US 3832053 A
An electrostatographic copying system in which an image is formed on an imaging surface and transferred at a transfer station to a copy sheet, where the copy sheet is transported through the transfer station on a belt which has a pattern of very closely spaced discrete conductive strips which are electrically biased to provide a pattern of electrostatic fringe fields holding the sheet onto the belt. The same conductors may be variably biased in the transfer station to effect tailored transfer fields.
Claims available in
Description (OCR text may contain errors)
Unlted States Patent 1 91 1111 3,832,053
Goel et al. Aug. 27, 1974 BELT TRANSFER SYSTEM 3,634,740 1/1972 Stevko 317/262 E 3,644,034 2/l972 Nelson 355/3 R  Y gfi g g' g ggt gggz 3,647,292 3/1972 Weikel 355/3 R  Assignee: Xerox Corporation, Stamford, Primary ExaminerRobert P. Greiner Conn.
 Filed: Dec. 3, 1973  ABSTRACT  App]. No.: 421,177 An electrostatographic copying system in which an image is formed on an imaging surface and transferred at a transfer station to a copy sheet, where the copy  Cl 355/3 5 sheet is transported through the transfer station on a  Int Cl j 15/14 belt which has a pattern of very closely spaced dis- 58] Fieid 96/1 crete conductive strips which are electrically biased to 317/262 f 9 provide a pattern of electrostatic fringe fields holding the sheet onto the belt. The same conductors may be 56] References Cited variably biased in the transfer station to effect tailored t f f ld UNITED STATES PATENTS fans er 1e 8 3194.131 7/1965 Robinson 355/3 R 12 Clam, 6 D'awmg F'gures 20 S l :II]: V
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- either mechanical or electrical forces.
BELT TRANSFER SYSTEM The present invention relates to an electrostatographic copying system in which copy sheets are electrostatically held and transported on a belt having a conductive pattern.
The accurate and reliable transport of copy sheets, particularly cut paper, through the work stations of electrostatographic copying systems is a particular problem due to the highly variable nature of such materials. Various sheet transporting devices, such as mechanical grippers, vacuum transport belts, feed rollers, wire guides, etc., are well known. It is also generally known that a copy sheet can be transported on a belt or other member which has been charged with an electrostatic charge pattern. The following U.S. Patents are exemplary of this general art of electrostatic tacking of paper to a paper transport belt by uniform or nonuniform electrostatic charging of the belt or paper: U.S. Pat. No. 2,576,882 to'P. Koole et a1; U.S. Pat. No. 3,357,325 to R. H. Eichorn; U.S. Pat. No. 3,642,362 to D. Mueller; U.S. Pat. No. 3,690,646 to .l. A. Kolivis,
U.S. Pat. No. 3,717,801 to M. Silverberg; and U.S. Pat. No. 3,765,957 to J. Weigl. The general concept of belts with alternating charged areas is suggested in these references, but not sufficiently close spacing to prevent interference with image transfer.
In the conventional transfer station in xerography, toner is transferred from the photoreceptor (the original support and imaging surface) to the copypaper (the final support surface). Such development material transfers are also required in other electrostatographic processing steps, such as electrophoretic development. in xerography, developer transfer is most commonly achieved by electrostatic force fields created by DC. charges applied to the back of the copy paper (opposite from the side contacting the toner-bearing photoreceptor) sufficient to overcome the charges holding the toner to the photoreceptor and to attract most of the toner to transfer over onto the paper. These xerographic transfer fields are generally provided in one of two ways, by ion emission from a transfer corotron onto the paper as in U.S. Pat. No. 2,807,233, or by a DC. biased transfer roller or belt rolling along the back of the paper. Examples of bias roller transfer systems are described in allowed U.S. Patent application, Ser.
No. 309,562 filed Nov. 24, 1972 by Thomas Meagher,
and in US. Pat. Nos. 2,807,233; 3,043,684; 3,267,840; 3,328,193; 3,598,580; 3,625,146; 3,630,591; 3,691,993; 3,702,482; and 3,684,364. U.S. Pat. No. 3,328,193 discloses a transfer system with spaced multiple rollers at different biases.
