Myth Buster!

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/.

It used to be that our supplier sales rep was a prime resource for help and technical insights. But the market has changed—since we commoditize most of the raw materials, vendors can no longer afford to share their insights door-to-door. The pittance we’ve saved in raw material costs (RMCs) is dwarfed by the inevitable increase in conversion cost, caused by these “would-be-savings” on supplies.

Nonetheless, absent our itinerant merchants today, we surf the ‘Net for insights and information. It’s free, after all… but all too often this price is considerably more than the information is worth. We are flooded with a proliferation of innocuous, incomplete or improper advice, not to be confused with insight.

Our Internet experience has ensured only one truth: it is way faster to get the wrong answers these days. Here, then is a review of some of the most misunderstood practices. Read on to debunk some of the ‘Net’s “conventional wisdom.”

Pre Press

Myth: Tension should be high as possible without damaging the mesh or distorting the frame.

Consider: Base the tension on the maximum gap within distortion limits in order to make a stable screen. Such a screen allows mesh extraction with zero lag when printing tacky inks on a flashed under base. The ideal is around 25N/cm²; anything higher can cause inconsistency and premature failure in the screen room and on press.

Myth: Tension in the screen room should be consistent at all points of the image.

Consider: The static tension should anticipate the change in tension during the print stroke (dynamic tension). The minimal tension suitable for stability and extraction is about 25N/cm².

Myth: Image detail is primarily controlled by the mesh.

Consider: The mesh only interferes with the image. The stencil is responsible for image detail. It is best to select the lowest count mesh with the thinnest thread that will supply the proper ink thickness. Then, use the stencil to control edge volume and definition.

Myth: Stretch in the stroke direction is best controlled with the use of a tension meter.

Consider: The preferred method of “tensioning” is by percentage of elongation, but stretch is most often due to inadequate off-contact, poor press calibration and soft, buckled, single-axis squeegee blades.

Myth: Count is the primary aspect of screen mesh.

Consider: The mesh count is hardly relevant because it hides the important part—the ratio between the thread and the tensioned mesh opening. Select a mesh that provides the desired thickness of ink then select the lowest open area at that deposit.

Myth: Lower mesh counts always deliver more ink volume.

Consider: The mesh that delivers the highest volume is a lower count… but one with the thinnest thread. Just opting for a lower count may literally reduce the ink volume but it also forces the blades to deliver even more ink.

Myth: Moiré is most likely due to improper angling on the mesh.

Consider: Most moiré is due to irregularities in the mesh and tensioning process, not the angular relationship between mesh and film or file.

Myth: Emulsion thickness on the garment side of the screen is the most important stencil variable.

Consider: The best stencils in typical applications are as thin as possible to make the substrate side flat. Any greater thickness (EOM) tends to interfere with all but fine lines and dots by causing a thicker deposit at the perimeter and thinner deposit in all other areas. 

Press Setup

Myth: Ideal off-contact should be as low as possible based on press calibration.

Consider: The ideal gap is one that is as high as possible within the limits of distortion. Lower gap causes image drag, which manifests in many print inconsistencies. If gap is too high, there is image error in both stroke and perpendicular directions.

Myth: The flood-bar should never preload the mesh with ink.

Consider: Metal flood-bars will rip the mesh but flexible (polymer) flood-bars can float, skim or pre-load to meter a wide range of ink volumes without damaging registration or the life of the mesh.

Myth: Platen deflection is an intrinsic part of the screen-printing process.

Consider: You can avoid deflection by using a bi-axial blade, and ensuring the optimal gap and downward pressure equals the resistance of the mesh. The result is a featherweight pressure on the platen.

Squeegees

Myth: Durometer is the best gauge of squeegee-blade performance.

Consider: Durometer is unstable and varies with time, use, ink and cleaning methods. We want to know the fit of the blade to the mesh (compression) and the deflection of the blade at zero off-contact gap (flexion).

Myth: Ideal blade angle is greater than 5º (or 85º)?

Consider: The ideal blade angle is zero, but not all conditions will permit it. Angle begets pressure, the pair begets buckling, and buckling compromises quality and consistency. Plus, it destroys productivity. Stay as close to zero as possible, minimize pressure and maximize speed.

Myth: A buckled blade delivers more ink in all cases.

Consider: The greatest ink volume delivered by the fill and print stroke is a product of a blade edge that fits the mesh, and runs at high speed and minimum angle. Buckling is a slippery slope; the amount of buckling changes from platen-to-platen and always varies from the center to the edge of the screen.

Myth: Squeegee edges should always be sharp and square.

Consider: Generally speaking, blades shouldn’t be sharpened for use on mesh counts lower than 100. Sharpen as needed on 100 to 200 meshes, and keep the blades factory-sharp for use on 230 and higher.

Myth: Faster blade speed reduces the volume of ink.

Consider: If the blade and pressure settings are more suited to each other and to the task, higher speed optimizes the ink volume. 

Inks

Myth: Some companies can’t afford premium materials (frame, mesh, emulsion, film, blades and ink).

Consider: I have never found this to be the case. Complacency and isolated purchasing practices are the shortcut for lower RMCs. But they have much greater conversion costs in terms of predictability, consistency, quality and output.

Myth: High-viscosity (thicker) inks are always more difficult to transfer.

Consider: Viscosity is not often the problem, but the tack of the ink can be an issue. Case in point—bees’ honey is pourable (has a low viscosity) but high tack (it’s sticky). Cold cream is high viscosity but with a low tack. I hate printing the former and love printing the latter.

Myth: Inks always build up on the underside of subsequent screens.

Consider: Buildup is unequivocally an ink problem, but such is not a property of a properly-manufactured product.

Myth: “One-hit” (single-pass) white inks are rare or non-existent.

Consider: White inks that achieve sufficient opacity in a single stroke are a product of an integrated printing system. It is not likely that any changing any single variable will improve products and methods sufficiently. Orchestrate and reap the benefits.

Myth: White ink is difficult to print primarily because it is dilatant (gets thicker when it’s pushed with a squeegee or stirred).

Consider: Although white ink is dilatant (it thickens when put under force), the real issue with white ink is its tacky nature. It sticks to the mesh and fights the filling, transfer and release processes.

Myth: Matte-down is a parameter built into a premium-priced ink.

Consider: Matte-down is a result of high print-stroke speed. Stroke speed is best facilitated by a match between ink tack and mesh volume, but is the result of using the correct flood-bar and printing blade.

Finally

Myth: You can believe everything you hear and read from experienced printers.

Consider: Although it may seem tenure and the holy-grail of “hands-on-experience” are one in the same, you may be sadly disappointed when you find tenacity never trumps technology. Be careful where and from whom you get advice. Trust, but verify in all cases.
Thanks for reading!