This article is developed from four aspects: the basics of fabric shrinkage (definition, principle, common…
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Twisting is an important technological means of spinning. Through twisting, the yarn is deformed, the fiber is transferred and compressed inside and outside, thereby forming twist. Twisting also plays an important role in yarn breaking strength, elasticity, hairiness, stretch rate and gloss. The twisting method is developed from the ancient twisting, manual spinning, drafting and twisting separation of the original hand twisting, and then developed to the twisting and winding spinning machine. Ring spinning is a revolution in spinning technology and has a milestone significance. In this process, there are also the progress and development of cotton spinning wing spindles and linen spinning cap spindles. The free-end rotor spinning developed in modern times is an extension and innovation of the ancient rope twisting technique, which has made a leap-forward contribution to the substantial increase in spinning output, and has developed friction spinning, electrostatic spinning, air-jet spinning, and vortex spinning. And other new free-end twisting spinning methods.
False twisting is another form of twisting. The spinning process can be optimized through false twisting, thereby forming new spinning methods such as self-twisting, texturing, elastic spinning, and low-torque spinning.
1 The mechanism of false twist
As shown in Figure 1, AC is a section of yarn, and the middle R is a false twister (or false twister), and its speed is n. The arrows and respectively indicate the direction of movement and rotation of the yarn. Suppose the moving speed of the yarn is V, the ends of the yarn 1 are fixed, and the yarn 2 to the yarn 5 can move, L1 represents the length of the yarn AB, and L2 represents the length of the yarn BC. Under the different conditions of the above parameters, there will be 5 different false twist states.
Figure 1 Five states of spinning false twist
False twist with both ends fixed
When R rotates in a clockwise direction, the AB section is Z twist, and the BC section is S twist. When R stops rotating, the AC section twist disappears, but because the yarn is plastic, false twist can make the yarn deform, compact and shrink , The fiber in the yarn will transfer to a certain extent. Yarn 1 is shown in Figure 1.
False twist distribution when the yarn is moving
Let t be the passing time of the yarn AC section, n be the rotation speed of the false twister, and V be the yarn running speed, calculated according to the false twist theory.
AB section twist:
BC section twist:
It can be seen that when t is quite small, there is a certain twist TB in the sliver BC called the residual twist. When t tends to ∞, TA = n/v, TB -> 0, and the relationship curve between t and TB is shown in Figure 2. Therefore, it can be understood as TB = 0 under long-term continuous operation.
Figure 2 Residual twist during false twist
False twist state where the yarn output end is thermally melted or bonded
As shown in the yarn sliver 3 in Figure 1, a heating device or a bonding device is added to the output end of the false twister to instantly fix the twist of the output yarn, so that the presence of false twist can be maintained, and the textured yarn or bonded yarn can be made.
The false twister keeps changing directions and stops continuously
As shown in the yarn sliver 4 in Figure 1, according to the 1.2 concept, two different twist directions and intermediate non-twisted cycles will appear when outputting the sliver. This is the mechanism of self-twisted yarn.
Air-jet spinning wrap yarn formation
Air-jet spinning passes through the air-jet nozzle to make the strands rotate at a speed of about 2500 r/min and undergo vortex torsion, so that the exposed free fiber ends on the surface of the yarn wrap the yarn body to form a wrapped yarn, as shown in Figure 1. 5 shown. It can be inferred that air-jet spinning is also produced in false twist, but its twisted wrapping fiber is only about 5%.
2 Common forms and functions of false twisters
Simple false twister with one turn and one twist
False twister generally refers to a device that does not turn at both ends of the control yarn but applies twist in the middle. The simplest false twister is shown in Figure 3. The rotor false twister is shown in Figure 3. The multifilament is wound on the rotor cross pin 3, and the rotor belt rotates with the multifilament to produce a twist. This kind of false twister will cause double The tension change of the output and input ends of the silk is too large, and it is rarely used now. Hook-shaped and perforated simple false twisters are often used.
Figure 3 Schematic diagram of simple false twister for multifilament
1-Feeding roller; 2-Rotor false twister; 3-Horizontal pin; 4-Output roller.
