Knitting is defined to be the formation of fabric by the intermeshing of loops of yarn. Unlike weaving, which requires two yarn sets, knitting is possible using only a single set of yarns. The set may consist of a single yarn (weft knitting) or a single group of yarns (warp knitting).
In weft knitting, the loops of yarn are formed by a single weft thread. The loops are formed, more or less, across the width of the fabric usually with horizontal rows of loops, or courses, being built one loop at a time.
In warp knitting, all of the loops making up a single course are formed simultaneously. Thus the lengths of each vertical column of loops, the wales, increase at the same time.
The knit loop may be characterized by its geometry or by the way in which it is viewed by an observer. Geometrically, an open loop is one in which the forming yarns do not cross at the bottom of the loop. In a closed loop, yarn crossing takes place.
The concept of face and back loops requires an observer. If the loop formation seems to be toward the observer, then a face loop is formed. If the loop formation seems to be away from the observer, then a back loop is formed. Although face and back terminology is not well-defined, it serves a definite purpose in the characterization and analysis of weft knitted structures.
In both warp and weft knitting, the principal mechanical elements used to form loops are needles. The most common type of needles, used in both warp and weft knitting, is the latch needle. The latch needle, developed in the mid 1800s, is so named because it can be closed using a latch which is activated without any special assistance during the knitting process.
The movements of a latch needle in forming a knit loop are as follows. In the running position the held loop rests on the top of the open latch. Clearing occurs as the held loop slips off the latch and onto the stem as the needle moves upwards. A downwards movement enables the needle hook to engage a new piece of yarn, this is known as feeding. As the needle continues downwards, the latch is forced to close under the influence of the held loop. Knockover occurs as the held loop disengages from the needle. Following knockover loop pulling occurs and a new knit loop is formed. The needle must now return to the running position to complete the cycle. It should be noted that the held loop remains at the same height during the cycle, this is essential otherwise clearing, knockover and loop pulling would not take place. Control of the held loop is usually achieved by the use of sinkers or by the application of the tension to the fabric. Note also that the fabric leaves the needle away from the hook. This is true for all needle designs.
Weft or filling knitting is a construction process in which the fabric is made by yarn forming loops across the width of the fabric or around a circle. Each yarn is fed at more or less a right angle to the direction in which the fabric is built. The term weft is taken from weaving terminology. In weaving the term is used synonymously with filling or pick to refer to the crosswise yams that are laid during the weaving operation. Weft knit fabrics can be made by machines, weft knitting is also the technique usually used in hand-knitting. A considerable amount of filling knit fabric is made on a circular knitting machine, in which a series of needles is arranged around the circumference of a circle. Fabric may be made in the shape of a tube. If a fiat fabric is desired, the tube can be cut open. Many fabrics, in fact, are made with a specific location for slitting the fabric open. Such knits could not be used in tube form. Other circular knits are designed so that they may be used in the cylinder or tube form in which they are made, adjustments are made for shaping to a figure or form for end use.
Knitting machines used for weft knitting may also be of the flat bed type. In these the needles are arranged parallel to each other in a flat plane. Flat bed machines that are used to make ribbed knit fabrics having two sets of needles arranged to form a V shape with the open end of the V parallel to the base of the machine.
Knitting requires four basic components: a yarn supply source; knitting elements or needles, which form the loops and build the fabric; fabric take down and fabric collections. The yarn supply for weft knitting may be located above the knitting area or on large creels at the side. If at the side, the yams are carried to the top of the knitting machine and are fed down to the knitting area. The knitting process includes the needles that permit the loops to be formed, held on the needle until a new loop is made, and then knocked off to gradually build the knit structure. After knitting, the fabric is carried to a roll at the base of the machine.
The actual knitting elements include the needles, sinkers, the needle bed or frame that holds the needles, and the yarn carriers that lay the yam in knitting position. There are two basic types of needles, latch and beard. Although either may be used for weft knitting, the latch needle is more common. The action of the needles is controlled by cams of various types, which determine the appearance of the knit fabric. Considerable weft knitting is done on circular machines. Flat bed machines are desired for knitting specific shapes and for producing full-fashioned knit pieces. Both circular and flat bed machines can produce highly patterned fabrics through the use of various types of controls such as computer controls. Attachments similar to those used for Jacquard weaves may be used to create elaborate knit patterns, which may be called Jacquard knits.
The term warp knitting is also adapted from weaving technology. Warp knitting differs from weft or filling knitting in that the loops are formed in a vertical or warpwise direction and yams lying side by side are interloped. Machines used for warp knitting tend to look somewhat like weaving machines. All of the yarns are placed on the beams and are located behind and above the actual knitting area. All yarns feed into the knitting area at the same time. Each yarn is manipulated by one specific needle as the interlooping proceeds; however, guide bars control the placement of the yarn, and the particular needle forming the loop may vary from one interlooping action to the next. Jacquard attachments can be used to provide for special needle controls and the making of highly patterned warp knits. Because of the knitting process used in warp knitting, the location of the yarn supply, and the interlooping of the parallel yarns, the machines used sometimes called knitting looms.
Essentially, warp knitting is a system for producing fabric that is flat, has straight side edges, and is manufactured rapidly and in large quantities. Warp knits are classified according to the type of equipment used and to special characteristics of the resulting fabrics. The types are tricot, raschel, Milanese, and simplex. The most common types are tricot and raschel.
Warp knit fabrics have some elongation or stretchability, although it may be less than for idling knits. The amount of extension is influenced by the type of construction ( such as single bar tricot, double-bar tricot, raschel); the tightness of the knitting ( the tighter the knit, the lower the amount of stretch ); finishing techniques, which can impart good dimensional stability; gage (number of needles per unit of width); and type of yarn (for example, core-spun stretch yarns, complex yarns with little or no stretch, single or ply yarns of high dimensional stability).
The strength of warp knitting fabrics may be increased by using yarns of strong fibers, yarns of balanced construction, and a high gage with strong free yarns. A combination of yarns of different fibers, or different types of yarns ( simple, ply, complex), and space between loops affects the strength of the fabric.