It is not possible to discuss the technology of knitted garment manufacture without describing the constituent fabrics that influence the construction and properties of garments made from them. At the same time it must be stated that this is not intended to be a comprehensive treatise on knitted fabric.
There are three recognised looped constructions: warp knitting, weft knitting and crochet (Fig. 2.1).
Warp knitting is characterised by the structural threads of the fabric running along the length of the fabric approximately parallel with the selvedge. One horizontal row of loops, or course, is made from many threads.
Weft knitted fabrics are characterised by the structural threads being perpendicular to the selvedge of the fabric. One horizontal row of loops (course) is made from one or very few threads.
Crochet, unlike the other two constructions, is solely hand-made. One thread is used which chains upon itself, and cross links are formed with previously formed chain to generate fabric.
Knitted fabrics are formed from loops. The constituent loop of weft knitting is of the general shape shown in Fig. 2.2. It is said to have length (€), i.e. the length of the thread forming it from a to b. This is its most important dimension and in fact decides the area the loop covers and its width and height within a construction.
The loop in fact consists of two bends, an upper one and a lower one, half of which is on either side of the overall construction. The loop can vary in size, that is its length (€) can alter. It is rather obvious that as the loop length increases the area the loop occupies gets larger. Such a relationship is independent of the diameter of the constituent yarn, although usually within a knitted construction the yarn size increases commensurate with the loop size (Fig. 2.3).
We can describe such loops by relating them to familiar articles, so that the small loop represents the constituent part of a ladies hose or tights fabric, (loop length 0.2 cm), the medium loop the constituent part of a ladies fully fashioned classic sweater (loop length 0.55 cm) and the coarse loop a man's heavy outerwear sweater (loop length 3 cm).
Loops can be related to one another and can be intermeshed with one another to form fabrics. In a horizontal direction the relationship is a simple one of a series of loops formed by the same thread diagram (Fig. 2.4). In the vertical direction loops can be joined together by intermeshing (Fig. 2.5), whereby individual loops are connected by drawing subsequent loops through previously formed loops. The result is a fabric of matrix-like construction, having vertical and horizontal series of loops (Fig. 2.6).
Course A horizontal row of loops is known as a course.
Wale A vertical row of loops is known as a wale.
Plain fabric
In the simplest fabric construction all the units are of the same sort, i.e. each loop is the same shape and is pulled through the previously knitted loop in the same manner or direction. This simplest fabric is called plain weft knitted fabric, usually abbreviated to plain fabric (Fig. 2.7). Because all the loops intermesh in the same direction the fabric has a different appearance on each side. The side to which the loops appear pulled through is known as the 'face' or 'technical face'. The side from which the loops appear pushed away is known as the 'back' or 'technical back'.
Another characteristic of constructional appearance is that it is impossible to differentiate between the top of the fabric and the bottom.
Properties
The fabric is extensible in a course-wise direction and in a wale-wise direction. However, the degree of extensibility is different when pulled top to bottom from when pulled side to side. The course-wise extension is approximately twice that of the wale-wise extension due to the degree of constraint imposed on each loop by ifs intermeshing.
The loop pulled vertically extends by half its length
while the loop pulled horizontally extends by its whole length, € (Fig. 2.8). The degree of recovery from stretch is not a property of the construction but depends on the nature of the raw material and yarn construction.
This is the first of the properties related to the Hsu-al and historical use of the fabric; garments made from plain fabric are constructed so that the minimum stretch is placed vertically on the human body and arms, and maximum stretch around the body. Present and past fashions usually demand garments that do not sag readily. Periodically there have been exceptions to this usage where the wales of the fabric have run from side to side of the garment. The Dolman sweater of the 1950s and more recently is one such usage.
The Dolman fashion was essentially draped, with folds developing under. the arms and around the tightly constrained waist. The draping was aided by another differential property of plain fabric: that the ease of bending the fabric is dependent on which side the bend is occurring, and whether the bend is wale wise or course wise (Fig. 2.9).
On the face of the fabric, bending takes place most readily along the wale outwards. On the back of the fabric bending takes place more readily along the course outwards. These subtleties of mechanical performance are very important in dictating the overall appearance of a garment. In a piece of unprocessed, unpressed plain fabric, the outer edges curl vigorously. The top and bottom curl in towards the face of the fabric and the sides towards the back of the fabric. Curling towards the face tends to diminish the forces causing the curling at the sides; likewise curling towards the back diminishes the tendency to curl towards the face. A few seconds play with a piece of fabric will show this to be true. In fact in the literal sense the fabric can be said to be most in a state of equilibrium when it is in a roll or sausage form.
Pressing or other heat/water processes are used to minimize or eliminate such curling which is caused by the directionality of the loop . formation. A cross section of the fabric cut vertically between wales (Fig. 2.10) shows that each loop bends in the same direction, towards the face, and is constrained in that form by being intermeshed with the loop below. However, the loop at the top is not constrained and is free to straighten, releasing the next loop, and so on, the guiding principle being that no loop or portion of yarn wishes to remain strained by bending.
