BACKGROUND TO 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.

In direct electrostatic printing methods a plurality of apertures, each surrounded by a control electrode, are preferably arranged in parallel rows extending transversally across the print zone, i. e. substantially perpendicular to the motion of the image receiving medium. As a pixel position on the image receiving medium passes beneath a corresponding aperture, the control electrode associated with this aperture is set on a print potential allowing the transport of toner particles through the aperture to form a toner dot at that pixel position.

Accordingly, transverse image lines can be printed by simultaneously activating several apertures of the same aperture row, and longitudinal image lines can be printed by sequentially activating at least one aperture when pixel positions in question passes beneath the at least one aperture.

Improvements to direct electrostatic printing methods have been made, for example dot deflection control (DDC), as is disclosed in U. S. Patent No.

5,847,733. According to the DDC method, each single aperture is used to address several dot positions on an information carrier by controlling not only the transport of toner particles through the aperture, but also their transport trajectory toward a paper, 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.

However, it can be considered a drawback of current direct electrostatic printing methods that it is difficult to print at a high speed and maintain a high printing quality. Therefore, there seems to still exist a need to improve the current direct electrostatic printing methods to attain higher printing speeds.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method of and a device for improving direct electrostatic printing methods.

Another object of the present invention is to provide a method of and device for improving the printing speed without a degradation of print quality.

A further object of the present invention is to provide a method of and device for reducing cleaning needs of a printhead structure thereby enabling higher printing speed.

A still further object of the present invention is to provide a method of and device for reducing an elongation of toner jets thereby enabling higher printing speed.

Said objects are achieved according to the invention by providing a direct electrostatic printing device and method for printing an image with improved tonerjet transport field control. It has been discovered that during print pulses unwanted electrical fields appear within the apertures, influencing a tonerjet transport field within the apertures. A tonerjet transport field is advantageously a combination of a background electrical field and a field created by the print pulse. According to the invention, the unwanted electrical fields within apertures, such as deflection fields, during at least a part of the print pulses, are controlled by controlling control voltages of electrodes, such as deflection electrodes, that create these unwanted electrical fields, to be at least substantially field neutral to the tonerjet transport field during at least a part of a print pulse.

The invention temporarily modifies control voltages of electrodes that generate unwanted electrical fields within an aperture during a print pulse time frame.

This will lessen the risk of aperture clogging, therefore lowering the need to clean, whereby the any necessary cleaning can be accomplished faster and thereby enabling a faster printing. Further this will also result in a more coherent, i. e. better held together, tonerjet, which in turn enables a tonerjet to reach an image receiving surface faster, thereby enabling a faster printing with still enough time for a tonerjet to reach an image receiving surface.

Said objects are also achieved according to the invention by a direct electrostatic printing device including a toner particle delivery, an electrical field creator, a printhead structure, and a control unit. The toner particle delivery providing toner particles. An image receiving surface and the printhead structure are arranged for relative movement between each other during printing. The electrical field creator creating an electrical field between the toner particle delivery and the image receiving surface for transport of toner particles from the toner particle

delivery toward the image receiving surface. The printhead structure being placed in the electrical field in between the toner particle delivery and the image receiving surface. The printhead structure including control electrodes connected to the control unit to thereby selectively open by a print pulse or close apertures through the printhead structure to permit or restrict the transport of toner particles during a print sequence in the form of toner jets. At least one print sequence is included in a print cycle. Thereby the formation of an image on the image receiving surface is enabled. According to the invention one or more control voltages, of one or more corresponding field generating sources, are controlled by the control unit to at least substantially, by a field or fields generated by the one or more field generating sources during at least a part of a print pulse time frame, attain a field neutral or synergetic influence on a tonerjet transport field within an aperture in question.

The tonerjet transport field is either the electrical field between the toner particle delivery and the image receiving surface, or an electrical field created by the control electrodes within a corresponding aperture during a tonerjet transport permission, or a combined electrical field within an aperture, the combined field comprising the electrical field between the toner particle delivery and the image receiving surface and an electrical field created by the control electrodes during a tonerjet transport permission.

In some embodiments the one or more control voltages of the one or more corresponding field generating sources are controlled such that a substantially field neutral field on the tonerjet transport field within an aperture in question is attained during at least a part of a print pulse time frame. In other embodiments the one or more control voltages of the one or more corresponding field generating

sources are controlled such that a with a print pulse synergetic field in an aperture in question is attained during at least a part of a print pulse time frame.

Advantageously the one or more control voltages are controlled by the control unit to at least partly follow a rise and fall of a print pulse during at least a part of the print pulse time frame.

Preferably the one or more control voltages of the one or more field generating sources are controlled by the control unit to at least substantially attain a field neutral or synergetic influence on the tonerjet transport field in the form of a respective pulse.

