US3866572A

US3866572A - Foraminous electrostatographic transfer system

Info

Publication number: US3866572A
Application number: US364463A
Authority: US
Inventors: Robert W Gundlach
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1973-05-29
Filing date: 1973-05-29
Publication date: 1975-02-18
Anticipated expiration: 1992-02-18
Also published as: BE815546A; CA1009503A; NL7407222A

Abstract

In electrostatographic apparatus for applying a high bias voltage between a roller electrode and a support surface to provide an electrical field for development material transfer between them without arcing or undesired corona, a roller electrode having an electrically conductive core to which said high voltage is applied and a normally thick roller body of foraminous open cell material highly compressed between the conductive core and the support surface.

Description

United States Patent Gundlach 1 Feb. 18, 1975 FORAMINOUS ELECTROSTATOGRAPHIC 3,633,543 1/1972 Pitasi 118/621 E 3,744,896 7/1973 Carreira 355/3 TRANSFER SYST M 3,776,723 12/1973 Royka ct a1 [75] Inventor: Robert W. Gundlac V NY. 3,796,183 3/1974 Thettu 118/70 73 A nee: Xerox Cor oration Stamford, 1 851g Conn p Primary Examiner-Mervin Stein Assistant Examiner-Leo Millstein [22] Filed: May 29, 1973 [21] Appl. No.: 364,463 [57] ABSTRACT In electrostatographic apparatus for applying a high 52 us. 01 118/637, 96/1.4, 117/175, bias voltage between a roller electrode and a pp 55 surface to provide an electrical field for development [51] Int. Cl G03g 13/00 material transfer between them Without arcing 0F 58 Field of Search 118/637; 117/175, 93.4; desired Corona, a roller electrode having an electri- 96/14; 55 cally conductive core to which said high voltage is ap plied and a normally thick roller body of foraminous [56] References Cited open cell material highly compressed between the UNITED STATES PATENTS conductive core and the support surface.

3,626,260 12/1971 Kimura et a1. 317/262 18 Claims, 3 Drawing Figures f -n 5/ 4- 54 u X/II PATENTEDFEBI8I9Y5 3,866,572 SHEET 1 OF 2 FIGJ FIG. 2 m

FORAMINOUS ELECTROSTATOGRAPHIC TRANSFER SYSTEM The present invention relates to the transfer of image developing charges or materials from one support surface to another in electrostatography, and more particularly to the use of endless foraminous members in connection with electrical fields for such transfers.

The best known example of such transfer is the conventional transfer step in xerography wherein toner is transferred from the photoreceptor (the original support surface) to the copy paper (the final surface). However, such development material transfers are required in other electrostatographic processing steps, such as electrophoretic development. In xerography, developer transfer is most commonly achieved by electrostatic force fields created by D.C. 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 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 D.C. biased transfer roller or belt rolling along the back of the paper. Examples of bias roller transfer systems are described in U.S. Pat. Nos. 2,807,233; 3,043,684; 3,267,840; 3,598,580; 3,625,146; 3,630,591; 3,691,993; 3,702,482; and 3,684,364. Also, French Pat. No. 2,065,390, German application OLS No. 2,102,634 and Brtish Pat. Nos. 1,210,666 and 1,302,922. U.S. Pat. No. 2,968,555 issued Jan. 17, 1961 to R. W. Gundlach in FIG. 3 and Column briefly discloses a xerographic transfer system utilizing a soft resilient sponge rubber coated transfer roller, preferably electrically conductive. However, no substantial deformation of this roller is shown or suggested.

Foraminous members have, of course, been utilized in other different applications in electrostatography, such as paper handling. Also, a planar, evenly compressed conductive pressure pad for intermittent latent (charge) image transfer is disclosed in U.S. Pat. No. 3,635,556 issued Jan. 18, 1972, to R. L. Levy.

The transfer of the development materials involves the difficult and critical physical detachment and transfer over of such particulate materials by high intensity electrostatic force fields from one surface into attachment with another surface, maintaining the same pattern and intensity as the original latent electrostatic image being reproduced without scattering or smearing of the developer material. This difficult requirement can only be met by careful control of the electrostatic fields, which must be high enough for transfer yet not cause arcing or excessive corona generation at undesired locations, since such electrical disturbances can easily cause scattering or smearing of the development materials.

It will be noted that in electrophoretic or photoelectrophoretic development that similar critical problems and requirements are present for sensitization and transfer of the image developer material. Of course, the function is somewhat different than for xerographic transfer since the image development materials (originally in a liquid suspension) are being selectively transferred from the conductive charge injecting surface to a blocking electrode surface, which may be the final support surface. Conventio'nally, however, the transfer to a final support surface such as paper or plastic sheets or webs of the image formed on the conductive surface is made in a subsequent transfer step. The electrically biased roller or web electrode in photoelectrophoretic imaging is conventionally called a blocking or imaging electrode rather than a bias transfer roll as in xerography. Further, there are important distinctions in the desired presence or absence of nip corona generation. The following U.S. patents are illustrative of electrophoretic systems and electrodes and various means by which control of their arcing or corona has been attempted: U.S. Pat. Nos. 3,384,565; 3,474,019; 3,551,320; 3,582,205 and 3,697,407. Of these, U.S. Pat. Nos. 3,474,019 and 3,551,320 illustrate deformable soft electrodes. Canadian Pat. No. 876,045 discloses a PEP blocking electrode with an open cell sponge-like layer for applying the liquid imaging material therefrom.

