WO2000069638A1

WO2000069638A1 - Direct printing device with cleaning unit

Info

Publication number: WO2000069638A1
Application number: PCT/SE1999/000818
Authority: WO
Inventors: Urban Albinsson; Bo RYDSTRÖM
Original assignee: Array Ab
Priority date: 1999-05-12
Filing date: 1999-05-12
Publication date: 2000-11-23
Also published as: WO2000069640A1; WO2000069641A1; WO2000069639A1; AU4790400A; WO2000069641A8; AU4809299A; AU4790600A; JP2004506533A; AU4790500A

Abstract

The invention provides a cleaning method, and a cleaning unit for performing that method, in which residual toner particles are quickly and efficiently removed from the apertures or from the vicinity of the apertures by an air pressure wave generated between subsequent image formation periods. The air pressure wave is produced by pressure changing means, which provide a temporary air pressure difference in the gap between the printhead structure and the image receiving medium. The air pressure wave has an amplitude, a rise time and a fall time dimensioned to release toner agglomerated within or around the apertures. The pressure wave is preferably produces by a vibrating element, for example a speaker membrane, arranged in conjunction with a nozzle disposed in the vicinity of the apertures.

Description

DIRECT PRINTING DEVICE WITH CLEANING UNIT

Field of the Invention

The invention relates to a direct printing apparatus in which a computer generated image information is converted into a pattern of electrostatic fields, which selectively transport electrically charged particles from a particle source toward a back electrode through a printhead structure, whereby the charged particles are deposited in image configuration on an image receiving substrate caused to move relative to the printhead structure. More specifically, the invention relates to a cleaning unit provided in such a direct printing apparatus in order to remove residual charged particles from the printhead structure after formation of the image. Background of the Invention

U.S Patent No. 5,036,341 discloses a direct electrostatic printing device and a method to produce text and pictures with toner particles on an image receiving substrate directly from computer generated signals. Such a device generally includes a printhead structure provided with a plurality of apertures through which toner particles are selectively transported from a particle source to an image receiving medium due to control in accordance with an image information. A drawback of direct electrostatic printing is that the aperture size has to be sufficiently small to permit high resolution printing, while sufficiently large to prevent clogging of the apertures due to toner agglomeration. Therefore, various solutions have been introduced to enhance cleaning means for removing residual toner from

CONFIRMATION COPY 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 in U.S. Patent No.5,374,949 in which an alternating electrostatic field in a space between the particle carrier and the back electrode gives toner a vibrational motion to prevent toner clog 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 for improving a cleaning unit, which efficiently dislodges residual toner from a vicinity of the apertures and conveys dislodged toner away from the printhead structure. Summary of the invention An object of the invention is to provide a cleaning method, and a cleaning unit for performing that method, in which 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. The air pressure wave is produced by pressure changing means, which provide a temporary air pressure difference in the gap between the printhead structure and the image receiving medium. The air pressure wave has an amplitude, a rise time and a fall time dimensioned to release toner agglomerated within or around the apertures. The pressure wave is preferably produced by a vibrating element, for example a speaker membrane, arranged in conjunction with a nozzle disposed in the vicinity of the apertures. Brief description of the drawings

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

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

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

Fig.4a is a partial view of a printhead structure of a type used in an image forming apparatus, showing the surface of the printhead structure that is facing the toner delivery unit. Fig.4b is a partial view of a printhead structure of a type used in an image forming apparatus, showing the surface of the printhead structure that is facing the intermediate transfer belt. Fig.4c is a section view across a section line I-I in the printhead structure of Fig.4a and across the corresponding section line II-II of Fig.4b. Fig.5a is a section view of a cleaning unit in accordance with a first embodiment of the invention. Fig.5b is a section view of a cleaning unit in accordance with another embodiment of the invention. Fig.6, 7, 8, 9 are section views of cleaning units according to alternate embodiments of the invention. Fig.10 is a diagram showing the pressure variation as a function of time, as a cleaning cycle is performed in the print zone .

Detailed description of the 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 the image receiving substrate to provide line-by line scan printing to form a visible image.

