Polypropylene PP material can cause shrinkage problems […]
Polypropylene PP material can cause shrinkage problems due to various external reasons after molding. If appropriate cooling temperature and post-treatment process are used in production, this shrinkage can be reduced. The problem of causing shrinkage of PP products is mainly manifested in the following aspects:
First, the mold
The multi-gate mold should adjust the filling speed of each gate and open the gate symmetrically.
The key parts of the mold should be effectively provided with cooling water channels to ensure that the cooling of the mold has a good effect on eliminating or reducing shrinkage.
The entire mold should be free of burrs and have mold-tightness, which can withstand high-pressure, high-speed, low-viscosity melt filling.
The screw or plunger is severely worn, and leakage of molten material occurs during *** and holding pressure, which reduces the mold filling pressure and material volume, resulting in insufficient molten material.
2. Nozzle hole is too large or too small
If it is too small, it will easily block the feeding channel, if it is too large, it will make the shooting force small, and mold filling will be difficult.
Third, the process
1. Increase pressure, hold pressure, and prolong time.
For high fluidity plastics, high pressure will cause flashing and cause pits. The material temperature should be appropriately reduced, the front section of the barrel and the nozzle temperature should be reduced to reduce the volume change of the melt entering the cavity, and it is easy to solidify. For high viscosity plastics, it should be increased. Barrel temperature makes filling easy. When shrinkage occurs in the gate area, the dwell time should be extended.
2. Increasing the speed makes it easier to fill the part and eliminate most of the shrinkage.
3. Thin-walled parts should increase the mold temperature to ensure smooth material flow; thick-walled parts should reduce the mold temperature to accelerate the curing and setting of the skin.
4. Extending the cooling residence time of the part in the mold, maintaining a uniform production cycle, increasing the back pressure, and retaining a certain cushion at the front of the screw are all conducive to reducing shrinkage.
5. Low-precision products should be ejected as early as possible to allow them to cool slowly in air or hot water, which can make the shrinkage depression be gentle without affecting the use.
Fourth, raw materials
The raw material is too soft and easy to sag. An effective method is to add a nucleating agent to the plastic to speed up the crystallization.
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