Weld Lines & Meld Lines


A weld line (also called a weld mark or a knit line) is formed when separate melt fronts travelling in opposite directions meet. A meld line occurs if two emerging melt fronts flow parallel to each other and create a bond between them. Weld and meld lines can be caused by holes or inserts in the part, multiple gates, or variable wall thickness where Hesitation or race tracking occurs. If weld or meld lines can't be avoided, position them at low-stress and low-visibility areas by adjusting the gate position and dimension. Improve the strength of weld and meld lines by increasing the local temperature and pressure at their locations.

How to tell the difference between weld and meld lines
Traditionally, the "meeting angle" is used to differentiate weld lines and meld lines. A meeting angle smaller than 135º produces a weld line; greater than 135º, a meld line. Note that the weld line surface mark disappears when the meeting angle reaches 120º to 150º. Normally, weld lines are considered to be of lower quality than meld lines, since relatively less molecular diffusion occurs across a weld line after it is formed.

Problems caused by weld lines
Weld lines are generally undesirable when part strength and surface appearance are major concerns. This is especially true with fiber-reinforced materials, because the fibers do not bridge the weld lines and often are oriented parallel to them.

 
Strength of weld lines

The exact strength of the weld line depends on the ability of the flow fronts to weld (or knit) to each other. The strength of the weld-line area can be from 10 to 90 percent as strong as the pure material used. With such a wide range possible, the conditions that are favorable to better weld-line quality are worth examining:

  • High injection pressure and speed.
  • High melt and mould-wall temperature.
  • Formation of the weld lines closer to the gate.
  • A temperature difference of less that 10ºC between the two emerging melt fronts.

If a weld line forms before the filling is complete and is immediately subject to additional packing pressure, the weld line will typically be less visible and stronger. For complex part geometry, flow simulation helps to predict the weld/meld-line position with respect to changes in the tool design, and to monitor the temperature difference.

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