Measurement of Yarn Twist:
Yarn twist is primarily used to hold the staple fibers together to form a strong and continuous yarn. The level of yarn twist has been traditionally measured on the yarn twist tester by using the untwisting or untwist–twist method. In a digital way, yarn twist can be estimated indirectly by measuring the angle between the tangent to the fiber helix and the axis of the yarn in images. According to the spatial location and color, the fibers available for such evaluation can be classified into two categories: surface fibers and tracer fibers. Surface fibers are those laid along the twisting curves on the main body of a yarn, while tracer fibers are an extremely small percentage of color fibers (preferably black) purposely introduced into a spun yarn during spinning. Different from the location of surface fibers, most tracer fibers are distributed inside the yarn structure with similar geometrical and migrational performances to other fibers. The tracer fibers were used to trace the actual fiber path inside a spun yarn and thus are very useful for the structural analysis of spun yarns. In order to capture the tracer fibers, the yarn sample usually needs to be immersed in a trough of special solvent so as to optically dissolve other fibers based on their different refractive indexes. Compared with the yarn sample with tracer fibers, no such effort is needed in the image acquisition of yarn surface fibers.
Fig: A tracer fiber image captured by CCD camera
Twisted yarns break when the central fibers, which are the most highly strained, reach their break extension. Consequently, yarn and fiber break extensions are similar and the strength reduction is similar to the modulus reduction. In this article I will discuss various twist measurement techniques.
Twist Measurement Techniques:
Simple/Direct counting method:
This test is performed at a temperature of 20°C ± 2°C and a relative humidity of 65% ± 4%.
Sampling:
Twenty-five meters of yarn is discarded from every package and each sample is withdrawn in the direction normally used. The most preferable method is the side end withdrawal because the over end withdrawal inserts a twist in the yarn. Twenty-five specimens of spun yarn and eight specimens of filament yarn are referred to in ASTM D1423-99.
Spun Single Yarn:
A moveable clamp gauge length is set as long as convenient but should be less than the staple length of the fiber. The counter is set at zero and a tension of 0.25 ± 0.05 cN/tex is maintained. Twist is removed completely by turning the rotatable clamp until the fibers in the yarn become parallel. The number of rotations for untwisting a yarn gives the twist in a specific length of clamp. The number of turns is counted and the turns per unit length are calculated.
Filament Single Yarns:
The clamp gauge length is adjusted to 250 ± 0.5 mm or 10 ± 0.02 in. The counter is set at zero and the specimen is mounted in the clamp at a tension of 0.25 ± 0.05 cN/tex and from both free ends. Twist is removed completely by rotating the clamp until the mono filaments in the strand become parallel and the parallelism is ensured by passing the needle through the strand. The number of rotations for untwisting the yarn gives the twist in the specific length of the clamp. The number of turns is counted and the turns per unit length are calculated.
Folded Yarn:
Five samples are referred to in ASTM D1423-99 for folded yarn twist measurement. The clamp gauge length is adjusted to 250 ± 0.5 mm or 10 ± 0.02 in. The counter is set at zero and the specimen is mounted in the clamp at a tension of 0.25 ± 0.05 cN/tex and from both free ends. A specified length of specimen is mounted in a twist device. One end is rotated until all the strands are free of twist. The number of turns is counted and the turns per unit length are calculated.
Continuous twist tester:
The continuous twist tester is designed to increase the number of tests performed per unit time. Yarn passed from a rotating jaw end is wrapped on a rotatable drum. Twist is measured by untwisting and twisting a specific length but, after removal of the twist, it is imparted back onto the yarn. By this method an instrument measures the twist per unit length of yarn.
Untwist and retwist method:
The tension-type twist tester is working on the principle of twist contraction. This method is also called untwist-re-twist method. When the level of twist increases, the length of yarn is contracted, and when the twist is removed, the length is increased; if all the twist is removed then the length reaches its maximum value. This method is used on equipment in which one end of the yarn is attached to a counter and the other is attached to a weight-pointer. When the yarn is untwisted, the pointer identifies the slight change in length. When the yarn is untwisting, the length of yarn is increased and the pointer moves from right to left; and when all the twist is removed then the length of yarn is at the maximum and the pointer does not move further from right to left but the rotating jaw continuously rotates in the same direction; further rotation causes length contraction due to twist insertion, at that point, before length contraction, the untwisted twist is the yarn twist. The level of twist is indicated by the instrument. This method is suitable for single yarns.
