Theoretical aspects of rotor spinning Main challenges

Theoretical aspects of rotor spinning

Main challenges of open end spinning systems Overcome the speed limit of the twisting element and continue yarn formation without interruption “Spindle less spinning” Interrupt the continuity of the fiber flow and to break inter fiber contact before twisting element. “Elementary spinning”

Open end rotor spinning system Rotor spinning system is consist of major three sections such as , Fiber separating section Twisting section Winding section

Fiber separation section It should open the fed sliver into very small groups of fibers or eventually into individual fibers as ideal, and to feed the separated fibers uniformly in to spinning rotor. It should not , impaired the required fiber characteristics. reduce degree of fiber straightening obtained by the prior operations.

The basic fiber separation operation take place at the following five points, Feeding Combing Transport Stripping Air Transport

Feeding section In this section the sliver is drawn from the supply package and it is fed at a constant speed for combing. Between 0 -1 : The sliver is reformed to a flat cross section suitable , after some minor adjustments for combing. The transformation of the sliver cross section causes increased density of fibers and inter fiber cohesion. increase the radial stress in the sliver increase the friction against the condenser wall These frictional forces must be overcome by the feed roller for a good fiber transferring and initial sliver strength is necessary for unwind the sliver. 0 1

• Between 2 -4 : In the second stage , the sliver is compressed once again between the feed roller and the pressure cradle. increases the fiber density. increases the width of the sliver. • The pressure in this area is unevenly distributed and correspond to the fiber density. • The control of fiber movement through the 2 4 feed device is proportional to the pressure acting on fibers.

The resistance force applies on the sliver by the pressure cradle is, F 1 = F cradle. μ 1 μ should be small as possible to reduce the force. The force applies on the circumference of the feed roller is, 1 F 2 = F fiber + F < F feed roller. μ 2 μ is the friction coefficient between sliver and the feed roller. 1 2 therefore, μfiber > μ 1 to prevent shifting of interim sliver layers > It is convenient to choose lf / ld ratio as big as possible in order to reduce periodical change on fibers in the cross section. lf = mean fiber length and ld= groove pitch length

Combing section • The actual fiber individualization occurs in this section. • Roller covering acts on the fibers within its reach and 5 removes from the sliver. • A fibrous fringe is formed within the arc of 5 -6. • The effect of the covering on the fiber fringe is twofold, 6 1. tooth acts on the fibers by its leading edge. 2. by friction against its sides. • The effect on fibers and its resistance to opening are determined by the position of the fibers in the fringe. Fibrous fringe

The fiber fringe is pulled by the cover and fibers are transported from a region of high cohesion to a law cohesion. The factors affect the degree of fiber release from the comb surface, - position of the fiber in the fiber fringe - clearance between the cover and the slot wall of the separating device Usually the fibers transported by the tooth in hooked form. The fibers are subjective to a velocity change when subject to pulling which generates a sudden force.

Mean velocity = V 0 + V 1 2 Force on the fibers = mk. (V 1 - V 0)/ ∆t The distance travelled by the fiber = ∆x = ∆t. mean velocity To prevent fiber damage the combing section should be suitable distance. Theoretical forms of fibrous assemblies according to the method of separation. a. single dimensional separation. b. two dimensional separation. c. three dimensional separation. Single dimensional separation is the ideal separation at the combing device.

Transport section The fibers pulled out from a fringe are transported by the cover from the transport section to the stripping section. In this section the, wall of the recess of the combing roller is set closely to the cover. The ideal expectation is to pass the fiber through the transport section. in order to fulfill that the friction of the wall should be smaller. It can be achieved by, reduce the friction coefficient of the wall. reduce the length of the transport section.

Stripping section The arc within which the opening roller intersects the transport channel and provides space for the discharge of the fibers. The contrary of the transport section The further movement of the fiber is depends on the configuration in the stripping section and on the air current. To be able to describe the fiber movement in simple mathematical terms, there are some assumptions to make. Via those assumptions the following equation can be built. V = V 1. exp (-Cx) = V 1. e (-Cx) This express the rapid exponential decrease of fiber velocity with the path length. Experimental assessment of the movement of fibers shows very similar scenario to above.

in the ideal case much space would be required to obtain smooth fiber movement without striking the wall. If the fibers are not opened by the cover in time , a separate knife is situated after the stripping section. A portion of fibers will again move in to the combing section.