Friction type false twister
The friction type false twister uses tension to rotate the yarn through the friction contact surface to form false twist, such as roving flyers, various types of false twisters used to produce false twisted textured yarns, and rotor spinning false twisters. . Its characteristic is that one rotation of the friction member can produce multiple twists, and the false twisting efficiency is high.
Figure 4 is a three-axis friction disc false twister for textured yarn. It is driven by 3 rotating friction discs to rotate the yarn and add false twist. When used, the component force of the included angle promotes the output of the yarn. The friction disc is made of materials such as diamond, ceramic or polyurethane.
Figure 4 Schematic diagram of three-axis friction disc false twister for textured yarn
Apron type false twister
The apron type false twister is shown in Figure 5, which consists of two sets of aprons that cross each other at a certain angle. Due to the large friction factor of the apron against the yarn, the wide contact surface and high twisting efficiency, but the structure is more complicated.
Figure 5 Schematic diagram of apron type false twister
The ring twisting false twister is shown in Figure 6. The false twister controls the twisting torque and yarn tension through external pressure; the apron speed and the yarn speed can be adjusted to keep the yarn twist stable.
Figure 6 Schematic diagram of ring twisting false twister
There are still many forms and patents of false twisters, which have a large choice.
3 Application examples of false twister
Carded wool spinning system for roving twisting
In the early wool spinning system, on the sliver forming machine behind the roller card, the wool net of the doffer needle surface was divided into multiple bundles with belt filaments, and then it was composed of two sets of upper and lower aprons, which could not only twist horizontally, but also move forward. The output mechanism is to apply false twist to the small sliver, twist it into a roving and then spin it into a spun yarn. The roller carding system is shown in Figure 7.
Figure 7 Schematic diagram of roller carding system
1-Cylinder; 2-Doffer; 3-Chopping knife; 4-Cutting mechanism; 5-Rubbing mechanism.
False twist of cooked sliver on super drafting sliver coiler
As we all know, the feeding method of ring spinning super draft is one of the keys to the success of super draft. In the 1970s, my country independently developed the A563 four-roller super-draft spinning frame. Four cotton slivers are used for the final drawing to reciprocate and wind into a horizontal sliver. The machine adopts a flat cross-section guide horn when winding, and the sliver is formed into false twist by reciprocating periodic motion to make a compressed sliver roll with alternating Z and S twists and no twist in the middle. The sliver volume is large and the transportation is convenient. It can be fed by the single-layer four-row creel of the spinning frame, which ideally solves the feeding problem of the super draft. Although the super-large drafting was not successful in the pilot test, the exploration of the feeding problem has certain reference significance.
In the 1980s, self-twist spinning was popular for a while. Self-twist spinning is mainly suitable for pure spinning of medium and long chemical fibers or blended with fibers such as wool, hemp, spun silk, etc. Because its output speed is 10 to 15 times that of general ring spinning, the process flow is simple, the labor productivity is high, and the cost is very low. , But only suitable for the strands of coarse special yarn in spinning. Since it is impossible to eliminate the untwisted weak loop of the yarn, only middle and low-end textile products can be produced.
The simplest self-twisting spinning machine is shown in Figure 8. After the yarn sliver or chemical fiber sliver is drawn, it is output by the drafting device, and false twist is added by a pair of rotating and moving rollers to make a self-twisted yarn with different twist at both ends and no twist in the middle. The early twisting mechanism consisted of a complicated planetary gear train. Currently, two servo motors can be designed to simplify the drive to achieve.
Figure 8 Schematic diagram of the simplest self-twisting spinning machine
In order to reduce the influence of the untwisted section of the self-twisted yarn, two self-twisted yarns with different phase differences can be twisted and combined, as shown in Figure 9. However, there are small untwisted sections in the spinning yarn, which is extremely unfavorable to the strength, evenness and end breakage of the product. Currently, self-twisted yarn is not used much.
Figure 9 Two self-twisted yarns with different phase difference twisted and plied
Roving flyer false twister
In the early 1950s, there was no false twister for the roving frame flyer, and the tip of the roving frame flyer was prone to “twisting”, which made the twist of the sliver (generally called the spinning section) from the front roller to the top of the flyer only It is 40% to 60% of the twist on the bobbin. The yarn strength of this section is very weak, and it is most prone to break ends and accidental drafts. When the flyer is rotating, the length of the yarn in the sleeve of the flyer changes, causing the yarn to shake and the weak ring to stretch. According to relevant literature, this section of yarn has an accidental elongation of up to 18% above 50 mm. In addition, the “twisting” of the front and rear rovings is different, which can easily cause variations in the density of the coarse yarns in the front and rear rows and increase the weight unevenness.
