Fabric and garment finishing : Basic washes in denim fabric


By : Noopur Shalini

NIFT, Hyderabad

THE HISTORY OF DENIMS

A popular conception of the etymology of the word denim is that it is a contraction or derivative of the French term, serge de Nmes. Denim was traditionally colored blue with indigo dye to make blue “jeans,” though “jean” then denoted a different, lighter cotton textile; the contemporary use of jean comes from the French word for Genoa, Italy (Gnes), from which the first denim trousers were made.

A similarly woven traditional American cotton textile is the diagonal warp-striped hickory cloth that was once associated with railroad mens overalls, in which blue or black contrasting with undyed white threads form the woven pattern. Hickory cloth was characterized as being as rugged as hickory woodnot to mention the fact that it was deemed to be worn mainly by “hicks”although neither may be the origin of that term [from a nickname for “Richard”]. Records of a group of New Yorkers headed for the California gold fields in 1849 show that they took along four “hickory shirts” apiece. Hickory cloth would later furnish the material for some “fatigue” pantaloons and shirts in the American Civil War.

clip_image001INTRODUCTION

Denim is a rugged cotton twill textile, in which the weft passes under two (twi- “double”) or more warp fibers, producing the familiar diagonal ribbing identifiable on the reverse of the fabric.

DENIM WASHING

Denim washing is the aesthetic finish given to the denim fabric to enhance the appeal and to provide strength.

Dry denim, as opposed to washed denim, is a denim fabric that is not washed after being dyed during its production.

Much of the appeal of dry denim lies in the fact that with time the fabric will fade in a manner similar to that which artificially distressed denim attempts to replicate. With dry denim, however, such fading is affected by the body of the person who wears the jeans and the activities of their daily life. This creates what many feel to be a more natural, unique look than pre-distressed denim.

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DENIM WASHES ARE OF TWO TYPES:

clip_image0031. Mechanical washes

  • Stone wash
  • Microsanding

2. Chemical washes

CHEMICAL WASHES

Denim bleach

In this process a strong oxidative bleaching agent such as sodium hypochlorite or KMnO4 is added during the washing with or without stone addition.

Discoloration produced is usually more apparent depending on strength of the bleach liquor quantity, temperature and treatment time.

It is preferable to have strong bleach with short treatment time.

Care should be taken for the bleached goods so that they should be adequately antichlored or after washed with peroxide to minimize yellowing. Materials should be carefully sorted before processing for color uniformity.

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Process cycle:

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Limitations:

– Process is difficult to control i.e. difficult to reach the same level of bleaching in repeated runs.

  • When desired level of bleaching reached the time span available to stop the bleaching is very narrow. Due to harshness of chemical, it may cause damage to cellulose resulting in severe strength losses and/or breaks or pinholes at the seam, pocket, etc.
  • Harmful to human health and causes corrosion to stainless steel.
  • Required antichlor treatment.

Problem of yellowing is very frequent due to residual chlorine.

Chlorinated organic substances occur as abundant products in bleaching, and pass into the effluent where they cause severe environmental pollution.

Enzyme Wash

It is environmentally friendly wash. It involves the Application of organic enzymes that eat away at the fabric, i.e. the cellulose.

When the desired color is achieved, the enzymes can be stopped by changing the alkalinity of the bath or its temperature. Post treatment includes final rinsing and softening cycle. The effects produced by the cellulose enzyme are—

  1. Use of cellulase making the seams, hems, and pockets more noticeable
  2. Salt pepper effect is color contrast effect.
  3. Faded garment with acid cellulase enzyme provides less color contrast in proportion to garment washed with neutral cellulase enzymes.

Garment load size of the machine is 35-40 jeans per machine and it cannot be overloaded.

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Acid wash

It is done by tumbling the garments with pumice stones presoaked in a solution of sodium hypochlorite or potassium permanganate for localized bleaching resulting in a non uniform sharp blue/white contrast.

In this wash the color contrast of the denim fabric can be enhanced by optical brightening. The advantage of this process is that it saves water as addition of water is not required.

Process cycle

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Limitations of acid wash:

– Acid washed, indigo dyed denim has a tendency to yellow after wet processing.

