Principles of Combing
Combing is the key process that makes the difference between an ordinary yarn and a quality yarn.It influence the following yarn quality-
- Yarn evenness
- Strength
- Cleanness
- Smoothness
- Visual appearance
The combing process therefore results in an improvement in the quality of the yarn and also enables the spinners to spin finer yarn counts.
Object: The basic objectives of the combing process are-
- Removal of predetermined amount of short fibres (Noil)
- Removal of neps & impurities
- Straightening of the retained long fibres(Hooks).
% Noil=Weight of noil*100/(Weight of noil+Weight of combed sliver)
The continuous assembly of long and parallel fibres delivered by the combing process is called as a comb.
Sliver-Just as long and well-aligned fibres, polymer makes strong fibres,long and straight fibres in the comb sliver will make strong and smooth yarns.
Noil-The material rejected in the combing process is called noil.Noil contains short fibres,neps and impurities.The amount of noil produced may be expressed as either percentage or tear ratio.
Percentage noil=Weight of noil*100/(Weight of noil+Weight of combed sliver)
Tear ratio = Weight of combed sliver/Weight of noil: 1
% noil = 100/(tear+1) or Tear = (100/percentage noil)- 1
A higher noils means longer fibres in the combed sliver,but less comb sliver will be produced.Yarns made from the combed slivers are called combed yarns.Without combing ,a carded yarn would be produced.Combed yarns,consisting of longer and more parallel fibres, are of better quality and command a higher price than carded yarns or yarn produced without the combing stage.
Cotton combed yarn versus cotton carded yarn–
| Cotton combed yarn | Cotton carded yarn | |
| Fibre used | Staples of 1 inch up, finer & more uniform; expensive fibre | Shorter staple ,loss uniform, Fibre less expensive |
| Yarn count | Finer | Coarser |
| Appearance | Not hairy ,smooth & clean, strong & lustrous | More protruding end ,Bulkier & softer |
| End uses | Shirting ,sewing thread | Washing cloth,trousers |
Combing Cycle and Mechanisms
1. Degree of Combing
The quality of combing yarn and degree of yarn fineness achievable depends upon the type and quality of raw material and degree of combing carried out during the combing process.Based upon the degree of combing ,following different combing processes can be employed in the production of a combed yarn.
1)Scratch combing-It is carried out to produce lower quality of coarser combed yarns,5% noil is removed.
2)Half combing-7 to 10% noil is removed.
3)Normal combing-Noil ranges from 10 to 15 %
4)Super combing or fine combing-It is used to make best possible fine yarns.Here up to 25% noil is removed.
5)Double combing-Here finest egyptian cotton is produced.Noil up to 25 % is removed.
2. Combing Cycle and Mechanism
The combing of cotton fibre is carried out in an intermittent cycle of operation called as combing cycle.In one combing cycle,the short fibres are removed and the long fibres are passed forward in to a sliver.One combing cycle can be divided in to many phases.
- Feeding
- Cylinder or half lap combing
- Piecing
- Top combing
- Sliver condensing & drafting
1. Feeding
Combing cycle begins with the feed of the lap.The lap sheet is fed to the comber between the feed roller and smooth cushion plate also called as bottom nipper.The top nipper and nipper knife moves down to fix on the bottom nipper (cushion plate). Hence the lap sheet is gripped between the top and bottom nippers.
2. Cylinder or Half Lap Combing
As the lap is held by the nippers, the half lap or the cylinder comb rotates and the protruding fringe of the fibres is combed with the help of needles mounted on the cylinder comb. Any fibres not held by the needle will be treated as short fibres and will be removed as a waste during the rotation of the circular comb. The waste (noil) is removed from the surface of the needle of the needle with the help of a revolving brush mounted just below the cylinder.The waste removed is then collected by suction at the back of the comber.
3. Piecing
As the half lap or circular comb has finished combing and has finished combing and has cleared the fibre fringe, the detaching roller remains stationary and does not rotate however only the back top detaching rollers rocks backward. At the same time the nipper is raised while the cushion plate or bottom nipper rocks forward. Also the feed roller rotates by a small and amount feeds a predetermined length of lap. This increases the length of the lap fringe and as the bottom nipper moves forward, the leading edge of the lap fringe is entered in to the nip of the detaching rollers and initiates piecing. During piecing the top comb also starts descending.
