The UV Ink Spitting Problem
UV flexo printing continues to achieve outstanding results in the flexo industry. However, despite the fact that many of the ink companies have dropped their viscosities for UV inks the phenomenon of UV ink spitting still persists.
Although there are numerous explanations as to the cause of ink spitting, the most common explanation is based on the fact that compared with water and solvent-based inks, UV inks can have a viscosity as much as five times that of other ink types within the transfer zone of the press which result in high hydraulic forces being applied to the doctor blade. The thicker UV ink creates a hydroplaning affect that actually lifts the doctor blade a very small amount and causes "extra" ink to come through under that portion of the blade and be transferred to the plate. This excessive ink then prematurely releases, especially at higher press speeds, and results in what can be a very troublesome print defect.
Another cause cited are the thixotropic properties (thixotropic fluids decrease in viscosity over time at a constant shear rate), which contribute to the spitting that occurs. As UV inks are released from the anilox roll, metered by the doctor blade, and finally transferred to the plate the ink becomes thinner and thus more likely to spray or spit. It has also been reported that contamination in the ink or if the ink wasn't ground properly can result in small amounts of pigment getting underneath the doctor blade which again can create the opportunity for poor blade metering and an uneven ink film.
Possible Solutions and Considerations
The key element is to reduce the risk of ink spitting without compromising print results. Pressroom experiences has proven that using a thicker or stiffer blade minimizes UV spitting as the stronger blade better resists the hydroplaning affect and does not allow too much ink to transfer to the anilox roll. However, too thick a blade can contribute to an uneven film thickness that will affect process dots, reverses, small positive type, and bar codes.
While many companies have found ways to minimize the problem such as using a "backer" blade (to back up your doctor blade with another blade), leading blade suppliers have attacked this spitting phenomenon by focusing on specific characteristics of the doctor blade composition. Doctor blade manufacturers such as Esterlam and Daetwyler have developed special materials and blade configurations designed to remain stiffer yet still meter ink uniformly to substantially decrease UV spitting without compromising print results.
These specialty doctor blades have been precision-engineered to be thicker at the base (where the blade is installed in the holder) and extend close to the contact metering point but with tips designed to maintain the quality and characteristics of a standard edge. These blades' unique design allows the thicker ink to meter itself just like it would with a normal doctor blade, but because of the blade's strengthened base, the amount of flex is decreased. Consistent ink-film metering is therefore accomplished while avoiding ink hydroplaning of the blade. This then substantially lessens UV spitting.
It is also important to set the blade at the correct angle to ensure the proper wipe. The set blade angle is the actual angle that results from the application of the blade to the cylinder by the blade holder. Set angles are typically between 25-35°. This angle has the ability to change due to blade or cylinder wear and even ink type. Having a range within a specified doctor blade angle allows the angle to be altered to achieve the best wipe under the current conditions. A steep angle is used to remove more ink from the cells, which is common when printing four-color process for which a heavy ink film thickness can increase dot gain. A flat angle is more common when printing solids or coatings requiring more ink in the anilox roll cells, resulting in greater transfer and SID.
A final consideration is making sure you are maintaining proper viscosity. Viscosity is one of the most important variables to control to achieve quality print. It not only affects color, final printability and drying but as mentioned, can contribute to ink spitting. Also, if contamination is a concern, consider using ink filters.
UV ink printers are making tremendous strides in printing high-quality graphics. Using the correct doctor blade and maintaining proper procedures can be the determining factor in achieving consistent high-quality print and increasing productivity.
All Printing Resources carries the most complete line of doctor blades from world leaders to allow us to assist our customers in carefully choosing the best blade for the application for printability, roll wear, blade life and increased productivity. We also offer free training on proper blade selection, use, and safety.