There’s never been a better time to hop aboard the water-based inks train.
By Lon Winters
Diazo-sensitized stencils hold up to the water, but they have minimal solvent resistance. The use of an emulsion hardener as a post treatment isn’t necessarily effective, as it only further improves the stencil’s water resistance, not solvent resistance.
Dual-cure emulsions are supposed to be solvent and water resistant, but in my experience, they don’t have the level of water resistance needed. The use of a hardener can increase its water resistance, but such a screen may not be reclaimable (though reclaimable hardeners are available).
It seems to me that hybrid photopolymer emulsions are the direction to go with these inks. Hybrid means they have been engineered to be solvent and water resistant. Photopolymer emulsions have a high solids content and are easy to coat. Their fast exposure enables the use of computer-to-screen technology and LED exposure.
Producing a durable screen for water-based inks also requires proper mesh preparation. Using a mesh prep and degreaser not only cleans the mesh, but also primes it to accept the coating for good adhesion. After coating, the stencil must be completely dried before exposure. Fully exposing the stencil is also very important with water-based inks. Underexposure will not work with these products; do exposure tests to be sure the emulsion is completely cured. After developing and fully drying the stencil, block out unwanted open areas with the same emulsion. I like to do a post exposure to help harden and fully crosslink the stencil. Taping the inside of the screen where the squeegee and flood bar travel, as well as where they “chop,” helps alleviate premature breakdown during the run.
Water-based ink is less opaque and will require some additional flashing. When you’re working out the color sequence, it’s important to put colors that are less opaque, have the biggest coverage, or are most important at the end or before a flash station. Water-based setups can be difficult and frustrating at times, especially when the humidity level in the shop is low. When HSA inks are exposed to the environment, the water in the ink begins to evaporate. This can be a problem if the registration process takes some time. New developments have improved the ink’s open screen time and re-wetting abilities.
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