4. Top Combing
Just as the piecing has started,a top comb with one row of needles is descended in to fibre fringe from the above. As the combed web of fibres is pieced or connected to the surface of the detaching rollers,the detaching rollers now start rotating forward and the combed web of fibres is pulled through the top comb.
As the fibre web is pulled through the top comb,short fibres, neps and entanglements not removed by the circular comb is removed here by the top comb. As the bottom reaches its maximum forward position, the detaching is completed because the detaching rollers continue to move forward while the bottom nippers begins to rocks backwards. As detaching finishes the comb is withdrawn upwards.
The feed roller again rotates and advances a new short section of the lap to be combed by the bottom circular comb and the same combing cycle continues.
On a modern comber the cycle of combing takes place 3 to 5 times in a second.
5. Sliver Condensing & Drafting
The combed web taken by the detaching rollers is delivered to web pan having a trumpet guide on its one side.The combed web is pulled through the trumpet guide with the help of a pair of calender rollers that converts the web in to a combed sliver.The combed slivers coming from all the heads of the comber are laid side by side and are passed through a draw box where a draft of 5 to 12 is given.Most commonly a 3 over 5 roller drafting is used where the rollers are inclined at an angle of 60.The drafted slivers are coiled in to sliver cans.On modern combing machines,the sliver cans are automatically doffed on completion.
The neps and impurities will not be able to pass through the closely pinned top comb and removed by the cylinder comb in the next combing cycle.The detaching rolls bring the combed fringe to the tail end of the previously combed material to make a joint or piecing. The combed fibres, from many combing heads, are then brought together and consolidated in to a sliver and coiled in to sliver can. The comb operates intermittently in cycles. Its speed is described in terms of cycles per minute or nips per minute. Modern cotton comb run in excess of 350 nips/minute.
3. Sequence of Operations
- Feeding-Lap is fed by feed roller
- Nipping-Feed lap gripped by the nipper
- Combing-Gripped lap is combed by circular comb
- Nippers forward-The nippers open again and move towards the detaching rollers
- Web return-The detaching rollers have returned part of the previously drawn off stock(web)
- Piecing-The forward movement of the nippers,the projecting fibre fringe is placed upon the returned web V
- Detaching-The detaching rollers begins to rotate in the forward direction again and draw the clamped fibres out of the sheet held fast by the feed rollers.
- Top combing-While the detaching roller delivers the material ,top comb come in to action to further clean the lap.
Recent developments in combing machine
1. Introduction
Before discuss the modern development, we will discuss the basic combing machine lap preparation are:
1. Comber Lap preparation (Sliver lap machine)
Since the main function of combing is to remove short fibres, if the carded slivers are directly fed to the combing machine, the waste extraction would be very high and also lot of fibres breakages takes place. This is due to the fact that fibre orientation in the card sliver has majority of the hooked surfaces. So it is desirable for the card sliver to be prepared in to such a form which is suitable for combing operation. For this reason a suitable lap with straight and parallel fibres is formed which is presented as a feeding material to the comber.
The majority fibres in the card slivers have trailing hooks (about 50%). It has been established with experiene and experiments that better combing results are obtained when majority of fibres presented to the comber has leading rather than trailing hooks. This is because the leading hooked fibres can be straightened out better and they pass in to combed sliver whereas the trailing hooked fibres have less chances of straightening and more chances of being removed as a noil. In order to straighten and align the fibres and also convert the trailing hooks in to leading position, even no. of machines must be used in the preparation of a comber lap and commonly two processes are used. Straightening and orientation of fibres is achieved by using roller drafting method in both of the preparatory operations. The total draft of both the machine ranges from 8 to 12. Since the drafting tends to cause imperfections and unevenness in the material, operations of doubling is also carried out. The doubling not only helps to improve the evenness of the material but also is necessary to form a sheet of material which is converted in to a lap.
The lap machine divided in to the following regions:
- Creel region
- Drafting region
- Winding region
Creel region– The creel of the machine consists of two feed arrangements,each usually holds 12 card sliver cans.All together it gives a doubling of 24. All card slivers after passing through series of guide rollers enter the drafting arrangement.
Drafting Arrangement-The sliver lap machine commonly uses a 4 over 4 roller drafting system with top roller pneumatically weighted. The pneumatic pressure can be adjusted up to 1600 Newtons.A total draft of 1.3 to 3 is given at this arrangement.
