Twisting design affects the physical and mechanical properties and appearance of the tow, yarn and thread, and then has a great influence on the property, appearance and use value of the fabric.
For short fiber yarns, twisting is a necessary means to obtain strength and other characteristics; for filament and strand yarns, twisting can form a tight structure which is not easily damaged by lateral external forces. Twisting can also form textured and fancy threads. The number of twists and the direction of twists not only affect the feel and appearance of the fabric, but also affect the internal quality of the fabric.
Purpose and requirements of twisting
Objective twisting is a necessary means to make the fiber strip become yarn. Before twisting, it is generally necessary to condense the loose fiber into fiber strip. After twisting, the outer fiber of the fiber is squeezed to the inner layer to produce centripetal pressure, so that the friction force of the fiber strip is obtained along the length direction of the fiber.
It is required to obtain the best strength, elongation, elasticity, flexibility, luster and hand feeling of the yarn, diversify the structure of the yarn, and improve the twisting efficiency of the yarn.
Index indicating the twist degree of yarn
Twist, twist angle, twist width and twist coefficient are the indexes to indicate the degree of twisting. The index indicating the twisting direction is called twisting direction.
The two sections of the yarn produce an angular displacement of 360 ° to form a twist, which is usually called a turn.
The number of twists per unit length of yarn is called twist. Our country's cotton yarn adopts the special twist system, that is, the twist number within the length of 10 cm yarn is used to express; combed wool yarn and chemical fiber filament adopt the metric twist system, that is, the twist number within each meter is used to express; in addition, the twist number within each inch is used to express the English twist system.
Before twisting, the fibers in the yarn are parallel to each other. After twisting, the fibers incline. The greater the twist degree of the yarn, the greater the inclination of the fiber. The twist degree can be expressed by the inclination angle of the fiber in the yarn - the twist angle β.
For two yarns with the same twist, the twist degree is different due to the different thickness. The twist degree of the thicker yarn is larger, and the twist angle β is larger.
If the yarn section is regarded as a circle, the angle between the fiber and the axial direction of the yarn at different radii is different. In order to express this situation, the twist index is introduced.
Twist can't be used to compare the twisting degree of different thickness yarn, because the inclination degree of coarse yarn is larger than that of fine yarn with the same twist. In practical production, the twist factor is often used to express the degree of twisting. Twist factor is the relative value of yarn twist degree, which is expressed by linear density. It can be used to compare the twist degree of different thickness yarn. Twist factor can be calculated according to twist and linear density of yarn.
Twist direction refers to the inclined direction of the single yarn in the single yarn or the single yarn in the strand after twisting. It is divided into Z-twist and S-twist. After twisting, the twist direction of the yarn inclines from the lower right corner to the upper left corner, and the twist direction which is consistent with the middle of "s" is called S-twist or hand twist; the twist direction of the yarn inclines from the lower left corner to the upper right corner, and the twist direction which is consistent with the middle of "Z" is called Z-twist or hand twist. Generally, Z-twist is used for single yarn and S-twist is used for stranded yarn.
The twisting direction of the strand is indicated by the twisting direction in sequence. For example, single yarn is Z-twist, initial twist is S-twist, and double twist is Z-twist. The twist direction is expressed by zsz.
The twist direction of the yarn has a great influence on the appearance and hand feeling of the fabric.
In plain weave fabrics, if the warp and weft yarns are of the same twist direction, the resulting fabrics will be of high strength, poor luster and hard hand feel. Twill weave fabric, if the twist direction of the yarn is opposite to the twill, the twill is clear and full.
When Z-twist yarn and S-twist yarn are arranged at intervals in the fabric, the hidden grid and strip effect can be obtained. Z-twist yarn and S-twist yarn are combined for twisting to form wrinkle effect, etc.
How to judge the twist direction of a yarn
Left hand fixed, right hand screw direction is s Twist
The left hand is fixed and the right hand is counter screwed in the direction of Z-twist
After twisting, the length of the yarn is shortened due to the inclination of the fiber, resulting in twisting shrinkage. The size of twist shrinkage is usually expressed by twist shrinkage, that is, the percentage of the difference between the length of yarn before and after twisting to the length before twisting.
