yarn quality part 3 yarn evenness

Yarn Evenness and Imperfections

By : Ravikeerthi Rao

Yarns of different quality levels result in fabrics of distinctly different quality levels, leading to problems in processing or variations in final value (of the fabric / garment).

These differences impact on prices and affect the costs and profitability of both fabric & apparel manufacturer.

There is an unavoidable link between quality and cost.

YARN EVENNESS

• Statistics tell us that yarn accounts for a major share of the total ‘quality cost’ of a garment.

• That includes not only direct price of the yarn, but also extra costs caused by substandard quality or wrongly-specified yarns, which may also have a knock-on economic effect through weaving or knitting and even into dyeing and finishing.

YARN QUALITY

Increased quality costs may be caused not only by wrong selection of the yarn for a certain article but also by poor performance of the yarn.

One of the most effective ways of managing increased quality costs and its related implications is to provide a better quality specification for yarns.

The most important parameter (specification) to be taken care is the yarn evenness.

YARN EVENNESS

Yarn evenness is a measure of the level of variation in yarn diameter along its length.

In other words, it refers to the variation in yarn linear density or mass per unit length of yarn.

A yarn with poor evenness will have thick and thin places along yarn length, while an even yarn will have little variation in mass or diameter along length.

It is the evenness of staple spun yarn that is of concern here.

Continuous filament yarns have virtually no variation in linear density, so, evenness is not an issue for those yarns.

YARN EVENNESS

Uneven fabric appearance is one of the most disturbing and most noticeable effects in a garment.

Irregular yarns and high imperfection levels in the yarn are the causes behind an uneven fabric appearance and to some extent, the mass variation of fabric (GSM).

It results in a cloudy surface which is considered off-quality and absolutely not acceptable.

By no means can this be hidden or mended once the fabric is produced.

YARN EVENNESS

Effect on Productivity

The yarn defects / imperfections such as thick places, thin places, slub, loose fluffs, etc. create difficulty during the process of knitting.

Any diameter higher than the average yarn diameter may create problem in performance of knitting due to the insufficient space available in between two sinkers.

In order to run a knitting machine smoothly the yarn should be of uniform diameter though out its lengths.

YARN EVENNESS

Effect on Productivity ( Cntd….)

With the use of even yarn the efficiency in Weaving and / or Knitting will be improved leading to higher productivity because of the lower incidence of weak places. Fewer end breaks are encountered with regular (more even) yarns.

YARN EVENNESS

Effect on Quality

Fabric defects and rejections are critically influenced by irregularity of yarns.

Periodic and quasi periodic irregularities in yarn result in warp way streaks and weft bars in woven fabrics and a non-periodical yarn irregularity gives cloudiness in the woven or knitted fabric, leading to fabric rejections.

Defects such as streaks, stripes, barre (moiré effect), or other visual groupings clearly apparent to eye and are usually compounded when the fabric is dyed or finished.

YARN EVENNESS

Effect on Quality ( Cntd…)

An irregular yarn will also vary in strength along the yarn.

More regular the yarn, better will be the appearance and aesthetic value of the product. As a result, better sale value can be achieved.

YARN EVENNESS


Regularity contributes to a smoother feel.

In apparel and most of other textiles, smoothness is most desired characteristic. Sale value of fabric is dependent, among other things, on smoothness.

Other fabric properties such as abrasion or pill-resistance, soil retention, drape, absorbency, reflectance, or luster may also be directly influenced by yarn evenness.

YARN EVENNESS

YARN FAULTS


Faults like spinners doubles are difficult to determine in the yarn, with the naked eye.

On the other hand, they can produce quite fatal results in the finished product.

A spinners double in the warp or in yarn for circular knitting can downgrade hundreds of meters of woven , or knitted fabric.


Neps•Apart from the strong influence on the optical appearance of textile surface structures, neps from a certain size upwards also lead to problems in the knitting machine sector. •In woven fabrics, the second most important yarn characteristic after yarn tenacity is the level of neps, since this adversely affects fabric appearance•Not only the size but also the number of neps are decisive criteria as to whether the yarn is usable or not.

YARN FAULTS

• Imperfections referred to as frequently occurring smaller yarn faults or irregularities.

• These can be subdivided into bellow three groups:

These are the conventional criteria that are considered in the evenness measuring equipment.

IMPERFECTIONS

IMPERFECTIONS

IMPERFECTIONS

•Typically, thin and thick places can be of up to one-inch length.

•Neps are typically of 3 to 10 mm length.

•Thick places exceeding the 100% limit are determined using the Classimat method.

YARN FAULTS

•It is still not possible to produce a yarn without faults for various reasons.•Stickiness of cotton can contribute to the formation of thick and thin places. •Fly liberation in Ringframe department is one of the major reasons for short faults in the yarn because of the fly gets spun into the yarn. •Hence it is not possible to have fault free yarn from ring spinning

YARN FAULTS

•Depending upon the raw material, the machinery set up, production and process parameters, there are about 20 to 100 faults over a length of 100 km yarn which do not correspond to the desired appearance of the yarn.

•This means that the yarn exhibits a yarn fault every 1 to 5 km. These faults are thick and thin faults, foreign fibers and dirty places in the yarn.

YARN FAULTS

It is obvious that the appearance of faults in a (grey) fabric depends on various items:

The cross-section of the fault compared to the mean value

The length of the fault

The count of the yarn

The yarn density in the fabric

The type of fabric (weaving or knitting)

Judging the yarn evenness through visual examination

The yarn has to be wound on black boards with help of suitable mechanical device and compared with the standard yarn appearance board for Grade (ASTM standard Yarn Appearance board).

