Patchy navy dyeing on cotton single jersey
A knit-dyeing unit in Erode processes a 300 kg batch of cotton single jersey, 160 gsm, to a navy shade for a school-uniform buyer. The approved lab dip has Delta E 0.6 against the buyer's standard. Bulk dyeing uses 3% owf reactive dye, 50 g/L salt added in one lot at the start, 15 g/L soda ash, a 1:8 liquor ratio and 60 degrees C. The water supply is 450 ppm hardness, and the fabric carries about 2.5% residual sizing and pectin. The bulk sample reads Delta E 2.3; the buyer's tolerance is Delta E 1.0. The fabric also shows a lighter centre and darker selvedges, and crocking fastness is 3 and washing fastness 3-4 against a required 4. The unit has 4,000 m of the same greige fabric and a repeat order of 8,000 m. The dyeing manager proposes adding 0.3% dye to deepen the shade and a cationic fixative to lift fastness.
1. Is the manager's proposal technically sound? Diagnose the bulk fault using the evidence.
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No. The evidence points to uneven absorption and fixation, not simply a shallow shade. Adding salt in one lot at the start drives rapid dye strike before the dye can migrate, which produces lighter centres and darker selvedges. Water at 450 ppm hardness complexes part of the reactive dye and reduces colour yield. Residual sizing and pectin at 2.5% block uniform wetting, so some areas absorb more dye than others. The low crocking and washing fastness show unfixed dye on the surface. Adding 0.3% dye will deepen the average shade but will not level the patchiness and may increase unfixed dye. A cationic fixative can improve washing fastness but will not correct crocking or patchiness, and it may shift the shade. The proposal treats the symptom, not the cause.
2. What correction would you authorise for the 300 kg, and what is the commercial logic?
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Authorise a corrective re-process, not a top-up. Hot-scour and rinse the batch to remove size and surface dye, add a sequestrant for the hard water, then re-dye at a longer liquor ratio with salt split into three doses and soda ash added in controlled stages. Soap at about 90 degrees C to remove unfixed dye, then verify Delta E and fastness. If one correction still gives Delta E above 1.0 or fastness below 4, reject the batch. Commercially, 300 kg at, say, Rs 500/kg is Rs 150,000 of fabric. A re-dye costing about Rs 65/kg plus 8% weight loss is roughly Rs 31,500, much less than a buyer claim, return freight and the risk to the 8,000 m repeat order. A fixative-only route is cheaper but fails the crocking test, so it is not a saleable correction.
3. What controls would you lock into the repeat order before production starts?
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Test water hardness before each batch and install or regenerate a softener; add sequestrant as a standard recipe item. Verify scouring by an absorbency drop test, not by visual appearance. Write a salt and alkali dosing profile with time and temperature stages, and record pH and temperature at each stage. Standardise lab-to-bulk correlation, including liquor ratio and fabric weight. Include a soaping step and a fastness test on every bulk lot before packing. Keep dye-lot and batch traceability so a fault can be traced to water, fabric preparation or dosing. These controls address the three causes shown by the evidence: hard water, poor preparation and uncontrolled salt addition.