When offset inking rollers deteriorate through age or neglect, they must be replaced with new or recovered rollers. before replacement time arrives however one or more printing problems may arise that are attributable to roller condition. Roller examination usually alerts the press operator to a need for replacement but occasionally the need for replacement is indicated through analysis of a problem.
The presswork problems may include the following, Roller stripping may appear in print as light streaks across the image accompanied by uneven ink distribution. If the uneven ink results in ink piling on the plate, wear may occur as the operator floods more ink to compensate for the light areas.
Stripping develops when a roller loses affinity for ink. Examination of the roller surface usually reveals a condition in which the ink appears to have been physically stripped away. In actuality, the ink-free band around the roller has rejected ink. An even closer look probably will disclose a tough glaze comprised of hardened gum and or fountain additive and dried ink on the rollers surface.
Preventative maintenance provides the best cure for roller strippng. In the case of inking rollers of an offset press, prevention of this and other presswork problems involves nothing more or less than adhering to a systematic programme of roller wash ups. Poor wash ups may be the greatest single cause of glazing, stripping, poor ink distribution and similar difficulties.
There are good solvents available and when applied as directed, the cleaning solvents remove ink from the rollers with ease and efficiency. A thoroughly cleaned roller should present a like new surface of velvety softness. If a hard pattern of dried ink and gum remains, the roller should be subject to a second or third application of cleaning solvent. Glaze that resists all attempts to remove it indicates removal of the rollers from the press for hand removal or the need for replacement.
The inking rollers that contact the plate can cause excessive plate wear to its surface if the rollers have been set with excessive pressure. Uneven distribution of the ink, brought on by a deteriorating roller surface, can also result in excessive plate wear. Another often unsuspected cause of plate wear is when the shore hardness of the roller increases.
Hardness may vary from roller to roller depending on press size, roller diameter, position in the press and age of the roller. To measure and control the hardness of the rubber the use of a Shore hardness meter is recommended. A hardness rating of 25 to 30 duro is judged best for most offset operations.
Several factors can cause a roller to become excessively hard. Chief among them is improper wash ups. Harsh cleaning solvents, often resorted to when hurried cleanups have left dried ink on the rollers, may work into the roller material and cause excessive hardness to develop.repeated or long exposure to alcohol also can cause rollers to harden .Alcohol tends to remove the plastercizers used in rubber compounds, robbing them of elasticity and increasing thier hardness beyond tolerable limits.
There is one cause of increased roller hardness beyond the control of the operator and that is aging . Since each type of roller ages at its own pace, increased surface hardness of different rollers will progress at different rates.