A printhead structure for use in direct electrostatic printing may take on many designs, such as a lattice of intersecting wires arranged in rows and columns, or an apertured substrate of electrically insulating material overlaid with a printed circuit of control electrodes arranged in conjunction with the apertures. Generally, a printhead structure includes a flexible substrate of insulating material such as polyimide or the like, having a first surface facing the particle carrier, a second surface facing the back electrode and a plurality of apertures arranged through the substrate. The first surface is coated with an insulating layer and control electrodes are arranged between the first surface of the substrate and the insulating layer, in a configuration such that each control electrode surrounds a corresponding aperture. The apertures are preferably aligned in one or several rows extending transversally across the width of the substrate, i.e. perpendicular to the motion direction of the image receiving substrate. According to such a method, each single aperture is utilized to address a specific dot position of the image in a transversal direction. Thus the transversal print addressability is limited by the density of apertures through the printhead structure. For instance, a print addressability of 300 dpi requires a printhead structure having 300 apertures per inch in a transversal direction. According to a preferred embodiment of the present invention, a direct electrostatic printing device includes a dot deflection control (DDC) . According to that embodiment, each single aperture is used to address several dot positions on an image receiving substrate by controlling not only the transport of toner particles through the aperture, but also their transport trajectory toward the image receiving substrate, and thereby the location of the obtained dot. The DDC method increases the print addressability without requiring a larger number of apertures in the printhead structure. This is achieved by providing the printhead structure with deflection electrodes connected to variable deflection voltages which, during each print cycle, sequentially modify the symmetry of the electrostatic control fields to deflect the modulated stream of toner particles in predetermined deflection directions. For instance, a DDC method performing three deflection steps per print cycle, provides a print addressability of 600 dpi utilizing a printhead structure having only 200 apertures per inch. According to a preferred embodiment, an improved DDC method provides a simultaneous dot size and dot position control. This method utilizes the deflection electrodes to influence the convergence of the modulated stream of toner particles thus controlling the dot size. Each aperture is surrounded by two deflection electrodes connected to respective deflection voltages DI, D2 , such that the electrostatic control field generated by the control electrode remains substantially symmetrical as long as both deflection voltages 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 medium, 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 order to clarify the method and device according to the invention, some examples of its use will now be described in connection with accompanying drawings. As shown in Fig.l, an image forming apparatus in accordance with a first embodiment of the present invention comprises at least one print station, preferably four print stations (Y, M, C, K) , an intermediate image receiving member 1, a driving roller 10, at least one support roller 11, and preferably several adjustable holding elements 12. The four print stations are arranged in relation to the intermediate image receiving member 1. The image receiving member, preferably a transfer belt 1 is mounted over the driving roller 10. The at least one support roller 11 is provided with a mechanism for maintaining the transfer belt 1 with a constant tension, while preventing transversal movement of the transfer belt 1. The holding elements 12 are for accurately positioning the transfer belt 1 with respect to each print station.

