WO1994022060A1

WO1994022060A1 - Liquid toner developing apparatus

Info

Publication number: WO1994022060A1
Application number: PCT/NL1993/000066
Authority: WO
Inventors: Dorron Levy
Original assignee: Indigo N.V.
Priority date: 1993-03-23
Filing date: 1993-03-23
Publication date: 1994-09-29

Abstract

Imaging apparatus including an imaging surface (12) having formed thereon a latent image, including image portions at a first voltage and background portions at a second voltage and a developer operative to develop the latent image with a liquid toner comprising charged toner particles and carrier liquid. The developer includes an electrified moving developer electrode (18), operatively engaged with the imaging surface to form a development region therebetween, whereat the latent image is developed and a resilient blade (32), having an operative edge urged against the outer surface (19) of the developer electrode (18) after it leaves the development region, for removing toner particles from the developer electrode. The resilient blade (32) includes a relatively thin base sheet (38) formed of a relatively hard material comprising the operative edge and having a back surface (42) facing generally outwardly from the development roller and a built-up portion (44) having a non-uniform, hydrodynamically shaped thickness profile and mounted on at least a portion of the back surface (42) adjacent the operative edge of the resilient blade, the built-up portion being operative to substantially reduce accumulation of the toner particles on the resilient blade.

Description

LIQUID TONER DEVELOPING APPARATUS FIELD OF THE INVENTION The present invention relates to imaging apparatus using liquid toner in general and, more particularly, to development apparatus of such imaging apparatus. BACKGROUND OF THE INVENTION There are a great variety of electrostatic imaging systems known in the art in which liquid toner is used to develop an electrostatic latent image. Such systems may include a photoconductive surface which is electrically charged and then selectively discharged, by exposure to an optical image, to form an electrostatic latent image on the photoconductor surface. Some imaging systems, using liquid toner, employ moving developer electrodes having electrically conductive surfaces such as endless metal belts or rollers in developing the image. A portion of the electrode is closely spaced from the imaging surface so as to form therebetween a development region that contains liquid toner. The closely spaced electrode enhances the electrostatic field at the imaging surface and aids in the removal of excess carrier liquid and unused toner particles from the development region. U.S. Patent 4,400,079 to Landa describes a developing system utilizing a roller electrode having a conductive outer surface, a portion of which is closely spaced from the imaging surface. A foam wiper pad mounted against the electrode surface is used to remove toner deposits therefrom. In some imaging systems employing metallic developer electrode surfaces, used toner particles adhering to the developer surface are removed therefrom downstream of the development region by one or more resilient blades which engage the cylindrical surface of the developer roller. Typically such blades consist of a thin sheet, having an operative edge which is urged against the outer surface of the developer electrode in either a "chiseling mode" (i.e. pointing to the direction generally opposite that of the electrode surface movement) or a "wiping mode" (i.e. generally parallel to surface and pointing in generally the same direction as the electrode surface movemen ) . The blades are operative either to completely remove the toner particles from the developer electrode's surface, typically in the "chiseling mode", or to meter the thickness of the toner layer on the developer electrode, typically in the "wiper mode". As the developer electrode rotates against the operative edge of the blade, in the "chiseling mode", used toner particles are separated from the developer electrode, slide along the back side of the resilient blade and are transported, generally by gravitational force, to a toner collection system. U.S. Patent 4,454,833, for example, describes a system employing a single doctor blade made of a flexible material such as urethane or Mylar, mounted against a portion of an applicator roll which is partly submerged in liquid developer. In other systems, such as the system described in U.S. Patent 4,839,688, thin hard blades (generally made of metal ) are used in order to achieve more precise and/or complete "chiseling" of toner off the developer electrode surface. For multi-color imaging systems, U.S. Patent 3,910,231 describes a system for removing used toner from a drum or belt including blades secured to rotatable shafts and disposed for operation in response to each color. U.S. Patent 4,264,191 describes a two-layered cleaning blade. According to this invention, a relatively hard layer, made of a material having a Shore A durometer hardness of between 60 and 95, is laminated with a relatively soft resilient layer, having a Shore A durometer hardness of between 5 and 50, wherein the soft layer is much thicker than the hard layer. Apparently, this combination yields a relatively flexible blade having a relatively hard edge. U.K. Patent 1,443,128 describes a generally metallic, thin, cleaning blade having an elastomer cleaning tip mounted only along the operative edge of the thin blade. The non-metallic tip is operative to reduce the friction between the cleaning blade and the surface of a photoreceptor drum while the surface is being cleaned by the blade. SUMMARY OF THE INVENTION The present inventor have found that when hard, thin, blades (typically, metal blades) are used for removing unused liquid toner from the developer electrode, agglomerations of toner particles tend to accumulate on the back side of the cleaning blade (i.e. the side not facing the developer electrode) forming a