A cross section through the fabric along the course (Fig. 2.11) shows a similar situation, all loops being curved towards the back. Again a release of the bending forces on the loops results in them straightening and curling towards the back of the fabric.
This property has a major influence on the design and construction of garments made from plain fabric. Obviously all the edges must be constrained in some way unless curling is deliberately desired. This is achieved by seams, welts and the use of other fabrics of a more stable
nature, especially ribs.
In addition, almost all garments made from plain fabric are constructed with the face outwards as the 'effect side', with the back inwards as the `reverse side'. This is primarily because of the difficulties of constructing side seams from pieces of unprocessed fabric. It is much more difficult to uncurl the edges of fabric when they are trapped between two layers, than when they are on the outside (Fig. 2.12). Neatness of appearance also plays a part in this choice.
The constituent loops of plain fabric can readily be disconnected from the structure, course by course, by merely pulling at the most exposed thread. This can take place either from the end first knitted or the end last knitted and is known as unroving. A related disconnection of loops leading to a breakdown of the structure is caused by a series of loops being sequentially unmeshed down one or a group of wales. The resulting fault is known as a ladder. Plain fabric incorporated into garments must be firmly locked into seams, or the structure changed into a different non/laddering, non/unroving form, such as the rib of a waistband or cuff of a sweater.
All knitted fabrics, including plain fabric, are relatively thick compared to the diameter of their constituent yarns, and are composed largely of air space. Because of this, knitted fabrics have excellent heat insulating properties. However their openness leads them to being very air/ water permeable, and therefore neither waterproof nor heat retentive under conditions involving air movement.
Plain fabric is the commonest weft knitted fabric and is produced by widely different sorts of knitting machinery in all forms from circular fabric piece goods to fully fashioned panels. Other structures are covered here using the same terms and concepts as in the discussion of plain
fabric.
Rib fabrics
Rib fabrics are composed of loops formed in opposite directions, so when viewed from one side both hack and face loops are apparent. All the loops of any one wale are of the same sort, either hack or face. The name rib is derived from the ribs of animals, whose contours rib fabrics resemble.
The simplest rib fabric is the 1 x 1 (one by one or one and one). This is formed by alternating wales of back and face loops. (Fig. 2.13a) is purely schematic and does not show the fabric in its normal relaxed form. When the fabric is relaxed (Fig. 2.13b) and under no strain in the direction of the courses, it collapses to a situation of alternate wales touching one another. The wales in between are hidden but show on the opposite side of the fabric.
The fabric therefore looks the same on both sides and appears to casual viewing to be composed solely of face loops. This collapse of the fabric is caused by the portion of the loops that bridges the face loops and the back loops within the structure. When the fabric is placed under strain by stretching along the course, the bridging portions are twisted into an S form by the legs of the loops. This twisting forms a storage of energy and the structure acts like a spring, quickly regaining its collapsed form when the strain is released.
Because of this, rib fabrics are used where portions of garments are expected to cling to the shape of the human form and yet be capable of stretching when required. Waistbands, cuffs and collars are typical applications, together with whole garments of a fitting nature.
Extensions of up to 120% can be obtained along the course, with normal constructions. Along the wale, rib fabric behaves very much like plain fabric, with very limited extensibility. As already mentioned, the fabric has no back or front, the appearance being similar on both sides. The fabric is also stable as a plane structure with no tendency to curl.
Other constructions of rib are possible and are widely used, such as two wales of face loops alternating with two wales of back loops to form
2 x 2 rib (Fig. 2.14). On the same basis there are 3 x 3, 2 x 1, 3 x 2 etc.
As the number of wales in each rib increases, the elasticity decreases
because the number of change overs from back to front diminishes. Over
3 x 3 rib the fabric more and more behaves like plain fabric, even curling in favour of the dominant rib. Such structures are known as 'broad ribs' (Fig. 2.15).
Rib fabrics will only unrove from the end last knitted and l x 1 rib will only ladder from the end last knitted (Fig. 2. 16). All other rib constructions will ladder from the end first knitted. Such a property reinforces the argument for using ribs on the extremities of garments.
Purl fabrics
The simplest type of purl fabric consists of courses of face loops alternating with courses of back loops. This is known as 1 x l purl (Fig. 2.17). This, like 1 x 1 rib fabric, is balanced: for every face loop there is a back loop generating equal and opposite forces. The fabric is stable with no tendency to curl. It does, however, have a relaxed and extended form, collapsing in a vertical dimension so that each course lies at an angle to the plane of the fabric. Looked at from the side, the fabric, and indeed each wale, appears like a concertina. The fabric therefore has a large vertical extensibility which is largely elastic, depending on the fibre used and the yarn construction. The fabric is very bulky and has excellent thermal insulation properties.
As with rib fabrics there are other combinations of simple purls, such as 2 x 2, 3 x 3 etc. These are uncommon, however, and not particularly useful.
Unlike the rib fabric however, the classification 'purl' covers any fabric with face and back loops in the same wale. This covers a vast range of fabrics with designs in back and face loops, known as 'fancy purls'. Another term used, particularly in the USA is 'links-links'.
