WO2001076880A1

WO2001076880A1 - Direct printing device and method

Info

Publication number: WO2001076880A1
Application number: PCT/EP2000/003115
Authority: WO
Inventors: Per Klockar; Agneta Sandberg
Original assignee: Array Ab
Priority date: 2000-04-07
Filing date: 2000-04-07
Publication date: 2001-10-18
Also published as: AU2000239654A1

Abstract

An electrostatic image forming apparatus in which computer generated image information is converted into a pattern of electrostatic fields, includes a particle source including a particle carrier (33) which holds a layer of electrically charged particles that are transported toward a back electrode via apertures in a printhead structure or structures (202-205). The particles are received on an image receiving member (201) to form an image. The image is formed by passing the image receiving member more than once past each printhead structure so that part of the image is formed on each pass. The printhead structures and/or image receiving member being moved relative to each other such that adjacent columns of print are not printed by the same aperture in different passes.

Description

Direct Printing Device and Method

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 source toward a back electrode through a printhead structure, whereby the charged particles are deposited in image configuration on an image receiving substrate caused to move relative to the printhead structure. More specifically, the invention relates to a direct printing apparatus arranged to print in more than one pass .

Background

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.

Summary of the invention

An object of the invention is to provide an image formingapparatus 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 toward a back electrode member, said image forming apparatus including: a background voltage source for producing a background electric field which enables a transport of charged toner particles from said particle carrier towards said back electrode member; a printhead structure arranged in said background electric field, 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,- and an image receiving member for intercepting the transported charged particles in image configuration; the image forming member and/or the printhead structure being caused to move in relation to each other; wherein, the relative movement of the image receiving member and the printhead structure is so arranged that each line on the image receiving member that is transverse to the direction of said relative movement passes the printhead in a longitudinal direction at least twice in order to form an image, the printhead structure printing only a part of each transverse line on each pass to form longitudinal columns of print, the printhead structure and/or the image receiving member being moved relative to each other either between consecutive passes or during a pass so that each time that the image receiving member passes the printhead structure transversely different parts of the image receiving member are positioned to receive charged toner particles, the image receiving apparatus being so constructed and arranged to operate that adjacent columns of print are not printed by the same aperture in different passes. Brief description of the drawings

The invention will now be described in more detail for explanatory, and in no sense limiting, purposes, with reference to the following drawings, wherein the dimensions in the drawings are not to scale, in which

Fig.l is a schematic view of an image forming apparatus ,

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

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

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

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

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

Fig. 5 is an illustration of the columns of print printed in a single pass in a two pass method, Fig. 6 is an illustration of the columns of print shown in Fig. 5 after the second pass,

Fig. 7 is an illustration of the effect of apertures which print with a lower density in a two pass printing method,

Fig. 8 is an illustration of the printing pattern of a first embodiment of the invention,

Fig. 9 is an illustration of the printing pattern of a second embodiment of the invention,

Fig. 10 is an illustration of the printing pattern of a third embodiment of the invention,

Fig. 11 is an illustration of the printing pattern of a fourth embodiment of the invention,

Fig. 12 is an illustration of the printing pattern of a fifth embodiment of the invention,

Fig. 13 is an illustration of the printing pattern of a sixth embodiment of the invention,

Fig. 14 is an illustration of an apparatus according to the invention in which the image receiving member is in the form of a drum.

Detailed description

To perform a direct electrostatic printing method in accordance with the present invention, a background electric field is produced between a particle carrier and a back electrode to enable a transport of charged particles therebetween. A printhead structure, such as an electrode matrix provided with a plurality of selectable apertures, is interposed in the background electric field between the particle carrier and the back electrode and connected to a control unit which converts the image information into control signals 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 the image receiving substrate to provide line-by line scan printing to form a visible image .

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

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

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

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

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

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

Although the above description refers to a fusing unit which includes a means for transferring the image from the transfer belt the transferring unit and the fusing unit may be provided as separate units .

