Need for Lap preparation:

The combers are fed with a small lap produced by combining several slivers. The raw material delivered by the card is unsuitable for as regards both form and fiber arrangement. If card slivers were just combined and fed to the comber, true nipping by the nipper plates would occur only on the high points, with the risk that the nippers could not retain the less firmly compressed edge zones of the slivers asshown in the below fig. This is because the slivers are not flattened.

The fibres could then be pulled out as clumps by the circular combs during combing operation. A sheet with the greatest possible degree of evenness in cross section, with uniform thickness is therefore required as in-feed to the comber.

Effect of fibre presentation:

The fiber arrangement must also be taken into account, i.e. in this case the disposition of the hooks. If the comber is to straighten hooks, as it is intended to, then the fibers must be presented to it with leading hooks. The carded slivers have trailing hooks as the majority hooks (more than 50%) as the emerges out of the calendar rollers in the machine. Each time the sliver is packed in a can and taken out, the majority hooks change. For example, as the sliver is withdrawn from the card can, the original trailing hooks (as the sliver went into the can) are now counted as the leading hooks as can be seen in Fig. Hence, at this stage majority hooks are the leading hooks.

As it can be seen from the above Fig., having even number of processes in between card and the comber will ensure that the majority hooks as they are presented to combing are leading hooks. Importance of Pre-comb Draft:

Fiber parallelization is the result of the pre-comb draft. Greater the pre-comb draft, better fiber parallelization and less load on the cylinder as well as top comb needles. The degree of pre-comb draft has significant effect on the comber waste extracted.

Depending upon the length, pre-comb draft between 25 to 40 is found enough for effective fraction at the comber.

Excessive pre-comb draft is also avoided although it increase the degree of fiber parallelization. It develops lap licking and hampers smooth working of comber. The materials become soft and there will be excessive sliver breakage between front roller & coiler roller.

Classification of lap former machine:

A. Conventional system 1. Sliver lap/ m/c 2. Ribbon lap m/c B. Modern system 1. Super lap former. In general, two systems are still in use 1)Conventional web doubling process employs a sliver lap machine followed by a ribbon lap machine. 2)(Modern)sliver doubling process, in which a normal draw frame (without leveling) provides the first passage and a sliver doubling machine follows as the second passage . Sliver lap/doubling M/C: The lap is obtained by doubling a certain number of slivers (from16 to 32)previously subject to a drawing passage. The slivers are fed side by side, passing through rollers and stop motion. The slivers enter the drafting section and then calendar section to produce a compact lap. Finally the lap is wound on to bobbin. In the lap former. The material under goes a light draft of around 1.5 to 2 .

Fig. – Top view of a sliver lap machine

Fig. – Sliver lap machine Features of Lap Former Machine:

 Batt weight up to 80 gms/mt  4/4 drafting system  Integral top roller release  Auto spool changer  Positively driven creel  Lap transport through lap trolley Necessity of lap former:

1. To reduce fiber damage. 2. To reduce chance of good fibers waste. 3. To reduce short fibers. 4. To reduce thick and thin places in the sliver. 5. To control wastage. 6. To parallel and straight of fiber in carded sliver 7. Not freely opening of fiber from sliver.

Ribbon Lap Machine: In case of ribbon lap former machine, the laps made from the sliver lap machine are drafted. After that, the webs from sliver laps are superimposed, calendared and finally wound on to the bobbin for creeling at the comber. Here, the usual draft applied is six.

Working Procedure of Ribbon Lap Machine

 The lap from the sliver lap machine is creeled here. This lap is unwound and fed to drafting rollers.  Ribbon lap machine is actually used to increase weight per unit length and uniformity of the lap obtained from sliver lap machine.  In ribbon lap m/c, 6-8 laps are fed. They form a web of same width but more thickness then doubling and drafting is done.  To unwind the lap two wooden fluted feed rollers are used here.  Generally, three over three or four over four drafting system are employed in this machine.  After drafting the lap is emerged out in web form passes via deflector plate’s web tables and calendar rollers.  Here, two pairs of calendar rollers are employed to compress the lap.  Comber lap is obtained and it is rolled on spool bobbin. Two fluted rollers are responsible to obtain a compact lap.

