WO2001017787A1 - Direct printing device and method - Google Patents

Direct printing device and method Download PDF

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Publication number
WO2001017787A1
WO2001017787A1 PCT/EP1999/006461 EP9906461W WO0117787A1 WO 2001017787 A1 WO2001017787 A1 WO 2001017787A1 EP 9906461 W EP9906461 W EP 9906461W WO 0117787 A1 WO0117787 A1 WO 0117787A1
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WO
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Patent type
Prior art keywords
deflection
back electrode
toner particles
image
apertures
Prior art date
Application number
PCT/EP1999/006461
Other languages
French (fr)
Inventor
Bo RYDSTRÖM
Original Assignee
Array Ab
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
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Publication date

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Classifications

    • BPERFORMING OPERATIONS; TRANSPORTING
    • B41PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41JTYPEWRITERS; SELECTIVE PRINTING MECHANISMS, e.g. INK-JET PRINTERS, THERMAL PRINTERS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/385Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by selective supply of electric current or selective application of magnetism to a printing or impression-transfer material
    • B41J2/41Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by selective supply of electric current or selective application of magnetism to a printing or impression-transfer material for electrostatic printing
    • B41J2/415Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by selective supply of electric current or selective application of magnetism to a printing or impression-transfer material for electrostatic printing by passing charged particles through a hole or a slit
    • B41J2/4155Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by selective supply of electric current or selective application of magnetism to a printing or impression-transfer material for electrostatic printing by passing charged particles through a hole or a slit for direct electrostatic printing [DEP]
    • GPHYSICS
    • G03PHOTOGRAPHY; CINEMATOGRAPHY; ELECTROGRAPHY; HOLOGRAPHY
    • G03GELECTROGRAPHY; ELECTROPHOTOGRAPHY; MAGNETOGRAPHY
    • G03G2217/00Details of electrographic processes using patterns other than charge patterns
    • G03G2217/0008Process where toner image is produced by controlling which part of the toner should move to the image- carrying member
    • G03G2217/0025Process where toner image is produced by controlling which part of the toner should move to the image- carrying member where the toner starts moving from behind the electrode array, e.g. a mask of holes

Abstract

In a direct printing device, charged toner particles are subjected to an electric field to transport them to an image-receiving medium (1). The electric field is generated by a back electrode (12) located below the image-receiving medium. A printhead (5) located in the electric field has a plurality of apertures through which toner particles are selectively transported. Ring electrodes (53) on each aperture control the opening and closing of the apertures. Deflection electrodes (54) are also provided on each aperture to selectively generate asymmetric electric fields around the apertures, causing toner particles to be deflected. By providing a deflection controller to alter the potential difference between the back electrode and deflection electrodes as a function of time and/or the distance between the printhead and image receiving member, and thereby the trajectory of the transported charged particles, a constant deflection across the printhead and thus improved print quality is obtained.

Description

Direct Printing Device and Method

Technical Field

The invention relates to a direct printing apparatus in which a computer generated image information is converted into a pattern of electrostatic fields, which selectively transport electrically charged particles from a particle source toward a back electrode through a printhead structure, whereby the charged particles are deposited in image configuration on an image receiving substrate caused to move relative to the printhead structure.

More specifically, the invention relates to a direct printing apparatus wherein the printhead structure causes at least some of the electrically charged particles to undergo a deflection prior to being deposited on the image receiving surface so as to increase the printable area of a receiving surface with a simplified printhead structure. The invention also relates to a method for improving the print quality and reducing manufacturing costs of such direct printing apparatus.

Background

U.S Patent No. 5,036,341 discloses a direct electrostatic printing device and a method to produce text and pictures with toner particles on an image receiving substrate directly from computer generated signals . Such a device generally includes a printhead structure provided with a plurality of apertures through which toner particles are selectively transported from a particle source to an image-receiving medium due to control in accordance with an image information. The printhead structure is formed by a lattice consisting of intersecting wires disposed in rows and columns. Each wire is connected to an individual voltage source. Initially the wires are grounded to prevent toner passing through the mesh. As a desired print location on the image receiving medium passes below an intersection, adjacent wires in a corresponding column and row are set to back potential to produce an electric field that draws the toner particles from the particle source. The toner particles are propelled through the square apertures formed by four crossed wires and deposited on the image-receiving medium in the desired pattern. A drawback with this construction of printhead is that individual wires can be sensitive to the opening and closing of adjacent apertures, resulting in imprecise image formation due to the narrow wire border between apertures .

