1.1 plastic varieties of thermoplastic plastic molding process due to the volume change is crystallization taxiing on, strong internal stress, residual stress within the frozen in plastics, molecular orientation factors such as the gender is strong, so compared with thermosetting plastic shrinkage is bigger, wide range of shrinkage and clear direction, also after molding shrinkage, shrinkage rate after annealing or wet processing generally are larger than the thermosetting plastic.
1.2 characteristics of plastic parts during molding, the outer layer in contact with the surface of the mold cavity is immediately cooled to form a low-density solid shell. Due to the poor thermal conductivity of plastic, the plastic parts of the inner layer slowly cooling and forming a large contraction of the solid density. So the wall thickness, cooling slow, high density layer of the thick shrinkage. In addition, the presence or absence of embedded parts and the layout and quantity of embedded parts have a direct impact on the direction of material flow, density distribution and the size of shrinkage resistance, etc., so the characteristics of plastic parts have a greater impact on the size and direction of shrinkage.
1.3 the form, size and distribution of the inlet directly affect the flow direction, density distribution, compaction and molding time. The direct inlet and the large section of the inlet (especially the thick section) have smaller shrinkage but larger directivity, while the wide and short sections of the inlet have smaller directivity. Those close to the inlet or parallel to the flow direction will have a large contraction.
1.4 molding conditions mold temperature is high, melting material cooling slow, high density, large shrinkage, especially for the crystallization of the material due to high crystallinity, large volume change, so the shrinkage is greater. The distribution of mold temperature is also related to the internal and external cooling and density uniformity of the plastic parts, which directly affects the shrinkage of each part and its direction. In addition, the holding pressure and time also have a great influence on the contraction. The injection pressure is high, the melt material viscosity difference is small, the interlayer shear stress is small, the elastic rebound after demolding is large, so the shrinkage can be reduced, the material temperature is high, the shrinkage is large, but the directivity is small. Therefore, in the molding process to adjust the mold temperature, pressure, injection speed and cooling time and other factors can also be appropriate to change the plastic parts shrinkage.
According to the shrinkage range of all kinds of plastics, the wall thickness and shape of the plastic parts, the form size and distribution of the inlet, the shrinkage rate of each part of the plastic parts will be determined according to the experience, and then the cavity size will be calculated. For plastic parts with high precision and difficult to master the shrinkage rate, it is generally advisable to use the following methods to design the mold:
(1) for the plastic parts of the outer diameter take smaller shrinkage, internal runoff larger shrinkage, to leave the room for correction after the trial mold.
(2) determine the mold casting system form, size and molding conditions.
(3) to be processed after the plastic parts after processing to determine the size change (measurement must be in 24 hours after demoulding).
(4) correct the mold according to the actual shrinkage.
(5) try again and can be appropriate to change the process conditions slightly modify the shrinkage to meet the requirements of the plastic parts.