WO2000069639A1

WO2000069639A1 - Image forming apparatus and method

Info

Publication number: WO2000069639A1
Application number: PCT/SE2000/000837
Authority: WO
Inventors: Ove Larsson; Bo RYDSTRÖM
Original assignee: Array Ab
Priority date: 1999-05-12
Filing date: 2000-05-03
Publication date: 2000-11-23
Also published as: WO2000069640A1; AU4790600A; WO2000069641A1; AU4790500A; WO2000069638A1; AU4790400A; WO2000069641A8; AU4809299A; JP2004506533A

Abstract

The present invention relates to an image forming apparatus in which image information is converted into a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier toward a back electrode member, said image forming apparatus including a background voltage source for producing a background electric field which enables the transport of charged toner particles, at least one printhead structure arranged in said background electric field, including a plurality of apertures and control electrodes; 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, and an image receiving member caused to move in relation to the printhead structure for intercepting the charged particles in an image configuration. The invention is characterized in that said image forming apparatus comprises a printhead cleaning structure having at least one opening in said image receiving member, the position of said opening being aligned with said apertures during a cleaning sequence, and an air pressure source adapted for supplying an air pressure during said cleaning sequence for forcing toner particles remaining in said apertures through said opening in the image receiving member and into the interior of said image receiving member.

Description

TITLE:

Image forming apparatus and method.

TECHNICAL FIELD:

The invention relates to an image forming apparatus in which image information is converted into a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier toward a back electrode member.

The image forming apparatus according to the invention includes a background voltage source for producing a background electric field which enables a transport of charged toner particles from said particle carrier towards said back electrode member, at least one 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 caused to move in relation to the printhead structure for intercepting the transported charged particles in an image configuration.

Furthermore, the invention relates to a method for operating an image forming apparatus of the above- mentioned type. Furthermore, the invention relates to an image receiving member for use in an image forming apparatus of the above-mentioned type. Also, the invention relates to a colour printing apparatus. 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 member due to control in accordance with image information.

A drawback of direct electrostatic printing is that the aperture size has to be sufficiently small to permit high resolution printing and sufficiently large to prevent clogging of the apertures due to toner agglomeration. Therefore, various solutions have been introduced for removing residual toner from clogged apertures. Such a solution, as disclosed in U.S. Patent No. 5,446,478, consists in creating an air flow during a cleaning cycle between two subsequent image formation, which air flow transports residual toner particles away from the printhead structure and back to the particle carrier. Another cleaning method is disclosed m U.S. Patent No. 5,374,949, in which an alternating electrostatic field m a space between the particle carrier and the back electrode gives toner a vibrational motion to prevent toner from clogging at the aperture. The field is also formed to repel the excess toner back to the particle carrier .

In order to meet the requirements of higher resolution printing with direct electrostatic methods, there is still a need improved cleaning units, so as to efficiently dislodge residual toner from a vicinity of the apertures and convey dislodged toner away from the printhead structure. SUMMARY OF THE INVENTION:

An object of the invention is to provide an improved arrangement for cleaning the apertures of a printhead structure in an image forming apparatus, in particular for reducing or preventing clogging of said apertures and consequently for improving the printing quality of said image forming apparatus.

Said object is accomplished by means of an image forming apparatus of the above-mentioned type, which comprises a printhead cleaning structure having at least one opening in said image receiving member, the position of said opening being aligned with said apertures during a cleaning sequence, and an air pressure source adapted for supplying an air pressure during said cleaning sequence for forcing toner particles remaining in said apertures through said opening in the image receiving member and into the interior of said image receiving member.

A further object of the invention is to provide an improved method for cleaning the apertures of a printhead structure in an image forming apparatus .

Said object is accomplished by means of a method for operation of an image forming apparatus of the above- mentioned type which comprises cleaning said printhead structure by supplying an air pressure forcing toner particles remaining in said apertures through an opening being provided in the image receiving member and into the interior of said image receiving member, the position of said opening being aligned with said apertures during said cleaning.

A further object of the invention is to provide an improved image receiving member for use in an image forming apparatus of the above-mentioned type. Said object is accomplished by means of an image receiving member which is provided with at least one opening, the position of which being arranged to be aligned with said apertures during a cleaning sequence in which an air pressure is generated for forcing toner particles remaining in said apertures through said opening in the image receiving member and into the interior of said image receiving member.

A further object of the present invention is to provide an improved colour printing apparatus in which a plurality of printhead structures can be sequentially cleaned by means of one single cleaning structure.

Said object is accomplished by means of a colour printing apparatus, in which image information regarding a plurality of colours is converted into a pattern of electrostatic fields for modulating a transport of charged toner particles from a plurality of particle carriers, corresponding to said colours, toward a back electrode member, said colour printing apparatus including a background voltage source for producing a background electric field which enables a transport of charged toner particles from said particle carriers towards said back electrode member, a plurality of printhead structures, corresponding to said colours, 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 carriers through the apertures and an image receiving member caused to move in relation to the printhead structures for intercepting the transported charged particles in an image configuration. Said colour printing apparatus comprises a printhead cleaning structure having at least one opening in said image receiving member, the position of said opening being sequentially aligned with the apertures of each of said printhead structures during a cleaning sequence, and an air pressure source adapted for supplying an air pressure during said cleaning sequence for forcing toner particles remaining in the apertures of said printhead structures through said opening in the image receiving member and into the interior of said image receiving member.

