Mechanical Finishing

Mechanical Finishing

Slide 1

Mechanical Finishing

As stated in the introduction to Chemical Finishing, the final properties of a textile fabric and the conformity to desired fabric specifications are ultimately affected by finishing steps, so the finisher is responsible for ensuring that the fabric meets those specifications for construction and functionality. This section concentrates on mechanical finishing, or processes in which the principle agent of change to the fabric is physical or mechanical in nature. As mentioned earlier, many mechanical finishing processes require steam or moisture to lubricate the fabric, but physical manipulation by mechanical action is the primary cause of the change in fabric properties. Examples of properties that can be enhanced by mechanical finishing are hand, appearance, luster, thermal insulation, dimensional stability and, for certain synthetic fibers, wrinkle resistance.

Copyright 20082011 North Carolina State University. All rights reserved.Mechanical Finishing

Slide 2

Systems for Mechanical Finishing

Heat setting

Napping

Shearing

Sueding

Calendering

Compacting

Relaxation Drying

Decatizing

Typical mechanical finishing processes are listed in this slide. A brief description will be given for each of the operations, but most of them will be covered in more detail in subsequent slides.

Heat setting is a process whereby the thermoplastic synthetic fibers like nylon and polyester can be stabilized by heat. The process lends shrinkage resistance and wrinkle resistance to fabric. Heat setting is generally done after dyeing, but it should be done as a preparation step in a fabric containing spandex or for dye formulations that are sensitive to high temperatures.

Napping raises surface fibers to create a fuzzy surface on a fabric and is accomplished by passing the fabric across rapidly turning, wire covered rollers. Fleece is an example of a napped fabric. Shearing uses a blade to remove surface fuzz from a fabric to create an even surface with uniform pile height. Sueding raises surface fibers on a fabric using sandpapercovered rolls. A fabric that has been sueded develops a hand similar to leather

suede.

Calendering imparts luster and smoothness to a fabric by passing it between pressurized rollers, one moving much faster than the other. In essence, calendering polishes the fabric.

Compacting forces fabric to shrink in its lengthwise direction by mechanical compression during processing, thereby minimizing shrinkage of the fabric during consumer use.

Relaxation drying minimizes tension on knitted fabrics during drying. The fabric is overfed into the dryer to allow for shrinkage during fabric processing rather than in garment form.

Decatizing is a process primarily for wool fabrics to improve luster and smoothness. The wool fabric is layered between heavy cotton fabrics on a roller and exposed to steam. The steam and pressure of the process flatten the wool fabric and improve its hand and luster.


Slide 3

Heat Setting Overview

Purpose

To impart stability to polyester and nylon fabrics by heating to 350400 degrees F for 2060 seconds.

Uneven moisture causes the fabric to dry unevenly and therefore to be subjected to uneven heatsetting. Not effective on cotton or rayon.

May be performed in fabric or garment form.

Differential dyeing, bowbias and yellowing can result.

May cause shade variation from sidetoside if done prior to dyeing. May cause variations in shrinkage.

Heatsetting has been covered previously in the Preparation Module, but it is often carried out after dyeing as a finishing step. It is effective only on certain synthetic fibers and has no effect on cotton, wool or rayon. The process is typically used for nylon and polyester but could also be used on triacetate. In order to be heatset, a fabric must be made of thermoplastic fibers that possess a glass transition temperature, or Tg. At temperatures above Tg, the polymer chains in the amorphous regions of the fiber become mobile and can rearrange into a structure that will be stable throughout the lifetime of the fabric as long as the heatsetting temperature is not exceeded. In other words, the fabric will retain a memory for its configuration during heatsetting, and the process imparts dimensional stability and wrinkle resistance to synthetic fabrics that can withstand typical laundering conditions.

Heatsetting is carried out at temperatures between 350 and 400 degrees F. The fabric will stabilize after 20 to 60 seconds at the heatsetting temperature, but it is important that the fabric itself, and not just the surrounding air, reach the desired temperature. For this reason, the fabric must be dried completely before effective heatsetting can occur. Blends of cotton and polyester can be heatset, but the process will stabilize the polyester component only, and the finisher must ensure that the process will not adversely affect the cotton component.

Poor control of the process can lead to quality problems. Heatsetting after dyeing can dull or yellow the shade. When done before dyeing, uneven temperature exposure can lead to differential dye uptake, a problem known as sidecenterside shading. Poor tension control during heat setting can lead to differential shrinkage and width variation.


