Transfer printing station for an electrographic device with a contact pressure element in the transfer printing area

A printer or copier has a transfer printing station for electrostatic transfer of toner from a photoconductor to the recording medium. A pressure contact element is provided on the recording medium guide elements to ensure contact of the recording medium with the photoconductor during transfer. Supporting rollers of a slightly greater diameter than the pressure contact element are provided at each end thereof bearing on the photoconductor as spacer elements to define a minimum spacing between the pressure contact element and the intermediate carrier. The contact pressure element(s) may also be mounted on pivotable levers movable by an eccentric.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention is directed to a transfer printing station with an
 electrostatic transfer printing means for transferring a toner image
 generated on an electrographic intermediate carrier onto at least one
 recording medium in a transfer printing area of the transfer printing
 station, with guide elements arranged in the transfer printing area for
 guiding the recording medium close to the intermediate carrier during the
 transfer printing event, and with a contact pressure element arranged
 between the guide elements that presses the recording medium against the
 intermediate carrier during the transfer printing event.
 2. Description of the Related Art
 Such a transfer printing station is disclosed by U.S. Pat. No. 5,400,125.
 It employs a leaf-shaped blade as a contact pressure element, the edge
 thereof pressing the web-shaped recording medium against the
 electrographic intermediate carrier, a photoconductor drum. Although
 relatively good practical results have been capable of being achieved with
 this solution, a few disadvantages nonetheless derive. Thus, the
 blade-shaped contact pressure element is a wear part whose service life is
 relatively short and that requires frequent replacement. Since the blade
 edge of the contact pressure element rubs against the recording medium,
 considerable contamination due to toner abrasion and paper abrasion
 derives in the transfer printing area. When different web widths of the
 recording medium are employed, then a contact pressure element matched to
 this web width must be utilized. Due to the blade-shaped contact pressure
 element, moreover, irregularities in the electrostatic field derive in the
 transfer printing area; these irregularities can produce stripes in the
 print image.
 Japanese Patent document discloses a transfer printing station with a
 contact pressure element in the transfer printing area. The contact
 pressure element is fashioned at a guide element, whereby the transfer
 printing station contains only a single guide element. The contact
 pressure element is fashioned a as contact pressure roller and has lateral
 supporting wheels that are supported on edge regions of an intermediate
 carrier that have no image-generating function. The recording medium is
 supplied with an edge close to a supporting wheel. The other supporting
 wheel has a larger diameter than the first-cited supporting wheel, as a
 result whereof a wedge-shaped gap forms between the surface of the contact
 pressure roller and the surface of the intermediate carrier. What is
 thereby achieved is that the contact pressure roller is not charged with
 toner material in the area of the enlarged gap.
 German Patent document discloses an electrographic printer wherein two webs
 of a recording medium lying side-by-side are simultaneously printed. One
 of the webs can have already undergone a fixing process.
 SUMMARY OF THE INVENTION
 An object of the present invention is to fashion a transfer printing
 station of the species initially cited such that, on the one hand,
 non-uniformly shaped recording media of different thickness or width can
 also be uniformly placed against the recording medium during transfer
 printing in order to produce a print image with high quality via the
 contact pressure element, and such that, on the other hand, it is assured
 in all operating conditions that the contact pressure element does not
 enter into contact with the intermediate carrier.
 This object and others are achieved by a transfer printing station for an
 electrographic printer or copier device having: an electrostatic transfer
 printing means for transferring a toner image generated on an
 electrographic intermediate carrier onto at least one recording medium in
 a transfer printing area of the transfer printing station, guide elements
 arranged in the transfer printing area for guiding the recording medium
 close to the intermediate carrier during the transfer printing event, at
 least one contact pressure element that is resistant to bending, arranged
 between the guide elements displaceable relative to the intermediate
 carrier opposite a spring power, and that presses the at least one
 recording medium against the intermediate carrier during the transfer
 printing event, and with at least one spacer element allocated to the
 contact pressure element and supported on the intermediate carrier for
 maintaining a defined minimum spacing between contact pressure element and
 intermediate carrier, whereby the contact pressure element is fashioned as
 contact pressure rail.
