sublimation and uv printing
We have been living in the “digital age” since the release of the first personal computer. Sublimation and UV printing have altered practically every area of our everyday lives, and the digital printing industry is no exception. Digital printing technology is pushing the edge when it comes to what we can print on and how to do so more efficiently. “Can I print on that?” has become the new motto as personalization has surpassed bulk production as the new norm.
Take, for example, a simple cellphone case. This generic chunk of plastic isn’t anything special. Yet, once painted, the perceived worth of the product grows tremendously. Although the case’s performance has not improved, customers are willing to pay more just because it has been personalized. Here are some introduction of sublimation and UV printing methods.
What is dye sublimation printing?
Dye sublimation printing is one of the most common methods for decorating plastic products. While most people associate dye sublimation with textiles and apparel, this is not the case; this process can also be used to decorate a wide range of plastic or rigid substrates.
What exactly is dye sublimation? Sublimation is defined as “a phase transition of a substance from a solid to a gas without passing through the intermediate liquid phase.” When dye sublimation printing onto rigid substrates, the disperse dye inks are printed onto a coated medium, which is most often “transfer paper,” but can also be “transfer film.” These water-insoluble dyes then dry and solidify as the water solution evaporates. Heat (typically 400°F), pressure, and time convert these solid dyes into gas, which then penetrates the plastic or polyester coating after being released from the transfer paper or film.
The sublimation process, in addition to dyeing the plastic or rigid substrate, transforms the dull colors that were originally printed on the transfer paper into incredibly vibrant hues.
Other dye sublimation factors
The applications for dye sublimation appear to be limitless. It is critical for users to remember that dye-sublimation occurs at the heat press, not the printer. To produce high-quality prints with accurate colors and ink drop placement, a high-quality printer with premium RIP software is required. Droplets of dye sublimation ink are extremely small. “The largest dye-sublimation ink droplet is smaller than the smallest eco-solvent ink droplet,” a colleague put it best. That is, these ink droplets are small, and they require precise control when they jet from the piezo print head. Fine lines and details, as well as smooth gradients, are impossible to achieve without proper control.
Dye sublimation necessitates the use of polyester or plastic in order for the inks to penetrate the substrate and dye. The blanks must be able to withstand temperatures of 400°F, pressure, and time (which can range from 60 seconds to minutes, depending on size and type of object).
How does UV printing ?
In many ways, UV printing differs from dye sublimation or traditional printing methods such as traditional pigment, solvent inkjet, and commercial offset. While it is still ink on “paper,” the ink cures in a completely different way, and the “paper” becomes something other than paper. UV inks dry through photoinitiators in the ink and are solidified by UV lamps, as opposed to solvents in the ink that evaporate into the air and absorb into the paper. When exposed to ultraviolet energy, the inks change from a liquid or paste to a solid. UV-curable inks are thus “cured” after being exposed to UV energy wavelengths.
This curing process is beneficial for a variety of reasons. One of the most significant advantages of UV printing is that it reduces emissions of volatile organic compounds into the environment because the solvents do not evaporate like traditional inks. UV printing also has the advantage of curing inks on plastic and other nonporous substrates. Because the inks dry through this photomechanical process, the ink solvent does not need to absorb into the stock. The UV process enables printing on virtually any surface. Printing is possible if the media or product can pass through or under the printer.
But – and this is my “but factor” – adhesion can still be a problem in UV printing. It is critical to understand that, while UV printers can print on virtually any surface, adhesion and durability issues may still exist.
Challenges of 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. UV inks dry or cure so quickly when exposed to UV energy that they have no time to soak into the media. As a cleaner, less contaminated dot, the ink dot sits on top of the uncoated sheet, allowing for a more vibrant and “crisp” color appearance.
Printing with UV inks requires the ability to expose the inks to enough ultraviolet energy for curing to occur without making the substrate too brittle, while also 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.
In UV printing, dye level or surface tension is the property of a UV ink that forms unbalanced molecular forces at or near the surface. If this is greater than a material’s surface energy, the liquid tends to form droplets rather than spread out. Surface energies of plastic materials can vary greatly depending on their composition and how they are formed. Surface tension is typically measured in energy units known as dynes/cm.
If the ink has a dyne level lower than the surface energy of the material, it will spread out in a uniform, wet layer over its entire surface. If the dyne level of the ink is equal to or greater than the dyne level of the material, the ink becomes cohesive and tends to remain in droplets, allowing for better adhesion to plastics.
So, how can we regulate the dyne levels in raw plastics or pre-formed plastic products? Adhesion promoters have significantly improved the adhesion of UV ink to plastic products. Adhesion promoters, also known as coupling agents, are bi-functional materials that strengthen the bond between the coating and the substrate. Adhesion promoters, unlike priming systems, are typically applied at thinner film thicknesses.The effectiveness of an adhesion promoter is determined by both the substrate and the adhesive used. Surface pretreatments such as solvent cleaning, mechanical etching, and corona treatment can be used as part of a pretreatment method with adhesion promoters. The functionality on the backbone of the molecule surface will vary depending on the resin system used and the substrate to which it is attached within a class of materials.
Many ink manufacturers have developed adhesion promoters to aid in the printing process. Polypropylene (PP promoters) and polyethylene (PE promoters) are two commonly used adhesion promoters in UV printing. Both of these promoters can aid in the formation of a chemical bond with various types of plastics. While these promoters are widely used in the UV printing industry, they do have varying success factors that must be tested.
There are two useful tests for evaluating adhesion with the UV printing process: a traditional “scratch” test and a “cross-hatch” test. Each of these tests evaluates the bond between the plastics product and the adhesion promoter and UV-cured ink printed on the object or raw material in a different way.