WO2002002340A1

WO2002002340A1 - Image forming apparatus and method

Info

Publication number: WO2002002340A1
Application number: PCT/SE2000/001437
Authority: WO
Inventors: Joakim Larvall
Original assignee: Array Ab; Matsushita Electric Industrial Co. Ltd.
Priority date: 2000-07-06
Filing date: 2000-07-06
Publication date: 2002-01-10
Also published as: AU2000260431A1; WO2002007981A1; AU2001225712A1

Abstract

The invention relates to an image forming apparatus in which image information is converted into a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier toward a back electrode member. The image forming apparatus includes a background voltage source for producing a background electric field which enables a transport of charged toner particles, a printhead structure arranged in said background electric field at a predetermined distance from said toner carrier, said printhead structure including a plurality of apertures and control electrodes arranged in conjunction to the apertures, each of said control electrodes having a predetermined area, control voltage sources for supplying control potentials to said control electrodes; said control potentials being arranged so as to allow said transport of toner particles, thereby forming a predetermined toner release pattern; and an image receiving member caused to move in relation to the printhead structure for intercepting the transported charged particles. The invention is characterized in said printhead structure presents a top surface facing said particle carrier and a bottom surface facing said image receiving member, at least one of said top surface and bottom surface being covered with a layer having a surface resistance which is chosen so that any occurring surface charges building up on said printhead structure will be conducted generally without influencing the optical density or the position control of the toner particles intercepted by said image receiving member. The invention also relates to a printhead structure and a method for manufacturing said image forming apparatus.

Description

TITLE:

Image forming apparatus and method.

TECHNICAL FIELD:

The invention relates to an image forming apparatus in which image information is converted into a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier toward a back electrode member. The image forming apparatus includes a background voltage source for producing a background electric field which enables a transport of charged toner particles from said particle carrier towards said back electrode member; a printhead structure arranged in said background electric field at a predetermined distance from said toner carrier, said printhead structure including a plurality of apertures and control electrodes arranged in conjunction to the apertures; control voltage sources for supplying control potentials to said control electrodes in accordance with the image information to selectively permit or restrict the transport of charged toner particles from the particle carrier through the apertures; said control potentials being arranged so as to allow said transport of toner particles, thereby forming a predetermined toner release pattern; and an image receiving member caused to move in relation to the printhead structure for intercepting the transported charged particles according to said toner release pattern.

Furthermore, the invention relates to a printhead structure for an image forming apparatus of the above- mentioned type. Furthermore, the invention relates to a method for manufacturing an image forming apparatus of the above- mentioned type.

BACKGROUND OF THE INVENTION:

U.S. Patent No. 5,036,341 discloses a direct electrostatic printing device and a method to produce text and pictures with toner particles on an image receiving substrate directly from computer generated signals. Such a device generally includes a printhead structure provided with a plurality of apertures through which toner particles are selectively transported from a particle source to an image receiving medium due to control in accordance with image information.

It is previously known to design a printhead structure of the above-mentioned type in the form of an apertured substrate of electrically insulating material overlaid with a printed circuit of control electrodes arranged in conjunction with the apertures. Generally, a printhead structure includes a flexible substrate of insulating material such as polyi ide or the like, having a first surface (i.e. a top surface) facing the particle carrier, a second surface (i.e. a bottom surface) facing a back electrode and a plurality of apertures arranged through the substrate. The first surface is coated with an insulating layer and control electrodes are provided between the first surface of the substrate and the insulating layer, in a configuration such that each control electrode surrounds a corresponding aperture. In this regard, each of said control electrodes has a predetermined area extending around the respective aperture. The apertures are normally arranged in two rows extending transversally across the width of the printhead structure. Furthermore, control voltage sources are provided for supplying control potentials to said control electrodes in accordance with the image information in question. In this manner, the transport of charged toner particles from the particle carrier through the apertures can be selectively permitted or restricted.

The control potentials are chosen in a manner so as to allow said transport of toner particles, thereby forming a predetermined toner release pattern with a desired optical density on an image receiving member caused to move in relation to the printhead structure for intercepting the transported charged particles according to said toner release pattern.

When the image forming apparatus is operated and toner particles are to be fed through said apertures, a background field between the particle carrier and a back electrode member is first established. In order to actually feed charged toner particles through a particular aperture, a "print pulse" is generated, i.e. a control voltage is supplied to a control electrode which is associated with the aperature in question. In this manner, the background field will be modulated in a manner so that the toner particles will be fed through the aperture and to the image receiving member.

A problem which is associated with said known image forming apparatus relates to the fact that an amount of surface charges will gradually accumulate and build up on the surfaces of the printhead structure. These charges will disturb the above-mentioned modulation of the background field provided by the control electrodes, thereby affecting the optical density in a negative way during printing. In order to solve this problem, it is previously known to provide a semiconductive layer on top of the above- mentioned electrically insulating layer. In this manner, the accumulated surface charges can then be guided away from the vicinity of the apertures and the control electrodes. In this manner, the modulated electric field generated by a control electrode will not be disturbed.

However, charged toner particles will accumulate on the printhead structure during use of the above-mentioned image forming apparatus. These particles will accumulate on both the top surface and the bottom surface of the printhead structure. Additionally, particles will also accumulate inside each aperture extending through the printhead structure. This means that surface charges being present on the semiconductive layer will become associated with the potential on the control electrode. This leads to a situation in which the field generated by the control electrode (for controlling the transport of toner particles to the image receiving member) will be gradually decreased. Eventually, the field generated by the control electrode will be sufficiently diminished so that the transport of toner particles will be prevented. Obviously, this constitutes a disadvantage in the field of image forming devices .

SUMMARY OF THE INVENTION:

An object of the invention is to provide an improved image forming apparatus by means of which the above- mentioned problem is solved.

This object is accomplished by means of an image forming apparatus of the kind initially mentioned, in which said printhead structure presents a top surface facing said particle carrier and a bottom surface facing said image receiving member, at least one of said top surface and bottom surface being covered with a layer having a surface resistance which is chosen so that any occurring surface charges building up on said printhead structure will be conducted generally without influencing the optical density or the position control of the toner particles intercepted by said image receiving member.

Said object is also accomplished by means of a printhead structure of the kind initially mentioned, which presents a top surface facing said particle carrier and a bottom surface facing said image receiving member, at least one of said top surface and bottom surface being covered with a layer having a surface resistance which is chosen so that any occurring surface charges building up on said printhead structure.