In some embodiments the control unit controls at least one of the one or more control voltages individually to at least substantially attain a field neutral or synergetic influence on the tonerjet transport field, for each individual aperture during each print sequence in dependence of if there is a corresponding print pulse or not. In other embodiments the control unit controls at least one of the one or more control voltages of the one or more field generating sources to at least substantially attain a field neutral or synergetic influence on the tonerjet transport field, for each aperture in the same manner irrespective if there is a print pulse or not during a print sequence.

In some embodiments the printhead structure further includes deflection electrodes connected to the control unit for controlling toner jets in transport by means of predetermined voltages. Preferably at least two print sequences are included in a print cycle, and where the deflection electrodes connected to the control unit are for controlling the deflection of toner jets in transport by means of predetermined deflection voltages to thereby be able to deflect toner jets

against predetermined locations, each aperture in question being arranged for placing toner jets at different dot positions in a direction substantially perpendicular to the relative movement between the printhead structure and the image receiving member during each print sequence, to thereby enable the formation of a pigment image on the first face of the image receiving member.

Preferably also the deflection electrodes connected to the control unit control the focusing of toner jets in transport by means of predetermined focusing voltages to thereby be able to focus toner jets. Advantageously two control voltages are connected to the deflection electrodes and in that at least two of the one or more control voltages that are controlled by the control unit to at least substantially attain a field neutral or synergetic influence on the tonerjet transport field, are the control voltages of the deflection electrodes. Preferably then the two control voltages connected to the deflection electrodes are controlled by the control unit to at least substantially attain a field neutral or synergetic influence on the tonerjet transport field by a same voltage potential, or then the two control voltages connected to the deflection electrodes are controlled by the control unit to at least substantially attain a field neutral or synergetic influence on the tonerjet transport field by raising or lowering the voltage potentials of each deflection electrode by a same vlaue.

In other embodiments the printhead structure further comprises one or more shield electrodes, one control voltage is connected to the one or more shield electrodes and in that at least one of the one or more control voltages that are controlled by the control unit, is the control voltage of the one or more shield electrodes. Then preferably the one or more shield electrodes is a single shield electrode covering an area of the printhead structure between the apertures, or the one or more shield electrodes are ring electrodes surrounding each aperture, or the

one or more shield electrodes are electrodes which surround at least a part of each aperture.

Advantageously either the one or more control voltages are controlled by the control unit to attain a potential level to at least substantially attain a field neutral or synergetic influence, at substantially the time a corresponding print pulse starts, or the one or more control voltages are controlled by the control unit to attain a potential level to at least substantially attain a field neutral or synergetic influence, at a time before a corresponding print pulse starts, or the one or more control voltages are controlled by the control unit to attain a potential level to at least substantially attain a field neutral or synergetic influence, at a time after a corresponding print pulse starts.

Preferably either the one or more control voltages are controlled by the control unit to terminate a potential level to at least substantially attain a field neutral or synergetic influence, at substantially the time a corresponding print pulse ends, or the one or more control voltages are controlled by the control unit to terminate a potential level to at least substantially attain a field neutral or synergetic influence, at a time before a corresponding print pulse ends, or the one or more control voltages are controlled by the control unit to terminate a potential level to at least substantially attain a field neutral or synergetic influence, at a time after a corresponding print pulse ends.

Advantageously the one or more control voltages are controlled by the control unit to at least substantially attain a field neutral or synergetic influence only in a direction at least substantially parallel to the electrical field between the toner particle delivery and the image receiving surface for transport of toner particles from the toner particle delivery toward the image receiving surface.

Preferably the one or more control voltages are controlled by the control unit to either at least substantially attain a same voltage as a print pulse attains during at least a part of a print pulse time frame, or attain a same voltage as a print pulse attains during at least a part of a print pulse time frame, or at least substantially attain a greater voltage than a print pulse attains during at least a part of a print pulse time frame, or at least substantially attain a voltage not more than 20 volts below the voltage a print pulse attains during at least a part of a print pulse time frame. These voltages are preferably used in a device where negatively charged toner is used, a back electrode is more positve than a particle delivery unit, the control voltages are controlling electrodes that are closer to the back electrode than the control electrodes. Then advantageously the voltage a print pulse attains during at least a part of a print pulse time frame, is either a maximum voltage that the print pulse attains, or a voltage that the print pulse attains during a larger part of a print pulse time frame, or a mean voltage that the print pulse attains during a print pulse time frame.

Preferably the device further comprises a back electrode, and where the image receiving surface is a first face of the back electrode, the image subsequently being transferred from the back electrode to an information carrier, or the image receiving surface is a first face of an information carrier which also acts as a back electrode, or the device further comprises an intermediate image receiving member and a back electrode, and where the image receiving surface is a first face of the intermediate image receiving member, and the back electrode being located facing a second face of the intermediate image receiving member, the image subsequently being transferred from the intermediate image receiving member to an information carrier, or the device further comprises a back electrode, and where the image receiving surface is a first face of an information carrier, and the back electrode

being located facing a second face of the information carrier. Then preferably the electrical field creator comprises a voltage source connected to the toner particle delivery and the back electrode.

