Injection Molding of Thermosets

Overview

In the mid-1960s, about two years after the introduction of automatic transfer molding machines and screw-preplasticizing/transfer molding machines, the concept of in-line screw injection molding of thermosets (sometimes referred to as direct screw transfer or DST) was developed. It had long been thought that this process, already highly successful with thermoplastic materials, would be impractical for thermosetting plastics because the critical time-temperature relationship would prove uncontrollable, and material would set up in the barrel.

Development progressed not only on screw and barrel designs but also on stability of ther-moset materials at elevated temperatures, finally leading to successful processing by screw injection molding. (See Fig. 8–1.)

This technique has had a significant influence on the thermoset molding business by virtue of reduced molding cycle time and the potential it offers for low-cost, high-volume production of molded thermoset parts.

Today, thermoset injection molding machines are available in all clamp tonnages up to 1200 tons and shot sizes up to 20 lb. (see Figs. 8–2, 8–3, and 8–4).

Thermoset in-line screw assemblies are fitted to horizontal or vertical clamp machines. Most horizontal clamp thermoplastic injection machines can be converted to injection of thermoset by changing the screw, barrel, and nozzle. Because most thermoset barrels are shorter than their thermoplastic counterpart, it may be necessary to reposition the injection assembly closer to the clamp fixed platen.

The thermoset molding materials developed specifically for injection molding have a long shelf life in the barrel at moderate temperatures (approximately 200°F), and react very rapidly when the temperature is brought up to 350 to 400°F as the material is forced through the sprue, runners, and gates, and fills the cavities. This unique development in materials helped to gain acceptance of injection molding as a reliable production process.