Plastic products are very common in our production, so let's talk about the reasons that affect the shrinkage of thermoplastic profiles.
1. The shrinkage rate of thermoplastic products is larger than that of thermosetting plastics because of the volume change of crystallization, the strong internal stress, the large residual stress frozen in the plastic parts and the strong molecular orientation. The shrinkage range is wide and the orientation is obvious. In addition, the shrinkage rate after forming, annealing or humidification treatment is generally also higher than that of thermosetting plastics. Solid plastics are large.
2. The outer layer of the molten material contacting the surface of the cavity is cooled immediately to form a low density solid shell. Due to the poor thermal conductivity of plastics, the inner layer of plastic parts slowly cooled to form a high-density solid layer with large shrinkage. So wall thickness, slow cooling and high density layer thickness shrink greatly.
In addition, the layout and quantity of inserts and inserts have a direct impact on the direction of material flow, density distribution and shrinkage resistance, so the characteristics of plastic parts have a greater impact on the shrinkage size and direction.
3. The shape, size and distribution of the inlet and outlet directly affect the direction of material flow, density distribution, pressure retaining and feeding effect and forming time. The large cross-section (especially the thicker cross-section) of the direct inlet and the inlet has smaller shrinkage but larger directivity, while the wide and short length of the inlet and the outlet have smaller directivity. Close to the inlet or parallel to the direction of the material flow, the shrinkage is large.
4. Mold temperature is high, melt material cooling is slow, density is high, shrinkage is large, especially for crystalline material because of high crystallinity and large volume change, so shrinkage is larger. Mold temperature distribution is also related to internal and external cooling and density uniformity of plastic parts, which directly affects the shrinkage and direction of each part.
In addition, holding pressure and time also have a greater impact on contraction, while large pressure and long time have smaller contraction but greater directionality. The injection pressure is high, the viscosity difference of melt material is small, the interlaminar shear stress is small, and the elastic rebound after demoulding is large, so the shrinkage can also be reduced appropriately, the material temperature is high, the shrinkage is large, but the direction is small. Therefore, adjusting the temperature, pressure, injection speed and cooling time can also properly change the shrinkage of plastic parts.
5. According to the shrinkage range of various plastics, the wall thickness and shape of the plastic parts, the size and distribution of the inlet and outlet forms, the shrinkage rate of each part of the plastic parts is determined by experience, and then the cavity size is calculated.
When it is difficult to control the shrinkage of high precision plastic parts, the following methods should be used to design the die:
The form, size and forming conditions of the gating system are determined by the trial die.
The size change of the plastic parts to be processed is determined by post-processing (measurement must be made 24 hours after demoulding).