BRIEF DESCRIPTION OF THE DRAWINGS:

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

Fig. 1 is a schematic side view of an image forming apparatus in accordance with a preferred embodiment of the present invention,

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

Fig. 3a 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 a particle carrier,

Fig. 3b 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 member, Fig. 3c is a section view across a section line I-I m the printhead structure of Fig. 4a and across the corresponding section line II-II of Fig. 4b,

Fig. 4 is a perspective view of an image receiving member, provided with a printhead cleaning structure according to the invention,

Fig. 5 is a cross-sectional view of a cleaning structure according to the invention,

Fig. 6 is a diagram showing the pressure variation provided by means of said cleaning structure as a function of time,

Fig. 7 is a perspective view of a part of an image receiving member, which is provided with guide elements,

Fig. 8 is a perspective view of a part of said image receiving member, including a helically shaped element for transporting residual toner particles,

Fig. 9 is a perspective view of said image receiving member, including an inner wall structure arranged inside the image receiving member,

Fig. 10 is a perspective view of said image receiving member according to a further embodiment,

Fig. 11 is a cross-sectional view of a section of an image recievmg member according to a particular embodiment, Fig. 12 is a perspective view of a section of an image receiving member, according to yet another embodiment,

Fig. 13 is an illustration of the columns of print printed in a single pass in a two pass method,

Fig. 14 is an illustration of the columns of print shown in Fig. 13 after the second pass,

Fig. 15 is an illustration of the effect of apertures which print with a lower density in a two pass printing method,

Fig. 16 is an illustration of the printing pattern of a first embodiment,

Fig. 17 is an illustration of the printing pattern of a second embodiment,

Fig. 18 is an illustration of the printing pattern of a third embodiment,

Fig. 19 is an illustration of the printing pattern of a fourth embodiment,

Fig. 20 is an illustration of the printing pattern of a fifth embodiment, and

Fig. 21 is an illustration of the printing pattern of a sixth embodiment.

PREFERRED EMBODIMENTS:

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

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

According to such a method, each single aperture is utilized to address a specific dot position of the image in a transversal direction. Thus, the transversal print addressability is limited by the density of apertures through the printhead structure. For instance, a print addressability of 200 dpi requires a printhead structure having 200 apertures per inch in a transversal direction.

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

As shown in a general, slightly simplified form in Fig. 1, an image forming apparatus in accordance with a first embodiment of the present invention comprises at least one print station, preferably four print stations la, lb, lc, Id, which are adapted for printing one colour each. Preferably, the colours being used are yellow, magenta, cyan and black. Each print station la-d is preferably in the form of a cartridge assembly which is removably arranged adjacent to a printhead structure 2a, 2b, 2c, 2d. Each printhead structure 2a-d, which will be described in greater detail below, is preferably in the form of an electrode matrix provided with a plurality of selectable apertures (not shown in Fig. 1), which is interposed in a background electric field defined between the corresponding cartridge and a back electrode which is constituted by a printing drum 3. The drum 3 is essentially cylindrally formed and is arranged so as to rotate during operation of the image forming apparatus. To this end, the drum 3 is powered by drive means (not shown in Fig. 1) . Furthermore, the drum 3 has a circumference which is slightly greater than the maximum vertical printed length, i.e. slightly greater than the length of the paper being used during printing. The drum is preferably manufactured from aluminium, but can also be made from other materials with suitable properties.

Each of the printheads 2a-d is connected to a control unit (not shown) 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 particles from corresponding cartridge la-d. In this manner, charged particles are allowed to pass through the opened apertures and toward the back electrode, i.e. the drum 3. In this manner, the charged particles are then deposited on the drum 3.

Due to the fact that the drum 3 is rotating during operation, the image being formed on the drum is then transferred onto an information carrier, such as a sheet of plain, untreated paper 4 or any other medium suitable for printing, which is fed from a paper delivery unit 5 and conveyed past the underside of the drum 3. To this end, each paper sheet 4 is pressed into contact with the drum 3 by means of a belt 6, which in turn is driven by means of two rollers 7, 8 around which the belt 6 extends. In this manner, the toner particles are deposited on the outer surface of the drum 3 and the superposed to the paper 4 form a four colour image.

After the image has been formed on the paper 4 by said charged particles, the paper 4 is fed to a fusing unit 9, in which the image is permanently fixed onto the paper 4. In particular, the fusing unit 9 comprises a fixing holder (not shown) which includes a heating element preferably 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 4. The fusing unit 9 further includes a pressure roller (not shown) arranged transversally across the width of the paper 4. Also, the fusing unit 9 is provided with means for feeding the paper 4 to an out tray (not shown) , to be collected by the user.

After passage through the fusing unit 9, the paper 4 can also 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 4, for removing untransferred toner particles from the paper sheet 4.

The print stations la-d and the printhead structures 2a-d and mounted in a generally cylindrically shaped housing element (not shown) which encloses the drum 3 and which is also provided with means for supporting said components and for maintaining the components in their correct positions. In this manner, the print stations and the printhead structures are supported in an accurate manner and maintained in a predetermined position with respect to the drum. For example, the printhead structures 2a-d are held in a predetermined position with respect to the peripheral surface of the drum and the print stations la-d.

As indicated in Fig. 2, a print station la forming part of an image forming apparatus in accordance with the present invention includes a particle delivery unit 10 preferably having a replaceable or refillable container 11 for holding toner particles, the container 11 having front and back walls (not shown) , a pair of side walls and a bottom wall having an elongated opening 12 extending from the front wall to the back wall and provided with a toner feeding element 13 disposed to continuously supply toner particles to a developer sleeve 14 through a particle charging member 15. The particle charging member 15 is preferably formed of a supply brush or a roller made of or coated with a fibrous, resilient material, and is arranged to be rotated as indicated by means of an arrow in Fig. 2. The supply brush is brought into mechanical contact with the peripheral surface of the developer sleeve 14 for charging particles by contact charge exchange due to triboelectrification of the toner particles through frictional interaction between the fibrous material on the supply brush and any suitable coating material of the developer sleeve. The developer sleeve 14 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 12 of the particle container 11. The developer sleeve 14 is arranged to be rotated as indicated by means of an arrow in Fig. 2.

Charged toner particles are held to the surface of the developer sleeve 14 by electrostatic forces essentially proportional to (Q/D) ², where Q is the particle charge and D is the distance between the particle charge center and the boundary of the developer sleeve 14. Alternatively, the charge unit may additionally include a charging voltage source (not shown) , which supplies an electric field to induce or inject charge to the toner particles. Although it is preferred to charge particles through contact charge exchange, the method can be performed using any other suitable charge unit, such as a conventional charge injection unit, a charge induction unit or a corona charging unit, without departing from the scope of the present invention.