The critical function of pre and post-nip corona in xerographic bias roll transfer is discussed, for example, in copending application Ser. No. 309,562 filed Nov. 22, 1972, by Thomas Meagher, entitled Constant Current Biasing Transfer System now U.S. Pat. No. 3,781,105, issued Dec. 25, 1973, and in U.S. Pat. No. 3,702,482 by C. Dolcimascolo et al., issued Nov. 7, 1972.

Perhaps the most difficult technical problem in all roll or belt electrode systems for transferring of electrostatographic imaging development materials or charges between supports is that of controlling or suppressing arcing and undesired corona generations. In practical systems the transfer of materials must be effected while the two surfaces between which the material is being transferred are both moving at the same speed and in relatively close contact." This as a practical matter requires the material transferring electrode to be an effectively endless surface of a cylindrical roller or small endless belt. This in turn means that the surface of the roller or belt electrode must continuously move in and out of contact with the original support surface. This creates varying width air gaps at each side of the actual contact area (the nip region). The upstream or entrance air gap is conventionally referred to as the prenip region and the downstream air gap is the post-nip region. Due to the fact that the breakdown voltage across an air gap is very non-linear with changes in the gap dimensions (this characteristic is known as the Paschen curve) control of arcing or ionization in such air gaps when there is a high biasing voltage on the electrode is very difficult. The higher the applied bias voltage the more difficult such control becomes, yet in many applications high voltages are either required or highly desirable for efficient transferring of the material from the original surface or liquid suspension to the second surface. Further, since the field intensity for material transfer is a function of the spacing as well as the applied potential the biasing voltage charge is desirably applied as closely as possible to the original support, again further increasing the difficulty of preventing voltage breakdown by arcing or excessive corona generation in the nip itself as well as the pre and postnip gaps. Further, both vector direction and intensity of the applied electrical fields varies at different locations and times relative to the roller electrode because the electrical fields are geometrically dependent upon the electrode configurations, and change as the electrode moves. The present invention provides a system in which such desired high biasing voltages and close spacings may be maintained with a simple and effective arrangement also providing desired suppression of arcing and suppression or control of corona emissions in all of the nip, pre-nip and post-nip regions, for more eflicient and reliable application of high fields for transfer of development materials and other operations.

Discussing in further detail the xerographic bias roller transfer process, the paper contact with the photoreceptor must precede the applied build up of high electrostatic fields by the transfer roller for two reasons. First, if excessive fields exist when the paper is still approaching the toner image on the photoreceptor, then toner particles can prematurely transfer, spreading as they jump the pre-nip gap, resulting in fuzzy images. Secondly, air ionization in the pre-nip gap can occur, reversing the polarity of toner particle charges and therefore preventing their subsequent transfer in the nip. The latter effect usually occurs intermittently, because it is self-quenching, and so manifests itself in what has been called zebra-stripe transfer. In the nip an electrostatic field of about volts per micron is sufficient to transfer loose charged toner particles from the photoreceptor to the paper surface. However, in order to establish a stable electrostatic bond between the toner and the paper after it is transferred, a net charge should be applied to the back of the paper 0pposite from the toner charge sufficient to tack the transferred toner to the paper so that it will not be dislodged in the subsequent paper handling, which includes the stripping of the paper from the photoreceptor. As disclosed in the Thomas Meagher U.S. Pat. No. 3,781,105 Dolcimascolo et al. U.S. Pat. No. 3,702,482, this tacking charge may be created by deliberately inducing, but controlling, corona generation in the postnip gap with a transfer roller of an electrically relaxable material. A constant current bias voltage supply can compensate for resistance charges in the relaxable material. Alternatively, if the roller and paper are conduc tive enough, this tacking charge may be applied to the paper in the nip by the contact between the roller and the paper. It will be noted that post-nip corona generation to generate toner tacking charges is not required for conventional xerographic corona transfer systems, where both transfer and tacking are effected by ionically depositing charges on the back of the paper.

The transfer of development material disclosed herein may take place in conjunction with, or subsequent to, the application of light to reduce the electrostatic forces retaining the toner on the original support surface, although this is not essential. It will also be appreciated that the system of the invention may be operated in conjunction with various subsequent means for the separation of the second support from the first support where desired. That is, various conventional sheet stripping devices and/or electrostatic detacking may be utilized. Further, various different surface configurations of the original support surface may be accommodated.