Fig: Tension-type twist tester
The tension-type twist tester as shown above has two jaws (fixed and rotating) for fixing the yarn. The position of the non-rotating jaw is variable according to the gauge length. The pointer connected to the rotating jaw moves over a scale having two graduations.
Working Procedure:
The two discs are disengaged from the worm and are rotated such that the pointer and zero mark coincide with the index mark. The yarn is first gripped in the fixed clamp. After being led through the rotating jaw, the yarn is pulled through until the pointer lies opposite a zero line on a small quadrant scale. The jaw is then closed. At this stage, the specimen is under a small tension and has a nominal length (gauge length).
As the twist is removed the yarn gets extended and the pointer reaches a vertical position. Eventually all the twist is taken out, but the jaw is kept rotating in the same direction until sufficient twist has been inserted to bring the pointer back to the zero mark. When the pointer coincides with the mark, the dial reading is noted and the twists per inch can be calculated using the following formula:
Twist per inch = dial reading / (gauge length × 2)
Automatic/motor-driven twist tester:
An automatic twist tester (Zweigle D302 and USTER ZWEIGLE TWIST TESTER-5) makes the largest number of tests for measuring the level of twist. These automatic twist testers are also governed by untwist–twist methods for measuring the twist levels with a special tensioning system. It is widely used twist measurement method.
Fig: USTER ZWEIGLE TWIST TESTER-5
Sample preparation:
From each packages, an equal number of specimens of adequate length shall be selected from different parts of the package.
The exact length may be either 350 or 500 mm, and the total quantity of test specimens is 10% of the test samples available.
Before cutting each test specimen from the package, a length of about 10 m of yarn should be discarded.
While drawing the specimens from the packages, care should be taken to avoid any changes in the twist of yarn.
Working Procedure:
The yarn specimen is placed on the yarn stand; keep the yarn stand on the left-hand side of the instrument.
Draw the yarn through the yarn guide which is in straight-line to the yarn bobbin, and draw through the instrument’s yarn guide, which is
fixed at the tail end on the left side of the instrument.
Depress the center screw of the disc yarn gripper using the thumb and lift the outer cone by force and middle finger of the left hand; then catch
the yarn by right hand, drawing through the opening of the disc yarn gripper and pull the yarn to required length (i.e., slightly more than the gauge length). Then arrest the yarn by simply releasing the outer cone.
After applying correct tension, release the hand from the revolving shaft gently and cut the protruding yarn by a sharp knife. Now, the specimen
yarn is ready for test.
Press the ‘Start’ button. Regulate the motor RPM to set the appropriate speed. While setting motor speed, the tester should ensure that the
following parameters are followed:
Set the motor speed at half-speed of maximum speed at which the motor can rotate; this is specifically applicable to all cotton yarns.
Set the motor speed at 75% speed for high-twist filament yarns.
Testing of single yarn:
For single yarns the test will stop automatically after the completion of untwist – retwist procedure. The display will show the direct result in TPI or TPM.
Testing of double and plied yarns:
For double and plied yarns only the untwist process is used. In this method test will stop automatically at a pre-set value. The pre-set value may be 2 to 3 less than the calculated TPI. Now insert a sharp-edged needle between the opened double yarn at the extreme end of the specimen yarn (from disk yarn gripper) and move towards the rotating jaw and press the ‘Start’ button once for releasing the remaining twist in the yarn. Reduce the motor’s speed to minimum. By doing this the motor will stop exactly after releasing the twist. The display will show the direct result in TPI.
References:
Advanced Textile Testing Techniques Edited by Sheraz Ahmad, Abher Rasheed, Ali Afzal, and Faheem Ahmad
A Practical Guide to Textile Testing By K. Amutha
Computer Technology for Textiles and Apparel Edited by Jinlian Hu
Fibers to Fabrics by Bev Ashford
The Substrates – Fibres, Yarn and Fabric by Mathews Kolanjikombil
Theory of Structure and Mechanics of Yarns By Bohuslav Neckář and Dipayan Das
Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.
Resp. Sir
Can we test TPI for 20s OPEN End yarn as compared to 20s CARDED YARN truly.