considerable buildup of toner waste. This toner build-up, generally referred to herein as "sludge", may result in serious malfunctioning of the development apparatus caused, for example, by occasional breaking away of toner agglomerates into the toner collection system. Such agglomerates can interrupt the free-flow of toner or even clog the system. Thus, prior art development systems using liquid toner cleaning blades require frequent maintenance including removal of the "sludge" gradually formed on the back side of the cleaning blade. Although it might be presumed that using an extremely thin blade would reduce sludge accumulation considerably, it has been experimentally shown that the thickness of the blade, per se, generally does not affect the accumulation of sludge. In fact, blades which are only 100 micrometers thick will be covered with sludge accumulations which are several millimeters thick. Other experiments have shown that coating the blade with a non-conductive material, in order to prevent discharge of toner particles on the surfaces of the blade, while it may reduce significantly the amount of accumulated sludge, does not completely solve the problem. It is, therefore, an object of the present invention to provide improved development apparatus for use with electrostatographic imaging systems employing liquid toner. More specifically, it is an object of the present invention to provide single color or multi-color electrostatic liquid toner imaging apparatus with reduced sludge formation within the development system of such apparatus. The present invention provides, in a preferred embodiment thereof, development apparatus having a cleaning assembly including one or more sludge inhibiting, resilient blades operatively associated with the developer electrode. Used toner particles are removed efficiently, without accumulation of toner residues or sludge on the blade surfaces, from the surface of the developer electrode and preferably transported to used toner collection apparatus. The present invention thereby provides, in a preferred embodiment, electrophotographic imaging apparatus combining a clean, relatively maintenance-free developer assembly with excellent image performance. The present inventor has found that by forming an appropriate hydrodynamic profile on at least a portion of the back-surface of the cleaning blade, sludge formation on the back-surface may be dramatically reduced and, for most practical purposes, completely eliminated. According to one aspect of the invention, the back- surface of a thin metal cleaning blade is provided with a sludge-inhibiting hydrodynamic profile on its back which is operative to reduce sludge accumulations on the back surface of the blades to negligible amounts so that the apparatus does not require maintenance for removal of the sludge. In a preferred embodiment of the invention, the hydrodynamic profile consists of a relatively thick layer of a preferably non conductive, preferably non-metallic, material. The hydrodynamic profile preferably varies in thickness such that the back surface profile is shaped to allow optimal flow of toner on the back surface. Applicant believes that when such flow is laminar, sludge formation is inhibited. It has been found by the present inventor that sludge accumulation on the back surface of a conventional cleaning blade generally converges to a typical near-equilibrium sludge-profile. Due to the nature of sludge and the dynamics of the system, the sludge profile does change with time, however it tends to a particular profile. Therefore, according to another aspect of the invention, the hydrodynamic profile attached to the back surface of the cleaning blade is shaped in accordance with the shape of the typical sludge-profile equilibrium. According to this aspect, toner particles moving along such a hydrodynamic profile will generally not adhere to the profile, if the shape of the hydrodynamic profile is approximately equivalent to the maximal sludge accumulation profile of conventional planar surfaces. Apparently, additional toner particles cannot be supported on the back surface of the blade and are swept away. In general elimination of sharp corners and other efforts to minimize turbulence will have the effect of minimizing sludge accumulation. Generally, the present invention includes electrostatic imaging apparatus for use with liquid toners, including an imaging surface, apparatus for forming on the imaging surface a latent image including image portions at a first voltage and background portions at a second voltage and a development apparatus for developing the latent image with a liquid toner including charged toner particles and carrier liquid . In accordance with a preferred embodiment of the invention, the development apparatus includes a developer electrode rol ler , electri f ied to a third voltage intermediate the first and second voltages, having a portion closely spaced f rom the imaging surf ace to form a development region therebetween, whereat the latent image is developed with the liquid toner. The development apparatus further includes , in accordance with a preferred embodiment of the invention, a toner removal assembly preferably including a resilient blade , urged against the outer surface of the developer roller, for removing the toner particles from the developer roller. Preferably, the resilient blade is formed of a thin hard base-sheet having attached to at least a portion of the back surface thereof a sludge inhibiting hydrodynamic profile. In a preferred embodiment of the invention, the sludge- inhibiting hydrodynamic profile is formed of a relatively thick layer of a relatively flexible material and , more preferably, the relatively flexible material is also a non- conductive material . Further, in a preferred embodiment, the hard base sheet is a sheet of metal such as, for example, stainless steel. In accordance with a preferred embodiment of the invention, the general shape of the hydrodynamic profile attached to the base-sheet is approximately equivalent to the shape of typical sludge formation equilibrium in the absence of the hydrodynamic profile. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood from the following detailed description of preferred embodiments of the invention, taken in conjunction with the following drawings of which: Fig. 1 is a schematic, cross sectional, illustration of electrostatic imaging apparatus constructed and operative in accordance with a preferred embodiment of the present invention; Fig . 2 is schematic, cross-sectional , illustration of part of the imaging apparatus of Fig . 1 , il lustrating particularly a development apparatus in accordance with a preferred embodiment of the invention; and Fig . 3 is a schematic, cross sectional, illustration of part of the development apparatus of Fig . 2 , illustrating particularly a cleaning assembly in accordance with a preferred embodiment of the present invention. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Reference is now made to Fig . 1 , which illustrates an electrostatic imaging system constructed and operative in accordance with a preferred embodiment of the present invention . The imaging system includes a rotating drum 10 having a photoconductive surface 12 of selenium or any other suitable photoconductor known in the art . A charging device 14 , which may be a corotron or a scorotron or any other charging device known in the art, is juxtaposed with surface 12 and is operative to charge surface 12 to a generally uni form predetermined voltage as drum 10 rotates past charging device 14. Further rotation of drum 10 brings charged photoreceptor surface 12 opposite an exposure station 15 which may include, for example, a laser scanner . Exposure station 15 is operative to selectively expose portions of surface 12 to light, thus forming on surface 12 an electrostatic latent image including image portions at a first voltage and background portions at a second voltage. Alternatively, an electrostatic master may be used to provide the electrostatic image in which case no exposure station is required. Other methods of providing a latent image are also useful in the present invention. Continued rotation of drum 10 brings photoconductor surface 12 into operative engagement with a developer electrode roller 18 forming a development region 20 between surface 12 and developer roller 18. The latent image is developed at development region 20 by application thereat of liquid toner, including charged toner particles and carrier liquid, which is preferably dispensed by a liquid toner dispensing assembly 16. Dispensing assembly 16 preferably includes a plurality of nozzles each of which sends a jet of liquid toner, preferably, toward the developer roller as shown in Fig. 2. The liquid toner is preferably supplied to dispensing assembly 16 through a liquid toner inlet 17. In a preferred embodiment of the invention, developer roller 18 is closely spaced from drum 10 and preferably rotates in the same direction as drum 10. Such preferred rotation provides that surface 12 and the surface of developer roller 18 move in opposite directions at development region 20. In a preferred embodiment of the invention, developer roller 18 is electrified to a third voltage intermediate the first and second voltages of the image and background portions, respectively, of the latent image on surface 12. As a result of the difference in voltage between the surface potential of developer roller 18 and the image and background voltages of the latent image on surface 12, the charged toner particles selectively adhere only to the image portions of the latent image thereby developing the latent image. Similarly, toner particles in development region 20 opposite the background portions of the latent image are attracted to roller 18. Dispensing assembly 16 and developer roller 18 form part of development apparatus 30 which is discussed in detail hereinbelow, in conjunction with figure 2. A preferred type of toner for use with the present invention is described in example 1 of U.S. patent No. 4,794,651, the teachings of which are incorporated herein by reference. Other carbon black toners may alternatively be used as well as colored liquid developers, in which the carbon black is replaced by color pigments as is well known in the art. Af ter the l atent image has been developed , photoconductor surface 12 is engaged by an excess liquid removal assembly 22 , such as a squeegee roller urged against photoconductive surface 12. The squeegee roller serves to compact the toned image , remove excess carrier l iquid therefrom and remove carrier liquid from the background portions of the latent image on surface 12 . The squeegee rol ler i s preferably formed of a res i l ient , slightly conductive , polymeric material , and is electrified to a voltage of several hundreds to a few thousand volts of the same polarity as that of the charge toner particles . Downstream of excess liquid removal assembly 22 there is provided a preferably electrified intermediate transfer member 24 in operative engagement with photoconductive surface 12. Various types of intermediate transfer members are known in the art , some examples of which are described in U . S . Patent 4 , 684 , 238 , PCT Publication WO 90/04216 and U. S . Patent 4, 974 , 027 , the disclosures of all of which are incorporated herein by reference . Although intermediate transfer member 24 is shown in Fig. 1 as a transfer drum, it may be equally suitable to use a transfer belt as transfer member 24. Alternatively, in a preferred embodiment of the invention , intermediate member 24 may be omitted and , in such a case , the developed image is directly transferred onto a final substrate . Transfer member 24 rotates in a direction opposite that of drum 10, as indicated by arrow 27 , such that there is substantially zero relative motion between their respective surface at their area of contact . Due to the simultaneous 1 rotation of drum 10, whose surface 12 bears the toner

