TECHNOLOGICAL FUNCTIONS
Introduction
Every aspect of the drawframe and the movement of the fibers can have an impact on the technological aspects of the delivered sliver. The challenge is to produce fault free high quality sliver at near maximum production speeds. The overall sliver quality is determined by:
The type and quality of the fibers being processed,
The machine condition and process control of previous operations,
The processing system variations such as combing, multiple drawing
steps, and the blending techniques for different fibers.
The results will be different for each of the many variables, but each will have an optimal drawing set up.
Unfortunately, any defects in the sliver leaving the finisher draw frame are going to cause subsequent processing stops or a yarn / fabric defects. The primary problems are associated with either the machine condition operator practice or the machine settings and include:
Drafting conditions that create uncontrolled sliver thick/thin places, clearer waste from the drafting rolls and drafting waves.
Uncontrolled sliver stretching in the coiling action, or any sliver stretching that cannot be removed by an auto leveler.
Sliver tension, throughout the machine should be low. Establish the tension by finding the point at which it is just too low and then increase it by one step.Sliver contamination due to the release of accumulated dust and fly.Sliver tags produced by rough spots in the sliver path or by defective cans. Damaged sliver caused by bad operator handling techniques..
Influence of fibers
The basic rules are:
Long, fine fibers of uniform length can be drafted at the high end of the draft range, carded cotton and blends should be limited to a maximum draft of eight.
With very short regenerated fibers the draft should be limited to four, and only one process of drawing. Multiple drawing processes create a very high level of drafting waves. Additionally, These fibers normally have to be run at speeds below 400 m/min.
Synthetic fibers with high fiber-to-fiber cohesion require higher drafting forces, create more heat and in general have to run at slower speed, in the range of 400 to 500 m/min.
The fiber finish on some synthetic fibers can create a finish build up on the machine, in the coiler tube and on the coiler plate. Care has to be taken to keep the machine clean.
Some bulky synthetic fibers need special components to run well. Check with the business unit if this is a problem.
Number of drawing processes
The number of drawing processes is normally established by the technological requirements because the cost of drawing is relatively low. However, there is always pressure to reduce processing steps to remain competitive.It is well established that the card produces a sliver containing both leading and trailing hooks that have to be eliminated prior to high draft ring spinning. This is normally done by using two passages of drawing
Single process drawing
These include:The spinning of coarse counts from blends containing very short fibers such as comber noil, regenerated fibers or gin motes.
An additional drawing process is not used with this type of material because it can create problems of drafting waves and reduced sliver cohesion.
Rotor spinning of coarse and some medium count yarns. Fiber hooks and fiber orientation do not greatly affect the spinning process but there will be a loss of strength by using only one process of drawing. This is not a problem for many pplications.
Coarse counts spun from synthetic fibers. Some carpet and upholstery yarns are produced on both ring and rotor spinning systems.
Two process drawing
The conventional short staple process, for “carded” yarns of cotton, man made fibers and intimate blends, includes two drawing processes.The primary reasons for two processes are:To reduce the number of fiber hooks in the sliver,Supply
roving, with the majority of hooks trailing, to the ring spinning, To improve
the degree of fiber orientation in the liver,To enable blending through doubling to improve yarn consistency,To gradually reduce the sliver weight from the card sliver to that needed by the roving or spinning machines.
Drawing for combed yarns
In the modern combed yarn spinning, there are three drawing operations:
First passage of preparation drawing is used to draw card sliver and produce sliver that can be wound onto laps for the comber.
The second drawing operation occurs at the comber to reduce the sliver weight. Relatively high drafts are needed at this stage and the drafting zone is very robust. The drafting system is normally a 3 over 5 roller arrangement to clearly separate the fiber control function of the break draft zone and the main draft zone.The third drawing operation is performed on the finisher drawframe.
The comber eliminates most of the fiber hooks, hence the remaining functions of the drawframe are to produce a very uniform sliver of the required weight. This “one process drawing” after combing has been made possible by the successful use of the RSB autoleveler drawframe.
Three process drawing for drawframe blends
When blending different fibers at the drawframe, multiple processes have to be performed to produce a homogeneous finished sliver. This is usually necessary for ring spinning.
An exception is in rotor spinning in which the yarn formation steps separate the fibers and reassemble them in the rotor to produce a well lended yarn. In some rotor spinning plants only two drawing processes are used)
Three process drawing for air jet and Vortex spinning
Three processes of drawing are normally used to prepare sliver for MJS and MVS.
The spinning machine drafts directly from sliver with drafts over 150. The fibers in the sliver have to be individualized and very well oriented. With three processes of drawing, the spinning machine functions more efficiently.
The machines need light sliver weights to be able to spin medium and fine yarn counts. Slivers in the range of 35 grains/yard are not unusual. Three drawing steps are helpful in converting relatively heavy card sliver into light finisher drawing sliver.
