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 coloured 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 dont 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 hand couldnt 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 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 designs 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 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 base 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 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 over 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.
Points to be noted
- Do not to overlap your separations if you want to print clean spot colors. Butt-to-butt separations are ideal for most discharge printing.
- Work with a negative background and allow the dark background of the garment to show through. If you choose to overlap the color separations, overlap the primary colors - red, blue and yellow - to achieve secondary and tertiary colors within the design.
- To create the screen for under-basing with discharge ink, sandwich all the color separations together (make a contact sheet or a mask) to allow the discharge ink to be printed under all subsequent colors. (Suitable plastisols can be printed over clear discharge bases. Consult your ink manufacturer for compatibility with plastisol.)
- Discharge printing is frequently used for all-over prints because of its soft hand.