Yarns as manufactured and packaged are not actually suitable for direct use to form fabrics. The package size, build and other factors make it necessary to prepare it to be handled efficiently during fabric formation. For weaving and warp knitting, many yarns are fed simultaneously in the form of warp sheet. These yarns are taken from packages called beams. Shuttle looms need a special weft yarn package, called “pirn”, which fits in the shuttle; while shuttle-less looms and weft knitting machines use yarn from large packages called cheeses or cones. Hence, after spinning the yarns must be repackaged through the process of winding to meet the particular needs and demands of the fabric forming system in which it is to be used.
The first step in yarn preparation for both weaving and knitting is “Yarn Winding“. The main objectives for winding are;
- To produce a package which is suitable for further processing, as per required form, build and density of the package.
- To inspect and clear the yarn from slubs, thick and thin places, etc.
Winding is divided into 3 principal zones:
- The unwinding zone
- The tension and clearing zone
- The winding zone
To rewind the yarn on a new package, it must first be removed from the old package. This is accomplished in unwinding zone. This zone merely consists of a creel which holds the old package in an optimum position for unwinding. For unwinding there are two common yarn withdrawal methods, side withdrawal and over-end withdrawal.
In side withdrawal, an advantage is that the yarn does not rotate upon withdrawal and therefore the yarn twist remains constant. it’s disadvantage is that the spool must rotate. Also, at high winding speeds, due to inertia, the rotation of the spool may lead to tension variations in the yarn. Furthermore, provision must be made to stop the spool if the machine stops. If this is not done the rotational momentum of the spool will cause it to remain in motion allowing yarn to be unwound without being taken up. Also, upon start-up, higher tensions are developed because the winding machine has to overcome spool inertia.
In the over-end withdrawal, the package must not be rotated as the yarn is pulled over the end of the package. This method is the simplest and most common method of yarn withdrawal. There are however, two factors which must be taken into account while using this method of yarn withdrawal. The first is known as ‘Ballooning’. As the yarn is unwound at high speed, centrifugal force causes it to follow a curved path. As the yarn rotates, it gives the illusion of a balloon above the package. This ballooning leads to uneven tensions being produced in the yarn. The second factor for consideration is that every time when one wrap of yarn is removed from the supply package, the twist in that length of changes by one turn. For most of the yarns, this insignificant change can be ignored. However, in cases where flat yarns of metal, polymers or rubber is used, the yarn must remain flat and therefore this is unacceptable. Such yarns cannot be unwound using over-end method and hence the side-withdrawal method must be used.
The next is tension and clearing zone, where the yarn receives proper tension to remove any thick and thin places and to provide an acceptable package density and build for further processing. This zone consists of a tensioning device, a device to detect thick spots or slubs in the yarn and a stop motion which causes the winding to stop in the case of yarn breaks or the depletion of a supply package. The yarn is directed into this zone by a guide. Guides fall into two categories, Closed, which require a yarn end to thread, and open which do not. Open guides, however, give less positive guiding. Guides are usually made of stainless steel or ceramics. Following the guide the yarn enters a tension device. The purpose of the tension device is to provide proper tension in the yarn in order to achieve a uniform package density. This tensioning device also serves as a detector for excessively weak spots in the yarn which break under the added tension induced by the device.
Tension devices fall into three categories:
- Capstan or multiplicative tensioner
- Additive tensioner
- Combined tensioner
The most common type of tensioning device found on winding machines is the combined tensioner. This device consists of a capstan tensioner and accepts weight discs and thus also functions as an additive tensioner. The capstan is added primarily as a post-type yarn guide rather than a tensioning device and, in general, tension is regulated by adding or taking off the weight discs.
Upon leaving the tension device, the yarn passes through a stop motion device. The purpose of the stop motion is to stop winding when the yarn breaks or runs out. This stop motion detector generally consists of a counter-weighted or spring loaded sensing device which is held in an inactive position when the yarn is present. Breaking or running out causes the absence of this restraining yarn and allows the sensing device to activate.
