SIZING
The weaving process requires the warp yarn to be strong, smooth and elastic or extensible to a certain degree. To achieve these properties on the warp yarns, a protective coating of a polymeric film forming agent (size) is applied to the warp yarns prior to weaving; this process is called slashing or sizing. This size material is later on removed in the finishing operation called desizing.
The main purposes of slashing are:
· To strengthen the yarn by causing the fibres to adhere together .
· To make the outer surface of the yarn smoother so that hairs protruding from one yarn in the warp should not become entangled with hairs protruding from a neighboring yarn .
· To lubricate the yarns so that there is less friction when they rub together in the weaving process . Lubrication also reduces the friction between the yarns and the loom parts . The reduction of friction reduces the forces acting on the yarns during weaving .
The ultimate goal of sizing is to eliminate or reduce warp breaks during weaving. Warp breaks are caused either by high tension or by low strength in the yam. High tensions in the warp are caused by large shed openings, lack of proper tension compensation, high beat- up force and inadequate let-off. Knots, yarn entanglement and high friction also cause tension buildup.
It is important that the size film must coat the yarn surface without excessive penetration into the body of the yarn, because if the size material is penetrated deep in the yarn, complete desizing would not be possible. Therefore, only enough penetration should occur to achieve bonding of the size film to prevent removal during weaving. The following terms are used related to sizing:
· Size concentration: the mass of oven dry solid matter in size paste
· Size take-up (Size add-on): the mass of paste taken up in the size box per unit weight of oven dry unsized yarn
· Size percentage: the mass of oven dry size per unit weight of oven dry unsized yarn
There is an optimum level of size add-on that d minimum warp end breakage as shown in figure. Excessive size makes the yarn stiffer and less extensible; yarns with too little size will not be strong and smooth enough for weaving. Therefore, too little or too much sizing causes an increase in warp end break. Optimum size add-on gives the best results for weaving.
Other points to consider in slashing:
· Slasher creel tension control is critical especially with open-end yarns. Maximum tension should not exceed 5% of breaking strength (15—20 g for ring spun yarns and 12—15 g for open-end yarns). With coarse yarns, sometimes 30 g is allowable.
· The amount of size picked up is affected by the viscosity of the size mix as well as the yarn structure. The viscosity of the mix is controlled by the recipe, amount of solid content in the size liquor and the type of sizing product, mechanical mixing level, temperature and time of boiling. Flat filaments, textured and spun yarns pick up size differently.
· Yarn spacing at the slasher size box and on the drying cylinders is very important.
· The choice of size for staple yarns is usually based on cost. For filament yarns, the size material is chosen based on the compatibility with the fiber.
· Running the slasher at creep speed, which is sometimes necessary, generates a very undesirable condition for proper sizing and should be minimized in every way possible.
· Stretch of warp yarns during slashing should be controlled accurately to maintain residual elongation in the yarn which is needed for good weaving. Back beam— size box stretch should not exceed 0.5%.
· Guide rollers should be kept free from scratches and sharp edges. They should be sanded/polished frequently.
· Pre-wetting yarns prior to sizing can reduce the amount of required size add-on for the same performance, especially for cotton yarns.
Main parts of sizing machines are :
· Beam Creel
· Size Box
· Dryer
· Yarn Splitting System
· Head Stock
Beam Creel
Beam arrangements in the creel are usually two types:
a) Groups of 2, 4, 6 or 8, one to four tiers
b) staggered, two-tier arrangement
SIZE BOX
Main parts of the size box are
– Immersion Roller
– Squeezing Rollers
– Steam Heating Pipes
• Immersion Roller
Position of the immersion roller depends on the quantity of the size required to apply . If heavy size is required then the roller is required to apply .
• Squeezing Rollers
The purpose of these rollers are to remove the extra amount of size chemical from the yarns by squeezing them
among two rollers .The squeezing rollers should be placed above the size box so that the extra size chemical should fall in the box .
• Steam Heating Pipes
The temperature of size box is maintained with the help of Steam Heating Pipes and the steam flow is regulated to control the temperature .
Explantion:-
The size box is probably the most important section of the sizing machine. During the sizing process, the sheet of yarns is passed through the size box which contains the hot water solution or mixtures of sizing agents. The yarns pick up the required quantity of size solution in the size box, any excess size is squeezed oft as the yarns pass through squeeze rolls. Depending on the size material, warp quality and density, single and double immersion rolls and single squeeze and double squeeze configurations are used. Multiple size boxes can also he employed. In general, single box sizing machines have two squeezing rollers and two box machines have a single roller in each box. It is important that the rollers provide uniform squeezing pressure. The squeezing system determines the degree of size pick up to a large extent. While providing size consistency, the roll pressure should be adjusted to get around 125—130% wet pick up for cotton yarns, 110—115% for poly/cotton and 95—105% for polyester. On average, open-end yarns pick up around 10—15% more wet size than a comparable ring spun yarn. Therefore, about 10% more water should be added to get the same add-on. The bottom rollers are usually made of steel and the top rollers are rubber coated.
