Method and device for detection of the speed difference between an image carrier and a carrier material

In a method and a system for generation of print images on a carrier material, an actual speed difference is generated in a transfer printing region between an image carrier and the carrier material. A deviation of the actual speed difference from a desired speed difference is determined with aid of a measurement arrangement.

BACKGROUND

The preferred embodiment concerns a method in which a speed difference is generated in a transfer printing region between an image carrier and a carrier material. The preferred embodiment also concerns a device for implementation of the method.

High-capacity printing systems for printing of a paper web are known from the documents DE 103 38 496 B3 and DE 103 38 497 A1, in which toner images present on the transfer belt are transfer-printed onto the paper web at a transfer printing point. The drive speed of the transfer belt is preset somewhat greater than the transport speed of the paper web, whereby a speed difference is present between the paper web12and the transfer belt. The speed difference is advantageously in the range from 0.1% to 10%, in particular in the range from 0.5% to 3% given a printing speed from 0.9 to 2 m/sec.

A printing system for printing of an endless paper web is known from the document U.S. Pat. No. 6,068,172, which printing system has a device for length compensation of a paper web to be printed in order to generate a preset tensile stress of the paper web in a transfer printing region.

A method and a device for transport of a web-shaped recording medium are known from the document DE 198 28 388 C2, in which method and device a length compensation element is provided with which a sensor is functionally coupled in order to determine the speed difference between a transport device arranged before the length compensation element and a transport device arranged after the length compensation element.

From the document U.S. Pat. No. 6,246,856 B1 a high-capacity printer and high-capacity copier is known in which toner images are generated that are transferred onto a recording medium. A plurality of developer stations are arranged along a photoconductor belt for generation of multi-color toner images, via which developer stations the color separations of a multi-color toner image are generated. The generated toner images of the color separations are successively transferred onto a transfer belt, whereby the individual color separations are thereby printed atop one another in registration. The color separations are thereby collected on the transfer belt. In a feed module what is known as a loop snag is provided via which the tensile stress of the carrier material to be printed is kept relatively constant.

A correct speed difference between the transfer belts serving as image carriers or the transfer belt serving as an image carrier and the carrier material to be printed is a decisive variable for the quality of the transfer printing of toner images onto the carrier material. If the speed difference is too great or too small, disruptions in the print image (for example what are known as wipes) arise or, for example, folds are formed in the carrier material to be printed. The carrier material can flutter in the region before the transfer printing point due to such folds, whereby the transfer printing conditions in the transfer printing region vary continuously. The actual speed difference does not remain constant due to various tolerances of arrangements for direction and propulsion of the carrier material as well as for direction and propulsion of the image carrier as well as variations of the direction and of the propulsion of the image carrier and of the carrier material during the print operation. The speed difference cannot be set or maintained exactly both over a longer image generation process and due to aging and/or wear of components of a printer or copier.

SUMMARY

It is an object to specify a method and an arrangement for generation of print images on a carrier material, in which method and arrangement the print images are transfer-printed from an image carrier onto a carrier material to be printed in a simple manner and in high quality.

In a method and a system for generation of print images on a carrier material, an actual speed difference is generated in a transfer printing region between an image carrier and the carrier material. A deviation of the actual speed difference from a desired speed difference is determined with aid of a measurement arrangement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a method and system for generation of print images on a carrier material, an actual speed difference is generated between an image carrier and the carrier material in a transfer printing region. A deviation of the actual speed difference from a desired speed difference is determined.

Via the method and the device of the preferred embodiment it is achieved that, via the determination of the deviation of the actual speed difference from a present speed difference, suitable measures can be taken in order (with the aid of suitable measures via the knowledge of the deviation) to set to a value that at least lies in a preset range in which a qualitatively high-grade transfer printing on the carrier material is possible. The speed difference can thereby be set exactly and kept constant even given variations of the direction and propulsion systems during the print operation as well as variations due to the aging or wear of elements of a printer or copier system.

