Printer or copier for simultaneously printing a supporting material on both sides

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a printer or copier with a transfer station for simultaneous both-sided printing of a carrier material. The invention is also directed to a corotron device that can be utilized in the transfer station.

2. Description of the Related Art

High-performance printers and high-performance copiers often have the capability of printing the front side and the back side of a carrier material, for example paper. This operating mode is also called duplex printing. It is known to first print one side, for example the front side, with a toner image and to subsequently turn the carrier material over. It is then reconveyed to the same printing station in order to then print the second side, usually the back side, with a second toner image. This type of duplex printing is known both for web-shaped material as well as for a single-sheet carrier material. In such a printing mode, the overall throughput is not high due to the additional transport and the turn-over of the carrier material. Given another known solution, a printer or copier system is given two printing units, whereby each printing unit prints one side of the carrier material. In this case, considerable space for the two printer units is required within the system and the technological outlay is high.

Given a printer device disclosed by U.S. Pat. No. 5,526,107, continuous form paper is supplied to a transfer printing location of a photoconductive cylinder that has electro-photographic units at two surfaces for producing differently colored toner images. At the transfer printing location, the continuous form paper is printed with a first color on the front side; subsequently, the continuous form paper is redirected and is supplied to a printing location at the same photoconductive cylinder lying opposite the transfer printing location and the back side is printed there.

European Patent Document EP-A-0 320 985 discloses that a transfer band is employed, this carrying toner images that have been transferred from a photoconductive drum onto the transfer band. German Patent Document DE-A-197 13 964, which is identical in content with U.S. Pat. No. 5,797,077, discloses a transfer station for simultaneous printing of both sides of a carrier material (duplex printing). The transfer station contains a pivotable transfer printing station that holds a transfer band away from the carrier material in a first position, so that no toner images are transferred onto this carrier material. In this position, toner images are produced superimposed on the transfer band in order to enable a multi-color printing. In a second position, the transfer station is pivoted against the carrier material and transfers the multi-color toner image.

Published PCT application WO 87/02792 discloses a corotron device having a corotron electrode whose cooperating electrode is implemented as a metal plate. This metal plate lies at ground potential. The electrical field generated between the corotron electrode and the cooperating electrode leads to a charge influencing of the toner particles.

SUMMARY OF THE INVENTION

An object of the present invention is to create a printer or copier that enables a simultaneous printing of front side and back side of a carrier material given low outlay and with high printing quality.

This object is achieved by a printer or copier having a transfer station for the simultaneous both-sided printing of a carrier material, whereby a first endless transfer band of a first transfer module carries toner particles of a first polarity in the region of a transfer printing location, a second endless transfer band of a second transfer module carries toner particles of a second polarity in the region of the transfer printing location, the carrier material is guided at the transfer printing location between the first transfer band and the second transfer band, an electrostatic field is generated at the transfer printing location that effects that the toner particles of each and every transfer band separate from the respective transfer band as a result of electrostatic forces and adhere to the surface of the carrier material lying opposite the respective transfer band, whereby each transfer module contains a switchable transfer printing station, that, in a first operating mode (a collecting and printing mode), initially keeps the respective transfer band at a distance from the carrier material, whereas a plurality of toner images are arranged on top of one another on the respective transfer band, and the carrier material does not move forward at the transfer printing location, and then conducts the respective transfer band close to the carrier material in order to transfer the toner images arranged on top of one another there onto in common, and that, in a second operating mode (a continuous printing mode) conducts the respective transfer band close to the carrier material in order to continuously print monochromatic toner images onto the carrier printer.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are explained below on the basis of the drawings.

FIG. 1

is a schematic sectional view of an electrophotographic printer device for monochromatic and/or colored, single-sided or both-sided printing of a web-shaped carrier material, whereby the transfer station of the invention can be utilized;

FIG. 2

is a schematic sectional view of a printer device according to

FIG. 1

that prints the carrier material on both sides;

FIG. 3

shows schematically, an arrangement of critical parts of the transfer station with charge reversal of the toner particles;

FIG. 4

is a detailed illustration of the arrangement according to

FIG. 3

for explaining the function;

FIG. 5

is an electrical equivalent circuit diagram that reflects the resistance conditions and conditions of the current at the transfer printing location;

FIG. 6

is a side view of an arrangement similar to that of

FIG. 3

with a negative toner system;

FIGS. 7

a

,

7

b

,

7

c

and

7

d

are electrical diagrams that show schematically, the possible relationships of potential at the transfer printing drums;

FIG. 8

is a side view of an exemplary embodiment wherein the transfer bands partially wrap around the transfer drums;

FIG. 9

is a side view of an exemplary embodiment with guide rollers;

FIG. 10

is a detailed illustration of the arrangement according to

FIG. 9

;

FIG. 11

is an arrangement similar to that of

FIG. 9

, whereby the electrical field required for the transfer printing is built up between the guide roller and the transfer drum;

FIGS. 12

a

and

12

b

are an electrical equivalent circuit diagram directed to the exemplary embodiment according to

FIG. 11 and a

perspective view of the drums;

FIG. 13

is an arrangement according to

FIG. 11

, whereby additional delivery rollers are provided;

FIG. 14

is an exemplary embodiment with deflection bows;

FIG. 15

is an enlarged side view showing the conditions of the current in the exemplary embodiment according to

FIG. 14

;

FIG. 16

shows the exemplary embodiment according to

FIG. 14

with insulated deflection bows;

FIG. 17

shows an exemplary embodiment having electrically conductive deflection bows that are conducted to ground potential via a resistor;

FIG. 18

is a side view of an exemplary embodiment similar to that of

FIG. 13

;

FIGS. 19

a

,

19

b

and

19

c

are perspective views of a plurality of exemplary embodiments for a transfer drum;

FIG. 20

id s perspective view of a transfer drum composed of high-impedance material having lateral electrode terminals;

FIG. 21

is a perspective view of a transfer drum having an electrically conductive core in a high-impedance coating;

FIG. 22

a

is a side view of a charge reversal corotron device having two corotron wires and two cooperating electrodes fashioned as blades and

FIG. 22

b

is an enlarged detail view of the blades;

FIG. 23

is a detail view of a charge reversal corotron having a corotron wire and a blade utilized as a cooperating electrode, whereby the field lines of the effective electrical field are indicated;

FIG. 24

is a perspective view of a cooperating electrode that is executed as a blade;

FIG. 25

is an enlarged illustration of a portion of a blade, whereby the blade is serrated;

FIG. 26

is a perspective illustration of a cooperating electrode that is composed of an arrangement of individual pins;

FIG. 27

is a perspective illustration of a cooperating electrode that is composed of a wire; and

FIG. 28

is a side view of a transfer printing corotron device having a corotron wire and having a cooperating electrode fashioned as a blade.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following preferred embodiments are presented without limitation to the scope of the claims.

