I remember a day when inks were innocuous—plastisol, also known as the Johnson’s Baby Shampoo of printable inks, was the mainstay in this country. Today, we’re catching up with international trends and, as a result, we’re printing more and more of the euphemistic water based inks (including emulsion-ravaging dischargeable inks). Seems these are about one step short of screen printing emulsion remover… which begs the questions: what do we unwittingly do to abbreviate stencil life and what we can do to extend it?
Water base and discharge prints are popular in the market, but the inks can be difficult to work with. (Image courtesy Forward Printing, Inc.)
Many rationalize that finer mesh saves ink, flashes faster, gets a better cure, gives a better hand or prints sharper without the need to put a lot of effort into the stencil area. Truth be known; this choice may be killing you on-press—coincidentally, the only place where you can really make any money. This mesh geometry is a primary cause of ink hang-up and subsequent drying-in.
Further, white discharge ink is dilatant (has a higher viscosity/is thicker), meaning it doesn’t liked to be pumped. This ink hates to be pushed through the screen even more when the nexus is small or the journey is long. Using a really fine mesh in this context usually means using one with a really thick thread or else it will rip prematurely. Yet, the opening in the really fine mesh is really small and the tunnel is really long. So we see why the preeminent psychologist Carl Jung may have referred to this mesh selection as self-sabotaging.
Since I don’t know precisely what screen mesh you are using, we’re going to bracket the best alternative in addressing drying-in problems. First, sample a mesh with the same thread diameter you’d normally use, but drop two mesh counts. Second, sample a mesh which drops by one mesh count but also has a thinner thread in diameter. Test print both of these alternative fabrics for on-press ink stability and coverage (once you’ve made the rest of the suggestions herein).
Water base and discharge ink can still yield good opacity when using the right inks within the right parameters. (Image courtesy Forward Printing, Inc.)
Both alternates have a larger mesh opening which improves the opacity and shortens the tunnel, and thus makes the white more cooperative on its journey to the center of the shirt. Best of all, these types of mesh will help mitigate residue and drying-in and they will be flatter, which creates a smoother finish and permits higher print-stroke speed.
Seven steps to ink transfer
It may seem a bit odd to discuss ink transfer and blade metrics in a stencil article, but the blade is the key to getting the ink through the mesh without drying in, while pampering the stencil. In that case, let’s take a look at how the ink gets from the top to the bottom of the screen.
The squeegee blade needs to capture the ink in the funnel between itself and the mesh. The print-stroke speed must be sufficient in order to raise the ink’s fluid pressure to where it will gladly enter the cells of the mesh to escape the pressure. The blade must also move fast enough to avoid the fluid pressure dropping at the next mesh opening. There is a slight fluid pressure increase (and proportional loss of fluid volume) as the ink transfers through the mesh. Once sufficient ink hits the shirt, the garment absorbs part of the ink. This absorption causes the ink to release the mesh. To create this eventuality:
1. Be certain the blade’s edge fits the mesh (dull it if necessary)
2. Select a chamfered or beveled edge for adequate deposit
3. Use a hard blade to maintain contact pressure and maximize the funnel
4. Minimize the initial angle of the blade (zero-angle is the ideal)
5. Apply downward force (a.k.a. squeegee pressure) to contact mesh to garment
6. Increase the speed until clearing is complete
7. If the mesh is not clear, increase downward force but do not allow the blade to buckle
Driving the ink into the shirt is like trying to push a rope up a cliff—it just doesn’t work that way. The “driving approach” stretches the stencil coating and makes it more susceptible to chemical attack from the water, polar-solvents and acids in the white discharge. (Image courtesy Kiwo)
Ejection versus extrusion
Water-based inks, particularly discharge bases, are ejectable inks and should be printed accordingly. Plastisols, on the other hand, are extrudable inks, which tend to climb the squeegee blade and create dry artifacts (mesh marks, pinholes, etc.). Ejectable water based inks will flow readily in front of the moving blade and are prone to creating wet artifacts (mottling, blurring, etc.).
Once you raise the fluid pressure with an ejectable ink, it will literally wrap around the mesh threads, lose volume as it squeaks through the mesh opening and form the shape of the image before transferring into the shirt. The lost volume hangs up in the mesh, plus, there’s no fluid momentum left to cause the ink to penetrate. The only solution is to use incremental stroke-speed to further reduce the viscosity and surface tension of the discharge ink. This will keep it thin enough long enough to penetrate into the construction of the shirt.
To do so, set the flood stroke speed so that it ends right before the print stroke begins and use a polymeric flood bar to preload white ink into the screen. The flood bar will allow you to pre-load the mesh with discharge white and thereby increase the deposit and the penetration. Combine this with the screen and stencil mentioned above and you’ll have a winner—higher print speed, brighter white, smoother and softer hand and less drying-in.
First, sample a mesh with the same thread diameter you’d normally use, but drop two mesh counts. Second, sample a mesh which drops by one mesh count but also has a thinner thread in diameter. (Images courtesy Kiwo)
Wiping the underside of a screen, however necessary, is worse than self-sabotage. Consider that the stencil is susceptible to attack from water, polar solvents, abrasives and acids—all four of which exist in our discharge white. When we wipe the screen, we’re adding more water and abrasion to the mix. As long as you are completely clearing the screen in every pass to eliminate the residue, there will be little need to wipe up. So wipe if you must, but use the recipe herein and wiping will become the exception, not the rule, for printing discharge and water-based inks.
Select an appropriate coating with your supplier’s assistance. Apply sufficient coating to the squeegee side of the screen and make sure it is completely exposed if you want to delay drying-in. Note: we can’t expose the water out of the stencil and we must expose the interior coating or it will wash down the drain during development. Additionally, don’t make the stencil too thick on the print side—you will note that mesh adds shearing force to thin the ink; stencils do not. If the stencil is too thick, we’ll never get penetration into the garment. In that case…
• Find a mesh with a larger percentage open area
• Use an appropriate stencil and watch the inside coating
• Make certain the blade fits the tensioned screen mesh
• Don’t let the squeegee buckle (get the ink to clear the mesh)
• Run at top stroke speed (will make the ink penetrate)
• Wipe up at the end of the run
Implementing these practices will make printing white discharge much easier. All of the other colors and series of water-based inks will accordingly benefit from the upgrades you’ve made.
White pigment is very hard and abrasive, the combination of chemical, the wetting ability of the acids and abrasive attack will continue to be very hard on any and all reclaimable stencils. (Images courtesy Forward Printing, Inc.)