How to Improve Weld Line Formation in Injection Molding

 

Injection molding plants, many new injection molding technologies are used. Weld lines in plastic products occur when molten resin from different directions fails to fully fuse at the joint during the injection process. During injection, the two resin streams at the fusion point do not mix; therefore, the lower the temperature, the more obvious the weld line and the lower the strength. Conversely, the higher the temperature, the stronger the adhesion. The adhesion at the fusion point also depends on the pressure applied. The lower the holding pressure, the worse the fusion and the lower the strength. The fusion point is the point where resin flows converge and may also be the end of the flow. If a proper vent is not provided at the weld line location to release gas, the appearance and strength of the weld will deteriorate.

 

Improvement Measures:

 

1) Increase resin temperature, increase mold temperature, increase injection pressure and speed; increase heating.

 

2) Pay attention to venting at the parting line. Adding inserts or ejector pins at the weld line location can also help with venting.

 

3) Choose a suitable injection point; mold flow analysis is best performed before mold opening for plastic products.

 

4) Increase the amount of process overflow and remove cold material after molding.

 

5) Stretch and shorten the gating system as much as possible to improve flow.

 

6) For textured products, choose a coarser texture, which also helps to reduce weld lines.

 

Technology is developing and improving; these are just a few examples. Further research and development of even better techniques are welcome!