Exemplary embodiments of the present invention are shown and described hereinbelow as incorporated in otherwise conventional exemplary electrostatographic apparatus and processes. Accordingly, said processes and apparatus need not be described in detail herein, since the above-cited and other references teach details of various suitable exemplary 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. Dessauer and Harold E. Clark, both first published in 1965 by Focal Press Ltd., London, England. All of the references cited herein are hereby incorporated by reference 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, which are substantially to scale, except as noted. wherein:

FIG. I is a schematic plan view of an exemplary photoelectrophoretic imaging system embodiment in accordance with the present invention;

FIG. 2 is the system and view of FIG. 1, showing the system in operation; and

FIG. 3 is another exemplary embodiment of the invention, in the form of a plan view, partially in crosssection, of a xerographic bias roller transfer system in accordance with the present invention.

Referring to the FIGS. 1-3, it may be seen that FIGS. 1 and 2 illustrate a foraminous bias roller system of the invention cooperatively improving an otherwise conventional photoelectrophoretic system 40 for electrophoretic development. Since the conventional details thereof are fully described in the above-cited references on electrophoresis, these details need not be described herein. FIG. 3 illustrates a xerographic transfer station incorporating a foraminous bias transfer roller in accordance with the invention. Here, also, other details of the transfer system 50 known in the art are taught in the above-incorporated references on bias roller transfer systems and need not be described in detail herein.

Considering first in general both the roller electrodes 42 and 15 of the

system

40 and 50, respectively, it may be seen from the partial cross-sectioning of these axial plan views that both rollers illustrated here are nor- .mally cylindrical and the vast majority of their crosssectional areas comprises a roller body of foraminous open cell material uniformly coaxially surrounding a much smaller central core of conductive material, such as a solid metal roller. The foraminous material here is shown as cylindrical and continuously bonded to the central core surface. However, it will be appreciated that the terms roller electrode or roller as used herein are not intended to be limited to integral cylindrical rollers. They are also intended to read broadly on equivalent structures such as moving endless belts of the same foraminous materials with either roller or stationary (sliding contact) arcuate conductive backing members. Examples of such equivalent structures are disclosed in the above-cited references.

The foraminous material of the rollers 42 and 15 is, for important reasons to be discussed subsequently, preferably open cell material rather than closed cell, i.e., having voids or pores which have openings to allow expulsion or transfer of the contents of individual cells when the cells are compressed. This material is preferably highly foraminous, i.e., the principle volume of the material in its normal uncompressed state comprises a multiplicity of convolute andseparated random voids or pores closely interspaced throughout the material,

so that the solid material itself can be considered primarily as discontinuous cell walls separating these voids and occupying only a minor portion of the total volume of the foamed material. Example of suitable materials are 25 pores or 45 pore (45 cells per inch) open celled polyurethane foam, which may be commercially obtained, for example, from the Scott Paper Company. The present invention is applicable to many different foraminous materials, many of which are commercially available, and given the criteria and teachings herein such materials may be readily selected by one of ordinary skill in the art of foraminous materials.

In the system 40 of FIGS. 1-2, the roller electrode 42 is shown with its foraminous body 44 normally uncompressed in FIG. 1 and operatively compressed in FIG. 2, It is compressively rotated by its conductive core 43 against an image support surface, here comprising a surface layer 45 of liquid electrophoretic developer material on a substrate 46, shown being conventionally optically image discharged. A high voltage bias supply 47 is connected between the substrate 46 and the conductive core 43 for continuous transfer of development material from the surface layer 44 onto the outer surface of the roller 42.

It may be seen that the foraminous body 44 of the roller 42 is highly compressed from its normal uncompressed radius 48 into close to the radius 49 of the conductive core 43. The maximum compression of the roller body 44, due to the curvature of the core 43 (which is not compressed), occurs at the normal nip area 51, and the compression is substantially less in the pre-nip area 52 and post-nip area 53. However, the low durometer resiliency of the foam roller body 44 provides a large area of roller surface contact 54 with the surface 45, extending well into the pre-nip and

post-nip areas

52 and 53, covering an area much larger than just the normal nip contact area 51.

The minimum distance 56 between the core 43 and the surface 45 is here (and also for roller less that. half the normal uncompressed thickness of the foraminous material, so that a substantial number of the normally open cells in the nip are closed by compression. In many cases substantially full compression is desired (closing of substantially all cells in the center of the nip). With suitably foraminous materials the spacing may be compressed to approximately only percent of the uncompressed spacing without excessive compression forces. With 50 percent compression (not practicably achieveable with a solid roller) the field in the nip can be twice the field in the pre and post-nip air gaps. As may be seen, the allowable deformation is such that the roller may act more like a rolling pillow or bag on the surface 45 than a normal solid roller. As shown, sufficient lateral roll bulging and deep chordal positioning can be achieved such that the surface contact area 54 is substantially as wide as the entire normal roll diameter. However, an uncompressed foraminous layer of only one/fourth to one/half inch in thickness has been successfully utilized.

In comparison, even very soft rubber rollers which are solid cannot achieve the desired forms and close nip spacings of the disclosed foraminous roller electrodes. Solid rollers can only be somewhat deformed, rather than compressed, causing severe internal stresses on the material in an attempt to bring the roller core close to the support surface. Further, with a foam roller a much lower effective durometer can be achieved than with a solid roller without having to go to a material which is so soft as to have poor strength and wear resistance properties. The much greater roller surface deformation which can be achieved with relatively light compression pressures in foam rollers provides a much greater surface contact areas for improved development or transfer and/or paper holddown, with much more even and reduced pressures for reduced wear and reduced distortion of components, in addition to the significant electrical advantages disclosed.

