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Fibre Dynamics in the Revolving-Flats Card - 2

carding revolving flat

 

Fig 3. shows the effect of the combing segment and the stationary flats on dust deposits in rotor spinning and on the imperfections in several types of ring spun yarn. The figure includes values for the effect of stationary flats above the doffer, but this will be considered in a later section. It would appear that the added components in the taker-in region might well reduce the dust deposit in the rotor, but the results showing improvements in yarn quality are not convincing, and in all cases the stationary flats above the doffer appear the most effective. Leifeld  reports that the cylinder – revolving flats carding action occurs when the fibre mass delivered to the cylinder is in a highly opened state.

Tandem cards are said to give a high standard of carding with low nep and trash levels in the card web. This is because a uniform web of almost discrete fibres is fed to the second cylinder of the tandem card and closer revolving flat settings with higher cylinder speeds can be used . Single taker-in systems, even with combing segments and stationary flats, cannot give as high a degree of opening. However, Leifeld reports that a triple taker-in system facilitates high taker-in speeds and, fitted to a single-cylinder card, feeds a uniform web of discrete fibres to the cylinder, thereby offering a more cost-effective process than the tandem card, but no comparative data for the two types of card are given. Although it may be reasoned that a triple taker-in action should improve nep removal, it is of importance to compare the web qualities with regard to dust and trash content, the level and type of fibre hooks, and the degree of fibre parallelism since these greatly influence yarn quality.

 

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Figure 3: Effect of Combing Segment and Stationary Flats

 

Contradicting the triple taker-in approach, Mills  states that the fibrous material fed to the card should not be broken down into individual fibres by the taker-in system. This is because the fibres would remain largely disoriented with a high proportion of them lying transversely to the direction of mass flow when transferred to the cylinder and subsequently to the revolving flats. This can result in fibre loss during transfer to the cylinder and an unevenness of the fibre mass across the cylinder width, causing neps to be formed and degrading the carding action between the cylinder and the revolving flats.

It is claimed that good carding requires a thin, uniformly distributed sheet of well-opened tuftlets fed to the cylinder from the taker-in. Fujino  reports results that would appear to confirm the view that as the level of opening increases through faster taker-in speed, the degree of fibre parallelism on transfer to the cylinder decreases. The nep level in the card web was, however, observed to decrease noticeably with increased taker-in speed. This was attributed to the reduced speed ratio of the cylinder and taker-in. Artzt found that reducing the takerin/ cylinder draft ratio from 2.4 to 1.4 caused yarn imperfections to increase. In contrast to these findings Harrison states that increasing taker-in speed did not affect the nep level in the card web, the exception being for low micronaire cottons. The apparent contradictions in these results suggest that a better understanding of the transfer mechanism may be needed which takes into account fibre properties. Fibre Mass Transfer to Cylinder

Two contrasting views have been reported on the mechanism of fibre transfer. Oxley  suggests that the fibre mass on the taker-in is ejected between the cylinder wire and the back plate. Whereas Varga  believes that the fibre mass is stripped from the taker-in in the following way. In the feed to the card, tufts and fibres lie randomly and by the action of the taker-in are brought into length-wise orientation in the direction of the roller rotation. The trailing ends of newly formed tuftlets protrude above the taker-in wire and are easily stripped by the cylinder wire clothing. This implies that the transfer involves a reversal of the leading and trailing ends of the fibres. Further orientation and parallelism of the fibre mass is thought to occur during the transfer onto the cylinder. No experimental work has been published which specifically involves a study of the transfer of fibres from the taker-in to the cylinder. Therefore it has yet to be established whether at the interface, the cylinder, which has the faster surface speed, strips the fibre mass with its clothing or the taker-in, through the action of centrifugal forces, ejects the tuftlets and single fibres onto the cylinder, or a combination of both occurs. It is also of interest to determine if the airflow in the region assists the fibre mass transfer. Whatever the case, the fibre mass is likely to be subjected to an uncontrolled drafting effect, which could introduce irregularities in the mass flow.

