In Addition to the Above, Combed Cotton Needs Less Twist Than a Carded Yarn

COMBER

INTRODUCTION

Combing is the process, which is used to upgrade the raw material. It influences the following yarn quality

1.  Yarn evenness

2.  Strength

3.  Cleanness

4.  Smoothness

5.  Visual appearance

In addition to the above, combed cotton needs less twist than a carded yarn.

OPERATIONS OF A COMBER

To produce an improvement in yarn quality, the comber must perform the following operations.

1.  Elimination of short fibres

2.  Elimination of remaining impurities

3.  Elimination of neps

·  The basic operation of the comber is to improve the mean length or staple length by removing the short fibres.

·  Since fineness of short fibres(noil) is low, the overall micronaire of the sliver after combing is high.

·  Because of combing, fibre parallelisation increases. Please note that this is a side effect, which is not an advantage always. The high degree of parallelisation might reduce inter-fibre adhesion in the sliver to
such an extent that the fibres slide apart while pulled out of the can. This may lead to sliver breaks or false draft.

PROCESSING SEQUENCE OF A COMBER

1.  Feeding, lap is fed by feed roller

2.  Fed lap gripped by the nipper

3.  Gripped lap is combed by circular comb

4.  Detaching roller grips the combed lap and moves forward

5.  While the detaching roller delivers the material, top comb comes into action to further clean the lap

6.  While going back, nipper opens and receives a new bit of lap

·  The raw material delivered by the carding machine can not be fed directly to the comber. Lap preparation is a must. A good lap fed to the comber should have

1.  Highest degree of evenness so that lap is gripped uniformly by the nipper

2.  A good parallel disposition of fibres so that long fibres will not be lost in the noil

3.  Trailing hooks from carding should be fed as leading hooks to reduce long fibre loss in the noil

·  Degree of parallelisation of lap fed to the combers should be optimum. If fibres are over parallelised lap licking will be a major problem. Because of fibre to fibre adhesion, mutual separation of layers within the sheet is very poor. Moreover the retaining power of the sheet can be strongly reduced that it is no longer able to hold back the neps as it usually does. Some of these neps also pass thro the top comb. Neppiness of the web is increased.

·  Retaining power of the fibres results in self cleaning of the lap during combing operation. A thick sheet always exerts a greater retaining power than a thin one. To certain extent, the bite of the nipper is more effective with a higher sheet volume. On the negative side , a thick sheet always applies a strong load to the comb and this can lead to uncontrolled combing. A compromise should be struck between quality and productivity.

·  If the sheet is more even across the width, clamping effect at the bite of the nipper will be better. Evenness of the lap is therefore of considerable significance. The most effective method of obtaining a high degree of evenness of the sheet is through more number of doublings in the web form (as it is done in RIBBON LAP).

·  Fibres must be presented to the comber so that leading hooks predominate in the feedstock. This influences not only the opening out of the hooks themselves, but also the cleanliness of the web. If the sheet is fed to the comber in the wrong direction, the number of neps rises markedly.

·  Both quantity and form of fibre hooks depend mainly upon the stiffness of the fibres. This rises to the second or third power with fine fibres. Fine and long fibres will always exhibit more and longer hooks than short and coarse fibres. Accordingly, the role of fibre hooks in the spinning process
becomes more significant as fibres become finer.

·  There are two types of feeds in COMBER

1.  Forward feed (concurrent feed):Feed of the sheet into the nippers occurs while the nippers move towards the detaching roller

2.  Backward feed (counter-feed) : Feed of the sheet occurs during return of the nippers

·  Higher Noil % always improves the imperfections in the final yarn. But the strength and other quality parameters improve upto certain noil %, further increase in noil results in quality deterioration.

·  In backward feed, the cylinder comb combs through the fibres more often than in forward feed. Therefore, the elimination of impurities and neps is always good. However the difference is usually undetectable
in modern high performance combers of the latest generation.

·  The FEED LENGTH has a direct influence on production rate, noil %, and the quality of combing. High feed length increases the production rate but cause deterioration in quality. Higher the quality requirement, feed length should be lower. To some extent , the feed length may be decided by the length of the fibre also.

·  Detaching length is the distance between the bite of the nippers and the nip of the detaching rollers. This distance directly affects the noil %. More the detaching distance, higher the elimination of noil.

·  Needles of the top comb have a flattened cross section and are used with a point density in the range of 22 to 32 needles per centimeter. More the needles, more the noil%.

·  The Depth of Penetration of top comb also affects the Noil %. If the comb depth is increased by 0.5mm, approximately 2% increase in noil will occur. When the depth is increased, the main improvement in quality is seen in neps. Over deep penetration of top comb disturbs fibre movement during piecing which will deteriorate the quality.

·  Since the web from detaching roller is intermittent because of the intermittent functions like feeding, combing and detaching, to have a continuous web from the comber, fibre fringes are laid on the top of each
other in the same way as roofing tiles. This process is called Piecing. This is a distinct source of fault in the operation of Rectilinear Combing. The sliver produced in this way exhibits a periodic variation.

·  As large a lap as possible with adapted lap weight which is as high and as uniform as possible must be positioned in front of the comber. The better the comber lap is prepared, the heavier the lap weight can be set on the comber and the less the resultant noil waste with the same degree of cleanliness of the yarn.

