Since the introduction of the first personal computer, we have been ushered into the “digital age.” Technology has transformed virtually every aspect of our daily lives, and this phenomenon has been no different in the digital printing arena. Digital printing technology is pushing the envelope when it comes to what we can print on and how to do so more efficiently. “Can I print on that?” is the new mantra, as customization increasingly replaces mass production as the new norm.
Let’s take, for instance, a plain cellphone cover. There’s nothing spectacular about this generic piece of plastic. However, once it’s decorated, the perceived value of the product exponentially increases. The performance of the case hasn’t increased, yet people are willing to pay more, simply because it has been personalized.
There are a variety of methods used to decorate plastic products, with one of the most popular processes being dye sublimation. While most people associate dye sublimation paper with textiles and apparel, this is not solely the case, as this process also can be used to decorate a broad array of plastic or rigid substrates.
What is dye sublimation? Sublimation is defined as “a phase transition of a substance from a solid state to a gas while bypassing the intermediate liquid phase.” In the case of dye sublimation printing onto rigid substrates, the disperse dye inks are printed onto a coated medium, which is most often a “transfer paper,” but also can be a “transfer film.” Once printed, these water-insoluble dyes then dry and turn solid as the water solution evaporates. Heat (typically 400°F), pressure and time turn these solid dyes into gas which, upon release from the transfer paper or film, then penetrates the plastic or polyester coating. In addition to dyeing the plastic or rigid substrate, the sticky sublimation paper transforms the dull colors that were originally printed on the transfer paper into incredibly vibrant hues.
The end-product and volume will determine the type of heat press that should be purchased. If decorating mainly flat objects, then a flatbed press is ideal. Flatbed presses can be small (16×20″) presses or large-format presses that can accommodate 4×8′ panels.
If sublimating on 3D objects, then a 3D oven press will be needed. Once again, the oven presses can range from smaller “desktop” units to machines the size of a walk-in room. Instead of printing onto transfer paper, the images will be printed onto transfer film, which is able to conform around the edges of the object being decorated. A fixture is needed to hold both the object and printed film in place. A vacuum hose is attached so the printed film thermoforms around the object and, when the oven heats up to 400°F, the Inkjet Sublimation Paper process takes place.
With dye sublimation, the applications are seemingly endless. The important thing for users to remember is that dye-sublimation takes place at the heat press, not at the printer. However, a high-quality printer with premium RIP software is necessary to produce quality prints with accurate colors and ink drop placement. Dye sublimation ink droplets are extremely small. A colleague best described it by saying, “the largest dye-sublimation ink droplet is smaller than the smallest eco-solvent ink droplet.” What that basically means is that these ink droplets are small, and they need proper control when they jet from the piezo print head. Without proper control, it is not possible to achieve fine lines and details or smooth gradients.
Dye heat sublimation paper requires polyester or plastic for the inks to penetrate into the substrate and dye. The blanks must be able to withstand 400°F heat, pressure and time (which can range from 60 seconds to minutes, depending on size and type of object).
How does UV printing differ from other printing methods?
Ultraviolet (UV) printing is different from tacky sublimation paper or conventional printing methods – including both traditional pigment, solvent inkjet and commercial offset – in many ways. While it is still ink on “paper,” the ink cures through a completely different process and the “paper” ends up being no longer just paper. Instead of having solvents in the ink that evaporate into the air and absorb into the paper, UV inks dry through photoinitiators in the ink and are solidified by UV lamps. When the inks are exposed to ultraviolet energy, they turn from a liquid or paste into a solid. Thus, UV-curable inks are “cured” once they are exposed to the wavelengths of UV energy.
This curing process is advantageous for many reasons. One of the biggest benefits of UV printing is that it lowers emissions of volatile organic compounds into the environment, as the solvents don’t evaporate like conventional inks. Another advantage of UV printing is that the inks can cure on plastic and other nonporous substrates. Because the inks dry through this photomechanical process, it’s not necessary for the ink solvent to absorb into the stock. The UV process allows for printing on just about anything. Essentially, if the media or product can get through or under the printer, it can be printed.
But – and this is what I call my “but factor” – adhesion can still be an issue in UV printing. It is important to understand that, while UV printers can print to virtually anything, there may still be adhesion or durablity issues that need to be overcome.
