Humidity corrected transfer apparatus

Abstract

A humidity compensated transfer apparatus in which charged particles are transferred from a support surface to a sheet of support material. The foregoing abstract is neither intended to define the invention disclosed in the specification, nor is it intended to be limiting as to the scope of the invention in any way.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to an electrostatographic printing machine, and more particularly concerns a humidity compensated transfer apparatus for utilization therein.

The process of electrostatographic printing involves the creation of an electrostatic latent image corresponding to an original document and the reproduction thereof in viewable form. Electrostatographic printing includes electrophotographic printing and electrographic printing. In the process of electrophotographic printing, as disclosed in U.S. Pat. No. 2,297,691 issued to Carlson in 1942, a photoconductive layer is charged to a substantially uniform potential in order to sensitize its surface. A light image of the original document is projected onto the charged photoconductive surface. The charge on the photoconductive layer is selectively dissipated in the irradiated areas in accordance with the light intensity reaching the photoconductive layer. In this way, an electrostatic latent image of the original document is created on the photoconductive layer. A developer mix comprising dyed colored thermoplastic powder, known in the art as toner particles, and coarser carrier granules, such as ferromagnetic granules, is brought into contact with the electrostatic latent image. The toner particles are attracted electrostatically from the carrier granules to the latent image recorded on the photoconductive layer. Thereafter the toner powder image developed on the photoconductive layer is transferred to a sheet of support material, such as plain paper or a thermoplastic sheet, amongst others. However, if the photoconductive surface is the final sheet of support material, the toner powder image will remain thereon. Subsequent to the formation of the toner powder image on the final support material, the toner powder image is suitably permanently affixed thereto, i.e. by heat. Electrographic printing differs from electrophotographic printing in that an insulating medium is utilized to form, without the aid of a light image, the electrostatic latent image. Other than that, electrographic printing is substantially identical to electrophotographic printing.

Heretofore, the toner powder image has been transferred to the sheet of support material by an electric field created by a corona generator, or by a transfer roll biased electrically to generate a high voltage discharge in the proximity of the support material. A typical corona generator is disclosed in U.S. Pat. No. 2,836,725 issued to Vyverberg in 1958. A corona generator of this type sprays ions on the back surface of the sheet of support material to induce transfer thereto. One type of suitable bias transfer roll is disclosed in U.S. Pat. No. 2,807,233 issued to Fitch in 1957. As described therein, a sheet of support material is interposed between a conductive roller and a surface having the toner powder image thereon. A charge of opposite polarity from the toner powder image is deposited on the back side of the sheet of support material. This charge attracts the toner powder image from the photoconductive surface to the support material.

Numerous factors effect the quality of the image transferred to the support material, the most significant factors being those which effect the uniformity of the toner powder image transferred thereto. The process of transferring multi-layered toner powder images, as exemplified in the process of multi-color electrophotographic printing, has produced various difficulties. In particular, transfer efficiency diminishes with variations in resistivity of the bias transfer roll. Transfer efficiency may be defined as the ratio of toner particles on the photoconductive surface to toner particles transferred to the support material. This produces a reduction in the density of the multi-color image reproduced on the support material. One factor that appears to influence the resistivity of the bias transfer roll is the relative humidity in the surrounding environment. As the relative humidity in the environment increases, the resistivity of the bias transfer roll decreases. The change in transfer roll resistivity effects the magnitude of the biasing potential applied thereto. Thus, the resultant image transferred is no longer optimum, and the characteristics thereof are degraded as the relatively humidity changes. Another factor influencing transfer efficiency is the change in resistivity for differing support materials. Generally, the support material may either be a sheet of plain paper or a thermoplastic sheet. The resistivity of the foregoing sheets differs substantially from one another. Hence, if the voltage applies to the bias transfer roll is optimum for a sheet of plain paper, it may no longer be optimum for a sheet of termoplastic material. Moreover, since the rate of change of resistivity is extremely slow, i.e. it may take days or weeks for the transfer roll to reach a stabilized resistivity, it is extremely difficult to manually compensate therefore, on a continuous basis during machine operation.

