halftones

Five Steps for Creating Clean Halftones

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/.

The conspicuous raw material cost of transparent film positives, the proliferation of point-‘n-click color separators, the plethora of computer-to-screen (CTS) systems, and even digital direct-to-garment has played its role in rendering high-end, service bureau film positives passé. This is good in terms of cutting material costs and saving an oil well, but we also lost a quaint value proposition—a set of films that could be inspected, a proof that occasionally represented a viable target, and the expertise of a specialist. The absence of the third in this trilogy renders the first and second points moot.

It seems we may have traded for quick and easy at the expense of predictable and consistent, and these are paramount when it comes to cost/benefit on press. We have abandoned the person arguably the best at assessing our on-press parameters and adapting films to fit. All of this just puts the pressure on us. So today, we must be more cognizant of the “square hole” (mesh opening) through which we push the round dot (halftone).

Mesh Selection

We always opt for a high volume mesh, but when we’re making dots for halftone printing, there is more to it than the veritable “thin thread and a low count” because we have to accurately shape the tiny dots. The minimum dot should be about the size of the mesh opening and the diagonal distance of the threads. So, a 280/35 high modulus mesh will do very well with a 50 minimum highlight dot and a 60 maximum shadow dot. Our criteria is easily met with a 65 lpi halftone.

Whether we use this parameter or any other, be sure to put the gloss side of the high-volume mesh on the garment side of the screen to best prepare it for the stencil. And don’t be fooled by any or all the tension meters on the market or pre-stretched mesh. Instead, use a thread counter to be sure the mesh is balanced—most aren’t (and we’ll never find the flaw with a meter).

Nonetheless, even the right mesh is the arch-enemy of the dot! Think of it this way: the mesh simply gives the blade something to push against and it gives the stencil somewhere to hang out. So we need to use the stencil and the blade to fix the inequities in the mesh.

Stencil selection

Consider the nature of the beastly mesh. It has a hole that limits the shadow dot, a knuckle to impair the highlight dot and a thickness that delivers too much ink. Plus, it is a classically stupid pump—its cross section is shaped like an hourglass, big up top, tiny in the middle and big on the bottom.

The good news is we can fix it with the stencil by using Capillex CX on the bottom of the screen. The emulsion over mesh (EOM) is near perfect for plastisol on T-shirt printing, and it partially fixes the dot by turning that hourglass shape the mesh creates into a funnel. To finish fixing the shape at the top, use an interior coating of Autosol 8000, which will “turn a funnel into a tunnel.”

Screen calibration

Eager as we are to test out the high-volume, high modulus mesh with the intellistencil, we first need to take time to calibrate them both. To do so, use an exposure calculator to precisely predict the exposure in time or units. 

Once that calibration is completed, monitor the processing—the application, exposure and development—of each screen. Do so by creating test images along the perimeter of the screen. Since a break in a line is much more obvious than a shift in the shape of one dot, we can inspect the screen for consistent flaws in a series of lines. 

The preliminary reason we can’t hold the dots on press is if we don’t select the right mesh/stencil. But once we pair them, we must give the stencil sufficient exposure to make it press-proof. 

The practice of slapping a couple coats of low-solids emulsion on a high count mesh, letting it sit in the room for a while and then under-exposing the stencil to maintain highlight dots is pathetic. The stencil must be sufficiently dried and then adequately exposed. If you vary from the path in pre-press, it makes it a long way home on press.

Press calibration

Four press planes must be parallel at all points in the stroke; carriage, blade-edge, mesh and platen on each head and bed. If they are parallel, congratulations, you have a calibrated press. But most practitioners don’t understand what this means, exactly. There are three common answers to the question is the press calibrated. The first: “Yep! It came in that way;” the second: “You bet! Jimmy just tuned it up last year;” or my fave: “Uh huh. I never thought it would take an hour and a half!” So, in order to actually verify that a press is in calibration, run a test to see for yourself.

I recommend a “Nine Zone” test. Here, think of dividing the screen in nine equal pieces: three zones across and three zones down. Create one small test image and print it in each of the nine areas. (See Figure 1, above.)

Now, to print the test, choose a blade that adapts to both stroke and perpendicular axes without distortion, one which is suited to a 280/35 mesh. Set the flood-bar at minimum clearance without touching the mesh and run it at maximum speed. Then, set the blade between 3 and 5 degrees, using only enough pressure to transfer the ink, clean the non-image and clear the open areas of the mesh, and run at maximum stroke speed.

If we look at the three columns that are parallel to the stroke, we can optimize our off-contact gap. This also allows us to minimize both blade pressure and angle and ensures the blade has not fatigued. The three rows that are perpendicular to the stroke allow us to balance left-to-right pressure. Once we print an excellent Nine Zone image, we know the press is in calibration.

Now that we’re calibrated (or know that we need to be), back to dots. So far, this package of mesh/stencil and blade is optimized for halftones. If the dots don’t look near perfect, it is due either to the ink or the press. Good news: if the press is at fault, the dots won’t look consistent. But if the dots are consistently bad, you know you need to fix the ink.

Ink doesn’t think

A couple words on the consistency of the ink: let’s think about it in terms of honey and Vaseline. A jar of honey has high tack (stickiness) but low viscosity (urge to flow)—it pours onto the screen. The same size jar of Vaseline is low tack but high viscosity; we needed to scoop it out onto the screen. Honey hangs up in the screen no matter what we do, so we can’t run fast. Conversely, Vaseline runs at top speed and whistles through the screen and onto or into the shirt.

Now to discover which you are printing. Stir the ink thoroughly on a mixer and then set the press at a print stroke speed of two on the controls, this is likely to be about 8" per second. Print a test design, cure it, and set it aside. Then do the same thing at a speed of four, six, eight and 10/maximum. Observe all the images and compare them for ink tack (the nemesis of ink transfer).

At slow speed, we see skipping and that canine culprit colloquially called “puppy paws.” If the ink transfers at the same pressure at maximum speed, with the recipe above, we’ll see pristine dots from highlights up to and including shadows. If, at any point, we can’t use the high-shear method to increase speed to maximum, we’ve hit the tack limit of the ink. Ask the supplier for a reducer which will reduce tack and surface tension with minimum effect on the viscosity. Remember: viscosity’s not our enemy, tack is!

Taking the time to test and ensure parameters allows us to print excellent 65 lpi halftones on white shirts with plastisol inks at maximum load and cure speeds. But also remember the most important equation: for every corner we cut it will take 30 percent longer to get where we’re going.