Fig. – The ribbon lap machine – cross-section

Unilap Machine:

The idea of creating a comber feeding lap by a single web forming process directly in front of the comber using a draw frame passage, called the unilap machine.

The web former always follows a normal draw frame. On the Web Former machine the material flow starts with the creel, consisting of two feed rails. In normal operation 12 cans are laid out under each roller-assisted feed table.

Altogether, finally gives a total of 24 doublings. The pre-draw frame slivers run over a guide bridge above the service alley and also over several guide rollers to the drafting system .

The web created by the draft of 1.3 to 2.5 passes over two deflecting plates onto the web table on which the webs are superimposed. Calendar rollers draw these superimposed webs from the table to the lap winding assembly.

The strong compression created between the calendar rollers forms a new web, which is rolled into a lap in the lap forming assembly.

Empty tubes are automatically exchanged for full laps. Transport of the laps to the combing machine is semi- automatic or fully automatic.

:-comber -:

IMPORTANCE-:

The short fibres, shorter than 12 mm do not contribute to the strength of the , but they only increase the hairiness of the yarn and adversely affect the appearance and other good properties of the and the fabrics made out of them. In the staple fibre yarn manufacturing process, only the trash particles, foreign matters and very short fibres are removed in the blow room and carding operations. In these processes, it is not possible to remove fibres which are shorter than a predetermined length. In the combing operation, fibres shorter than certain fixed percentage of short fibres are removed. Since the shorter fibres are generally coarser, removing the shorter fibres leaves behind fibres which are relatively finer. Hence, fabric made from combed cotton fibres has a very soft touch.

Combed cotton is also stronger since shorter and weaker fibers have been removed through combing operation. The straightened fibers lie together in a more compact manner after combing, making combed cotton yarn less likely to fray and unravel. These aspects make the garments made with combed cotton yarns an excellent choice for garments .

Objectives :

1)Elimination of a precisely predetermined quantity of short fibers.

Elimination of short fibers improves the staple length of the combed cotton. The short fibres removed in the combing process is called as “”.

Noil percentage = (Weight of produced / Total weight of cotton fed) x 100

The amount of noils removed depends upon the industry and the needs ranging from 8 to 25%.

2)Elimination of the remaining impurities.

There is fibre-to-fibre rubbing and action of combing needles on the surface of fibres. These actions enable the release of fine dust particles still adhering on to the fibre surface.

3)Elimination of neps in the fiber material.

In the combing process, the neps get either removed or straightened due to the action of the combing needles or inter fibre rubbing and sliding. 4)Formation of sliver having maximum possible fibre parallization

It should also be noted that combing increases the parallelization of the fibers

Types of Combers :

Works with stationary or oscillating nippers and is mainly used in the short staple Rectilinear comber : .

Circular combers: Used in the English process.

Rotary comber: Used in the production of Schappe spun yarns

Hackling machines: Used in the processing of bast fibers.

Steps involved in Combing-:

Combing Cycle :

Feed rollers move the lap by 4-6.5 mm .

Upper nipper plate Z0 is lowered and the fibers are nipped

Combing segment k combs the fiber fringe and Carries away those fibers not held by nippers

Nippers open again and move towards the detaching rollers.

Detaching rollers rotate backwards and bring back a part of the material

The new fiber fringe is placed over the returned material .

The top comb thrusts its single row of needles into the fibe fringe.