Said object is also accomplished by means of a method for manufacturing an image forming apparatus of the kind initially mentioned, said method furthermore comprising: providing said printhead structure with a top surface facing said particle carrier and a bottom surface facing said image receiving member, and covering at least one of said top surface and bottom surface with a layer having a surface resistance which is chosen so that any occurring surface charges building up on said printhead structure will be conducted generally without influencing the optical density or the position control of the toner particles intercepted by said image receiving member.

BRIEF DESCRIPTION OF THE DRAWINGS:

The invention will now be described in more detail for explanatory, and in no sense limiting, purposes, with reference to the following drawings, wherein like reference numerals designate like parts throughout and where the dimensions in the drawings are not to scale, in which: Fig. 1 is a schematic view of an image forming apparatus in accordance with a preferred embodiment of the present invention,

Fig. 2 is a schematic section view across a print station in an image forming apparatus, such as, for example, that shown in Fig. 1,

Fig. 3 is a schematic section view of the print zone, illustrating the positioning of a printhead structure in relation to a particle source and an image receiving member,

Fig. 4a is a partial view of a printhead structure of a type used in an image forming apparatus, showing the surface of the printhead structure that is facing the toner delivery unit,

Fig. 4b is a partial view of a printhead structure of a type used in an image forming apparatus, showing the surface of the printhead structure that is facing the intermediate transfer belt,

Fig. 4c is a section view across a section line I-I in the printhead structure of Fig. 4a and across the corresponding section line II-II of Fig. 4b,

Fig. 5 is an enlarged cross-sectional side view, which shows the principles of the present invention, and

Fig. 6 is an enlarged top view of a section of a printhead structure according to the invention.

PREFERRED EMBODIMENTS: To perform a direct electrostatic printing method in accordance with the present invention, a background electric field is produced between a particle carrier and a back electrode to enable a transport of charged particles therebetween. A printhead structure, such as an electrode matrix provided with a plurality of selectable apertures, is interposed in the background electric field between the particle carrier and the back electrode and connected to a control unit which converts the image information into a pattern of electrostatic fields which, due to control in accordance with the image information, selectively open or close passages in the electrode matrix to permit or restrict the transport of charged particles from the particle carrier. The modulated stream of charged particles allowed to pass through the opened apertures are thus exposed to the background electric field and propelled toward the back electrode. The charged particles are deposited on an image receiving member to provide line-by line scan printing to form a visible image.

A printhead structure for use in direct electrostatic printing may take on many designs, such as a lattice of intersecting wires arranged in rows and columns, or an apertured substrate of electrically insulating material overlaid with a printed circuit of control electrodes arranged in conjunction with the apertures. Generally, a printhead structure includes a flexible substrate of insulating material such as polyimide or the like, having a first surface facing the particle carrier, a second surface facing the back electrode and a plurality of apertures arranged through the substrate. The first surface is coated with an insulating layer and control electrodes are arranged between the first surface of the substrate and the insulating layer, in a configuration such that each control electrode surrounds a corresponding aperture. The apertures are preferably aligned in one or several rows extending transversally across the width of the substrate, i.e. perpendicular to the motion direction of the image receiving member.

According to such a method, each single aperture is utilized to address a specific dot position of the image in a transversal direction. Thus, the transversal print addressability is limited by the density of apertures through the printhead structure. For instance, a print addressability of 300 dpi requires a printhead structure having 300 apertures per inch in a transversal direction.

According to a preferred embodiment of the present invention, a direct electrostatic printing device includes a dot deflection control (DDC) . According to that embodiment, each single aperture is used to address several dot positions on an image receiving substrate by controlling not only the transport of toner particles through the aperture, but also their transport trajectory toward the image receiving substrate, and thereby the location of the obtained dot. The DDC method increases the print addressability without requiring a larger number of apertures in the printhead structure. This is achieved by providing the printhead structure with deflection electrodes connected to variable deflection voltages which, during each print cycle, sequentially modify the symmetry of the electrostatic control fields to deflect the modulated stream of toner particles in predetermined deflection directions. For instance, a DDC method performing three deflection steps per print cycle, provides a print addressability of 600 dpi utilizing a printhead structure having only 200 apertures per inch.

According to a preferred embodiment, an improved DDC method provides a simultaneous dot size and dot position control. This method utilizes the deflection electrodes to influence the convergence of the modulated stream of toner particles thus controlling the dot size. Each aperture is surrounded by two deflection electrodes connected to respective deflection voltages Dl, D2, such that the electrostatic control field generated by the control electrode remains substantially symmetrical as long as both deflection voltages Dl, D2 have the same amplitude. The amplitude of Dl and D2 are modulated to apply converging forces on toner particles as they are transported toward the image receiving medium, thus providing smaller dots . The dot position is simultaneously controlled by modulating the amplitude difference between Dl and D2 to deflect the toner trajectory toward predetermined dot positions.

A printhead structure for use in DDC methods generally includes a flexible substrate of electrically insulating material such as polyimide or the like, having a first surface facing the particle carrier, a second surface facing the back electrode and a plurality of apertures arranged through the substrate. The first surface is overlaid with a first printed circuit including the control electrodes and the second surface is overlaid with a second printed circuit including the deflection electrodes. Both printed circuits are coated with insulative layers. Utilizing such a method, 60 micrometer dots can be obtained with apertures having a diameter in the order of 160 micrometer.

In order to clarify the apparatus and method according to the invention, some examples of its use will now be described in connection with the accompanying drawings.

As shown in Fig. 1, an image forming apparatus in accordance with a first embodiment of the present invention comprises at least one print station, preferably four print stations (Y, M, C, K) , an intermediate image receiving member 1, a driving roller 10, at least one support roller 11, and preferably several adjustable holding elements 12. The four print stations are arranged in relation to the intermediate image receiving member 1. The image receiving member, preferably a transfer belt 1, is mounted over the driving roller 10. The at least one support roller^' 11 is provided with a mechanism for maintaining the transfer belt 1 with a constant tension, while preventing transversal movement of the transfer belt 1. The holding elements 12 are for accurately positioning the transfer belt 1 with respect to each print station.

The driving roller 10 is preferably a cylindrical metallic sleeve having a rotation axis extending perpendicular to the motion direction of the belt 1 and a rotation velocity adjusted to convey the belt 1 at a velocity of one addressable dot location per print cycle, to provide line by line scan printing. The adjustable holding elements 12 are arranged for maintaining the surface of the belt at a predetermined gap distance from each print station. The holding elements 12 are preferably cylindrical sleeves disposed perpendicularly to the belt motion in an arcuated configuration so as to slightly bend the belt 1 at least in the vicinity of each print station in order to, in combination with the belt tension, create a stabilization force component on the belt. That stabilization force component is opposite in direction and preferably^' larger in magnitude than an electrostatic attraction force component acting on the belt 1 due to interaction with the different electric potentials applied on the corresponding print station.