Air transport of the fibers At the entry to the feed tube fibers travel at different velocities and in varying directions. According to Bernoulli's equation there is a pressure difference between the inner and outer air layers which tends to push the fibers towards the tube axis. fibers Feed tube

Yarn Formation • Fibre flow to “Rotor” • Formation of the Fibre Ribbon (Ring) • Back Doubling Rotor • Influence of rotor • Rotor groove • Rotor diameter • False Twist Effect • Rotor rpm • Wrapping Fibre Yarn Withdrawal & winding • Direction of the withdrawal • Navel • Withdrawal type

Fibre flow to the rotor Axial feed Tangential feed

Formation of the fibre ribbon • Fibres flow in to rotor strike on the rotor wall above the groove • Rotor wall peripheral surface) speed >> speed of fibres collide on the wall • Results a drafting • Fibres deposits on the collection groove in the rotor • No air turbulence in the air flow between feed tube and rotor wall to ensure forwarding fibres are in straighten conditions Why not fibre directly deposit in to the collection groove?

Back Doubling • Fibres are fed and deposited individually • Higher evenness than equivalent ring spun(even for higher amount of short fibre content in the feed stock) Degree of evenness determined by the rotor diameter, If Dr is 35 mm Controllable length is 110 mm(∏Dr)

Back Doubling Contd. Back Doubling = Rotational speed of the rotor Lead of the yarn at separation point = Withdrawal speed of the yarn Rotor groove circumference(∏Dr) Withdrawal speed of the yarn = Rotational speed of the rotor TPM Back Doubling = (∏Dr) x TPM 1000 Example, Rotor diameter 35 mm, TPM 800 Back Doubling = (∏x 35) x 800 = 88 1000

Formation of the yarn • Open yarn end pressed in to rotor wall to contact with the fibre ring • Each revolution of the yarn at this pint insert 1 turn twist • This yarn twist transfer/penetrates in to fibre ring deposited in the collection groove • Thus these fibres bound together to form the yarn Importance of binding zone(a) • Too short-lees twist torque(has a minimum twist coefficient) • Too long-tight twist structure, many wrap fibres

Formation of the yarn Contd. . TPM = Rotational speed of the yarn – Speed of the separation point Withdrawal speed Since lead relative to rotor is negligible, TPM = Rotor rpm point Withdrawal speed How is it imparting the Twist?

False Twist Affect During Withdrawal, yarn rolls continuously on trumpet shaped nozzle(Navel), Imparting a false twist in the section b • Section b shows more twist than section a • Maximized by the tension variation a b

Wrapping Fibres formation • Fibres cannot be bound to the strand • Identification feature of rotor spg • Giving bi-partite structure Number of wrap fibres depends on • The position at which the fibres land on the rotor wall • The length of the binding zone • The ratio of the fibre length to the rotor circumference • The false twist level • Rotor rotational speed • Fibre fineness

Wrapping Fibres formation contd Core- fibres that are aligned with the helix of the inserted twist Sheath- occurring irregularly along the core length The twisting torque is in the direction for inserting S-twist into the fibre ribbon as a fibre slides down the rotor wall into the rotor groove to become a bridging fibre its leading end will be caught by the twist insertion point. This causes the length landing on the peripheral twist extent to become wrapped in the Ztwist direction around the yarn

Hearls’s 3 principle states of Twist A - Loose B - Tight C - Over lapping A B C Resulting yarn structure shows that one side of the ribbon is more on the surface, whereas the other side is mainly in the yarn core From loose to overlapping gives better anchoring of fibres in the yarn, thus quality improvement

Hearls’s 3 principle states of Twist Relationship among width of the fibre ring, twist coefficient and yarn count

Can Rotor Spinning Produce Yarns of Fine Count? • A reduction in yarn tex will result in a reduction in the spinning tension • Spinning tension the, controlling factor of the fiber flow • A reduction in yarn tex will also result in a smaller area of yarn/navel contact. • This will reduce the coefficient of friction, leading to a further reduction in spinning tension. More importantly, less area for friction heat imposed by the high rotational speed to dissipate. This is critical in spinning synthetic fibers or cotton/synthetic blends.