A metering element 16 is positioned proximate to the developer sleeve 14 to adjust the concentration of toner particles on the peripheral surface of the developer sleeve 14, to form a relatively thin, uniform particle layer thereon. The metering element 16 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 16 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 developer sleeve.

As indicated in Fig. 2, a printhead structure 2a is arranged adjacent to said developer sleeve 14. The printhead structure 2a will be described in greater detail below with reference to Figs. 3a-c. The particle delivery unit 10 is provided with a spacer element 42 for defining a predetermined distance between the developer sleeve 14 and the printhead structure 2a. Furthermore, a section of the drum 3 is also indicated in Fig. 2. It is to be understood that the embodiment according to Fig. 1 includes four print stations la-d and four printhead structures 2a-d of the same type as shown in Fig. 2, wherein said print stations la-d are intended for toner particles of different colours.

It can be noted that the printhead structure 2a is maintained in an arcuated shape along at least a part of its longitudinal extension. That arcuated shape has a curvature radius so as to maintain a part of the printhead structure 2a curved around a corresponding part of the peripheral surface of the developer sleeve 14. In this manner, the printhead structure 2a is arranged so that the distance between the printhead structure 2a and the peripheral surface of the developer sleeve 14 remains constant along the whole transverse direction at every moment of the print process, regardless of undesired mechanical imperfections of the developer sleeve 14.

All four print stations la-d can be used simultaneously for printing on the drum 3. This means that all four colours (CMYK) are printed on the drum 3 simultaneously, and with a print density which corresponds to the resolution defined by the dimensions of the apertures in the printhead structures 2a-d. Normally, the printhead structures 2a-d are provided with 200 equally spaced apertures per inch, said apertures being aligned parallel to the axial direction of the drum 3. For example, said apertures can be arranged in two rows each comprising 100 apertures per inch.

During printing with a printhead structure, the resolution of the printed image (i.e. the number of printed dots per inch) generally depends on the number of apertures per inch. If a higher resolution than 200 dots per inch is desired, some form of multiplexing method is required for using one single aperture in a printhead structure for producing several dots on the image receving member. According to a first embodiment of the present invention, such a multiplexing method is accomplished by rotating the drum 3 three revolutions, between which revolutions the drum 3 is diplaced sideways

(i.e. in its axial direction) a certain distance. Said distance then corresponds to a pitch which equals one single dot. This means that a printhead structure having

200 apertures per inch will produce undeflected centre dots in three consecutive complete revolutions of the drum 3. As a result, the printhead structure having 200 apertures per inch can be used for producing an image on the drum 3 which presents a resolution of 600 dots per inch. Consequently, this "multi pass" method, which will be described in detail below with reference to Figs. 13- 21, increases the print addressability of the printhead structure without requiring an increased number cf apertures in the printhead structure.

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

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

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

In the following, a printhead structure provided with deflection electrodes for said DDC method will be described. However, in the case of the above-mentioned first embodiment involving said ulti pass method, no such deflection electrodes are necessary.

As shown in Figs. 3a, 3b, 3c, a printhead structure 2a in an image forming apparatus in accordance with the present invention (and being intended for dot deflection control) comprises a substrate 17 of flexible, electrically insulating material such as polyimide or the like, having a predetermined thickness, a first surface facing the developer sleeve (cf. Fig. 2), a second surface facing the drum, a transversal axis 18 extending parallel to the rotation axis of the developer sleeve of the print station across the whole print area, and a plurality of apertures 19 arranged through the substrate 17 from the first to the second surface thereof. The first surface of the substrate is coated with a first cover layer 20 of electrically insulating material, such as for example parylene. A first printed circuit, comprising a plurality of control electrodes 21 disposed in conjunction with the apertures, and, in some embodiments, shield electrode structures (not shown) arranged in conjunction with the control electrodes 21, is arranged between the substrate 17 and the first cover layer 20. The second surface of the substrate is coated with a second cover layer 22 of electrically insulating material, such as for example parylene. A second printed circuit, including a plurality of deflection electrodes 23, is arranged between the substrate 17 and the second cover layer 22. The printhead structure 2a 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 22, facing the drum 3.

The printhead structure 2a is brought in cooperation with a control unit (not shown) comprising variable control voltage sources connected to the control electrodes 21 to supply control potentials which control the amount of toner particles to be transported through the corresponding aperture 19 during each print sequence. The control unit further comprises deflection voltage sources (not shown) connected to the deflection electrodes 23 to supply deflection voltage pulses which controls the convergence and the trajectory path of the toner particles allowed to pass through the corresponding apertures 19. 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 21 from one another, preventing electrical interaction therebetween. In a preferred embodiment of the invention, the substrate 17 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 17, respectively, using conventional etching techniques. The first and second cover layers 20, 22 are 5 to 10 microns thick parylene laminated onto the substrate 17 using vacuum deposition techniques. The apertures 19 are made through the printhead structure la using conventional laser micromachining methods. The apertures 19 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 19 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.

The first printed circuit comprises the control electrodes 21 each of which having a ring shaped structure surrounding the periphery of a corresponding aperture 19, 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 21 may take on various shape for continuously or partly surrounding the apertures 19, preferably shapes having symmetry about the central axis of the apertures. In some embodiments, particularly when the apertures 19 are aligned in one single row, the control electrodes are advantageously made smaller in a transverse direction than in a longitudinal direction.

The second printed circuit comprises the plurality of deflection electrodes 23, each of which is divided into two semicircular or crescent shaped deflection segments 24, 25 spaced around a predetermined portion of the circumference of a corresponding aperture 19. The deflection segments 24, 25 are arranged symmetrically about the central axis of the aperture 19 on each side of a deflection axis 26 extending through the center of the aperture 19 at a predetermined deflection angle d to the longitudinal direction. The deflection axis 26 is dimensioned in accordance with the number of deflection sequences to be performed in each print cycle. 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 drum motion, the second dot is undeflected and the third dot is deflected slightly downstream with respect to the drum motion, thereby obtaining a transversal alignment of the printed dots on the drum. Accordingly, each deflection electrode 23 has an upstream segment 24 and a downstream segment 25, all upstream segments 24 being connected to a first deflection voltage source Dl, and all downstream segments 25 being connected to a second deflection voltage source D2. Three deflection sequences (for instance: DKD2; D1=D2; D1>D2) can be performed in each print cycle, whereby the difference between Dl and D2 determines the deflection trajectory of the toner stream through each aperture 19, and thus the dot position on the toner image .

