Does this look familiar? Your beautiful print job is covered with a fine spray of dotted and slung ink and looks like it was part of the local high school’s modern art project.
Just about every label printer using UV inks has experienced or will experience the maddening issue of UV ink spitting at one point or another. What can you do about it?
You could embrace the spitting – enjoy the beautiful speckled designs. Or choose another industry where “spitting” means cooking meat over an open fire.
Kidding of course, but sometimes this frustrating issue is enough to make you lose your mind. Here are some realistic tips to help eliminate UV ink spitting from your life:
1. Clean and maintain the press
Smart people start with the easiest, most economical solution. As with any press issue, first make sure the equipment is properly maintained and cleaned. Inspect the anilox roll for damage, and remove dried ink from the cells and end of the roll. Check that the chamber is aligned and the doctor blade is installed correctly – at the proper angle (at least 30°) with the appropriate amount of pressure (the least possible to obtain a clean wipe) with the bevel facing away from the roll (See the doctor blade installation infographic). Replace old end seals and clean blade holders.
Sometimes, if you’re lucky, a simple adjustment works wonders. But if you’ve inspected, maintained, cleaned and aligned to no avail, proceed to tip #2.
2. Try a new doctor blade
The second most economical and practical solution is to try a different doctor blade. We all know that the general objective in flexo printing is to use the thinnest blade that can achieve a consistent wipe. However, oftentimes a thin steel blade is not stiff enough to withstand the force of the higher-viscosity UV inks, especially at high press speeds. The ink tends to build up behind the doctor blade, causing the blade to flex and let excess ink pass underneath. The excess ink then slings off or “spits” onto the web. A thicker blade can reduce deflection, but at the expense of contact area/print quality – a larger contact area leaves too much ink on the roll, producing dot gain, dirty print and other undesirable results.
What’s needed in these cases is a doctor blade that has a thick base and a fine tip that can achieve a small contact area with the roll. Long-life, tool steel doctor blades with laser-hardened thin beveled edges have performed well in these applications. Today’s next generation polymer doctor blades are another good option due to their mechanical and chemical properties. The specially formulated material is thicker at the base with enough rigidity to support a state-of-the-art MicroTip® edge. The material also has a lower surface tension than steel, decreasing the attraction of the ink to the blade and allowing a smoother transfer to the plate. (More on this in a later post.)
In our experience, tips one and two resolve almost all issues with UV ink spitting. But if you’re still having problems, there are some other things you can try.
3. Use a different anilox roll engraving
Traditional anilox engravings can hinder ink release due to the conical shape of their cells and even more so when transferring highly viscous UV chemistries. By allowing ink to pass through the channels, new elongated hexagonal open cell engraving designs relieve pressure on the doctor blade and offer improved ink flow and decreased spitting. Due to the high cost of replacing or re-engraving an anilox roll, using a different roll surface is a somewhat expensive but still viable option to correct spitting.
4. Convert to lower-viscosity inks
As we said, UV inks can have up to 7 times higher viscosities (and solid loads) than water or solvent-based inks, and their thixotropic properties, or their ability to thin out when agitated and then return to their more viscous state, make them transfer differently on press. Opting for the latter ink formulas will eliminate spitting, but at the cost of sacrificing the sharper graphic resolution, vivid color intensities and process efficiencies that UV inks can deliver.
Last but not least…
5. Slow down press speeds
Their high relative viscosities mean that UV inks place excess pressure on the doctor blade, especially when run at high press speeds. By slowing presses to below 100m/min, excess ink doesn’t have enough force to lift the edge of the doctor blade and pass underneath. But slowing down presses reduces efficiency (and profits) so this is a desperate measure!
Combating UV ink spitting can be easier than you think. Short of simple press cleaning and maintenance, finding the right doctor blade for your application is often all it takes to eliminate this frustrating problem from your pressroom. The cost of this little consumable pales in comparison to the high cost of anilox rolls, ink and press time. So, just when you think you can’t take anymore UV spitting, take a deep breath and try these tips. Or give us a call – we can help! After all, there is a much better way to think about spitting. ?