 The contact pressure element, which is fashioned as a contact pressure
 roller or as a contact pressure rail and essentially extends over the
 entire width of the electrographic intermediate carrier, for example a
 photoconductor drum, is fashioned resistant to bending and comprises a
 spacer element, preferably in the form of a supporting roller, supported
 on the intermediate carrier for maintaining a defined minimum spacing
 between the contact pressure element and the intermediate carrier.
 The force of the contact pressure can be increased due to the resistance to
 bending in combination with the distance-securing function of the spacer
 element, so that a rippled recording medium can also be pressed smoothly
 against the electrographic intermediate carrier. What the increased
 pressing power also effects is that no air cushion can form between the
 recording medium and the intermediate carrier, as has often occurred in
 the Prior Art devices.
 When the contact pressure element is fashioned as a contact pressure
 roller, it rolls over the recording medium and only rolling friction
 arises between the recording medium and the contact pressure roller.
 Accordingly, the abrasion at the recording medium is reduced and the risk
 of contamination is low.
 According to a development of the invention, the surface of the contact
 pressure element is electrically conductive and has a predetermined
 electrical potential, preferably ground potential. In this way, electrical
 charges that arc generated on the surface of the recording medium by
 electrostatic charging arc collected and eliminated to ground. A
 disturbance of the electrical field in the transfer printing area is thus
 avoided.
 A preferred exemplary embodiment of the invention is characterized in that
 the contact pressure element has two supporting rollers whose diameters
 are dimensioned such that a minimum spacing between the contact pressure
 element and the intermediate carrier is not downwardly transgressed. The
 rolling of the supporting rollers on defined sections of the intermediate
 carrier outside the actual writing area assures that the contact pressure
 element is always held in a defined position relative to the recording
 medium even given a lack of roundness and ripple of the intermediate
 carrier. Deviations in shape of a photoconductor drum such as, for
 example, lack of roundness and ripple do not, given the present invention,
 lead to the pressing position of the recording medium against the
 photo-sensitive surface of the intermediate carrier changing.
 A preferred employment of the transfer printing station is characterized in
 that the web has already undergone a transfer printing procedure, i.e. is
 provided with a toner image, and has been conveyed over a longer conveying
 path and been potentially subjected to a thermal fixing. Ripples and
 grooves can form on the recording medium due to the renewed thermal
 fixing, these potentially leading to a deteriorated print image. By
 employing the invention, these ripples and grooves are smoothed in the
 transfer printing area, so that the recording medium lies flush against
 the intermediate carrier for the transfer of toner.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 1 shows a part of an electrophotographic printer means that contains a
 transfer printing station schematically shown in the Figure. This transfer
 printing station serves the purpose of transferring a toner image
 generated on a photoconductor drum 10 onto a web-shaped recording medium,
 a smooth or pre folded continuous form paper 12 in the present case. This
 procedure is referred to as transfer printing. The transfer printing
 ensues within the scope of an electrophotographic process, whereby the
 photoconductor drum 10 is first uniformly charged to approximately 500 V
 with the assistance of a charging device (not shown) and is then
 discharged into the region of approximately 70 V in locations with the
 assistance of an illumination means that is controlled
 character-dependent. A latent character image generated in this way is
 then inked in a developer station (not shown) in the standard way with the
 assistance of a two-component mixture of toner particles and ferromagnetic
 carrier particles.
 The toner particles are triboelectrically positively charged. A field
 arises between a developer drum of the developer station, which lies at a
 basic potential of approximately 220 V, and the regions of the latent
 character image discharged to approximately 70 V, as a result whereof the
 toner particles agglomerate on the discharged regions. The toner particles
 are repelled by the non-exposed area having a charge voltage of
 approximately 500 V.
 The toner image of loose toner particles generated in this way is then
 transferred onto the web-shaped recording medium 12 with the assistance of
 a highly negative electrostatic field generated by a corotron wire 14. The
 corotron wire 14 is shielded at one side by a corotron shield 16; its
 electrical field takes effect through the aperture between two transfer
 printing jaws 18 and 20, passing through the recording medium 12, as a
 result whereof the toner particles are stripped from the photoconductor
 drum 10 and transferred onto the recording medium 12 having touch contact
 with the photoconductor 10, electrostatically adhering thereto.
