Textile Printing Process, Type of Printing, and Printing Machinery
Types of Textile Printing
Discharge Printing is also called Extract Printing. This is a method of applying a design to dyed fabric by printing a color-destroying agent, such as chlorine or hydrosulfite, to bleach out a white or light pattern on the darker colored ground. In color-discharge printing, a dye impervious to the bleaching agent is combined with it, producing a colored design instead of white on the dyed ground.
Discharge printing has been around for decades. But only in the past 7-8 years screen printers in the industry have recognized it seriously. In the early years of discharge printing, the finished discharge print needed to be steamed during the drying process. This discouraged the use of discharge systems in the finished garment arena. The newly developed discharge ink systems are chemically reactive and don’t need to be steam-neutralized. This advancement opened the door to discharge printing for the average screen-printer.
Discharge printing has the ability to make bright, opaque colors on dark fabrics with a soft hand. Years ago the idea of opaque colors on dark fabrics and soft hands couldn’t co-exist.
Successful light-on-dark printing with plastisol relies on increased pigment loads, fillers, and other additives to block out the color of the garment. Discharge inks modify the garment color by removing the garment color and replacing it with the new ink color. In simple terms, the discharge ink “bleaches” out the dye in the garment, thus allowing the pigment in the ink to absorb into the shirt fibers.
The real magic of discharge printing can be witnessed when printing the four-color process on black 100% cotton shirts. The print before curing appears very transparent. One can barely see the print until the garment exits the oven chamber, where the results can be quite remarkable: bright, vivid colors with a soft hand.
The graphics on the casino gaming tables are printed with discharge inks to avoid the interference of the printed line with the roll of the dice. If the ink on these tables were printed with plastisol, the ink film (because it is a surface print) would change the speed and direction of the dice, thus changing the way the dice land. Discharge ink, on the other hand, provides a dyed-in-the-fabric result, keeping the playing surface smooth.
The decrease in the production time is the biggest bonus of all. The fact that you can skip flash curing completely saves hours of production time and eliminates registration problems between the design’s colors and the white printer under base used in normal printing on blacks.
However, flash curing can be used in conjunction with discharge printing when printing discharge as an under the base.
Cleaner and more transparent inks can also be printed onto dark garments with the help of discharge additives. Early discharge additives were designed only for water-based inks, but plastisol additives are available.
Characteristics that indicate a garment will work with discharge ink
- The garment has to be made of natural fibers (100% cotton)
- The dye used in the garment must be dischargeable. The best results are achieved with garments that are 100% cotton and dyed with a reactive dye.
- The garment should not have been over-dyed (when the fabric is re-dyed to another color). This often happens because of a shortage of a certain fabric color or, in many cases, because quality control rejected the fabric color. These rejected colors are then dyed with a black dye, which will bring nightmares to life when trying to use discharge inks. The discharge ink might discharge the black dye – only to reveal a phantom color underneath.
Always test your garment to see if it is suitable for discharge printing. If you are a major printer doing large-volume printing, be aware that the shirts you order from the mill are tracked by lot numbers and it is possible that a completely different dye may be used from one lot to the next. Let your sales representative know that you are doing discharge printing and mention in writing that you need a dischargeable garment.
Discharge printing is frequently used for all-over prints because of its soft hand
A direct print is one in which the design is printed on a white cloth or a previously dyed fabric. The latter is called overprint and here the printed design must be of a darker color than the background design. This style is also called application printing because the design is directly printed over the fabric.
It is the most common approach to apply a color pattern on fabric. It can be done on white or colored fabric. If done on colored fabric, it is known as overprinting. The desired pattern is produced by imprinting 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. Earlier corn starch was preferred as a thickening agent for cotton printing. Nowadays gums or alginates derived from seaweed are preferred because they are easier to wash out, do not themselves absorb any color, and allow better penetration of color. Most pigment printing is done without thickeners as the mixing up of resins, solvents, and water itself produces thickening.
Resist or reserve printing is related to discharge printing in that the end results are often indistinguishable. The resist style, however, offers the advantage that dyes of great chemical stability, which could not be discharged, can be resisted to give prints of high fastness standards.
Resist printing involves a two-step procedure:
- Printing a pattern design on fabric with a chemical or wax-like resinous substance that will prevent or resist the penetration of dyes.
- Piece dyeing the fabric
The justification for both styles lies in the aesthetic appeal of a white or colored pattern on colored grounds, an effect that very often could not be reproduced by any other technique. The difference, therefore, between discharge and resist printing is not one of appearance, but of process. In discharge printing, the discharging agent is applied to the fabric after it has been dyed and the dye in the printed areas is destroyed during subsequent processing.
In resist printing, the resisting agent is printed onto the undyed fabric and effectively prevents the fixation or development of the ground color, which is subsequently applied by an appropriate ‘dyeing’ technique, such as dyeing, padding, or overprinting. The result can be either a white resist or a colored resist, where a selected dye or pigment is added to the resist paste and becomes fixed to the fiber during subsequent processing.
Virtually every class of colorant is capable of being resisted, as is borne out by reference to older publications on textile printing. Many of the techniques they describe are too complex and time-consuming to be of commercial interest today, but they do illustrate the wide scope of the style, with the proviso that it has little application to synthetic-fiber fabrics.
The resisting agents employed, then as now, function either mechanically or chemically or, sometimes, in both ways. It is used where background colors in a fabric cannot be discharged. It is usually not possible to visually differentiate between the discharge and resist printed fabrics since both of them produce the same results.
The mechanical resisting agents include waxes, fats, resins, thickeners, and pigments, such as china clay, the oxides of zinc and titanium, and sulfates of lead and barium. Such mechanical resisting agents simply form a physical barrier between the fabric and the colorant. They are mainly used for the older, coarser, and, perhaps, more decorative styles in which breadth of effect and variety of tone in the resisted areas are of more importance than the sharp definition of the pattern.
A classical, and nowadays almost unique, example of a purely mechanical resist is to be found in the batik style, using wax applied in the molten state. In a true batik, the wax is applied by hand, but the process has been developed and mechanized for the production of those styles which now come under the general heading of ‘Africa prints’. It is not possible to apply an illuminating color with wax resist but, after removal of the wax, another color can be printed within the resisted area. A mechanical resist is usually used in conjunction with a chemical resist, so improving the overall effect.
Chemical-resisting agents include a wide variety of chemical compounds, such as acids, alkalis, various salts, and oxidizing and reducing agents. They prevent fixation or development of the ground color by chemically reacting with the dye or with the reagents necessary for its fixation or formation. The actual choice of chemical-resisting agent depends, therefore, on the chemistry of the dye being used and its fixation mechanism. Consequently, as in discharge printing, working knowledge of the relevant chemistry is necessary when choosing effective resisting agents.