The degree of twist shrinkage directly affects the yarn density and twist, which must be considered in the process design of spinning and twisting. The twist shrinkage of cotton yarn is generally 2% ~ 3%. The twist shrinkage is related to the twist coefficient, spinning tension, workshop temperature and humidity, yarn thickness and other factors.
True twist and false twist
After the true twist is obtained on the whisker, the outer fiber will produce an inclined helix to twist back, the fiber will twist and deform, and the yarn will be tightly packed, which changes the structural form and mechanical and physical properties of the fiber group, as shown in Figure 9-3 (a).
When there is an angle of encirclement on the yarn, the centripetal pressure of the fiber on the yarn is increased. The greater the angle of encirclement, the greater the centripetal pressure. Due to the existence of centripetal pressure, the outer fiber is extruded to the inner layer, which increases the tightness of the yarn and the friction between the fibers, thus changing the structure and physical and mechanical properties of the yarn, which is the essence of true twist yarn.
The twist, twist angle, twist width, twist coefficient, twist direction and twist shrinkage are all indexes of true twist degree.
Hold the two ends of the multifilament and twist it through the twisting device in the middle of the two ends of the multifilament. The two ends of the multifilament get a twist diagram with the same number and opposite twisting direction: one end is S-twist and the other end is Z-twist. The total twist of the whole yarn is 0.
Generally speaking, the two ends of a rope are fixed and twisted in the middle. One side is more and more tightly twisted, while the other side is more and more tightly twisted, but the twisting direction is opposite. Therefore, the total number of twists on the rope has not changed and is still zero, so the added twists are called false twists.
[principle of true twisting]
One end of the strand is held, the other end rotates around its own axis, and the relative rotation angle displacement - twisting occurs between each section of the strand.
In a broad sense:
In the spinning process, the yarn (sliver, yarn, thread, silk) is twisted around its axis to twist or wind axially, so that the yarn can be twisted back, wrapped, intertwined or networked.
When ∠θ = 360 °, it is necessary to rotate the strip around its axis once to obtain a twist back. β is the twist angle.
As a result of twisting, the outer fiber is extruded to the inner layer, and the extrusion pressure is Q, which changes the structure of the yarn, increases the friction between the fibers, thus increases the tightness and strength of the yarn, and changes the physical and mechanical properties of the yarn.
Measurement of true twist
The angular displacement of the unit length yarn relative to the rotation on the section. Number system twist: TT = the number of twists on 10cm long whiskers, British twist: TE = the number of twists on each inch of length whiskers, metric twist: TM = the number of twists on 1m long whiskers. However, twist can only be compared with the twist degree of the same roving, but not the twist degree of different roving directly. As shown in the figure below, the twist angle (degree of twist) of different roving strips with the same twist is different.
2. Twist factor
The twist angle β reflects the inclination degree of the twisted yarn. But it is not convenient to use, so it is expressed by twist coefficient α. The relationship between α and twist angle β can be deduced as follows:
When the unit length yarn is twisted, the arc length of any point on the section relative to the rotation on the section.
4. Twist vector
According to the twist direction of the yarn, the twist on the yarn can be divided into "s" twist and "Z" twist.
The formation process of true twist
Twisting area and twist
1、 Twisting area
(1) Static state: the angular displacement of Twister B rotating relative to grip a is θ L = ω T, and the same reverse twist is obtained in BC area of twisting degree.
(2) Dynamic time: after t time, the twist of L-section yarn in AB area is t in T + DT time. The yarn with VDT length is input into AB twisting area, and the twist of L-section yarn in AB area is increased by DT. At the same time, VDT yarn length leaves AB area with T + DT twist and enters BC area.
2、 Instantaneous twist and stable twist theorem
The additional twist response of the twister to the AB twisting area in DT time is equal to the twist response of the twister added to the AB twisting area minus the twist taken from point B at the same time.
Stable twist theorem: when the twist is stable, the number of twists added to the AB section by the continuous rotation of the twister is equal to the number of twists taken away from the AB section at the same time. N-tv = 0 if the yarn fed from point a has t0 twist.
The acquisition of true twist
1. Non free end twisting
① There is no twist on the final yarn (as shown in the top right figure).
② Obtain the case of true twist (right figure below) AB area: n-vt1 = 0, T1 = n / v. BC area: vt1-vt2 = 0, T2 = T1 = n / v. The twist is n / v.
