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PRINTING

Printing is a process of decorating textile fabrics by application of pigments, dyes, or other related materials in the form of patterns. Although apparently developed from the hand painting of fabrics, such methods are also of great antiquity. There is evidence of printing being carried out in India during the 4th century BCE, and a printing block dated at about 300 CE has been unearthed in the burial grounds of Akhmīn in Upper Egypt. Pre-Columbian printed textiles have been found in Peru and Mexico.

PRINTING METHODS

The four main methods of textile printing are block, roller, screen, and heat transfer printing. In each of these methods, the application of the colour, usually as a thickened paste, is followed by fixation, usually by steaming or heating, and then removal of excess colour by washing.

BLOCK PRINTING

Block printing is known to have been used in India since at least the 12th century, although this method is thought to be around 2,000 years old.

Indian artisans borrowed a technique from China, one theory states, and turned it into a culturally distinct art form. Different types of dye and patterns became synonymous with different regions of the country. Over the centuries some portions of the process were changed or improved. Sometimes it is even eventually replaced by screen printing, but many small workshops hold true to traditional block printing.

The process

Wood blocks are typically prepared as a relief pattern, motifs carved carefully by hands using sharp knives or chisels. The patterns are carved in a way that each impression specifies where the next stamp must take place ensuring clean and even efforts. Small holes are made across the height of the block to prevent air pockets and ensure even impressions. Ready blocks are then treated to an oil bath in order to prevent pigments from permeating deep. Two to three weeks into the bath, a block is ready for printing. Each block has to be pressed firmly onto the cloth so that the print can take effect at one go. Multiple impressions of the same dye are avoided to ensure consistent impressions.

But before a fabric can be printed, it's cleaned and bleached to remove impurities and ensure that it takes the dye evenly. In traditional facilities, use of camel dung, soda ash, and castor oil is still prevalent for treating the fabric, the process repeated multiple times until the desired state of the cloth is reached.

2. BRUSHING
3. ROLLER

This technique is used whenever long runs of fabric are to be printed with the same design. The modern machine, based on one originally devised in 1783, consists of a large central cast-iron cylinder over which passes a thick endless blanket providing a resilient support for the fabric. Backing fabrics, called back grays, are placed between the blanket and the fabric to prevent undue staining of the blanket. Although formerly made of cotton fabric, most modern back grays are continuous belts of nylon. The blanket and back gray are appropriately tensioned, so that the fabric moves through the machine as the central cylinder rotates. Engraved printing rollers, one for each colour, press against the fabric and the central cylinder. The pattern on the roller is etched on the surface of a copper shell supported on a mandrel. High-quality engraving is essential for good printing. Each printing roller is provided with a rotating colour-furnishing roller, partially immersed in a trough of printing paste. Finely ground blades (doctor blades) remove excess colour paste from the un-engraved areas of these rollers, and each also has a lint blade. The printed fabric passes from the main cylinder and through a drying and steaming chamber to fix the colour. Although this machine prints only one side of the fabric, the Duplex roller machine, essentially a combination of two roller machines, prints both sides. Modern printing machines are smooth-running precision machines fitted with carefully designed roller bearings and hydraulic or pneumatic mechanisms to ensure uniform pressure and flexibility. Pressure is regulated from an instrument panel, and each roller is controlled independently. Automatic registration is effected by electromagnetic push-button control, and modern electric motors provide smooth-running, variable-speed drives. The washing of back grays and printer’s blankets has also been automated.

Spray printing is the application of colour from spray guns through stencils and has limited but occasionally profitable use.

4. SCREEN

Screen printing may be a hand operation or an automatic machine process. The cloth is first laid on a printing table, gummed in position or pinned to a back gray, and then the design is applied through a screen made of silk or nylon gauze stretched over a wooden or metal frame, on which the design for one colour has been reproduced. This is usually a photographic process, although hand painting with a suitably resistant blocking paint is an alternative. A screen is placed over the fabric on the table against registration stops, ensuring accurate pattern fitting. Print paste is poured on to the screen edge nearest the operator and is spread with a squeegee over the surface of the screen so that colour is pushed through the open parts. The screen is moved until one colour has been applied to the cloth. For application of other colours, the process is repeated with different screens.

With the growing importance of screen printing, the hand operation has been largely replaced by mechanical methods. In some machines, the screens are flat, as in hand printing; others employ rotary screens.

5. HEAT TRANSFER PRINTING

The popularity of polyester fabrics led to the development of a completely new form of printing: heat transfer printing, which prints the pattern on paper with carefully selected dyes. The paper is then applied to the fabric by passing the two together through a type of hot calender, and the pattern is transferred from one to the other. This method opens up new possibilities, such as the production of halftone effects.

In all textile printing, the nature and, particularly, the viscosity of the print paste are important, and the thickeners employed must be compatible with all the other components. For conventional methods the thickeners are such reagents as starch, gum tragacanth, alginates, methyl cellulose ethers, and sodium carboxymethyl cellulose. Many types of dye can be applied, including direct cotton, vat, mordant, and reactive dyes, as well as pigment colours. Most dyes are fixed by steaming or aging, by a batch or continuous method, and more rapid fixation is effected by flash aging—e.g., allowing a shorter steaming period by employing smaller machines. After steaming, the fabric must be thoroughly washed to remove loose dye and thickener, ensuring fastness to rubbing.