As shown in Fig.2, a print station in an image forming apparatus in accordance with the present invention includes a particle delivery unit 3 preferably having a replaceable or refillable container 30 for holding toner particles, the container 30 having front and back walls (not shown) , a pair of side walls and a bottom wall having an elongated opening 31 extending from the front wall to the back wall and provided with a toner feeding element 32 disposed to continuously supply toner particles to a toner carrier 33 through a particle charging member 34. The particle charging member 34 is preferably formed of a supply brush or a roller made of or coated with a fibrous, resilient material. The supply brush is brought into mechanical contact with the peripheral surface of the toner carrier 33 for charging particles by contact charge exchange due to triboelectrification of the toner particles through frictional interaction between the fibrous material on the supply brush and any suitable coating material of the toner carrier. The toner carrier 33 is preferably made of metal coated with a conductive material, and preferably has a substantially cylindrical shape and a rotation axis extending parallel to the elongated opening 31 of the particle container 30. Charged toner particles are held to the surface of the toner carrier 33 by electrostatic forces essentially proportional to (Q/D)², where Q is the particle charge and D is the distance between the particle charge center and the boundary of toner carrier 33. Alternatively, the charge unit may additionally include a charging voltage source (not shown) , which supply an electric field to induce or inject charge to the toner particles . Although it is preferred to charge particles through contact charge exchange, the method can be performed using any other suitable charge unit, such as a conventional charge injection unit, a charge induction unit or a corona charging unit, without departing from the scope of the present invention.

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

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

As shown in Fig.4a, 4b, 4c, a printhead structure 5 in an image forming apparatus in accordance with the present invention comprises a substrate 50 of flexible, electrically insulating material such as polyimide or the like, having a predetermined thickness, a first surface facing the toner carrier, a second surface facing the transfer belt, a transversal axis 51 extending parallel to the rotation axis of the toner carrier 33 across the whole print area, and a plurality of apertures 52 arranged through the substrate 50 from the first to the second surface thereof. The first surface of the substrate is coated with a first cover layer 501 of electrically insulating material, such as for example parylene. A first printed circuit, comprising a plurality of control electrodes 53 disposed in conjunction with the apertures, and, in some embodiments, shield electrode structures (not shown) arranged in conjunction with the control electrodes 53, is arranged between the substrate 50 and the first cover layer 501. The second surface of the substrate is coated with a second cover layer 502 of electrically insulating material, such as for example parylene. A second printed circuit, including a plurality of deflection electrodes 54, is arranged, in this embodiment, between the substrate 50 and the second cover layer 502. The deflection electrodes are not however essential . The printhead structure 5 further includes a layer of anti-static 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 502, facing the transfer belt 1. The printhead structure 5 is brought in cooperation with a control unit

(not shown) comprising variable control voltage sources connected to the control electrodes 53 to supply control potentials which control the amount of toner particles to be transported through the corresponding aperture 52 during each print sequence. The control unit further comprises deflection voltage sources (not shown) connected to the deflection electrodes 54 to supply deflection voltage pulses which controls the convergence and the trajectory path of the toner particles allowed to pass through the corresponding apertures 52. The control unit, in some embodiments, even includes a shield voltage source (not shown) connected to the shield electrodes to supply a shield potential which electrostatically screens adjacent control electrodes 53 from one another, preventing electrical interaction therebetween. In a preferred embodiment of the invention, the substrate 50 is a flexible sheet of polyimide having a thickness on the order of about 50 microns. The first and second printed circuits are copper circuits of approximately 8-9 microns thickness etched onto the first and second surface of the substrate 50, respectively, using conventional etching techniques. The first and second cover layers (501, 502) are 5 to 10 microns thick parylene laminated onto the substrate 50 using vacuum deposition techniques. The apertures 52 are made through the printhead structure 5 using conventional laser micromachining methods. The apertures 52 have preferably a circular or elongated shape centered about a central axis, with a diameter in a range of 80 to 120 microns, alternatively a transversal minor diameter of about 80 microns and a longitudinal major diameter of about 120 microns. Although the apertures 52 have preferably a constant shape along their central axis, for example cylindrical apertures, it may be advantageous in some embodiments to provide apertures whose shape varies continuously or stepwise along the central axis, for example conical apertures .

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

It should be noted that each aperture serves to provide an access for the toner particles to be directed from the toner carrier to a desired location. The direction of the toner particles is dictated by the prevailing electric fields. The apertures do not physically direct the particles in the sense that the shape of the borders of the aperture have no effect on the trajectory of the particles other than the effect that they may have on the electric fields. The toner particles pass in general via a central area of the apertures without touching the sides of the apertures. Thus the borders of the apertures provide the means for supporting the electrodes which provide the electric fields

