Methods of using discharge
Discharge inks require an activator/catalyst to work; there are two different systems available. The predominant system relies on active ingredient Zinc-Formaldehyde-Sulfoxylate (ZFS). The newer, and less used system relies on Thiourea Dioxide as its active ingredient. There are different name determinations dependant on what the ink company calls it, in most cases formaldehyde is the active ingredient. In both systems, the ink has a limited discharge life once the activator is added. There are two methods of discharge printing, both systems can be used.
1. The first and most traditional printing method is to discharge every color in the print; there is no need for an underbase screen. In this format you mix PC pigments into the discharge to give it color when discharging occurs. PC pigments can be ordered in a variety of different colors and are typically mixed at about 10% into the discharge base. This method saves a screen and does away with flashing between colors. The exception to this rule is when a black-ink screen is needed. Theres no need to use discharge if the black will cover without it. When printing on black, any black that is on the design is reversed, that part of the design will use the shirt color itself.
2. The second method is to use discharge strictly as an underbase. With this method, you can use either white discharge or natural discharge, which contains no pigment and reveals the natural color of the fabric. The following colors are printed with regular plastisol with or without flashing. Some prefer not to flash the discharge underbase. This saves the head used for the flash and any cool down heads. The end result is that printers can increase the amount of colors they can print on dark shirts by one or two.
The white discharge underbase works well for most design types, especially spot-color work. Even though the other colors are printed using plastisol, the overall print has a less heavy feel because the underbase is a water-based product. If a design contains halftones or other areas with very thin ink deposits, then a natural discharge would work much better. The pigment in the white discharge underbase might mix with the process inks and shift their color. This is usually a problem with spot colors.
Discharge underbasing makes true 4 color process printing on dark fabrics possible. For process printing do not use a white discharge underbase. When the white pigment mixes with the transparent process inks, they will turn pastel and muted. Work with a natural discharge underbase that will reveal the natural cotton background color using a highlight white to make the design pop. Process-ink systems can be beefed up using triple-strength versions to compensate for the natural background thus overcoming the off-white background color. Again if the design contains any white color of its own, print a white highlight that is designed to print with the process inks using this application.