Industry representatives offer their perspective on pad printing onto textures.
What I found was that the frequency of the texture played a significantly larger role in achieving an acceptable print than its depth or the angle of the side walls. One texture with a depth of 0.0055 in. and 8° draft (fewer peaks and valleys) was successfully single and double printed, whereas another texture that was only 0.0015 in. deep with 2.5° of draft (more peaks and valleys) couldn't be successfully printed at all. My conclusion: The higher the texture frequency, the lower the likelihood of complete ink coverage.
Note, however, that the data from my experiment only covers a limited number of known textures in a sea of millions. Unless you're printing the same plaques I used in my experiment, its results don't help much. You'll need to conduct similar tests on the textures you print in order to determine their printability.
When conducting such an experiment, make sure to use a pad press, cliche, ink (including hardener/thinner ratio), and textured image size and depth that closely resemble the conditions you'll face in production. Finally, use the same pad shape and durometer as you'll use in production. Set up your test machine to run with typical production settings for speed, and set pad compression on the cliche and the substrate at the minimum levels necessary to pick up and transfer the image.
If you can't get an acceptable print using the settings and tools you typically use, you can change several things. The easiest thing to try is a harder pad, which will penetrate the texture further than a soft pad before the ink releases.
If a harder pad alone doesn't do the trick, try using one that has steeper side angles, which increases the chance of the pad making contact with the substrate in recessed areas of the texture. You may also try slowing the speed at which the pad compresses on the substrate. This can make it easier to displace the air that leads to pinholes.
Allowing the pad to dwell on the surface of the substrate for a few seconds may achieve the same result, and some machines can be programmed to provide the extra dwell time. But note that overcompressing the pad on the part can actually stall the press, resulting in image distortion, or, worse yet, leading to undue wear and tear on the machine, the pad, and the part being printed.
On finely grained textures, if you fail to cover the texture with a single pass, the chances of covering it with a second pass aren't very good. This is because the thickness of the ink layer that you lay down only makes the valleys (and resulting voids) even deeper than they were initially. When this occurs, you may consider "bridging" the texture, rather than continue trying to fill it in.
You can bridge shallow, high-frequency textures by changing how the ink film releases from the pad. If you're lucky, you can achieve this by using less thinner or slowing down the machine to allow more solvent to evaporate from the ink film while it is on the pad. Directing low-velocity air at the pad surface after it picks up the ink is another means to accelerate solvent evaporation. Allowing more solvent to evaporate increases the ink's tackiness, making it leave the pad sooner in favor of the substrate. With the right degree of tackiness, the entire ink film will transfer intact, even though it's only making contact with the peaks of the texture.
Note, however, that even if the ink film transfers without pinholes, after drying it will have less mechanical resistance than a full-contact print, especially with deeper textures. Since the ink only adheres to the peaks, you'll have tiny voids in the valleys under the ink film. The ink above these voids can fracture more easily, so you need to keep the end use of the part in mind when you're deciding whether or not to bridge.
Finally, some textures are just plain impossible to completely cover, and the texture will need to be modified to make it printable. But pad printers generally don't make the parts they're printing, so this is rarely a viable option. If you've conducted the simple experiment outlined previously, you'll be better able to communicate to customers the reasons why the texture isn't printable. And your findings can be an asset in determining what the texture should be.
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