Slide 4

Tenter Frame

A tenter frame is typically used for heatsetting fabric in openwidth form. The fabric is introduced to a moving chain on each side that carries clips and pins for gripping the fabric on its selvages. Sensors on the machine allow for proper width adjustment. Clips are generally utilized for woven fabrics and pins for knitted fabrics. After the fabric is gripped on both sides, it moves through the sequential heating zones, which can be heated by steam, natural gas, propane or oil. The temperature in each zone can be set independently to provide the necessary drying and heatsetting conditions for the particular fabric being processed. The temperature profile for the fabric will also be affected by its speed through the frame. A profile of the actual fabric temperature and the dwell time at various temperatures is an important indicator of whether the tenter frame settings will produce proper drying and heatsetting effects on the fabric.


Slide 5

Napper Overview

Purpose

To lift or raise a layer of fibers from the surface yarns of a fabric to form a raised surface pile to produce warmth and soft hand.

Key Components

Wire Covered Rolls

Speed

Tension

Softener

Fabric construction and yarn has big effect on the pile.

Important to have a napping lubricant on the fabric to aid in pile raising.

Types of Nappers

Single acting which gives a long parallel pile.

Double acting which gives a full, heavy nondirectional pile.

Knit goods nappers are designed to hold tensions on knits and tuck nap back into ground.

Napping produces a raised surface pile on a fabric, producing thermal insulation and softness. Fleece, flannel, and fake furs are examples of fabrics that are napped. A napping machine, or napper, includes wiredcovered rollers that rotate against the fabric to pluck and raise surface fibers. The napper wires are extremely sharp to enable them to grab the fibers. Napping action is affected by machine type, speed of fabric and rolls, condition of wires on the rolls, fabric and yarn construction, fabric tension, and the presence of softener on the fabric. The yarns that contact the napper wires will be weakened by the napping action, so these yarns should not be responsible for the strength of the fabric or be an integral part of the base fabric. The high friction between the wires and the fabric necessitates the application of a lubricating softener to the fabric before napping. In addition to precise control and attention to the factors listed above, skill and operator experience are critical to producing the desired results on the fabric.

The type of napping machine chosen depends on the fabric type and on the type of pile desired. Single action nappers contain rotating wire covered rolls that have all wires angled in the same direction to produce a long, onedirectional pile. Double action nappers contain rollers that alternate direction of wire angle to produce a nondirectional pile. An illustration of a doubleaction napper appears in the next slide. Finally, special nappers are designed for knitted fabrics to maintain the width tension on the fabric and to tuck the pile back into the base fabric to improve pile durability.


Slide 6

Double Action Napper

The double action napper illustrated in this animation is the most commonly used type of napper in the industry. It would typically be used for producing fleece. The heart of the machine is a large cylinder containing 24, 30 or 36 worker rolls. The large cylinder rotates in the same direction the fabric is moving, while the small worker rolls turn in the opposite direction against the fabric. The worker rolls are designated as pile and counterpile rolls, which alternate in sequence. The pile rolls contain napper wires angled in the same direction as the rotation of the cylinder, while the counterpile rolls contain wires angled in the opposite direction. Not shown in the animation are cleaning rolls, called fancies, that are located underneath the large cylinder away from the fabric path. They rotate against the worker rolls to clean lint from the wires.


Slide 7

Napper Video

This video shows a double action napper in operation. The first view is of the entry side of the machine. The fabric seen is a tubular knit, but flat knits and woven fabrics are also napped on this machine. A typical reason for napping the fabric seen here would be to produce a sweatshirt fabric with a soft, fleecy side on the inside of the shirt. A spreader roll with a spiral design smoothes and flattens the fabric on the entry side of the cylinder. Note the wirecovered worker rolls turning in the opposite direction to the motion of the fabric. These rollers are covered with wires that grab and raise the surface fibers of the fabric. Adjacent rollers will have wires angled in opposite directions to produce a nondirectional pile. The final segment of the video shows the fabric before and after napping, with the original fabric on the right and the napped fabric on the left.


Slide 8

Napper Issues and Problems

Problems associated with napping can be related to the fabric, to the condition of the machine, or to the process itself. The fabric construction should lend itself to napping, and, upon entry into the napper, should not contain wrinkles, creases, or folds, because the fabric surface in the folded areas will not be exposed to the napper wires. If the napping process is too severe or is overdone, the fabric will be weakened.

