Patent Publication Number: US-10331055-B2

Title: Inner resistive film with ductile particles and outer resistive film

Description:
BACKGROUND 
     Electrophotographic printing devices, such as laser printing devices, form images on media like paper. In general, a photoconductive cylinder is charged over its entire surface, and then selectively discharged in accordance with the image to be formed. Charged colorant such as toner adheres to locations on the cylinder that have been discharged, and the toner is then directly or indirectly transferred from the cylinder to the media. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an example electrophotographic printing device. 
         FIG. 2  is a diagram of an example charge roller for an electrophotographic printing device. 
         FIG. 3  is a flowchart of an example method for applying a resistive film having at least two layers to a substrate like a charge roller for an electrophotographic printing device. 
         FIG. 4  is a diagram of an example thermally sprayed resistive film on a substrate. 
     
    
    
     DETAILED DESCRIPTION 
     As noted in the background section, in electrophotographic printing devices, a photoconductive surface is charged prior to being selectively discharged with an image to be formed on media. Printing devices employ either a charge roller or a corona wire to charge the photoconductive cylinder. The charge roller has a cylindrical conductive substrate to transfer a charge to the entire surface of the photoconductive cylinder. 
     In some electrophotographic printing devices, the charge roller is in direct physical contact with the photoconductive cylinder while charging, and has an outermost material made of a compliant, conductive rubber so as not to physically damage the photoconductive cylinder. In other electrophotographic printing devices, the outermost material of the charge roller is a hard ceramic. As such the charge roller is usually positioned with a physical air gap between the photoconductive cylinder and the charge roller to minimize potential damage to the photoconductive cylinder. 
     There are at least two issues involved with the charge roller&#39;s functionality of charging the photoconductive cylinder in such a way that ensures optimal print quality of the formed images on media. First, during charging of the photoconductive surface by the charge roller, high intensity discharges, which are referred to as streamers, can occur. Such high intensity discharge events can negatively affect print quality, because the photoconductive surface may not be uniformly charged. 
     Second, there is an optimal window of the physical gap between the charge roller and the photoconductive cylinder. If the roller-cylinder gap is less than the minimum gap specified by the window, the charge roller may contact the photoconductive cylinder if the machining tolerances of printing device components that affect the gap exceed the minimum gap. If the gap is greater than the maximum gap specified by the window, print quality is impaired. 
     As to the former issue, it has been found that the intensity of the streamers can be decreased by coating the metal charge roller core with a resistive film. As such, high intensity discharge events are reduced. Specifically, the intensity of the streamers is a function of the thickness of the film applied to the conductive surface, as well as electrical properties of the film. 
     As to the latter issue, it has been found that coating the metal charge roller core with a resistive material likewise increases the size of the optimal window of the physical roller-cylinder gap in which print quality remains high. Specifically, the maximum gap of the window is increased as the thickness of the resistive coating is increased. This is advantageous, because manufacturing tolerances and other challenges can make it difficult to precisely position the charge roller vis-à-vis the photoconductive cylinder within the confines of a small gap window. 
     For many types of electrophotographic printing devices, charge rollers are disposable components built into toner cartridges that are periodically replaced, or are otherwise considered periodically replaced consumable items. These types of charge rollers generally have a rubber coating as their resistive film. The rubber coating degrades relatively quickly over time, but the charge roller is regularly replaced when the toner of the toner cartridge of which the charge roller is a part is depleted and a new toner cartridge inserted into the printing device, or when separate replacement is performed to maintain print quality. 
     However, for commercial production environments, charge rollers are not built into toner cartridges, and further are not considered disposable components that are to be frequently replaced. This is at least because in many such production environments, the electrophotographic printing devices are treated as digital printing presses and run nearly constantly, such that downtime is undesirable. Therefore, it is desirable for the charge rollers to be considered nearly permanent components that are not normally replaced, or at most are infrequently replaced, within the printing devices. 
     In such electrophotographic printing devices, rubber-coated charge rollers are disadvantageous due to the impermanence of their rubber coatings. A more permanent resistive film is desirably employed, such as ceramic materials like various metal oxides, nitrides, and carbides. A noted shortcoming of such so-called permanent resistive ceramic films is that they are difficult to apply with great thickness. Above about 500 microns, the brittle ceramic films tend to crack during application to the charge roller&#39;s metal core and further may delaminate from the substrate. 
