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To solve the stress problem, which injection molding process parameters need to be adjusted?Posted on:2021-04-26 Browse：830 Source: Suzhou Lan
In the molding process of plastic products, any molding factor that can reduce the orientation of polymer molecules in the product may reduce the orientation stress; but any process conditions that can uniformly cool the polymer in the product can reduce the internal cooling stress; The processing methods of demolding plastic products are all conducive to reducing the internal stress of demolding.
The main processing conditions that have a greater impact on the internal stress are as follows:
?、貱ylinder temperature
Higher barrel temperature is conducive to the reduction of orientation stress. This is because at higher barrel temperature, the melt is plasticized uniformly, the viscosity drops, and the fluidity increases. During the process of filling the cavity with the melt, the molecular orientation effect is small. , So the orientation stress is smaller. However, at a lower barrel temperature, the melt viscosity is higher, the molecular orientation is more during the filling process, and the residual internal stress after cooling and shaping is greater. However, too high a barrel temperature is not good either. Too high will easily cause insufficient cooling and easily cause deformation during demolding. Although the orientation stress is reduced, the cooling stress and demolding stress increase instead.
②Mold temperature
The high or low mold temperature has a great influence on the orientation internal stress and the cooling internal stress. On the one hand, if the mold temperature is too low, it will cause the cooling to be accelerated, and it is easy to make the cooling uneven and cause a large difference in shrinkage, thereby increasing the cooling internal stress;
On the other hand, if the mold temperature is too low, after the melt enters the mold, the temperature decreases faster, and the melt viscosity increases rapidly, which causes the mold to be filled with high viscosity and the degree of orientation stress is significantly increased.
The mold temperature has a great influence on the crystallization of plastics. The higher the mold temperature, the more conducive to the compact packing of the crystal grains, the reduction or elimination of the defects inside the crystal, thereby reducing the internal stress.
In addition, for plastic products of different thickness, the mold temperature requirements are different. For thick-walled products, the mold temperature should be appropriately higher.
?、跧njection pressure
The injection pressure is high, and the shearing force during the melt filling process is large, and the chance of generating orientation stress is also greater. Therefore, in order to reduce the orientation stress and eliminate the demolding stress, the injection pressure should be appropriately reduced.
?、蹾olding pressure
The influence of holding pressure on the internal stress of plastic products is greater than that of injection pressure. In the pressure holding stage, as the melt temperature decreases, the melt viscosity increases rapidly. If high pressure is applied at this time, it will inevitably lead to the forced orientation of the molecular chains, thereby forming greater orientation stress.
?、軮njection speed
The faster the injection speed, the more likely it is to increase the degree of orientation of the molecular chain, thereby causing greater orientation stress. However, if the injection speed is too low, after the plastic melt enters the mold cavity, it may be layered successively to form melting marks, resulting in stress concentration lines, and stress cracking is easy to occur.
So the injection speed should be moderate. It is best to use variable speed injection and stop filling the mold when the speed is gradually reduced.
?、轍olding time
The longer the pressure holding time, the greater the shearing effect of the plastic melt, which will produce greater elastic deformation and freeze more orientation stress. Therefore, the orientation stress increases significantly with the increase of the dwell time and the increase of the filling amount.
?、?Residual pressure of mold opening
The injection pressure and the holding time should be adjusted appropriately so that the residual pressure in the mold is close to the atmospheric pressure when the mold is opened, so as to avoid greater internal stress during demolding
The main processing conditions that have a greater impact on the internal stress are as follows:
?、貱ylinder temperature
Higher barrel temperature is conducive to the reduction of orientation stress. This is because at higher barrel temperature, the melt is plasticized uniformly, the viscosity drops, and the fluidity increases. During the process of filling the cavity with the melt, the molecular orientation effect is small. , So the orientation stress is smaller. However, at a lower barrel temperature, the melt viscosity is higher, the molecular orientation is more during the filling process, and the residual internal stress after cooling and shaping is greater. However, too high a barrel temperature is not good either. Too high will easily cause insufficient cooling and easily cause deformation during demolding. Although the orientation stress is reduced, the cooling stress and demolding stress increase instead.
②Mold temperature
The high or low mold temperature has a great influence on the orientation internal stress and the cooling internal stress. On the one hand, if the mold temperature is too low, it will cause the cooling to be accelerated, and it is easy to make the cooling uneven and cause a large difference in shrinkage, thereby increasing the cooling internal stress;
On the other hand, if the mold temperature is too low, after the melt enters the mold, the temperature decreases faster, and the melt viscosity increases rapidly, which causes the mold to be filled with high viscosity and the degree of orientation stress is significantly increased.
The mold temperature has a great influence on the crystallization of plastics. The higher the mold temperature, the more conducive to the compact packing of the crystal grains, the reduction or elimination of the defects inside the crystal, thereby reducing the internal stress.
In addition, for plastic products of different thickness, the mold temperature requirements are different. For thick-walled products, the mold temperature should be appropriately higher.
?、跧njection pressure
The injection pressure is high, and the shearing force during the melt filling process is large, and the chance of generating orientation stress is also greater. Therefore, in order to reduce the orientation stress and eliminate the demolding stress, the injection pressure should be appropriately reduced.
?、蹾olding pressure
The influence of holding pressure on the internal stress of plastic products is greater than that of injection pressure. In the pressure holding stage, as the melt temperature decreases, the melt viscosity increases rapidly. If high pressure is applied at this time, it will inevitably lead to the forced orientation of the molecular chains, thereby forming greater orientation stress.
?、軮njection speed
The faster the injection speed, the more likely it is to increase the degree of orientation of the molecular chain, thereby causing greater orientation stress. However, if the injection speed is too low, after the plastic melt enters the mold cavity, it may be layered successively to form melting marks, resulting in stress concentration lines, and stress cracking is easy to occur.
So the injection speed should be moderate. It is best to use variable speed injection and stop filling the mold when the speed is gradually reduced.
?、轍olding time
The longer the pressure holding time, the greater the shearing effect of the plastic melt, which will produce greater elastic deformation and freeze more orientation stress. Therefore, the orientation stress increases significantly with the increase of the dwell time and the increase of the filling amount.
?、?Residual pressure of mold opening
The injection pressure and the holding time should be adjusted appropriately so that the residual pressure in the mold is close to the atmospheric pressure when the mold is opened, so as to avoid greater internal stress during demolding