general categories:

Plastic Molds

Flexible Molds

Vacuum formed plastic molds are used extensively when a low-cost, two-dimensional mold is desired. Several thousand mold designs are available, varying in size from a few inches to two by four feet. If handled with care they produce 25 to 100 plaster castings. Most molds require back-up support during casting. Plastic molds could use either a plaster back-up shell or a sand box as a back-up support. When using the latter, the sand must be dry and free of cracks or lumps. The mold must be pressed down carefully into the sand, until the mold is level. Then industrial plaster or gypsum cement is poured into the mold.

Flexible molds may be made of gelatin, latex, cold compounds, hot melt, polysulfide, urethane elastomers and other materials having good strength and elasticity. Flexible molds are a necessity for art work involving complicated figurines. They are expensive and may require several days to several weeks to complete. A back-up shell is almost always necessary to support the shape when pouring the plaster.

Flexible molding materials falls into three major groups. Each material is ideally suited for a specific application and usage. Which to use is determined by job requirements.

Latex rubber - These have a low raw-material cost, because less material is used than other flexible material. If properly cared for, latex molds last for years and hundreds of casts. They require a back-up shell of plaster or glass fiber to hold the mold shape while pouring. In some cases, latex molds take several weeks to make because the time between coat applications is 10 to 12 hours. Multiple coats (25 to 50) may be needed to get the desired thickness for a particular model. The most common way to apply latex is by brush; however it may be sprayed or dipped. Dipping reduces the time required to make the molds, but is limited to simple non-seam glove molds. Latex stretches but regains its original shape. Latex molds are used where high-tolerance and dimensional stability are not important, such as in the hobby and craft industry.

Cold molding compounds - silicones, polysulfides and urethanes are included in this category. Each comes in different grades, strengths, hardness, flexibilities and colors. Some are more dimensionally stable than others. Cold molding compounds offer better dimensional stability and faster mold preparation than latex. Molds can be made in days instead of weeks. Cold molding compounds, usually consist of 2 or 3 components. These must be weighed carefully and mixed properly in order to achieve good results. When selecting a cold molding compound to meet the needs of a project, discuss your job requirements with the manufacturer or distributor in your area. New materials are being developed every year. Store molds in a cool, dark place and in immediate contact with the model that has been sealed with high-quality orange shellac (not lacquer). A porous model will cause mold shrinkage due to the migration of the plastisizer from mold to model. Molds not stored in contact with model may become distorted.

Hot Melts - Similar to cold molding compounds in the style of mold produced, hot melt molding materials must be heated to a liquid before pouring to make the mold. With recent improvements in cold molding compounds, hot melts are used in only a very few operations.

Plaster Master Industries
4308 Shankweiler Rd. Orefield, PA 18069
Phone (610) 391-9277 · Fax (610) 391-0340

U.S. Gypsum Mold Making

U.S. Gypsum
Mold Making