The condition of the wires will have a direct effect on the napping process. As mentioned earlier, cleaning rolls called fancies remove lint from the worker rolls to prevent clogging of the wires. The wires will wear or bend over time and will need to be replaced to prevent napper streaks. When napping fabrics of different widths, the wires will not wear uniformly, so wider fabrics should be napped before narrow fabrics. Narrow fabrics can be fed into different locations across the width of the cylinder to accomplish more uniform wear on the wires.


Slide 9

Shearing Overview

PurposeTo smooth a fabric surface by cutting raised fibers to a uniform height.Napped fabrics are often sheared to give an even pile height.

Effects of ShearingImproved pilling resistance

Smooth surface or patterns (highlow or random)

Problems in ShearingDefects caused by folds, creases, and heavy edges

Uneven shearing caused by misaligned blades

Destruction of sewn seams or damage to blade by not jumping seams

Blade damage by foreign metallic objects

Whereas the napping process raises fibers to produce a fuzzy fabric surface, shearing cuts raised fibers to produce a smoother fabric surface and is somewhat analogous to mowing a lawn. Removal of surface fibers by shearing also contributes to better pilling resistance in the fabric, and the process could be considered as an alternative to biopolishing or singeing. Shearing often follows napping to produce a uniform pile height, but it is also used for fabrics that have not been napped. An example would be shearing of the terry loop ends in towels to produce a smooth, plush surface. Shearing blades could also be designed to cut a pattern into pile fabric.

Shearing is accomplished by passing the fabric across a rest bar in close proximity to a revolving spiral blade in contact with a ledger plate. The contact between the spiral blade and ledger plate cuts the surface fibers with the same action as the blades in a pair of scissors. The gap between the blades and the rest bar is adjusted based on the fabric width to give the desired pile height or surface smoothness. This gap can be adjusted to the nearest onethousandth of an inch.

The gap and the blade alignment must be carefully adjusted to give uniform shearing across the fabric width. Folds or creases in the fabric can result in a double fabric thickness passing though the gap, which causes fabric damage. A sewn seam in a fabric must be jumped, which can be done manually or automatically, depending on the design of the machine. Another potential problem would be the presence of a foreign metallic object in the fabric that could damage the blades. Many shearing machines incorporate a metal detector to automatically stop the machine if a foreign object is detected.


Slide 10

Shearing Schematic

A typical shearing machine is illustrated in this diagram. The fabric enters from the left side and passes through expander feed rolls to align and flatten it before passing across a brush roll that teases the surface fibers and causes them to lie in the same direction to facilitate cutting. The fabric then lies on the rest bar as it passes under the spiral shear blade. The ledger plate can not be seen in this diagram, but it would be located on the back side of the revolving spiral blade to provide an edge for the blade to cut against. The gap between the shear blade and the rest bar must be accurately controlled and adjusted for fabric thickness, especially for the presence of seams or yarn knots. The expanded picture of the rest bar shows a newer design containing lamella that can automatically adjust for fabric thickness. Most shearing machines also incorporate a vacuum system to remove lint from the cutting region.


Slide 11

Shearing Machine

This is a video of a sample shearing machine with a terry fabric being processed. Notice the cylindrical blade used in the shearing process. The lint being collected is removed through a vacuum process. The fabric passes over a rest bar which can be adjusted to the pile height desired. Finally the sheared fabric is collected.


Slide 12

Sueding Overview

Purpose

To cut surface fibers and produce a fuzz, a suedelike surface on the fabric. Fabric is abraded with sand paper covered rolls (emery cloth, etc.)

Types of MachinesLarge single roll Series of small diameter rolls

VariablesGrit of sandpaper

Speed of the sanding roll Pressure of the sandpaper on the cloth

Speed of the fabric Number and direction of passes through process

Course paper gives heavier pile whereas fine grit gives a lighter pile.

The process of sueding is sometimes called sanding because the fabric is passed across sandpapercovered rollers. The process raises surface fibers to produce a fuzzy fabric surface with a very low pile, thereby improving softness and hand. The resulting surface is described as similar to suede leather, often using the term peach skin.

Variables of the sueding process include type and coarseness of the sandpaper, speed of the sanding roll and of the fabric, pressure between sandpaper and fabric, and type of machine utilized. Two types of sueding machines are currently used: single cylinder and multicylinder. Each of these will be described in the next slides.

