Fibre Dynamics in the Revolving-Flats Card - 3
Fibres that are deeply embedded in the flats, and cannot be reached by the cylinder wires become flat strips. For this reason the closeness of the flats setting to the cylinder is important. It may be assumed that closer flats/cylinder setting and faster cylinder speeds will give more effective carding and combing actions as described by Varga and thereby improve web quality through reduced neps and trash . Cylinder diameters vary and Karasev showed mathematically that for a given cylinder rotational speed the carding power will be greater for a larger cylinder diameter with a higher number of working flats. However, because of lower mechanical stresses, smaller cylinders can be rotated at higher speeds than larger cylinders.
The above advantage is therefore reduced the higher the speed of the smaller cylinder. Artzt studying the influence of card clothing parameters and cylinder speeds on yarn imperfections, report that the teeth density of the flats and cylinder, and the speed of the cylinder must prevent tuftlets lying within the spiral pitch of the cylinder clothing. If this occurs the tuftlets generally become the thick places in the yarn. It was found that high teeth densities and low cylinder speeds were as effective as lower teeth densities and high cylinder speed. High teeth densities with high cylinder speeds did not give effective carding, but no reason was reported for this.
Since the action of the cylinder in this region is to individualise fibres, the wire clothing on the cylinder has a steeper rake and a higher point density than the wire clothing of the flats. Thus, with closer settings and higher cylinder speeds greater forces may be involved and may result in fibre breakage. However, the work of Li indicates that the withdrawal forces needed to separate an entangled fibre mass was largely dependent on the density of the fibre mass and the contact angle fibres made with the wire clothing, than on the machine speeds.
Van Alphen reports that increasing cylinder speed causes more fibre breakage than increasing taker-in speed and that this is reflected in the yarn properties. Rotor yarn tenacity was reduced by up to 5% with increasing cylinder speeds between 480 –600 r/min. Whereas ring yarns showed a 5% reduction for speeds between 260 – 380 r/min and 10% at 600 r/min. The higher sensitivity of ring yarns to fibre breakage was attributed to the negative effect of short fibres during roller drafting. Krylov reports that no fibre shortening was observed for cylinder speeds up to 380 r/min.
It may be reasoned that the smaller the tuftlets and the more parallel fibres in tuftlets are to the direction of mass flow the lower the probability of fibre breakage. Honold attributes fibre damage to the cylinder/flat interaction and suggested that the degree of damage depends on the size of the tuftlets entering the working area; the smaller the tuftlets, the closer the setting that can be used and the lower the fibre breakage . Hodgson’s work showed fibre length is also an important factor. For cottons, fibre breakage was only found to have occurred when the staple length was greater than 25mm. Increasing the flat speed appears to have no effect on fibre breakage. However, the amount of flat strips increased proportionally with the flat speed and the mean fibre length of the strips increased significantly. This means that faster flat speeds result in larger amounts of useable fibre in the waste. Interestingly, when carding cottons, immature fibres were not readily found in the flat strips. The coarser rigid fibres seem more easily retained by the flats. The effectiveness of the carding and combing actions within the cylinder/flats area is, inter alia, dependent on the quantity of fibre mass on the cylinder, and this includes the recycling layer, Q2. It is of interest therefore to consider how the Q2 is formed during fibre transfer from cylinder to doffer, and its importance to the card web quality.
Zone 3: Cylinder / Doffer Interaction
Varga reports that the action of fibre mass transfer to the doffer is similar to the transfer at the input to the cylinder-flats zone. The regions above and below the line of closest approach of the cylinder to the doffer (i.e. the setting line) are important to the mechanism of fibre mass transfer and the transfer coefficient, K. The two regions may be termed the top and bottom co-operation arcs or top and bottom zones. Simpson claims that transfer can occur in both zones and that the particular region in which transfer actually occurs influences the fibre configuration and the nep level of the card web, although cylinder-flats action is more important in reducing neps. Which zone transfer occurs in is dependent on the cylinder-doffer surface speed ratio, C/D. For high C/Ds, transfer occurs in the top zone and results in a larger number of trailing than leading hook fibres and a low nep level. The reverse occurs when transfer takes place in the bottom zone owing to lower C/Ds. Simpson does not however say at what C/D value transfer changes from one zone to the other. Although reference is made to other authors who have proposed a mechanism for fibre transfer in the top zone, no mechanism or experimental evidence is given to support the idea of fibre transfer in the bottom zone. Lauber and Wulfhorst used laser-doppler anemometry and high-speed cine photography to study fibre behaviour in the bottom zone, i.e. up to110 mm below the setting line. Their findings showed no evidence of fibre transfer within the bottom zone.
