By Olivia Cahoon
UV ink chemistry and the ability to cure quickly enables direct printing 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 custom corn hole boards.
Printing to Wood
UV ink’s specific chemistry is ideal for printing to wood-based substrates. With this technology, a non-traditional substrate becomes a custom graphic with unique embellishments.
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 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 it offers advantages like strong adhesion, durability, chemical resistance, longer outdoor life, and instant drying. “UV-curable inks combine ink with polymer chemistry. They offer advantages of printing ink and polymer coating in a 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, due to evaporation.”
According to Phil Jackman, global product manager, digital, Sun Chemical, some analog printing techniques work for direct printing. However, these are primarily used to manufacture high volumes of wood-based panels found 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
The process of how UV ink adheres to wood is different from other substrates, due to photo initiators and wood fibers.
As previously noted, 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 RIP control the exposure and 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 on the surface.”
Bill Brouhle, senior application specialist, Agfa Graphics, believes that UV ink cures to wood similarly to traditional print products. He advises increasing the output ink’s density—to offer vibrant, beautiful colors that are not washed out.
Several different wood-based applications are created with UV ink. 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 a level of personalization.
Another possibility is printing directly on older wood to simulate 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 placed away from human reach—like an art piece—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 used to increase outdoor durability, appearance, and abrasion protection. In 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 effects of weathering and UV exposure. For digitally printed wood placed outdoors, special precautions are taken.
To avoid moisture and water, a post-coating or clear paint can be applied 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. Wood Types
A 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 rough wood can have a negative effect. “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 best to clean the inkjet’s printheads daily when printing to raw wood. Outside of wood fibers, Roberts says he rarely encounters any issues in this application.
The wood should also have a 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.
Depending on the shade, precautions need to be taken to hide appearance and marks. If the wood is dark, Gusse suggests laying 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.
To avoid a lengthy finishing process, sandblasting resurfaces and shapes wood in minutes. “It’s efficient and typically done for creating art pieces,” explains Juan Kim, CEO, VALLOY Inc.
Aside from aesthetics and printability, price is another consideration as some wood types are costlier than others. Jackman finds that not real wood but wood-based panels such as MDF are favorable. “They are not only less money but more consistent in color and surface properties, which aids in consistent printing.”
Art to Furniture
UV ink allows manufacturers to take advantage of printing to non-traditional substrates like wood—creating products from art décor to furniture. Before offering this service, it’s important to consider wood type, finishing methods, and the application’s end use, which determines if a pre- or post-treatment is required.
Feb2019, Industrial Print Magazine