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Friday, January 23, 2015

Combing Faults with Causes and Remedies

Different faults and their possible causes in comber

Cutting across
This fault indicates the thick and thin bars which occur from the one side of web to the other. The possible causes are –
  • Incorrect roller setting, excessive draft, roller slippage or loose drafting rollers at the sliver and ribbon lap formers; lap former and comber drafting rollers.
  • Incorrect timing of the detaching roller cam.
  • Too deeply setting of the top comb.
  • Top comb touching the back detaching roller.
  • Lack of lubrication of top detaching roller.
  • Stretching of the web in the condensing plate.
Combing faults
Detaching roller lapping
The probable causes are –
  • Oil on roller.
  • Higher relative humidity percentage.
  • Improper weighting of the top roller.
  • Sticky dirt on roller.
The following are the probable causes –
  • Incorrect timing of the detaching cam.
  • Faulty top detaching roller covering.
  • Dirt in the flutes of steel detaching roller.
  • Air current originating in the aspirator, excessive brush speed or too low  fan speed
  • Incorrect atmospheric conditions; air may be too dry or too humid.
Longer fibres in the waste
The possible causes are as follows –
  • Defective back covered detaching roller.
  • Foreign matters wedged between the nipper jaws.
  • Faulty nipper setting.
  • Presence of tailing hook in the fibres is a major cause.
  • If the already detached fringe is fed back into the last raw of cylinder needles during backing off.
  • If the detachment setting is wider than the required length of fibres.
Waste flocking on comber needles
The causes are –
  • Damaged brush.
  • Hooked comber needles.
  • Brush set too far from cylinder.
Lap running stack on wooden lap rollers
  • The pawl operates on too many teeth in the ratchet wheel on the wooden roller.
  • The feed roller ratchet pawl operates on less no. of teeth.
Cotton not combing through at one head
The causes are –
  • Improper gearing of the pawl with the feed roller ratchet.
  • Incorrect nipper setting or foreign matters wedged between the nipper jaws.
  • Back covered detaching roller requiring lubrication.
Damage of the combing needles
The probable causes are –
  • Incorrect setting of comb with the adjacent parts i.e. too close to catch.
  • The making of a thick overlay when a new lap is being put in for combing.
Short fibres in all slivers
The probable causes are –
  • Poorly oriented fibres and hooked fibres which cause running of short fibres with the longer fibres and fibres breakage.
  • Damaged needles.
Irregular draw box sliver
The causes are –
  • Unequal weighting of top drafting roller.
  • Incorrect distribution of draft in the drafting zone.
  • Unequal setting of detaching roller, rocking mechanism.
  • Neps in the sliver.
  • Tension draft.
  • Grooved or worn roller covering.
Combing faults remedies
The remedies to the causes of various faults in combing are opposite. Steps i.e. if the fault is due to the incorrect setting; the setting should be corrected, if for improper lubrication, it needs correct lubrication and if for damage needles, they should be replaced and so on.

Thursday, January 22, 2015

Actions take place in Nasmith comber during combing

The following actions take place –
During combing by the cylinder
  • Nippers are closed
  • Nippers begin to rock backward as the cylinder combing is completed.
  • Feed rollers are stopped.
  • Top comb is lifted.
  • Detaching rollers are stopped holding previously by combed fringe of cotton ready for attaching.
During attaching
The nippers move forward with the combed fringe for piecing up with the early combed fibres to maintain continuity of strand.
  • Nippers open and rock forward to carry the fringe into the nips of the detaching rollers.
  • Feed rollers are feeding.
  • Top combers begin to drop down wards to enter into the fringe.
  • Detaching rollers turn backward holding the previously combed materials attaching.
During combing by the top comb
  • Nippers are in forward position and opened.
  • Top comb down into the fringe.
  • Detaching rollers delivering the fringe by drawing the fibres through the top comb.
  • Feed rollers are stopped.
During detaching
When the detaching rollers move backward with the previously combed fringe and become ready for next cycle is called detaching.
  • Nippers rocking back and also the top comb.
  • Nippers are opened but are about to close when detaching is completed.
  • Detaching rollers are stopped.
  • Top comb is about to lift out of the fringe.
  • Feed rollers are stopped.

