The Evolution of Glass Decorating—From Traditional Inks to Digital Printing
A changing legislative landscape and the introduction of alternatives to ceramic inks have resulted in a shift in commercial glass decoration methods.
Greek philosopher Heraclitus once said, “The only thing that is constant is change.” When it comes to printing onto glass, no truer words have been spoken. That may seem like an odd statement for a material that has been decorated for hundreds of years, but the reality is that the changing legislative landscape and the introduction of alternatives to ceramic inks have resulted in a shift in commercial glass decoration methods.
In addition to advancements in screen inks, we can no longer ignore the impact of digital printing methods for glass decoration. As printhead technologies have advanced and inkjet chemistry improved, systems are being designed, built, and now installed that rival traditional screen printing speeds and durability. While the largest print runs are still dominated by screen printing, digital is now a viable option for nearly any market segment.
In 2010, McDonald’s recalled 12 million Shrek drinking glasses when lab tests reportedly indicated the presence of toxic levels of heavy metals (lead and cadmium) in the decorated image. Under U.S. law at the time of printing, the levels of these materials would have been considered within specification if the final product was deemed to be marketed primarily to adults. However, the Consumer Product Safety Commission has much more stringent regulations for items considered “children’s toys.”
The federal standard for toy safety is a document called ASTM F963-16, which provides limits for a host of potentially hazardous materials and requires annual certification by an independent lab. While not all printed glassware is subject to these standards, the potential for many of these items to find their way into the hands of children is causing many printers to seek alternatives to ceramic screen inks, which are known to contain harmful heavy metals. Organic ultraviolet (UV) ink is the technology to which most are turning.
Are all UV inks safe? Not necessarily. The only way to be sure whether the inks you’re using are certified to all applicable standards is to ask your supplier for the test results from an independent lab. In addition to ASTM F963-16, your ink should be tested annually for the presence of a full list of potentially harmful raw materials. In some cases, the requirements may come from a particular state, such as California’s Proposition 65.
Any reputable supplier should be happy to provide all of the information. However, be careful about a concept called “compounding.” Based on the limits of some of the hazardous materials and the threshold of detection for the equipment in use, the maximum number of individual ink shades that can be combined in a single test is three. For some tests, it is only one. If you receive a report that shows only a single sample for a given ink series, find another supplier.
Understanding Organic Inks
If organic inks have so many advantages, what is the reluctance to change? The answer is simple: durability. Despite the continued improvements in organic inks, they still have not achieved the level of scratch resistance that is available with inorganic pigments.
Ceramic inks are made up of metal oxide pigments and finely ground glass particles that literally fuse with the surface of the glass or ceramic when exposed to the high-temperature curing process. Ovens capable of raising the surface temperature of the glass to well over 1,000°F are a requirement. This time-consuming and energy-intensive process results in a printed image that is nearly as durable as the glass itself. In some cases, this most basic performance requirement outweighs all other criteria. However, as manufacturers improve the adhesion of UV-curable organic ink, the case for this environmentally friendly alternative gets much stronger.
As previously mentioned, the pigments used in UV organic ink are free of heavy metals. In addition to this primary benefit, organic pigments are less abrasive, more brilliant and offer some unique options that ceramics do not. For example, metallic, fluorescent and phosphorescent effects are all possible with organic inks. In addition, organic inks do not experience a color change during the curing process and are cured instantaneously, making color matching a much easier task.
Despite the initial durability of a ceramic ink, the chemical resistance limitations of ceramic pigments have led to disappointing results when drinkware is exposed to the rigors of commercial and home dishwashing environments. As Figure 1 indicates, ceramic inks lose their vibrancy over time when washed on a regular basis. By contrast, UV organic inks have virtually zero change when subjected to the same environments.
Two primary cost drivers are involved when comparing ceramic and UV organic inks: energy and materials (screen and ink).
Depending on your local utility costs, a typical lehr gas oven used for firing ceramic inks will cost nearly $100/hr in gas and electricity. Unfortunately, this type of oven must be operated 24 hrs a day due to the long time it takes to reach and maintain required temperatures.
By contrast, the electrical requirements of a typical UV printer are roughly $15/hr. Assuming a printer with six UV lamps at a cost of $500 per lamp with a 1,500-hr operating life, we must add an additional $2/hr to account for the cost of the lamps, for a total of $17/hr. When you consider that these costs are applicable only when the printer is operational, it becomes clear that the energy requirements of UV organic inks are a fraction of those of ceramics.
In addition, recent developments in LED-curable inks could further improve the UV cost structure. The jury is still out as to whether the much-more-expensive LED lamps can realize an effective ROI vs. traditional broad spectrum lamps, but this advancement has made significant progress in the last few years.
Due to the nature of ceramic inks, the mesh used for screen printing is typically constructed of stainless steel. Depending on the mesh count used, the cost can exceed $10/ft2. Comparatively, organic inks use screens of polyester fibers. Even at the highest mesh counts, costs are typically less than half that of stainless steel. When the durability of polyester vs. stainless steel (40,000 prints per screen vs. 30,000) is taken into consideration, screen cost savings can reach approximately 40% overall (see Figure 2).
In calculating ink costs, it is important to consider both the cost of ink and the corresponding coverage. While UV inks do cost about 30% more than ceramics, that is only part of the story. When the physical characteristics of the mesh (e.g., screen thickness and mesh opening size) are considered, it becomes evident that, for a given volume of ink, UV curable will print more than three times the area of a ceramic ink. Figure 3 provides a comparative view of these combined factors in overall ink cost.
Digital Glass Decorating
As with virtually every other decorating technology, advances in digital printheads and digital inks are making digital printing on glass a reality. Print systems utilizing industrial heads such as Ricoh, Xaar and Konica Minolta are entering the market at a staggering pace. Whether using an entry-level system such as Engineered Print Solution’s Bottle Jet with an output of two units per minute or Kammann’s fully automatic digital press capable of 200 units per minute, digitally printed drinkware and bottles are now meeting the rigorous demands of bottlers, bars, restaurants, and home use.
A typical digital system involves a flame/silane pretreatment step followed by a liquid adhesion promoter, which is currently applied by spray or wipe. However, a digitally applied adhesion promoter is currently in beta testing that will significantly improve the workflow of most digital systems.* Following the pretreatment steps, a combination of white and CMYK process colors is printed. The inks are typically cured (full cure or pin) after each color. The final step is an overprint varnish that provides an additional level of protection for the digital ink.
Until recently, digital inks were unable to achieve the durability of UV screen inks. This is a function of the raw materials that are available. For screen printing applications, a variety of monomers and oligomers can be selected based on the desired final product characteristics. The resultant ink viscosity changes are not a significant challenge for screen printing. However, digital inks require a much tighter and lower viscosity range. This limits the selection of monomers and essentially eliminates oligomers from the equation.
Fortunately, development advances have produced inks that meet all commercial requirements. The glasses shown in Figure 4 were subjected to over 400 home dishwasher cycles in accordance with EN 12875. The temperature profile of the wash cycle is shown in Figure 5.
Commercial glass and ceramic decorators have more options today than they’ve ever had. Technological advancements are now providing significant cost savings, process improvements and safety benefits. Capital investments will be necessary, as with any change, but the ROI is measurable and the expense fully justified.
*In development by Marabu.