Q: What pull strength should a #5 zipper have?
As a working range, a #5 nylon coil should withstand roughly 400 to 600 newtons of longitudinal pull before the chain or tape fails, while a #5 metal chain typically reaches 600 to 900 newtons. Molded resin sits between the two, commonly 500 to 700 newtons. These are order-of-magnitude figures; the actual value depends on tape strength, stitch pattern, slider condition and test speed.
The failure mode matters as much as the number. A chain that separates at the slider indicates a tolerance or wear problem, while a tape that tears at the stitch line points to seam construction. Test both dry and after washing, because lubricants and finishes change friction, and test at the temperature the product will actually see.
Quote pull strength together with the test method, sample size and acceptance criterion. A single peak value from a supplier's best sample is not a specification. Ask for the lower confidence bound across a production lot, and set the garment requirement from the load the opening will carry, not from a competitor's marketing figure.