The Rotor Influence on • Yarn quality • Yarn properties • Working performance • Productivity • Costs, etc Rotor form • Made of aluminum or steel, coting is given to have a longer life time • Issues-Uneven wearing leads to quality issues • Is highly subjected to wear (mainly the groove) • Angle varies from 12 -50(smaller the higher speed)

The Rotor contd… The rotor groove Where fibre strand forms via back doubling(more parallel, straighten compact even arrangement of fibres in the strand) Influences on • Compactness • Hairiness Angle varies(width) from 30 - 60 • Strength • handle Wide grooves Soft, bulk yarns with low strength Narrow grooves Compact, strong, low hairiness yarns with hard handle

The Rotor contd Rotor diameter Influence on • Yarn characters • Yarn properties • Required yarn twist • Selectable rotor speed and productivity Smaller the rotor • Higher the speed • Lower the energy consumption But it needs • Higher the necessary yarn twist(high twist coefficentmin • Harder handle • Higher wrap fibres

The Rotor contd • Large diameters enables to form fibre strand in the groove w/o any difficult • Since space need to occupy fibre mass coarser the yarns larger the diameter • Long fibres must able to lie in the groove in stretched state • Rd(min)=staple length x 1. 2 70% of rotor spg covered by rotor 48 mm/43. 5(for denim yarns)

The Rotor contd Rotor speed • Ties with productivity and all other economical aspects • Higher the speed lower the rotor diameter Rotor rpm max = 106 x (6 R)1/2 Dr R-exploitable yarn breaking strength

Yarn Withdrawal & winding Withdraw through the rotor shaft Withdraw opposite the rotor shaft Navel • Like a nozzle having a trumpet shaped mouth piece • Generates false twist affect • Its surface parameters Influence other parameters like

Yarn Withdrawal & winding contd Navel formation • Mouth piece(Navel) made of steel, whereas surface contact of the yarn made of Steel(better heat conduction) Chrome plated steel Ceramic(more resistant to wear) • Its surface characteristics influence the yarn properties Rough Navel Surface gives • Rougher, hairier, more voluminous yarn • Enables spg with lower twist coefficient Smooth navel gives Better yarn properties(low hairiness, etc)

Yarn Withdrawal & winding contd Influence of the radius of the arc of wrap on the navel Larger radius-enables spg with low twist coefficient Smaller radius-gives higher strength(but need to give higher twist coefficient to maintain practicable running conditions Spiral Navels Grooved Navels Bulky & Smooth yarn with low twist level, fewer ends down Bulky & hairier Better twist distribution along the yarn Smooth navel

Yarn Withdrawal & winding contd The Withdrawal Tube • Used to guide the yarn(change of horizontal doffing disposition in rotor to vertical disposition for winding) • Angle of the bend is being optimized(interfere the twist) • Consist of integrated twist blocking elements(False twist theory to give the spg stability)

YARN STRUCTURE

Fibre orientation Fibre migration Twist structure Packing Fraction Comparison between rotor and ring yarn structural characteristics

Classification of rotor spun yarn Rotor spun yarn is classified in to seven classes based on scanning electron micrographs. Class I : Ordered : There are no wrapper fibre and has the appearance of unformly twisted yarn. Class II : Loosely wrapped fibres are superimposed on to the uniform structure of core yarn and their wrapping angles differ from the twist angle of core the core fibres. Class III : Hairy: Outer zone fibres are loosely attached to the yarn and appear entangled. They give a hairy appearance to the surface structure. Class IV : Multiple wraps Parts of wrapping fibres are lightly coiled around the core whereas their remainders adopt lower wrapping angle Class V : Opposingly wraps: The outer zone fibres have an opposing helix to the twist helix of core fibres and there wrap angle can be up to 75. Class VI : Tightly wrap: Surface fibres appear tightly and closely wrapped around the core. These sections of yarn look uniform and have no protruding fibre ends or loops. Class VII : Belt: Fibres are wrapped very tightly around the core at 90 in a narrow length.

Rotor spun yarn is basically comprised of two distinct regions. 1. 2. The core characterised by the helical twisting of fibres Sheath constituted by individual or a thin ribbon of wrapper fibres

Fibre orientation and extent Fibre orientation is the arrangement of fibres relative to the yarn axis. Fibre extent is the actual length of fibre in the yarn. Spinning-in coefficient= fibre extent initial length Good orientation of fibres results in higher fibre extent and Spinning-in coefficient. Binding fibres which are integrated with in hooked fibres exhibit very low and varying wrapper extent

Spinning-in coefficient Eg: Tested values for spinning-in coefficient in 20 Ne cotton yarn is as follows. Combed Carded Ring 0. 81 0. 77 Rotor 0. 68 0. 64 This shows that the orientation of fibres in rotor spun yarns is inferior to that in ring spun yarns

Fibre Migration Radial change of fibre positions along the yarn length. In rotor spinning, the most common fibre collecting surface is vshaped. The twisting from inside to outwards enables rounding up of the corners of the traingular shape. As the fibres are attached to the yarn tail one at a time, there can be tension variation among individual fibres. Then it follows different helix envelopes in the yarn.