2 developed image, and member 24, the developed image is

3 transferred to the surface of intermediate member 24 mainly

4 by electrophoretic or electrostatic transfer as is well

5 known in the art.

6 After transfer of the developed image from surface 12

7 to intermediate transfer member 24 as described above, the

8 image is transferred, in a further transfer procedure aided

9 by heat and pressure, from transfer member 24 to a final

10 substrate 26, such as a sheet of paper.

11 After transfer of the developed image from

12 photoconductive surface 12, surface 12 is engaged by a

13 cleaning station 28 which may be any suitable cleaning

14 station as is known in the art. A lamp (not shown) completes

15 the cycle by exposing surface 12 to intense light, thus

16 removing any residual charge which may remain on surface 12.

17 Drum 10 then returns to its starting position, ready for an

18 additional imaging cycle.

19 Reference is now made to Fig. 2 which illustrates

20 development apparatus 30 in more detail. As described above,

21 with reference to Fig. 1, development apparatus 30 includes

22 developer electrode roller 18 whose outer surface 19 is

23 operatively associated with photoconductor surface 12 of

24 drum 10. Also associated with surface 19 of developer roller

25 18, there is a cleaning assembly 32 which may be selectably

26 brought into operative engagement with surface 19 of roller

27 18. In case of multicolor development systems, a plurality

28 of cleaners (not shown) such as cleaning assembly 32 are

29 used for a plurality of respective different color toners

30 and, in such a case, each of the plurality of cleaners is

31 selectably engaged with the surface of roller 18 when toner

32 of the respective color is supplied to development region 20

33 by toner dispensing assembly 16. Thus, in multicolor

34 systems, contamination by mixing of the various color toners

35 can be prevented. Such multicolor systems are shown for

36 example in PCT publication W0 90/14619, which is equivalent

37 to U.S. Patent Application 7/864,655, filed November 15,

38 1991, the disclosures of which are incorporated herein by reference. According to a preferred embodiment of the present invention, cleaning assembly 32 includes a resilient blade member 34 having an operative edge 35 and is preferably pivoted about a pivot 37. When member 34 is activated, as described hereinbelow, operative edge 35 is urged against the cylindrical outer surface 19 of developer roller 18 thereby to remove used liquid toner from surface 19 and to direct the removed used toner into used toner collection unit 36. In a preferred embodiment, the removed toner collected by collection unit 36 is later recycled, as is well known in the art, so as to be ready for reuse by toner dispensing assembly 16. In addition to blade member 34, cleaning assembly 32 preferably includes two side wiping portions 38 which are arranged to engage the two end portions of outer surface 19 of roller 18. Blade member 34 is mounted on a linkage which is selectably positioned by a conventional actuator 40. Further details of developers of this type can be found in WO Publication 90/14619, the disclosure of which is incorporated herein by reference. The inventor of the present invention has found that, generally, when undeveloped liquid toner is removed from the developer roller by conventional blades, the toner particles tend to agglomerate and stick to the surfaces of blades forming a relatively large concentration of residual toner particles generally referred to as "sludge". The inventor has discovered that such "sludge" build up appears mainly on the back surfaces of those conventional blades (i.e. the surface facing away from the developer roller). It has been also found by the inventor that sludge accumulation on the back surfaces of conventional cleaning blades is generally self-limiting, and the equilibrium shape is not random, although due to the unstable nature of sludge, the shape will change with time as sludge accumulations break-off and are rebuilt resulting in a somewhat arbitrarily jagged shape. Experiments have shown that sludge accumulations tend to generally converge, over time, to a distinct equilibrium shape and form. Thus, the final shape of sludge formation may be accurately predicted, experimentally, for any given flat cleaning blade. However, it should be appreciated that conventional sludge accumulations are highly unstable and, therefore, substantial portions of the accumulated sludge tend to break off the blades and be swept into the toner collection system, interrupting the free flow of toner in the development apparatus. The typical shape of equilibrium sludge accumulation will be hereinafter referred to as the "typical sludge profile" or "the real sludge profile". It is believed that at the equilibrium shape, the flow is laminar and also that sludge formation is minimized for laminar flow. Furthermore, the positive effect of the back- profile does not critically depend on reproducing the exact equilibrium shape. It is important that the back-profile shape be such that the has also been found that the exact shape of the profile is not critical, since other shapes which reduce the turbulence substantially (for example by avoiding sharp corners)also result in reduction of sludge accumu1ations. It should be appreciated that sludge accumulation can not be generally reduced by simply using a very thin cleaning blade, as may be presumed. Similarly, the amount of accumulated sludge can generally be only partially reduced by coating the cleaning blade with a non- conductive or very toner-abhesive material . On the other hand sludge formation can be reduced by nearly an order of magnitude i f the appropriate hydrodynamic shape is used. Reference is now made to Fig . 