★ What is yarn? Yarn: a generic term for yarn and thread, it is a…
Table of Contents
★ What is yarn?
Yarn: a generic term for yarn and thread, it is a long, thin, soft and continuous strip with a certain strength, it is made of textile fibers, including single yarns and ply yarn.
Single yarn: A yarn made from staple fibres that have been spun so that the staple fibres or filaments are arranged in an axial direction and twisted.
Ply yarn: it is made of two or more single yarns by combining and twisting, which are stronger and more resistant to abrasion than single yarns, while the ply yarn can also be combined and twisted in a certain way to obtain double-twisted ply yarn.
★ What is twisting?
If one end of a fiber strand is held and the other end is turned back to form a yarn, this process is called twisting.
For staple fibers, twisting is necessary to turn the fibres into yarn. Before twisting, the loose fibres usually coalesce into a fibre strip, and after twisting the outer fibres are squeezed towards the inner layers to create centripetal pressure, which causes the strand to gain friction along the direction of the fibre elongation.
For filament yarns and ply yarn, twisting creates a compact structure that is not easily damaged by lateral external forces, and twisting also creates deformed filaments and fancy threads. The amount and direction of twisting affects not only the feel and appearance of the fabric but also its intrinsic quality.
After twisting, the outer fibres are twisted back in an inclined spiral, the fibres are twisted and deformed and the yarns are held together, changing the collective structural form and mechanical and physical properties of the fibres, as shown in the diagram below.
When there is an wrap angle on the yarn strip, the fibres exert centripetal pressure on the yarn strip, the greater the wrap angle, the greater the centripetal pressure. The greater the angle of encirclement, the greater the centripetal pressure. The centripetal pressure, which squeezes the outer fibres towards the inner layer, increases the tightness of the yarn strip and the friction between the fibres, thus changing the structural form, physical and mechanical properties of the yarn strip, which is the essence of yarn twisting.
★ Assessment of yarn twisting: twist direction, twist, twist factor, twist contraction rate
Twist direction: indicates the direction of twisting.
The twist direction is the tilting direction of the fibres in a single yarn or of the single yarn in a ply yarn. It is divided into two types: Z-twist and S-twist.
After twisting, the twist direction of the yarn from the lower right corner to the upper left corner, and the tilting direction is in the same direction as the middle of the “S”, which called the S-twist. When the twist direction of the yarn from the lower left corner to the upper right corner, with the tilting direction is in the same direction the middle of the “Z”, which called the Z-twist. In general, single yarns are often twisted with the Z twist and ply yarn often twisted with the S twist.
The twist direction of ply yarn is expressed in the order of successive twists. For example, in a ply yarn, the single yarn with a Z-twist, the first twist with an S-twist and the re-twist with a Z-twist, which twist direction is indicated by ZSZ.
The twist direction of the yarn has a great influence on the appearance and feel of the fabric. By using the twist direction of the warp and weft yarns in conjunction with the fabric organisation, different styles of fabric can be woven.
Twist: The number of twisting rounds per unit length of yarn.
Two sections of the yarn produce an angular displacement of 360°, which becomes a twisting round, commonly referred to as one turn.
Cotton yarns are generally twisted in the Tex system, expressed as the number of twisting rounds per 10cm of yarn length. For combed woollen yarns and chemical filaments, use the metric count system, expressed as the number of twisting rounds per metre. In addition, there is the English count system, expressed as the number of twisting rounds per inch.
Twist affects the strength, flexibility, elasticity and shrinkage of the yarn. As the twist increases, the strength of the yarn increases, but the twist must not exceed a certain value, otherwise the strength decreases, which is called the critical twist of the yarn. The critical twist is different for yarns made from different raw materials. In general, as long as the strength requirements are met, the less twist the yarn has, the better, because an increase in twist makes the yarn stiffer to the touch, less elastic and more shrinkage, this is the reason why filament yarns are twisted as little as possible.
The two commonly used methods of twist testing are the direct counting method and the untwist re-twist method. Staple fibres and ply yarn are generally tested by the direct counting method, while spun yarns are tested by the untwist re-twist method. In addition, there are also twice and thrice untwist re-twist method, slip methods, etc.
Direct counting method
Under a certain tension, clamp the ends of a length of yarn, one end is fixed and the other end is turned around axially in the direction of the re-twist until the single yarn in the ply yarn are perfectly parallel or the single fibres in the single yarn or compound yarn are perfectly parallel, the number of twist returns receded is the twist of the yarn specimen at that length.
Untwist re-twist method
Under a certain tension, clamp the yarn at both ends of a known length.
One end is held in place and the other end is turned back in the direction of the re-twist. When the yarn is returned to its starting length after re-twisting and reverse twisting, test the number of twist rounds, which is twice the twist in the length of the specimen.