– The major cause is residual manganese due to incomplete neutralization, washing or rinsing.

Remedy:

  • Manganese is effectively removed during laundering with addition of ethelene-diamine-tetra-acetic acid as chelating agent.
  • Acid washing jeans avoided some of problems of stone wash, but came with added dangers, expenses, and pollution.

MECHANICAL WASHES

Stone wash:

In the process of stone washing, freshly dyed jeans are loaded into large washing machines and tumbled with pumice stones to achieve a soft hand and desirable look.

Variations in composition, hardness, size shape and porosity make these stones multifunctional. The process is quite expensive and requires high capital investment.

Pumice stones give the additional effect of a faded or worn look as it abrades the surface of the jeans like sandpaper, removing some dye particles from the surfaces of the yarn.

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Process cycle:

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Selection of stone

Stone should be selected of the proper hardness, shape, and size for the particular end product. It should be noted that large, hard stones last longer and may be suited for heavy weight fabrics only.

Smaller, softer stones would be used for light weight fabrics and more delicate items.

Stone wt. /fabric wt. = 0.5 to 3 /1

It depends on the degree of abrasion needed to achieve the desired result. Stones can be reused until they completely disintegrate or washed down the drain.

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Problems caused by stones:

  • Damage to wash machineries and garment due to stone to machine and machine to stone abrasion
  • Increase in labor to remove dust from finished garments.
  • Water pollution during disposal of used liquor.
  • Back staining and re deposition.

Back staining or Re-deposition:

The dye removed from denim material after the treatment with cellulose or by a conventional washing process may cause “back staining or “redeposition. Re-coloration of blue threads and blue coloration of white threads, resulting in less contrast between blue and white threads.

Remedy of back staining —

  • Adding dispersion/suspension agent to wash cycle.
  • Intermediate replacement of wash liquor.
  • Using alkaline detergent like sodium per borate with optical brightener as after wash.

Limitations of stone washing:

  • Quality of the abrasion process is difficult to control Outcome of a load of jeans is never uniform, little percentage always getting ruined by too much abrasion.
  • The process is non-selective.
  • Metal buttons and rivets on the jeans in the washing machines get abraded.
  • This reduces quality of the products and life of equipment, and increases production costs.
  • Stones may turn into powder during the process of making the garment grayish in color and rough too
  • Provides rougher feel than enzyme wash
  • Stone may lead the harm to the machine parts

Microsanding

There are 3 ways for this technique:

  1. Sandblasting
  2. Machine sanding
  3. Hand sanding or hand brushing

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Used in various ways:

  • Flat surfaces (tables, ironing boards)
  • On the dummy (inflatable dummies, sometimes standing, sometimes flat, sometimes ‘seated’)
  • Various templates can be used to create a 3D effect.

SAND BLASTING

Sand blasting technique is based on blasting an abrasive material in granular, powdered or other form through a nozzle at very high speed and pressure onto specific areas of the garment surface to be treated to give the desired distressed/ abraded/used look.

  • It is purely mechanical process, not using any chemicals.
  • It is a water free process therefore no drying required.
  • Variety of distressed or abraded looks possible.
  • Any number of designs could be created by special techniques.

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WHISKERING

  • Also known as Cat’s Whiskers
  •  Crease lines around the crotch.
  • Industrially done with laser, sandblasting, machine sanding, hand sanding and abrasive rods.
  • Also used for ‘knee whiskers’ (whiskers on the sides of knees) and ‘honeycombs’ (crease marks on the back of the knee)

Other chemical washes:

  • Rinse wash
  • Cellulase wash
  • Ozone fading
  • Snow wash
  • Salt water denim
  • Flat finish
  • Over dye
  • Sun washing
  • Super dark stone

RINSE WASH

– Chemically bleaching jeans so that the color fades away

  • Breaks down the fibers of jeans and creates white streaks or spots on denim
  • Gives a unique rugged look, also called snow wash
  • Earlier involved the use of pumice stone
  • Presently process involves spraying chemical and removing it immediately
  • Come in colors like blue, black, green, brown, grey etc.

