Control of molding conditions. In the molding process of plastic products, any molding factor that can reduce the orientation of polymer molecules in the product can reduce the orientation stress; Any process conditions that can evenly cool the polymer in the product can reduce the internal cooling stress; Any processing method that helps with demolding plastic products is beneficial in reducing the internal stress during demolding.

The processing conditions that have a significant impact on internal stress mainly include the following:

①Barrel temperature. A higher barrel temperature is beneficial for reducing orientation stress. This is because at a higher barrel temperature, the melt plasticizes uniformly, viscosity decreases, and flowability increases. During the process of filling the mold cavity with the melt, the molecular orientation effect is small, resulting in lower orientation stress. At lower barrel temperatures, the viscosity of the melt is higher, and there is more molecular orientation during the filling process. After cooling and shaping, the residual internal stress is greater. However, if the temperature of the material barrel is too high, it is not good as it can easily cause insufficient cooling and deformation during demolding. Although the orientation stress decreases, the cooling stress and demolding stress actually increase.

②The temperature of the mold has a significant impact on the orientation internal stress and cooling internal stress. On the one hand, if the temperature of the mold is too low, it will cause accelerated cooling, which can lead to uneven cooling and significant differences in shrinkage, thereby increasing the internal cooling stress; On the other hand, if the mold temperature is too low and the melt enters the mold, the temperature drops faster and the viscosity of the melt increases rapidly, resulting in a significant increase in the degree of oriented stress formed during mold filling at high viscosity. The mold temperature has a significant impact on the crystallization of plastics. The higher the mold temperature, the more favorable it is for the tightly packed grains, reducing or eliminating internal defects in the crystals, and thus reducing internal stress. In addition, for plastic products with different thicknesses, the mold temperature requirements vary. For thick walled products, the mold temperature should be appropriately higher. Taking PC as an example, the relationship between its internal stress magnitude and mold temperature is shown in Table 5-5.

③Injection pressure. When the injection pressure is high, the shear force experienced during the melt filling process is large, and there is also a greater chance of generating orientation stress. Therefore, in order to reduce orientation stress and eliminate demolding stress, the injection pressure should be appropriately reduced. Taking PC as an example, the relationship between its internal stress and injection pressure is shown in Table 5-6.

④Maintaining pressure The impact of holding pressure on the internal stress of plastic products is greater than that of injection pressure. In the pressure holding stage, as the temperature of the melt decreases, the viscosity of the melt increases rapidly. If high pressure is applied at this time, it will inevitably lead to forced orientation of the molecular chains, thereby forming greater orientation stress.

⑤Injection speed. The faster the injection speed, the more likely it is to cause an increase in the degree of molecular chain orientation, resulting in greater orientation stress. But if the injection speed is too low, the plastic melt may layer and form melting marks after entering the mold cavity, resulting in stress concentration lines and easy stress cracking. So the injection speed should be moderate. It is best to use variable speed injection and end the mold filling as the speed gradually decreases.

Holding time. The longer the holding time, the greater the shear effect of the plastic melt, resulting in greater elastic deformation and freezing of more orientation stress. Therefore, the orientation stress significantly increases with the prolongation of holding time and the increase of replenishment amount.

⑦Residual pressure during mold opening. The injection pressure and holding time should be adjusted appropriately to ensure that the residual pressure inside the mold is close to atmospheric pressure during mold opening, thereby avoiding the generation of greater internal stress during demolding.