Printing on textile products
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 is the method of applying colour to fabric to make decorative patterns in a uniform way. There are many printing methods available.
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Fabrics are often printed with colour and patterns using a variety of techniques and machine types. Of the numerous printing techniques, the most common is the 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 per cent 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 fibres. 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.
This is a traditional printing method where a block made from wood, rubber, sponge or metal is shaped into a relief pattern (usually using a laser cutter or by hand), then dye is applied to the block and it is hand printed, or stamped, on to fabric. The print can then be repeated several times to achieve a pattern.
This method requires a screen, which is a frame with a fine mesh fabric tightly stretched over it. A pattern is either in stencil form or is blocked off on the screen itself, using a screen for each colour to be printed. Dye is pushed through the mesh fabric with a squeegee tool to evenly disperse the dye into the fabric below in the areas that have not been blocked out. Flat-screen printing is done in a similar way but machines operate each stage and often the pattern is applied to the screen digitally. Digital printing is a favoured method for small batches of fabric, as screen printing is an expensive process requiring space and specialist equipment.
Engaved Roller Printing
Engraved roller printing is an industrial method for large print runs; metal rollers are engraved with a pattern and dye reservoirs apply the color as needed. As in screen printing, a roller is needed for each color; the method is therefore too expensive for short runs of fabric, but it can be very cost-effective for large print runs as hundreds of metres can be printed per minute.
Transfer dyes are applied to paper and then heat transferred to the fabric. You can do this with transfer paints and an iron. These chemical transfer dyes are beter suited to synthetic fabrics, on which they give a better depth of colour.
Stencils are made from card or acetage; the pattern is cut out and then the dye is sponged or brushed into the cut-out areas. Each stencil can be used several times. Today many stencils are made using computerised cutting machines for more accuracy than hand-cut stencils.
A design can be made on the computer using computer-aided design (CAD) packages and then either transferred on to paper (sublimation paper), which can be transferred to the fabric with heat, or it can be directly printed on to the fabric using a textile printer and then steam heated to fix the design. In direct printing the fabric may need to be thickened using a special chemical agent, which can be washed out afterwards, to help it feed through the printer.
Rotary screen printing
Rotary screen printing uses seamless cylindrical screens made of metal foil. The machine uses a rotary screen for each colour. 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.
In direct-printing, a large cylindrical roller picks up the fabric, and smaller rollers containing the colour are brought into contact with the cloth. The smaller rollers are etched with the design, and the number of rollers reflects the number of colours. Each smaller roller is supplied with colour by a furnisher roller, which rotates in the colour trough, picks up colour, 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 rubberized 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 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 the 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 encompasses several hands 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 is a non-contact printing method in which droplets of colorant solution are propelled toward a substrate and directed to the desired spot. Inkjet is an emerging technology in the textile industry and has not yet been adopted for widespread commercial use. The dye types most amenable to the ink-jet printing of textiles are fiber reactive, vat, sulfur, and naphthol dyes.
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.