WO2002053387A1

WO2002053387A1 - Direct printing apparatus

Info

Publication number: WO2002053387A1
Application number: PCT/SE2000/002697
Authority: WO
Inventors: Mattias Svensson; Anders Boman
Original assignee: Array AB
Current assignee: Array AB
Priority date: 2000-12-29
Filing date: 2000-12-29
Publication date: 2002-07-11
Anticipated expiration: 2003-06-29

Abstract

A direct printing image forming apparatus arranged to provide at least a first printed image of intercepted toner particles within a printable area on a first information carrier in accordance with an image configuration. The first printed image has a print quality being a function of print settings of the image forming apparatus which include at least printing zone geometry, toner delivery system, and electric field characteristics. Thereby, the print settings have been set in such a way that, when the image information descrives a matrix of 8 x 6 pixels each 100 % of a single color selected from yellow (Y), magenta (M), cyan (C) and black (K) and in which pixels of the same color are present in at least 6 rows and in at least 5 columns of the matrix giving the first printed image (IM¿1) 8 x 6 print blocks (Y, M, C, K) having an area close to 25.4 x 25.4 mm, the difference in optical density between any two measurement positions (MP64¿1?, MP822) within the print blocks becomes smaller than 0.7 units when measured by means of Macbeth® RD914.

Description

Title

Direct printing apparatus.

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 invention relates to an image forming apparatus arranged to provide a printed image on an information carrier in accordance with the image information, wherein the printed image has a print quality which is a function of print settings of the image forming apparatus such as printing zone geometry, printing sequence parameters, toner delivery system, and electric field characteristics.

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 coπesponding 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.

When developing and manufacturing different types of printing devices, it is important to be able to evaluate the print quality of the produced images. This is usually accomplished by means of providing a suitable image information and printing a test image based on said image information. Thereafter, the print quality of the printed image can be evaluated using appropriate test methods, wherein the results from the evaluation can be utilized for modifying the printer settings or as a base for a quality approval or rejection.

However, the present test images and methods are less suitable for direct printing, since they do not provide all the information required for modifying the settings of an image forming apparatus or a method for direct printing under development, or for verifying that the settings of a direct printing apparatus which is to be delivered to a customer are correct. When direct printing is concerned, it has hitherto proved difficult to meet the very high demands on print quality. One reason for this has been undesired "white lines" in the printing direction of the printed image. Such white lines can be caused e.g. by a local lack of toner particles in the toner layer on the particle carrier, by clogging of the apertures in the printhead structure, by undesired triboelectrical charge phenomena, or by a non-uniform pressure applied onto the toner layer by a spacer element or the like.

When multi-color printing is concerned, the print quality can be impaired e.g. by a positioning error between the different colors, or by an accidental contact between toner particles of a first color on an image receiving surface and a subsequent printhead structure depositing toner particles of a second color.

Another problem with the previous direct printing technique is so-called scattering, which means that the toner particles "bounce" away from their intended positions on an image receiving surface, e.g. as a result of a too high impact velocity. Scattering can impair the print resolution and result in a blurred printed image.

Another occurring problem with direct printing has been a lack of repeatability, i.e. it has been difficult to keep the print quality at a satisfactory level for an extended period of time, when printing a large number of prints in succession, or when resuming printing after a change of toner cartridge or the like. This lack of repeatability manifests itself e.g. in undesired variations in the optical density across a printed image, between subsequent prints, or between different colors included in the printed image.

One fundamental reason for the above-mentioned problems is the large number of parameters capable of influencing the print result, and the complex relationships between these parameters. Furthermore, a viewer's perception of the print quality of a printed image is individual and difficult to quantify. Therefore, when designing, manufacturing or fine-tuning a direct printing apparatus, it can be very difficult to evaluate if the apparatus really is set to produce printed images of the desired high print quality and high degree of repeatability. Furthermore, as mentioned above, the present test images and methods do not provide all the information needed when setting the print settings in an image forming apparatus for direct printing.

