US2912586A

US2912586A - Xerographic charging

Info

Publication number: US2912586A
Application number: US693866A
Authority: US
Inventors: Robert W Gundlach
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1957-11-01
Filing date: 1957-11-01
Publication date: 1959-11-10
Anticipated expiration: 1976-11-10

Description

1959 R. w. GUNDLACH 2,912,586

XEROGRAPHIC CHARGING ,v. Q a s L Filed Nov. 1, 1957 POWER SUPPLY FIG. 3

INVENTOR. Robert W. Gundlach ATTORNEY United States Patent XEROGRAPHIC CILARGING Robert W. Gundlach, Spencerport, N.Y., assignor to Haloid Xerox Inc., a corporation of New York Application November 1, 1957, Serial No. 693,866

6 Claims. (Cl. 250-49.5)

This invention relates in general to methods and means for selectively depositing electrostatic charge on insulating surfaces.

This invention is concerned with the application of a pattern of charges to an insulating surface. Once deposited the charges may be used as through scanning or the like or they may be developed with charged particles, charged droplets, or the like.

It is therefore an object of this invention to provide new and improved means of selectively depositing electrostatic charge on an insulating surface.

It is another object of this invention to provide new and improved methods of selectively depositing electrostatic charge on an insulating surface.

It is a further object of this invention to provide new and improved electromechanical methods and apparatus for forming an electrostatic charge pattern on an insulating surface.

Other objects and advantages of the present invention will be more readily apparent in view of the following detailed disclosure and description thereof, especially when read in conjunction with the accompanying drawings, wherein:

Figure 1 is a schematic perspective view of one form of apparatus according to this invention;

Figure 2 is a schematic perspective view of another form of apparatus according to this invention; and

Figure 3 is a schematic perspective view of a third form of apparatus according to this invention.

For a better understanding of this invention reference is now had to Figure 1, wherein charging through selective deposition of electrostatic charge on an insulating surface is shown. A chargeable member is positioned on conductive backing plate 14. Conductive rollers 18 permit backing plate 14 to move back and forth over conductive base plate 11 and establish a conductive path between plate 11 and plate 14. Support frames 12 are rigidly mounted to base plate 11 and act to support and position charging cylinder 15 above base plate 11. Although only one support frame 12 is fully illustrated, both contain the same elements. Charging cylinder 15 is supported in frames 12 through axles 13 which are journaled in electrical insulating blocks 16 which in turn are slidably mounted in frames 12. Charging cylinder 15 is biased against chargeable member 10 by springs 17 pressing against insulating blocks 16, whereby chargeable members 10 of varying thickness may be accommodated. Charging cylinder 15 comprises an inner conductive core 20 surrounded by an outer covering layer 21 of poor conductive material in which an insulating pattern 22 of insulating material is imbedded. Although preferably pattern 22 is imbedded in covering layer 21 as in this embodiment, it may also extend outward above the surface of covering layer 21. A power supply 23 is connected to and supplies potential to conductive core 20 of charging cylinder 15 with respect to backing plate 14. Extending outward from one of the axle extensions 13 is an electrically insulating crank handle 19.

The mechanism of this figure is operated by positioning a chargeable member 10 on backing plate 14 with its leading edge against charging cylinder 15. A potential difference is supplied from power supply 23 between the conductive core '20 of charging cylinder 15 and blocking plate 14 through base plate 11. An operator may then turn handle 25 which will cause the outer surface of charging cylinder 15 to rotate against a surface of chargeable member 10. As the chargeable member 10 comes beneath charging cylinder 15 backing plate 14 rolls on rollers 18 thereby allowing chargeable member 10 to move freely as motion is imparted to it by rotating charging cylinder 15. Springs 17 press on blocks 16 causing cylinder 15 to make pressure contact with chargeable member 10. If initially an uncharged chargeable member is placed into the mechanism as illustrated in Figure 1 and is moved beneath charging cylinder 15, all areas of chargeable member 10 corresponding or adjacent to areas of poor conducting covering layer 21 of charging cylinder 15 become charged following movement beneath charging cylinder 15. In areas on the chargeable member which correspond with and contact areas of insulating image 22 of charging cylinder 15 no charge will deposit. In this figure a positive potential is supplied from power supply 23 to conductive core 20 with respect to backing plate 14 and the charges which deposit on chargeable member 10, following movement of charging cylinder 15 in the direction indicated, define an image pattern or electrostatic charge pattern of no charge in areas of image and positive charge in areas of background on the surface of the chargeable member corresponding to the pattern on the covering layer 21 of charging cylinder 15. If it is desirable to charge areas of background negatively, negative potential is supplied to conductive core 20 of charging cylinder 15 from power supply 23, and operation of the mechanism is carried out as has already been described.