According to the present invention, two transfer bands reside opposite one another at the transfer printing location, the toner particles thereof having different polarities. An electrostatic field is then generated that is directed such that the toner particles are repelled both from the first transfer band as well as from the second transfer band and agglomerate onto the respective surfaces of the carrier material. In this way, a simultaneous transfer printing is achieved. The transport path of the carrier material remains short since the carrier material need not be conducted past two printing stations or, respectively, past one printing station twice. Additionally, an interim fixing of the toner image which is transferred onto the carrier material is eliminated, as a result whereof the technical outlay is reduced and the printing quality remains high.

In a preferred exemplary embodiment, both transfer bands have toner images with toner particles of the same polarity in a section preceding the transfer printing location as seen in the delivery direction of the carrier material, whereby a transfer corotron is arranged preceding the transfer printing location along one of the transfer bands, this generating an electrical field that reverses the polarity of the toner particles on this transfer band as a result of the charge reversal. As a result of these measures, a uniform toner system, for example a positive or negative toner system with positive or, respectively, negative charging of the toner image, can be employed for both transfer bands. Accordingly, the printing quality is nearly identical on both sides of the carrier material.

Another exemplary embodiment is characterized in that two transfer drums reside opposite one another at the transfer printing location, and in that a DC voltage is applied to the transfer drums that generates the electrical field for the transfer printing of the toner particles. The transfer drums assure, on the one hand, a precise guidance of the carrier material and of the transfer bands in the region of the transfer printing location. On the other hand, they enable the build-up of an electrical field in the region of the transfer printing location in a simple way.

In practice, an exemplary embodiment has proven itself wherein two guide elements are arranged in front of the transferred drums as viewed in the delivery direction of the carrier material, the transfer bands and the carrier material being conducted between the two guide elements. In this way, the transfer bands and the carrier material are guided along a relatively great path distance given mutual touching. The fogging effect is reduced at the transfer printing location since the toner particles have only a slight or no spacing from the surface of the carrier material and a locationally exact transfer printing thus ensues.

According to another aspect of the invention, a corotron device is provided, this corotron device can be advantageously employed in conjunction with the transfer modules.

For printing a final image carrier, for example paper, a transfer of a toner image existing on an intermediate carrier onto the final image carrier is undertaken mechanically, thermodynamically or electrostatically. For an electrostatic transfer of the toner image from a photoconductive band onto an intermediate carrier or onto a final image carrier, the toner particles must have a certain voltage potential. The electrostatic transfer of the toner particles ensues due to forces in the electrical field and is based on a difference in potential between the toner particles and the final image carrier onto which the toner image is to be transferred. The force as a result of the electrical field must thereby be greater than the bonding forces with which the toner particles are held on the intermediate carrier for toner images from which they are to be transferred.

In electrographic printer and/or copier devices, dry toner particles are utilized for the electrographic transfer with a suitable voltage potential, so that the transfer of the toner particles onto a material can be implemented without additional charge influencing of the toner particles in the printer or copier. When the final image carrier is to be printed on both sides (duplex printing), the final image carrier must be turned over or a simultaneous or time-offset transfer of the toner particles ensues from both sides onto the final image carrier. In order to realize the transfer without interim fixing of the toner image transferred onto the final image carrier, the toner particles on the first side of the intermediate carrier must have a difference in potential compared to the toner particles of the second side. Preferably, the toner particles are reversed in charge from a positive voltage potential to a negative voltage potential with reference to the ground potential. The toner particles can thus be transferred simultaneously or time-offset without interim fixing, being transferred from the intermediate carrier onto the final image carrier from both sides. The toner particles on both sides of the final image carrier attract each other through the final image carrier due to their different potentials and/or are attracted by the difference in potential compared to the final image carrier, so that they adhere on the final image carrier.

After the transfer process, toner particles remain adhering to the intermediate carrier from which they are to be transferred, i.e. they have not been successfully transferred. This is thereby a matter of toner particles of a few percent of the toner image, usually substantially less than 20%. These toner particles that have not been transferred usually have a low or an incorrect voltage potential. In order to carry out a further transfer of these untransfered toner particles, for example for cleaning the intermediate carrier, with a high efficiency, it is necessary to charge the toner particles to a defined potential. This charging event ensues with a corotron device. The intermediate carrier thereby forms the cooperating electrode for the corotron device. When the intermediate carrier is of a conductive material having a specific resistance of less than 10

6

ohms cm, then the intermediate carrier is applied to ground potential or to some other suitable voltage potential and thus serves as a cooperating electrode. When the intermediate carrier, for example in the case of a photoconductor, is provided with a light-sensitive cover layer whose dark resistance is an extremely high-impedance (for example, above 10

6

ohms cm), a cooperating electrode must be arranged at the back side of the intermediate carrier. Cooperating electrodes are preferably implemented as metal plates or as conductive deflection rollers. Since deflection rollers involve high mechanical outlay, increased space requirements and high costs, metal plates are mainly utilized as the cooperating electrodes.

