By Cassandra Balentine
Digital printing is used to decorate objects made of various materials. UV inks are commonly utilized for direct to object (DTO) printing. UV printing in particular as well as the subsequent UV curing technologies—traditional or LED—enable high-resolution printing and deliver sharp, vibrant images. There is also the capability for print embellishments with specialty inks, coatings, and varnishes.
Many Uses of UV
Digital UV printing supports a variety of DTO materials from cylindrical hollow bodies like bottles, cans, tubes, cartridges, and cups; to flat surfaces like flooring, tiles, signs, promotional items, and decals, as well as non-packaging industrial applications such as printing on solder masks and industrial parts in automotive, lists Stacy Hoge, Phoseon marketing manager, Excelitas Technologies Corp.
Hugo Gonzalez, senior segment specialist, industrial products, Mimaki USA, Inc., notes that UV curing in the industrial DTO print space is a transformative technology that enables printing of high-quality images and designs directly onto a variety of substrates.
“UV curing systems in digital DTO printing have expanded rapidly in the last few years,” comments Paul Edwards, VP – digital division, INX International Ink Co. “Today, DTO is commonly used to print glass bottles, cups, tubes, formed boxes or cartons, cans, and containers of all types.”
The potential doesn’t stop there. For instance, the technology prints onto planks of wood flooring. “Considering the breadth of substrates, UV ink technology makes a lot of sense as it adheres well to all of the substrates mentioned,” comments Edwards.
Juan Kim, CEO, Valloy Inc., suggests that UV curing for imaging and coating on industrial substrates is quite popular and is easily found in various fields already today. “Many interior glasses are decorated by UV inkjet and displays from cell phones to larger TVs are coated by UV-curable inkjet technology. Anodized and chemical treated aluminum plates are digitally printed by solvent ink. Metal cans are also digitally produced,” he offers.
Thomas Lang, digital applications manager, Inkcups, also reports growth in industrial applications where DTO printing is replacing adhesive labels, such as for industrial parts and medical items.
Outlining the Process
UV curing is integral to industrial DTO inkjet printing.
The process involves applying tiny droplets of UV-curable ink onto plastic, glass, metal, and other materials through a printhead. These are then instantly “cured” in UV light. “It is the instantaneous nature of this drying process that is so important, because it ensures that each ink droplet stays exactly where it should, with no bleeding or spreading. This makes for a sharper image, allowing for precise reproduction of vividly colored digital images or fine lines, text, and numbering. UV-cured ink fits in perfectly with the fast-paced customization market, with instantly cured products coming off the machine making for a perfect process for print on demand that can accommodate next-day or even same-day shipping,” explains Lang.
Michael Perrelli, sales and marketing director, Innovative Digital Systems, points out that without UV curing, there would be no UV printing. “The lamps, the power, and the speed in which the ink can be cured on materials that span such a wide range continues to grow and evolve.”
The process typically employs UV LED curing technology due to its efficiency and precision. “In inkjet printing, LED pinning is used for initial curing, while a final cure is often achieved using either LED or traditional UV lamps, depending on the desired surface characteristics and finish. The choice between LED and traditional curing hinges on factors like the ink formulation, the nature of the substrate, and the specific surface properties required,” explains Chris Davis, head of sales, web and industrial, IST America.
LED based UV curing is ideal for applications that require heat-sensitive substrates due to the low heat, shares Hoge, adding that the longer UV LED wavelengths naturally penetrate deeper into the ink delivering stronger color, greater adhesion, and a better depth of cure.
Popular Materials
As previously discussed, UV printing technologies in the industrial DTO space enable printing images and designs directly onto a variety of substrates including plastics, metals, glass, and wood.
Because UV curing fixes each ink droplet into place instantly, Lang says a DTO UV inkjet printer can be configured to print on almost any type of substrate, although in some cases, pretreatment may be needed and specialized inks are used to further improve adhesion. “This versatility in terms of materials is one of the major factors driving the growth in popularity of UV inkjet printing.”
Plastic substrates dominate the DTO printing market, according to Davis, necessitating careful consideration of surface conditions and the material’s inherent energy properties. Surface modification techniques are often employed to enhance ink adhesion.
Plastic tubes and containers are popular. “Depending upon the type of plastic and any materials used for processing on the surface, it can be more challenging. With UV inks, most substrates can be effectively addressed. Hence, the growth into many new applications has been observed,” says Edwards.
Gonzalez adds that plastic materials are often used as manufacturing materials. “Plastics are favored for their versatility and cost effectiveness in promotional items and consumer goods.”
Perrelli argues that the term “plastic” covers a range of materials and products. “However, that isn’t meant to imply that metal and wood aren’t equally viable and extremely popular materials to print.”
Kim believes plastic and wood stand out in terms of top materials for UV DTO printing. “This is because special ink is preferred for metal and glass/ceramic due to durability issues.”
Davis admits that metal decoration is also prevalent, valued for its durability and aesthetic appeal.
“Metals are commonly used for industrial components and high-end promotional products due to their durability and aesthetic appeal,” echos Gonzalez. “Wood is often chosen for custom, rustic, or artisan products,” he adds.
“Glass is popular for its premium look in decorative items and awards,” comments Gonzalez.
Lang agrees that glass is a popular DTO material, but admits that it needs to be pretreated before inkjet printing.
Perrelli explains that powder-coated materials and glass are more popular on a rotary printer than a flatbed printer.
