In weaving, no matter what type of weaving is being done, four major operations are performed in sequence and are continuously repeated, they are Shedding, Picking, Beating Up, Taking Up and Letting Off
Preparing Warps and Wefts for Weaving
Weaving is the most basic process in which two different sets of yarns or threads are interlaced with each other to form a fabric or cloth. One of these sets is called warp which is the lengthwise yarn running from the back to the front of the loom. The other set of crosswise yarns are the filling which are called the weft or the woof.
The warps form the basic structure of fabrics. As such, they are made to pass through many operations before actual weaving is done. These operations include spooling, warping and slashing. In spooling, the yarn is wound on larger spools, or cones, that are placed on a rack known as creel. From the creel, the yarns are wound on a warp beam, which looks like a huge spool. These lengths of hundreds of warped yarns lie parallel to one another. These yarns are unwound for slashing, or sizing. The yarn is coated with sizing with the help of slasher machine.
Slashing prevents chafing or breaking of yarns during weaving process. Sizing is either starch based or a synthetic like polyvinyl alcohol or a water soluble acrylic polymers. The sized yarns are then wound on a final warp beam and are ready for the loom.
The filling yarns experience less strain during the weaving process. Their preparation includes spinning them to the required size and giving them just the right amount of twist desired for the kind of fabric they will be used.
Basic Weaving Operations
No matter what type of weaving is being done, four major operations are performed in sequence and are continuously repeated.
In shedding, alternate warp yarns are raised to insert the filling yarn into the warp to form a shed. Shedding is automatically performed by the harness on the modern weaving looms. Harness is a rectangular frame to which a series of wires, called heddles, are attached. As each warp yarn comes from the warp beam, it passes 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 warp yarns are raised through shedding, the weft 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. Different methods are used for carrying the filling yarn through the shed in different kinds of looms. There are many types of looms including shuttle loom, shuttle less loom, and circular loom.Shuttle Loom: The shuttle loom is the oldest type of weaving loom which uses a shuttle which contains a bobbin of filling yarn that appears through a hole situated in the side. The shuttle is batted across the loom and during this process, it leaves a trail of the filling at the rate of about 110 to 225 picks per minute (ppm). Although very effective and versatile, the shuttle looms are slow and noisy. Also the shuttle sometimes leads to abrasion on the warp yarns and at other times causes thread breaks. As a result the machine has to be stopped for tying the broken yarns.
This weaving operation is also called battening. In it, all warp yarns pass through the heddle eyelets and through openings in another frame that looks like a comb and is known as reed. With each picking operation, the reed pushes or beats each weft yarn against the portion of the fabric that has already been formed. It results in a firm and compact fabric construction.
Taking Up and Letting Off
As the shedding, picking and battening processes are being operated, the new fabric is wound on the cloth beam. This is known as 'taking up'. At the same time, the warp yarns are released from the warp beam which is known as 'letting off'.
The pattern of the weave depends on the manner in which groups of warped yarns are raised by the harnesses to allow the insertion of the weft yarn. These differences are responsible for producing different types of fabric weaves. Weave patterns can create various degrees of durability in fabrics apart from their utility and looks.
Shuttle less looms
Many kinds of shuttle less looms are used for weaving such as Projectile Looms, Rapier Looms, Water Jet Looms and Air Jet Looms.
- Projectile Loom: It is sometimes called missile loom as the picking action is done by a series of small bullet like projectiles which hold the weft yarn and carry it through the shed and then return empty. All the filling yarns are inserted from the same side of the loom. A special tucking device holds the ends of the wefts in place at the edge of the cloth to form the selvage. This loom needs smooth, uniform yarn which is properly sized in order to reduce friction. Projectile loom can produce up to 300 ppm and is less noisier then the shuttle loom.
- Rapier Loom: Rapier loom comes in many types. Early models of it use one long rapier device that travels along the width of the loom to carry the weft from one side to the other. Another type of rapier loom has two rapiers, one on each side of the loom. They may be rigid, flexible or telescopic. One rapier feeds the weft halfway through the sheds of warp yarns to the arm on the other side, which reaches in and carries it across the rest of the way. Rapier looms are very efficient and their speed ranges from 200 to 260 ppm. These looms can manufacture a variety of fabrics ranging from muslin fabric to drapery fabrics and even upholstery fabrics.
- Water Jet Loom: In it, a pre measured length of weft yarn is carried across the loom by a jet of water. These looms are very fast with speeds up to 600 ppm and very low noise. Also they don't place much tension on the filling yarn. As the pick is tension less, very high quality of warp yarns are needed for efficient operation. Also, only yarns that are not readily absorbent can be used to make fabrics on water jet looms such as filament yarn of acetate, nylon, polyester, and glass. However, it can produce very high quality fabrics having great appearance and feel.
- Air Jet Looms: In the air jet weaving looms, a jet of air is used to propel the weft yarn through the shed at speeds of up to 600 ppm. Uniform weft yarns are needed to make fabrics on this loom. Also heavier yarns are suitable for air jet looms as the lighter fabrics are very difficult to control through shed. However, too heavy yarns also can't be carried across the loom by air jet. In spite of these limitations, air jet loom can produce a wide variety of fabrics.
- Circular Looms: These looms are particularly used for making tubular fabrics rather than flat fabrics. A shuttle device in it circulates the weft in a shed formed around the machine. A circular loom is primarily used for bagging material.