Ink/substrate adhesion issues: why contact angle is decisive
In digital printing, the final result depends directly on how the ink interacts with the material surface. As soon as the droplet is deposited, several phenomena come into play: spreading, adhesion, distribution, and the ability to form a sharp, uniform dot.
Among the parameters used to analyse this interaction, the contact angle plays a central role. It provides information about the wettability of the substrate, in other words, its ability to allow an ink, varnish, or primer to spread correctly.
When properly controlled, this indicator helps anticipate the risk of adhesion defects and optimise print quality even before testing begins.
Understanding the origin of adhesion defects
Adhesion problems often occur when the ink is unable to interact properly with the substrate. In this case, the droplet may remain too pronounced on the surface, spread unevenly, or create a “beading” effect.
These behaviours can lead to various visible or functional defects:
- Poor ink adhesion,
- Irregular print rendering,
- Uneven distribution,
- Difficulty stabilising the print process,
- Insufficient mechanical resistance.
Before correcting a defect, it is therefore essential to understand how the droplet behaves when it comes into contact with the material.
Measuring the contact angle with a goniometer
To analyse this phenomenon accurately, we use a dedicated measuring instrument: the goniometer.
This device measures the angle formed between a droplet deposited on a material and the surface of that material. Depending on the project being studied, the droplet may be an ink, a varnish, or a primer.
This measurement, carried out in the laboratory, provides a direct indication of the substrate’s wettability. The lower the angle, the more the droplet spreads. Conversely, the higher the angle, the more the droplet remains on the surface.
This is essential data for assessing the compatibility between a liquid and a material before printing.
Interpreting contact angle values
Observing the droplet makes it possible to quickly characterise how the substrate behaves in relation to the ink.
Angle ≈ 30°: very good wettability
When the contact angle is close to 30°, the droplet spreads easily across the material.
This indicates very good wettability. The ink adheres easily and is distributed evenly over the surface.
This behaviour is favourable to uniform printing, with a good level of adhesion.
Angle ≈ 55–60°: suitable wettability
When the angle is around 55 to 60°, the droplet spreads moderately.
Wettability is considered suitable. This behaviour generally provides a good balance between ink adhesion and print quality.
This is often a favourable situation when both good adhesion and high print definition are required.
Angle ≈ 80–85°: low wettability
With an angle of around 80 to 85°, the droplet remains more pronounced on the substrate.
Wettability is lower. The ink spreads less easily, which can lead to adhesion defects or visual irregularities.
In this case, further analysis may be required to adapt the ink/substrate combination or consider surface preparation.
Angle > 90°: very low wettability
When the contact angle exceeds 90°, the droplet "beads" on the material.
This indicates very low wettability. Printing then becomes difficult without corrective action, particularly without surface treatment.
This type of behaviour can compromise adhesion and the stability of the final print result.
Measuring the contact angle is not only about observing a behaviour. Above all, it helps anticipate the necessary actions: adjusting the choice of varnish or primer, planning surface treatment, adapting printing conditions, and more.
An analysis carried out in Ardeje Printing’s laboratory
Ardeje Printing has a dedicated laboratory to support you in the analysis, qualification and technical validation of your digital printing projects.
Our team carries out tests on your own substrates to verify the feasibility of your project and identify the best printing conditions.
Do you have a digital printing project on a specific substrate? Contact us!