OEcotextiles

It is well known that the “finishing” of a fabric is where a great deal of the environmental impact occurs –  it uses the most water, chemicals and energy.

“Finishing” includes not only the application of treatments to make fabric perform in a certain way (i.e., to be free of something, such as static, wrinkles, or odor, or perhaps be waterproofed or flameproofed).  It also includes the decoration of the fabric.  This decoration can be simply dyeing the fabric a vibrant color.   But glorious designs on fabrics have always been popular.  Applying colored patterns and designs to decorate a finished fabric is called ‘printing’ – and we sure do love them!   Humans have been printing designs onto fabric for centuries.  It has been found on cloth in Egyptian tombs dating to about 5000 B.C. and India exported block prints to the Mediterranean region in the 5th cent. B.C., demonstrating that the Indus Valley civilization knew well the art of dyeing and use of mordents 5,000 years ago.

Printing on fabric is still very much in use today – we could even say it’s wildly popular –  and there’s a lot of talk about the sort of printing inks and dyestuffs used to print fabrics.   So I thought we’d take a look at textile printing and try to find out what the consequences of printing may be to us and the planet.  Printing is one of the most complex of all textile operations, because of the number of variables and the need for a high degree of precision, particularly since there is no way to correct a bad print.  So we’ll be looking at this topic over several weeks.

Technically, printing on textile can be defined as the reproduction of a decoration by application of one tool loaded with coloring material on a textile support. Early forms of textile printing are stencil work, highly developed by Japanese artists, and block printing. In the latter method a block of wood, copper, or other material bearing a design in intaglio (or relief)  with the dye paste applied to the surface is pressed on the fabric and struck with a mallet. A separate block is used for each color, and pitch pins at the corners guide the placing of the blocks to assure accurate repeating of the pattern.

Another style of fabric printing documented in Nuremberg, Germany, was the application of gold or silver dust on still wet fabric. This was an inexpensive way for lesser monasteries and churches to copy the expensive brocades from the Near and Far East, which arrived in Europe via the silk routes. These silk routes most often started in Italy, Venice in particular, and travelled over both land and sea. To economize further in the copying process, color was often filled in areas with a brush, reducing the number of blocks needed.  Velvet  effects were also added sometimes, this was accomplished by spreading powered wool on the gummed ink pattern. The document found in Nuremberg gave specific directions for duplicating the flowers and animals from the brocades.  These procedures could only be used on tapestries, church vestments and table furnishings because the colors weren’t fast. Because they couldn’t be washed these ornate fabrics were not used for clothing.

There are 5 basic steps in printing a fabric:

1. Preparation of the print paste.
2. Printing the fabric.
3. Drying the printed fabric.
4. Fixation of the printed dye or pigment.
5. Afterwashing.

We’ll begin with taking a look at  step #2, printing the fabric:  today, a decorative pattern or design is usually applied to constructed fabric by roller, flat screen, or rotary screen methods.

Cylinder or roller printing was developed around 1785.  In the roller printing process, print paste is applied to an engraved roller, and the fabric is guided between it and a central cylinder. The pressure of the roller and central cylinder forces the print paste into the fabric. Because of the high quality it can achieve, roller printing is the most appealing method for printing designer and fashion apparel fabrics.

Screen printing is by far the most popular technology in use today. Screen printing consists of three elements: the screen which is the image carrier; the squeegee; and ink. The screen printing process uses a porous mesh stretched tightly over a frame made of wood or metal. Proper tension is essential for accurate color registration. The mesh is made of porous fabric or stainless steel. A stencil is produced on the screen either manually or photochemically. The stencil defines the image to be printed in other printing technologies this would be referred to as the image plate.

In flat bed screen printing, this process is an automated version of the older hand operated silk screen printing. For each color in the print design, a separate screen must be constructed or engraved.