     A resistive coating of 500 microns or less in thickness does not result in optimal print quality, unfortunately, because high intensity discharge events are not minimized as much as desired. Furthermore, a charge roller having a resistive film 500 microns or less results in a relatively small roller-cylinder gap window in which print quality remains high. This small window can be difficult to achieve in electrophotographic printing devices without undue expense and redesign of the printing devices. 
     In the patent application entitled “resistive film with ductile particles,” filed on Jan. 31, 2017, and assigned application Ser. No. 15/500,907, techniques are presented to alleviate these shortcomings associated with permanent charge rollers. A charge roller for an electrophotographic printing device includes a cylindrical conductive substrate and a resistive film applied thereto to reduce high intensity discharge events. Ductile particles are disposed substantially uniformly throughout the resistive film to reduce the film&#39;s brittleness, thus reducing the likelihood of cracking and delamination, even for films greater than 500 microns in thickness. Extending film thickness beyond 500 microns increases the roller-cylinder gap window in which print quality remains high and improves print quality at operating conditions relative to a thinner film by a further reduction in high intensity discharge events. 
     However, it has been found that in implementations in which the ductile particles are conductive, such as metal, the ductile particles at the surface of the resistive film may in some situations themselves cause high intensity discharge events. Particularly, when the charge roller is subjected to high voltage, free electrons resident in the conductive particles may respond to an air Paschen discharge, enhancing the resulting electric field, and further accelerating Paschen discharge. This cycle between air discharge and field enhancement proceeds through a positive feedback cycle, and eventually can potentially cause high intensity discharge events that can affect print quality. 
     Disclosed herein are techniques to alleviate high intensity discharge events resulting from the inclusion of conductive ductile particles within a resistive film-coated charge roller. A charge roller for an electrophotographic printing device includes a cylindrical conductive substrate and two resistive films. An inner resistive film is applied to the substrate to reduce high intensity discharge events, primarily by recessing highly conductive substrate from the discharge region, and includes conductive ductile particles disposed substantially uniformly therein to reduce brittleness. An outer resistive film applied to the inner resistive film does not have conductive particles therein, to further reduce high intensity discharge events, primarily those resulting from the conductive ductile particles at or near the surface of the inner resistive film. 
     The outer resistive film thus buries the reservoirs of electrons within the conductive ductile particles at the surface of the inner resistive film, minimizing the effect that such electrons can have during charging. The resulting electric field is also reduced, for at least three reasons. First, the distance between the conductive substrate of the charge roller and the photoconductive cylinder is increased due to the additional thickness of the outer resistive film. Second, the distance between the reservoirs of electrons within the conductive particles in the inner resistive film and the photoconductive cylinder is increased. Third, thin, high curvature metal features that may be present in the conductive ductile particles are further distanced from the photoconductive cylinder. 
       FIG. 1  shows an example electrophotographic printing device  100 . Cylindrical components, such as rollers, of the device  100  rotate in the directions indicated by their arrows. A photoconductive cylinder  102 , which may also be referred to as a drum, rotates to receive a charge transferred by a rotating charge roller  104  across its photoconductive surface. The photoconductive cylinder  102  and the charge roller  104  are separated by a gap  122  that is within an optimal gap window in which print quality remains high. 
     An optical discharge mechanism  106 , such as a laser, selectively discharges the photoconductive cylinder  102  in accordance with an image to be formed onto media  116 , such as paper, as the cylinder  102  continues to rotate. At least one rotating dispensing roller  108  transfers toner to the photoconductive cylinder  102  as the cylinder  102  continues to rotate. The toner is deposited onto the photoconductive cylinder  102  typically just where the cylinder  102  has been discharged, and thus in accordance with the image to be formed. 
     As the photoconductive cylinder  102  continues to rotate with the selectively transferred toner thereon, a rotating transfer roller  112  transfers the toner from the cylinder  102  onto the media  116  that is advancing from left to right between the transfer roller  112  and a rotating impression roller  114 . The photoconductive cylinder  102  rotates past a cleaning mechanism  120  to completely discharge its photoconductive surface and remove any remaining toner still thereon before repeating the described process via being charged by the charge roller  104 . 
       FIG. 2  shows an example of the charge roller  104  in more detail. The charge roller  104  has a cylindrical conductive substrate  202 , which may be steel. The conductive substrate  202  receives a charge to transfer to the photoconductive surface of the photoconductive cylinder  102  of the electrophotographic printing device  100 . The charge roller  104  further includes a resistive film  204  or coating, such as a ceramic film or coating, applied thereto to reduce high intensity discharge events while the photoconductive surface of the printing device  100  is being charged. 
