Fibre Dynamics in the Revolving-Flats Card
A Critical Review
C.A. Lawrence, A. Dehghani, M. Mahmoudi, B. Greenwood and C.Iype
School of Textile Industries
University of Leeds
Over the last 30 years numerous developments have taken place with the cotton card. The production rate has risen by a factor of 5 with the main rotating components running at significantly higher speeds.
Triple taker-in rollers and modified feed systems are in use, additional carding segments are fitted for more effective fibre opening, and improved wire clothing profiles have been developed for a better carding action. Advances in electronics have provided much improved monitoring and process control. Most of these developments have resulted in enhanced cleaning of cotton fibres, reduced neppiness of the card web and better sliver uniformity.
Despite the various improvements made to the card a commonly held view is that more is known about the cleaning processes on the card than about the carding process itself . For instance, modern cards can achieve an overall cleaning efficiency of 95%. It is well established that the cleaning efficiency of modern taker-in systems is a round 30%, that the cylinder/flats action with the latest wire clothing profiles gives 90% cleaning efficiency and that effective cleaning is associated with lower neps in the card web .
However, even though the nep content and the sliver Uster CV% are used as quality measures of carding performance they are not satisfactory indicators for anticipating yarn quality. This is because some fibre arrangements in the sliver may lead to nep formation and imperfections during up-stream drafting processes .
In addition to the removal of trash and neps, important aspects of the carding process in relation to yarn quality and spinning performance are the degree of fibre individualisation, the fibre extent and the fibre hook configurations in the sliver. With regard to these factors, increased production rate can reduce carding quality . It is therefore of importance that a better understanding is established of the effect that carding actions have on such quality parameters, particularly at high production rates.
The most widely accepted view of how fibres are distributed within the card under steady-state conditions is illustrated in Figure 1 . Reported studies into the fundamentals of the carding process have largely been concerned with how the principal working components of the card affect this distribution of fibre mass and interact with the mass to achieve:trash and nep removal from cottons; the disentangling of the fibre mass into individual fibres, with minimal fibre breakage; and the alignment of the fibres to give a sliver suitable for drafting in down stream processes.
These actions occur at the interface of the card components within the three zones indicated in Figure 1. This paper therefore gives a critical review of published research on the:
· mechanisms by which the fibre mass is broken down into individual fibres,
· mechanisms of fibre transfer between the component parts of the card
· effect of the saw-tooth wire geometry on these actions
Figure 1: Distribution of Fibre Mass during Short-Staple Carding
Q1: fibre mass transferred from cylinder to doffer
K : transfer coefficient
Q2: recycling layer
QL: fibre mass transferred from taker-in to cylinder
Qf : flat strips
Qo : operational layer
(where Q is mass per unit time)
Zone 1: Fibre-Opening
Separation and Cleaning of the Input Fibre Mass:
The taker-in has effectively a combing action , which results in the breakdown of the tufts, consituting the fed fibre mass, into single fibres and smaller size tufts (tuflets), and in the liberation of trash particles ejected from the mass flow by the mote knives positioned below the taker-in. To effectively breakdown the fibre mass feed into tuftlets with minimal fibre breakage, the taker-in wire has to be coarse, with a low number of points per unit area (4.2 to 6.2 pcm-2) and not too acute an angle of rake. The objective is to obtain gentle opening of the fibre mass feed and easy transfer of the tuftlets to the cylinder. Angles of 80o – 85o are used for short and medium length cottons to give effective opening and cleaning. For longer cottons and synthetics, a 90o or negative rake may be needed to facilitate gentler opening and satisfactory fibre transfer to prevent lapping of the taker-in .
Fibres, usually very short fibres, which are not adequately held by the teeth or present in the interspaces of the clothing are ejected causing fibre loss. However, it is the mote knives that govern the amount of fibre to trash (i.e. lint) in the extracted waste. Experimenting with the settings of two mote knives below the taker-in, Hodgson found that the absence of the knives greatly increased the lint content with little increase in trash. With the knives present, the best setting was that which gave the least waste since increasing the amount of waste did not improve cleaning. Artzt found that irrespective of teeth density and tooth angle the waste increased with taker-in speed but the increase was attributed to higher lint content.
