On the whole the component shapes of knitted garments are simpler than those made of woven fabrics. This applies even when the garments are made of jersey fabrics and are intended to occupy the same role as woven fabric garments, e.g. jersey dresses.
This simplicity of shape arises out of the natural extensibility of knitted fabrics that enables three dimensional forms to he generated by deformation. Such forms in woven fabric are obtained by darts, tucks, gores etc. To illustrate this, Fig. 3.1 shows a raglan sleeve head where the shape of the shoulder is generated in woven fabric by cutting, whereas in knitted fabric in a fully fashioned jumper, the wearer creates the shoulder shape by deformation of the fabric.
Not all knitted fabrics, nor garment types, respond to this approach and this chapter examines some of the principles of generating shape in the fully cut classes of knitted garments already outlined. The production methods applicable to the cutting stages will also be outlined.
The process of cutting knitted fabric varies considerably depending on the particular branch of the industry. Practices range from the single garment handling of fully fashioned — where a cardigan front is slit or a V-neckline is cut out by hand on each garment individually, when the garment is already in an advanced state of make up — through to the enormous scale of cutting underwear from multiple lays.
Production
Included in fully cut garments is a wide range of differing types of garment, including men's, women's and children's underwear, swimwear, sportswear and leisurewear. This range is generally regarded as within the scope of one type of company which differ largely in the definition of the market place in which they operate. A different sort of company is involved with the product range covered by the term jersey fabric.
The companies producing the fully cut garment product range are usually characterised by being vertical in organisation. The company will knit its own fabric, often wet finish and dye it, cut, make-up and market the finished product. The specific product will decide the scale of operations and the organisation of production. At one end the making of men's or children's underwear is very much mass production, with a small variation in product design and size range. At the other extreme some leisurewear is now highly stylized and subject to the vagaries of fashion. The production is organised on small production runs, quick response, a large range of designs and multiple variations within a style.
Jersey dresses, suits and other ladies outerwear garments are largely made by firms organized on a similar basis to firms making similar products in woven fabric. Some companies produce both jersey articles and the woven equivalent, although the cutting patterns are somewhat different and the making up techniques very different.
The cutting process for fully cut knitted garments is largely the same as for woven garments. The garment itself is built up of two dimensional shaped portions of fabric, which are, after cutting, assembled by seams into a three dimensional shape to fit the human body. The shapes themselves are evolved from an interpretation of the design of the garment by a process known as pattern cutting. The pattern cutter is a highly skilled person who uses a mixture of geometry, experience and creative inspiration to arrive at the forms of individual pieces that make up a garment to look like the design. Often the pattern cutter and the designer is the same person.
The pattern cutter may start with a 'basic block' which represents a simple interpretation of, say, an upper body outerwear garment and contains the correct sizing dimensions and a particular type of sleeve insertion. The geometry of the block is manipulated to generate details of shape, seam locations etc.
Trials (toiles) of the garment are produced until the correct fit is obtained. The initial garment is of course only in one size, usually an intermediate size in the range that form the market for that particular garment.
For example, in a normal womenswear size range the prototype would be designed in size 10 or 12. Other sizes would be generated by a process known as grading. This is another mixture of geometry and creativity, the object being to increase the sizes of each portion of the pattern while maintaining the general feel of the design on a variety of human body shapes. The pattern grader applies 'grade rules' that stretch or contract each portion of the pattern to a pre-determined formula.
Such stretching or contraction is not uniform in all dimensions. A given increase in body width will not be accompanied by the same proportionate increase in length of sleeves. Important details like maintaining a fit between sleevehead and armhole require particular attention.
This process of pattern making and grading results in a series of shaped pieces for each size of a particular style. Such shapes exist either as a series of strong cardboard or plastic cut-outs, or increasingly as shapes within a computer memory. It is from these shapes that a 'marker' is planned and a cutting order assembled.
The marker
A marker portrays the way in which pieces of a garment are laid out on the fabric for cutting. The marker is laid out to a particular width of fabric and within an optimum length, and may represent only one size or a mixture of two or more sizes.
