15 Weft Knitting

T. Palani Rajan

1. INTRODUCTION

Knitting is the method of manufacturing fabric by means of intermeshing of yarns by looping. Loops are formed by needle action while a loop is taken through one more loops and the stitch is produced. Loops / Stitches possibly created in a horizontal direction or in vertical direction. Horizontal direction of loop formation is called weft knitting and vertical direction of loop formation is called warp knitting, both are the main classifications of knitting technology.

2. WEFT KNITTING

A weft knitted fabric contains horizontal / parallel row of courses of yarn are knitted. It requires just a single yarn to form a fabric but more number of yarns is fed with respect to machine capacity. Weft knitted fabrics is used widely in the clothing industry for example hosiery, sweaters, foundation garments and active wear. The most important benefit of weft knitted fabric is its construction techniques. The fabric is capability to stretch and recover thus match to body shape. To achieve a desired fabric performance properties, the selection of suitable fibers and yarns are taken on to consideration.

3. CLASSIFICATION OF WEFT KNITTING

Figure 1 clearly shows the classification of weft knitting based on the machine frame, needle and fabric produced

Fig. 1. Classification of weft knitting

The following points are taken consideration in the weft knitting classification based on the different approaches.

Machine frame and needle bed arrangement

• Circular knitting
• Flat knitting

Number of needle bed

• Single jersey
• Double jersey

End product

• Fabric machine
• Garment machine

Basic fabric structure

• Single jersey or plain
• Rib
• Interlock
• Purl

Type of needle used

•   By using latch needle

1.  Circular knitting

2.  Flat bed knitting

•  By using spring bearded needle

1.  Circular knitting

2.  Straight bar knitting

4.  KNITTING ELEMENTS

A knitting machine is an equipment for converting yarn into knitted loop structures by means of mechanical moving parts either manual or power operated. These mechanical or electro mechanical parts are called as knitting elements to convert yarn into knitted fabric. Circular knitting machines elements are possibly divided into:

• Primary knitting elements
• Secondary knitting elements

The primary knitting elements are actually forming the stitches by means of needles and sinkers. The secondary knitting elements are activating the needles and sinkers. Such knitting elements are cylinder, dial, cams, and pattern wheels. Expect dial, remaining all the knitting elements are common to all kind of weft knitting machine and dial is used only in double jersey knitting machine.

4.1 Needles

Needle is the primary element of the knitting machine and it is a hooked metal piece. The knitting needle gets the yarn from the yarn feeder and converts in to a knitted loop. This loop is  enclosed in the hook and finally drawn through the old loop. The needle has to rise from the normal position to clear already held loop and to collect the new loop from the feeder. The new feed yarn form a loop in the needle hook since the needle starts to slide down to retain the new loop and to knock off the old loop. For that reason all needles should have various methods of closing and opening the needle hook to keep hold of the new loop and leave out the old loop. As a result, needles are classified in to three types shown in figure 2.

• The latch needle,
• The spring-beard needle and
• The compound needle

                                                  Fig 2. Three main types of knitting needle

The needle parts can be divided into four main parts:

•  The hook – This receives and retains the thread to be looped.
•  The hook opening and closing device – It permits the hook to get a new yarn and release the old loop.
•  The but – This is inserted with CAM system to lift and descend the needle for loop formation.
•  The stem – A system allowing the loop to slide on its surface ant the of needle move up and down.

The spring beard needle

The needle is a shape of round wire stem with a beard hook end. Opposite the point of the beard, there is a groove in the shank which permits the needle eye / point forced by auxiliary mechanism to close the needle hook. The spring beard needle needed an auxiliary mechanism i.e. presser to seal the needle hook. The opposite end of the needle is equipped with but to raise and move down the needle for knitting action.

The latch needle

Latch needle is called as self acting needle. This type is shaped into flattened wire shank with a latched hook end. The latch swings by the rivet and close the tip of the hook. But is placed in the opposite end of the needle and inserted with CAM to move up and down. There is no auxiliary device is required to form a loop.

The compound needles

The compound needle has the advantages of both the bearded and latch needles. It consists of two individually controlled parts i.e. shank and tongue. Both part of the compound needle required independent and precise control system. This system is not a key trouble in warp knitting because all the needles activated together. However in weft knitting, it may create problems at the time of loop formation.

4.2 Sinker

The sinker is a thin metal piece of material which is arranged between the needles and it is a second primary knitting element. During the loop formation, sinker moving at right angles (forward and backward) to the needles by the special sinker CAM and executes one or more of the following operations.

•  Help for loop formation – Sinker is to go under the surface and bend the newly laid yarn into a loop
• Holding down – Sinker hold down the old loops at a lower level on the needle stems
• Knocking over – Sinker upper surface or belly supports the old loop as the new loop is drawn through it.

