The Challenges of Water-Based Inks
Though not always as production-friendly as plastisols, water-based inks can benefit you in the long run if you have the right skills.
(Continued from Mastering Water-Based Inks)
Water-based inks aren’t as production-friendly as plastisols. You can’t just switch out the inks using the same prepress and printing procedures. But with some important changes at each stage of production and attention to detail, you can overcome these challenges and get consistent results.
Ink Department First, your production management needs to focus carefully on color matching and mixing. The vast majority of water-based inks are pigment and base systems. This is definitely a plus because you use one set of pigments for all the different types of water-based inks. Generally, the pigment load is between six to 12 percent by weight, which compares to 20-25 percent with a typical plastisol. Thus, when weighing small batches, a significantly higher degree of accuracy is required, as the example in Figure 8 shows. Purchase a good scale – preferably one that is accurate to 0.01 grams to allow for consistent, predictable, and repeatable color matching.
Second, housekeeping is extremely important. Plastisol printers are used to leaving lids off buckets of inks. While that’s not recommended, not a lot of harm is caused short of forcing you to clean out dust and dead bugs from time to time. Water-based inks need to be sealed when in storage. Once water is exposed to the environment, contamination can result in bugs and mold living and growing in the container. Most manufacturers use biocides and fungicides to reduce this risk, but you still need to make sure ink containers are sealed.
Third, water-based pigments (especially fluorescent ones) tend to separate over time. Shake the pigment containers on a regular basis – monthly, at a minimum.
Finally, numerous additives are available for water-based inks, including retarders, penetrants, crosslinkers, softeners, thickeners, thinners, and de-tackifiers. Always follow the manufacturer’s recommendations, and be sure to label the ink buckets accordingly. This way, there will be no confusion about which additives an ink contains when pulling a bucket off the shelf.
Screen Department As a friend of mine says, “Plastisol printers can get away with murder in stencil preparation.” Not so with water-based inks. Most importantly, you must use a water-resistant emulsion. The screens must be thoroughly exposed, and at each step in the process (and especially before going into production), the screens must be thoroughly dry. Any residual moisture in the stencil or in an undercured emulsion layer will lead to premature stencil breakdown during the print run. The use of exposure calculators and emulsion hardeners for longer print runs are recommended. Post-exposing the screens after the stencils have been washed out and dried is beneficial to one degree or another with every type of stencil system.
Also, especially when using high-solids water-based ink for opacity printing, ensure that the screen has a good EOM (emulsion over mesh) ratio. Remember, 30-40 percent of a high-solids ink is water and will evaporate out.
Work with your distributor and/or manufacturer’s rep to ensure your screenmaking products and processes are correct for water-based printing. The time you spend prepping screens correctly will pay huge dividends on the production floor.
On Press The biggest issue facing production is ink drying in the screen during the run, especially with high-solids inks. Water wants to evaporate, and ambient conditions play a critical role in how fast or slow this will occur. A cold, dry climate (think Denver in winter) is the worst scenario; a hot and humid environment is best. Very simply, if the air is extremely dry, it will pull the water from the ink through evaporation much faster.
Be sure to set the cycle on your press to print and then flood, keeping ink over the image area at all times. Medium to soft squeegees work best with slightly more angle (approximately 30-35 degrees) than you are accustomed to with plastisol. When possible, set the job up with an empty printhead after each flash unit as a cool-down station. Large vector artwork with high-solids ink generally requires three heads per color (print-flash-cool), which means that more heads are required with multicolor jobs. To print a four-color job with a white highlight, you can get by with an eight- to 10-color machine with plastisol. Running the same job with a high-solids water-based ink could require as many as 16 print stations.
Use only a small amount of ink as you register the job to help prevent ink from drying in the screen. Minimize air movement over the screens and be sure no fans are blowing on them. Keep a spray bottle at the press to help keep the stencils open.
Curing All of your hard work in prepress and on the production floor will be for naught if the inks are not fully cured. Plastisol users are accustomed to specifications that state “the entire ink film must reach 320 degrees Fahrenheit.” Note that they don’t specify the ambient temperature in the dryer nor the dwell time, however. With water-based ink, having the proper dryer temperature, dwell time, and airflow within the drying chamber is critical to remove all of the water from the ink film, which must happen before the binders in the ink can reach the required temperature to bond with the fabric and become washfast. A general rule of thumb is two to three minutes at 330 F.
So can you run water-based inks on your current drying equipment? Let’s do the math for a typical scenario. Suppose you have a forced hot-air dryer with a 12-foot heating chamber. The common dwell time for plastisol in such a unit is 45 seconds (though many shops run at faster speeds), which equates to a belt speed of 16 feet per minute. In order to get two to three minutes of dwell time in such a unit, the belt would need to be slowed to four to five feet per minute. Dryers with longer heating chambers are highly recommended for water-based printing in order to keep production speeds up.
Fortunately, if your current equipment doesn’t allow you to cure for two to three minutes, an alternative is to add a crosslinker to the ink. These additives allow you to run at “normal” production speeds while ensuring that the ink film fully cures over time.
Discharge Printing These popular inks bring some additional considerations. Like other water-based inks, they require thorough drying because the discharge effect occurs only after the water has been thoroughly evacuated from the ink film. Be sure that the garment you’re working with is suitable for discharge printing. Use only 100-percent cotton garments with reactive dyes. Avoid garments that have been re-dyed. Know that some colors (such as kelly green, gold, and purple) generally do not discharge well, and that you might see a change in how a certain garment discharges due to variations in dye lots. For best results, print wet on wet and avoid the use of flash units.
Today’s water-based inks are not your grandfather’s chemistry. They still require extra attention to detail compared to printing with plastisol, but the inks are much more user-friendly today than they were even a few years ago. With customer demand showing no sign of abating, this is a great time to become familiar with water-based chemistry and processes. Don’t be complacent – that has led to the demise of plenty of businesses in this industry. Remember, if you aren’t able to meet your customers’ needs, there will always be a competitor who can.