Injection shot volume

Some injection moulding machines are designed according to the First-In, First-Out (FIFO) principle. The material first plasticised is also the first material to be injected so that the thermal state of the entire shot volume is identical. 

Simple ram machines are cheaper than screw machines having equivalent shot volumes. Although the simple ram injection moulding machine was once quite widely used it was generally obsolescent by 1977. Plasticisation by screw is quicker and more controllable than by heat conduction in simple ram machines. A higher and more uniform compound temperature and thus a more uniform viscosity is achieved. 

Modem designs fitted with non-retum valves through which the plasticised rubber compound passes into the injection cylinder portion are capable of giving precise shot volume. The non-retum valve is activated by the injection pressure. The injection ram/non-retum valve unit ensures complete purging of the injection chamber. 

The injection molding screw diameter chosen should be matched to the part s shot volume, which - as a general rule - should be not more than about 70% of the screw s theoretical swept volume. If the screw (diameter) chosen is too small, the swept volume will tend towards the shot volume, leaving insufficient leeway for correcting process fluctuations. On the other hand, if the injection unit is too large, only a small part of the swept volume will be employed per shot and the remainder of the melt in the barrel will possibly be exposed to too much heat. Furthermore, even small variations in the screw stroke would cause considerable variation in the shot weight. 

Injection moulding is completed by conveying the feed materials via a single-screw extruder into a shot volume and then injecting the shot volume into the mould cavity (Figure 6.25). 

Short shots are injections of polymer of insufficient volume to fill the mould. In these examples the short-shot volume is progressively increased on successive moulding cycles. 

Overpacking near gate Reduce shot volume Reduce injection pressure Reduce injection time Reduce heating Check runner and gate sizes... 

Injected compound volume too small (e.g., shot volume too small)... 

For most molding runs the amount of screw rotation must be adjusted so that there is always a pad, or cushion, of material left after the screw has finished injection this ensures that the screw forward time is effective and a constant injection pressure is being applied. On small injection molding machines this cushion may be 3mm (O.llSin) on larger machines it may be 9mm (0.354in) it is usually approximately 10% of the shot volume. Such a screw cushion is necessary on most injection molding... 

An emulsion of the hapten (i.e., conjugate) in normal saline is prepared by mixing with an equal volume of Freund s complete adjuvant. The prepared emulsion is injected subcutaneously into four different sites in New Zealand albino rabbits. Six weeks after the initial injection, all the animals are placed on a regimen of weekly booster shots. After a period of six months, antiserum from these animals are harvested and dilutions of 1 10,000 to 1 30,000 produced 50% binding or more and is employed in the RIA. 

Several useful ideas and conclusions can be obtained by studying the flow of an unreactive tracer, such as a coloured dye, through a series of tanks each of which is assumed to be ideally mixed. So that we may readily compare the effects of increasing the number of tanks, let us consider a volume V which can be made up of one tank only or two tanks each of volume V/2 or three tanks each of volume K/3 or, in general, i equally sized tanks each of volume V/i (Fig. 2.7). Through this series of tanks, a pure liquid flows at a volumetric flowrate v. Suppose that at time t - 0 when all the tanks are full and the whole system is in a state of steady flow, a shot of 0 moles of tracer is injected into the feed. The tracer is assumed to be... 

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