In addition to the problems of transfer efficiency, hollow characters, i.e. the periphery of the image rather than the entire image is transferred, and blurred characters may occur when the transfer system remains uncorrected for resistivity changes therein. The problem of hollow characters is most pronounced in line copy reproduction. However, hollow characters frequently occur in solid area copy as well. Hence, uncorrected variations in resistivity will degrade transfer efficiency as well as increasing the occurence of hollow characters and blurred characters.

Accordingly, it is a primary object of the present invention to improve transferring of the toner powder image from an image bearing member to a sheet of support material by correcting for changes in resistivity of the transfer member and the support material.

SUMMARY OF THE INVENTION

Briefly stated and in accordance with the present invention, there is provided a humidity compensated apparatus for transferring charged particles from a support surface to a sheet of support material.

In the preferred embodiment, the apparatus includes a transfer member and electrical biasing means. One of the features of the present invention is to have the sheet of support material secured to the transfer member. The transfer member is adapted to cooperate electrically with the support surface to attract the charged particles therefrom to the sheet of support material. Further, in accordance with the invention, the electrical biasing means biases the transfer member to a potential of sufficient magnitude to attract the charged particles from the support surface to the sheet of support material secured thereon. In the preferred electrical biasing means, the biasing potential applied to the transfer member is adjusted automatically for the resistivities of differing support materials, as well as for resistivity changes produced in the transfer member due to the relative humidity variations in the surrounding environment.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the drawings, in which:

FIG. 1 is a schematic perspective view of a multi-color electrophotographic printing machine incorporating the present invention therein;

FIG. 2 is a schematic perspective view of the apparatus of the present invention as employed in the FIG. 1 printing machine;

FIG. 3 is a fragmentary perspective view of a corona generator utilized in the FIG. 2 apparatus;

FIG. 4 is a graph illustrating the optimum transfer voltage applied to the transfer apparatus as relative humidity increases, and the approximations thereto by the electrical circuitry of the present invention; and

FIG. 5 is a schematic diagram of the electrical circuitry used in conjunction with the FIG. 2 transfer member to compensate for resistivity changes therein.

While the present invention will be described in connection with the preferred embodiment, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

With continued reference to the drawings, like reference numerals have been used throughout to designate like elements. FIG. 1 schematically illustrates a multi-color electrophotographic printing machine having the present invention incorporated therein. Although the humidity compensated transfer apparatus of the present invention is particularly well adapted for use in a multi-color electrophotographic printing machine, it should become evident from the following discussion that it is equally well suited for use in a wide variety of electrophotographic printing machines and is not necessarily limited in its application to the particular embodiment shown herein.

As shown in FIG. 1, the printing machine utilizes an image bearing member having a drum 10 with a photoconductive surface 12 secured to and entrained about the exterior circumferential surface thereof. Drum 10 is mounted rotatably within the machine frame and driven about its longitudinal axis by a drive motor (not shown) in the direction of arrow 14. U.S. Pat. No. 3,655,367 issued to Sechak in 1972 describes a suitable photoconductive material for use as photoconductive surface 12. As drum 10 rotates in the direction of arrow 14, photoconductive surface 12 passes sequentially through a series of processing stations. A timing disc (not shown) is mounted on one end of drum 10 and is adapted to coordinate the machine logic with the rotation thereof. The machine logic corrdinates the sequence of operations at each station to produce the proper events thereat.

Initially, drum 10 rotates photoconductive surface 12 through charging station A. A corona generating device, indicated generally at 16, extends in a generally longitudinal direction transversely across photoconductive surface 12. This readily enables corona generating device 16 to generate a spray on ions which charge photoconductive surface 12 to a relatively high substantially uniform potential. The foregoing corona generating device 16, is, preferably, of a type described in U.S. Pat. No. 2,778,946 issued to Mayo in 1957.

