Rotor spinning has a great potential to reduce the twist factor, which is the basic prerequisite for the production of knitted fabric with a soft touch; by support of a new spin box generation and its components, explains Nitin T Patil, Prof Dr SP Borkar, Dr Stephan Weidner-Bohnenberger and Marc Schnell.
The yarn structure is dependent primarily upon the raw material, spinning process, spinning principal, machine settings, twist, etc.
The structure can be voluminous or compact, high or low hairiness; soft or hard; round or flat.
Yarn structure has a greater or lesser influence on hand, strength and elongation, resistance to abrasion and dye absorption.
Rotor and airjet yarn compare to ring spinning, offer the advantages such as high production rates, elimination of processing stages, considerable reduction in personnel and space and energy consumption and relative ease of automation.
On other side, yarns produced from new spinning systems like rotor and airjet spinning have harsher feel compared to ring spun yarn due to the different outer structure of the yarn. With the proper selection of raw material, spinning components and machinery setting, now it would be possible to compensate in order to improve softness even on new spinning systems.
With this the gap between new spinning system and ring spun yarn can be narrowed down without compromising the parent benefits like better abrasion and pilling resistance of rotor and airjet spun yarn respectively.
Ring spinning: In ring spinning, the fibre mass in cross section of the roving is reduced by a drafting unit. The twist inserted by the turning spindle moves back and reaches the fibres leaving the drafting unit. The ring yarn process is characterised by high flexibility in the use of the raw material, the yarn count and the yarn character. Ring yarn possesses a high degree of strength and yarn hairiness.
Rotor spinning: In rotor spinning, the input fibre strand is a drawn sliver. A sliver may have more than 20,000 fibres in its cross section. This means that a yarn of 100 fibres per cross section will require a total draft of 200. This amount of draft is substantially higher than that applied in ring spinning. The separated fibres are fed into the turning rotor by an air stream. The fibres are collected in the rotor groove and collected from there by the rotating open yarn end and bound in by twisting.
The yarn then is pulled out of the spin box via the nozzle. Loose fibres in this moment are wrapped around the yarn, resulting in the characteristic wrapper fibres or belly belt fibres on the surface of the yarn. Depending on the shape of the rotor groove the yarn shape results more or less compact. The higher the centrifugal force due to rotor speed and rotor diameter, the yarn needs to be stronger and has less elongation.
Airjet spinning: In airjet spinning, a sliver is fed to the drafting system; the drafted sliver enters a spinning nozzle. The leading end of the fibres forms the parallel yarn core; the free fibre ends are wound around the yarn core by the air in the spinning nozzle. The airjet yarn is then wound onto a package. The airjet-spun yarn structure consists of core fibres without significant twist and covering fibres with a genuine twist, which ultimately produces the corresponding yarn tenacity. The specific yarn structure results in yarn tenacity between that of a ring-spun yarn and that of a rotor-spun yarn.
Rotor spun yarn
The study is carried out on rotor yarn with same material, same count and same conditions:
Latest rotor spin box technology: The possibility to reduce twist in spinning is limited by the spinning stability. The cohesion of fibres gets less with reduced twist, so that the spinning process gets interrupted by a yarn break more and more frequently resulting in loss of efficiency of the process.
The newest spin box technology applied for the trials in this respect allowed additional twist reduction compared to older technologies, which are still present in the market.
This better spinning stability is due to:
- Reduced spinning tension thanks to latest spin box geometry, new TWISTstop arrangement.
- Lower spinning tension allows to reduce possible twist multiplier
- With the correct spinning component it is possible to produce soft yarn
- TWISTstop insert with three twist-retaining bars for high twist-retaining effect
- High-spinning stability
Effect of twist reduction:
- Less twist typically results in softer yarn
- Less twist at same rotor speed results in higher productivity
- Evenness and count variation gets better, as well as the imperfections
Rotor yarn properties evaluation: The basic study is conducted with Ne 30’s cotton yarn with different twist levels to see the impact on physical yarn properties such as yarn strength, elongation, yarn hairiness and yarn packing density.
Yarn strength and elongation: Table 2 and graph 4.1 show the increase of yarn strength and elongation of yarn with increase in twist multiplier (TM) value. In the other hand, graph 4.2 shows also how the TM of yarn influences the hairiness of yarn. The hairiness has great impact on yarn softness. Low twist causes a more open yarn structure, with the result that single fibers spread out of the surface of the yarn structure and thus achieve better softness. In rotor spinning, the yarn hairiness can be designed by means of a wide range of spinning elements like rotor groove and draw-off nozzle. It has been verified during this study that today with latest spin box there is the possibility to produce yarn with a lower twist multiplier without affecting spinning stability.
