How to Solve Poor Adhesion of Ink on Film-Based Self-Adhesive Materials!

In the production of self-adhesive labels, one of the biggest differences between film-based materials and paper-based materials is that the surface of film does not absorb ink. Therefore, when using UV inks for printing, ultraviolet light is typically used to instantly cure the ink so it quickly adheres to the surface of the film material.

However, in actual printing processes, poor adhesion of UV ink on film-based self-adhesive materials still occurs frequently, causing significant challenges for label printers. So, what are the reasons behind this poor adhesion, and how can it be resolved? Let's take a closer look.



01 False Drying of Ink Surface Causes Poor Adhesion
During the printing process, the phenomenon of "false drying" of UV ink is often encountered. When tested with adhesive tape, the ink can easily be peeled off from the surface of the film material. Generally speaking, there are three main causes of false drying:

1-1 Ink Layer Too Thick
According to the principle of ink curing, when the ink layer is too thick, although the surface may appear cured instantly under UV light, the UV rays have difficulty penetrating through the already-cured top layer to reach the bottom. As a result, the lower layer of ink does not fully complete the photopolymerization reaction, leading to the false drying phenomenon.

Once this issue occurs, it is recommended to use high-color-strength inks and apply them in thinner layers — known as "printing deep colors in shallow layers." This ensures proper curing of each ink layer and also improves production efficiency.

1-2 UV Lamp Has Been Used Beyond Its Lifespan
The typical lifespan of a UV lamp is around 1,000 hours. Once this limit is exceeded, the spectral output of the lamp changes, and the emitted UV light no longer meets the requirements for ink curing. At the same time, the infrared energy increases, which may cause heat deformation of the material and make the ink brittle.

Therefore, during operation, it is important to accurately record the usage time of UV lamps and replace them in a timely manner. In daily production, the cleanliness of the UV mercury lamp should also be checked regularly, and the reflector should be cleaned. Typically, only one-third of the UV lamp’s energy reaches the material directly, while the remaining two-thirds relies on reflection from the reflector.

1-3 Printing Speed Is Too Fast
In production, some film-based self-adhesive materials show good ink adhesion at normal printing speeds, but once the speed increases, the adhesion performance deteriorates.

One reason is that faster printing reduces the contact time between the ink and the substrate, resulting in insufficient molecular bonding and thus poor adhesion. Another reason is that faster speeds reduce the duration of UV exposure, lowering the total energy received by the ink, which prevents full curing and leads to poor adhesion.

If this issue arises, the printing speed can be appropriately reduced to allow better contact between the ink and the substrate and ensure sufficient UV curing time, thereby improving ink adhesion.



02 Different Inks Show Different Adhesion Performance on the Same Material
Sometimes during production, it is found that a certain ink has excellent adhesion, but when switching to another brand or type of ink, the adhesion performance decreases significantly. This is because the type of coating on the substrate affects ink adhesion. Different inks may contain different resins, so their adhesion performance can vary on the same type of substrate. Similarly, the same ink may exhibit different adhesion performance on different substrates. Therefore, it is recommended to conduct thorough print testing between the substrate and the ink before use.



03 Poor Ink Adhesion on Uncoated PE/PP Film Materials
We know that for uncoated PE/PP materials, it is generally recommended to use inline corona treatment. Corona treatment helps increase the surface tension of the PE/PP material. However, the effect of corona treatment is temporary and can be affected by storage and transportation conditions. If the surface tension of the film is below the required level before printing, poor ink adhesion may occur after printing.

At this point, the dyne level (surface tension) of the film material should be checked. To ensure good adhesion of UV ink, the surface tension should be at least 38 dynes/cm. Therefore, for equipment without inline corona treatment, it is recommended to purchase coated film-based self-adhesive materials.

Conclusion
In actual production, if you encounter issues with poor ink adhesion on film-based self-adhesive materials, it is important to correctly identify the root cause in order to find an effective solution.

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