Shrinkage thermoplastic molding shrinkage form and calculation As mentioned earlier, the factors affecting the shrinkage of thermoplastic molding are as follows.

① Plastic varieties

Thermoplastic molding process due to the existence of crystallization in the form of volume changes, strong internal stress, frozen in the plastic part of the residual stress is large, molecular orientation and other factors, so compared with thermosetting plastics is a larger shrinkage, shrinkage of a wide range of directionality is obvious, in addition to molding shrinkage, annealing or humidity adjustment after the treatment of shrinkage are generally larger than thermosetting plastics.

② Characteristics of plastic parts

Molding the molten material and cavity surface contact with the outer layer immediately cooled to form a low-density solid shell. Due to the poor thermal conductivity of the plastic, the inner layer of the plastic slowly cooled to form a high-density solid layer with a large shrinkage. So the wall thickness, slow cooling, high density layer is thick shrinkage. In addition, the presence or absence of inserts and inserts layout, the number of direct impact on the direction of the material flow, density distribution and shrinkage resistance, so the characteristics of the molded part of the shrinkage size, direction of the greater impact.

③ Form, size and distribution of feed opening

These factors directly affect the direction of material flow, distribution density, pressure retention and shrinkage effect and molding time. Directly into the mouth, the mouth of the large cross-section (especially thick cross-section) is small shrinkage, but the direction of the large, the mouth of the wide and short length of the directionality of the small. If the inlet is close to the inlet or parallel to the direction of material flow, the shrinkage will be large.

④ Molding conditions

High mold temperature, slow cooling of molten material, high density, shrinkage, especially for crystalline material due to high crystallinity, volume changes, so the shrinkage is greater.

The distribution of mold temperature is also related to the cooling and density uniformity inside and outside of the molded part, which directly affects the size and direction of the shrinkage of each part. In addition, the holding pressure and time on the shrinkage also has a greater impact on the pressure, long time shrinkage is small, but the direction of the big.

Injection pressure is high, the viscosity difference between the molten material is small, the interlayer shear stress is small, the elasticity after the release of the mold jumps back to the big, so the shrinkage can also be reduced, the material temperature is high, the shrinkage is big, but the directionality is small. Therefore, in the molding to adjust the mold temperature, pressure, injection speed and cooling time and other factors can also be appropriate to change the plastic shrinkage.

Mold design according to the shrinkage range of various plastics, plastic wall thickness, shape, inlet form size and distribution, according to experience to determine the shrinkage rate of each part of the plastic, and then calculate the cavity size.

For high-precision plastic parts and difficult to grasp the shrinkage rate, it is generally appropriate to use the following methods to design molds:

(1) the outer diameter of the plastic part to take a smaller shrinkage rate, the inner warp to take a larger shrinkage rate, in order to leave room for correction after the trial mold.

② test mold to determine the form of casting system, size and molding conditions.

③To post-processing of the molded part diameter post-processing to determine the size change (measurement must be 24h after demolding).

④ Correct the mold according to the actual shrinkage.

⑤ Try the mold again and change the process conditions appropriately to slightly correct the shrinkage value to meet the requirements of the molded part.