Shot capacity

The system incorporates a screw and a ram of small diameters with a reduced shot capacity ... 

Shot capacities cover a large range, for example, a few grammes up to more than 50 kg. 

Very few of the installed IMMs run shot sizes anywhere near the full shot size capacity of the injection unit (Chapter 4). Typical usage is from 25 to 60%. Most suppliers of injection machines offer several sizes of injection plasticating units for any given press tonnage. The problem of having too much shot capacity can render some IMM unusable for certain plastics and applications. An example is excessive residence time for the plastic particularly the engineering materials. Any plastic that... 

When a molding machine is purchased, then the buyer is offered a choice of screw sizes three are usually available and these may be referred to as A, B or C. They have different screw diameters and allow an appropriate match of shot size (the feed system volume plus the molding volume) to barrel capacity. However, because they have different screw diameters, not only will the shot capacity vary but so will other things, for example, injection pressure and plasticizing capacity. Normally t e screws have the lowest shot capacity, and the highest injection pressure, whereas C type screws have the highest shot capacity and lowest injection pressure. 

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

Rated capacity usually 60 to 80%. If the shot capacity is too low e.g. less than 30%, then degradation can occur reduce risk by using low screw speeds, barrel temperatures and back pressures. 

The shot capacity should ideally be between 80 to 95% of the machine s capacity to prevent the material cooking in the barrel. As degradation depends on residence time as well as temperature, mold this material on a machine which has a small a shot capacity as possible. Yellowing of the material occurs (i.e. where the molding has a distict yellow cast, or hue) if the barrel residence time is too long, or the shot capacity of the machine is too large. 

Shot capacity n. The maximum weight of plastic that can be delivered to an injection mold by one stroke of the ram or screw. In the case of screw-injection molding machines that are not equipped with backflow-preventing valves at the end of the screw, slippage of material may occur in the screw flights and may not be reckoned with in calculations of shot capacity that are based on cubic displacement. 

Typical temperature and pressure settings are based on Ref. 3. Settings are based on studies using a reciprocating screw, general-purpose screw, clamp capacity of 175 tons, and rated shot capacity of 10 oz (280 g). Molded specimen thicknesses ranged from 0.065- to 0.125-in (1.7- to 3.2-mm) thickness. 

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