Fabric (all types)SpinningWeavingKnitting

Slub (Unwanted Thick Place)

A short, thick place in the yarn that shows as a lump on the fabric surface. In plain fabrics it is a defect; in slub yarn fabrics, controlled slubs are part of the design.

By Published 4 min read

https://textilenotes.com/quality/defects/slub

Diagram of a yarn with a thick lump and the same lump visible as a raised spot in woven fabric.

Likely causes at a glance

  • Machine

    • Drafting faults in spinning (roller or apron problems)Check: Check yarn from the same spindle or machine
    • Fly and lint accumulation spun into the yarnCheck: Inspect machine cleaning and suction systems

  • Material

    • Fiber waste or short fibers in the mixCheck: Check fiber quality and mixing
    • Yarn clearing settings too looseCheck: Review winding clearer settings with the spinner

  • Operator

    • Poor piecing after yarn breaksCheck: Look for thick places at piecing points

  • Method

    • Inadequate cleaning schedule in spinningCheck: Review cleaning frequency
    • No incoming yarn quality checkCheck: Check yarn test reports for imperfections
On this page
  1. What is a slub?
  2. What it looks like
  3. Where it occurs
  4. Likely causes
  5. Machine-related causes
  6. Material-related causes
  7. Operator and process-related causes
  8. Method-related causes
  9. How to confirm the cause
  10. Immediate containment
  11. Corrective action
  12. Preventive action
  13. Inspection notes
  14. Designed slubs versus defects
  15. Example investigation
  16. Factory checklist

What is a slub?#

A slub is a short thick place in a yarn. When the yarn is knitted or woven, the slub shows as a small lump or raised spot on the fabric. In fabrics made from plain yarns, slubs are defects. In fabrics deliberately made from slub yarn, the thick places are controlled and are part of the fabric's look; those are not defects unless they differ from the approved standard.

What it looks like#

  • Short thick lumps in the yarn, often a few millimeters to a few centimeters long.
  • Raised spots on the fabric surface that catch the light.
  • After dyeing, thick places can look darker or lighter than the surrounding fabric.
  • In wovens, a thick place along a single warp or weft yarn.

Where it occurs#

  • Plain woven fabrics (poplin, sheeting) where the smooth surface shows every fault.
  • Single jersey in plain dyed shades.
  • Yarn from spinning mills with poor cleaning or drafting control.

Likely causes#

Slubs are formed in spinning, when a bunch of extra fiber is drafted into the yarn. That happens when fly or lint collects on machine parts and is pulled in, when drafting is uneven, or when a yarn break is pieced up badly. Yarn clearing at winding should remove large slubs; loose settings let them through.

  • Drafting system faults: worn rollers, damaged aprons, wrong settings.
  • Fly accumulation on machine parts that is released into the yarn.
  • Poor suction and cleaning systems.
  • Fiber quality: short fibers and waste in the mix.
  • Clearer settings: electronic clearers on winding machines set too loosely.
  • Piecing: joining broken ends with extra fiber.
  • Cleaning practice: cleaning machines while running can throw fly into the yarn.
  • Cleaning schedules that do not match the fiber and machine.
  • No yarn inspection or imperfection testing on receipt.

How to confirm the cause#

  1. Check whether the slubs follow one yarn in the fabric (one wale in knits, one warp end or pick in wovens).
  2. Cut out a slub and look at it under magnification: fly and fiber bunches look different from a piecing.
  3. Compare yarn lots and suppliers.
  4. Review the spinner's yarn test reports for thick places and classimat faults, where available.
  5. Knit or weave a test piece from the suspected yarn lot.

Immediate containment#

  • Mark slubs during fabric inspection.
  • Grade them in the 4-point system. See 4-point inspection.
  • Cut around heavy slubs or replace affected panels.
  • Hold the yarn lot if slubs are frequent.

Corrective action#

Cause Corrective action
Fly in yarn Improve machine cleaning and suction at the spinner
Drafting faults Repair rollers and aprons; reset drafting
Loose clearing Tighten clearer settings for thick faults
Poor piecing Train on piecing; use splicing where available
Bad yarn lot Replace the lot; claim with the spinner

Preventive action#

  • Yarn specifications with limits on imperfections.
  • Incoming yarn testing for imperfections and thick places.
  • Supplier audits of spinning cleanliness and clearing.
  • Fabric inspection of every lot before cutting.

Inspection notes#

  • Distinguish designed slubs (slub yarn fabric) from defects using the approved standard.
  • Record slubs by length for 4-point grading.
  • Slubs in prominent positions on a garment may be classed as major; small slubs elsewhere as minor. Follow the buyer's manual.

Designed slubs versus defects#

In slub yarn fabrics, the slubs are produced on purpose by the spinning machine at controlled lengths, thicknesses and spacing. Problems in those fabrics are different:

  • Slub pattern too regular, creating visible repeats or stripes in the fabric.
  • Slub size varying between yarn lots, so garments from different lots look different.
  • Slub density too high or low compared with the approved sample.

Approve slub fabrics against a physical standard, and keep rolls from different yarn lots apart in cutting.

Example investigation#

A plain white poplin shows many short slubs in the weft direction only. Warp yarns are clean. The weft came from a different spinner than usual. Cutting out several slubs shows fiber bunches wrapped into the yarn, typical of fly being spun in. The spinner's clearing settings were checked and tightened, and the remaining weft lot was replaced.

Factory checklist#

  • Yarn imperfection limits in specification
  • Yarn test reports reviewed on receipt
  • Fabric inspected and slubs graded
  • Heavy slubs marked for cutting
  • Supplier informed and corrective action requested

Related guides: nep, fabric defects.

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