CELLULASE WASH

  • This is done to achieve a wash down appearance without the use of stones or with reduced quantities of stones.
  • Cellulase enzymes are selective only to the cellulose and will not degrade starch.
  • Under certain conditions, their ability to react with cellulose (cotton) will result in surface fiber removal (weight loss).
  • This will give the garments a washed appearance and soft hand.

Factors influencing cellulase performance

  • pH
  • Temperature
  • Time
  • Dose
  • Mechanical action

OZONE FADING

  • By using this technique, the garment can be bleached.
  • Bleaching of denim garment is done in washing machine with ozone dissolved in water.
  • Denim garments can also be bleached or faded by using ozone gas in closed chamber.
  • In the presence of UV light, there is an interaction between the hydrocarbons, oxides of nitrogen and oxygen that causes release of ozone.
  • Indigo dyestuff tends to fade or turn yellow due to ozone reaction.

The advantages associated with this process are:

  • Color removal is possible without losing strength.
  • This method is very simple and environmentally friendly because after laundering, ozonized water can easily be deozonized by UV radiation.

FLAT FINISH

It is a special process done to impart fabric with an even wash down effect and very clean surface. Originally liquid ammonia was used, but now use mercerization plus calendering processes to achieve the flat surface.

Mercerization swells up the cotton fibers and allows the calendering to press flat the surface.

They consider this as an imitation process to the use of ammonia, which is toxic and not allowed in commercial use in most countries

OVERDYE

  • Dyeing over the fabric or jeans to add another tone of color
  • Most often used is a ‘yellowy’ overdye to create a ‘dirty’ look
  • Also can be applied with spray gun or paintbrush for local coloring

SUNWASHING

  • A very light shade by bleaching and stoning
  • Looks as if the sun faded the fabric

SUPER DARK STONE

  • Commercial term for an extra dark indigo color
  • Results from a double-dyeing technique

SNOW WASH DENIM

Denim treated with a variation of acid wash that imparts bright white highlights.

QUICK WASH DENIM

  • Aims at minimizing wash cycle time
  • Results in more economical washes and solving many other washing problems faced by launderes during fashion wash cycles
  • The yarns are ring dyed using indigo giving 25 to 30% less fixed dye to obtain a given shade
  • During wash cycle,indigo dye can be removed quickly,giving washed look

clip_image014Advantages of quick wash denim

1. Streaks develop in garments after washing process due to differences in dye concentration of denim fabrics are avoided using a modified alkali-ph controlled system giving uniformity of shade.

2. Amount of indigo dye required is less thus making it an economical process

3. Time required for washing is 20-30% less than that required for conventional denim.

4. Lesser enzymes and oxidising agent used

5. Environment friendly process

6. Back staining is minimised due to less concentration of of indigo dye in the wash liqour.

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Other Mechanical washing

  • Whiskering
  • Shot gun denim
  • Water jet fading
  • Super stone wash
  • Ice wash
  • Thermo denim
  • Laser technology finish

WATERJET FADING

  • Hydrojet treatment is used for enhancing the surface finish, texture, durability of denim garment.
  • Hydroject treatment involves exposing one or both surfaces of the garment through hydrojet nozzles.
  • The degree of colour washout, clarity of patterns, and softness of the resulting fabric are related to the type of dye in the fabric and the amount and manner of fluid impact energy applied to the fabric.
  • As this process is not involved with any chemical, it is pollution free.

LASER TECHNOLOGY

  • It is a computer controlled process for denim fading.
  • This technique enables patterns to be created such as lines and/or dots, images, text or even pictures.
  • It is water free fading of denim.
  • Being an automatic system, chances of human error are slim.
  • Also called spray painting in denims.
  • This technique has relatively high cost.

SUPER STONEWASH

  • Prolonged stonewashing, up to six hours or more.