The paper tension is advantageously detected as a measure for the actual speed difference and the actual speed difference is thereby regulated to a defined preset value. The paper tension can be detected in a simple manner with the aid of known measurement devices.

An electrophotographic high-capacity printing system10for printing of an endless paper web12with a printing speed in the range from 0.9 m/s to 2 m/s is shown inFIG. 1. A printing module14comprises a first image generation and transfer printing unit16for printing of the front side of the paper web12as well as a second image generation and transfer printing unit18for printing of the back side of the paper web12. The image generation and transfer printing units16,18are designated in the following as printing units16,18. The printing unit16is essentially structurally identical to the printing unit18. Coinciding components of the first printing unit16and of the second printing unit18are therefore identified with the same reference characters, whereby the reference character for identification of the components of the second printing unit18are additionally provided with an apostrophe. Differences between the first printing unit16and the second printing unit18are explained in further detail below.

The printing module14furthermore comprises a paper feed20, a control unit22, a toner reservoir and preparation system24, an image data processing unit26as well as a paper web propulsion and direction system28.

The paper web12is transported through the printing system10in the direction of the arrow P1with the aid of the paper web propulsion and direction system28, whereby the paper web12is supplied to a fixing station30in the printing module14after the printing, in which fixing station30the toner images generated on the paper web12by the printing module are fixed. The paper web direction and monitoring system28comprises deflection rollers32through40as well as a roller pair with a drive roller42and a contact pressure roller44situated opposite the drive roller42. Two marker sensors46,48are also provided that monitor the entrance of synchronization markers applied on the paper web. Furthermore, a margin hole sensor49is provided that determines the entrance or the position of the margin holes contained in the paper web12, whereby the margin holes and/or synchronization markers are brought into a desired position and/or held in this desired position with the aid of a regulation via corresponding activation of the drive motor of the paper web12.

The first printing unit16and the second printing unit18are arranged on opposite sides of the paper web12. With the aid of the drive roller42the paper web12can be transported in the arrow direction of the arrow P1and in the opposite direction, whereby in the following the transport of the paper web12in the arrow direction of the arrow P1is designated with “forward movement” and the transport of the paper web12in the direction opposite the arrow P1is designated with “backward movement”. The functions of the printing module14and of the fixing station30are described in detail in the documents WO 00/34831 and DE 198 27 210 C1, whose content is herewith incorporated by reference into the present specification.

The first printing unit16comprises a first belt drive66with a photoconductor belt68(for example an organic photoconductor belt) that is typically also designated as an OPC belt. The photoconductor belt68is driven in the arrow direction of the arrow P2with the aid of the belt drive66. With the aid of a cleaning and charging unit70the photoconductor belt68is discharged, toner residues are removed from the photoconductor belt68and the photoconductor belt68is charged to a predetermined uniform potential. With the aid of a character generator72that is executed as an LED character generator, regions of the surface of the photoconductor belt68that are charged to the same potential are (dependent on the employed electrophotographic principle) partially (i.e. per image point) discharged to a lower potential or charged to a higher potential corresponding to the signals supplied to the character generator72by the image processing unit26, whereby a charge image is generated on the surface of the photoconductor belt68. The charge image located on the surface of the photoconductor belt68is a latent print image. With the aid of a developer unit74the charge image on the surface of the photoconductor belt68is developed (i.e. inked with toner) into a toner image.

The first printing unit16furthermore comprises a second belt drive76with a transfer belt78that is driven in the direction of the arrow P3. The photoconductor belt68contacts the transfer belt78at a first transfer printing point80(meaning that the surface of the photoconductor belt68touches the surface of the photoconductor belt78), whereby a toner image located on the photoconductor belt68is transferred to the surface of the transfer belt78.