The transfer belt 1 is preferably an endless band of 30 to 200 microns thick composite material as a base. The base composite material can suitably include thermoplastic polya ide resin or any other suitable material having a high thermal resistance, such as 260°C of glass transition point and 388°C of melting point, and stable mechanical properties under temperatures in the order of 250°C. The composite material of the transfer belt has preferably a homogeneous concentration of filler material, such as carbon or the like, which provides a uniform electrical conductivity throughout the entire surface of the transfer belt 1. The outer surface of the transfer belt 1 is preferably coated with a 5 to 30 microns thick coating layer made of electrically conductive polymer material having appropriate conductivity, thermal resistance, adhesion properties, release properties and surface smoothness.

The transfer belt 1 is conveyed past the four different print stations, whereas toner particles are deposited on the outer surface of the transfer belt and superposed to form a four color toner image. Toner images are then preferably conveyed through a fuser unit 13 comprising a fixing holder 14 arranged transversally in direct contact with the inner surface of the transfer belt. The fixing holder includes a heating element 15 preferably of a resistance type of e.g. molybdenium, maintained in contact with the inner surface of the transfer belt 1. As an electric current is passed through the heating element 15, the fixing holder 14 reaches a temperature required for melting the toner particles deposited on the outer surface of the transfer belt 1. The fusing unit 13 further includes a pressure roller 16 arranged transversally across the width of the transfer belt 1 and facing the fixing holder 14. An information carrier 2, such as a sheet of plain untreated paper or any other medium suitable for direct printing, is fed from a paper delivery unit 21 and conveyed between the pressure roller 16 and the transfer belt. The pressure roller 16 rotates with applied pressure to the heated surface of the fixing holder 14 whereby the melted toner particles are fused on the information carrier 2 to form a permanent image. After passage through the fusing unit 13, the transfer belt is brought in contact with a cleaning element 17, such as for example a replaceable scraper blade of fibrous material extending across the width of the transfer belt 1 for removing all untransferred toner particles from the outer surface.

As shown in Fig. 2, a print station in an image forming apparatus in accordance with the present invention includes a particle delivery unit 3 preferably having a replaceable or refillable container 30 for holding toner particles, the container 30 having front and back walls

(not shown) , a pair of side walls and a bottom wall having an elongated opening 31 extending from the front wall to the back wall and provided with a toner feeding element 32 disposed to continuously supply toner particles to a developer sleeve 33 through a particle charging member 34. The particle charging member 34 is preferably formed of a supply brush or a roller made of or coated with a fibrous, resilient material. The supply brush is brought into mechanical contact with the peripheral surface of the developer sleeve 33 for charging particles by contact charge exchange due to triboelectrification of the toner particles through frictional interaction between the fibrous material on the supply brush and any suitable coating material of the developer sleeve. The developer sleeve 33 is preferably made of metal coated with a conductive material, and preferably has a substantially cylindrical shape and a rotation axis extending parallel to the elongated opening 31 of the particle container 30. Charged toner particles are held to the surface of the developer sleeve 33 by electrostatic forces essentially proportional to (Q/D)², where Q is the particle charge and D is the distance between the particle charge center and the boundary of the developer sleeve 33. Alternatively, the charge unit may additionally include a charging voltage source (not shown) , which supply an electric field to induce or inject charge to the toner particles. Although it is preferred to charge particles through contact charge exchange, the method can be performed using any other suitable charge unit, such as a conventional charge injection unit, a charge induction unit or a corona charging unit, without departing from the scope of the present invention.

A metering element 35 is positioned proximate to the developer sleeve 33 to adjust the concentration of toner particles on the peripheral surface of the developer sleeve 33, to form a relatively thin, uniform particle layer thereon. The metering element 35 may be formed of a flexible or rigid, insulating or metallic blade, roller or any other member suitable for providing a uniform particle layer thickness. The metering element 35 may also be connected to a metering voltage source (not shown) which influence the triboelectrification of the particle layer to ensure a uniform particle charge density on the surface of the developer sleeve.

As shown in Fig. 3, the developer sleeve 33 is arranged in relation with a positioning device 40 for accurately supporting and maintaining a printhead structure 5 of the kind previously mentioned in a predetermined position with respect to the peripheral surface of the developer sleeve 33. The positioning device 40 is formed of a frame 41 having a front portion, a back portion and two transversally extending side rulers 42, 43 disposed on each side of the developer sleeve 33 parallel with the rotation axis thereof. The first side ruler 42, positioned at an upstream side of the developer sleeve 33 with respect to its rotation direction, is provided with fastening means 44 to secure the printhead structure 5 along a transversal fastening axis extending across the entire width of the printhead structure 5. The second side ruler 43, positioned at a downstream side of the developer sleeve 33, is provided with a support element 45, or pivot, for supporting the printhead structure 5 in a predetermined position with respect to the peripheral surface of the developer sleeve 33. The support element 45 and the fastening axis are so positioned with respect to one another, that the printhead structure 5 is maintained in an arcuated shape along at least a part of its longitudinal extension. That arcuated shape has a curvature radius determined by the relative positions of the support element 45 and the fastening axis and dimensioned to maintain a part of the printhead structure 5 curved around a corresponding part of the peripheral surface of the developer sleeve 33. The support element 45 is arranged in contact with the printhead structure 5 at a fixed support location on its longitudinal axis so as to allow a slight variation of the printhead structure 5 position in both longitudinal and transversal direction about that fixed support location, in order to accommodate a possible excentricity or any other undesired variations of the developer sleeve 33. That is, the support element 45 is arranged to make the printhead structure 5 pivotable about a fixed point to ensure that the distance between the printhead structure 5 and the peripheral surface of the developer sleeve 33 remains constant along the whole transverse direction at every moment of the print process, regardless of undesired mechanical imperfections of the developer sleeve 33. The front and back portions of the positioning device 40 are provided with securing members 46 on which the toner delivery unit 3 is mounted in a fixed position to provide a constant distance between the rotation axis of the developer sleeve 33 and a transversal axis of the printhead structure 5. Preferably, the securing members 46 are arranged at the front and back ends of the developer sleeve 33 to accurately space the developer sleeve 33 from the corresponding holding element 12 of the transfer belt 1 facing the actual print station. The securing members 46 are preferably dimensioned to provide and maintain a parallel relation between the rotation axis of the developer sleeve 33 and a central transversal axis of the corresponding holding member 12.

