Knitting and Knit Fabrics

Knitting and Knit Fabrics

INTRODUCTION

Knitting is defined to be the formation of fabric by the intermeshing of loops of yarn. Unlike weaving, which requires two yarn sets, knitting is possible using only a single set of yarns. The set may consist of a single yarn (weft knit) or a single group of yarns (warp knit).

In weft knitting, the loops of yarn are formed by a single weft thread. The loops are formed, more or less, across the width of the fabric usually with horizontal rows of loops, or courses, being built up one loop at a time.

In warp knitting, all of the loops making up a single course are formed simultaneously. Thus, the lengths of each vertical column of loops, the wales, increase at the same time. Figures 3.1 and 3.2 illustrate the weft knit and warp knit structures.

The knit loop may be characterized by its geometry or by the way in which it is viewed by an observer. Geometrically, an open loop is one in which the forming yarns do not cross at the bottom of the loop. In a closed loop, yarn crossing takes place. Open and closed loops are illustrated in Figures 3.3 and 3.4.

The concept of face and back loops requires an observer. If the loop formation seems to be toward the observer, then a face loop is formed. If the loop formation seems to be away from the observer, then a back loop is formed. Although face and back terminology is not well-defined it serves a definite purpose in the characterization and analysis of weft knitted structures. Figures 3.5 and 3.6 illustrate the face and back loop characterization.

KNITTING ELEMENTS

Needles and Knitting Action

In both warp and weft knitting, the principal mechanical elements used to

Figure 3.1: Weft knit structure

Figure 3.2: Warp knit structure

Figure 3.3: Open loops

•RARELY FOUND

Figure 3.4: Closed loops

•RARELY FOUND

Figure 3.4: Closed loops

Knitting and Knit Fabrics 83

form loops are needles. In modern knitting, three major needle types exist. The most common type of needle, used in both weft and warp knitting, is the latch needle, illustrated in Figure 3.7. The latch needle, developed in the mid-18005, is so named because it can be closed using a latch which is activated without any special assistance during the knitting process.

Figure 3.8 illustrates the movements of a latch needle as it forms a knit loop. In the running position the held loop rests on top of the open latch. Clearing occurs as the held loop slips off the latch and onto the stem as the needle moves upwards. A downwards movement enables the needle hook to engage a new piece of yarn; this is known as feeding. As the needle continues downwards the latch is forced to close under the influence of the held loop. Knockover occurs as the held loop disengages from the needle. Following knockover loop pulling occurs and a new knit loop is formed. The needle must now return to the running position to complete the cycle. It should be noted that the held loop remains at the same height during the cycle; this is essential otherwise clearing, knockover and loop pulling would not take place. Control of the held loop is usually achieved by the use of sinkers or by the application of tension to the fabric. Note also that the fabric leaves the needle away from the hook. This is true for all needle designs.

The spring beard needle, or, simply, the bearded needle, is the oldest and simplest of the needle types. The bearded needle, illustrated in Figure 3.9, does not have the self-closing feature of the latch needle. Like latch needles, bearded needles are found in both weft and warp knitting.

Figure 3.10 illustrates the loop forming sequence of a spring beard needle. The held loop is shown, initially, just below the tip of the beard in the running position. An upwards movement by the needle repositions the held loop further down the stem, at which time a new yarn can be fed to the needle, i.e., feeding occurs. The needle moves down until the newly fed yarn enters the hook. An auxiliary element, known as a presser, closes off the hook to enable the held loop to leave the needle by deflecting the beard tip into a recess cut in the needle stem. The moment when the held loop is securely located on the outside of the beard is known as the landing position. As the needle continues downwards knockover and loop pulling take place after which the needle returns to the running position to complete the cycle.

The newest of the needle types, the compound needle, is found almost exclusively in warp knitting. This needle, illustrated in Figure 3.11, is so named because it consists of two distinct elements, a hook and a tongue. The purpose of the tongue is to act as a closing device for the needle.

Figure 3.12 illustrates the loop forming cycle of a compound needle. The held loop is shown resting on the needle stem in the running position. The hook and tongue elements move upwards so that a new yarn may be presented to the hook and feeding occurs. Both elements descend, although at different velocities, which causes the tongue to close-off the hook. Thus, the held loop is free to leave the needle (knockover) and loop pulling occurs. The needle now returns to the running to complete the cycle. As the needle returns to its starting position, the hook and tongue elements

once again move at different velocities resulting in the opening of the hook.

