14 Warp Knitting

S. Natarajan

1. Introduction

Warp  knitting  is  definitely  the  most  versatile  fabric  production  system  in  fabric manufacturing. Warp knitted fabrics can be produced by elastic or stable yarn, with an open or closed structure. They can be produced flat, tubular or three-dimensional. Fabric width can be over 6 m without seams or even up to a multiple of this width if it is a net construction.

Warp knitting can be defined as a loop‐forming process in which the yarn is fed into the knitting zone, which is parallel to the fabric selvages. In warp knitting, fabric is made by forming loops from yarns coming in parallel sheet form which run in the direction of fabric formation

Warp knitting techniques started only a couple of centuries back which are virtually unknown to the textile consumer. The loop structure of warp knitted fabrics is similar in appearance to that of weft knitted structures. The mechanical properties are in many cases similar to those of a woven fabric and even better for certain applications.

In warp knitting, yarns in the form of sheet are fed from beams rather than from yarn packages. The beams are called warp beams. All the yarns are knitted simultaneously rather than in a sequence as in weft knitting. Most warp knit fabrics are knitted open width. Because all the needles knit simultaneously across the needle bar, warp knitting is more productive compared to weft knitting.

Warp knitting has many unique advantages over other fabric formation systems. Some of these are as follows.

1. Higher production rates than weaving .
2. A wide variety of fabric constructions.
3. Large working widths.
4. Low stress rate on the yarns which allows the use of fibers such as glass, aramid, and carbon, particularly in weft‐inserted fabrics.
5. Fabrics can be directionally structured.
6. Three dimensional structures can be knitted on double needle bar raschel machines.

The fundamental construction of a warp knitting machine is similar to that of a weaving machine, especially in the yarn supply system from warp beams (1) and the fabric the take-up mechanism(3). The fabric is produced by intermeshing loops in the knitting elements such as Needles, Holding-down Sinker, Knock-over bar and Yarn guide.

arrangement generally restricts the maximum number of beams and guide bars to four. 28 and 32 gauge and 84 and 168 inches width machines have been popular for two‐bar tricot and locknit structures.

3. Warp Knitting Elements

Guides

Guides are thin metal plates drilled with a hole in their lower end through a warp end may be threaded if required. The guides are held together at their upper end in a metal lead of 1 inch width and are spaced in it to the same gauge as the machine. Guide bars supplies yarn from each warp beam. All the yarns from one row or shaft of beams are typically threaded through one individual guide bar. For example, if a machine has three shafts or rows of beams, it will also have three guide bars. Shown is a section of a guide bar in a metal lead placed in the foreground of the picture. The minimum number of guide bars and warp sheets for commercially acceptable structures is usually two.

Needles

All the three types of needles such as bearded, latch & compound are used in warp knitting. Whatever may be the type of needle, all the needles move up and down together for loop formation, i.e., all the loops in a course are made simultaneously. So instead of giving motion to the individual needles, all the needles are connected or fixed to a bar called needle bar and the needle bar is lifted up and lowered down by means of a cam fitted outside the machine, generally at the driving side. Needles are set in tricks cut in the needle bed of the machine.

The latch of the needle depends, for its knitting operation, on the yarn. The loop within the hook opens the latch when the needle rises to the clearing position and closes it when the needle descends for knockover. A broken yarn causes a needle to be void of yarn, and hence, the latch stays closed, so that no loops can be formed. Such a needle has to be opened manually in order to allow loop formation to resume.

Beard needle

It is the cheapest and simplest type of needle to manufacture and commonly found in machine gauges as fine as 60 needles per inch. The bearded needle has a stem, around which the needle loop is formed. The needle head is where the stem is turned into a hook to draw the new loop through the old loop. The beard is the curved downward continuation of the hook that is used to separate the trapped new loop inside from the old loop as it slides off the needle beard. The eye or groove of the needle is cut in the stem to receive the pointed tip of the beard when it is pressed, thus enclosing the new loop. The needle shank may be bent for individual location in the machine or cast with others in a metal lead.

