Shade Drift in a Reactive-Print Lot
A small printing unit is producing 200 m of 100% cotton poplin with a six-colour reactive print for a school-uniform order. The paste is prepared with sodium alginate thickener, urea, alkali and a bifunctional reactive dye. The first 50 m match the approved shade. From about 55 m the ground shade becomes lighter, and from 90 m there are pale horizontal bands about 30 cm wide. The screens are 100 mesh, the squeegee is 70 Shore A, and the operator is making two passes. The steamer is set at 102°C for 8 minutes. The water supply has changed to a borewell source with 420 ppm hardness. The dryer is a loop dryer. The unit has no spectrophotometer, but the supervisor says the last 50 m look 10% lighter.
1. List the probable causes of the lighter ground shade and the pale bands, and explain how you would distinguish between paste exhaustion, screen clogging, inadequate steaming and water hardness.
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Ranked causes. (1) Steaming/fixation: 8 min at 102°C may be marginal for a bifunctional reactive on cotton; poor steam saturation or air pockets gives lower colour yield and banding. (2) Paste exhaustion/viscosity drift: as paste is consumed, dye and alkali concentration can fall; this gives gradual lightening, not sharp bands. (3) Screen clogging: partial blockage of 100-mesh openings reduces paste deposit and can produce bands or streaks. (4) Water hardness: 420 ppm calcium/magnesium can destabilise alginate and reduce dye solubility, causing patchiness.
Distinguishing tests. Print a 1 m sample with fresh paste and a cleaned screen. If it is still light, the fault is in steaming or fabric preparation. If fresh paste fixes it, the fault is paste exhaustion. If cleaning the screen fixes it, clogging is the cause. Measure band spacing against screen or roller circumference. Check steamer temperature with a probe and use distilled water for a lab paste.
2. Design a controlled trial to find the cause within one shift, using the available equipment.
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One-shift trial. Use the same fabric lot and cut 1 m samples. Prepare four pastes: A, current formula with borewell water; B, current formula with softened or distilled water; C, fresh paste with 10% extra urea; D, fresh paste with alkali checked by pH. Print each with a cleaned screen and the same squeegee, two passes. Steam A and B for 8 min; steam C and D for 8 and 10 min. Keep one unsteamed control. Wash off together and compare against the approved shade in daylight. Record paste viscosity, pH, steamer temperature, screen condition and band spacing.
If B is better, hardness is implicated; if C or D at 10 min is better, fixation is implicated; if all fresh pastes are better, exhaustion or clogging is implicated. Repeat the best condition on 5 m before releasing the lot.
3. The buyer rejects the lot. Compare stripping and re-dyeing, overprinting, and selling as second quality, and recommend a commercial decision.
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Options. (1) Strip and re-dye: reactive dye is difficult to strip fully from cotton; strong oxidative or reductive stripping damages strength and adds effluent cost. (2) Overprint: only viable if the variation is a uniform lightening and the design can take a darker ground; bands will still telegraph through and registration must be perfect. (3) Second quality: discount and sell with full disclosure.
Decision. Quantify: if the order is 200 m at ₹180/m = ₹36,000, a 25% second-quality discount loses ₹9,000; reprinting 100 m may cost ₹12,000–15,000 plus delay. Unless the buyer accepts a reprint of the faulty last 100 m, negotiate a second-quality settlement rather than strip. Preserve 1 m samples, record the trial, and correct the steamer and paste control before the next lot.