The following factors are taken into account when planning the marker:
(1) the width of the fabric from which the garments are to he cut;
(2) in knitted fabric, whether it is tubular, flat open width or folded on one side;
(3) the normal length of the lay, which is connected to the type and length of the cutting table;
(4) the need to minimise the amount of waste between the marked out garment portions;
(5) the need to ease the path of the cutter blade without it getting into impossible corners;
(6) the need at all times to maintain the grain and directionality of the fabric;
(7) the alignment of patterns and checks etc.
Sometimes it is convenient to make a particular marker for a specific order, although it is more usual to standardize the marker/markers and vary the number of layers in the lays and the number of lays assembled.
The marker itself can be prepared in a number of different ways. At the most primitive the individual portions of the garments are represented by cardboard cut-outs. These are assembled on the top layer of fabric and the outline drawn round using a piece of tailors chalk. This method is very time consuming, requires a high level of skill and is open to errors through movement of the pieces and deformation of the fabric. The chalk needs to be kept sharp at all times to aid accuracy. This system is used for highly patterned fabrics that have to align within the garment, and also for short runs and prototype garments.
The hand marking system is advanced one stage further by being carried out on a sheet of paper duplicated by various methods to form repeats of the marker which can be laid on top of the fabric and cut along with it.
The marker maker lays out the pattern pieces on the paper according to a pre-determined plan. The width of the paper is the same as the fabric to be subsequently cut. The outline of the pattern pieces is drawn using a pencil or pen. To duplicate the marker several methods are available. The paper may be the top of several pieces interleaved with carbon paper, or it may be made of the type of paper that produces a line in response to localised pressure on its own surface. Carbon copies are of course limited due to the number of copies that can be obtained legibly from such a process.
In other processes the first marker is a master copy which is duplicated. Similar methods are used to those in office type duplicators, such as spirit duplicating, xerographic duplicating and pressure transfer. A form of photographic process using ultra-violet light sensitive paper has also been used. Clarity of line and accuracy are very important factors which, assessing these systems, as well as the cost factor. Increasingly computers are taking over the task of producing the master .copy of the marker, or are bypassing the processes completely with the aid of automatic cutters.
Marker making by computer
The past 15 years have seen a revolution in pattern making, grading and marker making in the form of computer systems. Such systems are produced by several manufacturers including Gerber, Investronica, Lectra and Cybrid.
Early systems were characterized by their high initial cost, often out of reach of all but the largest manufacturers. In the past four years another revolution has taken place with the introduction of the first low cost system, Ormus, of the British company Concept II. This system also has the advantage of being, it is claimed, designer friendly, allowing a creative approach to pattern making. Gerber have responded to this challenge with their own low cost system, the Acumark 300, and Investronica with the lnvesmark DS. Such computer systems do much more than marker making. Lectra, for example, claim that their systems will perform the following tasks:
(1) digitising and storage of master patterns;
(2) independent input and storage of grade rules;
(3) modification of master pattern via digitizer or colour graphic screen;
(4) creation of new styles via digitizer or colour graphic screen;
(5) interactive logical lay planning;
(6) plotting and storage of markers;
(7) plotting of single or nested shapes;
(8) automatic lay planning;
(9) automatic digitizing and grading utilizing scanners;
(10) automatic laser cutting of card pattern;
(11) automatic knife cutting of multi-ply lays;
(12) automatic laser cutting of single-ply fabric.
Such a list covers a range of several differently configured systems; not all systems are teamed with autocutters for example.
Lectra's list could be matched in full or in part by the other principal manufacturers.
Markers are produced using computer systems as follows:
The garment portions themselves are established within the memory of the computer, either by creating them via the keyboard, digitizer and VDU screen, or by inputting existing pattern shapes using a full scale digitizer and reading off points around the pattern, or by scanning devices
that 'read' the shape of pattern pieces placed on a special table. The latter system is the speciality of Cybrid Ltd.
The garment. pieces can be graded within the computer system. Grade rules are provided by the system manufacturers but companies often prefer to use their own particular formulae. Markers can be planned within the computer systems via a display of a miniature length of the fabric on the VDU, and the placement of called-up, scaled portions of the garment on to it. Endless manipulation of the pieces is possible to achieve the most economical marker either by the operative or automatically by the computer working ceaselessly to achieve the best fit and to produce the minimum of waste.
