Injection molding machine shot

To ensure consistency, the two critical areas for control specific to injection molding are shot size control and temperature control on the injection system, i.e., screw portion for the reciprocating screw machines, and both the screw and ram sections for the screw/ram machines. As mentioned previously, for temperature and cure control, the use of microprocessor controls has significantly improved the performance of injection molding machines, including control of the shot size. 

An ounce is a unit used to represent the capacity of an injection molding machine, showing the polystyrene equivalent weight of polymer for one shot . 

Injection mold machines are rated by their ability to mold polystyrene in a single shot [3], Mold pressures can range from 55 to 275 MPa and cycles as low as 15 s. It is possible to injection mold a thermoset resin, although it is very difficult. Compression molding or transfer molding is used in these cases ... 

The injection molding machine should be capable to deliver a shot size of 50% or less of the injection stroke. Typically, the machine should be able to inject its rated capacity in less than 2 seconds, except for high viscosity resins or when warping or jetting must be prevented by slower injection rate. Depending on the grade, the cylinder temperature setting should be between 190 and 220°C. 

Figure 11.7 Schematic diagrams of a screw-type injection-molding machine (top) screw retracted, before injection (bottom) screw forward, at end of shot. (From Weir, C.L., Introduction to Molding, Society of Plastics Engineers, CT, 1975. With permission.)...

Example 11.5 An injection molding machine is rated 100 oz per shot. What is the plasticizing capacity (Ib/h) of this machine if the total cycle time is 30 s What is the average residence time of the material in the heating cylinder for an inventory weight of 50 oz Estimate the plasticizing capacity of the heating cylinder for low density polyethylene. 

Injection-molding machines are rated by their capacity to mold polystyrene in a single shot. Thus a 2-oz machine can melt and push 2 oz of general-purpose polystyrene into a mold in one shot. This capacity is determined by a number of factors such as plunger diameter, plunger travel, and heating capacity. 

Injection-molding machines (1) plasticize (heating and melting of the plastics), (2) inject (injecting under pressure a controlled volume shot of melt into a closed mold with solidification of the plastics beginning on the mold s cavity... 

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