Further variations of the device according to the invention will be described below. The described enhancements are possible to mix arbitrarily in view of a desired specific embodiment.

Said objects are also achieved according to the invention by a method for printing an image to an information carrier. The method comprises a number of steps. In a first step toner particles are provided from a toner particle delivery. In a second step an image receiving surface and a printhead structure are moved relative to each other during printing. In a third step an electrical field is created for transporting toner particles from the toner particle delivery toward the image receiving surface, the printhead structure being placed in the electrical field in between the toner particle delivery and the image receiving surface. In a fourth step apertures through a printhead structure are selectively opened by a print pulse or closed to permit or restrict the transporting of toner particles during a print sequence in the form of toner jets, at least one print sequence is included in a print cycle, to thereby enable the formation of an image on the image receiving surface. And in a final fifth step controlling one or more control voltages of one or more corresponding field generating sources to at least substantially attain a field neutral or synergetic influence on a tonerjet transport field within an aperture in question by a field or fields generated by the one or more field generating sources during at least a part of a print pulse time frame.

The tonerjet transport field is preferably either the electrical field between the toner particle delivery and the image receiving surface, or an electrical field created

by the control electrodes within a corresponding aperture during a tonerjet transport permission, or a combined electrical field within an aperture, the combined field comprising the electrical field between the toner particle delivery and the image receiving surface and an electrical field created by the control electrodes during a tonerjet transport permission.

Advantageously in the step of controlling one or more control voltages, either controlling the one or more control voltages of the one or more corresponding field generating sources such that a substantially field neutral field on the tonerjet transport field within an aperture in question is attained during at least a part of a print pulse time frame, or controlling the one or more control voltages of the one or more corresponding field generating sources such that a with a print pulse synergetic field in an aperture in question is attained during at least a part of a print pulse time frame.

Preferably in some versions of the method in the step of controlling one or more control voltages, the one or more control voltages are controlled by the control unit to at least partly follow a rise and fall of a print pulse during at least a part of the print pulse time frame.

In some versions in the step of controlling one or more control voltages, controlling the one or more control voltages of the one or more field generating sources to at least substantially attain a field neutral or synergetic influence on the tonerjet transport field in the form of a respective pulse.

Preferably in some versions in the step of controlling one or more control voltages, controlling at least one of the one or more control voltages individually to at least substantially attain a field neutral or synergetic influence on the

tonerjet transport field, for each individual aperture during each print sequence in dependence of if there is a corresponding print pulse or not. In other versions in the step of controlling one or more control voltages, controlling at least one of the one or more control voltages of the one or more field generating sources to at least substantially attain a field neutral or synergetic influence on the tonerjet transport field, for each aperture in the same manner irrespective if there is a print pulse or not during a print sequence.

Advantageously in the step of controlling one or more control voltages, controlling the one or more control voltages to attain a potential level to at least substantially attain a field neutral or synergetic influence either, at substantially the time a corresponding print pulse starts, or at a time before a corresponding print pulse starts, or at a time after a corresponding print pulse starts.

Advantageously in the step of controlling one or more control voltages, controlling the one or more control voltages to terminate a potential level to at least substantially attain a field neutral or synergetic influence either, at substantially the time a corresponding print pulse ends, or at a time before a corresponding print pulse ends, or at a time after a corresponding print pulse ends.

Preferably in the step of controlling one or more control voltages, controlling the one or more control voltages to at least substantially attain a field neutral or synergetic influence only in a direction at least substantially parallel to the electrical field between the toner particle delivery and the image receiving surface for transport of toner particles from the toner particle delivery toward the image receiving surface.

Advantageously in the step of controlling one or more control voltages either, controlling the one or more control voltages to at least substantially attain a same voltage as a print pulse attains during at least a part of a print pulse time frame, or controlling the one or more control voltages to attain a same voltage as a print pulse attains during at least a part of a print pulse time frame, or controlling the one or more control voltages to at least substantially attain a greater voltage than a print pulse attains during at least a part of a print pulse time frame, or controlling the one or more control voltages to at least substantially attain a voltage not more than 20 volts below the voltage a print pulse attains during at least a part of a print pulse time frame. Advantageously the voltage a print pulse attains during at least a part of a print pulse time frame is either, a maximum voltage that the print pulse attains, or a voltage that the print pulse attains during a larger part of a print pulse time frame, or a mean voltage that the print pulse attains during a print pulse time frame.

Further variations of the method according to the invention is described above and others will be described below. The enhancements are possible to mix arbitrarily in view of the desired specific version.

Said objects are also achieved according to the invention by a method for printing an image to an information carrier. The method comprises a number of steps. In a first step toner particles are provided from a toner particle delivery. In a second step an image receiving surface and a printhead structure are moved relative to each other during printing. In a third step an electrical field is created for transporting toner particles from the toner particle delivery toward the image receiving surface, the printhead structure being placed in the electrical field in between the toner particle delivery and the image receiving surface. In a fourth step apertures through a printhead structure are selectively opened by a print

pulse or closed to permit or restrict the transporting of toner particles during a print sequence in the form of toner jets, at least one print sequence is included in a print cycle, to thereby enable the formation of an image on the image receiving surface. And in a final fifth step temporarily modifying control voltages of electrodes that generate an unwanted electrical field within an aperture during at least a part of a print pulse time frame such that the unwanted electrical field at least substantially attains a field neutral or synergetic influence on a tonerjet transport field within an aperture in question during at least a part of a print pulse time frame.