Figure 3.7: The latch needle

Figure 3.9: The bearded needle

Figure 3.11: The compound needle

9® Fabric Forming Systems

The three needles considered above, while differing in design, have the following points in common:

(1) Hook — to take and hold the newly fed yarn

(2) Hook Closing Mechanism — to allow the held loop to leave theneedle

(3) Stem

(4) Control Butt — for individual or collective movement.

Sinkers

Sinkers are thin steel elements which are, on some knitting machines, placed between each pair of needles. Their purpose is to control the fabric movement during needle activation. Included in this is the holding of the fabric as the needle rises, supporting the fabric as the needle descends and oushing the fabric away from the needle after the new loop has been formed. The design oT a sinker varies according to its application as may be observed in Figures 3.13 and 3.14. There is, however, a certain similarity that can be observed in all sinkers as they all perform approximately the same function.

WEFT KNITTING

Weft knitting and weft knit fabrics can be classified as single or double depending upon the number of independent needle sets required to produce them. Although both bearded and latch needles can be found in weft knitting, the vast majority of weft knitting is done using latch needles. With this moti-vation, only latch needles will be used to illustrate weft knitting. However, it should be kept in mind that fabrics such as fully fashioned sweaters and, more importantly, knit fleece fabrics are often formed using bearded needles.

Single Knitting

Single knitting is weft knitting in which one set of needles is used. The needles are arranged in a needle bed which may be either linear or, as is most often the case, circular as shown in Figure 3.15. The needles are mounted in grooves, or tricks, cut into the needle bed. The number of tricks per inch of needle bed determines the cut, or gauge, of that machine. In weft knitting gauge is most often expressed in terms of needles per inch (npi).

When sinkers are used in single knit machines, they are mounted in tricks cut into a separate sinker bed and the sinker bed is mounted in such away that the sinkers and needles are able to mesh together as illustrated in Figure 3.16. Naturally, the number of sinkers and needles must be identical.

Knitting is achieved by a combination of vertical needle movements and horizontal sinker movements. To understand how these movements are con-trolled it is useful to reexamine Figure 3.16. It should be noted that both the latch needle and the web-holding sinker are provided with butts. In mounting needles and sinkers, these butts protrude from their respective tricks and are engaged by positive cams. Figure 3.17 shows a section of needle cylinder and

Figure 3.16: Latch needles and web holding sinkers

• Figure 3.17: Section of a needle cylinder and sinker ring

96 Fabric Forming Systems

sinker ring with cams and yarn feed in place The movement of the needles up and down is the consequence of the needle butt following the groove in the cam system. The sinker movement is achieved in the same manner A more detailed outline of the needle cam system is shown in Figure 3 18

Circular weft knitting machines come in two possible arrangements In some cases the yarn creel, feeders and cam systems are stationary while the cylinder, including the needles and sinkers, and the fabric take up system ro -tates Such a system, known as a rotating cylinder system, is the most common. When the cylinder is stationary and the yarn creel, feeders and cam systems rotate, the system is said to have a rotating cam box A typical circular knitting machine is shown in Figure 3 19.

Stnqie Knit Fabrics

The simplest of all weft knit structures is the single jersey, illustrated in Figure 3.20 It is composed entirely of face loops (or entirely of back loops). The single jersey stitch, or basic structural unit, is, therefore, one face loop (or one back loop)

The single jersey structure exhibits the following characteristics

(1) The appearance of the face and back differ

(2) Extensibility widthwise is approximately twice that of the lengthdirection

(3) The edges of the fabric tend to curl or roll

(3) A run (collapse of a wale) will occur if a cut or exposed loop isstressed The direction of collapse can be either from top tobottom or vice versa

(4) 1 he fabric can be unravelled, course by course, from either end

(4) The fabric thickness is approximately 2 times the diameter ofthe yarn used

Single jersey, being a single knit, is made on machines with one set of needles The sinker top machine is the most common type of single jersey ma-chine There are a large number of single jersey machines in place in the U S, estimated at over 20,000 These are in a variety of cuts and diameters, producing both underwear and outerwear, plain and fancy fabrics Recent trends are to finer cut machines, up to 28 cut, in piece goods diameters 26 to 30 inches, and with a high productive capacity, up to 5 feeds per diametral inch This high productive capacity is due to each needle being capable of independent control allowing many feeders to be placed around the circumference of the machine Each needle thus produces a loop each time it undergoes activation and many courses may be formed simultaneously

Double Knitting

Double knit fabrics are weft knits which require two needle sets to be produced For circular machines, the second needle set, the dial needles, are located in a needle bed whose tricks radiate outward like the spokes in a wheel.

