By Terry Clayton, Ph.D., chemist, Summit Analytical

There are many companies kicking off inkjet integration projects to shift to or combine digital printing and decoration with analog printing methods. Inkjet already has been successfully integrated into display graphics, ceramic tile, product decorating and marking, and steadily is moving into textiles and packaging.

Over the years, I routinely have worked with companies that had the following vision: Integrate a digital printing solution into an existing analog printing or manufacturing process. Ideally, the client would search out a digital “print bar,” which easily could be mounted into an existing product line. This would add to or even replace analog print stations.

A first step in this endeavor is to make sure that the ink delivery system can maintain adequate feed rates to the printheads and that the chosen printhead has the resolution and throughput needed for the required productivity. Equally important is the use of the appropriate ink or coating solution for the specific application. In most cases, trying to use an “off the shelf” ink or coating is going to spell disaster.

This article details just a few of the problems I have run into when trying an “off the shelf ink” in an industrial inkjet application, along with some common strategies to overcome those problems, which do NOT include “ink development.”

THE INK ISN’T DRY
Since a discussion of ink chemistry is beyond the scope of this article, there are several other areas to look at if there are drying issues. First, check that there is not too much ink being applied. Evaluate if the press speed is running faster than the ink was designed for and if slowing the press down can solve the drying issue. Also, checking the curing system or oven, making sure it is not under-powered, is vitally important. If this is the case, a combination of reengineering the curing/drying unit or potentially reducing the ink coverage will solve the issue.

THE INK IS FLAKING OFF
If the ink is drying but not adhering to the substrate, there are several areas to check. First, pretreatment may be necessary to increase the surface energy (by flame, plasma or corona) of the substrate. Or could a primer be used on the surface of the plastic material? Also, it may be worth exploring if an alternate plastic material can be used for the application. Cold temperatures in wintertime weather can make the ink more brittle. Making sure the ink is stored in a climate-controlled environment is important. I have seen companies try to “under cure” UV inks to boost adhesion and increase flexibility. I do not recommend this approach due to chemical migration concerns.

THE INK HAS AN ODOR
The raw materials used in digital inkjet inks, many times, have lingering odors that can be accelerated by heat, such as an automobile interior on a hot sunny day. Adding an odor mask to the ink may destabilize the ink and create other problems. Boosting the cure time sometimes can eliminate the odor. Another approach is to encapsulate the ink with an over-varnish providing the application allows.

PRODUCT FAILURE THROUGH CHEMICAL MIGRATION
Product failure through chemical migration potentially can be solved through increased curing, as well. Also, slowing down the speed of the press can help. Or as mentioned above, a flood coat over-varnish to keep residual from migrating may be necessary.

THE INK IS TOO EXPENSIVE
Be sure to explore all options with a potential lower-priced ink that is qualified in the digital print bar. In several applications, I have seen inks being specified that simply are an “over kill” for the job. For instance, an automotive-grade pigment is not necessary for a packaging application where the longevity of the print is not an issue.

THE INK HAS NO DURABILITY
A similar challenge to ink adhesion, durability may be increased through an adjustment to ink profiles and the curing/drying of the ink. Also, certain ink sets are formulated for durability including scratch and chemical resistance, so if it is an issue, a change in the ink may be necessary. Again, an overcoat can be used to protect the printed ink on the substrate. This has become a popular strategy in packaging where inkjet inks cannot include large- particle wax dispersions.

THE INK IS FOAMING
First, check the ink system and find out where the air is coming from. A single-pass application requires recirculation to ensure air doesn’t deprime the print heads. It may be necessary to dial down recirculation or tank agitators. Next, make sure there are no leaks in the ink systems by running pressure tests. Adding a defoamer to the ink could be an option; however, as stated earlier, options are being considered on a press without ink development and/or changes to the ink formulation.

THE PRINTHEADS ARE CONSTANTLY CLOGGING
To help eliminate clogging, it is recommended to engineer a closed environment to protect the heads, which includes adding climate control and humidity to the print zone of the digital print production. The press itself always should have a robust head cleaning system to prevent clogging and downtime on the press. It also is important to create Standard Operating Procedures (SOPs) to clean the print heads every few hours as needed for quality. Finally, it should be noted that a print operation utilizing inkjet printing needs to be kept much cleaner as compared to an analog print or decorating operation. Remember to keep 20-micron nozzles clean!

RELIABILITY AND UPTIME ARE TOO LOW
It is recommended to run extensive operating windows on every print mode and identify the ideal environment for the digital inkjet solution. Temperature, humidity, static, print gap, substrates and pressroom cleanliness all should be evaluated to help keep the inkjet printheads running at optimum speeds and minimize nozzle outs.

These are just a few of the ink issues that come up when integrating inkjet into an existing manufacturing process. Although ink development/formulation has not been discussed, it also is an important element to consider when working with a particular digital inkjet application for plastics or other materials.

Robust solutions for digital inkjet are a combination of engineering, process and chemistry. All three equally are important elements for a successful project.

Terry Clayton, Ph.D., is founder of Summit Analytical, a product development lab that creates and scales innovative formulations for inks, coatings and smart materials. Learn more at www.summit-analytical.com.