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 combing 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 sliver emerges out of the calendar rollers in the carding 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 For example, as the sliver is withdrawn from the card can, the original trailing hooks (as the sliver went into the can) are now counted as the leading hooks as can be seen in Fig.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 staple 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/doubling 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 yarn, but they only increase the hairiness of the yarn and adversely affect the appearance and other good properties of the yarns 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 cotton 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 “noils”. Noil percentage = (Weight of noil 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 : spinning. Circular combers: Used in the English worsted 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 .