The cooperating electrode should have a low transfer resistance compared to the intermediate carrier. The intermediate carrier is conducted passed the stationary cooperating electrode in a non-contacting fashion. In order to achieve the low transfer resistance, the intermediate carrier must be conducted past the fixed cooperating electrode at a slight distance. This distance preferably amounts to 0.2 mm through 1.0 mm. The forces between two bodies whose difference in potential generates an electrical field are comparable to the forces between two plates of a plate capacitor, whereby one plate of the plate capacitor is formed by the cooperating electrode and the other plate is formed by the underside of the intermediate carrier. This force leads to deflection of the web-shaped intermediate carrier in the direction of the cooperating electrode at the cooperating electrode, so that the carrier touches and adheres to it. As a result of the contact between the moving web-shaped intermediate carrier and the stationary cooperating electrode, adhesion and sliding friction arise. The mechanical energy required in addition to the drive of the intermediate carrier due to this friction between the intermediate carrier and cooperating electrode must be provided by the drive unit of the intermediate carrier. Moreover, the intermediate carrier and/or the cooperating electrode becomes worn as a consequence of the sliding friction.

A corotron device is provided wherein low attractive forces occur between the intermediate carrier for toner images and the cooperating electrode and the charge carrier exchange is assured.

According to the new corotron device, the cooperating electrode has conductive elevations whose end points project in the direction of the corotron wire and that lie in a plane parallel to the longitudinal axis of the corotron wire. As a result of this fashioning of the cooperating electrode, what is achieved is that the attractive force between the intermediate and the cooperating electrode is substantially reduced. This attractive force is critically dependent on the effective area. The critical effective area is the area of the cooperating electrode facing toward the intermediate carrier. As a result of the arrangement of electrically conductive elevations, whose end points represent the critical effective area, it is assured that the effective area and, thus, the attractive forces between the intermediate carrier and the cooperating electrode are low. What is also achieved as a result of this arrangement is that an intensified exchange of charge carriers as a consequence of a spike discharge occurs due to the curvatures at the elevations.

A preferred embodiment provides that the elevations of the cooperating electrode are arranged along the longitudinal axis of the corotron wire. What is thereby achieved is that the electrical field for influencing the charge of the toner particles is uniformly formed and the arrangement of the cooperating electrode is possible in a space-saving fashion. Another embodiment is characterized in that the cooperating electrode contains individual pins as elevations. What is thereby achieved is that the cooperating electrode can be cost-beneficially manufactured of standardized component parts.

Another preferred embodiment is comprised therein that the cooperating electrode contains acutely tapering elevations. What is thereby achieved is that the effective area of the cooperating electrode and, thus, the attractive force between the intermediate carrier for toner images and the cooperating electrode is reduced further.

According to another aspect, it is provided in a corotron device for an electrographic printer and/or copier device that the cooperating electrode is fashioned like a blade having a cutting edge, whereby the cutting edge is arranged in parallel to the longitudinal axis of the corotron wire. What this development achieves is that, for example, a sheet metal plate that is arranged perpendicular to the intermediate carrier for toner images and proceeds over the width of the intermediate carrier for toner images, is utilized as the cooperating electrode. Such an arrangement is space-saving and cost-beneficial. What is also achieved by this arrangement is that an automatic exchange of charge carriers (to a spike discharge) arises due to the curvatures at the cutting edge.

Another beneficial embodiment of the corotron device provides that the cutting edge of the cooperating electrode is serrated and that the serrations taper in the direction of the corotron wire, so that the end points and/or end surfaces of the serrations project in the direction of the corotron wire and lie parallel to the longitudinal axis of the corotron wire. What is thereby achieved is that the effective area on which the amount of the attractive force between the intermediate carrier for toner images and cooperating electrode is dependent is reduced compared to the continuous blade, as a result whereof the attractive force is reduced farther. The spike discharge is promoted further.

According to another aspect, it is provided in a corotron device for an electrographic printer and/or copier device that the cooperating electrode is formed with a wire whose longitudinal axis is arranged parallel to the longitudinal axis of the corotron wire. What is achieved with this development is that, for example, a corotron wire is also provided as a cooperating electrode. This wire proceeds over the width of the intermediate carrier. Such an arrangement is space-saving and reduces the number of component parts utilized.

FIG. 1

shows a printer device for monochromatic and/or colored, single-sided or both-sided printing of a web-shaped carrier material, for example a paper web. The printer device is modularly constructed and has a delivery module M

1

, a printer module M

2

, a fixing module M

3

and a post-processing module M

4

. The delivery module M

1

contains elements for delivering a continuous form paper taken from a stacker to the printing module M

2

. This printing module M

2

contains the transfer station that prints the carrier material that is subsequently fixed in the fixing module M

3

and cut and/or stacked in the post-processing module M

4

.

The printer module M

2

contains the units required for printing a web-shaped carrier material with toner images, these units being arranged at both sides of a transport channel

11

for the carrier material

10

. These units essentially comprise two differently configurable electrophotographic modules E

1

and E

2

with appertaining transfer modules T

1

and T

2

that, together, form the transfer station T. The modules E

1

and T

1

are allocated to the front side of the carrier material

10

; the modules E

2

and T

2

are allocated to the back side of the carrier material

10

.

The essentially identically constructed electrophotographic modules E

1

and E

2

contain a preferably seamless photoconductive band

13

conducted over deflection rollers

12

and electro-motively driven in an arrow direction that, for example, is an organic photoconductor, also referred to as OPC. The units for the electrophotographic process are arranged along the light-sensitive outside of the photoconductive band

13

. These units serve the purpose of generating toner images allocated to the individual color separations on the photoconductive band

13

. To this end, the photoconductor

13

moving in the arrow direction is first charged to a voltage of approximately −600 V with the assistance of a charging device

14

and is discharged to approximately −50 V dependent on the characters to be printed and with the assistance of a character generator

15

composed of an LED comb.

The latent charge image generated in this way and situated on the photoconductor

13

is then inked with toner with the assistance of developer stations

16

/

1

through

16

/

5

. Subsequently, the toner image is loosened with the assistance of the intermediate illumination means

17

and is transferred onto a transfer band

19

of the transfer band module T

1

in an intermediate transfer printing region

18

with the assistance of a transfer corotron device

20

. Subsequently, the entire photoconductive band

13

is discharged over its entire width with the assistance of the discharge corona device

21

and is cleaned of adhering toner dust via a cleaner device

22

having cleaning brushes. A subsequent intermediate illumination device

23

sees to a corresponding charge-wise conditioning of the photoconductive band

13

which, as was already set forth, is then uniformly charged with the assistance of the charging device

14

.