Further variations of the method according to the invention is described above and others will be described below. The enhancements are possible to mix arbitrarily in view of the desired specific version.

The present invention satisfies within an aperture a tonerjet transport field control not previously met, by actively controlling unwanted electrical fields within the aperture during at least a part of a print pulse.

The present invention relates to an image recording apparatus including an image receiving surface conveyed past one or more, so called, print stations to intercept a modulated stream of toner particles from each print station. A print station includes a particle delivery unit, a particle source, and a printhead structure arranged between the particle source and the image receiving member. The printhead structure includes means for modulating the stream of toner particles from the particle source and possibly means for controlling the trajectory of the modulated stream of toner particles toward the image receiving member.

According to a preferred embodiment of the present invention, the image recording apparatus comprises four print stations, each corresponding to a

pigment color, e. g. yellow, magenta, cyan, black (Y, M, C, K), disposed adjacent to an intermediate image receiving surface for example formed of either a seamless transfer belt made of a substantially uniformly thick, flexible material having high thermal resistance, high mechanical strength and stable electrical properties under a wide temperature range, of an information carrier such as paper, or of a drum.

The toner image is formed on the image receiving surface according to the invention and thereafter, if it is not an information carrier, brought into contact with an information carrier, e. g. paper, in a fuser unit, where the toner image is transferred to and made permanent on the information carrier, preferably by means of heat and pressure. After image transfer, the image receiving surface is brought in contact with a cleaning unit removing untransferred toner particles.

Other objects, features and advantages of the present inventions will become more apparent from the following description when read in conjunction with the accompanying drawings in which preferred embodiments of the invention are shown by way of illustrative examples.

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 section view across a print zone of a prior art image recording apparatus,

Fig. 2 is a schematic section view across a print zone of an image recording apparatus according to a preferred embodiment of the invention, Fig. 3 illustrates a printpulse, Fig. 4A illustrates a first example according to the invention of a waveform on one or more electrodes which creates an electrical field in one or more apertures, Fig. 4B illustrates a second example according to the invention of a waveform on one or more electrodes which creates an electrical field in one or more apertures, Fig. 4C illustrates a third example according to the invention of a waveform on one or more electrodes which creates an electrical field in one or more apertures, Fig. 4D illustrates a fourth example according to the invention of a waveform on one or more electrodes which creates an electrical field in one or more apertures, Fig. 4E illustrates a fifth example according to the invention of a waveform on one or more electrodes which creates an electrical field in one or more apertures, Fig. 5A illustrates a waveform on a first deflection electrode,

Fig. 5B illustrates a waveform on a second deflection electrode, Fig. 6A illustrates a first example according to the invention of a waveform on a first deflection electrode, Fig. 6B illustrates a first example according to the invention of a waveform on a second deflection electrode, Fig. 7A illustrates a second example according to the invention of a waveform on a first deflection electrode, Fig. 7B illustrates a second example according to the invention of a waveform on a second deflection electrode, Fig. 8 illustrates a control unit, Fig. 9 illustrates a high voltage control electrode driver, DESCRIPTION OF PREFERRED EMBODIMENTS An image recording apparatus according to the invention, comprises at least one print station, preferably four print stations (Y, M, C, K). The four print stations (Y, M, C, K) are arranged in relation to an image receiving surface, preferably an intermediate image receiving member. An intermediate image receiving member can either be a transfer belt mounted over driving rollers, or a drum. In other embodiments toner particles are deposited directly onto an information carrier without first being deposited onto an intermediate image receiving member. The image receiving surface and a print station move relative to each other at a velocity of one addressable dot location per print cycle, to provide line by line

scan printing. Each print station comprises a printhead structure that has a plurality of apertures extending perpendicular to the relative motion. A transfer belt is conveyed, or a drum is rotated, past the four different print stations (Y, M, C, K), whereby toner particles are deposited on the image receiving surface and superposed to form a toner image.

Also a so-called multi-pass technique can be utilized. When utilizing such a technique, the image forming apparatus is provided with moving means causing the image receiving surface and the printhead structure to move in relation to each other. i. e. the image receiving surface or the printhead structure, or both, are movable. Thereby, the relative movement is so arranged that each line on the image-receiving surface that is transverse to the direction of the relative movement passes the printhead structure at least twice in order to form an image.

