THE SEGMENTED PRINCIPLE OF MULTISTAGE INJECTION MOLDING

The following principle of speed segmentation is recommended:

1) The velocity of the fluid surface should be constant.

2) Rapid injection should be used to prevent melt freezing during injection.

3) The injection speed should be set to allow for rapid filling in critical areas (such as flow channels) while slowing down at the inlet.

4) The injection speed should ensure that the mold cavity is immediately stopped after filling to prevent overfilling, flash and residual stress.

Material properties are very important because polymers may degrade due to different stresses, increasing the molding temperature may lead to violent oxidation and degradation of the chemical structure, but at the same time the degradation caused by shear becomes smaller because the high temperature reduces the viscosity of the material and reduces the shear stress. There is no doubt that the multi-stage shooting speed is very helpful for the molding of heat sensitive materials such as PC, POM, UPVC and their mixing.

The geometry of the mold is also a determining factor: the thin-walled area requires the maximum injection speed; Thick-walled parts require slow-fast-slow speed curves to avoid defects; In order to ensure that the quality of the parts meets the standard, the injection molding speed should be set to ensure that the melt front flow rate is unchanged. Melt flow speed is very important, because it will affect the molecular orientation of the part and the surface state.

When the front of the melt reaches the cross-zone structure, it should slow down; For the complex mold with radiant diffusion, the melt flow should be ensured to increase evenly; The long runner must be filled quickly to reduce cooling of the melt front, but the injection of high-viscosity materials such as PC is an exception, because too fast a speed can bring cold material into the cavity through the water inlet.

Adjusting the injection speed can help eliminate defects caused by slower flow at the inlet. By the time the melt reaches the inlet through the nozzle and the runner, the surface of the melt front may have cooled and solidified, or the sudden narrowing of the runner may cause the melt to stagnate until sufficient pressure is built up to push the melt through the inlet, which will cause the pressure through the inlet to appear.