BY|B. PURUSHOTHAMA
INDRODUCTION
Nep is a tiny ball like substance produced by entangled fibre mass which is embeded in the yarn, and is very difficult to remove. The neps no t only give a bad appearance to the yarn and cloth, but also affect the dyeing quality. The neppy yarns give raise to problems like breaking of needles in sewing and knitting operations. The cotton neps would cosiderably mar the appearance of the fabric, particularly those dyed in contrast shades, in case of Polyester Cotton suiting.
It is therefore essential to have a target of nep-free yarn of course, by maintaining productivity and with minimum acceptable cost. According to Uster Manual, the number of neps is influenced to a large extent by the raw material used. # 2
However, the neps generated are also influenced to a significant effect by the process parameters and the condition of the machinery. Enormous research has been made on this problem of neps and a number of valuable articles have been published However, due to one or the other reasons, the problem of neps is increasing every year in our country. In this article, an attempt has been identify the real reasons for neps and to find out remedial actions. It is also a fact that the consumers are also becoming more quality concious because of the high priced fabrics. GENERATION OF NEPS
The neps are generated when fibres are strained in an-uncontrolled region allowed to curl.
Very fine and immature fibres, especially when the staple length is more, are prone to neppiness. The quality parameters such as surface finish, fibre crimp etc., which determine the loading behavior of polyester fibres on card wires, influence the generation of yarn faults like heds. # 3
The unginned cotton does not contain neps, where as the neps are seen in ginned cotton, it clearly shows that the fibers get ruptured because of the harsh mechanical action in ginning process. Similarly at the Blow Room the harsh beating excessive bends in conveyor pipes, projected surface inside the pipes etc. are responsible for the generation of neps. It is often see that hand opened cotton when fed to card directly gives very few neps compared to cotton processed through blow room.
The blunt wire points, improper settings etc. are responsible for generation of neps in carding. Factors in processing, which cause thick places and neps are also likely to be responsible for long length faults (2 cm and above) in the case of Polyester Cotton yarns, but not in the case Of cotton yarns. # 3
All the above facts are well established and known to almost all. Inspire of this the neps are still playing havoc in the industry. The probable reasons are as follows.
1) Increasing costs of cottons, polyesters viscose staple etc. which have forced the mill managements and cotton grower’s for the following actions.
(a) Early picking of cottons, without allowing the fibers to mature.
(b) Adding of more soft wastes, including the recovered short fibres from Droppings etc. The fibre recovery units are becoming more popular now a days.
(c) Using of Filament Cut wastes, Toe Cut wastes and other purchased wastes in polyester blends and in other manmade fibres
(d) Restriction in the Salable Soft wastes extracted in Spinning preparatory processes.
2. Increase in Labor Costs, which has resulted in:
(a) Reductions of hands at all process even with sacrificing the house keeping, machinery maintenance etc.
(b) More concentration on productivity at all places, including that of blow room carding and draws frames, which are key points in the maintenance of quality.
3. Increase in Stores/Capital Costs, which contribute for delays in maintenance aspects like re-clothing of cards, replacement of worn out parts etc. Installation of a good blowroom or Comber is always given last preference when compared to Ring frames, because of their low rate of returns on investment very often low quality spares including reconditioned are purchased because of their low price compare to standard items.
Whatever may be the circumstances, the technician working is supposed to try his level best for the reduction of neps and give a good appearance for the yarn spun. A number of trials have been conducted in recent years for the reduction of neps. The results of some of the successful trials have been explained here with.
BLOW ROOM
Blow-Room contributes significantly for the generation of neps. Hence a control on Blow-Room helps in reducing the neps in yarn.
There is a practice in a number of mills to spray water and hygroscopic agents to cotton mixing. In the case of manmade fires, they are tinted by spraying aqueous solution of fugitive dye. It should be ensured that the materials fed to Blow-Room should be thoroughly dried, or else it gives problem of chocking up, which finally results in neps. The dyestuff used for tinting should be selected in such a way that it does not give raise to lapping and loading troubles in carding and further process.
In some mills, L type Bahnson Fans are used for humidification. These fans, as they are installed with in the working area attract fluff. A lot of fluffs are accumulated in the fans and in water nozzles, which does not allow the water to atomize This results in the nusting of metal parts and accumulation of fluff, which become hard pressed. The rusted parts, burns and projected portions contribute for neps. It is suggested to use 15 Air Washer plants for humidify hation and not the L type fans.
