Injection Molding of Thermoplastics

Overview

Injection molding is a major processing technique for converting thermoplastic materials, accounting for almost 20% of U.S. resin production.

The process was patented by John and Isaiah Hyatt in 1872 to mold camphor-plasticized cellulose nitrate (celluloid). The first multicavity mold was introduced by John Hyatt in 1878. Modern technology began to develop in the late 1930s and was accelerated by the demands of World War II. A similar surge in the technology of materials and equipment took place in the late sixties and early seventies.

This section will discuss the machinery and the practice of injection molding.

The basic concept of injection molding is the ability of a thermoplastic material to be softened by heating, formed under pressure, and hardened by cooling. In a reciprocating screw injection molding machine (Fig 5–1), granular material (the plastic resin) is fed from the hopper (a feeding device) into one end of the cylinder (the melting device). It is heated and melted (plasticized or plasticated), and it is forced out the other end of the cylinder (while still melted) through a nozzle (injection) into a relatively cool mold (cooling), held closed by the clamping mechanism. The melt cools and hardens (cures) until it is fully set up. Then the mold opens, ejecting the molded part (ejection).

Thus, the elements of injection molding are:

• 1. The way in which the melt is plasticized and forced into the mold (the injection unit).

• 2. The system for opening and closing the mold and holding it closed with a force (the clamping unit).

• 3. The type of machine controls.

• 4. The injection mold, which forms the part and acts as a heat exchanger.

The author gratefully acknowledges the permission of John Wiley & Sons, Inc., New York, to use material from his book Injection Molding, Theory and Practice (1973). Mr. Rubin is president of Robinson Plastics Corp.