2. Free end twisting
Transmission, entrapment and stranding of twists
（1） Transfer and distribution of twist back
The twister rotates so that the twist moment of the sliver is generated and transmitted from the twister along the axial direction to the grip point. The factors that affect the twist transmission are: twist stiffness, sliver thickness, rotational moment of inertia, roundness of sliver, tightness of sliver, length of sliver (absorbed energy), twist of sliver.
The methods of twist fast transfer are: yarn string vibration, yarn rotation, direction vibration and axial vibration.
The methods to prevent transmission are: wetting, heat setting and additional friction boundary.
The distribution of twist is mainly related to the rigidity of the section of the yarn. If the torsion rigidity of the coarse part is large, the twist will be less; if the torsion rigidity of the fine part is small, the twist will be more. The final yarn reaches the balance of torque.
The conveying direction of the sliver is opposite to that of the twist back, and the friction piece is located between the twisting point and the holding point. The phenomenon that the twist on the yarn segment AC is less than the normal twist due to the friction piece C. That is: twist transfer efficiency η < 1.
The twisting area AB, the middle friction part C, and the twisting transmission direction are the same as that of the yarn. The friction resistance moment of Part C prevents the twist from returning to the AC section, resulting in the increase of T2 twist. But it has no effect on the twist of the product.
True twist structure
1. Solid twist
The twisted whiskers are basically cylindrical in shape. For example, filament, strand and other monofilament are cylindrical helix.
Because of the centripetal pressure TiSiN θ I, the edge fiber θ I is large, TiSiN θ I is large, the center fiber θ I → 0, TiSiN θ I → 0, so the edge fiber is squeezed to the center, the center fiber is squeezed to the outer edge, the center fiber is squeezed out, and the inner and outer fibers are repeatedly transferred to each other, forming a conical spiral line in the yarn.
The fiber is twisted while agglomerating, agglomerating one layer and one layer, agglomerating more twisting at first, then agglomerating and less twisting, forming a layered twisting state such as friction spinning and rotor spinning.
Some fibers are wrapped around the main body of the yarn, such as: air-jet spinning, parallel spinning, covering spinning, etc.
The yarn is rolled in a circle, such as self twist spinning, woolen twist roving, etc.
No matter which method is used, the fiber will be produced after twisting:
(1) Screw transfer and displacement of fiber points;
(2) The fibers are stressed and squeezed against each other. When the yarn is subjected to a certain tension to produce radial pressure, the fibers are tightly packed with each other, which is not easy to slip off, showing a certain strength.
[application of false twist]
Formation process of false twist
Static false twist process
The number of twists in section AB and section BC is equal and the direction is opposite.
The false twist process of the sliver moving along the axial direction
1. Two twisting areas
When t →∞, T2 = 0.
Applying the stable twist theorem:
AB section: n-vt1 = 0 T1 = n / V
BC section: n-vt1-vt2 = 0 T2 = 0
The yarn in the first zone (part) has twist, and the stable twist of the final product is zero. This twisting process is called false twisting.
2. Multiple lay zone systems
AB section: nb-t1v = 0 T1 = Nb / V
BC section: NC Nb + t1v t2v = 0 T2 = NC / V
CD section: vt2 NC VT3 = 0 T3 = 0
3. False twist conclusion
(1) When the sliver moves axially, no matter how many twisters there are between the two holding points and how they turn, the stable twist of the last twister is zero.
(2) The stable twist and twist direction of each twisting zone depend on the speed and turn of the twister at the exit of the twisting zone, but not on other twisters.
False twist effect
The function of false twist
(1) Twist the yarn of section AB (local) to prevent accidental drafting and reduce breakage.
(2) False twist of chemical fiber tow can be used to make textured yarn, elastic yarn, etc.
Example: point B on the top hole of roving spindle tube. Additional rotation of the sliver. N ′ - tab ′ v = 0 tab ′ = n ′ / V
In this way, n ′ / V false twist is produced, which enhances the strength of the upper end segment.
Application of false twist
Twist can be transferred from point B and false twist can be produced here, which enhances the strength of yarn segment.
Enhanced false twist:
(1) Enlarge the diameter of the top hole and increase the additional rotation of the yarn.
(2) Slot the top hole of the ingot wing or install a false twister to increase the friction f (multiple grooves in the front row).