ICE WASH

  • Ice washing in denim fabrics is done to remove more than half the dye during washing

THERMO-DENIM

  • Also called double denim. A lightweight fabric (either plain, fancy or colored) is glued to the denim. The glue comes off after washing and the trousers look like they’ve been lined

VINTAGE

  • Applies heavy stonewashing or a cellulose enzyme wash, with or without bleach
  • Gives an old and worn look

CHEMICALS ON DENIMS

1. Bleach fast Indigo

  • Value addition to denim
  • Retains indigo on certain parts
  • Kind of resist effect
  • Chemical applied by brush, cured at 150C
  • Ex. Indigofix AXN

2. Anti-depositing agent

  • Prevents back staining of fabric by loose indigo during washing
  • Improves contrast in denim
  • Used in stone wash step

3. Dye stuffs with softener

  • – To carry dyeing and softening in one step
  • – Soft and supple hand
  • – Saves time, money and energy as added to final rinse
  • – Gives used and worn out effect

4. Anti creasing agent

  • Provides fabric to fabric lubrication
  • Prevents formation of crack marks and streaks
  • Minimizes abrasion and gives strength

5. Wrinkle formation

  • Creating smooth and permanent wrinkle
  • Cross linking concept
  • Ex. DMDHEU
  • White pigment
  • Can be applied by brush, spray or screen
  • Then cured at 150C
  • Washed and treated with softener

6. White pigment

  • Can be applied by brush, spray or screen
  • Then cured at 150C
  • Washed and treated with softener

CONCLUSION

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Denim is unique in its singular connection with one colour. The warp yarn is traditionally dyed with the blue pigment obtained from indigo dye. Until the introduction of synthetic dyes, at the end of the 19th century, indigo was the most significant natural dye known to mankind, linked with practical fabrics and work clothing. The durability of indigo as a color and it’s darkness of tone made it a good choice, when frequent washing was not possible.

The old mass market has segmented, fragmented, shattered into a multitude of mini, micro and niche markets. The last generation has a vast quantity of brands to choose from, a different perception of the cult value of owning small insider labels and a fanatical loyalty only to what’s hot on a daily basis.

Freed of all social and creative restrictions, denim is assuming any number of disguises and contexts to be worn in and has broken through almost any limitation on price. It can also be found in home collections, appearing in cushions, bed spreads and furniture-coverings.

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Magnetic Ring-Spinning-Revolutionizing the Tradition


BY:- Faissal Abdel-Hady, leader; Yehia El Mogahzy
(Auburn Textile Engineering)

“By replacing the traveler in ring spinning with a disc that rotates in a magnetic field, we hope to maintain the high quality of ring spun yarn, but at much higher speeds.”

In today’s spinning technology, at least 4 types of spinning systems are commercially available. These are the tradi-tional ring spinning, rotor spinning, air-jet spinning and friction spinning. Among them, ring spinning stands alone in providing high quality yarn suitable for any type of tex-tile end product. Other more recent systems enjoy much higher production speed than traditional ring spinning, but yarn quality restricts their use to only narrow ranges of tex-tile products. The primary technological limitation of ring spinning lies in the speed of the ring-traveler system. The traveler is a C-shaped thin piece of metal that is used for a limited period of time, disposed and replaced on a frequent basis. Three specific issues must be addressed to overcome this limitation:image
Close View of Magnetically Suspended Spinning Ring
• the dependence of the yarn linear speed (or delivery speed) on the rotational speed of the traveler
• the continuous need to stabilize yarn tension during spinning and the dependence of this stability on the traveler speed
• the impact of traveler speed on fiber behavior in the spinning triangle
Research to date has only provided about a 15% improve-ment in traveler speed without affecting the traveler/ring contact thermal load capacity. Ring spinning is still at a production rate disadvantage of 15 to 20 times in compari-son with other spinning systems. Therefore, the challenge is how to break the traditional paradigm of ring spinning and revolutionize its principle in such a way that very high speeds can be achieved without sacrificing the traditional quality of ring spun yarns.
Our design approach is to totally eliminate the traveler from the ring spinning system and replace it with a mag-netically suspended lightweight annular disc that rotates in a carefully pre-defined magnetic field (See Figure below). By creating a non-touching environment of the rotating element for ring spinning, this system provides super high spinning rotation without the limitations of the current trav-eler system.

image

In the magnetic ring spinning system a bias flux is gen-erated from both permanent magnets across the air gap (shown in blue paths in Figure above), supporting the weight of the rotating disk in the axial direction. In case the floating ring is displaced from its central position, the permanent magnets will create a destabilizing force that at-tracts the ring even further away from the center. The con-trol system allows the current in the system to be controlled by feeding back information on the position of the rotor (obtained using four displacement sensors mounted radially to the floating ring) and adjusting the control currents based on this information. In simple terms, the control system re-duces the upper system current when the rotor is above the center position and increases the current when the rotor is below the center position. The total magnetic force will tend to bring the floating ring to its central position. We have now constructed the first prototype (See Photo below) and are optimizing its performance. The system was mod-eled using Simulink, to test how it performs. And the yarn balloon was analyzed using this model.

