Q: Why is the zipper wavy after sewing it into a garment?
Waviness after sewing is nearly always a seam tension or feed problem, because the zipper is the stiffest thing in the seam and does not stretch with the fabric. Check the seam before asking for a tape change.
- Lay the garment flat and see whether the waves follow the stitch line or the tape edge.
- Check stitch length and thread tension; a tight short stitch pulls the tape into waves.
- Confirm the zipper was not stretched while being fed, which is common when the operator pulls the tape.
- Measure tape length before and after sewing a scrap seam to see whether the seam shortened it.
- Check presser foot pressure and feed dog height, since heavy pressure distorts a narrow tape.
If the tape is straight before sewing and wavy after, the cause is in the seam. If it is wavy out of the box, stop and measure it against the maker's flatness requirement before sewing more. Correct one setting at a time so the improvement can be attributed to a specific change.