Q: Why is ink control more challenging in process printing?

Steve Utschig, Consulting Technical Editor. 920/735-4882. utschig@foxvalley.tec.wi.us -- Converting Magazine, 1/1/2001

A: Ink control can be more challenging in process printing for many reasons, such as the thickness of the ink film transferred, pigment concentration, and the amount of ink being used. An understanding of how the total system works will help give the press operator a better chance for success when printing process jobs of four, six or even seven colors.

In process printing the thinnest ink film possible, transferred under control and to the proper density, will yield the best print result. When working with the thinner ink films required by process printing, a thorough understanding of the ink systems transfer mechanisms is needed.

A flexo ink contains four component parts: pigments (which are solids in suspension), vehicle, additives and solvents. In order for the ink to perform well, these different components must be kept in balance with each other. Process inks, by definition, contain high pigment loads. The more pigment an ink contains, the harder it can be to work with, because the solids must be transported across the ink train.

As the ink is transferred from the anilox to the plate and then to the substrate, some is left behind in the cell and on the plate. This is referred to as an ink split. When this leftover ink (now beginning to dry) again comes into contact with fresh ink it must be rewet or resolubilized and reenter the ink stream to ultimately end up on the substrate. Without the proper balance of ink components this is compromised and the ink ends up staying in the anilox cell, affecting density; or on the plate, resulting in dirty print or bridging.

The vehicle is the major player in the transport of pigments across the ink train. When the ink begins to dry the vehicle tends to not fully surround the pigment. The pigment particles begin to touch, causing them to be deposited in the cell and plate surface and stay there. It is up to the press operator to reverse this reaction so the pigment ultimately ends up where it needs to be-on the substrate.

In solvent-based ink it is the acetate in the solvent which acts to keep the reaction reversing. Maintaining proper viscosity levels alone will not work, because the solvents in the blends don't evaporate at the same rate. Acetate evaporates must faster than alcohol, so when adding make up solvent to maintain viscosity, a higher concentration of acetate must be used.

When using waterbased ink the pH is the key player in the resolubility mechanism, so it must be maintained by adding a pH adjuster or amine when necessary.

Process inks are classified as small take-off colors, which means that not a lot of ink is used during any particular press run. For this reason, not only do the solvent blends and pH need to be diligently monitored and adjusted for, but the other components must be replaced periodically for the ink to remain in balance and perform. The key here is to run lower sumps for the process colors and make more frequent additions of fresh ink to control performance and resolubility.

Balance of the component parts of any ink system is critical to successful printing. The press operator must understand the mechanism of resolubility and its control. For information regarding control of pH range, viscosity, solvent blends, etc., contact your ink vendor.

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