A seal coating 57 may be provided on the circumferential exterior surface of the roller electrode 42. The seal coating 42 is liquid impervious in the surface contact area to prevent the entrance of external liquid into the cells of the foraminous material 44, and to retain any liquid therein, yet is sufficiently elastic and resiliently conformable to the compression of the foraminous material. An example is 10 mil polyurethane. A seal coating of non-compressable material can be used to permit surface speed synchronization of the system.

Considering now the exemplary xerographic transfer system 50 to FIG. 3, the similar bias roller 15 thereof has a foraminous roller body 21 with a conductive core 22 connected to a transfer bias voltage source 23. The bias source 23 is connected through common grounds 13 to a conductive backing roll 12 against which the roller 15 is highly compressed with moderate pressure. Through the nip 17 between the two rollers passes a flexible belt photoreceptor 11 and paper or other final support sheet 16. Negatively charged toner particles 10 previously developed onto the photoconductor l1 surface are retained thereon by positive latent image charges 14 until transfer is effected. Here, transfer by the bias source 23 occurs from the high fields in the nip area 17 created by the foraminous roller body 21 being highly compressed so that the conductive core 22 is closely spaced in the nip from the photoconductor.

The outer surface 24 of the bias roller 15 may be that of a seal coat 20 corresponding to the seal coat 57 of FIGS. 1 and 2. However, this is not required if the foraminous material is not liquid filled, or does not externally contact liquid inks, etc. The seal coating material can be the same, or different from, the foraminous material, and may be provided for wear resistance and cleanability properties in lieu of, or in addition to, liquid sealing.

As discussed in the introduction here, and in the above-cited references, particularly the pending application Ser. No. 309,562 by Thomas Meagher, in the transfer system 50 it is important to suppress corona in the pre-nip gap W, while in contrast the generation of a controlled corona in the post-nip gap Y is desired in order to apply a positive tacking charge 30 on the paper 16 to retain the transferred toner 10 on the paper when the paper 16 is stripped from the photoreceptor 11 (forming the gap Z th'erebetween).

The disclosed structure provides greatly improved control capabilities as compared to an ordinary transfer roller which maintains a generally cylindrical configuration and simple pre and post-nip air gap configurations. Not only is the shape and contact area of the nip different here, but also the foraminous material fills the space between the conductive core and the contacting supportsurface with a multiplicity of small discontinuities provided by the cells in the material, thereby providing a greatly improved air ionization control barrier. The foraminous material is significantly less compressed (less dense) in the pre-nip and post-nip areas than in the nip area. That is, it has a much greater thickness and porosity between the conductive core and the support surface in the normal pre-nip and postnip areas. The foraminous material extends much further laterally into the pre-nip and post-nip areas, due to its greater deformation, i.e., it has a much larger surface contact area. This increasing thickness and porosity of the foraminous material can be utilized to provide a varying ionization control barrier in the pre-nip and post-nip areas. Further, as previously noted, even further ionization control variability between the nip area and the pre and post-nip areas can be provided by' the degree of compression of the foraminous material in the nip. As the compression is increased, the individual cells may be collapsed, whereby the tops and bottoms of the cell walls contact one another. As more cells are collapsed the insulating pockets of air are eliminated and the electrical properties of the roller in the nip can change dramatically from those of the normal uncompressed porous material to those of a thinner solid roller in the same material. This greatly adds to the effect of the increase in field strength due to the closer conductor spacing geometry of the nip.

The motor 31 in FIG. 3 driving the roller 15 illustrates a means for eccentrically distorting the configuration of the foraminous material 21. A driving torque is applied by this motor 31 or any other support surface against which the roller is being deformed, causing the foraminous material to bulge out somewhat further in the pre-nip area and to be withdrawn somewhat in the post-nip area. Thus, both the area of surface contact and the thickness of the foraminous material in the prenip area may be made substantially greater than that in the post nip area. This provides a greater corona control barrier of such foraminous material in the pre-nip region than the post-nip region, which can be used to assist in providing a desired (above-discussed) suppres sion of pre-nip gap corona while simultaneously inducing post-nip gap corona, with the same bias level, as

shown.

It will be noted that the deformed radius of curvature of the roller 15 or 42 surface at the post-nip exit is much less than the normal roll radius. This sharp curvature, together with the paper beam strength, assists in assuring stripping of the paper from the transfer roll.

