WO2001045955A1

WO2001045955A1 - Direct electrostatic printing method and apparatus

Info

Publication number: WO2001045955A1
Application number: PCT/SE1999/002441
Authority: WO
Inventors: Agneta Sandberg; Karin Bergman; Thomas Oskarsson
Original assignee: Array Printers Ab; Matsushita Electric Industrial Co., Ltd.
Priority date: 1999-12-21
Filing date: 1999-12-21
Publication date: 2001-06-28
Also published as: AU2704100A

Abstract

An improved direct electrostatic printing device and method for printing an image to an information carrier. By controlling the transport of toner particles, dots can be adjusted from the exact alignment of a dot grid to thereby disable imperfections to propagate and make continuous patterns in a direction parallel to the direction of information carrier movement.

Description

DIRECT ELECTROSTATIC PRINTING METHOD AND APPARATUS

FIELD OF THE INVENTION

The present invention relates to direct electrostatic printing methods in which charged toner particles are transported under control from a particle source in accordance with an image information to form a toner image used in a copier, a printer, a plotter, a facsimile, or the like.

BACKGROUND TO THE INVENTION

According to a direct electrostatic printing method, such as that disclosed in U.S. Patent No. 5,036,341, a background electric field is produced between a developer sleeve and a back electrode to enable the transport of charged toner particles therebetween. A printhead struc- ture, such as an electrode matrix provided with a plurality of selectable apertures, is interposed in the background electric field and connected to a control unit which converts an image information into a pattern of electrostatic control fields which selectively open or close the apertures, thereby permitting or restricting the transport of toner particles from the developer sleeve. The modulated stream of toner particles allowed to pass through opened apertures impinges upon an information carrier, such as paper, conveyed between the printhead structure and the back electrode, to form a visible image. According to such a method, each single aperture is utilized to address a specific dot position of the image in a transverse direction, i.e. perpendicular to paper motion. 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.

A new concept of direct electrostatic printing, hereinafter referred to as dot deflection control (DDC) , 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. This is achieved by providing the printhead structure with at least two sets of 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 200 apertures per inch.

With or without DDC 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. at a substantially right angle 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.

However, it can be considered a drawback of current direct electrostatic printing methods that print imperfections propagate in the direction of relative movement of an image receiving medium which can cause an appearance of stripeness. Therefore, there seems to still exist a need to improve the current direct electrostatic printing method.

SUMMARY OF THE INVENTION

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

A further object of the present invention is to provide a method of direct electrostatic printing which reduces the effect of print imperfections.

Still a further object of the present invention is to provide a method of and a device for improving control over dot placement in direct electrostatic printing methods . Yet a further object of the present invention is to provide a method of and a device for removing or at least reducing print imperfections from propagating in a direction parallel to an in image receiving medium's relative movement.

Still another object of the present invention is to provide a method of and a device for trajecting toner particles to predetermined positions in view of an image which is to be recorded.

Said objects are achieved according to the invention by providing an improved direct electrostatic printing device and method for printing an image to an information carrier. By controlling the transport of toner particles, dots can be adjusted from the exact alignment of a dot grid to thereby disable imperfections to propagate and make continuous patterns in a direction parallel to the direction of information carrier movement. The control of toner particle transport can, for example, be effectuated by modification of deflection voltages.

Said objects are also achieved according to the invention by an image printing device and method for printing an image onto an information carrier. The direct electrostatic printing device includes a pigment particle source, a voltage source, a printhead structure, a control unit, and an image receiving member. The pigment particle source provides pigment particles . The image receiving member and the printhead structure moving relative to each other during printing. The image receiving member has a first face and a second face. The printhead structure is placed inbetween the pigment particle source and the first face of the image receiving member. The voltage source is connected to the pigment particle source and the back electrode to thereby create an electrical field for transport of pigment particles from the pigment particle source toward the first face of the image receiving member. The printhead structure includes control electrodes connected to the control unit to thereby selectively open or close apertures through the printhead structure to permit or restrict the transport of pigment particles for placement of pigment particles at dot positions nominally in a grid comprising nominal dot positions. The dot positions are arranged adjacent each other in rows along a direction substantially perpendicular to the relative movement between the printhead structure and the image receiving member, to thereby enable the formation of a pigment image on the first face of the image receiving member. According to the invention the control unit offsets the position of at least two adjacent dot positions, of at least one row, an equal amount and direction along the row in question, in relation to the nominal dot positions in the grid, thus modifying the placement of pigment particles in a first manner.

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 pigment particles are provided from a pigment particle source. In a second step an image receiving member and a printhead structure are moved relative to each other during printing. In a third step an electrical field is created for transporting pigment particles from the pigment particle source toward the first face of the image receiving member. In a fourth step apertures through a printhead structure are selectively opened or closed to permit or restrict the transporting of pigment particles, for placement of pigment particles at dot positions nominally in a grid comprising nominal dot positions, the dot positions being arranged adjacent each other in rows along a direction substantially perpendicular to the relative movement between the printhead structure and the image receiving member, to thereby enable the formation of a pigment image on the first face of the image receiving member. And in a final fifth step offsetting the position of at least two adjacent dot positions, of at least one row, an equal amount and direction along the row in question, in relation to the nominal dot positions in the grid, thus modifying the placement of pigment particles in a first manner .

Said objects are also achieved according to the invention by an image printing device and method for printing an image onto an information carrier. The direct electrostatic printing device includes a pigment particle source, a voltage source, a printhead structure, a control unit, and an image receiving member. The pigment particle source provides pigment particles . The image receiving member and the printhead structure moving relative to each other during printing. The image receiving member has a first face and a second face. The printhead structure is placed inbetween the pigment particle source and the first face of the image receiving member. The voltage source is connected to the pigment particle source and the back electrode to thereby create an electrical field for transport of pigment particles from the pigment particle source toward the first face of the image receiving member. The printhead structure includes control electrodes connected to the control unit to thereby selectively open or close apertures through the printhead structure to permit or restrict the transport of pigment particles for placement of pigment particles . The printhead structure further including deflection electrodes connected to the control unit for controlling the deflection of pigment particles in transport by means of predetermined deflection voltages to thereby be able to deflect pigment particles against predetermined locations for placement of pigment particles at dot positions nominally in a grid comprising nominal dot positions . The dot positions are arranged adjacent each other in rows along a direction substantially perpendicular to the relative movement between the printhead structure and the image receiving member, to thereby enable the formation of a pigment image on the first face of the image receiving member. According to the invention the control unit, for at least dot positions which are addressed by two apertures, along at least one row, modifies the deflection voltages in such a way that each such dot position that is addressed by an aperture is only adjacent dot positions along the row in question that are addressed by another aperture or apertures, thus modifying the placement of pigment particles in a second manner.

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 pigment particles are provided from a pigment particle source. In a second step an image receiving member and a printhead structure are moved relative to each other during printing. In a third step an electrical field is created for transporting pigment particles from the pigment particle source toward the first face of the image receiving member. In a fourth step apertures through a printhead structure are selectively opened or closed to permit or restrict the transporting of pigment particles. In a fifth step the control unit further controlling deflection of pigment particles in transport by means of predetermined deflection voltages to thereby be able to deflect pigment particles against predetermined locations for placement of pigment particles at dot positions nominally in a grid comprising nominal dot positions, the dot positions being arranged adjacent each other in rows along a direction substantially perpendicular to the relative movement between the printhead structure and the image receiving member, to thereby enable the formation of a pigment image on the first face of the image receiving member. And in a final sixth step, for at least dot positions which are addressed by two apertures, along at least one row, the control unit modifying the deflection voltages in such a way that each such dot position that is addressed by an aperture is only adjacent dot positions along the row in question that are addressed by another aperture or apertures, thus modifying the placement of pigment particles in a second manner.