Reference is now had to Figure 2 wherein there is illustrated another embodiment of electrostatic charge image formation. In this embodiment two

rollers

24 and 25 formed of conductive material, each with axles 28, are mounted in support frame 27. The lower roller 25 is journaled in fixed insulating blocks 37; whereas the upper roller 24 is journaled in slidably mounted blocks 38 to allow roller movement upward and downward to accommodate between the rollers material or layers of varying thicknesses. Springs 34 bias blocks 38 and roller 24 downward. Although in this figure there is illustrated only one support frame 27 supporting and positioning the

rollers

24 and 25, this has been done for purposes of clarity in the illustration and it is to be realized that a frame similar to frame 27 is positioned on the other ends of the rollers. Motor 32 drives roller 25 through belt 31 and insulating pulley 30 mounted on axle 28. When the lower roller 25 is rotating motion is imparted to the material between the rollers and the upper roller 24 rotates freely as motion is imparted to it by moving material between the rollers. Both

rollers

24 and 25 are connected to power supply 33 which supplies a positive potential to one roller and a negative potential to the other. Desirably potential is supplied only while material is between the rollers or the rollers are positioned at a slight distance from one another to prevent a short circuit brought on by contact made by the two conductive rollers while electricity is being supplied. In this embodiment the upper roller is supplied with negative potential.

Passing between

rollers

24 and 25 are chargeable member 10 and charging master 35. Charging master 35 comprises a layer of poorly conducting material 39 on which or in which there is formed insulating pattern 36. In this embodiment positive charge exists on the surface of chargeable member prior to movement between the rollers. Movement between

rollers

24 and 25 causes negative charge fiow from upper roller 24 to the surface of chargeable member 10 through the poor conducting areas 39 and no charge fiow through the insulating pattern areas 36 of the charging master. The new charges reaching chargeable member 10 discharge the pre-existing positive charge except in areas of member 10 corresponding to pattern 36 where there is no effect. Thus, there is produced, following movement of a chargeable member 10 against a charging master 35 through

rollers

24 and 25, a chargeable member carrying charge only in areas corresponding to insulating pattern 36. It is to be realized of course that the charge ble member may be charged negatively prior to ins'e'r'ti betw een

rollers

24 and 25, and are'aeaf charge may be discharged by biasing the uppef' ro pditiii'fl'fbr by placing the sandwich or assembly b'tw fftlie rollers so that the charging master contacts the posit ixg roller. It is also to be realized that selective in pattern configuration may also be carried out in connection with the embodiment illustrated in Figure 1 by precharging the chargeable member. Similarly, deposition of charge in background areas only, i.e., those not corresponding to insulating pattern 36, would be carried out in the mechanism of Figure 2 if the chargeable member were not precharged prior to movement between the rollers. It is also to be realized that the mechanism of this figure may be used to further charge rather than discharge the background areas. Thus, for example, if a negatively precharged chargeable member were moved through

rollers

24 and 25 with the charging master 35 against the roller connected to the negative end of power supply 33, additional negative charge would flow through the poor conducting areas of master 35 resulting in areas of differing negative potential on the surfaceGFc'HaYEeEBIETrIcmber 10. Areas of background in such an instance would have more negative charge than areas corresponding to insulating pattern 36.

Although a motor drive is illustrated in connection with the apparatus of Figure 2, it is to be realized that the apparatus may be adapted for manual drive and may be tied in with other equipment. Similarly, the apparatus in Figure 1, which is shown to be manually driven, may be driven by a motor or the like.

