Burn Marks & Weld Line Issues

It is always critical to check your first stage short shot since it affects both mold filling and part packing.


Please provide reasons & remedies of burn-mark or weld line problems in plastic (HDPE) moulded product during the injection moulding process.

My Response:

There are many possible causes, but injection should be checked first:

First, the mold must not be full during 1st stage injection.

If the mold fills all the way during 1st stage injection:

  • The gas will be trapped in the mold and may cause burning
  • The screw will bounce back which will cause:
    • Melt pressure loss in the cavity
    • Reduced pressure at the melt front
    • Reduced weld or meld line strength

If you short shot the mold, verify the burning is not present, and then finish filling the mold cavity with packing pressure, you may get better weld line and and burn marks.

You should also consider reducing the clamp tonnage to allow the mold to vent better.

Additional Comments: You can also download our reference guide for more assistance.

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