How to stop UV spitting
The numerous advantages offered by UV inks, such as sharper graphics and richer colors, have made it the preferred ink by most narrow web printers. Unfortunately, a phenomenon commonly known as ‘spitting’ that only occurs when printing with UV inks remains a major problem in many pressrooms.
06 Apr 2016 | 4122 Views | By Sonia Arcos
Spitting is when the ink is sprayed onto the web in an uncontrolled manner. This is caused when the doctor blade lifts because of excessive doctoring pressure, allowing the UV ink to pass under the blade and spit on the web.
Spitting is a characteristic specific to UV inks as water or solvent-based inks never encounter this problem. But what does cause UV spitting?
Many studies have been carried out through the years without any conclusive answer. One contributing factor would be the higher viscosity of UV inks that can be five to seven times higher than water- or solvent-based inks. Higher viscosity puts more pressure on the doctor blade making it to
lift.
Another contributor is the thixotropic nature of UV inks. The viscosity of UV inks decreases when subjected to stress. Stress is applied to the ink when it comes into contact with the doctor blade. With the sudden drop in viscosity at the doctor blade, the ink may be allowed to flow underneath the blade, resulting in spitting.
Understanding the possible contributing factors of spitting will help find the solution to this costly problem.
The main contributing factors to UV spitting are:
• High ink viscosity (1000-1900cp)
• Print speed above 100m/min
• Blade material, tip type, and thickness
• Anilox volume and type of engraving
Changing Ink viscosity and print speed might not be a commercially viable option, but changing doctor blade and anilox type is, and it’s a far more effective way to prevent and eradicate spitting.
Doctor blade, the main things to consider are:
Angle and pressure
Recommended Blade contact angles for flexographic printing are between 25 degrees to 40 degrees with the optimum angle being near 30 degrees. Contact angles lower than 30 degrees are not very efficient and may leave some surface ink on the anilox that could contribute to spitting. For UV inks the contact angle should be at least 30 degrees or higher to minimize UV spitting as a higher angle will resist deflection better.
Applying too much pressure to the blade will create a very flat angle that will cause the tip of the blade to lift off the anilox surface. This will allow the ink to pass under this portion of the blade and transfer to the plate and to the substrate, causing spitting.
Thickness
A general rule of thumb in flexographic printing is always to use the thinnest blade possible while still being able to achieve a consistent wipe.
With high viscous solutions, however, thicker blades have always been used as they resist deflection better, minimizing spitting. Using them in print applications could compromise quality, however, as the thicker blade will provide a thicker ink film, which will affect the definition and sharpness of the printed images.
New blades have been developed which are thicker at the base to reduce deflection but still offer the same quality of wipe, as the tip of the blade remains the same.
By stiffening the base of the blade, the blade holds its position, allowing for a correct metering of the ink and withstanding better the pressure of the UV ink. The blade flexes against the ink pressure, significantly reducing spitting.
Material
New polymer blades have been developed to combat UV spitting from a different field - that of the material surface energy.
The new polymer blades have a lower surface tension than steel. This decreases the attraction between the doctor blade and ink and reduces the amount of ink accumulated behind the doctor blade, which can cause the blade to lift allowing the ink to pass through and causing spitting.
Anilox
The majority of anilox rolls used today are engraved with a hexagonal 60 degrees angle. While its performance advantages over previous cell patterns are well known, so are its limitations in terms of ink release and spitting. Because of the ink release limitations of the hexagonal 60-degree pattern, a new range of elongated hexagonal cell engravings has flooded the market.
This new elongated hexagonal engraving is vastly better at ink release and transfer and therefore has become the preferred choice of many printers. However, on the spitting front, these engravings increase the amount of ink transferred at the blade/ anilox interface, thus increasing the chance of spitting.
In both types of engraving, the pattern created is a hexagonal closed cell. As the cells are not linked, the ink cannot flow across the engraved anilox surface as it rotates, and the only way out from the cell is upwards. Due to its high viscosity, UV ink tends to accumulate behind the doctor blade causing the blade to lift and allowing the UV ink to pass underneath, causing spitting.
The use of channel engravings has proven to be an effective way to eliminate and prevent spitting. A channel engraving, either a continuous channel or linked-in cells allows the ink to flow within the engraved channel across the anilox surface, decreasing the build-up of ink accumulated behind the blade, thus reducing the hydraulic pressure inflicted on the blade.
Changing the anilox specification from closed cell to channel engraving eliminates UV spitting in 99 percent of cases.
A study carried out in the last quarter of 2014 by Cheshire Anilox Technology revealed that the use of MaxFlo UV channel engravings effectively eliminated UV spitting.
Fifteen different UV label printers took part in the trial, where four types of engraving were tested. All fifteen printers that participated in the trial were experiencing spitting in their operations.
The different Anilox engravings tested had the same cell volume, leaving the rest of the print variables unchanged: the same doctor blade, substrate, backing tape, plate and print impression were maintained in each test.
The trial was double-blind, meaning that neither the printers nor the researchers knew which anilox engraving was being used at the time of the print trial. The different engravings were referred as type A, B, C and D in order to differentiate and record data.
Different print speeds and viscosity of ink were tested to see results using different anilox engravings. See chart for the results.
UV ink spitting is a common problem amongst UV label converters that can be successfully managed not just by following good operating and housekeeping practices, but by selecting
the right anilox.
Work with your anilox, ink and blade suppliers to determine the best system that works for you in order to minimise the potential of UV spitting.