Twist Structure With the bipartite strucrure of rotor spun yarn, the core exhibits twist structure similar to that in ring spun yarn. Sheath is characterised by belts having varying intensity and direction of wrapping. The fibres in the core are twisted helically. The fibres near the yarn axis show low helix angle while the fibres near the surface have greater helix angle. center of open-end yarn is highly twisted unlike that of ring-spun yarn and twisted core gives the open-end yarn great flexural rigidity.

Since open-end yarn already has high flexural rigidity, it is less affected by changes in twist level. Therefore the effect of increasing abrasion with increasing twist is essentially indiscernible in rotor yarns. Due to the differential twist structure, yarn cannot be completely untwisted to measure the twist To determine the twist, three different methods have been used. 1. Optical method based on tracer fibre technique. 2. Detwist-retwist method 3. Twist to break method

Packing of fibres in yarn Packing fraction represents the density of packing of fibres in the yarn. Packing fraction= Total area of fibres in the yarn Area of yarn cross section As in rotor spun yarns, helix angles vary from yarn axis to its surface, the surface, packing density wil also vary across the yarn cross section. The rotor spun yarns have greater density of fibre packing near the yarn axis and lower at the surface.

Structural difference between Rotor and Ring spun yarns Rotor yarn has core twist and ring yarn has envelope twist. Rotor yarn is more voluminous, open and rougher than ring yarn. Fibres in the envelop layer of a rotor yarn, partly escape from twisting action. They contribute relatively little to yarn strength. Rotor yarn can easily be rubbed together axially to form slubs. Rotor spun yarn generally needs rather more turns of twist than ring spun yarn. the fibres in a rotor-spun yarn are less parallel than those in a ringspun yarn. The core twist structure & the lower degree of parallelism are the causes of lower strength of rotor-spun yarn, & also of most other character differences.

Rotor-spun yarn compared with ring spun yarn Properties Rotor Spun Yarns Compared With The Ring Spun Yarns Breaking strength Lower Coefficient of variation of strength Better Imperfections Lower Resistance To Abrasion Higher Stiffness Higher Handle Harder Surface Rougher Hairiness Higher Luster Duller

Important Differences In Further Processing Properties Rotor Spun Yarns Compared With The Ring Spun Yarns Tendency to form slubs under axial force Greater Capacity To Take Up Dye Higher End Breaks In Warping Lesser By 50% Warp Breaks In Weaving Lesser By 70% Weft Breaks In Weaving Lesser By 25% Coefficient Of Friction Higher

Fabric Properties Rotor Spun Yarns Compared With The Ring Spun Yarns Tensile Strength Lesser Tearing Strength Lesser Bursting Strength Lesser Handle Harsher Or Crisper Thermal Insulation 10 -15% Better Air Permeability 15 -25% Better Absorption Of Water Much Better Take Up Of Dyes Better Shrinkage Same

YARN CHARATERISTICS 1. 2. 3. 4. 5. 6. 7. 8. 9. Strength Extension Evenness Imperfection Flexural Rigidity Hairiness Abrasion Resistance Pilling Moisture absorption

Rotor spun structure Bi-particle structure Core have higher twist and sheath have low twist. Fibers wrapped around the yarn. Rotor spun yarn Ring spun Yarn

Strength Rotor spun yarn are relatively weaker than the corresponding ring spun yarns( 80 -85%). Its mean low strength. The strength of the yarn is mainly derived from helical twisted core fibers and amount of wrap in the yarn.

Strength cont…. . Rotor spun Yarn is bipartite structure which has high twist in the core of the yarn and less twist in the outer layer of the yarn. And amount of wrap also low when compare with ring spun yarn. Ring spun is uniformly twisted along the cross section of the yarn and wrap tightly.

Extension of rotor spun yarn slightly higher than ring spun yarn at the break point. Rotor spun yarn has higher twist in the core cause to straighten and extend more before it break. Ring spun yarn less twist uniformly along the cross section. Due to the less twist it has less extent before break.

Evenness The rotor spun yarn is relatively evener than its equivalent ring spun yarn The arrangement of the fibers one over the other as layer in the rotor groove. this effect known as “back doubling”. So it reduces the short term irregularity. Ring spun yarn are prepared through the roller drafting system and helically twisted. so is not even as rotor spun yarn.

Imperfection Rotor spun yarn has less imperfection ( Thin, Thick & neps) than ring spun Yarn. The imperfection take place due to the un-open fibers. Rotor spinning system first open into individual fibers and spin, so it will reduce the un-open tuft to form the neps and thick places.

