Q: How is colorfastness to washing rated on zipper tape?
The tape is stitched to a multi-fibre adjacent fabric, washed under a defined temperature, time, detergent and mechanical action, then rinsed and dried. The change in the specimen's own colour and the staining of each adjacent fibre strip are assessed against the grey scales, in grades from 1 to 5 with half steps. The result is reported as two sets of numbers, not one.
For zippers, the common methods are the ISO 105-C series and the AATCC equivalents; the test temperature, the liquor ratio, the number of steel balls and the cycle time differ between them, so the same tape can produce different numbers under different methods. Always quote the method, the temperature and the number of washes with the grade. Where a care label promises repeated home washing, test the number of cycles claimed.
Acceptance is commonly set at 4 or better for colour change and 3-4 or better for staining on the fibres that will touch the garment, but the right figure is the one your customer's specification states. Judge the lowest adjacent-fibre result, because that is what shows as a mark on a light lining.