Figure 3.18: Typical cam system for a single knit-single jersey

STITCH CAM

Figure 3.19: Circular weft knitting machine

Figure 3.20: Single jersey


The needle bed is positioned over the cylinder with the dial needles at right angles to the cylinder needles. Figure 3.21 illustrates the relationship between dial and cylinder needle sets.

In the case of flat bed machines, the usual configuration is to place the needle beds so that when viewed from the end, they form an inverted "V." Machines of the type illustrated in Figure 3.22 are classified as vee bed machines.

Independent of the needle bed geometries as described above is the relative positioning of the needles within one bed with respect to the needles in the other. This positioning is termed the gaiting of the machine. If the needles are aligned, as in Figure 3.23, so that, if cleared, the needles of one set pass through the spaces between the needles of the second set then the machine is said to have rib gaiting. If, on the other hand, the two needle sets may not be cleared simultaneously, as in Figure 3.24, then the machine is said to have interlock gaiting. Gaiting is important because it affects the knitting sequence, and, therefore, the resultant fabric properties and production rates (see below 1 x 1 rib versus interlock).

Because two sets of needles are used, it is possible to produce structures having an identical appearance on both sides.

Double Knit Fabrics

The simplest double knit fabric produced using rib gaiting is the 1 x 1 rib fabric. It consists of alternate face wales and back wales, where a face wale is composed entirely of face loops and a back wale is composed entirely of back loops. The needles used to produce these wales are usually of the same design and most commonly of the latch needle types. The numerical designation of a rib fabric gives the number of face and back loops in the repeat. Thus, the 1 x 1 rib structure, illustrated in Figure 3.25, has a repeating stitch consisting of one face loop and one back loop, plus the connecting yarn.

This fabric has the following characteristics:

(1) The appearance of the face and back are identical.

(1) The extensibility of the fabric widthwise is approximately twicethat of single jersey. The lengthwise extensibility is essentiallythe same as in single jersey.

(2) The fabric does not curl at the edges.

(2) A run will develop in the fabric if an exposed loop is cut, as isthe case for single knits, except that the direction of collapsewill be from top to bottom only.

(3) The fabric can be unravelled course by course but only fromthe end last knitted.

(3) The fabric thickness is approximately twice that of single jersey.

The simplest fabric produced on an interlock gaited machine is called, simply, interlock, an illustration of which is shown in Figure 3.26. Close ex-amination of this structure reveals that it is composed of two 1 x 1 rib fabrics locked together. This interlocking phenomenon exists because of the knitting sequence necessitated by the needle gaiting used.

Figure 3.2"U Cylinder and dial

Figute 322: Vee bed machine

Figure 3.23: Rib gaiting

Figure 3.24: Interlock gaiting

Figure 3.25: 1 x 1 rib

Figure 3.26: Interlock


The characteristics of an interlock fabric are summarized below:

(1) The appearance of the face and back is the same.

(2) Extensibility widthwise and lengthwise are approximately thesame as single jersey. In practice, interlock will most probablybe more firm and rigid overall than single jersey because it isusually knitted tightly.


(4) A run will develop in the fabric the direction of which will befrom the end last knitted. An interlock fabric will run less freelythan single jersey or rib structures.

(5) The fabric can be unravelled from the end last knitted. Twoyarns must be removed to unravel a complete knitted course.

(6) The thickness of the fabric is approximately twice that of singlejersey.

Interlock is made on cylinder and dial machines which differ from rib machines. The difference is that each needle bed is equipped with two different types of needles, long and short, being set out in alternate tricks around the bed. In each case, opposite a long needle in one bed will be a short needle in the other, as may be seen in Figure 3.27. These two lengths of needle require two separate cam tracks both in the cylinder and in the dial to activate them. These cam systems are arranged such that only the long needles (cylinder and dial) knit at the first feed and only the short needles at the second. This alternating sequence is repeated all the way around the machine. (Note: Two feeders are required to knit a single course.) Figure 3.27 shows the needles and their asso-ciated yarns during the manufacture of the interlock stitch. This represents the one repeat of interlock produced by the two knitting feeds.

Purl Knit Fabrics

Purl fabrics are characterized by having both face knit loops and back knit loops in one wale. The 1 x 1 purl stitch consists of one face loop and one back loop intermeshed in a single wale. Since the purl structure has both face and back loops in the same wale, it is usually made on machines fitted with double hooked needles. Figure 3.28 shows the cross section of the knitting elements of a purl machine. The two needle beds are set at 180 degrees to each other (in a straight line) with the gap between straddled by the double ended latch needle. The needle is activated by one of the two jacks which lie one in each needle bed. Figure 3.28 illustrates the transfer action which occurs as the needle passes from the control of the one jack to the other. The needle can thus knit on either bed to produce face and back loops in the same wale. The transfer action can be initiated by a jacquard selection device for the production of fancy purl.