A particular copy sheettransport problem is the ac-' curate and positive transporting'of sheets into, through, and out of a xerographic or other electrostatographic transfer station. The copy sheet must be maintained'in accurate registration with the toner image to be transferred; The transfer electrostatic fields and transfer contact pressure are critical for good transferred'image quality. Further, the sheet typically acquires a tacking charge and the imaging surface has a charge on it as well. Thus, the copy sheet must be either. mechanically or electrostatically stripped (separated) from the imaging surface at the exit of the transfer station or process, yet without disrupting the transferred image whichis typically unfused at that point and easily disturbed by It may be seen that it is desirable to fully support and positively retain the copy sheet on the same transport through the entire transfer station, particularly including the removal of the sheet from the imaging surface. The present invention provides electrostatic means for continuously positively retaining a copy sheet, including its passage through a transfer station, on a single moving belt surface. Thus, the present system does not require a vacuum sheet retaining system, although it will be appreciated that a vacuum may be additionally applied in combination therewith if so desired.
Considering particularly references to prior transfer belt systems, U.S. Pat. No. 3,332,328 issued July 25, 1967 to C. F. Roth, Jr. discloses a xerographic transfer station including an endless loop belt for carrying the copy sheets through the transfer station, including contact with the xerographic drum, and corona charging means for placing a transfer charge on the back of the endless transfer belt.
U.S. Pat. No. 3,357,325 issued Dec. 12, 1967 to R. H. Eichom et al., also contains these same basic features, plus additional D.C. corona charging means to charge the sheet of copy paper on the belt prior to transfer, so as to hold the paper on the belt electrostatically. It should be noted however, that the charging of the paper (or belt) in this manner contributes to the total transfer potential, which is generally undesirable unless this additional charge can be held constant. A transfer corona generator is tilted relative to the back of the belt to provide the Eichorn transfer field.
U.S. Pat. No. 3,647,292 issued Mar. 7, 1972 to D. J.-
Weikel, Jr. discloses a uniform transfer belt system for carrying a copy sheet through the transfer station, vacuum means for holding the sheet on the belt, and transfer field generating means, which in one embodiment includes multiple stationary transfer electrodes in a stationary segmented plate with different (increasing) applied potentials acting at the back of the transfer belt. This reference is, therefore, particularly relevant to the present invention.
U.S. Pat. No. 3,644,034 issued Feb. 22, 1972 to R. L. Nelson discloses a segmented wide conductive strip transfer belt to which two different bias potentials are applied by two support rollers to those segments passing ovr the rollers. The conductive segments are separated by l/l6 inch insulative segments.
The most desirable aspects of a transfer system are high transfer efficiency with no image defects and high reliability, including insensitivity to external machine variables (relative humidity, paper type, etc.) where both are achieved with minimal complexity and cost.
As noted, an important aspect of reliability associated with the'transfer system is reliable paper handling. This must include good paper-to-photoconductor contact before application of an electric field sufficient for transfer. A bias belt transfer system offers the possibilityv of a reliable paper handling system with high transfer efficiency and less image defects. A belt-transfer system can take many forms. Its main distinguishing feature is the presence of a belt to which the paper is tacked reliably and then is carried through the transfer system and eventually on to the fusing system Thev corona or bias roll transfer systems. Belt transport into the transfer region can also remove the critieality of the paper lead-in configuration found in corona systems. Continuous sheet transport in and through the transfer region on the belt minimizes the chance of defects due to speed mismatch. The problem of insuring good paper-to-photoreceptor contact with thick papers and small photoconductor radii in corona transfer systems is eliminated in belt systems since it is only required to tack the paper to the infinite radius (substantially flat) belt, not the photoconductor. Further, lower nip pressures may be designed with more flexibility than a bias roller transfer system. Subsequent stripping of the copy sheet from the belt can be accomplished by using a sharp exit path radius; e. g. running the belt around a small radius roller, to make use of the inherent beam strength self-stripping action of the copy sheet.
In addition to paper transport gains, a belt transfer system offers potential special features. Among them are: simultaneous duplex, by initial toner image transfer to the belt and then reversing the charge of the toner (by corona treatment) before the next transfer pass; carrying paper directly to or through the fuser; and image preservation i. e. multiple copies from the same latent image.
