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Exploring the Secrets of Discharge Printing

(January 2001) posted on Wed May 30, 2001

"Bright" and "soft-handed"--a pair of terms you don't usually associate with screen-printed images on dark garments. But if you use discharge inks, these qualities can describe every dark-garment graphic you produce. Find out why so few printers try their hand at discharge printing and how to put discharge inks to work in your shop.

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By Terry Combs

A few years ago, I wrote an article on a little known but innovative process. Discharge printing was an interesting concept and had the potential to be revolutionary for the textile screen-printing industry. At the time, I happened to be one of a handful of screen printers who were actually using discharge ink systems to one extent or another. Since those days, not much seems to have changed--most of the printers who were using discharge inks then are the ones still using it today. The number of new printers climbing aboard the discharge-printing bandwagon have been relatively few, and it seems that the pros and cons of the system have resulted in little movement one way or another by the vast majority of garment-printing shops. As a result, discharge printing remains something of a "secret" screen-printing process. The process of discharge printing relies on a special water-based ink that undergoes a chemical reaction and "discharges" certain textile dyes from garments, leaving the white or natural garment color where it is printed. Discharge inks are primarily for dark-garment printing, where the discharge agent in the ink breaks down or bleaches the dye from the print area under heat and forced air, leaving the soft ink deposit you'd expect from a water-based ink. What is discharge printing? Discharge ink systems offer some definite benefits. First, you have the opportunity to print brightly colored designs, wet on wet, without the added tie and heavier hand of a flash-cured white underbase. With discharge printing, you totally eliminate the need for a plastisol underbase. Most importantly, you can achieve the most desirable characteristic of a water-based ink system--an ultra-soft hand on the finished print, something that was previously reserved for white and light-colored garments. Other water-based inks can be printed on dark garments and fabrics with successful results. But until the advent of discharge inks, plastisols were the dominant solution for printing bright designs on dark garments and textiles. Water-based inks formulated for use on dark fabrics have, for the most part, been reserved for cut-piece printing, where heavier plastisol ink deposits would interfere with the subsequent sewing process. Most textile printers are experienced in printing plastisols, rather than water-based formulations, which is definitely one factor that has discouraged more widespread use of discharge inks. Also detracting from their use is the discharge activating agent they contain, a material called zinc formaldehyde sulfoxylate (ZFS), which can pose health risks if not handled properly. While some ink manufacturers have researched formaldehyde-free discharge products, most still use ZFS as an activator in their inks. Consequently, printers must take certain precautions. It's important to educate employees who work with these inks about the potential hazards and proper handling, which can eliminate the hazards altogether. The most important issue to cover during training is that the heating and curing of discharge inks in the dryer creates formaldehyde and sulfur dioxide as by-products. These gases must be properly exhausted from your forced-air dryer and out of the production area. If you use discharge inks, it's a good idea to have your shop's air quality tested to make sure that formaldehyde levels in the air are within acceptable limits. The Occupations Safety and Health Administration regulates the threshold-limit value (TLV) of formaldehyde as 0.75 parts per million. Independent testing has shown that most discharge ink evaporation falls well below this level, provided that a shop used effective and adequate ventilation. Also note that the sulfur dioxide byproduct of discharge inks carries with it a strong, unpleasant odor, which may be the most obvious drawback for production-floor employees. But this odor can also be kept to a minimum with proper ink usage and ventilation. How discharge printing works Several ink manufacturers offer discharge ink systems (see below "Discharge Ink Manufacturers" ), and no two systems are exactly alike. Depending on the manufacturer, you will need to mix from two to five components (base, pigment, activator, and/or other additives) to prepare the ink for production. Talk with your supplier about ink makeup and mixing instructions to determine if you'll be comfortable with that brand. You should only mix enough discharge ink for a single press run. To get the ink ready, you will stir in a small, premeasured portion of the discharge activator before the start of the production run. Unlike "open and use" plastisols, you need to take special care when preparing discharge inks, closely following the manufacturer's measuring and mixing instructions when adding the activator. Inexact mixing may still create the discharge effect, but you might not achieve the maximum degree of bright and vibrant color in the finished prints. After mixing, the activated discharge ink has a limited pot life, commonly