The pipe lines used for conveying cottons should have minimum bends, and also the length of the pipes should be minimum.
The reduction in beater speeds reduce neps to a certain extent. However, a very low beater speed (less than 650 RPM) results in less opening of cottons, which in turn results in loading of Cylinder and flats in cards.
The use of Krischner beater reduce neps when compared to the three bladed beater at the some speed, provided the wire points are in a good condition.
The step Cleaner, Porcupine opener and Axi-flow cleaner - increases neps where as Crighton opener does not contribute for neps to that extent.
In the case of blended material containing Polyester and cotton, the blow - room blending gives consistently better performance in carding because of the absence of Cylinder loading. This reduces neps. However, with finer and immature cottons, - double treatment given for cot tons increases the number of neps.
Preopening and stacking of Polynosic fibres before processing in blow-room, helps in reducing the neps compared to the feeding of fibres directly from bales.
A coarse lap hank even though gets lesser beats per fibre, results in the loading on Cylinder wire, especially for finer cottons. Similar is the case with Polyester blends.
Lesser Calendar roller pressure, which contributes for-lap licking while unwinding, gives raise to double laps and subsequently loading on Cylinder at cards.
CARDING
Carding is a very important operation in the processes of spinning, and it is also a major contributing factor for the control of neps. The effects of poor carding cannot be evened out by combing or by using a good drafting unit.
The profile characteristics of wire points are to be rightly chosen in relation to fibre fineness, maturity, trash and length. Very often, the right type of clothing cannot be used for a given material because most mills process multiple counts.
SITRA conducted studies comparing flexible card clothing with metallic card clothing; which ruled out the thinking that flexible clothing gives better quality compared to metallic-clothing for fine counts. It claims that for fine and superfine mixing also, metallic clothing can yield as good a quality as that with flexible clothing, with some savings in waste, provided the choice of clothing, setting, speeds etc. are properly done. # 4
They also claim that the fibre arrangement in card silver improved with metallic clothing compared to the flexible clothing. Further improvement was noticed while using finer rib metallic wire.
Fine wire with more number of teeth per sq. inch gives a better web quality with fewer neps for both High speed and semi High speed cards in all counts. #5
The neps generated by the fine wire even at highest production rate was claimed to be lower than the neps for lowest production rate with coarse wire in all counts. # 5
The improvement with fine wires was attributed to the better fibre separation achieved with more points per unit area on the Cylinder surface.
SITRA also compared the indigenous metallic wire with imported wires, and concluded ^ that the profile characteristics were more important than the make or origin of the wires. It was found that there was no significant difference in neps and foreign matters in the card web between indigenous and imported wires, while the wire profile characteristics were maintained same. # 5
For a given card web quality, finer wire is preferable to achieve higher level of production. However, for trashy cottons, coarse clothing is preferred though it has less fibre penetration and less carding action, only to avoid loading of fibres and trash particles. In the case of synthetic fibres like Polyesters, even though they do not contain any trash, the coarse wires are recommended by both fibre manufacturers and card clothing manufactures. Lakshmi recommend their Matta Wires and ICC their 212 wire, which are both coarse wires and meant for coarse and trashy cottons. #6, 7
It is a common finding that higher production rates at carding give rise to more imperfections in the yarn. The studies conducted by Shri. Y. B. Maliikarjunappa 8 showed that for same hank silver, even with newly mounted wire, the nep counts were on HP and SHP cards compared to conventional cards. This was attributed to the coarse hank in the case of HP and SHP cards in web, and the final hank of silver was adjusted by higher tension drafts in HP and SHP cards. The nep per 100 metres of yarn was claimed to be significantly lower with the material produced on HP cards and SHP cards. However, no mention was made on actual productions.
SITRA's studies indicate that even though the higher production rates at carding increased imperfections in yarn, it did not influence the lea strength and unevenness much. 5 #
However in our studies, it was found that in the case of long staple fibres, the doffer speed as it increased, resulted in higher end breakages in Ring frames and also a drop in lea strength.
Removal of more flat strips by way of increased flat speeds helped in reducing the neps, however the reduction in neps was not that significant when the flat strips were increased by adjusting the front plate setting.
The tandem card gives very significant improvement in yarn quality, particularly with reference to neps and foreign mattes.
The tandem card, when compared to high production card reduces yarn faults, particularly of the shorter length. 3 #
Fibre retriever and comb segment in lickerin undercasing, which are alternative to mote-knives, are claimed to be more effective than mote-knives. Comb segment in lickerin undercasing reduced neps to a considerable extent in our studies.
