Compared to solvent and ecosolvent inks, UV curable inks have several advantages. No VOCs to pollute the air; the inks dry fast but don’t clog in the print head; and good adhesion to a wide range of substrates, including styrene, expanded PVC, polycarbonate, acrylic, gatorboard and corrugated polypropylene.
Differences in Adhesion.
Solvent inks and UV inks adhere to a substrate in different ways. Solvent inks are designed to partially solvate or dissolve the outer surface of a pressure-sensitive vinyl film in order to chemically bite into it. Printed on board stock or other porous materials, solvent inks adhere to the substrate through absorption.
Free radical UV inks work another way. They don’t solvate the substrate and don’t bond by being absorbed. Instead, these UV inks anchor to a material mechanically, in much the same way as a pressure sensitive adhesive does. After the ink is printed, it wets out and fills in the microscopic pores on surface of the substrate, holding on for dear life as the ink cures and becomes a solid sheet.
Another major factor in adhesion is the surface tension difference between the ink and the substrate. The ink should always be lower in surface tension than the substrate. Some substrates, Coroplast in particular, change dramatically in surface tension over relatively short periods of time. As the surface tension or dyne level of the substrate drops, ink adhesion can become problematic.
Differences in Curing/Drying.
Most inkjet inks dry by evaporation, with the exception of UV inkjet inks. To dry, solvent inks evaporate. Because VOCs pose an air quality problem in the shop, they must be ventilated. Although solvent inks dry faster than ecosolvent inks, and both inks dry faster than water-based inks, all inks take time to dry. To fully cure, solvent and ecosolvent inks can require at least 24 hours – often longer.
The solvents in an inkjet ink performs several functions. First and foremost the solvent acts as the carrier for the colorant. A secondary function is to control the drying time of the ink. The type of solvent used also determines how well the ink adheres to the substrate. Finally, the solvent alters the viscosity of the ink, thereby controlling the flow characteristics of the ink on the surface of the print media. Low viscosity is also required so the ink does not clog the print heads.
Ecosolvent inks uses a less aggressive, slower drying solvent. To aid the drying of the ink, ecosolvent printers generally require more heat in curing. Preheating the substrate opens the material’s pores up to accept the less aggressive solvents in the ecosolvent-type inks. The milder ecosolvent inks do not bite into the vinyl as well as an ink with a more aggressive solvent. The standard solvent inks also tend to better withstand solvent spillage.
When printing on vinyl, water-based inks require a top coat to absorb and encapsulate the ink. Solvent inks don’t require a top coating on pressure sensitive vinyl. Instead the vinyl absorbs the vinyl ink.
If you intend to print on a wide range of materials, UV-curable inks are your best choice. UV inks have a number of advantages over solvent based inks. Solvent-based inkjet inks take time to dry. The rule of thumb is that you should allow 24 hours after printing, before clear coating or laminating. The long time to cure these inks slows production down. That can be a problem when you have an anxious customer who is in a rush.
One of the major benefits of UV inkjet systems is that the inks dry almost instantly, after being exposed to intense UV light. Exposure to certain wavelengths of intense UV light initiates a chemical reaction, which cures or hardens the ink.
You will note that I hedged my bet by saying that the inks dried “almost” instantly. After exposure to UV light, prints are dry to the touch and can be handled. Not all prints, however, are totally dry. When printing with some UV inks, you should wait an hour or two before rolling up a print, just to play it safe. Some UV inks can continue to chemically crosslink and cure for as long as three days after the initial exposure. As this crosslinking process continues, the inks will become harder and harder.
More flexible inks.
Early generations of UV curable inks were somewhat brittle and prone to cracking. However, the once brittle inks have improved significantly. Later generations of UV inks are flexible and durable enough for fleet graphics applied to vehicle surfaces with compound curves, rivets and corrugations.
Heavier ink deposit.
UV inkjet printers lay down a heavier deposit of inks than solvent or ecosolvent printers. That should be no surprise. Solvent-based inks are 80% to 90% solvent. Most of ink that is printed evaporates. UV curable inks, on the other hand, are 100% solids. That means that 100% of the ink, that is printed, is cured and remains on the substrate. Because the ink deposit is thicker, the durability is arguably better. The heavier deposit of ink is also more abrasion resistant.
Another unique advantage is that many of the new UV-curable inkjet printers print white as a base color in addition to CMYK. Although that doesn’t sound like a big deal, imagine printing a four-color process image on a metalized vinyl film, such as RTape’s VinylEfx films, or a dark colored pigmented vinyl or a clear film. A background color will show through, and the printed image will be indiscernible. On a clear film, the image will be washed out.