This effect is mitigated in an arrangement described in US patent No. 5 847 733 by the present applicant. This proposes a control electrode array formed on an apertured insulating substrate. A ring electrode is associated with each aperture and is driven to control the opening and closing of the apertures to toner particles. Each aperture is further provided with deflection electrodes. These are controlled to selectively generate an asymmetric electric field around the aperture, causing toner particles to be deflected prior to their deposition on the imag -receiving medium. This process is referred to as dot deflection control (DDC) . This enables each individual aperture to address several dot positions. The print addressability is thus increased without the need for densely spaced apertures .

However, the actual position of a deflected dot relative to a dot formed by undeflected toner particles on the image receiving medium is affected not just by the electric field profile around the aperture, but also by the distance between the aperture, or printhead, and the image receiving medium. Accordingly variations in this distance, for example resulting from unevenness in the image receiving medium or the back electrode supporting this medium, or unparallelity between the printhead and the image receiving medium, will result in the relative positions of dots varying across the surface of the image. The print quality will thus be seriously degraded. Manufacturing measures designed to reduce unevenness or unparallelity are costly and furthermore not capable of eliminating the problem altogether. Similarly if paper is used as the image-receiving medium, significant differences in print quality will be observed for different paper thicknesses.

Thus there is a need for an direct electrostatic image forming arrangement and method that while permitting dot deflection control provides improved print quality that is independent of the paper type utilised and without an attendant increase in manufacturing costs .

Summary of the invention

According to the invention there is provided an image forming apparatus in which image information is converted into a pattern of electrostatic fields for modulating the transport of charged toner particles from a toner carrier towards an image receiving member. A back electrode for attracting charged toner particles is connected to a voltage source. A printhead structure is disposed between the toner carrier and the image-receiving member and includes plurality of apertures having associated control electrodes and deflection electrodes. Variable voltage sources are connected to the control electrodes to permit or restrict the transport of charged toner particles from the particle carrier through the apertures. Variable deflection voltage sources are connected to the deflection electrodes to generate asymmetric electric fields about an associated aperture. A deflection controller is provided to control the potential difference applied between said deflection electrode and said back electrode as a function of time and/or the distance between said printhead structure and image receiving member, in order to modify the trajectory of the transported charged particles. The present invention enables the deflection trajectory of the toner particles to be modified to render the printed image substantially insensitive to variations in the spacing between the printhead and the receiving medium.

By altering the deflecting electric field during flight of the toner particles, the shape of the trajectory can be changed. In accordance with a preferred embodiment of the invention the controller varies the amplitude of the applied potential with time during the transport of the charged toner particles such that the final portion of the particle trajectory is substantially normal to the receiving surface. In this manner the actual deflection experienced by a dot is rendered substantially insensitive to variations in the distance between the printhead and the image-receiving surface.

By modifying the deflecting electric field as a function of the spacing between the electrode and the image receiving member, the degree of deflection imposed on a dot can be varied to ensure that the absolute deflection of a dot on the receiving surface is substantially constant over the image receiving member, or between separate printing procedures. Hence variations in this distance can be actively compensated for this may be accomplished for individual apertures or groups of apertures across the printhead, or may be utilised to vary the deflection associated with all apertures for each separate print job. As compensation of this kind may become necessary over time, the controller may also vary the applied deflection potential with time for each aperture, or group of apertures, independently. The deflecting electric field may also be altered by varying the back electrode voltage relative to the deflecting electrode voltages . Thus the controller preferably also or alternatively controls the back electrode voltage. The back electrode may also be divided into segments, allowing the potential difference between the back electrode and deflection electrodes to be altered by different degrees in different areas.

Preferably a means are provided whereby the distance between the printhead and the image-receiving surface can be measured. The controller then controls the potential applied to the deflection electrodes and/or back electrode in accordance with the measured distance to obtain the desired displacement of a printed dot relative to the aperture in a direction transverse to the motion of the image receiving surface.

It is further advantageous to provide the controller with the ability to alter both the shape of the particle trajectory and its initial angle of deflection by modifying the potential amplitudes applied. In this way, the apparatus would be insensitive to small variations in spacing, for example due to surface unevenness, while permitting the apparatus to accommodate further spacing variations, for example due to different image receiving surfaces and/or thicknesses, without detriment to the print quality.

Brief description of the drawings

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

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

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

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

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

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

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

Fig.5 depicts three sequences of dot deflection,

Fig.6 illustrates a control and deflection pulse for obtaining the three sequences of dot deflection shown in Fig. 5 in accordance with the present invention,

Fig.7 illustrates the trajectory of a toner particle deflected in accordance with one embodiment of the invention,

Fig.8 illustrates the trajectory of a toner particle deflected in accordance with a further embodiment of the present invention,

Fig.9 is a block diagram schematically illustrating the deflection control, and Fig.10 schematically depicts an embodiment of a back electrode according to the invention permitting control of the degree of deflection.

