Warp Knitting Machines

Basics of knitting Warp Knitting Machines VASANT R KOTHARI has done Master’s in Textiles Technology from DKTE’s Textile and Engineering Institute, Ichalkaranji (Shivaji University, Kolhapur), Maharashtra. He has also done Diploma in Export Management (Apparel Export) from the Indian Institute of Export Management, and Garment Export and Merchandising Management from NIFT, Bangalore. Presently, he’s working as an Assistant Professor in Department of Fashion Technology, NIFT, Bangalore. (This is his fifteenth input from the series of articles in Knitting Views) T he history of warp knitting machine is closely associated with two names – William Lee and Karl Mayer. Unlike weft knitting machines, most of the warp knitting machines is open width/ flat type. As the name implies, loop formation is warp wise i.e. parallel to fabric selvedge. In warp knitting, fabric is made by forming loops from yarns coming from warp beam, which run in the direction of fabric formation. Every needle is fed by separate yarn for loop formation. In order to connect the loops into a fabric, the yarns are shifted (shogged) between the needles. In this manner the needle draws the new loop through the loop formed by another yarn in the previous knitting cycle. This unique feature of the loop continuity in upward direction makes the warp knitting fabrics more special with respect to their characteristics, production and applications. Warp knitting machines produce the widest range of fabric types and qualities of any fabric forming technology. Though the machine initiation has started very long back, in the middle of 20th century only the major developments in the manufacture of warp knitting machines has taken place. The warp knitting machines have gained their importance due to advent of manmade fibres such as nylon, polypropylene, polyester, acrylic, etc. Today, there is a vast range of machine sizes, types and configurations, ranging from 10 cm-wide crochet machine to a 5 mtr-wide geotextiles machine are available in the 32/KNITTING VIEWS/MAY-JUNE 2012 market. Modern warp knitting machines are engineered to operate at high knitting speeds (upto 3,000 cycles/min) and these machines may produce in excess of 5 sq mtr/min. Consequently, it is difficult to encapsulate such a range within a simple description. The given figure shows a typical knitting machine producing fabric for apparel. The main machine frame is constructed from sturdy cast steel or welded vertical side frames held together and stabilised by a large welded steel box section transverse girder. The needle bar and yarn guides are mounted transversely above box section girder in middle of the machine and run virtually full width of machine. Machine widths range from 1 mtr to 5 or 6 mtr depending on type and end-use of fabric. The yarn supply may be carried on warp beams situated above the knitting elements on beam control systems mounted on the side frames. Alternatively the beams may be mounted on Aframes behind the machine to permit greater beam capacities, or machine may be supplied from individual yarn packages mounted in creels behind machine. The fabric is taken away downwards and to the front of the machine to a take-up roller, or it may travel under a walkway for the operator, to be taken-up on a bulk fabric roller that is remote from the machine. Basic structure of a single bar knitting machine Types of warp knitting machines The classification of warp knitting machines is based on a number of factors such as • The type of the needles used • Numbers of guide bars • Machine speed • Machine complexity All three types of needle can be used on warp knitting machines. The needle are not independent for their action, but are mounted on a common needle bar. Number of needles bars as well as their width also takes part in classifying the warp knitting machines. Increase in number of guides bars increases the machine complexity; there by reduce the speed, which also influences machine classification. In general, warp knitting machines are divided into two classifications: Tricot and Raschel, each of which uses a different configuration of knitting elements and is suitable for producing different types of fabric structure. Both Tricot and Raschel may be made with either single needle bar or double needle bar. Conventionally, it was usual to differentiate between Tricot and Raschel by the needle used in each machine type. Tricot machines were equipped with bearded needles while Raschel machines only used latch needles. With the production of modern warp knitting machines, the compound needle replaces both the bearded and latch needles. However, an accurate differentiate can be made by regarding type of sinkers with which machine is equipped and the role they play in the loop formation. The sinkers used for Tricot knitting machines controls the fabric throughout knitting cycle. Whereas in Raschel knitting machines, the sinkers are only used to ensure that the fabric stays down when the needles rise. Both Tricot and Raschel machines use multi-guide bars. Though maximum number of warp beams and guide bars are four with conventional Tricot machines, majority of Tricot machines use only two guide bars. And in case of Raschel machines maximum 48 guide bars are possible Below table shows the major differences between Tricot and Raschel machines Tricot machine Raschel machine Bearded needle or compound needles are used Latch needles are commonly used Normally finer gauge – 28-32 needles per inch Coarser gauge – 8-16 needles per inch Machines are suitable for finer filament Machines are suitable for spun yarn, coarser filament or coarser, decorative staple spun yarn Less number of guide bars (2 to 4) More number of guide bars (6-48) Less numbers of warp beams are required More number of warp beams are required Sinkers control the fabric throughout the knitting cycle Sinkers only ensures that the fabric stays down when the needle rise Warp beams are positioned at back of the machine Warp beams are positioned at top of the machine The angle between needle and fabric take down is 90° The angle between needle and fabric take down is 160° Can produce simple fabrics Can product complicated fabrics as well Machine speed is high Machine speed is less Knitting tension is lower Knitting tension is higher Width of the machine is more Width of machine is less Produces light weight fabric Produces heavy weight fabric Less versatile machine More versatile machine Tricot fabric is more resilient, better drape, higher bursting strength, better dimensional stability. Soft hand and even opaque and tight Raschel fabric is less resilient, poor drape, lower bursting strength, and poor dimensional stability. Hard hand, uneven and loose (In the next session, we would be discussing about Tricot machine) KNITTING VIEWS/MAY-JUNE 2012/ 33