Accordingly, the printhead structure prints only a part of each transverse line on each pass. When utilizing multi-pass technique, the moving means further includes means to move the printhead structure and the image receiving surface relative to each other (i. e. the printhead structure or the image receiving surface, or both, are moved) between consecutive passes or during a pass, so that each time that the image receiving surface passes the printhead structure, different parts of the image receiving surface are positioned to receive charged toner particles. The multi-pass technique increases the print addressability without requiring a larger number of apertures in the printhead structure which, if desired, can eliminate the need for special deflection electrodes or the like.

In many cases, the multi-pass technique results in an improved resolution and print quality in comparison to when an image is printed in one single printing pass.

Multi-interlacing (MIC) is a further developed technique where an image forming apparatus utilizing multi-pass techniques is so constructed and arranged that adjacent columns of print are not printed by the same aperture in different passes. When utilizing MIC-technique, columns of print from different passes (partial images) are"interlaced"with each order in order to form the completed image. It has been found that MIC-techniques can improve print resolution and print quality even further in comparison to conventional multi-pass techniques.

Also dot deflection techniques (DDC) can be utilized in an image forming apparatus of the types in question. According to the DDC-method, each single aperture is used to address several dot positions on an image receiving surface by controlling not only the transport of toner particles through the aperture, but also their transport trajectory towards the image receiving surface, and thereby the location of the obtained dot. The DDC-method increases the print addressability without requiring a larger number of apertures in the printhead structure.

By means of utilizing a combination of multipass-, MIC-and/or DDC- techniques, the print addressability/number of apertures, and the print resolution can be optimized.

In some embodiments the transfer belt or drum can comprise at least one separate image area and at least one of a cleaning area and/or a test area. The image area being intended for the deposition of toner particles, the cleaning area being intended for enabling the removal of unwanted toner particles from around each of the print stations, and the test area being intended for receiving test patterns of toner particles for calibration purposes. The transfer belt or drum can also in certain embodiments comprise a special registration area for use of determining the position of the transfer belt or drum, especially an image area if available, in

relation to each print station. If the transfer belt or drum comprises a special registration area then this area is preferably at least spatially related to an image area.

Each print station comprises a particle delivery unit. The particle delivery unit preferably has a replaceable or refillable container for holding toner particles which is disposed to continuously supply toner particles to a surface of a particle carrier through a particle charging member. Toner particles are retained on the surface of the particle carrier by an adhesion force which essentially is related to the particle charge and to the distance between the particle and the surface of the particle carrier. The electrostatic field applied onto a control electrode to initiate toner transport through a selected aperture is selected to be sufficient to overcome the adhesion force in order to cause the release of an appropriate amount of toner particles from the particle carrier. The electrostatic field is applied during the time period required for these released particles to reach sufficient momentum to pass through the selected aperture, whereafter the transported toner particles are exposed to the attraction force from the back electrode and are intercepted by the image receiving surface.

Properties such as charge amount, charge distribution, particle diameter etc. of the individual toner particles have been found to be of particularly great importance to the print performance in a direct printing method. Accordingly, the size and size distribution of the toner particles affect the printing result, since larger toner particles have a tendency to cause clogging of the apertures in the control electrode array. In addition, the toner particles allowed to pass through selected "opened"apertures are accelerated towards the image receiving under the influence of a uniform attraction field from the back electrode. In order to control the distribution of the transported particles onto a printing surface, the particles

may be deflected by the application of a deflection pulse, resulting in an increase in the addressable area on the image receiving surface. Thereby, small particles having a low surface charge exhibit poor deflection properties.

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

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

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

multi-component type.

Advantageously, the toner particles have an irregular surface structure and an average diameter within the range of 3-8 microns. Depending on the application in question, electrically conductive, electrically non-conductive, or magnetic toner particles can be provided and utilized.

The printhead structure is preferably formed of an electrically insulating substrate layer made of flexible, non-rigid material such as polyamide or the like.

The printhead structure is positioned between a peripheral surface of a particle carrier and a bottom portion of a support device. The substrate layer has a top surface facing a toner layer on the peripheral surface of the particle carrier, and a bottom surface facing the bottom portion of the support device. Further, the substrate layer has a plurality of apertures arranged through the substrate layer in a part of the substrate layer overlying a elongated slot in the bottom portion of the support device. The printhead structure further preferably includes a first printed circuit arranged on the top surface on the substrate layer. The first printed circuit includes a plurality of control electrodes, each of which, at least partially, surrounds a corresponding aperture in the substrate layer.

According to some embodiments of the invention printing is performed in print cycles having three subsequent print sequences for addressing three different dot locations through each aperture, i. e. a dot location is addressed during each print sequence. Each print sequence comprises a print period tb followed by a recovering period tw during which new toner is supplied to the print zone. In other embodiments each print cycle can suitably have fewer or more addressable dot locations for each aperture. In still further embodiments each print cycle has a controllable number of addressable dot locations for each aperture. During the

whole print cycle an electric background field is produced between a first potential on a surface of a particle carrier and a second potential on a back electrode, to enable the transport of toner particles between the particle carrier and an image receiving surface. During each print sequence, control voltages are applied to control electrodes to produce a pattern of electrostatic control fields which due to control in accordance with the image information, selectively open or close the apertures by influencing the electric background field, thereby enhancing or inhibiting the transport of toner through the printhead structure.