Challenges in decorating with UV inks
UV printers can print on a variety of unusual substrates, ranging from wood and wooden veneer, glass and sheets of metal to fabrics and plastics of all shapes and sizes. Since UV inks dry or cure so quickly when exposed to UV energy, there’s no time for them to soak into the media. The ink dot sits on top of the uncoated sheet as a cleaner, less contaminated dot, ultimately allowing for a more vibrant and “crisp” color appearance.
Printing successfully with UV inks depends upon being able to expose the inks to enough ultraviolet energy for curing to take place without making the substrate too brittle, and at the same time ensuring an acceptable level of adhesion to the substrate. This can be extremely difficult, as each type of substrate has different surface tensions or dyne levels.
Dyne level or surface tension in UV printing is the property of a UV ink forming unbalanced molecular forces at or near the surface. If this is higher than the surface energy of a material, the liquid tends to form droplets rather than spread out. Plastic materials can have very different surface energies based on their composition and how they are formed. The surface tension is normally measured in energy units called dynes/cm.
If the ink has a dyne level lower than the material’s surface energy, then the ink will spread out over its entire surface in a uniform, wet layer. If the ink’s dyne level is equal to or higher than a material’s dyne level, the ink becomes cohesive and tends to remain in droplets, thus allowing for better adhesion to plastics.
So, how do we control the dyne levels of raw plastics or pre-formed plastic products? The development of adhesion promoters has dramatically increased the adhesion of UV ink to plastic products. Adhesion promoters, sometimes referred to as coupling agents, are bi-functional materials that increase adhesive strength between the coating and the substrate. Unlike priming systems, adhesion promoters are generally applied at thinner film thicknesses. An adhesion promoter?s effectiveness depends on both the substrate and the adhesive being used. Surface pretreatments, such as solvent cleaning or mechanical etching and corona treatment, can be used with adhesion promoters as part of a pretreatment method. Within a class of materials, the functionality on the backbone of the molecule surface will vary based on the resin system employed as well as the substrate to which it is attached.
Many ink manufacturers have developed adhesion promoters to aid in the printing process. Polypropylene (PP promoters) and polyethylene (PE promoters) are two adhesion promoters that are widely used in UV printing. Both of these promoters can help create a chemical bond to different types of plastics. While they are widely used within the UV printing industry, these promoters do have varying success factors that must be tested.
With the UV printing process, there are two useful tests that can be employed to evaluate adhesion – a traditional “scratch” test and a “cross-hatch” test. Each of these tests provides a different evaluation of the bond between the plastics product and the adhesion promoter and UV-cured ink printed to the object or raw material.
The scratch test is similar to a traditional abrasion test. A metal object, such as a penny, is vigorously rubbed on top of the UV print. Depending on the number of passes and vigor, it can be analyzed and given a “score” or rating. The better it performs against the abrasion, the higher the durable score.
The “cross-hatch” test is performed again after the adhesion promoter and UV-cured print have been output. Using a razor blade, knife or other sharp cutting tool such as a special crosshatch cutter, two cuts are made all the way through the UV print down to the substrate, forming an “X” mark with a 30- to 45-degree angle between the angles of the cuts.
An aggressive tape is placed at the center of the “X” and then quickly removed. The area is then examined to see if any UV ink has been removed. Again, it can be analyzed and given a “score” or rating. The better it performs against the “cross-hatch,” the higher the durable score.
While adhesion is a common issue with UV printing because of the vast amounts of printable objects or substrates, the acceptable level must be identified and tested on the final products the user is printing. Because UV printers can print onto just about anything, although they still may have adhesion or durability issues, it is important for the print provider to show the end user test prints to gauge the acceptable durability on the product based on two questions: 1) How long will the product be used, and 2) Will the product be handled for extended periods of time? The answers to these questions will provide the acceptance levels, which helps to ensure customer satisfaction.
UV print technology makes it simple to print on a vast array of rigid materials, including plastics. Being able to print directly to plastic objects – whether raw or pre-manufactured ? reduces both operation and delivery time. It also allows the user to incorporate white and gloss inks in glossy or matte finishes to create unique textures – enhancements that add value and sophistication to the end product. In addition, UV technology helps print providers expand their applications and product offerings by enabling them to print on wider variety of substrates.
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