by Rob Haak, Spikix B.V., www.spikix.com
Rob Haak, is an inkjet consultant and free-lance journalist based in The Netherlands. He is considered an authority on single pass inkjet worldwide.
UV inkjet is deemed to stay one of the preferred choices of ink types for future industrial print applications both for wide-format and also in single pass printing solutions. Wide-format multi-pass UV inkjet systems (flatbed, roll-to-roll or hybrid) in industrial environments are typically used for micro runs for sampling, proofing, prototyping, customization and small run productions. Single pass UV inkjet implementations on the other hand encompass the known advantages of digital printing systems: the flexibility of cost-effective short run production, a streamlined work process, lower labor costs and faster production times in heavy duty direct-to-material production.
Finding the right balance
As always, there are no secrets in life - you have to respect the interaction of all elements in the golden triangle of inkjet printing: optimal jetting properties, right balance between ink and media or substrate surface and perfect pre-treatment/curing/drying implementation. Inkjet systems, particularly single-pass systems, only perform well if the inkjet head and the ink meet all criteria for heavy-duty production. It is after all a delicate process. The ink must meet chemical and physicochemical requirements to achieve the correct jetting properties (particle size, latency, viscosity, surface tension, stability and shelf life). If this is not the case the ink may not jet at all, maybe failing nozzles will show in the printed image, or the nozzle jet direction may be slightly angled.
The interaction between ink and substrate define the color gamut, densities, adhesion, coalescence, light stability, durability and flexibility, highly impacting the quality of the printed result.
At the latest IJC, the InkJet conference, in Düsseldorf, we interviewed Marc Graindourze, Agfa Graphics’ ink expert for industrial UV inks, to understand what it takes to develop UV inks and to make them perfectly integrate into industrial printing applications.
UV Inks dissected
UV inks are composed of a mixture of monomers (as liquid carrier), a photo initiation system, a pigment dispersion (color pigment and polymeric dispersant) and additives (surfactants for wetting, stabilizers, adhesion promoters, anti-scratch, gloss modifiers, etc.). After printing the UV ink must be cured by exposure to UV light.
UV inks offer the flexibility to jet on a wide range of substrates such as paper, carton board, vinyl, plastics, films, foils and metal sheeting, and also on rigid materials such as glass (given a dedicated primer and/or pre-treatment), ceramic tiles, polyester, wood, metal and others. Commercial graphics industries today benefit from UV-curable inkjet for retail signage, POP displays, exhibits, banners, architectural signage, interior decoration products, vehicle graphics and packaging products.
UV-curable inkjet also allows users to expand their scope and to produce industrial applications, such as packaging, interior decoration and object printing. UV inkjet printing shows similar advantages to conventional offset printing with UV inks with regards to printing performance, low dot gain and print quality, as well as good physical properties (adhesion, solvent resistance, scratch resistance).
In general, UV inkjet systems are very reliable and require less maintenance because they dry less easily in the print heads, compared to water based and solvent based inks. Thanks to the ink curing with UV light, there is no VOC emission and the prints are instant dry.
A ‘fit-for-purpose’ recipe for UV inkjet inks
Rob Haak: Marc, can you share with us your approach how you handle ink projects for your partners that need to integrate new print technologies in their industrial manufacturing?
Marc Graindourze: Industrial inkjet printing systems all have their specific image width, performance and image quality expectations, so we have to follow a complete system approach. This embraces workflow and software (print files, color handling, proofing, etc.), the choice of ink and ink formulations, the option to use of pre- or post processing (with primers, coatings, varnishes), all integration aspects (off-line versus in-line printing for example), the integration in the manufacturing process, all specific functional requirements and maintenance aspects.
Each project is more like a partnership in which we take a very active position. Obviously ink is the key element but it needs more parties to come to the integration in the manufacturing process. Our know-how to develop the ink sets and competence in matching the UV ink to the production process are crucial to fulfill the functional requirements of the printing step.
Rob Haak: Please elaborate on some of the UV ink developments you have implemented so far, perhaps starting with any single color implementation?
Marc Graindourze: We developed UV inks for the marking and coding market (please refer to the full story on Agfa website).
