Symptom
Color patch in dyed zipper tape appears as a visibly darker or lighter region that does not follow the design. It may be a patch in the middle of a roll, a difference between the two faces of the tape, a difference between the inside and outside of a package, or a shade step at a splice. In a garment the defect is most visible where two zips meet at a centre front, and it cannot be corrected by pressing or by sorting at the sewing line.
Because tape is narrow and long, dyeing unevenness is easily hidden in a folded bundle and only revealed when the zip is sewn flat. That is why the defect is often detected by the garment factory rather than by the dye house, and why the resulting dispute focuses on where the fault occurred rather than on what caused it.
- Delta E above 0.8 between two positions in the same roll.
- Visible shade step at a splice or at a package turnaround.
- Face-to-back difference visible under D65 lighting.
- Patch boundaries that match beam perforation patterns or tape contact points.
- Shade variation that increases from the outside to the inside of a package.
Root Causes
Even dyeing requires that fibre, liquor and time be uniform everywhere in the package. Any restriction to flow, any variation in fibre chemistry and any variation in temperature creates a patch.
- Too-dense beam loading. Tape wound too tightly prevents liquor from penetrating, so the inner layers dye lighter and the outer layers darker. Winding tension is the controlling variable.
- Low liquor flow or reversed flow time. Insufficient flow differential across the package leaves a permanent gradient; the flow reversal schedule must match the package density.
- Fibre blend variation. Polyester and recycled polyester, or two supplier lots of the same polymer, can differ in dye uptake by a visible amount even when the specification is identical.
- pH drift in the dye bath. Disperse dye uptake is pH-sensitive; a drift of 0.3 can shift shade across a long bath.
- Uneven heating. A package that heats more slowly at the centre reaches temperature late and dyes lighter, producing a radial patch.
- Improper soaping or reduction clearing. Incomplete removal of unfixed dye leaves surface colour that rubs off and reads as patchiness.
Troubleshooting Sequence
Map the patch before changing the recipe. The geometry of the defect identifies the subsystem: radial patches indicate flow or heating, longitudinal patches indicate beam winding, and face-to-back differences indicate penetration or fibre chemistry.
- Cut specimens from outside, middle and inside of the package and measure Delta E at three points each under D65.
- Photograph the roll flat, not folded, so the patch boundary is visible.
- Review the beam winding tension and package density records for the lot.
- Check the flow reversal log and the actual pump pressure against the recipe.
- Measure pH at the start, middle and end of the bath.
- Verify that the fibre lot used matches the approved lot, and check the supplier's dye uptake certificate.
- Re-dye a retained specimen in a laboratory bath to determine whether the fault is in the substrate or the production machine.
Corrective Actions
For flow-related patchiness, reduce package density, extend the flow reversal interval and increase liquor ratio within the machine's capability. For substrate-related patchiness, quarantine the fibre lot and require dye-uptake certification at receiving, because no recipe change will make two different substrates dye the same.
For pH-driven shade drift, add automatic dosing with in-line pH measurement rather than manual addition at the start of the bath. For incomplete clearing, extend the reduction clearing cycle and verify with a rub test and a wash fastness test, since a patch that is actually unfixed surface dye will return after the first wash even when it looks acceptable at the warehouse.
| Patch geometry | Cause | Correction |
|---|---|---|
| Radial, inside lighter | Dense winding or low flow | Reduce density, extend reversal |
| Longitudinal stripe | Beam winding pattern | Rewind with controlled tension |
| Face-to-back difference | Poor penetration | Raise liquor ratio |
| Whole-lot shift | Fibre blend or pH | Quarantine lot, control pH |
Prevention
Control the inputs that dyeing cannot correct: fibre lot, package density and bath pH. A receiving check that records dye uptake on every fibre lot, and a winding specification that states tension in newtons rather than "medium", remove most of the variability before the machine is loaded.
Donglong Zipper measures Delta E on every package at three positions and holds a master shade card under D65 lighting, so a patch is caught at the dye house rather than at the sewing line. YCC sourcing teams should ask to see the Delta E distribution for a lot, not a single measurement: an average of 0.4 with a maximum of 1.6 is a different risk from an average of 0.6 with a maximum of 0.7, even though the averages are close.
Finally, lighting matters. A patch judged under warm warehouse light may disappear under D65 and reappear under retail lighting. Agree the viewing condition in the specification and use the same condition for every inspection, or the argument will be about the lamp rather than the tape.
Use a spectrophotometer for approval, not for every piece. The instrument gives an objective number for the lab dip and the master standard; the trained eye then judges the tape on the machine, because a patch that is invisible to the instrument can still be visible as a boundary line. Approve each dye lot from a laboratory dip on the actual substrate, record the recipe adjustment, and keep the dip alongside the production record. When the next lot arrives, the question becomes a comparison rather than an opinion, and the shade argument loses the ambiguity that makes it expensive.