Analysis of Common Defects in Plastic Products-Size Change

Basic Reasons: From the perspective of plastics, the reasons for the size change are as follows:
The change of pressure distribution in U-shaped cavity will cause the change of size.
For crystalline plastics, the change of crystallinity due to the change of cooling rate will cause the change of size.
After-treatment of U products, changes in ambient temperature and humidity can also cause size changes.
Next, the change of pressure distribution in the cavity and the size change are analyzed and discussed. The causes of defects will vary with the size changes.
First, 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 meet the cavity pressure required by qualified products.
2. Defects: Overall product size is too large
Cause analysis: the plastic pressure in the cavity is too high.
Solution: Reduce the feeding pressure to achieve the cavity pressure required by qualified products.
3. Defect phenomena: the size near the gate of the product is too small, and the size of other positions is normal.
Reason analysis: The pressure near the gate is too low. The main reason is that the gate is not frozen and plastic reflux.
Solution: Increase the holding time, or adjust the process according to other reasons that lead to the gate not frozen (such as the gate not frozen due to the increase of plastic or mould temperature).
4. Defect phenomena: the size near the gate of the product is too large, and the size of other positions is normal.
Cause analysis: the pressure near the gate is too high.
Solution: If the three-stage forming process is adopted, the feeding rate can be changed. Check the effective viscosity (injection pressure integral at filling stage), measure the plastic temperature and adjust it. If two-stage forming process is adopted, the filling rate, feeding rate and pressure can be changed.
5. Defect phenomena: the size of products fluctuates back and forth and is unstable.
Cause analysis: This is caused by cavity pressure fluctuation. Observe the fluctuation of cavity pressure curve. If the pressure distribution of each mold cavity varies, will the prosecutor change with time? If there is a trend of change, indicating changes in temperature or batch of materials, conversely, if a mold is different, it is indicated that the change of the shrinkage process leads to the freezing change of the gate, some gates are frozen, some gates are not frozen, and the pressure changes due to the leakage of the reverse ring.
Solution ideas: do gate freezing test and dynamic check ring leakage test, determine the cause, and solve the problem pertinently.

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