Problems associated with sueding can be caused by fabric folds and creases, yarn knots and slubs, and softener buildup in the sandpaper, thus reducing its effectiveness. Wear on the sandpaper necessitates its replacement on a routine basis to obtain product consistency.


Slide 13

Single Roll Sander

This animation of a single roll sander shows the fabric passing between the sandpapercovered roll and a rubber pressure roll. The clearance between these two rolls is a critical setting and can be adjusted to the nearest onethousandth of an inch. The accuracy of this setting would allow removal of print from a newspaper page without damaging the paper. The sensitivity of this setting requires the machine to be mounted into the floor to prevent vibration. The friction of the process generates heat, so the cylinder is often cooled by water circulating in its interior. In some cases, two passes of the fabric through the machine are necessary to achieve the desired results.


Slide 14

Sueding Machine

This is a video of a sample sueding machine sometimes referred to as a sanding machine. This machine configuration passes the fabric over a roll covered with sandpaper. Notice that the sandpaper roll oscillates back and forth. In addition the degree of sanding that occurs, can be controlled first by the angle of the fabric as it passed over the sanding roll, secondly by the speed of the fabric and the sanding roll and third by the type and coarseness of the sand paper used.


Slide 15

Multiroll Sander

A multicylinder sander is usually composed of five cylinders, each independently driven, that can be rotated either clockwise or counterclockwise. Idle rolls on each side of the cylinders control the fabric pressure against the cylinders. This machine design is better suited for fabrics that may contain knots, slubs, or thick selvages because the fabric is not compressed against the cylinder by a rubber roll as it is with the single roll machine. Another advantage to the multiroll design is that alternating cylinders can be made to rotate in opposite directions to minimize the production of a directional pile.

The replacement of sandpaper on a multiroll machine can be scheduled so that each roll is changed in a programmed sequence to maintain an average condition of wear at all times. To ensure consistent results, fabrics can be judged against a hand standard after the sueding process.


Slide 16

Calendering Overview

Purpose

To flatten fabric, thereby increasing luster and smoothness.

Key Components

Composition of the calender roll.

Pressure

Heat

Moisture

Types

Friction

Schreiner

Embossing

Calendering is a continuous physical finishing process that has traditionally been used on woven fabrics but has also been used on certain knit fabric styles in recent years. In the basic process, heavy pressure rolls are used to flatten the fabric. For most fabric styles, the calendering process also results in increased fabric luster and smoothness. From a practical viewpoint, the process of calendering is very similar to ironing a fabric. The fabric can be either dry or moist as it enters the machine. When the entering fabric is moist, the process is similar to steam ironing the fabric. Normally, the exact temperature of the pressure rolls and the moisture content of the entering fabric will be chosen based on such factors as the fiber content of the fabric and the end result expected from the calendering process.

For the basic process, flat openwidth fabric is passed between heavy pressure squeeze rolls. Different types of calenders use different roll types and arrangements. The most popular commercial machine is known as a threeroll friction calendar. In this machine, a composition roll is placed between two heated, heavy pressure steel rolls. The purpose of the composition roll is to cushion the fabric slightly as it travels through the calender. A variety of materials, including highly compressed cotton fabric, pressed paper, or synthetic materials, are used for the covering of the composition roll. The temperature of the heated rolls typically ranges from room temperature to 500 degrees F, while the pressure on the fabric can range from 200 to 2500 psi. As mentioned earlier, the fabric may be damp, moistened with steam, or dry as it enters the machine. In the case of a threeroll calender, the fabric is threaded around the composition roll so that it is pressed twice as it passes through the device, giving extra flattening, luster, and smoothness to the fabric from a single pass through the machine. Key control parameters for the calendering process include the moisture content of the fabric, the exact temperature of the heated rolls, the exact pressure applied to the fabric by the rolls, and the exact type of covering material used for the composition roll.

Three different general types of calenders are currently used by the modern textile industry. These are the friction, Schreiner, and embossing calenders. Each is designed to achieve a different result in the finished fabric.


Slide 17

Friction Calendering Diagram

This diagram illustrates the alignment of a typical threeroll friction calendar. When most manufacturers discuss the process of calendering, this is the machine to which they are referring. Other commercial machines, such as the fiveroll and tworoll friction calenders, have been available, but the threeroll machine is by far the most widelyused. As can be seen from this diagram, the flat openwidth fabric is laced around the center composition roll. As the fabric travels through the calender, it is pressed between the bottom steel roll and the composition roll, followed by pressing again between the top steel roll and the composition roll. To increase the luster on the fabric, the two steel rolls are turned at very high speeds compared to the center composition roll.

