WO2001017788A1

WO2001017788A1 - Direct printing device and method

Info

Publication number: WO2001017788A1
Application number: PCT/EP2000/008274
Authority: WO
Inventors: Bo RYDSTRÖM
Original assignee: Array Ab
Priority date: 1999-09-02
Filing date: 2000-08-24
Publication date: 2001-03-15
Also published as: AU7648300A; AU5744799A; WO2001017787A1

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 during the movement of toner particles towards the back electrode the toner trajectory can be changed to impact on the image-receiving medium substantially normally. The device is thus rendered largely insensitive to variations in distance between the printhead and the image-receiving medium and improved print quality is obtained.

Description

Direct Printing Device and Method

Technical Field

The invention relates to a direct printing apparatus in which 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 image-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 hg invpnt-ion

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 the deflection electrodes and the back electrode as a function of time during the transport of the charged toner particles towards the back electrode member such that the toner particles describe a deflected trajectory that is substantially normal to the receiving surface over at least a final portion of their trajectory.

By altering the deflecting electric field during flight of the toner particles to cause the toner particles to impact essentially perpendicularly with the image- receiving surface, the printed image is rendered substantially insensitive to variations in the spacing between the printhead and the receiving medium.

Preferably at least two deflection electrodes are arranged in opposed relation about each aperture, and the controlling means are arranged to control the application of a varying potential difference between each set of two deflection electrodes during at least part of the transport of the charged toner particles towards the back electrode member.

The varying potential difference between pairs of deflection electrodes may be a ramped potential difference. Alternatively, the variation may be applied discontinuously during the transport of the charged toner particles towards the back electrode member.

In a preferred embodiment of the invention, the variation in potential difference across said two electrodes is substantially exponential or logarithmic.

Preferably, the variation in potential difference is such as to exert an electrostatic force on the charged toner particles varying from a high deflecting force in one direction to a low deflecting force in the same direction.

The invention further resides in a method for converting image information into a pattern of electrostatic fields for modulating the transport of charged toner particles towards an image-receiving surface. The method includes: providing a source of charged toner particles, generating a background electric field for attracting the charged toner particles towards an image receiving surface, providing apertures and a control electric field for controlling the passage of charged toner particles from the toner source through the apertures towards the image receiving surface and selectively applying an asymmetric field about the apertures for causing deflection of charged toner particles passing through the apertures. The method further includes: during the passage of charged toner particles towards the image receiving surface, varying the strength of the asymmetric electric field to cause the toner particles to describe a deflected trajectory that is substantially normal to the image receiving surface over at least a final portion of the trajectory.

Further advantageous embodiments are set out in the dependent claims .

Rripf HPHΓT-H pi- ion of rhp 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-1 in the printhead structure of Fig.4a and across the corresponding section line II-II of Fig.4b,

Fig.5 is a block diagram schematically illustrating the deflection control,

Fig.6 depicts three sequences of dot deflection,

Fig.7 illustrates a control and deflection pulse for obtaining the three sequences of dot deflection shown in Fig. 5 in accordance with the present invention, and

Fig.8 illustrates the trajectory of a toner particle deflected using the control and deflection pulses shown in Fig. 7.

Detailed r ar-r t-.i on

As shown in Fig .1 , an image forming apparatus in accordance with a first embodiment of the present invention comprises at least one print station, preferably four print stations (Y, M, C, K) , an intermediate image receiving member 1, a driving roller 10, at least one support roller 11, and preferably ? several adjus_ta_ble 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 U 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.

Fig. 5 schematically illustrates an arrangement for controlling the voltages applied to the deflection electrodes. In this arrangement a deflection control unit 65 is provided connected to variable voltages sources Dl and D2 that in turn are connected to the deflection electrode segments 541 and 542 disposed on the printhead 5. Specifically, all upstream segments 541 are connected to a first deflection voltage source Dl and all downstream segments 542 are connected to a second deflection voltage source D2. The deflection control unit 65 may take the form of a dedicated deflection controller or be incorporated in a general voltage controller utilised to control both the deflection electrodes 54 and the control electrodes 53. While the deflection voltage sources Dl and D2 are illustrated as separate elements from the control unit 65, the equivalent function may be incorporated into a single or two elements.

In operation, three deflection sequences (for instance: D1<D2; D1=D2; D1>D2) can be performed in each print cycle, whereby the difference between Dl and D2 determines the deflection trajectory of the toner stream through each aperture 52, and thus the dot position on the toner image .