The driving roller 10 is preferably a cylindrical metallic sleeve having a rotation axis extending perpendicular to the motion direction of the belt 1 and a rotation velocity adjusted to convey the belt 1 at a velocity of one addressable dot location per print cycle, to provide line by line scan printing. The adjustable holding elements 12 are arranged for maintaining the surface of the belt at a predetermined gap distance from each print station. The holding elements 12 are preferably cylindrical sleeves disposed perpendicularly to the belt motion in an arcuated configuration so as to slightly bend the belt 1 at least in the vicinity of each print station in order to, in combination with the belt tension, create a stabilization force component on the belt. That stabilization force component is opposite in direction and preferably larger in magnitude than an electrostatic attraction force component acting on the belt 1 due to interaction with the different electric potentials applied on the corresponding print station. The transfer belt 1 is preferably an endless band of 30 to 200 microns thick composite material as a base. The base composite material can suitably include thermoplastic polyamide resin or any other suitable material having a high thermal resistance, such as 260°C of glass transition point and 388°C of melting point, and stable mechanical properties under temperatures in the order of 250°C. The composite material of the transfer belt has preferably a homogeneous concentration of filler material, such as carbon or the like, which provides a uniform electrical conductivity throughout the entire surface of the transfer belt 1. The outer surface of the transfer belt 1 is preferably coated with a 5 to 30 microns thick coating layer made of electrically conductive polymer material having appropriate conductivity, thermal resistance, adhesion properties, release properties and surface smoothness. The transfer belt 1 is conveyed past the four different print stations, whereas toner particles are deposited on the outer surface of the transfer belt and superposed to form a four color toner image . Toner images are then preferably conveyed through a fuser unit 13 comprising a fixing holder 14 arranged transversally in direct contact with the inner surface of the transfer belt. The fixing holder includes a heating element 15 preferably of a resistance type of e.g. molybdenium, maintained in contact with the inner surface of the transfer belt 1. As an electric current is passed through the heating element 15, the fixing holder 14 reaches a temperature required for melting the toner particles deposited on the outer surface of the transfer belt 1. The fusing unit 13 further includes a pressure roller 16 arranged transversally across the width of the transfer belt 1 and facing the fixing holder 14. An information carrier 2, such as a sheet of plain untreated paper or any other medium suitable for direct printing, is fed from a paper delivery unit 21 and conveyed between the pressure roller 16 and the transfer belt. The pressure roller 16 rotates with applied pressure to the heated surface of the fixing holder 14 whereby the melted toner particles are fused on the information carrier 2 to form a permanent image. After passage through the fusing unit 13, the transfer belt is brought in contact with a cleaning element 17, such as for example a replaceable scraper blade of fibrous material extending across the width of the transfer belt 1 for removing all untransferred toner particles from the outer surface.

