Circular Knitting Machine

Circular Knitting Machine

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 [2]. 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 [25]. 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 [26]. Other developments in machinery were

aimed to increase the efficiency, productivity [27, 28] and quality of pantyhose [29].
    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.