Said objects are also achieved according to the invention by an image printing device and method for printing an image onto an information carrier. The direct electrostatic printing device includes a pigment particle source, a voltage source, a printhead structure, a control unit, and an image receiving member. The pigment particle source provides pigment particles . The image receiving member and the printhead structure moving relative to each other during printing. The image receiving member has a first face and a second face. The printhead structure is placed inbetween the pigment particle source and the first face of the image receiving member. The voltage source is connected to the pigment particle source and the back electrode to thereby create an electrical field for transport of pigment particles from the pigment particle source toward the first face of the image receiving member. The printhead structure includes control electrodes connected to the control unit to thereby selectively open or close apertures through the printhead structure to permit or restrict the transport of pigment particles for placement of pigment particles . The printhead structure further including deflection electrodes connected to the control unit for controlling the deflection of pigment particles in transport by means of predetermined deflection voltages to thereby be able to deflect pigment particles against predetermined locations, each aperture in question being arranged for placing pigment particles at at least three different dot positions nominally in a grid comprising nominal dot positions. The dot positions are arranged adjacent each other in rows along a direction substantially perpendicular to the relative movement between the printhead structure and the image receiving member, to thereby enable the formation of a pigment image on the first face of the image receiving member. According- to the invention the control unit for at least dot positions which are addressed by two apertures, along at least one row, modifies the deflection voltages in such a way that each such dot position that is addressed by an aperture in question is adjacent at least one dot position along the row in question that is addressed by a different aperture, thus modifying the placement of pigment particles in a third manner.

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 pigment particles are provided from a pigment particle source. In a second step an image receiving member and a printhead structure are moved relative to each other during printing. In a third step an electrical field is created for transporting pigment particles from the pigment particle source toward the first face of the image receiving member. In a fourth step apertures through a printhead structure are selectively opened or closed to permit or restrict the transporting of pigment particles. In a fifth step the control unit further controlling deflection of pigment particles in transport by means of predetermined deflection voltages to thereby be able to deflect pigment particles against predetermined locations, each aperture in question being arranged for placing pigment particles at at least three different dot positions nominally in a grid comprising nominal dot positions, the dot positions being arranged adjacent each other in rows along a direction substantially perpendicular to the relative movement between the printhead structure and the image receiving member, to thereby enable the formation of a pigment image on the first face of the image receiving member. And in a final sixth step, for at least dot positions which are addressed by two apertures, along at least one row, the control unit modifying the deflection voltages in such a way that each such dot position that is addressed by an aperture in question is adjacent at least one dot position along the row in question that is addressed by a different aperture, thus modifying the placement of pigment particles in a third manner.

Said objects are also achieved according to the invention by an image printing device and method for printing an image onto an information carrier. The direct electrostatic printing device includes a pigment particle source, a voltage source, a printhead structure, a control unit, and an image receiving member. The pigment particle source provides pigment particles . The image receiving member and the printhead structure moving relative to each other during printing. The image receiving member has a first face and a second face. The printhead structure is placed inbetween the pigment particle source and the first face of the image receiving member. The voltage source is connected to the pigment particle source and the back electrode to thereby create an electrical field for transport of pigment particles from the pigment particle source toward the first face of the image receiving member. The printhead structure includes control electrodes connected to the control unit to thereby selectively open or close apertures through the printhead structure to permit or restrict the transport of pigment particles for placement of pigment particles at dot positions nominally in a grid comprising nominal dot positions. The dot positions are arranged adjacent each other in rows along a direction substantially perpendicular to the relative movement between the printhead structure and the image receiving member, to thereby enable the formation of a pigment image on the first face of the image receiving member. According to the invention the control unit offsets the positions of at least two adjacent dot positions, of at least one row, independently and in a random manner with regard to direction and amount along the row in question, in relation to the nominal dot positions in the grid, thus modifying the placement of pigment particles in a fourth manner.

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 pigment particles are provided from a pigment particle source. In a second step an image receiving member and a printhead structure are moved relative to each other during printing. In a third step an electrical field is created for transporting pigment particles from the pigment particle source toward the first face of the image receiving member. In a fourth step apertures through a printhead structure are selectively opened or closed to permit or restrict the transporting of pigment particles, for placement of pigment particles at dot positions nominally in a grid comprising nominal dot positions, the dot positions being arranged adjacent each other in rows along a direction substantially perpendicular to the relative movement between the printhead structure and the image receiving member, to thereby enable the formation of a pigment image on the first face of the image receiving member. And in a final fifth step offsetting the positions of at least two adjacent dot positions, of at least one row, independently and in a random manner with regard to direction and amount along the row in question, in relation to the nominal dot positions in the grid, thus modifying the placement of pigment particles in a fourth manner .

A modification of the dot placement of the fourth manner can be by having the printhead structure further including deflection electrodes connected to the control unit for controlling the deflection of pigment particles in transport by means of predetermined deflection voltages to thereby be able to deflect pigment particles against predetermined locations . The control unit can then offset at least two adjacent dot posisitons of at least dot positions which are addressed by deflection by two apertures, along at least one row, in relation to the respective nominal dot positions in the grid, thus modifying the placement of pigment particles in a fifth manner. Preferably only dot positions that are addressed by deflection are offset independently and in a random manner with regard to direction and amount along the row in question, in relation to the nominal dot positions in the grid.

Further variations of the methods and the devices are possible in view of the application of the invention, some of which will be disclosed in further detail below. The present invention relates to an image recording apparatus including an image receiving member 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, such as a developer sleeve, 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 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 inven- tion, the image recording apparatus comprises four print stations, each corresponding to a pigment colour, e.g. yellow, magenta, cyan, black (Y,M,C,K), disposed adjacent to an image receiving member formed of 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. The toner image is formed on the transfer belt and thereafter brought into contact with an information carrier, e.g. paper, in a fuser unit, where the toner image is simultaneously transferred to and made permanent on the information carrier upon heat and pressure. After image transfer, the transfer belt is brought in contact with a cleaning unit removing untransferred toner particles.

The present invention also relates to a direct printing method performed in consecutive print cycles, each of which includes several development periods having specific deflection modes. During each development period, control voltages are applied to control electrodes to produce electrostatic control fields which, due to control in accordance with the image information, open or close apertures through the printhead structure, thus enhancing or inhibiting the transport of toner particles from the particle source toward the image receiving member. Deflection voltages are simultaneously applied to the deflection electrodes to influence the symmetry of the electrostatic control fields to deflect the transported toner particles in predetermined directions, such that several dot locations are addressable through each aperture during each print cycle. The deflection length, i.e. the distance between a deflected dot and a central axis of the corresponding aperture, is optimized to obtain uniformly spaced dot locations across the entire width of the image receiving member. According to the invention the deflection lengths can be modified to thereby reposition determined dots to improve the apparent print quality.

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

Figure 1 is a schematic section view across an image recording apparatus according to a preferred embodiment of the invention,

Figure 2 is a schematic section view across a particular print station of the image recording apparatus shown in Figure 1,

Figure 3 is an enlargement of Figure 2 showing the print zone corresponding to a particular print station,

Figure 4a is a schematic plan view of the top side of a printhead structure used in a print station such as that shown in Figure 2 ,

Figure 4b is a schematic section view along the section line I-I through the printhead structure shown in Figure 4a,

Figure 4c is a schematic plan view of the bottom side of the printhead structure shown in Figure

4a,

Figure 5 is a schematic view of a single aperture and its corresponding control electrode and deflection electrodes,

Figure 6a illustrates a control voltage signal as a function of time during a print cycle having three subsequent development periods,

Figure 6b illustrates a first deflection voltage signal as a function of time during a print cycle having three subsequent development periods

Figure 6c illustrates a second deflection voltage signal as a function of time during a print cycle having three subsequent development periods

Figure 7a illustrates the transport trajectory of toner particles through the printhead structure shown in Figures 4a,b, c according to a first deflection mode wherein Dl > D2 ,

Figure 7b illustrates the transport trajectory of toner particles through the printhead structure shown in Figures 4a,b, c, according to a second deflection mode wherein Dl = D2 ,

Figure 7c illustrates the transport trajectory of toner particles through the printhead structure shown in Figures 4a,b, c, according to a third deflection mode wherein Dl < D2 ,

Figure 8a illustrates a grid with six rows of nominal dot positions from three apertures,

Figure 8b illustrates a grid with six rows of nominal dot positions from three apertures with deflection to two dot positions from each aperture,

Figure 8c illustrates a grid with six rows of nominal dot positions from three apertures with deflection, each aperture addressing three dot positions,

Figure 9a illustrates an offset on every other row for three apertures on six rows,

Figure 9b illustrates an offset on every other row for three apertures, with deflection to two dot positions, on six rows,

Figure 9c illustrates an offset on every other row for three apertures, with deflection where each aperture addresses three dot positions, on six rows,

Figure 9d illustrates an offset on every other row for three apertures on six rows according to a second embodiment,

Figure 9e illustrates an offset on every other row for three apertures, with deflection to two dot positions, on six rows according to a second embodiment ,