     A portion  206  of the resistive film  204  of the charge roller  104  is shown in magnified fashion in  FIG. 2 . Specifically, the resistive film  204  is made up of an inner resistive film  210  and an outer resistive film  212 . One or more of the resistive films  210  and  212  may be a ceramic material, such as alumina-titania (Al2O3-TiO2). That is, the resistive films  210  and  212  may be the same or different material. The inner resistive film  210  includes conductive ductile particles  208  disposed substantially uniformly therethrough. By comparison, the outer resistive film  212  does not include any conductive particles. 
     The conductive ductile particles  208  disposed substantially uniformly throughout the inner resistive film  210  to reduce brittleness of the overall resistive film  204 , reduce potential for delamination of the film  204  from the conductive substrate  202  during application, and permit the thickness of the film  204  to be increased without cracking of the film  204 . The dispersal of the ductile particles  208  throughout the inner resistive film  210  further increases the maximum operating gap  122  between the photoconductive cylinder  102  and the charge roller  104  while maintaining or ensuring print quality. 
     In one implementation, the conductive ductile particles  208  are a metal or metal alloy, such as a nickel aluminum (NiAl) alloy. Testing has shown that when such ductile particles  208  are dispersed within a resistive film  210  of Al2O3-TiO2 at five percent by weight, which is 2.5% by volume, brittleness of the overall film  204  is greatly reduced. Specifically, brittleness of the resistive film  210  is reduced sufficiently to avoid cracking and delamination during application on the conductive substrate  202  at thicknesses up to two millimeters. This represents an increase of more than 300% as compared to an Al2O3-TiO2 resistive film  210  that does not have such NiAl ductile particles  208  dispersed substantially uniformly therein. 
     More generally, the conductive ductile particles  208  can be of a particular resistivity, size, and/or density that permits the thickness of the resistive film  204  to be increased to achieve the advantages and benefits associated with such increased thickness. Metals may have a resistivity in the range 5×10 −6  to 100×10 −6  Ohm-centimeters. Furthermore, metal silicides and amorphous metal-based alloys, which are in the class of metals, can have higher resistivity than crystalline metals, but with resistivity generally less than 1×10 −3  Ohm-centimeters; such metal inclusions may affect print quality. Examples of metal silicides include molybdenum silicide (MoSi2), tungsten silicide (WSi2), titanium silicide (TiSi2), magnesium silicide (Mg2Si), chromium silicide (Cr3Si), and NiSi. Examples of amorphous metal-based alloys include cobalt zirconium (CoZr), cobalt zirconium boron (CoZrB), molybdenum tungsten silicon (MoWSi), molybdenum tantalum boron (MoTaB), and cobalt hafnium silicon (CoHfSi). 
     The diameter of metal particles in the resistive film is desirably less than about ten microns, such as about two microns. Particle geometry depends on the method of film deposition, such as thermal spraying, as is described in detail later in the detailed description. The sizes mentioned herein are for disk diameters of thermally sprayed materials, and sphere diameters may be somewhat lower. 
     The volume density of ductile particles within the high resistivity coating is desirably below the percolation threshold for creating a continuous string of ductile particles across the thickness of the film, which is a function of particle geometry and orientation within the film. For spherical inclusions, the percolation threshold is usually about 25%, and for randomly oriented oblate ellipsoids with an aspect ratio of ten, the percolation threshold drops to generally 10%. Ductile metal particle concentration in thermally sprayed resistive coatings is desirably between 2% and 10% by volume. 
     The outer resistive film  212  is applied to and makes contact with the inner resistive film  210  that is applied to and makes contact with the conductive substrate  202 . The outer resistive film  212  ensures that there are no conductive particles on the exterior surface of the resistive film  204  as a whole. Any conductive ductile particles  208  that are at the outer surface of the inner resistive film  210  are covered, or buried, with application of the outer resistive film  212 . 
     The outer resistive film  212  serves to reduce high intensity discharge events in two ways during charging. First, along with the inner resistive film  210 , the outer resistive film  212  increases a thickness of the resistive film  204  as a whole. Because high intensity discharge event reduction is a function of increasing thickness, adding the outer resistive film  212  to the inner resistive film  210  makes for a thicker overall resistive film  204 . In this way, too, the outer resistive film  212  provides for an increase in the maximum operating roller-photoconductive cylinder gap, which is also a function of increasing thickness of the resistive film  204  as a whole. Second, because the presence of the outer resistive film  212  ensures that there are no conductive ductile particles  208  at or near the outer surface of the overall resistive film  204 , high intensity discharge events that would otherwise result from such exposed particles  208 , or particles  208  proximate to the surface, are reduced. 
