Glass Decorating at the Speed of Light
UV inks for glass and ceramic decorating
For many years, UV-curing technologies have had profound effects on a variety of imaging processes. As screen printers, we are aware of the substantial and growing impact of UV curing on our industry, but the offset and flexographic markets have been similarly affected. Applications such as roller-coating and curtain-coating have also taken advantage of the benefits of UV curing.
In glass and ceramic screen printing, advances in UV-curable ink technology are addressing the significant interest in this "alternative" to traditional ink systems. In this article, we will examine the issues involved in integrating UV curing into the glass and ceramic markets, as well as the equipment and training issues to consider before taking the plunge into UV curing.
UV curing 101
Before delving into the benefits of UV curing, let's briefly look into the chemistry of the inks. Regardless of their intended applications, all UV-curable inks have a number of things in common. Chemically, they are monomers (thin, reactive liquids), oligomers (we can regard these as "resins"), photoinitiators, and pigments. When exposed to high-intensity UV energy, the photoinitiators initiate a reaction between the monomers and oligomers, which link to form the "cured" ink film. This initial chemical crosslinking occurs in fractions of a second and enables the cured prints to be handled immediately after exposure.
Note that the crosslinking reaction continues for various lengths of time after exposure. It is not unusual for the reaction to continue for 48-72 hours. However, once the reaction has begun, the crosslinking can be considered irreversible. During the crosslinking period, the ink acquires its full resistance and adhesion properties, but decorated pieces already can be handled and shipped during this period. Generally, this is a major benefit of UV inks.
Traditional UV inks are regarded as 100%-solid systems. This means that during the curing process, very little evaporation takes place. In contrast, solvent- and water-based inks evaporate significantly when dried, yielding a much thinner ink layer in the dried state than in the wet state. While it is true that some UV-curable inks contain water or even small amounts of solvent, this article will focus on 100%-solids UV systems that contain no water or solvent. We will focus more on the benefits UV inks later on.
Snapshot of the glass and ceramic market
It's nearly impossible to find any area of our homes--including our garages-- that don't contain some kind of decorated glass or ceramic item. If we can really say there is one all-encompassing glass and ceramic "market," it is a market so diverse that it presents real and significant challenges to ink manufacturers.
The large number of diverse industries encompassed by the glass and ceramic market include the following: Flat Glass architectural automotive furniture gaming machines mirrors Hollowware beer and soft drink cosmetic drinkware food wine and liquor Ceramics tableware tiles sanitaryware (not much decorating)
The end-use requirements of all these industries are quite varied: The gaming-machine industry wants strong, transparent colors, opaque whites, and blacks with reasonable adhesion and alcohol resistance. The cosmetic market wants chemical resistance to widely varied test criteria. The beer and soft-drink market requires good abrasion and water resistance. For tableware, resistance to commercial and residential dishwashing is essential, and the print must also be suitable for direct food contact. Tile decoration must be very durable and have excellent abrasion resistance. Inks for mirrors require a certain amount of water resistance, as the mirroring process takes place in an aqueous environment.
Needless to say, developing inks that fulfill all these demands requires a specialized approach to each market. Another interesting point is that sometimes we aren't really printing directly on glass at all, because it is very common for glass containers to have a protective coating that inhibits scratching (see "Glass Is Glass, Isn't It?").
Let's look at the various decorating systems that have been traditionally used in glass and ceramic decorating:
Two-part epoxies These heat-cured solvent-based inks usually have good adhesion and resistance properties. They are not typically used for applications with direct food contact.
Frit-ink direct decoration Actually made of ground glass or ceramic plus organic or inorganic pigments, frit inks are very durable, and when properly fired, are as long-lasting as the glass or ceramic substrate itself. The drawbacks to decoration with frit inks are major energy consumption, slow production times, and difficulties in matching colors, just to name a few. Frit inks can be based in oil, hot-melt wax, or UV-curable carriers.
Frit-ink transfer applications Frit inks are printed onto water-slide or heat-release transfer paper and then transferred to the substrate and subsequently fired. The common usage is for applications where the contour of the substrate prohibits direct decorating. A final cover coat must be applied to the image in order to transfer multicolor prints intact. The cover coat is usually solvent based, even though the actual inks can be oil based, wax based or UV-carrier based.
Benefits of UV in glass and ceramic decorating
Although the traditional inks for glass and ceramic decorating serve their purposes very well, they also present some problems and expenses that printers would just as soon do without. UV-curable inks possess many benefits to all kinds of screen printers, and glass and ceramic printers are no exception. The specific advantages these inks offer for glass and ceramic decorators (many of which apply to all UV inks and applications) are detailed in Table 1.