As shown in Figs. 4a, 4b, 4c, a printhead structure 5 in an image forming apparatus in accordance with the present invention comprises a substrate 50 of flexible, electrically insulating material such as polyimide or the like, having a predetermined thickness, a first surface (i.e. a top surface) facing the developer sleeve, a second surface (i.e. a bottom surface) facing the transfer belt, a transversal axis 51 extending parallel to the rotation axis of the developer sleeve 33 across the whole print area, and a plurality of apertures 52 arranged through the substrate 50 from the first to the second surface thereof. The first surface of the substrate 50 is coated with a first cover layer 501 (not shown in Figs. 4a and 4b) of electrically insulating material, such as for example parylene or polyimide, or another material with suitable properties (for example as regards its relative dielectricity constant) . A first printed circuit, comprising a plurality of control electrodes 53 disposed in conjunction with the apertures, and, in some embodiments, shield electrode structures (not shown) arranged in conjunction with the control electrodes 53, is arranged between the substrate 50 and the first cover layer 501. Furthermore, each one of the control electrodes 53 is formed with a first section which surrounds or extends around the corresponding aperture 52, and a second section which connects the first section with the appropriate control voltage sources. The second surface of the substrate is coated with a second cover layer 502 (not shown in Figs. 4a and 4b) of electrically insulating material, such as for example parylene or polyimide, as mentioned above. The purpose of the insulating layers 501, 502 is to provide highly resistive layers which are adapted to the relatively high voltage levels which normally occur between two adjacent control electrodes 53 during printing. A second printed circuit, including a plurality of deflection electrodes 54, is arranged between the substrate 50 and the second cover layer 502.

The first cover layer 501 is covered by a further, third layer 503 of semiconductive material, whereas the the second cover layer 502 is covered by a further, fourth layer 504 of semiconductive material. The semiconductive layers 503, 504 are not shown in Figs. 4a and 4b.

The printhead structure 5 is brought in cooperation with a control unit (not shown) comprising variable control voltage sources connected to the control electrodes 53 to supply control potentials which control the amount of toner particles to be transported through the corresponding aperture 52 during each print sequence. The control unit further comprises deflection voltage sources (not shown) connected to the deflection electrodes 54 to supply deflection voltage pulses which controls the convergence and the trajectory path of the toner particles allowed to pass through the corresponding apertures 52. The control unit, in some embodiments, even includes a shield voltage source (not shown) connected to the shield electrodes to supply a shield potential which electrostatically screens adjacent control electrodes 53 from one another, preventing electrical interaction therebetween. In a preferred embodiment of the invention, the substrate 50 is a flexible sheet of polyimide having a thickness on the order of about 50 microns. The first and second printed circuits are copper circuits of approximately 8-9 microns thickness etched onto the first and second surface of the substrate 50, respectively, using conventional etching techniques. The first and second cover layers 501, 502 are 5 to 10 microns thick parylene laminated onto the substrate 50 using vacuum deposition techniques.

The semiconductive layers 503, 504 are preferably arranged on the insulating layers 501, 502 by means of sputtering, which is a deposition technique which is known per se. Preferably, silicon is used for forming said semiconductive layers. According to a further embodiment, chemical vapor deposition (CVD) can also be used for providing the semiconductive layers. In such case, silicon or germanium is preferably used for forming said layers.

Although not shown in the drawings, the inner surfaces of the apertures 52 are preferably provided with a layer of semiconductive material. In this case also, chemical vapor deposition or sputtering is suitable used for forming said layer inside the apertures .

The apertures 52 are made through the printhead structure 5 using conventional laser micromachining methods. The apertures 52 have preferably a circular or elongated shape centered about a central axis, with a diameter in a range of 80 to 120 microns, alternatively a transversal minor diameter of about 80 microns and a longitudinal major diameter of about 120 microns. Although the apertures 52 have preferably a constant shape along their central axis, for example cylindrical apertures, it may be advantageous in some embodiments to provide apertures whose shape varies continuously or stepwise along the central axis, for example conical apertures.

In a preferred embodiment of the present invention, the printhead structure 5 is dimensioned to perform 600 dpi printing utilizing three deflection sequences in each print cycle, i.e. three dot locations are addressable through each aperture 52 of the printhead structure during each print cycle. Accordingly, one aperture 52 is provided for every third dot location in a transverse direction, that is, 200 equally spaced apertures per inch aligned parallel to the transversal axis 51 of the printhead structure 5. The apertures 52 are generally aligned in one or several rows, preferably in two parallel rows each comprising 100 apertures per inch. Hence, the aperture pitch, i.e. the distance between the central axes of two neighbouring apertures of a same row is 0,01 inch or about 254 microns. The aperture rows are preferably positioned on each side of the transversal axis 51 of the printhead structure 5 and transversally shifted with respect to each other such that all apertures are equally spaced in a transverse direction. The distance between the aperture rows is preferably chosen to correspond to a whole number of dot locations.

The first printed circuit comprises the control electrodes 53 each of which having a ring shaped structure surrounding the periphery of a corresponding aperture 52, and a connector preferably extending in the longitudinal direction, connecting the ring shaped structure to a corresponding control voltage source. Although a ring shaped structure is preferred, the control electrodes 53 may take on various shape for continuously or partly surrounding the apertures 52, preferably shapes having symmetry about the central axis of the apertures. In some embodiments, particularly when the apertures 52 are aligned in one single row, the control electrodes are advantageously made smaller in a transverse direction than in a longitudinal direction.

The second printed circuit comprises the plurality of deflection electrodes 54, each of which is divided into two semicircular or crescent shaped deflection segments 541, 542 spaced around a predetermined portion of the circumference of a corresponding aperture 52. The deflection segments 541, 542 are arranged symmetrically about the central axis of the aperture 52 on each side of a deflection axis 543 extending through the center of the aperture 52 at a predetermined deflection angle d to the longitudinal direction. The deflection axis 543 is dimensioned in accordance with the number of deflection sequences to be performed in each print cycle in order to neutralize the effects of the belt motion during the print cycle, to obtain transversally aligned dot positions on the transfer belt. For instance, when using three deflection sequences, an appropriate deflection angle is chosen to arctan(l/3), i.e. about 18,4°. Accordingly, the first dot is deflected slightly upstream with respect to the belt motion, the second dot is undeflected and the third dot is deflected slightly downstream with respect to the belt motion, thereby obtaining a transversal alignment of the printed dots on the transfer belt. Accordingly, each deflection electrode 54 has a upstream segment 541 and a downstream segment 542, all upstream segments 541 being connected to a first deflection voltage source Dl, and all downstream segments 542 being connected to a second deflection voltage source D2. Three deflection sequences (for instance: DKD2; Dl=D2; D1>D2) can be performed in each print cycle, whereby the difference between Dl and D2 determines the deflection trajectory of the toner stream through each aperture 52, and thus the dot position on the toner image .