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PROCESSING STICKY COTTON


In spinning mills, sticky cotton can cause serious problems. It contaminates the textile machineries like blow room , card, drawing, roving, and spinning frames. These contaminants are mainly sugar deposits produced either by the cotton plant itself (physiological sugars) or by feeding insects (entomological sugars), the latter being the most common source of stickiness.
Seventeen mixes having a moderate level of stickiness were evaluated in both ring and rotor spinning. High-performance liquid chromatography tests were performed on residues collected from the textile machinery to identify the types of sugars present. It was shown that among the sugars identified on raw fiber, only trehalulose exhibits higher percentages in the residues than on the fiber. During the fibers-to-yarn transformation, the flow of lint is submitted to different friction forces; consequently, the temperature of some mechanical elements may increase significantly and affect the thermal properties of the contaminated lint. After a sugar becomes sticky, the other sugars present on the lint, as well as other substances such as dusts, silica, etc., will stick to the lint and could cause unevenness in the flow of lint being drawn, such as lapping up on the rolls, nep-like structures, and ends-down….

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Effect of Weft parameters on Weaving Performance and Fabric Properties


Abstract:

Threads per inch and yarn count are some of the most important parameters that affect both weaving performance and fabric property.Experimental studies were conducted by weaving fabrics with three different picks per inch (PPI)and weft counts. The study shows that weaving performance is affected by the too high cover factor. Cover factor was calculated by dividing the threads/inch by the square root of the English cotton count and end breakage was taken as an indication of weaving performance. It was observed that when the count as well as threads/inch of one series of yarn changes the crimp% i.e. the consumption of both series of yarns are affected. It was also observed that, as expected, when the threads/inch increases the fabric strength also increases but at higher threads/inch the gain in strength is relatively more.

Keywords: Threads, cover factor, EPI, PPI,crimp, strength, weaving performance.

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Terry Weaving-PAPER


Terry Towel:
A terry towel is described as a textile product which is made with loop pile on one or both sides generally covering the entire surface or forming strips, checks, or other patterns (with end hems or fringes and side hems or selvages)

History of Terry Weaving:
The name “terry” comes from the French word “tirer” which means to pull out, referring to the pile loops which were pulled out by hand to make absorbent traditional Turkish towelling. Latin “vellus”, meaning hair, has the derivation “velour”, which is the towelling with cut loops. In research conducted on terry weaving by the Manchester Textile Institute, it was concluded that original terry weaving was likely the result of defective weaving. The research indicates that this development occurred in Turkey, probably in Bursa City, one of the major traditional textile centres in Turkey. Terry weaving construction is considered a later development in the evolution of woven fabrics. Terry towelling is still known as “Turk Fabric”, “Turkish Towelling” or “Turkish Terry”

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Feature based 3D garment design through 2D sketches


Charlie C.L.Wang* Yu Wang Matthew M.F.Yuen
Department of Mechanical Engineering, Hong Kong University of Science and Technology,
Clear Water Bay, Kowloon, Hong Kong

 

Abstract
This paper presents a new approach for intuitively modelling a 3D garment around a 3D human model by 2D
sketches input. Our approach is feature based – every human model has pre-defined features, and the
constructed garments are related to the features on human models. Firstly, a feature template for creating a
customized 3D garment is defined according to the features on a human model; secondly, the profiles of the 3D
garment are specified through 2D sketches; finally, a smooth mesh surface interpolating the specified profiles is
constructed by a modified variational subdivision scheme. The result mesh surface can be cut and flattened into
2D patterns to be manufactured. Our approach provides a 3D design tool to create garment patterns directly in
the 3D space through 2D strokes, which is a characteristic not available in other computer aided garment design
systems. The constructed garment patterns are related to the features on a human model, so the patterns can be
regenerated automatically when creating the same style of garment for other human models. Our technique can
greatly improve the efficiency and the quality of pattern making in the garment industry.