The difference in turning rate of the rolls results in a polishing action on the surface of the fabric. The name friction calender comes from the fact that this type of machine is capable of both polishing and pressing the fabric simultaneously. If the fabric is composed of synthetic thermoplastic fibers, the calendering effect can be permanent as long as the roll temperature is above the heatsetting temperature of the fibers. The typical thermoplastic fibers processed in this way are polyester and nylon. For fibers such as cotton, rayon, or flax, it is necessary to use a permanent press resin on the fabric for the calendering effect to be durable. For a temporary effect, starches and waxes can be used instead of resins. The calendering effect on cotton or similar fibers usually lasts through only one laundering cycle without the use of resin or starch. In the case of blends, combinations of both heat setting and resins may be required, depending on the fiber content and blend level of the specific fabric processed.

Typically, the positive effects of calendering are that the fabric becomes thinner, becomes less air and water permeable, has more cover, and exhibits increased luster. Excessive heat and pressure will cause the fabric to become papery or too lustrous. Great care must be taken to properly maintain the surface of both the composition and steel rolls. Any defect or crack in the surface of the rolls will be imprinted into the fabric.


Slide 18

Schreinering Diagrams

Schreiner calenders are specialty machines. When properly operated, the process imparts a soft, silky luster to the fabric. The machine was originally designed for cotton and linen fabrics. However, it also has been used with fabrics made from synthetic fibers such as Qiana nylon. Soft, opaque lingerie fabrics can be produced from tricot knits by processing them through a Schreiner calender.

The machine in the diagram is a tworoll calender. The key component of the Schreiner calender is the large metal roll engraved with very fine lines oriented at a 26 degree angle. There are as many as 250300 lines per inch engraved into the roll. These lines are so fine that the roll feels perfectly smooth to the touch. However, when looking at the roll, it is evident that it is not smooth because of the way it reflects light. As shown in the diagram, engraved lines can be saw tooth in nature with sharp edges. In some rolls, these lines can be somewhat serrated. In other rolls, the engraved lines have a rounded, wavy appearance. Each type of engraved line will give a slightly different luster to the fabric after processing.

The flat openwidth fabric is crushed under high pressure between the heated smooth steel roll and the engraved Schreiner roll. Various effects can be obtained on the fabric by varying the temperature of the rolls, the pressure on the fabric, and the moisture content of the fabric. The diagram on fabric modification illustrates the typical effect of Schreiner calendering on a woven fabric. In this drawing, the warp yarns are represented by the horizontal line while the filling yarns are represented by the circles. In the initial condition, the light reflects in a diffuse manner from the fabric surface, which results in low, dull luster due to the sharp angles between the filling and warp yarns. After calendering, the fabric is flattened and the angular difference between the warp and filling yarns is greatly reduced. The result is that light reflects in a more mirrorlike manner, yielding higher luster to the fabric surface. As stated earlier, cotton, rayon, and linen must be pretreated with resins in order to make the Schreiner effect durable. Thermoplastic synthetics require only that the temperature of the rolls be above their heatsetting point to make the effect durable.


Slide 19

Embossing Calender

Similar to the Schreiner calender, the embossing calender is a specialty machine. It is a tworoll calender consisting of a heated metal roll with an engraved or raised pattern and a composition roll with the negative of the pattern. It is also possible to have a composition roll that has no pattern at all. The composition roll can be covered with paper or an elastic covering layer.

A gear tooth design is shown in this animation. As the fabric is passed between the two rolls, the pattern is set into the fabric by heat and pressure. In order for the pattern to be durable on styles made from cotton, rayon, or linen, the fabrics must be pretreated with resin and calendered, followed by curing of the resin. Patterns embossed into fabrics made from thermoplastic synthetic fibers, such as polyester or nylon, are made permanent merely when the temperature of the embossing roll exceeds the heatset temperature of the fibers.


Slide 20

Compaction Overview

Purpose

To reduce fabric shrinkage mechanically by forcing the structure of the fabric to compress upon itself.

Process

May be compacted by a rubber blanket and Palmer unit or by a series of rolls and shoes. Steam and proper lubricants are necessary to allow the yarns to slip by each other.

Fabric Characteristics

A way of mechanically reducing fabric shrinkage.

