A recent quote from an industry expert claims, "UV cure ink will work with any substrate," which was immediately followed by, "UV cure inks don’t have any VOC’s so they are perfectly safe." Both of these statements which are heard commonly at industry trade shows and in sales presentations have some degree of truth and a fair amount of misleading information. UV-curable ink does work amazingly well on a large number of substrates, however it works poorly or not at all on others.
UV-curable ink certainly prints well on a much greater variety of media compared to aqueous- and solvent-based inks. This is especially true for printing onto substrates without an inkjet receptive coating layer. Other than the natural cross linking that occurs with true aggressive solvent inks and flexible vinyl banner media, most substrates require some kind of top-coat for optimum performance with aqueous and solvent inks, especially if the media is non-porous by nature. The water and solvent that carry the pigment have to go somewhere until they evaporate, so having an IRL provides for better uniformity in drying. Aqueous and solvent inks go into the surface of the media, whereas in most cases UV-curable ink gets cured on top of the substrate. The fact that UV cure inks can print directly to many non-treated rigid and flexible surfaces helps keep the cost of the media down to the same as for analog screen printing.
We have found no two media, even when designed for the same application, exactly alike. UV-curable inkjet technology requires the proper control of a number of factors to enable quality results on a wide variety of media types. These factors include the obvious such as ink and UV cure lamps, and the not-so-obvious such as media transport issues, drop placement, dither/screening, and color management technologies. Different brands of PVC, foam boards, corrugated plastic, vinyl, and polystyrene often require a unique set-up of the system to produce optimum results.
One of the best examples of varied results in substrates is on corrugated plastic boards. Some UV printers crush the flutes with the printer feed rollers causing an uneven print surface. The worst part is that you simply cannot look at a substrate and know it will work well. For example, most brands of PVC tend to work with many UV cure inkjet systems, but range greatly in color and uniformity, thereby requiring good color management to produce consistent color results.
The Universal Ink Myth
Screen printers often use a number of specialty UV cure inks, sometimes up to ten or more, for different media application types, while digital inkjet UV tends to try and make one ink, and sometimes two, work with a greater variety of substrates. While this means some compromise for certain substrates, it would be very impractical to flush and fill new ink sets into a wide format printer for each job during a shift. On average, the time to develop a good UV-curable inkjet ink set is 12 to 18 months.
While not necessarily a universal ink, a good quality UV ink set does offer very good performance on many media types, and is acceptable for
a number of others. The key is that before first time buying a load of media and kicking off a digital production run, it is a very good idea to do a quick test for some of the fundamental factors such as adhesion, scuff resistance, media transport, and color calibration. This is especially true when printing on the more difficult substrates, such as acrylic, glass, polystyrene faced media, and any light weight flexible media.
The one application that is simply still done better with a good aggressive solvent ink is vinyl for vehicle wraps. Typically, UV cure ink provides excellent outdoor durability, hardness, and adhesion on non-treated media, but most is not flexible enough to be good at stretching vinyl around shapes and contours.
The Safety Factor
VOC’s may not be an issue with UV, however, printing UV on any media requires several steps to ensure safety for the print operation and for the finished printed product. When properly 100 percent cured, the media comes out of the printer fully dry and acceptable to handle, stack, or move to post print finishing. If the media comes out still wet, tacky, or easily pulled off in a film-like layer, then the ink is not fully cured.
Components in non-cured UV-curable ink can be a skin sensitizer and should not be handled or inhaled without the proper protection, such as gloves and proper ventilation. Always refer to the manufacturers MSDS for instructions. This emphasizes the importance of confirming the settings on the printer to ensure a full UV cure. For some substrates this means slowing down the printer and adding more passes to add ink interlacing and longer UV lamp dwell time. This safety factor is also why so many high quality UV cure inkjet systems have a fully enclosed design to offer protection from the harmful UV light, and to capture and provide direct ventilation.
A good UV-curable inkjet system can print directly to a very wide variety of substrates, just not all of them without challenges. Safety considerations are a factor in every type of digital printing. Work closely with the manufacturers and distributors of the ink and media to test and confirm compatibility for each application. Direct digital UV cure already offers excellent results with many substrates and continues getting better every year.