EP0895867A2

EP0895867A2 - A device for direct electrostatic printing with an edge electrode and an AC-field on the surface of the toner delivery means

Info

Publication number: EP0895867A2
Application number: EP98202302A
Authority: EP
Inventors: Guido Desie; Frans Backeljauw
Original assignee: Agfa Gevaert NV
Current assignee: Agfa Gevaert NV
Priority date: 1997-08-07
Filing date: 1998-07-08
Publication date: 1999-02-10
Also published as: EP0895867A3

Abstract

A device for direct electrostatic printing is provided comprising means for creating a flow of charged toner particles from a means for delivering charged toner particles, having a surface bearing toner particles, to an image receiving substrate, applying a DC potential difference between the means for delivering charged toner particles and the image receiving substrate and a printhead structure having control electrodes, interposed in the flow of toner particles for image wise controlling the flow of toner particles, the printhead structure controlling the flow of toner particles only from one side, wherein means for applying an AC-field on the surface bearing toner particles are incorporated, AC-field having a frequency between 1.5 and 3 kHz.

Description

FIELD OF THE INVENTION

This invention relates to an apparatus used in the process of electrostatic printing and more particularly in Direct Electrostatic Printing (DEP). In DEP, electrostatic printing is performed directly from a toner delivery means on a receiving substrate by means of an electronically addressable printhead structure.

BACKGROUND OF THE INVENTION

In DEP (Direct Electrostatic Printing) the toner or developing material is deposited directly in an image-wise way on a receiving substrate, the latter not bearing any image-wise latent electrostatic image. The substrate can be an intermediate endless flexible belt (e.g. aluminium, polyimide etc.). In that case the image-wise deposited toner must be transferred onto another final substrate. Preferentially the toner is deposited directly on the final receiving substrate, thus offering a possibility to create directly the image on the final receiving substrate, e.g. plain paper, transparency, etc. This deposition step is followed by a final fusing step.

This makes the method different from classical electrography, in which a latent electrostatic image on a charge retentive surface is developed by a suitable material to make the latent image visible. Further on, either the powder image is fused directly to said charge retentive surface, which then results in a direct electrographic print, or the powder image is subsequently transferred to the final substrate and then fused to that medium. The latter process results in an indirect electrographic print. The final substrate may be a transparent medium, opaque polymeric film, paper, etc.

DEP is also markedly different from electrophotography in which an additional step and additional member is introduced to create the latent electrostatic image. More specifically, a photoconductor is used and a charging/exposure cycle is necessary.

A DEP device is disclosed in e.g. US-A-3 689 935 This document discloses an electrostatic line printer having a multi-layered particle modulator or printhead structure comprising :

a layer of insulating material, called isolation layer ;
a shield electrode consisting of a continuous layer of conductive material on one side of the isolation layer ;
a plurality of control electrodes formed by a segmented layer of conductive material on the other side of the isolation layer ; and
at least one row of apertures.

Each control electrode is formed around one aperture and is isolated from each other control electrode.

Selected electric potentials (only DC potentials) are applied to each of the control electrodes while a fixed potential is applied to the shield electrode. An overall applied propulsion field between a toner delivery means and a support for a toner receiving substrate projects charged toner particles through a row of apertures of the printhead structure. The intensity of the particle stream is modulated according to the pattern of potentials applied to the control electrodes. The modulated stream of charged particles impinges upon a receiving substrate, interposed in the modulated particle stream. The receiving substrate is transported in a direction orthogonal to the printhead structure, to provide a line-by-line scan printing. The shield electrode may face the toner delivery means and the control electrodes may face the receiving substrate. A DC-field is applied between the printhead structure and a single back electrode on the receiving substrate. This propulsion field is responsible for the attraction of toner to the receiving substrate that is placed between the printhead structure and the back electrode.

A DEP printer wherein the printhead structure is a mesh instead of a insulating base with printing apertures trough this base has been disclosed in US-A-5 036 341. In this disclosure it is taught to introduce an AC-field with frequency between 2 and 5 kHz and peak voltages between 500 and 2000 V on the toner delivery means in order to speed up the printing.

One of the recognised problems with both of these type of printhead structure is the fact that the printing apertures are easily clogged by toner particles when the printhead structure is used for a longer period of time.

