Weld Lines vs. Meld Lines

In a recent conversation, I got this age old processing question…

Some of my techs call them weld lines, some call them meld lines… what is the difference?
My Response
Both Weld and Meld Lines occur where melt fronts intersect, yet they are different…
Weld Lines are formed when two melt fronts meet with little or no flow afterwards.
Meld Lines are formed when a melt front is divided by a mold feature, such as a core pin, and then meets afterward to continue flowing. Meld lines tend to be much stronger with better appearance.
The strength and appearance is directly affected by the molecular interaction and gas removal at the point of melt front intersection. 
Additional Thoughts
Many part designers will manipulate the part design and gate location to transform weld lines into meld lines. Additionally, in rare cases, you can adjust injection speeds to alter the formation of weld and meld lines.
  1. MJ left a comment on 2009/09/21 at 1:01 pm

    When weld lines occurs, which injection parameters are most helpful.

  2. Andy Routsis left a comment on 2009/09/21 at 1:11 pm

    The answer is… It Depends.
    For example, if the weld line is caused by gas entrapment, then improved venting and reduced injection velocity may help.
    Adversely, if poor molecular entanglement is the issue, then increased injection velocity, mold temperature, and melt temperature might help.
    As with any other defect, understanding the ‘root cause’ is critical to good troubleshooting.

Leave a Comment