     The outer resistive film  212  provides an additional benefit in that it decouples film surface topography, internal film morphology, and film surface chemistry from the inclusion of ductile conductive particles. Surface topography of a resistive ceramic charge roller coating may be affected by inclusion of the conductive ductile particles  208 . For example, the addition of the conductive ductile particles  208  into the inner resistive film  210  may increase the surface roughness thereof by disparate morphologies of ceramic versus metal portions or by the creation of more voids within the film. Rougher films are more likely to damage the photoconductive cylinder by incidental contact; the addition of the outer resistive film  212  ensures a uniform composition at the coating surface, thus mitigating any roughening associated with a mixture of ceramic and metal. 
     Adding the conductive ductile particles  208  to the inner resistive film  212  results in exposed particles  208  at the surface as well. In some situations, the conductive materials of the particles  208  may be more chemically reactive than the resistive ceramic coating of the inner resistive film  210 . As such, the exposed conductive materials of the particles  208  may react with the chemistry of the printing environment, leading to increased contamination of the charge roller or photoconductive cylinder surfaces. Overcoating the inner resistive film  210  with the outer resistive film  212  prevents these potentially detrimental effects. 
     The thickness of the outer resistive film  212  is sufficiently thin so as not to add undue brittleness to the resistive film  204  as a whole. As such, the thickness of the outer resistive film  212  is desirably thinner than that of the inner resistive film  210 . For instance, the thickness of the inner resistive film  210  may be in the range of 400 to 3,000 microns, whereas the thickness of the outer resistive film  212  may be in the range of 100 to 600, or even up to 1,000, microns. 
     In one example implementation, the inner resistive film  210  is an Al2O3-TiO2 23% alloy with a thickness of 1,500 microns, and the outer resistive film  212  is a Al2O3-TiO2 26% alloy with a thickness of 300 microns, which does not cause any meaningful brittleness to the overall resistive film  204 . In this implementation, the conductive ductile particles  208  within the inner resistive film  210  can be NiAl, where the outer resistive film  212  does not have any conductive particles. It has been found that print quality with this implementation is improved, due to the reduction of high intensity discharge events, as compared to a comparably thick resistive film  204  that includes just the inner resistive film  210  with the conductive ductile particles  208  and not the outer resistive film  212  without any conductive particles. 
       FIG. 3  shows an example method  300  for forming a resistive film including an inner film having conductive ductile particles dispersed substantially uniformly therein and an outer film without any conductive particles on a substrate. The method  300  can be employed, for instance, to prepare the charge roller  104  that has been described. A first material is prepared that includes a first base resistive material and conductive ductile particles dispersed substantially uniformly therein ( 302 ). The first base resistive material may be ceramic particles, such as Al2O3-TiO2, and the conductive ductile particles may be NiAl. 
     Preparing the first material can include adding the conductive ductile particles to the first base resistive material particles ( 304 ), and thoroughly mixing the resulting first material to disperse the ductile particles substantially uniformly throughout the first material ( 306 ). Substantially uniformly means that the conductive ductile particles are uniformly distributed throughout the first material as much as possible. Perfect uniformity is unachievable due to randomness, entropy, and so on, but thoroughly mixing the first material after the conductive ductile particles have been introduced for a sufficient length of time results in substantial uniformity. 
     The resulting first material is applied as an inner film to a substrate at a desired thickness ( 308 ), where the conductive ductile particles reduce the brittleness of the film, permitting greater thickness than otherwise would be possible. Application can be performed by thermal spraying of the first material onto the substrate to coat the substrate with the inner film. Thermal spraying includes flame spraying, plasma spraying, arc spraying, and high velocity oxy-fuel deposition techniques. The first material is fed in powder form, in diameters of five-to-fifty micron, into a high temperature flame that melts the particles and propels them towards the substrate, where the molten particles spread into “splats” and are quickly quenched into solid form as disks. Orientation of the disks is parallel to the substrate plane, so the percolation threshold that has been described is higher than for randomly oriented disks. Extreme temperature gradients and cooling rates lead to stresses in thermally sprayed films, which increase in magnitude with film thickness. However, negative effects of such stresses are reduced by the introduction of the ductile particles, as has been described. 