Toner images allocated to individual color separations are generated with the electrophotography module E

1

or, respectively, E

2

, the totality of these color separations forming the color image to be printed. To this end, the developer stations

16

/

1

through

16

/

5

are fashioned to be switchable. They respectively contain the toner allocated to an individual color separation. For example, the developer station

16

/

1

contains black toner, the developer station

16

/

2

contains toner having the color yellow, the developer station

16

/

3

contains toner having the color magenta, the developer station

16

/

4

contains toner having the color cyan and, for example, the developer station

16

/

5

contains blue toner or toner of a special color. Both single-component as well as two-component toner/developer stations can be employed as developer stations. Preferably, single-component toner developer stations are utilized, these working with fluidizing toner as known, for example, from U.S. Pat. No. 477,106 (Applicant: Fotland). The subject matter of this U.S. Patent is likewise a component part of the present disclosure and is incorporated herein by reference.

In order to achieve the switchability of the developer stations, i.e. in order to be able to individually actuate each individual developer station, these stations, given employment of fluidizing toner, can be fashioned in conformity with German Patent Application DE 196 52 866. The switching of the developer station accordingly ensues by changing the electrical bias voltage of the transfer drum or, respectively, by changing the electrical bias voltage of the applicator drum. It is also possible to switch the developer stations in that they are mechanically shifted and are thereby brought into contact with the photoconductive band

13

. Such a principle is known, for example, from German Patent Document DE-A-196 18 324.

During operation of the printer device, a toner image that is allocated to an individual color separation is respectively generated by a single developer station with the assistance of the developer stations

16

/

1

through

16

/

5

. This toner image is then electrostatically transferred onto the transfer band

19

via the transfer printing device

18

in combination with the transfer corona device

20

. The transfer module T

1

contains this transfer band

19

, which is composed of a rubber-like substance, is conducted around a plurality of deflection devices and is motor-driven. The transfer band

19

is fashioned as an endless band and without a seam similar to the photoconductor band

13

. It is moved in the arrow direction, namely proceeding from the transfer region with the drum

18

and the transfer corona device

20

to a transfer printing station

24

with transfer drums, is moved therefrom further around the deflection roller

25

to a cleaning station

26

and from the latter is in turn moved to the transfer region

18

and

20

with the deflection roller

27

arranged thereat.

The transfer band

19

in the transfer module T

1

serves as a collector for the individual toner images allocated to the color separations that are transferred onto the transfer band

19

via the transfer device

18

and

20

. The individual toner images are thereby arranged above one another, so that an overall toner image corresponding to the color image arises. In order to be able to generate the overall toner image and in order to be able to then transfer it onto the front side of the carrier material

10

, the transfer module T

1

contains a switchable transfer station

24

. This, corresponding to the illustration in

FIG. 1

, can contain a plurality of mechanical displaceable transfer printing drums

28

with appertaining transfer printing corona device

29

. In the operating condition of “collecting”, the transfer printing drums

28

and the transfer printing corona device

29

are shifted upward according to the arrow direction, so that the transfer band

19

is spaced from the carrier material

10

. The individual toner images are taken from the electrophotographic module E

1

in this condition and are superimposed on the transfer band

19

. The cleaning station

26

is deactivated by being pivoted out. The carrier material

10

is at rest in the region of the transfer printing station

24

in this operating condition.

The electrophotography module E

2

and the transfer module T

2

for the backside of the recording medium

10

are constructed corresponding to the modules E

1

and T

1

. Here, too, an overall toner image is generated on the transfer band by collecting individual toner images for the backside, whereby the corresponding transfer printing station

24

is also pivoted away here in the operating condition of “collecting”.

For a simultaneous printing of the front side and back side of the carrier material

10

, the transfer bands

19

of the transfer module T

1

and T

2

are simultaneously brought into contact with the carrier material

10

in the region of the transfer printing stations

24

and the carrier material

10

is thereby moved. At the same time, the cleaning stations

26

of the transfer modules T

1

and T

2

are pivoted in and activated. After transfer of the two toner images onto the front side or, respectively, the back side of the carrier material

10

, toner image residues adhering to the transfer bands

19

are removed by the cleaning stations

26

. This is again followed by a collecting cycle for generating new toner images, whereby the transfer bands

19

are pivoted out and the carrier material

10

is at a standstill. The transfer of the toner images from the transfer modules T

1

and T

2

onto the carrier material

10

thus ensues given a start-stop operation of the carrier material

10

.

The carrier material

11

is moved in the paper transport channel with the assistance of motor-driven transport drums

38

. In the region between the transport drums

38

and the transfer printing stations

24

, charging or, respectively, corona devices

39

can be arranged for paper conditioning so that the paper

10

is, for example, uniformly set in terms of charge before the transfer printing.

So that the carrier material

10

composed of paper does not tear given the start-stop mode and can also be continuously supplied, the delivery module M

1

contains a loop forming means

30

. This loop forming means

30

functioning as a web storage buffers the carrier material

10

which is continuously taken off from a stack holding device

31

.

After the transfer printing of the two chromatic toner images in the region of the transfer printing stations

24

onto the carrier material

10

, these must still be fixed. The fixing model M

3

serves this purpose. It contains an upper and a lower row of infrared radiators

32

between which the paper transport channel for the carrier material

10

proceeds. The toner image located both on the front side as well as on the back side of the carrier material

10

and fixed by the infrared radiators

32

is still hot and soft and is guided free in non-contacting fashion over a deflection drum

33

arranged at the output side following the region of the infrared radiators

32

. The fixing ensues with the heat generated by the infrared radiators

32

. A cooling of the carrier material

10

as well as a smoothing, for example via corresponding decurler devices, ensues in a cooling path with cooling elements

34

and deflection rollers

35

following the infrared radiators

32

. Lower-driven air chambers can serve as cooling elements

34

. After the fixing of both toner images and cooling, a corresponding post-processing of the carrier material

10

ensues within the post-processing module M

4

that, for example, can contain a cutter device

36

with stacking device

37

.