Detailed description

As shown in Fig.l, an image forming apparatus in accordance with a first embodiment of the present invention comprises at least one print station, preferably four print stations (Y, M, C, K) , an intermediate image receiving member 1, a driving roller 10, at least one support roller 11, and preferably several adjustable holding elements 12. The four print stations are arranged in relation to the intermediate image-receiving member 1. The image-receiving member is mounted over the driving roller 10. In the illustrated embodiment the image receiving member is a transfer belt 1, however, it will be understood that toner particles could be projected directly onto paper, or alternatively, that a solid drum be provided for receiving the image and subsequently transferring this to paper or other final medium. The at least one support roller 11 is provided with a mechanism for maintaining the transfer belt 1 with a constant tension, while preventing transversal movement of the transfer belt 1. The holding elements 12 are for accurately positioning the transfer belt 1 with respect to each print station.

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

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

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

As shown in Fig.2, a print station in an image forming apparatus in accordance with the present invention includes a particle delivery unit 3 preferably having a replaceable or refillable container 30 for holding toner particles, the container 30 having front and back walls

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

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

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

As shown in Fig.4a, 4b, 4c, a printhead structure 5 in an image forming apparatus in accordance with the present invention comprises a substrate 50 of flexible, electrically insulating material such as polyimide or the like, having a predetermined thickness, a first surface facing the developer sleeve 33, a second surface facing the transfer belt 1, a transversal axis 51 extending parallel to the rotation axis of the developer sleeve 33 across the whole print area, and a plurality of apertures 52 arranged through the substrate 50 from the first to the second surface thereof. The first surface of the substrate is coated with a first cover layer 501 of electrically insulating material, such as for example parylene . A first printed circuit, comprising a plurality of control electrodes 53 disposed in conjunction with the apertures, and, in some embodiments, shield electrode structures (not shown) arranged in conjunction with the control electrodes 53, is arranged between the substrate 50 and the first cover layer 501. The second surface of the substrate is coated with a second cover layer 502 of electrically insulating material, such as for example parylene. A second printed circuit, including a plurality of deflection electrodes 54, is arranged between the substrate 50 and the second cover layer 502. The printhead structure 5 further includes a layer of antistatic material (not shown) , preferably a semiconductive material, such as silicon oxide or the like, arranged on at least a part of the second cover layer 502, facing the transfer belt 1. The printhead structure 5 is brought in cooperation with a control unit (not shown) comprising variable control voltage sources connected to the control electrodes 53 to supply control potentials which control the amount of toner particles to be transported through the corresponding aperture 52 during each print sequence . The control unit further comprises deflection voltage sources (not shown) connected to the deflection electrodes 54 to supply deflection voltage pulses which controls the convergence and the trajectory path of the toner particles allowed to pass through the corresponding apertures 52. The control unit, in some embodiments, even includes a shield voltage source (not shown) connected to the shield electrodes to supply a shield potential which electrostatically screens adjacent control electrodes 53 from one another, preventing electrical interaction therebetween. In a preferred embodiment of the invention, the substrate 50 is a flexible sheet of polyimide having a thickness on the order of about 50 microns. The first and second printed circuits are copper circuits of approximately 8-9 microns thickness etched onto the first and second surface of the substrate 50, respectively, using conventional etching techniques. The first and second cover layers (501, 502) are 5 to 10 microns thick parylene laminated onto the substrate 50 using vacuum deposition techniques. The apertures 52 are made through the printhead structure 5 using conventional laser micromachining methods. The apertures 52 have preferably a circular or elongated shape centered about a central axis, with a diameter in a range of 80 to 120 microns, alternatively a transversal minor diameter of about 80 microns and a longitudinal major diameter of about 120 microns. Although the apertures 52 have preferably a constant shape along their central axis, for example cylindrical apertures, it may be advantageous in some embodiments to provide apertures whose shape varies continuously or stepwise along the central axis, for example conical apertures.