The packing density, i.e., the fibre distribution in a yarn cross section, can significantly influence the properties and quality of the yarn. Thus, the need for precise and concise information about packing density becomes a must for an in-depth understanding of yarn structure and hence yarn mechanics. It is also clear that twist factor is responsible for packing density changes. For a given yarn count, fibres in the yarn with a higher twist factor are distributed in less scattered way, and the peak of packing density curves will shift towards the yarn axis. Similar impact has been observed with diameter change: Increased twist results in increased yarn density and reduced yarn diameter. During the twisting process, an external uniaxial tensile load P along the axis of yarn must exist, since twisting cannot process without tension. This tensile load can be conveyed to the constituent fibres of the yarn. Assuming that the fibres are perfectly flexible members, incapable of resisting any axial compressive forces, the stresses M (a bending moment), V (shear force) and T (a torsional moment), will all vanish. Therefore, the only fibre force that needs to be considered is the tension acting in the direction parallel to the fibre axis Pr. Since the individual contribution of Pr to total load Pc in compression is, where, m is the total amount of fibers in the yarn cross section. Equation two states that if the axis tension carried by each fiber is multiplied by the sine of the helix angle of the fiber and all such products are summed. The total will represent the compression load acting on the yarn in the direction normal to the yarn axis. Apparently, the higher the twist level, the higher the helix angle F and in turn the more prominent the compressive load. This is how a more compact yarn structure is created.
Role of spinning systems in improving softness: Yarns produced from new spinning systems like airjet have harsher feel as compared to ring spun yarn due to their different outer structure. With the proper selection of raw material, spinning components and machinery setting, now it would be possible to compensate in order to improve softness even on new spinning systems. With this, the gap between new spinning system and ring spun yarn can be narrowed down without compromising the parent benefits like better abrasion and pilling resistance of airjet spun yarn.
Development of soft yarn: Softness is an important characteristic for those products which come directly in contact with skin .Ways to produce soft yarns.
- Raw material, i.e., to use fine fibres.
- Process parameters, i.e., spinning under lesser tension.
- Incorporation of low twist and plying.
- Production hollow yarns using polyvinyl alcohol (PVA) fibres.
Improvement in softness during spinning
- To improve the softness of airjet yarn with modified yarn structure with newly developed components and process parameter
- To improve fabric feels of airjet yarn by chemical finishing, e.g.: softeners to match with ring.
Comparative analysis of yarns, fabrics and garments with different subjective and objective tests
Spin nozzle housing: The spin nozzle is the heart of the airjet spinning technology. The bottom part of the spin nozzle consists of a fibre-feeding element (FFE), to which a drafted and condensed sliver is delivered. The prepared sliver is spun into yarn due to the effect of pressurised air inside the spinning nozzle. In the nozzle housing, there is a twist element with holes through which pressurised air is supplied at a volume equal to that of the diameter of the spin air holes and air pressure. Air is discharged at an angle to generate a whirl wind. Fibres are fed through the spin tip with the diameter of nozzle. Air rotates around the spin tip and creates a twist in the fibres.
Airjet yarn properties evaluation: Basic study is conducted with Ne 30’s cotton yarn with different twist levels to see the impact on physical yarn properties such as yarn strength, elongation, yarn hairiness and yarn packing density and influencing together the softness of the yarns.
Yarn strength and elongation: It is an overview of the different trials to improve the softness of the airjet yarns. Different delivery speeds, different air pressure and spin tip diameter influencing the yarn properties. E5 and E6 with low delivery speed and high air pressure results in highest yarn strength and elongation. The yarn packing density is also on the high side.
Lower speed and/or higher air pressure increases simultaneously elongation and strength due to the higher twist effect on the yarn. The soft feel of fabric can be better if ?1+2 hairiness value is higher.
Low twist causes more hairiness in the yarn due to the open structure, visible as well in the yarn packing density.
Yarn hairiness: However, low hairiness is a drawback where a large number of protruding fiber ends contribute to a soft hand. First of all, airjet spun yarns can now be produced with significantly lower twist multipliers than previously, without any adverse effect on spinning stability.
Airjet yarn packing density and diameter: The impact of air pressure ,delivery speed and spin tip is visible on yarn packing density and yarn diameter. Factors affecting real yarn diameter are essentially those that affect yarn density or fibre compactness. In general, yarn become softer as the density become lower and diameter become higher.
An enumeration of various research revealed that all the studies were interested in the improving the fabric hand for rotor and air-jet yarn. The objective of this work is to validate and analyses the impact on the yarn packing density and yarn diameter of rotor and air-jet yarns with determining the more influential intrinsic yarn parameters like strength, elongation, etc. In this study, the authors provided a base to carry out further investigation in knitted fabric for fabric hand due to spinning and processing. The twist increase results in a free air volume reduction hence proportionally impact yarn packing density. A statistical study allowed the authors, by using ANOVA test to retain the most influential factors in the yarn packing density, the yarn count and the twist in yarn by keeping the strength and elongation at practical acceptable level as per requirement of downstream process.
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Nitin T Patil is from Veermata Jijabai
Technological Institute (VJTI), Mumbai.
Prof Dr SP Borkar is from Veermata Jijabai
Technological Institute (VJTI), Mumbai.
Dr Stephan Weidner-Bohnenberger and
Marc Schnell are from Rieter Machine Works Ltd