Fabric becomes heavier and yardage yield is reduced.

As long as the fabric is not stretched, the fabric is stable.

Over compacting distorts fabric.

Blanket Compactor

Fabric must be smooth and wrinkle free before entering the rubber blankets. Tension control into the Palmer unit is essential.

The fabric must be dry coming out of the Palmer to set the new configuration.

Takeoff and windup tensions must be minimal to get the full benefit of the run.

Heat Roll and Shoe

Fabric must be smooth and wrinkle free going into the compactor.

Distance between the roll and the shoe must be exact.

Tensions in takeoff and windup must be held at a minimum

The mechanical finishing process of compaction is also known as compressive shrinkage or mechanical preshrinkage. The purpose of this process is to reduce linear fabric shrinkage by mechanically forcing the structure of the fabric to compress upon itself. However, this process does not affect width shrinkage. Compaction is a continuous process that can be very effective on either knit or woven fabrics.

As stated earlier, the chief goal of the compacting process is to mechanically reduce fabric shrinkage. The fabric also becomes heavier and thicker, and the overall fabric yardage is reduced. Once compacted, the fabric must not be subjected to lengthwise tension, or the compaction will be pulled out of the fabric. If not stretched, the compacted fabric will remain stable. If the fabric is over compacted, it will become distorted and have a corrugated or orange peel appearance. The distortion can be corrected by stretching the fabric using a tenter frame, followed by the compaction of the fabric a second time.

Several different types of machines can be used for the compaction process. The original compactor was designed for woven fabrics. It was known by the trademark Sanforizer. Today, this device is known as a blanket compactor. However, some modern blanket compactors are designed for use with tubular knit fabrics. Additionally, there are other types of compactors that are designed primarily for knit fabrics. The two most popular machines are the heated roll and shoe compactor and the blade compactor.

The blanket compactor is the original machine used for the compaction process. The Sanforizer compactor for woven fabrics employs a fourinch thick continuous rubber blanket. During the process, the entering woven fabric must be openwidth, smooth and wrinkle free. Many, but not all, fabric styles are moistened before entering the compactor by passing them through a mister or injecting them with steam. Extremely accurate tension control of the fabric is critical as it passes through the compactor. To set the compacted configuration, the fabric must be dry coming out of the Palmer unit. Takeoff tension on the compacted fabric must be kept to a minimum to avoid negating the compaction effect. The details of the process will be discussed later.

The major difference between the blanket compactor for knits and the type used for woven fabrics is the thickness of the rubber blanket. As mentioned earlier, the typical thickness of the rubber blanket used for woven fabrics is approximately four inches. The typical rubber blanket used for knit fabrics is in the range of onehalf to one inch thick. Additionally, the knit fabric blanket compactors are designed to handle either tubular or openwidth fabrics.

The heated roll and shoe compactor was designed to process tubular knit fabrics. The basic machine consists of a feed roll and takeoff roll that are each covered with a nonslip surface. A small gap exists between the two rolls. Above and below this gap are two heated shoes. Each shoe has the general shape of the letter L. The function of the shoes is to form a small gap compacting zone between the rolls. The distance between the rolls and the shoes must be exact. The fabric must be smooth and wrinkle free going into the compactor. Great care must be taken to minimize takeoff and windup tensions on the fabric or the compaction effect will be negated.

The blade compactor is similar to the heated roll and shoe compactor. However, it was designed to be used for either tubular or openwidth knit fabrics. It also employs a feed and takeoff roll, each covered with a nonslip surface. This machine also has a small gap between the rolls. Above


the rolls and the gap is a heated shoe. Below the gap is a blade. The heated shoe and the blade form the compacting zone between the rolls. The major difference between the heated roll and shoe compactor and this machine is the path of the fabric as it is travels through the compactor. This machine has also been used for lightweight woven fabrics. All of the precautions for the other compactors also apply to this machine.


Slide 21

Heated Roll and Shoe Compactor

This is an animation of the heated roll and shoe compactor. This device is also known as an L and L due to the shape of the shoes. As the fabric enters the compactor, the feed roll is turning at a high rate of speed compared to the turning rate of the takeoff roll. Often the fabric is damp or moistened with steam for lubrication as it enters the machine. This lubrication minimizes fabric distortion and enhances the compacting effect.

The heat from the feed roll and shoe gives a steam ironing effect to the moist fabric as it is processed. The fabric enters the compactor by traveling over the top side of the feed roll. It continues through the compacting zone and exits the compactor by traveling over the bottom side of the take off roll.