Figure 9f illustrates an offset on every other row for three apertures, with deflection to where each aperture addresses three dot positions, on six rows according to a second embodiment,

Figure 9g illustrates an offset on every other row for three apertures on six rows according to a third embodiment,

Figure 9h illustrates an offset on every other row for three apertures, with deflection to two dot positions, on six rows according to a third embodiment,

Figure 9i illustrates an offset on every other row for three apertures, with deflection where each aperture addresses three dot positions, on six rows according to a third embodiment,

Figure 10a illustrates a dot position cross-over on each row from three apertures with deflection to two dot positions from each aperture,

Figure 10b illustrates a dot position cross-over on each row from three apertures with deflection where each aperture addresses three dot positions,

Figure 10c illustrates a dot position cross-over on every other row from three apertures with deflection to two dot positions from each aperture,

Figure lOd illustrates a dot position cross-over on every other row from three apertures with deflection where each aperture addresses three dot positions,

Figure lOe illustrates a dot position cross-over according to a second embodiment on each row from three apertures with deflection where each aperture addresses three dot positions,

Figure lOf illustrates a dot position cross-over according to a second embodiment on every other row from three apertures with deflection where each aperture addresses three dot positions,

Figure 11a illustrates a random offset on every row for three apertures on six rows,

Figure lib illustrates a random offset on every row for three apertures, with deflection to two dot positions, on six rows, Figure lie illustrates a random offset on every row for three apertures, with deflection where each aperture addresses three dot positions, on six rows,

Figure lid illustrates a random offset on every row for three apertures, with deflection where each aperture addresses three dot positions, on six rows, of only the deflected dot positions,

Figure 12 illustrates a control unit,

Figure 13 illustrates a high voltage control electrode driver.

DESCRIPTION OF PREFERRED EMBODIMENTS

In order to clarify the method and device according to the invention, some examples of its use will now be described in connection with Figures 1 to 13.

Figure 1 is a schematic section view of an image recording apparatus according to one embodiment of the invention, comprising at least one print station, preferably four print stations (Y, M, C, K) , an intermediate image receiving member, a driving roller 11, at least one support roller 12 , and preferably several adjustable holding elements 13. The four print stations

(Y, M, C, K) are arranged in relation to the intermediate image receiving member. The intermediate image receiving member, preferably a transfer belt 10, is mounted over the driving roller 11. The at least one support roller 12 is provided with a mechanism for maintaining the transfer belt 10 with at least a constant surface tension, while preventing transversal movement of the transfer belt 10. The preferably several adjustable holding elements 13 are for accurately positioning the transfer belt 10 at least with respect to each print station.

The driving roller 11 is preferably a cylindrical metallic sleeve having a rotational axis extending perpendicular to the belt motion and a rotation velocity adjusted to convey the transfer belt 10 at a velocity of one addressable dot location per print cycle, to provide line by line scan printing. The adjustable holding elements 13 are arranged for maintaining the surface of the transfer belt 10 at a predetermined distance from each print station. The holding elements 13 are preferably cylindrical sleeves disposed perpendicularly to the belt motion in an arcuated configuration for slightly bending the transfer belt 10 at least in the vicinity of each print station. The transfer belt 10 is slightly bent in order to, in combination with the belt tension, create a stabilization force component on the transfer belt 10. The stabilization force component is opposite in direction and preferably larger in magnitude than an electrostatic attraction force component acting on the transfer belt 10. The electrostatic attraction forces at a print station are created by induction charging of the belt and by different electric potentials on the holding elements 13 and on the print station in question.

The transfer belt 10 is preferably an endless band of 30 to 200 μm 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 tranfer belt 10 preferably has a homogeneous concentration of filler material, such as carbon or the like, which provides a uniform electrical conductivity throughout the entire surface of the transfer belt 10. The outer surface of the transfer belt 10 is preferably overlaid with a 5 to 30 μm thick coating layer made of electrically conductive polymere material such as for instance PTFE (poly tethra fluoro ethylene) , PFA (tetra flouro ethylene, perflouro alkyl vinyl ether copolymer) , FEP (tetra flouro ethylene hexaflouro, propylene copolymer) , silicone, or any other suitable material having appropriate conductivity, thermal resistance, adhesion properties, release properties, and surface smoothness. To further improve for example the adhesion and release properties a layer of silicone oil can be applied to either the transfer belt base or preferably onto a coating layer if it is applied onto the transfer belt base. The silicone oil is coated evenly onto the transfer belt 10 preferably in the order of 0.1 to 2 μm thick giving a consumption of silicone oil in the region of 1 centiliter for every 1000 pages. Silicone oil also reduces bouncing/-scattering of toner particles upon reception of toner particles and also increases the subsequent transfer of toner particles to an information carrier. Making use of silicone oil and especially coating of the transfer belt with silicone oil is made possible in an electrostatic printing method according to the present invention as there is no direct physical contact between a toner delivery and a toner recipient, i.e. the transfer belt, in this embodiment.

In some embodiments the transfer belt 10 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 10 can also in certain embodiments comprise a special registration area for use of determining the position of the transfer belt, especially an image area if available, in relation to each print station. If the transfer belt comprises a special registration area then this area is preferably at least spatially related to an image area.

The transfer belt 10 is conveyed past the four different print stations (Y, M, C, K) , whereby toner particles are deposited on the outer surface of the transfer belt 10 and superposed to form a toner image. Toner images are then preferably conveyed through a fuser unit 2 , comprising a fixing holder 21 arranged transversally in direct contact with the inner surface of the transfer belt. In some embodiments of the invention the fuser unit is separated from the transfer belt 10 and only acts on an information carrier. The fixing holder 21 includes a heating element preferably of a resistance type of e.g. molybdenium, maintained in contact with the inner surface of the transfer belt 10. As an electric current is passed through the heating element, the fixing holder 21 reaches a temperature required for melting the toner particles deposited on the outer surface of the transfer belt 10. The fuser unit 2 further comprises a pressing roller 22 arranged transversally across the width of the transfer belt 10 and facing the fixing holder 21. An information carrier 3, such as a sheet of plain, untreated paper or any other medium suitable for direct printing, is fed from a paper delivery unit (not shown) and conveyed between the pressing roller 22 and the transfer belt 10. The pressing roller 22 rotates with applied pressure to the heated surface of the fixing holder 21 whereby the melted toner particles are fused on the information carrier 3 to form a permanent image. After passage through the fusing unit 2, the transfer belt is brought in contact with a cleaning element 4, such as for example a replaceable scraper blade of fibrous material extending across the width of the transfer belt 10 for removing all untransferred toner particles. If the transfer belt 10 is to be coated with silicone oil or the like, then preferably after the cleaning element 4, and before the printing stations, the transfer belt 10 is brought into contact with a coating application element 8 for evenly coating the transfer belt with silicone oil or the like. In other embodiments toner particles are deposited directly onto an information carrier without first being deposited onto an intermediate image receiving member.

Figure 2 is a schematic section view of one embodiment of a print station in, for example, the image recording apparatus shown in Figure 1. A print station includes a particle delivery unit 5 preferably having a replaceable or refillable container 50 for holding toner particles, the container 50 having front and back walls, a pair of side walls and a bottom wall having an elongated opening extending from the front wall to the back wall and provided with a toner feeding element (not shown) disposed to continuously supply toner particles to a developer sleeve 52 through a particle charging member. The particle charging member can preferably be formed of a supply brush 51 or a roller made of or coated with a fibrous, resilient material. The supply brush 51 can suitably in some embodiments be brought into mechanical contact with the peripheral surface of the developer sleeve 52 , 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 51 and any suitable coating material of the developer sleeve 52. The developer sleeve 52 is preferably made of metal which can, for example, be coated with a conductive material, and preferably have a substantially cylindrical shape and a rotation axis extending parallel to the elongated opening of the particle container 50. Charged toner particles are held to the surface of the developer sleeve

52 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 52. Alternatively, the charging unit may additionally comprise 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 by 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 53 is positioned proximate to the developer sleeve 52 to adjust the concentration of toner particles on the peripheral surface of the developer sleeve 52, to form a relatively thin, uniform particle layer thereon. In some embodiments the metering element

53 also suitably contributes to the charging of the toner particles . The metering element 53 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 53 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 distribution and mass density on the surface of the developer sleeve 52.