A microprocessor-controlled control means ST coupled to the device controller GS serves the purpose of being able to realize the various operating conditions, the control means ST being in communication with the components delivery module M

1

, printer module M

2

and fixing module M

3

or, respectively, post-processing module M

4

to be controlled and regulated. Within the modules, it is coupled to the individual units, thus, for example, to the electrophotography modules E

1

and E

2

and to the transfer modules T

1

and T

2

. A control panel B via which the various operating conditions can be input is connected to the device controller GS or, respectively, to the control ST, which can be a component part of the device controller. The control panel B can contain a touch screen picture screen or, respectively, a personal computer PC with a coupled keyboard. The control itself can be conventionally constructed.

Given the embodiment according to

FIG. 2

, the electrophotography modules E

1

and E

2

contain two devices B

1

and B

2

that work independently from one another and generate images. The first image generating device B

1

contains a character generator

15

, a charging device

14

, an intermediate illumination device

23

, a cleaning device

22

, a discharge corotron device

21

and a developer station

16

/

1

. The second image-generating device B

2

is constructed analogous thereto with a charging device

14

, character generator

15

, a development station

16

/

2

and an intermediate illumination device

17

. The developer station

16

/

1

can be allocated to a first color, for example black, and the developer station

16

/

2

can be allocated to a second color, for example blue or some other color. With the assistance of the electrophotography modules E

1

or E

2

, it is thus possible to generate a first toner image having the color black and to superimpose a toner image having the additional color on this black toner image with the second image-generating device B

2

. The toner image (spot color toner image) superimposed is in this way is then transferred onto the transfer modules T

1

and T

2

and is transferred from the latter directly onto the carrier material

10

. It is thus possible to apply two-color toner images on both sides to the continuously moved carrier material

10

. When only one of the image-generating devices B

1

or B

2

is activated, monochrome printing is continuously carried out. In both operating modes, the transfer modules T

1

and T

2

serve merely for the transfer of the toner images without needing the operating mode of “collecting”. However, one can also imagine that both image-generating devices B

1

and B

2

be actuated in alternation and the transfer modules T

1

and T

2

in the operating mode of “collecting”, as initially set forth.

The transfer devices T

1

and T

2

shown in

FIGS. 1 and 2

belong to the transfer station T, whose critical parts are explained below with reference to

FIGS. 3 through 21

.

FIG. 3

shows an exemplary guide example of the transfer station T, whereby two transfer drums are utilized. The toner image

44

for the front side of the carrier material

43

is located on the transfer band

41

. The toner image

45

for the backside of the carrier material

43

, which is preferably a paper web, is located on the second transfer band

42

. The two toner images

44

and

45

have, for example, been transferred onto the transfer bands

41

and

42

with the assistance of the electrophotographic devices E

1

and E

2

according to FIG.

1

. In the present instance according to

FIG. 3

, a positive toner system is employed, i.e. the toner particles have positive electrical charges after the application of the toner images

44

and

45

, as indicated in FIG.

3

. The carrier material

43

, which is conveyed in the direction of the arrow P

1

, is located between the two transfer bands

41

and

42

. Two electrically conductive transfer drums

49

a

and

49

b

guide the transfer bands

41

and

42

such that they touch the carrier material

43

. An electrical DC voltage U is applied to the transfer drums

49

a

and

49

b

, the voltage being supplied from a DC voltage source

40

. The transfer printing process ensues in the region of the transfer drums

49

a

and

49

b

facing toward one another, whereby toner particles are transferred from the transfer bands

41

and

42

onto the respective surface of the carrier material

43

. This region is also referred to as a transfer printing location. A transfer printing corotron

47

a

is arranged at the transfer band

42

preceding the transfer printing location, the corotron

47

a

being supplied with negative DC voltage compared to ground from a DC voltage source

48

. A ground electrode

47

b

resides opposite the transfer charge reversal corotron

47

a.

Fundamentally, the transfer bands

41

and

42

can be composed of an insulating material or of a conductive material. The aim is that the transfer bands

41

and

42

as well as the carrier material

43

have the same surface velocities. Too great a relative motion of the surfaces relative to one another would cause a mechanical smearing of the toner images

44

and

45

and could thus negatively influence the printing quality.

FIG. 4

shows the functioning of the simultaneous transfer printing given employment of a positive toner system. Due to the electrical field generated by the charge reversing corotron

47

a

(shown in FIG.

3

), the polarity of the toner particles arranged on the lower transfer band

42

is reversed, i.e. the toner particles

46

no longer have a positive charge but a negative charge, as indicated in

FIGS. 3 and 4

. The toner particles of the toner image

44

continue to be positively charged. As a result of the voltage U applied to the transfer drums

49

a

and

49

b

, an electrostatic field F forms whose field lines proceed dependent on the shape of the transfer drums

49

a

and

49

b

, i.e. particularly dependent on the radius of curvature. It is indicated in

FIG. 4

that the electrical field F is largely uniform in the plane of the middle axes of the transfer drums

49

a

and

49

b

and becomes less uniform toward the edge along the plane of the carrier material

43

. Dependent on the electrical field strength that can be set with the voltage U, the toner particles of the upper toner image

44

separate from the transfer band

41

and deposit on the front side of the carrier material

43

. Since the potential of the upper transfer drum

49

a

is positive, a repellant force derives for the toner particles of the toner image

44

that effects the agglomeration of the toner particles on the surface of the carrier material

43

. The lower transfer drum

49

b

has a negative voltage potential referred to the potential of the toner particles

46

having a negative charge. Accordingly, these toner particles

46

are repelled from the surface of the lower transfer band

42

, migrate opposite the direction of the electrical field F to the back side of the carrier material

43

and agglomerate thereat.

Isolated toner particles can already separate early in the non-uniform region, for example in the region of the field line F

1

, of the electrical field F. Due to the inhomogeneity of the field and due to the increased distance between the surfaces of the carrier material

43

and the transfer bands

41

and

42

, the point of incidence of the toner particles on the carrier material

43

is not exactly defined; a fogging effect can occur that is known under the technical term of “fogging”. This effect is discussed in greater detail later.

