By Cassandra Balentine
Printing to glass can be tricky, as many factors play a role in attaining proper ink adhesion. When you add curves to the mix, it becomes even more complex.
“Flat glass has its own issues, but they are not nearly as problematic as printing on glass containers,” admits Sean Lanigan, president, Applied Surface Technologies, LLC.
The first article in this four-part series looked at printing to flat glass and ceramic products. In part two, we dig a little deeper to investigate how best to print to cylindrical and/or pre-manufactured glass or ceramic items.
Factors to Consider
When printing to cylindrical glass or ceramic objects, digital printing can offer graphic flexibility, fast changeover, unlimited production numbers, and short delivery times, says Bas Buser, consultant printing applications, Plasmatreat GmbH.
However, it also presents challenges. One of the biggest challenge is maintaining consistent surface energy. “When you have consistent surface energy, you get a better surface to print on,” offers Wilson Lee, director of business development, Enercon Industries.
Evan Reutling, production team lead, LogoJET Inc., agrees, noting that glass and ceramic can often be inconsistent from one piece to another. “Their surfaces can be uneven, which can be an issue when printing on a flatbed, but become even more of a factor when rotary printing. If using a tray/jig to print round ceramic ornaments, measuring your product to find a tolerance for the diameter is best practice as sizes can differ up to one to two millimeters.”
While flat glass tends to be more true to shape/form/flat, drinkware is molded and can have more defects that the pretreatment needs to fix or get down into. “When you look at a pint glass for example, you tend to see waves, lumps, or bumps. Flat sheets tend to be flawless unless a crack is present,” explains Michael Perrelli, marketing director, Innovative Digital Systems.
Operators need to keep this in mind when reviewing products prior to printing. “Glass drinkware can sometimes be dirtier—for lack of a better word—than their flat glass counterparts. The flame treatment process usually helps in these instances, but a proper chemical clean has been used in the past to clean items that posed persistent issue,” adds Perrelli.
It is also important to ensure the cylindrical or conical items are actually round and not slightly oval. “In the manufacturing process some variances are allowed. But when printing one to two mils from the surface any slight variation could cause distortion or even hit the printhead. So, you need to be careful and check that the objects are round,” explains Adam Tourville, director of sales, North America, Direct Color Systems.
He also suggests making sure the mold release is not silicone based. “If any silicone is left on the object it will make it incredibly difficult to get adhesion with a LED or UV inkjet ink,” cautions Tourville.
When it comes to selecting ink for cylindrical glass and ceramic products, it is mainly similar to flat glass.
“The challenge with a multi-dimensional—not a flat—part is getting consistent treatment,” suggests Lee. “If this treatment is achieved, any ink should work well.”
He adds that many companies have moved towards water-based ink formulations and, generally speaking, these inks require some form of surface preparation. “Solvent inks are more aggressive, but also benefit from ensuring the surface it is bonding with is optimized for adhesion.”
Always consider what the product is made of, its final use, and expected performance when selecting ink. “For instance there are hard, scratch-resistance inks; flexible inks for flexible substrates; and electric conductive inks,” offers Buser.
Ray Walker, digital inkjet application engineer, INX International Ink Co., finds UV inks to be best suited for these types of products, as they generally have better adhesion to the surface treatments available.
“UV-curable inks allow for quick production and turnaround of your printed products,” adds Reutling. “There’s virtually no dry time since inks cure as they’re applied to the surface, so products can be created and passed on to a customer instantly.”
Tourville agrees, adding that LED and UV inks are great for printing to cylindrical glass and ceramic objects, stressing that you will need to use a wipe-on adhesion promoter or flame treatment.
If the printed object is destined for the dishwasher, the recommendations change a bit.
For dishwasher-level durability, a flame treatment like Pyrosil is generally recommended.
Lanigan says while there are no industry specifications or standards for glass or ceramics to be “dishwasher safe,” it is generally accepted that 300-plus consumer dishwashing cycles qualifies as such. This represents 1,000-plus commercial dishwashing cycles. “This is considerably more than consumer dishwashings because the glass or ceramics are washed faster and have less time exposed to water and heat.”
Tourville says a wipe-on adhesion promoter will not last very long. But, Pyrosil with an adhesion promoter will last way longer in a residential dishwasher cycle, which is harder on an item when compared to a commercial dishwasher.
“Commercial dishwashers run wash and steam cycles very quickly, while residential dishwasher cycles last for two and half hours of hot water and steam. Because LED and UV inks are made from petroleum-based chemicals and dishwaters use water, they do not like each other. This is why it is so hard to achieve dishwasher safe,” explains Tourville.
Pretreating glass prior to printing makes or breaks the quality and durability of the print. There are a few options for printing to cylindrical objects.
“Flame plasma, wet chemistries, and roughening treatments are the most common surface pretreatments for digital printing to cylindrical glass products,” offers Lee. He points out that regular plasma is not the best option for a cylindrical surface. “The goal is to get consistent treatment and this is easiest with a flame plasma. Less variance in the treatment helps reduce defective products.”
Walker notes that flame, Pyrosil, and chemical promoters that are either wiped or sprayed on are utilized for printing to cylindrical glass and ceramics. “Some of the treatments require either a rest period of time or baking for improved results. A few applications have used combinations of flame and chemical promoters, and in others just a chemical treatment is applied. There are other cases where the treatment is a regular flame before adding Pyrosil flame,” he explains.
Perrelli stresses that Pyrosil treatment yields the dishwashability that many drinkware decorators seek.
“We use Pyrosil as a promoter for printing on cylindrical glass products,” shares Reutling. “It’s a form of flame treatment that is simple to apply. We have found that it makes a world of difference for ink adhesion, providing scratch resistance and extra durability during dishwashing when applicable.”
With plasma pretreatment, Lee says the distance to the surface and the treatment speed are important if consistent surface energy is required. “Flame plasma is generally used in this application for two reasons. First, it is more economical to pretreat with flame. Second, the distance between the burner and the surface is not as critical to getting the desired surface energy,” he offers.
Whether flat or cylindrical, glass and ceramics are challenging to achieve good adhesion. Like many substrates, a treatment is needed to promote ink adhesion to these surfaces. “Where flame and corona treating work with many types of plastics and metals, glass usually requires an additional chemical to be applied as an adhesion promoter,” concludes Walker.
When printing to glass or ceramic cylindrical objects, pretreatment is of utmost importance.
Mar2023, Digital Output