In the embodiment shown in Figs. 3a-c, the printhead structure 2a is suitable for performing 600 dpi printing utilizing three deflection sequences m each print cycle, i.e. three dot locations are addressable through each aperture 19 of the printhead structure during each print cycle. Accordingly, one aperture 19 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 18 of the printhead structure la. The apertures 19 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 18 of the printhead structure la and transversally shifted with respect to each other such that all apertures are equally spaced in a transverse direction. The distance between the aperture rows is preferably chosen to correspond to a whole number of dot locations.

As explained above, the invention can be implemented with a multiplexing method involving said multi pass method. In such cases, the deflection electrodes described with reference to Figs. 3a-c are not necessary.

As initially mentioned, a problem which may occur after repeated use with an image forming apparatus as described relates to the fact that toner particles may gradually cover the printhead structure 2a or may agglomerate on the apertures walls, obstructing the apertures 19. This may result in clogging of the apertures 19, which in turn may lead to a deterioration of the print quality, for example in the form of uneven printing or by excess toner being deposited on the paper. Consequently, the apertures 19 and their surrounding areas will need to be cleaned from residual toner particles which agglomerate there. For this reason, the image forming apparatus in accordance with the present invention preferably comprises a printhead cleaning structure, which now will be described in detail.

Referring back to Fig. 1, the present invention relies on the fact that an air pressure source, preferably in the form of an audio loudspeaker 27, is arranged adjacent to the drum 3. According to a first embodiment, shown in Fig. 1, the loudspeaker 27 is arranged in one end section of the drum 3, i.e. in the gable of the drum 3, covering essentially the entire cross-sectional area of said drum 3 and being directed towards the interior of the drum 3.

With reference to Fig. 4, the loudspeaker 27 is shown in a perspective view which also shows the drum 3. Fig. 4 also indicates a printhead structure 2a (i.e. one of the four printhead structures shown in Fig. 1). However, for reasons of simplicity, no print station is shown in Fig. 4. Neither is the above-mentioned housing (used for supporting the printhead structure 2a) shown in Fig. 4. The loudspeaker 27 is arranged at a first end section 3a or gable of the drum and is arranged for generating a pulse of air pressure inside the drum 3 during a cleaning sequence, for forcing toner particles remaining on the printhead structure 2a, i.e. in its apertures and the areas surrounding the apertures, into the interior of the drum 3. As will be described in greater detail below, the loudspeaker 27 is connected to the above-mentioned control unit (not shown) and adapted for producing said air pulse during the cleaning sequence.

The opposite end section 3b or gable of the drum 3 is preferably provided with means 28 for rotating the drum 3 in a direction indicated by means of an arrow in Fig. 4.

Said means 28 for rotating the drum 3 preferably comprises a drum drive gear which in turn is connected to a motor (not shown) being controlled so as to rotate the drum 3 in the desired manner. This operation is not described in detail here.

Furthermore, the wall of the drum 3 is provided with an opening, preferably in the form of a slot 29 which extends generally in the longitudinal direction of the drum 3. The slot 29 is formed as a longitudinally extending recess having a depth which is slightly less than the thickness of the drum 3. The bottom surface of the recess is provided with a number of holes 30, which connect the interior of the drum 3 with the surrounding air outside the drum 3. The purpose of the holes 30 is to be aligned with the apertures of the printhead structure 2a as the slot 29 sequentially passes all the printhead structures during a printing sequence. Preferably, the width of said recess is sufficiently small so that the printhead structure is kept out of contact with the drum due to said air pressure acting upon the printhead structure during the cleaning sequence. Also, the recess is sufficiently deep for the cleaning of said printhead structure to be generally evenly distributed along said printhead structure. Furthermore, the distance between any two of said holes is preferably less than the depth of the recess, said depth being defined as the distance from the outer surface of the drum to the opening of each hole facing the outer surface of the drum.

According to an alternative embodiment (not shown in the drawings), the width of said recess is at least twice the distance between said printhead structure and said drum. By making the recess sufficiently deep and wide, the printhead will be partly forced into the recess when said air pressure is applied to the printhead structure in question. In this manner, the apertures of the printhead structure will be exposed to a higher air pressure as compared with the above-mentioned embodiment, since the printhead structure is forced to assume a position closer to the interior of the drum. When the air pressure pulse is no longer acting upon the particular printhead structure, the printhead structure will be released, thereby resuming its position above the recess. In this manner, the drum is then allowed to continue its rotating motion.

In Fig. 5, the operation of the loudspeaker 27 is described in greater detail. The loudspeaker 27 comprises a pressure changing element 31 in the form of a membrane of the type used in conventional loudspeakers. A coil 32 is fixed at the center of the diaphragm 31 that is free to move in an annular gap 33. A magnetic field produced either by a magnet 34 or an electromagnet (not shown) is applied across the gap 33. As a cleaning signal is input to the coil 32 as alternating current, causing it to move in the magnetic field as a result of electromagnetic induction. The diaphragm 31 is thus caused to vibrate at the same frequency as the cleaning pulse, and an air pressure wave is produced by the diaphragm 31.

The loudspeaker 31 generates a pressure wave, i.e. a prompt pressure difference, transferred through the holes 30 in the slot 29 to produce a suction force in the air gap between the printhead structure 2a and the drum 3 as the drum 3 rotates past the printhead structure 2a.

According to a preferred embodiment, the control unit is adapted to actuate the loudspeaker 27 so as to generate a short air stream pulse of very high speed. This air pulse is generated in a synchronized manner depending on the position of the drum in relation to the four printhead structures 2a-d. In other words, the air pulse from the loudspeaker 27 is generated at those occasions when the slot 29 is aligned with the lines of apertures of the respective printhead structures 2a-d. Furthermore, in the case multi pass multiplexing is used, the air pulses from the loudspeaker 27 are preferably generated during each revolution of the drum 3 during the three-revolution sequence described above.