The foraminous bias roller systems disclosed herein may be utilized in at least three different material modes, with different operational properties andfunctions, although the above-described features and advantages are applicable to all three. One mode is to provide a foraminous material which is highly insulative, and therefore non-conductive to the bias voltage supply. Another mode is to provide a foraminous material which is resistive, but at least semi-conductive, such as an electrically relaxable material as disclosed in the above-cited references on bias transfer rollers. In this second mode at least part of the transfer bias charge will be conducted out toward the outer surface of the transfer roller, especially during compression. In a third mode, the foraminous material may be liquid filled. That is, the open pores of the material may be filled with a selected liquid. This latter mode has advantages in better controlling the resistivity of the foraminous material, since in the case of a liquid filled material the cell pores are not being exposed to humidity changes of ambient air as they open after the compression in the nip. As will-be subsequently noted, liquids of a formulated constant resistivity may be utilized to provide a selected bias charge transfer throughthe foraminous material by means of the liquids electrical conductivity.

Considering first the mode wherein the foraminous roller body is insulative and the pores are air filled, it may be seen that in this case the foraminous material does not affect the vector direction or intensity of the transfer fields. These fields will be controlled entirely by the geometry of the spacing between the conductive core and the conductive support surface. In this case the above-described function of the foraminous material in breaking up the air gap into many smaller individual air gaps separated by cell walls provides an important function. The foraminous material allows the application of biasing potentials of over 1,000 volts between the conductive core and the support surface with very close nip proximity therebetween to provide very high field intensities. The foraminous material can allow such high field intensities while either totally suppressing ionization in the entire air gap, or allowing some ionization in post-nip and suppressing it in the pre-nip and nip areas. However, in the case of either highly insulative or highly conductive foraminous materials it will be appreciated that the fields in the prenip and post-nip gaps will be symmetric for any applied charge, and that therefore it is not possible to simultaneously induce corona in one but not the other.

As will be recalled, the electrical field intensity at which any air breakdown occurs is a function of the air gap distance, and a smaller gap will support a much higher field intensity without breakdown. This is represented by the characteristic Paschen curve. The wide lateral extent of the roller contact area and its relatively even pressure insures that the air gap between the outer surface of the roller and the support surface is small and substantially constant to well outside of the normal nip areas, thereby suppressing arcing or undesired corona. No larger air gaps which would induce ionization are formed until the distance from the conductive core is so great that the field intensity or stress in the air gaps is below the ionization potential, i.e., the field intensity is greatly reduced by the time the larger pre or post-nip air gap is formed. With the foraminous roller, the normal position of the pre-nip and post-nip gaps can be greatly laterally displaced, yet simultaneously the distance between the conductors forming the transfer field can be made very small. These two inter-related desired criteria cannot be effectively met by a solid roller. They can be readily met by a foraminous roller body which is sufficiently thick and sufficiently compressible.

Considering now the materials mode in which the foraminous material of the bias roll is resistive rather than insulative, as noted above, this mode can be used to provide unsymmetrical fields. That is, the internal resistivity relaxation properties of the material can provide suppression of ionization in the pre-nip air gap while simultaneously encouraging it in the post-nip air gap. Suitable material resistivities for such relaxable transfer operations are discussed in above-cited references such as the Dolcimascolo, et al., U.S. Pat. No. 3,702,482 and the Thomas Meagher application. There a nonsymmetrical field distribution arises from relaxation of fields within the resistive roller over time. Here we have the additional advantage of changes in the effective bulk resistivity itself.

The foraminous material of the invention can provide significant improvements in such systems because the bulk resistivity (resistance per unit volume) of the foraminous material can be changed substantially by its compression. Thatis, as the foraminous material is compressed in the nip the actual resistance between the conductive core and the roller surface decreases. Therefore, the roller resistance and relaxation time in the nip is greatly lower than in the uncompressed prenip area of the roller. This allows a faster relaxation of the material in the nip, which in turn provides higher fields between the roller surface and the support surface in the nip and in post-nip over a much greater area. Accordingly, the effective latitudes for transfer are much greater electrically as well as mechanically. This allows a relatively higher resistivity material to be used, which material can be less humidity sensitive as to its resistivity and, therefore, more reliable. Such higher resistivity material, where foraminous, can insure pre-nip corona suppression even with substantial variations in humidity and paper, yet also prevent inadequate relaxation (excessive roll internal fields) which could cause inadequate transfer fields or inadequate post-nip gap ionization fields. In fact, the foraminous relaxable material can effectively act as a complete insulator on the uncompressed entrance side of the nip.

Resistivity changes under compression for foraminous relaxable materials of several orders of magnitude have been experimentally observed, which clearly allows greatly improved design and operating control over charge relaxations. That is, once the tops and bot toms of the cell walls touch one another in compression the resistivity has been observed to sharply drop immediately. Conductivity changes of 100 to 1,000 times have been measured between the fully expanded foam and a practical degree of high compression achievable with low pressures.

Charge build-up from ionization inside an open cell foam structure can present a problem. However, one solution is to utilize a liquid filling of the foam material, as further discussed below.

A much larger and more uniform mechanical contact area of a foraminous roller surface with any paper between it and a photoreceptor provides greatly improved mechanical tacking of the paper to the photoreceptor. Thus, the chances of premature toner transfer across any significant air gap between the paper and the photoreceptor are greatly reduced, since the paper is already mechanically held against the photoreceptor before it can be subjected to fields sufficient for toner transfer, either from the transfer roll or from charges deposited on the paper from pre-nip corona. This is even more true of the fully insulative foraminous material mode previously described. Uniform and positive paper/photoreceptor contact, especially at the leading and trailing edges, is, of course, one of the principal advantages of a bias roll transfer system as opposed to a corotron transfer system and a foraminous roll is superior in this regard.