Hardware and software
The minimum for a clothing computer system consists of the following items of computing equipment, interlinked and controlled by a series of software programs:
(1) The computer itself provides the facility to manipulate information input and provide rapid output. The computer also provides memory capacity to retain a certain amount of information placed into it or generated during its activities. Information can also be output in a transferable form, usually a floppy disc.
Computers are classified according to their memory handling capacity and their speed of activity. Most of those used in clothing application are classed as micro or mini.
(2) Inputs into the computer are of two sorts: keyboard symbols; or two dimensional special, e.g. digitizer or scanner. The keyboard is used to type in command functions and information in the form of words, numbers and symbols. It can also be used interactively to generate new software programmes or sub routines. The digitizer is an essential part of a graphic design system. It consists of a magnetic board which can sense the position of a pen or stylus and change that information instantly to co-ordinates that the computer can understand, locate and store.
The same function can be carried out by a 'mouse' — a small box with control buttons and a small protruding window engraved with a cross. The mouse is held in the hand, glides easily over a smooth surface via a roller in its base and transmits the co-ordinate locations of the cross to the computer without the need for a digitizer board.
Some systems, e.g. Ormus, combine the function of the keyboard with the digitizing pad so that most of the commands can be given without changing devices during working. Such a facility makes the system more user friendly and particularly designer friendly.
(3) The interaction with the computer is displayed to the user stage by stage on VDUs (visual display units), otherwise known as monitors. These are television-like screens, monochrome (black and white) or colour. Most clothing systems use two monitors, one largely displaying commands and the other largely displaying graphical representations in miniature of the manipulated two dimensional shapes.
(4) The output device of a graphic clothing system is the 'plotter'. Individual pattern pieces, nests of graded pieces or full size markers are produced by a two dimensionally controlled pen tracing the outlines on a piece of paper whose movement is also controlled. Plotters fall into two
categories:
(1) flat bed plotters where the paper is on a large table and moves in one direction when required, with the pen moving in two dimensions over the whole surface of the paper;
(2) roller plotters where the paper is stretched over a roller and moves backwards and forwards over the roller, while the pen moves in one dimension only from side to side.
Plotting speeds can be very fast. Gerber plotters, for example, can draw at 2.3 m per second.
Scanners
Some systems employ large scanners to imput patterns or block patterns into the computer system. A British firm, Cybrid, specialises in such systems, arguing that digitizing a series of existing pattern pieces is unnecessary and time wasting.
Their scanner is a box-like table 1 m x 1.7 m. On to this up to 15 pattern pieces can be laid, forming the basis of a lay plan. The individual pieces are aligned along the length of the box, as with the fabric grain. More than one arrangement of pieces can be scanned if the lay consists of more than 15 pieces or if the scanner table cannot accommodate the size of the pieces.
When the pieces have been arranged on the scanner bed, the lid is lowered and the pieces scanned optically/electronically so that they appear within a computer memory and can be displayed on a VDU screen. The individual pieces can then be reproduced in mirror image or multiplied.
Lay planning is now carried out automatically by the Cybrid computer, which is capable of working away, literally overnight, generating 'best fit' solutions that can be 'dumped' on to a disc and printed out on a miniature plotter for consideration before marker making on a full size plotter is carried out. Such a system has distinct appeal to firms that start with a stored pattern in full size on paper card.
Fig. 3.2 gives a summary of the possible outputs and inputs of a design/manufacture computer as used in the clothing industry. Not all the items listed are necessarily used on the same system.
Spreading
The scale of production of fully cut knitted garments is such that, with one or two 'exclusive' exceptions, cutting is carried out on multiple layers of fabric. To arrive at a 'lay' a process of spreading is carried out.
Spreading a lay of knitted fabric involves similar technology to that of woven fabric spreading, with one large exception. Knitted fabric is extensible and is readily distorted in width and length. Great care must be exercised in handling the fabric at all stages, whether the spreading is carried out manually or by machine. The fabric must finish on the table in as relaxed a state as possible.