The difficulties of successful image transfer are well known. In the pretransfer (prenip) region, before the copy paper contacts the image, if the transfer fields are high the image is susceptible to transfer across the air gap, leading to decreased resolution and, in general, to fuzzy images. Further, if there is prenip ionization, it may lead to strobing defects, loss of transfer efficiency, of splotchy" transfer and lower latitude of system operation. In the postnip region, at the photoconductor-paper separation area, if the transfer fields are low (say, less than approximately 12 volts per micron for lines and 6 volts per micron for solid areas) hollow characters may be generated, especially with smooth papers, high toner pile heights and high nip pressures (greater than approximately one pound per square inch). On the other hand, if the fields in the postnip region are improper, the resulting ionization may cause image instability and paper detaching from the belt. 1n the nip region itself, to achieve high transfer cfficiency and permanent transfer, the transfer field should be as large as possible (greater than approximately 20 volts per micron). To achieve these different fields in.
adjacent regions consistently and with appropriate transitions is difficult.
It will be noted that'the use of a fine charge pattem produced on. the imaging surface itself, for increased toner retention. by fringe field effects, e. g., for improved half-tone solid area image reproduction, is known. The fine charge pattern may be placed on the photoreceptor imaging surface by an optical screen, or by the photoreceptor construction itself, or by contact with a charging roller having a patterned or textured surface for .transferring a fine electrical pattern to the photoreceptor. For example, the imaging surface may be pattern charged by a contacting electrically charged wire screen or knurled conducting rubber roller at a suitable voltage. However, this type of structure is utilized for increasing the-quantity or uniformity of toner retained on a given area of the photoreceptor prior to its transfer to the copy sheet, and not forretention of acopy sheet. Thus, it affects the transfer by changing the image which is transferred. In contrast, the copy sheet transport system of the invention does not affect the imaging surface and does not affect the transfer process or the transferred image pattern.
The sheet transport system of the invention may be utilized in any desired path,'orientation or configuration. It may be utilized for transfer with an imaging surface which has any desired configuration, such as a cylinder or a belt. Belt imaging surface photoconductors in electrographic copying systems are examplified by US. Pat. No. 3,093,039 to Rheinfrank; US. Pat. No. 3,707,138 to Cartright, and US. Pat. No. 3,719,165 to Trachienberg, et al.
The above-cited and other references teach details of various suitable exemplary xerographic structures, materials and functions to those skilled in the art. Further examples are disclosed in the books Electrophotography by R. M. Schaffert, and Xerography and Related Processes by John H. Dessauerand Harold E. Clark, both first published in 1965 by Focal Press Ltd., London, England. All references cited herein are incorporated in this specification.
Further objects, features and advantages of the present invention pertain to the particular apparatus, steps and details whereby the above-mentioned aspects of the invention are attained. Accordingly, the invention will be better understood by reference to the following description and to the drawings forming a part thereof,
FIG. 1 is a schematic perspective view of an exemplary belt transfer system in accordance with the present invention, in an otherwise conventional xerographic copying system, with part of the upper belt surface broken away to show the conductors therein:
FIG. 2 is a magnified cross-sectional view taken along the line 22 of FIG. 1;
FIG. 3 is another embodiment of the. invention in a schematic side view;
FIG. 4 is a top view of the transfer station of FIG. 3;
FIG. 5 is a magnified cross-sectional view taken along the line 55 of FIG. 4; and
FIG. 6 is a magnified cross-sectional view taken along the line 6-6 of FIG. 4.
The embodiment of FIGS. M is preferred. Referring first to the other embodiment of FIGS. 1 and 2, there is schematically shown a belt transfer system 10 as an exemplary embodiment of the present invention. Since various details thereof are well known and fully described in the above-cited and other references relating to copy sheet handling, transfer, and xerography, those conventional details, for improved clarity, will not be described herein.