The toner particles allowed to pass through the opened apertures are then transported toward their intended dot location. The control voltage pulse (control) can be amplitude and/or pulse width modulated, to allow the intended amount of toner particles to be transported through the aperture. For instance, the amplitude of the control voltage varies between a non-print level Vw of approximately-50V and a print level Vb in the order of +350V, corresponding to full density dots. Similarly, the pulse width can be varied from 0 to tb.

The back electrode member or members utilized in an image forming apparatus can be of a number of different types, e. g. a stationary or rotating roll or sleeve, or a movable belt arranged in an endless loop by means of guide rolls. Depending on the application, the back electrode member can be made of different materials, e. g. a suitable metal alloy or another electrically conductive material. Furthermore, a back electrode member can be arranged behind a belt constituting an intermediate image receiving member.

It is also conceivable with embodiments where a suitable information carrier, such as a printing paper, passes across the back electrode when printing so that an image is printed directly onto the information carrier, or where the information

carrier also constitutes the back electrode by means of being electrically conductive.

In other applications, an intermediate image is formed directly onto the surface of the back electrode member, whereafter the image is transferred to a suitable image receiving substrate such as a printing paper. It is particularly advantageous to print directly onto the back electrode in applications utilizing so-called multi- interlacing (MIC) techniques.

Furthermore, it is conceivable with applications where the electrical field, by means of which the toner particles are transported, is generated by another means than a pair of electrodes, e. g. applications where the electrical field is generated by means of a suitable charge carrier which in itself is able to generate an electrostatic field.

In order to clarify the method and device according to the invention, some examples will now be described in connection with Figures 1 to 9.

Figure 1 is a schematic section view across a print zone of an image recording apparatus. An aperture 61 in a printhead structure 6 is shown between a particle carrier 52 with a toner layer 7 and an image receiving surface 10. A background electrical field is created by a voltage source 80 being connected to a back electrode arrangement and the particle carrier 52. Usually the toner particles are negatively charged, which results in the back electrode arrangement being supplied with a voltage which is positive in relation to a voltage supplied to the particle carrier 52. In some embodiments the back electrode arrangement is behind the image receiving surface, in other embodiments the image receiving surface is the back electrode arrangement shaped as a drum, or possibly a belt,

with possibly only a coating, which in turn may comprise one or more layers.

The image receiving surface can in some embodiments be an intermediate image receiving member 10 such as a belt or a drum. The printhead structure 6 comprises a substrate layer which carries control electrodes 62 in the vicinity of, i. e. proximate, corresponding apertures 61. The printhead structure 6 further usually comprises deflection or shield electrodes 64,65 also proximate corresponding apertures 61 but preferably located on an opposite side of the printhead structure in relation to the control electrodes 62. Normally the control electrodes 62 are located on a side of the printhead structure 6 facing the particle carrier 52, and the deflection or shield electrodes 64,65 are located on a side of the printhead structure 6 facing the image receiving surface 10.

According to the invention it has been discovered that a tonerjet transport field 70, either being the background electrical field, or a field created by print pulses on the control electrodes 62, or a combination of both, through the apertures for transport of a toner jet from the toner layer 7 to the image receiving surface 10, can be influenced by unwanted fields 78,79 within an aperture in question.

These unwanted fields 78, 79 are other electrical fields than the background electrical field and the fields generated by print pulses on the control electrodes 62. These unwanted fields 78,79 within an aperture 61 cause problems since they counteract the tonerjet transport field 70. Apart from just causing unwanted disturbances to the tonerjet transport field 70, unwanted fields 78,79 can cause toner particles of a tonerjet to slow down causing delays before all, or most, of a tonerjet reaches the image receiving surface 10. This will cause a degradation in print quality if an increased printing speed is desired. Further, toner particles can, due to the unwanted fields 78,79, get totally detached from a tonerjet when travelling within an aperture 61 and adhere to the inside walls of the aperture in question. This can cause clogging of the aperture, increasing the

need for thorough cleaning which steals time which could have been used for printing. Most commonly these unwanted fields 78,79 are caused by either deflection electrodes 64,65 or other electrodes (other than the control electrodes 62) which are arranged in relation to an aperture in question. It has further been discovered that the negative effects occur mainly during the time of the control electrode voltage pulses, so called print pulses. According to the invention the control voltages of field generating sources are controlled to at least substantially attain a field neutral or synergetic influence on a tonerjet transport field within an aperture in question by a field or fields generated by the one or more field generating sources during at least a part of a time frame of a print pulse, i. e. for example shield electrodes 64,65 are at least during part of a time frame of a print pulse put at a voltage potential which generates an electrical field from the shield electrodes which is at least substantially field neutral or synergetic with a tonerjet transport field within an aperture in question.

