Q: What causes skipped teeth in a molded zipper?
Skipped teeth in a molded chain usually point to the injection process. A short shot, caused by low melt pressure, low temperature or a partially blocked mold cavity, leaves a tooth that is undersized or missing. Mold wear and contamination do the same over time, and the defect often appears in a repeating pattern at the same spacing as the cavity.
The chain can also be forced out of engagement by the slider. If the slider is too large, worn, or pulled at an angle, the interlock opens and the teeth pass each other without engaging. A chain that has stretched, or a tape that has shrunk unevenly during dyeing, changes the pitch and makes engagement unreliable even with a good slider.
Separate the causes with two checks. Run a new slider of the correct size on the suspect chain; if skipping stops, the slider was at fault. Then inspect the tooth row for a repeating gap. A repeating pattern is a molding or tooling issue and belongs with the supplier; a random pattern is more likely handling damage or a stretched chain.