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

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

The second printed circuit comprises the plurality of deflection electrodes 54, each of which is divided into two semicircular or crescent shaped deflection segments 541, 542 spaced around a predetermined portion of the circumference of a corresponding aperture 52. The deflection segments 541, 542 are arranged symmetrically about the central axis of the aperture 52 on each side of a deflection axis 543 extending through the center of the aperture 52 at a predetermined deflection angle d to the longitudinal direction. The deflection axis 543 is dimensioned in accordance with the number of deflection sequences to be performed in each print cycle in order to neutralize the effects of the belt motion during the print cycle, to obtain transversally aligned dot positions on the transfer belt. For instance, when using three deflection sequences, an appropriate deflection angle is chosen to arctan(l/3), i.e. about 18,4°. Accordingly, the first dot is deflected slightly upstream with respect to the belt motion, the second dot is undeflected and the third dot is deflected slightly downstream with respect to the belt motion, thereby obtaining a transversal alignment of the printed dots on the transfer belt. Accordingly, each deflection electrode 54 has an upstream segment 541 and a downstream segment 542, all upstream segments 541 being connected to a first deflection voltage source DI, and all downstream segments 542 being connected to a second deflection voltage source D2. In accordance with the invention, the deflection voltage sources DI and D2 are controlled by a control unit 65 (see Fig. 9), which will be discussed in more detail below. Three deflection sequences (for instance: D1<D2; D1=D2; D1>D2) can be performed in each print cycle, whereby the difference between DI and D2 determines the deflection trajectory of the toner stream through each aperture 52, and thus the dot position on the toner image.

Fig- 5 shows the dot positions obtained in each deflection sequence. In this figure the motion of the transfer belt is given by the arrow 61. In the first sequence, the deflection forces act on the toner particles in the direction indicated by the arrow 62. The first dot is thus deposited on the transfer belt 1 upstream of the aperture 52. In the second sequence a substantially symmetrical electric field is generated about the aperture 52 causing the toner particles to be centred in the aperture and reach the transfer belt 1 undeflected. In the final sequence, the toner particles experience a deflection force 63, and reach the transfer belt 1 at a position downstream of the aperture 52. The distance between adjacent dots is given as .

Fig. 6 depicts the control pulse from the various voltage sources DI , D2 during the three print sequences D1<D2; D1=D2 ; D1>D2 illustrated in Fig. 5 in accordance with a preferred embodiment of the invention. In the first sequence, the selected control voltage sources supply a positive voltage pulse Vb to the associated control electrodes 53 for a period tb. For a back electrode potential of between around 0.5 kV and 1.5 kV the control voltage pulse is typically of the order of about 300 V. During this first period the voltage sources DI, D2 likewise supply positive potentials Vd to the associated deflection electrode segments 541, 542, however the voltage source DI to the deflection electrode segment 541 applies a higher voltage pulse at the beginning of the period tb to generate a higher initial field. Toner particles located on the developer sleeve 33 above the selected control electrode 53 are accordingly propelled towards the control electrode 53 and centred through the aperture 52. In the subsequent period, denoted by tw in Fig. 6, the potential on the control electrode 53 is pulled negative, causing any lagging negatively charged toner particles still in flight between the printhead structure 5 and the developer sleeve 33 to be repelled back to the developer sleeve 33. At around the same time, the deflection voltage source DI is pulled negative to exert a deflecting force on the negatively charged toner particles that have passed through the aperture 52. The voltage change on the deflection electrodes 541, 542 need not occur simultaneously with that on the control electrode, but may occur with some delay or advance. The deflection voltage source D2 associated with the deflection segment 542 is also reduced in amplitude in the period tw. This segment 542 is preferably set to a voltage that has a neutral influence on the electric field generated between the annular control electrode 53 and the back electrode. This voltage will be slightly positive with respect to the voltage of the control electrode due to the small distance separating these two electrodes. For example, if the back electrode is at 800 V and the control electrode at -50 V, then a suitable voltage for the deflection electrode voltage D2 would be 178 V. Alternatively, the deflection electrode segment 542 could be set to a negative voltage relative to a neutral voltage. This has the effect of focussing the beam of charged toner particles to obtain a sharper dot. Preferably, this focussing effect is generated towards the end of the period tw as shown in the figure. The voltage source DI gradually increases the potential applied to the electrode segment 541 in the period tw until the electrode is at a potential that exerts a focussing affect on the toner particles together with the electrode segment 541. In the illustrated embodiment this voltage is ground potential . The resulting force exerted on the charged toner particles propelled towards the transfer belt 1 varies from a high repelling force to a low repelling force relative to the deflecting electrode during the flight of the toner particle towards the transfer belt 1.

In the next sequence, a second voltage pulse is applied to the control electrode 53 and also to the two deflection electrode segments 541, 542. Subsequently the control electrode 53 is again restored to the slightly negative voltage, and so are the two deflection electrode segments 541 and 542. This focuses the beam of charged toner particles propelled through the toner particle beam through the aperture 52 in order to obtain a sharper dot. In the third sequence, the control electrode 53 is driven as for the first two sequences and the voltages supplied by the voltage sources DI and D2 are effectively reversed compared to the first sequence. Accordingly, the toner particles experience first a strong repelling force away from the deflection electrode segment 542; this repelling force is subsequently reduced.