In this manner, residual toner particles 35 are removed periodically during a cleaning cycle, preferably be generating said air pulse during each occation the slot 29 passes a printhead structure.

After being dislodged from a vicinity of the apertures, residual toner particles are transported away from the printhead structure. When an underpressure wave is produced by the loudspeaker 27, the transport of residual toner is directed toward the loudspeaker 27, and preferably also towards a waste toner container 36, which connects to the interior of the drum 3 via an opening 37 in the wall of the drum 3. In this manner, residual toner particles 35 will be collected in the waste container 36. The user may then empty or replace the waste container 36 when the image forming apparatus has been used a predetermined time period or when the waste container 36 can be assumed to be full. Furthermore, the air being forced through the slot 29 and into the waste container 36 can be forced out to the surrounding atmotsphere via an outlet 38, preferably via a reverse valve (not shown) . Furthermore, according to another embodiment, a filter (not shown) is provided in connection with the waste container 36, for filtering particles of desired shape or size when forcing said air through the waste container. In this case, the filter can be arranged in the container itself or in its inlet or outlet. As shown in Fig. 6, the air pressure P in the vicinity of the apertures 19 of the printhead circuit 2a (cf. Fig. 4) is changed from a initial, ambient pressure P₀ to a cleaning pressure P_cι., preferably lower than the ambient pressure P₀, during a predetermined fall time T_f, typically in the order of 5 ms . Thereafter, the pressure in the air gap recovers its initial, ambient value P₀ during a stabilization time T_s, typically in the order of 20 ms . The pressure fall, for example in the range of 100 to 500 Pa, produces a pressure wave propagating in the slot 29. The pressure wave applies suction forces on residual toner particles. Consequently, by means of the invention, residual toner particles are quickly and efficiently removed from the apertures or from the vicinity of the apertures by a air pressure wave generated between subsequent image formation periods.

Consequently, the air pressure wave is produced by said pressure source, which provides a temporary air pressure difference in the gap between the printhead structure and the drum. The air pressure wave has an amplitude, a rise time and a fall time dimensioned to release toner agglomerated within or around the apertures.

Fig. 7 is an enlarged perspective view of a section of the drum 3 according to a further embodiment. According to said embodiment, the slot 29 is provided with a plurality of guide elements 39, bridging the slot 29 and acting as support elements for the printhead circuits during the cleaning sequence. The guide elements 39 are suitably in the form of a plurality of thin T-shaped components which are placed along the slot 29 and arranged to extend generally flush with the drum surface. The guide elements 39 are particularly useful if the printhead circuit being cleaned has a tendency to be released from being in contact with its corresponding developer roller during said cleaning sequence.

As mentioned above, the slot 29 can alternatively be sufficiently narrow so that the printhead structure is not forced into the slot 29 when air pressure is applied during the cleaning sequence.

Fig. 8 is a perspective view of the first end section 3a of the drum 3. However, the loudspeaker described above is not shown in Fig. 8. As indicated in Fig. 8, the transport of residual toner particles can be further improved by providing the interior surface of the drum 3 with a tubular, helix-shaped element 40. More precisely, said element 40 is suitably in the form of a cylindrical sleeve-shaped element made for example from plastics and being inserted inside the drum 3. Said element 40 is provided with an inner surface having a helically shaped recess which then forms said helix-shaped element 40.

Alternatively, a spiral-shaped element (not shown in the drawings) can be used in essentially the same manner as said helically shaped element, i.e. for forcing toner particles through the interior of the drum towards a waste container.

Fig. 9 is a perspective view of the drum 3, i.e. not showing any components such the printhead structures or the print stations. According to a further embodiment, the cleaning action inside the drum can be further improved by arranging an inner wall structure 41 inside the drum 3. More precisely, this wall structure 41 is in the form of an elliptic disc which prevents standing-wave phenomena from occuring inside the drum 3, which otherwise would affect the efficiency of the invention negatively. The invention is not limited to the wall structure 41 according to Fig. 9. According to other embodiments, other forms of wall structures can be used. Generally, the inner wall structure - which defines an available cross-sectional area for the flow of the air pressure pulse along the interior of the drum - is designed so that said area is relatively small at a first position distant from the loudspeaker and relatively large at a second position which is closer to the loudspeaker than said first position.

As an alternative to the wall structure 41 shown in Fig. 9, the inside of the drum can be provided with a plurality of wall elements, the positions and dimensions of which are chosen so as to eliminate standing-wave phenomena, by reflecting air pressure pulses in a predetermined manner.

Fig. 10 is a perspective view of another embodiment of the present invention, in which an air pressure source - preferably in the form of a conventional loudspeaker 27' - is arranged inside the drum 3. Furthermore, a nozzle element 43 is arranged inside the drum 3, connecting the loudspeaker 27' to the inner surface of the drum. More precisely, the nozzle element 43 is generally shaped as a funnel or horn which extends along and covers the particular section of the inner surface of the drum 3 where the slot 29 is positioned. The toner particles being drawn out from the printhead structures (not shown in Fig. 10) and into the nozzle element 43 are fed towards a tubular conduit 44 which leads said toner particles from the nozzle element 43 and into a waste container 36^'. To this end, the conduit 44 preferably extends along the axis of symmetry about which the drum 3 rotates, whereas the waste container 36' is arranged in a fixed, i.e. non-rotating, manner. In this manner, the toner particles are fed to the waste container 36', via an inlet 45 in said waste container 36', in a direction indicated by means of an arrow in Fig. 10. In a manner similar to the embodiment shown in Figs. 4 and 5, the waste container 36^' can be provided with a filter and a reverse valve. Also, the embodiment according to Fig. 10 can be provided with a helically shaped or spiral-shaped element for removing the waste toner particles.

