Q: Why does the zipper tape pucker after sewing?
Puckering is a tension and shrinkage problem. If the tape is fed under tension while the shell fabric is relaxed, the seam contracts when the pressure foot lifts. Differential feed, a Walking foot and correct presser pressure reduce it, but the underlying cause is often that the tape and the fabric shrink differently in washing or pressing.
Polyester tape typically shrinks 1 to 2 percent, cotton tape 3 to 5 percent, and some coated tapes relax when heated. If the shell shrinks more than the tape, the fabric pulls the seam into waves; if the tape shrinks more, the zipper shortens and the opening cups. Needle heat on synthetic tape can also stretch it locally, producing a wavy section that only appears after sewing.
Test before cutting: wash a sewn sample through the intended care cycle and measure the seam. Preshrink the tape where the supplier offers it, stabilize the seam with a fusible tape, and keep the stitch line parallel to the teeth. For coated and laminated tapes, avoid over-pressing, because heat can both shrink the tape and damage the barrier.