It is reasonable to assume that the smaller the tuftlet size and the greater the mass ratio of individual fibres to tuftlets the better the cleaning effect of the taker-in. Supanekar and Nerurkar suggest that the takerin breaks down the fibre feed into tuftlets of various sizes and mass, conforming to a normal frequency distribution. In the case of cotton, some tuftlets may consist of only fibres whilst others will contain seed or trash particles embedded among the fibres, these tuftlets constituting the heavier end of the distribution curve. Thus, the mean of the distribution would depend on the trash content of the material, as well as on the production rate, the taker-in speed and the wire clothing specification. However, the authors did not report any data to support their ideas.
Little detailed information has yet been published on the mass variation of tuftlets or on the relative proportion of discrete fibres to tuftlets resulting from the combing action of the taker-in. Nittsu using photographic techniques studied the effect of process variables on tuftlet size. It was found that the total number of tuftlets decreases the closer the feed plate setting, the lower the feed rate, the smaller the steeper rake of the saw-tooth clothing and the higher the licker-in speed. Since th licker-in opens the batt into both tuftlets and individual fibres , a decrease in the total number of tuftlets suggests an increase in the mass of individual fibres. Liefeld calculated estimates of the opened fibre mass at various stages through the blowroom and gives a value of 50mg for tuftlets on the taker-in. Mills claims that the calculated optimum number of fibre per tooth is one, and that this should be maintained at increased production rates by increasing the taker-in speed. There is, however, the question of fibre damage at high taker-in speeds.
Figure 2: Frequency distribution of tuftlet mass
N: Taker-in speed (rpm), P: Production rate (kg/hr)
Honold and Brown found no fibre damage occurred at speeds of up to 600 r/min. Krylov reports the absence of fibre breakage at speeds up to 1,380 r/min, and Artzt’s work shows taker-in speeds to have a negligible effect on fibre shortening and subsequently on yarn strength. In all cases cotton fibres of 26.5- 30.2 mm (2.5% span length) and 3.8 – 4.9 micronaire were processed. The level of fibre breakage, however, would seem to depend on production rate and the batt fringe setting to the licker-in. High production rates achieved by increased sliver counts and a close setting of the batt fringe result in significant fibre breakage.No fundamental studies have been reported on the forces involved in the fibre-wire interaction of revolvingflat card components. However, Li and etal report a simulated study of fibre-withdrawal forces for wool in high-speed roller- clearer cards. Although impact forces could cause damage , it was found that card component speeds had no significant effect on the withdrawal-force, and that fibre configuration and entanglement were the important factors.
The importance of producing small size tuftlets is evident form the various components fitted in the fibreopening zone on modern short-staple cards. Saw-tooth wire covered plates, termed combing segments, fitted below the taker-in or built into the taker-in screen are claimed to give improved trash removal. Reportedly , the stationary flats fitted between the taker-in and the revolving flats provide extra opening of the tuftlets transferred to the cylinder from the taker-in.
They also act as a barrier to large, hard, trash particles such as seed coats, protecting the wire of the revolving flats from damage, particularly at high cylinder speeds. This enables finer wire to be used for the revolving flats and thereby improves the cleaning effect of the interaction between cylinder and revolving flats. The chances are also reduced of longer length fibres becoming deeply embedded in the revolving flats to become part of the flat strips. These attachments are widely accepted by the industry as beneficial, particularly at high production speed. However, there is no published systematic study of their effectiveness in reducing tuft size, and the effect of stationary flats on the recycling layer, Q2, is unknown. The little information that is available attempts to illustrate the effectiveness of these components on yarn quality, but there is no evidence of analytical rigour in the way the data were obtained.
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