Avoid Moiré and get Consistent Results in the Process

How to Print 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/.

Writing a how-to essay about four-color process, simulated-process or index color would be a lot sexier than one about halftones. But recent inquiries, feedback and observations all indicate we should walk before we run. So, here we’ll look at how to print a single-color halftone.

Similarly, printing a 300-line halftone on a garment is nifty, but not what’s going to pay the bills for the mainstream audience. In that spirit, we’re also going to touch all the bases for the components and the construction of a more pragmatic line count in a single color. This paper will address what you can enlist from Photoshop and how to predictably translate the digital information to a knitted garment.

Types of halftones

In simple terms, halftone printing is the process of reproducing an image with dots of ink. Each dot puts down one of many tones of color (or gray). The dots are typically one of three different shapes—round, square or elliptical. In screen printing, there is a tradition of using elliptical dots. At our 65 lines per inch (LPI) line count, this dot shape is fine, but be forewarned; at higher line counts, the typical transforming elliptical shape can lead to tonal moiré in the quarter-tones. Elliptical dots smooth the optical contrast jump in the quarter-tones, mid-tones and three-quarter-tones. (Note: LPI is the resolution of a halftone image—the higher the LPI, the higher the density of dots, and thus, the higher the resolution of the image. Moiré is a visual pattern in the image.)

The highlight dots should be clipped at 6 percent. Below this dot level, it is very difficult for the observer to distinguish and even more difficult to maintain consistently, shirt after shirt. We want some punch out of the shadows, so plug the maximum tonal range at 97 percent if you want the image to pop; push the 50 percent dot to 60 percent and then smooth the tonal curve.

There are dozens of patterns of moiré and many causes for each of them. The answer, then, to the question “how do I get rid of moiré?” might take a while. In the meantime, we already know the mesh count, thread diameter and tension level, so we’re going to run the halftone at 14 degrees to mitigate any signs of radial moiré. (We’ll also take the necessary precautions to eliminate the other types along the way.)

Screen mesh

High-modulus (that with a high elasticity), high-volume screen mesh in a 280/35 geometry, when used properly, is not directionally dependent and thereby prevents frequency moiré. It also allows the ink to whistle through with minimum pressure, minimum drag and maximum stroke speed with our intended blade. This mesh can deposit ink to produce a soft hand and intense contrast and fits nicely with the pigment-loading of our ink selection.

To screen print most garments efficiently, we need to end up with tension levels in the high-20s. Therefore, if we get to press at low 20s, we’ll need too much off-contact gap to make it fly. Better to get to press in the mid- to high- 20s. The modulus and construction of the 280/35 mesh will allow this tension level to print without a hitch.

Two caveats: if using retensionable frames, do not retention—it will cause progressive moiré and there is no angle out of it. Also, if using pre-mounted mesh to plug into the frame, be careful. All mounting is not equal—if the moiré pattern looks like the grain in a fine piece of oak, the mesh may be erratic during the mounting process.

Stencil…and then some

At one time or another, all of us have fought with dots on Ts. There are two primary reasons. First, we relied on the mesh to control the dots and, second, we used a hunk of rubber instead of a printing blade. This pair of stencil-and-blade should be selected to dovetail each other in order to upgrade the state of all woven screen mesh. Remember, the mesh creates an hourglass shape—wide up top, skinny in the middle and wide again at the bottom. This causes fluctuations in the volume of ink delivered to the specific tones and this imbalance can’t be remedied in Photoshop. So, we must fix the mesh with the stencil.

The primary goal of the stencil is to be consistently flat on the garment side. Nothing is better at achieving this than capillary film. If you like a thick stencil, you likely don’t understand ink transfer. If you want the optimal thickness for our mesh and intended halftone use, we want a minimal flatness on the garment side (RzS1) and a minimal emulsion over mesh (EOM) or “thickness” on the garment side of the screen. An interior coating of emulsion to the squeegee side of the screen will balance the tonal range and eliminate any contrast jumps due to fluctuations in the volume of ink delivered. It will also do wonders for eliminating stoppage on press.

Press Parameters

Given our tension level and the ink we’re about to choose, ensure the press is calibrated (that is, that the carriage, blade edge, mesh and platens are all parallel) so we can set the off-contact distance to a consistent 0.150" (between 1/8" and 3/16"). This will give us sufficient printing tension to transfer the ink.

For this mesh and the generic halftone, we’ll leave a gap between the mesh and the flood bar a couple thousandths of an inch. When we pre-load we won’t use a metal flood bar and we don’t ever preload halftones—it causes an erratic shift in the tonal range. Run the flood bar at 80 percent of maximum speed—it reduces the lag time between first and last flood.

Blade Metrics

The edge of the squeegee blade should be 50 micros at a 2 to 5 degree initial angle in order to fit the mesh. The profile of the blade should be hinged so a 16" blade fits the resistance differential of a 23" X 31" frame. (Note that all screens are more elastic at the center than at the edges.)

With this mesh, tension, press settings and blade, the pressure (downward force) can be minimal. If you are using pneumatics, I suggest to dial the pressure in by position then use the minimal amount of air pressure (25 PSI should do it) to balance the pressure platen-to-platen. In addition, the minimal pressure eliminates drag between the blade and mesh.

The key to a quality image is to use the fastest possible print stroke. We’ll set ours at 80 percent of maximum speed. With the vertical angle, the blade acts like a snow plow. It pushes the ink so it climbs higher along the plow. The mass (weight) of the ink puts pressure on the ink nearest the point where the blade and mesh meet, and causes this ink to thin. This means it takes itself through the mesh without all the squeezing, stretch and dot gain, and progressive moiré intrinsic to the traditional process.

When using “economical” shirts (those with a short staple length and low fabric mass), we need lots more ink volume. So, we’ll use the beveled edge of the blade, keeping the angle, pressure and speed the same. On that rarest of occasions that we get to use a top-grade shirt (one with lone staple length and high fabric mass), we use the square edge. Again, keep all settings the same.

Ink is last but not least

We always prefer an ink with low tack—i.e. has a consistency more like cold cream than honey. For halftones, we like a short fluid momentum—i.e. runs more like Crisco than cold-cream. Since we have a strong, thin mesh with large capacity, a stencil to throttle the volume, and a blade to deliver it, Crisco it is!

In ink-maker jargon, this means we want a “high absolute viscosity,” the lowest possible yield point and low plastic viscosity. Follow all these steps, and we’ve matched the pump to the task at hand, and the ink to the pump, which makes for predictable and consistent halftones at top production speeds.