The developer sleeve 52 is arranged in relation with a support device 54 for supporting and maintaining the printhead structure 6 in a predetermined position with respect to the peripheral surface of the developer sleeve 52. The support device 54 is preferably in the form of a trough-shaped frame having two side walls, a bottom portion between the side walls, and an elongated slot arranged through the bottom portion, extending transversally across the print station, parallel to the rotation axis of the developer sleeve 52. The support device 54 further comprises means for maintaining the printhead structure in contact with the bottom portion of the support device 54, the printhead structure 6 thereby bridging the elongated slot in the bottom portion.

The transfer belt 10 is preferably slightly bent partly around each holding element 13 in order to create a stabilization force component 30. The stabilization force component 30 is intended to counteract, among other things, a field force component 31 which is acting on the transfer belt. If the field force component 31 is not counteracted it can cause distance fluctuations between the transfer belt 10 and the printhead structure 6 which can cause a degradation in print quality.

Figure 3 is an enlargement of the print zone in a print station of, for example, the image recording apparatus shown in Figure 1. A printhead structure 6 is preferably formed of an electrically insulating substrate layer 60 made of flexible, non-rigid material such as polyamide or the like. The printhead structure 6 is positioned between a peripheral surface of a developer sleeve 52 and a bottom portion of a support device 54. The substrate layer 60 has a top surface facing a toner layer 7 on the peripheral surface of the developer sleeve 52. The substrate layer 60 has a bottom surface facing the bottom portion of the support device 54. Further, the substrate layer 60 has a plurality of apertures 61 arranged through the substrate layer 60 in a part of the substrate layer 60 overlying a elongated slot in the bottom portion of the support device 54. The printhead structure 6 further preferably includes a first printed circuit arranged on the top surface on the substrate layer 60 and a second printed circuit arranged on the bottom surface of the substrate layer 60. The first printed circuit includes a plurality of control electrodes 62, each of which, at least partially, surrounds a corresponding aperture 61 in the substrate layer 60. The second printed circuit preferably includes at least a first and a second set of deflection electrodes 63 spaced around first and second portions of the periphery of the apertures 61 of the substrate layer 60.

The apertures 61 and their surrounding area will under some circumstances need to be cleaned from toner particles which agglomerate there. In some embodiments of the invention the transfer belt 10 advantageously comprises at least one cleaning area for the purpose of cleaning the apertures 61 and the general area of the apertures 61. The cleaning, according to these embodiments, works by the principle of flowing air (or other gas) . A pressure difference, compared to the air pressure in the vicinity of the apertures, is created on the side of the transfer belt 10 that is facing away from the apertures 61. The pressure difference is at least created during part of the time when the cleaning area is in the vicinity of the apertures 61 of the print station in question during the transfer belt's 10 movement. The pressure difference can either be an over pressure, a suction pressure or a sequential combination of both, i.e. the cleaning is performed by either blowing, suction, blowing first then suction, suction first then blowing, or some other sequential combination of suction and blowing. The pressure difference is transferred across the transfer belt 10 by means of the cleaning area comprising at least one slot/hole through the transfer belt 10. The cleaning area preferably comprises at least one row of slots, and more specifically two to eight interlaced rows of slots. The slots can advantageously be in the order of 3 to 5 mm across . The pressure difference appears on the holding element 13 side of the transfer belt 10 through a transfer passage in the holding element 13. The transfer passage can advantageously suitably extend transversally across the printhead structure as an elongated slot with a width, in the direction of the transfer belt 10 movement, that is equal to or greater than the minimum distance between the printhead structure 6 and the transfer belt 10. In some embodiments it can be advantageous to have a controllable passage which can open and close access of the pressure difference to the transfer passage. Thereby a suction pressure will not increase the transfer belt's friction on the holding element 13 more than necessary. The controllable passage will preferably open and close in synchronization with the movement of the transfer belt 10 to thereby coincide its openings with the passage of the cleaning area of the transfer belt 10. The means for creating the pressure difference is also not shown and can suitably be a fan, bellows, a piston, or some other suitable means for creating a pressure difference. In some embodiments according to the invention the transfer passage is substantially located symmetrically in relation to the apertures . In other embodiments according to the invention the transfer passage is shifted in relation to the direction of movement of the transfer belt 10.

Although, a printhead structure 6 can take on various embodiments without departing from the scope of the present invention, a preferred embodiment of the printhead structure will be described hereinafter with reference to Figures 4a, 4b and 4c. A plurality of apertures 61 are arranged through the substrate layer 60 in several aperture rows extending transversally across the width of the print zone, preferably at a substantially right angle to the motion of the transfer belt. The apertures 61 preferably have a circular cross section with a central axis 611 extending perpendicularly to the substrate layer 60 and suitably a diameter in the order of lOOμm to 160μm. Each aperture 61 is surrounded by a control electrode 62 having a ring-shaped part circumscribing the periphery of the aperture 61, with a symmetry axis coinciding with the central axis 611 of the aperture 61 and an inner diameter which is equal or sensibly larger than the aperture diameter. Each control electrode 62 is connected to a control voltage source (IC driver) through a connector 621. As apparent in Figure 5a, the printhead structure further preferably includes guard electrodes 64, preferably arranged on the top surface of the substrate layer 60 and connected to a guard potential (Vguard) aimed to, among other things, decrease the influence on the toner layer and to electrically shield the control electrodes 62 from one another, thereby preventing undesired interaction between the electrostatic fields produced by two adjacent control electrodes 62. Each aperture 61 is related to a first deflection electrode 631 and a second deflection electrode 632 spaced around a first and a second segment of the periphery of the aperture 61, respectively. The deflection electrodes 631, 632 are preferably semicircular or crescent-shaped and disposed symmetrically on each side of a deflection axis extending diametrically across the aperture at a predetermined deflection angle to the motion of the transfer belt, such that the deflection electrodes substantially border on a first and a second half of the circumference of their corresponding aperture 61, respectively. All first and second deflection electrodes 631, 632 are connected to a first and a second deflection voltage source Dl, D2 , respectively.

Figure 5 is a schematic view of a single aperture 61 and its corresponding control electrode 62 and deflection electrodes 631, 632. Toner particles are deflected in a first deflection direction Rl when Dl < D2 , and an opposite direction R2 when Dl > D2. The deflection angle δ is chosen to compensate for the motion of the transfer belt 10 during the print cycle, in order to be able to obtain two or more transversally aligned dots.

A preferred embodiment of a dot deflection control function is illustrated in Figures 6a, 6b and 6c respectively showing the control voltage signal (V_control) , a first deflection voltage Dl and a second deflection voltage D2, as a function of time during a single print cycle. According to some embodiments of the invention and as illustrated in the figure, printing is performed in print cycles having three subsequent print sequences with corresponding development periods for addressing three different dot locations through each aperture. 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 the surface of the developer sleeve and a second potential on the back electrode, to enable the transport of toner particles between the developer sleeve and the transfer belt. During each development period, control voltages are applied to the 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 along a trajectory which is determined by the deflection mode.

The examples of control function shown in Figures 6a, 6b and 6c illustrates a control function wherein the toner particles have negative polarity charge. As is apparent from Figure 6a, a print cycle comprises three development periods t_b, each followed by a recovering period t_w during which new toner is supplied to the print zone. The control voltage pulse (V_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 V_w of approximately -50V and a print level V_b in the order of +350V, corresponding to full density dots. Similarly, the pulse width can be varied from 0 to t_b.

The control of the position of a dot location can be increased to thereby enable an apparent increase of the print resolution. A method/part method of achieving this according to the invention is to individually control the timing of each developer period, i.e. individually control the timing of the opening and closing of the apertures . By individually controlling the timing for each developer period for each aperture, each dot location can be repositioned in a direction which is mainly parallel to the direction of travel of the image receiving member, information carrier, or transfer belt. Thus according to the invention individual dot positions can be moved/adjusted forward or backward, i.e. in a direction parallel to the direction of travel of the information carrier, by time displacing the opening and closing of the apertures .

As apparent from Figures 6b and 6c, the amplitude difference between Dl and D2 is sequentially modified for providing three different toner trajectories, i.e. dot positions, during each print cycle. The amplitudes of Dl and D2 are modulated to apply converging forces on the toner to obtain smaller dots. Utilizing this method enables, for example, 60μm dots to be obtained utilizing 160μm apertures. Suitably the size of the dots are adjusted in accordance with the dot density (dpi) and thus also dynamically with the number of dot locations each aperture is to address.

