1, Causes of shrinkage
- Thermal expansion and contraction.
- Melt crystallization (the higher the crystallinity, the more severe the melt shrinkage).
- Molecular orientation (in general, molecules are always oriented along the flow direction, for unreinforced materials, the melt shrinkage in the flow direction is always greater than the vertical direction; for reinforced materials, the opposite is true).
2, The stage of shrinkage: The shrinkage begins with the gradual cooling of the melt from the beginning of injection. It includes three stages:
- From the start of injection to the end of pressure maintenance.
- From the start of cooling time to demoulding.
- After demoulding.
3, Deformation: The root cause of deformation is uneven shrinkage.
Causes of uneven shrinkage:
- Uneven cooling (temperature distribution).
- Uneven wall thickness.
- Uneven pressure distribution.
- Molecular orientation.
- Uneven demoulding force.
Crystallization of plastic
1, The concept of crystallization: Crystallization refers to the ordered arrangement of molecules.
2, Influencing factors of crystallization: The influencing factors of crystallization are mainly the cooling rate, the faster the cooling rate, the lower the degree of crystallization.
3, Effect of crystallization on product performance: the higher the crystallinity, the higher the density, the greater the shrinkage, the better the finish, the higher the strength, but the toughness of the product becomes worse.
1, Viscosity is a property of fluid itself, its size is a measure of fluid flow performance, the larger the value, the worse fluid flow performance.
2, Factors affecting viscosity:
- Cutting speed.
It is worth noting that the viscosity is often the result of the combined effect of temperature, shear speed and pressure. Different materials have different sensitivity to temperature, shear speed and pressure, and which one starts at different injection speeds and the leading role is also different. Normally, for high temperature injection materials such as PA, PC, etc., the shear rate plays a leading role (hence, high-speed injection should be used for thin-walled plastic parts or products with thin-walled parts).