Moulding Plant for Plastics

Abstract

Commencing with a general outline of the method of pressure moulding as it may be applied to moulding powders or laminated material, the author passes on to a description of the usual types of hydraulic presses, divided into upstroke, downstroke, angle presses, and sheeting presses. The hydraulic system as applied to moulding presses, is then discussed, with a description of hydraulic pumps, and both weight-loaded and air-loaded accumulators. The relative merits of water and oil as the hydraulic medium are also considered. Modern practice favours self-contained hydraulic pump units where only a few presses are installed in a factory, and also for very large units which would put an undue strain on the accumulator system.

The preheating of material in gas- or steam-heated ovens is described, also preheating by high-frequency high-voltage electric currents. Moulds and platens are heated by steam, gas, or electricity. The injection moulding of materials both of the thermo-plastic and thermo-setting type is discussed, with particular reference to the injection of thermo-plastic materials such as cellulose acetate, and to the injection and transfer moulding of thermo-setting materials such as wood-flour “filled” phenol formaldehyde. Screw-type extruders are used for certain materials such as casein, and for many of the newer thermo-plastic materials, such as “P.V.C.”

The author then considers the manufacture of tools and dies, and gives a typical layout of the mould and ejector pins. He also describes a 500-ton hobbing press, and the moulds which can be hobbed on this machine. Attention is also given to preforming machines for making the plastic moulding powder into “preforms”. In the manufacture of laminated sheet, paper or fabric material is impregnated with resin, dried in ovens, and then cut into sheets for pressing. A short description is given of the equipment required.