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Weld Lines In The Plastic Injection Molding Industry

Weld Lines

A weld line is a visible line appearing on the mold surface where two or more streams of material meet then fuse together. Weld lines are very hard to detect and may cause mechanical failure during the application of the part. Weld lines occur where two or more plastic flow streams within a mold meet one another as the mold is filling. The individual flow patterns ‘cool down’ as they approach each other and do not totally fuse together as one solid region of the part. The appearance of a weld line may be unacceptable. Depending upon the design it may be possible to reduce, but not eliminate weld lines.

Typical solutions:

  • Vent at the weld line.
  • Raise the melt and mold temperature or localize temperature.
  • Add a run off well at the weld location.
  • Increase part thickness at the weld.
  • Change the gate location.
  • Lubricate the material used.



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