The system 10 here comprises a copy sheet transport belt 12 which is supported and rotatably driven between rollers 14 and 16. The transport belt 12 is preferably constructed from a relatively thinand uniform conventional dielectric material such as 5 to 25 mil Mylar, (polyetheleneterethalate) for example. (An additional relaxable or semiconductive backing layer may also be provided, as subsequently noted). The belt 12 has, over its upper surface, a very fine (closely adjacent) patternof interdigitated conductive stripes 13 extending linearly perpendicular the direction of belt movement. These conductors 13 may be placed on the belt 12 by conventional flexible printed circuit techniques. The conductors 13 may be protectively over- The copy sheet transport belt 12 positively supports, holds and carries the copy sheet 18 into and out of contact with an imaging surface 20 of a xerographic copying system 22 at a transfer station 24. Transfer is provided here at the transfer station 24 by three differently biased transfer rollers extending uniformly under 7 the belt in fixed positions. The'xerographic copying system 22 shown here also schematically includes the conventional stations, in order, for cleaning, charging, optical imaging and toner development of the imaging surface 20.
The transport belt 12, by an electrostatic fringe field charge pattern generated by differentially biasing the conductors. 13, provides positive retention of the copy sheet 18 at all desired points along the path of the transport belt 12, until it is desired to strip the copy sheet therefrom by any suitable conventional sheet stripping meansJWith the disclosed system the copy sheet 18 can be positively retained through the entire transfer station 24 without affecting the normal xerographic transfer in any way.
A highly desired feature of the electrostatic paper tacking pattern formed on the belt is that the adjacent conductive areas are sufficiently closely spaced, i. e., sufficiently fine, to form a very fine fringe field electrostatic pattern which will not affect the image transfer at the transfer station. Preferably the spacing between conductors is not substantially greater than the thickness of the copy-sheet or not greater than the thickness of the copy sheet plus the intervening belt material thickness if that is substantial. Such close or fine spacing will cause the fringe fields to extend primarily inside the copy sheet from the supported back surface thereof, and not extend appreciably outside of the front. or image-receiving, surface of the copy sheet. Thus, they will not affect transfer. Note FIG. 2 in this regard. For most conventional copy sheet thicknesses the preferred conductor pattern isthus approximately 0.13 millimeters (5 mils) in spacing between the conductive areas, with comparable conductor area widths, which provides 40 50 parallel conductors per centimeter. With this spacing the fringe fields generated on the underlying transport belt 12 will not significantly affect the transfer fields in the transfer nip of the transfer station. and thereby will not affect the transfer of toner to the upper or exposed surface of the copy sheet 18. Further, they will not disturb the toner once it is transferred to the copy sheet. This substantially, eliminates the chances for any observable toner print-out of the transport belt charge pattern onto the copy sheet. I
It will be noted that the adjacent conductors of the transport belt 12 do not have'to be biased to an opposite polarity. One can be grounded, or both can beof the same polarity but different levels. For paper tacking it is only necessary that adjacent conductors be charged or discharged to a substantially different, i. e., higher or lower, voltage level than so as to create fringe fields of appropriate intensity for retention of the particular copy sheets.
For applying the desired tacking bias voltages to the conductors 13 in the belt 12, the conductors are divided into two interdigitated sets, that is, each altemating conductor (one set) is brought out to one side, and the other set is brought out to the other side or edge of the belt 12. This may be seen in the broken-away area of the belt of FIG. 1. For better contacts and wear resistance the conductors may take the form at each edge of an exposed strip of thicker conductive pads such as more heavily plated copper or gold. These pads, however, must be spaced from one another so that each individual conductor remains electrically discrete. Since the alternating conductors 13 are thus provided with a line of contact pads moving linearly in an endless loop along with the rest of the belt surface, it may be seen that they may be easily electrically connected conventionally to any desired electrical bias source by any conventional sliding or rolling electrical contactor. This is illustrated inFlG. 1 by the extended linear bars or blocks 28 and 29 along opposite sides of the belt 12 which may be of copper, brass, carbon or other suitable contactor materials. The blocks 28 and 29 here apply opposite bias potentials to all of the conductors l3 thereunder, thus providing a copy sheet tacking field coextensive with their length over the belt surface between the blocks! in the embodiment of FIG. 1 the belt 12 is desirably wider than the imaging surface 20. Thus, even though here the conductor contact pads and the engaging blocks 28 and 29 are on the copy sheet carrying sideof the belt facing the imaging surface 20, the contact blocks 28 and 29 will not interfere with the imaging surface 20. The blocks 28 and 29 are interrupted (not present) in the transfer station 24 so that the conductors 13 are electrically floating there and will not form a Faraday shield blocking the transfer fields. However, the paper tacking charges already applied to the conductors will remain on them through transfer. Thus, copy sheet retaining fringe fields can be produced and maintained continuously on the belt 12 from the point where the copy sheet first engages the belt to the point after transfer where the copy sheet is to be stripped from the belt. Thus, the copy sheet is positively fully retained on the belt at all times, including transfer, yet without interference with the normal image transfer process.