A further method of increasing the apparent print resolution is to control the size of the individual dots not only in view of the dot density but also according to the image which is to be printed. Thus by being able to increase or decrease the size of individual dots, in dependence upon the image which is to be printed, especially edges can be improved, giving an improved image print quality. This can be used on its own or in combination with the improved dot location control. Figures 7a, 7b and 7c illustrate the toner trajectories in three subsequent deflection modes . The figures 7a, 7b and 7c illustrate a cross section of a substrate layer 60 with apertures 61 with corresponding control electrodes 62. Also illustrated are deflection voltages Dl and D2 that are connected to respective deflection electrodes 631, 632. During a first development period illustrated in Figure 7a, the modulated stream of toner particles is deflected to the left by producing a first amplitude difference (Dl > D2) between both deflection voltages. The amplitude difference is adjusted to address dot locations 635 located at a deflection length L_d to the left of the central axes 611 of the apertures 61. During a second development period illustrated in Figure 7b, the deflection voltages have equal amplitudes (Dl = D2) to address undeflected dot locations 636 coinciding with the central axes 611 of the apertures 61. During a third development period illustrated in Figure 7c, the modulated stream of toner particles is deflected to the right by producing a second amplitude difference (Dl < D2) between both deflection voltages. The amplitude difference is adjusted to address dot locations 637 located at a deflection length L_d to the right of the central axes 611 of the apertures 61. As is apparent from the Figures 7a-c, the toner particles in question are negatively charged.

According to the invention an increased dot control is used to diminish the effects of a symmetrical and perpendicularly arranged grid of resulting dot positions . The effects result from the relative movement of a printhead structure and a print receiving medium. Figure 8a illustrates a grid with six rows 821, 822, 823, 824, 825, 826 of nominal dot positions from three apertures 801, 802, 803. The respective nominal dot positions of each row are addressed in sequence row by row 821, 822,

823, 824, 825, 826. This creates lines 801, 802, 803 of dots that run in a direction parallel to the releative movement between the print receiving medium and the printhead structure with the corresponding apertures 801, 802, 803, each of these lines 801, 802, 803 being addressed by a single respective aperture 801, 802, 803. Any defects or any other problems with a single aperture will propagate in straight lines 801, 802, 803 which are substantially perpendicular to the sequentially addressed rows 821, 822, 823, 824, 825, 826.

Figure 8b illustrates a grid with six rows 821, 822, 823,

824, 825, 826 of nominal dot positions from three apertures 801, 802, 803 with deflection to two dot positions 809, 811 from each aperture, and Figure 8c illustrates a grid with six rows 821, 822, 823, 824, 825, 826 of nominal dot positions from three apertures 801, 802, 803 with deflection, each aperture addressing three dot positions 809, 810, 811. As was explained in connection with Figure 8a, deviations of apertures 801, 802, 803 will propagate in a direction parallel to the relative movement of the print receiving medium. The difference is that with deflection more dots on a row 821, 822, 823, 824, 825, 826 are affected by an aperture. Also there might be deviations of the right deflection 811 in comparison with ^' the left deflection 809 in addition to the deflections not being what they should be. For these reasons aperture characteristics and deflection characteristics may result in repetitive patterns that propagate in a direction parallel to the relative movement between the printhead structure and a print receiving medium.

According to the invention the effects of these varying aperture characteristics and deflection characteristics in combination with the relative movement between the printhead structure and the print receiving medium can be eliminated or at least reduced. According to the invention the dot positions addressed by an aperture are spread out, i.e. varied instead of being arranged in columns .

According to a first manner of modifying the placement of pigment particles according to the invention the position of at least two adjacent dot positions, of at least one row, are offset, i.e. diplaced, an equal amount and direction along the row in question, in relation to the nominal dot positions in a dot position grid. Figure 9a illustrates such an offset on every other row for three apertures on six rows 821, 822, 823, 824, 825, 826. Figure 9b illustrates such an offset on every other row for three apertures, with deflection to two dot positions, on six rows 821, 822, 823, 824, 825, 826. Figure 9c illustrates such an offset on every other row for three apertures, with deflection where each aperture addresses three dot positions, on six rows 821, 822, 823, 824, 825, 826. In Figures 9a, 9b and 9c every other row 822, 824, 826 comprises an offset and every other row 822, 826 that comprises an offset offsets the addressed dot positions one dot position to the right and the other row that comprises an offset offsets the addressed dot positions one dot position to the right. By repeating the offset or offsets according to the invention there will thus not be any continuous repetitive patterns running parallel to the relative movement of the printhead structure and the print receiving medium, created by a same aperture addressing dot positions in straight columns .

Further Figures 9d, 9e, and 9f illustrate a different variation, of the first manner, where every other row 822, 824, 826 comprises a same directional offset. Figure 9d illustrates such an offset on every other row for three apertures on six rows 821, 822, 823, 824, 825, 826. Figure 9e illustrates such an offset on every other row for three apertures, with deflection to two dot positions, on six rows 821, 822, 823, 824, 825, 826. And Figure 9f illustrates such an offset on every other row for three apertures, with deflection where each aperture addresses three dot positions, on six rows 821, 822, 823, 824, 825, 826. In Figures 9d and 9e the offset is one dot position to the right and in Figure 9f it is two dot positions to the right.

According to another aspect of the invention a printhead structure used is capable of addressing additional needed dots. In Figures 9a, 9b and 9c there is an additional dot 805 needed on the left side on two rows 822, 826, and an additional dot 807 needed on the right side on one row 824. Figures 9d and 9e illustrate where an additional dot 805 is needed on the left side on three rows 822, 824, 826 and Figure 9f illustrate where two additional dots 805 are needed on the left side on three rows 822, 824, 826. Where and if one or more additional dots 805, 807 are needed is dependent on the amount of the offset and the direction of the offset. It is preferable, if a whole row is offset and if the amount of the offset is in the range of more than half a dot position, to insert one or more dot positions as needed. It is also preferable that if one or more dot positions are added to also adjust a mapping of a bit map that is to be printed to a grid of addressed dot positions instead of to the original nominal dot positions .

Figures 9g, 9h, and 9i still further illustrate a different variation, of the first manner, an offset amount which is smaller than approximately one half a dot position. Figure 9g illustrates such an offset on every other row for three apertures on six rows 821, 822, 823, 824, 825, 826. Figure 9h illustrates such an offset on every other row for three apertures, with deflection to two dot positions, on six rows 821, 822, 823, 824, 825, 826. And Figure 9i illustrates such an offset on every other row for three apertures, with deflection where each aperture addresses three dot positions, on six rows 821, 822, 823, 824, 825, 826. Figures 9g, 9h and 9i can also be seen as comprising a smaller offset on every row 821, 822, 823, 824, 825, 826, the offset being different in direction with regard to two adjacent rows 821, 822, 823, 824, 825, 826.

As mentioned previously the first manner of modifying the placement of pigment particles according to the invention involves the position of at least two adjacent dot positions, of at least one row. As is illustrated in the Figures 9a to 9i there are a number of different possibilities. Further possibilities are possible, a few of which will be described in the following, these can be combined in any arbitrary manner as long as they are not conflicting possibilities. Offsetting substantially all dot positions of a row an equal amount and direction. Offsetting a section of adjacent dot positions an equal amount and direction, a section of adjacent dot positions comprising more than two adjacent dot positions and less than substantially all dot positions of a row. That if the amount of offset is more than approximately 0.5 times the distance between two adjacent nominal dot positions in the grid, then the furthest dot position of the row in the direction of the offset is not used and an additional dot position is added and used at the opposite side of the row. That if the amount of offset is more than approximately 0.5 times the distance between two adjacent nominal dot positions in the grid, then the image to be printed is shifted approximately an equal amount and in the opposite direction as the offset. That the amount of the offset is less than a distance between two adjacent nominal dot positions in the grid. That the amount of the offset is substantially equal a distance between two adjacent nominal dot positions in the grid. That the amount of the offset is more than a distance between two adjacent nominal dot positions in the grid. That the printhead structure has the capability to place pigment particles at dot positions outside the grid with nominal dot positions. That the printhead structure comprises at least one additional aperture in relation to the number of apertures needed to place pigment particles at the nominal dot positions of the grid. That the offset of two adjacent rows is different as to amount and/or direction. That the direction of the offset is different from row to row with an offset. That only predetermined rows comprise an offset. That every other row comprises an offset. That the offset varies randomly as to amount and/or direction. That the offset varies systematically as to amount and/or direction. That all rows of the grid comprise an offset with the same amount and direction. That the control unit to enable the offset, controllably adjusts the deflection voltages to thereby enable deflection of pigment particles against predetermined dot position not in correspondence with the nominal dot positions of the grid. That the modified placement of pigment particles is so arranged that a matrix with the size D by D arbitrarily placed within the grid always comprises dot positions onto which at least two apertures will place pigment particles according to the modified placement of pigment particles, where D is the number of dots a single aperture can place pigment particles in a row. That there is a repetition D as to the number of rows comprising an offset with the same amount and direction, and e.g. rows not comprising an offset, where D is the number of dots a single aperture can place pigment particles in a row. This will, for example, give D rows of a right offset, D rows of no offset, D ^'rows of a left offset D rows of a rigth offset and so on, or for example D rows of a right offset, D rows of no offset, D rows of a right offset and so on, or for example D rows of a left offset, D rows of a right offset, D rows of a left offset and so on.

