Plastic processes injection-type

Polypropylene can be fabricated by almost any process used for plastics (see Plastics processing). The extmsion of pipe and injection mol ding of fittings present no unusual problem. However, there is no way to bond the fittings to the pipe except by remelting the polymer, which is impractical on most constmction sites. The resin can be reinforced by glass fibers, mineral fillers, or other types of fillers and can be pigmented readily. 

One of the most cotrunon processing methods for plastics is injection moulding. Nowadays every home, every vehicle, every office, every factory contains a multitude of different types of articles which have been injection moulded. These include such things as electric drill casings, yoghurt cartons, television... 

The process of injection molding (IM) is used principally for processing unreinforced or glass fiber reinforced thermoplastics (TPs) and thermosets (TSs) (Figure 4.1). Up to at least 90wt% of all plastics processed are TPs. There are many different types or designs of IM machines (IMMs) that permit molding many different products based on factors such as quantities, sizes (such as auto bumpers to medical micro products), shapes (simple to complex), product performances, and/or economics.1,150>157,173 176 476... 

Fluoropolymers can be injection-molded in plunger or ram-type equipment but a screw machine works best with this family of plastics. The screw-type injection molding machines have a number of advantages including those listed in Table 6.23. Various components of screw-type machines for processing fluoropolymers are described in this section. 

Although it is one of the oldest types of machinery for plastics processing, the single-screw extruder has not yet reached the end of its development—as can be seen in improvements over the past few years, based on the continuing industrial demand for improvements. Output, a major focus years ago, remains paramount but qualities such as wear and versatility are of equal importance. Improvements in all equipment are ongoing, with additional new developments occurring in injection molding machines (Chapter 2). 

There are many old machines in operation, particularly in the United States so there is ample room to improve and simplify plastics processing. In the United States it is estimated that about 80,000 injection molding machines, 12,000 extruders, and 6,000 blow molders are operating. For each type of machine, about 30 percent are under five years old, at least 35 percent are five to ten years old, and the rest are over ten years old. Present... 

Melt temperature n. The temperature of molten or softened plastic at any point within the material being processed. In extrusion and injection molding, melt temperature is an important indicator of the state of the material and the process. Many types of instruments, most of them based on thermocouples or resistance thermometers, have been employed in extruders, where melt temperature is usually measured in the head and sometimes in the die. In thermoforming, temperatures of softened sheets are measured with infrared pyrometers. 

Several factors affect any type of plastics processing including injection molding. They include viscosity, heat, temperature, thermal stability, thermal conductivity, crystallinity, and moisture. 

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