Compression Molding. This is the most common method of forming thermosetting materials. (It is not general used for thermoplastics.) Compression molding is simply squeezing a material into desired shape by heat and pressure to the material in a mold (see figure 12). Plastic molding powder, mixed with such fillers as wood flour, cellulose, and asbestos, is put directly into the open, heated mold cavity. The mold is then closed, pressing down on the plastic and causing it to flow throughout the mold. While the heated mold is closed, the thermosetting material undergoes a chemical change which permanently hardens it into shape of the mold. The there compression-molding factors- pressure, temperature, and time the mold is closed- vary with the design of the finished article and the material being molded.

Blow molding. This method of forming thermoplastics consists of stretching and then hardening a plastic against a mold. There are two general ways this is done: the direct and the indirect method, each with several variations. In the direct, old, a gob of molten thermoplastic is formed into the rough shape of the desired finished product. This shape is then inserted into a female mold, and air is blown into it, as into a balloon, to force it against the sides of the mold (see figure 13). The formed material is then cooled before removal from the mold. In the indirect method, a thermoplastic sheet or special shape is first heated, and then clamped between a die and cover. Air pressure-forced-between the plastic and the cover forces the material into contact with the die, which has the contour desired in the finished product. The plastic is cooled before removal from contact with the die.

Plastics are so durable that they will not rot or decay as do natural products such as those made of wood. As a result great amounts of discarded plastic products accumulate in the environment as waste. It has been suggested that plastics products could be made to decompose slowly when exposed to sunlight by adding certain chemicals to them. Plastics present the additional problem of being difficult to burn. When placed in an incinerator, they tend to melt quickly and flow downward, clogging the incinerator’s grate. They also emit harmful fumes.

Plastics are relatively to the field of engineering materials. The first commercial plastic was developed in 1868 to replace ivory for billiard balls. During World War II plastics were mainly a substitute for metals. Today high-performance plastics are responsible for the successful operation of parts and products in the critical environment of electronics, communications, and aerospace systems. And, in recent years, thousands of plastics have been developed and have shown that in many industrial and consumer applications they can do an equal or better job at lower cost than other materials.

Birley, Arthur, et.al., Plastic Materials: Properties and Applications. New York: Leonard Hill, 1982.

Braun, Dietrich. Simple Methods for Identification of Plastics. New York: MacMillan publishing Co., Inc. 1982.

Clauser, Henry R. Industrial and Engineering Materials. New York: McGraw-Hill, Inc. 1975.

Cordon, William A. Properties, Evaluation, and Control of Engineering Materials. New York: McGraw-Hill, Inc., 1979.

DeGarmo, Paul, et.al. Materials and Processes in Manufacturing. New York: John Wiley and Sons., 1999.

Kaufman, Morris. Giant Molecules: The Technology of Plastics, Fibers, and Rubber. New York: Doubleday and Company, Inc., 1968.

Van Vlack, Lawrence H. Materials for Engineering: Concepts and Applications. Massachusetts: Addison-Wesley, Publishing, 1982.

The next page includes a manufacturers plastics website and brochure.

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