Purl machines are either flat bed or circular. The flat bed variety are usually known as links-links machines. The circular type have two cylinders, one above the other, and are thus referred to as superimposed cylinder machines.

Figure 3.27: Knitting sequence for interlock

Figure 3.28: Purl knitting elements


Purl fabrics can be manufactured on certain rib machines (dial and cylinder) which are fitted with special needles to facilitate loop transfer. This is not the main purpose of rib transfer machines, however, and the production of purl fabrics on them would be secondary to the production of other products.

The simplest purl, the 1 x 1 purl is illustrated in Figure 3.29. The 1 x 1 purl exhibits the following characteristics:

(1) Same appearance, face and back (simitar to the back of singlejersey).

(2) Highly extensible in all directions. Approximately twice asextensible as single jersey in the length direction because of thelengthwise fabric contraction which occurs to form the course-wise ribs. Figure 3.30 represents a vertical cross section of 1 x 1purl illustrating the mechanism of fabric extension.


(4) The fabric will run in the wale direction starting from either end.

(5) The fabrib may be unravelled course by course starting fromeither end.

(6) The fabric tends to be two or three times thicker than singlejersey.

1 x 1 purl is used in end uses which utilize its great length extensibility and good width extensibility. Two principal end use areas are golf sweaters and infants' and children's wear. In both cases, easy extensibility is important and in the latter, the ability of a garment to "grow" lengthwise with the child is a definite advantage.

KNIT, TUCK AND FLOAT LOOPS

In addition to knit loops, tuck and float loops also constitute very impor-tant elements of knit fabric structure. It is the judicious combination of these three elements that allows the formation of a wide range of knit fabrics.

Both tuck and float loops are produced by modifying the yarn-to-needle relationship existing in the normal knitting sequence. The modifications are achieved by altering the profile of the clearing cam in the standard cam system.

(1) Knit Loop — The basic loop, the knit loop, is shown in Figure 3.31.The shape of this loop is relatively independent of the loop length and all knitloops will be similar in shape to that shown in Figure 3.31.

We have seen previously that the knit loop is produced by clearing the old loop below the latch (by raising the needle) and feeding a new yarn into the hook.

(2) Tuck Loop — A tuck loop is formed if the needle is raised only partially by the clearing cam so that the held loop does not clear but rises sufficiently for feeding to take place. This results in two yarns being held in thehook (the held loop plus the new yarn which will form the tuck loop), as shownin Figure 3.32.

Figure 3.29: 1 x 1 purl

Figure 3.30: Extension of 1 x 1 purt

CONTRACTED EXTENDED


Figure 3.31: Knit loop

Figure 3.32: Tuck loop formation


At the next feed the needle will go through a normal knitting cycle and both loops will clear and latch and, eventually, knockover together. The tuck loop is typically shaped like an inverted "u" and is more open at the neck than a knit loop because it is not pulled through another loop. The appearance of a tuck loop in the fabric is shown in Figure 3.33.

(3) Float Loop - A float loop is produced if the needle neither clears nor is fed a new yarn, i.e., the needle remains at the run position. Thus while ad -jacent needles form a new loop, the needle making the float loop merely retains the held loop. This can be seen in Figure 3.34.

If the needle forming the float loop goes through a normal knitting cycle at the next feed then the appearance of the float will be as illustrated in Figure 3.35.

Tuck and float loops represent the main ways of modifying fabric structure to achieve diversity. Although each affects the properties of knit structures in a number of ways, the major effects can be summarized as follows:

A tuck loop makes a basic knit fabric:(1) Wider(2) Thicker(3) Slightly less extensible

A float loop makes a basic knit fabric:(1) Narrower(2) Thinner(3) Much less extensible

In addition to these structural effects, both tuck and float stitches lie be -hind associated knit loops and so can be used to hide unwanted yarn In this manner complex designs produced by selectively hiding colored yarn from the fabric surface can be created. These structures are known as Jacquard knit fabrics.

Multiple tucks and floats are possible, but the maximum number of consec-utive tucks or floats either vertically (on the same needle) or horizontally (across adjacent needles) is limited for structural reasons and is fewer than 10.

It should also be noted that no fabric can be produced entirely from tuck or float loops and that a basic fabric of knitted loops is required.