Considering the liquid filled mode, all or part of the open cells may be filled with a suitable liquid. In addition to changing the electrical properties the liquid filling can change the mechanical tacking properties since the weight of the liquid and its hydrostatic characteristics causes it to apply a uniform pressure over a large contact area between the outer insulating skin of the roller and the supportsurface. Electrically, in addition to the reduction of resistivity changes due to humidity, the liquid filling of the foraminous material has been observed to provide more uniform and higher density images.

With a liquid filled foam material the material does not have to be as critically compressed for good transfer, rather the volume resistivity of the liquid is the controlling factor. This can be advantageous because it does not require as large a compression to achieve its effect. A severe roller deformation is less desirable because it may present problems in permanent distortion of the roller or roller surface speed synchronization with the support surface. However, the same open cell material properties are utilized here to provide sufficient nip compression to take advantage of the volume resistivity of the fluid and therefore to provide a substantially lower resistance in the nip region. The substantial nip compression effectively prevents the use of such a roller as a liquid loaded development material applicator, which is not desired in any case. The seal coating prevents intermixture of the internal ionization control liquid with any developer material such as P.E.P. liquids.

Several different liquid materials have been utilized for the filling of the foraminous material. It will be appreciated that these noted here are merely exemplary. Examples are silicone oils doped, for example, with tin salts to a selected conductivity. Several drops of Bis- Tri-N-Butyl Tin Maleat doping has been utilized. However, a mixture of 65 percent GANEX and 35 percent Butyl Stearate has been found to be more reliable. Another example is a Sohio mix consisting of 3440 Sohio and isopropyl alcohol.

In conclusion, it may be seen that there has been described herein a foraminous roll system providing greatly improved operating properties and capable of overcoming many problems in electrostatographic systems. It will be obvious that the disclosed system is applicable to many other electrostatographic systems than those specifically discussed above. One example is in TESI systems wherein latent electrostatic image charge patterns are transferred from one support surface to another and the surfaces are subsequently separated. The rollers of the invention can overcome some of the similar ionization control problems in such TESI systems. Another possible application area would be induction charging systems in which a photoconductor on a transparent substrate is image exposed in a temporarily high field region generated in the bias roller nip.

The exemplary embodiments described herein are presently considered to the 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 electrostatographic apparatus wherein a transfer bias voltage is applied between a roller electrode and a first support surface to provide an electrical field for transfer therebetween while relative movement is provided between said roller electrode and said first support surface and said roller is variably deformed by said first support surface at a roller nip area with prellll nip and post-nip areas at each side of said nip area, the improvement in said roller electrode comprising:

an electrically conductive core to which said bias voltage is applied, spaced from said first support surface; and a thick highly compressible roller body of foraminous open cell material extending between said conductive core and said first support surface and having a substantial normal uncompressed thickness;

said foraminous material occupying the space between said conductive core and said first support surface with a multiplicity of small discontinuities provided by said cells in said material and providing an ionization control barrier;

said foraminous material being highly compressed between said conductive core and said first support surface in said nip area to a thickness at least approximately one-half of said normal uncompressed thickness;

said foraminous material being much less compressed in said pre-nip and post-nip areas than in said nip area, having a much greater thickness and porosity between said conductive core and said first support surface in said pre-nip and post-nip areas than in said nip area, and said foraminous material lying over a substantial area of said first support surface, for ionization control in said prenip and post-nip areas.

2. The apparatus of claim 1 wherein said foraminous material is substantially fully compressed in the center of said nip area and said conductive core closely approaches said first support surface in said nip area.

3. The apparatus of claim 1 wherein said first support surface carries electrostatically attractable image development material in a liquid, and wherein said roller electrode is a blocking electrode for electrophoretic imaging development thereof.

4. The apparatus of claim 1 wherein a second support surface member is engaged between said first support surface and said roller electrode, and said roller electrode is a bias transfer roller for electrostatic image transfer of developer material from said first support surface to said second support surface.

5. The apparatus of claim I wherein the exterior of said foraminous roller body has a substantially liquid impervious seal coating which is resiliently conformable to said compression of said foraminous material.

6. The apparatus of claim 1 wherein said foraminous roller body is non-conductive to said bias voltage and electrically insulates said conductive core from said first support surface and does not affect the electrical field therebetween.

7. The apparatus of claim 1 wherein said conductive core and said foraminous roller body are curvalinear and said core has a radius greatly smaller than the normal uncompressed radius of said foraminous roller body.

8. The apparatus of claim 1 wherein said open cells of said foraminous roller body are air filled.

9. The apparatus of claim 1 wherein said open cells of said foraminous roller body are filled with a resistive liquid material.

10. The apparatus of claim 9 wherein said liquid material is electrically conductive for said transfer bias voltage.

11. The apparatus of claim 1 wherein said foraminous material in said nip is compressed sufficiently to collapse a substantial portion of said open cells in said nip.

