Circular Knitting Machine
The term knitwear includes two main textile techniques, weft and warp knitting (Spencer, 2001; Weber and Weber, 2008) (Table 7.1). After weaving, it is the most common method of manufacturing textile fabrics. Because of the interlooped structure of the knitted fabric, the properties are completely different to woven fabrics. The difference in weft and warp knitting originates in the way the needles move during the production and in the way the yarn is supplied. Weft knitting is a one fibre technique, which means that only one fibre is needed to build the stitches. The needles are moved separately, whereas the warp knitting needles are moved simultaneously. Therefore, all needles need the fibre material at the same time. For this reason, the yarn is supplied with the help of warp beams. The most important knitwear fabrics are circular knitted, warp knitted, flat-knitted fabrics and fully-fashioned fabrics.
The specific features influencing yarn delivery on large-diameter single jersey circular knitting machine are high productivity, continuous knitting and a great number of simultaneously processed yarns. Some of these machines are equipped with a striper (yarn guide exchange), but only a few enable reciprocated knitting. Small diameter hosiery machines have up to four (or occasionally eight) knitting systems (feeders) and an important feature is the combination of rotary and reciprocal movement of the needle bed (beds). Between these extremes are the middle diameter machines for ‘body’ technologies.
Figure 4.15 shows the simplified yarn supply system on a large-diameter double jersey circular knitting machine. Yarns (1) are brought from the bobbins (2), passed through the side creel to the feeder (3) and finally to the yarn guide (4). Usually the feeder (3) is equipped with stop-motion sensors for yarn checking.
The knitted textile structure evolves from loops that are intermeshed row after row. The needle hook is responsible for the formation of a new loop with the supplied yarn. During the upward movement of the needle in order to catch the yarn to build a new loop, the old loop slides down the needle (Fig. 7.20). This causes the opening of the needle. The needle hook is now open to catch the yarn. The newly built loop is drawn through the old loop from the previous knitting circle. During this movement, the needle is closed. Now the old loop can be released as the new loop remains in the needle hook.
The creel of the knitting machine controls the placement of yarn packages (bobbins) on all machines. Modern large-diameter circular machines use separate side creels, which are able to hold a large number of packages in a vertical position. Floor projection of these creels may differ (oblong, circular, etc.). If there is a long distance between the bobbin and the yarn guide, the yarns may be threaded pneumatically into tubes. The modular design facilitates the changing of the number of bobbins where required. Small-diameter machines with a smaller number of cam systems use either side creels or creels designed as integral to the machine.
Modern creels make it possible to use double bobbins. Each pair of creel pins is centred on one thread eye (Fig. 4.16). The yarn of a new bobbin (3) may be linked to the end of the previous length of yarn (1) on bobbin (2) without stopping the machine. Some of the creels are equipped with systems for blowing off dust (fancreel), or with air circulation and filtration (filtercreel). The example in Fig. 4.17 shows the bobbins (2) in six rows, closed in a box with internal air circulation, provided by fans (4) and tubes (3). A filter (5) clears dust from the air. The creel can be air-conditioned. When the machine is not equipped with a striper, this can be supplied by yarn exchange on the creel; some systems enable the knots to be positioned in the optimal area of the fabric.
The sinker is also important for the production of knitwear (Fig. 7.21). It is a thin metal plate, which can have different shapes. Each sinker is positioned between two needles and its main purpose is to help build the loop. Furthermore, it holds the loops that were formed in the previous circle down when the needle moves upwards and downwards to build the new loops.
Both single set und double set machines also exist as Jacquard machines, which are needed for special designs. In these machines, the movement of each needle can be controlled from each cam. Common products that are produced with circular knitted fabric are T-shirts. For production, nearly every material can be used. The form varies from filament to staple fibre yarn. For special purposes, also monofilaments and wires are used.
Machines that possess just one set of needles are only able to produce plain- knitted structures (Fig. 7.22). In these structures, one side of the fabric shows right loops and the other side rib loops. The following picture shows the loop structure of a plain knitted fabric.
