Synthetic resins and their raw materials / A survey of the types and uses of synthetic resins, the organization of the industry, and the trade in resins and raw materials, with particular references to factors essential to tariff consideration. Under the general provisions of section 332, title III, part II, Tariff act of 1930.

by United States Tariff Commission

TAR-ACID RESINS FOR MOLDING

The tar-acid resins were first developed for molding and they are still used in large volume in this way. An article produced in large quantity is more likely to be made of molded resin. The cost of the mold, which may amount to several thousand dollars, then becomes very small per unit produced. If the article is of such a shape that it would require a great deal of labor to produce in metal or wood, it may be produced in quantity much more cheaply from resin, since it will come from the mold almost in finished form.

A few of the large molders find it economical to make their own resins when they use one type in large volume or desire some special modification. Most of the molders buy resins for molding in the form of either powder or pre-formed pellets ready for use.

Molding powders and pellets.

Molding powder is made from B-stage resin (see p. [13]), a filler, a pigment, a lubricant, and a plasticizer. These materials are mixed and put through rolls at a moderate heat and pressure. The resin softens and amalgamates with the other materials. It hardens upon cooling and is ground to powder. A pre-formed pellet may be made from the powder by pressure; use in this form saves the time of the molder when filling the mold, since he is not required to measure the powder.

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.

Preform Press Making Pellets for Use in Molding.

Source: Bakelite Corporation, 247 Park Avenue, New York, N. Y.

Vacuum Cleaner Parts of Tar-Acid Resin Illustrating the Intricate Molded Shapes Possible.

Source: Bakelite Corporation, 247 Park Avenue, New York, N. Y.

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.

Ordinarily the molds used are made of hardened steel, highly polished. They must stand working pressures of several thousand pounds per square inch. The mold is placed in a hydraulic press, heated by steam, electricity, or gas, and the molding material is placed in the mold. The press is closed and heat and pressure are applied. The temperatures used range between 250° F. and 365° F., and the pressures between 1,000 and 8,000 pounds per square inch. The molding time depends on the shape and size of the article and on the composition of the molding material. As little as one-half minute is required for small objects and as long as 10 minutes for large objects. Average molding time is about 3 minutes. The article is removed from the mold, allowed to cool, and is then trimmed, sanded, filed, or polished. Since the mold is highly polished, the finishing operation is usually needed only to remove the flash. Inserts, such as metal parts (binding posts, electrical contacts, etc.), or inlays of polished metal in name plates, and signs, are often molded in; gear shift knobs are molded over a hollow metal core; rubber inserts are used in castors, electrical plugs, and similar objects.

The molding operation is an art, and has made remarkable progress in recent years. Many articles molded of tar-acid resins are well-known to the public. The automotive industry is the best customer, using such molded parts as gear shift knobs, horn buttons, accelerator pedals, light switches, ignition parts, and distributor heads. Other well-known applications are builders’ hardware, electrical switch plates, switches and fixtures, fountain pens, radio parts, telephone parts, handles for stoves, vacuum cleaners, and other appliances, buttons, buckles, costume jewelry, camera cases, radio cabinets, small containers, and hundreds of others.

The importance of tar-acid resins in molded articles is shown by the fact that more than 75 percent of all synthetic resin molded articles made in 1937 used this type of resin as a binder.

Production of tar-acid molding resins.

Domestic production of tar-acid molding powders and pellets was reported to the Tariff Commission by 15 makers in 1937. Most of these firms have specialized in resin development and manufacture. Among the well-known brands are Bakelite, Durez, Durite, Resinox, Indur, and others (see p. [153] for list of trade names).

Statistics of production and sales of tar-acid resins used in molding were collected separately for the first time in 1935. They show a net resin output of about 21,000,000 pounds, with sales of 18,000,000 pounds or about 40 percent of the total tar-acid resins. The average unit value was 17 cents per pound. In 1937 the production of tar-acid resins for molding exceeded 32,000,000 pounds, again about 40 percent of the total. These statistics are based on net resin and do not include fillers, modifiers, pigments, or inert material of any kind.