Since Morton and Summers’ work in 1949 other researchers have confirmed that the values given in Table 1 for the five classes of fibre configuration observed in slivers. It is of interest to note that the hooked lengths are greater for leading than trailing hooks. Although, the calendar draft can be used to change the relative proportions, Gosh and Bhaduri showed that the method of removing the web from the doffer does not influence the propensity of any class of configuration. It is the mechanism of transfer that is seen as principally responsible for the shape fibres have in the sliver.
Table 1: Classification of Fibre Configuration in Card Sliver
Several studies have been reported on the fibre-mass-transfer mechanism. A number used tracer fibres with one end of a fibre dyed a different colour from the other. The reported findings suggest that fibre mass transfer occurs by fibres acting independently and not as a web of fibres. Observations showed that prior to transfer, nearly 70% of fibres on the cylinder had leading hooks, only 9% had trailing hooks. On transfer the relative proportions changed as indicated in Table 2. Half the number observed underwent reversals, with greater than 70% changing their configurations [e.g. leading hooks becoming trailing hooks]. Of those that transferred without reversals ca 90% did so with a change of configuration.
Table 2: Mode of Fibre Transfer from Cylinder to Doffer
Ghosh and Bhaduri report that tracer fibres were noted generally to go around with the cylinder for several revolutions before being transferred by the doffer. On occasions transfer only happened when the cylinder speed was increased. Debar and Watson’s work with radioactive viscose tracer fibres showed that a fibre on the cylinder wire passes the doffer up to a maximum 20 times before being removed by the doffer, sometimes interchanging several times between the cylinder and flats, during the 20 revolutions on the cylinder. Hodgson found that cotton fibres make between 10 and 25 cylinder revolutions before being removed by the doffer. With the continuity of fibre mass flow through the card, this means that the doffer web is built up over many cylinder revolutions and that the recycling layer, Q2, is comprised of multiple fractional layers of the fibre mass transferred from taker-in to cylinder during these cylinder revolutions .
Figure 5: Mechanism of fibre transfer for trailing hook formation
A proposed hypothesis for the mechanism of fibre transfer is illustrated in Figure 5. Here the trailing ends of fibres are lifted from the cylinder surface by centrifugal forces and become hooked around the teeth of the doffer clothing. The frictional drag of the doffer clothing eventually removes these fibre from the cylinder clothing. This mechanism only explains the formation, without reversal, of trailing hooks in the doffer web. However, the importance to fibre transfer of the relative angles and tooth lengths of the cylinder and doffer is self evident from the figure. Baturin developed equations that showed the importance of tooth angle and teeth density of the cylinder and doffer wires to the value of K and thereby Q2. Other investigators have reported experimental data that verify Baturin’s equations. It was found that the more acute the working angle of the doffer wire compared to the cylinder wire, the higher the value of K, and the lower Q2, and that higher teeth densities on the doffer increased K. These findings would tend to suggest that the proposed mechanism is a principal action by which fibres are removed from the cylinder. However, this mechanism of fibre transfer does not explain the change of fibre configuration with reversals and the formation of leading hooks in the doffer web. It also does not explain how fibres forming the recycling layer, Q2, are subsequently removed, even though an input layer of fibre mass is added to Q2 each time it passes the taker-in.
The above studies did not take account of the degree of fibre parallelism on the cylinder prior to transfer, nor the number of fibres per tooth on the cylinder and consequently the likelihood of fibre interaction during transfer. Fujino and Itani used a microscopic technique to observe the orientation of fibres on the cylinder surface above the taker-in and just before the doffer, and in the doffer web. They found that fibres showed the highest degree of parallelism when on the cylinder surface just above the doffer. The degree of parallelism decreases on transfer to the doffer, and further deteriorates when the web is removed from the doffer to form the sliver, even though the calendar draft helps to maintain some degree of parallelism. Grimshaw and others report the use of fixed flats just before the cylinder/doffer top transfer zone, to improve fibre parallelism in the card web.; up to 20% reduction in fibre hooks and 25% improvement in fibre parallelism were obtained in the card web, resulting in improved yarn properties. Figure 3 shows that the fixed flats in this region are more effective in improving yarn properties compared with the fixed flats above the taker-in. The action of the flats fitted above the doffer is not fully understood. It is assumed that they tend to lift the fibres to the tip of the cylinder wire for more effective transfer to the doffer, particularly at high cylinder speed. Lauber and Wolfhorst , Kamogawa, report that in this region aerodynamic forces affect the parallelism of the fibres and the way they are transferred to the doffer. However, no details are given.