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

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

Since the apertures 52 and their surrounding areas will under some circumstances need to be cleaned from residual toner particles which agglomerate there, an image forming apparatus in accordance with the present invention preferably further includes a cleaning unit 6 which is used to prevent toner contamination. Due to undesired variations in the charge and mass distribution of the toner material, some of the toner particles released from the toner carrier 33 do not reach sufficient momentum during a print sequence to be deposited onto the transfer belt 1 and contribute to image formation. Some toner particles having a charge polarity opposite to the intended, so called wrong signed toner, may be repelled back to the printhead structure 5 after passage through the apertures under influence of the background field, and adhere on the printhead structure 5 in the area surrounding the apertures 52. Some particles may be deviated during transport and agglomerate on the apertures walls, obstructing the aperture 52. Residual toner particles have to be removed periodically during an appropriate cleaning cycle, for example after a predetermined number of image formation cycles or due to control in accordance with a sensor measuring the amount of residual toner.

The resolution achieved by the printhead structure 5 for a given number of apertures 52 may be increased without necessarily the use of deflection electrodes 54. In order to achieve a printing resolution greater than the number of apertures in the printhead structure 5 the printing takes place in two or more passes of the transfer belt 1. By a pass is meant a movement of the transfer belt which passes a section of the transfer belt 1 to be printed with a movement relative to the printhead structure 5 and allows the printhead structure 5 to deposit a plurality of longitudinal columns of printing. A column of printing is a longitudinal line of the transfer belt which is subject to printing of dots by an aperture or apertures even if not all the parts of the line receive dots due to the content of the image being formed requiring some parts of the column to be left without dots. A transverse line of printing is a transverse line of the transfer belt which is subject to printing of dots from a plurality of apertures, even if not all the parts of the line receive dots due to the content of the image being formed requiring some parts to be left without dots. The closest distance between two adjacent columns or lines of print is defined as the pitch or the distance between two addressable pixel locations. After the first pass the next passes may be in the same or opposite longitudinal directions to that of the first pass.

The transverse direction is the direction which in the case that the image receiving member is a 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 the case of a transfer belt it is the direction in the plane of the belt perpendicular to the direction of movement of the belt, the said movement being the movement required to allow the belt to move around the rollers 10, 11. Thus, the transverse direction will normally be parallel to the axes of these rollers 10, 11. The longitudinal direction is the direction perpendicular to the transverse direction and in the plane of the surface of the image receiving member, i.e. transfer belt or drum. In the case of the drum the longitudinal direction is the direction perpendicular to the transverse direction and along the circumference so the drum. In the case of a transfer belt the longitudinal direction is the direction at any point on its surface in the direction perpendicular to the axis of rotation of the rollers and in the plane of the surface of the drum.

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 transfer belt 5. 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 reference is not limiting as the image could be the size of A3 or A5 paper or other paper sizes or any other chosen size.

In order to better understand the invention an example will first be described with respect to performing just two passes with each pass taking place in the same direction. In this case the number of apertures 52 per unit length is half that needed to achieve the desired resolution with a single pass . In a first pass a first half of the image is formed on the transfer belt 1. This first half of the image comprises alternate longitudinal columns of print of the intended final image, i.e. alternate columns are printed and alternate columns are not printed. The transfer belt 1 and printhead structure 5 are then moved relative to each other in the direction transverse to the direction of movement of the transfer belt, but in the plane of the belt, preferably by moving the belt transversely. This relative movement may be carried out by any suitable means known to the person skilled in the art. Then, in a second pass, the remaining columns of print are printed to form a complete image. The second pass can carried out with the belt traveling in the same longitudinal direction as the first pass or the opposite longitudinal direction. This effect is illustrated in Figs. 5 and 6. Fig. 5 represents a section of the transfer belt 1 after the first pass. The areas that are printed in the first pass are shown as hatched areas 61. Fig. 6 represents the same section of the transfer belt 1 after the second pass. The areas that are printed in the first pass are shown as hatched areas 61, whilst the areas that are printed in the second pass are shown as differently hatched areas 62.

The density of a dot, i.e. the quantity of toner particle used to form the dot, may vary according to the position of the aperture on the printhead structure due to insufficient toner particles being available. 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 example there is just one row of apertures 52 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. 7 in which the first positions of the apertures is shown by the reference numeral 70 and the positions of the same apertures during a second pass are shown by the reference numeral 71. Each aperture then prints a double column of print by printing two adjacent columns. If, by way of example, every fourth aperture suffered from starvation effect then every fourth aperture would produce columns which have less 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 also be printed in less 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 72 and columns of greater density each produced by a single aperture in two passes are heavier shaded and are indicated by reference numeral 73 in Fig. 7.