Drum 10, thereafter, rotates to exposure station B. At exposure station B, a color filtered light image of the original document is projected onto the charged photoconductive surface 12. A moving lens system, generally designated by the reference numeral 18, and a color filter mechanism, shown generally at 20, are disposed at exposure station B. As shown in FIG. 1, an original document 22, such as a sheet of paper, book or the like, is placed face down upon transparent viewing platen 24. Lamp assembly 26, filter mechanism 20 and lens 18 move in a timed relation with drum 10 to scan successive incremental areas of original document 22 disposed upon platen 24. Hence, a flowing light image of original document 22 is created and projected onto the charged photoconductive surface 12. Filter mechanism 20 interposes selected color filters into the optical light path to produce a single color flowing light image of the original document 22. The appropriate color filter operates on the light rays passing through lens 18 which record an electrostatic latent image on photoconductive surface 12. The latent image corresponds to a single color light image having light rays in a pre-selected spectral region of the electromagnetic wave spectrum. The electrostatic latent image formed by the single color light image will hereinafter be referred to as a single color electrostatic latent image. U.S. Pat. No. 3,062,108 issued to Mayo in 1962 describes a suitable moving lens system. A suitable color filter mechanism is described in copending application Ser. No. 830,282, filed in 1969.

After exposure, drum 10 rotates the single color electrostatic latent image recorded on photoconductive surface 12 to development station C. Three individual developer units, generally indicated by the reference numerals 28, 30 and 32, respectively, are disposed at development station C. A suitable developer unit is described in co-pending application Ser. No. 255,259, filed in 1972. Preferably, the developer units are all of a type referred to as magnetic brush developer units. In general, a magnetic brush developer unit utilizes a magnetizable developer mix having carrier granules and toner particles therein. The developer mix is continually brought through a directional flux field to form a brush thereof. The single color electrostatic latent image recorded on photoconductive surface 12 is developed by bringing the brush of developer mix into contact therewith. Each of the respective developer units contain discretely colored toner particles corresponding to the complement of the spectral region of the wave lengths of light transmitted through filter 20. For example, a green filtered electrostatic latent image is rendered visible by depositing green absorbing magenta toner particles on the charged regions of the photoconductive surface. Similarly, blue and red latent images have yellow and cyan toner particles, respectively, deposited in the charged regions of the photoconductive surface.

Drum 10 is, next, rotated to transfer station D. At transfer station D, the toner powder image adhering electrostatically to photoconductive surface 12 is transferred to a sheet of support material 34. Support material 34 may be plain paper or a sheet of thermoplastic material, amongst others. It is evident that the resistivity of plain paper is appreciably different from that of a thermoplastic material. Accordingly, it is desirable to correct the transfer apparatus for the changes in resistivity due to the varying support materials utilized thereat. Transfer station D includes corona generating means, indicated generally at 36, and a transfer member, designated generally by the reference numeral 38. Corona generator 36 is energized with an alternating current and arranged to spray ions on photoconductive surface 12 to pre-condition the toner powder image adhering electrostatically thereto. Hence, the pre-conditioned toner powder image will be more readily transferred from photoconductive surface 12 to support material 34 by transfer member 38. Corona generator 36 will be described hereinafter in greater detail with reference to FIG. 3. Electrical biasing means 40 biases transfer member 38 to a potential of sufficient magnitude and polarity to attract electrostatically the pre-conditioned toner particles from photoconductive surface 12 to support material 34. As will be described hereinafter in greater detail with reference to FIGS. 4 and 5, electrical biasing means 40 is adapted to adjust the biasing potential applied to transfer member 38 in an inverse relationship with variations in the relative humidity of the surrounding environment. Moreover, electrical biasing means 40 corrects automatically for the resistivity of differing support materials. Transfer member 38 is a roll adapted to recirculate support material 34 and rotates in synchronism with drum 10. In this case, transfer roll 38 rotates in the direction of arrow 42 at substantially the same angular velocity as drum 10. Inasmuch as support material 34 is secured releasably on transfer material 38 for movement in a recirculating path therewith, successive toner powder images may be transferred thereto, in superimposed registration with one another. Transfer member 38 will be described hereinafter in greater detail with reference to FIG. 2.

