YARN EVENNESS - 2
DETERIORATION IN EVENNESS DURING PROCESSING:
In processing in the spinning mill, the unevenness of the product increases from stage to stage after drawframe. There are two reasons for this
- The number of fibres in the cross section steadily decreases. Uniform arrangement of the fibres becomes more difficult, the smaller their number.
- Each drafting operation increases the unevenness
Each machine in the spinning process adds a certain amount to the irregularity of finished yarn. The resultant irregularity at the output of any spinning process stage is equal to the square root of the sum of the squares of the irregularities of the material and the irregularity introduced in the process.
Let us assume that,
CVo - CV of output material
CV1 - CV of input material
CV - irregularity introduced by machine
CVo = sqart(CV1 + CV)
UNEVENNESS OVER DIFFERENT CUT LENGTHS:
A length of yarn, for example of 10mm, contains only few fibres. Every irregular arrangement of only some of these fibres has a strong influence on the unevenness. In a length of yarn of 10m, incorrect arrangement of the same fibres would hardly be noticed against the background of the large number of such fibres. Accordingly, the CV value of the same yarn can be, for example, 14% based on, 8mm, and only 2% based on 100 m. The degree of irregularity is dependent upon the regerence length.
Unevenness is therefore discussed in terms of short lengths(uster tester):medium lengths(seldom used):long lengths(count variation).
Fabric stripiness and barre have been problematic fabric defects in the textile industry for many years. Though direct quantification has not been possible, the causes for such fabric defects have been studied. It has been shown that raw material quality and yarn mass variations (particularly medium and long term variations) contribute significantly to the guidance of such faults. Of these causes, there has been a general neglect of the control of medium term variations (variations over 1m, 3m,10m, etc). A mill needs to control the cut length variations of the yarn produced in order to ensure a fault free fabric.
If the variation of cut length C.V.% of 1 meter, 3 meters, 10 meters is high , when different cops are tested , the fabric appearance will be very badly affected. It will result in fabric defects such as stripiness.
Yarns spun from staple fibres contain "IMPERFECTIONS" . They are also referred to as frequently occurring yarn faults. They can be subdivided into three groups
- Think places
- Thick places
The reasons for these different types of faults are due to rawmaterial or improper preparation process. A reliable analysis of these imperfections will provide some reference to the quality of the raw material used.
Thick places and thin places, lie in the range of +-100% with respect to the mean value of yarn cross-sectional size.The Neps will overstep +100% limit.
Thick places over +100% are analysed by the CLASSIMAT system, are cut by the clearers in winding depending upon the end use of the yarn.
Imperfection indicator record imperfections at different
- Thin place
- -30% : yarn cross section is only 70% of yarn mean value
- -40% : yarn cross section is only 60% of yarn mean value
- -50% : yarn cross section is only 50% of yarn mean value
- -60% : yarn cross section is only 40% of yarn mean value
- Thick place
- +35% : the cross section at thick place is 135% of yarn mean value
- +50% : the cross section at thick place is 150% of yarn mean value
- +70% : the cross section at thick place is 170% of yarn mean value
- +100%: the cross section at thick place is 200% of yarn mean value
- 400%: the cross section at the nep is 500% of the yarn mean value
- 280%: the cross section at the nep is 380% of the yarn mean value
- 200%: the cross section at the nep is 200% of the yarn mean value
- 140%: the cross section at the nep is 140% of the yarn mean value
Thick places and thin places which overstep teh minimum actuating sensitivity of +35% and -30% , respectively, correspond to their length to approximately the mean fibre length. Medium length or long thick and thin places are to be considred as mean value variations and are not counted by the instrument.
The standard sensitive levels are as follows
- Thin place : -50%
- Thick place : +50%
- Neps : 200% ( 280% for open-end yarns)
The reason for reducing the sensitivity of nep counting in rotor spun yarns is due to the fact that with these yarns, the neps tend to be spun into the core of the yarn and therefore are less visible to the human eye in the finished product. With ring spun yarns, on the other hand, the neps, in general tend to remain on the surface of the yarn. Due to the above reasons, while a nep is considered serious
for a ring spun yarn even if its size exceeds +200%, it becomes serious only after its size exceeds +280% for open end yarns.
It is however worth mentioning here that, though the imperfection values at standard sensitiviy levels i.e. +50% for thick places and -50% for thin places indicate the acceptable quality levels in terms of fabric appearance, the quality of processing in terms of optimization of process parameters will be better indicated by imperfections at higher sensitivity levels. It is commonly observed that while the thin places may be '0' for any two mills at the standard sensitivity level of -50%, the thin places at -40% sensitivity may show a big difference.
Thin places and thick places in a yarn can, on the one hand, quite consdierably affect the appearance of a woven or knitted fabric. Furthermore, an increase in the number of thin places and thick places refer to a particularly valuable indication that the raw material or the method of processing has become worse. On the other hand, it cannot be concluded from the increased number of thin place faults that this yarn, the downtime of weaving or knitting machines will be increased to a similar degree.
Thin places usually exhibit a higher yarn twist, because of fewer fibres in the cross-section resulting in less resistance to torsion. The yarn tension does not become smaller proportionally with a reduced number
of fibres. With thick plalce faults the contrary is the case. More fibres in the cross-section result in a higher resistance to torsion. Thic places have therefore, in many cases, a yarn twist which is lower than the average. The yarn tension in the yarn at the position of the thick place is only in very few cases proportional to the number of fibres. These considerations are valid primarily for ring-spun yarns.
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