Summary of the invention

Accordingly, a first object of the present invention is to provide an image forming apparatus for direct printing, which apparatus solves the above-mentioned problems since the parameters vital to the print result have been identified, have been made possible to evaluate and control, and have been set within specified intervals in order to give the image forming apparatus the ability to print any image with an excellent print quality and with a high degree of repeatability.

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 caπier 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. Thereby, 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. Furthermore, the image forming apparatus is arranged to provide at least a first printed image of the intercepted toner particles within a printable area on a first information carrier in accordance with said image configuration, wherein the first printed image has a print quality being a function of print settings of the image forming apparatus which include at least printing zone geometry, toner delivery system, and electric field characteristics. According to the invention, the print settings have been set in such a way that, when the image information describes a matrix of 8 x 6 pixels each 100% of a single color selected from yellow, magenta, cyan and black (K) and in which pixels of the same color are present in at least 6 rows and in at least 5 columns of said matrix giving said first printed image 8 x 6 print blocks having an area close to 25.4 x 25.4 mm, the difference in optical density between any two measurement positions within the print blocks becomes smaller than 0.7 units when measured by means of

Macbeth^® RD914.

The instrument for optical density measurements, i.e. Macbeth^® RD914, can be obtained from Macbeth^® Process Measurements, KOLLMORGEN Co, Newburg, N.Y., U.S.A.

An image information of the above-described type will provide all the information necessary in order to set the print settings of the image forming apparatus in a correct way or in order to verify that the settings are correct. The image information is produced by means of PhotoShop^® 5 computer software from Adobe Systems Inc., California, USA. Thereby, the print resolution is set to be 1 pixels per inch before sending the image information to the image forming apparatus which is to be evaluated. In the present context, the expression "100% of a single color selected from yellow (Y)..." means that the respective pixels have been set to be a 100% pure yellow (Y), magenta (M), cyan (C) or black (K) by means of the default CMYK- model provided with the PhotoShop^® 5 computer software. The "ink options/ink colors" are set to be "SWOP (coated)". It is essential that the pixels of each color are distributed across a major portion of both the length and width of the matrix and that each color has a distribution comparable with the distributions of the other colors. This will give a printed image providing information which is particularly useful when setting the print settings of an image forming apparatus for direct printing. For example, defects in the printed image originating from insufficient toner delivery will show up as an increased variation in optical density between print blocks in the printing direction (PD, Fig. 15) of the printed image IMi. Defects originating from clogging of apertures in the printhead structure will instead show up as an increased variation in optical density between print blocks transversely to the printing direction PD. Defects originating from an incorrect transverse positioning of one or several printhead structures will show up as a displacement between print blocks of different colors, whereas defects caused by a non-uniform distance transversely to the printing direction between the particle carrier and the printhead structure, will show up as a an increase or decrease of the optical density for print blocks in a transverse row of the matrix.

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 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 an 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.

Fig. 14 is a schematic illustration of a sixth printing pattern.

Fig. 15 is a schematic view of a printed image IMi obtained by means of an image forming apparatus according to the invention, wherein the image information has been a matrix of 8 x 6 pixels each 100% of a single color selected from yellow Y, magenta M, cyan C and black K and in which pixels of the same color are present in at least 6 rows and in at least 5 columns of the matrix.

Fig. 16 is a schematic sectional view of a portion of the printing zone of an image forming apparatus according to the invention.

Fig. 17 is a schematic side view of an image forming apparatus according to a preferred embodiment of the invention. Fig. 18 is a schematic illustration, partially in cross-section, of a printhead structure intended for used in an image forming apparatus according to a particularly advantageous embodiment of the invention.

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 DI, D2, such that the electrostatic control field generated by the control electrode remains substantially symmetrical as long as both deflection voltages DI, D2 have the same amplitude. The amplitude of DI 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 DI 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.l shows an image forming apparatus 1000 which comprises at least one print station, preferably four print stations (Y, M, C, K), an intermediate member 1001 providing an image receiving surface, a driving roller 1010, at least one support roller 101 1, 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 101 1 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.l 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.l 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 and superposed to form a four color 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 butylene. 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(l/3), i.e. about 18.4°. Accordingly, the first dot is deflected slightly upstream with respect to the belt motion, the second dot is undeflected and the third dot is deflected slightly downstream with respect to the belt motion, thereby obtaining a transversal alignment of the printed dots on the transfer belt. Accordingly, each deflection electrode 1054 has a upstream segment 1541 and a downstream segment 1542, all upstream segments 1541 being connected to a first deflection voltage source DI, 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 DI 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 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 1 162.

