Three terms that often get mixed up in injection molding are weld, meld and knit lines. Meld and knit lines are actually both different types of weld lines. Let’s review what causes meld and knit line imperfections, how they’re different, and how they affect part durability.
What Causes Knit and Meld Lines?
Like many part inconsistencies, these unwanted features are rooted in product design. Since material is injected through a gate, it must flow through the cavity and around various features, such as holes or bosses.
Think of it like water flowing down a river with a rock protruding through the surface. Once the water hits the rock, the flow must split, continue around and converge on the opposite side. You want to focus on is that converging of resin to determine if it’s creating a knit or meld line.
Why does this matter? If a knit line is present in a screw boss, the boss will likely crack when a screw is driven into it, leading to part inconsistencies. For automotive parts, this results in parts that bump, squeak or rattle. For electronics, the broken screw boss will not allow proper compression of a seal, causing a PCB (printed circuit board) to be susceptible to water damage, for example.
For plumbing parts, if these occur in an O-ring groove, there is a possibility that there will be a weeping of fluid causing a slow drip. In a pipe fitting, if these issues are not managed well, a fitting will not pass burst or crush testing, yielding product field failures.
A meld line is defined as the re-merging of two flow fronts after the plastic flow has been split by a feature within the part design. One way to think of this is to visualize merging onto the freeway after a long day at the office—all the vehicles are going the same direction and have to figure out how to get there without damage. In the accompanying figure, we can see how the flow front splits due to the rectangular core-out and merges again on the opposite side. Since there is more space inside the mold cavity, the molten resin continues onward, creating a new flow front.