Analysis of Common Defects in Plastic Products: Dimensional Changes

Basic reason: From the perspective of plastic, the reasons for size changes are as follows:
1. The change in pressure distribution within the mold cavity can cause dimensional changes.
2. For crystalline plastics, changes in crystallinity due to changes in cooling rate can cause dimensional changes.
3. Changes in product post-processing, environmental temperature, and humidity can also cause dimensional changes.
The following is an analysis and discussion on the changes in pressure distribution in the mold cavity that cause size changes. The reasons for forming defects may vary depending on the size changes.
1、 Defect phenomenon: The overall product size is too small
Reason analysis: The plastic pressure inside the mold cavity is too low.
Solution: Increase the feeding pressure to achieve the required cavity pressure for qualified products.
2、 Defect phenomenon: The overall product size is too large
Reason analysis: The plastic pressure inside the mold cavity is too high.
Solution: Reduce the feeding pressure to achieve the required cavity pressure for qualified products.
3、 Defect phenomenon: The size near the product gate is too small, and the size at other positions is normal
Reason analysis: The pressure near the gate is too low. Mainly due to the non freezing of the gate and plastic backflow.
Solution: Increase the holding time or adjust the process accordingly based on other reasons that cause the gate to not freeze (such as plastic or mold temperature rise causing the gate to not freeze).
4、 Defect phenomenon: The size near the product gate is too large, and the size at other positions is normal
Reason analysis: The pressure near the gate is too high.
Solution: If a three-stage forming process is adopted, changing the feeding rate can solve the problem. Check the effective viscosity (injection pressure integration during the filling stage), measure the plastic temperature, and adjust it. If a two-stage molding process is adopted, changing the filling rate, feeding rate, and pressure can solve the problem.
5、 Defect phenomenon: The size of the product fluctuates back and forth, causing instability
Cause analysis: This is caused by the fluctuation of cavity pressure. Observe the fluctuation of the cavity pressure curve. If the cavity pressure distribution of each the first mock examination is changing, check whether there is a change trend over time. If there is a trend of change, indicate temperature changes or material batch changes; On the contrary, if the the first mock examination is different from the the first mock examination, it means that the feeding process has changed, leading to the freezing change of the gate. Some gates are frozen, and some gates are not frozen; It is also possible that pressure changes may occur due to leakage of the check ring.
Solution: Conduct gate freezing test and dynamic check ring leakage test to determine the cause and solve the problem in a targeted manner.