Keywords: sketched input, 2D strokes, 3D design, computer-aided design, garment industry

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Quality Control Aspects of Garment Exports


  • Introduction

For every industry or business, to get increased sales and better name amongst consumers and fellow companies it is important to maintain a level of quality. Especially for the businesses engaged in export business has to sustain a high level of quality to ensure better business globally. Generally quality control standards for export are set strictly, as this business is also holds the prestige of the country, whose company is doing the export. Export houses earn foreign exchange for the country, so it becomes mandatory to have good quality control of their products. In the garment industry quality control is practiced right from the initial stage of sourcing raw materials to the stage of final finished garment. For textile and apparel industry product quality is calculated in terms of quality and standard of fibres, yarns, fabric construction, colour fastness, surface designs and the final finished garment products. However quality
expectations for export are related to the type of customer segments and the retail outlets

There are a number of factors on which quality fitness of garment industry is based such as – performance, reliability, durability, visual and perceived quality of the garment. Quality needs to be defined in terms of a particular framework of cost. The national regulatory quality certification and international quality programmes like ISO 9000 series lay down the broad quality parameters based on which companies maintain the export quality in the garment and apparel industry. Here some of main fabric properties that are taken into consideration for garment manufacturing for export basis:
• Overall look of the garment.
• Right formation of the garment.
• Feel and fall of the garment.
Physical properties.
• Colour fastness of the garment.
Finishing properties
• Presentation of the final produced garment.

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Technical Textiles in India – A dormant volcano prepares to erupt…


ABSTRACT:-

India is rising and moving ahead with opportunities in every sector. For the past four years India’s GDP has grown up to 8%, and is assumed to remain consistent at 8-9% for coming years. According to Goldman Sachs, India’s economy will exceed the economy of Europe and Japan by 2030 and that of the US by 2045.Such a growth is possible because of the increase in household incomes and the predicted growth in agriculture, manufacturing and service sectors. Also the consumer spending level is growing over 5% per annum which has resulted in the on-going growth of organized retail sectors.

Talking about the technical textile industry in India, it is said to be its initial stage as it contributes only 3% of total consumption. But, it would be wrong to say that India’s technical textile industry is still sleeping. It has woken up to the enormous potential of the technical textile sector and is predicted to grow faster in next two decades than the growth withstand by US and Europe in last three decades. This is said to become possible with the growing middle class, young and educated population. And Technical Textile would be one of the most promising sectors in this growth.

And the factors like, the global economic change, strong government support, the introduction of appropriate legislation, the development of tests and standards, and widespread recognition of the need for more trained personnel, etc. also playing the valuable role in driving the industry to the farthest destination. Thus it won’t be wrong to say that, “Technical Textiles in India- A sleeping volcano prepares to erupt.”

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NEEDLE PUNCHING TECHNOLOGY


Abstract:
Needle punching is the oldest method of producing nonwoven products. The first needle punching loom in U.S. was made by James Hunter machine co. in 1948. Then in 1957, James Hunter produced the first high speed needle loom, the Hunter model 8 which is still used today.

The needle punching system is used to bond dry laid and spun laid webs. The needle punched fabrics are produced when barbed needles are pushed through a fibrous web forcing some fibres through the web, where they remain when the needles are withdrawn. If sufficient fibers are suitably displaced the web is converted into a fabric by the consolidating effect of these fibres plugs or tufts. This action occurs in needle punching occurs around 2000 times a minute.

Needle punched fabrics finds its applications as blankets, shoe linings, paper makers felts, coverings, heat and sound insulation, medical fabrics, filters and geotextiles.

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Optical Fibers


  • Introduction to Optical Fibresclip_image001

Optical Fibres are thin rods of glass wrapped in a low density plastic and cabling. In modern days, it is used as an instrument in micro surgery to project images from inside the body and help surgeons see in hard to reach places. It is also used widely in communications, both in computer networks as a fast Internet connection source and in telecommunications both transcontinentally and transoceanically.

We’ll now discuss some concept that are relevant to Optical Fibres, starting with Reflection and Refraction.

