Screen Print Methodology: Pumping White

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/.

There are two generally accepted beliefs in screen printing. The first is “the surface to be printed must be flat” and the second is “you can’t print on air.” Most T-shirts, a substrate that is 2/3 to 3/4 air-space, are highly compressible, anything but flat and nearly all of them use white ink. 

Screen printing is a hydraulic pumping process—the action of moving a fluid under pressure from Point A to Point B. Opaque white ink is by far the most difficult color to pump. So, borne from the burden of printing opaque white ink on T-shirts, a new axiom appears: “use discharge whenever possible.” But, to discover what to do when discharge is not the best option, read on.

Cost at hand

If you get to press and you’ve brought the wrong tools, the time and materials cost is staggering—clean up, rework the files, films, stencils and screens and try again. It may be the case that the art department selected the mesh to get the detail they wanted without regard to the fact that the job calls for detail and the deposit of white. It may be that, for soft-hand’s sake, you have selected a thinner mesh count. It may be the ink you get for no-bleed is akin to arctic-circle roofing tar and you can’t make it look right. But whatever the cause, it often happens on press that the print operator can’t get the detail, hand, coverage or deposit required of the task at hand.

Consider: Ink goes through the screen in one of two ways—either by ejection or extrusion. Run this very quick test to determine which: An ejectable ink deposits more ink when printed at higher speeds while the deposit of an extrudable ink decreases with an increase in print speed. 

You probably already know which is which. Certain white inks are ejectable; a lot more fun to print but not as opaque and do not matte down fibers as well. Extrudable white inks are those opaque inks that bury the fibers but, when complete, the texture of the print is something you could refinish furniture with. Nevertheless, even when the mesh is filtering too much ink or when the ink refuses to flow, we need to make the print. Going back to films or screens is rarely tempting so we resort to the only two on-press tools—the flood bar and the squeegee.

The great flood

In “The Before Time,” screen printing was done on cheap, temporary window signs with a thin layer of low-solids, very runny ink. When the first semi-automatic presses arrived on the scene, they brought with them the need to get ink from the back end of the screen to beyond the front end of the image between each stroke. Finding that these “poster inks” dried in the screen, we reasoned we could reduce the evaporation rate of the ink residue in the screen by intermittently covering the top of the screen with fresh ink. But the (multifilament yarn) meshes were so open that a thick flood of ink on a single-color automatic press would cause the ink to drip through the mesh before the print stroke. So we engineered an inflexible, completely rigid, sharp-edged blade to skim a wafer-thin layer of ink over the mesh between strokes.

Enter the imprinted T-shirt onto which we’re applying prints with the same old, metal poster-ink blade. Only now we print ridiculously close to the inside edges of the frame, white inks are never runny and we rarely want a “wafer-thin” layer of ink (we’re talking about printing on a darker background).

Setting the bar

There are two keys to successfully transfer white ink. First, pre-load the mesh with ink before the print stroke and don’t push so hard. The best way to pre-load is with a semi-rigid, softer-than-PET polymer flood bar to protect the screen mesh. There are three positions or modes in which a flood bar can perform—float on top of the mesh to return the ink from end to start; ski with a flat edge onto the surface of the mesh to pre-load a minimum amount; and snow-plow with a beveled edge to maximize the pre-load of ink into the mesh. 

With an open mesh and an ejectable fluid white ink, floating the flood bar to just skim the surface is preferred. With an extrusion white and the same mesh, you will find the ski mode works very well. With the extrusion white and a more closed mesh you will like the results of a snow-plow flood stroke.

With most white inks, you will find it is very helpful to pre-load the mesh. This facilitates matte-down, greater opacity, a smoother finish and allows a higher print speed. Now that we understand the options with a flexible flood bar, the second set of options come from the squeegee blade. 

There are two primary drivers for squeegee blade performance—footprint (the amount of blade in contact with the screen mesh during the print stroke) and funnel (the cross-sectional shape formed between the screen mesh and the squeegee blade). These can be adjusted for printing needs utilizing the central and extreme profiles offered only in variable-hinge blades.

The footprint of the squeegee blade should always be minimized. The best funnel is formed with a vertical or near-vertical blade with a consistent progressive curvature nearest the screen mesh. The footprint as described should approximate one mesh period to permit minimal pressure and to eliminate dot gain and smearing. 

The funnel as described regulates the volume of ink. The funnel should never have an hourglass shape. If the flow of ink causes a neck or restriction in the funnel, the fluid pressure rises and the volume of ink that gets transferred diminishes. And the printed image will be ink-starved forever. (This is why, on press, you have seen that more pressure might help the volume but only to a point until it creates an 86 to 160-grit sandpaper finish.) The best method of regulating white on press is with both the flood bar and squeegee—not pressure.

The beveled edge

Printing with soft blades is like printing with a ripe banana—they buckle over at the slightest touch, have a large inconsistent footprint and a very delicate funnel. Just when the coverage starts to look good… bam! The funnel gets crimped and opacity and finish are lost. Hard blades pinch the mesh at the ends and must be run at steep angles. This forces us to print at slower speeds and thereby produces the sandpaper finish we all object to.

A bevel uses leverage to cause the very end of the blade to slightly curl to maximize the ink volume. If we run such a blade near vertical (no angle whatsoever), it’s possible to maintain excellent detail at top print speeds while getting the coverage and opacity we seek. Only the variable-hinge blade allows the footprint and funnel to be consistent even when printing close to the edge. By varying the position of the bevel with respect to the variable hinge, the deposit and pre-load can be adjusted on flood bar and squeegee based on the needs of the print and the constraints of the mesh.

The white ink story

If the desired result is for the finish to be satin-smooth, maximize pressure between the blade and the mesh; not between the blade and the platen (make certain your off-contact distance is sufficient) and print at top speed. To maintain the sharpness of the printed white, the footprint of the blade must be minimized (zero-angle and minimal pressure). If the goal is for the ink to matte down the surface, pre-load the mesh with a blade that won’t harm it. If more opacity is desired, a beveled edge on the back side of the hinge is best. 

Paradoxically, if you eject an extrusion-white or if you extrude an ejection-white, they’ll both end up feeling like sand! Finally you can combine the blade configurations and flood bar modes for varying levels of deposit. 

The next time you are about to reach for shape, curable reducer, soft-hand extender, or new screens and stencils, consider instead adding the right flood and squeegee… and you’ll be much better off in the long and short run. Next month we’ll discuss the nature of and options with white inks.