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 1 172 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 transversely 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 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 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 1 183 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 1 194. 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 1 196 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. 1 1 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. 1 1 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. 1 1, 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 121 1 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 121 1 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.

In the following description a preferred embodiment and a number of alternative embodiments of the image forming apparatus according to the invention will be described in greater detail with particular reference to the attached Figs. 15 - 18.

Fig. 15 is a schematic view of a printed image IMj obtained by means of an image forming apparatus according to the invention, wherein the image information has been a matrix of 8 x 6 pixels each 100% of a single color selected from yellow Y, magenta M, cyan C and black K and in which pixels of the same color are present in at least 6 rows and in at least 5 columns of the matrix. As described above, the matrix has been obtained by means of a PhotoShop^® 5 computer software. Fig. 16 is a schematic sectional view of a portion of the printing zone of an image forming apparatus according to the invention. Fig. 17 is a schematic side view of an image forming apparatus according to a preferred embodiment of the invention. Fig. 18 is a schematic illustration, partially in cross-section, of a printhead structure intended for used in an image forming apparatus according to a particularly advantageous embodiment of the invention.

In the image forming apparatus according to the invention, 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. The apparatus further includes 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. Thereby, 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. The image forming apparatus is arranged to provide at least a first printed image of said intercepted toner particles within a printable area on a first information carrier in accordance with the image configuration, wherein the first printed image has a print quality being a function of print settings of the image forming apparatus which include at least printing zone geometry, toner delivery system, and electric field characteristics.

According to the invention, the print settings have been set in such a way that, when the image information describes a matrix of 8 x 6 pixels each 100% of a single color selected from yellow Y, magenta M, cyan C and black K and in which pixels of the same color are present in at least 6 rows and in at least 5 columns of the matrix. This image information gives the first printed image IMj 8 x 6 print blocks Y, M, C, K having an area close to 25.4 x 25.4 mm, wherein the difference in optical density between any two measurement positions MP64j, MP82₂ within the print blocks becomes smaller than 0.7 units when measured by means of Macbeth^® RD914. This ensures that the image forming apparatus will produce images of a print quality which makes a favorable impression on most users.

The Macbeth^® RD914 instrument should be well known to the skilled person, and enables a very simple and rapid evaluation of the print quality level the image forming apparatus in question is able to produce. By means of measuring the optical density level in a few positions within the printed image IM_{1 }} normally no more than 20 positions per color, all the information necessary in order to set the print settings correctly can be obtained within a few minutes.

In a preferred embodiment of the image forming apparatus according to the invention, the print settings have been set in such a way that the difference in optical density between any two measurement positions MP82_l5 MP82₂ within any single one of the print blocks becomes smaller than 0.5 units when measured by means of Macbeth^® RD914. The preferred embodiment ensures that the print settings have been set in such a way that most users will perceive any image formed by the apparatus as having a satisfactory uniformity.

In another advantageous embodiment of the invention, the print settings have been set in such a way that, when said image information also is utilized to produce a second image, subsequent to and intended to be identical to said first image, the difference in optical density between any measurement position within a print block in the first image and any measurement position within a corresponding print block in the second image becomes smaller than 0.7 units when measured by means of Macbeth^® RD914. In this embodiment, it is ensured that the print settings have been set in such a way that the print quality is consistent also when forming a plurality of images in a succession.

The image forming apparatus according to the invention can be intended for black and white prints, for multi-color prints, or both.

