Source: The Concrete Producer/Concrete Journal
Publication date: April 1, 1994
By Charles Biggs
Abstract: Polyurethane molding compounds have revolutionized the decorative concrete industry, enabling high-quality statues and artwork to be reproduced at relatively low costs. Before polyurethane molding materials, the concrete and cast-stone industries relied on latex, hot-melt vinyl, and rigid multipart fiberglass or metal molds. Latex and hot-melt vinyl both distort easily over time, creating deficiencies in the final product. The introduction of polyurethane skin molds supported by rigid multi-part cases enables manufacturers to produce lifelike forms over many uses. The creation of the mold is an intricate process: First a pouring case must be formed to contain the liquid molding material. This is accomplished by coating the model in clay, and then pouring plaster around the clay. The clay is removed and the plaster cleaned. Then the plaster case is replaced and the polyurethane poured into the space previously occupied by the clay. When the polyurethane has cured, the plaster case is removed and a fiberglass production case is made. The initial cost of urethane molding materials is greater than vinyl or latex because of the labor required to produce the mold. However, once the mold is created, costs decrease. Urethane also offers a time advantage in terms of making a new mold--one can be created in a few days, and a typical mold can be used several hundred times.
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