Mechanical Ghosting on Web Offset Presses

During printing on web offset presses and particularly with a blanket to blanket configuration, coated grades are sometimes disturbed by a dot distortion. Halftone dots in some areas, on one side, have a lower dog gain due to an image on the opposite side. This phenomenon is a mechanical ghosting and has not been described previously. We have tested a number of coated grades, different web offset presses and printing conditions to try to solve this problem. We have tested a number of coated grades, different web offset presses and printing conditions to try to solve this problem. Results to be developed are: Halftone dots on one side are more influenced by block prints than by dots on the opposite side. The loss in print density increases with running time and after cleaning the blanket, the normal situation is recovered. Some coated papers are more sensitive than others in relation to their ability to cause a build-up on the blanket. The variation of ink formulation affects this behavior. During commercial printing, additives in fountain solution have been able to save critical situations. Decreasing the viscosity of the ink on the opposite side has also been sufficient to eliminate the problem. Analysis of this phenomenon reveals a microscopic build-up on the blanket between printing dots where local defects are appearing. In these areas, blanket pressure against the sheet is modified and automatic cleaning by abrasion of the web is not sufficient.