Reference is now had to Figure 3 wherein another embodiment of means and methods of selective charge deposition is illustrated. A base plate 11 of conductive material supports on its surface chargeable member 10. Hinged at hinges 42 to base plate 11 is pressure plate 43 comprising conductive layer 44, poorly conductive layer 45, and handle 47. Handle 47 is made of insulating material and is used to open and close pressure plate 43 away from and into contact with the upper surface of chargeable member 10. Imbedded in poorly conductive layer 45 is a pattern of insulating material .46. Base plate 11 is connected to power supply 48 as is conductive layer 44 of pressure plate 43. Power supply 48 supplies a difference of potential to each of these members thereby creating fields of force between these members. Optionally, a switch may be positioned against the upper surface of base plate 11 to close the power circuit when pressure plate 43 is closed against base plate 11.

In this embodiment, as in the previous embodiments, when potential is applied between the conductive layer 44 of pressure plate 43 and base plate 11 and when a chargeable member 10 is in position on base plate 11 with the pressure plate 43 in closed position against chargeable member 10 an electrostatic charge pattern corresponding to the insulating pattern 46 will be formed on chargeable member 10. If negative potential is applied to conductive layer 44 and base plate 11 is grounded or placed at a positive potential and chargeable member 10 is not precharged, then areas of background will be charged negatively; if a positive potential is applied to conductive layer 44 and support table 11 is grounded or placed at a negative potential and chargeable member 10 is not precharged, then background areas of chargeable member 10 will be charged positively. If chargeable member 10 is precharged positively and a negative field generating potential is applied to conductive layer 44, then areas of background Will be discharged and areas corresponding to insulating pattern 46 will remain charged on chargeable member 10. Similarly, if chargeable member 10 is precharged negatively and a positive field generating potential is applied to conductive layer 44 areas of background will be discharged while other areas on chargeable member 10 will remain negatively charged. If a precharge is given to chargeable member 10 which is of the same polarity as the potential placed on conductive layer 44, areas of background will have the precharge strengthened while other areas will continue to hold the same amount of charge. Thus, it is possible, following the techniques described and illustrated in connection with Figures 1, 2, and 3 of this invention and the modifications which will readily occur to those skilled in the art, to create on a chargeable member an electrostatic charge pattern of differing charges corresponding to image and nonimage areas of an original pattern. In some cases, particularly where the insulating pattern is only a thin insulating coating on the poorly conductive layer, the chargeable member may acquire charge in areas C01- responding to the insulating pattern. This effect will disappear after the charging master or cylinder has been used a few times.

The term chargeable member used throughout this specification is intended to mean any member on which an electrostatic image pattern may be formed. Preferably, the chargeable member has good insulating qualities such as a resistivity in the order of above 10 ohm-centimeters and, preferably, it is uniform in thickness through out. It may be homogeneous throughout, or it may have a layered or laminated construction, provided that at least an outermost layer thereof has the necessary insulating qualities.

Experiments conducted in carrying out selective charge deposition according to this invention have shown that the resistive qualities of the poor conducting material, whether it be the covering layer 21 of the charging cylinder in Figure 1, the background poorly conductive layer 39 or 45 of the master layer in Figure 2 or Figure 3, should be within a particular range. The use of a too highly conductive material will prevent quality charge image formation by distorting selective charge deposition. The chargeable member, which is an insulating layer, will generally have localized spots of high conductivity. When contact is made with a spot of high conductivity in the chargeable member, the entire charge will travel through the path of least resistance if the resistant qualities of the poorly conductive material are too low. When the resistant qualtities of these areas are low, lateral conductivity takes place readily, and substantially no charge deposits on the chargeable member in contact with the poorly conductive layer when a point of high conductivity exists along the line or area of contact. Thus, following selective charge deposition using a poorly conductive layer of excessively low resistance, the chargeable member will be charged properly in some areas, but in lines, in the case of roller contact, or entire areas, in the case of the pressure plate type of contact illustrated in Figure 3, along which there exists localized points of high conduc tivity in the chargeable member substantially no charge will be deposited, thereby disrupting charge deposition according to this invention. Furthermore, the high current flow created by the low resistance at the point of high conductivity tends to destroy and break down the insulating material of the chargeable member in adjacent surrounding areas further destroying its subsequent useful' value. In addition, the high current flow through a small area of the poorly conductive layer is often suflicient to cause breakdown in this layer itself. Such breakdown will result in nonuniformity of charge deposition as the poorly conductive layer is used for subsequent cycles. Such destructive effects could be eliminated by a suitable resistor in series with the power supply, but this would not eliminate the nonuniformity of charging. For these reasons, it is desirable to avoid using a poorly conductive or cover layer made up of excessively conductive material, but in the unlikely event that the chargeable member is altogether free from conductive flaws there is no upper limit to the conductivity of the cover layer.