Fig. 11 is a cross-sectional view showing a slot 29^' according to an alternative embodiment of the invention. The slot 29' is arranged along a section of the circumference of the drum 3 and is partly filled with filling material 46 so as to form a cross-sectional area which is tapered in the direction towards the outer surface of the drum 3. This means that the cross-section is narrowing in a direction from the inner surface of the drum 3 and towards the outer surface of the drum 3. In other words, the area of the slot 29^', as regarded along the outer surface of the drum 3, is smaller than the area of the slot 29' at the bottom of the recess. In this manner, the pressure resulting from the air pressure pulses during said cleaning sequence will be concentrated towards the outer surface of the drum 3, i.e. close to the printhead structure (not shown in Fig. 11) which is located just above the drum 3.

Preferably, the cross-sectional area of this tapered recess (as regarded in the cross-sectional view of Fig. 11) is greater than the area formed by the distance between two of said holes 30 in the axial direction of the drum multiplied by the height h of said recess. In this manner, the pressure resulting from said air pressure pulses will be evenly distributed along the axial direction of the slot 29^". Furthermore, the area of the opening of the recess along the outer surface of the drum is preferably less than the total area of all of said holes.

Fig. 12 shows yet another embodiment which is similar to that which is shown in Fig. 11, but in which at least one voltage electrode 47 is arranged in the above- mentioned filling material 46. According to the embodiment, said filling material 46 is constituted by an electrically isolating material, for example a suitable type of plastics. The electrode 47 is electrically connected to voltage source (not shown) which in turn is arranged so as to supply a voltage. This results in a movement of toner particles in the vicinity of the slot 29^'. According to a preferred alternative, said voltage is an AC voltage. This means that the toner particles are set in motion. Said motion of the toner particles will then depend on the frequency of said AC voltage. During a cleaning sequence, said AC voltage is then preferably combined with the above- mentioned air pressure pulse for providing an improved cleaning operation.

According to another embodiment, the above-mentioned voltage is a DC voltage. Preferably, the DC voltage is in this case of opposite voltage as compared with the background voltage applied for producing the background electric field described above. In particular, this means that so-called "wrong sign toner particles", i.e. particles not having been correctly charged, will be drawn towards the drum. If an air pressure pulse is then applied, the toner particles will be drawn into the drum, in a manner as described above.

Furthermore, the methods involving applying AC and DC voltages can be combined in one single embodiment, for improving the cleaning of the image forming apparatus according to the invention. Such a combined AC and DC voltage operation is in turn preferably combined with the above-mentioned air pressure pulse.

In the following, the above-mentioned "multi pass" method, which be described in detail below with reference to Figs. 13-21. By means of said method, the resolution achieved by a printhead structure 2 for a given number of apertures 19 may be increased without necessarily the use of deflection electrodes 23. In order to achieve a printing resolution greater than the number of apertures in the printhead structure 2 the printing takes place in two or more passes of the image receiving member, i.e. the drum 3 according to the above-mentioned embodiment. By a pass is meant a movement of the image receiving member which passes a section of the drum 3 to be printed with a movement relative to a given printhead structure 2 and allows the printhead structure 2 to deposit a plurality of longitudinal columns of printing. A column of printing is a longitudinal line of the image receiving member which is subject to printing of dots by an aperture or apertures even if not all the parts of the line receive dots due to the content of the image being formed requiring some parts of the column to be left without dots. A transverse line of printing is a transverse line of the image receiving member which is subject to printing of dots from a plurality of apertures, even if not all the parts of the line receive dots due to the content of the image being formed requiring some parts to be left without dots. The closest distance between two adjacent columns or lines of print is defined as the pitch or the distance between two addressable pixel locations. After the first pass the next passes may be in the same or opposite longitudinal directions to that of the first pass.

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

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 drum. 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 of 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 19 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 drum. 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 drum and printhead structure are then moved relative to each other in the direction transverse to the direction of movement of the drum, but in the plane of the drum. 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 drum traveling in the same longitudinal direction as the first pass or the opposite longitudinal direction. This effect is illustrated in Figs. 13 and 14. Fig. 13 represents a section of the drum after the first pass. The areas that are printed in the first pass are shown as hatched areas 61. Fig. 14 represents the same section of the drum 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 19 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. 15 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. 15.

A first embodiment of the multi pass method is illustrated in Fig. 16. 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. 16. In Fig. 16 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. 16. As can been seen from Fig. 16 the columns of less density are of narrower width than those in Fig. 15 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 multi pass method is illustrated in Fig. 17. In this example the printing is carried out in three passes. In Fig. 17 the row of apertures has moved from the first position indicated by reference numeral 91 in which the first pass took place to the position indicated by reference numeral 92 in which the second pass took place and then to the position indicated by reference numeral 93. The number of apertures 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 drum. This first third comprising one third of the columns of print indicated by reference numeral 94 of the intended final image. The drum and printhead structure are then moved relative to each other in the direction transverse to the direction of movement of the drum, but in the plane of the drum, preferably by moving the drum 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 drum 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 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 drum traveling in the same direction as the first pass or in the opposite direction. This embodiment is illustrated in Fig. 17 which shows the drum 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 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 in Fig. 18. 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. 18 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 a 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. 19) 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. 19 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 drum 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 drum, but in the plane of the drum. 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. W /

4 0

In a still further embodiment using DDC control (see Fig. 20) each aperture prints two columns of print which are separated from each other by four times the pitch length. In Fig. 20 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. 21. In Fig. 21 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 image receiving member 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.

The multi pass method which now has been described will now be further explained with reference to Fig. 1.

The image receiving member in this embodiment is a drum 3. The drum 3 rotates about an axis. Around the periphery of the drum 3 are arranged four print stations la, lb, lc and Id. The print stations respectively contain differently coloured toner particles to allow colour printing. One of the print stations may contain black toner particles to allow black and white printing. There is also provided a transfer station 6 for transferring the image to another medium such as a paper sheet 4. Transfer may be effected by electrostatic attraction or by pressure transfer. A cleaning structure is provided for cleaning the printhead structures of toner particles as required. The cleaning station comprises a vacuum source, for example m the form of an audio loudspeaker. The vacuum source acts through one or more transversely aligned rows of apertures m the drum so that a suction force may be effected on a printhead structure.