Since the apertures 52 and their surrounding areas will under some circumstances need to be cleaned from residual toner particles which agglomerate there, an image forming apparatus in accordance with the present invention preferably further includes a cleaning unit (not shown) which is used to prevent toner contamination. Due to undesired variations in the charge and mass distribution of the toner material, some of the toner particles released from the developer sleeve 33 do not reach sufficient momentum during a print sequence to be deposited onto the transfer belt 1 and contribute to image formation. Some toner particles having a charge polarity opposite to the intended, so called wrong signed toner, may be repelled back to the printhead structure 5 after passage through the apertures under influence of the background field, and adhere on the printhead structure 5 in the area surrounding the apertures 52. Some particles may be deviated during transport and agglomerate on the apertures walls, obstructing the aperture 52. Residual toner particles have to be removed periodically during an appropriate cleaning cycle, for example after a predetermined number of image formation cycles or due to control in accordance with a sensor measuring the amount of residual toner.

Fig. 5 is a cross-sectional side view which is enlarged as compared with that shown in Fig. 4c. According to Fig. 5, the preferred embodiment of the present invention comprises a printhead structure 5 with an electrically insulating substrate 50. The top surface of the substrate 50, i.e. the surface facing the developer sleeve 33, is covered with an insulating layer 501, whereas the bottom surface of the substrate 50, i.e. the surface facing the image receiving member 1, is covered with a further insulating layer 502. Furthermore, as mentioned initially, a problem related to image forming devices is that surface charges may build up on the surface of an insulating surface layer of a printhead structure. For this reason, the top insulating layer 501 is covered with a first semiconductive layer 503, whereas the bottom insulating layer 502 is covered with a second semiconductive layer 504. The printhead structure 5 is also provided with a plurality of apertures, of which one aperture 52 is shown in Fig. 5. A control electrode 53 is disposed around the aperture 52, according to what has been described above with reference to Figs. 4a-c.

The printhead structure 5 is arranged in a manner so that a predetermined distance L_k is defined between the a layer of toner particles 56 being formed on the developer sleeve 33 and the upper surface of the first semiconductive layer 503.

Furthermore, the printhead structure 5 according to the preferred embodiment is provided with deflection electrodes 53, as discussed previously with reference to Figs. 4a-c. However, the image forming apparatus according to the invention can also be implemented without such deflection electrodes.

When a background field is activated (as described above) and the control electrode 53 is also activated, a certain amount of toner particles 56 will be drawn from the developer sleeve 33, through the aperture 52 and to the image receiving member 1. However, as mentioned initially, a problem associated with image forming devices is that toner particles will accumulate on the surface of the printhead structure, which in turn means that surface charges building up on the semiconductive layer will become associated with the charges defining the potential on the control electrode. In previously known image forming devices, this may lead to a diminished electric field and a risk that the toner particles are not drawn through the apertures and deposited on the image receiving member in a manner as desired. This is particularly serious if the control electrodes are thin, i.e. if the printhead structure is arranged with only one row of apertures arranged next to each other.

In order to eliminate the risk for the electric field generated by means of the control electrode being distorted (which may seriously affect the transfer of charged toner particles from the developer sleeve 33 to the image receiving member 1) , the present invention is based on the fact that said semiconductive layers 503, 504 are provided on said top surface and bottom surface of the printhead structure 5. By means of these semiconductive layers 503, 504, any surface charges resulting from toner particles accumulating on the printhead structure 5 can be guided away from the semiconductive layers 503, 504 in a manner so that the print quality is not affected. To this end, the semiconductive layers 503, 504 are manufactured in a way so that they present a predetermined surface resistance which is suitable for guiding away said charges, without negatively affecting the optical density or the position control of the toner dots deposited on the image receiving member.

Fig. 6 shows an enlarged top view of a printhead structure 5 according to the invention. In order to guide charges away from the printhead structure, the first semiconductive layer 503 is electrically connected to a first conductive bar 57. Also, the second semiconductive layer 504 is provided with a second conductive bar 58

(which is shown by means of a broken line in Fig. 6) . The conductive bars 57, 58 present a generally elongated shape and are arranged as thin strips of electrically conductive material on the surface of the semiconductive layers 503 and 504, respectively. The conductive bars 57,

58 are preferably formed by aluminium or another suitable conductive material, such as chromium, titanium or nickel, and are suitably deposited by means of sputtering. However, all deposition methods which are suitable for forming said conductive bars 57, 58 from any of said materials can be used.

The invention is based on the insight that the surface resistance defined by the material of the semiconductive layer 503 (as regarded from the area around the control electrodes 53 and extending to the conductive bar 57) together with the capacitor which is formed between each control electrode 53 and the semiconductive layer 503 can be said to form a so-called RC circuit. The RC constant of said RC circuit determines in what manner the charges on the semiconductive layer 503 will be guided away. In particular, it can be noted that the voltage across said capacitor will rise to a maximum value during a particular time period which is determined by-the surface resistance of the semiconductive layer 503. If said surface resistance is relatively low, the voltage across the capacitor will quickly rise to a value which is generally equal to the voltage applied to the control electrode. This means that the semiconductive layer 503 will then act as a "shield" which seriously affects the print pulse which is applied in order to feed charged toner particles through the aperture 52. On the other hand, if the surface resistance is relatively high, the surface charges which accumulate on the semiconductive layer 503 (as described initially) will be guided away from the area around the aperture 52 "too slowly", i.e. these accumulated charges will then distort the modulated electric field generated by means of the control electrodes 53.

In this regard, it can be noted that the area of each control electrode determines a particular electric current which can be fed betweeen the two "plates" forming part of said capacitor, i.e. from the control electrode 52 to the first semiconductive layer 503.

A corresponding situation will occur as regards the RC circuit formed by the semiconductive layer 504 on the bottom surface of the printhead structure 5.

Consequently, the invention relies on the principle that the surface resistance of the semiconductive layers 503, 504 must be chosen to a value which is within a relatively narrow interval in order to provide an optimum optical density during printing and in order to provide correct positioning of the toner dots on the image receiving member. This is particularly important in case the above-mentioned dot deflection method (DDC) is used.

According to the invention, the surface resistance of the semiconductive layers 503, 504 is in the magnitude of 10⁸ ohm per square or more, preferably more than 10¹¹ ohm per square, and most preferably approximately 10¹² ohm per square. The actual values of the surface resistance of the semiconductive layers 503, 504 must be also adapted to the materials and the dimensions chosen for the semiconductive layers 503, 504. Furthermore, the surface resistance of the semiconductive layers 503, 504 is not higher than 10¹⁵ ohm per square, preferably not higher than 5*10¹² ohm per square, and most preferably not higher than 10 ohm per square.