WEFT KNIT DESIGN

Stitch Notation

The purpose of stitch notation is to record in a readily understandable form the layout of the loop, or loops, which form the basic repeat of a particu-lar structure. It is usual to show just one repeat of the structure (sometimes referred to as the structural unit cell). If more than one repeat is shown then the highlighting of one repeat is recommended.

Weft knitted fabric may be represented by using any one of two stitch notational systems. In this text only one of these systems will be considered, namely, the diagrammatic method, because it is relatively easy to understand

Figure 3.33: Tuck loop

Figure 3.34: Float formation

Figure 3.35: Float


and it offers considerable flexibility, an important feature when developing more complex fabric designs.

In the final form the notational system will provide exact information concerning the knitting sequence at each feeder on the machine. As previously mentioned, it is normal practice to represent just one repeat of the structure and it is assumed that this sequence will be repeated around the machine until all feeders have been programmed.

In the diagrammatic system dots are used to represent needles in the ma-chine. In the simple case of a single knit machine adjacent dots represent ad-jacent needles in the machine, with each horizontal row of dots representing a group of needles at the individual feeder.

If the structure requires more than one feeder to produce a single repeat then additional rows of dots are drawn above the first, to represent the same needles as they pass by each additional feeder.

Feeder #1 . . . .

When dealing with fabrics made on two sets of needles (double knits) an addi-tional row of dots must be drawn at each feeder to represent the second set of needles. The position of dots relative to one another is used to indicate the gaiting of the needles. Labelling the rows of dots to clearly identify the two sets of needles is strongly recommended.

Feed #1

For multiple feed structures additional pairs of rows are drawn directly above the first.

Rib Gaiting Interlock Gaiting

D . . . D . . .

Feed #2 C . . . Feed #2 C . . .

D . . . D . . .

Feed #1 C . . . Feed #1 C . . .

A modification to this system uses lines instead of dots to represent the needles. The advantage of this is that needles of different length may be repre-

A modification to this system uses lines instead of dots to represent the needles. The advantage of this is that needles of different length may be repre-

Using the dots, or lines, to represent the needles, basic loop configurations are indicated as in Figure 3.36.

Weft Knit Fabrics

In diagrammatic notation, the four fabrics-single jersey, 1 x 1 rib, simple interlock and 1 x 1 purl—are represented in Figure 3.37.

Some common derivatives of single jersey, incorporating tucks and/or floats are shown or described below.

"" (1) LaCoste® — Often made with cotton yarns and used chiefly for sportswear because of its "cellular" appearance. LaCoste® is shown in Figure 3.38.

(2) Design effects by floating and knitting — Two or more coloredyarns can be knit into a patterned fabric which is basically plainjersey. A needle will knit the yarn which is to appear on theface of the fabric, but will float the remaining yarn, or yarns,to the back of the fabric where they are hidden.

(3) Accordian type fabrics — To produce large designs, it is essentialto bind potentially long floats of yarn into the structure withoutcausing them to appear on the fabric face. This is achievedusing a combination of floating and tucking as shown in Figure3.39.

(4) Laying-in — Extending the principle used in accordian fabrics,it is possible to bind yarns to the back of plain jersey fabricusing tuck stitches so that this yarn never knits. This is achievedusing a combination of floating and tucking as shown in Figure3.40.

Since the laid-in yarn never knits, it is possible to use a wide variety of yarns for this purpose, particularly very thick soft and relatively weak yarns. Fleecy fabrics for use as sweat shirts and dressing gowns are made this way.

WEFT KNITTED FABRIC PRODUCTION

The rate of fabric production is usually calculated in terms of linear yards

sented by lines of different lengths. For example: 1 set of needles, needles set out 2 long, 2 short

Figure 3.36: Stitch notation-diagrammatic form

Figure 3.37: Stitch notation for simple weft knits

Figure 3.38: Stitch notation-LaCoste® knit

Figure 3.40: Laying in in a weft knit fabric


(or meters) per unit time. (Note: A linear yard is a measure of fabric length and independent of fabric width.)

Course density is a fabric parameter and is used as a measurement of loop size. It is normal to express the course density in terms of courses per inch (cpi) or courses per centimeter (cpcm).

The number of courses produced per unit time is a function of the fabric structure, the number of feeders on the machine and the machine speed in terms of revolutions per minute (rpm) or traverses per minute (tpm).

The efficiency (i?) is calculated by obtaining data concerning machine running time and downtime.

Example: A single jersey fabric is produced on a machine having 32 feeders and a rotational speed of 20 rpm. If the fabric being produced has 28 cpi calculate the production (yards) over a 4-hour period if the machine is usually stationary for 3 minutes each hour.