With the aid of a roller pivoting device82whose rollers are connected with one another via levers, the transfer belt78is directed onto the paper web12(i.e. pivoted onto the paper web12) and directed away from this (i.e. pivoted away from the paper web12) at a second. transfer printing point84, whereby the transfer belts78,78′ are pivoted onto the paper web12inFIG. 1. The transfer belts78,78′ of the printing unit16and of the printing unit18are essentially simultaneously pivoted onto the paper web12, whereby a contact pressure is generated between two opposite rollers or roller pairs of the belt drives76,76′ of the transfer belts78,78′. In the pivoted-towards state the transfer belt78contacts the surface of the paper web12on its front side and the transfer belt78′ on its back side, whereby the toner images located on the transfer belts78,78′ are transferred from the transfer belts78,78′ onto the front side as well as onto the back side of the paper web12. The pivoting of a transfer belt78,78′ towards and the pivoting of the transfer belt78,78′ away. from the paper web12is described in detail in the document WO 00/54266; the content of which is herewith incorporated by reference into the present specification.

In contrast to the second printing unit18, a reloading unit86for reloading of the toner image located on the transfer belt78is arranged at the belt drive76of the first printing unit16. The toner image on the transfer belt78is reloaded with the aid of a reloading unit79that is embodied as a corotron arrangement. The toner particles of the toner images of the front side and back side have different charges due to the reloading of the toner image on the transfer belt78, such that the transfer of the toner images onto the paper web12at the transfer printing point84is enabled by the attraction forces between the oppositely-charged toner particles through the paper web12. The toner images are thereby transfer-printed from the transfer belts78,78′ onto the paper web12at the second transfer printing point84.

The paper web12with the toner images is subsequently supplied to the fixing station30. The fixing station30comprises a first fixing unit54and a second fixing unit56that are arranged on the opposite sides of the paper web12, whereby the first fixing unit54fixes the toner images on the front side of the paper web12and the second fixing unit56fixes the toner images on the back side of the paper web; the toner images are thereby permanently bonded with the paper web12. The fixing units54,56are executed as radiation fixing units, whereby the fixing units54,56respectively comprise an occlusion unit58,60that masks the radiation of the fixing units54,56during operating states in which no fixing of the toner images on the paper web12should occur. Viewed in the transport direction of the paper web12, cooling elements62,64that cool the paper web12before the exit from the fixing station30are provided after the fixing units54,56in order to prevent a damage to the paper web12, in particular as a consequence of too little paper moisture. A further drive roller50with opposing contact pressure roller52is provided in the fixing station30for paper extraction.

The drive speed of the transfer belts78,78′ contacting the paper web12is set somewhat higher than the transport speed of the paper web12, such that a speed difference between the paper web12and the transfer belts78,78′ is preset. The speed difference is advantageously in the range from 0.1% to 10% (advantageously 0.5% to 3%) when the transfer belts78,78′ are pivoted onto the paper web12and contact this. The speed difference serves to hold the relatively elastic paper web12under tension at the transfer printing point84and therewith to avoid paper travel problems such as paper fluttering. If the transfer belt78is pivoted onto the paper web12as described, a tensile force of the transfer belt78acts on the paper web12in its transport direction P1as a consequence of the higher speed.

In a region before the transfer printing point84in the transport direction P1of the paper web12, a measurement arrangement90is arranged between the roller pair42,44and the transfer printing point84, which measurement arrangement90comprises a roller92that is pressed from below against the paper web12. The measurement arrangement90is designed such that the roller92is pressed with a relatively constant force against the underside (back side) of the paper web12and the deflection of the roller92is detected. The deflection of the roller92is dependent on the deflection of the paper web12that is effected by the force exerted on the paper web12by the roller92. Depending on the tensile stress of the paper web12in the region between the roller pair42,44and the transfer printing point84, the paper web12is deflected only slightly by the force introduced by the roller92given a relatively large tensile stress and relatively significantly given a relatively slight tensile stress. The force exerted on the paper web12by the measurement arrangement90via the roller92acts essentially orthogonal to the plane in which the paper web12is conveyed in the region between the rollers42,44and the transfer printing region84. The tensile stress of the paper web12in the region between the roller pair42,44and the transfer printing point84significantly depends on the speed difference between the transfer belts78,78′ and the paper web12in the transfer printing region84. Due to the deflection of the paper web12as a result of the force exerted on the paper web12by the measurement arrangement90, the tension of the paper web12can thus be detected as a measure for the speed difference between the transfer belts78,78′ and the paper web12.