FIG. 5

shows an electrical equivalent circuit diagram that is shown as a circuit having series resistors R. The flowing current i derives from Ohm's law, i.e. the current i is the quotient of the voltage U divided by the sum of the individual resistors R. The aim is that the resistors R of the two transfer drums

49

a

and

49

b

are as small as possible. This can be realized with the assistance of conductive materials, i.e., for example, transfer drums of metal are employed. It is also to be provided that the resistors R of the transfer bands

41

and

42

are as large as possible so that the overall current i remains small. Given a great overall current i, namely, the wear of the transfer bands

41

and

42

is increased. The resistance R of the transfer bands

41

and

42

must, however, assume a finite value so that the electrical field F forms with high intensity at the surface of the respective transfer bands

41

,

42

. When, namely, the resistance R of the transfer bands

41

and

42

is too high, then the effective distance for the electrical field F is increased; it extends from the surface of the transfer drum

49

a

up to the surface of the transfer drum

49

b

. Given the same voltage U, the field strength within the field F is then attenuated. Given a certain conductivity of the transfer bands

41

and

42

, the effective distance for the electrical field F between the transfer bands

41

and

42

is reduced and, thus, the field strength is increased given a voltage U that remains the same.

FIG. 6

shows critical parts of the transfer station T given employment of a negative toner system, i.e. wherein the charges of the toner particles are negatively charged after application onto the transfer bands

41

and

42

. The polarity reversal is again effected by the charge reversing core

47

a

that, however, has positive potential in this case. Likewise, the transfer drums

49

a

and

49

b

are driven with a voltage U, so that an electrical field arises whose field strength reverses compared to the exemplary embodiment of FIG.

3

. The function of the transfer printing corresponds to that described up to now but merely with an inverted operational sign of the charge and of the field strength.

FIGS. 7

a

,

7

b

,

7

c

and

7

d

show the possible relationships of potential at the transfer drums

49

a

and

49

b

. One of the transfer drums

49

a

and

49

b

is at ground in

FIGS. 7

a

and

7

b

. Likewise, one electrode of the DC voltage source

40

is applied to ground.

FIG. 7

c

shows a symmetrical voltage drive whereby the voltage mid-point is applied to ground.

FIG. 7

d

shows an asymmetrical voltage drive for the transfer drums

49

a

and

49

b.

FIG. 8

shows a development of the arrangement according to FIG.

3

. The carrier material

43

to be printed is guided such by the transfer drums

49

a

and

49

b

that it wraps around the transfer drums

49

a

and

49

b

by a respective, predetermined wrap angle. In this way, the region wherein the respective toner image

44

or, respectively,

45

lies against the surfaces of the carrier material

43

is enlarged. Inhomogeneities of the electrical field F at the edge thereof have a less pronounced effect; the fogging effect is reduced.

FIG. 9

shows an exemplary embodiment which is shown in detail in

FIG. 10

wherein two guide rollers

49

c

and

49

d

between which the transfer bands

41

and

42

as well as the carrier material

43

are guided are arranged preceding the transfer drums

49

a

and

49

b

as shown in the feed direction of the carrier material

43

. The two guide rollers

49

c

and

49

d

are applied to ground potential, whereas the voltage U for generating the electrical field is applied to the transfer drums

49

a

and

49

b

. The two guide rollers

49

c

and

49

d

bring the transfer bands

41

and

42

into contact with the carrier material

43

or, respectively, reduce the spacing to a minimum. When, given forward conveying, the toner images

44

and

46

reach the non-uniform region (see

FIG. 4

) of the electrical field and the first toner particles are transferred onto the surface of the carrier material

43

, then the flight path for these toner particles is minimal or, respectively, equal to zero, and a topically exact toner transfer ensues. The fogging effect is avoided in this way and a high print quality is achieved.

FIG. 11

shows a modification of the exemplary embodiment according to FIG.

9

. The lower transfer drum

49

b

and the upper guide roller

49

c

are charged with a voltage potential such that the electrical field F takes effect between the drum

49

b

and roller

49

c

. The transfer drum

49

a

and the guide roller

49

d

are seated in an insulated fashion and have a floating potential. As a result of these measures, the electrical field F required for the transfer printing is effective over a longer distance, so that the transfer printing process proceeds more gently since the effective area on which the transfer of the toner particles from the transfer bands

41

and

42

onto the surface of the carrier material

43

ensues is enlarged.

FIG. 12

a schematically shows the physical relationships on the basis of an electrical equivalent circuit diagram. When the specific material resistance ρ of the transfer bands

41

and

42

employed is low, then relatively high current i result as a result of Ohm's law. Given a permanently applied voltage U, this can yield an undesirably high electrical power P according to the relationship:

P=U·i.

Due to inhomogeneities of the materials for the transfer bands

41

and

42

, local current spikes are possible that allow the electrical field to briefly collapse and, thus, disturb the process of transfer printing. As a result of an increase of the spacing between the drums that form the electrodes for the electrical field, the electrical resistance R of the transfer bands

41

and

42

is increased, as is that of the carrier material

43

. The current i that flows reduces correspondingly on the basis of the relationship

R=ρ·l/A,

wherein R is the overall electrical resistance, ρ is the specific electrical material resistance of the transfer bands, l is the effective material length and A is the effective material cross-section, as shown in

FIG. 12

b.

FIG. 13

shows a combination of the exemplary embodiments according to

FIGS. 9 and 11

. Two delivery rollers

49

e

and

49

f

between which the transfer bands

41

and

42

and the carrier material

43

are guided are arranged preceding the guide rollers

49

c

and

49

d

. The delivery rollers

49

e

and

49

f

carry ground potential, whereas the arrangement of the drums

49

a

and

49

b

and rollers

49

c

and

49

d

as well as the carrying of the potential thereof corresponds to that of FIG.

11

. In this way, an electrically neutral zone arises in the region of the delivery rollers

49

e

and

49

f

, whereby the attractive forces of the toner particles with different potential can be left out of consideration. A premature jumping of toner particles in the region of increased distance between the transfer bands

41

and

42

is thus avoided.

FIG. 14

shows a modification of the arrangement according to FIG.

