Maintaining Your Balance

by Dennis Cheeseman

Oil and water don't mix, right? Wrong--they can and do. Furthermore, your lithographic press depends on their mixing in precisely the right way.

Lithographic inks must emulsify fountain solution to work properly. This means the oil-based inks must hold water from the fountain solution in a stable liquid suspension. On presses with ink-train dampening, emulsification helps transport fountain solution, in fine and large droplets, from the dampening system to the plate.

The plate is composed of two areas. The image area attracts oil, while the nonimage area attracts water. In ink-train dampening, this means that the ink and fountain-solution mixture should break down when exposed to the plate, with the ink adhering to the image area while the water keeps the nonimage area clean.

Unfortunately, it doesn't always work that way, since all lithographic inks have a "window" for optimal press performance. Thus, ink-and-water balance must be properly maintained.

Too much ink per unit of fountain solution will cause scumming--a condition in which ink gets on the nonimage area because there is not enough moisture on plate. Too much fountain solution will cause water marking, snowflaking in solid areas and missed dots in the halftones.

There is no "universally correct" ratio of ink to fountain solution. The correct ratio depends on the liquids being used and on the various interacting systems. The following are just some of the factors:

• The size of the image area
• Solid vs. screened image area
• Printing area across the cylinder
• Temperature and humidity in the press room, which affect evaporation.

Regardless of their ratio, optimal ink transfer usually occurs when ink and water are at their minimal levels. When an ink over-emulsifies, or takes up too much water, its flow properties change, inhibiting the transfer of ink to the plate. Press operators sometimes try to compensate by running an excessive ink film just to approach the correct ink density.

The following steps should help:

1. Reduce the water level five points at a time. Allow 3-to-5 minutes for the press to react.
2. Observe the ink level and reduce as necessary to maintain proper ink density.
3. Continue this process until you get toning in the background and ink density is in or just shy of specification.
4. Bring water level up slightly.
5. Adjust the ink to its proper density. Don't guess--check it with a densitometer.

If you are diligent, you can expect the following benefits:

• More consistent ink settings
• Better trapping of solids
• Better single-color signatures
• Smooth laydown (not mottled)
• Better four-color reproduction
• Reduced build-up on ink rails
• Reduced misting
• Reduced lint buildup on blankets
• Reduced plate wear
• More pages printed per pound of ink
• Reduced ink ruboff
• Lower pipe-roller buildup
• Reduced setoff
• Reduced dot gain.

Dennis Cheeseman is director of customer services for US Ink Corp. Phone is (201) 935-8666; fax, (201) 933-2291.

TechNews Volume 2, Number 6: November/December 1996

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