The first step in obtaining good adhesion and print quality is to assure that the ink wets out on the substrate fully and evenly.
How Flexo Surface Tension and Surface Energy Affect Wettabilty
Proper wetting of the substrate is largely governed by two factors:
- Surface tension of the ink
- Surface energy of the substrate
What is Flexo Surface Tension?
Surface tension is the tendency of liquids in contact with air to act as if they are surrounded by an elastic membrane under tension. This force is what causes droplets to form into a spherical shape, and also prevents small particles from sinking into liquids, such as water, even if they're heavy enough.
Surface tension arises due to the tenancy for molecules of liquid to stick together more strongly than they stick to air.
Surface tension is expressed in dynes/cm.
What is Flexo Surface Energy?
Surface energy is a very similar concept that applies to solids, and refers to the excess energy at the surface of a material compared to its bulk.
Surface energy is in joules/sq. meter.
Effect on Wettability
In flexo printing, surface tension and surface energy affect wettability.
If you have a liquid with a high surface tension, it tends to bead up and not wet the surfaces it comes in contact with.
If you have a surface with a low surface energy, it will have the same effect. This is not generally the ink substrate interaction that you want.
Flexo Printing Surface Tension and Surface Energy
To produce a successful print, the components of the flexographic inking system must have appropriate surface tensions and energies. More specifically the ink needs to have a low enough surface tension compared to the image carrier so it will adhere to it. The substrate must then have a higher surface energy to fully pull the ink off the image carrier.
In general as the ink moves through the inking system, the surface tension/energy of each successive component must increase.
Measurement of Flexo Surface Tension and Energy
There are a variety of ways surface tension and energy can be measured due to different test conditions and materials.
In flexography typically measurement is done using a method called goniometry. This entails measuring the contact angle formed between a surface, and a droplet of liquid placed on it. Surface tension can then be calculated from this measurement.
Editor's Note: This blog post was originally published April 25, 2017 and was updated on August 27, 2019 to reflect updated insight and resources.