Weld line

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In manufacturing, the Weld line or Knit line or Meld line is the line where two flow fronts meet when there is the inability of two or more flow fronts to "knit" together, or "weld", during the molding process. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part.[1] Knit lines are considered molding defects, and occur when the mold or/and material temperatures are set too low: thus the materials will be cold when they meet, so that they do not bond perfectly. This can cause a weak area in the part which can cause breakage when the part is under stress. There are many Computer Aided Engineering tools that are available that can predict where these areas could occur.


Knit lines could be caused by different causes:

  • Low temperature of molding machine barrel
  • Inadequate back pressure
  • Injection pressure or injection speed is too low
  • Low mold temperature
  • Small injection gates and/or runners
  • Improper location of injection gate
  • Excessive gate land length
  • Improper flow rate of injected materials
  • Inconsistent process cycle