  • Reflection and Refraction

The concepts of reflection and refraction are very important in the design of Optical Fibres and has a key part in how Optical fibres work. The law of reflection states that a beam of light striking a flat surface (the incidence ray) would be reflected at the same angle at the normal, the medium dividing the incidence ray and the reflective ray and is perpendicular to the surface of reflection. That would also mean that the angle of incidence (from the incidence ray to the normal) is also equivalent to the angle of reflection (from the reflective ray to the normal). Refer to Diagram 1.

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Diagram 1

Snell’s Law, named after the Dutch mathematician Willebrord van Roijen Snell, states that the product of the refractive index and the sine of the angle of incidence of a ray in one medium is equal to the product of the refractive index and the sine of the angle of refraction in a successive medium. This can be represented algebraically by n1 sinclip_image0031 = n2 sinclip_image003[1]2, where n1, n2 are the two values of refractive index and clip_image003[2]1,clip_image003[3]2 are the angles of the incidence and refraction. Refer to Diagram 2.

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Diagram 2

We shall now move on to Critical Angles.

  • Critical Angles

Generally, the refractive index of a denser transparent substance is higher than that of a less dense material, which would mean that light would travel slower in the substance. A general rule is that when a ray travels from a medium to another with a higher refractive index, it’ll bend towards the normal. When a ray travels from a medium to another with a lower refractive index, it’ll bend away from the normal.

Due to this fact, as the angle of incidence of a ray increases in a medium with a higher refractive index, the ray passing through to a medium with lower refractive index will bend away from the normal until it meets 90 deg. with the normal, and on the boundary between the two mediums, which is then referred to as the critical angle. Refer to Diagram 3.

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Diagram 3

Next we’ll be looking at the physics behind Optical Fibres.

Physics in Optical Fibres

Optical Fibres relies heavily on two concepts of physics, the concepts of refraction, refractive indexes, critical angles and Total Internal Reflection. The concepts of refraction states that a ray of light travelling from a medium with a higher refractive index to a medium with a lower refractive index would bend away from the normal. With this in mind, it also states that a critical angle would be reached when the ray of light increases to an angle that will bend it 90 deg. away from the normal. The concept of Total Internal Reflection is apparent when the ray of light travelling from a higher refractive index medium to a lower refractive index medium has an angle so great it is able to refract the light greater than the critical angle, resulting in the ray reflecting back into the first high refractive index medium.

The main function for Optical Fibres is to send information through it by transmitting a beam of light from one end to another, trying to have as little quality loss as possible. The design of modern Optical Fibres reflect this and relies much on the physics involved.

Optical Fibres are more commonly used in surgery as a means of a tiny camera for surgeons in a patient’s body and used together in their thousands in Fibre Optic communication cables for telephones and high-speed Internet connections. Each Optical Fibres has a core of thin glass, which is wrapped then by a cladding (usually a plastic with a lower refractive index than glass) and is then finally all wrapped in a harder plastic buffer to prevent damage. Refer to Diagram 4.

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Diagram 4

When transmitting a signal, an L.E.D. or laser is used to send a beam of light in on/off pulses (digital) down the Optical Fibre to be received on the other end. It is either transmitted straight down the Optical Fibre, resulting in longer distance travel, or it is transmitted at an angle which is calculated to be greater than critical angle of the glass to the plastic, resulting in Total Internal Reflection and the ability for the beam of light to turn corners. Refer to Diagram 5.

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Diagram 5

The angle of transmission of the beam into the glass is usually greater than 82 deg, which is required to achieve Total Internal Reflection when the light hits the plastic cladding. If the angle is less than 82 deg, the beam of light would be refracted out of the Optical Fibre. Refer to Diagram 6.

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Diagram 6

That concludes the project on Optical Fibres, and other relevant information. To find out more, go to the next section ‘Relevant Links’.

  • Relative Links

Here are some relevant links to provide you with more information on Optical Fibres and related material.

HowStuffWorks – comprehensive site on all you need to know about Optical Fibres, along with other interesting things.
Corning Optical Fibres – discovery centre with animated explanations.
What are Optical Fibres made of? – detailed site on what optical fibres are made of.
Light and Optics: Arizona State University – brief explanation of how optical fibres work.