3 which illustrates in more detail the cross section of cleaning assembly 32 which is also seen in Fig . 2 as part of development apparatus 30. In accordance with a preferred embodiment of the invention, blade member 34 includes a base- sheet 38 , which i s preferably made of sheet metal and whose thickness is approximately 100 micrometers . On back surface 42 of base sheet 38 there is preferably attached a prefabricated hydrodynamic back-profile 44 which covers at least a portion of back surface 42 , near operative edge 35 of blade 34. Back profile 44 is preferably formed of relatively flexible , preferably plastic, material and has a cross-sectional shape as can be generally seen in Fig. 3. It has been found by the inventor that when the cross-sectional shape of profile 44 is similar to that of the "typical sludge profile" (i.e. the shape of typical sludge accumulation on the back surface of a conventional flat blade) or other turbulence reducing profile, sludge accumulation on the back surface of blade member 34 is dramatically reduced and, with the possible exception of a thin layer of toner on a face portion 47 of profile 44, even completely eliminated. When other, less ideal, hydrodynamic profiles are used, a lesser, but still striking, reduction of sludge formation is achieved. In general, shapes which have no sharp corners are most desirable. It has been experimentally shown that when back profile 44 is shaped in accordance with a turbulence reducing profile such as the "typical sludge profile", as shown in Fig. 3, its back surface 46 remains substantially free of sludge accumulation. It is a feature of an embodiment of the present invention that only negligible amounts of toner "sludge" accumulate on surface 46 of profile 44, when profile 44 is formed in accordance with this proper hydrodynamic shape. It should be appreciated that, unlike the typical sludge profile, profile 44 is formed of a physically stable material which is not likely to break- apart during operation of the development apparatus. Thus, clogging of the toner collection system by detached portions of the profile, which is typical of "the real sludge profile", is highly unlikely. It should be appreciated that the reduction in sludge accumulation, achieved by the present invention, is not attributed to specific surface properties of surface 46 and, therefore, various materials may be used to form hydrodynamic back profile 44. It should also be appreciated that the cleaning capability of blade 34 is unaffected by the addition of back profile 44, because the typically sharp, hard, edge of base sheet 38 is not generally covered by profile 44. Rather, it is only a portion of the back surface 42 of base sheet 38 which must be covered by profile 44. In a preferred embodiment of the invention , hydrodynamic profile 44 is formed of a non-conductive material . In such a case , discharge of toner particles flowing on back surface 46 of profile 44 is prevented and sludge accumulation on surface 46 may be reduced even further . Similarly developer electrode roller 18 may be coated with a non-conductive layer 19 which is operative to prevent discharge of the toner particles on the roller. In a preferred embodiment of the invention, base sheet 38 is typically 100 micrometers thick and the total thickness of the blade i s between 1 - 5 mm thick , more preferably between 2-4 mm thick. A typical desired thickness is about 3 mm. It is appreciated that the "typical sludge profile" is dependent upon features of the specific development apparatus which is used. For example, there is generally more sludge accumulation in line with the toner jet (not shown in the Figures) which are generally employed by toner dispensing assembly 16. Other parameters of development apparatus 30, such as toner viscosity, toner particle size, etc., affect the accumulation of sludge and, therefore, the "typical sludge profile". But, since all these parameter are generally constant, the specific "typical sludge profile" of any development apparatus can be accurately predicted. Thus, a specific hydrodynamic profile can be "tailored" for any specific imaging system. While the invention has been described with respect to a developer roller, other moving developer surfaces, such as moving belt developers may be used in the practice of the invention. It will be appreciated by persons skilled in the art that the present invention is not limited to what has been thus far described but, rather, the scope of the present invention is limited only by the following claims:

Claims

C I M S 1. Imaging apparatus comprising: an imaging surface having formed thereon a latent image , comprising image portions at a first voltage and background portions at a second voltage; and a developer operative to develop the latent image with a l iquid toner comprising charged toner particles and carrier liquid, comprising : an electri f ied moving developer electrode , operatively engaged with the imaging surface to form a development region therebetween, whereat the latent image is developed; and a resilient blade, having an operative edge urged against the outer surface of the developer electrode after it leaves the development region, operative to remove toner particles from the developer electrode, comprising: a relatively thin base sheet formed of a relatively hard material comprising the operative edge and having a back surface facing generally outwardly from the development roller; and a built-up section having a non-uniform, hydrodynamically shaped thickness profile mounted on at least a portion of the back surface of the base sheet adj acent the operative edge of the resilient blade, the built-up section being operative to substantially reduce accumulation of toner particles on the resilient blade.

2. Imaging apparatus comprising: an imaging surface having formed thereon a latent image , comprising image portions at a first voltage and background portions at a second voltage; and a developer operative to develop the latent image with a liquid toner comprising charged toner particles and carrier liquid, comprising: an electri f ied moving developer electrode , operatively engaged with the imaging surface to form a development region therebetween, whereat the latent image is developed; and a resilient blade, having an operative edge urged against the outer surface of the developer electrode after it leaves the development region, operative to remove toner particles from the developer electrode, comprising: a relatively thin base sheet formed of a relatively hard material comprising the operative edge and having a back surface facing generally outwardly from the development roller; and a built-up section mounted on at least a portion of the back surface of the base sheet adjacent the operative edge of the resilient blade and having a thickness profile which approximates in size and shape the profile formed by accumulations of toner particles over at least part of a resilient blade not having such a built up section.

3. Apparatus according to claim 1 or claim 2 wherein the overall thickness of the resilient blade is between 1-5 mm.

4. Imaging apparatus comprising: an imaging surface having formed thereon a latent image, comprising image portions at a first voltage and background portions at a second voltage; and a developer operative to develop the latent image with a liquid toner comprising charged toner particles and carrier liquid, comprising: an electrified moving developer electrode, operatively engaged with the imaging surface to form a development region therebetween, whereat the latent image is being developed; and a resilient blade, having an operative edge urged against the outer surface of the developer electrode after it leaves the development region, operative to remove toner particles from the developer electrode, comprising: a relatively thin base sheet formed of a relatively hard material comprising the operative edge and having a back surface facing generally outwardly from the development roller; and a non-conducting built-up portion mounted on at least a portion of the back surface of the base sheet adjacent the operative edge of the resilient blade, wherein the overall thickness of the resilient blade is between 1 and 3 mm.

5. Apparatus according to any of the preceding claims wherein the relatively thin base sheet is electrified to a third voltage intermediate the first and second voltages .

6. Apparatus according to any of the preceding claims wherein the relatively thin base sheet is formed of sheet metal.

7. Apparatus according to any of the preceding claims wherein the built-up section is formed of a relatively flexible material.

8. Apparatus according to any of the preceding claims wherein the built-up section approximates in size and shape the profile formed by toner accumulations over at least part of a resilient blade not having such a built up portion.

9. Apparatus according to any of the preceding claims wherein the built up section is non-conductive.

10. Apparatus according to any of the preceding claims wherein the operative edge is not covered by the built-up section.

11. Apparatus according to any of the preceding claims wherein the outer surface of the developer electrode is non- conductive.

12. Apparatus according to any of the preceding claims wherein the developer electrode is a roller developer electrode.

13. Apparatus according to any of the preceding claims wherein the overall thickness of the resilient blade is between 2-4 mm.

14. Apparatus according to claim 13 wherein the overall thickness of the resilient blade is about 3 mm.