Q: Why does my sewn zipper twist to one side?
A twisted zipper usually follows the grain of the tape or an uneven seam, and it is often visible before the garment is finished if the operator checks. Catching it early avoids a repair that cannot hide the distortion.
- Lay the zipper on a table before sewing and note whether it already curls or twists.
- Sew a test seam on scrap and check twist with the seam relaxed for a few minutes.
- Compare left and right seam allowances; an uneven allowance pulls one edge.
- Check that the tape was not stretched on one side during feeding.
- Inspect the tape weave for a diagonal bias that makes it curl under tension.
Twist present before sewing is a zipper defect and should be rejected at incoming inspection. Twist that appears only after sewing is a process issue and is corrected at the machine, not by ironing the tape flat. Ironing hides the symptom and can add heat deformation on top of the original twist.