Alternatively, the paper web12can be deflected by a preset amount with the aid of the measurement device90, Whereby a different force is required for deflection dependent on the tensile stress of the paper web12in the region of the measurement arrangement90. The force required by the measurement arrangement90for deflection of the paper web12is detected with the aid of the measurement arrangement90, whereby the required force serves as a measure for the tension of the paper web and thus for the speed difference between the transfer belts78,78′ and the paper web12.

A block diagram for adjustment of the speed difference between the transfer belts78,78′ and the paper web12according to a first embodiment of the invention is shown inFIG. 2. The same elements are provided with the same reference characters. The value (detected with the aid of the measurement arrangement90) of the tensile stress of the paper web12is supplied to the control unit22, which comprises a regulator module for regulation of the difference speed between the transfer belts78,78′ and the paper web12.

The control unit22compares the value (determined for the first time with the aid of the measurement arrangement90) for the speed difference between the transfer belts78,78′ and the paper web12with a predetermined desired value and determines the control deviation between the determined real value and the predetermined desired value. The control unit controls the drive speeds of the photoconductor belts68,68′ as well as of the transfer belts78,78′ and/or the drive speed of the paper web12dependent on this deviation. The drive speeds of the respective transfer belt78,78′ and of the photoconductor belt68,68′ associated with the same printing unit14,16are respectively altered identically, advantageously by the same amount. The drive speeds of the transfer belts78,78′ and of the photoconductor belts68,68′ of the upper first printing unit16and of the lower second printing unit18are also varied in the same manner, such that the association and positioning of the print images on the front side of the paper web12and the back side of the paper web12are further retained and the print images on the front side and the back side are arranged in register relative to one another. It is in particular to be observed that, in the high-capacity printing system according toFIG. 1, the path which the toner image travels on the transfer belt78of the upper first printing group is longer than the transport path of a toner image on the transfer belt78′ of the second, lower printing unit.

Via the alternative or additional modification of the drive speed of the paper web12the form length that specifies the length of the print image of a print side is changed slightly since then the drive speed of the paper web12no longer coincides with the writing speeds of the character generators72,72′. The speed changes to be expected for regulation of the speed difference between the transfer belts78,78′ and the paper web12occur only by some thousandths or sub-thousandths, whereby the form length likewise changes only in this range and thereby lies within the required precision.

In addition to or as an alternative to the change of the drive speed of the paper web12, the change of the drive speed of the transfer belts78,78′ as well as of the photoconductor belts68,68′ can in particular occur when the print images on the paper web12are regulated in an unregulated print operation without a regulation of the position of the print image to margin holes provided on the edge of the paper web12or position markers printed on the paper web.

A block diagram for adjustment of the speed difference between the transfer belts78,78′ and the paper web12is shown in regulated print operation inFIG. 3according to a second embodiment of the invention. In the regulated print operation the positions of the print images printed on the paper web12are controlled or regulated with the aid of margin holes provided in the lateral margin regions of the paper web12and/or with the aid of position markers printed on the paper web12. In this embodiment the drive speed of the paper web12cannot be varied to adjust or regulate the speed difference between the transfer belts78,78′ and the paper web12since the drive speed of the paper web12is predetermined by the marker or hole regulation for positioning of the print images. Thus only the drive speed of the transfer belts78,78′ as well as, in the same manner, the drive speeds of the photoconductor belts68,68′ can be varied to adjust the predetermined speed difference.