The effect of the generated electric field on the charged toner particles will be described with reference to Figs. 7 and 8.

Figs. 7 and 8 show a sectional view through an aperture 52 at an angle substantially perpendicular to the deflection axis 543 shown in Fig. 4b. The printhead is represented schematically by the control electrode 53 and the deflection electrode segments 541, 542. Fig. 7 illustrates a simplified trajectory v of charged toner particles experiencing a substantially constant net deflecting force away from the deflecting electrode segment 541. This deflection corresponds to a first print sequence. This deflecting force is caused by applying a constant deflecting and/or attracting potential across the deflection electrodes during the period tw shown in Fig. 6. In other words, the voltage levels applied to deflection electrodes 54 are constant in the period tw. The trajectory shape is substantially straight and inclined at a constant angle from an orthogonal projection of the aperture axis A. While many different factors influence the actual path described by charged toner particles, including variations in mass and charge, the illustrated trajectory is considered to be a good approximation of those observed experimentally.

It is apparent from this drawing that any variation in the distance D between the printhead structure 5 and the transfer belt 1 will affect the deflection distance L if the same deflection force, and therefore the same deflection angle, is maintained. For example, as illustrated in the figure, if the distance between the printhead and the transfer belt 1 is reduced to D1, the deflection length will also be reduced to L' . Such a variation in length distorts the image obtained, since the deflected dots are printed closer or further away from the central dot with decreasing or increasing distance D, respectively. Constant variations in the distance D transverse to the motion of the transfer belt 1 result in dark and/or blank lines across the printed image .

Fig. 8 shows the trajectory of a toner particle beam deflected in accordance with the deflection voltages illustrated in Fig. 6. From this figure it is evident that the majority of the desired deflection occurs close to the printhead. This is caused by the initial strong deflecting force pushing the toner particles away from the electrode segment 541. With the reduction in voltage from voltage source Dl, the deflecting force weakens and the trajectory tends to describe path that is substantially normal to the transfer belt 1. This form of trajectory advantageously permits some change in the distance D between the printhead and the transfer belt 1, without altering the actual deflection length L. In effect, the deflection imposed on a toner particle beam is insensitive to variations in the distance D.

While the deflection voltage variations shown in Fig. 6 are stepped, it will be appreciated that other forms of ramped voltage variations will alter the trajectory in the desired manner. Specifically, the variable voltage waveform may be continuous, for example, varying linearly with time with an appropriate slope. Since the trajectory is dependent on the mass and charge of the toner particles, the toner material will obviously strongly influence the choice of pulse waveform that gives the best results in any individual case. Thus the voltage pulse may be most effective when it has an exponential or logarithmic variation with time. It is also possible the particular toner materials may require a differently varying deflection pulse to describe the desired trajectory illustrated in Fig. 8. In particular, the particles may require an initial net repelling force followed by an attracting force relative to the deflecting electrode segment. It will appreciated that while in Fig. 6 the pulse waveform is controlled to vary for only the deflecting electrode segment, the voltage waveform of the attracting electrode segment could be varied to the same effect instead, or even in addition.

Returning again to Fig. 7, if the same deflection length L is required for a different distance D', the deflection angle must be varied. This is illustrated by the dashed trajectory v' which terminates at the transfer belt 1 at a distance D' from the printhead. The trajectory v' is correspondingly shorter than the trajectory v. In accordance with a further embodiment of the invention, this selective change in trajectory length is utilised to compensate for variations in the distance D. More specifically, the degree of deflection of a dot (i.e. the angle of deflection of the particle) can be altered by increasing or decreasing the amplitude of the constant deflecting voltage applied to the deflecting electrodes 54 in the period tw. Thus if the spacing between the printhead 5 and the transfer belt is known to decrease progressively in one direction transverse to the motion of the transfer belt, this variation can be compensated for by progressively increasing the deflecting voltages applied to the deflecting electrodes 54 in the same direction. Similarly, the effects of an increase in the spacing in a particular area can be mitigated by decreasing the deflection voltage of electrodes 54 in that area. This flexibility further allows the deflection across the whole printhead structure (5) to be varied on a per print job basis. This is of particular advantage when the toner particles are propelled directly onto paper without the intermediary of the transfer belt . In this way a uniform print quality can be obtained for paper or other print media of varying degrees of thickness .