Compound needles consist of two separately controlled parts which are the open hook and the sliding closing element. The two parts rise and fall as a single unit but, at the top of the rise, the hook moves faster to open the hook and at the start of the fall the hook descends faster to close the hook. The preferred type of compound needle is the open-stem slide needle which has a closing tongue that slides externally along a groove on the edge of the flat hook member. The slim construction and short hook of the compound needle make it particularly suitable for producing fine warp knitted structures at high speed. It can knit chain stitches without the loops rising up the needles, and its sturdy construction resists the deflection generated by elastic yarns or thick places in spun yarns.

The sinker is a thin plate of metal which is placed between each needle. The sinkers are usually cast in units one inch long which in turn are screwed into the sinker bar. The sinker has various parts with individual functions. This photograph shows the neb of the sinker and the throat which is used to hold down the fabric. The belly of the sinker is used as a knocking-over platform. The sinkers are given almost linear horizontal (forward and backward) motion through the sinker bar. The drive generally comes from a crank or eccentric arrangement. The neb and the throat of the sinker are used to hold down the fabric while the belly of the sinker is used as a knocking over platform

Presser bar

In order to close the hook for casting-off of the old loop in Tricot machine, some closing element (Presser bar) is must. The elements needed in Tricot machine are set in a separate bar across the full width of the machine which also get motion from a cam or crank fitted on the main shaft. The presser bar closes the hook of the bearded needle when the same moves downward after catching of the new yarn for loop formation.

Trick plate

The other name of needle bed is trick plate. Tricks or grooves are made on the bed for properly accommodating the needles so that they can move up and down freely without having any lateral tilt.

Guide Bar Movement

In order to feed the yarn to the needle for loop formation as well as to connect the adjacent wales, guide bar are required to execute a dual movement which are called swinging motion and a shogging movement. They act at right-angles to each other in order for their yarns to form overlap and underlap paths that combine as one yarn path around the needles. The swinging motion of the guides takes place either from the front of the needles to the back or from the back of the needles to the front. It occurs between adjacent needles and is a fixed, collective, and automatic action for all the guide bars as they pivot on a common rocker-shaft. The shogging movement of the guide bar is the lateral motion of the guides which occur parallel to the needle bar. The sideways shogging produces the underlaps or overlaps. The occurrence, timing, direction and extent of each shog is separately controlled for each guide bar by a pattern chain or pattern wheel. A shogging movement can occur when the guides have swung clear of the needle heads on the back or front of the machine. On the hook side, it produces an overlap and on the side remote from the hook it produces an underlap. The timing of the shog during the 360 degree of the main cam-shaft revolution determines whether an overlap or underlap is produced.

The combined effect of underlap and overlap is the lapping of yarn around the needle. Depending upon the relative direction of underlap and overlap there are two types of laps – closed lap and open lap. The loops made of closed and open lap are shown in Fig.

A closed lap is produced when an underlap follows in the opposite direction to the overlap and thus laps the thread around both sides of the needle. An open lap is produced either when the underlap is in the same direction as the overlap, or it is omitted so that the next overlap commences from the space, where the previous overlap finished. Closed laps produce heavier, compact and less extensible fabric than open lap produces.

Needle bar movement

The needle bar is lifted up and lowered down for the purpose of loop formation. During upward movement, the old loop is cleared and needle catches the yarn wrapped around it by the guide and forms the new loop during the downward movement. Such movement is imparted on the needle bar by means of a cam or eccentric fitted on a shaft called eccentric shaft. The shaft extends to the full width of the machine and the cam is located outside the machine, generally at the driving side.

Uniform warp yarn feeding and proper yarn tension control is made possible by supplying flanged beams which are attached to shafts that turn to unwind the warp yarns in a parallel arrangement. Each shaft of beams generally feeds its yarns to a separate guide bar which operates independently from other guide bars.

Every needle is fed by a separate end of yarn, from which a loop is formed. In order to connect the loops into a fabric, the ends shog between the needles – meaning that the guide bars through which these yarns are fed move from one needle position to another. In this manner each knitting needle draws a new loop through the loop formed by another end of yarn in the previous knitting cycle. The accompanying diagram shows four complete wales. Notice how the red yarn is knitting in combination with two other yarns. From this basic section of fabric, it can be seen that at least one set of ends of yarns, equaling the number of needles in the machine, is necessary to produce the fabric.

