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FLEXO Magazine : June 2014
Are Your UV Inks Spitting? Minimize by Selecting & Using Proper Doctor Blades Bill Warner U V inks are known for their remarkable printing characteristics that lead to better graphics and rich colors. Unfortunately, the same inks are also known for the problem that is commonly referred to as “spitting.” Spitting is a condition where the UV ink will get past the doctor blade in an uncontrolled manner and ul- timately be transferred to the printed web. The result is commonly seen in the form of lines or other odd shapes on the printed web; they can vary from just a few to too many to count (See Image 1). So what are the causes of UV ink spitting? Many studies have been conducted over the years to try to determine the cause without a definitive answer. Fundamentally it is a characteristic specific to UV ink, as water and solvent based inks rarely exhibit the same spitting phenomenon. So what makes UV inks prone to spitting? There are two leading contributors: UV inks are much more viscous than water based inks, typically five times or more. The higher viscosity puts more pressure on the doctor blade and can cause the blade to flutter or otherwise deflect and allow ink to get past the blade if it is not set or chosen correctly. The second contributor to UV ink spitting is the thixotropic nature of UV inks. The dictionary defines thixotropic as a fluid or gel “having a viscosity that decreases when stress is applied, as when stirred.” The stress applied to the UV ink is a result of the shearing forces that occur when the ink comes in contact with the doctor blade. The drop in viscosity at the doctor blade/anilox roll interface can allow some ink to flow under the doctor blade. The shear forces on the ink are reduced when the ink moves past the doctor blade and the viscosity again increases, giving the ink the tendency to either spit or spray 62 FLEXO | JUNE 2014 TECHNOLOGY & TECHNIQUES Anilox Roll, Doctor Blade, Ink Selection Guide Image 3