Furthermore, the product of the surface resistance and the distance from the apertures to said conductive bar is preferably greater than 3*10^s meter ohm per square.

The surface resistance on the semiconductive layer 503 on the top surface of the printhead structure 5 can be different than that of the bottom surface of said printhead structure 5. Preferably, the ratio between the surface resistance of the layers is not more than 100.

The distance from the line of apertures 52 defined along the printhead structure 5 to the conductive bars 57, 58 defines a particular surface resistance. In a corresponding manner, the distance between any two adjacent apertures 52 also defines a surface resistance. Preferably, the distance from a line as regarded in a direction along said apertures to said conductive bar is less than 10 mm, preferably less than 5 mm, most preferably approximately 3 mm. According to the preferred embodiment, it is desired that the resistance from the apertures 52 to the conductive bars 57, 58 is considerably lower than the resistance between the apertures 52. For this reason, the semiconductive layers 503, 504 are preferably treated in a manner so as to provide a relatively high resistance in the areas between the apertures 52. This can be implemented for example by removing a certain part of the semiconductive material in the areas between the apertures 52, or by using a semiconductive material in said areas having a doping which is different than that of the rest of the semiconductive layers. In this manner, the surface resistance as regarded in a direction along the apertures can be made considerably higher than the resistance in a direction generally perpendicular to that direction. In order to guide away the charges which accumulate on the printhead structure, the first conductive bar 57 is connected to a first conductive pattern 59 (for example made from copper) which is suitably formed on the top surface of the substrate 50, separated from the pattern which defines the control electrodes 53. Furthermore, the second conductive bar 58 is connected to a second conductive pattern 60 which is formed on the bottom surface of the substrate 50. As shown in Fig. 6, the conductive pattern 59 is preferably arranged so as to partially overlap with the conductive bars 57, 58. Furthermore, the printhead structure 5 is provided with a suitably arranged hole (not shown) extending through the first insulating layer 501 and allowing electrical contact between the first conductive bar 57 and the first conductive pattern 59. The printhead structure 5 is also provided with a hole (not shown) extending through the second insulating layer 502 and allowing electrical contact between the second conductive bar 58 and the second conductive pattern 60.

The electrical contact between the conductive bars and the conductive patterns are preferably provided by sputtering aluminium (or another suitable material) in the holes and on the semiconductor layers, thereby filling the holes and forming the conductive bars at the same time.

The conductive patterns 59, 60 are in turn electrically connected to connection 61 defining a predetermined electrical potential. Suitably, the connection 61 can be connected to earth, but the invention is not limited to this. In this manner, the charges accumulating on the semiconductive layers 503, 504 can be conducted away from the area around the apertures 52 on the printhead structure 5.

According to a further embodiment, the printhead structure can be formed with one single conductive pattern (for example, formed as the above-mentioned first conductive pattern 59) which is connected to both the conductive bars 57, 58 via suitably arranged holes through the insulating layers and the substrate.

In order to further improve the image forming apparatus according to the invention, the area of the control electrode 53 is preferably greater than a predetermined value which in turn corresponds to a time period during which a desired toner release pattern can be upheld in the presence of any surface charges building up on the printhead structure 5. This means that charged toner particles may still accumulate on the printhead structure 5 to a certain extent, but due to the fact that the area of the control electrode 53 is greater than said value, the surface charges building up on the semiconductive layer (as a result of the accumulated toner particles) will not seriously affect the electric field generated by the control electrode 53. There will still be time for the toner particles to be fed through the aperture 52 before the electric field is affected by said surface charges. According to such an embodiment, said value is chosen so as to correspond to a time period during which a desired toner release pattern can be upheld in the presence of any such accumulated surface charges building up on the printhead structure 5. In this regard, the area of the control electrode 53 constitutes a measure of the amount of surface charges which have been accumulated on the printhead structure 5.

The actual value of said area of the control electrode 53 may vary due to a number of parameters, for example the dimensions of the printhead structure 5, the diameter of the aperture 52 and the distance between the developer sleeve and the printhead structure 5. Additionally, there are other factors which affect the size of the area of the control electrode 53. In particular, it can be noted that the surface resistance of the semiconductive layers 503, 504 will affect the manner in which surface charges accumulate on the printhead structure 5 and consequently affect the electric field generated by the control electrode 53. For this reason, the value of the surface resistance of the semiconductive layers 503, 504 can be adapted to the magnitude of the area of the control electrodes (or vice versa) in order to provide correct operation of the image forming apparatus.

In particular, it can be noted that the design of the printhead structure 5 is preferably chosen so that the radial extension r of the control electrode 53 (cf. Fig. 5), i.e. the width of the first, generally circular section of the control electrode 53 which encloses the aperture 52, is greater than the distance L_k between the layer of toner particles and the upper surface of the printhead structure 5.

Also, the square root of the area of said first section of the control electrode is preferably greater than 15% of the circumference of the control electrode or the shortest path which extends around the control electrode. Also, the width of the control electrode is preferably, and at least partially, greater than 3% of the circumference of the control electrode or the shortest path which extends around the control electrode.

Furthermore, the area of a control electrode 53 is preferably more than 25% of the area of a corresponding aperture 52. Also, the width of the control electrode 53 is preferably, and at least partially, greater than 10% of the diameter of the aperture 52. Furthermore, the product of the area of a control electrode 53 and said surface resistance is greater than 1 m² ohm per square.

Furthermore, in the field of image forming devices, there is a general requirement to design the control electrodes with as small area as possible, which in turn would make it possible to produce printhead structures using only one row of apertures. In this regard, it should be noted that even if the control electrodes were of sufficiently small area, said area is still preferably greater than said predetermined value, which in turn corresponds to a time period during which the toner release pattern can be upheld even though surface charges are building up on the printhead structure 5. In this manner, the electric field can be provided during a sufficiently long time period for the toner particles to be allowed to be fed (by means of the background field cooperating with the electric field generated by the control electrodes) from the developer sleeve 33, through the apertures and onto the image receiving member.

The invention is not limited to the embodiments described above but may be varied within the scope of the appended patent claims. For example, the image forming apparatus according to the invention can be implemented with or without dot deflection electrodes.

As an alternative to the above-mentioned dot deflection control method, another type of multiplexing method can be used if a high resolution of the printed image (i.e. the number of printed dots per inch) is required. If, for example, a higher resolution than 200 dots per inch is desired, some form of multiplexing method is required for using one single aperture in a printhead structure for producing several dots on the image receving member. One such alternative multiplexing method can be accomplished by passing the image receiving member 1 three times, between which passes it is diplaced sideways a certain distance. Said distance then corresponds to a pitch which equals one single dot. This means that a printhead structure having 200 apertures per inch will produce undeflected centre dots in three consecutive complete revolutions of the image receiving member 1. As a result, the printhead structure having 200 apertures per inch can be used for producing an image on the image receiving member 1 which presents a resolution of 600 dots per inch. Consequently, this "multi pass" method increases the print addressability of the printhead structure without requiring an increased number of apertures in the printhead structure.

By means of the "multi pass" method, the resolution achieved by a printhead structure for a given number of apertures may be increased without necessarily the use of deflection electrodes. In order to achieve a printing resolution greater than the number of apertures in the printhead structure 2 the printing takes place in two or more passes of the image receiving member, i.e. the image receiving member 1 according to the above-mentioned embodiment. By a pass is meant a movement of the image receiving member which passes a section of the image receiving member to be printed with a movement relative to a given printhead structure and allows the printhead structure to deposit a plurality of longitudinal columns of printing. A column of printing is a longitudinal line of the image receiving member which is subject to printing of dots by an aperture or apertures even if not all the parts of the line receive dots due to the content of the image being formed requiring some parts of the column to be left without dots. A transverse line of printing is a transverse line of the image receiving member which is subject to printing of dots from a plurality of apertures, even if not all the parts of the line receive dots due to the content of the image being formed requiring some parts to be left without dots. The closest distance between two adjacent columns or lines of print is defined as the pitch or the distance between two addressable pixel locations. After the first pass the next passes may be in the same or opposite longitudinal directions to that of the first pass.

The image receiving member can be for example in the form of a transfer belt, as described above with reference to Fig. 1, or a drum. In the case that the image receiving member is a transfer belt, as mentioned above with reference to Fig. 1, the transverse direction is the direction in the plane of the belt perpendicular to the direction of movement of the belt, the said movement being the movement required to allow the belt to move around two rollers (not shown) . Thus, the transverse direction will normally be parallel to the axes of these rollers .

In the case the image receiving member is a drum, the transverse direction is the direction which is perpendicular to a radial vector of the cylinder towards the printhead structure at the surface of the drum and parallel to the axis of rotation of the drum along the surface of the drum.

The longitudinal direction is the direction perpendicular to the transverse direction and in the plane of the surface of the image receiving member, i.e. transfer belt or drum. In the case of the drum the longitudinal direction is the direction perpendicular to the transverse direction and along the circumference of the drum. In the case of a transfer belt the longitudinal direction is the direction at any point on its surface in the direction perpendicular to the axis of rotation of the rollers and in the plane of the surface of the drum.

In order to better understand the principles of the "multi pass" method, an example will first be described with respect to performing just two passes with each pass taking place in the same direction. In this case the number of apertures per unit length is half that needed to achieve the desired resolution with a single pass. In a first pass a first half of the image is formed on the image receiving member. This first half of the image comprises alternate longitudinal columns of print of the intended final image, i.e. alternate columns are printed and alternate columns are not printed. The image receiving member and printhead structure are then moved relative to each other in the direction transverse to the direction of movement of the image receiving member, but in the plane of the image receiving member. This relative movement may be carried out by any suitable means known to the person skilled in the art. Then, in a second pass, the remaining columns of print are printed to form a complete image. The second pass can carried out with the image receiving member traveling in the same longitudinal direction as the first pass or the opposite longitudinal direction.

The "multi pass" method can also be carried out using three or more passes for each printing sequence.

Furthermore, one or both of the insulating layers 501, 502 can be omitted. In this case, the corresponding semiconductive layers are then formed directly on the substrate 50. Also, the invention can be implemented with only one semiconductive layer, arranged on the top or bottom surface of the printhead structure. According to a further embodiment, the printhead structure can be provided with one or two layers of photoconductive material instead of the above-mentioned semiconductive material. Photoconductivity is the ability of a material to conduct electricity in a manner which depends on light which strikes the material in question. Certain semiconductor materials (for example germanium and gallium) can be used for this purpose. According to such an embodiment of the invention, the photoconductive layers are preferably arranged so that their surface resistance, when lit by a suitable source of light (which for example can be a infrared or visible light source) , decreases to a relatively low value as compared with its unlit condition. In this manner, an image forming apparatus using such photoconductive layers can be operated so that these layers are lit for example after a sheet of paper has been completely printed. Alternatively, the layers can be lit at one or several occasions during printing of a sheet of paper. In this manner, the surface resistance of the photoconductive layers can be controlled depending on whether any surface charges accumulate on the printhead structure .

Claims

CLAIMS :

1. An image forming apparatus in which image information is converted into a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier toward a back electrode member, said image forming apparatus including: a background voltage source for producing a background electric field which enables a transport of charged toner particles from said particle carrier towards said back electrode member; a printhead structure arranged in said background electric field at a predetermined distance from said toner carrier, said printhead structure including a plurality of apertures and control electrodes arranged in conjunction to the apertures; control voltage sources for supplying control potentials to said control electrodes in accordance with the image information to selectively permit or restrict the transport of charged toner particles from the particle carrier through the apertures; said control potentials being arranged so as to allow said transport of toner particles, thereby forming a predetermined toner release pattern; and an image receiving member caused to move in relation to the printhead structure for intercepting the transported charged particles according to said toner release pattern; characterized in that said printhead structure presents a top surface facing said particle carrier and a bottom surface facing said image receiving member, at least one of said top surface and bottom surface being covered with a layer having a surface resistance which is chosen so that any occurring surface charges building up on said printhead structure will be conducted generally without influencing the optical density or the position control of the toner particles intercepted by said image receiving member.

2. An image forming apparatus as defined in claim l,a characterized in that said layer presents a predetermined surface resistance which is more than 10⁸ ohm per square or more, preferably more than 10¹¹ ohm per square, and most preferably approximately 10¹² ohm per square.

3. An image forming apparatus as defined in claims 1 or 2, characterized in that said layer presents a predetermined surface resistance which is less than 10¹⁵ ohm per square, preferably less than 5*10¹² ohm per square, and most preferably less than 10¹³ ohm per square.

4. An image forming apparatus as defined in any one of the preceding claims, characterized in that said top surface and bottom surface are covered with a first semiconductive layer and a second semiconductive layer, respectively.

5. An image forming apparatus as defined in any one of the preceding claims, characterized in that the respective inner surfaces of said apertures are covered with a semiconductive layer.

6. An image forming apparatus as defined in claim 4 or 5, characterized in that said first semiconductive layer is of a different surface resistance than said second semiconductive layer.