A powerful dust extraction unit and pits below carding machines for the collection of dust helps in maintaining the cards in a cleaner way, which reduces fluff entering the web. These fluffs, even though not noticed on card web, are seen boldly on the yarn surface, which gives a nep like appearance. A circular nylon brush on the doffer surface in preferred where suction units are not fitted.
Roller doffing devices, which are replacing doffer combs, must be accompanied with a good suction cleaning unit, or else it is found to damage the doffer wires by the fluff balls generated.
Reduced lickerin speeds are found to reduce neps in manmade fibres. In case of cottons, a very low speed of licker in gives raise to Cylinder loading and flat loading because of less opening. It is found that a higher lickerin speed breaks the immature fibres, and makes them either fall or to out along with flat strips. Trials with negative wires for fine cotton increased neps at all speeds, compared to normal wires
For very fine denier Polynosic fibres (1DN-51mm), removing of alternate row of Lickerin wires, and a speed of 450 RPM, gave a considerably good performance from the neps point of view.
Removal of one grid bar in Lickerin under-casing helps in removing the short fibre and trash particals, which reduces the neps. Similarly lickerin undercasing with large diameter perforation helped in reducing the neps level.
Neps were found to be minimum immediately after stripping and was found to increase gradually after 6 to 8 hours working, even though, no loading was noticed on the Cylinder surface. The same phenomena was found with manmade fibres also. It is therefore advisable to strip the cards once in a shift irrespective of the loading problems.
Neps were also found to reduce after grinding of the wires.
Long nose feed plates are recommended for long staple fibres to avoid fibre rupture and subsequently neps. Shri. G. V. Anturkar #9 Suggested a modification in short nose feed plate so that it could be used for long staples also. No further work has been reported in this regard.
Wider width flats are being tried for long staple fibres however, the detailed reports are not yet available.
Modification in lickerin zone are being tried more for reducing neps. The loading developments include Double Lickerin system, Additional cleaning roller on Lickerin, fibre retriever etc.
It was found that the neps were generated at the soldered portions of metallic clothing at the sides of Cylinder and doffer. It is therefore suggested to keep lap width less by 3" compared to card width.
POST CARDING OPERATIONS
It is observed in a number of cases that the neps found on yarn surface are considerably more compared to the neps found in the care web, which clearly indicate that there are some other factors which also contribute for the generation of neps.
It was found that higher speeds at draw-frame and closer settings in superfine cottons generate neps or give neppy appearance to the yarn.
The clearer wastes, if not removed continuously, also give a neppy appearance to the yarn.
Combing helps in reducing the neps in yarn by removing the neppy portion along with comber noil, However the damaged needles of Cylinder and top Combs generate neps
ATIRA'S studies # 2 show that the type of drafting system did not affect the neps as well as the yarn appearance
BTRA'S studies # 10 indicate that the reduction of apron spacing improved the evenness and imperfection count of arn in all mixings.
Broken condensers, burrs inside the flyers damaged aprons etc. also contribute for neppy appearance of the yarn.
CONCLUSION
The problem of neps is getting more importance now a days however the problem is also increasing due to various reasons.
A proper selection of fibres and processes helps in reducing the problem of neps.
REFERENCES
1 Selection of Indian Cottons for Polyester Blends by B. R. Dole - 31st All India Textile Conference.
2 How Fibre Properties & Processing Machines influence imperfections in Cotton Yarns - By A. R. Garde and J. Sethi - JTA - June 1970.
3 Infrequent Yarn, Faults, their incidence, causes and Remedies - By K. Kumaraswamy and I. Sharieff.
4 Carding- Still Universal area for improving yarn quality By M. Ramachandran & R. Sripathi -35th All India Tex. Conference.
5 Quality and Performance of Metallic card clothing for higher production- By R Sripathi - JTA Oct - 1975.
6 Booklet of Lakshmi Card Clothing Co. Ltd. Coimbatore.
7 Booklet of Indian Card Clothing Co. Ltd. Poona.
8 Nep count on card web - By - Y. B. Malli-karjunappa JTA - Jan. 75.
9 Modification in feed plate to improve yarn appearance by - G. V. Anturkar JTA - Jan. 1979.
10 Effect of input Hank, Roving Twist and Apron Soaking upon yarn quality and Ring frame and breaks - By Dr. N. Balasubramaniam & B. N. Bhanot - JTA Oct. 1974
(Nguồn Textitleknowled4u)
