Technical field The invention relates to a direct printing apparatus in which a computer generated image information is converted into a pattern of electrostatic fields, which selectively transport electrically charged particles from a particle carrier towards an image receiving surface through a printhead structure, whereby the charged particles are deposited in image configuration on the image receiving surface caused to move relative to the printhead structure.

More particularly, the present invention relates to an image forming apparatus where the particle carrier is included in a toner delivery unit for delivering the charged toner particles to the printhead structure, wherein the apparatus comprises a plurality of the toner delivery units and the printhead structures arranged in a succession along the image receiving surface. Furthermore, the invention relates to a method for direct printing.

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 an image information.

The printhead structure according to US 5,036,341 is formed by a lattice consisting in intersecting wires disposed in rows and columns. Each wire is connected to an individual voltage source. Initially the wires are grounded to prevent toner from passing through the wire mesh. As a desired print location on the image receiving substrate passes below an intersection, adjacent wires in a corresponding column and

row are set to a print potential to produce an electric field that draws the toner particles from the particle source. The toner particles are propelled through the square aperture formed by four crossed wires and deposited on the image receiving substrate in the desired pattern. A drawback with this construction of printhead structure is that individual wires can be sensitive to the opening and closing of adjacent apertures, resulting in imprecise image formation due to the narrow wire border between apertures.

This effect is mitigated in an arrangement described in US Patent No. 5,847,733 by the present applicant. This proposes 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 aperture to toner particles. Each aperture is further provided with deflection electrodes. These are controlled to selectively generate an asymmetric electric field around the aperture, causing toner particles to be deflected prior to their deposition on the image-receiving member. 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.

Today, more than 15 pages per minute (ppm) is regarded as a high speed when direct printing is concerned. However, it has proved difficult to increase the printing speed further since toner delivery units in accordance with the present technology cannot provide a sufficient amount of toner. As should be well known to the skilled person, an insufficient amount of toner will cause an undesired variation in the optical density of the printed image.

Due to lack of space and manufacturing problems, it has also proved difficult to manufacture printheads constituted of flexible printed circuits with a sufficient number of apertures and control electrodes to reach a higher print resolution than about 200 dots per inch (dpi). It has previously been suggested (e. g. in U. S. Patent No.

5,515,084) that a matrix comprising a plurality of apertures and control electrodes in rows and columns should be included in the flexible printed circuit (FPC) instead of providing a single row of apertures and control electrodes. As mentioned above, also dot deflection control (DDC) can be utilized in order to increase the print resolution.

Even if these previously disclosed technical solutions are able to provide flexible printed circuits enabling the print resolution to be increased, they cannot solve the above-mentioned problem of an insufficient toner supply.

Since there still are market demands for higher printing speeds with maintained or even improved print resolution, there is a need for an improved direct printing apparatus and method.

Summary of the invention Accordingly, a first object of the present invention is to provide an image forming apparatus for direct printing, which apparatus enables a maintained or even improved print resolution at a higher printing speed than what previously has been possible.

In accordance with claim 1, this first object is achieved by means of an image forming apparatus in which an image information is converted into a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier towards an image receiving surface. The image forming apparatus includes a background voltage source for producing a background electric field which enables a transport of charged toner particles from the particle carrier towards the image receiving surface, a printhead structure arranged in the background electric field including a plurality of apertures and control electrodes arranged in conjunction to the apertures, and control voltage sources for supplying control potentials to the 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.

The image receiving surface is arranged for movement in relation to the printhead structure for intercepting the transported charged toner particles in an image configuration, and the particle carrier is included in a toner delivery unit for delivering the charged toner particles to the printhead structure. Thereby, the image forming apparatus comprises a plurality of the toner delivery units and the printhead structures arranged in a succession along the image receiving surface.

According to the invention, the relative movement of the image receiving surface in relation to the printhead structures causes each line in the image configuration that is

transverse to the direction of the relative movement to pass several of the printhead structures in order to form the image configuration, so that at least two of the printhead structures print only part of each transverse line in order to form longitudinal columns of print, and so that columns of print from different printhead structures together form the image configuration in one single pass of the image receiving surface past the succession.

A second object of the present invention is to provide an improved method for direct printing, which method ensures an excellent print quality also at high printing speeds.

In accordance with claim 20, this second object is achieved by means of a method for direct printing comprising to convert an image information into a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier towards an image receiving surface and to produce a background electric field which enables a transport of charged toner particles from the particle carrier towards the image receiving surface. The method further comprises to provide a printhead structure including a plurality of apertures and control electrodes arranged in conjunction to the apertures in the background electric field, to arrange the image receiving surface for movement in relation to the printhead structure, to include the particle carrier in a toner delivery unit for delivering the charged toner particles, and to arrange a plurality of the toner delivery units and the printhead structures in a succession along the image receiving surface. Furthermore, the method comprises to supply control potentials to the control electrodes in accordance with the image information to selectively permit or restrict the transport of charged toner particles from the particle carriers through the apertures of the printhead structures, and to intercept the transported charged toner particles in an image configuration on the image receiving surface.