The processing of combed cotton for MVS is a challenge because spinning results are good with only one process of autoleveler drawing after the comber. This requires drafts much higher than normal range.
Sliver quality
As frequently mentioned throughout the chapter on Settings, the sliver quality is of primary importance. The sliver quality is normally judged by consideration of the Test Lab Data produced under controlled conditions.
Sliver weight
The weight of the sliver is usually measured in g/m or grains/yard. Obviously, the shorter the length of the individual samples, the more the variation is revealed. For example, it is not recommended to reel long lengths of sliver to check the autoleveler function.
The following checking procedures are suggested.
Delivered sliver weight of a machine - 5 x 10 yd (m).
( Not consecutive lengths.) Machine to machine sliver weight control – 5 x 10 yd (m) lengths from each machine. “Sliver Test” for leveling intensity…3 x 10 yd (m).
Care has to be taken when manually preparing and cutting lengths of sliver. The sliver is easily stretched and if different operators perform the task there can be manual errors introduced.
When a reel is used, care should be taken to prevent sliver tension variations that could cause uncontrolled sliver stretch.
Sliver evenness CV%
The evenness is normally measured by passing the sliver through a measuring head that records a capacitive signal measuring the mass of every 12 mm of sliver length. The values are represented in a continuous chart that shows the variation of the sliver above and below a mean value. This chart is called the “Diagram” in this text.
The data is used to calculate the mathematical value of mass variation known as the coefficient of Variation (CV%). The lower the CV%, the more uniform is the sliver.
Additionally, a Spectrogram is produced that shows the periodic occurrences of variations.
It is normal for customers to request the lowest possible sliver CV%. To do this, the draft distribution has to optimal and the roll settings close. These tight roller settings tend to:
Over control the sliver and break the longest fibers, Reduce the yarn strength,Increase the ends down rate in spinning.Create small “spikes” visible in the evenness diagram that cause Increased thick places in the yarn.
Evenness diagram of a sliver with a low CV% but with spikes in the diagram
The ultimate judgment of the best CV% can be made only by considering the
Diagram, the Spectrogram, the processing performance at roving and spinning
and the yarn quality. Unfortunately a considerable length of time is needed to
perform these extensive evaluations; hence, the customer should be encouraged
to assist.
Effects of tight roller settings
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Effect of drawframe roller spacing on yarn quality Ne 30/1 – 100% combed cotton
ring yarn. (41 mm staple) |
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|
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Too tight (lowest CV% 2.7%) |
Optimized setting (CV 3.0%) |
|
Roller spacing (HVD/VVD) |
40 / 46 |
42 / 47 |
|
Single end strength |
420 g |
412 g |
|
Strength variation -Vo |
7.8% |
6.2% |
|
Single end elongation |
6.5% |
6.5% |
|
Elongation variation -Vb |
2.4% |
1.9% |
|
|
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|
Uster CV % |
12.2 |
11.7 |
|
Thins |
0 |
1 |
|
Thicks |
16 |
10 |
|
Neps |
11 |
10 |
|
|
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Classimat minors |
80 |
28 |
|
Majors and long thicks |
22 |
2 |
|
|
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ITT quality index (higher is better) |
137 |
146 |
Recommendation – In many cases the best settings are found to be:
Roller distances for lowest CV% but with the main draft zone opened up by 1 - 2 mm, and the break draft zone opened 2 – 4 mm.
Spectrogram
The spectrogram is extremely important in judging the sliver quality in that the periodic mechanical faults and the sliver irregularity waves can be seen. Additionally, if the leveling action point is not correctly optimized, a hump in the spectrogram will be visible somewhere between 35 cm and 50 cm.
The appearance of sliver can indicate the quality, and is an important element in judging how well a machine is performing.
When the can is full and doffed the following points should be observed:
The sliver should not be “scuffed” from over filling. This leads to matted fibers that are difficult to draft in subsequent processes. DO NOT OVERFILL the cans.
If the sliver is folded during the coiling action the tension is too low.
Fiber tags on the surface of the sliver usually means that the coiler tube or coiler plate has a rough place that needs to be repaired or replaced.
Trash accumulations in the top coils, sometimes referred to as “mice”, are due to trash collecting in the coiler tube during running and being released as the coiler slows down at a stop. The coiling tension should be reduced slightly to enable the sliver to have a more complete contact with the coiler tube.
Sliver cohesion
If the sliver has low strength, the trumpet, calender roller pressure and coiler tube inside diameter should be checked to see if they are correct for the sliver weight and material being processed.
With combed cotton, the strength of the drawn sliver is influenced by the piecing produced at the comber. Many times the optically best piecing seen in the combed web is too delicate for the subsequent drawing steps. It is suggested that heavier overlaps at combing produce a stronger sliver. The associated mass increase can be subsequently corrected by the leveling action of the auto leveler draw frame.
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