By Olivia Cahoon
UV ink chemistry and the ability to cure quickly enable print service providers (PSPs) to print on wood-based products. UV ink features photo initiators, which instantly solidify ink to wood—allowing for a variety of digitally printed wood-based applications like décor, directional signage, and keepsakes.
Depending on the application’s end use, a pre- or post-treatment may be required to improve ink adhesion and durability. The type of wood is also a consideration when printing direct with digital technologies.
Above: Available from Advanced Color Solutions, Roland VersaUV flatbed printers print with UV ink directly onto wood to create items such as custom corn hole boards.
Printing to Wood
UV ink’s specific chemistry is ideal for printing to wood-based substrates. By using this technology, PSPs take a non-traditional substrate and create custom graphics as well as special effects.
While UV ink is not the only ink set that allows for printing to wood, it is unique in its ability to create special effects. UV formulations are manufactured with photo initiators in the ink. These photo initiators are cured instantly to wood—and other substrates—by UV lamps, shares Jay Roberts, product manager, UV printers, Roland DGA Corporation. “This process allows for some eye-catching effects on many different types of wood,” he continues.
Wood is a non-traditional substrate, so end users are already looking for a custom, one-of-a-kind piece when they select this medium. UV inks further satisfy these needs by incorporating custom effects on or under CMYK prints using white or gloss. By putting white under specific areas, Roberts says print providers can control color intensity and the way the wood comes through the print. Gloss UV ink also makes it easy to add texture or a glossy embossed effect to enhance wood prints.
In addition to special effects, UV ink is ideal for wood because the ink offers advantages like strong adhesion, durability, chemical resistance, longer outdoor life, and instant drying. “UV-curable inks combine ink chemistry with polymer chemistry. They offer advantages of printing ink and polymer coating in one single operation,” explains Nitin Goswamy, president, A.T. Inks.
The ink is flexible or hard depending on the substrate’s requirements. Due to the mix of polymer chemistry, Goswamy believes the design space for UV-curable ink is relatively large and tailor made to suit a specific substrate like wood, glass, or metal. “It is this unique ability to customize the ink to the application that makes UV-curable inks the fastest growing inkjet ink globally in the last few years,” he adds.
Other Ink Technologies
While UV inks are typically the preferred method to print on wood-based products, other ink technologies are also capable of printing to wood.
This includes solvent inks, which Goswamy believes may not provide strong enough adhesion to wood or enough color fastness. “Relatively high porosity of wood can cause the solvent ink to lose color vibrancy. Water-based inks cannot be printed onto wood directly either.”
According to Phil Jackman, global product manager, digital, Sun Chemical, some analog printing techniques direct print. However, these are primarily used to manufacture high volumes of wood-based panels used in décor, furniture, flooring, and medium density foamboard (MDF) boxes. “Inkjet has the advantage of being a non-contact process,” he adds.
Roberts agrees, pointing out that other technologies such as screenprinting offer limited capability for printing on wood. While screenprinting can be utilized to create multiple prints, it’s a method often used when hundreds or thousands of prints require simple graphics.
The Curing Process
How UV ink adheres to wood is different from traditional substrates, partly due to photo initiators and wood fibers.
As noted previously, UV formulations have photo initiators inside the ink. These photo initiators are activated when exposed to UV light from the printer’s lamps—instantly solidifying the ink, says Roberts. Because of this, he believes the UV process is similar to the photographic process—the ink is exposed and preserved immediately upon cure. The printer and the RIP control the exposure and the ink loads.
UV ink is typically exposed to UV light immediately after the printhead fires, which means the ink does not have as much opportunity to be absorbed by the wood and therefore stays closer to the surface, explains Michael Maxwell, senior manager, Mimaki USA, Inc. However, in the short amount of time the liquid ink is introduced to the wood surface it has just enough time to wrap around and get between the wood fibers. “This short window increases the ink adhesion and makes for a more durable finish then other substrates that repel liquids, forcing the ink to remain just on the surface.”
Bill Brouhle, senior application specialist, Agfa Graphics, believes that UV ink cures to wood similarly to traditional print products. He advises PSPs to keep in mind that digitally printing onto wood requires increasing the ink density—to offer vibrant, beautiful colors that are not washed out. “Just like in a backlit film where you would want to increase the density and volume of the ink being output, as the nature of the backlit material is that it will look washed out,” he recommends.
Several different wood-based applications are created with UV ink, including art pieces, décor, directional signage, furniture, keepsakes, jewelry boxes, and toys.
For example, Maxwell says printed wood in a residential setting can be used to enhance room décor or create a decorative element on a desk, cabinet, or bar. There are also other applications for wood decoration, such as taking an ordinary item and adding personalization.
