Moldflow Making Accurate Plastic Parts

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Plastic melts are very compressible at the pressures used in injection molding. As the ram moves forward, the material in the barrel is compressed so that the flow rate in the cavity is less than indication by the ram movement. As the ram slows down, the plastic expands under pressure.
Melt compressibility causes a smooth transition from mold filling to packing.
The molding process is frequently divided into two phases. Commonly injection molders will talk about the filling and holding stages because this corresponds to machine settings.
Experiments on an instrumented mold show this concept is far from the truth. Figure 1?C2 illustrates a simple mold with pressure transducers PT1, PT2, PT3 positioned as shown. The lines labelled PT1, PT2 and PT3 show the pressures recorded by these transducers during filling of the mold.
Figure 1?C2: Pressure Traces for Simple Molding