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Screen Print Methodology: Standard Prints

Joe Clarke has spent the past 47 years in the lab and in the engineering department, in pre-press and on-press, as an R&D / technical researcher and as a manager of screen print production. Clarke has held executive positions as President of M&R Printing Equipment and as Vice-President at Wilflex [Poly One]. He has been granted a growing number of print-related patents, including one for High-Shear printing with Smilin'Jack - he is a member of the ASDPT, is an Associate Editor for NBM and an SGIA Fellow.

Clarke has presented hundreds of technical papers, written a couple books and published over 600 technical / management articles for which he has been awarded five Swormstedts; the international standard for excellence in technical writing.

Currently Joe Clarke is the President of CPR, a Chicago-based corporation which manufactures Synergy Inks including NexGen; environmentally & financially responsible T-Shirt inks. For more information on CPR, visit http://www.cprknowsjack.com/.

In the last installment in our series, we discovered that raw material costs—frame, mesh, stencil, films, ink and blade—are a very small percentage of the total cost to print a shirt, but that they are the keys to minimizing the total costs. Specifically, with the right-stuff—raw materials that facilitate faster production of higher quality garments—total costs are reduced. Now that the raw materials do not have to be the problem and are likely to be the solution, the question becomes “what do we do about it?” Or, more specifically, “how do we select and use our raw materials to our advantage?”

Tale of two inks: phase separation

Both water-based and plastisol inks each have charming characteristics that mature into true margin killers. That is, water-based inks often dry in the screen and plastisol inks usually build-up on the underside of them. Interestingly, both phenomena (drying and building) are due to phase separation that takes place in the screen. When inks are shaken, stirred or squeegeed, the liquids and solids contents move at different speeds which results in those dried and built up problems; this phase separation.

But water-based inks don’t have to dry in and plastisol inks don’t have to build up. It is very likely inexpensive versions of them will do just that, so perhaps instead of pounding suppliers for a better price, you need to start asking for a better value. There is also some value in understanding why phase separations occur in order to avoid it. 

Phase separation will occur if the ink contains too much liquid, and will occur faster at higher temperatures and under higher levels of shearing forces. Shearing forces are divided into two discrete groups; shear stress and shear rate. Both of these are capable of changing the viscosity (thickness) and therefore the phase stability of the ink. 

Shear stress, in our context, is a result of the force applied divided by the area covered. It is not about how hard you push, but about the shape created between the blade and the mesh during the print stoke. There are two terms that come into play here: footprint­— the amount of blade in contact with the mesh during the print stroke; and funnel—the shape formed between the blade and the mesh during the print stroke. 

When creating prints where the main objective is color, the footprint should be bare-bones minimum, no more than one mesh period. The funnel should be formed as close to the ideal zero-angle as possible. Angling only forces phase separation and slows print speed. The ideal pressure in PSI is about half of the printing tension, so the ideal is between 20 PSI and 35 PSI.

Shear rate, in our context, is a result of the clearance between blade and mesh divided by the velocity of the blade. Unlike most conventional pumps (ways to move ink through a screen), ours is open at the top and sides which is good news; the velocity can be extremely high without harming the ink and speed actually improves the print.

But, in order to use shear rate to your quality and productivity advantage, you must minimize the shear stress (pressure). A reduction in squeegee pressure is the key to increasing speed, but speed is good for profits. And, at higher speeds, ink systems used for colored prints transfer the mesh based on ink ejection. When the inks are ejected versus extruded, the finish of the printed ink is optimal. Mesh marks, streaks, pinholes and mottling are non-existent and, if printed at the zero-angle, there is no abnormal dot gain even at top speeds… and even on top of an under base.

So the key to reducing phase separation once on press is to minimize blade pressure and optimize shear-rate. But before you get to press, there are a few things you can do to maximize quality and productivity.

Adjusting off-contact

Colored prints in essence are shear-thinning, fluid-like prints that have a relatively soft finish and are intended for light-colored backgrounds. They print best on a calibrated press (the likelihood your press is set within reasonable tolerances is rare; check yours). Once the platens and screens are on parallel planes, we can begin to discuss the off-contact distance. If the distance is too high, you will need to use a variable-hinge squeegee blade. But if the distance is too low, ink transfer is based on extrusion and this causes phase separation with this type of print. The liquids in the ink are absorbed into the garment… water-based inks dry, plastisol inks build.

To help upgrade your screen mesh be certain of the count and thread diameter of the given mesh. You will find the mesh manufacturers often use one thread diameter for three or four mesh counts. Get a sample of mesh with the same thread diameter at one count lower than you are currently using.  This mesh will not plug as fast with water-based and will be less likely to cause build-up with plastisol. It will also have comparable durability, reduce ink consumption and improve the hand of the shirt. (Editor’s note: See “Thin Thread Mesh,” Printwear June page 90 for more on mesh count and thread diameter.)

Printing tension needs to be between high 20s and high 30s for virtually all ink systems. Printing tension is a combination of static tension and the tension increase as the blade puts the mesh into contact with the garment. Avoid too much printing tension as it will directly cause phase separation.

Since you are likely to use both water-based and plastisol ink systems, select an emulsion that has the highest water-resistance and wet strength and is suited to your exposure needs. It is always best to re-calibrate your exposure, but it should remain about the same. The thinner-thread mesh will have improved exposure on the squeegee-side of the stencil and make it more resistant to water-based inks. 

Select a flood-bar that is not rigid and will conform to your screen mesh without damaging it. This flood-bar will allow you to pre-load the mesh, improving screen stability, and permit high speed printing with excellent coverage. Select a squeegee blade which will run at or near a zero-angle—straight up and down with constant pressure from center to edge. These blades will maximize water-based and plastisol stability. Colored prints are shear-thinning and can and should be transferred with blade speed instead of blade pressure. If your blade is set to an angle or if it is buckling, you’re the one to blame for the drying in and building up.

Next month we’ll get into the bane of the existence of most press operators and ink companies alike: opaque white inks.

Commoditize This

Calibrate your press, look into ancillary materials to see if they are meeting your needs, then check for the following to make your colored screen prints less of a commodity:

• Once the integrity of an ink is off course it is impossible to adjust, you can tolerate it or replace it, but no one can alter it once it’s made. 

• Water-based ink’s stability in the screen is diametrically opposed to its cure speed… pick one because you can’t have both at any price.

• Build-up is a preventable ink issue. If ink builds in the center; print faster. If it builds at the edges, reduce the printing tension. If ink builds in both areas simultaneously, either switch ink series or brands.

• Colored T-shirt prints are most often seen as commodities. If you apply the same label to the inks, you are likely to find their usage unaffordable.

• When you find an ink that is screen stable, it has immense value. If using one that is not screen stable, it costs too much… even if it’s free.