Solution:

32= —r- = 32 courses/revolution

Courses/4 hours = Courses/rev, x rpm x min/hr x 4 hr

= 32 x 20 x 60 x 4 =

153,600 courses/4 hours

57Efficiency = 17 =■ -^r = 0.95

oO153,600 x 0.95

Production = ---------rg---------

= 5,211.4 inches = 144.76 yards/4 hours

WARP KNITTING

Introduction

Warp and weft knitting are similar fabric manufacturing processes in that they both utilize needles to form and intermesh loops. The main difference between these two systems lies in the manner in which the yarn is fed to the needles. In weft knitting a single yarn end may be fed to all the needles and knitting progresses around, or across the machine. In warp knitting, however, each needle is supplied with a yarn (or yarns) and all the needles knit at the same time producing a complete course at once.

A general view of a warp knitting machine is shown in Figure 3.41. In com-mon with weft knit machines, there are four basic zones:

Figure 3.41: Warp knitting machine


(1) Yam supply(2) Knitting elements(3) Fabric takedown(4) Fabric collection

Unlike weft knit machines, the great majority of warp knitting machines (over 99%) are rectilinear, i.e.. straight needle bed, not circular. Thus they are tsd from warp beams and make fabric which is knitted and collected in open width, not tubular, form.

Major Machine Classification ,

Tricot Machine: A tricot machine is a warp knitting machine which uses a single set of spring beard or compound needles. The fabric is removed from the needles at approximately 90 degrees.

Figure 3.42 shows a cross section of the knitting elements found on a tricot machine.

Raschel Machine: A Raschel machine is a warp knitting machine which uses a single set of vertically mounted latch needles. The fabric is removed from the needles at approximately 150 degrees.

Figure 3.43 shows a cross section of the knitting elements found on a Raschel machine

Simplex Machine: A simplex machine is a warp knitting machine equipped with two sets of spring beard needles. The fabric is removed from the needles vertically downwards between the two-needle beds

Figure 3.44 shows a cross section of a simplex machine.Two-Needle Bar Raschel: The two-needle bar Raschel machine is a warp

knitting machine equipped with two sets of vertically mounted latch needles. The fabric is removed from the needles vertically downwards between the two-needle beds.

Figure 3.45 shows the cross section of the two-needle bar Raschel machine.

Note: In all four of the figures mentioned, the view shown is that obtained when looking from the side of the machine. When one needle is illustrated it is assumed that there exists a whole set of similar needles which cannot be seen because they are all exactly aligned. This situation !S also true for the guides and sinkers.

Knitting Elements

Warp Beams: Yarn is supplied to the needles in the form of warp sheets. Each individual warp sheet is usually supplied from its own beam, which may consist of several section beams, as shown in Figure 3.41.

The number of beams used on a machine is normally equal to the number of guide bars.

Guide Bars: The guide bars extend across the complete width of the ma-chine and their function is to wrap yarn around the needles (i.e., feed). Each guide in the guide bar is usually provided with a single end of yarn. Warp knit-ting machines are usually equipped with two or more guide bars. (Note: Each guide bar has its own warp beam.)

Figure 3.42: Tricot elements

Figure 3.43: Raschel elements

Figure 3.44: Simplex elements

Figure 3.45: Two needle bar Raschel elements


The guides themselves are essentially thin metal pressings through which a hole has been drilled to facilitate threading. The guides are usually mounted in leads one-inch wide to ensure accurate guide separation. These leads are then attached to a horizontal bar to complete the guide bar assembly, as shown in Figure 3.46. in general, the more guide bars a machine is equipped with the more complex the fabric it will ptoduce.

During the knitting cycle the guide bars experience two modes of move-ment:

—A forward and backward movement in which the guides pass through the needle spaces and carry their yarns to the opposite side of the machine.

—Sideways movements on the hook and reverse sides of the needle. These control the wrapping of yarn around each needle and the repositioning of the guides for the following knitting cycle. The sideways movements of the guide bars are controlled by a pattern chain, mounted on a pattern drum, and located at one side of the machine. Each guide bar has its own pattern chain which enables independent lateral movements to occur.

A guide bar pattern chain is built up from a series of links, with two links being required to produce one course. Successive links in a particular chain differ only with respect to their heights which enables the guide bar, with which it operates, to be displaced different distances to the left or right. Figure 3.47 shows a series of links and Figure 3.48 illustrates the relationship between the guide bars that pattern chain and pattern drum.

Needle Bar: Needles, either mounted individually or in leads, are clamped to the needle bar which extends across the complete machine width, as shown in Figure 3.49. The needle separation is normally equal to that of the guides and must also be accurately controlled.

