Neps and Noil in a Combed Cotton Mill
A 20,000-spindle spinning mill produces 40s combed hosiery yarn from a mix of Shankar-6 and MCU-5 cotton. For the last eight weeks, the quality report shows card sliver neps of 210 per gram against a normal 90, comber noil of 23.5% against a normal 17%, yarn neps up 280% on the USTER IPI, and end breakage of 38 per 1,000 spindle-hours against a normal 20. The mill has tried reducing card production from 22 to 18 kg/h and increasing comber feed to 6.5 mm; neps fell only 8% and noil rose to 25%. The card cylinder wire is 8 years old and the flat tops were last ground 14 months ago. The comber half-lap is 2 years old; nipper and detaching roller settings are standard. Cotton Micronaire is 4.6, short fibre content is 12.5%, and trash is 3.2%. The mill produces 3,000 kg/day of combed sliver and sells comber noil at ₹60/kg; cotton costs ₹140/kg. Management wants to stop combing to save noil and increase production.
1. Identify the most likely source of the nep problem and explain why increasing comber aggressiveness has not solved it. Refer to the data.
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The card is the most likely source. Card sliver neps are already 210/g against a normal 90/g, so the problem exists before combing. A comber can remove some neps, but it cannot correct a badly opened and poorly carded web. Increasing comber feed and noil has simply overloaded the comber: noil rose from 23.5% to 25% while neps fell by only 8%. Aggressive combing can also create fresh neps and remove good long fibre. The worn card cylinder wire, 8 years old, and flat tops last ground 14 months ago are strong mechanical causes of poor fibre individualisation and ineffective nep removal. The high trash (3.2%) and short fibre (12.5%) add to the load. Stopping combing is not a valid solution for 40s hosiery yarn; it would remove the little nep and short-fibre control that remains and would damage yarn quality and saleability. The priority is to restore carding action, not to comb harder or stop combing.
2. Calculate the daily cost of the excess noil above the 17% norm, and compare it with the apparent value of cotton saved if combing is stopped. State your assumptions.
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Assume the 3,000 kg/day figure is combed sliver output and that noil percentage is calculated on comber feed. At 23.5% noil: feed = 3,000 / (1 − 0.235) = 3,921.6 kg/day; noil = 921.6 kg/day. At 17% noil: feed = 3,000 / 0.83 = 3,614.5 kg/day; noil = 614.5 kg/day. Excess noil = 921.6 − 614.5 = 307.1 kg/day. Loss per kg of excess noil = cotton value ₹140 − noil sale value ₹60 = ₹80. Excess noil cost = 307.1 × ₹80 = ₹24,568/day, about ₹24,600/day.
If combing were stopped, the mill would need 3,000 kg of cotton instead of 3,921.6 kg, saving 921.6 kg × ₹140 = ₹129,024/day of cotton. It would, however, lose noil sale revenue of 921.6 × ₹60 = ₹55,296/day, giving an apparent net saving of about ₹73,700/day. This is not a real saving because the mill would no longer be making saleable 40s combed hosiery yarn. The quality loss and customer rejection cost would far exceed the noil cost.
3. Recommend a corrective action plan with priorities and a way to verify improvement within four weeks. Include one quality and one cost metric.
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Priority 1: inspect the card cylinder wire, flat tops, licker-in, feed plate and mote knives. Replace or regrind the wire and flats; 8-year-old wire and 14-month-old flats are not acceptable for a mill chasing neps. Until the card is repaired, reduce card production further and stop pushing the comber. Priority 2: reset the comber to standard feed and settings; do not chase noil. Check the half-lap condition, nipper and detaching roller, and suction. Priority 3: tighten bale selection and mixing to reduce short fibre and trash; check whether the ginning source has changed. Verify with card sliver neps per gram measured twice per shift, comber noil %, yarn neps on USTER IPI, and end breakage per 1,000 spindle-hours. Quality metric: card sliver neps at or below 110/g and comber noil back to 16–18% within four weeks. Cost metric: excess noil cost per day, which should fall from about ₹24,600/day towards zero. A successful trial should show yarn neps falling before noil is reduced, proving that carding, not combing, was the root cause.