eight hours. Any activated ink left at the end of the day will have to be disposed of properly. Dispose of this ink in accordance with your own local or state regulations, just as you would with other ink or solvent products from your facility. As with all water-based ink systems, a water-resistant stencil is required for printing discharge inks. This should not be a major concern for your screenmaking department, since water-resistant stencil systems are very similar to solvent-resistant stencils. Garments When discharge printing was introduced, it was like the proverbial "cart before the horse." In theory, the concept seemed like a great idea, but printers quickly discovered a limited supply of garments that were ideal for the process. Not all textile dyes are fully dischargeable, and garments made of cotton/polyester-blends can also complicate the discharge process. With cotton/poly garments, each fiber type is colored with a different family of dyes to achieve the final garment color. When such garments are discharged, the assorted dyes lead to an inconsistent bleaching effect. To achieve the brightest, most consistent results, you should use discharge-ready, 100%-cotton garments. To create these garments, another step is added in the garment-manufacturing process. Essentially, the garments are bleached white before they are dyed to their finished color. This bleaching process is a time-consuming and unnecessary step when the fabric will be printed with plastisol inks. But the availability of discharge inks has encouraged some manufacturers to offer such specially processed, discharge-ready garments. It's also becoming increasingly common for printers to order their garments custom dyed (or even to dye them in house), so getting prebleached 100%-cotton garments is easier than ever. But the added cost may still put off some shops. Dyes used in discharge-ready fabrics break down easily when discharge-inks are activated in the dryer. However, not all dyes have this property, so you must be careful to specify dischargeable garments (or dyes) when placing garment or fabric orders. If you use 100%-cotton garments that haven't been prebleached, they probably don't have discharge-ready dyes, and the best result you can expect is a return to the original predyed color of cotton, which has an off-white, yellowish caste. You also can't depend on achieving consistent color across the discharged areas of standard, non-prebleached garments. For example, if you print a process-color design over discharged areas of a standard cotton T-shirt, you can expect the final image to be influenced by the base color of the discharged fabric (just as you would if you printed process colors on a yellow or other light-colored garment). Additionally, you have to consider variations in color from batch to batch of dyed garment. In applications involving fine-detail process color, the only option that guarantees consistent success is to use discharge-ready garments. Basically, the less particular you are about the colors in your final image, the more garment options you have available. The basics of printing discharge inks are quite simple. Images are printed as with standard plastisols and water-based inks, then the garments are delivered through a dryer. If you already use other water-based inks, then you're familiar with their drying requirements and potential for clogging screens, so adapting to discharge inks will be easier. But discharge printing does have one unique aspect that makes printing more challenging, regardless of the types of ink you now use: You cannot see the image area you printed with discharge ink when you pull the garment off the press platen; you won't see it until the garment leaves the dryer, which will be your first opportunity to inspect print quality. Immediately after printing, discharge inks are virtually invisible, appearing as little more than a wet area on the garment. This means that pinholes or other problems resulting from dried ink in the screen might not be detected until the dryer belt is full of garments and the first spoiled pieces exit the dryer hood. The inability to gauge print quality on the press is another factor that has caused some garment printers to steer clear of discharge inks. If you have no experience with water-based inks, the most important thing to remember is not to leave a screen unattended. After using versatile and forgiving plastisols, it's easy to think you can walk away from the press for a little break or a little conversation. When a press stops moving and is loaded with any water-based ink, unattended ink can quickly dry in the screen and make life difficult. But you can avoid this problem with extra attention and discipline on the production floor. As with plastisol inks, maintaining a proper curing temperature for discharge inks is very important. Even though discharge is a water-based system, the substrate must reach the proper curing temperature in order for the ink to set. And, of course, with any water-based system, good air flow within the dryer is important to ensure complete evaporation of the water. After printing, you should treat discharge prints as any other printed garment and perform wash testing on sample prints from every production run. Properly cured discharge-printed garments will have excellent washability, and, unlike traditional plastisol, these garments can be dry cleaned. You can even iron directly across the print area without adversely affecting the decoration. Water-based discharge with plastisol