We can’t stress enough that flexographic printing is a pressure-sensitive process. If pressure is not applied correctly and uniformly from your print cylinder to the substrate, you’re bound to experience flexo printing defects.
The halo effect is one of 9 defects we commonly recognize with pressure as a probable cause. Its symptom? The ink extends beyond the edges of printed areas on the substrate (see above), creating a distinct, halo-like outline.
It’s similar to excessive dot gain, another common flexo printing defect, but its causes aren’t quite so numerous and overwhelming.
OK, enough with the puns. Halo is a serious flexo printing defect that can waste time and substrate (and thus money) if not corrected quickly.
The most common causes of the halo effect include:
These things are simple and relatively easy to fix, once you determine what the problem is. Let’s take a look at what you’re seeing.
Even inks with the highest viscosity (think: maple syrup) have nowhere to go other than out when under pressure. So ink creating halos around your printed areas is a telltale sign the pressure is too high.
How do you solve it? Simple: Lighten the load. Lower the pressure from the print cylinder to the substrate.
How certain are you that pressure is uniform in all areas where your print cylinder contacts the substrate? Well, the halo effect could be present only in one area of your printed substrate, or the severity of the halo could differ from one spot to the next.
Irregular pressure may cause visible wobbling during press runs, or it could be hardly noticeable at all.
Solution? Check the uniformity of the pressure across the web and adjust it accordingly.
You do need enough ink to wet out completely on the substrate. But in this case, you may have too much. This could be a question of:
But you’re probably dealing with an anilox with too much cell volume or a problem with ink viscosity.
Anilox cell volume: If there’s too much cell volume, too much ink will be transferred from the anilox to the image carrier and subsequently to the substrate. Choose an anilox with a lower cell volume or a higher cell count.
Ink viscosity: An ink with low viscosity will spread more easily across the substrate. That means it could extend beyond the areas you intended to print. You can adjust the water, alcohol, or solvent content of the ink to control viscosity.
Are your impression cylinder and web traveling at a uniform speed? They must be in sync. If not, you’ll see ink skewing off edges of the printed image, much like in “doughnuts,” another common flexo printing defect.
This should be an easy fix if you’re diligent about monitoring press operation.
Don’t let easy press mechanics and ink transfer issues hamper your print quality. Halo is entirely preventable by monitoring pressure, ink transfer, and speed.
Still, flexo printing defects can leave you scrambling for answers. We hope you find them without wasting too much time and materials.
We’re always here to offer a second opinion on troubleshooting solutions. Call (888) 545-2270 to speak with a Luminite customer service representative. The free guide below offers a more comprehensive view on printing problems.
Editor's Note: This article was originally published in February 2018 and has recently been updated.