What are the Differences in Drying and Curing Pad Printing Inks?
by John Kaverman
Pad Print Pros LLC
When you read the technical data for a pad printing ink, you’ll notice that the recommended drying and curing parameters generally are called out separately. This is because drying and curing are two distinctly different things.
Drying simply is evaporating the solvents from the printed ink film. Curing is the chemical reaction that takes place until the ink film has reached its maximum level of performance for adhesion, as well as for abrasion- and chemical-resistance.
Most pad printing inks dry very rapidly and are "dry to touch" within a few minutes of printing. Drying can be accelerated by subjecting the printed ink film to increased (preferably heated) airflow to aid in the evaporation of solvents.
Curing of pad printing inks can take anywhere from a few hours to several days. Conventional, solvent-based, single-component pad printing inks typically cure by oxidation as the binder (resin system) reacts with oxygen, creating a hard film.
Conventional, solvent-based, two-component (2K) inks require a catalyst (hardener). Most hardeners used in pad printing inks are isocyanates. These catalysts react with the resin system to cure by polymerization. In order for this polymerization to occur, the printed ink film needs time and exposure to humidity (water vapor) in the air.
Inevitably, nearly everyone using a two-component pad printing ink asks what can be done to accelerate the cure of the ink. The answer is, "Practically nothing." Really, all that can be done is to wait for the chemical reaction to take its course. In limited cases, storing printed parts at elevated temperatures can accelerate the process, but that only will be known by thoroughly testing to determine exactly what the curing parameters must be in order to meet requirements. It’s a science project.
UV- and LED-cured inks
Immediately upon hearing that answer, everyone asks, "What about UV inks?" This is a logical question to ask, since UV inks cure almost instantaneously when exposed to UV radiation. However, unlike conventional solvent-based pad printing inks which can be printed "wet on wet" on top of or in close proximity to each other without creating image quality or color changes, UV inks require that each color be cured before subsequent colors can be over-printed or printed in close proximity. In most cases, this makes UV-cured pad printing inks viable only for single-color and/or small-format applications because the curing systems usually have to be small, shuttered systems that easily can fit between print stations.
Additionally, for many applications, UV inks still require a hardener to meet performance requirements, thus requiring that they, too, be allowed to cure. The same limitations exist for LED inks, which are a new ink technology that has a ways to go before it is robust enough for industrial pad printing use.
Any time a wet ink film transfer process is being worked with, the drying and curing environment needs to be considered. As I mentioned, isocyanates need to be exposed to the air/humidity in order to cure. Therefore, sealing recently printed parts in plastic bags or applying protective films over printed areas are not suggested until the recommended cure schedule is completed.
Ink manufacturers’ technical data also (normally) recommends a "minimum temperature," below which parts printed with two-component inks should not be stored prior to the termination of their cure. Low temperatures can stop the chemical reaction, and once stopped, it cannot be restarted.
Finally, two-component inks normally have a "window of opportunity" within which they can successfully be over-printed. After the ink film is cured to a certain point, it will become impossible to get successive coatings to adhere.