Common Narrow-Format UV Inkjet Printing Issues

What We’ve Learned While Qualifying Our Products

Print technology is constantly evolving, with new advancements coming to the industry regularly. In response, substrate suppliers must qualify their products with these new technologies and/or develop new ones that are compatible. Because the ink delivery mechanism for narrow-format UV inkjet does not touch the substrate, different issues arise that affect ink flow-out and print quality than with conventional print methods. Therefore, print qualification has been critically important because (a) there are many printers on the market, and (b) each one responds differently to the print media. To ensure printing success, Flexcon works closely with printer OEMs to qualify our products on narrow-format UV inkjet equipment. This article will review the most common issues we’ve experienced and provide some suggestions for resolving them.

To Treat or Not to Treat

We suggest starting with the corona treater off to get a baseline for the substrate on which you’ll be printing. What we’ve discovered is that while corona treatment may increase ink adhesion, it can also have an adverse effect on ink flow-out and, of course, it doesn’t matter how well the ink sticks if the image is lousy. You must strike a balance to achieve both the image quality and ink adhesion required. If you decide to try adding a corona treatment, start with the lowest possible setting and work your way up.

Ink Flow Out Issues

This is a new challenge for the industry. Surface energy plays a more prominent role because the ink is not applied directly to the print media surface as it is with conventional printing. Rather, it is “jetted” onto the surface, making print quality much more reliant on surface energy. Here are some of the challenges we’ve encountered and some possible remedies.

• Crawl-back of ink: If you’re seeing small “holes” in the ink, it may be crawling back before it has a chance to cure. This could be due to surface energy variations of the film. It may help to speed the printer up so that the ink cures before it can crawl. Pinning levels can also affect ink flow-out. If it’s possible to adjust them on your machine, using a higher setting may resolve the issue. Since pinning is a light surface cure, like a partial cure, it may help to freeze the ink where it is before getting to the final cure, thus preventing it from crawling back. Increasing the surface energy with corona treatment is hit or miss. It may help depending on the surface chemistry of the substrate, but it could also make things worse.
• Inadequate flow-out of ink: If you’re experiencing a grainy or mottled appearance, the ink may be curing before it has the chance to flow out properly. In this case, you may need to slow down the printer. You could also try lowering the pinning levels to allow the ink to flow out better. If you set it too low, however, you may get blurred lines or hard-to-read small text.

It’s important to distinguish whether you think the ink is crawling back or not flowing out enough in order to determine the correct fix. And remember - it’s all about striking a balance to achieve the best possible outcome.

Smearing of Ink

This occurs when there is no clearance between the print head and the substrate. It could be caused by one of two things: (1) the material is curling, causing the edges to touch the print heads. If this is the case, try increasing the tension on the material; or (2) the print heads are set too close to the substrate. This issue can be resolved by simply resetting the print heads to a greater distance above the substrate.

Cross Web Chatter Marks

This can be caused by a loss of tension and the material slipping in the press. Adjust tension and try again.

Fine Lines in Machine Direction

A nozzle is either clogged or misdirected and must be cleaned.

In a nutshell, pinning levels, final cure levels, corona treatment levels, tension settings and run speeds are all “knobs” you can turn to try and remedy an issue or maximize print quality. Of course, these are only suggestions based on Flexcon’s experiences in qualifying our products. Furthermore, every printer is different, even multiple machines of the same model, so reach out to your printer OEM for assistance with any printing issues you are unable to resolve on your own.