Interesting, as they say, your mileage will vary. We have been printing on coroplast for longer than I wish to remember. Our old Halogen UV curved machines, forget it... Our new LED UV Cured machines, are good. Here is what we have learned and apply in our shop...
1. We clean boards to ensure no finger prints (no need to debate the many processes used, just be sure the board is clean, dry, and free of any oils from handling the board to avoid fingerprint on light colors).
2. We try to keep our stock fresh. We prefer to purchase by the skid, but sometime adjust seasonally to ensure we do not end of with a bunch of stock that is aging on the floor.
3. Print on @ 50% carriage speeds using 6 or 8 pass profiles (we want to lay the ink down in thin layers, curing as we go). Depending on colors, adhesion test results, and use, we can in many cases speed it up.
4. UV lamps between 30-40%. For us, this is key, we are curing the ink to maximize adhesion, not trying to bake it and create a new ink layer on top of the coroplast. Think of it lay spay paint, it cannot penetrate the surface, so build up lighter layers to establish as much grip as possible.
Adhesion tests: First, when we start with new lots since we do not know how long the boards were sitting before we received them. If we find a lot that is problematic, we adjust the UV exposure. Per discussions with several of the chemical guys at the factory, (this is their claim not mine) the corona chemical can be partly reactivated via UV exposure. If we find boards that are not adhering well, we adjust the printing to expose UL lamps prior to any ink being applied. I have no understanding why, only know that this clearly improves adhesion, and is a quick and simply way to address potential adhesion issues. Again, not a scientifically based fact, but we do notice fresh boards have a distinct order that dissipates as the boards age; not sure if this is the corona treatment or not but just something we notice.
We only print on full sheet in production. End runs of course on scraps. Trimming/ automated cutting requires solid adhesion to ensure no ink cracking/chipping. Prior to putting a lot of material into finishing, we test the adhesion on the first few prints and adjust as needed. For standard adhesion applications, we fold the material to ensure the ink stretchers and verify the ink cannot be pulled off with painters tape. For applications that require higher adhesion, we cut the ink and then pull a tape test to be use we cannot pull off the ink.
For applications requiring maximum adhesion, we have never been able to use UV cured inks to meet the requirements. For those, (i.e. coroplast packing that is cut, folded, and typically used as retail displays), we have only seen success using Laxtex inks on Coroplast. I know our approach seems very different than most comments in the thread (tons of ink at 100% UV vs our opposite approach), but I can only report, if we did that, I could peel the ink off the board with little effort and it would never pass any adhesion testing.
Of, and not to state the obvious, but make sure the board you are purchasing is Corona treated; it is more expensive and many supply houses and sales people do not know the difference between the two; only that to win your business, they quote the non-treated board... Sign grade is sufficient (it is what we use), although the Medical Grade stuff is the best (and very expensive).
Tim