Fig. 6 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 L.

Fig. 7 depicts the control pulse from the various voltage sources Dl, D2 during the three print sequences D1<D2; D1=D2; D1>D2 illustrated in Fig. 6 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 Dl, D2 likewise supply positive potentials Vd to the associated deflection electrode segments 541, 542, however the voltage source Dl to the deflection electrode segment 541 applies a higher voltage pulse at the beginning of the period t_b 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. 1?

In the subsequent period, denoted by t_w in F g. 7, 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 Dl 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 t„ as shown in the figure. The voltage source Dl gradually increases the potential applied to the electrode segment 541 in the period t_w 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 Dl 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 Fig. 8.

Fig. 8 shows 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. The trajectory of the charged toner particles passing through the aperture in a single deflection sequence is shown as a solid line. The trajectory starts at a point substantially centrally between the opposing deflection electrodes 541, 542, and impacts on the transfer belt 1 at a deflection distance L from a dotted line extending normally towards the transfer belt from a central point in the aperture . This dotted line represents the axis of the aperture 52. From this figure it is evident that the majority of the desired deflection occurs close to the printhead structure 5. 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 a path that is substantially normal to the transfer belt 1. This form of trajectory advantageously permits some change in the distance between the printhead and the transfer belt 1, this change being limited to the portion of the trajectory that is substantially perpendicular to 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. 8 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. 8 the pulse waveform is controlled to vary for only the deflecting electrode segment, the voltage waveform of the attracting electrode segment, in this case the left-hand segment 542, could be varied to the same effect instead, or even in addition.

The invention is not limited to the embodiments described above but may be varied within the scope of the appended 20 patent claims.

Claims

What is claimed is;

1. 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 arranged between said back electrode member and said printhead structure for intercepting the transported charged particles in image configuration; characterised in that the image forming apparatus further includes means (65) for controlling said deflection voltages sources to modify the asymmetric electric field about said apertures (52) as a function of time 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.

2. An image forming apparatus as defined in claim 1, characterised in that at least two deflection electrodes (541, 542) are arranged in opposed relation about each aperture (52) , wherein the controlling means (65) are arranged to control the application of a varying potential difference between said two deflection electrodes during at least part of the transport of the charged toner particles towards the back electrode member (12) .

3. An image forming apparatus as defined in claim 2, characterised in that said controlling means (65) are arranged to control the application of a ramped potential difference between said two deflection electrodes (541, 542) during at least part of the transport of the charged toner particles towards the back electrode member (12) .

4. An image forming apparatus as defined in claim 2, characterised in that the controlling means (65) are arranged to control the application of a discontinuously varying potential difference between said two deflection electrodes (54) .

An apparatus as claimed in claim 1 or 2, characterised in that said varying potential difference across said two electrodes varies substantially exponentially or logarithmically.

An image forming apparatus as defined in any one of claims 2 to 5 previous claim, characterised in that the controlling means (65) are adapted to control the application of a potential to said two deflection electrodes (54) that exerts an electrostatic force on said charged toner particles varying from a high deflecting force to a low deflecting force.

7. An image forming method, for converting image information into a pattern of electrostatic fields for modulating the transport of charged toner particles towards an image receiving surface, said method further including: providing a source of charged toner particles, generating a background electric field for attracting said charged toner particles towards an image receiving surface, providing apertures and a control electric field for controlling the passage of charged toner particles from said toner source through said apertures towards said image receiving surface, selectively applying an asymmetric field about said apertures for causing deflection of charged toner particles passing through said aperture, characterised by during the passage of charged toner particles towards said image receiving surface, varying the strength of said asymmetric electric field to cause the toner particles to describe a deflected trajectory that is substantially normal to said image receiving surface (1) over at least a final portion of said trajectory.

8. A method as defined in claim 7, characterised by applying a continuously varying asymmetric electric field about each aperture (52) at least during a final portion of said trajectory.

9. A method as defined in claim 7, characterised by applying a discontinuously varying asymmetric electric field about each aperture (52) at least during a final portion of said trajectory. 2A

10. A method as defined in any one of claims 7 to 9, characterised by applying an asymmetric electric field about each aperture (52) that exerts a high net deflection of said charged toner particles in one direction followed by a low net deflection in said direction during the passage of charged toner particles towards said image receiving surface at least during a final portion of said trajectory.