Paper stripping and cleaning of the belt is preferably accomplished in uncharged areas of the belt, which can be provided wherever desired with the disclosed commutative belt structure. A grounding contact may be provided for the conductive pads in the desired stripping area to remove all tacking charges from the belt.
Considering now the transfer system of the embodiment of FIGS. 1 and 2, this is accomplished with three spaced apart and differently biasedtransfer electrodes 30, 31 and 32, which are respectively located under the belt 12 in'the prenip, transfer nip and postnip areas of the transfer station 24. The electrodes 30-32 are all mounted at-a fixeddistance from the imaging surface, basically determined by the thickness of the belt 12. Preferably they will ride against the back of the belt. 12, although they may vary in spacing or contact'with the belt 12 depending on the copy sheet presence and thickness. The electrodes are electrically insulated from the belt 12 here by the intervening dielectric backing of the belt.
The transfer'electrodes 30-32 here are shown as conductive rollers. However, they may also be fixed electrodes of any desired configuration, for example, rods of a diameter of approximately 1 centimeter or less, but preferably not so small as to act as corona generators with the applied voltages. The electrodes 30-32 preferably have the same diameter extending fully trans'- versely under the belt-so as to provide transversely uniform transfer fields.
As schematically illustrated, the electrical transfer biases applied to each electrode 30, 31 and 32 are from the same power source, but differ, so as to apply tailored (selectively varying) transfer field potentials to the imaging surface copy sheet interface as the copy sheet moves through the transfer station, i. e. along the belt path. The use of multiple transfer electrodes allows this to be accomplished without requiring the use of special electrically relaxable materials for the belt or the transfer electrodes. Typically, the voltage on the prenip electrode 30 may be only a few hundred volts, while the nip electrodes 31 may have approximately 5,000 volts bias, and the postnip electrode 32 a different bias again. A prenip field of less than approximately 2 volts per micron can be tolerated with a copy sheet to imaging surface air gap of greater than approximately 1 mil.
It will be appreciated, of course, that a different transfer system can be designed in which the transfer electrodes contact the back of the belt continuously, held thereagainst by a spring bias force or the like. They may be spaced along the belt by approximately one to one and a half centimeters, contacting a relaxable or resistive material layer overlying the back of the belt, which provides transfer field tailoring between electrodes. The same paper holding advantages of the present system may be provided by the use of the conductive pattern 13, since it will not interfere with any type of transfer system described herein.
Another desirable transfer electrode system for use with a transfer belt system is disclosed in a U.S. patent application Ser. No. 421,178 filed concurrently with this application by Walter C. Allen, commonly assigned. entitled Conductive Block Transfer System.
Considering now the transfer belt embodiment 50 of FIGS. 3-6, it may be seen that it has a belt 52 similar in construction and function to the belt 12 as described above. The pattern and spacing of the conductors 54 therein to achieve paper tacking fringe fields is preferably similar.
The system 50 differs in several respects, however. Here the lower surface of the belt carried the copy sheets 56 through engagement with the similar photoconductive imaging surface 58, and the pattern of conductors 54 here is on the oppositeor upper surface of the belt. However, the arrangement of FIGS. 1 and 2 could also be utilized here instead. The principal distinction of the system 50 is that in the transfer station 60 here transfer is accomplished by a constant transfer charge tailored transfer field system in which the transfer bias voltages are commutatively applied to selected belt-conductors 54 themselves by the transfer electrodes. Therefore, the transfer fields are created between thoseconductors 54 which are transfer biased and the imaging surface 58. These transfer biases may be applied to the conductors 54 by sliding or rolling (as shown) contacts at the edges of the belt in the transfer station 60.
