Drying is, of course, used to remove moisture from the fabrics. However, the way in which the heated air is applied can have more far-reaching effects. Controlling the mechanical action of the drier, including the air velocity and other mechanisms, can result in bulking of the fabric and shrinkage control.
High temperatures after drying can be used for heat setting of thermoplastic fibers. Various types of dryers are used in the textile finishing house.
Tenter Frames & Loop Dryers
Tenter frames are the most common method used for drying. Used for open-width fabrics, tenter frames provide absolute control of the fabric width and can reduce shrinkage. Tenter frames can also be used to heat set thermoplastic fiber or blend with cotton. You can also use tenter frames to dry and cure cellulosic fabrics treated for wrinkle resistance. At the entrance of the frame, the fabric is in pins or clips located in rails. Both edges (or selvages) of the fabric are gripped which constrains and controls the width while the fabric is transported through the tenter. The fabric can be fed on the pins at a rate faster than the pins move, this results in an overfeeding of the fabric for subsequent drying and/or curing in the ovens or sections of the frame. As the fabric dries, it shrinks. Inside the frame, a high velocity of air is blown through nozzles placed above and below the fabric. As the fabric is transported through this frame, the mechanical action of blown air causes it to dry and shrink. In essence, the fabric moves through on a cushion of air. The series of ovens or sections in a tenter frame can be heated to different temperatures. When tenter frames are used to dry the water out of the fiber, temperatures are kept as low as possible. In other applications, requiring chemical reactions, such as curing, temperatures are higher to achieve the action.
Unlike a tenter frame, where the fabric passes horizontally through the machine, a loop dryer carries the fabric up and down around rollers so that fabric forms vertical loops as it passes through the dryer. Fabric spends more time in this dryer but drying is more gentle because air velocities are lower than in a tenter frame.
Another drying method relieved fabric distortion and results in fabric that is pre-shrunk. This is referred to as relaxation drying and usually occurs after chemical finishing and before compaction. Performed on pen width or tubular fabrics and used primarily for knits, relaxed drying makes use of mechanical action to reduce shrinkage.
Conveyor Belts & Drum Dryers
Two different systems are used: conveyor belts and drum dryers.
Belt dryer systems are designed to overfeed and relax fabric. A relaxed fabric transport is coupled with mechanical action achieved by airflow and, in some systems, the bouncing action of the conveyor belt. The belt is typically made of chemically resistant fabric material, but it can be metal. With knit fabrics, as fabric modulates, shakes, and shimmies, it shrinks. Combined with overfeed, fully staggered nozzles achieve a sine wave effect on the fabric causing it to shake. As a result, the fabric shrinks as tensions are removed from the structure.
Drum dryers incorporate a series of large perforated metal drums, these are inside an enclosed oven where fabric is wrapped around most of the outside surface of each drum and is transferred from drum to drum as it passes through the dryer. The greater the air velocity, the faster the fabric dries. The conventional drum dryer is designed so that air flows from inside the drums out through perforations and through to the fabric. High air velocities can blow the fabric off the drums, stretch and distort its structure so another method has been devised. The suction drum drying method pulls the heated air from outside the drum to the inside. The suction causes the fabric to lie against each drum. Because air is pulling the fabric to the drum, rather than pushing it away higher velocities can be used without stressing the structure of the fabric. As the fabric moves up, around, and over these drums, it vibrates, shrinks, and flattens. This is a less stressful way to dry fabric.
Drum dryers are commonly used to dry knits in tubular form, however open-width fabrics, knit or woven, can also be accommodated by this system.
A fabric’s tendency to shrink is affected by its structure, with more open structures having a greater propensity to shrink.
Compaction, a mechanical process, can be used to reduce fabric length shrinkage by mechanically compressing the structure of the fabric. Compacting machines are available in various designs for knit and woven fabrics.
Yarns are flexible and during fabrication are bent into non-linear configurations.
In the case of knits, fabrication and processing tensions tend to elongate the loops. If the shape of individual loops can be made more round, the fabric is made more compact and the length can be reduced.