Reference will now be made to the method of advancing successive sheets of support material 34 to transfer roll 38. Feed roll 46, in association with retard roll 48, advances and separates the uppermost sheet from stack 44 disposed on tray 50. The advancing sheet moves into chute 52 which directs it into the nip between register rolls 54. Thereafter, gripper fingers, indicated generally at 56, mounted on transfer roll 38 secure releasably thereon support material 34 for movement in a recirculating path therewith. After a plurality of toner powder images have been transferred to support material 34, gripper fingers 56 release support material 34 and space it from transfer roll 38. Stripper bar 58 is then interposed therebetween to separate support material 34 from transfer roll 38. Thereafter, endless belt conveyor 60 advances support material 34 to fixing station E.

A fuser, indicated generally at 62, is disposed at fixing station E. Fuser 62 is adapted to coalesce the transferred powder image to support material 34. One type of suitable fuser is described in U.S. Pat. No. 3,489,592 issued to Moser et al in 1970. After the fixing process, support material 34 is advanced from fuser 62 to catch tray 68 by endless belt conveyors 64 and 66. At catch tray 68 the copy sheet is removed from the machine by the operator.

After the transfer of toner particles from photoconductive surface 12 to support material 34, residual toner particles remain on photoconductive surface 12. These residual toner particles are removed from photoconductive surface 12 as it passes through cleaning station F. At cleaning station F, the residual toner particles are initially brought under the influence of a cleaning corona generating device (not shown) arranged to neutralize the electrostatic charge remaining thereon. The neutralized toner particles are then mechanically cleaned from photoconductive surface 12 by rotatably mounted fibrous brush 70. A suitable brush cleaning device is described in U.S. Pat. No. 3,590,412 issued to Gerbasi in 1971. Rotatably mounted brush 70 is positioned at cleaning station F and maintained in contact with photoconductive surface 12. Brush 70 removes residual toner particles remaining on photoconductive surface 12 after each successive transfer operation.

Referring now to FIG. 2, there is shown therein transfer roll 38 and corona generator 36. Transfer roll 38 includes an aluminum tube 72, preferably, having about 1/4 inch thick layer of urethane 74 cast thereabout. A polyurethane coating 76, preferably about 1/2 mil thick, is sprayed over the layer cast urethane 74. Electrical biasing means applies a direct current bias voltage to aluminum tube 72 via suitable means such as a carbon brush and brass ring assembly (not shown). Transfer roll 38 is substantially the same diameter as drum 10 and is driven at substantially the same angular velocity. Contact between photoconductive surface 12 of drum 10 and transfer roll 38, with support material 34 interposed therebetween, is, preferably, limited to a maximum of about 1.0 lb. total linear force. A synchronous speed main drive motor rotates transfer roll 38. The drive motor is coupled directly to transfer roll 38 by a flexible bellows coupling 78 which permits the lowering and raising of transfer roll 38. Synchronization of transfer roll 38 and drum 10 is accomplished by precision gears (not shown) coupling the main drive motor to transfer roll 38 and drum 10. Preferably, transfer roll 38 has a durometer hardness ranging from about 10 units to about 30 units on the Shore A scale. The resistivity of transfer roll 38 preferably ranges from 10.sup.8 to about 10.sup.11 ohm-centimeters.

Referring now to FIG. 3, corona generating device 36 is shown therein in detail. Corona generator 36 includes an elongated shield 80 made preferably from a conductive material, i.e. an aluminum extrusion. Elongated shield 80 is substantially U-shaped and may be grounded or, in lieu thereof, biased to a suitable electrical voltage level. A corona discharge electrode 82 is mounted in the chamber defined by U-shaped shield 80. Discharge electrode 82, is, preferably, a coronode wire approximately 0.0035 inches in diameter extending longitudinally along the length of shield 80. Coronode wire 82 is made, preferably, from platinum. Discharge electrode 82 is energized to produce a flow of ions adapted to pre-condition the toner particles deposited on photoconductive surface 12. Pre-conditioning the toner powder image improves the efficiency of transferring the toner powder image from photoconductive surface 12 to support material 34. Preferably, discharge electrode 82 is excited to about 110 micro-amperes A.C. by a voltage source of about 4400 volts RMS A.C. The alternating current output from coronode wire 82 to photoconductive surface 12 with the toner powder image thereon is, preferably, about 4.0 micro-amperes.