Flexural Rigidity Flexural rigidity is defined as the force couple required to bend a rigid structure to a unit curvature. Rotor spun yarn shows higher Flexural rigidity than its equitant ring spun yarns. The flexural rigidity of the rotor spun yarn is grater influence by the nature of wrappers. Ring spun yarns are helically arranged not subjected to wrapper.

Flexural rigidity cont……. Rotor spun yarns have tight wrapper fibers which cause difficult to inter fiber movement during the bending. So it has high flexural rigidity. The flexural rigidity of the rotor spun yarn may be 1. 5 to 2. 5 times higher than ring spun yarn.

Hairiness The Rotor spun Yarn are expected to be less hairiness than ring spun yarn due to the wrapper fibers cover the main body. It prevent protruding out of the yarn. In the ring spun yarn fibers are arranged helically so it will come out and create hairiness. The hairiness can be varied through the selection of different nozzle and groove. Rotor spun yarn Ring spun Yarn

Abrasion Resistance The rotor spun yarn are more likely to grater abrasion resistance then ring spun yarn. The bipartite structure of the yarn ensure the movable sheath to relieve the abrasive stress. Ring spun yarn twist is higher in the surface of the yarn so less mobility to fiber so it has low abrasion resistance.

Pilling The pilling resistance of the rotor spun yarn is higher than ring spun yarn. Rotor spun yarn surface fibers are just wrap so when subjected to rub the wrapped fibers come out and go away from yarn. The ring spun yarn fiber migration is good so when subjected to rub there will be entanglement and form pilling.

Moisture Absorbency Moisture absorbency of the rotor spun yarns are high than ring spun yarn. Due to the less twist in the rotor spun yarn surface it can absorb moisture easily. But ring spun yarn has higher twist in the surface than ring spun yarn so it has less moisture absorbency.

New Developments

SPEEDpass unit A fiber insertion unit Specially developed for spinning man-made fibers and coarse cotton yarns Permits more air to be pulled through the fiber channel Enhances the fiber separation

Optimized Spinning Geometry Location of the spinning box in the machine is important because it affects the spinning stability Distance between rotor groove and takeoff roller is critical for optimal spinning conditions A higher spinning stability facilitates either the application of higher rotor speeds or the reduction of the yarn twist and therefore the increase of the delivery speed

Draw-off Nozzles A new development of nozzles is “Nano 4” nozzle Made out of very fine grained ceramic This offers productivity increases of more than 10% when processing viscose fibers, due to an optimized friction behavior

Energy Saving Rotor Design A new generation of rotors called “X-Rotors” to save energy This has an optimized external contour and reduces the energy consumption in rotor operations It consumes 2 -5 watts per rotor (1 -2% less energy compared to the previous rotor generation)

X rotor enables up to 12% higher speeds and up to 5 watts lower energy consumption with better acceleration and performance.

Efficient Rotor Cleaning Trash particles which affects yarn quality, in the rotor grooves must be removed efficiently A rotating cleaning head with two scrapers and three compressed air jets nozzles is integrated to the rotor Rotor groove is cleaned with the scrapers and the rotor groove & the entire rotor wall is cleaned by compressed air

Easy Settings Machine settings on touch screen Opening roller speeds, winding angle, tension draft and anti patterning etc. can be set easily and quickly

Longer Machines Longer machines with individual spinning units Individual control of spinning units make it possible to process various orders at the same time

Automated Piecing Automatic robot units for piecing Yarn like piecing with AEROpiecing technology Can achieve shorter piecing times Consists of laser guided positioning with millimeter precision

Yarn Clearer Systems Monitor the yarn in terms of its quality within predefined limits Includes optical sensors with foreign fiber detection option Replaces the recognized yarn faults with AEROpiecing

Automatic Sliver Feeding & Package Removal Automated units in feeding section can change empty cans with full cans automatically Robotic feeding unit takes the sliver from the can, brings it into the spinning position and starts the spinning process

Automated package removal units enable the package removal without operator availability

Water Rotor Spinning Using for synthetic fibers like Aramid Used for Gloves, Helmets, Bullet proof vests and tires etc… The liquid polymer leaves a rotating disc which is located inside the cylinder horizontally Polymer hits the cylinder and drains away with water that is falling down the cylinder wall

Quick Recap… Three major sections of rotor spinning Fiber separating section (at five points) Twisting section Winding section Yarn Formation About rotor Yarn withdrawal & winding Yarn structure Yarn characteristics New developments

Q & A Session

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