In embodiments where the image forming apparatus is intended for multi-color printing, the print settings preferably have been set in such a way that the difference in optical density between any two measurement positions within the print blocks where the toner particles are yellow Y becomes smaller than 0.6 units when measured by means of Macbeth^® RD914. The difference in optical density between any two measurement positions within the print blocks where the toner particles are magenta M or cyan C preferably becomes smaller than 0.5 units, whereas the difference in optical density between any two measurement positions within the print blocks where the toner particles are black K preferably becomes smaller than 0.4 units. The reason why the variation in optical density can be larger for yellow than for magenta and cyan, and larger for magenta and cyan than for black, is that defects in a yellow print are less visible against a white printing paper (the most common information carrier) than defects in a print which is magenta, cyan, or black. In cases where the information color has a dark color such as black this relationship will be the opposite.

This contrast effect also affects the minimum level of optical density required in order to give the viewer an impression of a pure, saturated color.

Accordingly, the print settings preferably have been set in such a way that the optical density in any measurement position within any one of said yellow print blocks Y becomes higher than 0.7 units when measured by means of Macbeth^® RD914.

In another embodiment, the print settings have been set in such a way that the optical density in any measurement position within any one of said magenta M or cyan C print blocks becomes higher than 0.8 units when measured by means of Macbeth^® RD914.

In still another embodiment, the print settings have been set in such a way that the optical density in any measurement position within any one of said black (K) print blocks becomes higher than 0.9 units when measured by means of Macbeth^® RD914. Accordingly, the optical density for black print blocks advantageously is higher for black print blocks than for other print blocks. In a particularly advantageous embodiment, the print settings have been set in such a way that no print block has a positioning error larger than 200 μm in any direction. Thereby, possible positioning errors easily can be measured from the printed image IMi by means of a light microscope or a magnifying glass.

In the image forming apparatus according to the invention, as illustrated in Fig. 16, the printing zone geometry preferably is such that the apertures 3052 are arranged transversely to a printing direction PD of the printhead structure 3005 with an average pitch p_av, and the apertures 3052 extend a first distance l_ap from a particle-receiving plane A to a particle-emitting plane B of the printhead structure 3005 which is positioned so that said particle-receiving plane A is located at a second distance I_k from the charged toner particles PA on the particle carrier 3033, and so that the particle-emitting plane B is at a third distance li from the image receiving surface.

Thereby, the printhead structure 3005 preferably is designed to set the first distance l_ap between 40 and 150 μm. The second distance l_k preferably is set to be below 40 μm, and the third distance li preferably is set to be between 50 and 300 μm. The toner delivery system preferably has been set to deliver the toner particles PA with an average volume diameter D_v smaller than 12 μm to the particle carrier 3033. The electrical field characteristics preferably are set in such a way that an average potential V_BE of the electrical field divided by a sum l_ap + lk + li of the first, second, and third distances becomes smaller than 7 V/μm. These settings will enable the apparatus to print any image with a sufficient optical density and a uniform print quality and prevent the flight speed of the toner particles form becoming so high that it causes extensive scattering problems.

The print settings advantageously further include printing sequence parameters, which preferably have been set in such a way that the relative movement of the image receiving surface 3001 and the printhead structure 3005 causes each line in the image configuration that is transverse to the direction of said relative movement PD to pass the printhead structure 3005 at least twice in order to form the image configuration, so that the printhead structure 3005 prints only part IM_part of each transverse line on each pass in order to form longitudinal columns of print, and so that columns of print from different passes together form the image c on fi gurat i on . P arti cu l arl y advantageously, the printing sequence parameters have been set so that said adjacent columns of print are not printed by the same aperture in subsequent passes. These embodiments will make defects e.g. caused by clogging of apertures in the printhead structure less visible in the printed image. Such defects will be detectable in the above-described optical density measurement and can be seen as an increased variation in optical density both between different print blocks transversely to the printing direction and within single print blocks. From the color of defective print blocks in the printed test image IMj, it will be possible to identify the printhead structure in which apertures are clogged.

In a first alternative embodiment of the image forming apparatus according to the invention, the apertures are provided in a single row extending transversely to the printing direction, wherein the average pitch p_av is set to be between 100 and 200 μm. A pitch which is too small can cause problems with short-circuits between the control electrodes or with interference between the electrical fields created by neighboring control electrodes, something which will result in defects in the printed image. A pitch which is too large might give an insufficient print resolution.