Knitted fabric before spreading may itself be deformed and thus have potential shrinkage. This is a particular problem with fabrics knitted from cotton and usually manifests itself in high shrinkage by length (along the wale) in garments after their first washing. These fabrics have in fact been distorted during dyeing and finishing and have not found it possible to relax with time, or to relax in the roll form in which most knitted fabric is presented. Very stringent quality control procedures are required both to sample fabric prior to spreading and to assess degrees of distortion during spreading.
The Starfish project
The International Institute for Cotton in 1984 introduced to the industry the results of an extensive research programme into the shrinkage of knitted cotton fabrics that has 'led to a practical system for reliably predicting the shrinkage and dimensional properties of finished knitted cotton fabrics'.
This research project was given the code-name Starfish. In this research the establishment of a stable state (i.e. fully relaxed and reproducible) was extremely important. This stable state was called reference state and was achieved on all the samples by the following procedure:
- wash in automatic machine at 60°C;
- tumble dry to constant weight;
- wet Out in a washing machine (rinse cycle);
- tumble dry to constant weight;
- repeat steps 3 and 4 three more times, making a total of five cycles;
- condition to normal regain, i.e. allow the sample to adapt to its normal moisture content in a standard atmosphere.
Further work showed that distortion of fabrics occurred during wet finishing and subsequent drying, and that differing treatments resulted in different reference states. Factors that need to be controlled during the knitting process are loop length and yarn count. In finishing, the beneficial effects of tumble drying or pseudo tumble drying, where the fabric is maintained in a state of unstressed agitation during the drying cycle, are noted.
Finally, the model relates the fabric finished state to the performance of the garment during wear and subsequent to laundering processes.
The package is marketed as a computer program and a hand held slide rule calculator. These arc prescriptive and predictive, forming valuable tools for all manufacturers of fully cut knitted goods made from cotton fibres.
Hand spreading
Spreading can be effected by hand or machine. Cutting tables for knitted fabrics must be particularly wide. Slit fabric from 30 in diameter knitting machines can commonly be 90 in (2.28 m) wide and this is by no means the largest diameter. Hand spreading requires at least two people standing on opposite sides of the table. They not only unroll the fabric but constantly vibrate or shake the fabric to position it. Any localised pulling will distort the fabric which will he prevented from recovery by friction with the adjacent layers. Inevitably with knitted fabric the edges of the fabric within the lay are less well aligned than with lays of woven fabric, and there is greater edge cutting loss. The overall width and length of the lay must also be constantly checked, as distortions of dimensions tend to be cumulative and once induced may affect every layer and be very difficult to eliminate once the lay is built up. As already mentioned, knitted fabric is often processed in circular form or in slit/folded form. Both these of their own handling problems in maintaining the alignment of the two layers.
Most knitted fabric is uni-directional — there is a definite top and bottom to the fabric. (Only plain fabric of the simplest type can be treated as hi-directional.) Fabric that is uni-directional must have each layer of the lay going in the same direction; fabric must be processed from the same end of the cutting table for every layer. For plain fabric that is bi-directional, building the layers can take place from each end of the table alternately. This is of particular advantage when spreading with automatic or semi-automatic machinery.
Knitted fabric is usually different in appearance front to back, with one of the surfaces being selected to he the effect side. With fabric finished in tubular or folded form within the lay, fabric can be positioned effect side up or reverse side up. This can also occur with spreading by machine where the uppermost side changes according to which end each traverse is made from.
This is relatively unimportant with most of the common garment portions that are either width symmetrical in themselves or occur in mirror image form, left and right. But where garments are constructed asymmetrically the fabric must be the same side up in the lay and of open width finish. Such garments are, however, extremely rare.
Knitted fabric can be patterned either in colour or structurally. Stripes, prints and knitted colour designs present alignment problems both in terms of the layers of fabric within the lay and in terms of the marker in relation to the lay. Inevitably extra care must be taken and a high degree of handling skills are required to spread such a lay.
Increasingly printed pattern alignment is ignored in leisure garments and underwear, much to the relief of production managers. There is also increasing use of printing processes applied to finished garments.
