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June 19, 2013

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Summary

Printing

Fabrics are often printed with colour and patterns using a variety of techniques and machine types. It is the process of transferring colour, pattern, motif or decoration of one or more colours in any one of a variety of methods or techniques to fabric. It involves the surface application of colour in a predetermined pattern, design or motif by manual or mechanical directed discharge, direct or resist methods.

Printing

Fabrics are often printed with color and patterns using a variety of techniques and machine types. Of the numerous printing techniques, the most common is rotary screen. However, other methods, such as direct, discharge, resist, flat screen (semicontinuous), and roller printing are often used commercially. Pigments are used for about 75 to 85 percent of all printing operations, do not require washing steps, and generate little waste (Snowden-Swan, 1995). Compared to dyes, pigments are typically insoluble and have no affinity for the fibers. Resin binders are typically used to attach pigments to substrates. Solvents are used as vehicles for transporting the pigment and resin mixture to the substrate. The solvents then evaporate leaving a hard opaque coating. The major types of printing are described below.

Rotary screen printing

Rotary screen printing uses seamless cylindrical screens made of metal foil. The machine uses a rotary screen for each color. As the fabric is fed under uniform tension into the printer section of the machine, its back is usually coated with an adhesive which causes it to adhere to a conveyor printing blanket. Some machines use other methods for gripping the fabric. The fabric passes under the rotating screen through which the printing paste is automatically pumped from pressure tanks. A squeegee in each rotary screen forces the paste through the screen onto the fabric as it moves along (Corbman, 1975). The fabric then passes to a drying oven.

Direct printing

In direct printing, a large cylindrical roller picks up the fabric, and smaller rollers containing the color are brought into contact with the cloth. The smaller rollers are etched with the design, and the number of rollers reflects the number of colors. Each smaller roller is supplied with color by a furnisher roller, which rotates in the color trough, picks up color, and deposits it on the applicator roller. Doctor blades scrape excess color off the applicator roller so that only the engraved portions carry the color to the cloth. The cloth is backed with a rub- berized blanket during printing, which provides a solid surface to print against, and a layer of gray cloth is used between the cloth and the rubber blanket to absorb excess ink.

Discharge printing

Discharge printing is performed on piece-dyed fabrics. The patterns are created through removal, rather than addition, of color, hence most discharge printing is done on dark backgrounds. The dyed fabric is printed using discharge pastes, which remove background color from the substrate when exposed to steam. Colors may be added to the discharge paste to create different colored discharge areas (EPA, 1996).

Resist printing

Resist printing encompasses several hand and low-volume methods in which the pattern is applied by preventing color from penetrating certain areas during piece-dyeing. Examples of resist printing methods include batik, tie-dyeing, screen printing, and stencil printing.

Ink-Jet printing

Ink-jet printing is a noncontact printing method in which droplets of colorant solution are propelled toward a substrate and directed to a desired spot. Ink jet is an emerging technology in the textile industry and has not yet been adopted for widespread commercial use. The dye types most amenable to ink-jet printing of textiles are fiber reactive, vat, sulfur, and naphthol dyes.

Heat-transfer printing

In heat-transfer printing, the pattern is first printed onto a special paper substrate. The paper is then positioned against the fabric and subjected to heat and pressure. The dyes are transferred to the fabric via sublimation.

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Types of Dyes-Classification based on chemical structure

The Dyes are classified based on the fibres to which they can be applied and the chemical nature of each dye. Dyes are complex unsaturated aromatic compounds fulfilling characteristics like intense colour, soluability, Substansiveness and fastness. Dyes can be defined as the different type of colouring particles which differ in each type from the other in chemical composition and are used for colouring fabrics in different colours and shades which are completely soluble in liquid media.

Finishing

Finishing encompasses chemical or mechanical treatments performed on fibre, yarn, or fabric to improve appearance, texture, or performance. Some of the fabric finishing methods are-Brushing, Shearing, Pressing, Raising, Beetling, Calendaring, Folding, etc. Finishing can be carried out in 3 stages i.e. Pre-treatment, coloration and Finishing.