If, on the other hand, too good an insulator is used in areas of background for the covering or poorly conductive layer, charge will be drawn through the chargeable member and will deposit on the surface of the covering or poorly conductive layer rather than on the surface of the chargeable member. Excessive resistance in the covering layer will also unduly increase the time required to effect charge deposition. The upper limit of insulating characteristics in these areas is therefore set by the resistance characteristics of the chargeable member. The resistance characteristics may be defined or determined by the re sistivity and the thinness or thickness of the particular layer of insulating material included in the chargeable member.

To avoid or minimize the effects of localized breakdown or effects attributable to spots of high conductivity in the chargeable member, it has been found that the resistivity of the poorly conductive layer should generally be above ohm-centimeters using as thin a layer of material in these areas as is possible. The preferred range of practical resistivity which has been found for areas of background in the covering or poorly conductive layer is between 10 ohm-centimeters and 10 ohm-centimeters. Localized breakdown on the chargeable member does not cause breakdown of the charging system or mechanism using layers of materials for the cover or poorly conductive areas in this range, and selective charge deposition takes place effectively. For purposes of illustration without any intent to limit this invention, it is suggested the various vinyl plastics, as well as black rubber, fabrics, or the like, may be used as this cover or poorly conductive layer of material and the chargeable member may comprise, for example, various photoconductive insulators such as vitreous selenium; good insulating plastics such as Mylar or polyethylene, dry insulating paper, or the like. The photocondutive insulator as the chargeable member is valuable when, for example, it is desired to sensitize a xerographic plate with a halftone pattern or to add a charge pattern to the pattern on the plate.

The insulating characteristics of the insulating pattern area is preferably in the order of at least 10 times greater than the resistivity elsewhere in the poorly conductive or covering layer.

It is generally desirable to utilize as thin a material as possible in the background areas. If a point of high conductivity exists in the insulating layer of the chargeable member, the effect is to create a cone of lower potential in the adjacent areas of the master or covering layer. The cone tends to distort charging in those areas of the chargeable member beneath areas of the cone in the master covering layer. When this layer is thin the cone size is held to a minimum; whereas, when it is thick the cone is larger and the distortion in charge deposition created by the cone is greater on the surface of the chargeable member. Another beneficial feature which flows from a thinner master layer is that selective charge deposition takes place within a shorter time period than is necessary if this layer is thicker. This feature allows quicker selective charge deposition. The lower limit of thinness is determined by finding a layer which will operate without dielectric breakdown in itself at a point at which the insulating layer of the chargeable member is broken down. Thus, for example, it has been found in charging a chargeable member having a resistivity in the order of 10 ohm-centimeters and having a thickness of 20 microns using as the poorly conductive or covering layer a material having a resistivity in the order of 10 that the thickness in the covering layer should be at least 4 mils. As the resistivity of the background area of material becomes lower a thicker layer is necessary, and as the resistivity of this material increases a thinner layer is possible.

Selective charging or charge deposition according to this invention takes place to a maximum at which point the charging stops; that is, charge flow according to this invention is self-limiting. When the charge flow to the surface of the chargeable member reaches a maximum set by the particular configuration of the system or apparatus involved no further charge flow will take place even though the chargeable member remains in contact with the poorly conductive layer while backed by the conductive electrode. Thus, once it is determined what particular potential should be applied because of theparticular arrangement involved and because of the charge deposition desired, no fear need exist that damage through breakdown will be caused since charge deposition will only take place to the maximum set by the potential applied to that circuit.