A preferred arrangement for accomplishing this compensation is schematically shown in Fig. 9. In this arrangement the deflection control unit 65, which may take to form of a dedicated deflection controller or be incorporated in a general voltage controller utilised to control both the deflection electrodes and the control electrodes, is connected to variable voltages sources Dl and D2 that supply deflecting voltages to the deflection electrode segments 541 and 542, respectively, disposed on the printhead 5. The deflection electrode segments 541, 542 are divided into n groups with all the segments 541 contained in a group and disposed on one side of deflection axis 543 being connected together. Similarly all electrode segments 542 of a group that are disposed on the other side of the deflection axis are connected together. Several pairs of voltage sources Dl1 to Din and O2. to D2n are provided, each being connected to one group of connected electrode segments. Thus, for example voltage source Dlt is connected to all electrode segments

541 contained in a first group and voltage source Dl2 is connected to all electrode segments 542 in the first group, and so on. Preferably, a measuring unit 66 is arranged to determine the distance between the printhead 5 and the transfer belt 1 and relay this reading to the deflection control unit 65. The deflection control unit 65 then controls the amplitude of the voltages supplied by the voltage supplies Din and D2n as a function of the measured distance. In this manner variations across the width of the transfer belt 1 can be compensated for.

The number of groups provided depends on the degree of variation in spacing expected. Also the size of the groups, i.e. the number of apertures 52 contained in each group may vary, depending on the likely spacing variations that will be encountered. For instance, if variations in spacing are more severe at the edges of the printhead 5 it might be preferably to provide smaller separately controlled groups of deflection electrodes at these edges than in the centre of the printhead. In this way, the differences in distance D at these edges may be compensated for more effectively. The measuring unit 66 preferably provides information on the spacing D at different positions along the printhead. This unit 66 may be configured to measure the distance between the printhead 5 and the transfer belt 1 either mechanically, or optically, for example using interference means. Preferably, however, the measuring unit 66 monitors the distance between deflected and undeflected dots deposited on the image transfer belt 1 or on the final information carrier 2. As discussed with reference to Fig. 5, the deflection between the dots deposited in sequences 1 and 3 relative to sequence 2 will depend on the gap between the developer sleeve 33 and the holding member 12, or receiving surface 1. The measuring unit 66 may include, but is not limited to, a series of photo-emitters and detectors directed parallel to the direction of motion of the transfer belt 1 to detect variations in the positions of the belt 1 and/or the printhead structure 5. These sensors could be located downstream of the printhead 5 relative to the direction of movement of the transfer belt 1. Alternatively, sensors could be integrated in the surface of the printhead 5 itself to measure the distance of electromagnetic radiation reflected from the belt 1. The degree or angle of deflection experienced by a toner particle is dependent on the field lines about the apertures 52. These field lines are varied by the deflection electrode potentials, but they may also be modified by the potentials applied by the back electrode 12. To enable the electric field to be altered from the lower end of the toner path, the deflection control unit 65 of Fig. 9 also controls the voltages applied to the back electrode 12 by a back electrode voltage source B. Thus if the deflection angles need to be modified by the same amount across the whole of the printhead 5, for example for a different thickness of paper, this may be achieved by varying only the back electrode potential. However, depending on the required variation, the deflection control unit 65 may alter both the deflection potentials and the back electrode potentials.

Turning now to Fig. 10, there is shown a further embodiment of the back electrode or holding element 12 which permits the deflection angle of the toner particles to be varied as described with reference to Fig. 7. In this embodiment the back electrode 12 is divided into a number of physically separate and electrically insulated segments 121. The deflection control unit 65 controls the voltage applied to each segment 121 via a number of back electrode voltages sources, Bl to Bn. Thus the deflection experienced by each toner particle as it emerges from an aperture depends not only on the deflection electrode potential but also on the back electrode segment potential. As for the groups of deflection electrodes, the number of segments depends on the likely degree of spacing variation expected, or the degree of compensation desired. Preferably at least two segments 121 are provided. While the provision of separate segments 121 is relatively easy to implement, the interface between adjacent segments may disrupt the local electric fields and cause anomalies in the deflection of particles. A smooth transition between areas of different potential would be preferable. This may be achieved by providing a back electrode of material with varying resistance along its length. The potential at a point on the surface of the back electrode would then depend on the position of this point from the voltage source.