The critical parameters to watch in the sizing process are size homogeneity, constant speed of the sizing machine, constant size concentrations and viscosity. Flooding or dry zones should be prevented in the size box. Temperature of the size box is important for proper size pick up. For 100% polyvinyl alcohol (PVA) sizing, a temperature of 70 – 75 °C is recommended. Constant size temperature can be obtained in two ways:
1. Direct heating in which steam is injected into the size.
2. Indirect heating in which steam flows in pipes around the double walled size box.
A cooker is used to prepare the size; powdered size is metered into weight stations and then transferred to the cooker. Size mix is also called size liquor. Liquor concentration and the amount of water absorbed by the warp after the squeezing rollers are measured using a microwave measuring head, which enables the degree of sizing to be calculated. The degree of sizing can be calculated as the absolute degree of sizing or as a relative value. If the measured degree of sizing is different than the set data, then the squeeze pressure in the box is changed with a controller. The squeeze pressure is also adjusted according to speed of the system.
Rotor cooker is normally used for size preparation, in a rotor cooker, the size is prepared without pressure. The ingredients are added cold and mixed with a rotor and raking agitator. Additional breaking elements can be used to obtain high shearing effect. The size ingredients are automatically fed-in to the size box.
• DRYER
The different drying techniques used in sizing are as
under :
– Cylinder Type
• Two Cylinder Type
• Multi Cylinder Type
– Hot Air Drying
– Infra-red Drying
After the size box, the yarns go through the dryer section. The wet yarns are dried by using hot air, infrared radiation or cylinder drying. Cylinder drying is done using steam heated hot rolls which are called drying cylinders. Sometimes, a combination of drying methods is used on the same machine.
The drying temperature is critical; excess temperature increases the penetration of size into the yam, which can lead to excessive hairiness and even yarn breakage. The typical temperature range is 80 – 105°C. In cylinder machines, the evaporation rate can be calculated as the mass of water evaporated per unit contact area between warp and drying cylinder per unit time. A typical evaporation rate in a modern slasher is around 13 kg/hr/m2. The maximum recommended machine speed is 120 m/min.
Splitting the warps after sizing and separate drying reduces the risk of adjacent yarns sticking together which reduces the number of yarn breaks. For air-jet weaving, a minimum of 75% open space on the dry cans is recommended for any type of yarn.
Sizing machines are classified based on the method of drying (cylinder, hot air or infrared) or according to the method of yarn supply (single end, direct and indirect). In single – end slashers, yarns are fed to the size box directly from the supply packages. This type of creel is generally used for a small number of yarns and textured yarns. In direct sizing, yarns are fed to the size box from a single creel beam or warper’s beam. In indirect sizing, several warper’s beams (“section beams”) are combined sheet to sheet, forming a final beam for weaving (weaver’s beam)
• YARN SPLITTING SYSTEM
Main parts involve in this section are :
– Lease Rods
– Lease Reed
YARN SPLITTING SYSTEM Lease Rods
· The function of the lease rods is to separate the individual yarns which are stuck together because of the drying of the size film in the drying section .
· To achieve this a series of lease rods , with one large diameter busting rod are used .
· These rods are generally chrome plated hollow cylindrical bars flattened at both ends .
· The yarn sheet emerging from the drying section is divided into two sections by one large lease rod and each section is further subdivided into two subsections by successive lease rods
YARN SPLITTING SYSTEM Lease Reed
• The function of the lease reed is to align the yarns to ensure uniform winding of the warp on the weaver’s beam .
• A V comb reed is used to help in providing an even distribution of the warp yarns and for avoiding abrasion
among yarns .
HEADSTOCK
• This is a take-up unit supporting the weaver’s beam and necessary drive gears .
• The drive equipment imparts necessary beaming tension for compact and straight winding of the warp yarn .
• Winding system is equipped with a positively driven roll commonly known as delivery roll or draw roll .
• This driven roller is placed between two heavy chrome plated nip rolls to assure that the yarn sheet being drawn is wrapped well around the draw roll .
SIZING INGREDIENTS
• For a size formula to approach perfection in a given application it should have several basic characteristics which will serve to eliminate warp breaks during weaving . For the size film to accomplish its primary tasks one would have to consider the following .
Tensile Strength Uniformity
· Adhesiveness Stability in Storage
· Flexibility Viscosity
· Elongation and elasticity
· Abrasion Resistance
· Penetration
· Uniformity
· Stability in Storage
· Viscosity
Base Materials
Since the prime function of sizing is to produce weavability in the warps the essential part of the size formula is the film forming ingredients . Base materials are the film forming ingredients or binders
Additive Materials
Additives are usually required to modify the base by increasing ( or decreasing ) one or more of the desirable properties .
Base Materials
Natural Base Materials
• Flour
– Maize Flour
– Rice Flour
– Wheat Flour
• Starches
– Rice Starch
– Wheat Starch
– Corn Starch
– Potato Starch
• Gum
• Dextrine
• Amylose
• Gelatin
Synthetic Base Materials
•Polyvinyl Alcohol ( PVA)
• Carboxylmethyl Cellulose ( CMC )
• PolyAcrylates
• Lubricants
Used to reduce friction and abrasion between adjacent yarns and the reed ,frames , drop wires
• Softeners
Used to give soft handle to the warp and the size film and to decrease the brittleness of the film
• Electrostatic Agents
Used to solve the static problem
• Deliquescent Substances
A small amount of hydroscopic agent or deliquescent in the size will prevent the size film from drying completely (Bone Dry ) and help the fibre to regain normal moisture after drying .