Zone 2: The Fibre Carding Zone

In the carding zone, it is the interaction of the fibre mass and the wire-teeth clothing of cylinder and flats that fully individualises the fibres and gives parallelism to the fibre mass flow. In considering how fibres enter and are individualised in the carding zone, Oxley   suggests that tuftlets are not strongly held on the cylinder clothing because the tooth angle faces the direction of cylinder rotation. They are, thus, easily removed and more firmly held by the opposing teeth of the flats. It is therefore assumed that as a flat enters the carding zone it becomes almost fully loaded with fibres, the airflow within the region assisting the fibre mass transfer. Having been stripped of fibre mass, subsequent following areas of the cylinder wire clothing move past the fully loaded flat and proceed to comb fibres from the flat, carrying them towards the doffer. The action of combing causes the fibres to be hooked around the cylinder wire points and prevents them from being easily removed by other flats.

Debar and Watson’s  experiments of the movement of radioactive tracer fibres through a miniature card showed that some fibres caught by the flats were often only removed by the cylinder-wire clothing after many revolutions of the cylinder. Varga  reports an alternative view to Oxley’s, stating that two types of action occur at the cylinder-flats interface. First, a carding action where the upper layer of a tuftlet or a loosely opened fibre group is caught and held by the flats whilst simultaneously the bottom layer is sheared away by the fast moving cylinder surface. This action causes the top to hang from the flats and to contact subsequent parts of the cylinder wire surface resulting in the second action which is combing, where the wire clothing of the cylinder hooks single or a small group of fibres and combs them from the top layer. A second flat catches the bottom layer on the cylinder and the actions are repeated. In this way tuftlets or groups of fibres are separated into individual fibres.

By making abrupt changes in the colour of the fibre mass fed to the card, Oxley  demonstrated that tuftlets from the load on a given flat are carried forward by the cylinder clothing and separated into individual fibres over a small number of preceding flats, typically 4. It was concluded that the interchange of fibres between cylinder and flats does not occur over the full carding zone. Sengupta ] made measurements of the carding/combing forces and showed that essentially these actions were on average confined to the first ten working flats.

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Figure 4: Relation of Flat Load and Working Time

A study by Hodgson   showed that moving in the direction of the cylinder rotation, a given flat acquires two-thirds of its final load directly it comes into position over the cylinder. The load then increases exponentially with time, reaching nine-tenths of the final value within 6-8 minutes. Completion of the load takes place slowly during the remainder of the working time. See Fig 4. As shown in the figure, with flats moving in the reverse direction the load first increases rapidly with time and then slows until the flat is about to leave the working area. Here it encounters the fibre layer being transported on the cylinder surface from the taker-in. The flat receives a sudden addition of fibre mass to become fully loaded, and, in agreement with other results , the load weighs more than for the forward direction of motion. Contrary to Oxley’s conclusions, it was found that 30% of the final load on a given flat resulted from fibre interchange between flats and cylinder over the full carding zone.

It may be reasoned that the number of flats involved in separating a tuftlet depends on the tuftlet size, the mass flow rate and the flat setting. Large tuftlets will be pressed into the cylinder wire during the carding action, whereas small tuftlets will be more easily carded and will remain at the top of the cylinder wire teeth. The larger the tuftet, the higher the production rate and the closer the flat settings, the greater the number flats involved in the separation of a given tuftlet. Bogdan  reports that flats tend to load quickly at the beginning of their cycle of contact with the cylinder. This, however, is only a partial loading, since the fibre mass tends to resist more fibres entering the space but, in the case of cotton, not the leaf and trash particles present.

Analysis of the trash in cotton flat strips showed that initially the percentage of trash in a given flat strip is low and increases slowly during the first 10 minutes of carding, then remains at almost a constant value . The final percentage depends on the trash content of the cotton. For a fixed production rate, the amount of flat strips was found to be directly proportional to the flat speed, but provided the speed was such that the working time was not less than 10 minutes, both the weight and composition of the flat strips remained approximately constant. Feil claims that a high degree of air turbulence exists in the flat/cylinder zone. A combination of centrifugal forces, mechanical contact with the flat wire and air turbulence causes the trailing ends of fibres attached individually to the cylinder clothing to vibrate and shake loose trash and dust particles. Short fibres which cannot adequately cling to the cylinder clothing will also be shaken free, and along with impurities become part of the flat strips.

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