In Figure 3.49 very little needle design detail is shown because any one of the three types of needles may be used.

Sinker Bar: Sinkers are positioned between each pair of needles in the needle bar and provide for fabric control during loop formation. The sinkers are normally mounted in leads to ensure correct spacing and to reduce vibration, see Figure 3.50.

Warp Knitting Action

In order to produce a warp knit fabric, the movements of the needle bar, guide bar(s) and sinker bar must be fully synchronized. The needle movements required to produce or knit stitch have been discussed previously and are the same for any type of knitting. However, because the warp knitting needles are mounted on a needle bar, they are not capable of independent movement. Therefore, every needle across the needle bar undergoes the same motion at the same time.

The independently controlled guide bars have two distinct motions, a swing-through motion serves to put the guide bar in position for the lapping motions. The overlap, in which the guide bar moves laterally in front of the needle, is

Figure 3.46: Guide bar detail

Figure 3.47: Warp knitting pattern links


the feeding motion and must occur during the feeding portion of needle activation. The underlap, in which the guide bar moves laterally behind the needle, allows the yarn to be part of different wales thereby producing a coherent warp knit fabric. Usually, the underiap allows the yarn to be fed to a different needle than the one from the previous course.

Because of the differing systems the guide bar movements are timed differently in the two systems—tricot and Raschel—but the basic principles remain intact. Figure 3.51 illustrates the guide bar movement in relation to the needle movement for a Raschel machine with one guide bar.

Comparison Between Tricot and Raschel

Over the years tricot and Raschel machines and the fabrics they produce have developed along two clearly definable lines. The reasons for this divergence are many but one of the most important is that related to needle design.

Traditionally tricot equipment has been supplied with a set of spring beard needles and Raschel with latch needles. The relatively simple design of a spring beard needle resulted in the

manufacture of fine gauge machines while Raschel machines, equipped with latch needles, tended to be produced in coarser gauges.

The demand for fine, even yarns for use on tricot machines could not be adequately satisfied by conventional spun yarns and as a result continuous filament yarn was used almost exclusively. Raschel machines, on the other hand, being produced in coarser gauges, were well equipped to handle spun yarns.

These and other differences between these two systems are summarized in Table 3.1.

Table 3.1: Comparison of Tricot and Raschel Technology

Tricot

Raschal

Needle type spring beard or compound latchMachine gauge fine 28-32 npi coarse 16-18 npiYarn type filament spunNo. of guide bars few 2, 3 or 4 many 6,8, 12-48Fabric complexity simple complexSpeed fast 1,200 cpm slow 600 cpmMachine width wide 168 inches narrow < 100 inches

It should be pointed out that in recent times the traditional differences between the fabrics produced on these two systems has become less distinct. Increasingly, tricot machines are being equipped with 4 guide bars and being supplied in coarser gauges. At the same time, similar equalizing changes have occurred in the design of Raschel machines.

WARP KNIT DESIGN

Point Paper Notation

Warp knitted structures are composed of vertical wales of loops, each wale

Figure 3.51: Guide bar movements


being produced by a single needle. The overlap movement of the guide bars provides the yarn fpr these loops.

If each thread guide always worked with one needle, then the result would be vertical rows of loops, or chains, with no lateral connections, and no fabric would be formed. To form a fabric, therefore, each waie must be connected to its neighbor. The connections are provided by traversing the guide bar be-tween overlaps, so that their threads wrap around different needles at differ-ent courses. These movements, known as underlaps, determine by their direction and distance the structure of the fabric produced.

Since it is the underlaps that decide the structure of the fabric, some form of notation must be used in order to record the movements of the guides for design purposes. Such a notation system must show the design pictorially, must show up points of construction, and must be easy to translate in terms of guide bar threading and pattern chain construction.

The movement of the guide bars is plotted on point paper which is paper with small dots placed equidistant to each other. Each horizontal row of dots represents the needles or needle bar at one course of the fabric working up the paper from bottom to top for each successive course. The path of each guide is shown by drawing a line around the dots as if looking down on the needle from above. If a line is drawn for each threaded guide, the design of the fabric may be built up.

The fabric shown in Figure 3.52 may be shown in the notation as follows'

(1) Guides swing back through the needles, as in Figure 3.53a.

(2) Guides move sideways to pass yarn over the needle, i.e., theymake their overlaps, Figure 3.53b.

(3) Guides swing forward through the needles to the front of themachine. Figure 3.53c.