In a second alternative embodiment, the apertures are provided in two substantially parallel rows extending transversely to the printing direction, wherein the average pitch p_av in each of the rows is set to be between 200 and 300 μm. This embodiment reduces the risk of short-circuits and interference.

In a third alternative embodiment, the apertures are provided in three or more substantially parallel rows extending transversely to the printing direction, wherein the average pitch p_av in each of the rows is set to be above 300 μm. This embodiment reduces the risk of short-circuits and interference even further.

The apertures preferably have a circular cross-section with an inside diameter set to be between 30 and 250 μm or, alternatively, the apertures have an elongate cross-section with a minor dimension and a major dimension, wherein the minor dimension is set to be between 30 and 200 μm. This aperture size will provide sharp dots without making the risk of aperture clogging too high.

Particularly advantageously, the second distance l is set to be between 10 and 30 μm. This can be accomplished by means of the image forming apparatus comprising a spacer element which delimits the second distance lk, and preferably by means of a spacer element 3004 which delimits the second distance lk by means of contacting the printhead structure 3005 via the toner particles PA on the particle carrier 3033 during the transport. Advantageously, at least a surface portion of the spacer element contacting the toner particles on the particle carrier has been manufactured with, or surface treated in order to obtain, an enhanced surface smoothness. This will minimize the disturbance of the toner layer on the particle carrier caused by the spacer element,

The above mentioned third distance li particularly advantageously is set to be between 70 and 200 μm. This can be accomplished by means of the image forming apparatus comprising a distance element which delimits the third distance 1;, e.g. by means of associating the printhead structure with the image receiving surface, or preferably by means of associating the particle carrier with the image receiving surface.

The toner delivery system particularly advantageously has been set to deliver toner particles PA with an average volume diameter D_v smaller than 8 μm to the particle carrier 3033. This enables the image forming apparatus to print dots with a very shaφ definition.

The toner delivery system advantageously includes at least one toner delivery unit comprising a toner container forming an opening in which the particle carrier is arranged and a particle delivery member inside the toner container, wherein the particle delivery member is arranged in relation to the particle carrier in order to form a toner particle input zone. Thereby, the particle carrier preferably is set to rotate with a first peripheral speed, wherein the particle delivery member is set to rotate with a second peripheral speed which is different from said first peripheral speed. This embodiment enables the toner particles to be uniformly distributed across the outer surface of the particle carrier, so that a very uniform print result can be obtained.

In one embodiment of the image forming apparatus according to the invention which is intended for multi-color prints, illustrated in Fig. 17, the toner delivery system comprises a plurality of the particle carriers 4033', 4033", 4033'", 4033"" and the printhead structures 4005', 4005", 4005'", 4005"" arranged in a succession along the image receiving surface 4001. Thereby, preferably one printhead structure is arranged at a smaller third distance l-_' than the remaining printhead structures li", li"', li"" in the succession for selectively permitting or restricting transport of charged toner particles from a particle carrier 4033' carrying black K toner particles. This allows areas of black color to be printed with a higher optical density than areas of remaining colors. As mentioned above, this is advantageous when the information carrier is of a light color. The third distance preferably is smallest lj' for the first printhead structure 4005' and largest li"" for the last printhead structure 4005"" in the succession. This reduces the risk that the last printhead structure accidentally contacts and disturbs toner particles already deposited on the image receiving surface by the first printhead structure. Particularly advantageously, the third distance increases in steps from the first li' to the last li"" printhead structure, wherein each step is larger than the average volume diameter D_v of the toner particles. This makes it possible to keep the third distance as small as possible for the respective printhead structures without any risk for accidental contact between printhead structures in the succession and already deposited toner particles.

