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Knitting & Hosiery
  Critical nep size & its influence on yarn

Fabric appearance and yarn imperfections are not directly correlated, says T G Sarathy, who backs up his claim with a case study.

In one of the famous hosiery yarn unit, manufacturers got market feedback that the knitting cloth appearance of their competitor is the best in comparison. The mill process test results found that the yarn imperfections and card neps/gram were very low when compared with their competitors and in spite of such low values, the knitting cloth appearance was not good. The mill has taken trial with high imperfection and still the fabric appearance was not comparable with the competitors.

When analysing this case study it was clearly revealed that card neps/gram and yarn imperfections are not directly related with yarn quality. And so, the relationship is arrived in this article in order to improve yarn quality. The critical nep size is contributing to fabric appearance and yarn neps. It causes short thick places in yarn resulting in uneven fabric appearance. Often erratic fibre orientation in these areas can cause weak places in yarn. This can lead to spinning efficiency loss, weaving & knitting machine stoppage and fabric defects. Neps in yarn can block the holes of yarn guides and needle hooks, resulting in yarn break and hole in knitted fabric.

Neps which are on the surface of the fabric, can cause undyed or unprinted spots during dyeing or printing. The most disturbing effect of this white spots is that they cannot be recognised until dyeing or printing. Especially in dark colours this problem is very critical and damaging. Neps some time contain immature fibres which are usually weaker than normal fibres. This weakness may lead to break-off of fibre fragments which creates excessive fibre dust fly and lint deposits.

Neps are generally classified according to structure and size into three groups:

  1. Biological neps: The neps containing immature or dead fibres. Cards and combers are effectively removing such neps.
  2. Seed coat neps: The seed coat fragments entangled with fibres. The quantity of seed coat neps depends quality of ginning process such as knife sharpening, blade alignment and delivery roller grinding. Seed coat neps are clearly visible in the finished grey fabrics as dark spots.
  3. Mechanical neps: The neps originate from production stages; fibre entanglement created by processing that starts with Ginning and usually ends with opening and carding. During carding damaged machine parts or improper machine settings can also create mechanical neps. These neps are usually formed from fine, immature fibres. Unlike coarse and mature fibres, fine and immature fibres tend to curl up and entangle with other similar fibres to form mechanical neps.

Small neps: The neps of which diameter size is less than 0.3 mm are the most difficult to remove because of their small size and low mass.

Medium sized neps: The neps of which diameter size is between 0.4 - 0.9 mm are usually removed efficiently during carding.

Large neps: The neps of which diameter size is 1 mm or over are the easiest to remove because of their relatively heavy mass reacting to the centrifugal forces of opening and cleaning machines and they are usually removed during carding.

The neps content of raw materials was between 80 to 260 neps/gram. Raw material and conditions of the technological process in spinning mills influencing number of neps in cotton yarn. AFIS (Advanced fibre Information System) is used to measure neps in cotton. Cotton fibres are exposed to two or more stages of lint cleaning in ginning process to meet demand for cleaner cotton and to improve cotton grade. However the lint cleaners increase the seed coat neps and reduce the fibre length. It becomes very difficult to remove in opening and cleaning lines. Blow room generates neps due to beating and cotton transportation by air stream. The blow room nep generation ranges between 60 - 100%. A first significant reduction in neps takes place in carding process. The nep removal efficiency is very critical for better yarn quality and consistency and ranges from 67% to 84% between various makes of carding machine.

NRE% shall be calculated for each carding machines as it is crucial for evaluating carding performance and selecting optimum machine settings.

For prediction of yarn neps the most important fibre property measured by High Volume Instrument (HVI) is the Uniformity Index, whereas the maturity ratio and short fibre content are the crucial properties measured by AFIS for determining yarn neps. The fibre properties measured by AFIS instrument enable determining yarn neps content more effectively compared with the fibre properties measured by HVI instrument. The graph indicates the critical nep size affecting yarn quality and fabric appearance for various count range.

Critical nep size for ring yarn

Hence trials are to be concluded based on critical nep size reduction. The number of neps above critical nep size shall be eliminated in carding by optimising setting, grinding frequency, wire condition/replacement, suction, process parameters, etc. It is to be noted that neps below critical nep size will not influence yarn quality/fabric appearance significantly. This analysis and controlling/minimising of critical nep size is highly significant for yarn dyeing quality requirements.

It is also advisable that any trials/R&D study should be concluded based on knitted fabric appearance/critical nep size in carding to impress/delight yarn buyers by manufacturing premium yarn quality.

T G Sarathy
Head - Technical Services (Spinning Operations)
NSL Textiles Limited
Veeravalli, Krishna district
Andhra Pradesh-521 110.
Mobile: 095814 12213.
Email: sarathy.tg@nsltextiles.com.

published September , 2013
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