In figure 3 clearly show the sinker, it has an exclusive shape and size. The upper section of the sinker is called the nose and lower one is the breast.

4.3 CAMs

A device which can convert the rotary force into a suitable reciprocating movement is called as CAM. The reciprocating action of knitting needles and sinkers are independently controlled by knitting cams.

Fig 4. Knitting CAM

Conventional knitting machines cams are mostly single track cam to produce only the basic knitted structures. Further increasing the complexity of knitted structure, multiple cam tracks are needed for the each set of needle movement. The cams are employed either independently or in a unit form of cam system and it depends upon the design of machine. Separate cam systems are necessary for each yarn feeder to receive the yarn by the needle. In a cam system various sections are available to control the needle movement and it shown in figure 4.

4.4 Cylinder

Cylinder is a metal plate which acts as a needle bed to support and hold the needles in the knitting action. Outside wall of the cylinder made up of tricks are cuts. Figure 5 shows that the needles that are placed into each trick and the arrangement of needles are depend upon the design features of the knitted structure. The but protrudes from the needles and needles are arranged perfectly vertical. This protruded but travels along the cam track, which provides the preferred movement to the needle.

4.5 Dial

Dial is a metal plate placed perpendicular to cylinder which acts as a needle bed to support and hold the horizontal needles in the knitting action. Like cylinder, outside wall of the dial made up of tricks are cuts. Dial is the main knitting element in double jersey knitting machine

5. CIRCULAR WEFT KNITTING MACHINES

Circular weft knitting is a predominant knitting machine producing verities of knitted fabrics according to the applications. It produces tubular knitted fabric with the help of mechanical or electro mechanical components. It can be classified in two types based on the number of needle bed. First one is single jersey knitting machine used one set of vertical needle produces plain knitted fabric and its derivates. The second one is double jersey knitting machine used two set of needles (vertical and horizontal) to produce rib and interlock fabrics

5.1 Single Jersey Knitting Machine

In single jersey circular knitting machine, the needles are placed in a vertical groove on the surface of the cylinder. The cylinder needle buts are protruding into a cam track which is provides reciprocating movement to the needles. The machine function can be divided into different zone i.e. creel zone, knitting zone and take down zone. The creel zone consists of creels, thread guide, tensioner, stop motion, feeders and yarn monitoring devices. The knitting zone consists of primary and secondary knitting elements. The take down zone has spreader and take up mechanism. The supply package cone is converted into knitted fabric and produced in tube form.

Three stronger cast iron legs are provided to hold all the parts and components of knitting machine and it anchor on the floor. A heavy duty motor is used to drive the cylinder and the necessary parts of a knitting machine.

                                                     Fig 6. Circular weft knitting machine

Passage of yarn

The supply package is cone, placed on the creel and the yarn is unwound than passed through the thread guides and top stop motion. Different stages of stop motions are used to stop the machine immediately to avoid the major defect. Subsequent to that, the yarn passes through the middle stop motion and tensioners. The positive yarn feeding system is placed in between the  stop motion to feed the yarn positively. This positive feeder may be a storage feeder that unwound the supply yarn from the creel continuously and carry the thread in an uniform tension. From the positive feeder, the yarn feed to the knitting zone in a constant rate and feed yarn tension is maintained. The knitting elements in a knitting zone begin to knit a fabric. The knitting needle raise and descent to form a loop and this process is supported by remaining knitting elements. After that the knitted fabrics is taken down in inside of the cylinder through take-up mechanism and finally wound on the fabric roller without loop distortion and creases. Before wound the tubular fabric on the cloth roller, the spreader is used to maintain the diameter of the knitted fabric and spreader positioned inside the tube of knitted fabric. The cloth roller is collected at the bottom of the circular weft knitting machine.

5.2 Double Jersey Knitting Machine

The basic principles of loop formation in double jersey knitting machine is same as single jersey knitting machine. Here the machine equipped with two set of needle, one is cylinder needle (vertical) and another one is dial needle (horizontal). Both are combined together to form a knitted loop structure. The arrangement of needle in cylinder and dial in a machine decide the type of fabric produced. Rib and interlock can be manufacture through double jersey knitting machine.

1. Rib Knitting Machine

Rib knitted fabric has a vertical cord like appearance for the reason that loops in wales direction form face loop and reverse alternatively or vice-versa. In rib knitted fabric manufacturing, two sets of needles are alternately used and it arranged in a set or gaited between each other. In rib gaiting, the needles are arrangements in the cylinder are between the needles of the dial it shown in figure 7.