Winding Region-The sheet or web of fibres after being drafted out is passed over a guide plate or deflecting the the plate which changes its direction towards four calender rollers. The high compression (up to 16000 newton) created by the calender rollers transforms the fibre web in to a lap. The lap then passes through two winding rollers that press against the lap tube with a pressure of 1000 newton and assists in winding of the lap on to the lap tube. The lap tube is placed on lap weighing devices which on modern machine automatically removes the lap roll when its required weight has been reached and is ejected on an automatic transport system that will take the lap roll directly to the next machine i.e. ribbon lap machine.
2. The Ribbon lap machine
The basic concept of ribbon lap machine is same as that of the sliver lap machine as shown below:
Here, instead of a feed table a lap unrolling table is provided for 6 lap rolls. The sheet unrolled from the lap passes through a standard 4 over 4 roller drafting system where a draft of 3 to 6 can be given. The drafted material after passing through calender roller is wound by the winding assembly on a lap tube. The ribbon lap machines have an advantage that it gives a doubling of 6 in web form and thus high degree of evenness in traverse direction is achieved.
The lap doubling system is considered to be old conventional system of preparing comber lap. But with the introduction of high performance modern machinery the trend is tilting towards using a sliver doubling system rather than lap doubling.
3. Super lap machine (Unilap)
The super lap machine has three feeding heads in the creel portion with each head holding 16 to 20 draw sliver cans. All slivers coming from each feeding head is passed through a vertically held 2 over 3 roller drafting system. The lap sheet formed by the combination of these drafted slivers are passed through a pair of calender rollers and is ultimately wound on the lap tube with the help of two working winding rollers.
4. The Combing Machine
Many different types of combers are used for different fibre materials, combers can be classified in to following types:
- Rectilinear combers(used for cotton)
- Circular comber(used for worsted)
- Rotary combers(used for spun silk)
- Hackling machine(used for bast fibres)
The rectilinear or cotton combers operates intermittently beacause the short length of the fibres does not allow continuous method of combing. One end of the cotton fibre bundle is combed with cylinder comb or half lap having half of its circumference covered with teeth while the other end is combed with single row of needles called as the top comb.
After both ends of the cotton fringe have been combed separately, the separated fringes are reunited by a piecing unit. The standard parts of the comber are shown in the figure below:
The combing of cotton fibres is carried out in an intermittent cycle of operation called as combing cycle. In one combing cycle can be divided in to many phases.
2. Recent developments in combing machine
Combers, speed frames and ring frames have gone through such major changes that today they meet not only the requirements of the manufacturers but also the demands of consumers.The comber has undergone a lot of changes. Consequently, the speed as well as the quality of the combed sliver and hence yarn is improved a lot. Following are the significant modern developments in comber.
1) Change in the kinematics to increase nips/min:
Most of the latest models of combers are able to run at higher speed. The reason behind this higher speed, apart from others things, is better manipulation of the comber technology, kinematics and air control.
2)Concentric synchronise movement of the nippers:
The combing will be better if the bottom nipper lower surface remains at fixed set distance from the bottom comb during the entire circular combing. This is achieved by using the standing pendulum principle. The Rieter E 65/E 75 comber, Trützschler Comber TCO 1, Marzoli Comber CM600N are equipped with concentric synchronise nippers movements.
3) Reducing the clamping distance:
By reducing clamping distance, long fibres going into the waste can be avoided. If this distance is wide, fibre control during combing is hampered, thus deteriorates combing efficiency. Rieter comber has designed this area to keep this distance minimum.
4) Self-cleaning top comb:
The top-combs, during operation, get loaded with short fibres and impurities. Thus, the machine is stopped intermittently for cleaning and results into production loss. Trützschler comber is equipped with self-cleaning top combs.An extremely short compressed air blast of a few milliseconds purges the needles from top to bottom and detaches the adhering fibres.
5) Easy setting of nipper gauge:
Toyota VC5A Comber offers one-point adjustment for nipper gauge. Thus it can be easily adjusted by moving top comb back and forth to align with setting gauge. Hence it is not required to adjust nipper pivot gauge and check cylinder gauge.
6) Constant Lap weight (g/m) during lap build-up: In Trützschler Superlap TSL 1, the draft, speed and pressure are manipulated in such a way that the lap g/m is kept constant through out the lap build.
7) Rieter Combers E 65/E 75 with Computer Aided Process Development:
Rieter has developed computer programmes entitled CAPD+ (subsequently CAPD 500), i.e., Computer Aided Process Development, for the optimisation of combing process. Through this programme, elements involve in the combing process were recalculated, optimised and checked for feasibility in several billion computer operations and simulated precisely.