The first embodiment of the invention is illustrated in Fig. 8. In order to reduce the effect of the problem of starvation mentioned above, the row of apertures is moved transversely by more than one dot pitch between passes . The row is moved transversely by an amount equal to 2N+3 number of times the transverse pitch length L, where NX 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 approx . 127 microns or a higher integer multiple as specified in the preceding formula. This is illustrated in Fig. 8. In Fig. 8 the row of apertures has moved from the first position indicated by reference numeral 80 in which the first pass took place to the position indicated by reference numeral 81 in which the second pass took place. 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. Columns of lesser density produced each by a single aperture in two passes are indicated by reference numeral 82 and columns of greater density each produced by a single aperture in two passes are indicated by reference numeral 83 in Fig. 8. As can been seen from Fig. 8 the columns of less density are of narrower width than those in Fig. 7 and spaced apart from each other. These narrower columns will be less visible to a viewer. As is evident from the figure at the areas at the lateral sides of the image 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 then suppressed.

A second embodiment of the embodiment of the invention is illustrated in Fig. 9. In this example the printing is carried out in three passes. In Fig. 9 the row of apertures has moved from the first position indicated by reference numeral 90 in which the first pass took place to the position indicated by reference numeral 91 in which the second pass took place and then to the position indicated by reference numeral 92. The number of apertures 52 per unit of length transverse is one third that needed to achieve the same resolution as with a single pass. In a first pass a first one third of the image is formed on the transfer belt 1. This first third comprising one third of the columns of print indicated by reference numeral 94 of the intended final image. The transfer belt and printhead structure 5 are then moved relative to each other in the direction transverse to the direction of movement of the transfer belt, but in the plane of the belt, preferably by moving the belt transversely. Then, in a second pass, a second set of columns of the image indicated by reference numeral 95 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 96 are printed. Between the second and third pass the printhead structure is moved transversely by an amount equal to 3N4-2 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 belt traveling in the same direction as the first pass or in the opposite direction. This embodiment is illustrated in Fig. 9 which shows the transfer belt at the end of the first 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. In accordance with this embodiment between each pass the row is moved transversely by an amount equal to 3N+2 number of times the transverse pitch length L. In an alternative 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. As in the first embodiment 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 5 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 a third, preferred, embodiment of the invention is illustrated in Fig. 10. In this embodiment the printhead structure includes two rows of apertures 101, 102. The apertures in one row 101 are transversely displaced relative the apertures in the other row 102. 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 103, 104 and 105 respectively. Between each pass the printhead structure and image receiving member are moved relative to each other by a distance equal to four times the pitch length L. In Fig. 10 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 106, 107 and 108 respectively. As can be seen in the figure 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. In accordance with the preceding embodiment no two adjacent columns of print are produced by the same row of apertures . 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. Similarly to the first and second embodiments 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.

In still further embodiment 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. 11) the DDC control is arranged to print two non-adjacent columns of print per pass from each aperture, in this embodiment the columns are separated from each other by a distance of twice the pitch length. In Fig. 11 the row of apertures has moved from the first position indicated by reference numeral

110 in which the first pass took place to the position indicated by reference numeral 113 in which the second pass took place. The columns printed by a single aperture

111 are indicated by shaded lines 112 in the drawing. The position of the aperture 111 producing the columns is indicated by shading. The aperture in this embodiment produces columns of print that are separated by a single column. The transfer belt and printhead structure 5 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 113 of the apertures in the second pass are indicated by the second row of apertures . The columns printed by the aperture 111 in the second pass are indicated by shaded lines 114 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. In this embodiment 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 a still further embodiment using DDC control (see Fig. 12) each aperture prints two columns of print which are separated from each other by four times the pitch length. In Fig. 11 the row of apertures has moved from the first position indicated by reference numeral 120 in which the first pass took place to the position indicated by reference numeral 123 in which the second pass took place. The columns printed by a single aperture 121 are indicated by shaded lines 122 in the drawing. The position of the aperture 121 producing the columns is indicated by shading. The position 123 of the apertures in the second pass are indicated by the second row of apertures. The columns printed by the aperture 121 in the second pass are indicated by shaded lines 124 in the drawing.

In another embodiment 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. 13. In Fig. 11 the row of apertures has moved from the first position indicated by reference numeral 130 in which the first pass took place to the position indicated by reference numeral 133 in which the second pass took place. The columns 132 printed from the aperture 131 on the first pass are printed in hatched shading and the columns 134 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 a 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 a yet 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 transfer belt or drum 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 the an aperture passes beyond the position of its neighbour either at the next pass or over next pass.

