Influence of Injection Molding Process Parameters on Internal Stress

General principle for reducing internal stress

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

The processing conditions that have a significant impact on internal stress are mainly as follows:

Material barrel temperature

A higher barrel temperature is beneficial for reducing orientation stress, because at higher barrel temperatures, the melt plasticizes uniformly, viscosity decreases, and fluidity 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, too high a temperature in the barrel is also not good. It can easily cause insufficient cooling and deformation during demolding. Although the orientation stress decreases, the cooling stress and demolding stress actually increase.

Mold temperature

The temperature of the mold has a significant impact on the orientation internal stress and cooling internal stress. On the one hand, if the mold temperature is too low, it will cause accelerated cooling, which can lead to uneven cooling and significant differences in shrinkage, thereby increasing the internal stress during cooling;

On the other hand, the mold temperature is too low, and after the melt enters the mold, the temperature drops faster, and the viscosity of the melt increases rapidly, resulting in filling the mold at high viscosity and significantly increasing the degree of orientation stress.

The mold temperature has a significant impact on plastic crystallization. The higher the mold temperature, the more favorable it is for the grains to stack tightly, reducing or eliminating defects inside the crystals, thereby reducing internal stress.

In addition, the mold temperature requirements vary for plastic products of different thicknesses. For products has thick wall, the mold temperature should be slightly higher.

Injection pressure

The injection pressure is high, and the shear force experienced during the melt filling process is large, which increases the chance of generating orientation stress. Therefore, in order to reduce orientation stress and eliminate demolding stress, the injection pressure should be appropriately reduced.

Holding pressure

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

Injection speed

The faster the injection speed, the more likely it is to cause an increase in the orientation of the molecular chain, resulting in greater orientation stress. However, 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.

Pressure holding time

The longer the holding time, the greater the shear force of the plastic melt, resulting in greater elastic deformation and freezing of more orientation stress. Therefore, the orientation stress significantly increases with the extension of holding time and the increase of feeding amount.

Residual pressure during mold opening

The injection pressure and holding time should be adjusted appropriately to make the residual pressure inside the mold close to atmospheric pressure during mold opening, thereby avoiding the generation of greater demolding internal stress. Please contact us for custom molding products.