In a further embodiment of the image forming apparatus, illustrated in Fig. 18, the printhead structure 5005 further includes a plurality of deflection electrodes 5054, 5054', 5054" arranged in conjunction to the apertures 5052, and deflection voltage sources (not shown) for supplying deflection potentials to the deflection electrodes in order to deflect trajectories of the charged toner particles during the transport. In this embodiment, the electrical field characteristics are set to produce toner-stream converging forces, acting on said toner particles during the transport, by means of the deflection voltage sources and deflection electrodes 5054, 5054', 5054". This embodiment will produce more distinct dots in the printed image. In one embodiment, the image forming apparatus further comprises a back electrode, wherein the image receiving surface is a first face of the back electrode from which the toner particles in the image configuration can be transferred to an information carrier. In another embodiment, the image receiving surface is a first face of an information carrier which also acts as a back electrode. In still another embodiment, the apparatus further comprises an intermediate image receiving member and a back electrode, wherein the image receiving surface is a first face of the intermediate image receiving member and the back electrode is located facing a second face of the intermediate image receiving member, so that the toner particles in said image configuration can be transferred from the first face of the intermediate image receiving member to an information carrier. In still another embodiment, the apparatus further comprises a back electrode, wherein the image receiving surface is a first face of an information carrier and the back electrode is located facing a second face of the information carrier.

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

Claims

1. 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, said image forming apparatus including: a background voltage source for producing a background electric field which enables a transport of charged toner particles from said particle carrier towards said image receiving surface; a printhead structure arranged in said background electric field, including a plurality of apertures and control electrodes arranged in conjunction to the apertures; control voltage sources for supplying control potentials to said control electrodes in accordance with the image information to selectively permit or restrict the transport of charged toner particles from the particle carrier through the apertures; said image receiving surface being arranged for movement in relation to the printhead structure for intercepting the transported charged toner particles in an image configuration, wherein said image forming apparatus is arranged to provide at least a first printed image of said intercepted toner particles within a printable area on a first information carrier in accordance with said image configuration, and said first printed image has a print quality being a function of print settings of said image forming apparatus which include at least printing zone geometry, toner delivery system, and electric field characteristics, c h a r a c t e r i z e d i n that the print settings have been set in such a way that, when said image information describes a matrix of 8 x 6 pixels each 100% of a single color selected from yellow (Y), magenta (M), cyan (C) and black (K) and in which pixels of the same color are present in at least 6 rows and in at least 5 columns of said matrix giving said first printed image (IMi) 8 x 6 print blocks (Y, M, C, K) having an area close to 25.4 x 25.4 mm, the difference in optical density between any two measurement positions (MP64j, MP82₂) within said print blocks becomes smaller than 0.7 units when measured by means of Macbeth^® RD914.

2. An image forming apparatus according to claim 1, characterized in that the print settings have been set in such a way that the difference in optical density between any two measurement positions (MP82j, MP82₂) within any single one of said print blocks becomes smaller than 0.5 units when measured by means of Macbeth^® RD914.

3. An image forming apparatus according to claim 1 or 2, characterized in that the print settings have been set in such a way that, when said image information also is utilized to produce a second image, subsequent to and intended to be identical to said first image, the difference in optical density between any measurement position within a print block in the first image and any measurement position within a corresponding print block in the second image becomes smaller than 0.7 units when measured by means of Macbeth^® RD914.

4. An image forming apparatus according to any one of the preceding claims, c h aracterized in that the print settings have been set in such a way that the difference in optical density between any two measurement positions within the print blocks where said toner particles are yellow (Y) becomes smaller than 0.6 units when measured by means of Macbeth^® RD914.

5. An image forming apparatus according to any one of the preceding claims, c h aracterized in that the print settings have been set in such a way that the difference in optical density between any two measurement positions within the print blocks where said toner particles are magenta (M) or cyan (C) becomes smaller than 0.5 units when measured by means of Macbeth^® RD914.

6. An image forming apparatus according to any one of the preceding claims, c h aracterized in that the print settings have been set in such a way that the difference in optical density between any two measurement positions within the print blocks where said toner particles are black (K) becomes smaller than 0.4 units when measured by means of Macbeth^® RD914.