12. The apparatus of claim 1 wherein said conductive core and said roller body are cylindrical and coaxially mounted with said conductive core uniformly wrapped with said foraminous material, and wherein the radius of said conductive core is less than approximately onehalf the normal radius of said foraminous material.

13. The apparatus of claim 11 wherein said foraminous material is compressed in said nip to approximately 20 percent of its normal uncompressed thickness.

14. The apparatus of claim ll wherein said foraminous material is an insulator and is non-conductive to said transfer bias.

15. In electrostatographic apparatus wherein a transfer bias voltage is applied between a roller electrode and a first support surface to provide an electrical field for transfer therebetween while relative movement is provided between said roller electrode and said first support surface and said roller is variably deformed by said first support surface at a roller nip area with prenip and post-nip areas at each side of said nip area, the improvement in said roller electrode comprising:

an electrically conductive core to which said bias voltage is applied, spaced from said first support surface; and

a thick highly compressible roller body of foraminous open cell material extending between said conductive core and said first support surface and having a substantial normal uncompressed thickness;

said foraminous material being highly compressed between said conductive core and said first support surface in said nip area to a thickness greatly less than said normal uncompressed thickness;

said foraminous material being much less compressed in said pre-nip and post-nip areas than in said nip area and having a much greater thickness and porosity between said conductive core and said first support surface therein over a substantial area of said surface for ionization control in said pre-nip and post-nip areas;

wherein driving torque means apply torque to said roller electrode, resisted by said first support surface, for eccentrically distorting the configuration of said foraminous material so that said foraminous material extends substantially further into said prenip region than said post-nip region over said first support surface, thereby providing a greater said corona control barrier in said pre-nip region then said post-nip region.

16. In electrostatographic apparatus wherein a transfer bias voltage is applied between a roller electrode and a first support surface to provide an electrical field for transfer therebetween while relative movement is provided between said roller electrode and said first support surface and said roller is variably deformed by said first support surface at a roller nip area with prenip and post-nip areas at each side of said nip area, the improvement in said roller electrode comprising:

said foraminous material occupying the space between said conductive core and said first support surface with a multiplicity of small discontinuites provided by said cells in said material and providing an ionization control barrier;

said foraminous material being much less compressed in said pre-nip and post nip areas than in said nip area and having a much greater thickness and porosity between said conductive core and said first support surface therein over a substantial area of said surface for ionization control in said pre-nip and post-nip areas;

wherein said foraminous material is electrically resistive and conducts said transfer bias, and wherein the resistivity of said material compressed in said nip is rendered substantially lower than its resistivity in said normal thickness by a substantial percentage of compressively collapsed cells in said nip, to provide increased nip transfer field strength.

17. The apparatus of claim 16 wherein foraminous material in said nip is compressed to at least approximately one-half of said normal uncompressed thickness.

18. In electrostatographic apparatus wherein a transfer bias voltage is applied between a roller electrode and a first support surface to provide an electrical field for transfer therebetween while relative movement is provided between said roller electrode and said first support surface and said roller is variably deformed by said first support surface at a roller nip area with prenip and post-nip areas at each side of said nip area, the improvement in said roller electrode comprising:

an electrically conductive core to which said bias voltage is applied, spaced from said first support surface; and a thick highly compressible roller body of foraminous open cell material filled with an electrically resistive liquid material extending between said conductive core and said first support surface and having a substantial normal uncompressed thickness; said foraminous material occupying the space between said conductive core and said first support surface with a multiplicity of small discontinuities provided by said cells in said material and providing an ionization control barrier; said foraminous material being compressed between said conductive core and said first support surface in said nip area to a thickness less than said normal uncompressed thickness; said foraminous material being much less compressed in said pre-nip and post-nip areas than in said nip area and having a much greater thickness between said conductive core and said first support surface therein over a substantial area of said surface for ionization control in said pre-nip and postnip areas; wherein said foraminous material with said resistive liquid material therein conducts said transfer bias, and wherein its resistivity in said nip is substantially lower than its resistivity in said normal uncompressed thickness, to provide increased nip transfer field strength; and wherein the exterior of said foraminous roller body has a substantially liquid impervious seal coating which is resiliently conformable to said compression of said foraminous material.

Claims

1. In electrostatographic apparatus wherein a transfer bias voltage is applied between a roller electrode and a first support surface to provide an electrical field for transfer therebetween while relative movement is provided between said roller electrode and said first support surface and said roller is variably deformed by said first support surface at a roller nip area with pre-nip and post-nip areas at each side of said nip area, the improvement in said roller electrode comprising: an electrically conductive core to which said bias voltage is applied, spaced from said first support surface; and a thick highly compressible roller body of foraminous open cell material extending between said conductive core and said first support surface and having a substantial normal uncompressed thickness; said foraminous material occupying the space between said conductive core and said first support surface with a multiplicity of small discontinuities provided by said cells in said material and providing an ionization control barrier; said foraminous material being highly compressed between said conductive core and said first support surface in said nip area to a thickness at least approximately one-half of said normal uncompressed thickness; said foraminous material being much less compressed in said prenip and post-nip areas than in said nip area, having a much greater thickness and porosity between said conductive core and said first support surface in said pre-nip and post-nip areas than in said nip area, and said foraminous material lying over a substantial area of said first support surface, for ionization control in said pre-nip and post-nip areas.

