Incremental printing of symbolic information – Electric marking apparatus or processes – Electrostatic
Reexamination Certificate
2001-05-17
2002-12-10
Pendegrass, Joan (Department: 2852)
Incremental printing of symbolic information
Electric marking apparatus or processes
Electrostatic
C358S001180, C399S301000
Reexamination Certificate
active
06493012
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to setting register on a multicolor printing machine having color printing units allocated to various printing inks and having image cylinders, equipment for producing images, in particular electrostatic latent images, on the image cylinders, a carrier for printing substrates and image transfer points for the transfer of the color separations from the color printing units to the printing substrates, a time independent allocation of the image productions on the image cylinders being carried out in order to achieve coincidence of register of the color separations in the print.
The invention further relates to apparatus for setting register in accordance with the above-described method on a multicolor printing machine having color printing units allocated to various printing inks and having image cylinders, equipment for producing images, in particular electrostatic latent images, on the image cylinders, a carrier for printing substrates and image transfer points for the transfer of the color separations from the color printing units to printing substrates, sensors for measuring position, and at least one setting device for allocating the positions of the image production points on the image cylinders to the printing substrates in order to achieve coincidence of register of the color separations in the print. Furthermore, the invention relates to an appropriately equipped multicolor printing machine.
BACKGROUND OF THE INVENTION
Printing color illustrations, in particular color images, is carried out 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 compositions can be achieved, the quality of the prints depending significantly on the in-register overprinting of the color separations. In conventional, non-automated printing processes, the printing plates are corrected by means of test prints and register marks printed at the same time as these until exact overprinting, that is to say, maintenance of register in the print, is achieved.
In digital printing processes, the image cylinders are written with image points by image production equipment in each case, by electrostatic charges being generated and these being provided with adherent colored pigments. The colored pigments are then transferred to a printing substrate. In digital printing processes, maintenance of register can be achieved by the image production equipment being controlled appropriately. Since the setting of an image is carried out anew for each print, it is not necessary, as in conventional printing processes, for a one-off setting to be made, but presetting and control can be provided, which performs corrections for each individual print. Of course, this does not apply only to the application of electrostatic latent images but also to all other printing processes in which image points are applied by a digital control system.
For an electrostatic printing process of the type mentioned at the beginning, U.S. Pat. No. 5,287,162 has, therefore, proposed to print register marks preferably onto the carrier for the printing substrates and to detect these by an apparatus. Here, the times, which the register marks need to pass from production by the image production equipment to a detection point are determined. These times are then used to determine the instants at which the image production equipment perform the image setting on the individual image cylinders, in order to achieve the maintenance of register after the images have been transferred to a printing substrate.
Since achieving coincidence with respect to image setting on the image cylinders leads to inaccuracies if there are differences in speed relating to the surfaces of the image cylinders, U.S. Pat. No. 5,287,162 has proposed to record calibration tables with times which are allocated to various angular positions of the image cylinders, in order, with the aid of these calibration values, to eliminate regularly occurring fluctuations—which are mostly caused by unroundnesses of the cylinders—and in this way to make the corrections for each individual print.
Since the maintenance of register required for high printing qualities requires extremely high precision, such calibration tables, in which time values are set, are inadequate, however. It is not possible to take into account irregularities, which are reflected in differences in time intervals, which cannot be allocated to rotational angles of the image cylinders. Nor is the latter helped either by a proposal in U.S. Pat. No. 5,287,162 to draw up the calibration tables again and again since by this means only the long-term and slow drift of the values can be taken into account, but no short-term differences that cannot be allocated to angular positions of the image cylinders.
A typical example of such irregularities, which are not reflected in differences between time intervals, are fluctuations in the speed of the drive system, since the allocation of the same to specific rotary angles of the image cylinders or other cylinders is not possible, since these fluctuations do not exhibit any synchronism with the angular positions of the image cylinders or other cylinders. Regulation by a calibration table of the type proposed with time values, which is allocated to the rotary angles of the image cylinders, would, thus, rather produce errors than eliminate errors. Thus, in investigations it has been determined, for example, that the poles of the electric drive motors occur as items which cause frequency-type speed fluctuations of the drive, which, because of the different transmission distances, also do not exhibit any synchronous occurrence on all the image cylinders and, therefore, lead to time/position differences on the individual image cylinders. These frequency-type fluctuations are sufficient to cause faults in the register setting. Faults of this type can occur as early as at the start of the image or can make themselves noticeable in the image quality as faults in subareas of images, for example, as register inaccuracies like transverse stripes. Since such frequency-like fluctuations of the drive system are superimposed on other faults, such as unroundnesses of image cylinders, it is no longer possible to draw up calibration tables for correction with a tolerable outlay. These tables could no longer be oriented to the angular positions of the image cylinders or other cylinders for one revolution or for a comprehensible sequence of revolutions, but it would be necessary—if this is at all possible as a result of the complexity—for a curve of calibration values relating to complex machine configurations as far as the occurrence of a repetition situation to be determined. However, drawing up correction values over relatively long time periods in this way is, in turn, opposed by the fact that there are also further causes of faults, such as irregularities in the guidance of the carrier and primarily also long-term changes such as temperature changes, the change in mechanical stresses in the machine, changes in the type of paper, the amount of toner, and so on. Such a “colorful mixture” of faults, which change in the short term and behave synchronously with respect to the angular positions of cylinders, with faults, which likewise change in the short term but not synchronously with the angular positions, and long-term asynchronous changes, oppose the achievement of high precision by a correction with the aid of the proposed calibration tables with time values.
SUMMARY OF THE INVENTION
The invention is, therefore, based on the object of configuring a method, and an apparatus in such a way that high precision of the register setting can be achieved with a tolerable outlay, in particular as far as possible without reject prints. At the same time, both the rapid and most exact possible presetting, as well as a continuous rapid correction of the register setting, is to be made possible.
According to th
Buch Donald
Dreher Ingo Klaus
Hunold Heiko
Liston Christopher
Metzler Patrick
Kessler Lawrence P.
Nexpress Solutions LLC
Pendegrass Joan
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