7. An image forming apparatus according to any one of the preceding claims, characterized in that the print settings have been set in such a way that the optical density in any measurement position within any one of said yellow print blocks (Y) becomes higher than 0.7 units when measured by means of Macbeth^®

RD914.

8. An image forming apparatus according to any one of the preceding claims, c h aracterized in that the print settings have been set in such a way that the optical density in any measurement position within any one of said magenta (M) or cyan (C) print blocks becomes higher than 0.8 units when measured by means of Macbeth^® RD914.

9. An image forming apparatus according to any one of the preceding claims, c h aracterized in that print settings have been set in such a way that the optical density in any measurement position within any one of said black (K) print blocks becomes higher than 0.9 units when measured by means of Macbeth^® RD914.

10. An image forming apparatus according to any one of the preceding claims, c h aracterized in that the print settings have been set in such a way that no print block has a positioning error larger than 200 μm in any direction.

11. An image forming apparatus according to any one of the preceding claims, wherein the printing zone geometry is such that said apertures (3052) are arranged transversely to a printing direction (PD) of said printhead structure (3005) with an average pitch (p_av), and said apertures (3052) extend a first distance (l_ap) from a particle-receiving plane (A) to a particle-emitting plane (B) of said printhead structure (3005) which is positioned so that said particle-receiving plane (A) is located at a second distance (l_k) from said charged toner particles (PA) on said particle carrier (3033), and so that said particle-emitting plane (B) is at a third distance (1,) from said image receiving surface, characterized in that the printhead structure (3005) is designed to set said first distance (l_ap) between 40 and 150 μm, that the second distance (l_k) is set to be below 40 μm, the third distance (1;) is set to be between 50 and 300 μm, the toner delivery system has been set to deliver said toner particles (PA) with an average volume diameter (D_v) smaller than 12 μm to said particle carrier

(3033), and that the electrical field characteristics are set in such a way that an average potential (VBE) of said electrical field divided by a sum (l_ap + l_k + li) of said first, second, and third distances becomes smaller than 7 V/μm.

12. An image forming apparatus according to claim 11 , wherein said print settings further include printing sequence parameters, characterized in that the printing sequence parameters have been set in such a way that the relative movement of said image receiving surface (3001) and said printhead structure (3005) causes each line in said image configuration that is transverse to the direction of said relative movement (PD) to pass said printhead structure (3005) at least twice in order to form said image configuration, so that said printhead structure (3005) prints only part (IM_part) of each transverse line on each pass in order to form longitudinal columns of print, and so that columns of print from different passes together form said image configuration.

13. An image forming apparatus according to claim 12, characterized in that the printing sequence parameters have been set so that said adjacent columns of print are not printed by the same aperture in subsequent passes.

14. An image forming apparatus according to any one of claims 11-13, wherein said apertures are provided in a single row extending transversely to said printing direction, characterized in that the average pitch (p_av) is set to be between 100 and 200 μm.

15. An image forming apparatus according to any one of claims 11-13, wherein said apertures are provided in two substantially parallel rows extending transversely to said printing direction, characterized in that the average pitch (p_av) in each of said rows is set to be between 200 and 300 μm.

16. An image forming apparatus according to any one of claims 11-13, wherein said apertures are provided in three or more substantially parallel rows extending transversely to said printing direction, characterized in that the average pitch (p_av) in each of said rows is set to be above 300 μm.

17. An image forming apparatus according to any one of claims 11 - 16, wherein said apertures have a circular cross-section with an inside diameter, characterized in that the inside diameter is set to be between 30 and 250 μm.

18. An image forming apparatus according to any one of claims 11 - 16, wherein said apertures have an elongate cross-section with a minor dimension and a major dimension, characterized in that the minor dimension is set to be between 30 and 200 μm.

19. An image forming apparatus according to any one of claims 11-18, characterized in that the second distance (lk) is set to be between 10 and 30 μm.

20. An image forming apparatus according to any one of claims 11-19, characterized in that the image forming apparatus comprises a spacer element which delimits said second distance (lk).

