What is Warp Knitting?

Technically speaking, it is the sequential formation of interlinking loops in an axial direction on a lateral array of needles, with at least one separate thread being supplied to each needle. In a width-wise direction, the loops are then joined by moving the threads back and forth between the adjacent needles.

Warp knitting is the fastest method of producing fabric from yarns. It differs from weft knitting as every needle loops its own thread, whereas, in weft knitting, the needles may loop over a few threads. This results in a parallel row of loops that are simultaneously interlocked in a zigzag pattern. In using one or more sets of warp yarns, the fabric is produced in a sheet or a flat form which are then fed from warp beams to a row of needles extending across the width of the machine.

There are two major types of warp knitting machines – the Raschel machine, which is made with latch needles, and the Tricot machine, using bearded needles. Karl Mayer is considerably related to the commencement of warp knitting. In 1947, the insightful entrepreneur and mechanic showed off his first warp knitting loom. The FM 48 was compiled with two guide bars and with bearded needles, attained a speed of 200 rpm. It marked the beginning of a technological era, pioneering leaps in the field of warp knitting.

Karl Mayer, in 1953 launched his company's first raschel loom into the factories. These warp knitting machines was working with up to four guide bars, used a four-roll take-up system to allow fabric beams alterations without stopping the machine. It had a pattern box with pattern discs or chain elements. The technology of these high-performing warp knitting machines was enhanced from one generation to the other, considering to improvise the product technology, equipping for added facilities like blind lap tools for producing knitted plisse, or sinkers with two pieces, and easily movable sinkers or pattern pressers.

The market witnessed inventions of new machines in the year 1954, the first elastic raschel machine and the first tulle raschel knitting machine. In 1955 'Super Garant' series marked its name on the market. 1956 saw the first warp-knitted lace machine featuring 12 guide bars. The first curtain raschel machine was introduced in 1958, and the first carpet raschel machine in 1959. The success story continued in 1967 with a launch of the first fall plate multibar raschel machine and jacquard raschel machine weft insertion, along with other series. Following gradual developments led to more diversity in the product range of lace raschel machines. A significant move in this process was the execution of the jacquard theory in warp knitting.

Generally speaking, courser yarns are used for raschel knitting, and recently there is a growing interest in knitting staple yarns on these machines. On the Raschel machine, the needles move on a ground steel plate, also known as the trick plate. The verge, which is the top of this plate, defines the level of completed loops on the needle shank. As the fabric is pulled down with the sinkers between the needles, the loops are prevented from moving upward. The guide bars then feed the yarn to the needles. On a cycle, the needles start at the lowest point. Once the proceeding loop has been cast off, and the new loop joins, the needle hooks to the fabric. As the needle rises, the new loop opens the latches and ends up on the shank below the latch. Subsequently, the guide bars then swing through the needles. The front bar moves a needle's space sideways. As the guide bar swings back to the front of the machine, the font bar has then laid the thread on the hooks. The needles fall, the earlier loops close the latch to trap the new loops, and the old loops are cast off. The Raschel machines are made in a variety of forms and are generally more open in construction and coarser in texture than other warp-knitting machines. Raschel machines are regarded to be more useful as they can process all yarn types in all forms (filament, staple, combed, carded, etc).

The other kind of popular warp-knitting machine is the Tricot machine. The tricot, a warp knit made with two sets of threads, is typically defined by the distinct ribs running vertically on the fabric and horizontally across on the back. The tricot machine is specifically adept at making light fabrics, weighing less than four ounces per square yard. The invention of the FNF compound needle stimulated its development as it was a sturdy device that later fell into disuse but, regardless, allowed for the advancement in production speeds. Even though roughly half the tricot machines un current use make plain fabrics on two guide bars, there is an increased interest in pattern knitting. In this type of cycle, warp knitting requires close control on the lateral bar motion, which is achieved by control chains made of thick metal links.

In terms of structure, warp knitting is defined as a stitch forming process whereby the yarns are supplied to the knitting zone parallel to the selvedge of the fabric. Every needle in warp knitting is supplied with at least one separate yarn. The yarns are deflected laterally between the needles to connect the stitches to form a fabric. In this way, the knitting needle often draws the new yarn loop through the knitted loop formed by another end of the yarn in the previous cycle.

A warp knitted structure is divided into two parts. Firstly, there is the stitch itself, which is formed by wrapping the yarn around the needle and drawing it through the previously knitted loop. This is known as the overlap. The overlapping movement wraps the yarn around the needle, ready for the knock-over displacement. The second part of the stitch formation is the length of the yarn linking the stitches together, which is also known as the underlap, formed by the lateral movement of the yarns across the needles.

The needle space defines the length of the underlap. The more it lies at right angles to the fabric axis, the longer the underlap. The longer the underlap of a knitted warp, the more the lateral fabric stability will increase. On the other hand, the shorter the underlap, the less strength and stability it will possess width-wise, but this will be increased lengthwise. The underlap's length will also influence the fabric's weight. While knitting with longer underlaps, more yarn must be supplied to the knitting needles. This then covers and crosses more wales on its way, resulting in the fabric increasing in weight, thickness, and density. As the underlap is connected by the root of the stitch, it causes a lateral displacement in the root due to the warp tension. The reciprocating movements of the yarn cause the stitch of each knitted course to incline in the same direction, alternately to the right and left.

To produce an improved fabric appearance and to control both the lateral and longitudinal properties, the second set of yarns is typically applied. The second set of yarns is moved in the opposite direction. This is to help balance the lateral forces on the needles. The length of both the yarns used for the underlap does not have to be the same.

Warp knitting is unique in that it can produce some particular special effects. The scope of what warp knitting can produce increased by the progression of procedures for laying nonknitted threads for colour, texture effects (inlaying), density, although not all these threads will be an essential part of the structure and may only be decorative. An example of this could be the form called "zigzagging across several pillars" whereby on the ground of most fabrics, the front bar makes crochet chains or "pillars" which are connected by the zigzag inlays. 

An extension of conventional warp knitting is the Co-We-Nit warp-knitting machine, which can produce fabrics with properties of both woven and knitted fabrics. These machines only require two warp-forming warps and provision for up to eight interloped threads between each chain of loops. These are then interlaced with a quasi-weft which forms a fabric resembling woven cloth on one of the sides.   

Interlaced fabrics, in general, have become more and more popular, which has led to the invention of other lace-making machines. Early models required intricate engineering mechanisms, and the modern lace industry all stems from when a machine was designed solely to produce laces that are identical to the Brussels lace.