by John Kaverman, president, Pad Print Pros LLC
About 20 years ago, we conducted some tests on more than 60 Mold-Tech texture plaques to determine “printability” of various textures using the pad printing process. A lot can change in two decades, and – in this case – what changed was the equipment. A pneumatic printer was used 20 years ago, but we’ve been working almost exclusively with stepper motor-driven pad printing machines since 2000. It was time for another test.
For this test, 42 different Tenibac-Graphion, Inc. textures and 79 different Mold-Tech textures were printed, using both a stepper motor-driven and pneumatic pad print for side-by-side comparison.
The plaques were black in color. Tenibac-Graphion’s plaques were molded using a generic polypropylene resin with no filler. Mold-Techs plaques were molded using ABS. No pre-treatment was performed because the testing process wasn’t looking for results relating to adhesion – only coverage of the texture/print quality.
For clichés, straight etched steel with a depth of 25 microns was used. The ink was a high-opacity white, conventional, solvent-based, two-component pad printing ink with 4:1 hardener. This ink is commonly specified for automotive interior applications, where a wide variety of different materials and textures are utilized. The pads used were molded from a high-grade silicone, in a medium shore (hardness). Plaques were single- and double-printed.
The respective machines were set up to have similar cycle times, with the stepper motor-driven machine programmed for a 0.05 second delay on the plaques at image transfer. After printing, each plaque was visually evaluated for ink coverage/print quality using standard industry parameters: over a neutral colored background and under uniform, non-directional CWF (cool, white, fluorescent) lighting for a period of five seconds at a viewing distance of 18″.
To help ensure unbiased results, two individuals inspected the plaques: an experienced pad printing technician and a front office administrator who has never touched a pad printer. The inspectors gave each plaque a numerical rating from 1 to 4:
1 = failure
2 = marginal
3 = pass
4 = excellent
The inspection results for both the pneumatic and stepper motor-driven machines then were averaged to determine “Pass/Fail” or “Marginal” printability.
Many of my previous articles have been about the benefits of stepper motor-driven machines. One of the benefits that Ive repeatedly mentioned is the ability to program delays in increments of 1/100th of a second at various points in the printing cycle. When printing on textures, the theory is that delaying the pad momentarily on the substrate (texture) allows the air within the texture more time to escape, resulting in fewer voids (pinholes) in the ink film after transfer.
Of the 121 plaques printed, the plaques printed with the stepper motor-driven machine (with the 0.05 second delay at transfer) had superior results on 64 out of 121 samples (52.9 percent). The plaques printed using the pneumatic machine had superior results on 13 out of 121 samples (10.7 percent). Results were equal on 44 of 121 plaques (36.4 percent).
Textures vary in three distinct ways: depth, sidewall draft and “frequency,” all three of which come together to describe the topographical characteristics of the texture. As with my initial experiment 20 years ago, the results of this latest testing indicate that the “frequency” (number of peaks and valleys within a given surface area) has a larger part in determining “printability” than depth or sidewall draft. For example, one texture with a specified depth of 0.007″ was given a passing grade for “printability” whereas another textured with a depth of 0.0015″ was given a marginal grade for a single print. There are instruments that can measure surface characteristics, but they’re not inexpensive.
Additionally, textures can be either negative (below the surface of the part) or positive (above the surface of the part). At first glance, the difference is not easily perceived. It stands to reason, however, that air can more easily escape from between the surface of the transfer pad and the substrate on a positive texture.
Tables 1 and 2 show the results of the testing.
Texture printing tricks
It is important to realize that even if you’re tasked with pad printing one of the textures that we’ve deemed to be marginal or worse for printability, you may still be able to work around it.
As indicated by our testing, it is possible in some cases to cover a marginally printable texture by simply multiple printing to build up the ink film thickness. It depends largely upon whether the texture in question is positive or negative. Over-printing on positive textures can sometimes increase the ink film thickness enough to provide sufficient coverage. To ensure optimal ink transfer efficiency, we recommend blowing air on the part in between passes or, if youre using a stepper motor-driven machine, programming a delay between passes to allow the first pass more time to dry before the second pass is transferred.
Printing multiple times on negative textures appears to have little or no effect because the tiny voids left behind after the first pass are difficult to fill on subsequent passes.
Sometimes it is possible to “bridge” textures by transferring the ink film only to the high points or “peaks” of the texture. If it is possible, program in a delay and/or blow clean, dry air on the pad before transfer to allow the ink film to become more cohesive (or drier) throughout while on the pad, so it essentially traps air in the texture, thus minimizing or eliminating voids.
It’s counterintuitive, but harder pads work better for printing on textures. The shore (hardness) of a pad is determined by the amount of silicone oil it contains. Harder pads have less silicone oil, which results in a higher surface energy than softer pads. The higher surface energy enables harder pads to penetrate the texture further before transfering the ink to the substrate. That also means that softer pads may help more effectively “bridge” difficult textures (as mentioned previously) because the pads have more oil, a lower surface energy and release the ink sooner (for example, on the high points of the texture).
Pads that have a higher contact angle during image transfer are more effective at penetrating textures. The pad’s function as it compresses onto the part is to roll out from its central point (or ridge), displacing air. The higher the degree with which it contacts the surface of the part, the more effectively it displaces air and the fewer voids that occur.
The worst-case scenario for printing on textures occurs when the pad printer used lacks sufficient force to effectively compress steep, hard pads, coupled with little or no ability to program delays within the cycle or blow clean, dry air on the pad and the part to optimize ink transfer efficiency.
The best-case scenario for printing on textures is to have a pad printing machine with ample force to efficiently compress and the ability to selectively delay the pad for optimal transfer characteristics, consistently. While the ability to program delays isn’t absolutely essential to printing some textures (after all, people successfully pad printed for decades without it), programmable delays definitely make life easier.
At some point, you probably will experience some challenges when pad printing on textured surfaces. Some textures are easy to print – others are impossible. Hopefully, the results of the testing that we conducted and the tips provided for printing on textures will provide useful in your applications.
Thank you to Mike Huber, Tenibac-Graphion in Grand Rapids, Michigan, and Ray Hamilton, Mold-Tech in Fraser, Michigan, for their cooperation in providing the visual standard plaques for the testing.
John Kaverman is president of Pad Print Pros LLC, an independent consulting firm specializing in pad printing. Kaverman has 28 years of combined industrial screen and pad printing process experience. To see the full results of the testing and/or have Pad Print Pros evaluate a texture for pad printing feasibility, email [email protected] or visit www.padprintpros.com.