Twist factor: relative value of the degree of twisting
The twist cannot be used to compare the degree of twisting of yarns of different thicknesses, because, for the same twist, thicker yarns have a greater inclination of the fibres than finer yarns. In practice, the twist factor is often used to indicate the degree of twisting of a yarn. The twist factor is a relative value that indicates the degree of twisting of yarn in combination with the linear density and can be used to compare the degree of twisting of yarns of different thicknesses. The twist factor can be calculated from the twist and the linear density of the yarn.
Conversion of twist factor: αt = 95.07×αe = 3.162×αm
The twist factor is a relative value that in combination with the linear density, indicates the degree of twisting of the yarn and can be used to compare the degree of twisting of yarns of different thicknesses, the higher the value the greater the degree of twisting.
Twist contraction rate
After twisting, the fibers tilt and the length of the yarn shorten, resulting in twist contraction, which is usually expressed in terms of the twist contraction rate, i.e. the difference in yarn length before and after twisting as a percentage of the length before twisting.
The twist contraction rate, which directly affects the linear density and twist of the spun yarn, must be taken into account in the design of the spinning and twisting process. The twist contraction rate of cotton yarn is generally 2% to 3%. The twist contraction rate is related to the twist factor, but also to the spinning tension, the temperature and humidity of the workshop, the thickness of the yarn and other factors.
★ Effect of twisting on the properties of the ply yarn
The properties of the ply yarn are related to the physical properties of the single yarn, ply number of yarn, the twist direction and the twist strength. Once the physical properties of the single yarn have been determined, the ply number, twist direction and twist strength are the main factors influencing the properties of the ply yarn. In general, the performance of the ply yarn is significantly better than that of the single yarn.
According to the principle of merging, when n single yarns are merged, the unevenness is reduced to 1/√n of the original rate. However, when the individual ply is separated, the level of evenness of the individual ply can still be distinguished on the outside. The ply yarn has a merging effect and sometimes even the appearance of the ply yarn is better than the theoretical calculation because the thicker knots or details on the yarn are always partly hidden inside the core cavity and not easily visible on the outside.
Increase in strength
After n single yarns have been combined, the untwisted strength generally does not reach n times the strength of the original single yarn, see the table below, this is because the elongation of each single yarn is not uniform and the stress is more concentrated where the elongation is small.
Strength effect of n single yarns combined without twisting
|Number of merges||1||2||3||4||5|
|strength utilisation of single yarn (%)||100||92.5||86.8||81.3||76.5|
The ply yarn is an integral whole with better evenness. During the twisting process, the original distortion level of the fibres and the imbalance of forces between the fibres can be improved, and increase the twisting pressure between the fibres and the yarn, thus improving the breaking strength resistance, so that the strength of the ply yarn often exceeds the sum of the strength of the individual yarns that make it up. Generally speaking, the strength of 2-ply yarn is 1.2 to 1.5 times the strength of the original single yarn (enhancement factor), and the enhancement factor of a 3-ply yarn is 1.5 to 1.7 times.
Changes in elasticity and elongation
When the ply yarn is twisted in the opposite direction, the elongation decreases slightly because the twist amplitude of the outer fibers decreases. As the twist factor increases, the twist amplitude of the outer fibres begins to increase, so elongation begins to increase again. When the ply yarn is twisted in the same direction, the average twist amplitude of the fibres increases with the twist factor, so the elongation of the ply yarn also increases and is numerically greater than when the ply yarn is twisted in the opposite direction, as shown in the graph below.
Increased abrasion resistance
During processing, the abrasion resistance of the yarn is mainly expressed in the degree of abrasion resistance when the yarn is in contact with the machine parts. As the ply yarn is evenly dried and rounded in cross-section, the friction with various machine parts is small. For the fabric of ply yarn, even if the surface fibres are partially worn, the fabric still has a certain strength due to its tight structure and therefore the fabric has good wear resistance.
Gloss and feel improvement
The gloss and feel of the ply yarn depend on the slope of the fibres on the surface of the thread. The greater the slope of the fibres, the worse the lustre and the harder the ply yarn; conversely, the better the lustre and the softer the feel. If the yarn is twisted too much, the axial inclination of the fibres is high and the stress on the fibres is high, the lustre is dull and the feel is hard. In this case, reverse twisting will improve the axial parallelism of the fibres on the surface of the ply yarn and reduce the inward compression, thus improving the lustre and feel of the ply yarn.
In addition, twisting has an effect on the volume, weight, and diameter of the yarn, and the twisting effect increases the tightness of the yarn. Within a certain range, the volume and weight of the yarn increase with an increasing twist, and the diameter of the yarn decrease with an increasing twist, resulting in changes in fabric coverage, comfort, etc.
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