The printhead structure provided with each print station is preferably of the type illustrated m Figs. 3a-c, i.e. two parallel rows of apertures with constant pitch between the apertures m a row. The apertures of one row are staggered m relationship to the apertures of the other row. The apertures of one row may be centered m the spaces between the apertures of the other row, though they could be arranged eccentrically.

Cleaning of the printhead structures 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 6. While the drum is rotating it is moved along its axis. The printhead structures and drum are thus moved continuously relatively to each other m 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 structure 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 m 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 m 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 drum 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 aluminium. 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 E 7

4 6

charged toner to the transfer belt or drum. The insulating materials may be any suitable material, m 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 multi pass 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. For example, the invention is not limited to use of an image receiving member in the form of a drum. For example, the image receiving member can be a drum of the kind mentioned above, or may alternatively be an endless belt.

Furthermore, said image recevmg member can be formed for example by a sheet-like or belt-like element which is bent so that its end portions meet. In the position where these end portions meet, the above-mentioned opening or openings can be formed by means of for example a joining element which joins said end portions while providing a slit or gap between the end portions.

The invention can be used for colour printing, as described above. It can also be used for black and white printing. In the latter case, a printing apparatus using only one printing station and only one cartridge with black toner particles can then be used.

The invention can be used with a multiplexing method, such as the above-mentioned multi pass method or DDC method, or may alternatively be operated without any multiplexing method.

Furthermore, the air pressure being generated according to the invention can be partly fed to the upper side of each printhead structure (i.e. the side of the printhead structure facing the corresponding developer sleeve) during the cleaning sequence. In this manner, the movement of the printhead structures during the cleaning sequence can be minimized.

The loudspeaker 27 can be arranged to be rotated together with the drum 3, or may alternatively be arranged so as to be standing still while the drum 3 rotates. Furthermore, the loudspeaker 27 does not have to be arranged adjacent to the drum 3. An an alternative, the invention may comprises a loudspeaker which is distant from the drum. In such a case an air connection is provided between the loudspeaker and the interior of the drum.

Claims

CLAIMS :

1. An image forming apparatus in which image information is converted into a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier toward a back electrode member, said image forming apparatus including: a background voltage source for producing a background electric field which enables a transport of charged toner particles from said particle carrier towards said back electrode member; at least one 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 caused to move in relation to the printhead structure for intercepting the transported charged particles in an image configuration; characterized in that said image forming apparatus comprises a printhead cleaning structure having: at least one opening in said image receiving member, the position of said opening being aligned with said apertures during a cleaning sequence, and an air pressure source adapted for supplying an air pressure during said cleaning sequence for forcing toner particles remaining in said apertures through said opening in the image receiving member and into the interior of said image receiving member.

2. An image forming apparatus according to claim 1, characterized in that said image receiving member is formed as an element which is bent so that its end sections meet, said opening being formed along a position where said end sections meet.

3. An image forming apparatus according to claim 1 or 2, characterized in that said image receiving member is constituted by a cylindrical drum.

4. An image forming apparatus according to claim 3, characterized in that said drum is provided with a plurality of holes in the periphery of said drum, said holes extending essentially along the axial direction of said drum.

5. An image forming apparatus according to claim 4, characterized in that said holes are provided in a recess extending essentially along the axial direction of said drum.

6. An image forming apparatus according to claim 5, characterized in that said drum is provided with a plurality of guide elements bridging said recess and being arranged for supporting said printhead structure.

7. An image forming apparatus according to claim 5, characterized in that the width of said recess is sufficiently small so that the printhead structure is kept out of contact with the drum during said cleaning sequence .

8. An image forming apparatus according to claim 5, characterized in that the width of said recess is at least twice the distance between said printhead structure and said drum.

9. An image forming apparatus according to any one of claims 5-8, characterized in that said recess is sufficiently deep for the cleaning of said printhead structure to be generally evenly distributed along said printhead structure.

10. An image forming apparatus according to any one of claims 5-9, characterized in that said recess presents a cross-section which is narrowing in a direction from a an inner surface of the drum and towards an outer surface of the drum.

11. An image forming apparatus according to claim 10, characterized in that the cross-sectional area of said recess is greater than the area formed by the distance between two of said holes multiplied by the height of said recess.

12. An image forming apparatus according to any one of claims 5-11, characterized in that the area of the opening of the recess along the outer surface of the drum is less than the total area of all of said holes.

13. An image forming apparatus according to any one of claims 5-12, characterized in that the distance between any two of said holes is less than the depth of the recess.

14. An image forming apparatus according to any one of claims 3-13, characterized in that said air pressure source is arranged in the gable of said drum.

15. An image forming apparatus according to any one of claims 3-13, characterized m that the air pressure source is arranged m the interior of said drum.

16. An image forming apparatus according to any one of claims 14 or 15, characterized m that the air pressure source is connected to said at least one opening via a nozzle element extending inside said drum.

17. An image forming apparatus according to any one of the preceding claims, characterized m that said air pressure source is constituted by an audio loudspeaker.

18. An image forming apparatus according to any one of the preceding claims, characterized m that said image receiving member comprises an inner wall structure arranged to prevent standing wave-phenomena during said cleaning sequence.

19. An image forming apparatus according to any one of the preceding claims, characterized in that said image receiving member comprises an inner wall structure which is adapted to define an available cross-sectional area for the flow of air along the interior of said image receiving member, said wall structure being designed so that said area is relatively small at a first position distant from said air pressure source and relatively large at a second position which is closer to said air pressure source than said first position.

20. An image forming apparatus according to any one of the preceding claims, characterized in that said printhead cleaning structure comprises a waste container for collecting toner particles being transported into the interior of said image receiving member.

21. An image forming apparatus according to claim 20, characterized in that said waste container is provided with an outlet.

22. An image forming apparatus according to claim 21, characterized that the waste container is provided a reverse valve associated with said outlet.

23. An image forming apparatus according to any one of claims 20-22, characterized in that a toner particle filter is arranged in connection with the waste container.

24. An image forming apparatus according to any one of the preceding claims, characterized in that the interior of said image receving member is provided with a generally helically shaped element for transporting toner particles.

