Patent Application
20050163521
1. Field of Invention
This invention relates to the field of electrophotographic image forming systems and power supplies used therewith.
2. Description of Related Art
Hybrid Scavengeless Development (HSD) is a process for ionographic or electrophotographic imaging and printing apparatuses designed to prevent scavenging of toner from the photoreceptor of the imaging device by subsequent development stations.
In general, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential to sensitize the surface. The charged photoconductive surface is exposed to a light image from either a scanning laser beam, an LED source, or an original document being reproduced. This records an electrostatic latent image on the photoconductive surface. After the electrostatic latent image is recorded on the photoconductive surface, the latent image is developed. Two-component and single-component developer materials are commonly used for development. A typical two-component developer comprises magnetic carrier granules having toner particles adhering triboelectrically thereto. A single-component developer material typically comprises toner particles. Toner particles are attracted to the latent image, forming a toner powder image on the photoconductive surface. The toner powder image is subsequently transferred to a copy sheet. Finally, the toner powder image is heated to permanently fuse it to the copy sheet in image configuration.
The electrophotographic marking process discussed above can be modified to produce color images. One color electrophotographic marking process, called image-on-image (IOI) processing, superimposes toner powder images of different color toners onto the photoreceptor prior to the transfer of the composite toner powder image onto the substrate. While the IOI process provides certain benefits, such as a compact architecture, there are several challenges to its successful implementation. For instance, the viability of printing system concepts such as IOI processing requires development systems that do not interact with a previously toned image. Since several known development systems, such as conventional magnetic brush development and jumping single-component development, interact with the image on the receiver, a previously toned image will be scavenged by subsequent development if interacting development systems are used. Thus, for the IOI process, there is a need for scavengeless or non-interactive development systems. For a thorough description of scavengeless development see U.S. Pat. No. 5,031,570, hereby incorporated by reference in its entirety.
Hybrid scavengeless development technology deposits toner via a conventional magnetic brush onto the surface of a donor roll and a plurality of electrode wires are closely spaced from the toned donor roll in the development zone. An AC voltage is applied to the electrode wires to generate a toner cloud in the development zone. This donor roll generally consists of a conductive core covered with a thin (50-200 μm) partially conductive layer. The magnetic brush roll is held at an electrical potential difference relative to the donor core to produce the field necessary for toner development. The toner layer on the donor roll is then disturbed by electric fields from a wire or set of wires to produce and sustain an agitated cloud of toner particles. Typical ac voltages of the wires relative to the donor are 600-900 Vpp at frequencies of 5-15 kHz. These ac signals are often square waves, rather than pure sinusoidal waves. Toner from the cloud is then developed onto the nearby photoreceptor by fields created by a latent image.
A problem inherent to developer systems using wires is a vibration of the wires parallel to the donor roll and photoreceptor surfaces. This wire vibration manifests itself in a density variation, at a frequency corresponding to the wire vibration frequency, of toner on the photoreceptor. Also, higher harmonics of vibration, being an integer multiple of the wire fundamental frequency, can be excited by the applied voltage frequency. Again these vibrations can cause a density variation, at a frequency corresponding to the wire vibration frequency to produce density variations that correspond to a harmonic standing wave patterns, of toner on the photoreceptor. The toner density variations and the wire vibrations that cause them are lumped together into a problem with the generic name of “strobing.” More specifically, fundamental strobing is the term used to describe the vibration and print defect associated with the fundamental mode of vibration, while harmonic strobing is used to describe the defect caused by the higher harmonics. Strobing does not occur at all hardware setpoints. For instance, it can often be reduced by decreasing the amplitude of the wire voltage, or varying the donor roll speed. Also, fundamental strobing is related to the applied wire frequency in a complex manner, and both types of strobing are sensitive to the frictional properties of the toner.
In various exemplary embodiments according to this invention, a power supply is separately provided for an HSD image forming system which includes frequency deviation capability for avoidance of wire strobing defects.
In various exemplary embodiments according to this invention, a power supply is separately provided for an HSD image forming system which uses square waves instead of sinusoidal waves in generating toner clouds to increase the average voltage applied to the toner without increasing the peak voltage.
In various exemplary embodiments according to this invention, a power supply is separately provided for an HSD image forming system which uses relatively low amplitude AC voltages, thus reducing power consumption and stress on toner concentration sensors.
In various exemplary embodiments according to this invention, a power supply is separately provided for an HSD image forming system which utilizes asymmetric waveforms.
The systems and methods according to this invention provides a power supply for a hybrid scavengeless development electrophotographic image forming system in which the donor roll and the wires are operated at the same AC voltage frequency without phase shifts, allowing the donor roll to be run at a higher voltage.
Referring initially to
As the photoreceptor belt moves, each part of it passes through each of the subsequently described process stations. For convenience, a single section of the photoreceptor belt, referred to as the image area, is identified. The image area is that part of the photoreceptor belt which is to receive the toner powder images that, after being transferred to a substrate, produce the final image. While the photoreceptor belt may have numerous image areas, since each image area is processed in the same way, a description of the typical processing of one image area suffices to fully explain the operation of the printing machine.