5. The apparatus of claim 1 wherein the exterior of said foraminous roller body has a substantially liquid impervious seal coating which is resiliently conformable to said compression of said foraminous material.

12. The apparatus of claim 1 wherein said conductive core and said roller body are cylindrical and coaxially mounted with said conductive core uniformly wrapped with said foraminous material, and wherein the radius of said conductive core is less than approximately one-half the normal radius of said foraminous material.

13. The apparatus of claim 1 wherein said foraminous material is compressed in said nip to approximately 20 percent of its normal uncompressed thickness.

14. The apparatus of claim 1 wherein said foraminous material is an insulator and is non-conductive to said transfer bias.

15. In electrostatographic apparatus wherein a transfer bias voltage is applied between a roller electrode and a first support surface to provide an electrical field for transfer therebetween while relative movement is provided between said roller electrode and said first support surface and said roller is variably deformed by said first support surface at a roller nip area with pre-nip and post-nip areas at each side of said nip area, the improvement in said roller electrode comprising: an electrically conductive core to which said bias voltage is applied, spaced from said first support surface; and a thick highly compressible roller body of foraminous open cell material extending between said conductive core and said first support surface and having a substantial normal uncompressed thickness; said foraminous material occupying the space between said conductive core and said first support surface with a multiplicity of small discontinuities provided by said cells in said material and providing an ionization control barrier; said foraminous material being highly compressed between said conductive core and said first support surface in said nip area to a thickness greatly less than said normal uncompressed thickness; said foraminous material being much less compressed in said pre-nip and post-nip areas than in said nip area and having a much greater thickness and porosity between said conductive core and said first support surface therein over a substantial area of said surface for ionization control in said pre-nip and post-nip areas; wherein driving torque means apply torque to said roller electrode, resisted by said first support surface, for eccentrically distorting the configuration of said foraminous material so that said foraminous material extends substantially further into said pre-nip region than said post-nip region over said first support surface, thereby providing a greater said corona control barrier in said pre-nip region then said post-nip region.

16. In electrostatographic apparatus wherein a transfer bias voltage is applied between a roller electrode and a first support surface to provide an electrical field for transfer therebetween while relative movement is provided between said roller electrode and said first support surface and said roller is variably deformed by said first support surface at a roller nip area with pre-nip and post-nip areas at each side of said nip area, the improvement in said roller electrode comprising: an electrically conductive core to which said bias voltage is applied, spaced from said first support surface; and a thick highly compressible roller body of foraminous open cell material extending between said conductive core and said first support surface and having a substantial normal uncompressed thickness; said foraminous material occupying the space between said conductive core and said first support surface with a multiplicity of small discontinuites provided by said cells in said material and providing an ionization control barrier; said foraminous material being highly compressed between said conductive core and said first support surface in said nip area to a thickness greatly less than said normal uncompressed thickness; said foraminous material being much less compressed in said pre-nip and post nip areas than in said nip area and having a much greater thicKness and porosity between said conductive core and said first support surface therein over a substantial area of said surface for ionization control in said pre-nip and post-nip areas; wherein said foraminous material is electrically resistive and conducts said transfer bias, and wherein the resistivity of said material compressed in said nip is rendered substantially lower than its resistivity in said normal thickness by a substantial percentage of compressively collapsed cells in said nip, to provide increased nip transfer field strength.

18. In electrostatographic apparatus wherein a transfer bias voltage is applied between a roller electrode and a first support surface to provide an electrical field for transfer therebetween while relative movement is provided between said roller electrode and said first support surface and said roller is variably deformed by said first support surface at a roller nip area with pre-nip and post-nip areas at each side of said nip area, the improvement in said roller electrode comprising: an electrically conductive core to which said bias voltage is applied, spaced from said first support surface; and a thick highly compressible roller body of foraminous open cell material filled with an electrically resistive liquid material extending between said conductive core and said first support surface and having a substantial normal uncompressed thickness; said foraminous material occupying the space between said conductive core and said first support surface with a multiplicity of small discontinuities provided by said cells in said material and providing an ionization control barrier; said foraminous material being compressed between said conductive core and said first support surface in said nip area to a thickness less than said normal uncompressed thickness; said foraminous material being much less compressed in said pre-nip and post-nip areas than in said nip area and having a much greater thickness between said conductive core and said first support surface therein over a substantial area of said surface for ionization control in said pre-nip and post-nip areas; wherein said foraminous material with said resistive liquid material therein conducts said transfer bias, and wherein its resistivity in said nip is substantially lower than its resistivity in said normal uncompressed thickness, to provide increased nip transfer field strength; and wherein the exterior of said foraminous roller body has a substantially liquid impervious seal coating which is resiliently conformable to said compression of said foraminous material.