The yarn is now ready to be put on a suitable package in the winding zone. This package may be one of many types, a cone, a cheese, a tube, a dye tube, a spool, depending upon the next operation the yarn must encounter. It is important that during winding, no twist change take place. Thus, physically wrapping the yarn around the package during winding should be avoided. The yarn is wound on the package by only rotating the package. This rotation may be accomplished in one of two ways:
- Spindle drive, where the spindle upon which the package is placed is driven directly; or
- Frictional drive, where the spindle upon which the package is placed is free to rotate and the package is driven, through friction, by contact with a driven drum. It is important that the yarn be wound under as uniform tension as possible. It is known that this tension varies with the tension applied on the yarn and the yarn speed. The applied tension, controlled by the tensioning device in the clearing zone can be considered to be constant. Thus the tension on the package is only a function of the yarn speed.
Spindle drive winders consist of two types: constant speed winder and variable speed winder
For the constant speed spindle winder, the angular velocity of the package is constant. As more yarn is wound onto the package, the package radius increases and gradually the winding speed increases. This gradual rise in yarn speed causes an increment in package tension and therefore the density is unequal throughout the package. To overcome this, variable speed winders are used. In variable speed winder the spindle speed is not constant but varies with the package radius. To have constant yarn speed on a spindle drive winder it is necessary to have a mechanism which causes the speed to vary. However there is a simpler way to accomplish the same task and that is the use of a friction drive winder. In this type of winder, the package is driven by a constant speed friction drum. The yarn passes between the friction drum and the package and is taken up by the package. At the point of contact of the package, drum and yarn, if no slippage occurs, all three of them have the same velocity.
Not only must the yarn be wound on the package but also it must be distributed evenly along the length of the package. This is the function of traversing mechanism. One of the methods of traversing found only on friction drive winders is the use of a traversing groove cut into the friction drum. In this method of traverse, the yarn rides in the groove in the friction drum and is laid back and forth throughout the length of the package being wound. All spindle drive winders and some friction drum winders use a reciprocating traverse, in which an externally driven guide carries the yarn back and forth across the package. The main advantage of this method of traverse is the ability to precisely lay the yarn onto the package.
The type of package which may be built depends upon a combination of winding speed and traversing speed. If the traversing speed is relatively fast, successive layers of yarn will be laid at distinct angles to each other. This produces what is known as a cross-wound package. Because of the angle between the successive yarn layers the shoulders of such a package are stable and do not need to be supported. The traversing necessary to build a cone or a cheese is slightly different. Due to the difference in geometry between the tubular cheese package and the conical cone package, the angle and spacing of the traverse are constant in the case of the cheese but vary in the case of cone.
If the traversing speed is relatively slow, successive layers will be very close to parallel to each other and a parallel-wound package will result. Parallel wound packages are not stable and the shoulders of these packages need to be supported by flanges. Thus for this type of wind, a spool is an inappropriate package.
In applications where the package wind angle is important, such as yarn for weft knitting and filling for shuttleless weaving, it is important to ascertain and maintain a critical wind angle to prevent, or atleast reduce, a condition wherein many coils of yarn unwind at a time from the package. This condition is known as sloughing-off. It is also important that the wind angle be such that the force required to remove the yarn remains constant.
If the fabric design calls for yarn dyeing then the yarn is wound on a special perforated tube which facilitates dye penetration into the package. After dyeing the yarn is normally rewound and sent to the next operation.
If the yarn is to be used as weft in shuttle looms it must be rewound on pirn, also called as quill. This pirn is designed in such a manner that it fits inside a shuttle. There are special winders available to wind these pirns known as pirn winding machine or “quill winder”. A pirn winder differs very little from a package winder, the differences being no need for clearing and a different traverse mechanism. In package winding, the traversing mechanism makes a full cycle in carrying the yarn completely back and forth along the package. In pirn winding, however the traverse only covers part of the pirn at a time. When one section is built up the traverse indexes to the next section. This is called a progressive reciprocating traverse. The method of pirn winding is used mainly:
- To reduce the tendency to balloon as the yarn is unwound from the pirn;
- To maintain uniform tension in the filling yarn; and
- To reduce the possibility of sloughing off.
