Yarns to Fabrics
Fabric is formed by networking of natural or synthetic yarn. Weaving and knitting are two of the major methods of fabric construction.
In the process of weaving, two distinct sets of yarns called the warp and the weft are interlaced with each other to form a fabric. The lengthwise yarns which run from the back to the front of the loom are called the warp. The crosswise yarns are the filling or weft. A loom is a device for holding the warp threads in place while the filling threads are woven through them. Yarns made from both, natural fibers and synthetic fibers, are used for weaving textile. Other fibers can also be used for weaving. Yarn intended for the warp goes through such processes as spooling, warping and slashing to prepare them to withstand the strain of the weaving process.
In spooling, the yarn is wound on larger spools, or cones, which are placed on a rack called a creel. From the creel, the yarns are wound on a warp beam, which is similar to a huge spool. These hundreds of warp yarns lie parallel to one another. These yarns are unwound for the process of slashing, or sizing, bath. The yarn is coated with sizing. Sizing is either starch based or a synthetic, such as polyvinyl alcohol or a water soluble acrylic polymer. This is done to prevent chaffing or breaking of yarn during the weaving process. The sized yarns are then wound on a final warp beam and are ready for the loom.
Four major operations are involved in any type of weaving- Shedding, Picking, Beating up (Battening), Taking up and letting off.
Each alternate warp yarn is raised to insert the filling yarn into the warp to form a shed. On the modern looms, simple and interrelated shedding operations are performed automatically by the harness. It is a rectangular frame to which a series of wires, called heddles, are attached. As each warp yarn comes from the warp beam, it must past through an opening in the heddle. The operation of drawing each warp yarn through its appropriate heddle eye is known as drawing in.
As the harnesses raise the heddles, which in turn, raise the warp yarns, the filling yarn is inserted through the shed by a carrier device. A single crossing of the filling from one side of the loom to the other is called a pick. The method used for carrying the filling yarn through the shed depends upon the kind of loom used. Different types of looms are used for this purpose-Shuttle loom, shuttle less looms, circular looms etc.
Beating up (Battening)
All warp yarns pass through the heddle eyelets and through openings in another frame that resembles a comb and is called a reed. With each picking operation, the reed pushes or beats each filling yarn against the portion of the fabric that has already been formed. It gives the fabric a firm, compact construction.
Taking up and letting off
With each shedding, picking and battening operation, the new fabric must be wound on the cloth beam which is called 'taking up'. At the same time, the warp yarns must be released from the warp beam which is called 'letting off'.
After weaving, the most frequently used method of fabric construction is knitting. Knitting industry has two main divisions- One manufactures knitted goods for apparel production, sewing centers, consumers and others. The other division manufactures finished apparel such as hosiery, sweaters and underwear.
Construction of Knitted Fabric
Knitted fabrics may be constructed with a single yarn that is formed into interlocking loops with the help of hooked needles. According to the purpose of the fabric, the loops may be either loosely or closely constructed. Crocheting is knitting in its simplest form. The interlocked loops permit the fabric to stretch in any direction.
The construction of knitted fabric is evaluated by the number of stitches or loops. When the interlocking loops run lengthwise, each row is called a wale. A wale can be compared with the warp in woven fabric. When the loops run across the fabric, each row is called a course. A course corresponds to the filling, or weft.
Weft and warp knitting
There are two major varieties of knitting: weft knitting and warp knitting. In weft knitting, one continuous yarn forms courses across the fabric. In warp knitting, a series of yarns form wales in the lengthwise direction of the fabric.
Weft knitting has three basic stiches
1.Plain- knit stitch
The other new stitches are the variations of these three stitches. The knitting done by hand is through weft knitting. On a machine, the individual yarn is fed to one or more needles at a time.
Plain- knit stitch
Plain- knit stitch is the basic form of knitting. It can be produced in flat- knit also called jersey stitch or in tubular (or circular) form. The knitting is done with a row of latch or beard needles arranged in a linear position on a needle plate or in a circular position on a cylinder. All the needles are placed side by side and are moved by shafts, which act on the needle butts. The spacing of the needles is referred to as gauge, gage or cut.