The amount of potential applied to the biased electrode or electrodes is dependent on the particular arrangement being used. For example, in the arrangement in Figure 1, if it is desired to flow 300 volts to the background areas of chargeable member 10, conductive core 20 of cylinder 15 should be placed at a positive DC. potential in the order of 900 volts with respect to backing plate 14. The additional 600 volts, it is believed, are necessary because of the gaps present in the arrangement. Thus, in the arrangement of Figure 1 there would be a gap, although minute, between the surface of chargeable member 10 and the surface of charging cylinder 15 and a gap, although again minute, between chargeable member 10 and backing plate 14. Where an additional gap is added, such as in Figure 2, due to the minute gap between roller 25 and the charging master 35 carrying insulating pattern 36, a few hundred additional volts would be necessary for selective charge deposition. The particular amount of additional voltage necessary in the particular instance will depend on such features as the pressure brought to bear by the rollers or pressure plates, atmospheric conditions, resistivity conditions, and characteristics of the poor conductive material and the insulating pattern areas, thicknesses involved, and the like. Generally, however, it may be stated that in the various embodiments of this invention more than about 600 volts of potential must be applied before charge migration will take place through the poor conductive material.

While the present invention as to objects and advantages, as has been described herein, has been carried out in specific embodiments thereof, it is not desired to be limited thereby, but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. Apparatus for depositing an electrostatic charge pattern comprising a charging roller having an electrically conductive core and an outer covering layer of material having first areas comprising material having a resistivity between 10 to 10 ohm-centimeters and second areas having a resistivity at least 10 times greater than that of said first areas, a support table of electrically conductive material adapted to support a chargeable member, means to cause the charging roller to move in rolling contact with respect to the surface of the chargeable member, and an electrical power supply to supply D.C. potential between the conductive core of the charging roller and the support table.

2. The method of depositing an electrostatic charge pattern on a chargeable member comprising flowing charge through poor conductive material in contact with a chargeable member in areas of background to deposit charge on the surface of the chargeable member while blocking charge flow in areas of image with insulating material in contact with the chargeable member.

3. Apparatus for depositing an electrostatic charge pattern on a sheet-like chargeable member, comprising a first electrode having an electrically conductive surface positionable against and in contact with a first surface of the chargeable member, a second electrically conductive electrode having a surface less conductive than adjacent underlying portions of said electrode and positionable against and in contact with a second surface of the chargeable member, said surface of said second electrode comprising first areas comprising material having a resistivity between 10 and 10 ohm centimeters and second areas comprising material having a resistivity at least 10 times greater than that of said first areas, an electrical power supply connected to said electrodes and adapted to supply a DC. potential therebetween, and pressure means connected to said electrodes to urge them toward each other and against the chargeable member when supported therebetween.

4. Apparatus according to claim 3 in which the electrical power supply is adapted to supply a DC. potential of at least about 600 volts.

5. Apparatus for depositing an electrostatic charge pattern on a sheet-like chargeable member, comprising a first electrode having an electrically conductive surface positionable against and in contact with a first surface of the chargeable member, a second electrically conductive electrode positionable against a second surface of the chargeable member and supporting a covering material less conductive than said second electrode and having a surface positionable against and in contact with the second surface of the chargeable member, said surface of said covering material comprising first areas having a resistivity between 10 and 10 ohm centimeters and second areas having a resistivity at least 10 times higher than the first areas, an electrical power supply connected to said electrodes and adapted to supply a DC. potential therebetween, and pressure means connected to said electrodes to urge them toward each other and against the chargeable member when supported therebetween.

6. Apparatus according to claim 5 in which the DC. power supply is adapted to supply a DC. potential of at least about 600 volts. I 1

References Cited in the file of this patent l UNITED STATES PATENTS 191,176 Randall May 22, 1877 2,573,881 Walkup et al Nov. 6, 1951 2,576,047 Schafi'crt Nov. 20, 1951