Yarn Faults in Knitwear: Neps, Slubs and Thick Places

By YZR Sweaters · 2026-09

A buyer receives a sweater and finds small raised specks across the surface. To them it looks like a fault. To the factory it is normal for the yarn — a characteristic of the spun fibre that the swatch showed all along. Neither is wrong, and the disagreement is entirely avoidable. Yarn faults are the clearest example in knitwear of a quality argument that is really a specification gap.

What causes surface imperfections in a knitted fabric?

Most visible imperfections come from the yarn rather than the knitting, and they arise from what spinning can and cannot eliminate.

• Neps. Small tangled knots of fibre, formed when short or immature fibres tangle during carding and spinning rather than lying flat in the yarn. They appear as tiny raised specks and are more common in shorter-staple fibres and in yarns spun from recycled or lower-grade fibre.
• Slubs. Deliberate or accidental thick places in the yarn — a locally heavier section. In some yarns slubs are designed in for texture, which is a different thing entirely from an accidental thick place.
• Thick and thin places. Variation in yarn diameter along its length. Some variation is inherent to spun yarn; excessive variation points to inconsistent spinning or a drafting fault.
• Foreign fibre contamination. Fibres of a different colour or type that should not be present, often from incomplete cleaning between lots. Unlike neps, this is nearly always a genuine defect.
• Over-twisted or kinky sections, which can appear as small tight loops standing proud of the fabric surface.

Which are character and which are defects?

The honest answer is that it depends on the yarn and the intended look — which is why the decision belongs in the specification rather than in a dispute.

• Character: neps in a rustic wool, a deliberately slubbed fancy yarn, or the natural unevenness of a woollen-spun yarn. Our notes on worsted and woollen spinning explain why woollen-spun yarns are inherently more irregular and hairier. These are features of the aesthetic, and a swatch approved with them cannot later be rejected for having them.
• Defect: foreign fibre contamination; slubs or thick places in a yarn specified as smooth or fine gauge; neps at a density beyond what the approved swatch showed; anything affecting the structural integrity of the fabric rather than its surface appearance.

The dividing line is usually the approved swatch. A swatch showing a certain level of surface irregularity sets the reference. Bulk production with visibly more than that has drifted, and that is a legitimate claim. Bulk production that looks like the swatch is not defective, however much the buyer may have hoped for a smoother surface than they approved.

Why does this cause so many disputes?

Because the buyer and the factory are looking at the same garment with different reference points. The buyer compares it to an imagined ideal; the factory compares it to the yarn they were told to use and the swatch that was signed off. Three specific gaps produce most of the arguments:

• The swatch was approved casually. A swatch signed off without close inspection at the time sets a reference nobody actually examined. Fault levels are much easier to agree on a knitted swatch than on a finished garment.
• The yarn was changed after approval. A substitute yarn with different fibre length or spinning route will show a different surface. Our notes on yarn substitution explain why this counts as a specification change rather than a minor swap.
• "Acceptable quality" was never defined. Without a stated fault level, every observation becomes negotiable, and the discussion shifts from the goods to the relationship.

How should a buyer specify yarn surface quality?

You do not need a laboratory programme to control this. Four practical steps cover most of the risk:

• Approve a swatch, and inspect it under the light you will use for bulk. Neps and thick places are far more visible in raking daylight than under office lighting, and the assessment should reflect how the customer will see the garment.
• State the yarn's spinning route and fibre content in the tech pack. Both determine the expected surface irregularity, and leaving them vague leaves the surface to chance. Our guide to the tech pack covers where these belong.
• Describe accepted character explicitly. A line such as "neps consistent with the approved swatch are acceptable; foreign fibre is not" removes the ambiguity without requiring technical measurement.
• Keep the approved swatch and the bulk reference on both sides, so a comparison is possible during inspection rather than being reconstructed from memory.

What should be checked at inspection?

Where surface faults are a concern, the check is comparative rather than absolute. Inspectors compare the bulk against the approved swatch for density and distribution of surface irregularity, look specifically for foreign fibre or colour contamination, and treat any fault that affects the fabric's structure — as opposed to its appearance — as a defect regardless of the yarn's character. Our guide to quality defects and inspection explains how critical, major and minor categories are applied, and it is worth classifying surface faults explicitly in the order so that inspection has something to judge against.

One related point worth separating out: pilling is a different phenomenon from yarn faults, arising from wear rather than from the yarn's construction, and it is covered in our notes on preventing pilling. A buyer who describes neps as "pilling" will get advice for the wrong problem.

What should you expect from a supplier?

A supplier who understands yarn will tell you upfront what surface irregularity to expect from the yarn you have chosen, show you a swatch that demonstrates it rather than an idealised sample, and flag it if your chosen yarn and your desired surface are in conflict. That conversation is much cheaper than a bulk claim.

At YZR Sweaters we work with partner factories holding BSCI, SEDEX and Oeko-Tex certifications, welcome factory audits, and support both full inspection and third-party inspection through firms such as SGS and BV. When a yarn is inherently irregular we say so at swatch stage, and we keep the approved swatch on file so that any later comparison has a reference both sides accepted.

Frequently asked questions