Printing
Printing is a method of decorating textile fabrics by applying pigments, dyes, and other materials. In recent years, textile printing has become highly sophisticated, requiring the skills of many artists and designers. The four main methods of textile printing are block, roller, screen, and heat transfer printing. In each of these methods, the application of color, usually as a thickened paste, is followed by fixation, usually by steaming or heating. Then, the removal of excess color by washing. Printing styles are classified as direct, discharge, or resist. In direct printing, colored pastes are printed directly on the cloth. For discharge printing, the cloth is first dyed with a background color, which is destroyed by reagents, or reducing agents, carried in a print paste. In the resist process, the cloth is first printed with a substance called a resist, protecting these printed areas from accepting color.
Wooden blocks, carved with a design standing out in relief, are made from solid pieces of wood or by bonding closely grained woods with cheaper ones. When designs include large areas, these are recessed and the space is filled with thick wool felt. Fine lines are usually built up with copper strips, and other effects are obtained with copper strips interleaved with felt. To facilitate registration of successive prints, or lays, each block has several pitch pins arranged to coincide with well-defined points in the pattern. The fabric is printed on a table covered with several layers of fabric or blanket, covered with a thick sheet of tightly stretched synthetic rubber. The cloth to be printed is spread on the rubber, either gummed in ink or pinned to a backcloth attached to the table. Colour is applied evenly to the block, and the pattern is stamped on the fabric to be printed. This is done using the handle of a small heavy hammer, or maul, to aid penetration of the paste. More color is then applied to the block and the process is repeated using a pitch pin to obtain true registration. After the fabric has been entirely printed with one color, other colors are added in the same way until the design is complete. Although block printing is becoming too laborious and costly for commercial use, some of the most beautiful prints have been made in this way.
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 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 color, 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 high-quality printing. Each printing roller is provided with a rotating color-furnishing roller, partially immersed in a trough of printing paste. Finely ground blades (doctor blades) remove excess color paste from the unengraved areas of these rollers, and each also has a lint blade. The printed fabric passes through the main cylinder and a drying and steaming chamber. This process fixes the color. 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 affected by electromagnetic push-button control, and modern electric motors provide smooth-running, variable-speed drives. The washing of back grays and printer blankets has also been automated.
The process of screen printing can be done by hand or by machine. The cloth is first laid on a printing table, gummed in, or pinned to a back gray. Then the design is applied through a screen made of silk or nylon gauze stretched over a wooden or metal frame. This is where the design for one color has been reproduced. This is usually a photographic process, although hand painting with suitably resistant blocking paint is an alternative. A screen is placed over the fabric on the table to prevent registration stops, ensuring accurate pattern fitting. The print paste is poured onto the screen edge nearest the operator and spread with a squeegee over the screen's surface. This ensures that color is pushed through the open parts. The screen is moved until one color has been applied to the cloth. The process is repeated with different screens for the application of other colors.
The popularity of polyester fabrics led to the development of a brand-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 calendar, 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 key, 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, reactive dyes, and pigment colors. Most dyes are fixed by steaming or aging, by a batch or continuous method, and more rapid fixation is affected 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 thickeners, ensuring resistance to rubbing.