Nipper assembly moves back. Top comb is withdrawn and new cycle begins. Combing segment is cleaned by the brush roller. Description-: Feed-:Feed rollers (S) move lap sheet (W) forward by a small amount (4.3 - 6.7 mm), while nippers (Zo/Zu) are kept open . Nipping-:Upper nipper plate Zo is lowered onto cushion plate (Zu) so that the fibers are clamped between them . Rotary combing-:Combing segment (K), mounted on rotating cylinder (Z), sweeps saw-teeth through fiber fringe and carries away anything not held by the nippers Nippers forward-:The nippers open again and move toward detaching rollers (A). Web return-:Meanwhile detaching rollers (A) have returned part of the previously drawn-off stock by means of a partial reverse rotation, so that the web protrudes from the back of the detaching device . Piecing -:In the course of the forward movement of the nippers, the projecting fiber fringe is placed on the returned web . Detaching-:The detaching rollers begin to rotate in the forward direction again and draw the clamped fibers out of web held fast by feed rollers (S) (inside the nippers).. Passive combing by the top comb-:Before the start of the detaching operation, top comb (F) has thrust its single row of teeth into the fiber fringe. As the fibers are pulled through the teeth of the top comb during detaching, the trailing part of the fringe is combed, thus making up for the inability of the circular combs to reach this part of the fringe . Withdrawal of top comb -:As the nipper assembly is retracted, the nippers will open for the next feeding step. During that time top comb will withdraw and the system is ready for new combing cycle The combing cylinder rotates continuously during this rotation and at a certain instant the combing segment is brought into the vicinity of a rapidly revolving brush mounted below the combing cylinder. During this processThis the brush removes the imperfections, dust etc., from the combing segment, and ejects them into an extractor that carries the noil away to a collecting filter system.

Degrees of Combing There are, particularly for cotton, four degrees of combing. 1) Scratch Combing-: where up to 5% noil is removed. This gives no great improvement in average yarn properties but has the benefit or reducing end breakage rates in spinning and winding. 2) Half-combing-:which involves around 9% waste, resulting in reduced yarn irregularity and improved spinning performance. 3) Ordinary combing-:involving between 10 to 18% noil, which is necessary for spinning yarns in the finer end of the count range. 4) Full combing-: resulting in greater than 18% noil. This often means double combing to obtain the highest quality yarns – 18% removed in the first combing and 7% in the second.

Type of Feed : There is no machine drive of the feed rollers as such; they are driven indirectly by the opening and closing of the nipper plates. Forward shift of the web by the feed roller into the opened nippers can be performed:

while the nippers move forward (described as forward feed); or

When the nippers swing back (described as backward feed)

Forward feed -:Forward feed used to be chosen for higher production rates when quality requirements were not too rigorous, mainly for “upgrading” with noil percentages of up to 12% (max. 14%).

Forward feed is mostly used over all staple ranges and to achieve better “self cleaning effect” during detaching and top combing action by generating higher retaining forces. Backward feed -:When higher demands were made on quality, backward feed had to be used with noil percentages in the range of 12 - 25%.

The Detachment Setting : -: This refers to the distance between the clamping line of the nippers and the nip line of the detaching rollers -:when these parts are at their closest spacing. -: The detachment setting provides the chief means for influencing the level of noil elimination. -: Wide detachment setting results in a high level of noil elimination; a closer setting is associated with a lower noil level. -: The detachment setting normally lies in the range of 15 - 25 mm. -: If the noil percentage varies for no reason while the machine settings (including the detachment setting) are kept constant, the cause lies not in the machine but in the raw material.

Cylinder Clothing-: -:A cylinder drive shaft (R) extends through the whole machine, and carries one combing cylinder (O) per combing head.

-:The combing cylinder in turn supports a combing segment called half-lap (S), which is bolted to the cylinder and is fitted with metallic clothing (K). -:Only metallic clothing is now used on high-performance combers, since clothing is more robust than the needles that were used formerly, needs no maintenance, is not liable to damage in use, and permits operation with a thick batt sheet. -:Nowadays metallic clothing is available with different point densities as many as three to five zones of point density, i.e. with fewer teeth at the start, a somewhat higher density in the central zone and a still higher density in the trailing zone.

. Speed: In combing machine, the speed is denoted as “nips/minute”, which is nothing but the rotational rate of the main combing cylinder. For every rotation of the main combing cylinder, one cycle of combing comprising feeding, combing, detaching and top comb operation are completed. There has been continuous increase in the combing speeds associated with improvement in quality as well. Generally for a given machinery conditions, the quality of the combed sliver will go down if the combing speed is increased.