In the regulated print operation the initial position of the print images to be generated is additionally changed in synchronization with the change of the drive speed of the transfer belts78,78′, and thus the desired value for marker or hole regulation is in particular adapted to the changed drive speeds of the transfer belts78,78′. This adaptation of the initial position of the print images is required since the time for transport of the print image from the character generators72,72′ up to the transfer printing of the print images from the transfer belts78,78′ onto the paper web12in the transfer printing region84are changed via the modification of the drive speeds of the photoconductor belts68,68′ and of the transfer belts78,78′. The print image thus reaches the transfer printing region84at a different point in time, i.e. too early or too late after adaptation of the drive speeds, and without a compensation of the initial position of the print images would thereby be incorrectly positioned.

In the described embodiment of the invention the respective belts68,68′,78,78′ and/or the paper web12can be stopped and the speed specifications can be adapted to change the drive speeds of the photoconductor belts68,68′ or of the transfer belts78,78′ as well as additionally or alternatively to change the drive speed of the paper web12. The high-capacity printing system10is subsequently restarted again to continue a begun printing process or to begin a further printing process. Alternatively or additionally, the drive speeds of the transfer belts78,78′ of the photoconductor belts68,68′ and/or of the paper web12can also be altered during the printing operation of the high-capacity printing system10, in particular between two print images to be generated in succession.

The measurement of the paper tension of the paper web12between the propulsion of the paper web12with the aid of the rollers42,44and the transfer printing point84occurs with the aid of the roller9of the measurement arrangement90, which roller92is abutting on the underside of the paper web12. Alternatively, the roller92can be arranged above the paper web12and press on the paper web12from above. Instead of a roller92that essentially extends over the entire width of the paper web12, only one roller can also be provided that presses on the paper web12in a relatively narrow region. The paper tension is determined with the aid of the measurement device90, either in that the force that the paper web12exerts on the roller92or with which the roller92must be pressed against the paper web12in order to deflect the paper web12is measured or in that the deflection of the roller92that is pressed against the paper web12with an essentially constant force is alternatively measured. The deflection can also be determined when the roller92is pressed against the paper web12with a force dependent on the deflection.

The speed difference between the transfer belts78,78′ and the paper web12can be detected and regulated continuously in the described exemplary embodiments. Changes in the speed difference occurring in the operation of the high-capacity printing system, which changes can in particular occur due to a marker regulation or a hole regulation in a regulated print operation, are detected and corrected with the aid of the regulation or are limited to an allowable deviation. The speed difference can thereby be set to a relatively small desired value since, via the continuous detection of a deviation of the speed difference from this preset value, an immediate correction of the deviation is possible and the speed difference thereby does not have to be set unnecessarily high in order to be able to ensure the presence of a higher speed of the transfer belts78,78′ relative to the drive speed of the paper web12even given disadvantageous conditions.

In a high-capacity printing system according toFIG. 1, the speed difference had to be set to a value of approximately 1% of the drive speed of the paper web12or of the transfer belts78,78′ without the detection of the deviation of the actual speed difference from a desired speed difference. Problems in the print image positioning and the print quality result due to such a relatively high speed difference. These problems can be significantly reduced or entirely corrected via a smaller speed difference. The speed difference can in particular be set to a value in the range from 0.1% to 0.95% of the drive speed of the transfer belts78,78′ or of the drive speed of the paper web12and thereby be regulated to this value.

The detection of the speed difference can also occur in printers or copiers that comprise only one printing unit16,18for generation of a print image on the paper web12. Other carrier materials (both web-shaped carrier materials and sheet-shaped carrier materials, in particular sheet-shaped paper) can also be printed both with the printing system10shown inFIG. 1and with alternative printer or copier systems. Given printing of individual sheets, the leading and/or trailing edge of a single sheet can be detected for print image positioning on the individual sheet.

In the explained exemplary embodiments transfer belts78,78′ are provided as image carriers between which and the paper web12to be printed wherein the speed difference between the image carriers and the paper web12to be printed is generated and detected. As an alternative to the transfer belts78,78′, other image carriers (such as, for example, transfer drums, photoconductor drums or photoconductor belts) can also be provided between whose surface (on which a toner image is arranged) and the side of the paper web12(or an alternative carrier material) to be printed a speed difference is generated.