Less washing shrinkage is a result of this process. In the case of woven fabric, more crimp is created in the warp yarn which reduces its length—the more the crimp, the lower the shrinkage. When knits or wovens are compacted, the fabric becomes heavier and thicker, and the yardage is reduced. Softeners on the fabric and steam in the process will allow the yarns to slip by each other and adjust themselves more easily.
There are two basic technologies for compaction: the heated roll and shoe compactor and the belt compactor.
Heated Roll & Shoe Compaction
Steam is applied as fabric wraps around the heated feed roll made of steel. Just enough moisture is applied so that the fabric will move freely. The fabric moves between two heated shoes which have special surfaces that grab the fabric in a controlled way. The gap between these shoes is set to achieve compaction. The delivery roll is made of rubber and turns at a slower surface speed than the feed roll, and along with the shoes slows the fabric down to allow for compaction. When the fabric hits the surface of the heated shoe, the fabric slows, shortens, and compacts based on the surface friction in the compaction zone. Over-compaction will result in a corrugated appearance. Note how the fabric coming out of this machine is slightly thicker than the fabric going in. A similar but different system design incorporates a compaction blade located below one heated shoe, positioned between the feed and delivery rolls. With this system, the compaction also takes place when the fabric movement is precisely slowed by surface contact in the compaction zone.
Belt compaction utilizes either a rubber or a felt belt that is stretched by flexing over a pressure roll. The fabric is laid on the belt while the belt is still stretched. Steam is applied as fabric enters the path where it is pressed between the belt and the Palmer unit which is heated. Compressive shrinkage of the fabric with the rubber belt is obtained by the recoil of the belt when it is flexed around the Palmer unit. After the fabric has been compacted, it’s preferred to flat fold the fabric because forming a roll can result in undesirable tension and stretch the fabric. After compaction, the fabric can go directly to cut and sew.
Calendering is a mechanical finishing process typically used to produce special effects in fabric, such as high luster, smoothness, or even embossing.
In this operation, primarily used on woven goods, the fabric is passed between heated rolls under heavy pressure to flatten the fabric.
Usually a three-roll process, a central cotton fabric or paper roll is sandwiched between two metal heated rolls rotating at faster speeds.
For friction calendaring, the cloth is unwound from a fabric supply roll. Steam is applied before it moves around the calendaring rolls which apply pressure, hundreds of pounds per square inch. As a result, this fabric is brought to a highly polished state.
Schreiner calendaring is a specialized system. In this scenario two rolls are used, a small roll and a larger, finely engraved roll. This large, heavy metal roll is engraved at a 36-degree angle with fine, or shallow, lines. The number of lines can range from 180 to 500 per inch but are most often 300 lines per inch. Heat is maintained as the fabric is guided under pressure between the engraved roll and a smaller, steel roll which is heated. The Schreiner roll has a greater surface feed than does the small steel roll. Notice the difference between fiber structure before and after it has been calendered. This flattened form allows more surface for light to be directed back to the viewer resulting in more luster. Because of this, fabric finished by the Schreiner calendar method appears to have a lot more luster. It is also less flexible because calendaring flattens the fiber and reduces the permeability
With embossing, another calendaring method, a pattern is embedded into the fabric. An embossing calendar consists of an upper engraved steel roll and a lower composition or paper roll with a negative pattern of the metal roll. As the fabric moves between these rolls, embossing occurs. Thermoplastic fiber can be set by heat while cellulosic fibers must have a resin finish applied to ensure a durable effect. This kind of calendaring produces many popular fabrics such as fake seersucker, crepes, and other relief fabrics.
Sanding, Napping, & Shearing
In contrast to calendaring, which normally makes a fabric smooth, firmer, and shiny, the mechanical finish of sueding makes knit or woven fabrics feel softer and appear more textured. One effect: the surface can be made to resemble sueded leather.