The arrangement of Fig. 9 may also be utilised to accomplish the modified trajectory of toner particles described with reference to Figs. 6 and 8. In this case, the measuring unit 66 would not be always required, since the modified trajectory is substantially insensitive to variations in the spacing D. Moreover, the deflection control unit 65 would not necessarily control the deflection electrodes separately, since the same, varying, potentials could be applied to all electrodes 54. However, it will be appreciated by those skilled in the art that the two forms of deflection control illustrated in Figs. 7 and 8 can very well be combined in a single system. This would be of particular interest when, for example, the toner particles are applied directly to paper or other image receiving media that may vary in thickness. Thus if the range of insensitivity obtained by the variation of deflection during the flight of the toner particles, i.e. the length of the lower portion of the trajectory illustrated in Fig. 7 that is substantially normal to the receiving surface 1, is insufficient to compensate for these differences in thickness, the deflection amplitude applied to all, or select ones, of the deflection electrodes could be modified by a constant amount throughout the deflection.

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
l.An image forming apparatus wherein image information is converted into a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier toward a back electrode member, said image forming apparatus including: at least one background voltage source coupled with said back electrode member for producing a background electric field; a printhead structure disposed between said particle carrier and said back electrode in said background electric field and including a plurality of apertures, control electrodes and deflection electrodes being associated with the apertures; control voltage sources for supplying control potentials to said control electrodes in accordance with the image information to selectively permit or restrict the transport of charged toner particles from the particle carrier through the apertures; deflection voltage sources for selectively applying potentials to said deflection electrodes relative to the back electrode member to generate an asymmetric electrical field about said apertures to cause said charged toner particles to be deflected when transported from said particle carrier towards said back electrode member; an image-receiving member caused to move in relation to the printhead structure for intercepting the transported charged particles in image configuration; characterized in that the image forming apparatus further includes means (65) for controlling said deflection voltages sources and/or said background voltage sources to modify the relative potential difference between said deflection electrodes (54) and said back electrode member (12) as a function of time and/or spacing between said printhead structure (5) and image receiving member (1) so as to modify the trajectory of the deflected charged particles.
2. An image forming apparatus as defined in claim 1, characterised in that said controlling means (65) are adapted to vary the amplitude of a potential applied to the deflection electrodes (54) during the transport of the charged toner particles towards the back electrode member (12) such that the toner particles describe a deflected trajectory that is substantially normal to said receiving surface (1) over at least a final portion of said trajectory.
3. An image forming apparatus as defined in claim 2, characterized in that the controlling means (65) are adapted to control the application of a ramped potential to said deflection electrodes (54) .
4. An image forming apparatus as defined in claim 2 or 3 , characterized in that the controlling means (65) are adapted to control the application of a discontinuously varying potential to said deflection electrodes (54) .
5. An image forming apparatus as defined in any one of claims 2 to 4, characterized in that the controlling means (65) are adapted to control the application of a potential to said deflection electrodes (54) that exerts an electrostatic force on said charged toner particles varying from a high deflecting force to a low deflecting force .
6. An image forming apparatus as defined in any one of claims 1 to 5, wherein said back electrode member is substantially elongate and said apertures (52) are arranged in an elongate pattern in said printhead structure (5) , the longitudinal directions of said pattern and said back electrode being substantially parallel and transverse to the direction of movement of the image receiving member, characterised in that said controlling means (65) controls the voltage amplitudes of said deflection electrodes and the back electrode member to generate a varying relative potential difference between the deflection electrodes (54) and the back electrode member (12) in said longitudinal direction.
7. An image forming apparatus as defined in claim 6, characterised in that the apertures (52) are divided into at least two groups in said longitudinal direction, wherein said controlling means (65) controls the amplitude of the deflecting control potential applied to the deflection electrodes (54) of each group separately.
8. An image forming apparatus as defined in claim 7, characterized in that each of said groups include adjacently disposed apertures (52) .
9. An image forming apparatus as defined in any one of claims 1 to 8 , characterised in that the back electrode is divided into at least two segments (121) in said longitudinal direction, wherein said controlling means (65) controls the amplitude of the back electrode potential applied to each segment (121) differently.
10. An image forming apparatus as defined in any one of claims 1 to 9, characterized in that means (66) are provided for measuring the distance between said printhead structure (5) and said image receiving member (1) , wherein said controlling means (65) controls the relative potential amplitude applied between said deflection electrodes (54) and said back electrode member (12) through said deflection voltages sources and/or said background voltage sources in accordance with the measured distance to obtain essentially the same degree of deflection on said image receiving member (1) in said longitudinal direction.
