COMPRESSION MOLDING

History

History has failed to establish definitely the date of origin of the art of molding. It might be said that the art of molding originated with prehistoric man when he learned how to form pottery from clay, using the pressure of his hands to form the shape and the heat of the sun to harden the clay.

The earliest application of compression molding as a manufacturing process was early in the 19th century, when Thomas Hancock perfected a process for molding rubber. The first patent on a molding process in theUnited States was issued in 1870 to John Wesley Hyatt, Jr., and Isaiah S. Hyatt.

Dr. Leo H. Baekeland’s development of phenol-formaldehyde resins in 1908 gave the industry its first synthetic molding material, which is even today one of the principal materials used in the compression-molding process.

As a result of recent developments in the art of molding thermosetting materials, the transfer or plunger method has to a certain degree replaced the original compression method and has made it possible to produce articles of more intricate design, and also to work to closer tolerances. However, compression molding still is, and will be for a long time to come, the standard conventional method of producing molded articles from thermosetting plastics.

Materials Used in Compression Molding

Both thermosetting and thermoplastic materials can be molded by compression, but thermoplastics can generally be molded more economically by injection, and hence compression molding is mostly confined to the thermosetting materials.

Thermosetting materials are softened by the initial application of heat, but further application of heat causes a chemical reaction within the materials, which hardens them permanently so that they cannot be remolded. The maintenance of pressure on the material during the molding prevents the development of porosity by any gases evolved by the reaction.

Most commonly known thermosetting materials are made from phenol-formaldehyde, urea-formaldehyde and mclamine-formaldehyde resins. Of late, a new important material has been produced from alkyd resins. Silicone resin with glass-fiber filler is another molding compound for highly specialized applications. Also, polyester resins are thermosetting, but these are used mostly in low-pressure molding, which is covered in Chapter 6 on Reinforced Plastics.

Thermoplastic materials soften when heated and remain soft until cooled. They do not undergo a chemical change during molding, and can, therefore, be remolded.

Examples of thermoplastic materials are cellulose acetate, cellulose acetate butyrate, ethyl cellulose, acrylics, vinyls, polystyrene, styrene copolymers, nylon, polyethylene and fluorocarbons.

General Description of Compression Molding

Compression molding is the art of producing an article of definite shape from a molding material by loading the material into an open heated mold, shaping it by dosing the mold under pressure, and hardening it in the mold under pressure 

In modem plastics practice, the molding material is supplied in comminuted form, and the softening of the material by heat in the mold serves to permit the pressure to weld it into a continuous mass and to force this mass to take the shape of the cavity of the mold. The hardening of the molded mass under pressure is accomplished, for thermoplastics, by cooling it, and for thermosetting materials by continuing the heating until the hardening has been effected by chemical change or “cure’

Advantages of Compression Molding

Thermosetting and thermoplastic materials can be molded by this method. Large production is possible through the use of multiple-cavity molds. Large articles, such as trays, radio cabinets and industrial switch bases, are molded by this method . The size of the article which can be molded is limited only by the tonnage and size of the available press equipment. This method is best suited for quantity production from thermosetting materials.

Limitations of Compression Molding

In the case of very intricately designed articles, containing undercuts, side draws and small holes, the compression method may not be practicable, because of the necessity of complicated molds and the possibility of distorting or breaking pins during the flow of the material under high pressure. Also, compression molding may be unsatisfactory for production of articles having extremely close dimensional tolerances, especially in multiple-cavity molds, particularly in relation to the parting line of the molded article. In both cases, it is recommended that the plunger or transfer method of molding be used in preference. For details regarding this method of molding, see the section in this chapter on Transfer Molding.

For thermoplastics, compression molding is usually economical only when articles are too large, in either weight or projected area, to be produced on available injection-molding machines. However, a combination of injecting the correct amount of the thermoplastic material into the mold and then applying pressure on the softened material through a compression force plug is being used with considerable success. This is especially desirable for thermoplastic moldings with very heavy cross-sections. The action of the compression force plug keeps the material under pressure while setting, and thus prevents development of air bubbles and shrink marks.

Molding of Phenol-Formaldehyde Compounds

The details of the procedure of molding thermosetting materials can conveniently be covered by a description of the molding of phenol-formalde-hyde compounds. The minor differences in procedure required for handling other thermosetting materials will be pointed out in later sections.

A typical mold is made in two parts which when brought together enclose a cavity representing the article to be molded. The parts are mounted, in register, in a hydraulic press which serves to open and close the mold and to apply pressure to its contents. Usually the mold is cored for circulation of steam under pressure, since steam provides the most satisfactory heating, especially in large complicated molds, because it automatically replaces itself as it condenses, and holds the mold at an even temperature.

Electrical heating is sometimes used, and gives good results with molds in which uniform distribution of heating elements of su伍cient capacity is possible. Heating of molds by other media, such as oil and hot water, has been successfully used in a limited manner.

In molding, an amount of molding material required for the molded article is placed in the lower part of the open heated mold, and the mold is closed by operation of the hydraulic press. The molding material becomes heated by the hot mold, and thereby plastified. The pressure consolidates it into a continuous mass and forces it to flow into the shape of the mold and completely fill the cavity formed between the two parts of the mold. A slight clearance is allowed between these, in order to allow a slight excess of molding compound to escape; usually a clearance of 0.002 to 0.005 in. is sufficient. The material filling the cavity is held under heat and pressure to harden it, and then the mold is opened and the molded article removed. Thermosetting materials, having been hardened by a chemical change through application of heat, can be ejected after the proper curing cycle without need of cooling the mold. Only occasionally is a slight cooling of the mold advisable with thermosetting materials in order to improve the dimensional stability of the articles.

The proper molding procedure produces properly cured articles of sound, uniform structure. Before these can be used, they must be finished, by operations described in Chapter 16 on Machining and Finishing.

General-purpose phenolic molding compounds are supplied in granular form, and can be loaded into the mold in this form, in weighed or measured charges. But in commercial operation there is economy in preforming such material into tablets of the correct size and weight by means of automatic preforming equipment. This is less expensive than weighing out individual