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Count, Tenacity and Evenness: Reading a Polyester Yarn Data Sheet Correctly
2026-09-24
A knitting mill in Egypt switched Polyester Yarn suppliers to save four cents a kilo and within two weeks had doubled its fabric rejection rate. The count was correct, the shade was correct, and the price was better. What had changed was invisible on the quotation: the tenacity had dropped from 4.2 to 3.4 cN per dtex and the evenness CV had gone from 1.8 to 3.1 per cent. Those two numbers produced more ends down on the machines and visible barre in the finished fabric. The mill had bought a count and received a different yarn. Every buyer in this position is one data sheet away from the same mistake.
Count Is the Headline; Count Conversion Is the Trap
Polyester Yarn is sold in several count systems and confusion between them is routine. The Ne or English cotton count expresses the number of 840 yard hanks per pound, so a higher Ne means a finer yarn. Denier and dtex are direct measures of mass per length, so a higher number means a coarser yarn. The conversion that matters in practice is Ne multiplied by denier equals roughly 5315, which makes 30 Ne about 177 denier. Confirm which system the quotation uses before comparing anything, and state the count system on your own enquiry so every supplier answers in the same units.
Ply matters just as much. A 32s/2 two ply yarn is not the same product as a 16s/1 single, even though the resultant count is similar, because the plying process changes evenness, hairiness and strength. Two ply yarns are stronger and more even but more expensive, and they are the usual choice where the fabric will be heavily brushed or where surface clarity matters. Standard programmes such as 32s/2 raw white polyester yarn exist because that balance suits a very large share of knitwear production.
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Tenacity and Elongation: The Numbers That Run Your Machines
Tenacity, quoted in cN per dtex or grams per denier, measures the force a filament can carry relative to its thickness. For standard polyester Spun Yarn, 3.5 to 4.5 cN per dtex is normal, and anything below about 3.2 cN per dtex will show up as breakage in high speed knitting and weaving. Elongation at break, typically 15 to 30 per cent for Spun Polyester, tells you how the yarn behaves under tension. Low tenacity combined with low elongation is the combination that stops machines, and it is never visible on a shade card.
Ask for the test method and the sample size. A single test on one cone tells you almost nothing; a credible mill reports an average over at least twenty breaks with the coefficient of variation. Where the yarn is destined for technical or heavy duty use, a high strength 100 percent polyester yarn is a different grade entirely, and it should be quoted with its tenacity range and its intended end use stated explicitly.
Evenness, Imperfections and What Shows in the Fabric
Evenness is measured as a coefficient of variation, or CV per cent, of mass along the yarn, usually with an Uster tester. Below 2.0 per cent is excellent for Spun Polyester, while above 3.5 per cent will usually show as barre or streakiness in piece dyed fabric. Alongside CV, ask for the imperfection counts: thin places, thick places and neps per thousand metres. Thick places are the ones that jam a knitting needle or a heddle, and neps are the ones a customer sees in a finished garment.
Do not treat these as a laboratory formality. A mill running fine gauge circular knitting should hold its supplier to a written evenness specification with a defined test frequency, ideally per production lot rather than per shipment. Where the product is a coarse yarn for heavier fabrics, the tolerance is wider; a 16s/1 raw white polyester yarn for heavier knit or woven work has different evenness expectations than a fine count.
Twist, Oil Content and Cone Build: The Practical Details
Three further items decide how the yarn behaves on your floor. Twist per metre affects strength, handle and pilling, and a change of ten per cent is noticeable in the fabric. Oil or spin finish content, normally 0.5 to 1.5 per cent, affects friction on knitting needles; too little and you get needle wear and breaks, too much and the finish migrates and causes uneven dyeing. Cone build and winding tension affect whether cones unwind cleanly at speed, and a badly built cone with poor transfer tails will cost more in machine stoppages than any yarn discount saves.
For dyed or solution dyed programmes, remember that colour is a separate specification. A 21s/1 black polyester yarn with colourfast performance should be quoted with its wash and light fastness ratings, since black is the shade where a poor dye job shows fastest.
A custom colour polyester yarn programme needs a lab dip approval process and a stated shade tolerance before bulk production starts.
The Data to Request on Every Enquiry
- Count and ply, with the count system named and the denier equivalent shown.
- Tenacity in cN per dtex and elongation at break, as averages over a stated sample size.
- Evenness CV per cent and thin, thick and nep counts per thousand metres.
- Twist per metre and the twist direction.
- Oil or spin finish content as a percentage.
- Cone weight, build and winding tension, plus the cones per carton.
- Fastness ratings and shade tolerance for any dyed programme.
The Bottom Line
A count is not a specification. Confirm the count system and ply, set tenacity, elongation and evenness limits in the purchase contract, agree twist and oil content, and require lot level test reports rather than samples. Mills that buy this way stop discovering quality problems at the finished fabric stage, and they end up paying less per usable metre than the buyer who chased four cents a kilo.