thermoplastic and thermosetting

All plastic articles are initially derived from molding compounds. These molding compounds consist of a resin or binder and one or more of the following components： fillers, plasticizers, dyes and pigments, and lubricants. The “resin，” as the principal component, gives the compound its name and classification, and imparts the primary properties to it It is the cohesive and adhesive agent which provides rigidity and binds together the filler particles. The “filler” is usually an inert, fibrous material which modifies the properties of the resin or imparts special properties to it. “Plasticizers” are added to the compound if the flow or softness of the compound must be regulated, while “dyes and pigments” are added to impart color to the molded part. Lubricants of wax or stearates are added occasionally to a molding compound to facilitate its removal from the mold. (Its degree of granulation also has a major bearing on molding qualities.)

Natural and synthetic rubber are plastics which are very important to industry. These incompressible elastic plastics present problems very similar to those encountered in the manufacture and application of other types of plastics. Some specialized equipment，such as callenders, is required for the preparation of raw material. Rubber-like plastics may be extruded into shapes, filaments, or sheets, and formed by the extrusion, transfer, and compression types of molds.

Some thermoplastic and thermosetting plastics are used as adhesives. They are tough, strong, and reliable, and can be applied between almost any combination of materials. One of the first and most successful applications was the use of transparent plastic sheets between two plates of glass to form our present-day safety glass. The tough plastic adheres to the glass and prevents splinters from flying. Plywood made with plastic adhesives withstands weathering and water and now can be used for concrete forms and outside sheathing for homes. Large wooden columns and thick panels can be built by curing the adhesive by induction heating. The combination of plastic and wood makes a strong structural member. Wood furniture and metal cabinets are held together by adhesives which simplify their design and reduce cost of manufacture.

Fabrics are made of many plastics in pleasing colors. They are durable, tough, and easy to clean. Natural fabrics such as cotton and wool are facing stiff competition from these new plastic filaments.

Reinforcing of plastics by metal and glass fibers has produced strong, flexible, and light materials, such as that used in bullet-proof vests for the armed services.

Plastics are manufactured under controlled conditions to give uniform raw material and finished products. Color, surface, strength, and size variations are minor. Failures usually are due to misapplication by the engineer. There is no such thing as a bad plastic； all plastics are good if compounded properly for a particular use. Sufficient information for making a proper choice is available from material suppliers and plastic-molding companies.

The cost of plastics is being reduced constantly as improvements are made and demand increases. (See Table 8-1 for relative costs of one type of plastic.) In general they are more expensive than metals on a “per-pound” basis. It will probably be some time before automobile bodies, kitchen cabinets, or refrigerator housings are predominantly made of plastics. By coating metals with plastics the benefits of both materials can be obtained. Plastics must do the same job as another material at less cost, or a better job for the same money, or be in such a position that a plastic is the only material that will do the job, before they are used more extensively.

The cost of metal molds for plastics is about the same as that of die-cast molds. A superior polish in a mold for plastic is transferred authentically to the molded piece. The molding operation in some cases may be slower than metal-die and permanent-mold castings； however, the trimming of flash associated with die casting is not usually as time consuming with plastics. (See Factors Affecting Costs under “Molding.”)