 Subsequently, the toner images transferred in this way are fused into the
 surface of the recording medium 12 between a heated fixing drum and a
 contact pressure drum in a thermal pressure fixing station (not shown).
 The remaining particles that still adhere to the photoconductor drum 10
 after the transfer printing are removed in a standard way via a cleaning
 station. The electrostatic process begins anew thereafter by charging the
 photoconductor drum 10 via the charging station (not shown). Further
 details about this transfer printing process can be derived from U.S. Pat.
 No. 5,179,417, which discloses the employment of two web-shaped recording
 media.
 In order to place the web-shaped recording medium 12 into contact or,
 respectively, into the immediate proximity of the surface of the
 photoconductor drum 10, two transfer printing jaws 18 and 20 are provided
 as a guide means, these being shown in FIG. 1 in their condition wherein
 they are pivoted toward the photoconductor 10.
 When no transfer printing event is to take place, then these transfer
 printing jaws 18 and 20 can be pivoted away from the photoconductor drum
 10, whereby the recording medium 12 is entrained. The type of drive of the
 transfer printing jaws 18 and 20 and the appertaining conveyor devices for
 the recording medium 12 are disclosed in greater detail in U.S. Pat. No.
 4,131,358.
 The end 22 of the transfer printing jaw 18 delivering the recording medium
 12 plays a critical part in the prior art. The feed direction for the
 recording medium 12 derives from the movement of the photoconductor drum
 10, as indicated with an arrow P1. This end 22 cannot be brought into
 intimate contact with the photoconductor drum 10 since the risk of damage
 to the sensitive surface of the photoconductor drum 10 is too high. On the
 other hand, the recording medium 12 must have this contact with the
 surface of the photoconductor drum 10 in order to transfer the toner
 particles. When, then, the carrier material has slight defects or
 irregularities, then these have an especially critical effect at the end
 22, so that the transfer of the toner particles does not ensue uniformly.
 For example, air bubbles can arise between the underside of the recording
 medium 12 and the surface of the photoconductor drum, these leading to a
 loss of contact or even preventing the toner transfer. Moreover, lack of
 roundness and ripple of the cylindrical surface of the photoconductor drum
 22 have a negative effect since the spacing between the end 22 and the
 surface of the photoconductor drum 10 fluctuates.
 FIG. 2 shows a solution from the prior art, as disclosed by U.S. Pat. No.
 5,400,125 of the same assignee. This document, which shows further details
 of the transfer printing station that is relevant here, is thus
 incorporated into this patent application by reference as a source of
 disclosure. The known solution employs a blade-like contact pressure
 element 24 that is arranged at the end of the transfer printing jaw 18 and
 produces a defined contact between the recording medium 12 and the surface
 of the photoconductor drum 10 with its blade edge. This solution has
 definitely proven itself in practice, but it should be cited as
 disadvantages that the contact pressure element 24 wears over time and
 must be replaced, as a result whereof the printing process is interrupted.
 Print image errors arise given a high degree of wear of the contact
 pressure element 24. Further, abrasion arises at the contact pressure
 element 24 and at the recording medium 12, which leads to contamination
 and to a further source of disruption.
 FIG. 3 shows an exemplary embodiment of the invention in a perspective
 view, whereby identical parts are identically referenced. As can be seen,
 a contact pressure roller 26 that extends over the entire length of the
 photoconductor drum 10 is seated on the transfer printing jaw 18.
 Supporting rollers 28 that rotate together with the transfer printing drum
 are attached to the outer ends thereof. The transfer printing jaw 18 can
 be pivoted away from the surface of the photoconductor drum 10 around a
 rotational axis 19 and can in turn be pivoted toward it, whereby the
 contact pressure roller 26 executes movements according to the arrows P2
 and P3. The supporting rollers 28 have a diameter that is dimensioned such
 that they roll on the outermost sections on the surface of the
 photoconductor drum 10 during the transfer printing event. In order to
 achieve this, the radius of the supporting rollers must be slightly larger
 than the sum of the radius of the contact pressure roller 26 and the
 thickness of the recording medium. For a paper having a thickness of less
 than 1/10 mm given a paper weight of 35, 70 through 160 g, the difference
 between the radii of supporting roller 28 and contact pressure roller 26
 should be about 15/100.+-.10%.