The knitting action:

• Clearing. The cylinder needles and dial needles move to clear the old loops formed in the earlier cycle.
• Yarn Feeding. The needles are reserved and the open latches cover the old loops, new yarn is supplied into the needle hooks portion.
• Knocking-over. The needles are reserved in action the old loops are knocking over and the fresh loops are hanged through them.

                                             Fig. 7 Rib knitting machine needle arrangement

2. Interlock knitting machine

Interlock knitted fabrics is produced mostly on particular type of cylinder and dial circular machines with two different heights of needles. To produce interlock fabric, the entire course involves two feeder courses systems. While considering the feeders individually, every feed yarn knits on alternate needles (either long needle or short needle) to its corresponding cylinder and dial to manufacturing two 1 x 1 rib structures.

At first, the interlock machine has two different needles height i.e. long and short needles for both cylinder and dial. The arrangement of interlock needle gaiting is long needle in cylinder exactly facing at right angles to short needle in dial and vice-versa. To construct this knitted structure, two individual cam tracks used to controlling the long and short needles in the cylinder and the same type of setup is applicable for dial.

In figure 8 shows that the odd yarn knit with the long needles of cylinder and dial produces first alternate wales and the even feeders yarn knit with short needles of cylinder anddial produces second alternate wales. While considering this structure individually it is 1 x 1 rib structures. But in combine together the operation of all the feeders and needles in interlock machine produces interlock structure. Here the appearance of fabric on both face and back are reflecting same structure

                 Fig. 8 Rib knitting machine needle arrangement

6. BASIC WEFT KNIT STRUCTURES

      Weft knitted structures is manufactured either by means of circular or by flat knitting machine. In both the knitting systems, knitted fabrics are produced by looping the feed yarn in a horizontal / course wise direction to the width of cloth. Here the horizontal loops (courses), vertical loops (wales), face loop and back loop are called the major deciding factor for the basic weft knitted structures. The following four weft knitted structures are taken as primary / basic parental structures.

• Plain structure (Single jersey)
• Rib structure
• Interlock structure
• Purl structure

1. Plain structure

• It is known as single knit structure or single jersey.
• Produced by means of single set of needles.
• Two type of structure can be possible i.e. technical face (face loop) and technical back (back loop).
• Technical face – Loops are formed by drawing the new loops through the old loop at front and the food of new loop is cover the head of old loop showing the appearance of ‘V’s columns in the wales direction (figure 9).
• Technical back – The new loops are formed under the old loop and the foot of the new loop is covered by head of old loop showing the appearance of inverted ‘U’s rows in the course direction.
• Plain structure weft knitted fabric (single jersey) produced with face loop stitches on the fabric face side and back loop stitches on fabric backside.

                                                        Fig. 9 Plain knit face loop structure

2. Rib structure

• Rib knitted structure is one of the double jersey knitted fabrics manufactured with alternate face wales and back wales or vice-versa by means of cylinder and dial.
• In figure 10 shows, that the fabric produced with face loop and back loop alternatively in wales direction shows vertical stripped appearance on either side of fabric.
• Because of the same appearance on face and back of the fabric, it is a reversible knitted structure.
• 1×1, 2×2, 3×3 are called balanced rib knitted structures. Unbalanced rib structure can be produced with small modification in knitting needle lifting action.

                                        Fig. 10 1 x 1 Rib structure

3. Interlock structure

• Interlock knitted structure can be produced by a special arrangement in the knitting needle and knitting action. It was initially derived from rib knitted structure.
• In figure 11, ‘A’ represent first 1×1 rib structure and ‘B’ shows the second 1×1 rib structure. With meshing of two 1 x 1 rib knitted structure produces regular interlock structure.
• By hiding the look of the reverse loops through knitting process, each side of the fabric shows only face loop.

Fig. 11 Interlock structure

                                                                 Fig. 11 Interlock structure

4. Purl structure

• Purl is a special kind of structure producing special knitting machine. Two cylinder knitting machine is used with two hook end needs. Latch needle is the most convenient double-ended needle for the manufacturing of purl structure.
• Loops are formed by alternate face courses and back courses or vice-versa by means of two cylinders (figure 12).

                                                                   Fig. 12 Purl structure

REFERENCES and URLs

1. David Spencer, J 2001, Knitting Technology – A Comprehensive Handbook and Practical Guide, Woodhead Publishing Limited, England
2. Ajgaonkar D.B., 1998, Knitting Technology, Universal Publishing Corporation, Mumbai.
3. Chandrasekhar Iyer, Bernd Mammel and Wolfgang Schach., 1995, Circular Knitting, Meisenbach GmbH, Bamberg,
4. Anbumani N, 2014, Knitting – Fundamentals, Machines, Structures and Developments, New Age International Publishers, New Delhi.
5. http://mytextilenotes.blogspot.com
6. www.knittingtradejournal.com