As the fabric is fed rapidly to the compacting zone, the fabric cannot slide due to the nonslip surface of the feed roll and the position of the top shoe. Once in the compacting zone, the fabric cannot slide due to the nonslip surface of the takeoff roll and the position. The force on the fabric generated by differences in turning rate between the feed roll and takeoff roll compresses the fabric structure onto itself, resulting in the compacting effect. The animation shows a closeup of the compacting zone as the fabric is being processed. The movement of the arrows illustrates the structure of the fabric as it compresses and is compacted.

The amount or percentage of compaction for any particular fabric style is controlled by the difference in turning rate between the feed and takeoff rolls. As mentioned earlier, this machine was designed to be used with tubular knit fabrics and was specifically targeted to be used with cotton and cotton blend fabrics. It has been successfully used for fabrics composed of other types of fibers as well.


Slide 22

Gull Wing Blade Compactor

The blade compactor was originally designed for processing both tubular and openwidth knit fabrics. It is similar in many aspects to the heated roll and shoe compactor, but the fabric processing path is different. In this machine, the fabric enters the compactor traveling over the top side of the feed roll and through the compacting zone. The fabric exits the machine by moving over the top side of the takeoff roll. This machine is known as a gull wing compactor due to the similarity of the fabric processing path and the typical artists sketch of a sea gull in flight.

As seen in the animation, the turning rate of the feed roll is much higher than the turning rate of the takeoff roll. When the fabric enters the machine, it cannot slide due to the nonslip surface on the feed roll and the position of the heated shoe. As with other compactors, the fabric is usually damp or moistened with steam prior to entering the compactor. The heat of the feed roll and the topside heated shoe give a steam ironing effect to the fabric as it is processed. The fabric is fed rapidly to the gap compacting zone, but it is prevented from traveling between the two rolls by the position of the compacting blade. This blade and the topside shoe guide the fabric onto the slowly turning takeoff roll. The fabric cannot slide due to the nonslip surface of the takeoff roll and the position of the topside shoe. The force generated by the difference in the turning rates between the feed and takeoff rolls causes the fabric structure to compress upon itself. This is illustrated in the animation by the closeup of the compacting zone. The actual compaction of the fabric is represented by the arrows as they travel through the compacting zone.

The amount or percentage of compaction for any particular fabric style is controlled by the difference in turning rate between the feed roll and the takeoff roll. This compactor was also specifically designed for use with cotton and cotton blend knitted fabrics, but it can be very effective for fabrics composed of other types of fibers.


Slide 23

Blanket Compactor

This is an animation of a typical blanket compactor designed for knit fabrics. It uses a continuous rubber blanket that is approximately one inch thick. A compactor designed for woven fabrics would have a continuous blanket approximately four inches thick because woven fabrics generally require a much greater force than knit fabrics to produce the compacting effect. In this process, the flat, wrinklefree fabric is usually damp or moistened with steam for lubrication prior to entering the compactor. This lubrication minimizes fabric distortion and enhances the compacting effect.

The moistened fabric is placed on the stretched continuous rubber blanket. It is held tightly in place between the rubber blanket and the large heated cylinder so that it cannot slide during the process. The heated cylinder, sometimes referred to as a Palmer unit, is normally covered with a nonslip surface. During the process, the heated cylinder gives a steam ironing effect to the fabric.

As the fabric travels through the compactor, it is held tightly against the rubber blanket by the heated cylinder, and the rubber blanket is allowed to slowly contract to a relaxed position. Since the fabric cannot slide on the blanket, the force generated by the relaxation of the blanket causes the fabric structure to compress upon itself, or compact. This compaction is illustrated in the closeup of the stretched blanket, fabric, and heated cylinder in the animation. The compaction of the fabric is illustrated by the movement of the arrows. Notice that the fabric gets thicker as compaction takes place.

Once the fabric is compacted, great care must be taken to control takeoff and windup tensions. The compaction effect can be easily pulled out of the fabric if it is stretched during any subsequent processing or storage, including cut and sew.


Slide 24

Tamdem Blanket Compactor

The purpose of compaction is to preshrink the fabric prior to cut and sew. Once the item is washed, the compaction relaxes out of the fabric. However, the fabric will normally shrink during drying back to the approximate dimensions of the compacted fabric. Therefore, the consumer does not realize any appreciable change in the dimensions of the item.

