Knit Defects: Spirality, Barré and Holes — Root Cause and Control

A fabric defect is never just a blemish; it is a signal from somewhere in the production chain. In 100% polyester knits the four most common defect families — spirality (skew), barré (lengthwise/widthwise streaks), holes/needle lines/drop stitch, and slubs (thick places) — originate at different stages: in the yarn, on the knitting machine, or in the dyehouse/finishing line. This guide traces each defect to its root cause and shows the four control levers: positive (storage) yarn feed, correct gauge and stitch length, stenter heat-setting, and ASTM D5430 4-point inspection. Why polyester is disperse-dyed at ~130 °C, how yarn is textured, and what a stenter does are covered in separate guides (disperse dyeing, DTY texturing, stenter/heat-setting); here the focus is where these defects come from and how they are controlled.

Spirality: single-jersey torque and balance

Spirality (or skew) is when, after washing, the courses of a single-jersey fabric deviate from the perpendicular and rotate into a spiral — showing up as seam displacement and distorted prints. The root cause is usually residual twist torque in the yarn: in a single-cylinder single-jersey structure, the loops have no opposing dial to release unbalanced twist energy against. The set/non-set state of the textured yarn (high-torque non-set DTY increases spirality), the yarn twist direction and the knitting tension are decisive. Control starts with balanced yarn selection (set DTY, appropriate S/Z balance); double-knit structures (rib/interlock) are structurally balanced and show almost no spirality. In finishing, the strongest correction is stenter heat-setting: PET chains recrystallize at ~180–210 °C (typical), fixing loop geometry; the stenter exit angle is set under control to reduce residual skew. Spirality is measured per AATCC 179 / ISO 16322, with a commercial target typically ≤5% after washing (representative).

Barré: yarn and dyeing streaks

Barré is a repeating fine widthwise (sometimes lengthwise) banding across the fabric, and it usually becomes visible only after dyeing — which makes it the most insidious defect. There are two root causes. The first is yarn-related: yarns from different batches/lots, with slightly different denier, twist, IV (intrinsic viscosity) or texturing history, take up disperse dye at different rates. The second is machine/feed-related: variable yarn tension or inconsistent stitch length produces fluctuation in fabric density. In polyester, barré ruthlessly exposes differences in disperse-dye affinity. The primary control is positive (storage) yarn feed: feeders such as Memminger-IRO, BTSR or LGL deliver a fixed length of yarn to every loop, suppressing tension-driven barré. The second defense is disciplined lot management — feeding a single dye batch only from a single production lot. Pre-testing IV and dye affinity on suspect yarns catches the risk early.

Holes, needle lines and drop stitch

This mechanical defect group comes directly from the knitting machine's needles and sinkers. A hole is usually a yarn break or a bent/broken needle latch cutting the yarn. A needle line is an open lengthwise streak — the signature of a single damaged, bent or mis-seated needle. A drop stitch forms where the needle fails to hold the yarn properly — mostly from insufficient yarn feed, wrong cam setting or weak sinker action. Root causes: worn needles (requiring periodic needle replacement from suppliers such as Groz-Beckert), incorrect yarn tension, insufficient knitting oil/cleanliness, or an out-of-spec shop climate (typically ~60–65% RH, ~24–26 °C). Control is preventive: positive feed ensures consistent tension, planned needle/sinker maintenance prevents breakage, and inline optical detectors (stop-motions) halt the machine on a break, preventing meters of faulty fabric. Whatever escapes is caught at the inspection table by 4-point.

Slubs and yarn-borne irregularities

A slub is a local thickening or irregularity of the yarn, appearing as a spot bump or streak in the fabric. In filament polyester, true slubs are relatively rare; they occur more in staple (spun) yarns — from ring, rotor or vortex spinning irregularity — or from a texturing/intermingle fault, an oil stain or lint build-up. The root cause is yarn unevenness (Uster evenness profile) or inconsistency in the intermingle knot distribution. Control is incoming-yarn quality acceptance (evenness, thin/thick places, neps testing), process stability on the spinning/texturing side, and cleanliness/lint management on the floor. Slubs are hard to detect during knitting; the real catch is again at inspection.

Prevent and catch: four levers

• Positive (storage) feed — Memminger-IRO/BTSR/LGL: fixed yarn length to every loop → consistent stitch length, less tension-driven barré and drop stitch.
• Correct gauge and stitch length: gauge matched to yarn count (typically E18–E28) and a set stitch length (typically ~2.1–2.9 mm) both hit the GSM target and avoid the over-tension that causes holes/lines.
• Stenter heat-setting — ~180–210 °C (typical): fixes PET loop geometry, reduces spirality and skew, locks in width and dimensional stability; the compactor then brings residual shrinkage to ~3–5% (typical).
• ASTM D5430 4-point inspection: 1–4 penalty points by defect length, max 4 per linear yard, normalized to 100 yd²; common acceptance threshold ~≤40 points/100 yd² (tight 20–28, representative). Lab backup: ISO 105 fastness, ISO 3801 GSM, ISO 6330/5077 dimensional stability, spectrophotometer ΔE for barré/shade control.

Defect → Cause → Control