As the photoreceptor belt 10 moves, the image area passes through a charging station A. At charging station A, a corona generating device, indicated generally by the reference numeral 22, charges the image area to a relatively high and substantially uniform potential. As an example, the image area will be charged to a uniform potential of about −500 volts. In practice, this is accomplished by charging the image area slightly more negative than −500 volts so that any resulting dark decay reduces the voltage to the desired −500 volts. While this description refers to the image area as being negatively charged, it could be positively charged if the charge levels and polarities of the toners, recharging devices, photoreceptor, and other relevant regions or devices are appropriately changed.
After passing through the charging station A, the now charged image area passes through a first exposure station B. At exposure station B, the charged image area is exposed to light which illuminates the image area with a light representation of a first color (say black) image. That light representation discharges some parts of the image area so as to create an electrostatic latent image. While the illustrated embodiment uses a laser-based output scanning device 24 as a light source, it is to be understood that other light sources, for example an LED printbar, can also be used with the principles of the present invention. In various exemplary embodiments, a voltage level of about −500 volts will exist on those parts of the image area which were not illuminated, while a voltage level of about −50 volts will exist on those parts which were illuminated. Thus after exposure, the image area has a voltage profile comprised of relative high and low voltages.
After passing through the first exposure station B, the now exposed image area passes through a first development station C which is identical in structure with development systems E, G, and I. The first development station C deposits a first color, say black, of negatively charged toner 31 onto the image area. That toner is attracted to the less negative sections of the image area and repelled by the more negative sections. The result is a first toner powder image on the image area. It should be understood that one could also use positively charged toner if the exposed and unexposed areas of the photoreceptor are interchanged, or if the charging polarity of the photoreceptor is made positive. In addition, it may be advantageous to first deposit a color other than black on the photoreceptor.
For the first development station C, development system includes a donor roll 40. As illustrated in
After the image area passes through the first development station C toner 76 (which generally represents any particular color of toner) adheres to the illuminated image area. This causes the voltage in the illuminated area to increase to, for example, about −200 volts. The non-illuminated parts of the image area remain at about the level of −500 volts.
Referring back to
After the image area passes through the first recharging device 36, the image area is overcharged by the first recharging device to more negative levels than that which the image area is to have when it leaves the recharging station D. For example, the toned and the untoned parts of the image area reach a voltage level of about −700 volts. The first recharging device 36 is preferably a DC scorotron. After being recharged by the first recharging device 36, the image area passes to the second recharging device 37. The second recharging device 37 reduces the voltage of the image area, both the untoned parts and the toned parts (represented by toner 76) to the desired potential of −500 volts.
After being recharged at the first recharging station D, the now substantially uniformly charged image area with its first toner powder image passes to a second exposure station 38. Except for the fact that the second exposure station illuminates the image area with a light representation of a second color image (say yellow) to create a second electrostatic latent image, the second exposure station 38 is the same as the first exposure station B. At this point, the non-illuminated areas have a potential of about −500 volts. However, illuminated areas, both the previously toned areas denoted by the toner 76 and the untoned areas are discharged to about −50 volts.
The image area then passes to a second development station E. Except for the fact that the second development station E contains a toner 40 which is of a different color (yellow) than the toner 31 (black) in the first development station C, the second development station is substantially the same as the first development station. Since the toner 40 is attracted to the less negative parts of the image area and repelled by the more negative parts, after passing through the second development station E the image area has first and second toner powder images which may overlap.
The image area then passes to a second recharging station F. The second recharging station F has first and second recharging devices, the devices 51 and 52, respectively, which operate similar to the recharging devices 36 and 37. Briefly, the first corona recharge device 51 overcharges the image areas to a greater absolute potential than that ultimately desired (say −700 volts) and the second corona recharging device, comprised of coronodes having AC potentials, neutralizes that potential to that ultimately desired.
The now recharged image area then passes through a third exposure station 53. Except for the fact that the third exposure station illuminates the image area with a light representation of a third color image (say magenta) so as to create a third electrostatic latent image, the third exposure station 38 is the same as the first and second exposure stations B and 38. The third electrostatic latent image is then developed using a third color of toner 55 (magenta) contained in a third development station G.
The now recharged image area then passes through a third recharging station H. The third recharging station includes a pair of corona recharge devices 61 and 62 which adjust the voltage level of both the toned and untoned parts of the image area to a substantially uniform level in a manner similar to the corona recharging devices 36 and 37 and recharging devices 51 and 52.
After passing through the third recharging station the now recharged image area then passes through a fourth exposure station 63. Except for the fact that the fourth exposure station illuminates the image area with a light representation of a fourth color image (say cyan) so as to create a fourth electrostatic latent image, the fourth exposure station 63 is the same as the first, second, and third exposure stations, the exposure stations B, 38, and 53, respectively. The fourth electrostatic latent image is then developed using a fourth color toner 65 (cyan) contained in a fourth development station I.