The spacing of 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.

A particularly advantageous embodiment is illustrated in Fig. 14.

The image receiving member in this embodiment is a drum 200. The drum rotates about an axis 201. Around the periphery of the drum are arranged four print stations 202, 203, 204 and 205. The print stations 202, 203 and 204 respectively contain differently colored toner particles, e.g. yellow, cyan and magenta respectively, to allow color printing. The fourth print station 205 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 206 for transferring the image to another medium. Transfer may be effected by electrostatic attraction or by pressure transfer. A cleaning station 207 is 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 provided with each print station is of the type illustrated in Fig. 10, 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 202 - 205 preferably is performed after each pass. Alternatively, the cleaning is performed after an image has been formed. In a further alternative the cleaning is performed after two or more images have been formed.

During a pass the 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 206. While the drum is rotating it is moved along its axis 201. 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 207 is preferably permanently so that cleaning of each printhead structure may be effected on each pass .

The image preferably occupies 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 starved 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 belt or drum 1 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 aluminium oxide. The aluminium oxide may be combined with any conducting material for the drum, but is particularly advantageous when used with a drum with an aluminium 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 printer, particularly those with high surface speeds of the drum or belt.

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

Claims

What is claimed is ;

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 (33) toward a back electrode member, said image forming apparatus including: a background voltage source for producing a background electric field which enables a transport of charged toner particles from said particle carrier towards said back electrode member; a printhead structure (5, 202-205) arranged in said background electric field, including a plurality of apertures (52) and control electrodes (53) 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 (52) ; and an image receiving member (1, 201) for intercepting the transported charged particles in image configuration; the image forming member (1, 201) and/or the printhead structure (5, 202-205) being caused to move in relation to each other; characterized in that, the relative movement of the image receiving member (1, 201) and the printhead structure (5, 202-205) is so arranged that each line on the image receiving member that is transverse to the direction of said relative movement passes the printhead structure (5, 202-205) in a longitudinal direction at least twice in order to form an image, the printhead structure (5, 202-205) printing only a part of each transverse line on each pass to form longitudinal columns of print, the printhead structure (5, 202-205) and/or the image receiving member (1, 201) being moved relative to each other either between consecutive passes or during a pass so that each time that the image receiving member passes the printhead structure (5, 202-205) transversely different parts of the image receiving member (1, 201) are positioned to receive charged toner particles, the image forming apparatus being so constructed and arranged to operate that adjacent columns of print (81, 82) are not printed by the same aperture (52) in different passes.

2. An image forming apparatus according to claim 1, wherein the image forming apparatus is so constructed and arranged to operate that adjacent columns of print (81, 82) are not printed by the same aperture (52) in any pass.

3. An image forming apparatus according to claim 1 or claim 2, wherein the printhead structure includes apertures arranged in two or more longitudinally separated transverse rows, the apertures in one row not being in longitudinal alignment with the apertures in another row, and the apparatus is arranged to print adjacent columns by apertures from different rows of apertures.

4. An image forming apparatus according to claim 3 , wherein the apertures in a row are transversely equidistantly spaced apart from each other.

5. An image forming apparatus according to claim 3 or claim 4, wherein the apertures in one row are transversely displaced relative to the apertures in another such that the apertures in one row are not in longitudinal alignment with the apertures in another row .

6. An image forming apparatus according to any preceding claim, wherein the apparatus is arranged to have a relative transverse movement between the image forming member and the printhead structure between consecutive passes which is greater than the transverse distance between the apertures in the printhead structure.

7. An image forming apparatus according to any preceding claim, wherein the apparatus is arranged to have a relative transverse movement between the image forming member and the printhead structure which is of the same amount between each of the passes in the formation of an image.

8. An image forming apparatus according to any of claims 3 to 5, wherein the transverse relative movement between passes is equal to PN+P+1 or PN+P-l times the pitch length where P is the number of passes used to form an image and N is an integer including 0.

9. An image forming apparatus according to any of claims 3 to 8, wherein the printhead structure includes apertures arranged in two transverse rows and the transverse spacing of the apertures is such that that either three or four passes are required in order to print an image .

10. An image forming apparatus according to claim 9 wherein, the step distance between passes is given by the formulae P + RxPxN + 1, or P + RxPxN - 1 times the pitch length, where P = number of passes, R = number of rows of apertures and N is an integer including 0.