Most textile materials can be printed without special pre-treatment, but wool cloths are generally chlorinated before printing. Tops (long, parallel wool fibres), printed in stripes, are used for mixed effects, and printed warps produce shadowy effects. Tufted carpets are printed by a process designed to ensure good penetration.

PRINTING STYLES

Printing styles are classified as direct, discharge, or resist.

In direct printing, coloured pastes are printed directly on the cloth. The print paste essentially consists of Pigment colour, Binder, Thickener and fixer.After printing the water from print paste at the printed portion is evaporated during drying. After the evaporation of water the pigment colour, binder and thickener are present on the fibre in the form of design.

For discharge printing, the cloth is first dyed with a background colour, which is destroyed by reagents, or reducing agents, carried in a print paste. This action may leave the discharged design white on a coloured background, although print pastes may also contain colouring matters not destroyed by the discharging agent, producing a coloured design.

In the resist process, the cloth is first printed with a substance called a resist, protecting these printed areas from accepting colour. When the cloth is dyed or pigment padded only those parts not printed with the resist are dyed. A special application of this technique, imparting plissé effects, is the printing of the fabric with a resist, followed by treatment with caustic soda.

DIGITAL PRINTING

Digital printing takes a different approach and assembles the images ready for print, from a complex set of numbers and formulas. These images are captured from pixels, and the digitalised image is used to control the deposition of ink, toner and exposure, to replicate the image you would like to print.

Introduction Digital printing is by far one of the most exciting developments in the textile industry. Not only does it open up endless opportunities for customization, small run printing, prototyping and experimentation but it also puts textile printing within the budget of your average illustrator.

Digital textile printing can reproduce unlimited colours and shades but - as with most forms of printing - what you see on screen is not necessarily what you get back.

DYEING

The Indian dyestuff industry plays a vital role in the economic development of the country. It is one of the core chemical industries in India. It is also the second highest export segment in the chemical industry. Maharashtra and Gujarat account for 90% of dyestuff production in India due to the availability of raw materials and dominance of textile industry in these regions. The major users of dyes in India are textiles, paper, plastics, printing ink and foodstuff industries. The textiles sector consumes around 80% of the total production due to high demand for polyester and cotton, globally.

India has been a home of dyes since a very long time. And as early as 2300-1750 BC, evidence being formed in the form of a piece of cotton dyed with madder - a vegetable dye having been found in the ruins of Mohenjo Daro.

Further testimony of the fact that dyes had been discovered early in India came from the Ajanta and Bagh paintings that date to the early Christian period. Paintings showed men in colored clothing, indicative of a progress made since the times of Mohenjo Daro.

Other colour shades and dyes like henna were very much on display during the Mughal period. Dyes like Indigo, Madder and Kermes were introduced to the world quite early by the Indian civilization.

There are two types of natural dyes. Adjective or additive dyes such as madder must use a mordant (a chemical that fixes a dye) to bond with fibers. These are the most common type and have been used for at least 2,000 years. Substantive dyes bond with a fiber without the use of a mordant or they contain tannin, a natural mordant. Examples of substantive dyes include safflower, cochineal, and black walnut. Mordants are chemical compounds that combine with the fiber and the dye forming a chemical bridge between the two. Madder, cochineal, and other commercial y important natural dyes are poly-chromic, meaning that they yield different colors with different mordants. Common mordants are weak organic acids, such as acetic or tannic acid, and metal salts including aluminum ammonium or potassium sulfate, ferrous sulfate, and copper sulfate. Usually, the textile to be dyed is simmered in a mordant solution before dyeing (pre-mordanting). Other options include adding the mordant to the dyebath or treating with another mordant after dyeing to shift the colour.

TYPES OF DYES

1. Acid Dyes

A class of dyes used on wool, other animal fibers, and some manufactured fibers. Acid dyes are seldom used on cotton or linen since this process requires a mordant. Acid dyes are widely used on nylon when high wash fastness is required. In some cases, even higher wash fastness can be obtained by after treatment with fixatives.

2. Natural Dyes

Direct Printing, it is the most common approach to apply a colour pattern onto a fabric. If done on coloured fabric, it is known as overprinting. The desired pattern is produced by pressing dye on the fabric in a paste form. To prepare the print paste, a thickening agent is added to a limited amount of water and dye is dissolved in it.

3. Basic (Cationic) Dyes

Basic dyes are water-soluble and are mainly used to dye acrylic fibers. They are mostly used with a mordant. A mordant is a chemical agent which is used to set dyes on fabrics by forming an insoluble compound with the dye. With mordant, basic dyes are used for cotton, linen, acetate, nylon, polyesters, acrylics and modacrylics. Other than acrylic, basic dyes are not very suitable for any other fiber as they are not fast to light, washing or perspiration.

