Ink Dryback: Other Factors

In the September issue of GAM, this column began a two-part discussion of the phenomenon called "ink dryback," defined as a loss of gloss and a shift in perceived color values between the time a sheet of paper is first printed and after the ink has dried on it. The result is a decrease in optical density. When this occurs, ink films may appear lighter than specified if they are transparent, and darker if they are more opaque.

Ink dryback occurs more often in sheetfed offset printing; contributing ink-related factors include formulations, on-press delivery systems, and the aforementioned ink/substrate interaction. Sheetfed printing is more prone to this problem because, depending on the type of conventional ink applied and the on-press drying mechanism, drying can extend from several hours to as long as a full day.

In web heatset printing, dryback is relatively rare because the ink is dried rapidly and consistently on press both along and across the web.

To prevent dryback, a cause of customer dissatisfaction, offset printers need to understand that the ink/substrate interaction is only one of many variables that can trigger or add to dryback's occurrence.

Listed below are some of these other variables to alert printers as to when preventive or remedial actions may be needed:

Press crew training: Many of the dryback trigger mechanisms are within the control of the press crew. There is no better way to assure optimum productivity and excellence of end products than investing in in-depth training for those working in the pressroom.

Pressroom environment: A recent Graphic Arts Technical Foundation (GATF) report on sheetfed printing attributes notes that temperature and humidity are pressroom variables that can influence color appearance on a job. Too much humidity, for example, can prevent fountain water from evaporating. This leaves too much phosphoric acid on the ink, which, by reacting with the driers, will slow the drying process, causing dryback when too much vehicle is absorbed into the paper.

For the pressroom, GATF recommends the Technical Association of the Pulp and Paper Industry standards of 50% humidity and about 72° F.

Fountain solution pH: Many printers are not fully aware of the importance of maintaining proper pH acidity levels in the fountain solution. This can prove to be critical in controlling dryback with certain colors, especially reflex blue and some of the rubine reds.

Ink dispensing systems: The press crew also controls the rate of ink flow from the dispensing system. Without due care, too heavy or too thin an ink film may be applied. Further, if an operator makes ink adjustments during the press run, color changes are possible because the other variables also may have been affected.

Plates: Plates on conventional presses usually are metallic; for waterless printing, they are silicone-based. For either, how consistent the plate material is, how its grain accepts both water and ink, and the plate's wear characteristics will all affect the quality of the transferred image.

Blankets: On the offset press, the type of blanket employed also may have its effect; the harder the blanket, the less chance that dryback may occur. This may be because a harder blanket can decrease the possibility of dot gain, preventing the dispersion of the ink film over a greater area. The degree of compressibility and absorbency of the blanket also may affect how the ink film is transferred and released.

Dryers: If the press-end infrared dryer is run too high, it heats the substrate. This causes the ink to penetrate more quickly, the result being that the ink film is thinned. Color values are lost because the white of the substrate is permitted to come through.

Paper stacks: If printed sheets coming off the delivery end of the press are permitted to stack too high, these high lifts prevent air circulation. The weight of the paper thus deprives the lower sheets of the oxygen needed for proper drying. With sheetfed inks, oxygen is critical for proper drier function.

Ultimately, many influences may result in ink dryback. Inadequate control of these variables may be an invitation to a customer's rejection of a job. By knowing how and why this phenomenon occurs, printers can take the actions necessary to reduce or eliminate its esthetic and economic effects.