PAD Print Ink Adhesion
Henniker Plasma were contacted after a company encountered issues with the PAD printing of certain medical devices. Pad printing involves placing a very thin layer of ink onto the surface, and these devices required steady, intricate printing & depth marks, difficult even with the most high-spec equipment. The medical devices being printed on were made of PEEK and included, among others, Catheters and Microcatheters, Nasogastric Feeding Tubes & Endotracheal Tubes.
Polyetheretherketone (PEEK) is an engineering thermoplastic with superior heat-resistance. The glass transition temperature is 144 °C, the melting point 335 °C and it is typically melt processed at 370 °C.
Although these properties are beneficial to a design engineer, they can cause secondary problems later in the process during bonding, printing, coating & painting.
In order to find a solution to these problems Henniker worked side by side with their team.
In order for ink to successfully adhere to a surface, it must spread across the surface, which will only happen if the surface tension of the ink is lower than that of than the free energy of the surface it is sitting on. Once the Henniker team had made sure that there was no issue of contamination, they analyzed the surface energy of several of the PEEK parts using a variety of in-house test methods. This is a typical step along their process development method.
Surface energy is defined as the energy associated with the intermolecular forces at the interface between two media. So if the molecules within the ink are more strongly attracted to each other than they are to the surface, then the ink won’t spread properly and wet the surface. This causes the ink to bead up and sit atop the surface.
The inverse of this effect would be if the ink were more strongly attracted to the surface, which would cause it to spread out and wet the surface more easily. Since the wettability is a simple way to define the adhesion of a surface, this second scenario is better if a surface needs to glued, printed or painted upon. The untreated PEEK devices were found to have surface energy values of between 30-35 dynes, whereas the typical inks used had surface tension values around 25% higher than this, clearly, this was the source of the ink adhesion issues.
Plasma Surface Treatment
So as to determine the fastest and most cost-effective process, a range of plasma treatments were tested on their ability to improve the surface wettability of the PEEK devices. Once these preliminary tests were done, Henniker showed how an easy and effective process could raise the PEEK surface energy to over 72 dynes with a total process cycle time of less than two minutes.
Contract Plasma Treatment
The company also required treatment for both small one-off requests and on-going production. This was supplied in a cost-effective manner through contract plasma treatment with Henniker and is an excellent route to use when buying specialist equipment is not necessary, usually because of small job sizes and short-term contracts.
All in all, the process was taken on and completed in a timely manner thanks to the professionalism of the Henniker Plasma team. Working with the team and using their experience with low surface energy materials along with their ability to perform rapid testing, a suitable process was quickly developed and continues to serve the client with a fast and cost-effective solution to their specific printing problem.
This information has been sourced, reviewed and adapted from materials provided by Henniker Plasma.
For more information on this source, please visit Henniker Plasma.