Owing to the higher speed and larger diameter of the cylinder, it is assumed that during transfer in the top zone the fibres are more substantially affected by the flow of air transported with the cylinder’s than by the doffer’s wire clothing. High-speed photographs showed that in the bottom zone the main flow of fibre mass was with the doffer at close to the doffer speed, even when the fibres were just below the cylinderdoffer setting line. However, some fibres were seen to be free of both the doffer and cylinder and tended to move with the air currents and eventually with the motion of the cylinder surface. From the above discussion, it can be seen that work is still needed to establish a more detailed understanding of fibre mass transfer between the cylinder and doffer. The results of such work may also help in better explaining how fibres remain on the cylinder to form the recycling layer Q2.
Varga suggests that with fibre transfer in the top zone, the thicker layer of web on the doffer surface protrudes above the doffer wire and into the gap setting between doffer and cylinder. The faster moving cylinder wire clothing combs through the doffer web and thereby pulls fibres back onto the cylinder surface. De Swann showed that fibres can be readily transferred from the doffer to the cylinder as well as from cylinder to doffer. In Hodgson’s study , changing cylinder/doffer setting affected the neppiness of the web but did not affect K, which seems to contradict Varga’s view. Baturin and Simpson however showed that K will increase if the region of interaction between the cylinder and doffer is reduced by decreasing the doffer or the cylinder diameter and this tend to supports Varga’s suggestion for a combing and robbing action of the cylinder. It is reasonable to assume that the combing action could lead to fibres in Class II and IV (Table 1), but there is still no verified explanation of how fibres in Classes I, III, and V are formed, with and without reversals.
Much of the research on the cylinder / doffer interaction concerns the effect of machine variables on the size of Q2 (or the operational layer, Qo), on the web quality and changes to the relative proportions of the classified configurations, and on ultimately the yarn quality. Sing and Swani developed a Markovian model for the carding process in order to determine the probabilities of fibre transfer between cylinder and flats and cylinder and doffer, taking into account the recycling of fibres. It was shown that the times spent by a fibre on the cylinder, Tr, and in the flats/cylinder region, Td, are given by:
Tr = 1 / K and Td = Tr . Pf …………. (1)
Where K = Q1 / Qo and Pf = Qf / Qo
Reported values for K would seem to vary between 0.2% to 20% , depending on doffer and cylinder speeds, on the relative profiles of the saw-tooth wire clothing, and on the sliver count. Simpson suggests that fibre properties are also of importance, in that there is a tendency for low micronaire cottons to give higher cylinder loading and for fibres with low shear friction and good compression recovery to result in higher K values. No physical explanation is given for these findings and no other studies are reported on the effect of fibre properties. Further work is therefore needed in this area.
Figure 6: Effect of Cylinder and Doffer Speed on K and Pf
A popular view is that a low fibre mass entering the cylinder/flats interface, i.e. a low fibre load on the cylinder, results in better quality carding . This would seem to imply that the higher the value of K the better the carding since less fibre mass is recycling to be added to the mass transferred from the taker-in. However, there are several ways of increasing K and not all of them result in improved carding quality. Figure 6, shows that for a given cylinder speed and sliver count, increased doffer speed increases K and reduces Pf , whereas keeping the doffer speed and sliver count constant and increasing the cylinder speed increase both K and Pf . For constant cylinder and doffer speeds, increased sliver count was found to reduce K and Pf. If the same up stream machinery is used, then the best measure of effective carding is the quality of the carded ring-spun yarns produced . Gosh and Bhaduri’s work showed that for a fixed carding rate, with increasing doffer or cylinder speed, K increases but Qo and the yarn imperfections decrease; no trend was found with yarn tenacity or irregularity. Singh and Swani studied the properties of yarns made from slivers corresponding to differing K and Pf values and found that Pf was the more important of the two parameters, in that the higher the value of Pf the better the yarn quality. Kaufman reports that the lighter the fibre load is on the flats, the better the carding quality. Thus, the use of Pf does not give an adequate understanding of the importance of the recycling layer nor of the size of the fibre mass load at the cylinder/flats interface.
Figure 7: Effect of Doffer Speed on Carding Parameters
Baturin reports an alternative approach to the above in which the following expression was derived for the number of cycles, Np, under steady state conditions that fibres on the cylinder clothing make pass the flats before being removed by the doffer:
Np = 1 + Vc/KVd ………….. (2)
Where K is the transfer coefficient
Vc and Vd are cylinder and doffer surface speeds (m/min).
