Contaminated Cotton Bales and a 30s Combed Yarn Lot
A co-operative spinning mill in western Odisha must supply 6,000 kg of 30s combed cotton yarn to a Kolkata circular knitter within 12 working days. The mill has three cotton lots in stock: Lot A, 40 bales, micronaire 4.6, staple 27 mm, trash 3.2%; Lot B, 20 bales, micronaire 4.0, staple 29 mm, trash 1.8%; Lot C, 10 bales, micronaire 3.6, staple 31 mm, trash 1.1%. Each bale weighs 170 kg. The knitter reports dark specks and a 12% rise in end breakage since the previous shipment. The mill's mixing register shows no change, but the blow room has been run at 850 kg/h instead of the rated 700 kg/h, and the comber noil extraction is still set at 14%. The mill cannot buy new cotton before the due date. The laboratory can test only three bales per day, and the comber has spare capacity.
1. Recommend a bale mix for the 6,000 kg order and justify it using the fibre properties. State any assumptions about waste.
Read the worked answer
Assume 8% process waste, so cotton required = 6,000 ÷ 0.92 ≈ 6,522 kg. Stock is 70 bales × 170 kg = 11,900 kg, so quantity is not the constraint; quality is.
A workable mix is 8 bales of A + 21 bales of B + 10 bales of C = 39 bales = 6,630 kg. That is about 20.5% A, 53.8% B and 25.6% C. Weighted micronaire = (4.6 × 0.205) + (4.0 × 0.538) + (3.6 × 0.256) ≈ 4.02. Weighted staple ≈ 29.1 mm. Weighted trash ≈ 1.9%.
This is suitable for 30s combed yarn: the high-micronaire, short, trashy A is limited to about one-fifth of the mix; B provides the bulk and consistent length; C improves yarn strength and reduces specks. Using more A would raise end breakage and dark specks. The assumption is that the 8% waste figure is realistic for this mill; if waste is higher, cotton requirement rises and the mix must be recalculated.
2. Explain the process changes you would make in the blow room and comber to reduce specks and end breakage, and how you would verify improvement.
Read the worked answer
First restore the blow room to about 700 kg/h. Running at 850 kg/h reduces cleaning efficiency and gives less uniform opening; the tufts become unequal and the comber cannot remove all trash. Keep the beater speed moderate and avoid over-beating, which damages fibre and creates neps. Increase cleaning points only if the machine can maintain even feed.
At the comber, raise noil extraction from 14% to about 16–18% for this mix. The extra noil removes short fibre and fine trash, improving yarn strength and reducing specks, but excessive noil wastes good fibre and raises cost. Check lap evenness and comb web condition.
Verification should not rely on a single yarn test. Test bales for contamination, then spin a trial lot and test count, CSP, unevenness, IPI and Classimat faults. Knit a 10 kg sample and inspect the fabric on a light box for dark specks. Compare against the previous shipment. Also verify that the polypropylene contamination is not coming from wrapping material in the bale feed.
3. The knitter demands a blanket 5% price reduction as compensation. What evidence would you collect before accepting or rejecting, and what alternative would you offer?
Read the worked answer
Collect evidence that separates mill fault from knitter fault: the type and frequency of dark specks, whether they are polypropylene, dyed fibre or seed-coat fragments; the bale numbers and test results; the knitter's storage, opening and knitting conditions; and the end-breakage record by machine and lot. Ask for a sample of the complained yarn and fabric.
If the fault is traceable to a specific lot, offer replacement of the affected quantity or a credit on that quantity, not a blanket 5% on the whole order. If contamination is widespread, offer 100% inspection of the next lot and a joint sorting protocol. If the evidence points to the knitter's housekeeping or machine settings, reject the claim politely and offer technical support.
A blanket discount rewards the complaint without proving the cause and sets a precedent. A better commercial offer is lot traceability, a pre-shipment sample, and a quality certificate with agreed test limits.