To condition the toner for effective transfer to a substrate, the image area then passes to a pretransfer corotron member 50 which delivers corona charge to ensure that the toner particles are of the required charge level so as to ensure proper subsequent transfer. After passing the corotron member 50, the four toner powder images are transferred from the image area onto a support sheet 57 at transfer station J. It is to be understood that the support sheet is advanced to the transfer station in the direction 58 by a conventional sheet feeding apparatus which is not shown. The transfer station J includes a transfer corona device 54 which sprays positive ions onto the backside of sheet 57. This causes the negatively charged toner powder images to move onto the support sheet 57. The transfer station J also includes a detack corona device 56 which facilitates the removal of the support sheet 52 from the printing machine.
After transfer, the support sheet 57 moves onto a conveyor (not shown) which advances that sheet to a fusing station K. The fusing station K includes a fuser assembly, indicated generally by the reference numeral 60, which permanently affixes the transferred powder image to the support sheet 57. Preferably, the fuser assembly 60 includes a heated fuser roller 67 and a backup or pressure roller 64. When the support sheet 57 passes between the fuser roller 67 and the backup roller 64 the toner powder is permanently affixed to the sheet support 57. After fusing, a chute, not shown, guides the support sheets 57 to a catch tray, also not shown, for removal by an operator.
After the support sheet 57 has separated from the photoreceptor belt 10, residual toner particles on the image area are removed at cleaning station L via a cleaning brush contained in a housing 66. The image area is then ready to begin a new marking cycle.
The various machine functions described above are generally managed and regulated by a controller which provides electrical command signals for controlling the operations described above.
Referring now to
With continued reference to
The electrode structure 42 is comprised of one or more thin (e.g., 50 to 100 micron diameter) conductive wires which are lightly positioned against the toner on the donor structure 40. The distance between the wires and the donor is self-spaced by the thickness of the toner layer, which may be approximately 15 microns. The extremities of the wires are supported by blocks (not shown) at points slightly above a tangent to the donor roll surface. Suitable scavengeless development systems for incorporation in the present invention are disclosed in U.S. Pat. No. 4,868,600 and in U.S. Pat. No. 6,101,357, both of which are hereby incorporated by reference in their entirety. As disclosed in the '600 patent, a scavengeless development system may be conditioned to selectively develop one or the other of the two image areas (i.e. discharged and charged image areas) by the application of appropriate AC and DC voltage biases to the wires 42 and the donor roll structure 40.
Referring again to
While in the development system 38, as shown in
The electrical sections of
Referring now to the present invention, as illustrated by
Referring again to
Asymmetric waves have the property that their positive voltage and negative voltage are not equal about the zero voltage axis. When a DC offset is added to a symmetric AC voltage, the applied DC shifts both the positive and negative voltages. The positive and negative values are no longer equal with respect to the zero voltage axis. The use of asymmetric waveforms allows use of all the available voltage space while avoiding air breakdown. That is to say that the magnitude of the positive voltage can be different from the negative voltage by choosing the appropriate level of asymmetry. In this way it is possible to maintain a DC offset with the same positive and negative voltage levels about the zero voltage axis. This allows use of all the available voltage space while avoiding air breakdown. Toner that has been aged by a developer housing needs the highest AC biases possible for good development latitude. Thus, using asymmetric waveforms allows the highest positive and negative voltage without the possibility of air breakdown in the air gap between the donor and the mag or the donor and the wire electrode.
Square waves are advantageous in generating toner clouds in electrophotographic systems because breaking toner adhesion on donor surfaces requires high electric fields that are very close to air breakdown levels. Thus, simply increasing the amplitude of sine wave AC biases is limited by air breakdown. The use of square waves allows a longer push-pull force on the toner for the same peak voltage than do sinusoidal waves, for example.
The signal from the master oscillator is then fed to each of the mag roll AC driver 220, the donor roll AC driver 230 and the wire electrode AC driver 240 to generate the AC component of the mag, donor and wire voltages. In the case of the mag voltage, the mag AC driver 220 and the mag DC power source combine to charge the mag roll 46 to a voltage level VM. The actual charge level of the mag roll 46 is not significant, but rather the relative AC voltage difference between the mag roll 46 and the donor roll 40, VDMAC is significant. It is the relative voltage difference VDMAC which causes toner to travel from the mag roll 46 to the donor roll 40.
The donor roll is charged to a combined voltage value of VD by the donor AC driver 230 and the donor DC power supply 235. In various exemplary embodiments, the mag bias is set lower than the donor bias to cause the toner to be attracted to the donor roll 40 from the mag roll 46.
The wire electrode 42 is charged by the wire AC driver 240 and the donor DC source 235. The combined voltage Vwd is the voltage which generates the toner cloud.
In the configuration of
While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, equivalents, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention, as set forth above, are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention.
