Ring-Spinning End Breakage After Cotton Lot Change
A 25,000-spindle ring-spinning mill is running 30 Ne carded knitting yarn at 15,500 rpm, 42 mm rings, 19.5 TPI, and 55% RH. After switching to a new cotton lot (micronaire 4.7, strength 29 g/tex, short-fibre index 11.8%, neps 280/g), end breakage rose from 18 to 42 per 1,000 spindle-hours. Yarn CV is 16.8%, thick places +50% are 120/km, thin places -50% are 45/km, neps 210/km, and count CV 2.1%. Traveller number, draft, and delivery speed were not changed. The new lot represents three days of inventory and a knitting customer shipment is due in five days; the contract carries a late-delivery penalty. The mill has an old lot of similar count available for blending, but only enough for a 40% blend. Quality assurance has one winding and one knitting trial machine available.
1. Rank the probable causes of the end-breakage increase and justify the ranking using the supplied data. Explain why traveller weight and humidity alone are not sufficient explanations.
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Ranking
- Fibre quality, especially short fibre and neps. Short-fibre index 11.8% and 280 neps/g are high for a 30 Ne carded knitting yarn; short fibres migrate to the yarn surface, reduce tenacity at the spinning triangle, and generate thick places. The +50% thick places at 120/km and 210 neps/km show that carding and drafting are not removing the disturbance.
- Spinning geometry and traveller selection for the weaker lot. With unchanged traveller number and spindle speed, the yarn is being spun at the same tension despite lower fibre quality. That does not create the CV and IPI jump by itself, but it amplifies breakage once tenacity falls.
- Carding or drafting fault. Possible, but the timing coincides with the lot change; a machine fault would usually affect more than one lot and would not raise neps/g in the raw cotton.
Why not traveller or humidity alone Traveller problems usually raise end breakage without a large simultaneous rise in thick places, thin places, and neps. Humidity at 55% RH is normal for carded knitting yarn; low RH would affect all lots and would show ballooning and static, not a sudden neps increase. The data point to raw-material neps and short fibre as the primary driver, with traveller/speed as a secondary multiplier. Position data (spinning triangle, balloon, ring/traveller) should still be collected to confirm.
2. Design a controlled trial plan with four conditions to decide how to run the new lot. Include measured responses and acceptance criteria that would protect the knitting customer.
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Use four conditions, each on the same machine group and same 1,000-spindle block, randomised to avoid position bias, with 4 hours per condition after 30 minutes stabilisation:
- A: 100% new lot, current traveller, 15,500 rpm.
- B: 100% new lot, one traveller number lighter, 14,800 rpm.
- C: 60% new lot/40% old lot, current traveller, 15,200 rpm.
- D: 60/40 blend, lighter traveller, 14,800 rpm.
Responses: end breakage per 1,000 spindle-hours, count CV, CV%, thick +50/km, thin -50/km, neps/km, single-end tenacity and elongation, and a 20-kg knitting trial checked for holes, needle lines, and bursts.
Acceptance criteria: end breakage at or below 25 per 1,000 spindle-hours; count CV at or below 2.0%; CV at or below 16.2%; thick +50 at or below 80/km; neps at or below 150/km; knitting trial with zero holes and acceptable fabric appearance. Condition D is preferred if it meets these limits because it uses the old lot without starving the order. If only A meets quality but breakage remains above 25, the lot should not be run at full speed.
3. Recommend whether to accept the new lot as-is, blend it off, or reject it, considering the five-day shipment and penalty. Give the commercial and technical reasoning.
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Do not accept the lot as-is and do not reject it outright. Rejection does not solve the delivery date, and the lot is not unusable if blended and spun at moderated speed. The evidence shows a fibre-driven quality loss, not a permanent machine fault, so a 60/40 blend with old cotton is technically defensible.
Recommended decision: run trial condition D. If it meets the acceptance limits, blend off up to 40% old cotton, reduce spindle speed to 14,800 rpm, use the lighter traveller, and release the shipment with a 10% quality buffer and enhanced knitting inspection. If D fails but B passes, run 100% new lot at reduced speed and accept a lower production rate; if neither passes, request a three-day extension and ship from the old lot first. Reject the balance only if the knitting trial shows holes or bursts, because a customer rejection and claim would exceed the late-delivery penalty. The commercial test is: penalty plus rework cost versus the cost of blending and lost production, not the lot price alone.