Bemould Plastic News

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Plastic Injection Overmolding Solution

 Tuesday, 11 April 2023

Plastic product information: 
  • Part Name:PC injection molding 
  • Part dimension:φ100*40mm
  • Part molding type:PC injection overmolding 
Material :
        the first shot :Black PC
        the second shot :PC(clear)

Common injection molding defects in two-shot injection molding :
  1. Plastic overflow 
  2. Sink mark 
  3. Poor weld line 
  4. Stress mark 
  5. Poor color mixing 
  6. Flow mark 

 At this case , the molding defect was color mixing. After molding, black burrs were found in the transparent second shot part, especially at the edge of the molten area. 
After discovering this issue, it was preliminarily speculated that a shot was produced by remelting and filling again. To verify the correctness of the guess, we run the mold flow analysis to see the details  :


The mold flow analysis found that the flow trend is consistent with the actual filling flow, which can be compared with the results shown in the following figure: 

It is mentioned in the theoretical research that there is scouring and friction between the second shot and the first shot in two-color molding. After the first shot molding is completed, the second shot plastic will produce secondary melting and molecular structure displacement and destruction on the cooled plastic surface under the scouring of high temperature, high pressure and high speed . From the filling results in the figure below, it can be seen that the second shot rubber material fills through the first shot surface, especially in the edge area of the first shot at a faster speed than in the middle area. Therefore, the scouring force of the second shot rubber material on the edge has a greater impact than in the middle area. Therefore, when the second shot rubber material is filled, the rubber material in the edge area of the first shot will melt and fill with the flow front of the second shot.

Conclusion: After molding verification, if the temperature difference between the two shots exceeds 15 degrees, the product molding effect is relatively ideal. When the temperature difference between the two shots is less than 15 degrees, the second shot product will melt off the first shot . Controlling the occurrence of erosion can reduce injection pressure and injection speed, reduce mold temperature and melt temperature, and effectively reduce erosion by widening the gap between the melt temperature of the first and second shots and the temperature of the mold.

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