According to a second manner of modifying the placement of pigment particles according to the invention, in embodiments comprising dot deflection where each aperture addresses more than one dot position on a row, for at least dot positions which are addressed by two apertures, along at least one row, the control unit modifies the deflection voltages in such a way that each such dot position that is addressed by an aperture is only adjacent dot positions along the row in question that are addressed by another aperture or apertures, i.e there is a dot positioning cross-over of different apertures in such a way that for the dot positions concerned there are not two adjacent dot positions that are addressed by the same aperture. Figure 10a illustrates such a dot position cross-over on each row 821, 822, 823, 824, 825, 826 from three apertures with deflection to two dot positions from each aperture. Figure 10b illustrates such a dot position cross-over on each row 821, 822, 823, 824, 825, 826 from three apertures with deflection where each aperture addresses three dot positions . Further Figure 10c illustrates such a dot position cross-over on every other row 821, 822, 823, 824, 825, 826 from three apertures with deflection to two dot positions from each aperture. Figure lOd illustrates such a dot position cross-over on every other row 821, 822, 823, 824, 825, 826 from three apertures with deflection where each aperture addresses three dot positions . As mentioned previously the second manner of modifying the placement of pigment particles according to the invention involves embodiments comprising dot deflection where each aperture addresses more than one dot position on a row, for at least dot positions which are addressed by two apertures, along at least one row. As is illustrated in the Figures 10a to lOd there are a number of different possibilities. Further possibilities are possible, a few of which will be described in the following, these can be combined in any arbitrary manner as long as they are not conflicting possibilities. The control unit can modify the deflection voltages of substantially all dot positions of a row in such a way that each of such a dot position that is addressed by an aperture is only adjacent dot positions along the row in question, that are addressed by a different aperture or apertures . That the control unit modifies the deflection voltages of all dot positions of a section in such a way that each such dot position that is addressed by an aperture is only adjacent dot positions, along the row in question, that are addressed by a different aperture or apertures, all dot positions of a section comprising more dot positions than are addressed by two apertures and less dot positions than substantially all dot positions of a row.

According to a third manner of modifying the placement of pigment particles according to the invention, in embodiments comprising dot deflection where each aperture addresses at least three dot positions on a row, for at least dot positions which are addressed by two apertures, along at least one row, the control unit modifies the deflection voltages in such a way that each such dot position that is addressed by an aperture in question is adjacent at least one dot position along the row in question that is addressed by a different aperture, i.e there is a dot positioning cross-over of different apertures in such a way that each dot position is adjacent at least one dot position that is addressed by a different aperture. Figure lOe illustrates such a dot position cross-over on each row 821, 822, 823, 824, 825, 826 from three apertures with deflection where each aperture addresses three dot positions. Figure lOf illustrates such a dot position cross-over on every other row 821, 822, 823, 824, 825, 826 from three apertures with deflection where each aperture addresses three dot positions .

As mentioned previously the third manner of modifying the placement of pigment particles according to the invention involves embodiments comprising dot deflection where each aperture addresses at least three dot positions on a row, for at least dot positions which are addressed by two apertures, along at least one row. As is illustrated in the Figures lOe and lOf there are a number of different possibilities. Further possibilities are possible, a few of which will be described in the following, these can be combined in any arbitrary manner as long as they are not conflicting possibilities. The control unit modifies the deflection voltages of substantially all dot positions of a row in such a way that each such dot position is addressed by a different aperture than an aperture at least one dot position adjacent to it along the row in question is addressed by. The control unit modifies the deflection voltages of all dot positions of a section in such a way that each such dot position is addressed by a different aperture than an aperture at least one dot position adjacent to it along the row in question is addressed by, dot positions of a section comprising more dot positions than are addressed by two apertures and less than substantially all dot positions of a row. According to a fourth manner of modifying the placement of pigment particles according to the invention the control unit offsets the positions of at least two adjacent dot positions, of at least one row, independently and in a random manner with regard to direction and amount along the row in question, in relation to the nominal dot positions in the grid. Figure 11a illustrates such a random offset on every row for three apertures on six rows 821, 822, 823, 824, 825, 826. Figure lib illustrates such a random offset on every row for three apertures, with deflection to two dot positions, on six rows 821, 822, 823, 824, 825, 826. Figure lie illustrates such a random offset on every row for three apertures, with deflection where each aperture addresses three dot positions, on six rows 821, 822, 823, 824, 825, 826.

As mentioned previously the fourth manner of modifying the placement of pigment particles according to the invention involves the position of at least two adjacent dot positions, of at least one row. As is illustrated in the Figures 11a to lie there are a number of different possibilities. Further possibilities are possible, a few of which will be described in the following, these can be combined in any arbitrary manner as long as they are not conflicting possibilities . That only predetermined rows comprise an offset. That every other row comprises an offset . That all rows of the grid comprise an offset . That substantially all dot positions of a row are offset independently in a random manner with regard to direction and amount. That a section of adjacent dot positions are offset independently in a random manner with regard to direction and amount, a section of adjacent dot positions comprising more than two adjacent dot positions and less than substantially all dot positions of a row. That substantially all dot positions of a row which are addressed by deflection are offset independently in a random manner with regard to direction and amount. That a section of adjacent dot positions which are adressed by deflection are offset independently in a random manner with regard to direction and amount, a section of adjacent dot positions adressed by deflection comprising more than two adjacent dot positions addressed by dot deflection and less than substantially all dot positions of a row addressed by dot deflection. That the amount of the offset of any dot position is less than a distance between two adjacent nominal dot positions in the grid.

According to a fifth manner of modifying the placement of pigment particles according to the invention in embodiments comprising dot deflection where each aperture addresses more than one dot position on a row, preferably at least three dot positions on a row, for at least two adjacent dot posisitons of at least dot positions which are addressed by deflection by two apertures, along at least one row, in relation to the respective nominal dot positions in the grid, preferably only dot positions that are addressed by deflection are offset independently and in a random manner with regard to direction and amount along the row in question. Figure lid illustrates such a random offset on every row for three apertures, with deflection where each aperture addresses three dot positions, on six rows 821, 822, 823, 824, 825, 826, of only the deflected dot positions 809, 811. The center, undeflected dot positions 810 are not offset.

As mentioned previously the fifth manner of modifying the placement of pigment particles according to the invention involves the position of at least two adjacent dot positions, of at least one row. As is illustrated in the Figure lid there is one possibility. Further possibilities are possible, a few of which will be described in the following, these can be combined in any arbitrary manner as long as they are not conflicting possibilities. That only dot positions that are addressed by deflection are offset independently and in a random manner with regard to direction and amount along the row in question, in relation to the nominal dot positions in the grid. That at least all dot positions that are addressed by two apertures are offset independently and in a random manner with regard to direction and amount along the row in question, in relation to the nominal dot positions in the grid. That the control unit to enable the offset, controllably adjusts the deflection voltages to thereby enable deflection of pigment particles against predetermined dot position not in correspondence with the nominal dot positions of the grid. That only predetermined rows comprise an offset. That every other row comprises an offset. That all rows of the grid comprise an offset. That substantially all dot positions of a row are offset independently in a random manner with regard to direction and amount. That a section of adjacent dot positions are offset independently in a random manner with regard to direction and amount, a section of adjacent dot positions comprising more than two adjacent dot positions and less than substantially all dot positions of a row. That substantially all dot positions of a row which are addressed by deflection are offset independently in a random manner with regard to direction and amount. That a section of adjacent dot positions which are adressed by deflection are offset independently in a random manner with regard to direction and amount, a section of adjacent dot positions adressed by deflection comprising more than two adjacent dot positions addressed by dot deflection and less than substantially all dot positions of a row addressed by dot deflection. That the amount of the offset of any dot position is less than a distance between two adjacent nominal dot positions in the grid.