The three classifications discussed in this chapter — plain, rib and purl — are absolute ones and all weft knitted fabrics can be categorised into one of these classes. To summarise:
(1) Plain — all the loops in the fabric are of the same sort, face or back, depending on which side is looked at.
(2) Rib --the loops are of two sorts, face and back, but in any wale all the loops are of the same sort.
(3) Purl — some if not all the wales contain loops of both sorts, front and back.
It is of course possible to manipulate, modify and displace the loops in a fabric, but this does not alter the basis of the above classification.
Within the classification it is possible to modify the structures by various means. It is also possible to introduce design into a fabric without changing the structure, by such means as yarn type, colour, lustre etc., or by striping and plating (see Glossary). Some structures can be described as hybrids with, in particular, combinations of rib and plain to form milano ribs or ripple (bourelet) fabrics.
Others are unique structures like interlock and eightlock which fall into the rib category. Interlock consists of two 1 x 1 rib fabrics knitted in so that they are locked together. Eightlock is similar in construction but involves 2 x 2 rib. Interlock fabric is extremely stable; knitted in soft cotton yarns it is widely used in men's underwear, and leisurewear.
The main modifications used to alter knitted structures fall into three categories: tuck stitches, miss stitches, and transferred and displaced loops. It is not the purpose of this book to be a knitted fabric manual, but brief descriptions of these three categories are given here with mention of their significance in garment construction.
Tuck stitches
A tuck loop is a loop that is incorporated into a knitted structure without actually passing through or intermeshing with the loop immediately below it, but is intermeshed with the succeeding loop. Because the tuck loop is missing from the structure, the loop below it is stretched slightly to bridge the gaps. This loop is known as the held loop (Fig. 2.18). When a series of tuck loops are formed one after another in one wale, the structure distorts and a `knop' occurs (Fig. 2.19). When a series of tuck loops are formed adjacent to one another in a course, they form a float on the back of the fabric (Fig. 2.20).
One of the most interesting consequences of incorporating tuck loops into a knitted structure of any classification is the widening of that fabric. The more tuck loops the more the fabric widens relative to a similar structure containing no tuck loops. This is caused by the sides of the tuck loop, which would normally be constrained by the previously knitted loop, straightening out and exerting outward pressure on the neighbouring loops. The widening effect can be used in garments to 'stitch-shape' portions of the garment where extra width is required.
Common constructions include 1 x 1 cross tuck in plain fabric with its variations, and half and full cardigan in 1 x 1 rib fabric. Tucking can also be a colour pattern method, particularly in plain fabric fully fashioned outerwear. The colour patterning is based on the premise that tuck loops side by side in a course generate a float which is not seen on the face side of the fabric.
Miss stitches
A miss loop is generated when a loop is missed out of a knitted structure altogether, and does not pass through the loop below nor intermesh with the subsequent loop. The yarn that would have formed the loop lies as a float across the back of plain fabric (Fig. 2.21). As in the tuck stitch, the loop that stretches across the gap is known as the held loop.
Miss stitches can be used for generating structural interest as floats on the technical back of plain fabric, or as held stitch designs on the face, or to create 'relief' effects on rib fabrics. Their main use, however, is as the colour patterning medium of knitted fabrics, both plain and rib. In the simplest situation two threads form complementary loops; where one knits the other misses, and vice versa (Fig. 2.22).
More complex variations of this simple structure are used to generate rib jacquards, in which one side of the fabric contains the pattern, and the other — the reverse side — has various structures to create balance or imbalance with the face. Some of the structures can be so imbalanced (i.e. more loops on one side than the other) as to cause the surface of the fabric to bubble within a pattern. Such fabrics are known as 'relief'.
The extensibility of fabrics is reduced but not eliminated by the introduction of miss stitches. Such reduction is considered desirable in jersey fabrics used as alternatives to woven fabrics in outerwear garments.
Transferred loops
A loop that is displaced after being formed so that it combines with an adjacent loop, or so that it appears in a different wale, is said to have been transferred.
Transferring is used to generate holes in fabric to form lace-like effects. Fig. 2.23 shows a structure and draft of a typical lace fabric, formed by transferring loops on a plain fabric. Transferring can be used to produce structural effects by inclining wales of both plain and rib fabrics. Fig. 2.24 shows a structure and draft of an inclined wale plain fabric. This is also used to produce cables by exchanging two or more groups of wales with one another.
Most structural and colour designs in weft knitted fabric fall into the above three categories of modification. These influence the nature of the garments subsequently produced from them, largely because they modify the physical properties of the basic fabrics. They also give a wealth of visual interest to the fabrics.
Other factors contribute to the complexity of knitted fabrics and to the appearance and properties that characterise them. The following is by no means a comprehensive list:
• fibre type, size, colour, lustre, cross-sectional shape etc.;
• yarn type, size, colour, surface nature, contrived irregularities (fancy yarns etc.);
• loop size;
• constructional details generating colour patterns or structural patterns;
• making a construction with specific properties of weight, insulation, abrasive or washing capabilities etc;
• wet finishes applied to fabrics, e.g. shrink resist, softening, stiffening, anti-creasing, moth proofing etc;
• dry finishes such as pressing, brushing, calendering, setting etc.