Knit Hem Puckering and Skipped Stitches on Lockstitch
On a lockstitch line sewing 95% cotton / 5% elastane single jersey, operators report skipped stitches, seam puckering and needle holes along the hem. Machine settings are: needle size 14 regular point, polyester core thread 40/2, stitch density 14 stitches per inch, tension dial 4, high presser-foot pressure. The fabric was compacted but not pre-washed. The first 60 pieces show 8% rejection. Production wants to increase tension to close the seam. QA reports that the seam elongation is only 10% while the fabric stretches about 40%. The hem is a simple double-fold hem, and the factory also has a 5-thread overlock and a coverstitch machine available. The buyer has complained previously about wavy hems on knit T-shirts. The line runs at 1,200 pieces per day and cannot afford long stoppages.
1. Diagnose the likely causes of skipped stitches, puckering and needle holes, using the machine and fabric data.
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The combination points to a seam that is too stiff and too dense for an elastane knit, plus a needle that is too large and too sharp.
- Needle: a size 14 regular point can cut the elastane and cotton yarns, producing needle holes and occasional skipped stitches. A ballpoint needle in size 11–12 is more suitable.
- Thread: 40/2 polyester core has limited stretch. The seam elongates only 10% while the fabric stretches 40%, so the thread locks the seam. When the fabric is pulled, the thread either breaks or skips.
- Stitch density: 14 stitches per inch is too dense for a knit. More needle penetrations increase hole damage and reduce stretch; 10–12 SPI is more appropriate.
- Presser-foot pressure: high pressure stretches the fabric during sewing. When it relaxes, the seam puckers.
- Fabric: compacted is not the same as pre-shrunk. Residual shrinkage after sewing can cause puckering after washing.
Raising tension would reduce thread elongation further and make the puckering and skipped stitches worse.
2. Recommend specific machine setting changes and a seam/stitch type for the hem, with reasons.
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Recommended changes:
- Needle: size 11 ballpoint, changed at least every 8 hours or after a needle strike.
- Thread: textured polyester or a stretch thread in the looper; if using a coverstitch, woolly nylon in the looper improves stretch and softness.
- Stitch density: 10–12 stitches per inch, not 14.
- Tension: balance to the minimum tension that gives a clean stitch; do not increase tension to close the seam.
- Presser-foot pressure: reduce so the fabric is not stretched as it feeds.
- Stitch type: use a 2-needle coverstitch for the hem, with a top cover for a clean finish, instead of a lockstitch double-fold hem. The coverstitch has built-in stretch. For side seams, a 5-thread overlock is more suitable than lockstitch.
Run a test strip on the actual fabric before restarting the line. If the buyer permits, pre-wash or shrink the fabric before cutting; otherwise test the finished garment after washing.
3. Explain why raising tension to close the seam is counterproductive, and state a quality check that would prove the fix.
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Increasing tension pulls the needle thread tighter and reduces the amount of thread available to stretch. On a knit, the seam then has even less elongation than the 10% already measured, so it will pucker more, skip more and may crack when the wearer moves. It may look closed on the machine but fail in wear or after washing.
A suitable check is a seam elongation test: cut a 10 cm × 20 cm strip across the hem, mark 100 mm across the seam, and stretch slowly until the fabric or seam fails. Record the extension at which the seam starts to crack or the thread breaks. The seam should stretch to a high proportion of the fabric’s extension - in practice, at least about 80% of the fabric stretch for a knit hem. Also wash a sewn sample at the buyer’s care conditions and inspect for puckering, needle holes and wavy hem. Only then release the line.