·  The higher degree of combing out are used in order to permit final spinning of ultra-fine yarns or to increase the strength of a yarn. Reducing the lint content improves the medium staple. However, not all cottons meet these requirements. Low degree of combing out , on the other hand, frequently serve to improve purity. When the card sliver is pulled through the needle bars, these separate off foreign bodies, large neps
and torn fibres. Light combing out has also been introduced to a greater extent owing to the impairments in cotton purity influenced by mechanical harvesting. Even when combing with minimum noil percentages, there
is a noticeable improvement in fibre parallelism in the sliver. Even the smoothness and shine of the yarn are improved. It must thus be anticipated that this method will become more and more popular in the future.

·  Production of the comber is dependent upon the following

1.  N- Nips per min

2.  S- feed in mm/nip

3.  G- lap weight in g/m

4.  K- Noil percentage

5.  A- tension draft between lap and feed roller(from 1.05 to 1.1)

6.  E- efficiency

Production = {E * N * S * G * (100-K) * 60 * 8} / (1000 * 1000 * A *100)

PROCESS PARAMETERS IN COMBING

Introduction

Combing is a process which is meant for upgrading the cotton raw material so that the following yarn properties will improve compared to the normal carded yarn.

·  U% of yarn

·  Tenacity gms/tex

·  Trash in the yarn(or kitties in the yarn)

·  Luster and

·  Visual appearance

Points to be Considered

Following parameters are very critical as far as the yarn quality of combed yarn is concerned

·  Noil percentage(waste percentage)

·  Type of feed

·  Feed length

·  Feed weight in grams per meter

·  Piecing length

·  Top comb penetration depth

·  The distance between unicomb to nipper

·  Unicomb specification

·  Number of needles in top comb

·  The cleaning of unicomb

·  Variation in nipper grip

·  Variation in noil percentage

·  Type of lap preparation

·  Total draft between carding and comber i.e. total draft employed in lap preparation

·  Drafting roller settings in comber

·  Drafting roller settings in lap preparation machines

·  No of doublings in lap preparation

·  Short fibre content

·  Fibre micronaire

·  The type and the amount of trash in the card sliver

Waste Percentage

The noil percentage from a comber depends upon the following

·  Short fibre content

·  Detaching distance

·  Feed length

·  Top comb penetration

·  The distance between unicomb to top comb

·  The basic idea of removing the waste is to remove the short fibres i.e to improve 50% span length or mean length.

·  The two important basic parameters to be considered in deciding the waste percentage are; 1.Yarn quality requirement and 2. Short fibre content in the raw material

·  Let us assume that the following cotton is used

2.5 span length = 28 to 30mm

Uniformity ratio = 50 to 53%

FFI % = 6 to 14

Micronaire = 3.8 to 4.2

Fibre strength = 24 to 28gms/tex and the quality requirement for

counts 30s to 40s, is to meet 5% uster standards in U%, imperfection, strength and classimate faults.

To meet this quality requirement with the above raw material ,the amount of noil to be extracted may be around 16 to 18% if E7/4(RIETER MAKE) comber is used or 15 to 16 % if E-62 (RIETER MAKE) comber is used. The above example is given to highlight the effect of noil removed and the quality achieved. This is just an approximate figure, the parameters may vary depending upon the application.

·  Combing efficiency is calculated based on the improvement in 50% span length, expressed as a percentage over 50% span length of the lap fed to the comber multiplied with waste percentage i.e.

{(S-L)/(L*W)}*100

where,
S - 50% span length of comber sliver L - 50% span length of comber lap W - waste percentage

·  Higher the noil %age , lower will be the combing efficiency.

·  Given a chance, it is better to remove waste more from top comb penetration than increasing the waste percentage by increasing the detaching distance. When the detaching distance is more the control
during detaching will be less.

·  Given a chance, it is better to work with backward feed than forward feed for the same waste percentage. Nep removal will be better, loss of long fibres in the waste during detaching will be less.

·  With backward feed, top comb penetrates into the fibre fringe which is already combed by the unicomb, therefore combing action done by top comb will be better and there will not be longer fibres in the
waste.

·  Waste percentage depends upon the feed length and type of feed. In backward feed, higher the feed length, higher the waste percentage. In forward feed, higher the feed length, lower the waste percentage.

·  With backward feed, the detaching distance will be less for the same waste percentage compared to forward feed. Therefore fibre control during detaching and during top comb action will be better.

·  Higher the noil, higher the yarn strength. But this is true upto certain level of waste. Further increase may not increase the yarn strength. Very high %age of noil will reduce the yarn strength and will increase the breakage rate in ring frames.

Top Comb And Unicomb

·  The number of needles in Top comb depends on the Fibre micronaire , the lap weight and fibre parallelisation in the lap. If the fibre Micronaire is less than 3.6, number of needles per centimeter in top comb can be 30. In general for fibres above 3.8 Micronaire, 26 needles per centimeter is used.

·  Top comb plays a major role in removing the waste. Around 40 to 60% of noil is removed by top comb. But top comb will get damaged very fast. Top comb damage will result in slubs in the sliver. Even 4 to 5 needle damages will result in bad webs. Top comb maintenance is very important to produce good quality yarn.

·  Different types of unicombs are used in different combers. The circumference of unicombs, the number of wire points and its variation in the unicomb are different. It is not true that 110 degree unicomb will produce good quality yarn compared to 90 degree unicomb.

·  In most of the cases, 75 degree unicomb has given better results compared to 90 degree unicomb in E7/4 combers, for different types of cottons.

·  Rieter has standardised 90 degree unicomb for its E-62 combers. 110 degree unicomb can not be used in this comber.

·  Unicomb action will be effective as long as nipper and unicomb moves in opposite direction. If unicomb and nipper move in the same direction, unicomb can not do its work properly. Moreover the finer
needles will not be utilised properly. That may be the reason why 90 degrees unicomb do not produce a good quality yarn compared to 75 degrees unicomb.