Selection of roving twist factor
(1) Purpose of twisting
① Bear the tension during processing
② Additional friction boundary in spinning drafting
(2) Factors of twist factor selection
① Fiber length and fineness: when the roving has a certain fineness, the fiber is long and fineness, and the twist coefficient is smaller.
② Roving fineness: when the fiber length and fineness are fixed, the roving fineness is coarse and the twist coefficient is smaller.
③ Additional friction boundary: the twist coefficient is large, and the additional friction boundary is strong, but the draft cannot be opened.
④ In the dry season of climate season, the fiber should be stiff and twist coefficient should be large.
1. Spinning frame twisting
2. Simulation diagram of ring spinning process and twist distribution
In addition to the upper twist, there is a NW / VR twist potential in the twist of the upper winding yarn. When unwinding, the actual twist on the yarn is:
In the process of twist transmission, there are stranding (balloon section) and stranding (spinning section). Spinning segment TS < tube segment TW < balloon segment TB, the twist of spinning segment itself also presents a certain distribution, and the twist at the near front roller is the least, which is called weak twist segment.
3. Twist change in one drop
(1) Twist change in one drop
(2) Twist change of ring plate in one lifting movement
4. Technological factors affecting twist of spinning section
(1) Length of spinning section
(2) Yarn guide angle
(3) Arc length of front roller
(4) Height of air ring
(5) Coil number
5. Relationship between twist (twist coefficient) and physical and mechanical properties of spinning
(1) Strong P = ∑ qicos β + ∑ fi
The sum of the breaking strength of some fibers and the friction resistance caused by the slippage of some fibers.
With the increase of twist, the elasticity increases and the yarn is fatigue resistant. But when the twist reaches the critical value, the elasticity decreases.
(3) Glossy feel
Large twist, hard handle, poor gloss.
Twist shrinkage (%) = (calculated length - actual length) / calculated length × 100%
(5) Twist direction
The twist direction of the yarn has a great influence on the appearance and hand feel of the fabric.
(6) Selection of spinning twist factor
① Objective: to meet the requirements of yarn use and finished products.
② Twist factor selection:
A. Yarn strength: high strength, high twist coefficient.
B. Hand feeling and cloth state
C. Decapitation rate
6. Strand twisting
In general, the production of strand shall go through winding, doubling, twisting and other processes. The process flow is as follows:
(1) The change of strand properties after twisting
① Improve unevenness of strip
② Increase strength
In general, the average strength of single yarn in double yarn is 1.2-1.5 times larger than that of single yarn, and that of three yarn is 1.5-1.7 times larger.
③ Change of elasticity and elongation
④ Increased wear resistance
⑤ Gloss change
⑥ Change of hand feel
(2) Theoretical analysis of twisting
① Double strand reverse twisting
A. Single twist
B. Strand twist
C. The overall actual lay width of strand is shown in Figure C. Outer layer (r = 2r0) lay length Pb = p0-p1
D. M-point twist
The vector sum of P0 'and P1' of any point m of strand section shall be used.
E. Some special cases
(1) When the twist of strand is equal to the twist of single yarn P1 = P0, the twist on the surface of strand is zero, so it has good handle and luster. The relationship between the twist coefficient α 1 of strand and the twist coefficient α 0 of single yarn
It can be calculated as follows:
(2) When the twist of strand is twice that of single yarn P1 = 2p0, the twist of all parts of the strand is equal (even), so the strength of strand is the best. At this time, the relationship between the twist coefficient of the strand and the single yarn can be calculated as follows: P = TG β = 2 π RT
(3) When the twist of strand is larger than that of single yarn P1 > P0, that is to say, α 1 > 0.707 α 0 strand has better strength and elasticity, and has certain luster and feel.
② Double strand twisting in the same direction
The twist width of the inner and outer fiber is very different, the twist width of the outer fiber is increased, and the feel of the strand is relatively solid.
The effect of twist direction of strand on the properties of strand
② Elongation and elasticity
③ Luster, feel and wear resistance
A. Luster: the outer fiber has good parallel luster with the axial direction.
B. Feel: low twist of outer layer, soft feel.
C. Wear resistance: even twist distribution, strong wear resistance.
D. The relationship between twist extension, twist contraction and twist.
If one end of the spinning is held and the other end rotates, the yarn can be formed. This process is called twisting.
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