The application of the paper tacking (fringe field generating) biases to opposite sides of the belt can be accomplished by sliding contacts 62 and 64 similarly to the blocks 28 and29 of FIG. 1. However, as shown,
these blocks 62 and 64 may be interrupted in the transfer station 60 so as to prevent conflicts with the transfer bias supplies. As previously noted, the belt preferably is wider than the imaging surface, and the contacts brought out to the edge of the belt so that all contacts can be made on either side of the belt. The arrangement of FIG. 3-6, desirably allows unimpeded access to the transfer nip since all electrodes are located on the side of the belt opposite from the imaging surface. Also, with this arrangement the belt conductors 54 may be left exposed on the back of the belt as shown. Normally, however, a thin dielectric coating would be applied over the conductors for protection and ease of cleaning except at the contact pad (side strip) areas.
Referring to the overall system 50 illustrated in FIG. 3, it may be seen that the belt 52 transports the copy sheets 56 without transfer from the input stack to a conventional heated roll fuser 68 from which the sheets exit to the output stack 70. In fact, if desired, the belt 52 may even be designed to carry the sheets right through the fuser, although this introduces certain material constraints and additional cleaning problems.
A retard sheet feeder 72, as described in U.S. Pat. No. 3,768,804 issued Oct. 30, I973 to K. K. Stange, for example, is shown feeding individual copy sheets from the input stack 65 into registered contact with the belt 52. From thereon the described electrostatic tacking forces hold the sheets in fixed positions on the belt surface until sheet stripping occurs at the sharp radius turn at the opposite end of the belt, at the fuser 68 entrance. An alternating current corona generator 74 may be positioned there, acting on the copy paper to neutralize any charges on the paper, thereby aiding stripping and preventing Lichtenberg figures (toner disruptions from air breakdowns at stripping). This corotron 74, like a detack corotron, preferably has a high output current sensitivity to the surface voltage for preferential neutralization.
After the copy sheets 56 are stripped from the belt 52, the return loop of the belt may be used for cleaning and charge neutralizing of the belt.
To prevent excessive toner build-up on the belt and to remove toner due to images transferred without paper moving through the transfer nip, the belt may be cleaned by one of the many standard cleaning systems, e. g., vacuum, brush, blade, web, biased fabric or magnetic brush. Due to low toner throughput, the requirements of the belt cleaning system are not as large as found in photoconductor (imaging surface) cleaning. FIG. 3 illustrates a cleaning system comprising a conventional pre-clean (toner neutralizing) corotron 76, followed by a fabric cleaning roller 78. This in turn is followed by a further belt surface charge neutralizing corotron to remove any belt surface charges, which could add to or-detract from the subsequently applied transfer fields. Although by proper choice of belt material cyclic surface charge buildup can be avoided, for
long term and low humidity reliability such a belt neutralizer may be desirable.
Also, for long term reliability it is desirable to provide a belt lift mechanism (not shown) for lifting the belt away from the imaging surface 58 during the shutdown periods of the copier. This could be provided by a solenoid retracted intermediate belt roller by way of example. Moving the entire belt system away by an appropriate releasable mounting of the belt end rollers could also be provided. v
Referring to FIGS. 5 and 6, these are enlarged cross sectional views through the belt 52 and a copy sheet 56 thereon, along the lines S-S and 6-6 of FIG. 4. Thus, FIG. is a cross sectional view along the longitudinal direction (of movement) of the belt at and beyond the postnip area, while FIG. 6 is cross sectioned perpendicularly through the rear edge (side) of the belt. The thickness of the printed circuit conductive strips 54 is somewhat exaggerated relative to the belt thickness for clarity here.
An important consideration for the thickness of the belt 52 here is that since the conductors 54 are on the back of-the belt, the dielectric material of the belt thickness is between these conductors and the imaging surface 58. Higher bias potentials on the conductors 54 are therefor needed for thicker belts in order to obtain the same transfer fields. A very high applied transfer voltage is undesirable, to avoid excessive air gap ionization occurring in pre or post nip air gaps.
However, this problem can be avoided both by thinner belts and by belts with a greater dielectric constant. Thus, a 20 volts per micron transfer field can be achieved with an applied conductor potential of only 3,000 volts witha belt having a dielectric constant of 5 and a thickness of 27 mils. Much thinner belts are practical with modern flexible dielectric materials. Of course, the conductors 54 do not have to be on the back of the belt, but can be sandwiched inside, closely adjacent the imaging surface, as previously noted.