As shown in Fig.2, a print station in an image forming apparatus in accordance with the present invention includes a particle delivery unit 3 preferably having a replaceable or refillable container 30 for holding toner particles, the container 30 having front and back walls (not shown) , a pair of side walls and a bottom wall having an elongated opening 31 extending from the front wall to the back wall and provided with a toner feeding element 32 disposed to continuously supply toner particles to a developer sleeve 33 through a particle charging member 34. The particle charging member 34 is preferably formed of a supply brush or a roller made of or coated with a fibrous, resilient material. The supply brush is brought into mechanical contact with the peripheral surface of the developer sleeve 33 for charging particles by contact charge exchange due to triboelectrification of the toner particles through frictional interaction between the fibrous material on the supply brush and any suitable coating material of the developer sleeve. The developer sleeve 33 is preferably made of metal coated with a conductive material, and preferably has a substantially cylindrical shape and a rotation axis extending parallel to the elongated opening 31 of the particle container 30. Charged toner particles are held to the surface of the developer sleeve 33 by electrostatic forces essentially proportional to (Q/D)², where Q is the particle charge and D is the distance between the particle charge center and the boundary of the developer sleeve 33. Alternatively, the charge unit may additionally include a charging voltage source (not shown) , which supply an electric field to induce or inject charge to the toner particles. Although it is preferred to charge particles through contact charge exchange, the method can be performed using any other suitable charge unit, such as a conventional charge injection unit, a charge induction unit or a corona charging unit, without departing from the scope of the present invention. A metering element 35 is positioned proximate to the developer sleeve 33 to adjust the concentration of toner particles on the peripheral surface of the developer sleeve 33, to form a relatively thin, uniform particle layer thereon. The metering element 35 may be formed of a flexible or rigid, insulating or metallic blade, roller or any other member suitable for providing a uniform particle layer thickness. The metering element 35 may also be connected to a metering voltage source (not shown) which influence the triboelectrification of the particle layer to ensure a uniform particle charge density on the surface of the developer sleeve. As shown in Fig.3, the developer sleeve 33 is arranged in relation with a positioning device 40 for accurately supporting and maintaining the printhead structure 5 in a predetermined position with respect to the peripheral surface of the developer sleeve 33. The positioning device 40 is formed of a frame 41 having a front portion, a back portion and two transversally extending side rulers 42, 43 disposed on each side of the developer sleeve 33 parallel with the rotation axis thereof. The first side ruler 42, positioned at a upstream side of the developer sleeve 33 with respect to its rotation direction, is provided with fastening means 44 to secure the printhead structure 5 along a transversal fastening axis extending across the entire width of the printhead structure 5. The second side ruler 43, positioned at a downstream side of the developer sleeve 33, is provided with a support element 45, or pivot, for supporting the printhead structure 5 in a predetermined position with respect to the peripheral surface of the developer sleeve 33. The support element 45 and the fastening axis are so positioned with respect to one another, that the printhead structure 5 is maintained in an arcuated shape along at least a part of its longitudinal extension. That arcuated shape has a curvature radius determined by the relative positions of the support element 45 and the fastening axis and dimensioned to maintain a part of the printhead structure 5 curved around a corresponding part of the peripheral surface of the developer sleeve 33. The support element 45 is arranged in contact with the printhead structure 5 at a fixed support location on its longitudinal axis so as to allow a slight variation of the printhead structure 5 position in both longitudinal and transversal direction about that fixed support location, in order to accommodate a possible excentricity or any other undesired variations of the developer sleeve 33. That is, the support element 45 is arranged to made the printhead structure 5 pivotable about a fixed point to ensure that the distance between the printhead structure 5 and the peripheral surface of the developer sleeve 33 remains constant along the whole transverse direction at every moment of the print process, regardless of undesired mechanical imperfections of the developer sleeve 33. The front and back portions of the positioning device 40 are provided with securing members 46 on which the toner delivery unit 3 is mounted in a fixed position to provide a constant distance between the rotation axis of the developer sleeve 33 and a transversal axis of the printhead structure 5. Preferably, the securing members 46 are arranged at the front and back ends of the developer sleeve 33 to accurately space the developer sleeve 33 from the corresponding holding element 12 of the transfer belt 1 facing the actual print station. The securing members 46 are preferably dimensioned to provide and maintain a parallel relation between the rotation axis of the developer sleeve 33 and a central transversal axis of the corresponding holding member 12. s shown in Fig.4a, 4b, 4c, a printhead structure 5 in an image forming apparatus in accordance with the present invention comprises a substrate 50 of flexible, electrically insulating material such as polyimide or the like, having a predetermined thickness, a first surface facing the developer sleeve, a second surface facing the transfer belt, a transversal axis 51 extending parallel to the rotation axis of the developer sleeve 33 across the whole print area, and a plurality of apertures 52 arranged through the substrate 50 from the first to the second surface thereof. The first surface of the substrate is coated with a first cover layer 501 of electrically insulating material, such as for example parylene. A first printed circuit, comprising a plurality of control electrodes 53 disposed in conjunction with the apertures, and, in some embodiments, shield electrode structures (not shown) arranged in conjunction with the control electrodes 53, is arranged between the substrate 50 and the first cover layer 501. The second surface of the substrate is coated with a second cover layer 502 of electrically insulating material, such as for example parylene. A second printed circuit, including a plurality of deflection electrodes 54, is arranged between the substrate 50 and the second cover layer 502. The printhead structure 5 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 502, facing the transfer belt 1. The printhead structure 5 is brought in cooperation with a control unit (not shown) comprising variable control voltage sources connected to the control electrodes 53 to supply control potentials which control the amount of toner particles to be transported through the corresponding aperture 52 during each print sequence. The control unit further comprises deflection voltage sources (not shown) connected to the deflection electrodes 54 to supply deflection voltage pulses which controls the convergence and the trajectory path of the toner particles allowed to pass through the corresponding apertures 52. The control unit, in some embodiments, even includes a shield voltage source (not shown) connected to the shield electrodes to supply a shield potential which electrostatically screens adjacent control electrodes 53 from one another, preventing electrical interaction therebetween. In a preferred embodiment of the invention, the substrate 50 is a flexible sheet of polyimide having a thickness on the order of about 50 microns. The first and second printed circuits are copper circuits of approximately 8-9 microns thickness etched onto the first and second surface of the substrate 50, respectively, using conventional etching techniques. The first and second cover layers (501, 502) are 5 to 10 microns thick parylene laminated onto the substrate 50 using vacuum deposition techniques. The apertures 52 are made through the printhead structure 5 using conventional laser micromachining methods. The apertures 52 have preferably a circular or elongated shape centered about a central axis, with a diameter in a range of 80 to 120 microns, alternatively a transversal minor diameter of about 80 microns and a longitudinal major diameter of about 120 microns . Although the apertures 52 have preferably a constant shape along their central axis, for example cylindrical apertures, it may be advantageous in some embodiments to provide apertures whose shape varies continuously or stepwise along the central axis, for example conical apertures .

