For several years there was only one choice for printing graphics on non-porous, rigid substrates using inkjet technology—UV-curable inks using piezo inkjet printheads. This has now changed. Today there are several additional choices, using both piezo and thermal inkjet technologies. Here we will discuss these alternatives and comment on their differences, advantages, applications and how they compare with UV-curable inks.
Clearly, unlike flexible substrates, printed roll to roll, rigid substrates must be printed in sheets requiring feeding them one at a time. Because UV curable inks require protection from pre-exposure to light, they must be immediately pinned as the printing takes place to avoid coalescence of the drops. The implementation today is often LED-based UV light sources attached to the shuttle containing the piezo printheads. For additional printing speed with good curing, an oxygen-free environment is necessary since oxygen inhibits the curing process. This technology has been the mainstay of companies like EFI for rigid substrates. Several aspects of this process, however, may be viewed negatively. The relatively thick ink layer can provide relief and may not result in a perfectly smooth surface since the drops become 100 percent polymer when cured. Also, an objectionable smell when used indoors and marginal adhesion to some untreated surfaces may be challenging. UV inks use ingredients that are expensive compared to non-UV inks. UV inks are 100 percent solids vs. water-based inks which are only about 10 percent solids.
Water-Based Pigmented Inks
For porous substrates, like corrugated paper board, water-based pigmented or dye-based inks in thermal inkjet printers can provide a good alternative since the ink vehicle, mostly water, can soak into the paper and dry quickly. If the paper surface is white the result can be more cost effective than UV-cure inks since the system is simpler and the inks' ingredients are less expensive. However, these inks cannot perform on non-porous substrates so they are generally dedicated to corrugated board and—if a hybrid printer—paper or coated roll media. Some substrates may require a pre-coat prior to printing to optimize hold-out of the pigment and improve image quality. HP and Durst offer high-volume, water-based flatbed printers aimed at corrugated packaging markets. Other advantages of water-based inks in this application include ease of de-inking compared to UV-curable inks, and a lack of odor for indoor applications.
The newest entrance into the rigid printer is the HP R2000 Latex hybrid printer first introduced in 2018 at FESPA in Europe. It provides water-based latex ink printing on porous and non-porous rigid media. In addition they offer white ink, a first for water-based inks from HP. The inks used in the R2000 are different from other HP Latex inks since they print two layers in addition to their preprint optimizer. This offers a very thin pigment layer which improves their ability to manage drying and maintain high image quality. The optional top clear layer provides durability. If the print is to be laminated the top layer is omitted to provide better adhesion of the laminate to the media. The inks are film formers, not chemically crosslinked like UV inks, and provide a less brittle image. The result is that ink cracking when cutting the substrate is reduced or eliminated. The white ink is whiter and more opaque than most UV white inks and does not yellow with age as some UV inks will, according to the company. Like water-based pigment inks there is no smell commonly associated with UV inks.
We now have multiple choices for printing onto rigid substrates. Selection of the best technology is application-dependent. LED UV ink technology has been the only choice for non-porous rigid media until now. The introduction by HP of a latex printer into this application offers a choice and some very attractive performance characteristics. Even though HP offers all three technology options in their product line I expect them to continue promoting latex technology further into nearly all applications since it offers a low cost, greener, safer alternative to LED UV technology.