Plastics Design Principles: Positioning Weld & Meld Lines

By Jay Shoemaker

A weld line is formed when two flow fronts meet head-on. A meld line is formed when the flow fronts meet while flowing in the same direction. Formation of weld lines and meld lines are shown in Figure 8: Weld lines and meld lines. Weld lines are generally weaker and more visible than meld lines, but they both should be avoided.

Every time a gate is added to the part, an additional weld or meld line is formed, and so eliminating extra gates is advisable. When the number of weld or meld lines cannot be reduced, they should be placed in the least sensitive or critical areas with regards to their strength and appearance. Depending on the application, a weld or meld line could be a problem in terms of either strength or appearance. The strength of a weld or meld line is generally improved when they are formed at higher temperatures and when the pressure to pack them out is higher.

Figure 8: Weld lines and meld lines

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