There still seems to be some misconceptions in the flexo industry concerning this issue. The confusion begins when trying to make assumptions or comparisons of similarity in ink travel across the ink train between two-roll metering versus enclosed chambered doctor-blade metering. Though both systems are intended to transfer a somewhat controlled ink film thickness across the ink train to the substrate, the similarity ends there.

The thinner the ink film, under control and to the proper density, the higher the print quality. With this axiom in mind, let us examine the two systems:

The two-roll metering system. With this metering system the ink transferring rollers, the rubber roll and anilox, are somewhat flooded with ink. Most of the ink that ends up being transferred to the surface of the plate comes not from the anilox cells, but from the land area between the cells. The rubber roller also exerts pressure against the anilox to help force ink into the cell and onto the land areas.

A true film of ink ends up being transferred to the face or print surface of the plate. The major benefit of this system is increased ink dwell time, which enhances rewetting of the ink film in the transfer process.

Hydraulic pressure is also an issue with this metering system. It is because of this phenomenon that the ink film thickness varies with viscosity shifts, operator set, and press speed. The major reason a wide range of densities can be measured using the same anilox is because of the variability of the ink film thickness within the system. But going back to the statement above, the greater the ink film thickness, the more chance there is for print quality degradation. With this system the mechanics of the mechanism play a major role in getting the ink to the substrate efficiently.

The enclosed-chambered, doctor-blade metering system. With this mechanism there is no longer a flooding of rollers with ink through the ink train, and there is little or no pressure applied to help fill the anilox cells with the ink. The overall ink film is thinner than most comparable two-roll metering systems. It is in fact not transference of ink film to the print surface but microscopic pockets of ink. This is because there is little or no transference from the land areas between the cells of the anilox.

The ink system itself must now be much more involved because of reduced dwell time, thinner ink film and the pockets of ink on the plate needing to better wet out and become a film before transferring to the substrate. For these reasons, more ink control and ink system balance must be maintained.

This metering system is desirable because it lessens the variability in the overall ink film thickness due to viscosity, operator set and speed, so shifts in density will be minimized. Implementing of a much thinner ink film will enhance print quality. The real only downside to this system is that more aniloxes will need to be inventoried based on the different job densities.

Doctor blade metering systems are by far the better mechanism for print quality improvement, but the system requires that the operator have a much better understanding of how the ink systems themselves work, and how to keep them working at their peak efficiency.

Steve Utschig

Consulting Technical Editor

920/735-4882

utschig@foxvalleytech.com