According to the invention the control of the position of a dot location can thereby enable a mixing of aperture and/or deflection characteristics . A method of achieving this according to the invention is to divide a print sequence into different parts with different deflection voltages by time multiplexing, i.e. during a first part time dots with normal deflection are printed and during a second or more part time(s) dots with a modified deflection are printed. Another method/part method of achieving this according to the invention is to individually control the deflection of each print sequence, i.e. individually control the deflection voltages Dl and D2 of the deflection electrodes of each aperture to thereby individually adjust L_d and possibly introduce a deflection of a center dot. By individually controlling the deflection voltages during each print sequence for each aperture, each dot location can be repositioned in a direction which is mainly perpendicular to the direction of travel of the image receiving member, information carrier, or transfer belt. Thus according to the invention individual dot positions can be moved/adjusted leftward or rightward, i.e. in a direction perpendicular to the direction of travel of the information carrier, by adjusting the deflection voltages of the apertures.

According to the invention, preferably a preprocessor, a format controller, will processan image to be printed to identify and determine if the image to be printed comprises one or more of text, image, greyscale printing or black and white printing. In view of this determination the control unit preferably optimizes the modified placement of pigment particles as to sharpness and total perception of a printed image of the image to be printed. The offset and/or additional offset amount, direction and/or type of manner of modifying the placement of pigment particles can accordingly be changed midrow or on a row by row basis in dependence on the determination.

The control functions of a printer according to the invention is handled by a control unit which is schematically illustrated in Figure 12. 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 13 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 .

Claims

WHAT IS CLAIMED IS

1. A direct electrostatic printing device including a pigment particle source, a voltage source, a printhead structure, a control unit, and an image receiving member, the pigment particle source providing pigment particles, the image receiving member and the printhead structure are moving relative to each other during printing, the image receiving member having a first face and a second face, the printhead structure being placed in between the pigment particle source and the first face of the image receiving member, the voltage source being connected to the pigment particle source and the back electrode thereby creating an electrical field for transport of pigment particles from the pigment particle source toward the first face of the image receiving member, the printhead structure including control electrodes connected to the control unit to thereby selectively open or close apertures through the printhead structure to permit or restrict the transport of pigment particles, for placement of pigment particles at dot positions nominally in a grid comprising nominal dot positions, the dot positions being arranged adjacent each other in rows along a direction substantially perpendicular to the relative movement between the printhead structure and the image receiving member, to thereby enable the formation of a pigment image on the first face of the image receiving member, characterized in that the control unit offsets the positions of at least two adjacent dot positions, of at least one row, independently and in a random manner with regard to direction and amount along the row in question, in relation to the nominal dot positions in the grid, thus modifying the placement of pigment particles in a first manner.

2. A method for printing an image to an image receiving member, where the method comprises the following steps :

- providing pigment particles from a pigment particle source; moving an image receiving member and a printhead structure relative to each other during printing;

- creating an electrical field for transporting of pigment particles from the pigment particle source toward a first face of the image receiving member;

- a control unit selectively opening or closing apertures through the printhead structure to permit or restrict the transporting of pigment particles, for placement of pigment particles at dot positions nominally in a grid comprising nominal dot positions, the dot positions being arranged adjacent each other in rows along a direction substantially perpendicular to the relative movement between the printhead structure and the image receiving member, to thereby enable the formation of a pigment image on the first face of the image receiving member; characterized in that the method further comprises the following step:

- the control unit offsetting the positions of at least two adjacent dot positions, of at least one row, independently and in a random manner with regard to direction and amount along the row in question, in relation to the nominal dot positions in the grid, thus modifying the placement of pigment particles in a first manner .

3. The direct electrostatic printing device according to claim 1 or the method for printing an image to an image receiving member according to claim 2 , characterized in that the printhead structure further includes deflection electrodes connected to the control unit for controlling the deflection of pigment particles in transport by means of predetermined deflection voltages to thereby be able to deflect pigment particles against predetermined locations, and in that the control unit offsets at least two adjacent dot posisitons of at least dot positions which are addressed by deflection by two apertures, along at least one row, in relation to the respective nominal dot positions in the grid, thus modifying the placement of pigment particles in a second manner .

4. The direct electrostatic printing device according to claim 3 or the method for printing an image to an image receiving member according to claim 3 , characterized in that only dot positions that are addressed by deflection are offset independently and in a random manner with regard to direction and amount along the row in question, in relation to the nominal dot positions in the grid.

5. The direct electrostatic printing device according to claim 3 or the method for printing an image to an image receiving member according to claim 3 , characterized in that at least all dot positions that are addressed by two apertures are offset independently and in a random manner with regard to direction and amount along the row in question, in relation to the nominal dot positions in the grid.

6. The direct electrostatic printing device according to any one of claims 3 to 5 or the method for printing an image to an image receiving member according to any one of claims 3 to 5, characterized in that the control unit to enable the offset, controllably adjusts the deflection voltages to thereby enable deflection of pigment particles against predetermined dot position not in correspondance with the nominal dot positions of the grid.

7. The direct electrostatic printing device according to any one of claims 1, 3 to 6 or the method for printing an image to an image receiving member according to any one of claims 2 to 6, characterized in that only predetermined rows comprise an offset .

8. The direct electrostatic printing device ' according to any one of claims 1, 3 to 6 or the method for printing an image to an image receiving member according to any one of claims 2 to 6, characterized in that every other row comprises an offset .

9. The direct electrostatic printing device according to any one of claims 1, 3 to 6 or the method for printing an image to an image receiving member according to any one of claims 2 to 6, characterized in that all rows of the grid comprise an offset.

10. The direct electrostatic printing device according to any one of claims 1, 3 to 9 or the method for printing an image to an image receiving member according to any one of claims 2 to 9, characterized in that substantially all dot positions of a row are offset independently in a random manner with regard to direction and amount .

11. The direct electrostatic printing device according to any one of claims 1, 3 to 9 or the method for printing an image to an image receiving member according to any one of claims 2 to 9, characterized in that a section of adjacent dot positions are offset independently in a random manner with regard to direction and amount, a section of adjacent dot positions comprising more than two adjacent dot positions and less than substantially all dot positions of a row.

12. The direct electrostatic printing device according to any one of claims 3 to 9 or the method for printing an image to an image receiving member according to any one of claims 3 to 9, characterized in that substantially all dot positions of a row which are addressed by deflection are offset independently in a random manner with regard to direction and amount .

13. The direct electrostatic printing device according to any one of claims 3 to 9 or the method for printing an image to an image receiving member according to any one of claims 3 to 9, characterized in that a section of adjacent dot positions which are adressed by deflection are offset independently in a random manner with regard to direction and amount, a section of adjacent dot positions adressed by deflection comprising more than two adjacent dot positions addressed by dot deflection and less than substantially all dot positions of a row addressed by dot deflection.

14. The direct electrostatic printing device according to any one of claims 1, 3 to 13 or the method for printing an image to an image receiving member according to any one of claims 2 to 13, characterized in that the amount of the offset of any dot position is less than a distance between two adjacent nominal dot positions in the grid.

15. The direct electrostatic printing device according to any one of claims 1, 3 to 14 or the method for printing an image to an image receiving member according to any one of claims 2 to 14, characterized in that the control unit additionally offsets the position of at least two adjacent dot positions, of at least one row, an equal amount and direction along the row in question, in relation to the nominal dot positions in the grid, thus modifying the placement of pigment particles in a third manner.

16. The direct electrostatic printing device according to claim 15 or the method for printing an image to an image receiving member according to claim 15, characterized in that substantially all dot positions of a row are additionally offset an equal amount and direction.

17. The direct electrostatic printing device according to claim 15 or the method for printing an image to an image receiving member according to claim 15, characterized in that a section of adjacent dot positions are additionally offset an equal amount and direction, a section of adjacent dot positions comprising more than two adjacent dot positions and less than substantially all dot positions of a row.

