Q: How does stitch density affect zipper strength?
Stitch density controls how the load moves from the tape into the garment. Too few stitches, below about six per inch, concentrate stress and let the tape edge separate under load. Too many, above roughly fourteen per inch, perforate the tape and weaken it, especially on coated or laminated products where each needle hole is also a leak path.
Eight to twelve stitches per inch is the usual working range for garment zippers, adjusted for tape weight and thread size. The thread should match the tape: a heavy bonded polyester thread in a fine tape tears it, while a light thread in a heavy tape breaks first. Needle size matters as well, because a large needle makes a large hole that never closes.
Keep the stitch line clear of the teeth. Sewing through the chain cuts the monofilament or cracks the molded tooth, creating a fatigue point that appears after a few washes. Use a zipper foot, adjust the seam allowance so the needle runs beside the teeth, and bar-tack the ends of the opening to spread the load. Then pull-test the sewn sample rather than trusting the trim specification.