If the pirns are not to be used immediately after winding, they are usually taken to a room to be conditioned with hot, humid air. This conditioning is done to allow the filling yarn to relax, reducing the twist liveliness of the yarn and preventing the formation of kinks.
If the fabric forming system is weaving or warp knitting, some or all of the yarns forming the fabric are presented in sheet form. It is therefore necessary to remove the yarns from the winding package and arrange the desired number on a package called “beam”. Before winding the required number of yarns on a beam, which comes around hundreds or thousands of yarns and for this there must be atleast one supply package for each of these yarns. These hundreds or thousands of packages are positioned to facilitate ease of removal of yarn. It is logical therefore to have a frame to hold these packages. This frame is known as a creel. To accomplish the purpose creels are equipped with package holders on which the supply packages are placed, tension devices to help maintain uniform tension throughout the creel, guides to direct the yarn and to help keep the ends apart, antistatic devices to eliminate static charges and stop motions to detect broken ends and/or empty packages.
In considering very large creel, it is obvious that some of the supply packages must be very much far away from the point where the beam is being formed. Also, the yarn must be supported to keep it from dragging on the floor and tangling. Each support acts as a capstan tension device. Thus, it is important to keep the packages in a distance range where the effect of yarn weight and the effect of supports as tensioners may be neglected. Hence the size and capacity of the creel is limited. In general, maximum creel capacity ranges from about 300 packages for very heavy yarns to 1,400 packages for fine yarns.
To achieve higher beaming efficiency, single package creels are often used in various combinations. If the winding head or headstock is fixed, the non-stationary single end creels are moved in and out of position as required. These creels are called as Truck creels. Another setup where the headstock is capable of being moved and creels remain stationary, are known as Duplicated creels.
In a multiple package type of creel, known as a magazine creel, more than one package is provided for each end. The packages are tied head-to tail so warping can continue when one package is exhausted. In another type of multiple package creel, known as travelling package creel, instead of moving creels or headstock when fresh packages are required, the packages themselves are moved into position. With a travelling package creel, the replacement of empty packages with full ones, or creeling, is done in the center while the packages in use are on the outside.
If the creel capacity is sufficiently high and the total number of ends required is sufficiently low or, if creel capacity is not sufficient to supply all the required ends and no distinct yarn pattern is required, then beam warping is generally used, where yarns are directly wound from the supply packages onto a beam. This beam is called a section beam since it contains only a section of the warp required. If however, with insufficient creel capacity, it is necessary to build a warp beam containing the total number of ends required or if the warp yarns have to be arranged in a definite order, then drum warping is used.
In drum warping, the warp is not wound directly from the creel onto the beam but rather sections of the warp are wound onto a pattern drum. In this manner the entire warp is built in a series of sections on the pattern drum. When the total numbers of warp ends required in the fabric have been wound on the pattern drum, they are all removed simultaneously and wound upon a beam. This beam contains the exact number of ends required in the warp.
In general for warp knitting, the yarn for the entire fabric is not put on a single beam, but rather put up on a series of smaller section beams which contain only a portion of the ends required for a full width fabric. These beams may be produced either by beam or drum warping methods. If, however the yarn is to be used in weaving, it generally must undergo one further operation called, slashing or sizing.
WARP SIZING OR SLASHING
In the weaving process, the warp yarns are subjected to high degree of abrasion and rubbing against metal as being threaded through drop wires, healds and reed; they are constantly being rubbed together during shedding; they are subjected to tension both constant, by the let-off and take-up, and intermittent, by the shedding and beat-up. All of these lead to conditions which favours yarn breakages and hence reduced weaving efficiencies. Thus it is desirable to produce a high quality of warp as much as possible, one which will withstand the rigours of weaving. This is the purpose of slashing or warp sizing. This is accomplished by applying an adhesive film on the yarn which protects them from the abrasive and frictional forces, rubbing and chafing, tangling against each other, etc.
