In the molding process of plastic products, all the molding factors that can reduce the orientation of polymer molecules in the products may reduce the orientation stress; all the process conditions that can make the polymer in the products cool evenly can reduce the cooling internal stress; all the processing methods that help to demould the plastic products are conducive to reduce the demoulding internal stress.
The main processing conditions which have great influence on the internal stress are as follows:
① Barrel temperature
Higher barrel temperature is beneficial to the decrease of orientation stress. This is because at higher barrel temperature, the melt plasticizes uniformly, the viscosity drops and the fluidity increases. In the process of filling the mold cavity with melt, the molecular orientation effect is small, so the orientation stress is small.
At lower barrel temperature, the melt viscosity is higher, the molecular orientation is more during filling, and the residual internal stress is larger after cooling. However, too high barrel temperature is not good, too high is easy to cause insufficient cooling and deformation during demoulding. Although the orientation stress decreases, the cooling stress and demoulding stress increase.
② Mold temperature
The high mold temperature has a great influence on the orientation stress and cooling stress. On the one hand, if the mold temperature is too low, the cooling speed will be accelerated, and the uneven cooling will easily lead to a large difference in shrinkage, thus increasing the cooling internal stress;
On the other hand, when the mold temperature is too low and the melt enters the mold, the temperature decreases rapidly and the melt viscosity increases rapidly, resulting in the mold filling at high viscosity and the formation of orientation stress.
Mold temperature has a great influence on plastic crystallization. The higher the mold temperature is, the more tightly the grains are stacked. The defects inside the crystal are reduced or eliminated, so as to reduce the internal stress.
In addition, for different thickness of plastic products, the mold temperature requirements are different. For thick wall products, the mold temperature should be higher.
③ Injection pressure
When the injection pressure is high, the shearing force in the process of melt filling is great, and the chance of producing orientation stress is also great. Therefore, in order to reduce the orientation stress and the demoulding stress, the injection pressure should be reduced appropriately. .
④ Packing pressure
The effect of packing pressure on the internal stress of plastic products is greater than that of injection pressure. In the packing stage, with the decrease of melt temperature, the melt viscosity increases rapidly. At this time, if high pressure is applied, the forced orientation of the molecular chain will be caused, and a greater orientation stress will be formed.
⑤ Injection rate
The faster the injection speed is, the easier it is to increase the degree of orientation of the molecular chain, resulting in greater orientation stress. However, if the injection speed is too low, the plastic melt may be layered one after another after entering the mold cavity, resulting in the formation of melt marks, stress concentration lines and stress cracking.
So the injection speed should be moderate. It is better to use variable speed injection, and stop filling when the speed gradually decreases.
⑥ Holding time
The longer the holding time, the greater the shear effect of plastic melt, resulting in greater elastic deformation and freezing more orientation stress. Therefore, the orientation stress increases obviously with the increase of holding time and feeding amount.
⑦ Residual pressure of die opening
The injection pressure and holding time should be adjusted properly to make the residual pressure in the mold close to the atmospheric pressure, so as to avoid producing greater internal stress in demoulding.