According to the invention, the image receiving surface is moved in relation to the printhead structure in such a way that each line in the image configuration that is transverse to the direction of the relative movement passes several of the printhead structures in order to thereby form the image configuration. Thereby, at least two of the printhead structures print only part of each transverse line in order to form longitudinal columns of print, and columns of print from different printhead structures

together form the image configuration in one single pass of the image receiving surface past the succession.

Brief description of the drawings Fig. 1 is a schematic view of an image forming apparatus where an image-receiving surface is provided on a belt arranged in an endless loop.

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

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

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 an image receiving surface.

Fig. 4c is a sectional 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 schematic perspective view of an image forming apparatus according to a first embodiment of the invention, wherein the apparatus is shown printing the capital letter H in black color at a high printing speed.

Fig. 6 schematically illustrates the principle for high speed printing according to the invention when printing the capital letter H in mono-color.

Fig. 7 is a schematic side view of an image forming apparatus according to a preferred embodiment of the invention, wherein the apparatus is intended for high speed multi- color printing.

Fig. 8 is a schematic side view of an image forming apparatus according to a first alternative embodiment of the invention, also intended for high speed multi-color printing.

Fig. 9 is a schematic side view of an image forming apparatus according to a second alternative embodiment of the invention, also intended for high speed multi-color printing.

Detailed description of embodiments In order to perform a direct electrostatic printing using the apparatus shown in Figs. 1- 4c, 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 surface 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. perpendicularly to the motion direction of the image receiving surface.

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.

Advantageously, a direct electrostatic printing device of the type in question includes a dot deflection control (DDC). Thereby, each single aperture is used to address several dot positions on an image receiving surface by controlling not only the transport of toner particles through the aperture, but also their transport trajectory toward the image receiving surface, 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.

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 D1, D2 have the same amplitude. The amplitude of D1 and D2 are modulated to apply converging

forces on toner particles as they are transported toward the image receiving surface, thus providing smaller dots. The dot position is simultaneously controlled by modulating the amplitude difference between D1 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.

Fig. 1 shows an image forming apparatus 1000 which comprises four print stations (K, K, K, K), an intermediate member 1001 providing an image receiving surface, a driving roller 1010, at least one support roller 1011, and preferably several adjustable holding elements 1012. The four print stations are arranged in relation to the intermediate member 1001. The intermediate member, in Fig. 1 a transfer belt 1001, is mounted over the driving roller 1010. The at least one support roller 1011 is provided with a mechanism for maintaining the transfer belt 1001 with a constant tension, while preventing transversal movement of the transfer belt 1001. The holding elements 1012 are for accurately positioning the transfer belt 1001 with respect to each print station.

The driving roller 1010 in Fig. 1 is a cylindrical metallic sleeve having a rotational axis extending perpendicularly to the motion direction of the belt 1001 and a rotation velocity adjusted to convey the belt 1001 at a velocity of one addressable dot location per print cycle, to provide line by line scan printing. The adjustable holding elements 1012 are arranged for maintaining the surface of the belt at a predetermined gap distance from each print station. The holding elements 1012 in Fig. 1 are cylindrical sleeves disposed perpendicularly to the belt motion in an arcuate configuration so as to slightly bend the belt 1001 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 1001 due to interaction with the different electric potentials applied on the corresponding print station.

The transfer belt 1001 in the apparatus shown in Fig. 1 is an endless band of 30 to 200 microns thick composite material as a base. The base composite material can 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 below temperatures in the order of 250°C. The composite material of the transfer belt has 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 1001. The outer surface of the transfer belt 1001 in Fig. 1 is 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 1001 is conveyed past the four different print stations, whereas toner particles are deposited on the outer surface of the transfer belt to form a toner image.

Toner images can then be conveyed through a fuser unit 1013 comprising a fixing holder 1014 arranged transversally in direct contact with the inner surface of the transfer belt. The fixing holder includes a heating element 1015, suitably of a resistance type of e. g. molybdenium, maintained in contact with the inner surface of the transfer belt 1001. As an electric current is passed through the heating element 1015, the fixing holder 1014 reaches a temperature required for melting the toner particles deposited on the outer surface of the transfer belt 1001. The fusing unit 1013 further includes a pressure roller 1016 arranged transversally across the width of the transfer belt 1001 and facing the fixing holder 1014. In the apparatus shown in Fig. 1, an information carrier 1002, such as a sheet of plain untreated paper or any other medium suitable for direct printing, is fed from a paper delivery unit 1021 and conveyed between the pressure roller 1016 and the transfer belt. The pressure roller 1016 rotates with applied pressure to the heated surface of the fixing holder 1014 whereby the melted toner particles are fused on the information carrier 1002 to form a permanent image. After passage through the fusing unit 1013, the transfer belt is

brought in contact with a cleaning element 1017, such as for example a replaceable scraper blade of fibrous material extending across the width of the transfer belt 1001 for removing all non-transferred toner particles from the outer surface.

Instead of a single unit performing a combined image transfer and fusing step, separate units for transferring the image to the information carrier and for fusing/fixating the image to the information carrier can be provided. The fusing unit normally is provided with means for feeding the paper to an out-tray, from which the paper can be collected by a user.

Toner particles are retained on the surface of a particle carrier (e. g. 1033 in Figs. 2-3) by an adhesion force which essentially is related to the particle charge and to the distance between the particle and the surface of the particle carrier. The electrostatic field applied onto a control electrode to initiate toner transport through a selected aperture is selected to be sufficient to overcome the adhesion force in order to cause the release of an appropriate amount of toner particles from the particle carrier. The electrostatic field is applied during the time period required for these released particles to reach sufficient momentum to pass through the selected aperture, whereafter the transported toner particles are exposed to the attraction force from the back electrode and are intercepted by the image receiving surface.

Properties such as charge amount, charge distribution, particle diameter etc. of the individual toner particles have been found to be of particularly great importance to the print performance in a direct printing method. Accordingly, the size and size distribution of the toner particles affect the printing result, since larger toner particles have a tendency to cause clogging of the apertures in the control electrode array. In addition, the toner particles allowed to pass through selected"opened"apertures are accelerated towards the image receiving under the influence of a uniform attraction field from the back electrode. In order to control the distribution of the transported particles onto a printing surface, the particles may be deflected by the application of a deflection pulse, resulting in an increase in the addressable area on the image receiving surface. Thereby, small particles having a low surface charge exhibit poor deflection properties.

Normally, toner particles are produced by the so-called melt-crushed method, which involves crushing and classifying coloured resin with dispersed colouring agents and other additives using a compounding process. However, this method is not ideally suited for producing small-particle toner since it has a relatively low yield, and tends to produce a great variety of particle sizes and toner particles with a non-uniform composition. A non-uniform toner results in a poor charge uniformity and may impair the print quality.

Toner particles can also be produced in a chemical polymerization process, which is better suited for producing small toner particles of a uniform size. There are three basic processes, i. e. the suspension polymerization method, the dispersion polymerization method, and the emulsion polymerization method. The suspension and dispersion polymerization methods produce full-shaped spherical toner particles with a size between a few and up to 10 microns. The emulsion polymerization method produces polymer particles of sub-micron size or smaller, which particles are aggregated by means of different methods, e. g. heat-welding or coagulation, in order to form micron-order particles. The shape of the aggregated particles can vary from grape cluster to spherical, depending on the conditions prevailing in the aggregation process.

Toner particles can comprise a number of ingredients, e. g. a binding resin based on a cyclic polyolefin e. g. a copolymer of an alicyclic compound with double bonds, such as cyclohexene or norbornene, and an alpha-olefin, such as ethylene, propylene or butylen. Accordingly, the toner particles can be of 2-component or multi-component type.

Toner for use in direct printing can be of a multi-component type comprising a suitable toner carrier, e. g. in the form of carrier beads which are recirculated within the toner supply system when printing. In addition to toner resin and toner carrier, multi-component toner can comprise different colorants, charge control agents, magnetic additives, bulk additives, surface additives, conductive additives, etc.

Advantageously, the toner particles have an irregular surface structure and an average diameter within the range of 3-8 microns. Depending on the application in question

electrically non-conductive, non-magnetic, or magnetic toner particles can be provided and utilized.

As shown in Fig. 2, a print station of an image forming apparatus, e. g. the one shown in Fig. 1, includes a particle delivery unit 1003 advantageously having a replaceable or refillable container 1030 for holding toner particles, the container 1030 having front and back walls (not shown), a pair of side walls and a bottom wall having an elongated opening 1031 extending from the front wall to the back wall and provided with a toner feeding element 1032 disposed to continuously supply toner particles to a sleeve 1033 through a particle charging member 1034. The particle charging member 1034 is advantageously 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 sleeve 1033 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 sleeve. The sleeve 1033 is advantageously made of metal coated with a conductive material, and advantageously has a substantially cylindrical shape and a rotational axis extending parallel to the elongated opening 1031 of the particle container 1030.

Charged toner particles are held to the surface of the sleeve 1033 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 sleeve 1033.

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 most advantageous to charge particles through contact charge exchange, the method can also be performed using any other suitable charge unit, such as a conventional charge injection unit, a charge induction unit or a corona charging unit.

A metering element 1035 is positioned proximate to the sleeve 1033 to adjust the concentration of toner particles on the peripheral surface of the sleeve 1033, to form a relatively thin, uniform particle layer thereon. The metering element 1035 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 1035 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 sleeve The particle charging member and the metering element can be of different designs and materials. The particle charging member, for instance, can be made of or comprise a polymeric foam material instead of a fibrous, resilient material.

In Fig. 3, the sleeve 1033 is arranged in relation to a positioning device 1040 for accurately supporting and maintaining the printhead structure 1005 in a predetermined position with respect to the peripheral surface of the sleeve 1033. The positioning device 1040 is formed of a frame 1041 having a front portion, a back portion and two transversally extending side rulers 1042,1043 disposed on each side of the sleeve 1033 parallel to the rotational axis thereof. The first side ruler 1042, positioned at a upstream side of the sleeve 1033 with respect to its rotation direction, is provided with fastening means 1044 to secure the printhead structure 1005 along a transversal fastening axis extending across the entire width of the printhead structure 1005. The second side ruler 1043, positioned at a downstream side of the sleeve 1033, is provided with a support element 1045, or pivot, for supporting the printhead structure 1005 in a predetermined position with respect to the peripheral surface of the sleeve 1033. The support element 1045 and the fastening axis are so positioned with respect to one another, that the printhead structure 1005 is maintained in an arcuate shape along at least a part of its longitudinal extension. That arcuate shape has a curvature radius determined by the relative positions of the support element 1045 and the fastening axis, and is dimensioned to maintain a part of the printhead structure 1005 curved around a corresponding part of the peripheral surface of the sleeve 1033. The support element 1045 is arranged in contact with the printhead structure 1005 at a fixed support location on its longitudinal axis so as to allow a slight variation of the printhead structure 1005 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 sleeve 1033. That is, the support element 1045 is arranged to make the printhead structure 1005 pivotable about a fixed point to ensure that the distance between the printhead structure 1005 and the peripheral surface of the sleeve 1033 remains constant along the whole transverse direction at every moment of the print process, regardless of undesired mechanical imperfections of the

sleeve 1033. The front and back portions of the positioning device 1040 are provided with securing members 1046 on which the toner delivery unit 1003 is mounted in a fixed position to provide a constant distance between the rotational axis of the sleeve 1033 and a transversal axis of the printhead structure 1005. Preferably, the securing members 1046 are arranged at the front and back ends of the sleeve 1033 to accurately space the sleeve 1033 from the corresponding holding element 1012 of the transfer belt 1001 facing the actual print station. The securing members 1046 are preferably dimensioned to provide and maintain a parallel relation between the rotation axis of the sleeve 1033 and a central transversal axis of the corresponding holding member 1012.

In Fig. 3, a spacer element 1004 delimits the minimum distance between the sleeve 1033 and the printhead structure 1005. The spacer element can be constituted of a thin foil of stainless steel or another suitable material.

As shown in Figs. 4a, 4b, 4c, a printhead structure 1005 in an image forming apparatus, e. g. of the type illustrated in Fig. 1, can comprise a substrate 1050 of flexible, electrically insulating material such as polyimide or the like, having a predetermined thickness, a first surface facing the sleeve (particle carrier), a second surface facing the transfer belt, a transversal axis 1051 extending parallel to the rotation axis of the sleeve 1033 across the whole print area, and a plurality of apertures 1052 arranged through the substrate 1050 from the first to the second surface thereof. The first surface of the substrate is coated with a first cover layer 1501 of electrically insulating material, such as for example parylene. A first printed circuit, comprising a plurality of control electrodes 1053 disposed in conjunction with the apertures, and, in some embodiments, shield electrode structures (not shown) arranged in conjunction with the control electrodes 1053, is arranged between the substrate 1050 and the first cover layer 1501. The second surface of the substrate is coated with a second cover layer 1502 of electrically insulating material, such as for example parylene. A second printed circuit, including a plurality of deflection electrodes 1054, is arranged between the substrate 1050 and the second cover layer 1502. The printhead structure 1005 further includes a layer of antistatic material (not shown), preferably a semiconductive material, such as silicium oxide or the like, arranged on at least a part of the second cover layer 1502, facing the transfer belt

1001. The printhead structure 1005 is brought in cooperation with a control unit (not shown) comprising variable control voltage sources connected to the control electrodes 1053 to supply control potentials which control the amount of toner particles to be transported through the corresponding aperture 1052 during each print sequence. The control unit further comprises deflection voltage sources (not shown) connected to the deflection electrodes 1054 to supply deflection voltage pulses which controls the convergence and the trajectory path of the toner particles allowed to pass through the corresponding apertures 1052. In some designs, the control unit also includes a shield voltage source (not shown) connected to the shield electrodes to supply a shield potential which electrostatically screens adjacent control electrodes 1053 from one another, preventing electrical interaction therebetween. The substrate 1050 is advantageously 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 1050, respectively, using conventional etching techniques. The first and second cover layers 1501,1502 are 5 to 10 microns thick parylene laminated onto the substrate 1050 using vacuum deposition techniques. The apertures 1052 are made through the printhead structure 1005 using conventional laser micromachining methods. The apertures 1052 have preferably a circular or elongated shape centered about a 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 1052 preferably have a constant shape along their axis, for example cylindrical apertures, it may be advantageous in some embodiments to provide apertures whose shape varies continuously or stepwise along the axis, for example conical apertures.

In one advantageous design, the printhead structure 1005 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 1052 of the printhead structure during each print cycle. Accordingly, one aperture 1052 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 1051 of the printhead structure 1005. The apertures 1052 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 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 1051 of the printhead structure 1005 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 control electrodes 1053 each of which having a ring shaped structure surrounding the periphery of a corresponding aperture 1052, 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 1053 may take on various shapes for continuously or partly surrounding the apertures 1052, preferably shapes having symmetry about the axis of the apertures. In some embodiments, particularly when the apertures 1052 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 a plurality of deflection electrodes 1054, each of which is divided into two semicircular or crescent shaped deflection segments 1541, 1542 spaced around a predetermined portion of the circumference of a corresponding aperture 1052. The deflection segments 1541,1542 are arranged symmetrically about the axis of the aperture 1052 on each side of a deflection axis 1543 extending through the center of the aperture 1052 at a predetermined deflection angle d to the longitudinal direction. The deflection axis 1543 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 (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 1054 has a upstream segment 1541 and a downstream segment 1542, all upstream segments 1541 being connected to a first deflection voltage source Dl, and all downstream

segments 1542 being connected to a second deflection voltage source D2. Three deflection sequences (for instance: D1<D2 ; D1=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 1052, thus the dot position on the toner image.

The printhead structure can be of a number of different designs and materials. For instance, instead of being deposited onto the substrate by means of vacuum deposition techniques, the cover layers may be constituted of a 5-20 micron thick film laminated onto the substrate. Furthermore, the printhead structure will of course need no deflection electrodes in applications where no dot deflection control is utilized.

Fig. 5 is a schematic, simplified view of an image forming apparatus according to a first embodiment of the invention where the image receiving surface is provided on a cylindrical drum 3001. The image forming apparatus comprises several print stations 3003', 3003", 3003"', 3003"" (toner delivery units). Even if not shown in Fig. 5, each toner delivery unit 3003'advantageously has the form of an elongated cartridge assembly and is arranged adjacent to a printhead structure 3005'providing an electrode matrix with a plurality of selectable apertures, which is interposed in a background electric field defined between the corresponding cartridge 3003'and a back electrode, which in the image forming apparatus in fig. 5 is constituted of the cylindrical drum 3001. The drum 3001 is arranged so as to rotate during operation of the image forming apparatus. To this end, the drum 3001 is powered by drive means (not shown in Fig. 5). Furthermore, the drum 3001 has a circumference which is slightly greater than the length of the paper 3002 (or other information carrier) used during printing. The drum 3001 advantageously is made of aluminum, but can also be made from other materials with suitable properties.

Each printhead 3005', 3005", 3005"', 3005""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. In this manner, charged particles are allowed to pass through the opened apertures and toward the back electrode, i. e. the drum 3001. The charged toner

particles are then deposited on the surface of the drum 3001. Accordingly, in the image forming apparatus in Fig. 5, the drum 3001 constitutes both back electrode and image receiving surface.

Due to the fact that the drum 3001 is rotating during operation, the image being formed on the drum is then transferred onto an information carrier 3002, such as a sheet of printing paper or any other medium suitable for printing. The paper sheet 3002 is fed from a paper delivery unit (not shown in Fig. 5) and is conveyed past the underside of the drum 3001. In order to transfer the image to the paper sheet 3002, it is pressed into contact with the drum 3001 by means of a belt (not shown), which in turn is driven by means of two rollers (indicated in Fig. 5) around which the belt extends. In this manner, the toner particles are deposited on the outer surface of the drum 3001 and then superimposed to the paper sheet 3002 to form an image.

Accordingly, the operation of the belt defines a transfer step, which advantageously is positioned in the lowest section of the image receiving surface on the drum 3001. 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 3001 to the paper sheet 3002 during operation.

After the image has been formed on the paper sheet 3002 by the charged toner particles, the paper sheet 3002 is fed to a fusing unit (not shown in Fig. 5), in which the image is permanently fixed onto the paper sheet 3002. In particular, the fusing unit can 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 3002. The fusing unit further includes a pressure roller (not shown) arranged transversally across the width of the paper sheet 3002. Additionally, the fusing unit is provided with means for feeding the paper 3002 to an out-tray (not shown) from which the paper 3002 can be collected by a user.

Furthermore, after passage through the fusing unit, the paper sheet 3002 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 3002 (or another suitable information carrier), for removing non-transferred toner particles from the paper sheet 3002.

As mentioned above, the toner delivery units 3003', 3003", 3003"', 3003"" normally will be provided in the form of cartridges, e. g. of the type shown in Fig. 2.

The toner delivery units and the printhead structures 3005', 3005", 3005"', 3005"" are mounted in a housing element (not shown in Fig. 5) or another supporting structure, so that they are maintained in predetermined positions with respect to the drum 3001.

An image forming apparatus of the type shown in Fig. 5 is particularly well suited for high speed direct printing in accordance with the invention. In order to achieve the desired high speed, the printing takes place in such a way that each printhead structure 3005', 3005", 3005"', 3005""deposits longitudinal columns of print on the image receiving surface provided by the drum 3001, wherein the different longitudinal columns of print deposited by the different printhead structures together form a complete image which then is transferred to the information carrier 3002.

In this context, a column of print C', C"is a longitudinal line or curved portion extending in the printing direction PD 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 L'is a transverse line (i. e. transverse to the printing direction PD) 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 or lines of print is defined as the pitch or the distance between two addressable pixel locations.

Accordingly, both a column of print and a transverse line of print can consist of one single visible dot of print in the completed image.

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. 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 according to the invention with at least two cooperating printhead structures in a succession, the number of apertures per unit length is half of or smaller than that needed to achieve the desired resolution with a single printhead structure. By means of a first printhead structure, 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. Thereafter, the remaining columns of print are printed by a second printhead structure, or by a plurality of further printhead structures in a succession, in order to form the complete image. This is illustrated in Fig. 5, where longitudinal columns of print printed by means of the first

printhead structure 3005'are denoted C', and longitudinal columns of print printed by means of the second printhead structure 3005"are denoted C". Fig. 6 is a detailed schematic illustration of mono-color high speed printing in accordance with the present invention, wherein the printed image (capital H) and the apertures of the printhead structures 3005', 3005", 3005"', 3005""are shown with different patterns indicating which aperture printed which column of print.

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 from the toner delivery unit associated with the particle carrier in question. This is known as the starvation effect. The variation in dot density may take place between apertures within the same row, between apertures in different rows, and/or between apertures of different printhead structures. It should be understood that several adjacent columns of lesser optical density will be more visible to a viewer than one single column of lesser density. In embodiments of the present invention where adjacent columns are printed by different printhead structures, the probability that adjacent columns simultaneously should suffer from toner starvation is reduced. Furthermore, in embodiments where a large number of printhead structures print columns of the same color, the instantaneous toner consumption per toner delivery unit is reduced and the starvation problem can be further reduced or even eliminated.

When printing by means of four identical cooperating printhead structures in a succession, as illustrated in Fig. 6, the number of apertures per unit of length transversely in each printhead is one fourth of that needed for achieving the same resolution by means of a single printhead structure. Thereby, one fourth of the image is formed by means of a first printhead structure 3005'. Then, by means of a second printhead structure 3005", a second set of columns of the image are printed. The remainder of the columns of the image are printed by means of subsequent third 3005"'and fourth 3005""printhead structures. The positions of cooperating apertures differ between the different printhead structures, e. g. in the way shown in Fig. 6. The number and transverse extent of the apertures in the rows are chosen such that not all the apertures in a printhead structure are needed to print the intended image.

Each printhead structure in the image forming apparatus according to the invention 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 PI, P2 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 can be in a staggered or non-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 be three or more rows of apertures per printhead.

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

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.

Within the scope of the present invention, so-called DDC control of the apertures may be used in applications where a very high print resolution is required. 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. It is also possible to use DDC control in combination with multiple rows of apertures. In applications where maximum speed is required, DDC control is less desirable since it requires a longer print cycle (i. e. the time it takes an aperture to print its dots).

In the previously discussed Fig. 5, the image receiving member is a drum 3001. The drum rotates about an axis. Around the periphery of the drum are arranged four print stations or toner delivery units 3003', 3003", 3003"', 3003"". In the embodiment shown in Fig. 5, all four print stations contain toner particles of the same color, e. g. black. There is also provided a transfer station (indicated in Fig. 5 by two rolls below the drum 3001) for transferring the image to an information carrier 3002. 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 3005'is of a similar type as illustrated in Figs. 4a-4c, i. e. two parallel rows of apertures with constant pitch between the apertures in a row.

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

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

During one single pass, each transverse line L'of the image to be formed on the drum passes the printhead structures 3005', 3005", 3005"', 3005""in turn. The transverse line then passes the transfer station. The drum 3001 is rotating about its axis so that the printhead structures and drum are moved relatively to each other. Each rotation of the drum constitutes a pass and completes an image. After one pass or rotation of the drum 3001 during which printing is effected, the transfer station starts to transfer the image to an information carrier 3002. 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 3001, in particular more than 50%, preferably more than 75%.

The transfer drum 3001 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.

In any of the above-discussed examples, the pitch (distance between centers of dots, denoted PI, P2 in Fig. 6) 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 positioning of the printhead structures in the succession in relation to each other. The vertical pitch can be varied by varying the amount of longitudinal movement between the printing of lines.

In a first embodiment of the invention, illustrated in Fig. 5, the image forming apparatus is intended for high speed printing with a single color (mono-color). In Fig.

5, the image forming apparatus is shown printing the capital letter H in black color at a printing speed which preferably is higher than 15 ppm. The image forming apparatus is of a type in which an image information is converted into a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier, e. g. 3033', towards an image receiving surface 3001. The image forming apparatus includes a background voltage source (not shown) for producing a background electric field which enables a transport of charged toner particles from the particle carrier towards the image receiving surface and a printhead structure, e. g.

3005', arranged in the background electric field. The printhead structure includes a plurality of apertures and control electrodes (not visible in Fig. 5, see Figs. 4a-4c) arranged in conjunction to the apertures. The image forming apparatus further

includes control voltage sources (not shown) for supplying control potentials to the 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. The image receiving surface 3001 is arranged for movement PD in relation to the printhead structure for intercepting the transported charged toner particles in an image configuration. The particle carrier 3033'is included in a toner delivery unit, (denoted 3003', see Fig. 2 for example of detailed design) for delivering the charged toner particles to the printhead structure 3005'. The image forming apparatus comprises a plurality of the toner delivery units 3003', 3003", 3003"', 3003""and the printhead structures 3005', 3005", 3005"', 3005""arranged in a succession along the image receiving surface 3001.

According to the invention, the relative movement PD of the image receiving surface 3001 in relation to the printhead structures causes each line L'in the image configuration IM that is transverse to the direction of the relative movement PD to pass several of the printhead structures 3005', 3005", 3005"', 3005""in order to form the image configuration IM, so that at least two of the printhead structures 3005', 3005"print only part of each transverse line L'in order to form longitudinal columns of print C', C", and so that columns C', C"of print from different printhead structures 3005', 3005", 3005"', 3005""together form the image configuration IM in one single pass of the image receiving surface 3001 past the succession.

As illustrated in Fig. 5, a first printhead structure 3005'of the at least two printhead structures preferably is arranged to print first longitudinal columns of print C', wherein a second printhead structure 3005"of the at least two printhead structures is arranged to print at least one second longitudinal column C"of print between the first columns of print C'. However, it is also conceivable with less advantageous embodiments of the invention where the columns of print are printed in another order.

In the image forming apparatus according to the invention, the first and second printhead structures have the apertures arranged with an average pitch in at least one row extending transversely to a printing direction of each printhead structure.

Thereby, as illustrated in Fig. 6, the average pitch Pi, P2 preferably is substantially identical for the first 3005'and second 3005"printhead structure. In embodiments

with more than two printhead structures printing the same color, preferably all printhead structures have the same average pitch.

In the first embodiment of the image forming apparatus according to the invention, illustrated in Figs. 5-6, the toner delivery units 3003', 3003", 3003"', 3003""all are arranged to deliver the same color of the charged toner particles (mono-color printing).

Fig. 7 is a schematic side view of an image forming apparatus according to a preferred embodiment of the invention, wherein the apparatus is intended for high speed multi- color printing. It should be noted that Fig. 7 is shematic and that only the components necessary in order to understand the invention are shown. In the preferred embodiment, the above-mentioned plurality of toner delivery units comprises several sets Y, M, C, K of the toner delivery units, wherein each set is arranged to deliver a different color of the charged toner particles, and at least one of the sets comprises more than one of the toner delivery units 4003', 4003", 4003"'.

In the preferred embodiment, all toner delivery units 4003', 4003", 4003"'of at least one of the sets are arranged next to each other Y, Y, Y, M, M, M, C, C, C, K, K, K.

This embodiment makes it possible to complete print areas of one color before starting to print the next color, and to mount all toner delivery units of at least one of the sets in an easily replaceable module. Such a modular design is particularly advantageous, since it facilitates the replacement of toner delivery units and makes it possible to preload modules with the desired toner delivery units already before they are needed. Furthermore, by means of a suitable control software and several different modules for different colors, the image forming apparatus according to the invention rapidly can be converted from a mono-color to a multicolor printer or vice versa.

In an alternative embodiment of the invention, illustrated in Fig. 8, the toner delivery units 5003', 5003", 5003"'of at least one of the sets are intermingled with toner delivery units of the other sets Y, M, C, K, Y, M, C, K, Y, M, C, K.

In another advantageous, alternative embodiment, illustrated in Fig. 9, a set 6003', 6003", 6003"'for one color K, K, K comprises a larger number of toner delivery

units than sets for remaining colors Y, M, C. This embodiment enables the same image forming apparatus to perform high speed printing with e. g. black color and printing of remaining colors at a lower speed. It should be understood that the speed of the relative movement can be different when printing a first color of an image than when printing a second color of the same image.

In another embodiment, a set for one color is arranged to deliver charged toner particles to a printhead structure or structures having a higher number of apertures per inch in the transverse direction than printhead structures to which remaining sets deliver remaining colors. This can provide a higher print resolution e. g when printing with black than with other colors. This embodiment is advantageous since black, due to the strong contrast against white printing paper, normally is the most critical color for a viewer's impression of the print result. In cases where the information carrier has another color than white, another color than black might become the most critical.

In another advantageous embodiment, at least one of the printhead structures can be set to print either a higher number of dots per aperture when high print resolution is required or a lower number of dots per aperture when high printing speed is required.

In this embodiment, dot deflection control (DDC) of the above-discussed type can be utilized when maximum print resolution is desired, e. g. for a high quality black and white image. When desired, the DDC can be switched off and the printing speed increased, e. g. for multi-color prints or in applications where a lower print resolution otherwise is sufficient (e. g. in so-called draft mode).

As discussed above, at least one of the printhead structures advantageously have the plurality of apertures arranged in several parallel rows.

In one embodiment, the above-mentioned particle carriers are cylinders, each rotatable about an axis, wherein the cylinders preferably have a diameter smaller than 40 mm. This small diameter enables a large number of toner delivery units to be arranged in the succession along the image receiving surface. However, within the scope of the present invention, it is conceivable with particle carriers, or other means performing the same task, of any suitable design.

Most present types of toner delivery units will only function properly if the particle carrier rotates in a predetermined direction of rotation. Therefore, in another embodiment of the image forming apparatus according to the invention, the particle carriers each are rotatable about an axis in a direction of rotation, wherein the image forming apparatus comprises at least one toner delivery unit 5003', 5003"where the particle carrier is arranged to rotate in a first direction of rotation, and at least one toner delivery unit 5003"where the particle carrier is arranged to rotate in a second direction of rotation which is substantially opposite to the first direction of rotation.

This embodiment makes it possible to utilize identical toner delivery units on both lateral sides of the"highest"point of an image receiving surface provided by a cylindrical drum or an endless belt loop. When mounted in the image forming apparatus, a toner delivery unit most advantageosuly has a larger extension in a direction perpendicular to the image receiving surface than in a direction parallel to the image receiving surface. This enables a larger number of toner delivery units to be arranged in the succession along the image receiving surface with a maintained toner capacity.

In the following, a preferred and a number of alternative embodiments of a method for direct printing according to the invention will be described in greater detail with reference to the attached drawings.

The method according to the invention comprises to convert an image information into a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier towards an image receiving surface, to produce a background electric field which enables a transport of charged toner particles from the particle carrier towards the image receiving surface, and to provide a printhead structure including a plurality of apertures and control electrodes arranged in conjunction to the apertures in the background electric field.

The method further comprises to arrange the image receiving surface for movement in relation to the printhead structure, to include the particle carrier in a toner delivery unit for delivering the charged toner particles, and to arrange a plurality of the toner delivery units and the printhead structures in a succession along the image receiving surface. The method also comprises to supply control potentials to the control

electrodes in accordance with the image information to selectively permit or restrict the transport of charged toner particles from the particle carriers through the apertures of the printhead structures, and to intercept the transported charged toner particles in an image configuration on the image receiving surface.

In the method according to the invention, the image receiving surface 3001 is moved in relation to the printhead structures in such a way that each line L'in the image configuration IM that is transverse to the direction of the relative movement PD passes several of the printhead structures 3005', 3005", 3005"', 3005""in order to thereby form the image configuration IM, wherein at least two of the printhead structures 3005', 3005"print only part of each transverse line L'in order to form longitudinal columns of print C', C', and columns of print C', C"from different printhead structures 3005', 3005", 3005"', 3005""together form the image configuration IM in one single pass of the image receiving surface 3001 past the succession.

In a preferred embodiment of the method according to the invention, a first printhead structure 3005'of the at least two printhead structures prints first longitudinal columns of print C', and a second printhead structure 3005"of the at least two printhead structures prints at least one second longitudinal column C"of print between the first columns of print C'.

In another embodiment, the first and second printhead structures have the apertures arranged with an average pitch in at least one row extending transversely to a printing direction of each printhead structure, wherein first 3005'and second 3005"printhead structures with a substantially identical average pitch Pi, P2 are mounted in the image forming apparatus before printing.

In one embodiment (Figs. 5 and 6), all toner delivery units 3003', 3003", 3003"', 3003""deliver the same color of the charged toner particles.

In the preferred embodiment of the method according to the invention (Fig. 7), several sets Y, M, C, K comprising one or several toner delivery units are provided, wherein each set delivers a different color of the charged toner particles, and at least one of the

sets comprises more than one of the toner delivery units 4003', 4003", 4003"'.

Thereby, preferably all toner delivery units 4003', 4003", 4003"'of at least one of the sets are arranged next to each other Y, Y, Y, M, M, M, C, C, C, K, K, K. Even more preferably, at least one replaceable module including all toner delivery units of a set is mounted in the image forming apparatus before printing.

Alternatively, the toner delivery units 5003', 5003", 5003"'of at least one of the sets are intermingled with toner delivery units of the other sets Y, M, C, K, Y, M, C, K, Y, M, C, K in the succession. However, it is also conceivable with embodiments where the toner delivery units are arranged in another suitable succession.

In another alternative embodiment, a larger number of toner delivery units 6003', 6003", 6003"'deliver the charged toner particles of one color K, K, K) than remaining colors.

In one embodiment, a set for one color delivers its charged toner particles to a printhead structure or structures having a higher number of apertures per inch in the transverse direction than printhead structures to which remaining sets deliver their charged toner particles of remaining colors.

In still another embodiment, at least one of the printhead structures is capable of printing either a higher number of dots per aperture to produce a high print resolution or a lower number of dots per aperture to enable a high printing speed.

The apertures of at least one of the printhead structures, can be arranged in several parallel rows. The particle carriers can be cylinders rotating about an axis, wherein the cylinders preferably have a diameter smaller than 40 mm.

Alternatively, the particle carriers each rotate about an axis in a direction of rotation, wherein the particle carrier of at least one toner delivery unit 5003', 5003"rotates in a first direction of rotation, and the particle carrier of at least one toner delivery unit 5003"rotates in a second direction of rotation which is substantially opposite to the first direction of rotation.

Advantageously, at least two toner delivery units having a larger extension in a direction perpendicular to the image receiving surface than in a direction parallel to the image receiving surface are mounted in the image forming apparatus.

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.

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.

Accordingly, when performing high speed printing in accordance with the present invention, it might be advantageous to provide the information carrier in a continuous form, e. g. as a roll of printing paper.

In addition to the embodiments described in the foregoing description, also the following non-limiting examples can be mentioned: A) Machine set-up with 12 printheads Color and Mono: 4 colors x 3 printheads x flexible printed circuits (FPCs) with 200 apertures per inch (api) X images with 600 dpi can be printed in Y, M, C and K B) Machine set-up with 9 printheads Color: 3 x 2 x 200 api FPCs 400 dpi in Y, M and C Mono: 1 x 3 x 200 api FPCs 600 dpi in K C) Machine set-up with 8 printheads Color: 3 x 2 x 200 api FPCs 400 dpi in Y, M and C Mono: 1 x 2 x 300 api FPCs 600 dpi in K D) Machine set-up with 8 printheads Color and Mono: 4 x 2 x 200 api FPCs => 400 dpi in Y, M, C and K By means of the present invention, the printing speed can be increased to 250-300 mm/s (approx. 50-60 ppm) or higher at 600 dpi or better print resolution. This high printing speed can be achieved with a sufficient and reliable toner supply to all printhead structures in the image forming apparatus, since the instantaneous toner consumption in the respective toner delivery units, in spite of the higher printing speed, can be maintained or even decreased in comparison to the previously disclosed direct printing assemblies printing at lower speeds.