Piston injection unit

In the piston injection unit, the material is transported step by step through the heated plasticating cylinder until the required temperature is reached. A dosage device replaces the consumed plastic by new material. As the dwell time in the cylinder is long, this method is not suitable for processing heat sensitive materials such as rigid polyvinyl chloride (PVC) and thermosets. Figures 1.3-1.5 illustrate the machine cycle. 

In screw piston injection machines, the material is plasticised more homogeneously and has to stand less thermal stress than in piston injection units, as the plasticising itself happens just shortly after the... 

This is the most common type of unit and will be the basis for further discussion in this section. Thermoplastics as well as thermosets and classical elastomers can be processed with screw piston injection units with the process cycle as described in Chapter 1. 

After the mould has closed, but before final clamping, a measured quantity of rubber is injected into the transfer port. The rubber forces the transfer port open until it contains the required volume. With a top injection system the mould is forced downwards and the transfer piston remains stationary. With a bottom injection unit the mould is stationary and the transfer piston is raised. In both cases the effective volume of the port is increased by the filling action of the rubber. 

Ram and piston injection molding units are used in the rubber iudustry [57]. The rubber stock is fed to a heated cyliuder, which is heated to the required temperature aud from there is forced by a hydraulic ram through a nozzle, mold runners, and... 

IMMs for TS polyester molding compounds can have either a flighted screw or a solid injection piston and are of modular design, allowing quick and effective change of injection unit fi om screw to piston. A unique aspect of one design is an automatic feed unit in three capacities of 100, 350 and 650 liters. 

Figure 1.79 (b) shows the mold for the transfer molding process from the bottom. A press with a separate injection unit is required for molds of this design. Most commonly, these are presses with a hydraulic cylinder that is centrally attached to the mold table. This actuates the transfer piston which is linked to the machine control (transfer press). 

In the early machines, the injection unit was simply a piston traveling in a heated cylinder. The raw material was fed into the cylinder by volume or by weight, and the material came to process temperature by thermal conduction. This approach had two severe limitations ... 

This pump is the same in principle as the piston type but differs in that the gland is at one end of the cylinder making its replacement easier than with the standard piston type. The sealing of piston and ram pumps has been much improved but, because of the nature of the fluids frequently used, care in selecting and maintaining the seal is very important. The piston or ram pump may be used for injections of small quantities of inhibitors to polymerisation units or of corrosion inhibitors to high pressure systems, and also for boiler feed water applications. 

The SCORIM process employs a unique hardware component that is retrofitted to a conventional injection molding machine. Located between the injection nozzle and the mold, this unit contains a pair of pistons that exert pressure on the melt after it is injected... 

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