The plain knit is made by needles intrlocking loops drawn to one side of the fabric. These loops form distinctive vertical wales on the right side ( or technical face) of the fabric. On the reverse side ( or technical back), the courses can be seen as interlocking rows of half circles.
Purl stitch is made on flat bed and circular machines by needles using hooks on both ends to alternately draw loops to the front of the fabric in one course and to the back in the next course. The fabric looks the same on both sides and resembles the back of the plain knit. It is slower and more costly method. The use of double hook needles enables ready changeover during fabric construction to include flat and rib stitches.
Rib knit fabrics have alternating lengthwise rows of plain and purl stitches constructed so that the face and back of the fabric appear alike. This may be produced on both, a flat rib machine or a circular rib machine. In the flat rib machine, one set of needles is placed opposite the other set of needles in an inverted V position of 45 degrees to the horizontal. In the circular rib machine, one set of needles is placed vertically in a cylinder and the other set of needles is placed horizontally on a dial. In both the systems, one set of needles pulls the loops to the front and the other set pulls the loops to the back of the fabric.
In warp knitting, each needle loops its own thread. The needles produce parallel rows of loops simultaneously that are interlocked in a zigzag pattern. The stitches on the upper side of the fabric appear vertical but if seen from a slight angle or from back, they appear horizontal as floats. These floats are called laps or underlaps. Warp knitting may be flat or tubular and can be produced in many varieties of patterns. There are various types of warp knitting- Tricot, Milanese, Simplex, Raschel, Ketten raschel, etc.
The warp loom intended for tricot knit has one or more warp beams. Each set of yarns from a warp beam is fed to a row of needles arranged across the width of the machine and is controlled by yarn guides set in a guide bar. Greater the number of guide bars, more flexibility is there for designing. The movements of the guide bars is controlled by chains linked to them. As the guide bar is raised and moved sidewise, it lays the warp yarns in their respective needle hooks to form a course of loops. There are many types of tricot knit fabrics- plain tricot, satin tricot, Mesh tricot, Outerwear tricot, upholstery tricot, Napped or brushed tricot etc. - used in various applications such as lingerie, sleepwear, blouses, shirts, slacks, automobile upholstery and others.
It is knitted on the flat bed machine and spring- beard needles are used. When using circular machine, latch needles are used. It is generally knitted from filament yarn. The resultant fabric is fine lightweight similar to tricot in appearance but can be differentiated from it by the fine rib on the face and the diagonal pattern on the back. They are also superior to tricot in smoothness, elasticity and tear resistance.
These fabrics are produced with spring beard needles on a machine which can be called two tricot machines arranged back to back. Simplex knit is relatively dense and thick, sometimes lightly napped. They are generally used for making gloves, handbags, sportswear, slipcovers etc.
These fabrics are made with latched needles and heavy yarns usually having an intricate, lacelike pattern. They are knitted on machines having a number of guide bars. Raschel machines can knit every type of yarn made of any kind of fiber, including metallic and glass fiber.
Ketten raschel Knit
It is also known as chain raschel. It is a variation of the tricot knit and produced with bearded needles. The machine can be additionally equipped to produce raised pattern effects. The fabric is fine with good elasticity.
New Developments in Warp Knits
Developments have been made in warp knitting and now non knitted threads of different color, density, and texture can be included in the knitted fabric ( inlaying), such as zigzag inlays. New machines have also been developed which can produce fabrics with the properties of both woven and knitted fabrics. These machines have only two warp-forming warps and provision for up to eight interlooped warp threads between each chain of loops. These warp threads are interlaced with a quasi weft, forming a fabric resembling woven cloth on one side.
Tufting is a type of textile weaving which is done by pushing extra yarns onto a fabric. Many needles simultaneously punch the fabric at predetermined distance. Each of these needles carry a yarn from a spool fixed in a creel. When the needles withdraw, hooks come forward to hold the loops. This forms an uncut pile. Sometimes, knives are attached to the hooks which move like scissors and cut the loops as the needles are drawn up. This produces cut piles. Tufting is mainly done to manufacture rugs, carpets, blankets and upholstery. Sometimes, the backs of rugs and carpets are coated with a binder to prevent abrasion. The method of tufting is rapid and very economical.
