If you’re in the flexographic printing industry, you know better than anyone that in order to get the perfect print there are hundreds of things to align, test, and measure.
These considerations are only increasing with the development of remarkable new technologies and tighter printing requirements -- making the quest for the ideal print seem nearly impossible at times.
If you’re having problems with your flexo prints and experiencing defects such as halos, dot gain, fill-in, and many others it might be because you’re overlooking a facet of flexo that’s been around since the beginning: surface tension.
Better Ink Bonding & Surface Tension
Oftentimes, when print after print is coming out defective due to poor ink adhesion (or ink bonding), it’s easy to blame the image carrier. What is too often forgotten is that the image carrier cannot work alone -- its surface energy must fit between the surface tension of your ink and your substrate.
If you’re not testing the surface tension and surface energy of your ink, your image carrier, and your substrate, you’re leaving yourself open to ink adhesion related defects.
When printing, you always want your ink to have the lowest dyne levels. It is impossible to give an exact range of dynes for ink due to the variation between ink type -- water based inks, solvent based inks, UV inks, oil based inks, and others all have unique dyne levels.
Many press operators mix additives with their ink; such as adding more solvent to counteract humidity, or adding a surfactant to break up the beading in water based inks. This changes the surface tension of the original ink, and can cause defects if the surface tension is not re-tested before print.
Note: If you’re sending your ink to your image carrier manufacturer to be tested, it is important to always send press ready ink that accounts for any additives.
It is important to always make sure your image carrier surface energy is higher than that of your ink, but lower than that of your substrate.
What does a press run look like if you don’t stick to that rule of thumb? If your image carrier surface energy is too high, it will do the opposite of what you’re hoping for and pull the ink away from the substrate. If it is too low, it will not pick the required amount of ink up off the anilox, yielding a defective final print.
Here are some general image carrier surface energy considerations:
- Typically, your surface energy will be between 20-38 dynes.
- Photopolymers one of the lower surface energy’s of image carrier materials, usually clocking in below 30.
- When printing flood-rolls or large areas of solid color, you’re aiming for a dyne level on the higher end of the spectrum.
- When printing fine detail work, you want it to be in the mid-range to lower end of the spectrum.
As we mentioned before, you want your substrate to have the highest surface energy in comparison with your ink and image carrier.
Substrate surface energy varies greatly depending on the substrate’s material. Typically, film substrates are treated between 38-45 dynes, though there are some outliers.
Similar to inks, you always want to test your substrate surface energy level before you print. Certain substrate surface energy treatments may degrade if they are left sitting for too long and are not used, especially if proper care and storage is neglected.What’s The Best Way To Test Surface Tension & Energy?
The best way to test your surface tension and energy is to send your press-ready samples to a testing facility, which can include image carrier vendors, such as Luminite, that offer the services to their customers. They can also recommend an image carrier that would be ideal for ink adhesion based on the test.
For a quicker method that offers a good estimation, you can run a test on your press. It involves using various test liquids with pre-determined surface tensions to compare with your pre-print surface tensions.
In case you missed it, we have a guide to measuring surface tension with an ink test that explains the different types of inks, how the test works in detail, and other important surface tension considerations.
If you tested your surface tension and are still experiencing frustrating flexo print defects, make sure to take a look at our Flexographic Printing Defects Guide: