Background of the Invention US patent No. 5 847 733 describes a direct electrostatic printing device and a method of generating text and pictures with toner particles on an image receiving substrate directly from computer generated signals. Such a device generally includes a printhead structure through which toner particles are selectively transported in accordance with image data. The printhead structure is generally constituted by a control electrode array formed on an apertured insulating substrate. A ring electrode is associated with each aperture and is driven to control the opening and closing of the apertures to toner particles. Each aperture is further provided with deflection electrodes which are controlled to selectively generate asymmetric electric fields around the apertures, causing toner particles to be deflected prior to their deposition on the image-receiving medium. This process is referred to as dot deflection control (DDC). This enables each individual aperture to address several dot positions. The print addressability is thus increased without the need for densely spaced apertures.

The toner is held and charged on a particle carrier prior to being projected through the printhead structure onto the print medium. Ideally the toner is given a uniform and constant charge. However, the charging process is complex and it often occurs that a minority of the toner particles are given an equal but opposite charge to the bulk of particles. Such toner is often called wrong sign toner or WST. When the toner is released from the particle carrier and accelerated through the printhead structure under

the influence of the back electric field, the wrong sign toner will be swept along with the bulk of toner particles, possibly as part of an agglomerated particle. However, at any point along the trajectory, or as a result of the impact of the toner on the print medium, the wrong sign toner particles may become separated. These particles are then accelerated in the opposite direction, that is towards the particle carrier. Most of this wrong sign toner will be returned along the same path. But some toner will miss the printhead apertures and be deposited on the underside of the printhead structure.

Depending on the amount of deposited wrong sign toner, the electric field around the aperture will be more or less modified, which in turn will affect the amount and direction of the right sign toner particles emitted by an aperture. Moreover, this wrong sign toner is often distributed unevenly around an aperture which will influence the electric field asymmetrically, so subjecting toner to an additional, unwanted and uncontrollable deflection.

Thus there is a need for a direct electrostatic image forming arrangement that provides an improved toner flow in the vicinity of the printhead structure.

There is further a need for a direct electrostatic image forming arrangement that provides an improved print quality by ensuring that the deflection across the printhead is controllable and substantially uniform.

Summary of the invention The above needs are met by a printhead structure for an electrostatic printing apparatus that includes first and second surfaces and a plurality of apertures extending between the first and second surfaces, the first surface including a first electrode layer having a first set of electrodes associated with each aperture and the second surface including a second electrode layer having a second set of electrodes associated with each aperture, wherein means are provided on said second surface for generating an electrostatic charge that is substantially equal and opposite to that of charged toner particles deposited thereon.

By generating an equal and opposite charge for each wrong sign toner particle deposited on the printhead structure, the deflecting electric field can be maintained

unmodified and intact. Moreover, the apparatus can continue to function reliably without the need for frequent cleaning.

Preferably, the above means include an electrically conductive layer arranged on the second substrate surface between the apertures. By using an electrically conductive layer, a mirror charge is induced virtually instantaneously. This conductive layer preferably has a surface resistivity of at most 108 Q/square.

The electrically conductive layer is formed into several portions associated with each aperture and each portion is separated from an aperture by at most 70 micrometers, preferably at most 50 micrometers and most preferably no more than 30 micrometers to ensure that charge of WST particles is reliably cancelled out.

The conductive portion may be continuous about an aperture. In this case they are appropriately ring-shaped and arranged to surround an associated aperture.

In an arrangement wherein each electrode of the second set of electrodes is arranged to surround an associated aperture and has a diameter that is greater than the aperture diameter, the conductive portions may advantageously be arranged between the electrode and the aperture. Such an arrangement greatly facilitates the fabrication of the printhead structure by doing away with one deposition step. In this case the electrode and conducting portions are advantageously of the same material.

When the electrically conductive layer is connected to a voltage source, the attraction and cancellation of WST particles can be better controlled. The voltage source may generate ground potential or another potential. The voltage source may alternatively generate a varying voltage to effect a cleaning function in addition to cancelling out the WST charge. The voltage source is preferably connected to the conductive portions through an impedance such as a resistor or diode.

When the second set of electrodes are deflection electrodes, an insulating layer is provided on these electrodes and the conductive portions are arranged on the insulating layer.

The invention further resides in an image forming apparatus, including a particle carrier for holding a source of charged toner particles, a back electrode for generating a background electric field for accelerating the transport of charged toner particles from said particle carrier towards said back electrode, means for transporting an image receiving member between the particle carrier and the back electrode for intercepting the transported charged particles, and a printhead structure including a first surface directed towards the particle carrier and a second surface directed towards the back electrode and a plurality of apertures extending between the first and second surfaces, wherein the first surface includes a first electrode layer having a first set of electrodes associated with each aperture and the second surface includes a second electrode layer having a second set of electrodes associated with each aperture, wherein means are arranged on the second surface capable of generating a charge that is substantially equal and opposite to that of charged toner particles deposited thereon for neutralising the electric field generated by charged toner particles on the printhead structure.

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 the figures Fig. l 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. l, 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 section view across a section line I-I in the printhead structure of Fig. 4a, Fig. 4c is a partial view of a printhead structure of a type used in an image forming apparatus across a section line III-III of Fig. 4b, Fig. 4dis a partial view of the printhead structure of a type used in an image forming apparatus according to the present invention showing the surface of the printhead structure that is facing the intermediate transfer belt, Fig. 5 is a schematic side view of an image forming apparatus where the image receiving surface is provided on a shell of a cylindrical drum, Fig. 6 is a schematic illustration of the columns of print printed in a single pass in a two-pass method, Fig. 7 is a schematic illustration of the columns of print shown in Fig. 6 but after a additional, second pass, Fig. 8 is a schematic illustration of the effect of apertures which print with a lower density in a two-pass printing method, Fig. 9 is a schematic illustration of a first printing pattern, Fig. 10 is a schematic illustration of a second printing pattern, Fig. 11 is a schematic illustration of a third printing pattern, Fig. 12 is a schematic illustration of a fourth printing pattern, Fig. 13 is a schematic illustration of a fifth printing pattern, and Fig. 14 is a schematic illustration of a sixth printing pattern.

Detailed description of embodiments 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 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 create a stabilisation force component on the belt in combination with the belt tension. That stabilisation force component is opposite in direction to, 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 holding elements 12 are provided with an electrically conducting surface which is connected to a voltage source for generating a background electric field. These elements 12 thus serve as back electrodes.

The transfer belt 1 is preferably an endless band of 30 to 200 microns thick having composite material as a base. The base composite material can suitably include

thermoplastic polyamide 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 colour 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 toner sleeve or carrier 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 toner carrier 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 toner carrier. The toner carrier 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 on the surface of the toner carrier 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 toner carrier 33. Alternatively, the charge unit may additionally include a charging voltage source (not shown), which supplies 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 toner carrier 33 to adjust the concentration of toner particles on the peripheral surface of the toner carrier 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 influences the triboelectrification of the particle layer to ensure a uniform particle charge density on the surface of the toner carrier.

As shown in Fig. 3, the toner carrier 33 is arranged in relation with a positioning device 40 for accurately supporting and maintaining the printhead structure 5 in a

predetermined position with respect to the peripheral surface of the toner carrier 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 toner carrier 33 parallel with the rotation axis thereof. The first side ruler 42, positioned at an upstream side of the toner carrier 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 toner carrier 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 toner carrier 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 toner carrier 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 eccentricity or any other undesired variations of the toner carrier 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 toner carrier 33 remains constant along the whole transverse direction at every moment of the print process, regardless of undesired mechanical imperfections of the toner carrier 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 toner carrier 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 toner carrier 33 to accurately space the toner carrier 33 from the corresponding holding element 12 of the transfer belt 1 facing the actual print station.

Figs. 4a, 4b, 4c and 4d show a printhead structure in accordance with the present

invention. Fig. 4a shows the printhead structure 5 viewed from above, that is from the side facing the toner carrier 33. Fig. 4b shows a sectional view of the printhead structure viewed through line I-I of Fig. 4a. Fig. 4c shows a view of the printhead structure 5 viewed through the line III-III of Fig. 4c, and Fig. 4d shows the underside of the printhead structure 5. As shown in Figs. 4a, 4b, 4c and 4d, 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 facing the toner carrier 33, a second surface facing the transfer belt 1, a transversal axis 51 extending parallel to the rotation axis of the toner carrier 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 is coated with a first cover layer 501 of electrically insulating material, such as for example parylene. 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. The second surface of the substrate is coated with a second cover layer 502 of electrically : insulating material, such as for example parylene. A second printed circuit, including a plurality of deflection electrodes 54, is arranged between the substrate 50 and the second cover layer 502. As shown in Figs. 4b and 4d, a further layer 60 of electrically conductive material is disposed around each aperture 52 on the second cover layer 502. This layer 60 is comprised of several electrically conductive portions 60 that are associated with each aperture and face the transfer belt 1 in the assembled printing apparatus. The electrically conductive layer 60 is exposed to the surrounding air.

The printhead structure 5 is coupled to 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. In some embodiments, the control unit may even include a shield

voltage source (not shown) connected to shield electrodes (not shown) to supply a shield potential which electrostatically screens adjacent control electrodes 53 from one another.

In a preferred embodiment of the invention, the substrate 50 is a flexible sheet of polyimide having a thickness of the order of about 50 microns. The first and second printed circuits are copper circuits of approximately 8-9 microns thick deposited and etched on the first and second surface of the substrate 50, respectively, using conventional 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 electrically conductive portions 60 associated with each aperture 52 are aluminium that is deposited by sputtering to a thickness of about 8-9 microns thick. However, these conductive portions 60 need not be aluminium. Any electrically conductive material may be used. Electrically conductive in this sense is understood to be a material layer that has a surface resistivity of at most 108 Q/square. The surface resistivity is defined as the resistance between two opposite sides of a unit square of the surface of a material.

The apertures 52 are made through the printhead structure 5 using conventional laser micromachining methods. The apertures 52 preferably have a circular shape about a central axis with a diameter in a range of 80 to 120 microns as illustrated in Figs. 4a, 4c and 4d. Alternatively they may be elongated with a transversal minor diameter of about 80 microns and a longitudinal major diameter of about 120 microns. Although the apertures 52 preferably have 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.

As shown in Fig. 4a, the first printed circuit comprises the control electrodes 53 each 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 as shown in Fig. 4a is preferred, the control electrodes 53 may take on various shapes for continuously or partly surrounding the apertures 52. Preferred shapes include those 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 smaller in a transverse direction than in a longitudinal direction.

Turning now to Fig. 4c, 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 oriented relative to the longitudinal direction of the printhead, i. e. the direction of movement of the print medium 1, in accordance with the number of deflection sequences to be performed in each print cycle. This effectively neutralises the effects of the belt motion during the print cycle and allows transversally aligned dot positions to be obtained on the transfer belt. For instance, when using three deflection sequences, an appropriate deflection angle is chosen to arctan (1/3), 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 an upstream segment 541 and a downstream segment 542, all upstream segments 541 being connected to a first deflection voltage source (not shown) which will be referred to as D1, and all downstream segments 542 being connected to a second deflection voltage source, which will be labelled D2.

In accordance with the invention, the deflection voltage sources D1 and D2 are controlled by a control unit (not shown). Three deflection sequences (for instance: DKD2 ; D1=D2 ; D1>D2) can be performed in each print cycle, whereby the difference between D1 and D2 determines the deflection trajectory of the toner stream through each aperture 52, and thus the dot position on the toner image.

During charging of the toner particles on the toner carrier 33, some toner particles will be given an opposite charge to the bulk of toner particles. These particles are generally referred to as wrong sign toner (WST). Correct sign toner particles will naturally be attracted to these WST particles the result being agglomerated toner clumps with an overall charge that is correct. These clumps will thus be accelerated towards the back electrodes 12 when the control electrodes are driven to electrically 'open'the apertures. However, during the flight of these clumps, or as result of the impact on the transfer belt 1, the clumps will break up, releasing the WST particles.

These particles are naturally repelled by the back electrode potential and will fly back towards the toner carrier 33. However, some WST particles will become stuck to the underside of the printhead structure, and remain there around the aperture openings under the influence of the electrode potentials. The presence of these WST particles on the underside of the printhead structure affects the strength of the deflecting electric field generated by the deflection electrodes 54 around the aperture 52.

Moreover, their deposition is quite uncontrolled and random and may be asymmetric in the vicinity of the apertures 52, which will alter the deflection axis 543 for some apertures. The result is non-uniform deflection across the printhead structure 5, leading to an inferior print quality.

As shown in Fig. 4d, the conducting portions 60 formed in the conducting layer

surround each aperture 52. Each conducting portion 60 may have the same diameter as the aperture 52 or have a slightly larger diameter. Preferably the conducting portions have a diameter that is no more than 50 microns greater than that of the aperture 52, in other word, the conducting surface formed by these portions is preferably spaced by no more than 50 microns from the edge of the aperture opening 52.

The conducting layer 60 does not serve to discharge the toner particles. The latter are insulating bodies and will give up their charge only after considerably time, possibly several days. However, when a charged toner particle is deposited on or near the conducting layer 60 this layer generates a mirror charge that is substantially equal and opposite to that of a toner particle. In other words, the presence of a charged toner particle induces an opposite charge in the conducting layer. This induced charge cancels out the effects of the charged toner particles on the surrounding electric field.

The layer 60 is of electrically conductive material to ensure that the cancellation effect is virtually immediate.

As schematically illustrated in Fig. 4d the conducting layer may be connected to a voltage source 62 over an impedance 61, preferably a resistance, or a diode. The voltage source 62 may be ground potential or at some other potential. Specifically, the voltage source can be selected to generate a charge of opposite sign on said conducting layer, so that the wrong sign toner particles are attracted to the conducting layer rather than to other portions of the printhead structure. Alternatively, a varying voltage may be applied to the conducting portions 60. This may serve both to attract the WST particles and simultaneously to repulse them periodically, so as to effect cleaning of the printhead structure. The voltage source 61 is preferably controlled by a control unit that also controls the voltage applied to the control and deflection electrodes 52,54.

Although in the above embodiment the conducting layer 60 is deposited on the insulating layer 502, it may alternatively be deposited in parallel with the deflection electrodes 54, specifically on the substrate surface between the aperture openings 52 and the deflection electrodes 54. Obviously, the conducting portions 60 and the deflection electrodes 54 must be insulated from one another.

Fig. 5 is a schematic, simplified view of an image forming apparatus 2000 where the image receiving surface is provided on a cylindrical drum 2001. The image forming apparatus comprises one or several print stations 2003, each adapted for printing one color. Normally, the colors being used are yellow, magenta, cyan and black. Each print station 2003 advantageously has the form of an elongated cartridge assembly and is arranged adjacent to a printhead structure 2005, providing an electrode matrix with a plurality of selectable apertures, which is interposed in a background electric field defined between the corresponding cartridge 2003 and a back electrode, which in the image forming apparatus in fig. 5 is constituted of the cylindrical drum 2001. The drum 2001 is arranged so as to rotate during operation of the image forming apparatus. To this end, the drum 2001 is powered by drive means (not shown in Fig.

5). Furthermore, the drum 2001 has a circumference which is slightly greater than the length of the paper (or other information carrier) used during printing.. The drum 2001 advantageously is made of aluminum, but can also be made from other materials with suitable properties.

Each printhead 2005 is connected to a control unit (not shown in Fig. 5) which converts the image information in question into a pattern of electrostatic fields so as to selectively open or close passages in the electrode matrix to permit or restrict the transport of charged toner particles from the corresponding cartridge 2003. In this manner, charged particles are allowed to pass through the opened apertures and toward the back electrode, i. e. the drum 2001. The charged toner particles are then deposited on the surface of the drum 2001. Accordingly, in the image forming apparatus in Fig. 5, the drum 2001 constitutes both back electrode and image receiving surface.

Due to the fact that the drum 2001 is rotating during operation, the image being formed on the drum is then transferred onto an information carrier 2002, such as a sheet of printing paper or any other medium suitable for printing. The paper sheet 2002 is fed from a paper delivery unit 2021 and is conveyed past the underside of the drum 2001. In order to transfer the image to the paper sheet 2002, it is pressed into contact with the drum 2001 by means of belt 2017, which in turn is driven by means of two rollers 2016 around which the belt extends. In this manner, the toner particles

are deposited on the outer surface of the drum 2001 and then superimposed to the paper sheet 2002 to form a four-color image.

Accordingly, the operation of the belt 2017 defines a transfer step, which advantageously is positioned in the lowest section of the image receiving surface on the drum 2001. As a result, the force of gravity acting upon the toner particles will contribute to the transfer of said particles from the image receiving surface to the paper sheet 2002 during operation.

After the image has been formed on the paper sheet 2002 by said charged particles, the paper sheet 2002 is fed to a fusing unit 2013, in which the image is permanently fixed onto the paper sheet 2002. In particular, the fusing unit 2013 comprise a fixing holder (not shown) which includes a heating element, advantageously of a resistance type of e. g. molybdenium. As an electric current is passed through the heating element, the fixing holder reaches a temperature required for melting the toner particles deposited on the paper sheet 2002. The fusing unit 2013 further includes a pressure roller (not shown) arranged transversally across the width of the paper sheet 2002. Additionally, the fusing unit 2013 is provided with means for feeding the paper 2002 to an out-tray (not shown) from which the paper 2002 can be collected by a user.

Furthermore, after passage through the fusing unit 2013, the paper sheet 2002 can be brought in contact with a cleaning element (not shown), such as for example a replaceable scraper blade of fibrous material extending across the width of the paper sheet 2002 (or another suitable information carrier), for removing non-transferred toner particles from the paper sheet 2002.

The printstations 2003 and the printhead structures 2005 are mounted in a housing element (not shown in Fig. 5), so that they are maintained in predetermined positions with respect to the drum 2001.

An image forming apparatus of the type shown in Fig. 5 is particularly well suited for direct printing with multi-pass methods by means of which the resolution given by a printhead structure for a given number of apertures may be increased. In order to achieve a print resolution greater than the number of apertures in the printhead

structure 2005, multi-pass printing takes place during two or more passes of the image receiving surface provided by the drum 2001, wherein a plurality of longitudinal columns of print are deposited in each pass. A column of print is a longitudinal line of the image receiving surface 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 columns to be left without dots. A transverse line of print is a transverse line of the image receiving surface 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 of 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 transverse direction is the direction which, in case the image receiving surface is provided on a cylindrical drum, 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. In case the image receiving surface is provided on a transfer belt, the transverse direction is the direction in the plane of the belt perpendicular to the movement of the belt, wherein said movement is 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. The longitudinal direction is the direction perpendicular to the transverse direction and in the plane of the image receiving surface, 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 surface of the drum. In the case of 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 belt.

With respect to the description which follows, reference is made to image or printable area. In the present context, an image is formed by the toner particles over an area of the image receiving surface. The image also includes those printable areas that could receive toner particles but do not receive the particles because the content of the

image does not require this. Typically, an image covers approximately the area of an A4 sheet of paper, though possibly reduced by a small area around the margins that is not printed. The image may for example comprise a plurality of pictures or printed areas which would be printed on the same sheet of paper. Although reference is made to A4 paper, this is not limiting as the image could be the size of A3, or A5 or any other chosen paper size.

When performing direct printing with two passes in the same direction, the number of apertures per unit length is half of 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 surface. 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 are not printed. The image receiving surface and the printhead in question are then moved relative to each other in the direction transverse to the direction of movement of the plane of the image receiving surface. 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 the complete image. The second pass can be carried out with the image receiving surface traveling in the same longitudinal direction as the first pass or in the opposite longitudinal direction. This effect is illustrated in Figs. 6 and 7. The areas that are printed in the first pass are shown in Fig.

6 as hatched areas 1161. Fig. 7 represents the same section of the image receiving surface after the second pass. The areas that are printed in the second pass are shown as differently hatched areas 1162.

The density of a dot, i. e. the quantity of toner particles used to form the dot, may vary according to the position of the aperture on the printhead structure due to an insufficient available amount of toner particles. This is known as the starvation effect.

The variation in dot density may take place between apertures within the same row and/or between apertures in different rows. In the discussed example, there is only one row of apertures and the row is moved transversely by one dot pitch between the two passes. In this case, pairs of adjacent rows will be printed by the same aperture. This is illustrated in Fig. 8 in which the first positions of the apertures are indicated by the reference numeral 1170, while the positions of the same apertures during a second pass are indicated by the reference numeral 1171. Thereby, each aperture prints a

double column of print by printing two adjacent columns. If, for example, every fourth aperture suffered from the above-mentioned starvation effect, these apertures would produce columns of print having a lower optical density than the columns produced by the remaining apertures. If the row of apertures is moved transversely by one dot pitch, then the adjacent column will be printed in lower density. The result is then a column of a width that is double the width which would be due to printing by a single aperture. Such a double width column is more visible to a viewer. Columns of lesser density produced each by a single aperture in two passes are lighter shaded and indicated by reference numeral 1172 in Fig. 8, while columns of greater density produced by a single aperture in two passes are heavier shaded and are indicated by reference numeral 1173 in Fig. 8.

In order to reduce this"starvation"problem, the row of apertures can be moved tranversely with more than one dot pitch between the passes, as illustrated in Fig. 9.

The row is moved transversely by an amount equal to 2N+3 number of times the transverse pitch length L, where N is an integer including 0. N will have a maxiumum value dependent upon the number of apertures and the width of the image to be printed such that the transverse movement leaves enough apertures available to print the image. The pitch length L is the distance between adjacent dots. For 600 dpi (dots per inch), the pitch length is approximately 42 microns. The movement for one row of apertures printing in two passes at 600 dpi is approximately 42 microns. The movement for one row of apertures printing in two passes at 600 dpi is approximately 127 microns or a higher integer multiple as specified in the preceding formula. This is illustrated in Fig. 9 where the row of apertures has moved from first position indicated by reference numeral 1180 in which the first pass took place, to the position indicated by reference numeral 1181 in which the second pass took place. The apertures that are lighter shaded represent apertures that produce dots having lower density, while columns that are lighter shaded represent columns of print that have a lower density.

Columns of lesser density produced by a single aperture in two passes are indicated by reference numeral 1182 and columns of greater density each produced by a single aperture in two passes are indicated by reference numeral 1183 in Fig. 9. As can be seen from Fig. 9, the columns of less density 1182 are of narrower width than those 1172 in Fig. 8 and therefore less visible to a viewer. As is evident from the areas at the lateral sides of the image in Fig. 9, not every column of print may be printable by

an aperture. The number of apertures in a row is chosen such that not all the apertures are needed to print the intended image. The printing from apertures at the end of the rows which are outside the area to be printed is the suppressed.

Fig. 10 is a schematic depiction of printing in three passes. In Fig. 10, the row of apertures has moved from the first position indicated by reference numeral 1191 in which the first pass took place to the position indicated by reference numeral 1192 in which the second pass took place and then to the position indicated by reference numeral 1193. The number of apertures per unit of length transversely is one third of that needed for achieving the same resolution in a single pass. In a first pass a first one third of the image is formed. This first third of columns of print is indicated by reference numeral 1194. The image receiving surface and the printhead structure are then moved relative to each other in the direction transverse to the printing direction but in the plane of the image receiving surface, preferably by moving the drum (or belt) transversely. Then, in a second pass, a second set of columns of the image indicated by reference numeral 1195 are printed. The printhead structure is moved transversely by an amount equal to 3N+2 number of times the transverse pitch length, where N is an integer including 0. N will have a maximum value dependent upon the number of apertures and the width of the image to be printed such that the transverse movement leaves enough apertures available to print the image. The second pass can occur with the belt traveling in the same longitudinal direction as the second pass or in the opposite longitudinal direction. In a third and final pass, the remainder of the columns of the image indicated by reference numeral 1196 are printed. Between the second and third pass the printhead structure is moved transversely by an amount equal to 3N+2 number of times the transverse pitch length L, where N is an integer including 0. N will have a maximum value dependent upon the number of apertures and the width of the image to be printed such that the transverse movement leaves enough apertures available to print the image. The third pass can occur with the image receiving surface traveling in the same direction as the first pass or in the opposite direction. Fig. 10 schematically depicts the image receiving surface at the end of the third pass. The apertures that are lighter shaded represent apertures that produce dots having lower density. The columns that are lighter shaded represent columns of print that have a lower density. As can be seen these columns of lower density are not adjacent each other. Between each pass the row of apertures has been

moved transversely by an amount equal to 3N+2 number of times the transverse pitch length L. Alternatively, the row could be moved transversely by an amount equal to 3N+4 number of times the transverse pitch length between each pass where N is an integer including 0. The number and transverse extent of the apertures in a row is chosen such that not all the apertures are needed to print the intended image. The printing from apertures at the end of the rows which are outside the area to be printed is then suppressed. The apparatus may be arranged such that the non-used apertures are at both ends of the row or rows of apertures and during a pass an aperture or apertures at both ends are simultaneously not used.

In general, for printhead structures having a single row of apertures the movement could at least be PN+P+1 or PN+P-1 times the pitch length where P is number of passes needed to complete an image, and N is an integer including 0. However for certain numbers of passes there may be more allowable movement possibilities. So for 5 passes the movements may be 5*N + X times the pitch length where X may be 2,3, 4 or 6, and N is an integer including 0. In this case the values of X = 4 and 6 correspond to the general formula, whereas the values of X = 2 and X = 3 are extra values. Furthermore for 7 passes the movements may be 7*N + X times the pitch length where X may be 2,3,4,5,6 or 8 and N is an integer including 0. In this case the values of X = 6 and 8 correspond to the general formula, whereas the values of X = 2, 3,4 or 5 are extra values. Extra values in particular occur where the number of passes is a prime number. In this case the number of pitch lengths may be neither 1 nor an integer multiple of 7.

The printhead structure may comprise one or more transverse rows of apertures. The number of apertures in each transverse row may be equal or unequal. The pitch between each aperture in a row may be equal or unequal. The pitch between apertures in a row may be the same in each row, or different rows may contain apertures with different pitches. The apertures in one row are in staggered relationship with the apertures of another row. In a printhead structure containing two rows of apertures the apertures in one row may be arranged to be centered between the apertures of the other row. Alternatively the apertures of one row may arranged to be off centre relative to the apertures of the other row, whilst avoiding being in longitudinal alignment. There may alternatively three or more rows of apertures per printhead. The

number of rows of apertures may be the same on each printhead or different.

This is illustrated in Fig. 11 where the printhead structure includes two rows of apertures 1201,1202. The apertures in one row 1201 are transversely displaced relative the apertures in the other row 1202. The apertures as shown are spaced apart from each other transversely by the same distance, though this is not essential. Each row of apertures includes one sixth of the number of apertures per unit length required to print the complete image so that the two rows of apertures together include one third of the number of apertures per unit length required to print the complete image.

The image is printed in three passes of the printhead structure. The positions of the rows of apertures for the first, second and third passes are indicated by 1203,1204 and 1205 respectively. Between each pass the printhead structure and image receiving surface are moved relative to each other by a distance equal to four times the pitch length L. In Fig. 11 the columns of print printed by the second row of apertures is indicated by shading. Columns printed during the first, second and third passes are indicated by 1206,1207 and 1208 respectively. As can be seen in Fig. 11, the movement by four times the pitch length results in adjacent columns of print being printed by apertures which belong to different rows.

The rows of apertures may not receive the same quantity of toner particles when printing. Since one row is always upstream or downstream of another row relative to the movement of the toner carrier the row which is upstream will have more toner available than the row which is downstream. The effect of this is that the downstream row or rows may produce dots of a lower density than other rows. If adjacent columns of print are printed by apertures in the same row then the effect of the lower density will be more visible as double width columns of low density will be produced.

Preferably, no two adjacent columns of print are produced by the same row of apertures, since this ensures that the columns of lower density are always spaced from each other and hence are less visible. Although, described with a relative movement between passes of four times the column width the movement could also be eight times the column width. The number and transverse extent of the apertures in the rows is chosen such that not all the apertures in each row are needed to print the intended image. The printing from apertures at the end of the rows which are outside the area to be printed is then suppressed. The apparatus may be arranged such that the non-used

apertures are at both ends of the row or rows of apertures and during a pass an aperture or apertures at both ends are simultaneously not used.

Furthermore, so-called DDC control of the apertures may be used. When DDC control is applied, each aperture is able to print more than one column of print in a single pass. The DDC control is preferably arranged to print columns from a single aperture which are not adjacent to each other, though in a less preferred embodiment they could print adjacent columns in a single pass. In an example (see Fig. 12) the DDC control is arranged to print two non-adjacent columns of print per pass from each aperture, wherein the columns are separated from each other by a distance of twice the pitch length. In Fig. 12 the row of apertures has moved from the first position indicated by reference numeral 1210 in which the first pass took place to the position indicated by reference numeral 1213 in which the second pass took place. The columns printed by a single aperture 1211 are indicated by shaded lines 1212 in the drawing. The position of the aperture 1211 producing the columns is indicated by shading. The aperture in this example produces columns of print that are separated by a single column. The image receiving surface and printhead structure are then moved relative to each other by 5 pitch lengths L in the direction transverse to the direction of movement of the transfer belt, but in the plane of the belt. Then, in a second pass, a second set of columns of the image are printed. The position 1213 of the apertures in the second pass are indicated by the second row of apertures. The columns printed by the aperture 1211 in the second pass are indicated by shaded lines 1214 in the drawing. The printhead structure is moved transversely by an amount equal to N*2 + 5 number of times the transverse pitch length L, where N is an integer including 0. N will have a maximum value dependent upon the number of apertures and the width of the image to be printed such that the transverse movement leaves enough apertures available to print the image. Thereby, N is equal to 0 so that the relative movement between passes is equal to 5. As can be seen, the relative movement is just sufficient to ensure that the columns printed by a single aperture are not adjacent each other.

The relative movement could however be greater than 5, e. g. 7,9 etc.

In another example using DDC control (see Fig. 13) each aperture prints two columns of print which are separated from each other by four times the pitch length. In Fig. 13 the row of apertures has moved from the first position indicated by reference numeral

1220 in which the first pass took place to the position indicated by reference numeral 1223 in which the second pass took place. The columns printed by a single aperture 1221 are indicated by shaded lines 1222 in the drawing. The position of the aperture 1221 producing the columns is indicated by shading. The position 1223 of the apertures in the second pass are indicated by the second row of apertures. The columns printed by the aperture 1221 in the second pass are indicated by shaded lines 1224 in the drawing.

In another example, the relative movement is less if the distance between the columns printed in a pass is at least six. In this case the relative movement may be only three pitch lengths. This is possible because the individual columns printed by a single aperture are sufficiently far apart to allow an intermingling of columns printed from different passes by the same aperture. This is illustrated in Fig. 14 where the row of apertures has moved from the first position indicated by reference numeral 1230 in which the first pass took place to the position indicated by reference numeral 1233 in which the second pass took place. The columns 1232 printed from the aperture 1231 on the first pass are printed in hatched shading and the columns 1234 printed on the second pass are printed in differently hatched shading. As is visible in the drawings, columns from one pass intermingle columns from the other pass.

In a further example each aperture prints two columns per line and pass, the distance between the two columns is three times the pitch length and the image is printed in three passes. In this case the relative transverse movement between passes may be 5,7 or more times the pitch length, according to the formula N*3 + 5 or N*3 + 7, where N is an integer including 0.

In yet another example each aperture prints three columns per line and pass, the distance between the three columns is two times the pitch length and the image is printed in two passes. In this case the relative transverse movement between passes may be 7,9 or more times the pitch length according to the formula N*2 + 7, where N is an integer including 0.

In a further example DDC control is used to print adjacent columns of print. Each aperture prints two adjacent columns so the spacing is one pitch length. The image is

printed in two passes. In this case the relative transverse movement between passes may be 6,10 or more times the pitch length according to the formula N*4 + 6, where N is an integer including 0.

In still a further example DDC control is used to print adjacent columns of print. Each aperture prints two adjacent columns so the spacing is one pitch length. The image is printed in three passes. In this case the relative transverse movement between passes may be 4,8 or more times the pitch length according to the formulae N*6 + 4 or N*6 + 8, where N is an integer including 0.

In a yet a still further example DDC control is used to print adjacent columns of print.

Each aperture prints three adjacent columns so the spacing is one pitch length. The image is printed in two passes. In this case the relative transverse movement between passes may be 9,15 or more times the pitch length according to the formulae N*6 + 9, where N is an integer including 0.

It is also possible to use DDC control in combination with multiple rows of apertures.

The amount of transverse movement of the printhead structure relative to the image receiving surface is normally greater than the transverse distance between the apertures in the printhead structure. This means that for any one aperture its transverse position during a subsequent pass is beyond the position of the aperture which was transversely adjacent to said one aperture in the previous pass. Alternatively, any one aperture is beyond the position of the aperture which was transversely adjacent to said one aperture in the previous pass plus one, i. e. two passes previously. This means that an aperture passes beyond the position of its neighbor either at the next pass or over next pass.

The transverse spacing of the apertures in the printhead structure may assume any suitable value. Preferably the value is between 1 and 9 times the pitch length more preferably it is between 2 and 6 times the pitch length or less. Even more preferably it is between 3 and 5 times the pitch length.

In the previously discussed Fig. 5, the image receiving member is a drum 2001. The

drum rotates about an axis 2201. Around the periphery of the drum are arranged four print stations 2003. The print stations 2003 respectively contain differently colored toner particles, e. g. yellow, cyan and magenta respectively, to allow color printing.

The fourth print station contains black toner particles to allow black and white printing. Alternatively, the black print station could be arranged before the color print stations. There is also provided a transfer station 2016,2017 for transferring the image to another medium. Transfer may be effected by electrostatic attraction or by pressure transfer. A cleaning station (e. g. of the type denoted by reference numeral 1061 in Fig. 2) can be provided for cleaning the printhead structures of toner particles as required. The cleaning station comprises a vacuum source. The vacuum source acts through one or more transversely aligned rows of apertures in the drum so that a suction force may be effected on a printhead structure.

The printhead structure 2005 provided with each print station is of the type illustrated in Figs. 4a-4c, i. e. two parallel rows of apertures with constant pitch between the apertures in a row. The apertures of one row are staggered in relationship to the apertures of the other row. The apertures of one row may be centered in the spaces between the apertures of the other row, though they could be arranged eccentrically.

Cleaning of the printhead structures 2005 preferably is performed after each pass.

Alternatively, the cleaning is performed after an image has been formed, or after two or more images have been formed.

During a pass each transverse line of the image to be formed on the drum passes the printhead structures in turn. The transverse line then passes the transfer station 2016, 2017. While the drum is rotating it is moved along its axis 2201. The printhead structures and drum are thus moved continuously relatively to each other in the transverse direction parallel to the axis of the drum. Each rotation of the drum causes a pass of the printhead structures. After two or more passes or rotations of the drum during which printing is effected the transfer station starts to transfer the image to paper as soon as the leading edge of the image reaches the fuser unit. This transfer may start before the other parts of the image have passed all the printhead structures.

The cleaning station (not shown) is preferably arranged so that cleaning of each printhead structure may be effected on each pass.

The image preferably occupies a major portion of the circumference of the drum, in particular more than 50%, preferably more than 75%. Where the image occupied a sufficient portion of the circumference of the drum, the start of a further pass for the leading edge of an image may start to be printed before the previous pass has been completed by all printhead structures.

The relative transverse movement between or during passes may take on the following values. In a first example for three or four passes and two rows of apertures per printhead structure a step distance of (P + RxPxN + 1) or of (P + RxPxN-1) times the pitch length give suitable values for the transverse movement, where P = number of passes, R = number of rows, N is an integer including 0. In a second example for five passes and two rows of apertures per printhead structure a step distance of (P + RxPxN + X) or of (P + RxPxN-X) times the pitch length give suitable values for the transverse movement, where X can take the values: +3, +1,-1, -3. In a third example for two passes and three rows of apertures per printhead structure a step distance of RxPxN-2 is possible. In a fourth example for three passes and three rows of apertures per printhead structure a step distance of RxPxN + X, where X has the values-7 or-5 are possible.

The above examples are particularly useful where the starvation effect leads to a variation in dot density between different rows of apertures on the printhead structure.

However, the starvation effect may occur over several adjacent apertures which are spaced from each other in the transverse direction. In this case it may be appropriate to have a larger transverse movement. For example it may be two or more times the extent of the starvation effect. The printhead structure or another part of the printer may include an instrument for measuring the optical density of the image. The instrument may detect the transverse extent of the starvation effect. The output of the instrument may be used to cause a transverse movement sufficient that that the apertures affected by the starvation effect do not print columns adjacent to columns which were formed by the"starving"apertures in a preceding pass.

After a number of passes the direction of movement of the drum relative to the printhead structures will be reversed. To effect this a pass without any printing is

performed during which the direction of movement is changed. Preferably the change in direction takes place after one image has been completed and before another image is commenced. A pass without printing may also be made where it is desired to change the speed and/or pattern of the transverse motion of the drum.

The transfer drum 2001 can be formed of an electrically conducting material. The material may optionally be covered on its surface facing outwardly towards the toner carrier with a thin layer of an electrically insulating material, preferably less than 100 microns thick. The electrically conducting material is preferably a metal though any material is possible so long as it conducts electricity. The metal is preferably aluminum. The thin layer of insulating material is sufficiently thin that the electric field lines pass through sufficiently to allow a mirror charge to be formed which mirrors the charge on the toner on the surface of the transfer belt or drum. This mirror charge increases the force holding the charged toner to the transfer belt or drum. The insulating materials may be any suitable material, in particular aluminum oxide. The aluminum oxide may be combined with any conducting material for the drum, but is particularly advantageous when used with a drum with an aluminum surface. The above form of drum is particularly useful when the transfer of the image is to be effected by pressure as the stronger material of the drum allows a higher pressure to be used.

This form of drum is particularly useful with a multipass printer as hereinbefore described, but may be used with other types of printers, particularly those with high surface speeds of the drum or belt.

In any of the above-discussed examples, the pitch (distance between centers of dots) may be varied. The distance between dots on the transverse lines (horizontal pitch) may be varied and/or the distance between dots in a longitudinal column (vertical pitch) may be varied. The horizontal pitch may be varied by varying the amount of relative transverse movement between passes. The vertical pitch can be varied by varying the amount of longitudinal movement between the printing of lines.

The back electrode member or members utilized in an image forming apparatus can be of a number of different types, e. g. a stationary or rotating roll or sleeve, or a

movable belt arranged in an endless loop by means of guide rolls. Depending on the application, the back electrode member can be made of different materials, e. g. a suitable metal alloy or another electrically conductive material. Furthermore, a back electrode member can be arranged behind a belt constituting an intermediate image receiving member.

It is also conceivable with embodiments where a suitable information carrier, such as a printing paper, passes across the back electrode when printing so that an image is printed directly onto the information carrier, or where the information carrier also constitutes the back electrode by means of being electrically conductive.

In other applications, an intermediate image is formed directly onto the surface of the back electrode member, whereafter the image is transferred to a suitable image receiving substrate such as a printing paper. It is particularly advantageous to print directly onto the back electrode in applications utilizing so-called multi-interlacing (MIC) techniques.

Furthermore, it is conceivable with applications where the electrical field, by means of which the toner particles are transported, is generated by another means than a pair of electrodes, e. g. applications where the electrical field is generated by means of a suitable charge carrier which in itself is able to generate an electrostatic field.

The present invention is not limited to the embodiments described above but may be varied within the scope of the appended patent claims.