Screen Print Methodology: Prepress Considerations

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/.

Editor’s note: This article is the first in a series on the methodologies of screen printing, where Joe Clarke will explore how to get the best possible quality prints by putting ink to mesh.

Since the dawn of T-shirt Time customers have wanted their screen-printed apparel perfect, free and now. Only recently it’s gotten harder to deliver on this trilogy of buyer demands. The tools of the trade are virtually same as they were two decades ago—maybe there’s a bell here and a whistle there but there is not a lot more process support. Conversely, the imagery has become more challenging, both dimensionally and technically. Run lengths are down because retailers’ SKUs are down. Alternatively, run time as a percentage drops because of shorter-run lengths and, in addition, set-up time increases because each order is now more complex and therefore each job becomes more of an on-press experiment. Add to this the fact that virtually all print shops downsize when demand changes to curtail negative cash flow which, even if it is temporary, often times leaves enough labor for the order to be late, and we see how this perfect-free-now trifecta is so difficult. Follow these general recommendations and you’ll have far fewer surprises and more often get what you would expect from your artistic efforts. 

Success by design

The number one priority for artists is to initiate the building of garments that flow through production and sell… at a profit. To pull off this formidable task the artist must have not only specifications for what production can reproduce but, even more important, they must have tolerances for every single one of those specs. In other words, art needs a target but you also need a practical amount of wiggle-room. 

Artists must realize there are two components in the screen-printing process—the ink and the pump that directs the ink to the garment (which includes everything except the ink).  The pump has to have the capacity and rate to handle the ink, so let’s first look at the artist’s palette of what can be achieved with their inks.

The image will be generated to achieve some combination of four styles of print—transparent, colored, opaque and textured (see fig. 1). In order to get a transparent style print, think of the deposit of ink as a stain with very little texture and excellent drape. The preferred substrate color for transparent prints is white, be it the garment or printing on an under-base. The inks will need to be fluid-like and very shear-thinning (shear describes applied energy or a disturbance to the ink which includes; stirring, shaking or squeegeeing). Now that we have the end-result identified we’ll begin to build the pump.

Priming the pump

Everyone talks about “count” when mesh count is really a result of the diameter of the thread and the dimension of the opening between the threads. To get a transparent stain type finish, the thinnest thread and the mesh with the largest opening (that will have the highest percentage of open area) will provide the best results. The mesh is the primary determinant of how much ink will be deposited. Its tension level needs to prevent image stretch and to lift the mesh out of the wet ink—that’s it, nothing else. Since we will have a fluid consistency of ink (see figure 1), a thin thread and a large opening we can run bare-bones pressure to eliminate distortion and it is a snap to lift the mesh out of the wet ink deposit. The constraint to your mesh selection is its durability on press but don’t allow a poorly-calibrated press to dissuade you from making the best fabric selection. 

The stencil needs to provide some level of resolution (area) and some level of edge definition (volume). Emulsion over mesh (EOM) does not drive the resolution, but rather is a result of building a specific stencil flatness which is the metric for resolution. Transparent prints will at times warrant very fine detail, so the RzS1 (stencil flatness [substrate] side one) must be very low or very flat. The RzS2 (stencil flatness [squeegee] side two) modulates the ink volume at the edge of the image which is not too critical even up to 65 Lpi halftone shadows. In the case of edge acuity or definition, the EOM increases the stencil thickness as needed for added ink volume. For transparent prints, the EOM can be minimal, the RzS1 should be minimum and the RzS2 can be moderate (see figure 2). 

Transfer station

It is time to discuss which of the two methods of ink transfer we will use when we get to press—ejection or extrusion—and, when we get to press, what the parameters of the blade are which support this type of transfer. A transparent print will fit the ejection method; the inks will be fluid and shear thinning (these become thinner as soon as they are pushed by the squeegee). The screen mesh selection indicates a thin wet ink film and not one which pushes the volume capacity of the mesh and stencil, such as with high-density printing. Ejection’s counterpart, extrusion, defines the transfer method required when the ink is not shear-thinning (usually very high viscosity) and/or when the mesh has a thick thread and a small opening, which is not at all the case for our highly fluid inks necessary for a transparent print. 

Most squeegee blades are bought (and beaten to death on press) based on their durometer. Once again in our attempt to improve communication between departments we define “durometer” as a result not a core variable. First, the durometer is a name, not a specification, and this name changes with age, temperature, usage and sharpening. Case in point, if you don’t have a meter, you can’t know the durometer. In addition “durometer” doesn’t tell us anything about how the blade intends to or will print on press (how it flexes and compresses).

Our transparent print needs a blade which has moderate flex and compression. The flex strikes the balance between the footprint and the funnel—the footprint is the amount of blade in contact with the mesh during the print stroke and the funnel is the shape of the divergent tunnel created between the blade and mesh during the stroke. 

There are a myriad of optional edges but for our purpose a flat edge is best suited. The blade is responsible for print speed. Since high print-stroke speed equates to both improved quality and profits we will try to minimize the initial angle to between 0º and 5º. Although it may look like the blade pushes the ink through the mesh it just ain’t so. The necessary amount of pressure must allow dimensional accuracy and allow the mesh to pull itself out of the wet ink. Adding any amount of pressure above the tension of the screen mesh puts the operator on a slippery slope, but we’re allowing an extra 5 N/cm² of pressure to allow for press calibration, screen tension and so on. 

Footprint is minimized by controlling how much contact the blade will have with the mesh. The mesh tension levels and speed of the print stroke optimize the length of our funnel to make a tight seal immediately after the ink is transferred. This means we can run as fast as we want to while improving the quality of the printed image in terms of color consistency, matte down and so on. The only constraint to print-stroke speed in our example is the screen mesh and it is more than flat enough to allow us to fly.

Reference Point: Methodology Terms at a Glance

Durometer—the metric which gauges the hardness of the blade (based on the Shore-A scale for elastomers).

Ejection—one method of transferring low viscosity inks through mesh.

EOM (emulsion over mesh)—stencil thickness as determined by the amount of emulsion used to expose a stencil.

Extrusion—another method, opposite of ejection, of transferring high viscosity inks through mesh.

Footprint—the amount of blade in contact with the mesh during the print stroke. 

Funnel—the shape of the divergent tunnel created between the blade and mesh during the print stroke.

Mesh count—a result of the diameter of the thread and the dimension of the opening.

Shear (thinning)—describing the viscosity of inks and how easily they can build momentum to move through mesh.

Editor’s note: As you review and then prepare your own chart with your specifications and tolerances, if you have any questions please email joeclarke@cprknowsjack.com.