Subsection principle of multistage injection molding

The basis for setting the speed segment must take into account the die geometry, other flow constraints and instability factors. The speed setting must have a clear understanding of the injection molding process and material knowledge, otherwise the product quality will be difficult to control. Because the melt flow rate is difficult to measure directly, it can be calculated indirectly by measuring the screw forward speed or cavity pressure (it is determined that the check valve has no leakage).

The following speed segmentation principle is recommended:

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

2) Rapid glue injection shall be adopted to prevent melt freezing during glue injection.

3) The setting of glue injection speed shall take into account the rapid filling in critical areas (such as flow channels) and slowing down at the water inlet.

4) The glue injection speed should ensure that it stops immediately after the mold cavity is filled to prevent overfilling, flash and residual stress.

Material properties are very important because polymers may degrade due to different stresses. Increasing molding temperature may lead to severe oxidation and degradation of chemical structure, but at the same time, the degradation caused by shear becomes smaller, because high temperature reduces the viscosity of materials and reduces shear stress. Undoubtedly, the multi-stage glue injection speed is very helpful for forming heat sensitive materials such as PC, POM, UPVC and their blending.

The geometry of the mold is also a decisive factor: the maximum injection speed is required at the thin wall; Thick wall parts need slow fast slow speed curve to avoid defects; In order to ensure that the quality of parts meets the standard, the injection speed shall be set to ensure that the melt front flow rate remains unchanged. Melt flow rate is very important because it will affect the molecular arrangement direction and surface state in the part.

When the melt front reaches the cross region structure, it should slow down; For complex dies with radial diffusion, the melt throughput should be increased evenly; The long runner must be filled quickly to reduce the cooling of the melt front, but the injection of high viscosity materials such as PC is an exception, because too fast will bring the cold material into the cavity through the water inlet.

Adjusting the injection speed can help eliminate defects caused by the slow flow at the water inlet. When the melt reaches the water inlet through the nozzle and flow channel, the surface of the melt front may have cooled and solidified, or the melt may stagnate due to the sudden narrowing of the flow channel until sufficient pressure is established to push the melt through the water inlet, which will make the pressure through the water inlet appear.