25. An image forming apparatus according to any one of the preceding claims, characterized in that the interior of said image receving member is provided with a spiral- shaped element for transporting toner particles.

26. An image forming apparatus according to any one of the preceding claims, characterized in that an electrode is arranged in connection with said opening, said electrode being arranged for applying a voltage for generating movement of said charged toner particles during said cleaning sequence.

27. An image forming apparatus according to claim 26, characterized in that said electrode is arranged in an electrically isolating element accommodated in said opening.

28. An image forming apparatus according to claim 26 or 27, characterized in that said voltage is a AC voltage.

29. An image forming apparatus according to claim 26 or 27, characterized in that said voltage is a DC voltage of opposite polarity as compared with said background electric voltage.

30. An image forming apparatus according to claim 26 or 27, characterized in that said voltage is a combined AC and DC voltage.

31. An image forming apparatus according to any one of the preceding claims, characterized in that 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 structure 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 forming apparatus being so constructed and arranged to operate that adjacent columns of print are not printed by the same aperture in different passes.

32. An image forming apparatus according to claim 31, wherein the image forming apparatus is so constructed and arranged to operate that adjacent columns of print are not printed by the same aperture in any pass.

33. An image forming apparatus according to claim 31 or claim 32, 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.

34. An image forming apparatus according to claim 33, wherein the apertures in a row are transversely equidistantly spaced apart from each other.

35. An image forming apparatus according to claim 33 or claim 34, 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.

36. An image forming apparatus according to any one of claims 31-35, 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.

37. An image forming apparatus according to any one of claims 31-36, 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 .

38. An image forming apparatus according to any of claims 33-35, wherein the transverse relative movement between passes is equal to PN+P+1 or PN+P-1 times the pitch length where P is the number of passes used to form an image and N is an integer including 0.

39. An image forming apparatus according to any of claims 33-38, 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.

40. An image forming apparatus according to claim 39 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.

41. An image forming apparatus according to any of claims 33 to 38, 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.

42. An image forming apparatus according to any of claims 33 to 38, 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.

43. An image forming apparatus according to claim 39, wherein the distance between adjacent apertures on a row is six times the pitch length required for the resolution.

44. An apparatus according to any one of claims 31-43, wherein the control electrodes 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.

45. An apparatus according to any one of claims 31-44, 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.

46. An apparatus according to claim 45, 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.

47. An apparatus according to any one of claims 31-46, 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 .

48. An apparatus according to claim 47, 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.

49. An apparatus according to any one of claims 31-48, 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.

50. An apparatus according to any one of claims 31-49, wherein the image receiving member is in the form of a generally cylindrical drum which rotates about the axis of the cylinder such that lines on the surface of the drum which are parallel to the said axis pass the printhead structure when the drum rotates.

51. An apparatus according to claim 50, 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.

52. An apparatus according to claim 51, 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.

53. An apparatus according to claim 52, 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.

54. An apparatus according to claim 53, 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.

55. An apparatus according to any one of claim 31-54, further comprising a transfer station for transferring the image formed on the drum to another medium, e.g. paper.

56. An apparatus according to claim 55, 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.

57. An apparatus according to any of claims 55 or 56, 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 .

58. A method for operating an image forming apparatus m which image information is converted into a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier toward a back electrode member and comprising at least one printhead structure arranged m said background electric field, including a plurality of apertures and control electrodes arranged in con unction to the apertures, said method including: producing a background electric field which enables a transport of charged toner particles from said particle carrier towards said back electrode member; supplying control potentials to said control electrodes m accordance with the image information to selectively permit or restrict the transport of charged toner particles from the particle carrier through the apertures; and intercepting the transported charged particles m an image configuration by means of an image receiving member caused to move m relation to the printhead structure; characterized m that said method further comprises cleaning said printhead structure by supplying an air pressure forcing toner particles remaining m said apertures through an opening being provided m the image receiving member and into the interior of said image receiving member, the position of said opening being aligned with said apertures during said cleaning.

59. A method for operating an image forming apparatus according to claim 58, characterized m that said air pressure is repeatedly generated for cleaning at least two printhead structures at different occasions.

60. Image receiving member for image forming apparatus in which image information is converted into a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier toward a back electrode member, said image forming apparatus including: a background voltage source for producing a background electric field which enables a transport of charged toner particles from said particle carrier towards said back electrode member; at least one printhead structure arranged in said background electric field, including a plurality of apertures and control electrodes arranged m conjunction to the apertures; control voltage sources for supplying control potentials to said control electrodes m accordance with the image information to selectively permit or restrict the transport of charged toner particles from the particle carrier through the apertures; wherein said image receiving member is arranged to move in relation to the printhead structure for intercepting the transported charged particles m an image configuration; characterized m that said image receiving member is provided with at least one opening, the position of which being arranged to be aligned with said apertures during a cleaning sequence m which an air pressure is generated for forcing toner particles remaining in said apertures through said opening in the image receiving member and into the interior of said image receiving member.

61. A colour printing apparatus m which image information regarding a plurality of colours is converted into a pattern of electrostatic fields for modulating a transport of charged toner particles from a plurality of particle carriers, corresponding to said colours, toward a back electrode member, said colour printing apparatus including: a background voltage source for producing a background electric field which enables a transport of charged toner particles from said particle carriers towards said back electrode member; a plurality of printhead structures, corresponding to said colours and being arranged in said background electric field, each of said printhead structures including a plurality of apertures and control electrodes arranged m conjunction to the apertures; control voltage sources for supplying control potentials to said control electrodes m accordance with the image information to selectively permit or restrict the transport of charged toner particles from the particle carriers through the apertures; and an image receiving member caused to move in relation to the printhead structures for intercepting the transported charged particles in an image configuration; characterized m that said colour printing apparatus comprises a printhead cleaning structure having: at least one opening m said image receiving member, the position of said opening being sequentially aligned with the apertures of each of said printhead structures during a cleaning sequence, and an air pressure source adapted for supplying an air pressure during said cleaning sequence for forcing toner particles remaining in the apertures of said printhead structures through said opening in the image receiving member and into the interior of said image receiving member.