Machine spreading
A spreading machine consists basically of a frame that bridges the table on which the lay is to be formed. The frame, mounted on rollers, supports and carries the fabric in roll or folded form, and the fabric is delivered as the frame passes along the length of the table (Fig. 3.3). Spreading machines vary considerably in their size and complexity ranging from simple hand-manipulated machines that contain one roll of fabric and are trundled by hand backwards and forwards along the length of the lay, to large, fully automatic, programmed machines that change rolls from a magazine and produce the correct sequence and number of different colours or fabrics in the lay (Fig. 3.4). There is a bewildering array of machines in between these extremes, with individual firms producing their own specification for machines to be made for them.
The machines vary in their manner of traversing the length of the table. In the simplest system the .frame, or carriage, is mounted on wheels that fit on to rails on either side of the table. As the machines advance in complexity and automation the wheels become driven, or become gear engaging with a rack replacing the rail. Location of the carriage relative to the length of the table becomes important, as also does delivery of the fabric with the machine positively driving the roll either on its surface or axially.
With all machine spreading it is impossible to remove the human element required to supervise the machine. Human tasks involve dealing with observed fabric faults, with rolls of fabric that run out in the middle of the lay, and, with knitted fabric in particular, undesirable stretching and skewing of the fabric.
All knitted fabrics need to be spread with the minimum of applied tension, and while machine makers claim that all their machines do this, there are some specialist machines particularly designed to handle knitted fabrics. CRA (Cutting Room Appliances Corporation) are one manufacturer of these. Their Systema series offers the ability to handle widths up to 3 m (120 in) with a load capacity of 182 kg (400 lb) on the roll. Synchronized feed rollers work in conjunction with the self adjusting dividers to compensate automatically for variations in the tension and width of the fabric. One version of the machine will handle folded fabric (cuttled or flopped).
Spirality
There is at least one other fabric distortion besides stretching, that occurs in knitted fabric and results in spreading difficulties. `Spirality' arises from twist stress in the constituent yarns of plain fabric, causing all loops to distort and throwing the fabric wales and courses into an angular relationship other than 90°. If the fabric is retained as a tube, the spirality throws the vertical alignment of the fabric awry so that the wales lie at an angle to the edges of the fabric and slowly spiral around the fabric. Garment portions cut from the fabric show obvious distortion and are worthless (Fig. 3.5).
If the fabric is slit along a wale line during the knitting process or immediately prior to finishing, the distortion still takes place but appears as a course distortion, with the courses lying at an angle to the cut edges of the fabric. Fabrics with this problem often appear in low cost underwear and tee shirts, angled courses appearing to the consumer to be much less of a fault than angled wales.
Plain knitted fabrics made from single cotton yarn are most prone to spirality, the degree being related to the number of twists/unit length in the yarn. Such yarn is said to be 'twist lively' and, unlike similarly constructed yarns produced from thermoplastic fibres, cannot be heat set in yarn or fabric form to eliminate spirality.
Spirality is measured by the number of degrees of distortion that the fabric is away from a 90° relationship of wale to course. Fabrics of around 10° spirality are commonly processed, although acceptability varies with the quality, price bracket, and end use of the particular goods.
Resin treatment known as cross linking is sometimes used to reduce the degree of distortion due to spirality. The resin is applied to the fabric in aqueous solution and is set by passing the fabric once through a high temperature stenter (see Glossary). Besides eliminating some or all of the spirality, improved dimensional stability, appearance and handle are claimed for the process. Its main drawback is a general weakening of the cotton fabric.
Spirality is minimised by the use of doubled (two-fold) yarns, but this pushes up the price prohibitively in all but the most expensive garments.
Spirality does not occur in 1 x 1 rib and interlock fabrics, the loops formed in opposite directions cancelling out the distortions.
Another mild form of spirality occurs in fabrics produced on multi-feeder circular machines, because the number of courses knitted in one revolution of the machine distorts the wale/course relationship (Fig. 3.6). For example, a 30 in diameter machine with 90 feeders of 20 gauge will knit approximately 3 in of fabric every revolution. This will produce, if the fabric is finished 90 open width, 2° of spirality.
Usually open width finishing with the fabric passing through a stenter will correct this. Finishing the fabric in tubular form will not.