21. An image forming apparatus according to claim 20, characterized in that the spacer element (3004) delimits said second distance (l_k) by means of contacting said printhead structure (3005) via said toner particles (PA) on said particle carrier (3033) during said transport.

22. An image forming apparatus according to claim 20 or 21, ch aract e rize d in that at least a surface portion of the spacer element contacting said toner particles on said particle carrier has been manufactured with, or surface treated in order to obtain, an enhanced surface smoothness.

23. An image forming apparatus according to any one of claims 11 - 22, characterized in that the third distance (1;) is set to be between 70 and

200 μm.

24. An image forming apparatus according to any one of claims 11 - 23, characterized in that the image forming apparatus comprises a distance element which delimits said third distance (li).

25. An image forming apparatus according to claim 24, characterized in that the distance element delimits said third distance by means of associating said printhead structure with said image receiving surface.

26. An image forming apparatus according to claim 24, characterized in that the image forming apparatus comprises a distance element which delimits said third distance by means of associating said particle carrier with said image receiving surface.

27. An image forming apparatus according to any one of claims 11 - 26, characterized in that the toner delivery system has been set to deliver toner particles (PA) with an average volume diameter (D_v) smaller than 8 μm to said particle carrier (3033).

28. An image forming apparatus according to any one of claims 11 - 27, wherein the toner delivery system includes at least one toner delivery unit, comprising a toner container forming an opening in which said particle carrier is arranged and a particle delivery member inside said toner container, and wherein said particle delivery member is arranged in relation to said particle carrier in order to form a toner particle input zone, characterized in that said particle carrier is set to rotate with a first peripheral speed, wherein said particle delivery member is set to rotate with a second peripheral speed which is different from said first peripheral speed.

29. An image forming apparatus according to any one of claims 11 - 28, wherein the toner delivery system comprises a plurality of said particle carriers (4033', 4033", 4033'", 4033"") and said printhead structures (4005', 4005", 4005'",

4005"") arranged in a succession along said image receiving surface (4001), c h a r a cterized in that one printhead structure is arranged at a smaller third distance (li') than the remaining printhead structures (V, li'", li"") in said succession for selectively permitting or restricting transport of charged toner particles from a particle carrier (4033') carrying black (K) toner particles.

30. An image forming apparatus according to claim 29, characterized in that the third distance is smallest (lj') for the first printhead structure (4005') and largest (1;"") for the last printhead structure (4005"") in said succession.

31. An image forming apparatus according to claim 30, characterized in that the third distance increases in steps from said first (li') to said last (li"") printhead structure, wherein each step is larger than said average volume diameter (D_v) of said tone particles.

32. An image forming apparatus according to any one of claims 11 - 31, wherein the printhead structure (5005) further includes a plurality of deflection electrodes (5054, 5054', 5054") arranged in conjunction to said apertures (5052), and deflection voltage sources for supplying deflection potentials to said deflection electrodes in order to deflect trajectories of said charged toner particles during said transport, char acterized in that the electrical field characteristics are set to produce toner- stream converging forces, acting on said toner particles during said transport, by means of said deflection voltage sources and deflection electrodes (5054, 5054', 5054").

33. An image forming apparatus according to any one of claims 1 - 32, characterized in that the apparatus further comprises a back electrode, wherein the image receiving surface is a first face of the back electrode from which the toner particles in said image configuration can be transferred to an information carrier.

34. An image forming apparatus according to any one of claims 1 - 32, ch aracter ized in that the image receiving surface is a first face of an information carrier which also acts as a back electrode.

35. An image forming apparatus according to any one of claims 1 - 32, characterized in that the apparatus further comprises an intermediate image receiving member and a back electrode, wherein the image receiving surface is a first face of the intermediate image receiving member and the back electrode is located facing a second face of the intermediate image receiving member, so that the toner particles in said image configuration can be transferred from the first face of the intermediate image receiving member to an information carrier.

36. An image forming apparatus according to any one of claims 1 - 32, ch aracterized in that the apparatus further comprises a back electrode, wherein the image receiving surface is a first face of an information carrier and the back electrode is located facing a second face of the information carrier.