Electrophotography – Control of electrophotography process – Responsive to copy media characteristic
Reexamination Certificate
2001-05-15
2003-02-11
Grainger, Quana M. (Department: 2852)
Electrophotography
Control of electrophotography process
Responsive to copy media characteristic
C399S301000
Reexamination Certificate
active
06519423
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a method and apparatus for setting registration in a multicolor printing machine having a number of exposure devices for the digital production of color separations, the actions of setting up and combining the color separations being controlled when printing substrates of a different grade are fed in, in order to correct the registration setting.
Printing colored illustrations, in particular colored images, is carried out, for example, by a number of color separations being printed over one another. These are generally the colors yellow, magenta and cyan, as well as black. If required, special colors are added. By overprinting these colors, all color combinations can be achieved, the quality of the prints depending significantly on the in-register overprinting of the color separations. In the case of digital printing processes, for example, electrostatic printing processes, the maintenance of the registration of the overprint is achieved by the image production devices being controlled such that the color separations meet one another in-register when they are transferred to a printing substrate.
A method and an apparatus for color printing are disclosed by U.S Pat. No. 5,689,757. The teaching of this document is to take account of the influence of the roughness of the paper on the registration when printing substrates of a different grade are fed in. For this purpose, the roughness of a paper, which is to be printed, is measured. The influence of the paper roughness on registration is then taken into account by the registration controller before the paper is fed to the printing process.
It has been shown that influencing the registration accuracy, as a result of changing the printing substrate grade, can be detected only inadequately in this way, so that a high print quality cannot be achieved. The reason for this is that the influence of the change in the substrate grade cannot be detected by a single property. In addition, the paper roughness is, first of all, not the property, which exerts the greatest influence on the registration. This leads to the situation where satisfactory registration accuracy can be achieved only by a correction, which is based on evaluating the printing result. The disadvantage of setting the registration by evaluating the printing result, for example, by registration marks, is, however, that in the event of a change in the printing substrate grade, counteractive control is only possible for the print which follows a print which is faulty because of the change, since the effect of the change in an influencing variable is only detected when it has already caused a registration fault. Changes to the substrate during a print job are, therefore, not possible.
SUMMARY OF THE INVENTION
The invention is, therefore, based on the object of a method and an apparatus such that in the event of a change in the printing substrate grade, the influence of this change on registration can be compensated for optimally before the printing substrate is fed to the printing process. According to the invention, the object is achieved with respect to the method and apparatus, in that when the printing substrate grade is changed, the influence of all the properties of the printing substrate that are relevant to registration is taken into account directly as the change is implemented, by correction values for the printing substrate that are available before printing is carried out. At least one controller is provided that when the printing substrate grade is changed, the influence of all the properties of the printing substrate that are relevant to registration is taken into account directly as the change is implemented, by available correction values for the printing substrate.
The invention is based on the finding that there are a number of properties of printing substrates which have an influence on registration, and optimum compensation of the influences of these properties when the printing substrate grade is changed is possible only when the sum of the influences of all the properties that are relevant in this regard is taken into account. If significant properties remain unconsidered, there remains only the subsequent correction, mentioned at the beginning, by evaluation of a proof result.
For instance, it has been shown that a significant influencing variable with regard to registration is the printing substrate thickness. This is because the movements of the elements carrying the color separations and substrates are linked to one another by friction. Here, as a result of a change in the printing substrate thickness, the radius, which is critical for the transmission ratio between the printing substrate carrier and a cylinder, is changed. This changing transmission ratio affects registration to a significantly greater extent than the paper roughness.
However, in addition to the printing substrate thickness and the surface roughness, there are still further properties of a printing substrate which have an influence on registration, such as the compressibility or the flexing of the printing substrate during printing, the stiffness or changes resulting from toner uptake or heating.
The invention achieves an optimum correction, since the sum of all these influences is taken into account, without it mattering which and how many properties of the printing substrate have what influence. In addition, it is possible to include changes in the properties of a printing substrate within the machine, if these properties change from one printing unit to the next as a result of heating, flexing or printing with one or more color separations. For this case, the correction values for the individual printing units are correspondingly different.
The invention, therefore makes, it possible for the occurrence of a registration fault to be prevented from the outset, so that proofs on printing substrates, for example, with registration marks, are normally no longer needed. It is precisely in the case of small jobs or for printing on printed pages with continually changing paper grades that this is of economic importance, since machine time and often also printing substrates can be saved. In addition, the measure according to the invention makes proofs on the printing substrate carriers possible without feeding printing substrates in order to correct other influences on registration, since the influence of the properties of the printing substrates is detected and compensated for in a satisfactory manner by the measure according to the invention.
As a result of the invention, a registration setting of known type can be optimized with the effect that, when a printing substrate grade is changed, it does not “get out of hand” initially in order to be corrected again later, but rather the registration setting remains within the range of tolerable fluctuations, so that during a change to a different grade of printing substrate, it is possible to continue to print without interruption. At the same time, the invention does not exclude additionally taking into account other influencing variables, which are necessitated by a printed page but not by the printing substrate grade. One example of such an influencing variable is the application of toner, which is determined by the motif in the image and results in a different toner profile with respect to each color separation. In the measure according to the invention, nor does it matter either whether the printing substrate carrier with the printing substrates cooperates directly with the image cylinders in order to transfer the color separations, or whether image transfer cylinders are interposed.
A development of the method provides that in the event of a registration correction because of a change in printing substrate grade, the retroactive influence of the properties of the printing substrate of printed pages which have gone before but are still in the printing process on the registration of the new printed page that is already in the printing process is taken into account.
Dreher Ingo Klaus
Liston Christopher
Metzler Patrick
Peter Karlheinz Walter
Grainger Quana M.
Kessler Lawrence P.
NexPress Solutions LLC
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