The better you understand the process variables that affect flexographic printing, the quicker you'll be able to remedy any problems that arise. The marriage between ink and film is subject to a slew of variables, and this article reviews application issues relative to the surface energy of the substrate.

The key equation for successful printing is the difference in the surface energy between the ink and the substrate. For a surface to be properly wetted by a liquid, the surface energy of the substrate must be higher than the surface tension of the liquid. Surface energy is measured in dynes per centimeter. Ideally, the surface energy of the plastic should be higher than the surface tension of the ink. For example, a printing ink having a surface tension of 30 dynes/cm would not adequately wet or bond to a material having a surface energy less than 37 to 40 dynes/cm.

Generally, plastics have chemically inert and nonporous surfaces with low surface tensions causing them to be non-receptive to bonding with printing inks. For example, polyethylene and polypropylene are known to have an extremely low surface energy.

To activate substrates and make them receptive to ink adhesion, printers can employ corona treaters. Corona treaters increase the surface energy of substrates by oxidizing the surface and creating positive and negative sites on the surface by adding and deleting electrons.

It is important to have a documented quality control plan that calls for routinely measuring the surface energy of your substrate before and after corona treatment. Collection of this data will allow you to compare results should you ever find quality issues with ink adhesion.

When you identify poor ink adhesion and suspect surface energy is the cause, you should first ask the question, “What has changed?” Is the ink the same? Is the substrate the same? Is the operating speed the same? Is the corona treater turned on? (Don't laugh, it has happened.)

Let's take an example where the ink is the same, but the substrate is somehow different. Realize that different doesn't necessarily mean a different type of film. A change in film suppliers or even a different batch of film might yield different characteristics that affect its surface energy. This is where your recorded data from past jobs will come in handy.

In some cases, you may be working with the same type of film, but your initial dyne level reading is much lower than your recorded history. This can often be attributed to slip additives in the film rising to the surface. You probably know that slip additives are used by film extruders to ensure films transport smoothly through converting operations. What you may not be aware of is that film with a high percentage of slip additives (measured in parts per million) has a significant impact on a film's ability to be treated, and to retain the effect of corona treatment. The initial impact of higher additive loading is to require increasing watt densities (treatment power) to raise the film's surface tension by a given amount. (See sidebar for more information on watt density)

Treatment power can be increased by either raising the power level of the treater or by slowing down your printing press to allow for more treatment dwell time. Keep in mind that your corona treater was designed for the application parameters that your company specified at the time of purchase. Printing on a different film or a film with a higher percentage of additives may or may not be possible. That's why it is critical to precisely define your application when requesting proposals from corona-treater suppliers.

The power of understanding your process and controlling variables is a huge benefit when it comes to troubleshooting. Take, for example, a situation where you notice ink picking after winding. If your team understands the corona-treatment process, then they can immediately check dyne-level readings to determine if the treater or treat level is the cause.

If the problem is occurring in only one spot, you might suspect backside treatment. Backside treatment occurs when air is entrapped between the backside of the web and the ground roll. The air beneath the web becomes ionized, and corona treatment occurs.

The cause of backside treatment can be attributed to either material imperfections or operational problems. Wrinkles in film, scalloped edges or “bagging” are all material conditions that promote backside treatment. Again a simple dyne test can tell you if you have backside treatment. Once you've determined that backside treatment isn't the cause of ink picking, you can turn your attention to other potential causes; ink formulations, drying time and winding tension may be the culprit.

If backside treatment is an ongoing concern you should consult your corona-treater supplier. They should be able to recommend in-field remedies as well as offer systems which are specifically designed to minimize the chances of backside treatment occurring.

This classic example shows the flexographic-printing results on a substrate where the upper portion was treated and the lower portion was not. The surface energy on the untreated area was too low for the ink to properly wet-out.

By controlling the substrate's surface energy, flexographic printers can eliminate a variable from the ink-to-film equation. Surface energy issues can be quickly identified by measuring and documenting untreated and treated surface dyne levels. Some substrates are more receptive to treatment than others, and a change in the percentage of additives can have a significant impact on the treating power required. In some cases, the treater can generate the increased power requirements; in others, alternative solutions may be required. Most importantly, it is critical to provide your surface-treater supplier with specific application criteria to ensure your system will be able to handle the range of materials and speeds your operation requires.