9

. Grounded deflection bows

49

g

and

49

h

are employed instead of the grounded guide rollers

49

c

and

49

d

. These deflection bows

49

g

and

49

h

can be arranged close to the transfer drum

49

a

and

49

b

, as a result whereof the length of the contact of the transmission bands

41

and

42

with the carrier material

43

is shortened. When the arrangement according to

FIG. 9

is compared to that according to

FIG. 14

, then it can be seen that the minimum path wherein there is contact between the transfer bands

41

and

42

and the carrier material

43

in

FIG. 9

is the sum of the radii of the transfer drums

49

a

or, respectively,

49

b

and of the guide rollers

49

c

or, respectively,

49

d

. When velocity differences dv between the velocity of the transfer bands

41

and

42

and the carrier material

43

occur, then this leads to a mechanical slip and, thus, to an undesired smearing of the toner images to be transferred. The smearing effect is all the greater the longer the contact path is or, respectively, the greater the velocity difference is. The reduction of the velocity difference between the transfer bands

41

and

42

and the carrier material

43

is hardly possible in practice since length tolerances given pre-print forms must be compensated. In order to nonetheless keep the smearing effect low, the length of the contact between transfer band

41

and

42

and the carrier material

43

is reduced according to the exemplary embodiment of

FIG. 14

in that narrow deflection bows

49

g

and

49

h

are employed whose sliding surfaces can be arranged close to the surface of the transfer drums

49

a

and

49

b

. In order to reduce frictional forces, it is meaningful to provide the deflection bows

49

g

and

49

h

with a friction-reducing layer, for example with a layer of a fluorine-containing plastic material, for example PFA, ETFE, FEP, PFDC, Teflon or polyimide (PI). The surface wear of the deflection bows

49

g

and

49

h

can be reduced in that hard, wear resistant materials, for example chromium nickel steel, VA steel, are employed or in that the deflection bows

49

g

and

49

h

are provided with a layer of a wear-reducing material, for example by nickel-plating, by employing silicate or with the assistance of a surface hardening.

FIG. 15

shows the relations of the current given the example of

FIG. 14

, whereby the deflection bows

49

g

and

49

h

lie at ground potential. The overall current Iges derives from the sum of the currents Ium at the transfer printing location and the quadrature-axis currents Iq

1

and Iq

2

. The aim is that

Ium>>Iq

1

+

Iq

2

applies or that

Iq

1

=

Iq

2

=0

applies. When the quadrature-axis current components Iq

1

and Iq

2

, which flow directly into the grounded deflection bows

49

g

and

49

h

through the transfer bands

41

and

42

are undesirably high, then the deflection bows

49

h

and

49

g

can also be arranged so as to be electrically insulated, so that they assume a floating potential (see FIG.

16

).

FIG. 17

shows an exemplary embodiment wherein the deflection bows

49

g

and

49

h

are electrically conductive but are connected to ground potential via a resistor R. In this exemplary embodiment according to

FIG. 17

, too, the quadrature-axis current components are reduced.

FIG. 18

shows a modification of the exemplary embodiment according to FIG.

13

. The delivery rollers

49

e

and

49

f

are replaced by deflection bows

49

i

and

49

j

. These deflection bows

49

i

and

49

j

can be electrically fashioned as indicated in the examples according to

FIGS. 16 and 17

.

FIGS. 19

a

,

19

b

and

19

c

show various embodiments of the transfer drums. In

FIG. 19

a

the transfer drum is cylindrically fashioned and fabricated of an electrically conductive metal, being fabricated as a solid component part. In

FIG. 19

b

, the transfer drum is tubularly fabricated of metal, i.e. is hollow on the inside. The

FIG. 19

c

shows a metallic core that can be composed of solid material or of a tube. This core is provided with a cladding of high-impedance material. The employment of a metallic core for the transfer drum is expedient since it must be fabricated very precisely with little out-of-roundness. In order to minimize concentricity errors, the circumference of the transfer drum and the length of the transfer band should have a whole-numbered ratio relative to one another. The transfer bands, however, have a certain thickness fluctuation that has a disturbing influence on the transfer printing process; for example, a local detachment of the transfer bands from the drum can occur. Advantageously, an elastic coating is therefore applied onto the transfer drum that can compensate slight mechanical tolerances of the component parts on the basis of elastic deformation. This coating should have an electrical conductivity in order to be able to build up a strong electrical field in the transfer printing zone at its outside skin. The electrical conductivity of the coating should lie in the range from 0.5×10

6

through 5×10

12

Ωcm but preferably in the range from 0.5×10

5

through 5×10

9

Ωcm. The elastic coating should have a Shore hardness in the range from 10 through 90 Sh(A), preferably lying in the range from 20 through 70 Sh(A). 0.2 through 15 mm, preferably 0.5 through 2 mm are to be set as thickness of the elastic coating. The elastic coating can additionally have a layer of fluorine-containing plastic material, preferably of PFA, ETFE, FEP, PVDC or Teflon or can be composed of a polyimide layer. The additional layer can also be electrically insulating and have a maximum thickness of 40 μm, preferably 0.1 through 20 μm. The elastic layer can have conductive fillers, preferably lampblack, silicates, oxides added to it, this enabling an increased layer thickness.

FIG. 20

shows a transfer drum that does not have a continuous metallic core but lateral metallic contacting cylinders

50

. The middle part

52

of the cylindrical drum is composed of a high-impedance material. The resistance R over the length 1 of the drum is entered in the figure. It can be seen that the resistance R increases with increase length l, as a result whereof the topical currents i drops over the length l when the voltage U is applied. Different potentials thus derive over the length l, this being undesired.

FIG. 21

shows an exemplary embodiment of a transfer drum having a low-impedance, metallic core

56

on which a coating

54

that is composed of a relatively high-impedance material is applied. The resistance R remains constant over the length l, as a result whereof a constant potential also derives on the surface of the high-impedance jacket coating along the length l. The core can also be manufactured of an electrically conductive plastic, for example of the material PA that contains lampblack particles.

FIG. 22

shows a charge reversing corotron device

110

having two corotron wires

112

and having two cooperating electrodes

114

fashioned as blades. A photoconductor band

116

is provided as an intermediate carrier. However, a transfer band can also be utilized.