What is combing? Describe the working principle of cotton combing

Definition of Combing

Removing short fibres, neps and impurities by using combs is called combing.
It is the process of separation of long desirable fibres of same length from the short fibres, neps and all remaining foreign materials of the fibre stuck. The fibres also become straight and parallel.

Working principle of cotton combing

All the combers operate intermittently, both ends of the fibres are combed separately by needles and the continuity of the strand is maintained by an ingenious method of piecing up the separated combed tufts, shortly, the process is operated as follows –

Combing machine
  • The lap prepared for combing is fed into the machine between the feed roller and the bottom nipper. At the front of the nipper, the top nipper is positioned to gripe the lap as it passes to be combed at the front end of a fringe.
  • The protruding fibres beyond the nipping point are combed by the passage of raws of needles fastened of a cylinder and situated below the lap. The cylinder with the needles revolves and carries away the short fibres, neps, and impurities from the front part of the fringe. The wastage are removed from the cylinder needles by a brush and collected around an aspirator.
  • When the cylinder comb completes its action, the top comb then comes down to perform its action at the back part of the fibre fringe. Meanwhile, the partly combed fibres are withdrawn through the top comb by detaching rollers.
  • When all the combed fibres have been withdrawn by the detaching roller, the top comb is withdrawn and the cylinder comb operates on new sets of fibres as before.
  • The top comb operates again but before the rollers draw away the newly combed fibres, then turn backwards a little so that, the new fibres can be over lapped with those withdrawn in the previous combing and in this way the continuity of the combed web of fibres is maintained.
  • The combed web delivered by the rollers is passed through a trumpet to consolidate the fibres and to make a sliver which is pulled along a table alongside other slivers from the other combers delivered by the same machine. 6 or 8 slivers are drawn into a single sliver by a drafting mechanism at the end of the sliver table and passes into a sliver can.

What are the Contributions of Combing of fibres on yarn quality?

Contributions of Combing of fibres

Combing process is quite expensive one and adds to the value of yarn by –
  • Improving the uniformity and strength.
  • Improving the spinning value.
  • Improving the smoothness and luster.
  • Reducing the neps in yarn.
  • Reducing the hairiness in the yarn.
  • Producing clear yarn.
With the removal of short fibres shorter than the predetermined length the average length of fibres are increased and there by the uniformity ratio which results in better control over the short fibres during the next drafting operation. Thus it results in production of more uniform sliver, roving and ultimately the yarn. The combing operation normally adds 1/8” to the average length of cotton depending upon the noil extracted. A card sliver with fibre average length of 0.97” produced a combed sliver with average length of fibres of 1.127” after extracting 5.8% noil at the comber. Therefore, it increases the average length of fibres, uniformly of the yarn and reduces the weak places in the yarn. Thus increases the C.S.P value depending upon the noil percentage removed.

Combing of fibres

Increase average length of cotton means, it is possible to spin higher comb for the same cotton. The straightening and parallelization of the fibres cause to lay in a regular pattern in the yarns. As the short fibres have the tendency to migrate toward the surface of the yarn is improved. This also results in the production of yarn with smooth and lustrous surface.
Since all the fibres are straightened and parallelized and all foreign matters presented in the card sliver are removed at the comber, thus it produces a much clean yarn with less neps. Another improved property that is affected is the improvement in the breaking elongation % of the yarn.
With the removal of short fibres, neps, foreign matters and production of even sliver and ultimately yarn the efficiency at the subsequent operation is greatly improved due to reduction in end breakage.

Wednesday, January 21, 2015

Conventional and modern lap preparation techniques

Sliver lap machine

20 carded slivers in cans are assembled at the back of the machine and led side by side along a polished table, passing through assisting rollers and a stop motion device. They enter the drafting section, which gradationally has three lines of rollers. The slivers are subjected to a draft of about 1.5. After being drafted, the sliver enters a calendar section where they are subjected to a considerable pressure. So, as to make a compact lap with projecting fibres pressed down well to reduce any bindings to licking. Finally in a lap forming section the slivers are wound on a wooden former under pressure. This section is similar to the lap forming unit in scutcher.