Mr. Shou Yimin of Shanghai first proposed that grooves on the top of the flyer could add false twist in the spinning section, which solved the above-mentioned problems. It was promoted in some factories in Shanghai and achieved certain results. However, the depth of the groove is difficult to control by manual operation, and the false twist between ingots varies greatly. During this period, there were reports on the research and development of spindle-end false twisters at home and abroad. The false twisters of different materials such as square hole type, guide vane type, metal, polyvinyl chloride were developed successively, and finally formed the current general metal striped flyer False twister.
Flyer false twister is actually a typical friction type false twister. The false twist TC (twist/10 cm) of the spinning section can be calculated by the following formula.
Where: D is the inner diameter of the flyer false twister, that is, the diameter of the contact yarn (mm); d is the diameter of the yarn (mm); n is the flyer speed (r/min); V is the running speed of the yarn ( 10 cm/min); k is the false twist coefficient, which is determined by the fiber properties of the yarn, the pressure and the friction state in contact with the false twister, because the false twister drives the yarn to rotate and is slippery, generally k>1.
This formula shows: because D / d ≥ 1, the TC twist is several times greater than the yarn twist T (T=n/v). To increase TC, you can increase D or increase the value of k. The former is limited by the specifications of the false twister, and the only way is to increase the friction or pressure between the yarn and the false twister. The pressure is determined by the spinning tension. Therefore, the only way to increase the friction of the false twister is to increase the number of groove stripes and the depth of the stripes.
The false twist in the spinning section is not as much as possible. Excessive twist will cause fiber shrinkage and fiber transfer in the cross section, resulting in increased hairiness and uneven mixing. According to the law of fiber twisting and transfer, twisting will make thick fibers, hard fibers, fibers with small initial modulus, fibers with large crimps, fibers with short lengths, and special-shaped fibers tend to the outer layer. Using its distribution law, you can Give full play to the characteristics of various fibers to meet product requirements, but the layering of fibers can also cause uneven fiber mixing, especially for products such as color spinning. In short, the false twister should be preferably used according to product quality requirements.
Application of false twist in rotor spinning
Similar to the roving flyer, the original rotor spinning did not have a false twister. It was later discovered that when the yarn sliver revolves around the twist stop disc, the rotation around the axis of the yarn itself will form a false twist, and the twist direction is consistent with the true twist. The greater the friction factor of the turntable, the greater the false twist effect. False twist can be transmitted to the peeling point A shown in Figure 10, thereby reducing the occurrence of broken ends. The larger the diameter of the twist stop disc, the larger the enveloping angle, the more false twists. The false twist is beneficial to increase the dynamic strength of the rotating yarn, but it is not good for the strength of the yarn. Too much twist will cause more riding fiber bundles to be involved Yarn, forming entangled fibers, reduces strength and increases hairiness. Therefore, it is necessary to select false twisters of different materials and shapes according to the characteristics of the product.
Figure 10 Rotor spinning false twister
False twisted textured yarn
Chemical fiber filaments are generally arranged in parallel and do not have cohesive force, so they are easy to break off and hook, causing difficulties in weaving, easy to produce uneven tension, and increase in breakage. For this reason, it is often necessary to make filaments into false twisted textured yarns, air textured yarns, network textured yarns, etc. False twist textured yarn is the most used and best one. False twisted textured yarn (yarn) has the characteristics of good elasticity, high elongation, good bulkiness, high strength, and high coverage. False twist texturing machine is the most successful example of false twist application. Its production speed can reach 1000 m/min or more, with high efficiency and low operating cost. Polyester textured yarns and nylon elastic textured yarns that have been widely produced at home and abroad are used in socks, outerwear, seamless clothing, functional sportswear and fancy yarn products. The key technology of false-twisted textured yarn is to select a false twister with high speed, stability and convenient operation.
