A New Spin on Water Based Inks
As the demand from brand manufacturers for non-PVC prints grows and intensifies, screen printers using traditional plastisols have found themselves at a crossroads. No one wants to turn away business, but what sort of adjustments will be necessary in the print shop to meet the customer’s requirements?
The biggest challenge for ink manufacturers has been the race to provide a non-PVC solution for their customers that acts like a plastisol ink in production. One solution is the development of a better, more user-friendly, high solids water-based ink. This ink has a look and feel similar to plastisol on press with improved open screen time, but the print processes are very different for the shop. The details outlined here reflect how ink formulation, pre-press processes and print parameters all play into a solution for this ink formulation.
Many printers think of water-based ink as a thin, runny product made primarily for printing on light or white garments to create a soft-hand image. High solids, water-based ink technology is quite different, as it is engineered to behave similar to a soft plastisol throughout the printing process and color mixing processes.
Ink manufacturers build pigment percentage guidelines into their formula guide to create curable color. The big difference is in the pigment loading: plastisol may average a 20 percent pigment load, while high solids, water-based comes in at around 6 percent. This difference affects the strategies for art separations, print rotation and flashing. The opacity difference also requires a strategy for under-base creation. It’s a common practice to “seal” the fabric with a first-down clear or low-opacity white base, flash, then overprint with a full high-opacity white to create a smooth, white under-base.
The basic art creation and separation process is similar to that of conventional plastisol printing. However, the final rotation and additional flashes impact the decision-making process. The separation artist must work closely with the production department to ascertain the amount of heads needed on press to achieve a quality print. Water-based ink is inherently less opaque than plastisol ink and will require more flash heads to build opacity and hold color on the print. Knowing this, the production department should build a strategy on rotation that puts any less-opaque colors last, or before a flash, to maintain full color. Some decisions for rotation may be based on which colors are more important to the design in terms of color or creating better contrast. Even the best laid plans can change once the first strike-off is created.
In terms of white-plate building in the separation, a good strategy is to create the first under-base plate for the clear or based-back white with all the color information included, except the black areas. The secondary under-base plate, for the high opacity (HO) white, will only include the bright, light and white color areas of the design. All dark colors, like navy or maroon, will hit color quicker if printed directly on the single-layer clear or based-back white. Florescent colors require different strategies, which are implemented on the press with additional screens and rotation set up.
The pre-press department also must make adjustments for water-based inks. The best practice for water-based is to use open mesh—either lower mesh count or thinner thread—to create a more open area. The emulsion must be water resistant to guard against breakdown during the run. All emulsion manufacturers offer a solution for this process. Some are straight water resistant (WR) emulsions and some are dual purpose emulsions with hardeners. When coating this emulsion, it is very important to coat the screens properly. The emulsion thickness percentage works best at around 20 percent EOM (emulsion over mesh). This stencil helps the printer achieve a thicker application of the lighter, short-flow ink, which, in turn, coats the surface of the substrate and helps build opacity and strengthen color.
Water-Based Set Up Tips
Water-based setups can be difficult and frustrating at times, especially when the humidity level in the shop is low. When water-based is exposed to the elements, the water in the ink begins to evaporate in the screen. This can be a problem if the registration process takes some time. New developments in high solids water-based inks have improved open screen time and re-wetting abilities. Knowing this, the traditionally accepted “flood-up” parameter is use sparingly. Since the ink will stay wet when it is gathered together in the screen, it makes more sense to keep the screen open, as if it were plastisol, so that there is less “skinning” of the ink in the screen. By keeping the screen open, there is less ink in the image area to dry, and the re-wetting nature of the ink will clear out the screen after a few strokes.
Setup tips for high solids water-based ink:
• Keep flood open.
• If leaving the machine alone for up to two hours, wet the area with a spray bottle that uses a misting action, rather than droplets.
• The first screen is used to seal the garment, and the rest are used to build opacity. Use as little pressure as possible for all subsequent screens.
• From time to time, there may be some drying in the screen. If striking a few times doesn’t work, soak a rag with water and touch the image from underneath the screen, pushing the water through the image. Don’t scrub the screen.
• Soft squeegees work best for flow.
• A fast, double stroke is better than a slow single stroke.
• Keep the flash temperature low.
• The flood bar should not contact the screen, allowing a thick flood of ink.
• If the print calls for a bright florescent color on a dark substrate, the best strategy is to print an HO white under-base, flash/cool and then print a screen with a tone color of 10 percent HO white mixed with 90 percent florescent color. Flash/cool again, then print a screen with 100 percent florescent color.
During the Run
Once the job is running, this ink will keep the image open unless the ink stops flooding. It is important to keep the screens fully loaded with ink. High solids water-based has a tendency to move to the sides of the squeegee, so it’s important to have a person tend the screens or install a “winged” flood bar to keep ink in the center of the screen. As the ink runs, it will be going through an evaporation process. Depending on the humidity level, this process may be fast or slow. If the ink starts to appear thick or dull, simply spray water in the back side of the flood during the print cycle to return water to the ink without affecting the screen image.
For water-based inks in general, the most effective flash unit will be an infrared (IR) flash unit with high air flow. This combination of heat and air flow creates a better situation for water evaporation, which is key to flashing and curing this type of product. The ink’s surface temperature should reach 200°F for 2.5 to 3.5 seconds. Use a ray-tech gun to record the platen temperature to help keep the correct settings on the flash units. The platen temperature should stay around 140°F to 160°F. If the ink reaches 220°F or higher on the surface during the flash, the surface may become tacky, which could be confused for under-cure. This can also contribute to the platens overheating, which could gel the ink in the screen.
Tip: If the flash units don’t have temperature, time or power controls, use other tricks like distance between the flash and the substrate, index time on the machine or multiple cool stations.
Possibly the biggest difference between water-based and plastisol is the way that each cures. Plastisol actually fuses when PVC resin absorbs the plasticizers in the fluid ink when it is exposed to high temperatures.
Water-based “cures” when all of the water evaporates from the product, leaving binder and pigment on the substrate. Knowing this, the best strategy is to expose the printed ink to constant heat for a longer period of time. For instance, plastisol needs to reach a predetermined temperature for a few seconds, while water-based cures better when it is exposed to heat for 90 seconds to two minutes. Once the belt speed is established, the oven should be set to reach about 340°F in the ink film for about 45 seconds. This is best measured with a thermo-probe; place the cross-hairs donut from the thermo-probe directly into freshly printed wet ink. Run the garment through and watch the temperature readout. If the ink does not reach temperature, repeat the procedure with higher temperature settings.
It’s possible for the traditional, plastisol-only screen print shop to meet customers’ requirements to produce non-PVC prints with minimal disruption. With a water-based solution that prints like a plastisol, and some minor adjustments in the art and setup processes, screen printers can open the doors to more non-PVC contract business.