Lap former

Ribbon lap machine

In this machine the laps made from the sliver lap machine are drafted, the webs from sliver laps are superimposed, calendered and then wound on to a bobbin for creeling at the comber. The usual draft in this machine is six. Change places are provided for the usual draft alternation. Lap wt/yd is up to 700 grains, total lap weight is up to 20 lbs.  

Lap machine

The whitin super lap machine

The carded slivers are first processed in a draw frame. There are three sliver creels for one machine. Each with twenty cans of sliver. The sets of twenty ends are separated into two sets of ten as they pass through the detector rollers and brought together again in a manner which gives a ten ends width, but of double thickness, as they pass through a vertically arranged 2 over 3 drafting system. After drafting the webs are led through pairs of calender rollers and along a table, where they are superimposed calendered and made into a lap.
The draft used in the initial draw frame is 8 and 3.66 at the super lap machine. The total draft is 29.28 and the no. of doublings 480, both being far higher than those on the old system and this is the reason for improvement.
Lap wt up to 35 lbs
Wt/yd up to 1000 grains
Production up to 500 grains

Why Carded sliver are prepared for Combing? How they are prepared

Reasons of preparing carded sliver for combing

Fibres which are to be combed must be prepared in the form of lap which must be even in thickness and the fibre must be straight and parallel to the length of the lap. Fibres in the carded sliver are arranged in the manner that 16% fibres have leading hook, 48% tailing hook, and 13% hooked at both end and 21% have no hooks. Moreover, the fibres in the carded sliver are oriented in all directions. The use of this sliver for combing action is the detriment of good combing and lead to an excessive amount of waste percentage. The sliver in the rope form contains the fibre more or less in round form will damage the combing needles. So, the fibres are combed in the form of lap of approximately about 10.5” width and weighting 100 – 1000 grams/yd. If the comber lap is uneven in thickness, combing needle may damage, there may be presence of longer fibres in the waste and combed sliver will be irregular in weight/unit length. Due to the above reasons preparation of the carded sliver is must be combed.

Methods of preparation

Roller drafting is employed to make the fibres parallel and straight and doubling to achieve uniformly in weight per unit length. In the conventional process, sliver lap and then ribbon lap machine are used for lap preparation for combing. In the modern process, high draft supper lap former is used for the comber lap preparation.

Tuesday, December 10, 2013

What is combing? Objects and necessity of combing

Definition of Combing

The combing process is carried out in order to improve the quality of the sliver coming out of the card. The process eliminates short fibres, it achieves better parallelization of fibres, it straightens curls, and it removes neps and residue impurities. The combing process is essentially aimed at obtaining excellent quality yarns. Depending on what is being produced, waste from combing varies from 12% to 25%, and this can be employed to obtain yarns with a medium-coarse count using the open-end process.
So we can say that, the process of straightening parallelizing and the removal of short fibres and impurities by using a comb assisted by brushes and rollers is called combing.
Combing machine
                                                                Fig: Combing machine

Objects of combing

                     i.            To remove the fibre shorter than a pre determined length so as to enable the spinner to produce  finer yarn.
                   ii.            To remove remaining impurities in pre-comber lap, this helps in producing cleaner yarn.
                   iii.            To remove neps in the carded sliver.
                   iv.            To make the fibre more parallel and straight, so that the yarn becomes more even and lustre.
                   v.            Finally produce a uniform sliver of required weight per unit length and collect into a can in coil form.

Necessity of combing

1.    For finer count, high draft is required but draft irregularity for presence of short fibre. After combing short fibre free product (sliver/roving) is ready for higher draft.
2.    Longer fibres are finer than short fibres. After combing higher count is possible to keep minimum no of fibre in yarn dia.
Ring spinning system at least 60-65 no of fibre.
Rotor spinning system more than 80 no of fibre.
Length ↑→ Yarn dia
Length ↑→ Yarn dia
3.    Long fibre yarn has less hairiness but more luster.
4.    Combing is necessary for better yarn appearance and regularity.
5.    Less twist for finer yarn but more twist required in presence of short fibre to remove this.