The form, size and distribution of feed directly affect the direction of material flow, the distribution of density, the effect of compression and shrinkage and the forming time. The direct feed inlet and feed port have a large section (especially a thicker section), but the shrinkage is small, but the directivity is large, and the width of the feed opening and the length of the material are short, and the direction is small. Near the inlet or in parallel with the direction of the material flow, the shrinkage is large.
When the material is formed, the molten material contacts the cavity surface and the outer layer is cooled immediately to form a low density solid shell. Because of the poor thermal conductivity of plastic, the inner part of the plastic part is cooled slowly to form a large solid layer with high density. Therefore, the wall thickness, cooling slow, high-density layer thickness is large contraction. In addition, the layout and number of inserts have direct influence on the direction of material flow, the density distribution and the shrinkage resistance. Therefore, the characteristics of plastic parts have great influence on the shrinkage size and direction.
Plastic thermoplastic plastic molding process because of the presence of the crystalline volume changes shape, inner stress, frozen in plastic residual stress in the large, strong molecular orientation and other factors, so compared with thermosetting plastic shrinkage shrinkage is large, wide range, obvious direction, also after molding the shrinkage, annealing or conditioning after shrinkage generally than thermosetting plastic.