Another possibility is printing directly on older wood to simulate faux vintage advertising signage, suggests Brouhle. This can be taken a step further and customized specifically to the customer. “Applications like custom table tops, chair seats, and directional signage all have a place in digital print offerings that add a layer of texture and warmth to surroundings.”
Pre- and Post-Treatments
Printing to wood with UV ink sometimes requires the use of a pre- or post-treatment depending on the application’s end use and wood type.
Wood is often printed without a primer or post coating. However, the efficacy of the print depends on the application’s end use. If the print is likely to be regularly used, Goswamy recommends a primer or post coating to provide protection from abrasion and adhesion failure. However, he says printed wood products displayed away from human reach—like an art piece—only require moderate adhesion and may be printed without a post coating.
Primers enhance adhesion to a range of wood types and prevent failures based on poor adhesion. “The main reason to use a primer is to neutralize the variability in the surface characteristics of wood with various surface qualities,” explains Goswamy. For example, most wood printing is completed on MDF, which has a surface quality that varies widely. With a primer/adhesion promoter, the substrate’s variability is neutralized while providing optimum print performance.
Post-treatments are mostly used to increase outdoor durability, appearance, and abrasion protection. In more demanding applications, such as wood flooring, a post coating provides the wear resistance needed to last 20 years or more, says Jackman.
Wood for the Outdoors
Any outdoor application is susceptible to the effects of weathering and UV exposure. For digitally printed wood placed outdoors, special precautions are taken.
To avoid moisture and water, PSPs can apply a post coating or clear paint to the digital print. Roberts suggests using a water-based polyurethane to seal the print as it is less aggressive, doesn’t react with inks, and won’t yellow or change the print’s appearance. “I typically suggest using water-based sealers from Miniwax or Golden, but many of our customers use brands from Lowes or Home Depot as well,” he offers.
According to Matt Gusse, VP of sales and marketing, Advanced Color Solutions, many UV printers are equipped with a gloss ink that can be used as an additional layer of protection. “If the UV printer does not have a gloss or clear ink channel, a third party clear ink could be used to enhance durability,” he advises.
A wide variety of wood types are compatible with UV printing. However, it is important to note that the type of wood affects appearance and the printing process.
In some cases, the use of very rough wood can be a negative influence. “It really isn’t the wood itself, but rather the fibers that come off the wood and cling to the printhead that can be problematic,” explains Roberts. He recommends using a good tack cloth to remove loose fibers from the wood prior to printing. It’s also good practice to clean the printer’s printheads daily when printing to raw wood. Outside of wood fibers, Roberts says he rarely encounters any issues in this application.
Depending on the wood’s shade, PSPs may need to take precautions to hide its appearance and marks. If the wood is very dark, Gusse explains that PSPs might need to lay a white coating underneath the art so that the image pops. If printing to light wood, he says its color can be used as part of the image or it may be light enough to not require a white undercoating.
Aside from aesthetics and printability, price is also considerable as some wood types are costlier than others. Jackman finds that PSPs typically prefer inexpensive materials, which are often not real wood but wood-based panels such as MDF. “This is not only costs less but is more consistent in color and surface properties, which aids in consistent printing.”
Tips and Tricks
By printing on wood, PSPs can expand their application offering. While this presents great opportunity, there are some important factors to consider.
Selecting the best wood type is the first step. To do so, PSPs should purchase a wood with a good level surface. “Many printers cannot deviate more than a couple of millimeters from the highest point measured and still produce a good image,” advises Maxwell. Purchasing a strong vacuum also helps hold down heavy wood sheets.
It’s also good practice for PSPs to be aware of the wood’s textured surface and how it could cause printhead strikes. According to Maxwell, wood can act as a harsh abrasive—ruining an expensive printhead. Furthermore, the wood should be relatively dust free and wiped down with a damp cloth the day before printing to avoid dust exposure to the printer, ink, and electronics, which can create maintenance issues. “To that point any cutting or prepping of the wood should be done in an area that is separated from the printer.”
To avoid a lengthy finishing process, sandblasting resurfaces and shapes wood in minutes. “It’s efficient and typically done for the sole purpose of creating art pieces,” explains Juan Kim, CEO, Valloy Incorporation.
Art to Furniture
UV ink allows PSPs to take advantage of printing to non-traditional substrates like wood—creating products from art pieces to décor and furniture. Before offering this service, it’s important that PSPs consider wood type, finishing methods, and the application’s end use, which determines if a pre- or post-treatment is required.
Jan2019, Digital Output