Figure 2 is a schematic section view across a print zone of an image recording apparatus according to a preferred embodiment of the invention. Here it can be seen that the unwanted fields are at least temporarily, i. e. during at least a part of a print pulse, changed to synergetic fields 781,791.

Figure 3 illustrates a printpulse 100, which, in this example, reaches a print pulse voltage level 31 during a print pulse time frame 33, a start time for the print pulse 100 being normalized to the origin of the time axis. A print pulse 100 is can also be called a control voltage pulse (Vcontrol) > a print pulse time frame 33 is usually equal a print period tb, and a print pulse voltage level 31 is usually equal a print level Vb. A print pulse 100 is only a part of a print sequence, several of which can be comprised in a print cycle, as mentioned previously. A print sequence is according to a preferred embodiment less than a milli-second long. A print pulse

100 can in some embodiments comprise a kick-pulse, which is used to facilitate toner particle release from the toner layer. The invention is concerned about a temporary modification of one or more control voltages during at least a part of a time frame of a print pulse. Figures 4A to 4E illustrate different waveforms 200, 210,220,230,240 on one or more electrodes, which electrodes would normally without the invention create unwanted electrical fields within the apertures during print pulse time frames. The waveforms 200,210,220,230,240 are in these examples illustrated with a voltage greater than the print pulse voltage level 31.

This is suitable in some embodiments, especially embodiments where these waveforms are applied to electrodes which are located closer to a positive back electrode than the control electrodes, such as the deflection or shield electrodes 64,65 of Figure 1 and 2 in relation to the control electrodes 62. This will normally lead to the generation of a synergetic field during the print pulses. In some embodiments it is sufficient that the waveforms attain a voltage which is the same voltage as a print pulse voltage level 31 or even a voltage level up to approximately 20 volts below the print pulse voltage level 31 during at least a part of the print pulse time frame and thereby attain at least a substantially field neutral field in relation to a toner jet transport field. In some embodiments a control voltage Vcv, of a waveform 200,210,220,230,240, can during at least a part of a print pulse be expressed as Vcv (Lcv/IVaE, where Lcv is a length from the particle carrier to an electrode in question where the control voltate Vcv is applied such as a shield or deflection electrode, LBE is a length between the particle carrier and the back electrode, and VBE is a voltage potential of the back electrode. In case the particle carrier is at a different potential than zero then it can also be expressed as Vcv-Vpc (LcvLBE) VBE-Ypc where Vpc is a voltage potential of the particle carrier. In other embodiments a control voltage Vcv, of a waveform 200,210,220,230,240, can during at least a part of a print pulse be

expressed as Vcv ! VcE, where VcE is a print pulse voltage 31. In still other embodiments the expression (vcv-VCE)/ (LCV-LCE) 2 (VBE-VPC)/LBE should be valid during at least a part of a print pulse, where additionally LCE is the distance from the particle carrier to a control electrode in question. In some embodiments the print pulse voltage level 31 is the maximum voltage of the print pulse during the print pulse time frame, in other embodiments it is the mean voltage value during the print pulse time frame, and in still other embodiments it is the value the print pulse has during the longest time during the print pulse time frame.

Figure 4A illustrates a first example according to the invention of a temporary modification waveform 200 on one or more electrodes which creates an electrical field in one or more apertures. As can be seen the first temporary modification waveform 200 follows the print pulse and substantially starts when a corresponding print pulse starts and ends when the corresponding print pulse ends 33. Figure 4B illustrates a second example according to the invention of a temporary modification waveform 210 on one or more electrodes which creates an electrical field in one or more apertures. As can be seen the second temporary modification waveform 210 starts before the start of a corresponding print pulse.

Figure 4C illustrates a third example according to the invention of a temporary modification waveform 220 on one or more electrodes which creates an electrical field in one or more apertures. As can be seen the third temporary modification waveform 220 starts after the start of a corresponding print pulse. Figure 4D illustrates a fourth example according to the invention of a temporary modification waveform 230 on one or more electrodes which creates an electrical field in one or more apertures As can be seen the fourth temporary modification waveform 230 ends after a corresponding print pulse ends 33. Figure 4E

illustrates a fifth example according to the invention of a temporary modification waveform 240 on one or more electrodes which creates an electrical field in one or more apertures. As can be seen the fifth temporary modification waveform 240 ends before a corresponding print pulse ends 33. A temporary modification waveform can start according to any one of Figures 4A, 4B or 4C and end according to any one of Figures 4A, 4D or 4E, i. e. a temporary modification waveform can start before, substantially at the same time as, or after a print pulse time frame starts and can en before, substantially at the same time as, or after a print pulse time frame ends.

The unwanted fields can, as mentioned in connection with Figures 1 and 2, be generated by deflection electrodes, which are intended to deflect toner jets in flight. Figure 5A illustrates a typical waveform 300 on a first deflection electrode, such as electrode 64 of Figures 1 and 2. Figure 5B illustrates a typical waveform 400 on a second deflection electrode, such as electrode 65 of Figures 1 and 2.

