Warp Stops and Loom Efficiency Collapse After a Sizing Change
A weaving mill runs 48 air-jet looms on 100% cotton poplin: 40 Ne warp and weft, 133 ends/inch by 72 picks/inch, 63-inch finished width, 700 picks/min. A new 10,000 m warp lot was sized with 8% maize starch, 2% PVA and 0.3% wax at 10% size add-on, 60 m/min sizing speed and 22 kN/m squeeze pressure. Sized-yarn abrasion resistance is 180 cycles, against 350 cycles for the previous lot; sized yarn elongation is 4.2%. Loom efficiency has fallen from 92% to 78%, with 4.5 warp stops per 100,000 picks versus 0.8 before. Stops concentrate at the back rest and drop wires, and the cloth shows short warp streaks and size shedding. Shed conditions are 68% RH at 30°C. The size kitchen reports starch paste viscosity varying by ±8 s between batches on a Ford cup.
1. Using the stop data, estimate the efficiency loss from warp stops and explain whether it accounts for the fall from 92% to 78%.
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At 700 picks/min, 100,000 picks take 100,000/700 = 142.9 min. At the old rate of 0.8 stops per 100,000 picks and 4 min per stop, lost time is 3.2 min, or 3.2/142.9 = 2.2%; the predicted efficiency from warp stops alone is about 97.8%, so the remaining gap to 92% comes from other stops and normal losses. At the new rate of 4.5 stops, lost time is 18 min, or 18/142.9 = 12.6%; predicted efficiency is about 87.4%. The actual 78% is 9.4 points lower still. Therefore warp stops explain most of the 14-point fall, but not all of it. Either the average stop takes longer than 4 min (at 5 min per stop the loss is 15.75%, predicting 84.25%), or weft stops, restart losses and speed reductions are adding to the problem. The calculation still points to warp preparation as the dominant cause, because 4.5 versus 0.8 stops is a 5.6-fold increase and the stops are at the back rest and drop wires, which are classic abrasion and size-shedding locations.
2. Diagnose the sizing problem and propose specific recipe and sizing-machine changes. Include the role of viscosity variation.
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The abrasion result of 180 cycles versus 350 is decisive: the new size film is brittle and has poor abrasion resistance. An 8% starch / 2% PVA recipe with only 0.3% wax is starch-rich; starch gives a stiff film with low flexibility, and the low wax level gives poor lubrication. A sized yarn elongation of 4.2% is low for a 40 Ne cotton warp, which increases breakage when the yarn is flexed at the back rest and drop wires. Size shedding confirms that the film is flaking rather than protecting the yarn.
The ±8 s Ford-cup viscosity variation is also serious. At fixed squeeze pressure and sizing speed, viscosity variation changes wet pick-up and size add-on along the beam, so some sections are under-sized and some are over-sized. Over-sized sections shed brittle film; under-sized sections abrade. Change the recipe to approximately 5–6% starch, 3–4% PVA or acrylate binder, 0.5–0.8% wax/lubricant and a penetrant. Control paste viscosity to ±2 s, temperature to 85–90°C, and use automatic size-box concentration control. Increase squeeze pressure to about 25 kN/m to improve penetration, and reduce sizing speed to 50 m/min if necessary to get penetration rather than a surface coating. Trial two beams and require abrasion ≥300 cycles, elongation ≥5% and add-on CV within the normal range before releasing the recipe.
3. The 10,000 m warp is already sized and on the loom beam. Decide whether to re-size, re-beam, or weave it with containment, and support the decision with a cost/loss calculation.
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Estimate the yarn on the beam. 40 Ne is 590.5/40 = 14.76 tex, or 14.76 g/km. With 133 ends/inch over 63 inches, the fabric has about 8,379 ends; allow for a 67-inch reed width, giving roughly 8,400 ends. Yarn mass = 8,400 × 10,000 m × 14.76 g/1,000 m = 1.24 t. At $2.20/kg the yarn is worth about $2,728. Re-sizing involves stripping, desizing and re-sizing, with perhaps 3% yarn loss and a re-sizing cost of $0.50/kg: about $37 of yarn loss plus $620 processing, say $700–$1,000 including downtime. Weaving the beam at 78% instead of 92% is more expensive: at 72 picks/inch the pick density is 2,835 picks/m; at 700 picks/min the loom advances 0.247 m/min, so 10,000 m needs 40,500 min, or 675 loom-hours at 100% efficiency. At 78% efficiency it takes 865 h; at 92% it takes 734 h. The extra 131 loom-hours at $20/h is about $2,620. On that basis, re-sizing is economically justified if the abrasion resistance can be restored to at least 300 cycles and yarn strength loss is under 5%. If the beam cannot be stripped without damage, weave it on a lower-pick-density style or at reduced speed with higher shed RH and accept the loss; do not simply run it at full speed and absorb the stops.