WINDING - 2
YARN FAULTS AND CLEARING:
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 ringspinning, it is necessary to have yarn monitoring system in the last production process of the spinning mill. As physical principle for electronic yarn clearing the capacitive and the optical principle have established. Both principles have their advantages in specific applications.
Depending upon the rawmaterial, the machiery 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 deisred 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, foregin fibres and diry places in the yarn.
The yarn faults which go into the woven or knitted fabric can be removed at very high costs or can not be removed at all. Therefore the yarn processing industry demands a fault free yarn.
The difference between frequent yarn faults and seldom occuring yarn faults are mainly given by the mass or diameter deviation and size. These faults are monitored by classimat or clearer installation on winding.
Each yarn contains, here and there, places which deviate to quite a considerable extent from the normal yarn corss-section. These can be short thick places, long thin places , long thick places or even spinners doubles. Eventhough such events seldom occur, they represent a potential disturbance in the appearance of the fabric or can negatively influnece subsequent processing of the yarn.
Short thick places are those faults which are not longer than approximately 8 cms, but have a cross-sectional size approx. twice that of the yarn. These faults are relatively frequent in all spun yarns. To an extent they are the result of the rawmaterial ( vegetable matter, non-seprated fibres, etc). To a much larger extent, these faults are produced in the spinning section of the mill and are the result of spun in fly. Short thick places are easily determinable in the yarn. In many cases, they cause disturbances in subsequent processing. Once they reach a certain size( cross-section and length) , and in each case accoridng to the type of yarn and its application, short thick place fults can considerably affect the appearance of the finished product.
Long thick places are much more seldom-occuring than the short thick places and usually have a length longer than 40cms. In some cases, their length can even reach many meters. Their cross sectional size approx. + 40% to +100% and more with respect of the mean cross-section of the yarn. Long thick places will affect the fabric apperance. 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.
Thin places occur in two length groups. Short thin places are known as imperfections, and have a length approx. three times the mean staple length of the fibre. Their frequency is dependent on the rawmaterial and the setting of the drafting element. They are too frequent in the yarn to be extracted by means of the electronic yarn clearing.
Long thin places have lengths of approx. 40cms and longer and a cross-sectional decrease with respect to the mean yarn cross-section of approx.30 to 70%. They are relatively seldom-occuring in short staple yarns, but much more frequently-occuring in long staple yarns. Long thin faults are difficult to determine in the yarn by means of the naked eye. Their effect in the finished product however, can be extremely serious.
The quite extensive application of electronic yarn clearing has set new quality standards with respect to the number of faults in spun yarns.
It is therefore necessary to evolve a method of yarn fault classification before clearing the faults in winding. The most important aspect is certainly the determination of the fault dimensions of cross-sectional size and length. With such a cross-section and length classification and by means of the correct choice of the class limits, the characteristic dimensions of the various fault types can be taken into consideration, then a classification system will result which is suitable primarily for satisfying the requirements of yarn clearing and yet allows, to quite a large extent, for a selection of the various types of faults.
The yarn faults are classified according to their length and cross-sectional size, and this in 23 classes.
FIG: CLASSIMAT FAULTS:
- The cross-sectional deviations are given +% or -% values. i.e theupper limit, respectively , lower limit with respect to the mean yarn fault cross-section is measure in %. The fault length is measured in cms.
FIG: YARN CLEARING CONCEPT OF USTER QUANTUM CLEARER
N - NEPS
S- SHORT FAULTS
CCP - COARSE COUNTS
The classes and their limits are set out according to the following:
- Short thick place faults: 16 classes with the limits, 0.1 cm, 2cm, 4cm, and 8cm for the lengths and +100%, +150%,+250%, and +400% for the cross-sectional sizes are provided. The classes are indicated A1...D4. The classes A4, B4,C4,D4 contain all those faults, according to their length, whose cross-sectional size oversteps +400%.
- spinners doubles: This refers to a class (with the indication E) for faults whose length oversteps 8cms and whose cross-sectional size oversteps +100 ( open to the right and upwards)
- Long thick place faults and thick ends: The long thin place faults are contained in 4 classes with the limits 8 cms and 32 cms for the lengths, and -30% , -45% and -75% for the cross-sectional sizes. The classes are designated H1.....I2. The classes I1 and I2 are open to the right. i.e they contain all those thin places having a size between -30 and -45%, respetively, -45% and -75% and whose lengths are longer than 32 cms. The classification of the shorter thin places is of no advantage in the analysis of the seldom-occuring faults.
FIG: A DIAGRAM FROM LOEPFE YARN CLEARER MANUAL
Types of Electronic Yarn Clearers
Electronic Yarn Clearers available in the market are principally of two types –capacitive and optical. Clearers working on the capacitive principle have ‘ mass’as the reference for performing its functions while optical clearers function with ‘ diameter’ as the reference. Both have their merits and demerits and are equally popular in the textile industry. Besides the above basic difference in measuring principle, the basis of functioning of both the types of clearers are similar if not exactly same. Since most of the other textile measurements like, U% / CV%, thick and thin places etc., in various departments take into account mass as the reference parameter, the functioning of the capacitive clearer is explained in some detail in the following sections.
The yarn is measured in a measuring field constituted by a set of parallely placed capacitor plates. When the yarn passes through this measuring field (between the capacitor plates), an electrical signal is produced which is proportional to the change in mass per unit length of the yarn. This signal is amplified and fed to the evaluation channels of the yarn clearing installation. The number and type of evaluation channels available are dependent on the sophistication and features of the model of the clearer in use. Each of the channels reacts to the signals for the corresponding type of yarn fault. When the mass per unit length of the yarn exceeds the threshold limit set for the channel, the cutting device of the yarn clearer cuts the yarn.
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