In a preferred embodiment of the present invention, the printhead structure 5 is dimensioned to perform 600 dpi printing utilizing three deflection sequences in each print cycle, i.e. three dot locations are addressable through each aperture 52 of the printhead structure during each print cycle. Accordingly, one aperture 52 is provided for every third dot location in a transverse direction, that is, 200 equally spaced apertures per inch aligned parallel to the transversal axis 51 of the printhead structure 5. The apertures 52 are generally aligned in one or several rows, preferably in two parallel rows each comprising 100 apertures per inch. Hence, the aperture pitch, i.e. the distance between the central axes of two neighbouring apertures of a same row is 0,01 inch or about 254 microns. The aperture rows are preferably positioned on each side of the transversal axis 51 of the printhead structure 5 and transversally shifted with respect to each other such that all apertures are equally spaced in a transverse direction. The distance between the aperture rows is preferably chosen to correspond to a whole number of dot locations. The first printed circuit comprises control electrodes 53 each of which having a ring shaped structure surrounding the periphery of a corresponding aperture 52, and a connector preferably extending in the longitudinal direction, connecting the ring shaped structure to a corresponding control voltage source. Although a ring shaped structure is preferred, the control electrodes 53 may take on various shape for continuously or partly surrounding the apertures 52, preferably shapes having symmetry about the central axis of the apertures. In some embodiments, particularly when the apertures 52 are aligned in one single row, the control electrodes are advantageously made smaller in a transverse direction than in a longitudinal direction.

The second printed circuit comprises a plurality of deflection electrodes 54, each of which is divided into two semicircular or crescent shaped deflection segments 541, 542 spaced around a predetermined portion of the circumference of a corresponding aperture 52. The deflection segments 541, 542 are arranged symmetrically about the central axis of the aperture 52 on each side of a deflection axis 543 extending through the center of the aperture 52 at a predetermined deflection angle d to the longitudinal direction. The deflection axis 543 is dimensioned in accordance with the number of deflection sequences to be performed in each print cycle in order to neutralize the effects of the belt motion during the print cycle, to obtain transversally aligned dot positions on the transfer belt. For instance, when using three deflection sequences, an appropriate deflection angle is chosen to arctan(l/3), i.e. about 18,4°. Accordingly, the first dot is deflected slightly upstream with respect to the belt motion, the second dot is undeflected and the third dot is deflected slightly downstream with respect to the belt motion, thereby obtaining a transversal alignment of the printed dots on the transfer belt. Accordingly, each deflection electrode 54 has a upstream segment 541 and a downstream segment 542, all upstream segments 541 being connected to a first deflection voltage source Dl , and all downstream segments 542 being connected to a second deflection voltage source D2. Three deflection sequences (for instance: Dl<D2 ; Dl=D2; Dl>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 52, thus the dot position on the toner image.

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

As shown in Fig.5a or 5b, a cleaning unit 6 in accordance with a preferred embodiment of the present invention includes an elongated nozzle 60 extending transversally across the print zone and positioned between the transfer belt 1 and the printhead structure 5. The elongated nozzle 60 is connected to a pressure changing element 61, preferably a membrane or other vibration generating element, for instance of the type used in conventional loudspeaker. The pressure changing element 61 generates a pressure wave, i.e. a prompt pressure difference, transferred through the nozzle 60 to produce a suction force in the air gap between the printhead structure 5 and the transfer belt 1. As shown in Fig.10, the air pressure in the vicinity of the apertures 52 is changed from a initial, ambient pressure P₀ to a cleaning pressure P_cl., 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 air gap through the nozzle 60. The pressure wave applies suction forces on residual toner. In a preferred embodiment of the invention, shown in Fig.5a, the substrate 50 of the printhead structure 5 is bent upwardly around the peripheral surface of the developer sleeve 33 and the transfer belt 1 is bent downwardly against the corresponding holding element 12. As a result, the substrate 50 and the transfer belt 1 are positioned adjacent to each other at a vicinity of the apertures 52 and their relative distance increases on both side of the developer sleeve 33, forming a wedge- shaped space between the substrate 50 and the belt 1. That wedge-shaped space is used as a channel through which the pressure wave is transferred from the print zone toward the pressure changing element 61. The cleaning unit 6 is preferably arranged on the downstream side of the developer sleeve 33 with respect to its rotation direction. The cleaning unit 6 comprises a chamber 62 having front and back walls, a first transversal wall 621 in conjunction to the substrate 50 and a second transversal wall 622 in conjunction to the transfer belt 1. The pressure difference is produced by a prompt volume variation in the chamber 62, for example created by a membrane, or any other appropriate movable element, such as a fan, bellows, piston or the like, suitable to create pressure changes . Fig. 6 to 9 illustrate different embodiments of the pressure changing element. The common feature of those embodiments is that they enable a sufficiently prompt volume variation in the chamber 62. This can be achieved by a translation of a movable free element (piston) such as that shown in Fig.6, a deformation of a fixed flexible element (membrane) such as that shown in Fig.7, a translation of a fixed rigid element (bellow) , such as that shown in Fig.8, or a rotation of an element having a cavity (pump), such as that shown in Fig.9.

