Q: What is burst strength in a zipper and why does it matter?
Burst strength measures how much outward pressure the closed chain and its seam can take before the two halves separate or the tape fails. It is tested by pulling the assembled zipper apart perpendicular to the chain, or by inflating a sewn panel until the closure gives way. The result is usually reported in newtons per unit length or as a pressure in kilopascals.
It matters because garments fail at openings, not in the middle of a panel. A tight jacket, a loaded bag or a pair of jeans under strain puts the zipper seam in tension across its width, and a chain that is strong in line can still be weak in burst if the tape is narrow, the stitch is sparse or the slider does not fully interlock.
Burst performance is improved by a wider tape, denser stitching, a correctly sized slider and a chain that engages deeply. It is reduced by needle damage, skipped stitches and over-pressing. For protective or load-bearing products, specify a burst requirement and test the finished seam, since the sewn assembly is always weaker than the loose trim.