11. An image forming apparatus according to any of claims 3 to 8, wherein the printhead structure includes apertures arranged in two transverse rows and the transverse spacing of the apertures is such that five passes are required in order to print an image and the step distance between passes is given by the formulae P + RxPxN + X, or P + RxPxN - X times the pitch length, where P = number of passes, R = number of rows, N is an integer including 0, and X can take on the values +3, +1, -1, -3.

12. An image forming apparatus according to any of claims 3 to 8, wherein the printhead structure includes apertures arranged in three transverse rows and the transverse spacing of the apertures is such that three passes are required in order to print an image and the step distance between passes is given by the formulae RxPxN + X, or RxPxN - X times the pitch length, where P = number of passes, R = number of rows, N is an integer including 0, and X can take on the values -7 or -5.

13. An image forming apparatus according to claim 9, wherein the distance between adjacent apertures on a row is six times the pitch length required for the resolution .

14. An apparatus according to any preceding claim wherein, the control electrodes (13) of one or more apertures are arranged to enable the aperture to print at two or more locations separated transversely in a single pass so as to produce two or more columns of print per aperture per pass.

15. An apparatus according to any preceding claim, wherein the number and transverse extent of the apertures in the row or rows of the printhead structure is more than is necessary to print the transverse width of the image and the apparatus is arranged not to use apertures which are only able to print transversely outside the image area to be printed during a pass .

16. An apparatus according to claim 15, wherein the apparatus is arranged such that the non-used apertures are at both transverse ends of the row or rows of apertures and an aperture or apertures at both ends are simultaneously not used during a pass.

17. An apparatus according to any preceding claim, wherein the image receiving member is either formed from electrically conducting material or has a layer of electrically conducting material, for example aluminum, surrounding the circumference of the image receiving member.

18. An apparatus according to claim 17, wherein the surface of the conducting material that is facing the at least one printhead structure is at least partly covered by an electrically insulating material, for example aluminum oxide, such that a mirror electrostatic charge is formed on the conducting material, the mirror charge corresponding to the charge on any toner particles on the surface of the layer of electrically insulating material.

19. An apparatus according to any preceding claim, wherein the image receiving member includes perforations in at least part of its surface which receives toner particles, there being a vacuum source

(207) provided on the side of the image receiving member which is opposite to the at least one printhead structure, the vacuum source and the image receiving member being arranged such that the vacuum source can remove toner particles from the printhead structure via the perforations .

20. An apparatus according to any preceding claim, wherein the apparatus is constructed and arranged to be capable of varying the distance between adjacent columns of print and/or adjacent transverse lines of print.

21. An apparatus according to any preceding claim, wherein the image receiving member is in the form of a generally cylindrical drum (200) which rotates about the axis (201) of the cylinder such that lines on the surface of the drum which are parallel to the said axis pass the at least one printhead structure when the drum rotates .

22. An apparatus according to claim 21, wherein the drum is arranged for movement parallel to its own axis to effect transverse movement relative to the at least one printhead structure and at least some of the apertures in the at least one printhead structure are arranged in at least one row which is arranged parallel to the axis of the drum.

23. An apparatus according to claim 22, wherein the transverse movement of the drum relative to the at least one printhead structure is continuous while the image to be formed passes the at least one printhead structure .

24. An apparatus according to claim 23, wherein a partial revolution, or one revolution, or more than one revolution of the drum is effected without printing during a printing operation, preferably while changing the direction of the transverse movement relative to the at least one printhead structure to the opposite direction.

25. An apparatus according to claim 24, wherein said partial revolution, or one revolution, or more than one revolution of the drum without printing is effected between the printing of successive images.

26. An apparatus according to any of claims 21 to 25, wherein there are four or more printhead structures

(202-205) arranged around the circumference of the drum and at least one of the printhead structures is arranged to transport black toner particles.

27. An apparatus according to claim 26, further comprising a transfer station (206) for transferring the image formed on the drum to another medium, e.g. paper .

28. An apparatus according to claim 27, wherein the transfer station is arranged such that the transfer of the image is started before the whole of the image has passed all of the printhead structures for the last time before the image is transferred by the transfer station.

29. An apparatus according to any of claims 26 to 28, wherein the apparatus is constructed and arranged such that one or more of the printhead structures starts the printing of a subsequent image before one or more of the other printhead structures have finished printing the preceding image.