YARN EVENNESS

YARN APPEARANCE BOARD WINDER

ASTM standard test method describes the yarn appearance into five grades – Grade A, B, C, D & E.

The appearance grade of yarn is based on the composite evaluation of several factors, such as unevenness, fuzziness, and neppiness.

YARN EVENNESS

STANDARD YARN BOARD

YARN EVENNESS - ASTM GRADES

Grade A:

No large neps, very few small neps, must have very good uniformity, less fuzziness.


Grade B:

No larger neps, few small neps, less than 3 small pieces of foreign matters per board, slightly more irregular and fuzzy than A.


Grade C:

Some larger neps and more smaller neps, fuzziness, foreign matters more than B, more rough appearance than B.


Grade D:

Some slubs (more than 3 times diameter of yarn). More neps, larger size neps, fuzziness, thick and thin places, foreign matters than Grade C yarn. Overall rougher appearance than C.


Grade E:

Below grade D, more defects and overall rougher appearance than grade D yarn.

The grading the yarn evenness based on visual examination lost its relevance with advent of instrumental grading, which is less prone to judgmental errors and the results are repeatable.

It is now become normal and more acceptable trade practice to declare the instrumental values, such as U% & CLASSIMAT values.

YARN EVENNESS -INSTRUMENTAL

GRADING

There are two methods to numerically express irregularity or unevenness :

1.The Irregularity U%2.The Coefficient of Variation ( CV% of

Count).

Note: I had already explained the influence of count CV% in earlier PPT


VALUES

• Obviously the higher the CV% value, the more irregular the yarn is.

• Mass variations in a yarn CV% were measured by using the Uster evenness tester.


VALUES

Uster Technologies AG, Switzerland (USTER) created a numeric value to describe the level of yarn unevenness, the so-called U%, deriving from the German word ‘Unregelmässigkeit’, which means irregularity.

USTER %

Uster Eveness in % (U%)

Uster% (U%) is a measure of unevenness of yarn.

Actually, this is a measure of variation in weight (in gms) of approximately 1 cm. of yarn.

U%, the statistical value representing the percentage variation of a yarn.

Lower the value the better the yarn, because less imperfections exist in the yarn.

USTER %

Infrequent yarn faults are long thick places, long thin places, thick ends, spinners doubles, etc. and are rare and hence a longer length of yarn needs to be analysed.

The classimat system enables a quick and objective estimate of these types of faults.

USTER® CLASSIMAT

The Uster Group is the leading high-technology instrument manufacturer of products for quality measurement and certification for the textile industry.

In 1968 USTER launched USTER CLASSIMAT, which classified yarn defects into classes – the CLASSIMAT values

USTER® CLASSIMAT

The instrument “USTER CLASSIMAT” classifies /categorize the yarn defects according to their size and length into 23 standard class and designated as A0, A1, A2, A3, and so on ….

The latest instrument “USTER CLASSIMAT 5” detects and quantifies unevenness for periodic faults, evenness, imperfections and hairiness, in addition to critical thick and thin places.

USTER® CLASSIMAT

Now a days, the classification of yarn defects according to their size and length into 23 standard classes is used extensively to certify yarn quality ( and to compare the quality of different suppliers).

As a buyer, we need to determine & define the values to a specific amount and range, in which the yarn tests must match.

USTER® CLASSIMAT

The 23 classes of yarn faults classified according to their length and cross-sectional size is given here

USTER® CLASSIMAT

USTER® CLASSIMAT

Here is the pictures of the faults corresponding to the previous chart

Classimat Faults refers to no. of faults / 100km (CMT3)

Objectionable faults/100 km CMT3(H2+I2)/100 km CMT3(E+F+G)/100 km CMT3(i.e. Imperfection / km) UT4

Similarly other imperfections like Thin place, Thick place and Neps also quoted per Km (UT4)

Objectionable faults are: (A4B4C3D3C4D4)(It is the sum of tested yarn values)

CLASSIMAT FAULT

EXAMPLE OF SPECIFICATION

It is impossible to set out general rules or directives to determine from yarn test results how the final fabric structure will appear - because of the decisive influence of subsequent yarn processing stages.

In addition, the woven fabric structure, the yarn count in weft and warp, the number of picks and ends down and the knitted fabric structure are all variables which affect fabric appearance. In a similar way, the dyeing and finishing processes also have a major impact on fabric appearance.

YARN EVENNESS

However, it is proven that testing of some of the most important physical characteristics of a yarn can give a good indication of the appearance of the finished fabric.

For example, a very uneven yarn can never result in a perfect fabric, at least as far as appearance is concerned.

YARN EVENNESS

One can optimize the costs in the supply chain by defining the specific needs with respect to quality parameters according to the quoted values, compare yarns from different suppliers.

With this information in hand, we can optimize the portfolio of the yarn producers and classify them into quality groups & pay the right price for the right quality.

YARN EVENNESS

• The retailer will always be aware that consumers expect a certain quality level, which is related to the brand image and reputation.

• Implementing Yarn Quality Profiles into the sourcing process is very important in reducing quality costs and strengthening the brand reputation.

YARN EVENNESS

Knowing the basics of raw material, yarn production process and the other factors influencing quality will put the sourcing manager at the same eye level as a spinner /supplier when negotiating quality issues.

As a consequence this puts the sourcing manager in the position to pay the right price for the corresponding quality level.

This kind of know-how supports a retailer enormously in his efforts to establish a reliable supply chain which is based on mutual understanding.

UNDERSTANDING YARN QUALITY – A WAY TO

PROFITABILITY

Note: This is an internal training document

yarn quality part 3 yarn evenness

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