GSM shortfall and spirality in single jersey
A hosiery unit in Malegaon knits 30s combed cotton single jersey on a 24-gauge, 30-inch diameter, 96-feeder machine. The buyer specification is 160 ± 5 GSM, maximum 5% length shrinkage and 5% spirality after three washes at 40 °C. Current results are 148 GSM, 9% length shrinkage and 7% spirality; stitch length is 3.4 mm. The unit proposes switching to 26s cotton to reach the GSM target and raising machine speed by 15% to meet delivery. The buyer has approved only 30s yarn and requires a pre-production sample. The floor manager argues that tighter loop formation will fix both weight and shrinkage, while the dyeing unit reports edge curling and needle lines in the greige roll. No lot has been knitted at the proposed settings yet.
1. Evaluate the proposal to switch to 26s cotton and raise machine speed by 15%.
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The yarn change breaks the approved specification. 30s Ne is about 19.7 tex and 26s is about 22.7 tex, so a 26s yarn is coarser and will add mass per loop at the same stitch length. GSM would rise, but the buyer has approved only 30s. Coarser yarn also changes hand, drape, cover and possibly spirality, so it needs a new approval and a fresh sample, not a production shortcut.
The speed increase is a quality risk, not a weight solution. Running 15% faster tends to increase and vary yarn tension, disturb loop length consistency, raise needle stress and worsen needle lines, curling, shrinkage and spirality. It may help delivery, but it works against the quality targets.
Use stitch length first, with a trial. At 30s, reducing stitch length from 3.4 mm towards about 3.1–3.2 mm should raise GSM by making the structure tighter. The trade-off is that tighter loops usually increase length shrinkage, so the gain must be balanced by yarn relaxation, controlled take-down and a finishing/compacting step. The correct answer is a small trial matrix, not two simultaneous large changes.
2. Explain why the fabric shows 7% spirality and suggest ways to reduce it while keeping 30s yarn.
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Spirality is loop inclination, not just shrinkage. In single jersey, each loop is inclined because the yarn is twisted and the machine rotates in one direction. A twist-lively 30s combed cotton can produce a diagonal displacement of wales and courses after washing. High machine speed and uneven take-down make it worse, and edge curling in the greige roll shows that the fabric is already stressed.
Reduce twist liveliness at source. Specify a lower twist multiplier or a steam-set yarn, and ask the spinner for twist and hairiness data. Where the machine allows, alternate S-twist and Z-twist yarns at feeders to balance torque. Check feeder geometry and take-down tension; excessive pull stretches the loop and increases the later recovery distortion.
Control wet processing. Avoid over-stretching the fabric during dyeing and finishing, and use a spreader or relaxation step where appropriate. Compacting can reduce length shrinkage but will not fully correct spirality, so it cannot be the only remedy. Measure spirality after the specified three washes on a marked square, not on the greige roll.
3. Design a pre-production trial and state the acceptance criteria you would use before bulk knitting.
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Trial structure: knit 10–15 m for each combination of stitch length (3.0, 3.1 and 3.2 mm) at the current machine speed and at no more than a 5% increase. Keep yarn lot and feeder arrangement constant, and include one steam-set yarn option if available. Dye and finish each sample by the same route, then wash three times at 40 °C using the buyer's method.
Measure: GSM, length and width shrinkage, spirality, edge curling, needle lines and hand. Test after relaxation as well as after washing, because some distortion appears only after wet relaxation.
Acceptance criteria: GSM between 155 and 165, length shrinkage at or below 5%, spirality at or below 5%, no visible needle lines or edge curling, and acceptable hand and cover. If no combination passes, do not bulk knit. Return to the buyer with a technical explanation and seek approval for a revised yarn or specification rather than shipping a non-conforming lot.