Since this gives the number of times the recycling fibre mass is subjected to the carding action, it may be a better indication than Pf of the importance of Q2. From the expression, Np decreases when K increases by increasing doffer speed. Figure 7 shows that for a constant production rate, web quality decreases when Np decreases with doffer speed, even though the cylinder load decreases and a high number of cylinder teeth per fibre is obtained. The last two parameters are usually taken as indicative of good carding. Figure 8 shows the effect of increased doffer speed and sliver count on web quality and there is a consistent trend which suggests that increasing the production rate by increasing the sliver count, instead of doffer speed, gives better web quality. With regard to sliver irregularity, several investigators report theoretical and experimental studies showing that increasing the recycling layer, Q2, reduces the short-term irregularity.
Figure 8: Effect of Doffer Speed and Sliver Count on Web Quality
Karasev attempted to show experimentally the importance of Q2 by removing it during carding using a suction extractor. It was found that without Q2 a large proportion of the fibre mass transferred from the takerin became embedded into the empty teeth of the cylinder clothing. Only the larger tuftlets and groups of individual fibres would then be subjected to the carding and combing actions. Hence, there is a greater chance of small groups of entangled fibres being removed by the doffer. Q2 therefore acts as a support to new layers of fibre mass being transferred form the taker-in, keeping the new fibre mass at the tips of the cylinder wire teeth and thereby promoting the interaction of tuftlets with the flats and cylinder clothing. This idea, however, does not facilitate an explanation of the mechanism by which fibres leave the recycling layer to form part of the doffer web, Q1 . Gupta suggest that the rotating cylinder could be considered as a large centrifuge that would cause fibres, impurities and seed fragments to migrate to the cylinder periphery and thereby make contact with the flats clothing and, presumably, the doffer teeth.
However, no experimental verification of this hypothesis is reported. Many of the authors have reported the effect of machine variables on fibre configurations within the card sliver and several have related yarn properties to the observed configurations. Generally it was found that for a fixed sliver count increasing the carding rate by increasing the doffer speed, increased the number of minority hooks and reduced the number of majority hooks, irrespective of cylinder speed. However, for a given doffer speed, increased cylinder speed gave the reverse trend for minority hooks, but no clear trend for majority hooks. Baturin and Brown showed that increased cylinder speed decreases cylinder load owing to the effect of centrifugal forces and Simpson showed that increased cylinder speed also increased minority hooks and decreased majority hooks. Bhaudri reports that when the fibres are forced nearer the surface of the cylinder teeth, either by increasing the fibre load or increasing the centrifugal force on the cylinder, the proportion of minority hooks increases. Simpson found that there was a direct relation between yarn imperfections and increased occurrence of minority hooks and that spinning end breakage rates and yarn imperfection increased with increased card production speed owing to minority hooks. Gosh and Simpson found that heavier slivers had fewer minority hooks. However, the increased draft needed to process the heavier slivers into yarn led to increased yarn imperfections.
1. The taker-in action separates the fed fibre mass into tuftlets and individual fibres. Although it is reported that the taker-in action gives a normal mass distribution of tuftlet sizes, this is speculation. Little research has been reported on the effect of taker-in parameters, fibre properties and the blowroom process on tuftlet size distribution and on the relative proportions of tuftlets to individual fibres.
2. The perceived benefits of combing segments built into the taker-in under-screen and of stationary flats fitted before and after the revolving flats are well known, but only limited experimental findings have been reported to support the use of these attachments. There are conflicting views on the benefits of triple taker-in systems, concerning whether the fibre opening by such systems would give a high misalignment of fibres to the direction of mass flow during transfer to the cylinder and degrade the subsequent carding action. A better understanding is therefore required of the fibre mass transfer from taker-in to cylinder, since the surface speed ratio of these components is seen as a key factor in the proper functioning of high production cards.
3. The cylinder-flats and cylinder-doffer interactions have been well researched. Published findings show that each flat acquires two-thirds of its load at the beginning of its cycle of contact with the cylinder, and that separation of a given tuftlet occurs over a few flats. With regard to clothing parameters and cylinder speed, high teeth densities and lower cylinder speeds gave similar results to the converse arrangement. However, a high teeth density and cylinder speed did not give effective carding. Results showed that high cylinder speeds caused more fibre breakage than high taker-in speed.
4. A high cylinder to doffer speed reduces cylinder load, gives a higher K value and a better web quality. Increasing doffer speed was also found to increase K, but the web quality deteriorated. The reported mechanism of fibre transfer from cylinder to doffer does not adequately explain the effect of the cylinder– doffer speed ratio, or the various reported changes in fibre configuration during transfer. Further work is therefore still needed in this area.
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