inks When I wrote about discharge inks back in the mid 90s, I discussed the approach my own shop took with these inks. That approach involved applying discharge ink as an underbase and directly overprinting the underbase with plastisols. Needless to say, I opened a can of worms by suggesting that water-based and plastisol inks can be printed--and cured--together. I received quite a few calls, faxes, and letters in response, some from printers who claimed, "That is absolutely impossible," others from printers who took the plunge and responded, "I tried it, too, and it really does work." Now I'd like to reopen that can of worms. What prompted my shop to use both ink types in the same job? Basically, I did not have the option of buying and using discharge-ready garments and was locked into printing on an existing garment line. My situation was further complicated by the fact that on a single print order with the same graphic, customers frequently requested multiple garment types--some 100% cotton (not pre-bleached before dying), some 80/20 blends, and/or some 50/50 blends. To determine if we could use discharge inks across this range of garment types, we decided to do a little experimenting with the discharge process. Although logic suggests that our plan had no real basis for working, we decided that combining water-based and plastisol in one job sounded simple and doable. So we replaced the white plastisol underbase with a base of discharge ink. Right after the discharge underbase (and without flashing), we printed each plastisol color required by the design, wet on wet, directly over the discharge ink. After a little tweaking with dryer temperature and belt speed, we ended up with a bright and vibrant, high-quality multicolor print on a dark garment without the heavy hand (and extra work) we would have faced with a flash-cured plastisol underbase. The finished garment had a hand similar to a conventional plastisol print on a white garment. Thereafter, we used discharge printing whenever we wanted an alternative to flashing and to heavier plastisol ink films on our garments. Two pieces of equipment are necessary to accomplish the direct, wet-on-wet, discharge/plastisol printing combination. The first is a good forced air dryer. You will need to move a heavy volume of heated air to evaporate water from discharge ink beneath the plastisol print. The second equipment requirement is an ink scale. If your ink department is like many in shops that print plastisol, shortcuts in mixing may be common due to the forgiving nature of plastisol inks. Discharge inks, on the other hand, will keep your ink department honest. The ink components must be measured precisely to achieve an effective discharging mixture. If you're already mixing water-based inks from pigments in your shop, the transition to discharge inks will be much easier. Value-added printing Some shops are beginning to re-evaluate discharge inks as a means of setting themselves apart from the competition. One way to accomplish this is in printing custom dyed garments--such as tie-dyed T-shirts--with discharge inks. Others have explored discharge effects on materials such as denim or other dark dyed-cotton fabrics. The right combination of dyed fabric, discharge ink, and overprinted graphics can lead to some impressive printed garments, and the potential combinations are almost unlimited. Discharge ink systems can also benefit belt printers involved in all-over printing, allowing them to broaden the range of possible effects without sacrificing productivity. Belt printers are limited to water-based ink because the ink is printed beyond garment edges and onto the belt itself. The excess ink is then scrubbed from the belt as it cycles under the machine, a process that is not possible with plastisols. However, the water-based limitation can make it difficult for belt printers to achieve bright graphics on dark garments, unless they turn to discharge inks. With discharge inks, they can print directly on dark fabrics, rather than trying to achieve the same effects by printing standard water-based inks on lighter garments. Discharge printing also offers opportunities beyond garment applications. For example, the ink has proven useful in decorating gaming-table surfaces. In this application, the soft hand of the discharge ink helps eliminate the drag and resistance that result when conventional inks, such as plastisols, are used. The result with discharge ink is vibrant detail with minimal hand on a print that is less likely to impede movement of cards, dice, or chips. Conclusion The challenges of working with water-based inks and concerns over the safe handling of discharge products may make you hesitant to adopt discharge printing. But a little practice and common sense when using discharge inks can lead you to stunning results and help set your business apart from the competition. Let discharge printing remain a secret to other garment printers while you make it a secret to new success. Discharge Ink Manufacturers * Jantex Inks & Beyond, Inc. Commerce, CA 562-806-9595 Fax: 562-806-9595 * Pavonine Products Lynchburg, OH 937-364-2933 / 800-817-8732 Fax: 937-364-2108 * Lancer Group Int'l Winnipeg, MB, Canada 204-885-7792 / 800-665-4875 Fax: 800-498-6675 * Polyone Corp. Waukesha, WI 262-932-6000 screen_prod_intro.htm * Manoukian North America Gloucester, MA 978-283-4564 Fax: 978-283- 5968 * Screen Colour Systems Ltd. London, England 44 081-997-1694 Fax: 44 081-997-2174


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