Contact between a common transfer bias voltage potential source 82 and the conductors 54 in the transfer 60 could be accomplished by direct sliding or rolling electrical contacts. However, series resistance is desired to prevent ionization or arcing, both at the contacts themselves as the conductors make and break contact, and also possibly between adjacent conductors where a high potential difference exists. These problems are resolved here by a continuous thick strip of resistive material 84 commonly interconnecting and overlying the ends of the conductive strips 54 which extend to each edge of the belt. The resistive material is not critical. A suitable bulk resistivity is to lO ohmcentimeters. It should act as a short time constant (purely ohmic) conductor in the direction of belt movement, but not cause an excessive power drain between contactors. Here the contact with both the paper tacking and transfer bias sources is made through this resistive layer, which thereby functions as an additional high series resistance in the bias supply leads to prevent contact arcing problems and to protect the conductors from contact wear.
An even more important function of the strip of resistive material'84 is that it uniformly distributes the applied voltage between adjacent bias supply contacts evenly over all the intervening conductive strips, assuming the bias is applied evenly to both sides of the belt in thesame transverse line. Thus, if the spacing along the side of the belt between two adjacent contacts on the resistive material 84 is l centimeter and there are parallel conductors per centimeter, even a 5,000 volt difference between the voltages applied by the twocontacts will cause a voltage drop between conductors of only 250 volts, which is well below ionization potentials even for the closely spaced conductors 54.
Further, it will be noted that with the described system, where the transfer bias is applied to the belt conductors by multiple contacts, that tailored transfer fields can be generated without requiring any critical relaxable or self leveling resistance properties of the belt. Likewise, since the resistance material 84 is not in the nip its durometer is not important either. Any suitable plastic, carbon or rubber resistance material may be utilized.
The transfer bias contacts are provided here by six conductive wheels making continuous contact with the strip of resistive material 84 in the transfer station. It will be appreciated that sliding block or other contactors could be used instead. The contactor wheels are in commonly biased pairs at opposite sides of the belt,
' comprising here a prenip wheel pair 86, a nip wheel pair 87, and a postnip wheel pair 88. Each pair is differently biased to the appropriate level to achieve the electrical transfer field in the transfer region in which it is correspondingly located. The slrips of resistive material 84 therebetween smooth the bias level transitions between the individual conductors between each wheel pair, and also between the outside wheel pairs and the adjacent sliding contacts 62 and 64 which are applying the paper tacking biases. Because the transfer bias contactor wheels are provided with a constant voltage level from the common bias source 82, each individual belt conductor is temporarily provided with a constant preset transfer voltage as it passes a given point in the transfer station 24.
This is assisted by the fact that the conductors 13 in the belt are fully insulated at all times from both the copy sheet 18 and the imaging surface 20. Thus, the conductor bias levels are not affected by changes in ambient conditions such as humidity, copy paper, conduction, etc. Likewise, there is no ion flow (discharge) path between the conductors and the other transfer station components.
As previously noted, completely sealing the conductors inside the belt is desirable, so that contaminants will not affect the above-described properties of the system. Although less desirable, it will also be appreciated that spaced multiple corotrons can be used to apply the transfer bias potentials to the belt conductors.
The above described transfer system utilizing the resistive connecting strips 84 between the conductors 54 provides a constant voltage on each individual conductor transfer system. A constant charge transfer system can be provided if the resistive strips 84 are not present, so that each individual conductor 54 is electrically isolated and is directly sequentially briefly connected to a biased contactor as the belt moves past the contactors. That is, the contactor (especially if it is connected to a constant current'power supply) will put a given predetermined charge on each conductor while they are in contact. After the individual conductor discon nects from the contactor the same electrical charge will remain on it, because it is electrically floating and insue lated from all of the other conductors and other system elements. This floating charge on the conductor will dissipate slowly due to leakage currents, but at typical belt speeds this leakage will not be significant, so that the charge on each conductor will effectively remain constant until it is deliberately reduced or discharged by subsequent discharge means. The voltage on the indifferent position in which the distance between the same individual conductor and the imaging surface (the transfer gap) is increased, (and/or the dielectric thickness of the intervening copy sheet has increased) the capacitance between the conductor and the imaging surface decreases, which correspondingly increases the voltage on the individual conductor. This capacitance-controlled change in the voltage on the individual conductors occurs without any change in the initial bias voltage or charge supplied to the conductors and tends to keep the transfer field more constant as the transfer gap increases or decreases. Thus, this provides another desirable system design. It will be noted that with such a constant charge system the initial charge should be put on the conductors at the maximum capacitance region, i. e. in the transfer nip, since a greater charge can be put on the conductors for a given connecting bias voltage in this region, and therefore a greater transfer field can be provided.
