The proper selection of the filler in a molding powder is important in influencing the quality of the molded article. Fibrous fillers improve the mechanical strength and shock resistance of the finished article. Wood flour is the most widely used filler in tar-acid resins as well as in other thermosetting resins. Pine, spruce, and fir are the principal kinds used, and consideration must be given to the bulk, gum content, color, and the size and shape of the wood particles. Color is the least important since most of the tar-acid resins give brown or black moldings. When the molding must withstand high temperatures, asbestos fiber is used as a filler. In articles requiring high shock resistance, such as golf club heads, a filler of paper pulp is used. Where high electrical insulation and dielectric properties are required, ground mica is used as the filler. Certain inorganic fillers such as powdered slate, gypsum, barium sulphate, calcium sulphate, china clay, zinc oxide, and infusorial earths, are sometimes used. Large proportions of these may be used where hardness is more important than strength, as in phonograph records. Other materials used include rubber, graphite, horn, bone, starch, pumice, and cork. Coloring matter used may be coal-tar dyes or pigments such as bone black, carbon black, and iron oxides. Pigments are usually more satisfactory, although dyes are sometimes preferred in articles for insulation. A lubricant is added to the molding mixture to overcome the tendency to stick in the mold. Metallic soaps, stearates, and stearic acid are those most commonly used. Sometimes a plasticizer is included, its function being to act as a solvent for the resin, thus increasing the flow of the material in the mold. The plasticizer should be one which will become infusible or at least remain solid in the molded article. [Illustration: PREFORM PRESS MAKING PELLETS FOR USE IN MOLDING. Source: Bakelite Corporation, 247 Park Avenue, New York, N. Y.] [Illustration: VACUUM CLEANER PARTS OF TAR-ACID RESIN ILLUSTRATING THE INTRICATE MOLDED SHAPES POSSIBLE. Source: Bakelite Corporation, 247 Park Avenue, New York, N. Y.] [Illustration: RADIO CABINET AND TELEPHONE SET OF MOLDED TAR-ACID RESIN. Source: Bakelite Corporation, 217 Park Avenue, New York, N. Y.] A typical molding powder or pre-form pellet will contain by weight: Resin 40 to 50 percent Filler 35 to 50 percent Plasticizer 5 percent Lubricant 1 percent Pigment 1 percent The molding of tar-acid resins.