Weft_Knitting

Circular Weft Knitting Introduction to circular knitting machines and production of interlock and single jersey Aim: * To investigate the production of interlock and single Jersey fabrics. * How to produce fabrics with horizontal and Vertical Stripes * Examine different types of yarn feed used on weft Circular knitting machines. Procedure: 1. Examine the cam systems of interlock and single jersey Circular knitting machines to produce the fabrics below * Horizontal stripes [Single Jersey] examine the variations in width of stripe and how this can be achieved. * Vertical stripes {interlock} * Small check patterns. 2. Examine the types of quality control on a weft knitting machines. * Negative feed. * Positive feed. * Assisted feed. 3. Investigate the stitch length, stitch density [Courses and wales] and yarn measuring devices used in quality control. Conclusion: Single Jersey: A single jersey fabric can be knitted using a plain circular knitting machine with one needle system , with help of latch needles specifically . This machine can only knit plain knits and hence is called a single jersey machine.It is called open top circulars since they have a cylinder and no dial i.e only one set of needles. Present day single jersey machines have 12- 60 gauges up to 48 inch diameter. Production of single jersey fabric on a circular latch needle machine: According to David J Spencer ,Knitting technology; The yarn is supplied from cones via a bobbin stand or a free standing creel it is further passed through tensioners , stop motions and guide eyes down to the yarn feeder guides. The fabric formed is of tubular from and is drawn downwards with the help of tension rollers inside the needle cylinder it is then wound onto the fabric batching roller. The winding down motion is in unison with the cylinder and fabric tube.Since the sinker cam plate are placed outside the machine it is referred as open top machine. Interlock: A interlock fabric has a technical face of plain fabric on both sides. There is no way of revealing the reverse meshed loop wales because the wales on each side are exactly opposite to each other and are locked together. An interlock pattern row requires two feeder courses each having separate yarns knitting on separate alternate needles.( this will be further explained in production of interlock fabric). An interlock structure is balanced and lies flat without a curl. Production of interlock fabric on a circular latch needle machine: An interlock fabric can be produced either on a special cylinder and dial circular machine or double-system flat v bed machine. However An interlock fabric would require the following * Interlock gaiting: That is when the short needles and long needles are exactly opposite each other hence only short or long needles can knit at a given time. * Cam systems: Interlock fabric would require individual cam system since this is the only way to produce alternating knitting. Hence one cam system will control the long needles while the other the short needles. However in modern machines nowadays the needles are of the same length, however the concept of two cam systems for each feeder remains. * Also the cam system controls diagonal needles and not the needles next to each other , hence it is alternating and opposite. Variation of stripes in an interlock fabric structure: Two different yarns can be used to form horizontal and vertical stripe patterns Horizontal stripes can be formed if the same colour is knitted at two consecutive feeders one after the other. Vertical stripes can be formed if odd feeders knit one colour and even feeders knit another colour. Description of the interlock cam systems: Usually an interlock would require eight cam systems to produce on complete course i.e two cam systems for each feeder in each needle bed. An interlock cam system consists of the Cylinder cam and Dial cam: Cylinder Cam System: * Clearing Cam to lift the needle to clear old loops A * Stitch cams and guard cams to vertically adjust the varying stitch length. B,C * Upthrow cam, To raise the cylinder needle whilst dial needle knocks over D * Guard cams to clear the track E,F * Guide cam to provide the track for idling needles. G,H Dial Cam System: * Raising cam to tuck positions 1 * Dial knock over cams(adjustable)2,3 * Guard cam to complete the track 4 * Auxilarry knock-over cam to provent the dial needle re-entering the old loop. 5 * Guide cams that provide the track for the idiling needls. 6,7 * Swing type clearing cam, which may occupy the knitting positions or tuck positions 8 Negative Feed: A yarn feeder is negative type when the needles takes the yarn directly form the package during stitch formation step and the feeding tension of the yarn cannot be controlled. Disadvantages of negative feed: * This negative feed generates difference in the yarn length used for stitch formation. * Due to this variable tension more or less yarn is unwond and fed in the machine * Lower machine speeds * Friciton on yarn increases and changes. * 15% of the stitch can be robbed by negative feed. Positive Feed: * The yarn feeder is a positive type when the feeder supplies precise amount of yarn to each needle. * Advantages of positive feed: * Highly consistent quality of product can be obtained. * Constant yarn length delivered to the needles. * Faster knitting speeds. * Substandard yarn can be used. * No firction upto the feeder. * Disadvantages of positive feed: * All cam systems must be knitting the same amount of yarn ,upto 4 belt drives can be used. Assisted feed: This system usually takes the form of a smooth polished metal roller, the metal roller runs at a higher surface speed than the maximum yarn speed during knitting.The yarn forms a loop around the metal roller such that as soon as the carriage starts to move ,further the needles draw the yarn from the package ,Due to the yarn guides theres a tension in the yarn path, hence the yarn tightens around the metal roller.Also the metal roller has a polished surface the yarn grips this polished surface and thus drives it forward towards the yarn feeder.the yarn tension falls back to zero as soon as the yarn speed exceeds the yarn demanded by the needles.hence the grip of the loop relaxes and the yarn speed drops. Disadvantages : * Short term fluctualtions in loop length due to changes in robbing back. Adavantages : * Reduces average yarn tension at athe needles and reduces cuts and holes. * Can knit any needle selection, all cam systems can knit different needles selections. * Higher machine speeds. * Little firction * Substandard yarn can be used. References: * Knitting Technology David J Spencer * www.why.knovel.com