In a preferred embodiment, shown in Fig.5a or 5b, the pressure changing element 61 is a diaphragm of a type conventionally utilized in loudspeakers. A small coil 63 is fixed at the center of the diaphragm 61 that is free to move in an annular gap 64. A magnetic field produced either by a magnet or an electromagnet (not shown) is applied across the gap 64. As a cleaning signal is input to the coil 63 as alternating current, causing it to move in the magnetic field as a result of electromagnetic induction. The diaphragm 61 is thus caused to vibrate at the same frequency as the cleaning pulse, and an air pressure wave is produced by the diaphragm 61. Preferably, the coil 63 is brought in cooperation with a conical diaphragm supported round its edges by a metal frame 65, and the coil 63 is maintained in position at the center of the annular gap by thin flexible supports 66.

In the embodiment shown in Fig.5b, , the chamber 62 is formed between a first transversal wall 621 positioned on the surface of the transfer belt that is facing away from the developer sleeve, and a second transversal wall 622 positioned parallel to the first surface of the substrate 50, so as to ensure no pressure difference between the first and second surface of the substrate 50. The first and second transversal walls 621, 622 extend at an appropriate angle to each other so as to form a wedge- shaped cavity having its narrowest part disposed in the vicinity of the apertures 52. The first transversal wall 621 can be used as a holding element for maintaining the transfer belt 1. The second transversal wall 622 extends at a small distance from the first surface of the substrate 50 for allowing a small air channel therebetween .

In some embodiments of the invention (not shown) , a single cleaning unit, common for all print stations, is connected to several channels, which can be sequentially selected. In other embodiments, each print station is provided with a corresponding cleaning unit.

After being dislodged from a vicinity of the apertures, residual toner particles are transported away from the printhead structure. The transport of residual toner is directed toward the chamber 62, when an underpressure wave is produced (suction forces), or away from the nozzle 60, when an overpressure is produced (blowing forces). In some embodiments, a filter (not shown) is provided in the nozzle 60 or in the chamber 62, in order to collect transported residual toner. The suction forces applied on residual toner are sufficiently high for allowing toner particles to pass through the filter. In some embodiments of the invention, the chamber 62 is provided with a filter for collecting the dislodged residual toner particles. In another embodiments (not shown) , an electric potential is produced to attract transported residual toner to an appropriate area on the second surface of the substrate. Thereafter, the collected residual toner are propelled onto the transfer belt by a sequentially applied electric field.

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

Claims

What is claimed is;

1. An image forming apparatus in which an image information is converted into a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier toward a back electrode member, said image forming apparatus including: a background voltage source for producing a background electric field which enables a transport of charged toner particles from said particle carrier towards said back electrode member; a printhead structure arranged in said background electric field, including a plurality of apertures and control electrodes arranged in conjunction to the apertures; control voltage sources for supplying control potentials to said control electrodes in accordance with the image information to selectively permit or restrict the transport of charged toner particles from the particle carrier through the apertures ; an image receiving member caused to move in relation to the printhead structure for intercepting the transported charged particles in image configuration; characterized in that : the image forming apparatus further includes a cleaning unit, which, after image formation, generates a pressure wave for dislodging residual toner particles from a vicinity of the apertures .