The photoconductive band

116

having a toner image

118

that has not yet been fixed and contains positively charged toner particles

120

or, respectively, negatively charged toner particles

122

after the charge reversal is conducted past between the two corotron wires

112

and the two cooperating electrodes

114

, whereby it is guided and driven by deflection rollers

124

. The blades

114

are secured to a holder

126

that also produces the electrical connection to the ground potential of the printer and/or copier device

128

. The corotron wires

112

are surrounded by two shields

130

at that side facing away from the photoconductive band

116

. The photoconductor band

116

is conducted past the cooperating electrodes

114

at a distance in the range from 0.2 mm through 4 mm, preferably in the range from 0.2 mm through 1 mm. The negatively charged toner particles

122

of the latent toner image

118

are reversed in charge due to the electrical field between the corotron wires

112

and the cooperating electrodes

114

.

FIG. 23

shows a charge reversal corotron device

110

with a corotron wire

112

and an individual blade utilized as a cooperating electrode

114

, whereby the field lines

132

and

134

of the effective electrical field are indicated. The effective area, on which the amount of the attractive force between photoconductive band

116

and cooperating electrode

114

is dependent, is referenced

136

. The cooperating electrode

114

has a connection to a ground electrode. Alternatively, the cooperating electrode can have negative potential with reference to the ground potential. An electrical field is formed between corotron wire

112

and the cooperating electrode

114

. This field

134

acts on the toner particles

122

, which have a negative potential. The toner particles

122

are discharged when they pass by the corotron wire

112

and are recharged to a positive potential. The amount of the potential of what is now positively charged toner

120

is dependent on the dwell time of the toner in the electrical field and on the density of the electrical field. The photoconductive band

116

is thereby attracted by the cooperating electrode

114

. The attractive force F is calculated from the relationship:

F = \frac{ε_{0} \cdot ε_{r} A \cdot U^{2}}{2 \cdot d^{2}},

wherein ∈

r

is the dielectric constant of the air between photoconductive band

116

and the cooperating electrode

114

, A is the effective area

136

of the cooperating electrode

114

in the electrical field, U is the difference in potential and d is the distance between the underside of the photoconductive band

116

and the cooperating electrode

114

.

FIG. 24

shows another cooperating electrode

114

that is implemented as a blade. This blade

115

has a rectangular cross-section and is secured in the printer and/or copier

128

with a holder

126

.

FIG. 25

shows a blade

114

whose cutting edge is serrated. The blade

114

is arranged such in the printer/copier

128

that the serrations

140

taper acutely in the direction of the photoconductive band

116

. The serrations

147

are arranged at equal spacings. As a result of this arrangement, a uniform charge reversal of the latent toner image

118

is assured. The holder

126

of the blade

114

is not shown in this FIG.

25

.

FIG. 26

shows a cooperating electrode

114

that is composed of an arrangement of individual pins

142

. The pins

142

are arranged at symmetrical spacings on a holder

126

. The holder

126

is arranged such in the printer and/or copier

128

that the ends of the individual pins

142

lie in a plane that is parallel to the photoconductive band

116

and parallel to the corotron wire

112

.

FIG. 27

shows a cooperating electrode

114

that is composed of a wire

144

. The wire

144

is arranged such in the printer and/or copier

128

with a suitable holding mechanism

126

that it lies in a plane that is parallel to the photoconductive band

116

as well as parallel to the corotron wire

112

. A shield

130

is arranged at that side of the wire

144

facing away from the photoconductive band

116

. A wire

144

similar to the corotron wire

112

is utilized as the wire

144

.

FIG. 28

shows a transfer printing corotron device

146

having a corotron wire

112

and a cooperating electrode fashioned as a blade

114

. Two photoconductive bands

116

a

and

116

b

are provided as the intermediate carrier. Alternatively, however, two transfer bands could also be utilized. A toner image

118

a

on the photoconductive band

116

a

that has not yet been fixed contains positively charged toner particles

120

. A toner image

118

b

on the photoconductive band

116

that has not yet been fixed contains negatively charged toner particles

122

. The photoconductive bands

116

a

and

116

b

as well as a paper web

148

are conducted past between the corotron wire

112

and the blade

114

without touching these, whereby the photoconductive bands

116

a

and

116

b

are guided and driven by deflection rollers

124

. The drive and the guidance of the paper web

148

is not shown in this figure. The corotron wire

122

has a positive potential and the blade

114

has a negative potential with reference to the ground potential. The corotron wire

112

is surrounded by a shield

130

at that side facing away from the photoconductive band

116

a

. The positively charged toner particles

120

of the latent toner image

118

a

are repelled by the positively charged corotron wire

112

and are attracted by the negatively charged toner particles

122

of the latent toner image

118

b

as well as by the negatively charged blade

114

. Analogous thereto, the negatively charged toner particles

122

of the latent toner image

118

b

are repelled by the negatively charged blade

114

and are attracted by the positively charged toner particles

120

of the latent toner image

118

a

as well as by the positively charged corotron wire

112

. The transfer printing corotron

146

exerts a force on the positively and negatively charged toner particles

120

and

122

that is greater than the bonding forces between the toner particles

120

and

122

and the photoconductive bands

116

a

and

116

b

. The positively and negatively charged toner particles

120

and

122

are transfer printed onto the paper web

146

by the field forces of the electrical field. The toner particles

120

and

122

remain on the paper web

146

due to the binding forces between the toner particles

120

and

122

and the paper web

146

as well as due to the attractive force between the positively charged toner particles

120

on the one paper side and the negatively charged toner particles

122

on the other paper side.

Although other modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.

Number	Name	Date	Kind
5797077	Samizo et al.	Aug 1998	A
5805967	De Bock et al.	Sep 1998	A
6246856	Kopp et al.	Jun 2001	B1

Number	Date	Country
11-212308	Aug 1999	JP
WO 9839691	Sep 1998	WO

Printer or copier for simultaneously printing a supporting material on both sides

Information

Patent Number

Date Filed

Date Issued

Inventors

Original Assignees

Examiners

Agents

CPC

US Classifications

Field of Search

US

International Classifications

Abstract

Description

Claims

Priority Claims (1)

PCT Information

US Referenced Citations (3)

Foreign Referenced Citations (2)