Yarn length control (positive feeding), when not used for patterned fabric knitting, must enable different yarn lengths to be fed into courses in different structures. As an example, in Milano-rib knit there is one double-faced course (1) and two single-faced (2), (3) courses in the repeated pattern (see Fig. 4.18). As a double-faced course contains twice as many stitches, the yarns must be fed at approximately twice the length per machine revolution. This is the reason why these feeders use several belts, individually adjusted for speed, whilst feeders using yarns of the same length are controlled by one belt. The feeders are usually mounted onto two or three rings around the machine. If a configuration with two belts on each ring is used (Fig. 4.5), yarns can be fed simultaneously at four or six speeds.
The interlock structure was derived from the rib structure (Fig. 7.23). For the production of this kind of fabric, two needle sets are necessary and the needles need to be arranged in a different way. The loops are formed in two different directions (Fig. 7.24). The result is a fabric with smooth surfaces on both sides. This is due to the right loop structure on each side. The rib structure shows rib loops on both sides of the fabric. These fabrics can be produced using loop- or needle transfer.
Normally, the machine for flat knitting has two stationary beds that are arranged in an inverted V formation. These beds possess tracks in which the needles can be moved. The fabrics produced by a flat-knitting machine are mainly coarse and intensely patterned. An advantage of flat-knitted products is that vertical and horizontal stationary threads can be integrated into the fabric. In this case, the fabric serves to fix these threads. Fabrics produced this way can be used for technical textiles. Common products produced on conventional flat knitting machines are outer-wear, such as jumpers that consist of staple fibre yarns.
In small diameter circular knitting machine spare parts as well as flat bed knitting machine, generally one yarn is fed at a time to the needle for loop formation through the desired feeding system or feeder. However, in medium to larger diameter circular knitting machines, more number of feeders are arranged/accommodated at regular intervals for supplying more number of yarns to the needles simultaneously for achieving higher production. Each feeder produces separate course in each revolution of the machine. Production can also be increased by increasing the machine speed. But there is a limitation in increasing the machine speed as vibration, jerk, noise, yarn breakage and ultimately power consumption increase to a great extent at higher speed. So instead of increasing the machine speed, attempts are being made to increase the number of feeders in the machine. Machines are available with up-to 152 feeders for 42 inch diameter . Number of feeders on a circular machine depends on machine diameter, type of machine (plain, rib etc.), patterning facility and machine gauge. Number of feeder is mostly even number.
Traditionally, loop cut circular knitting machine were used for producing pantyhose . In 2002 Karl Mayer introduced the RDPJ 6/2 warp knitting machines for making seamless, jacquard patterned tights and fish-net pantyhose. Karl Mayer’s MRPJ43/1 SU and MRPJ25/1 SU jacquardtronic raschel knitting machines can manufacture pantyhose with relief-like and lace patterns . Other developments in machinery were aimed to increase the efficiency, productivity [27, 28] and quality of pantyhose .
Matsumoto et al. have also carried out some studies on the control of sheerness in pantyhose fabrics [18,19,30,31]. They produced an experimental hybrid knitting system composed of two experimental covering machines and a circular knitting machine. Each covering machine had two sections of single covered yarn. The pantyhose samples were knitted under a constant condition, while the single covered yarns were produced by controlling the covering levels of 1500 turns per metre (tpm) and 3000 tpm in nylon yarn with a draw ratio of 2 = 3000 tpm/1500 tpm for the core polyurethane yarn. The lower covering level produced a higher sheer in the pantyhose. Four different pantyhose samples were produced with different covering levels of tpm in different leg regions. The results showed that the aesthetics and sheerness of pantyhose fabric were greatly influenced by changing the covering level of the single covered yarn in the leg parts, and the mechanical hybrid system could improve the aesthetic properties of pantyhose fabric.
Weft knit fabrics are produced predominantly on cut pile circular knitting machine. The simplest of the two major weft knitting machines is a jersey machine. Generally, the terms circular knit and plain knit refer to jersey goods. The loops are formed by knitting needles and the jersey machine has one set of needles. Typical fabrics are hosiery, T-shirts, and sweaters.
Rib knitting machines have a second set of needles at approximately right angles to the set found in a jersey machine. They are used for the production of double-knit fabrics. In weft knits, design effects can be produced by altering needle movements to form tuck and miss stitches for texture and color patterns, respectively. Instead of a single yarn, several yarns can be used in the production of these structures. This increases the design possibilities.
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