Turning now to FIG. 4, there is shown a graph of the optimized voltage applied to transfer roll 38 as a function of relative humidity. As the relative humidity increases, the voltage applied to transfer roll 38 should decrease. Relative humidity decreases the resistivity of transfer roll 38. Hence, if the voltage applied thereto remains constant, the magnitude of the electrostatic field applied to the toner particles will increase. It is, therefore, evident that it is desirable to decrease the voltage applied to transfer roll 38 as the resistivity thereof decreases to maintain the electrostatic field applied to the toner particles substantially constant. Curve 84 depicts the ideal change in voltage applied to transfer roll 38 as the relative humidity increases. Curve 84 is shown for a sheet of plain paper having a discrete resistivity. However, when a thermoplastic sheet of support material is utilized the resistivity thereof is greater than that for a plain sheet of paper and the voltage applied to transfer roll 38 should correspondingly increase. For example, a transparent polymeric non-fibrous sheet of support material 34 made from a polysulfone thermoplastic available in sheets of approximately 3 mils thickness under the trademark Rowlex from Rowland Products, Inc., Kensington, Connecticut, will require a voltage increase ranging from about 400 to 600 volts over that of plain paper. Thus, the voltage increases when transparencies rather than opaque copies are being formed. Electrical biasing means 40 is adapted to provide a best straight line fit to curve 84. When support material 34 is a plain sheet of paper, electrical biasing means 40 will approximate straight line 86. However, when support material 34 is the exemplified sheet of thermoplastic material, electrical biasing means 40 will approximate straight line 88. Straight line 88 is parallel to straight line 86 and separated therefrom by about 400 to 600 volts, i.e. the voltage differential required to effect transfer when the exemplified thermoplastic sheet rather than plain paper is utilized as support material 34.

Referring now to FIG. 5, there is shown therein the detailed structural configuration of electrical biasing means 40. Electrical biasing means 40 includes a voltage source 90 and a resistor 92 connected in series therewith. Preferably, voltage source 90, and resistor 92 are connected in parallel with transfer roll 38. Voltage source 90 preferably produces an open circuit output voltage of about 5000 volts D.C. Register 92 is preferably about 200 meg-ohms. In this configuration, as the relative humidity increases the resistivity of bias transfer roll 38 decreases, and the voltage applied to transfer roll 38 will also decrease. By a judicious selection of the magnitude of resistor 92 and the voltage level of voltage source 90, a best fit straight line approximation to curve 86 may be obtained for a sheet of plain paper support material. Similarly, a best fit straight line approximation to curve 88 will also be formed for a thermoplastic sheet of support material. Hence, the large series resistor with transfer roll 38 provides a quasi-self-regulating system. As the resistivity of transfer roll 38 decreases with increasing relative humidity, resistor 92 tends to compensate for the reduced transfer roll resistivity. The 5000 volt voltage source 90 with the 200 meg-ohm series resistor 92 is utilized because it provides a best fit to the characteristic curve of FIG. 4. Hence, it is evident that electrical biasing means 40, as depicted in FIG. 5, automatically corrects for changes in relative humidity by decreasing the voltage applied to transfer roll 38 as the relative humidity increases. Moreover, the foregoing circuit automatically corrects for the resistivity of different support materials. This is achieved by voltage source 90 in conjunction with resistor 92 automatically adjusting the voltage applied to transfer roll 38 as the resistivity thereof changes due to differing support materials being utilized and/or changes in the relative humidity.

In recapitulation, it is apparent that the transfer roll cooperating with the electrical biasing means hereinbefore described substantially maximizes transfer efficiency and minimizes hollow characters and blurred characters by adjusting the voltage applied to the transfer roll as the resistivity thereof changes. Any resistivity changes due to variation in the relative humidity of the surrounding environment or due to the use of a different support material is automatically corrected by the electrical circuitry of the electrical biasing means. Thus, it is feasible to optimize the voltage applied to the toner particles on the photoconductive surface to insure maximum transfer thereof to the support material. In this manner, transfer efficiency is optimized and hollow characters and blurred characters are minimized such that substantially the entire area of the toner powder image is transferred to the support material.