11. An image forming apparatus as claimed in claim 10, characterized in that said measuring means (66) monitor the distance between dots of deflected charged toner particles and non-deflected charged toner particles deposited on said image receiving member (1) or on an information carrier (2) adapted to receive a transferred image from said image receiving member (1) to determine variations in the spacing between said printhead structure (5) and image receiving member (1) .
12. An image forming apparatus as claimed in any previous claim, characterized by including sets of at least two deflection electrodes (541, 542) associated with each aperture (52) , wherein said controlling means (52) are adapted to control the amplitude of a voltage applied to each deflection electrode in a set independently, to alter the degree of deflection of said charged toner particles .
13. An image forming method, wherein image information is converted into a pattern of electrostatic fields for modulating the transport of charged toner particles from a particle carrier toward a back electrode member, said method further including: providing a printhead structure having a plurality of apertures between said particle carrier and said back electrode member; providing an image receiving member between said printhead structure and back electrode member for receiving deposited charged toner particles; providing control electrodes associated with said apertures, providing deflection electrodes associated with said apertures ; applying background voltage to said back electrode member for producing a background electric field which enables a transport of charged toner particles from said particle carrier towards said back electrode member; selectively applying variable voltages to the control electrodes to influence the background electric field, thereby permitting or restricting transport of charged toner particles towards the back electrode member; selectively applying voltages to the deflection electrodes to generate asymmetric electric fields about the apertures to produce deflection forces acting on the charged toner particles; characterized by modifying the relative potential difference between said deflection electrodes (54) and said back electrode member (12) by controlling the potential applied to said deflection electrodes (54) and/or said back electrode member (12) as a function of time and/or spacing between said printhead structure (5) and image receiving member (1) so as to modify the trajectory of the deflected charged particles.
14. A method as defined in claim 13, characterised by varying the amplitude of the potential applied to the deflecting electrodes (54) during the transport of the charged toner particles such that the toner particles describe a deflected trajectory that is substantially normal to said receiving surface (1) over at least a final portion of said trajectory.
15. A method as defined in claim 13 or 14, characterized by applying a ramped potential to said deflection electrodes (54) .
16. A method as defined in any one of claims 13 to 15, characterized by applying a discontinuously varying potential to said deflection electrodes (54) .
17. A method as defined in any one of claims 13 to 16, characterized by applying a potential to said deflection electrodes (54) that exerts an electrostatic force on said charged toner particles varying from a high deflecting force to a low deflecting force.
18. A method as defined in any one of claims 13 to 17, including providing the apertures (52) in a substantially elongate pattern in said printhead structure, the elongate pattern defining a longitudinal direction, characterized by varying the relative potential difference between said deflection electrodes (54) and said back electrode (12) in said longitudinal direction to obtain a substantially constant deflection of toner particles across said longitudinal direction.
19. A method as claimed in claim 18, characterised by dividing the deflection electrodes into at least two groups in said longitudinal direction and controlling the voltage amplitude applied to at least two groups of deflection electrodes (54) separately.
20. A method as claimed in any one of claims 13 to 19, characterised by dividing said back electrode member (12) into segments 121) in said longitudinal direction and controlling the voltage amplitude applied to said segements (121) separately.
21. A method as defined in any one of claims 13 to 20, characterized by measuring the distance between said printhead structure (5) and said image receiving member (1) and controlling the potential difference between said back electrode member (12) and said deflection electrodes (54) in accordance with the measured distance.
22. A method as defined in any one of claims 13 to 21, characterized by determining the distance between said printhead structure (5) and said image receiving member
(1) by measuring the distance between deflected charged toner particles and non-deflected charged toner particles deposited on said image receiving member (1) .
PCT/EP1999/006461 1999-09-02 1999-09-02 Direct printing device and method WO2001017787A1 (en)

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AU5744799A AU5744799A (en) 1999-09-02 1999-09-02 Direct printing device and method
PCT/EP1999/006461 WO2001017787A1 (en) 1999-09-02 1999-09-02 Direct printing device and method
AU7648300A AU7648300A (en) 1999-09-02 2000-08-24 Direct printing device and method
PCT/EP2000/008274 WO2001017788A1 (en) 1999-09-02 2000-08-24 Direct printing device and method

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EP2262268A2 (en) 2002-06-28 2010-12-15 Dolby Laboratories Licensing Corporation Improved interpolation of video compression frames
EP2458864A2 (en) 2002-06-28 2012-05-30 Dolby Laboratories Licensing Corporation Improved interpolation of compressed video frames
EP2458863A2 (en) 2002-06-28 2012-05-30 Dolby Laboratories Licensing Corporation Improved interpolation of compressed video frames
EP2782345A1 (en) 2002-06-28 2014-09-24 Dolby Laboratories Licensing Corporation Improved transmission of compressed video frames

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