In the conventional nonwoven carding practice, the major carding zones include the area with feed roller/lickerin, worker/cylinder and cylinder/doffer. On the cylinder surface, the fibers are taken by the cylinder carding wire teeth moving at very high surface speed and radical forces with “combing effect” are exerted on the fibers when the fibers encounter and are transferred to the slower moving worker carding wire teeth which are set very close to cylinder teeth at certain positions. During this process, the fibers are opened, individualized, straightened and aligned, or simply stated, carded.
However, it is well known and proven in the textile industry that the fiber transfer from cylinder to doffer is inefficient, i.e., a decent amount of residual fibers remain held by cylinder carding wire teeth to constitute the recycled fibers. Some measurements even show that doffer efficiencies can be as low as 0.1 (J.P. Rust and E. Koella, Carding Fiber Load Measurement, Textile Research Journal. 1994; 64 (6), pp. 364 – 369).
Although beneficial fiber blending and mixing is generated from the fiber recycling, the inefficient transfer from cylinder to doffer brings about the following major negative effects:
• Quality. Recycling may likely damage or break the fibers, which generates short fibers and consequently excessive neps out of the short fiber rolling, poor tuft opening and poor web quality as well as more waste.
• Output. Recycling reduces the amount of fiber transferring which may impact the output of a card. Moreover, in the presence of quality issues mentioned above, the only solution for the nonwoven producers has been to reduce the speed of the worker/doffer rollers until the expected web quality appears. This correspondingly shrinks the output of the card and boosts the manufacturing cost.
• Efficiency. Increased damaged fibers and reduced fiber transfer impacts the cleanliness of the cylinder and as a consequence the machine stops and cleaning cycles need to be introduced with a negative impact on efficiency as a result.
With the similar configuration, workers are sometimes considered as a smaller version of doffers. The higher speed setting of workers than doffers in the modern nonwoven production process together with the smaller roller diameter brings about a greater centrifugal force of fibers on worker wire teeth and correspondingly a larger tendency of uncontrolled fibers or even flying fibers, which significantly impacts the level of waste and web quality. This phenomenon is more pronounced in certain applications, for example, PPS and P84 fiber processing for filtration products, or recycled PET for geotextiles
Besides, limitation is set on the performance of workers by the fiber taking and holding capability or carding power, reflected by the amount of fibers taken and held together with the area of the carding zone. The effective carding zone is often measured by the spanning central angle, beyond which, carding is not happening. The bigger the central angle (given identical other parameters, such as roller diameter, gauge settings, roller speed, carding wire specs, etc.), the more carding time a fiber receives, or in other words, the more times a fiber gets carded by the teeth of the carding wires throughout the carding zone. In nonwoven production, the conventional worker or doffer wires, taking the main section for instance, usually have a front angle of 50 to 65 degrees with the height often ranging between 3.5 mm and 5.3 mm, which leads to the fixed dimension of the carding zone, given all the other aforementioned parameters unchanged. Such circumstances set a limitation on the size of this carding zone and the carding power. Insufficient carding power may bring about issues such as poor web quality with unopened fiber tufts, patches and neps.
It is commonly observed in nonwoven production that the aforementioned negative effects have constituted a bottleneck for nonwoven producers to further improve web quality, line output and fiber control performance. The root cause behind this bottleneck is the design of the conventional worker and doffer carding wire. As discussed above, the limitation is set by the conventional shape of the wire with a certain front angle and height. Although measures were taken to lower the front angle or add striations onto the conventional shape of the wire or make rougher contact surface, only limited benefits are observed in the practice and the bottleneck of web quality, output and fiber control remains.
Web quality improvement and output increase as well as fiber control optimization, eagerly wanted by nonwoven producers, are however blocked by the conventional worker and doffer design. In response to this market need Bekaert Carding Solutions has developed a new generation of worker/doffer wires.
Three years ago Bekaert Carding Solutions successfully introduced SiroLock and high performing doffer/worker wire which features a substantially horizontal step undercut on the tooth front. This horizontal step enables the ‘locking’ of fibers, which results in superior fiber taking and holding capabilities and enhanced carding power almost independent from roller speed. Many implementations, especially in the area of high speed carding for spunlace applications (direct nonwoven manufacturing), have proven the high added value of this product.
Based on this buildup of experience in advance fiber control, Bekaert Carding Solutions is now further broadening its product portfolio of worker/doffer wires with EvoStep. This new wire is featured with an evolutionary step undercut on the tooth front with the greater aggressiveness than the angle of the overhang part below the tip. Compared to conventional wires, this type of step provides better fiber control that is initiated by a locally increased friction at the undercut. Therefore this makes this doffer/worker wire very suitable for medium speed carding typically used in indirect manufacturing of nonwovens (crosslapping and needlepunch).
The delicately designed undercut step in EvoStep brings the following benefits:
1. Increase of the fiber taking and holding capability or the carding power.
a. Compared to conventional wires, EvoStep is able to hold notably more fibers with the undercut step section.
b. Compared to conventional wires, carding zone is significantly enlarged with EvoStep. For instance, in the case where fiber length = 70mm, worker roller diameter = 220mm, cylinder roller diameter = 1500mm, EvoStep brings 17% increase of carding zone compared to 50° conventional worker wire. This means EvoStep still well holds the fibers at the positions where conventional wires have lost control.
2. Improved fiber control. Fiber control improvement is reflected in 2 aspects.
a. First, the aggressiveness of the step enables a better fiber behavior on the wire’s tooth front at the undercut step area. The friction exerted on the fiber of interest (= fiber retaining force) by conventional wire tooth is F1=μ*T*cos(θ) is notably lower than that by EvoStep wire tooth F2=μ*T*cos(γ) where μ is the coefficient of friction, N is the normal pressure force exerted on fiber by the tooth front, γ is the angle of the “step” and is significantly lower than θ.
b. Second, due to the increase of the carding zone, the fibers can be taken and held throughout a bigger range. It is clearly seen from the nonwoven cards equipped with EvoStep worker wire that the web is taken deep down to the nip between worker and stripper rollers, which facilitates significantly the stripping of the web. The improvement of fiber control enhanced the threshold value of the roller speed where the fibers start flying, compared to the conventional wires.
3. Increase of the transfer ratio. Due to the much stronger fiber taking and holding capability of EvoStep, more fibers will be transferred from cylinder to worker and to the doffer so that the transfer ratio is notably increased compared to conventional wires.
4. Further benefits include a lower risk of fiber recycling, loading and melting. It also reduces flying fibers, which leads to less consumption and lower waste. EvoStep wires are easier to clean and facilitate a smooth and quick start-up of the card, limiting downtime to an absolute minimum.
Due to the horizontal undercut step design on the tooth front, SiroLock “locks” the fibers with the geometric constraint instead of with the friction between tooth front and fibers, while EvoStep retains fibers with the locally increase friction on the delicately slanted evolutionary step aiming at the “fit for use” aggressiveness and correspondingly optimum fiber retaining capabilities. As a result, although SiroLock offers superior fiber taking and holding capabilities and the highest carding power, the extension of our product portfolio with EvoStep enables our customers to find the optimal solution for their specific nonwoven applications. Carding speeds (linear roller speeds) as a function of bonding technology, fiber types and fiber range are taken into account to determine the optimal solution.
For more information please visit www.bekaert.com/en/Product%20Catalog/BU%20News/Carding/Bekaert%20introduces%20Evostep.aspx