With some fabric styles, the inherent shrinkage potential is too great to correct with a single run through a belt compactor. This is especially true with some knitted fabric styles. For these products, two passes through a belt compactor are required to achieve an acceptable compacting effect. The tandem belt compactor was designed to achieve the desired effect on these types of fabrics in one continuous run through the compacting machine. It is, quite simply, two belt compactors placed in position so that the fabric can be compacted twice in a single pass through the machine. All the other factors and precautions necessary for the other types of compactors apply to this machine. However, great care must be taken so that the operating speeds of these two belt compactors are exactly in sync. If there is any difference in the speed of the two units, tension will be created on the fabric that will diminish the compacting effect or distort the fabric. This compactor is normally used for specific knit fabrics with high shrinkage potential.


Slide 25

Relaxation Drying Overview

Purpose

To minimize shrinkage in knitted fabric during drying by overfeeding fabric into dryer, minimizing tension, and allowing fabric to preshrink during the drying process.

Factors Affecting Shrinkage During Drying

Mechanical Action

Overfeed

Spreading

Moisture Content

Temperature

Relaxation drying is a process used to help minimize shrinkage of knitted fabrics. These are continuous machines used for large production volumes. During the drying process, the wet knitted fabric is allowed to relax out any stretch or strain placed into it during wet processing operations. A number of different designs for the relaxation drying of knit fabrics exist, but the most popular and most successful are the conveyer belt relaxation dryers.

Several key parameters must be controlled to minimize shrinkage and distortion during the wet processing of knitted fabrics. The mechanical action necessary to achieve desired results from the preparation and dyeing operations builds stress into the knitted fabric structure. Additionally, the tension required to transport the wet fabric from one process to another builds additional stress into the fabric that is aggravated by the pull of gravity on the wet fabric. All of this processing stress leads to high final fabric shrinkage if not corrected.

One way to correct this added shrinkage due to wet processing is to use relaxation drying. During the drying of the fabric, stress is allowed to relax out of the fabric. The removal of this stress is accomplished by overfeeding the fabric onto a continuous conveyer belt that carries the fabric into the drying oven. Overfeeding is the mechanical technique of feeding fabric into the dryer faster than it is taken out. Since this knitted fabric is not held tightly in place, overfeeding does not compact the fabric but allows the knit loops to rearrange or relax back to their original knitted configuration. The presence of moisture in the fabric also lubricates the yarns forming the loops and enhances the relaxation effect. This drying process does not remove all shrinkage potential from the fabric, but it does restore it to a stable dimension and stable knitted loop configuration. After this process, additional shrinkage potential can be removed from the fabric by using the compacting process.

Key factors to monitor and control during relaxation drying include the mechanical action of the dryer, the amount of overfeed, the spreading of tubular knits prior to entering the dryer, the entering moisture content of the fabric, the temperature of the dryer, the consistency of temperature inside the dryer, and the presence of any fabric softeners on the wet fabric.


Slide 26

Tensionless Dryer

The most popular type of relaxation dryer is known as the tensionless conveyer dryer. The diagram represents the basic components of the relaxation dryer. The dryer housing is not shown but is represented by the air recirculation units drawn behind the conveyer belt of the dryer. The wet fabric is overfed onto the conveyer belt. During the drying process, the fabric forms a series of waves that are similar in appearance to sine waves. This mechanical action on the fabric is brought about by alternating the direction of the hot drying air within the dryer housing. This action can be enhanced by simultaneously vibrating the conveyer belt. The combination of the overfeeding and mechanical action of the wave formation allows the loops of the knitted fabric to rearrange and relax out the stress that was placed in the fabric during wet processing. The relaxation process can be enhanced by the use of proper amounts of fabric softener added to the wet fabric prior to relaxation drying. The diagram shows that as drying proceeds, the stretch or strain in the fabric relaxes out, and there is a reduction in the overall fabric yardage. Proper control of any windup or takeoff tension is a major key to the success of the relaxation drying operation.

Relaxation drying was designed originally for cotton tubular knit fabrics. Once dried, these fabrics are normally processed through a compactor to minimize shrinkage potential. These two processes have been most commonly used for cotton fabrics that are not normally chemically finished with permanent press resins. In recent years, some textile finishers have used relaxation drying for certain loosely woven fabrics as well as for some nonconventional or nonwoven fabric styles.

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Mechanical Finishing

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