Basic reasons: From the perspective of plastics, the reasons for the size change are as follows:
1, the change of pressure distribution in the cavity will cause the size change;
2. For crystallines plastic, the change of crystallinity due to the change of cooling rate will cause the size change;
3. After-treatment of products, changes in ambient temperature and humidity will also cause dimensional changes.
The following is an analysis and discussion on the change of the pressure distribution in the cavity, which causes the dimensional change. The causes of defects will be different with different dimensions.
I. Defect phenomenon: the overall product size is too small.
Cause analysis: the plastic pressure in the cavity is too low.
Solution: Increase the feeding pressure to reach the cavity pressure required by qualified products.
Second, the defect phenomenon: the overall product size is too large
Cause analysis: the plastic pressure in the cavity is too high.
Solution: Reduce the feeding pressure to reach the cavity pressure required by qualified products.
Third, defect phenomenon: the size near the product gate is too small, and the size at other positions is normal
Cause analysis: the pressure near the gate is too low. The main reason is that the gate is not frozen and the plastic flows back.
Solution: Increase the holding time, or adjust the process pertinently according to other reasons that cause the gate not to freeze (such as the gate not to freeze due to the temperature rise of plastic or mold).
4. Defect phenomenon: the size near the product gate is too large, and the size at other positions is normal
Cause analysis: the pressure near the gate is too high.
Solution: If the three-stage forming process is adopted, changing the feeding rate can solve the problem. Check the effective viscosity (injection pressure integral in filling stage), measure the plastic temperature and adjust it. If two-stage molding process is adopted and filling rate, feeding rate and pressure are changed, it can be solved.
V. Defect phenomenon: the product size fluctuates back and forth and is unstable.
Cause analysis: This is caused by the fluctuation of cavity pressure. Observe the fluctuation of cavity pressure curve. If the cavity pressure distribution of each mold is changed, check whether there is a change trend with time. If there is a change trend, explain the temperature change or material batch change; On the contrary, if it is different from one mold to another, it means that there is a change in the feeding process, which leads to the change of gate freezing, some gates are Be frozen and some gates are Not be frozen; Pressure changes may also be caused by leakage of check ring.
Solution: Do gate freezing test and dynamic check ring leakage test to determine the cause and solve the problem pertinently.