The advantage of printing an opaque white is that it blocks out the background. Without it, the image is lost, especially if you are printing on metallized specialty films. By printing white first, before printing the process colors, the white ink serves as the foundation for the printed image. On pigmented films, the printed image will visually pop off of the background.
Virtually No VOCs.
UV inks are 100% solids. What this means is that 100% of the amount of ink that is deposited on a substrate is cured and turns into a solid. No solvents flash off to pollute the atmosphere. No solvents penetrate the substrate or attack the adhesive system of vinyl films. (NOTE: Some types of monomers are aggressive and can compromise the integrity of pressure sensitive vinyl films.) And there are no harmful volatile organic compounds (VOCs) to threaten anyone’s health.
Because UV curable inks are 100% solids, they can have better opacity that solvent-based inks, dependent, of course, on the pigment load that of the UV ink. This is a big advantage, when you print on colored substrates, because the ink has more hiding power, which helps prevent the background from overpowering the printed image.
Printer systems with a white ink option give you the ability to lay down white first as a block out on a specific area of the substrate, before you print the image. Or, when you are printing subsurface, you can print a wrong-reading image on the second surface of a clear substrate and then back it up with white. While some solvent inkjet printers also have the white ink option, their inks typically don’t have the same hiding power (opacity) to knock out a background. The downside to either of these approaches is that the printer must image the material twice, therefore print speed is cut in half.
Remember that UV inks adhere to the substrate by mechanically bonding to the surface of the media, rather than adhering by absorption into the substrate, such as water-based inks. With ink systems that adhere to the media through absorption, the appearance of the print can vary as the amount of ink that is absorbed, varies. Because all of the UV ink stays on the surface of the media, the color is more opaque and print quality is more consistent from one print to another.
Unless the print nozzles are exposed to UV light from the curing lamps, the printheads do not clog as readily as those of solvent or ecosolvent inkjet printers.
Because UV-curable inks won’t cure without UV light, the printheads rarely dry out and clog, as they often do with inkjet systems using solvent-based or water-based inks. This minimizes equipment maintenance. Maintenance for the UV ink jets includes daily cleaning of the printheads and periodically changing the UV lamps. The average lifespan of a lamp is about 1000 hours. Regular maintenance also includes cleaning the lamps, reflectors, light shield and air filters.
While UV inkjet systems are low maintenance, that’s not the same as no maintenance. Printers should follow the manufacturer’s recommended preventative maintenance programs covering the periodic cleaning and replacement of lamps, reflectors and filters. Failure to service the curing units can result in inks failing to cure properly.
Printing on Reflective Surfaces.
With some UV inkjet systems, printing on reflective and metalized vinyl films can be problematic. Reflective and mirror finish surfaces can reflect UV light back to the printheads. This can cause the ink to dry in the printhead. Once the UV ink is cured in the head, you cannot clean it with solvents or other cleaner. It’s time to replace the printhead. This can be a costly proposition. UV inkjet printheads can be much more expensive then solvent or water inkjet printheads, with costs ranging from a little less than $1000 to more than $3000.
While it’s true that the UV inks shouldn’t dry in the print head, the nozzles can get clogged if they are exposed to UV light. This can happen, in rare cases, if UV light reflects back to the print head off a mirrored finish, such as a reflective vinyl or a smooth metalized film. Today’s printheads are engineered to prevent this occurrence. Printhead failure rarely occurs as a result of dried ink. They fail because of wear and tear.
Compatible With A Wide Range Of Substrates.
Compared to solvent and ecosolvent printers, UV curable inkjet inks adhere to a wider range of substrates.
Flatbed systems, which use UV-curable inks and high heat lamps can print on a wide range of flexible and rigid substrates, including pressure-sensitive vinyl, rigid vinyl, expanded PVC, polycarbonate, acrylic, polystyrene, polyester, polyethylene, polypropylene, metal and glass.
UV curable inks cure almost immediately. Solvent inks dry though evaporation, which can take time. Because UV inks cure faster, productivity is better.
High Production Speeds.
The production speeds of some UV inkjet printers are mind boggling. In a visit to a Point-Of-Purchase printer, one of their flatbed printers was capable of printing 5382 square feet in an hour. That equates to more than one hundred 5’ x 10’ sheets in an hour. Now that’s smokin’.
This rapid cure of the UV inks after printing, means that the dots have no chance to spread. The end result is that the integrity of the dot is maintained, which means that the printed image looks crisper.