How does weld line occur and how to eliminate this problem?

A weld line is formed when separate melt fronts traveling in opposite directions meet. A meld line occurs if two emerging melt fronts flow parallel to each other and create a bond between them. The formation of weld and meld line can be caused by holes or inserts in the part, multi-gate cavity system, or variable wall thickness where hesitation or race tracking occurs.

Remedies can be increasing the size of gate and runners. Place a vent in the area of the weld/meld line to eliminate entrapped air which would further weaken the weld/meld-line strength. Increase the melt temperature, injection speed, or injection pressure.
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