Q: What shade tolerance is normal between zipper lots?
Shade tolerance between zipper lots is normal and should be defined rather than assumed. Dyed tape varies with the fibre, the bath and the drying, so a small difference is expected even when the recipe is the same. The practical approach is to agree a tolerance in a standard colour-difference system and a light source, and to approve a master swatch as the reference. Ask whether the tolerance applies to the tape only or to the assembled zipper, and whether it is judged visually or instrumentally. A common rule is that a difference visible as a distinct edge when two lots are placed side by side is outside tolerance, while a difference visible only under close inspection is inside. Metallic and coated tapes need their own tolerance because gloss changes the reading. Record the light source, because a match under one light can fail under another. Put the tolerance in the specification so a later lot can be judged against a rule, not an opinion.