Stitch Bonding or sewing- knitting technique is used to manufacture different kinds of constructions such as flat fabrics, piles, nonwovens etc. It has many variations. In Malimo technique, a layer of warp yarns is placed on a layer of filling yarns. Theselayers are then locked together using a multiple threads and needles system through chain stitch. It is a very fast process which is done at the rate of nearly 1400 stitches per minute. The fabrics produced by this technique are generally used for making coats, blankets, and industrial fabrics such as interlinings for conveyor belts. Malimo fabrics are very strong and have exceptional resistance to tearing because of their construction method.
The other variation of Stitch Bonding is Malivlies technique in which no yarn is used. A web made of criss- crossed layers of carded fibers is put into the machine which has needles that pierce through this pile. These barbed needles catch tufts of fibers in their hooks and stitch them into chain of loops. Thus, the fibers are transformed into fabric. As there is no need to produce yarn for this method, a considerable cost is cut thus making this method relatively inexpensive.
The felted fabrics are produced directly from fibers matted together. Wool is the usual raw material for making felted fabrics although other fibers like fur are also used sometimes for the purpose. When using wool, the fibers are converted into a fine web by making them parallel and evenly thickened through carding operation. Several webs are formed until the desired weight and thickness is obtained. These mass or batts are then cut in required widths. Warm water is sprinkled over the batts and then they are passed over a steam box to warm them. Then theses warm batts are passed between two rollers. After this process, the felt is allowed to cool down. Then the felt is dampened with a suitable lubricant, such as soap and soda combination for the fulling process where it is pounded with hammers. The time taken for fulling process is crucial as the longer it takes, finer becomes the felt but excessive fulling may destroy the quality of the felt. Finally, the felt is washed, stiffened, ironed, brushed to raise a nap and trimmed to produce a smooth surface.
Felts are used for making hats, slippers, shoes, insoles, earmuffs, table paddings etc. Since these fabrics have insulating and noise absorptive properties, they are extensively used for industrial purposes.
The nonwoven fabrics are made by bonding or interlocking of fibers through mechanical, chemical, thermal or solvent means. The methods of manufacturing differ on the basis of fibers used, techniques of laying these fibers and the bonding agents used in the process. Many types of fibers are used for nonwovens such as cotton, wool, rayon, acetate, nylon, polyester, acrylic, modacrylic, polypropylene, and polyethylene. The fibers are processed through various operations like opening, conditioning and blending. Then webs of fiber are made and arranged into layers. Various methods are used for affixing these fibers which include heat, resin, latex or other bonding agents.
Many techniques are adopted for web formation. Parallel-laid web, Cross-laid web, random-laid web, air-laid web, wet-lay web are some of the methods used for web formation.
Methods of Bonding
Several methods of bonding are there for forming nonwoven fabric from the fibers.
Just before the web is passed between the two rollers, it is saturated with a suitable bonding agent through spraying, printing, padding or by immersing in a bath. While going through the rollers, the excess liquid is squeezed out.
Latex is applied to the web by pressure- spreading, spraying, rotary screening, roller dripping or through perforated drum.
A substance of gelatin class may also be used for bonding fibers which coagulates at the intersections of the fibers in the web after which the web is compressed and dried.
A web of polymeric fiber is deposited on a moving belt and subsequently bonded by a physical or a chemical process. Examples of polymeric spunbonded fabrics are Reemay made of polyester fiber, Tyvek made from polyethylene, Typer, made of polypropylene etc.
Bicomponent fibers can be bonded by this technique of melding. The bicomponent fibers are made of two polymer components which melt at different temperatures. Through carefully controlled heating process, the component with the lower melting point melts and is affixed to the other fiber.
The web is exposed to electron-beam radiation as it is moved on a conveyer belt. The cross linking agent which is applied earlier is activated which bonds the filaments together.