PROCESS CONTROL IN COMBER

In combers, the important aspects to be controlled are waste level, combing efficiency and sliver irregularity. Mills generally fix the amount of comber waste to be extracted only by past experience and not on the fiber length distribution of the cottons processed which primarily determines the amount of waste to be removed to achieve the desired fiber length uniformity.
Best results of combing of Indian cottons are generally achieved by suitable adjustments of detaching distance, even though the top comb penetration is not very deep. However, the cottons have a high fiber length uniformity but the lap contains too many impurities and neps, a deep penetration of top comb is desirable, without fiber rupture.
Mill process all classes of cottons in the same comber without regard to the suitability of the needling arrangement. Coarsening of the needling scheme helps to reduce the comber waste by 2% for coarser cottons and by 1% for finer cottons. Incase of heavier laps, it increases the comber waste by 2 to 3% as compared with lighter laps.
Sliver Variation
Sliver variation in comber is due to short term irregularity of combed sliver and Control of this short-term irregularity is very important since, a high sliver U% can lead to increase in within-bobbin lea count variation. The long-term variation of the comber sliver should be maintained within a C.V of 3% for one meter wrappings. For quality control purposes, it is however not necessary to take routing wrappings in the combing department.
Combing Performance
The performance of combers has also considerable bearing on yarn quality as well as the amount of waste extracted. The poor combing performance arises due to improper settings and timings, poor upkeep and inadequate maintenance, unsatisfactory lap preparation and unsuitable process parameters. Based on digital Fibrograph tests, SITRA has evolved criteria for judging combing performance. The improvement effected in the various measures of fiber length are found to be interdependent and it will suffice to assess only 50% span length. If for every 1% comber extraction, 50% span length improves by 1.7% or more relative to card sliver, then the combing performance can be considered to be good.
Generally, for comber waste extractions upto 10%, all cottons respond well to combing. For levels beyond 10%, the law of diminishing returns operate and the improvement in yarn quality is not commensurate with the additional cost of production. Higher levels of waste should be extracted only in such cottons which indicate satisfactory combing performance or where the end use requires yarns of very high quality.
The head-to-head and comber-to-comber variations in waste should not exceed ± 1.5% and ± 0.5% respectively from the average value. A few short fibers in comber sliver and a few long fibers in noils would be unavoidably present. If the ratio of the scanned mean length measured by digital fibrograph of the noil to that of combed sliver is more than 0.4, it can be taken to indicate the presence of more long fibers in the noil.
As per the studies, while using high speed combers, the modified unicomb half lap yields 10 – 15% higher combing efficiency, 30 to 40% reduction in comber sliver neps and 0.5 U% improvement in sliver evenness as compared to that obtained using conventional half lap at a comber speed of 200 nips/minute. Yarns produced using modified unicombs are stronger (5-12%) and more even.
It is also observed that use of high speed combers with modified unicomb and modified top comb results in a significant improvement in infrequent faults (to the tune of 30%). In the modified comb, density of the needle is higher compared to normal top combs. If the normal top comb has 66 needles/inch, then modified comb will have 76 needles/inch.

In high speed combers, when the speed is increased from 160 to 240 nips per minute, the combing efficiency reduces of 20% and classimat faults increase by 10 to 20%. When the % noil extracted during combing is increases from 12 to 20%, the combing efficiency reduces by 0.55% and the classimat faults by about 60%.
Sources of count variation in combers
The contribution by combed sliver U% and variation in sliver weight to yarn lea count variation would be of the same order as that of carded material in the case of carded counts. The short-term irregularity U% of the comber sliver has a significant influence on within-bobbin count variation, whilst long-term variation of about 0.15 to 0.3m. Sliver is also affected by between-bobbin count variation. Variation in waste between heads of a comber as well as between combers will not have any significant effect on yarn lea count variation.
The causes for high sliver U% include,
➢ Eccentric top and bottom rollers
➢ Misaligned and bent nippers
➢ Improper needle spacing
➢ Broken or bent needles
➢ Variation in detaching roller diameters and improper timing of top combs
➢ Damaged or improperly meshed gears
➢ Phasing of piecing waves,
➢ Chocked aspirator systems
➢ Defective weighing and improper trumpet size for the hank.
Defects and Causes in Comber
Higher comber sliver Variation
➢ Differences in waste extraction between heads.
➢ Variation in the settings between back detaching roller and nipper.
➢ Improper cam setting depending upon the staple length of the material.
➢ Unicomb chocked with seed coats or immature cotton.
➢ Wider setting between unicomb and comb cleaning brush.
➢ Eccentric top and bottom rollers.
➢ Mis-aligned and bent nippers.
➢ Improper needle spacing
➢ Broken or bent needles
➢ Variation in detaching roller diameters and improper timing of top combs
➢ Damaged or improperly meshed gears
➢ Phasing of piecing waves,
➢ Chocked aspirator systems
➢ Defective weighing and improper trumpet size for the hank.
Thick and Thin Bars in Comber Web
➢ Incorrect timing of the detaching roller cam.
➢ Top comb set too deeply.
➢ Top comb touching the back detaching roller.
➢ Improper pressure on nippers.

Differences in Noil between Heads
➢ Variation in top comb penetration between the heads of the same comber.
➢ Variation in setting between unicomb and bottom nipper.
➢ Improper setting of the cam which decides the length of overlapping after combing.
➢ Uneven and insufficient nipper grip.
➢ Variation in diameter and pressure of top detaching rollers.
➢ Obstruction in the movement of aspirators.
Poor Combing Efficiency
➢ Presence of fiber hooks or disorderly arranged fibers due to non-standard preparatory process.
➢ Timing of the combing cycles not adjusted properly.
➢ Top comb chocked with short fibers/ immature fibers.
➢ Inadequate penetration of top comb.
➢ Half-lap cleaning brush loose on shaft or set too far from the cylinder.
➢ Excessive variation in short fiber content in the mixing.
➢ Wider setting between unicomb and nipper.
➢ Inconsistent mixing of soft waste.
Lap Running Slack
➢ Improper feed-ratchet movement.
➢ Lap loose on the shaft.
➢ Insufficient tension draft between lap roller and feed roller.
Poor Nep Removal Efficiency in Combers
➢ Wider setting between half lap and bottom nipper.
➢ Wider setting between cleaning brush and stripping rail.
➢ Low penetration of brush with half lap.
➢ Insufficient penetration of top comb.
➢ Uneven nipper grip.
➢ Damaged needles in unicomb.
➢ Damaged/absence of top roller clearer cloth.
➢ Incorrect atmospheric conditions.