How do sanders work? Fabric, open to its full width, is put in contact with one or more abrasive rolls which move at much higher surface speeds than the fabric.
There are basically two types: the multi-roll sander and the pressure or gap sander.
The sanding action associated with the multi-roll sander results from only surface or tension contact. The surface path for the multi-roll sander reveals that the fabric undulates under an idler roll and over an abrasive roll, then continues on to the next repeating sanding unit so it becomes subject to a series of abrasive rolls. Where idler rolls are located affects the result—the lower the idler rolls are in relationship to the sanding rolls, the greater the angle of wrap on the fabric which results in more abrasion. Multi-roll sanders allow for different levels of aggressiveness and can be adapted to use only one grit or different levels on each of the rolls. This system is less forgiving but more precise in application.
Pressure or Gap Sander
A pressure or gap sander has only one roll of sandpaper but added pressure is applied as fabric runs through a precisely set gap between the sanding roll and the pressure roll. This gap must be less than the thickness of the fabric. The fabric is pushed with force onto the sandpaper. The action of the faster moving abrasive roll against the fabric cuts the fabric to a precisely set thickness, to ten-thousands of an inch.
Another common mechanical finishing method that appears to rip and tear at the surface rather than cut is napping. Napped fabric produces pile, a fabric effect where fibers are caused to protrude in a dense or high population of the surface. This imparts a feel that is pleasant and plush. To achieve this effect, the napping system employs up to 30 raising or worker rolls that are covered with wire points. Wires are bent and arranged so points strike the surface of the fabric in one of two ways. One roll contains wires with points that directly strike the surface of the yarns in the fabric in a piercing way. This roll, referred to as the Counter Pile roll, pulls fibers from the surface in an unorganized manner. A second roll uses the backs of the bent wire points to strike the fabric in a more combing manner. This roll, referred to as the pile roller, organizes the fibers that have already been lifted from the fabric and makes them parallel. By combining the action of the two rollers, you can get more counter pile action which causes fibers to be random and uneven. Or you can get more pile action, which causes fibers to tuck back into a smoother surface. The speed relation between the counter pile and pile rolls and the type of wire has a big impact on the type of effect achieved.
Rollers can be in combinations other than pairs, producing single action, double action, and nappers for knit goods. To give napped fabrics a uniform and even pile height, random length fiber can be removed by shearing. Here is how shearing is achieved: A fabric with fibers, yarns, or loops protruding from the surface is precisely presented to the cutting point of a rotating cylinder and a rigid shearing blade. On the cylinder, multiple spiraling cutting blades create a scissor cutting action as they cross the rigid and straight shearing blade. As a result, the tips of the fiber or yarns are accurately cut across the width of the fabric. With this system, it is important that the blades are aligned properly, or the fabric and blades can be damaged.
For example, knitted velour is a terry looped fabric that has been sheared. Tips of the loops have been cut off, so yarns are even in height and spacing. This processing creates a lustrous fabric. More elaborate, sculptured effects where the textured pile is high and low can be produced when shearing machines are specially modified. The smoothness or texture of shearing, the piles of napping, and the sueded appearance of sanding are effects that enhance the appeal of many different popular fabrics.
TERMS TO KNOW (click to flip)
Removal of moisture from a substrate.view in glossary
A mechanical process in finishing that causes the fabric to be reduced in length resulting in shortening and thereby less…view in glossary
A compaction system that places a fabric for shrinking onto a stretched belt that when the belt is compressed, the…view in glossary
A compaction machine that uses frictional forces to shrink the fabric in a compaction zone consisting of a heated roll…view in glossary
Mechanical finishing process in which fabric is passed between two rollers (one metal and one cotton fabric) under heavy pressure…view in glossary
The abrading of a fabric surface with sandpaper or some abrasive substance. Can be performed on all types of fabrics.view in glossary
Mechanical finishing process that raises the fibers on the face of a fabric by the continuous application of the fabric…view in glossary
Mechanical finishing process that precisely trims off protruding fibers on the surface of a fabric.view in glossary