Figure 6A illustrates a first example according to the invention of a temporary modification waveform 310 on a first deflection electrode, such as electrode 64 of Figures 1 and 2. Figure 6B illustrates the first example according to the invention of a temporary modification waveform 410 on a second deflection electrode, such as electrode 65 of Figures 1 and 2. In the first example of temporary modification waveforms on deflection electrodes, both deflection electrodes are raised by the same voltage during at least a part of a print pulse time frame. The voltage that both electrodes are raised with is such that both waveforms 310,410 attain at least a voltage such that at least during part of a print pulse time frame an electrical field is generated from the deflection electrodes, which electrical field is at least substantially field neutral or synergetic with a tonerjet transport field

within an aperture in question. The illustrated attained voltage of the deflection electrodes according to this first example is the print pulse voltage level 31. An advantage of this method according to the invention is that the mutual voltage difference between the deflection electrodes is preserved even during the temporary modification waveform.

Figure 7A illustrates a second example according to the invention of a temporary modification waveform 320 on a first deflection electrode, such as electrode 64 fo Figures 1 and 2. Figure 7B illustrates the second example according to the invention of a temporary modification waveform 420 on a second deflection electrode, such as electrode 65 of Figures 1 and 2. In the second example of temporary modification waveforms 320,420 on deflection electrodes, the deflection electrodes are raised individually to a voltage level such that at least during part of a print pulse time frame an electrical field is generated from the deflection electrodes, which is at least substantially field neutral or synergetic with a tonerjet transport field within an aperture in question. The control voltages of the deflection electrodes are raised to at least approximately the same voltage level. The illustrated voltage level of the deflection electrodes according to this second example is the print pulse voltage level 31. An advantage of this method according to the invention is that the a voltage level on a single deflection electrode does not reach unsuitable values during the temporary modification waveform.

The temporary modification waveforms 310,320,410,420 according to Figures 6A, 6B, 7A, 7B can of course start and end according to any of Figures 4A to 4E and have suitable voltage levels as previously discussed..

The control functions of a printer unit according to the invention is handled by a control unit which is schematically illustrated in Figure 8. The illustration of the control unit 900 is merely to give an example of one possible embodiment of the control unit 900. All the different parts may be separate as illustrated or more or less integrated. The memories 902,903,930 may be of an arbitrary type which will suit the embodiment in question. The control unit 900 comprises a computing part which comprises a CPU 901, program memory ROM 902, working memory RAM 903, a user I/O interface 910 through which a user will communicate 951 with the printer for downloading of commands and images to be printed, and a bus system 950 for interconnection and communication between the different parts of the control unit 900. The control unit 900 also suitably comprises a bitmap 930 for storage of the image to be printed and one or more I/O interfaces 911,912 for control and monitoring of the printer. Further, if necessary, one or more power-high voltage drivers 921,922,923,924,925 are connected to the hardware of the printer illustrated by an interface line 999.

The one or more I/O interfaces 911, 912 for control and monitoring of the printer can logically be divided into one simple I/O interface 912 for on/off control and monitoring and one advanced I/O interface 911 for multilevel control and monitoring, speed control, and analog measurements. Typically the simple I/O interface 912 handles keyboard input 969 and feedback output 968, control of simple motors and indicators, monitoring of different switches and other feedback means. Typically the advanced I/O interface 911 will control 954,955 the deflection voltages 964 and guard voltages 965 via high voltage drivers 924,925.

The advanced I/O interface 911 will typically also speed control 966 one or more motors with a control loop feedback 967.

A user, e. g. a personal computer, will download, through the user'I/O interface 910, commands and images 951 to be printed. The CPU 901 will interpret the commands under control of its programs and typically load the images to be printed into the bitmap 930. The bitmap 930 will preferably comprise at least two logical bitmaps, one which can be printed from and one which can be used for download of the next image to be printed. The functions of the preferably at least two logical bitmaps will continuously switch when their previous function is finished.

In a preferred embodiment the bitmap 930 will serially 952 load a plurality of high voltage drive controllers 921,922,923 with the image information to be printed. The number of high voltage drive controllers 921,922,923 that are necessary will, for example, depend on the resolution and the number of apertures, i. e. control electrodes, each controller 921,922,923 will handle. The high voltage drive controllers 921,922,923 will convert the image information they receive to signals 961,962,963 with the proper voltage levels required by the control electrodes of the printer.

Figure 9 illustrates one possible schematic of a high voltage drive controller 940.

The image information is received serially via a data input 971. The image information is clocked 972 into a serial to parallel register 941. When the serial to parallel register 941 is full the image information is latched 973 into a latch 942 at an appropriate time, thus enabling new image information to be clocked into the serial to parallel register. The controller preferably comprises high voltage drivers 943,944,945,946,947 for conversion of the image data in the latch to signals 983,984,985,986,987 with the appropriate voltage levels required by the control electrodes of the apertures. The high voltage drive controller can also

suitably comprise a blanking input 974 to enable a higher degree of control of the outputs 983,984,985,986,987 to the control electrodes.

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