7. An image forming apparatus as defined in claim 6, characterized in that the ratio of the values of the surface resistances of the two semiconductive layers is less than 100.

8. An image forming apparatus as defined in any of claims 4-7, characterized in that said semiconductive layers support at least one conductive bar which is in turn connected to a voltage source having a predetermined electric voltage, or to an earth connection.

9. An image forming apparatus as defined in claim 8, characterized in that said printhead structure is manufactured with openings allowing electrical contact between said conductive bar and an electrical connector being connected to said potential.

10. An image forming apparatus as defined in any one of the preceding claims, characterized in that a first insulating layer is arranged between said substrate and said first semiconductive layer and/or a second insulating layer is arranged between said substrate and said second semiconductive layer.

11. An image forming apparatus as defined in claim 10, characterized in that said insulating layer or layers are manufactured from parylene.

12. An image forming apparatus as defined in any one of the preceding claims, characterized in that each area of said control electrodes is greater than a predetermined value which corresponds to a time period during which said toner release pattern can be upheld in the presence of any occurring surface charges building up on said printhead structure.

13. An image forming apparatus as defined in any one of the preceding claims, characterized in that said layers are constituted by photoconductive layers which are associated with a light source arranged for influencing the surface resistance of said layers in dependence of any occurring surface charges building up on said printhead structure.

14. An image forming apparatus as defined in any one of the preceding claims, characterized in that the surface resistance as regarded in a first direction along said apertures is higher than the resistance as regarded in a second direction generally perpendicular to said first that direction.

15. An image forming apparatus as defined in any one of the preceding claims, characterized in that it is adapted for dot deflection control, involving control of the transport trajectory of said toner particles toward said image receiving member, thereby controlling the location of dots forming said toner release pattern.

16. An image forming apparatus as defined in any one of claims 1-14, characterized in that it is adapted so that a single printing sequence involves multiple passing of said image receiving member in relation to said printhead structure, said image receiving member being displaced a predetermined amount in a direction generally perpendicular to its direction of travel between passes.

17. A printhead structure for an image forming apparatus in which image information is converted into a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier toward a back electrode member, said image forming apparatus including a background voltage source for producing a background electric field which enables a transport of charged toner particles from said particle carrier towards said back electrode member; said printhead structure being arranged in said background electric field at a predetermined distance from said toner carrier, said printhead structure also including a plurality of apertures and control electrodes arranged in conjunction to the apertures; said printhead structure also being provided with a plurality of control voltage sources for supplying control potentials to said control electrodes in accordance with the image information to selectively permit or restrict the transport of charged toner particles from the particle carrier through the apertures; said control potentials being arranged so as to allow said transport of toner particles, thereby forming a predetermined toner release pattern on an image receiving member; characterized in that the printhead structure presents a top surface facing said particle carrier and a bottom surface facing said image receiving member, at least one of said top surface and bottom surface being covered with a layer having a surface resistance which is chosen so that any occurring surface charges building up on said printhead structure will be conducted generally without influencing the optical density or the position control of the toner particles intercepted by said image receiving member.

18. An printhead structure as defined in claim 17, characterized in that said top surface and bottom surface are covered with a first semiconductive layer and a second semiconductive layer.

19. A printhead structure as defined in claim 17 or 18, characterized in that the respective inner surfaces of said apertures are covered with a semiconductive layer.

20. A printhead structure as defined in any one of claims 17-19, characterized in that the semiconductive layers support at least one conductive bar which is in turn connected to a voltage source having a predetermined electric voltage, or to an earth connection.

21. A printhead structure as defined in claim 20, characterized in that it is manufactured with openings providing electrical contact between said conductive bar and an electrical connector being connected to said potential .

22. A printhead structure as defined in any claims 18-21, characterized in that each area of said control electrodes is greater than a predetermined value which corresponds to a time period during which said toner release pattern can be upheld in the presence of any occurring surface charges building up on said printhead structure.

23. A printhead structure as defined in any one of claims 18-22, characterized in that a first insulating layer is arranged between said substrate and said first semiconductive layer and/or a second insulating layer is arranged between said substrate and said second semiconductive layer.

24. A printhead structure as defined in claim 23, characterized in that said insulating layer or layers are manufactured from parylene.

25. A printhead structure as defined in any one of claims 20-24, characterized in that the distance from a line as regarded in a direction along said apertures to said conductive bar is less than 10 mm, preferably less than 5 mm, most preferably approximately 3 mm.

26. A printhead structure as defined in any one of claims 17-25, characterized in that the product of the area of a control electrode times said surface resistance is greater than 1 m² ohm per square.

27. A printhead structure as defined in any one of claims 17-26, characterized in that the product of the surface resistance and the distance from the apertures to said conductive bar is greater than 3*10⁵ meter ohm per square.

28. A printhead structure as defined in any one of claims 17-27, characterized in that the value of said surface resistance is chosen depending on the value of the area of the control electrode.

29. A method for manufacturing an image forming apparatus in which image information is converted into a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier toward a back electrode member, said method comprising the assembly of: a background voltage source for producing a background electric field which enables a transport of charged toner particles from said particle carrier towards said back electrode member; a printhead structure arranged in said background electric field at a predetermined distance from said toner carrier, said printhead structure including a plurality of apertures and control electrodes arranged in conjunction to the apertures, each of said control electrodes having a predetermined area; control voltage sources for supplying control potentials to said control electrodes in accordance with the image information to selectively permit or restrict the transport of charged toner particles from the particle carrier through the apertures; said control potentials being arranged so as to allow said transport of toner particles, thereby forming a predetermined toner release pattern; and an image receiving member caused to move in relation to the printhead structure for intercepting the transported charged particles in accordance with said toner release pattern; characterized in that said method comprises: providing said printhead structure with a top surface facing said particle carrier and a bottom surface facing said image receiving member, and covering at least one of said top surface and bottom surface with a layer having a surface resistance which is chosen so that any occurring surface charges building up on said printhead structure will be conducted generally without influencing the optical density or the position control of the toner particles intercepted by said image receiving member.

30. A method as defined in claim 29, characterized in that it comprises covering said top surface and bottom surface with a first semiconductive layer and a second semiconductive layer.

31. A method as defined in claim 30, characterized in that it comprises covering the the respective inner surfaces of said apertures with a semiconductive layer.

32. A method as defined in claim 30 or 31, characterized in that said semiconductive layers are deposited by means of chemical vapor deposition (CVD) or sputtering.

33. A method as defined in any one of claims 30-32, characterized in that it providing said semiconductive layers with at least one conductive bar which in turn is adapted to be connected to a voltage source having a predetermined electric voltage, or to an earth connection, said conductive bar being deposited by means of sputtering.