It is, therefore, evident that there has been provided in accordance with this invention a humidity compensated transfer apparatus that fully satisfies the objects, aims and advantages set forth above. While this invention has been described in conjunction with specific embodiments thereof, it is apparent that many alternatives, modifications and variations will be evident to those skilled in the art. Accordingly, it is intended to embrace all alternatives, modifications and variations as fall within the spirit and broad scope of the appended claims.

Claims

What is claimed is:

1. A humidity compensated apparatus for transferring charged particles from a support surface to a sheet of support material, including:

a transfer member having the sheet of support material secured thereto, said transfer member cooperating electrically with the support surface to attract the charged particles to the sheet of support material;

means for electrically biasing said transfer member to a potential of sufficient magnitude to attract the charged particles from the support surface to the sheet of support material;

means for correcting automatically said electrical biasing means to compensate for the resistivity of differing support materials and for resistivity changes produced in said transfer member by variations in the relative humidity of the surrounding environment; and

corona generating means disposed adjacent to the support surface and adapted to apply an alternating charge potential to the support surface pre-conditioning the charged particles thereon to readily facilitate the transfer thereof from the support surface to the support material by said transfer member.

2. An apparatus as recited in claim 1, wherein said correcting means adjust the biasing potential applied by said electrical biasing means to said transfer member in an inverse relationship with relative humidity variations in the surrounding environment.

3. An apparatus as recited in claim 2, wherein:

said biasing means includes a voltage source; and

said correcting means includes includes a resistance element electrically coupled in series with said voltage source, said voltage source and said resistance element being electrically coupled in parallel with said transfer member.

4. An apparatus as recited in claim 3, wherein:

said voltage source generates, preferably, about 5000 volts; and

said resistance element is, preferably, about 200 meg-ohms.

5. An apparatus as recited in claim 1, wherein said corona generating means includes:

an elongated shield defining an open-ended chamber; and

a corona discharge electrode mounted in the chamber of said shield and arranged therein to generate ions for pre-conditioning and charged particles on the support surface.

6. An apparatus as recited in claim 1, wherein said transfer member includes:

a cylindrical core of electrically conductive material;

a first layer of resilient material entrained about said cylindrical core and being substantially in contact therewith; and

a second layer of resilient material entrained about said first layer of resilient material and being substantially in contact therewith.

7. An electrostatographic pringint machine of the type wherein toner particles are transferred to a sheet of support material forming thereon a copy of the original document being reproduced, including:

an image bearing member having toner particles deposited thereon in image configuration;

a transfer member having the sheet of support material secured thereto, said transfer member cooperating electrically with said image bearing member to attract toner particles therefrom to the sheet of support material;

means for electrically biasing said transfer member to a potential of sufficient magnitude to attract the toner particles from said image bearing member to the sheet of support material;

means for correcting automatically said electrical biasing means to compensate for the resistivity of differing support materials and for resistivity changes produced in said transfer member by variations in the relative humidity of the surrounding environment; and

corona generating means disposed adjacent said image bearing member and adapted to apply an alternating charge potential to said image bearing member pre-conditioning the toner particles thereon to readily facilitate the transfer thereof from said image bearing member to the support material by said transfer member.

8. A printing machine as recited in claim 7, wherein said correcting means adjusts the biasing potential applied by said electrical biasing means to said transfer member in an inverse relationship with relative humidity variations in the surrounding environment.

9. A printing machine as recited in claim 8, wherein:

said biasing means includes a voltage source; and

said correcting means includes a resistance element coupled in series with said voltage source, said voltage source and said resistance element being electrically coupled in parallel with said transfer member.

10. A printing machine as recited in claim 9, wherein:

said voltage source generates, preferably, about 5000 volts; and

said resistance element is, preferably, about 200 meg-ohms.

11. A printing machine as recited in claim 7, wherein said corona generating means includes:

an elongated shield defining an open-ended chamber, and

a corona discharge electrode mounted in the chamber of said shield and arranged therein to generate ions for pre-conditioning and toner particles on said image bearing member.

12. A printing machine as recited in claim 7, wherein said transfer member includes:

a cylindrical core of electrically conductive material;

a first layer of resilient material entrained about said cylindrical core and being substantially in contact therewith; and

a second layer of resilient material entrained about said first layer of resilient material and being substantially in contact therewith.