|Cost and Performance Advantages of UV Inks|
|Increased production capabilities||UV inks can be processed, then handled immediately after curing. Frit inks must go through lengthy firing/cooling schedules, and epoxies must be heat cured or air dried for several hours. The use of UV curables cuts production times, thereby freeing machinery for other jobs.|
|Enhanced print quality||UV inks are typically printed through mesh counts of 305-420 thread/in. (150-215 thread/cm ) or finer. Epoxies and frit inks traditionally are printed through coarser meshes.|
|Excellent screen stability||Because they are 100% solids, UV inks do not dry in the screen. Four-color process and fine-detail jobs are more easily accomplished. Because of their evaporative nature, solvent-based inks can clog a screen when printing fine detail.|
|Excellent color stability||UV colors are stable before and after curing, unlike frit inks that can change dramatically after firing. UV inks are stable on press because there is no evaporation. With solvent-based inks, the evaporation can affect color stability on long print runs.|
|Heated screens not required||With hot-melt wax inks, screens are typically heated to ensure the ink flows through the screen. There is no need for this with UV-curable glass and ceramic inks.|
|Space savings||UV-curing units are very compact compared to kilns and lehrs or even conventional dryers used for epoxies.|
|Enhanced color range||Because no firing is required, wide ranges of pigments are suitable for UV inks. Bright, intense colors that are difficult to achieve with frit technologies are common with UV inks.|
|Special effects are possible||Metallic colors and clears that simulate acid etching and sandblasting are possible with UV.|
|Positive impact on the environment||Because UV inks have very low (or no) VOC emissions, UV technology helps printers comply with various environmental regulations. Although we still use solvents to clean up UV inks, those emissions are far less than if we were printing solvent-based inks. Furthermore, it is quite common for UV inks to be free of heavy-metal pigments. This addresses concerns about the heavy metals used in frit inks. Looking at energy consumption, just imagine the amount of energy--and cost--needed to fire an oven up to 1475°F (800°C) compared to running a small UV-curing lamp.|
|Reduced ink consumption||Because finer mesh counts are used, it is not unusual to increase ink mileage 75-200% over conventional glass and ceramic inks.|
|Multicolor printing||UV inks can be interstation cured, so multicolor jobs can be achieved without delay between colors. Hot-melt-wax frit inks can also be printed inline (since they resolidify after leaving the screen), but other technologies require drying between colors.|
Considerations before a UV conversion
Despite the benefits of UV inks, we must consider several other issues before switching to UV technology. UV curing requires the light to penetrate through the ink film and reach the substrate to achieve proper cure and adhesion. So the very nature of UV curing limits the amount of opacity we can achieve with the inks.
For decorating on dark-colored glass, printing a first-down white may be required when trying to print certain colors. This technique is commonly used in decorating plastic containers as well as optical discs and can easily be used in the glass market. Ink and equipment technologies have improved over the years, and achievable opacity has increased, but limited opacity in some colors is worth mentioning here.
We are all familiar with the myriad variables in the screen-printing process. To get proper performance from UV inks, the factors that affect ink deposit must be controlled more so than with other ink technologies. For example, we must pay attention to squeegee and mesh selection, screen construction and tension, stencil thickness, and much more to ensure we do not have too high an ink film or insufficient cure.
Because UV inks don't dry, we also must follow a few basic housekeeping rules. Simple things like keeping work areas and equipment clean, as well as keeping your hands and clothes clean, become more important with inks that don't dry.
So what about equipment?
UV lamps vary in wattage, peak energy output, recommended useable life, cost, etc. Many of the same commercially available lamps used to cure UV screen inks for other applications can be used to cure inks for glass. It's best to check with ink manufacturers to get recommendations for cure requirements. For flat-glass applications, lamps can easily be added to existing conveyer lines. For hollow-ware applications, purpose-built multicolor UV presses are available. These presses can have up to eight print stations, making multicolor jobs possible. UV retrofits to existing hollowware presses may be achievable, but at the sacrifice of print stations. With retrofits, the number of print stations would typically be cut in half to accommodate the UV-curing units.
Not for every application
As we discussed earlier, frit inks are the most durable since they essentially become part of the printed object during the firing process. Markets like automotive glass, tiles, and tableware (plates, china) will not likely use UV inks and will instead turn to frit systems.
Having said that, there is potential in those markets for UV frit carriers, which are designed to allow decoration of glass and ceramics with frit, but use UV energy to cure prior to firing the frit. They essentially facilitate handling and multicolor printing, serving as a temporary carrier that burns off during the firing process.
Some of the benefits of direct UV printing can be realized here, but the process still requires firing and the use of frit pigments. This technology is currently used in various markets, such as automotive glass and decal decorating for tableware. But that is a topic for another day.
There are certainly markets where the performance of UV inks is well suited. Gaming-machine panels have been decorated for quite some time with UV inks, and decorative mirrors are another suitable application. In the one-trip beer and soft-drink market, UV inks are also gaining acceptance. UV inks will certainly exceed the durability of the paper labels used for beer, wine and liquor bottles, and there is growing interest in this market sector. Cosmetic bottles are another area where UV is getting serious attention.
How about the possibility of screen printing large-format architectural glass with UV-cured etched-look clears instead of acid etching? Also, simulated sandblast effects can be screen printed. The cost savings can be significant here, but again, acid etching and sandblasting is permanent, where as screen printing would not be. It comes down to end-use requirements and performance expectations in each market sector.
It's safe to say that UV inks offer some exciting opportunities for glass and ceramic decorators. But making the transition to UV inks is more than just a matter of switching inks. When you move from a traditional ink system to UV curables, you face a true conversion, and every area of production must be controlled in order to get a consistent and acceptable result. But with the help of your ink supplier, you can shorten the learning curve and maximize the benefits of UV technology as it applies to a large assortment of the products you print.
Editor's note: This article was expanded from a version that originally appeared in The FESPA Magazine, Spring 2001.