 What is achieved by the supporting rollers 28 is that, on the one hand, the
 recording medium 12 is brought into a defined proximity of the
 photoconductor drum 10, whereby the rolling friction between recording
 medium 12 and contact pressure roller 26 is minimal. On the other hand,
 what the supporting rollers 28 effect is that, even given a recording
 medium 12 whose width is less than the length of the photoconductor drum
 10 viewed in longitudinal axis, damage to the light-sensitive surface does
 not occur. Arbitrary web widths can thus be transfer-printed, which is
 advantageous particularly given a great length of the photoconductor drum
 10.
 As can also be seen on the basis of FIG. 3, the recording medium has two
 webs 12A and 12B that are arranged side-by-side. The illustrated transfer
 printing station is thus suited for utilization in what is referred to as
 duplex mode, whereby one web of the recording medium is printed on the one
 side, the web--following thermal fixing--is turned over by 180.degree. and
 supplied to the same transfer printing with the other side and then
 transfer-printed. Given such an operating mode, thus, a first, as yet
 unprinted web 12A and a web 12B that has already been printed are supplied
 to the transfer printing station. Due to the thermal fixing, the web 12B
 that has already been printed often has ripples and a different surface
 quality. What is achieved by the contact pressure roller 26, then, is
 that, even given two webs with different surfaces and different properties
 that lie side-by-side, both webs nonetheless lie uniformly and smoothly on
 the surface of the plhotoconductor drum 10, so that an immaculate transfer
 printing can ensue. Since the supporting rollers 28 roll on the surface of
 the photoconductor drum, deviations in shape from the ideal cylindrical
 shape have no influence; on the contrary, the contact pressure roller 26
 is always held at the same distance from the surface of the photoconductor
 drum 10 even given ripple and lack of roundness thereof. The width of the
 transfer printing jaw 18 and of the contact pressure roller 26 as well as
 the length of the photoconductor drum 10 arc typically dimensioned such
 that two DIN-A4 webs in width format can be simultaneously
 transfer-printed side-by-side. It must also be pointed out that the
 arrangement with the contact pressure roller 26 also proves advantageous
 in other operating modes, for example in simplex printing with recording
 medium webs lying side-by-side and spot color printing without turning the
 recording medium webs over or, on the other hand, in duplex printing on a
 paper web with two printers that are coupled via a turn-over station.
 In order to eliminate electrostatic charges from the surface of the
 recording medium 12A and 12B, the contact pressure roller 26 has a
 conductive surface of, for example, metal. This conductive surface is
 electrically connected to ground potential, as indicated at 30 in FIG. 3.
 FIG. 4 shows an illustration of the embodiment of FIG. 3 seen from the
 side. It can be seen that the supporting rollers have a slightly larger
 diameter than the contact pressure roller 26 itself.
 FIG. 5 shows a further embodiment of the invention wherein the contact
 pressure roller 26 is resiliently seated at the transfer printing jaw 18
 via a turning ant 32. Identical parts are again identically referenced in
 this Figure, too. The right-hand transfer printing jaw has a salient guide
 end 34. As can be seen in FIG. 5, the contact pressure roller 26 is
 arranged at the transfer printing jaw 18 instead of a guide end. A
 pressure spring 35 biases the contact pressure roller 26 in the direction
 of the photoconductor drum 10. A relatively large angle a over which the
 recording medium 12 is in contact with the surface of the photoconductor
 drum IO derives due to the defined pressing power by the contact pressure
 roller 26. What this large angle a effects is that the transfer of toner
 particles onto the recording medium ensues uniformly and without spacing
 fluctuations between the carrier material 12 and surface of the
 photoconductor drum 10. A high printing quality is achieved in this way.
