Printing – Processes
Reexamination Certificate
2002-08-28
2004-02-17
Hirshfeld, Andrew H. (Department: 2854)
Printing
Processes
C101S484000, C250S339070, C250S339100, C226S002000
Reexamination Certificate
active
06691620
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to a process and a device for the detection of the position or register position of a paper web, especially in wet offset printing.
BACKGROUND OF THE INVENTION
Newspapers are produced predominantly according to the offset process. A plurality of paper webs are wound off from rolls, printed in the printing units and finally folded in the folding apparatus and cut. While one paper web is running through the press, it is continuously in a stretched state. Paper comes in contact with water and ink during wet offset printing, whereby the stretching properties of the papers are changed. The paths over which the individual paper webs run have different lengths. The addition of water and ink is not the same for all printing mechanisms. Various guide elements are used in order to turn the webs and to optimize the run of the web. The stretching properties also vary between different grades of paper and different paper weights, and there are tolerances in the elastic properties of the paper from one roll to the next and even within one roll even within the same grade of paper.
The printed webs are first folded into bundles in the folding apparatus, and the bundles are subsequently cut such that the cut is located outside the printing area of the pages. Errors in the cutting position make it impossible to sell the newspapers. The cutting position can be adjusted by the printer by adjusting the web length between the printing mechanism and the cutting position by means of so-called compensator rollers. It is also possible to set the cutting position by an equal adjustment of the printing positions of all printing mechanisms and thus to do away with a register roller (which is also called virtual main crop mark). A separate compensator roller is necessary for one half of the divided web in the case of double-width webs that are divided and placed one over another before the entry into the folding apparatus. The so-called secondary register is thus set.
Difficulties arise during the correct cutting of the bundled webs in the folding apparatus from the different paths of each web and the different stretchings, which the paper webs undergo during the run through the printing press. When similar productions are repeated, the cutting position is approximately known and can be roughly preset from the beginning. In fact, the printer must, however, accurately set the cutting position at the beginning of each production. This results in a certain number of spoiled copies and a loss of time during the production.
Processes that make possible the rapid, automatic regulation of the cutting position have been known. Measuring fields of various sizes are printed along, which are detected by suitable sensors shortly before the folding apparatus and thus provide a measuring reference, which can be used to regulate the cutting position. A large measuring mark, which is placed at the edge of the page that is not used for printing, can be detected, e.g., by a photodiode, but it compromises the overall visual impression. Smaller marks can be recognized by video cameras in conjunction with a digital pattern recognition. The drawback of both processes is that additional marks must be printed along. The technical effort for the recognition of these marks increases, in general, with decreasing size of these marks.
Another possibility of recognizing errors in the cutting position has been known from DE 199 10 835 C1, in which the printed image in two areas are scanned. The first scanning at the outlet of the printing mechanism forms the reference, with which the scanning before the entry into the folding apparatus is compared. The position errors can be determined from the correlation of the signals and used for the regulation. The drawback of this process is that a plurality of sensors are necessary and these must be placed suitably.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a process and a device especially for wet offset printing, which are able to automatically recognize the position or register position.
According to the present invention, the position or register position of a paper web, especially in wet offset printing, is detected by detecting strips of the paper web that are free from moistening agent. In addition or as an alternative, it is also possible to detect a strip in which there is no printing ink.
The device according to the present invention for detecting the position of a paper web, especially in wet offset printing, has a sensor, with which a moistening agent, e.g., water, or the water content in a paper web can be detected. According to a second embodiment, the same sensor or another sensor is used to detect printing ink on the paper web. An evaluating unit, with which the sensor signals can be evaluated and the moistening agent content of a paper web can be detected, is preferably provided.
According to another aspect, the present invention pertains to the regulation of the position of a paper web using the detected position, so that the cutting length of the paper web can be set, e.g., based on the detected printing ink-free or moistening agent-free strip, e.g., by the automatic actuation of a compensator roller.
The principle of function of the present invention will be described below.
Wet offset printing is based on the different surface tensions of the moistening agent and the printing ink. The process has been described several times and shall not be explained in greater detail here. The only thing that should be referred to here is the circumstance that water also comes into contact with the web besides the printing ink during the printing on the paper web, doing so on the area that approximately corresponds to the printing area. However, no water is transferred to the paper web in the area of the clamping device of the printing plates. The amount of moisture entering the paper depends on the number of printing mechanisms that come into contact with the web and, in addition, on the amount of moistening agent that is used in the particular printing mechanism. A page of a newspaper is usually printed on the front and the back. Consequently, depending on the number of colors used, 2 to 8 printing mechanisms are in contact with one paper web. Since each printing mechanism is set in good register, the zone free from printing ink or moistening agent forms a strip across the web, which is always located in the same area, for all printing mechanisms.
The present invention is based on the recognition of this printing ink-free or moistening agent-free strip. In fact, this area, which will hereinafter be called a channel strip, forms in a certain way an invisible marking or mask. Even though the marking formed by the moistening agent-free strip disappears after a few minutes, is it nevertheless present shortly after the printing operation and can be used to recognize or regulate the position.
The moistening agent consists mainly of water. Water is transparent, i.e., visible light is hardly absorbed by water. By contrast, water shows a marked absorption for some wavelengths in the infrared range and in the range close to the infrared. At a wavelength of 2.95 &mgr;m, water has an absorption, at which the depth of penetration for electromagnetic radiation is only about 1 &mgr;m, i.e., if such infrared light passes through a layer of water with a thickness of 1 &mgr;m, more than 60% of the radiation is absorbed. Less marked absorption maxima are located at half and one third of this wavelength. Furthermore, additional absorption peaks are found for water at the harmonic multiples of the vibration frequency of the electromagnetic radiation of the wavelength of 2.95 &mgr;m. For example, it is possible to measure absorption maxima at about 1.48 &mgr;m, 980 nm, etc. Even though the absorption is less intense there, faster and more sensitive detectors are available at these shorter wavelengths. In addition, there are powerful light sources, e.g., semiconductor lasers, which can be advantageously used for the measurin
Evans Andrea H.
Hirshfeld Andrew H.
Maschinenfabrik Wifag
McGlew and Tuttle , P.C.
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