Pre-treatment for glas printing
Published: SIP, 05-2018
Author. Balthasar Mayer
How can you prepare glass for digital printing to create a durable product? We provide a brief overview of pre-treatment for glas printing. Glass is well known for being difficult to print on. Often the inks just don’t last well enough to make a saleable print product. The fact that the ink does not remain on the substrate – but runs – and then makes a good save well is an exciting topic for the entire printing industry. As Jochen Christiaens, industrial consultant in the LMNS network, explains, there is a whole range of pre-treatment methods that can be used to achieve better adhesion: “Machine manufacturers integrate pre-treatment or primer stations directly into their plants in order to offer a complete process in which all parameters are under control”. However, these stations require additional integration space and make the process more complex. Especially for companies that do not manufacture on an industrial scale or do not print only on glass, this is often not the optimal solution. But there are also possibilities for these companies. We present some of the most common pre-treatment methods, which can be used both independently and partially inline.
One of the best known methods is the prior application of a primer, which serves as an adhesion promoter between glass and ink. In addition to primers supplied by printer manufacturers and applied through the print heads, there are primers that are applied before printing. These include the glass primer from Salon Iris (see also SIP 5.16, page 50). The product, which is now also available in five-litre canisters, works both with UV inks that cure with LEDs and with those that use UV mercury vapour lamps. And not only flat glass can be treated with it. According to Managing Director Stefan Fiedler, many customers report very good adhesion results for UV inks on container glass. However, there are a few things to keep in mind when using these inks: If, for example, the containers are coated with a wafer-thin plastic film, as is often the case with glass bottles, this must first be burnt off. Another question that arises in the case of printed glasses is their suitability for dishwashers. “If used correctly, they are basically dishwasher-safe,” explains Stefan Fiedler. “It should be noted, however, that UV inkjet inks generally absorb warm water or water vapor, swelling and softening them. This makes the ink layer vulnerable to damage such as abrasion or scratches. Stefan Fiedler emphasizes that the problem has nothing to do with the glass primer, but with the inks themselves. “As soon as the inks have dried, they become hard again and scratch-resistant as before. To counter this problem, Salon Iris is currently developing a transparent, UV-curing protective coating to protect the ink from moisture absorption and scratching.
Print pre-treatment with plasma
Plasma is a particle mixture of highly reactive species, including electrons, ions and chemical radicals. In plasma treatment, this mixture causes the hydrophilicity of the glass surface. As a result, the surface energy increases, allowing liquids such as inks, paints or adhesives to wet the surface better. There are several ways to generate a plasma, on which the composition of the gas mixture depends. The Regensburg-based company Relyon Plasma offers both pulsed arc generation and piezoelectric discharge. Both plasma generators are suitable for the pre-treatment of glass.For the former, Relyon has the high-performance system Plasmabrush PB3 in its portfolio, which can be used in a wide temperature range from a few hundred to several thousand degrees Celsius. At the other end of the power spectrum is the Piezobrush PZ2, whose function is based on a piezoelectric discharge. “This produces a typical ‘cold’ non-equilibrium plasma with a temperature of around 50 degrees Celsius,” explains Corinna Little, Application Technology and Product Development at Relyon. According to Corinna Little, the advantage of treatment with atmospheric pressure plasma is that liquid chemicals are not required. Only electricity and a carrier gas are required. “Compressed air or other harmless gases such as nitrogen or forming gas can be used, as can in the case of piezoelectric discharge inert gases and even ambient air. According to Relyon, the effect produced by the plasma treatment lasts for at least a few hours.