 For comparison, FIG. 6 shows an arrangement in a traditional fashion,
 without the contact pressure roller. The angle o over which contact ensues
 between the recording medium 12 and the surface of the photoconductor drum
 10 is clearly reduced.
 FIGS. 7 through 9 show another advantageous example of a transfer printing
 station, whereby a contact pressure rail 36 is provided instead of the
 contact pressure roller. The contact pressure rail 36 is a component part
 of the transfer printing jaws 18 and 20 and is composed of a profile of
 polished steel with a glide surface 37 (see FIG. 9) for the recording
 medium formed thereon. It extends over the entire width of the writeable
 area of the intermediate carrier composed of a photoconductor drum 10.
 Supporting rollers 28 are arranged at both sides of the contact pressure
 rail, these being rubberized at their circumference. They interact with
 corresponding areas of the photoconductor drum arranged outside the actual
 writing area. The contact pressure rail is especially resistant to bending
 as a result of the profiling. The transfer printing gap of approximately
 0.15 mm in this case can be adhered to all the more exactly the more
 resistant to bending the profile of the contact pressure rail or--in the
 case of the exemplary embodiment of FIG. 3--the contact pressure roller 26
 is. The glide surface 37 has a large delivery radius, particularly in the
 region of the transfer printing jaw 18 that supplies the recording medium
 12 to the transfer printing area. In this way and corresponding to the
 illustration of FIG. 8, it is possible to displace the contact pressure
 point 38 of the recording medium 12 (paper) approximately 4.6 mm closer to
 the transfer printing area with reference to the position of the contact
 pressure point 39 given the employment of a contact pressure roller 26
 (FIG. 3). Creases in the paper (recording medium) are thus smoothed to a
 significantly greater extent. This is particularly advantageous given what
 is referred to as PTL paper transport (pinless transport) wherein the
 paper does not have any transport holes and is transported via friction
 rollers. The wrap forces are significantly higher here than given tractor
 paper conveyors.
 As can be seen from FIG. 7, the contact pressure rail 36 together with the
 supporting rollers 28 can be a component part of the two transfer printing
 jaws 18 and 20; the recording medium 12 is thus guided especially well in
 the area of the transfer printing location or, on the other hand, can also
 only be a component part of the transfer printing jaw 18 that supplies the
 recording medium to the transfer printing location. Analogous to the
 exemplary embodiments with a contact pressure roller, it is also possible
 to arrange the contact pressure rail 36 separately from the transfer
 printing jaws 18 and 20.
 As a result of the flexible yet rigid contact pressure rail 36 in
 combination with the supporting rollers, it is possible to adhere
 risk-free to and guarantee an extremely narrow transfer printing gap
 (paper transport gap) of--in this case--approximately 0.15 mm between
 transfer printing jaws 18 and 20 and photoconductor drum 10. Given paper
 tears and given absence of paper, the supporting rollers prevent damage to
 the photoconductor drum 10.
 As can particularly be seen from the sectional view of FIG. 7, the transfer
 printing jaws 18 and 20 are seated so as to be pivotable as illustrated by
 the (arrows) around the turning axes 44. Via lever arms 40 and rollers 41
 arranged thereon, they are supported--under the influence of a tension
 spring 42--on an eccentric 43 fashioned as cam plate. By turning the
 eccentric 43, the transfer printing jaws 18 and 20 are pivoted together
 with the contact pressure elements 36 arranged thereon. The tension spring
 42 sees, on the one hand, to the contact with the eccentric, particularly
 when being pivoted out; on the other hand, it presses the recording medium
 12 against the photoconductor drum 12 with a predetermined spring power
 via the contact pressure rails 36 or, respectively, their glide surfaces
 37, whereby the supporting rollers prevent a downward transgression of the
 transfer printing gap width. The transfer printing jaws are thus seated
 displaceable relative to the surface of the photoconductor drum 12
 opposite the spring power of the spring 42, so that recording media having
 the greatest variety of thicknesses can be processed.
 The invention was described above on the basis of exemplary embodiments
 wherein web-shaped recording media are employed; however, given
 appropriate modification, it can also be employed in transfer printing
 stations that process single sheets.
 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.