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). 

Form (e.g., liquid, solid, fragments, pea gravel L/shot (volume of the inner energetic destruction. Liquids neutralents and... 

The maximum shot capacity, or shot volume, is given by -... 

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. 

Screw and barrel specifications deal with type of material to be processed, shot volume, and plastic imiformity. Shot volume (the part, runner, and sprue)... 

Shot volume is limited by the residence time. PP has a tendency to oxidise and is routinely protected by the inclusion of antioxidants in the polymer production process. Hence, PP cannot be exposed to very high residence times. Allowable residence time depends on the material temperature and the nature of heat stabilisation. Longer residence time is possible at lower temperature. The shot volume may be up to 85% of the machine maximum shot volume. The minimum shot volume depends on the residence time and can be as low as 15% of the maximum shot volume of the machine. 

Increase the available shot volume through sprue elimination (it should be remembered, however, that the compressibility of the melt in HR demands an increase in the available shot volume by around 20% of the HR volume) ... 

A variety of nozzle types of different lengths are available, and depending on the application and size of the shot volume, different kinds of nozzles can be used. 

Nominal Shot Volume (cm ) Equiv. Volume (m) Equiv. Shot Weight in GPPS (gm) Equiv. Shot Weight in GPPS (oz)... 

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

Non-fills, appear as depressed areas on the surface of the molded part. This may happen if the preform or the shot volume is too light. The operator has to adjust and optimize the volume of rubber to mold. Non-fills also occur when the rubber begins to cure before the cavity is fiiU. In that case, cure of the product must be slowed, either by lowering mold temperature or slowing cure, by addition of bisphenol AF, reduction of the accelerator or Ca(OH)2, or eventually the addition of CaO. 

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