In order to make a fabric, the guides must now make an underlap so that the threads may lap around another needle. On the point paper, however, the plot must be transferred to the second row of dots, to show that the first course of loops has been completed, and knitting of the second course is commencing. This is shown in Figure 3.53d.

The plot is thus continued as follows:

(4) Guides swing through the needles. Figure 3.53e.

(5) Guides make their overlap, this time in the opposite direction,as in Figure 3.53f.

(6) Guides swing forward, Figure 3.53g.

In order that the method may be simplified in practice, lines are smoothed out and arrows omitted; thus, the plot shown in Figure 3.54a is drafted as shown in Figure 3.54b, and if repeated for more courses, appears as in Figure 3.54c.

The lapping movement on point paper not only shows the movement of the guides on the machine, but also gives a diagrammatic representation of the path of the yarn in the fabric.

If the same needles are lapped as before, but in opposite directions, the result is known as a 1 x 1 "open loop" structure as shown in Figure 3.55a. An

Figure 3.52: Half tricot


open loop is formed when the underlap is made in the same direction as the preceding overlap and a closed loop is formed when the underlap is made in the opposite direction of the preceding overlap.

The notation process may be extended to cover more than 2 adjacent needles, and a 1 x 2 lap is shown in Figures 3.55b and 3.55c.

Single Bar Fabric

Warp knitted fabrics, in which all the yarn follows exactly the same lapping movements, are normally made with a single guide bar controlling the yarns. These structures are known as single bar fabrics.

As a class these fabrics have little commercial importance because of their low cover and lack of stability. Therefore, these structures will be ignored at this time.

Two Bar Fabrics

The use of two guide bars gives a wider scope for patterning than is available with single guide bar fabrics, and these fabrics form the basis of the commercial trade, using continuous filament materials in most cases. There are, however, several important basic technical features, illustrated by the use of two bars, which must be understood and which form the basic technology of warp knitting. These are as follows:

(1) If the underlaps of the two bars move in opposite directions,the loops will lie straight in the fabric.

(2) If the underlaps of the two bars move together

(in parallel),loops will lie at an angle in the fabric, the direction of inclination depending on the direction of movement of the underlap.

(3) If a large underlap is made by the front bar with a short one onthe back bar, the fabric will contain widthwise elasticity.

(4) If the underlaps of the front bar are over 3 or 4 needle spaces,the technical back of the fabric will be of a lustrous nature.

(5) If a large underlap is used on the back bar and a short one onthe front bar, with the bars moving in opposite directions, arigid and stable fabric will be produced. This is due to the factthat the back bar underlaps will be trapped in the center of thefabric by the front bar underlaps, thus restricting the yarn movement.

(6) The underlaps of the front bar will appear at the top of thetechnical back of the fabric.

Figures 3.56, 3.57 and 3.58 illustrate the structures of three standard fabrics, namely, full tricot, locknit and reverse locknit. In each case the technical back has been illustrated.

Using the six points listed above, the following observations can be made:

(1) In all three cases, the guide bars are moving in opposition and, therefore, the wales in the fabric will be straight.

Figure 358: Reverse locknit

(2) The large front guide bars underlap present in the locknit fabricwill cause it to have the greatest widthways elasticity.

(3) The locknit fabric will also be more lustrous than the othertwo structures.

(2) The most stable fabric will be reverse locknit.

By further modifications to the lapping movements of each bar, additional structures can be produced. The behavior of these fabrics will depend upon the size and direction of the underiaps and overlaps and also their position, relative to one another, in the fabric.

WARP KNIT FABRIC PRODUCTION

To determine the rate of fabric production in the case of warp knitting, certain machine and fabric parameters, similar to those in weft knitting, must be known. These are:

(1) The number of courses produced per unit time (course/min) —This may be varied on any given machine to suit any set ofknitting conditions; for example, it is usual to run a machineat a slower speed if the yarn in question has a low breakingstrength. This parameter is controlled by the rotational speed(rpm) of the main cam shaft where one course is produced percam shaft revolution.

(1) The course density in the fabric (cpi or cpcmj —

(2) The number of needles in the machine — This can be obtained,given knowledge of the knitting width (inches) and the machinegauge. The gauge of tricot machines is expressed in needles/inch, while that of Raschel machines is expressed in neeales/2 inches.

(2) The efficiency of the process —

Needles knitting = needles/inch x knitting width (inches)

Example: A 168-inch, 28 gauge tricot machine runs at 1,100 courses/min. The fabric has 58 courses/inch and 60 wales/inch in the final state and the process efficiency is 0.92 (92%).

Documents

Knitting and Knit Fabrics