2. An image forming apparatus as defined in claim 1, in which the cleaning unit includes a hollow part having at least one opening, and a movable element arranged in the hollow part, characterized in that the movable element can be activated to produce a volume variation in the hollow part, thereby generating a pressure wave through the opening, for dislodging residual toner particles from a vicinity of the apertures.

3. An image forming apparatus as defined in claim 2, characterized in that the movable element can be activated to produce a volume expansion in the hollow part, thereby generating a pressure wave propagating toward the opening, said pressure wave applying suction forces for dislodging residual particles from a vicinity of the apertures.

4. An image forming apparatus as defined in claim 2, characterized in that the movable element can be activated to produce a volume contraction in the hollow part, thereby generating a pressure wave propagating away from the opening, said pressure wave applying blowing forces for dislodging residual toner particles from a vicinity of the apertures.

5. An image forming apparatus as defined in any one of claims 2 - 4, characterized in that the opening is arranged in the vicinity of the printhead structure.

6. An image forming apparatus as defined in any one of claims 2 - 5, characterized in that the opening is arranged between the printhead structure and the image receiving member in a direction substantially perpendicular to the motion of the image receiving member .

7. An image forming apparatus as defined in any one of claims 2 - 5, characterized in that the opening is arranged between the particle carrier and the image receiving member in a direction substantially perpendicular to the motion of the image receiving member .

8. An image forming apparatus as defined in any one of claims 2 - 7, in which the printhead structure includes a substrate having a first surface facing the particle carrier, a second surface facing the image receiving member, and a plurality of apertures arranged through the substrate, characterized in that the opening is arranged near said second surface in a vicinity of said apertures.

8. An image forming apparatus as defined in claim 8 in which the apertures are arranged in at least one row extending in a direction substantially perpendicular to the motion of the image receiving member across a width of the substrate, characterized in that the opening extends substantially parallel to said row of apertures.

9. An image recording apparatus as defined in any one of claims 2 - 4, characterized in that the movable element is a flexible partition, preferably a membrane, activated by a vibration generating element .

10. An image recording apparatus as defined in any one of claims 2 - 4, characterized in that the movable element is a rigid partition, activated by a translation in a direction away from or toward the opening.

11. An image recording apparatus as defined in any one of claims 2 - 4, characterized in that the movable element is a rotating member having a cavity.

12. An image recording apparatus as defined in claim 2, in which the volume variation in the hollow part has a duration in the range of X to Y microseconds

13. An image forming apparatus in which an image receiving member is caused to move in relation to at least one print unit, said print unit including a substrate of electrically insulating material having a first surface facing a particle carrier, a second surface facing the image receiving medium and a plurality of apertures through which charged toner particles are selectively transported from the particle carrier to the image receiving member during image formation characterized in that said image forming apparatus further including a cleaning unit having at least one opening arranged in conjunction to said second surface of the substrate, and pressure changing means for producing a pressure wave which dislodges residual toner particles from the substrate after image formation.

14. An image forming apparatus as defined in claim 13, having several print units and a cleaning unit common to said several print units, characterized in that the cleaning unit has several openings, each of which is arranged in conjunction to said second surface of the substrate of a corresponding print unit.

15. An image forming apparatus as defined in claim 13, having several cleaning units, characterized in that each print units cooperates with a corresponding cleaning unit.

16. An image forming apparatus as defined in claim 13, in which an air gap is provided between said second surface of the substrate and the image receiving member, characterized in that the cleaning unit comprises pressure changing means connected to said opening for changing the air pressure in said air gap from an ambient pressure to a predetermined cleaning pressure.

17. An image forming apparatus as defined in claim 16, characterized in that said cleaning pressure is lower than said ambient pressure, thereby producing a pressure wave propagating toward the opening.

18. An image forming apparatus as defined in claims 16, characterized in that the air pressure difference from said ambient pressure to said cleaning pressure is produced during a pulse period shorter than 20 ms , preferably shorter than 10 ms .

19. An image forming apparatus as defined in claim 1, in which the printhead structure further includes deflection electrodes for controlling the trajectory of toner particles.