The belt transfer system disclosed herein is presently considered to be preferred; however, it is contemplated that further variations and modifications within the purview of those skilled in the art can be made herein.
The following claims are intended to cover all such variations and modifications as fall within the true spirit and scope of the invention.
What is claimed is:
1. In an electrostatographic copying system in which an image is formed on an imaging surface and trans ferred at a transfer station to a copy sheet by electrical transfer fields generated by electrical transfer means, the improvement comprising:
copy sheet transport means including a copy sheet supporting belt,
said supporting belt having a multiplicity of integral electrically discrete closely spaced adjacent conductors forming an extensive pattern over said belt;
support means for moving said supporting belt intimately past said imaging surface at said transfer station; and biased electrode means for differentially electrically charging adjacent said conductors of said supporting belt for generating over said supporting belt a fine charge pattern of alternating closely adjacent differentially charged areas providing copy sheet retaining electrical fringe fields; the spacing of said conductors being sufficiently close so that said image transfer at said transfer station is unaffected by said charge pattern of electrical fringe fields. Y 2. The copying system of claim 1 wherein said spacing between said adjacent conductors is not substantially greater than the thickness of said copy sheet.
3. The copying system of claim 1 wherein said spacing between said adjacent conductors is less than apfield gap with said imaging surface.
5. The copying system of claim 4 wherein there are three of said transfer electrodes, spaced apart and differently biased and positioned at a prenip, nip, and postnip areas of said transfer station respectively, and wherein said supporting belt passes freely between said transfer electrodes and said imaging surface.
6. The copying system of claim 1 wherein said electrical transfer means comprises a set of differently biased contactor means electrically connecting different electrical biases sequentially to selected said conductors of said belt with movement of said belt at said transfer station to provide tailored transfer fields between said selected conductors and said imaging surface.
7. in an electrostatographic copying system in which an image is formed on an imaging surface and transferred at a transfer station to a copy sheet by electrical transfer fields generated by electrical transfer means, the improvement comprising:
copy sheet transport means including a copy sheet supporting belt;
said supporting belt having a multiplicity of integral electrically discrete closely spaced adjacent conductors forming an extensive pattern over said belt;
support means for moving said supporting belt intimately past said imaging surface at said transfer station; and biased electrode means for differentially electrically charging adjacent said conductors of said supporting belt for generating over said supporting I belt a fine charge pattern of alternating closely adjacent differentially charged areas providing copy sheet retaining electrical fringe fields;
wherein said electrical transfer means comprises a set of differently biased contactor means electrically connecting different electrical biases sequentially to selected conductors of said belt with movement of said belt at said transfer station to provide tailored transfer fields between said selected conductors and said imaging surface.
8. The copying system of claim 7 wherein said transfer electrode means comprises at least three fixed and spaced apart electrical contact members making said electrical connections sequentially with said conductors as said conductors move with said belt through said transfer station.
9. The copying system of claim 7 wherein said conductors are interconnected by electrical resistance material on said belt which smoothly distributes said electrical bias appliedby said electrical contact members to said conductors. I v
10. The copying system of claim 7 wherein said electrical contact members are in commonly biased pairs at opposite sides of said belt.
11. The copying system of claim 8 wherein said contact members are spaced apart along said. belt by a multiplicity of said conductors.
12. The copying system of claim 9 wherein said resistive material extends along at least one edge of thebelt, and said electrical contact members contact said resistive material whereby said resistive material provides a series resistance betweensaid electrical contact members and said conductors.