If the design has four colors, then four separate screens must be engraved. The modern flat-bed screen-printing machine consists of an in-feed device, a glue trough, a rotating continuous flat rubber blanket, flat-bed print table harnesses to lift and lower the flat screens, and a double-blade squeegee trough. The in-feed device allows for precise straight feeding of the textile fabric onto the rubber blanket. As the cloth is fed to the machine, it is lightly glued to the blanket to prevent any shifting of fabric or distortion during the printing process. The blanket carries the fabric under the screens, which are in the raised position. Once under the screens, the fabric stops, the screens are lowered, and an automatic squeegee trough moves across each screen, pushing print paste through the design or open areas of the screens. Remember, there is one screen for each color in the pattern. The screens are raised, the blanket precisely moves the fabric to the next color, and the process is repeated. Once each color has been applied, the fabric is removed from the blanket and then processed through the required fixation process. The rubber blanket is continuously washed, dried, and rotated back to the fabric in-feed area. The flat-bed screen process is a semi-continuous, start-stop operation. Flat screen machines are used today mostly in printing terry towels.

Many factors such as composition, size and form, angle, pressure, and speed of the blade (squeegee) determine the quality of the impression made by the squeegee. At one time most blades were made from rubber which, however, is prone to wear and edge nicks and has a tendency to warp and distort. While blades continue to be made from rubbers such as neoprene, most are now made from polyurethane which can produce as many as 25,000 impressions without significant degradation of the image.

From a productivity standpoint, the process is slow with production speeds in the range of 15-25 yards per minute. Additionally, the method has obvious design limits. The design repeat size is limited to the width and length dimensions of the flat screen. Also, no continuous patterns such as linear stripes are possible with this method. However, this method offers a number of advantages. Very wide machines can be constructed to accommodate fabrics such as sheets, blankets, bedspreads, carpets, or upholstery. Also, this technique allows for multiple passes or strokes of the squeegee so that large amounts of print paste can be applied to penetrate pile fabrics such as blankets or towels. Currently, approximately 15-18% of printed fabric production worldwide is done on flat-bed screen machines.

Rotary screen printing is so named because it uses a cylindrical screen that rotates in a fixed position rather than a flat screen that is raised and lowered over the same print location. Rotary presses place the squeegee within the screen. These machines are designed for roll-to-roll  printing on fabric ranging from narrow  to wide-format  textiles.

In rotary printing, the fabric travels at a consistent speed between the screen and a steel or rubber impression roller immediately below the screen. (The impression roller serves the same function as the press bed on a flatbed press.) As the fabric passes through the rotary unit, the screen spins at a rate that identically matches the speed of substrate movement.

The squeegee on a rotary press is in a fixed position with its edge making contact with the inside surface of the screen precisely at the point where the screen, substrate, and impression roller come together . Ink is automatically fed into the center of the screen and collects in a wedge-shaped “well” formed by the leading side of the squeegee and the screen’s interior surface. The motion of the screen causes this bead of ink to roll, which forces ink into stencil openings, essentially flooding the screen without requiring a floodbar. The squeegee then shears the ink as the stencil and substrate come into contact, allowing the ink to transfer cleanly to the material.

By converting the screen-printing process from semi-continuous to continuous, higher production speeds are obtained than in flat bed printing. Typical speeds are from 50-120 yards per minute  for rotary screen printing depending upon design complexity and fabric construction.  Rotary screen machines are more compact than flat screen machines for the same number of colors in the pattern. Therefore, they use less plant floor space.

Also with rotary screens, the size of the design repeat is dependent upon the circumference of the screens. This was initially seen as a disadvantage, because the first rotary screens were small in diameter. However, with today’s equipment, screens are available in a range of sizes and are no longer considered design limited. Today’s rotary screen machines are highly productive, allow for the quick changeover of patterns, have few design limitations, and can be used for both continuous and discontinuous patterns.

Estimates indicate that this technique controls approximately 65% of the printed fabric market worldwide. The principle disadvantage of rotary screen printing is the high fixed cost of the equipment. The machines are generally not profitable for short yardages of widely varying patterns, because of the clean-up and machine down time when changing patterns. Flat screen printing is much more suitable for high pile fabrics, because only one squeegee pass is available with rotary screen. However, rotary machines are used for carpet and other types of pile fabrics.  Most knit fabric is printed by the rotary screen method, because it does not stress (pull or stretch) the fabric during the process.

The rotary garment screen printing machine, developed in the 1960s,  is the most popular device for screen printing in the industry. Screen printing on garments currently accounts for over half of the screen printing activity in the United States. [i]

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