18. The direct electrostatic printing device according to any one of claims 15 to 17 or the method for printing an image to an image receiving member according to any one of claims 15 to 17, characterized in that if the amount of additional offset is more than approximately 0.5 times the distance between two adjacent nominal dot positions in the grid, then the furthest dot position of the row in the direction of the additional offset is not used and an additional dot position is added and used at the opposite side of the row.

19. The direct electrostatic printing device according to any one of claims 15 to 18 or the method for printing an image to an image receiving member according to any one of claims 15 to 18, characterized in that if the amount of additional offset is more than approximately 0.5 times the distance between two adjacent nominal dot positions in the grid, then the image to be printed is shifted approximately an equal amount and in the opposite direction as the additional offset.

20. The direct electrostatic printing device according to any one of claims 15 to 19 or the method for printing an image to an image receiving member according to any one of claims 15 to 19, characterized in that the amount of the additional offset is less than a distance between two adjacent nominal dot positions in the grid.

21. The direct electrostatic printing device according to any one of claims 15 to 19 or the method for printing an image to an image receiving member according to any one of claims 15 to 19, characterized in that the amount of the additional offset is substantially equal a distance between two adjacent nominal dot positions in the grid.

22. The direct electrostatic printing device according to any one of claims 15 to 19 or the method for printing an image to an image receiving member according to any one of claims 15 to 19, characterized in that the amount of the additional offset is more than a distance between two adjacent nominal dot positions in the grid.

23. The direct electrostatic printing device according to any one of claims 15 to 22 or the method for printing an image to an image receiving member according to any one of claims 15 to 22, characterized in that the printhead structure has the capability to place pigment particles at dot positions outside the grid with nominal dot positions.

24. The direct electrostatic printing device according to claim 23 or the method for printing an image to an image receiving member according to claim 23, characterized in that the printhead structure comprises at least one additional aperture in relation to the number of apertures needed to place pigment particles at the nominal dot positions of the grid.

25. The direct electrostatic printing device according to any one of claims 15 to 24 or the method for printing an image to an image receiving member according to any one of claims 15 to 24, characterized in that the additional offset of two adjacent rows is different as to amount and/or direction.

26. The direct electrostatic printing device according to any one of claims 15 to 25 or the method for printing an image to an image receiving member according to any one of claims 15 to 25, characterized in that the direction of the additional offset is different from row to row with an additional offset.

27. The direct electrostatic printing device according to any one of claims 15 to 26 or the method for printing an image to an image receiving member according to any one of claims 15 to 26, characterized in that only predetermined rows comprise an additional offset.

28. The direct electrostatic printing device according to any one of claims 15 to 27 or the method for printing an image to an image receiving member according to any one of claims 15 to 27, characterized in that every other row comprises an additional offset.

29. The direct electrostatic printing device according to any one of claims 15 to 28 or the method for printing an image to an image receiving member according to any one of claims 15 to 28, characterized in that the additional offset varies randomly as to amount and/or direction.

30. The direct electrostatic printing device according to any one of claims 15 to 28 or the method for printing an image to an image receiving member according to any one of claims 15 to 28, characterized in that the additional offset varies systematically as to amount and/or direction.

31. The direct electrostatic printing device according to any one of claims 15 to 25 or the method for printing an image to an image receiving member according to any one of claims 15 to 25, characterized in that all rows of the grid comprise an additional offset with the same amount and direction.

32. The direct electrostatic printing device according to any one of claims 3 to 31 or the method for printing an image to an image receiving member according to any one of claims 3 to 31, characterized in that the control unit to enable the offset and or the additional offset, controllably adjusts the deflection voltages to thereby enable deflection of pigment particles against predetermined dot position not in correspondance with the nominal dot positions of the grid.

33. The direct electrostatic printing device according to any one of claims 15 to 32 or the method for printing an image to an image receiving member according to any one of claims 15 to 32, characterized in that the additional offset is so arranged that a matrix with the size D by D arbitrarily placed within the grid always comprises dot positions onto which at least two apertures will place pigment particles according to the offset, where D is the number of dots a single aperture can place pigment particles in a row.

34. The direct electrostatic printing device according to any one of claims 3 to 33, characterized in that for at least dot positions which are addressed by two apertures, along at least one row, the control unit modifies the deflection voltages in such a way that each such dot position that is addressed by an aperture is only adjacent dot positions along the row in question that are addressed by another aperture or apertures, thus modifying the placement of pigment particles in a fourth manner .

35. The direct electrostatic printing device according to claim 34 or the method for printing an image to an image receiving member according to claim 34, characterized in that the control unit modifies the deflection voltages of substantially all dot positions of a row in such a way that each of such a dot position that is addressed by an aperture is only adjacent dot positions along the row in question, that are addressed by a different aperture or apertures.

36. The direct electrostatic printing device according to claim 34 or the method for printing an image to an image receiving member according to claim 34, characterized in that the control unit modifies the deflection voltages of all dot positions of a section in such a way that each such dot position that is addressed by an aperture is only adjacent dot positions along the row in question, that are addressed by a different aperture or apertures, all dot positions of a section comprising more dot positions than are addressed by two apertures and less dot positions than substantially all dot positions of a row.

37. The direct electrostatic printing device according to any one of claims 3 to 33 or the method for printing an image to an image receiving member according to any one of claims 3 to 33, characterized in that for at least dot positions which are addressed by two apertures, along at least one row, the control unit modifies the deflection voltages in such a way that each such dot position that is addressed by an aperture in question is adjacent at least one dot position along the row in question that is addressed by a different aperture, thus modifying the placement of pigment particles in a fifth manner.

38. The direct electrostatic printing device according to claim 37 or the method for printing an image to an image receiving member according to claim 37, characterized in that the control unit modifies the deflection voltages of substantially all dot positions of a row in such a way that each such dot position is addressed by a different aperture than an aperture at least one dot position adjacent to it along the row in question is addressed by.

39. The direct electrostatic printing device according to claim 37 or the method for printing an image to an image receiving member according to claim 37, characterized in that the control unit modifies the deflection voltages of all dot positions of a section in such a way that each such dot position is addressed by a different aperture than an aperture at least one dot position adjacent to it along the row in question is addressed by, dot positions of a section comprising more dot positions than are addressed by two apertures and less than substantially all dot positions of a row.

40. The direct electrostatic printing device according to any one of claims 1, 3 to 39 or the method for printing an image to an image receiving member according to any one of claims 2 to 39, characterized in that the offset is enabled by deflection means.

41. The direct electrostatic printing device according to any one of claims 1, 3 to 40 or the method for printing an image to an image receiving member according to any one of claims 2 to 40, characterized in that an image to be printed is processed to determine if the image to be printed comprises one or more of text, image, greyscale printing or black and white printing and in that in view of this determination the control unit optimizes the modification of placement of pigment particles as to sharpness and total perception of a printed image of the image to be printed.

42. The direct electrostatic printing device according to claim 41 or the method for printing an image to an image receiving member according to claim 41, characterized in that the offset and/or additional offset amount, direction and/or type of manner of modifying the placement of pigment particles changes midrow or on a row by row basis in dependence on the determination.

43. The direct electrostatic printing device according to any one of claims 1, 3 to 42 or the method for printing an image to an image receiving member according to any one of claims 2 to 42, characterized in that the image receiving member is an information carrier .

44. The direct electrostatic printing device according to any one of claims 1, 3 to 42 or the method for printing an image to an image receiving member according to any one of claims 2 to 42 , characterized in that the image receiving member includes a transfer belt positioned at a predetermined distance from the printhead structure, the transfer belt being substantially of uniform thickness, whereby a pigment image is subsequently transferred to an information carrier.

45. The direct electrostatic printing device according to any one of claims 1, 3 to 44, characterized in that the image printing device is capable of printing color images and includes four pigment particle sources .

46. The direct electrostatic printing device according to any one of claims 1, 3 to 44 or the method for printing an image to an image receiving member according to any one of claims 2 to 44, characterized in that the printing device includes at least two pigment particle sources with corresponding control electrodes and apertures on and in at least one printhead structure.

47. The direct electrostatic printing device according to any one of claims 1, 3 to 44 or the method for printing an image to an image receiving member according to any one of claims 2 to 44, characterized in that the image printing device includes four pigment particle sources with corresponding control electrodes and apertures on and in at least one printhead structure.