This application is considered a sample for automation of industrial processes: tracking products in plants and warehouses for logistic purposes, adding manufacturing data such as batch code, expiry date, barcode and QR codes is now mainly inkjet based. You can imagine why UV inks are used: adhesion on a wide range of substrate is very important as well as printing on coated and uncoated stock and plastics in an industrial environment. Because reliability and durability are basic requirements, the ink formulation needs to facilitate high solvent- and scratch resistance. A unique selling point of Agfa’s UV Black inks for this application is our high-density neutral-black ink, which is available for the full range of industrial piezo print heads. It features very high contrast printed codes even on colored substrates. Marking and coding inks should print a crisp, sharp image, especially for security codes etc. at production speeds that increase year after year. This also implies ink curing at speeds of more than 100 meters per minute preferably with curing LEDs. No need to explain that all this requires a good system understanding to select the right ingredients and to tune the ink to support this coding and marking applications. In essence it down to jetting black UV inks on a variety of materials at incredible speeds in a harsh manufacturing environment.
Rob Haak: How about full-color implementations for object printing?
Marc Graindourze: This market is quite demanding. Final products include phone covers, plastic tubes, containers and promotional items. A good example is industrial short-run and customized printing of white goods panel displays. Traditionally screen printing and pad printing were used to print front panels of dish washers or like. Now that inkjet technology is flexible and robust, it allows suppliers to print these customized front panels for white goods. Our UV inks produce excellent full-color print results, both in multi-pass systems (wide-format and flatbed), and in high-speed single-pass implementations. To minimize the number of pre- and post printing steps, UV inks are jetted in full color registration directly onto the panel, ensuring sound adhesion of the ink after curing. Of course these customers are looking at high image quality, industrial reliability and low maintenance.
Rob Haak: What about the largest market, the packaging market, where different products, multiple colors and food safety need to be addressed?
Marc Graindourze: In the packaging market the challenges are of another order of magnitude, because of the food safety requirements. Many guidelines and legislations apply for printing food packaging, implying the need of very specific UV ink formulation, which can not be obtained just by changing a few of the ink compounds. It requires so-called low migration UV inks with a completely different conceptual design. Low migration inks have an extremely high curing degree and –as the name says– low migration of ink compounds and breakdown products. For inkjet inks, the need for low viscosity means an extra hurdle, which Agfa managed to overcome by the combination of special monomers and photo-initiators.
Another challenge is tuning of the low-migration ink for a specific packaging applications. We have developed low-migration UV inks to jet directly onto PET bottles, plastic food cups/tubs and closure caps. Here the focus is on mechanical requirements, including adhesion/scratch/scuff resistance, light fastness, sealing fastness, solid/acid resistance, etc.
Besides those, also high image quality requirements are set, including line width, color gamut, density, bleeding, gloss, tactility, ink layers, etc. Obviously the use of white ink is mostly required, with a high opacity and allow receptive to the four-color ink set for overprinting. To guarantee uniform color appearance, the ink does contain chemicals to ensure that the ink film spreads in a controlled way. Viscosity, wetting performance, surface energy and careful control of drop spreading behavior contribute to the control of dot gain, which is vital for image quality. So, it is a very complex technology indeed. Not to forget the finishing requirements such as lamination, cutting, sealing, heat/sterilization resistance and, last but not least, ink setoff or migration.
On the Agfa Graphics website you can download a white paper about low-migration UV inkjet inks. It explains the application, guidelines, migration principles and testing of low-migration UV inkjet ink.
Industrial printing trends
It is good to understand the complexity of ‘fit-for-purpose’ of (UV curing) inks, and bearing in mind the proof of concept of early direct product decoration. We all expect an accelerated growth in industrial print due to the need for shorter runs, versioning, customization, fast-turnaround, and the desire to optimize the supply chain and move to lean manufacturing. The deployment of flexible and scalable inkjet technology and UV curing inks will be important enablers to produce decorated end products in a very cost effective way.
The industry is rapidly adopting industrial print and will incorporate digital workflows in their manufacturing processes, offering decorate-on-demand technology and just-in-time production, with reduced inventory, reduced waste and logistics.
Innovative direct product decoration techniques will create new opportunities for marketing but also in the supply chain, as customization of goods is leading to smaller yet more diverse production runs.
It will be interesting to follow this evolution, finding a good balance between conventional volume decoration versus digital short run alternatives making print in manufacturing processes sustainable and environment friendly for the future.