Field of the Invention
The invention relates to a modular printing machine system for printing on sheets, including a first printing machine of satellite construction type having a central first impression cylinder and at least four printing devices assigned thereto, a second printing machine having a second impression cylinder, and a coupling device for coupling the printing machines to one another for in-line operation thereof.
The published German Patent Document DE 197 43 770 A1, which corresponds to U.S. Pat. No. 6,101,944, describes such a printing machine system, which includes a satellite printing-unit group as the first printing machine, and imprinting-unit group as the second printing machine and a connecting element as a coupling device. A point of separation in the coupling runs between a transfer drum belonging to the satellite printing-unit group and a transfer drum belonging to the imprinting-unit group.
A drawback of the aforedescribed system is the low maintenance of register thereof. For example, in the imprinting-unit group, it is barely possible to provide four-color images printed in the satellite printing-unit group with a frame of a special color adjoining the four-color image seamlessly, on the one hand, without any overlap and, on the other hand, without any free space. Between the adjacent colored objects, in the example the four-color image and the frame, small register differences occur, which manifest themselves to the viewer for the most part as disruptive so-called “flashes” of the paper background in the printed product. A significant cause of the register differences is the so-called transfer register of the printing machine system. Because of the necessary clearance and the wear of the connecting element that is caused by the use thereof, accurate-reproduction coupling of the groups is not possible, so that the position of the groups relative to one another at the point of separation turns out slightly differently each time they are coupled. In other words, sheet grippers belonging to the transfer drum have a slightly different position in relation to sheet grippers belonging to the acceptance drum, each time they are coupled. Because of the necessity to be able to couple the satellite printing-unit group to the imprinting-unit group and to uncouple them from one another, in the system described, no adequately good transfer register accuracy, of for example ±10 μm (standard deviation), is ensured, as is usual in the case of other modern sheet-fed offset printing machines with printing units which are coupled rigidly and cannot be uncoupled from one another. In the case of the system described in the foregoing German patent document, the pressman has no possibility of compensating for poor transfer register accuracy.
Further prior art is described in U.S. Pat. No. 5,660,108, in the published German Patent Documents DE 43 03 797 A1 (which corresponds to U.S. Pat. No. 5,479,856) and DE 195 03 619 A1.