Cotton lot selection under a regain clause and an immature-fibre constraint
Two cotton lots arrive for a 40s combed hosiery yarn. Lot A contains 120 bales of 170 kg with micronaire 4.6, maturity 0.86 and regain 6.2%. Lot B contains 80 bales of 170 kg with micronaire 3.4, maturity 0.71 and regain 9.8%; it also shows immature seed-coat fragments and sticky spots. The purchase contract is settled on conditioned weight at 8.5% standard regain. The mill needs a 34,000 kg mix, the comber is already running at 16% noil, and the customer requires neps below 60 per 1,000 m with consistent dye lots. The mill proposes a 50:50 mix because Lot B is cheaper. The humidification plant is working, but the mixing room has no separate bale-conditioning space. Lot A is clean but noticeably coarse, and the spinning master is worried about both yarn strength and dye uptake.
1. Calculate the conditioned invoice weight of each lot at 8.5% regain and the water carried per 100 kg of dry fibre. Which lot is commercially disadvantaged by the regain clause, and by how much?
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Use dry mass as the anchor. Lot A actual mass = 120 × 170 = 20,400 kg. Dry mass = 20,400 / 1.062 = 19,209 kg. Conditioned mass = 19,209 × 1.085 = 20,842 kg, or directly 20,400 × 108.5/106.2 = 20,842 kg.
Lot B actual mass = 80 × 170 = 13,600 kg. Dry mass = 13,600 / 1.098 = 12,386 kg. Conditioned mass = 12,386 × 1.085 = 13,439 kg.
Water per 100 kg dry fibre is the regain itself: 6.2 kg for A and 9.8 kg for B. On a moisture-content basis, A is 5.84% water and B is 8.93%, so B is delivering nearly 4 percentage points more water.
Lot B is disadvantaged: its invoice weight falls from 13,600 kg to 13,439 kg, a loss of 161 kg, about 1.2% of actual mass. Lot A gains 442 kg against its actual mass. The commercial point is that a cheap wet lot can be expensive per kilogram of dry fibre, especially when the price is quoted per invoice kilogram.
2. Calculate the blend micronaire and maturity for a 50:50 mix and for a 70:30 A:B mix. Recommend a maximum Lot B percentage for 40s combed hosiery, supporting the choice with the data and the neps and dye-risk mechanisms.
Read the worked answer
50:50: micronaire = (4.6 × 0.5) + (3.4 × 0.5) = 4.00; maturity = (0.86 × 0.5) + (0.71 × 0.5) = 0.785.
70:30 A:B: micronaire = (4.6 × 0.7) + (3.4 × 0.3) = 4.24; maturity = (0.86 × 0.7) + (0.71 × 0.3) = 0.815.
A 50:50 mix looks acceptable only if micronaire is treated as the sole criterion. A maturity of 0.785 means roughly half the mix is immature; thin-walled fibres are weak, crimp poorly, form neps at carding and combing, and take dye unevenly. The mean micronaire of 4.00 hides that half the blend is the fine, immature component.
At 30% B the maturity rises to about 0.815 and micronaire to 4.24, which is a more defensible combed hosiery mix. I would set the maximum at 25–30% B, not 50%. Lot A supplies maturity and strength; Lot B supplies fineness but must be diluted. The 30% option should be released only after a neps count on the comber sliver and a dye-uptake check on a pilot lot.
3. The mill wants to avoid extra comber noil. Propose a practical laydown and process-control plan if 30% Lot B is used, including two measurable release criteria.
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Use a controlled laydown rather than a simple bale count. Condition Lot B for at least 24 hours in a separate area, open it in a separate blowroom line or at a different shift, and distribute it across the laydown so no card sees a pure B pocket. A practical laydown is 10–12 bales with 3–4 B bales, giving about 30% B, and the B bales should not be placed together at one end.
Reduce carding damage by keeping flats and wire in good condition, and monitor card web neps. At the comber, do not raise noil blindly; first check sliver neps and short-fibre content. If neps rise, increase noil by 1–2 percentage points only as a temporary measure. Segregate yarn lots by laydown so dye-lot traceability is preserved.
Two measurable release criteria are: (1) combed sliver neps not exceeding an agreed mill limit, for example 60 neps per 1,000 m on the evenness tester; and (2) CSP of the 40s yarn at least 2,300 with U% not more than 12.5. If either fails, reduce B to 20% before changing the comber setting.