4. Synthetic Dyes

Synthetic dyes are classified based upon their chemical composition and the method of their application in the dyeing process.

5. Direct (substantive) Dyes

Direct dyes colour cellulose fibers directly without the use of mordants. They are used for dyeing wool, silk, nylon, cotton, rayon etc. These dyes are not very bright and have poor fastness to washing although they are fairly fast to light.

6. Disperse Dyes

Disperse dyes are water insoluble. These dyes are finely ground and are available as a paste or a powder that gets dispersed in water. These particles dissolve in the fibers and impart colour to them. These dyes were originally developed for the dyeing of cellulose acetate but now they are used to dye nylon, cellulose triacetate, and acrylic fibers too.

7. Sulfur Dyes

Sulfur Dyes are insoluble and made soluble by the help of caustic soda and sodium sulfide. Dyeing is done at high temperature with large quantities of salt so that the colour penetrates into the fiber. After dyeing the fabric is oxidized for getting desired shades by exposure to air or by using chemicals. Excess dyes and chemicals are removed by thorough washing. These dyes are fast to light, washing and perspiration and are mostly used for cotton and linen.

8. Pigment Dyes

Although pigments are not dyes in a true sense, they are extensively used for colouring fabrics like cotton, wool and other manmade fibers due to their excellent light fastness. They do not have any affinity to the fibers and are affixed to the fabric with the help of resins. After dyeing, the fabrics are subjected to high temperatures.

9. Mordant Dyes

The mordant or chrome dyes are acidic in character. Sodium or potassium bi-chromate is used with them in the dyebath or after the process of dyeing is completed. This is done for getting the binding action of the chrome. They are mostly used for wool which gets a good colour fastness after treatment with mordant dyes. They are also used for cotton, linen, silk, rayon and nylon but are less effective for them.

10. Vat Dyes

Vat dyes are insoluble in water and cannot dye fibers directly. However, they can be made soluble by reduction in alkaline solution which allows them to affix to the textile fibers. Subsequent oxidation or exposure to air restore the dye to its insoluble form. Indigo is the original vat dye. These dyes are the fastest dyes for cotton, linen and rayon. They are used with mordants to dye other fabrics such as wool, nylon, polyesters, acrylics and modacrylics.

11. Reactive Dyes

Reactive dyes react with fiber molecules to form a chemical compound. These dyes, they are either applied from alkaline solution or from neutral solutions which are then alkalized in a separate process. Sometimes heat treatment is also used for developing different shades. After dyeing, the fabric is washed well with soap so as to remove any unfixed dye. Reactive dyes were originally used for cellulose fibers only but now their various types are used for wool, silk, nylon, acrylics and their blends as well.

12. Macromolecular Dyes

A group of inherently coloured polymers. They are useful both as polymers and as dyes with high colour yield. The chromophores fit the recognized CI classes, i.e., azo, anthraquinone, etc., although not all CI classes are represented. Used for mass dyeing, hair dyes, writing inks, etc.

13. Metallized Dyes

A class of dyes that have metals in their molecular structure. They are applied from an acid bath.

14. Naphthol Dyes

A type of azo compound formed on the fiber by first treating the fiber with a phenolic compound. The fiber is then immersed in a second solution containing a diazonuim salt that reacts with the phenilic compound to produce a coloured azo compound.

15. Pre-metallized Dyes

Acid dyes that are treated with coordinating metals such as chromium. This type of dye has much better wet fastness than regular acid dye. Pre-metallized dyes are used on nylon, silk, and wool.

16. Gel Dyeing

Passing a wet-spun fiber that is in the gel state (not yet at full crystallinity or orientation) through a dyebath containing dye with affinity for the fiber. This process provides good accessibility of the dye sites.

17. Developed Dyes

Dyes that are formed by the use of a developer. The substrate is first dyed in a neutral solution with a dye base, usually colourless. The dye is then diazotized with sodium nitrate and an acid and afterwards treated with a solution of B-naphthol, or a similar substance, which is the developer.

18. Azo Dyes

Dyes characterized by the presence of an azo group (-N=N-) as the chromophore. Azo dyes are found in many of the synthetic dye classes.

19. Aniline Dyes

Dyes derived chemically from aniline or other coal tar derivatives.

20. Anthraquinone Dyes

Dyes that have anthraquinone as their base and the carbonyl group (>C=O) as the chromophore. Anthraquinone-based dyes are found in most of the synthetic dye classes.

TECHNIQUES

1. Shibori Tie-Dye with Rubber Bands Fabric Dyeing Technique

2. Shibori Tie-Dye with Stitch Resist Fabric Dyeing Technique

3. Shibori Pole Dyeing Technique

4. Shibori Folding Technique

5. Ombre Dip Dye Technique

6. No-Wax Batik Technique

7. Low Water Immersion Fabric Dyeing Technique

8. Painting Fabric with Dye – Watercolor Technique

PRACTISED IN-HOUSE

1. SHIBORI TIE-DYE WITH STITCH RESIST

2. GRADIENT DYEING

3. WHOLE FABRIC ONE COLOUR DYEING