What causes shrinkage and depression of plastic molds?

2020-03-04

Many people who have been working in this industry for many years have found this. In our previous work, the plastic mold after processing will have shrinkage, but it is not very large. What is the reason for this? Is there any way to avoid this?

Machine tool platform: Nozzle holes are too large, which will not cause backflow and shrinkage, resistance is too large, and material shrinkage is large. Insufficient clamping force can cause edge shrinkage and should be checked if there is a problem with the locking system. The amount of plasticization is not enough to select a machine with a large amount of plasticization, and check the screw and barrel for wear.

For plastic molds: the design should be uniform and even. The cooling and heating system of the plastic mold should ensure the temperature uniformity of each component. The gating system should be smooth and the resistance should not be too large. For example, the size of the main channel, the shunt and the gate should be appropriate, the smoothness should be sufficient, and the transition zone should be a circular arc transition. For thin parts, increase the temperature to ensure smooth flow, and thick-walled parts should reduce the temperature of the plastic mold. The gate should be opened symmetrically, try to produce in thick-walled parts, and increase the volume of the cold trap.

Plastic: The crystallinity of plastic is less than the crystallinity of non-crystalline plastic. During the processing, the amount of material needs to be increased, or a modifier is added to the plastic to accelerate crystallization and reduce shrinkage.

Processing process: The temperature of the cylinder is too high, and the volume changes greatly, especially the temperature of the original furnace. The plastic with poor fluidity should be appropriately raised in temperature to ensure stability. The injection pressure, speed, back pressure is too low, and the injection time is too short, so that the material or density is insufficient, the pressure, speed, and back pressure are too large, and the time is too long to cause edge shrinkage. When the mat is too large, it consumes injection pressure.

For precision parts that are not needed, the injection pressure and the outsole are condensed and hardened, and the interlayer is soft. Extrude the part and remove it from the plastic mold as soon as possible. Let it slowly cool in air or hot water to make the shrinkage smooth , Not so prominent, does not affect the use.

Cup mould

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