During the injection phase the hot molten plastic material is first pushed as fast as possible into a cool closed mold by the screw. Once the mold is full the injection unit packs and holds extra material into the mold to compensate for shrinkage. It does this until the gate freezes off. The function of “fill” is to push plastic into the mold cavity as fast as possible until about 95% full. The function of “pack” is to fill the last 5% of plastic into the mold and to put extra plastic into the cooled mold. This is desirable because as thermoplastics cool they shrink. The function of “hold” is to hold the extra plastic in the cooled mold until the gate freezes off. This prevents backflow of material through the gate.
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