Meeting the requirements of a shaping machine

In order to knit shaped panels or integral garments, it is necessary to meet a number of exacting requirements which can only be achieved with a specially designed fully computerized V-bed flat machine having the characteristics set out in Sections 19.11.1 to 19.11.7.

19.11.1 The shaping control programme

The shaping control programme needs to have sufficient memory to include the data for all the parts of a garment, whether integrally knitted or sequentially knitted shaped-pieces, in the complete range of sizes.

Shaping in width can only be achieved on machines freed from the constraints of constant-width traverse. On electronic machines, the computer is linked to the cam-carriage whose variable traverse and speed is driven from a belt. The traverse distance is varied by the belt drive, which transports the yarn carriers so that they follow the selvedge edge.

19.11.2 Variable-width carriage traverse

One of the most important features of shaping is keeping the cam-carriage traverses to the minimum width using a lightweight compact cam-carriage and belt drive, combined with knitting/transfer cams, and needle butts that are sunk when out of action.

19.11.3 The shaping method

Fashion shaping using loop transfer is the most satisfactory method of introducing shape into garment blanks. It is employed on straight bar frames in the form of plain loop transfer, using a set of rackable fashioning points. Although separate loop transfer fashioning points are employed on some V-bed machines, the most common method is to use the needles to rib loop transfer from needle bed to needle bed, combined with needle bed racking to move the selvedge loops inwards or outwards. Care must be taken to ensure that receiving needles are empty of loops.

19.11.4 Modern take-down systems

Modern machines have a computer-programmed, positively-driven takedown system whose operation is synchronised with that of the requirements of the knitting programme and provides pre-determined fabric tension as required. Sometimes, small sub-rollers provide a nip immediately below the gap in the needle beds. The main control is provided by the nip formed by the takedown roller and the counter roller that presses against its surface. The counter roller is segmented, consisting of individual rollers that are each spring-adjusted.

The roller drive speed can be selected from as many as 31 possibilities and can be stopped during needle bed racking and rib loop transfer, or it can be reversed to achieve zero fabric tension whenever required during the knitting programme.

19.11.5 Control of the fabric during knitting

The production of width-shaped garment pieces requires different or additional facilities to those used when knitting constant-width garment pieces joined by draw-thread separation. No one device alone appears to provide for all conditions of fabric takedown when knitting to shape.

When changing from a narrow width at the end of one garment panel and recommencing on a wider starting width for the next panel, with normal takedown rollers there will be a lack of takedown tension and fabric control at the selvedges, even with a draw-thread connection. If the pieces are not connected together, there will be no takedown tension. The most common solution is to employ a takedown comb in addition to the conventional takedown rollers; this rises to engage its pins with the set-up courses of the new garment piece. As knitting continues, it guides the fabric until it engages with the takedown roller, which then takes over control of the knitted panel. With separated garment piece knitting it is also necessary to employ thread cutters and trappers, otherwise yarn ends will wrap around the rollers.

Shima have a new computer-controlled pull-down system for their FIRST Whole-Garment machines. The front and back of the garment each has a separate takedown panel of tiny pins, each section of which can be individually controlled for specific tension. This results in a more dimensionally-accurate garment; for example by allowing shoulder lines for set-in sleeves to be positioned over the shoulders and towards the back.

19.11.6 Stitch pressing-down devices

The object of the presser foot and other similar devices (such as knock-over bits and holding-down sinkers) is to keep the old (fabric) loops low down on the needle stems. They are thus prevented from rising ('riding-up') and staying on the latch spoons as the needles rise for clearing or yarn feeding. This ensures a 'clean' knitting action, irrespective of the variable tensions within the knitted structure or the lack of takedown tension operating onto the fabric from below.

Interest in this concept was regenerated in 1968 by the development work of Frank Robinson and Max Betts of Courtaulds, whose 'presser foot' patents were licensed by Dubied, Bentley-Cotton and Shima Seiki for use on their flat knitting machines. Other companies also employed stitch pressing-down devices of various types on their machines.

The original presser foot consisted of a piece of wire bent at either end to form a foot (Fig. 19.5).The centre of the wire is carried on the underside of a pivoted arm that hangs downwards from a cross member so that it brushes against the upper surface of the fabric loops as it moves with the cam-carriage. At the end of each traverse, the pivoted arm is tilted to incline in the opposite direction, lifting one foot out of action and lowering the foot on the other end to trail across the needle beds for the return traverse

There is a device working with each cam system and its yarn carrier. Different

Fig. 19.5 Action of the presser foot.The foot is shown during a right-to-left traverse pressing down the loops on the needles in advance of the yarn carrier. (The cam carriage has been removed for purposes of clarity) [Knitting International].

diameters of wire can be employed for varying machine gauges and yarn counts, and it is possible to fit specially-angled feet of triangular cross-section for use during single-bed knitting or loop transferring, if necessary.

The foot acts slightly in advance of the yarn carrier and the rise of the needles for tucking or clearing. It enters the space between the needle beds to gently stroke the old loops down the needle stems as it trails, at a slight decline to their upper surface. Accommodation to differing degrees of knitting tightness can be achieved with a spring-loaded, self-compensating presser foot, which rides-up the support arm when the structure is knitted to a tighter quality.

As the presser foot does not create tension on loops already formed, loops may be held on inactive needles for many knitting cycles and stitch concentrations can be varied across the fabric width. It also enables separate garment panels to be commenced on empty needles and to be pressed-off on completion. The reduced takedown tension removes the problem of shape distortion and the bowing of courses caused by relaxation of the structure, often eliminating the need for first pressing. The structures tend to be heavier, and rib knitted on two-cam systems shows a slightly racked appearance because the presser foot causes yarn to flow into the first limb of each loop that it contacts. Two courses made in the same direction of traverse emphasise the inclination of the loops. To produce a conventional elongated loop instead of a round loop it is important to maintain some take-down tension.

The presser foot principle provides scope for the use of holding of loops, pressing-off, and part-course knitting in the production of unconventional integrally-knitted garments, which require less seaming and virtually no cutting. Amongst the garment shapes are cruciform, tubular plain articles, and garment parts in varying course lengths, knitted as shaped single pieces of fabric in a spiral formation, similar to the principles of the Basque beret or the ideas of MacQueen or Pfauti. Early attempts employing these techniques met with limited success until the development of computerised V-bed machines with full facilities for integral garment knitting, which could exploit the design potential offered in this area. The original presser foot was less precise than the modern computer-controlled stitch pressers and was susceptible to tension deflection and contact with the needles.

19.11.7 Needle bed racking

A maximum racking distance of 2 inches, in some cases on both beds, is available. This includes 1/4 pitch and 1/2 pitch. An over-racking facility stretches the loops, making their transfer easier.

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