     A second material that includes a second base resistive material but without any conductive particles dispersed therein is applied as an outer film to the inner film that has been applied to the substrate ( 310 ). The second base resistive material—and thus in some implementations the second material as a whole—may be ceramic particles, such as Al2O3-TiO2. Application of the second material can be performed in the same manner as that of the first material, such as by thermal spraying of the second material onto the already applied inner film of the first material on the substrate to coat the inner film with the outer film. 
       FIG. 4  shows an example of a thermally sprayed resistive film  204  on the conductive substrate  202  of a charge roller  104 . The thermally sprayed film  204  is grown on the substrate  202  by successive deposition of particles, those of the inner resistive film  210  and those of the outer resistive film  212 . The particles of the inner resistive film  210  include the particles  402  that make up the bulk of the resistive film  210 , and the conductive ductile particles  404 . The particles of the outer resistive film  212  can include just the particles  406 , since the outer resistive film  212  has no conductive particles. The particles  406  of the outer resistive film  212  may be of the same material as the particles  402  of the inner resistive film  210 . 
     It is noted that the film  204  as depicted in  FIG. 4  is exaggerated for illustrative clarity. In actuality, the particles may be considered as being more pancake-shaped and randomly stacked, with fewer voids therebetween. Furthermore, the aspect ratio of the particles  402  and  406  that are ceramic is usually between 10:1 and 50:1, whereas the aspect ratio of the ductile particles  404  that are metal is usually between 2:1 and 10:1. It is also noted that more generally, particles can be of variously different and random shapes, in addition to those described herein. 
     The sizes of the conductive ductile particles that have been referenced above can refer to the diameter of the disks created in the thermal spraying process. The thickness of the disks is generally on the order of one micron, independent of disk diameter. Particles having a diameter of less than five micron are difficult to produce by some processing techniques like thermal spraying. Therefore, the conductive ductile particles may have a diameter of as close to five microns as possible, such as within the range of five to ten microns. Powder source material used in thermal spray systems is typically greater than five microns in diameter. 
     It is noted that in the implementations that have been described above, the ductile particles within the inner resistive layer or film have been stated as being conductive ductile particles, such as metal such particles. However, the ductile particles in other implementations may be ductile particles that have higher resistivity than metal, and therefore may not be considered conductive per se. The foregoing description is applicable to such implementations as well. The inclusion of any type of ductile particles may introduce undesirable features, such as increased roughness and voids, as well as greater chemical reactivity. Therefore, having an outer resistive layer or film that encapsulates the ductile particles of the inner resistive layer or film can be beneficial even if the ductile particles are primarily resistive and not highly conductive. 
     Examples of resistive ductile inclusions include non-stoichiometric metal oxides having a resistivity in the range of 10 −4  to 10 3  Ohm-centimeters. Thus, ductile metallic materials, such as NiAl, may be replaced with a high electrical resistivity material that still has sufficient ductility to afford the advantages associated with inclusion of the ductile particles within the inner resistive film to reduce brittleness. As noted above, most metals have electrical resistivity in the range of 5×10 −6  to 100×10 −6  Ohm-centimeters. Electrical resistivity of stoichiometric metal oxides range from about 10 3  to 10 13  Ohm-centimeters, but stoichiometric metal oxides are not usually ductile. However, ductility can be improved by adding metal beyond the stoichiometric composition, although doing so reduces resistivity. Still, the resistivity of non-stoichiometric metal oxides can be many orders of magnitude higher than for metals. 
     The diameter of resistive ductile particles in the resistive film is desirably within the range of about two to fifty microns. The sizes that have been noted are for disk diameters of thermally sprayed materials, and sphere diameters may be somewhat lower. 
     The volume density of resistive ductile particles within the high resistivity coating is desirably between 2% and 15% by volume. 
     Taking the above into account, examples of non-stoichiometric metal oxides that can be employed as the ductile particles include magnesium oxide (MgO x ), titanium oxide (TiO x ), zirconium oxide (ZrO x ), hafnium oxide (HfO x ), tantalum oxide (TaO x ), chromium oxide (CrO x ), cobalt oxide (CoO x ), iron oxide (FeO x ), copper oxide (CuO x ), aluminum oxide (AlO x ), and zinc oxide (ZnO x ). The resistivity range of such ductile particles  208  is within 10 −4  to 10 3  Ohm-centimeters.