Ilisna Lane, sales manager at NUtec Digital Ink, outlines the importance of understanding ink adhesion. Something that has popped up a lot in discussions with customers, especially since UV ink technology has become a very popular method of printing, is ink adhesion. A topic that she feels is still very misunderstood and deserves to get some time in the spotlight.
Ink adhesion does not only apply to UV-curable inks, but it also applies to solvent ink. In this article I mainly focus on UV printing considering the number of substrates that are available in the market to print onto.
Not only is the ink adhesion a contributing factor to a strong bond, but the quality of the substrate and its surface energy play just as important roles.
With high volume printing, cost is often the driving factor for converters sourcing similar product types from different suppliers or simply looking for the most cost-effective substrate to maximise profit for each job.
Even though the product class might be the same, it does not always mean that the adhesion properties between the ‘similar’ substrates are going to be the same. No two acrylics are the same, just as no two vinyls are the same.
By understanding ink adhesion, converters are able to put checks in place prior to securing work to ensure that the substrate together with the ink adhesion meets the requirements for the job. Having this type of check in place also safeguards the converter from expensive, costly re-prints and possibly consequential losses in terms of labour, transport, etc.
As an ink manufacturer, each ink set undergoes a stringent set of tests to rate the adhesion of the selected ink onto a particular substrate.
One of the most widely recognised methods is the ISO 2409 Crosshatch Adhesion Test:
– A lattice pattern is scored through the printed ink film.
– Pressure-sensitive adhesive tape is firmly applied over the cuts.
– The tape is removed rapidly.
– The amount of ink removed is assessed and assigned an adhesion rating.
The ISO 2409 rating ranges from 0 to 5, where:
– 0 = Excellent adhesion (no coating removed).
– 5 = Very poor adhesion (significant coating removal).
Testing is typically performed immediately after printing and repeated after 24 hours to evaluate adhesion once the ink has fully cured.
What are some of the influencing factors that can affect ink adhesion?
Surface Energy
I have mentioned this in many articles I have written in the past because this is one of the KEY things when it comes to ink adhesion. The surface free energy on the substrate you are printing onto must be high. If the surface free energy is low, as found on many types of substrates like, plastics, ceramics etc., adhesion will be very tricky.
Substrate Cladding
Many substrates come with a protective cladding that’s applied on one side or occasionally on both sides of the substrates to protect the material from dirt, dust and other contaminants during shipping, transportation and handling of the media. These cladding liners may leave a residue behind when removed prior to printing that can affect the surface energy of the media.
Surface Preparation
Whether the material has a cladding or no cladding, ensuring that the substrate is clean is crucially important. We recommend cleaning the surface of the substrate by wiping it down with IPA with 98% purity. This will ensure that any traces of contaminants, dirt or dust is removed prior to printing, ensuring the surface is optimised.
Curing
With many different types of UV-LED lamps in the market, it’s important to check adhesion of the ink on a substrate depending on the ink deposit. Not having the right intensity or weak lamp settings can affect curing of the ink layer, resulting in adhesion challenges. Ageing lamps can also contribute to improper curing.
Coatings
Many manufacturers of low surface energy substrates, like Correx for example, will treat the substrate surface (Corona treatment) to ensure good ink adhesion onto their substrates. The corona treatment on a substrate will be most effective directly after application. As the stock ages while in storage, the treatment efficiency will deteriorate and could impact ink adhesion.
Adhesion Promoters
There are many different types of liquids that can be used to optimise the surface of a substrate for printing.
Adhesion promoters, also known as primers, can optimise the surface of a substrate for printing. Using jettable primers will allow the print operator to first print a primer layer onto the substrate followed by the CMYK graphic.
Hand applied promoters or primers can also be used in some cases. However, compatibility between primer and ink must always be tested because chemical incompatibility between two different chemistries may occur.
Some printing systems make use of inline plasma, flame or corona treatment options that can also optimise the print surface.
Checking the surface energy of a substrate using dyne pens is a quick and effective way to determine whether the surface is suitable for good ink adhesion. If the dyne level is too low, the ink may not wet the surface effectively, which can result in poor adhesion.
It’s also important to keep your dyne pens in good condition. Replace them regularly and follow the manufacturer’s recommended shelf life, as ageing or contaminated pens can produce unreliable test results.
Expired Ink
Always ensure that the batches of inks on your printer are within the use by date from the manufacturer. Using expired inks can cause challenges not only with adhesion and curing, but it can also be catastrophic to the printhead on your printer. Always keep track of your ink lot numbers to ensure that you have traceability, and train operators to check the expiry dates before pouring the inks into the main tank.
In Practice
Ink adhesion should never be assumed simply because a substrate has printed successfully in the past or because another material appears to be similar that it will have the same result.
Small differences in substrate formulation, surface treatment, age, contamination, curing conditions, or print preparation can have a significant impact on the final result.
By understanding the factors that influence adhesion and performing simple validation tests before production, converters can dramatically reduce costly failures, improve print quality, and deliver more reliable products to their customers.
At NUtec Digital Ink, adhesion testing forms an integral part of our product development process. By combining stringent testing with extensive substrate evaluation, we ensure our ink systems deliver reliable performance across a broad range of print applications.
| MANUFACTURER NUTEC DIGITAL INK Tel: +27 21 763 6990 Email: info@nutecdigital.com Website: http://www.nutecdigital.com  |