Edwards finds it difficult to really point to a material which is most favored. “Glass bottle printing is used quite often due to the wonderful looks which can be achieved on a glass bottle, and the structure which can be created with the use of clear inks. Equally, printing onto two-piece aluminum (metal) cans has recently become very popular. It is mostly being driven by the rise and popularity of craft breweries and the unique looks which can be achieved.”
Hoge stresses that UV LED curing enables users to process a variety of materials, including heat sensitive and thin substrates, at maximum production speeds with low input power requirements. “UV LEDs allow high adhesion on covering inks, enabling strong colors on the more difficult substrates. Due to narrow, high UV-A wavelength, UV LEDs produce significantly less heat. The UV-A wavelength also allows for a more penetrative cure, which is an advantage when faced with thicker sections. Digital printing is often short run with frequent stops, so immediate ‘on/off’ with UV LED curing technology yields greater productivity and revenue.”
“Each material presents unique challenges and opportunities, requiring tailored surface preparation and curing strategies to achieve optimal print quality and durability,” concludes Davis.
Special Effects
In addition to standard CMYK printing, UV curing systems enable special effects on printed objects.
Kim confirms that digital embellishment is a hot topic and UV curing is at the center of it, from spot varnishing to glossy and matte varnish effects, and cold foiling with varnish ink.
“When it comes to UV printing, the time in which the print is cured, or the segmentation of the UV lamp, or the amount of lamp power used, can yield varying effects in regards to the finish of the ink,” explains Perrelli.
Special effect possibilities offered by UV range from full, seamless wraps or raised varnish for a 3D look and feel, largely because of the technology’s ability to instantly cure inks and varnishes, comments Lang.
Many DTO UV printers use UV ink layering to create raised relief textures, as well as matte and gloss effects, adding a tactile dimension to the printed objects. “Additionally, the use of opaque white inks enhance color vibrancy on dark substrates, while primers expand the range of printable materials by improving ink adhesion. These capabilities enable unique finishes that elevate the final product’s visual and tactile appeal,” notes Gonzalez.
Hoge adds that digital printing processes that utilize UV LED curing achieve matte and gloss effects, tactile surfaces, cold foil, and other special effects, creating a multi-sensory experience. “Users have the option for pinning to ‘freeze’ ink drops and immediate overprint.”
Many effects require the precise application of LED light to penetrate opaque or optically challenging media. “Understanding how UV energy interacts with the embellishment is crucial. This involves determining the correct energy dosage and distribution to ensure the desired visual and tactile effects are achieved without compromising the integrity of the substrate or the print,” notes Davis.
To create these layer effects, Gonzalez recommends that the layer be fully cured before applying the subsequent layers. “If the layer below, for example with white ink, is not cured to the touch, it may blend with and ’muddy’ the color ink over it instead of giving it a vibrant pop.”
Working through the Process
Davis admits that UV curing can integrate at various stages of the printing process, including inline, offline, or nearline, depending on the application’s requirements. “Inline curing is common for high-volume, continuous production, such as beverage cups, where curing is synchronized with the printing process. Offline or nearline curing is more suitable for complex or lower volume items like washing machine displays, where curing occurs downstream to accommodate additional processing steps or to ensure uniformity across complex geometries.”
“Unlike solvent- and water-based inks, which typically dry through evaporation using heat, UV inks are cured through a chemical reaction that occurs when they are passed under UV light. In addition to allowing operators to package and ship products directly after decorating, an added benefit of this is that UV curing doesn’t generate pollutants and also consumes less energy,” says Lang.
The exact process is dependent upon the application. “The curing of the ink is almost always done inline with the printing process and relatively close to it. Sometimes, due to the configuration of the printing system, you can pin the inks first. This means you cure just enough for the inks to stop being mobile, and then the final cure can take place a bit later. This is mostly done inline,” explains Edwards.
The way in which a UV ink is cured is very important to the process. “It controls the spread of the drops, which in turn has a big effect on print quality as the size of and interaction between the drops needs to be well controlled,” shares Edwards.
Sometimes in DTO printing, the user may want to print one color at a time or print after all the colors are applied as part of the process. “There is flexibility in how and where the UV inks get cured. However, it is essential that the system is designed in such a way as to maximize the print quality and adhesion. Curing too late in the process can lead to challenges in handling the product. The inks may still be quite soft and prone to damage,” adds Edwards.
When curing inline, Lang says images can hold a tighter registration for high-quality imagery.
Perrelli adds that inline curing allows operators to go directly from print to package or retail shelf.
UV curing inline allows printers to quickly change the design printed on a product or its packaging without disrupting the process. “The inline process is a flexible technique that allows a digital printer to print on surfaces with different textures. It is designed to be agile without compromising on the production speed, quality, or reliability,” states Hoge.
Many digital printing machines come equipped with multiple printing stations that utilize UV curing for pinning, full curing, or a final coating. “A pinning station utilizes a UV light source for the pinning process to ‘freeze’ ink drops before overlaying other colors or doing a full cure. A full cure station would typically include a four-color process with CMYK or single-color inkjet printing. Sometimes both pinning and full cure UV light sources are integrated into one station. A final coating station utilizes UV curing for top coating,” shares Hoge.
Gonzalez says curing inline also ensures that design lines and boundaries are maintained. “If the wet ink is allowed to settle before curing, edges and boundaries may spread and be blurred reducing the overall quality of the print.”
The Name of the Game
When it comes to the benefits of UV DTO printing, versatility stands out. From plastics to glass, metals, and wood, UV inks and varnishes are cured directly onto a usable product. In addition to CMYK printed output, the UV process allows for specialty finishes including matte and gloss, metallics, and even textures.
Oct2024, Digital Output