Low torque spinning
The first low-torque spinning at the Hong Kong Polytechnic University in my country is an example of the application of false twist in spinning technology, which has epoch-making significance. The idea is to install a high-speed false twister similar to false twisted textured yarn on the guide hook of ring spinning, like the roving flyer false twister, to increase the twist of the spinning section, thereby reducing the twist of ring spinning. The potential value of the development of high-end textiles with soft, low-twist, low-shrink high-end spun yarns and torque, is that it can reduce end breaks, increase vehicle speed, reduce energy consumption, increase strength, and reduce fabric weft skew. It is especially suitable for knitwear, extra-fine yarn, hollow core yarn, core-spun yarn, denim, untwisted towels, etc.
Application of false twist of wrapping yarn and fancy yarn
Suessen’s parallel spinning is a typical wrapping yarn.
The false twist device is shown in Figure 11. After the yarn enters the hollow spindle, the hollow spindle is introduced into the hollow spindle hole after a quarter of a revolution, and a false twist is applied to the spindle end after the spindle rotates once. False twist can prevent this section of spindle from loosening before the filament is wrapped, so as to improve the strength and reduce the hairiness. Parallel spinning wrapping yarn has good coverage and small shrinkage rate, suitable for medium and thick special yarns and carpet fabrics, and the cost is higher.
Figure 11 False twist device in Suessen wrap spinning process (Parafill)
Similar to the above principle, the hollow spindle fancy twisting machine is equipped with a simple false twister with twisting hooks under the spindle, as shown in Figure 12.
When the spindle rotates, false twists are applied to the upper and lower yarns at point B; the twist of the finished yarn does not change, but the fancy shape of the decorative yarn when the false twist is removed on the core yarn can be fixed, and the fancy twist of the ring spinning can be maintained The machine is also soft and fluffy. In Figure 11, A is the core yarn, and A’is the decorative yarn.
Figure 12 Hook-shaped false twister
Utilization of false twist in bonded spinning
Bonded spinning has a long history, such as the Dutch Twilo process, the Swiss Reiterpavena process, and the Canadian Bobtex process. Bonded spinning uses adhesives, bonding fibers, or polymers to bond fibers together to increase fiber strength. The fabric produced has a hard hand feel, poor softness, high coverage and poor washing performance. However, the output is as high as 500 m/min ~ 600 m/min, without spinning processing, but in order to improve the softness, it often needs softening treatment, and the cost is not low.
Figure 13 shows the spinning process of Twilo in the Netherlands. Cooked sliver 1 consists of 5% to 11% of water-soluble vinylon (PVA) mixed with cotton sliver; the former can be water-soluble and mixed with mixed fibers at about 70 ℃. The cooked sliver first passes through the first drafting zone and is drawn by 5 to 10 times, and then passes through the wetting zone with the first false twisting device 2 to spray water and false twist the strands. Finally, in the untwisted state, After the second drafting zone, the draft is about 40 times. The drafting mechanism is connected to the second false twisting device 3 to heat and bond the yarns, then the drying roller 4 at 140 ℃ sets the shape, fixes the twist, strengthens, and finally winds it into a cone. The cross-section of the yarn is flat, so it has good coverage. The product is suitable for making bath towels, interlining cloths, coated cloths, etc.
Figure 13 Twilo spinning principle
1-cooked strip; 2-first false twisting device; 3-second false twisting device; 4-drying roller.
Twisting or false twisting can change the shape and fiber arrangement of the strands or the cross section of the yarn during the spinning process, increase the strength of the yarn, reduce the accidental elongation of the spinning, enhance the elasticity of the yarn, reduce the hairiness and improve the surface properties. Reduce the torque and twist of post-processing, improve fabric quality, etc., such as false twist of flyer, false twist of hollow spindle, false twist of rotor spinning, etc. Through the false twister, it can be spun into self-twisted yarn, carded woolen roving, super-draft sliver, etc. by continuously changing the direction, stopping and opening of the false twister. The output end of the false twister can be spun out of false twisted textured yarns and bonded yarns by adding a heating melting device or a binder. Low-torque spinning is a major breakthrough in the utilization of false twist. It has mature technology, less investment, shorter payback period, convenient operation and simple maintenance, and it has the greatest potential for development.