Sequencing and Textures

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/.

What happens when a run-of-the-mill job turns mysteriously frustrating? Many print practitioners have fallen victim to the curious case of “disappearing” ink and maddening moiré. The scenario is set as such: a black T-shirt is to receive an over-sized, full-coverage print. A white under-base plays host to the colors—it’s flashed to where the under-base can accept a print on top. Next, the customer’s corporate color—a clean, dark blue image—goes on top of almost all the under-base area and it looks awesome.

Then, without flashing the royal, we print the second corporate color, PMS 032 red, on the white under base and viola…the white’s fine, the red looks pristine, but what little trace is left of the blue has shifted colors.

Being skilled technicians, we look at the underside of the red screen for the missing blue ink… nope, not there. In desperation, we look to see if the print fattened, but once again, nobody’s home. Now we’re fairly certain the under base is not absorbent, but the royal ink is most definitely gone.

This illustration depicts what happens when we use the wrong components to print a white, flash it, then print a low-value, super saturated, highly-transparent blue wet-on-wet and, finally, when we attack the blue with a red. The result is that the white looks great, as does the blue… until we step on it with the wrong red screen, blade and ink. (All illustrations courtesy the author)

So we make a second print, making darn (d-a-r-n!) certain the under base is bone-dry and completely sealed the shirt below. No less, when we follow suit, the final results are worse. Some major portion of the royal is nowhere to be found… so do we applaud Houdini or contact Holmes?

Elementary, my dear printer

Determined to find the final destination of our elusive blue, we must first look at the composition of our print. We assembled the worst raw materials we could find for our three-color disappearing act. For the white, we used a 110/80 low-volume screen mesh. We used tried-n-true conventional wisdom with respect to tensioning, yanking the 110/80 mesh up to 35N/cm² in the center and all four image corners, which then dropped to 30N/cm².

At the top of this figure, the blue we intend to print is represented. But, when the blue is over-printed wet-on-wet, the red screen will alter the color and leave “peaks and valleys” in the blue. The second box shows the peaks that appear darker blue because they are thicker deposits. Below it, we see a patch of light blue where the pressure from the 156/64 red screen “embossed” the blue. When we texturize a low-value transparent color on a white under base, the valleys overpower the peaks in area and change in intensity, as seen in the lowest box.

We’ll need to deposit more blue ink on the flashed and textured white than would be required if the surface were flat and smooth. So we sadly opted for a second thick, low-volume 156/64 mesh, using the same tensioning method as with the white. By the same convention, we always put a thick emulsion coating on the garment side of the screen and check the emulsion over mesh (EOM) to ensure the sharpest image edges.

Of course, before getting to press, we added “a good bit” of curable reducer so the blue would flow out without looking so choppy. But this “ad-asmuchasyouwant-ditive” didn’t make the printed color more uniform.

Finally, we look to the vast inventor of different colors of squeegee-rubber we’ve been using, but we seem to like the red more than the pink ones. Our automatic press is only two years old and it was calibrated by the factory authorized technician at the time of installation.

Pulling moiré out of thin air

The aforementioned mesh and tension ensure that we can print white with a really rough finish and a texture equivalent to a 100 lines per inch (lpi) halftone—in every inch of the mesh, there are 100 “hills” with 100 “valleys” in between. The blue’s also going to have a pattern, but not as much since it’s going through mesh with about 150 lpi resolution. But, since the blue is very transparent and prints on top of the white, the resulting pattern (called moiré) will be more complex.

The overprint is not going to look perfectly uniform because it is so transparent—where the blue ink fills in the valleys of the white, it will be darker blue; where it sits on top of the hills of white, it will be lighter blue. With the textured white printing through a 100 mesh and the undulating transparent blue on a 156, the two patterns will recur approximately every 4" and create large, wavy dark-to-light bands. This repetition/conflict creates a large field of optical moiré pattern, both from north to south and east to west regardless of the fact it is a composite of two solid images.

 

Ink don’t think… so the operator must

Now that we understand exactly what’s going on in the print, we can attempt to fix it. The initial step: create a better surface in the under base. The under base white should have matte-down first and smoothness second. The best way to matte the fibers is to pre-load the mesh with a beveled-edge flood stroke, and print with a beveled-edge and a very fast print stroke. The key to smoothness is to have sufficient off-contact (between mesh and garment) and setup with minimum contact between a “low-drag” blade and the mesh; print at maximum stroke speed.

Next, let’s look at the color. The corporate blue we chose is highly transparent with high saturation and low value. Blues tend to be very thixotropic—that is, the consistency is more fluid when it is agitated (stirred or squeegeed) and returns to a thicker state when it is allowed to sit. We need to modify it in order to behave more uniformly, but less expensive inks don’t really allow for this modification.

Also consider that the particles in blue ink are also shaped like tiny knitting-needles; very lightweight and high in oil absorption. They are best served through a high-volume screen mesh (higher modulus, thinner thread, lower count), so what we selected in the 156/64 mesh is precisely the opposite of what we need. To orient the pigment particles in the direction they need to be to transfer through the screen mesh, the stroke speed should be at or near 100 percent with near zero-angle and minimal pressure.

The red is every bit as mission critical as the blue, in this case. But the red is somewhat easier to print via bi-axial blades at maximum stroke speed and minimum angle and pressure through a high volume screen mesh.

For our final trick

It is always best to use the highest volume mesh for the required deposit to transfer at minimal pressure. Excessively high tension may make a “better” screen, but it usually ruins the production print. A nice, thick EOM on the stencil will increase the contrast at the perimeter of the blue on top of the white. However, this doesn’t help add clarity to larger, solid areas of blue.

To ensure this edge-acuity, be certain there is sufficient emulsion coating the squeegee side of the screen. Expose it sufficiently to where the interior coating won’t wash away during development.

Moving on to the print stage of production, minimize the residual pressure on-press by using a bi-axial blade for the blue ink. This will transfer beautifully with minimum angle, minimum pressure with maximum stroke speed. The white should be flat, not gloss, printed at sufficient off-contact gap and not over flashed.

Ask your ink supplier for a pseudo plastic (not thixotropic) blue—don’t be tempted to go with the “econo” shade of blue, thinking that curable reducer will fix it; it will do the opposite and make it worse. Using the recommended blade and mesh should minimize the necessity to use additives in the ink.

Follow the same recipe for the red (mesh, stencil, blade and ink) and be certain the residual pressure (also known as platen deflection) is zero. If you feel a certain need to adjust the red, ask for a surfactant (wetting agent) versus a viscosity depressant (thinner) or worse; curable reducer (the ad-asmuchasyouwant-ditive).

This prescription should clarify how to use some of the more troublesome inks—blue, white and red—and ease production frustrations. It’s essential to comprehend how each aspect of production affects the final product.