Nipper Assembly The construction of the nipper assembly The mass of the nippers should be chosen carefully because it is accelerated and decelerated back to rest twice per nipper cycle (up to 7 times per second in modern machines). A low-mass nipper assembly – for example, made of aluminum alloy – is therefore advantageous. At the same time it should also clamp a relatively thick lap batt (up to 80 ktex) firmly and evenly. Therefore nipper plates must be made of steel (at least the clamping region), and the upper plate must be stiff while the lower plate may slightly springy. The upper nipper is mounted so that it can pivot on the lower nipper on pivot axis and can therefore be raised and lowered. Two springs one each to right and left of the nipper assembly, generate the required contact pressure for the nipper closing. The nose is designed to press the fiber fringe downward during clamping, so that the fringe cannot escape the action of the circular combs.

The Nipper Movements The lower nipper plate (5) is supported at the front by two pivot levers (6), on the left and right respectively, pivoted on comb axis (7) of the circular combs, and also by two swing arms (2) screwed onto

nipper shaft (1) and rotatable at point 8. During rotation of the nipper shaft – which is less than a full revolution – in the course of each combing

cycle, the whole nipper assembly is moved back and forth about point (8) by swing arm (2). ‘Forth’ means the nippers are moved closer to the detaching rollers as far as the position of closest

approach (the detachment setting), and then withdrawn again. The upper nipper is movably supported on the lower nipper at point 10, and is also suspended from shaft

(12) by means of spring (11). Therefore, as the nipper assembly is moved forward, the upper nipper is raised relative to the lower nipper

owing to the different lengths (different leverage) of the lever mechanism, and the nippers are opened. As the nippers are withdrawn, spring (11) presses the upper nipper back against the lower nipper (due to

the different length of the levers) Movement of the nippers should be monitored as they are not closed suddenly and sharply, but gently

pressed together with gradually increasing pressure. This gentle closure of the nippers is effected by an eccentric (12). During continuous rotation of the eccentric, the spring is periodically compressed and then released.

The Top Comb The replaceable top comb (F) is arranged between nippers (Z) and detaching rollers (A) so that the fiber

fringe can be drawn through the needles of the top comb during detaching process. The top comb usually comprises a holder ( H) to which needle bar (B) is secured by screws. The needle bar

consist of set of needles which are soldered over the bar. The holder mounts the top comb on the lower nipper plate so that the top comb swings with that plate.

The needles have a flattened cross-section and a bend. Apart from its participation in the swinging movements of the nippers, the top comb is fixed, i.e. it is not subject to any additional and complicated up-

and-down movements. During detaching the fiber fringe is pressed into the needles of the comb automatically. The depth of

penetration is very important.

The top comb assembly

The top comb (with needles)

The Operation of the Combs The circular combs can treat only the forward portion of the fiber fringe to be processed, since the comb clothing do not penetrate exactly to the bite of the nippers and also because the rear ends of the fibers are

located within the nippers. The fairly long, trailing portion has therefore to be combed out by another device called– the top comb .

This could lead to the false impression that the trailing portion of the fringe is not processed as effectively as the front portion, because it is not passed through a complete combing zone (circular combs), but only

through a single row of needles. In fact, the quality of processing of both portions is the same.

Cleaning and elimination of short fibers is, of course, performed by the top comb, but also at the same time

by the retaining effect (self-cleaning effect) of the batt in the nippers. During detaching less than 20% of the fibers in the nippers are pulled out of the batt .

This low percentage of fibers is unable to take the impurities within the batt with it, because the retaining

force of the more than 80% of fibers of the thick batt that remain is too strong. Impurities, neps, and short fibers therefore remain in the sheet as the other fibers are detached. It goes

without saying that this retained material also has to be eliminated somehow, somewhere. It occurs when the fringe is treated by the circular comb during the next combing cycle, or the following

one. So that elimination of impurities is always performed by the circular comb.

Self-cleaning effect of the batt

Salient features of modern comber-:

1) Change in the kinematics to increase nips/min 2) Concentric synchronise movement of the nippers 3) Reducing the clamping distance 4) Self-cleaning top comb 5) Easy setting of nipper gauge