Patterning mechanism

As discussed earlier in topic guide bar movement, the guides have two types of motions one is shogging and another one is swinging. Such motions not only produce lap of the yarns around the needles but also shift the yarns from one needle to other. The ultimate pattern or structure of the fabric depends on the nature (direction, relative position and extent) of movements of the guides. So, control of nature of movements of the guides is very much important. The following pattern controlling mechanisms are generally used in warp knitting machines for imparting the necessary motions to the guides

A pattern wheel is a steel disc as shown in figure which has different slopes on its circumference. These slopes stroke a shogging roller or bowl as the pattern wheel revolves and the shoggingmotion is transmitted to the guide bar through a push rod. The pattern wheel is just like a cam with curves or slopes made on its circumference according to the pattern. These curves which are required for the overlapping and underlapping of the needle bar are smoothly shaped and have a well formed transition to and from each other. This ensures quiet and smooth running and makes the pattern wheel suitable for high speed machine. The heights of the slopes decide the extent of lateral displacement of the guide bars.

Pattern wheel provides accuracy and smooth running even at high speed. But pattern wheel is economical for producing longer fabrics of simple structure. A pattern wheel has restricted utility because pattern can not be changed to produce some other structure and it is not interchangeable between machines of different kinds.

4. Raschel machine knitting cycle

   (a) The guide bar is at front of the machine completing its underlap shog. The sinker bar moves forward to hold the fabric whilst the needle bar starts to rise from knock-over.

(b) The needle bar rises to its full height and the old overlaps slip down onto the stems after opening the latches which are prevented from flicking closed by latch wires.

(c) The guide bars swing to the back of the machine and then shog for the overlap. The sinker bar then starts to withdraw for allowing the guide bar to overlap.

(d) The guide bar swings to the front to wrap the warp threads into the needle hooks.

(e) The needle bar descends; the old overlaps contact and the latches are closed. The sinker bar now starts to move forward.

(f) The needle bar continues to descend and its head passes below the surface of the trick plate drawing the new overlap through the old overlap which is cast-off and as the sinkers advances over the trick plate.

Raschel machine knitting cycle

There are two different lap forms used in warp knitting, depending on the way the yarns are wrapped around the needles to produce an overlap. An open lap, illustrated in the top row of loops, is formed when the overlap and the next underlap are made in the same direction. When the overlap and the following underlap are in opposing directions, a closed lap is formed, illustrated in the bottom row of loops. The most common lap used, in most warp knit structures, is the closed lap. The open lap is used when special fabric properties are needed or when technical limitations exist. Special attention must be paid to the overlap direction because it affects the fabric properties significantly.

5.Types of stitches and structures

The popular warp knitted structures are mainly produced with two full guide bars. The structures are based on two-course repeat cycle and direction of lapping changes in every course. The two guide bars should invariably make different lapping movement otherwise the resultant structure would be equivalent to the structure produced with single guide bar. The proportion of yarns in the fabric is influenced by the extent of underlap and overlap of the guide bars. The presence of yarns in the face or back side of the fabric depends on the controlling guide bar. Under normal conditions the threads of the front guide bar dominate on both face and back sides of the fabric. Considering two guide bars (front guide bar and back guide bar), the nature of guide bar lapping movement is shown in Fig. 14.1 for producing some of the popular warp knitted structures

6. Conclusion

The production of technical textiles is a rapidly developing trade in textile industry. New end-uses are developed daily and in most cases technical textile structures are used to replace expensive, heavier or technically inferior constructions traditionally produced from other materials. To achieve the objective of a favourable performance/cost ratio, flexibility of warp knitting techniques makes them attractive both to the designer and to the manufacturer of technical textiles

REFERENCES and URLs

1. Sadhan C. Ray, “Fundamentals and Advances in Knitting Technology”, WPI Publishing, March, 2012
2. http://nptel.ac.in/courses/116102008/
3. David J Spencer, “A comprehensive handbook and practical guide Knitting Technology”,WPI Publishing, 2001