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.