Discharge Ink and Discharge Screen Printing: How It Works

Water base discharge inks:

Discharge ink printing can be a valuable addition to your operation. Discharge ink is not a use-it-and-forget-it product. Discharge printing comes with safety precautions, issues with finished goods and procedures for a healthy shop environment. These issues are not that difficult to deal with and cannot be ignored. Be prepared to properly handle the discharge products or don’t even try them.

Methods of using discharge ink:

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 under base screen. 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. There’s 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 under base. 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 ink 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.

Safety issues in production:

Water-based ZFS-activated discharge is the most used and the most versatile method of discharge printing. ZFS has an unpleasant odor and should be handled carefully in its crystal form then blended into the ink by a properly trained employee. Always blend the ZFS slowly until it is well mixed in the ink to prevent dust, ZFS is relatively safe once in solution. Formaldehyde is a skin irritant. Proper procedure dictates dryers should be properly vented, never use an unvented dryer for discharge curing. Shirts printed with ZFS discharge have measurable levels of formaldehyde. Garments that are allowed to sit for a time after printing in an unconfined state will disperse most of the formaldehyde within days. If at all possible, avoid folding and packing ZFS-discharged garments immediately after printing.

Water-based Thiourea dioxide discharge should also be handled with care; it’s a strong oxidizer that must be kept in a sealed container. The same precautions that are recommended for ZFS apply to Thiourea dioxide as both are strong oxidizers to be kept in sealed containers. Efforts should be made to minimize dust when pouring the crystalline powder into the mixing container, occasional direct contact is not considered an immediate health threat but advisable to avoid excessive contact.