In general, singles spun yarns must be slashed, while continuous filament yarns, if they are slashed at all, usually need adhesive to protect the filaments from breaking, and the plied yarns are generally not sized if at all done will be only for some lubrication.
Since slashing is non-productive and protective measure which adds to the cost of production, it is important to carefully select the size ingredients. Some factors which must be considered are:
- cost of the ingredients
- non-degrading to the yarn
- compatibility with the equipment
- easily removable
- Non-hazardous and biodegradable
- least amount of dusting-off during weaving
- minimum number of end-breaks during weaving
If the yarn contains too much size by weight it will tend to be brittle and, as a result, an excessive number of end breaks will occur. If the yarn contains little or no size then none of the benefits of sizing the yarn will be realized and again there will be excessive end breakage. For most spun yarns, depending upon the fiber type and size recipe, the minimum warp breaks occurs when 5 to 15% size by weight is applied. At the slasher sizing machine, many warp beams are combined to form a single weaver’s beam. In case of multiple section beams, prior to the yarn’s entry in the size box, the yarn passes over a series of rods called “lease rods”. The lease rods help the yarns coming from many different warp beams to flow together smoothly. The yarn next enters the size box containing the size solution, known as size liquor. the yarn is fed into the size box by means of a guide roll. It then passes under a dip or immersion roll. This roll is capable of being moved up or down allowing the yarn to be held in the size liquor for a desired period of time. The warp sheet then passes through two rolls known as squeeze rolls, where they squeeze out excess size; and physically drive the size into the yarn for better penetration.
The percentage of size by weight is controlled by the yarn’s exposure to the solution, governed by the speed of the machine and the immersion roll depth, the yarn structure and the pressure applied by the squeeze rolls. The size box also contains pipes which supply size liquor at constant flow rate and steam for heating. In this way, the temperature and concentration, and hence the viscosity, of the size is kept as constant as possible to assure both correct and uniform size pick-up by the warp yarns.
Next the warp yarns are dried which is done either by exposing the yarns to hot air, or by passing them over heated cylinders, or by exposing them to infra-red, or by using combination of these methods. The most common method of drying is the use of cylinder drying range. Time of contact, temperature of the cylinders, and the yarn speed is important in order to dry the warp sheet efficiently, however high speed can be achieved by increasing the number of cylinders. While using cylinder drying method precautions must be taken that the size do not cause the yarn to stick to the cans as it is drying. Precautions must be made to prevent excess size from building up on the cylinders. For these reasons, the first three or four cylinders are usually coated with Teflon.
The warp yarns in weaving required to be separated with each other as due to adhesive present in size liquor causes individual yarns to stick to each other. Therefore, care must be taken to separate individual ends. This task is performed by the burst rods. The warp yarns are threaded through these burst rods in such a way that alternate ends are sent in alternate directions. This process may be repeated as many times to achieve total separation. Often in the case of continuous filament yarns, there is a splitting section prior to the drying section. This is done in order to reduce any effects to the yarns from the initial shock of bursting a completely dried sheet.
DRAWING-IN and TYING-IN
If the new warp has different no. of warp yarns and a different combination or weave pattern, or the loom does not have a warp to begin with, then drawing-in is performed. Drawing-in is the process of providing each end with a drop wire, a heddle in the proper harness and a dent in the reed. In short it is the process of threading each end through the drop wire, heddles and the reed. Drawing-in does not generally occur at the loom, but rather in another area in the mill.
If the new warp is similar no. of warp yarns and a similar combination or weave pattern, that is if the beam is meant for the same fabric being woven on the loom than the operation of tying-in is performed. Tying-in is merely the cutting-off of the old warp and the end-to-end tying of the yarns from the new beam to the corresponding warp yarns already in place on the loom. This operation generally occurs at the loom. When the mill is producing long runs of the same fabric, tying-in is most prevalent.
In warp knitting, the yarns are taken from the warp beam and manually threaded through the machine. Tying-in, even if the new beams are replacements, is not normally practiced.