|Rubber lithographic rollers have tiny mountain like protrusions ground into the surface, which helps to work a uniform emulsion of fountain solution and ink flow evenly down through the inking system. Rollers should have a dull, satin appearance, and feel smooth to the touch when dragging your finger across the roller surface. Lithographic rollers that look shiny and have a glassy smooth feel to them have become glazed.
Glaze is a combination of embedded ink pigment, dried ink vehicle, gum from the fountain solution, paper coating and paper dust that accumulates on the surface of a resilient roller. It is generally a result of the unavoidable effects of press chemistry compounded by insufficient cleaning. Glazing seriously reduces the efficiency of the inking system.
When rollers are only slightly glazed, their condition can be improved by hand scrubbing them with roller compatible solvent, hot water and a commercial paste cleaner, and using a blanket and roller pad. Deglazing should reduce the hardness of the roller by about 3-4 durometer units. If the hardness does not decrease, the roller must be reground or replaced. Regrinding should be performed by a roller manufacturer according to press specifications.
Improper cleaning of the rollers results from poor roller settings, a worn wash-up blade, or inadequate solvents. If the rollers are not contacting each other properly, dissolved inks will not transfer properly up the roller system to the wash-up blade. A worn wash-up blade should not be used. The moderate cost of blade replacement can be lost many times over in long wash up times, increased glaze and roller deterioration, and the eventual scoring or damage to the adjacent oscillator due to use of excessive blade pressure.
A frequent cause of glazing is an improperly formulated solvent. As a cost-saving measure, a general-purpose industrial solvent may be used and may appear to do the job properly. However, the removal of ink from the system is not enough. Paper coating, ink additive compounds, and emulsified fountain solution components, especially gum can be left by common solvents, and contribute to roller glazing. Wash-up solutions that have been specially formulated for lithographic rollers are recommended. Several of these are two-step solutions; the first should be a water-miscible solvent. The extra cost of these solutions is quickly recovered in faster wash up times, the use of less solvent, and easier color changes. If low-grade solvents are used, a good detergent type glaze removing solvent should be applied to the rollers once or twice a week.
Along with the proper solvents, the wash-up technique is important. We recommend the following steps:
1. Gum and dry the plate to protect it against stray drips.
2. Turn the press on and run it at medium speed, usually about 6,000 – 7,000 iph on newer presses.
3. Thoroughly wet the rollers away from the wash-up blade with solvent to soften the ink.
4. Allow the press to idle for a couple of minutes, and then gently engage the wash-up blade to the oscillator roller, increasing pressure until contact is achieved.
5. Add solvent to one side of the ink system at a time, being careful not to allow the wash-up blade on the other side to become too dry. In this way, roller skidding, drips, and excessive buildup of ink at roller ends are avoided.
6. Apply solvent with a squirt bottle to various points of the roller system away from the wash-up blade. To avoid waste and drips, do not apply more solvent than can be held at the nips of the rollers.
7. Continue applying solvent periodically until the fluid running into the drip tray appears clear, and the rollers are clean.
8. Apply a water–miscible solution to the rollers as a final flushing of the rollers. An additional rinse of hot water and white vinegar could also be done.
9. When the rollers are satisfactorily clean, release the wash-up blade while the press is still operating. Occasionally, if the wash-up blade is released on the run, a buildup of ink behind the blade will run back into the rollers, meaning that they have to be washed again.
10. To evaluate the thoroughness of the wash-up, stop the press, put the safety on, and then rub cheesecloth across the rollers. Any residue remaining on the rollers will be easily seen on the cheesecloth.