This problem of clogging of the printing apertures has been addressed in several ways. In e.g. US-A-4 491 855 different measures are disclosed to overcome some of the mentioned problems. Means are disclosed for realising a stable and uniform supply of toner particles to the printhead structure and for avoiding clogging of the apertures in the printhead structure by toner particles. Therefore a conveying member is provided on which a layer of toner particles is deposited and an AC voltage is applied between the toner conveying member and the continuous layer of conductive material on the printhead structure. Due to this AC voltage the toner particles "jump" between the toner conveying member and the surface of the printhead facing said toner conveying member, forming a "toner-cloud". The AC-voltage (in this disclosure 300 V peak to peak and frequency of 4.5 kHz) is adjusted such as to allow the toner particles to reach the printhead structure, thus enabling the overall DC voltage laid between the printhead structure and the image receiving substrate member to extract said toner particles from said powder cloud. The overall DC voltage propels the toner particles onto the image receiving substrate interposed between the printhead and a backing electrode.

It is believed that the "touching" toner particles will assist in delaying the contamination of the printhead structure and clogging of the apertures. At the same time a special design of the apertures in the printhead structure and a special selection of the material from which the printhead structure is made is claimed to assist in delaying the clogging. A last measure which is proposed is to 'clean' the printhead structure by periodical electrical bursts (spark discharges).

In US-A-4 478 510 said spark discharge is used to remove toner particles adhered to the printhead, in order to set if free again. For that purpose the printing time is divided in a writing time (during which an image is written to the receiving material) and a cleaning time. During the cleaning period the voltage applied to the back electrode is enhanced so that a spark discharge occurs from printhead to back electrode. Toner particles adhered to the printhead become dislodged and are gathered upon the back electrode. Another possibility that has been described is a spark discharge between shield and control electrodes providing the same effect, namely cleaning of clogged apertures in the printhead.

In US-A-4 755 837 an AC voltage is used for the backing electrode during the cleaning cycle. In a preferred embodiment the AC voltage on the back electrode is phase shifted by 180° if compared with the AC field (400 V peak to peak, no frequency disclosed) that is used upon the charged toner conveyor which is needed to obtain a high toner mist production, leading to high optical densities and short printing times. Further on the AC voltage can also have a certain DC-offset.

In US-A-4 876 561 clogging of the printhead is prevented by making the apertures large enough and/or the thickness of the isolating layer small enough.

In US-A-4 903 050 an AC voltage is applied to the back electrode as in US-A-4 755 837, but a shutter and vacuum system is added in order to prevent the dislodged toner to fall onto the receiving substrate.

In US-A-5 095 322 clogging of the apertures is prevented by applying to the shield electrode a pulsed DC-voltage which is 180° out of phase if compared with the AC-voltage applied to the charged toner conveyor. In an other embodiment a DC-biased AC voltage with the same frequency as the AC voltage applied to the charged toner conveyor but 180° out of phase is used to prevent clogging of the apertures in the printhead.

In US-A-5 153 611 an ultrasonic vibration is applied to the printhead, yielding a better performance regarding prevention of clogging of the apertures. The same idea has also been described in US-A-5 202 704 where is disclosed wherein the toner cloud is mechanically produced and the printhead is vibrated so as to free the apertures of the printhead from toner particles sticking within the apertures.

In US-A-5 233 392 a better performance in preventing clogging of the apertures is disclosed by using an ultrasonic vibration applied to the printhead, the improvement being changing within the writing time for each individual pixel the resonant frequency of the oscillation used by a small amount, resulting in a much better prevention of clogging.

In US-A-5 256 246 a printhead structure is made from a thin ceramic insulating member with control electrodes applied to said ceramic member by thin film techniques such as sputtering, vacuum deposition, ion plating, chemical vapour deposition and screen printing. It is claimed in this patent application that the absence of a sticky coating layer under the conductive layer does make the printhead structure less sensitive to clogging. A big drawback of this technique, however, is the reduced adhesive power of the conductors to the substrate.

In US-A-5 283 594 the level of vibration applied to the printhead is different during writing time and cleaning time. During writing time the oscillation is large enough to prevent clogging of the apertures for a great amount, during cleaning time the amplitude of the oscillating vibration is large enough to dislodge the toner particles that have partially clogged the apertures during the writing cycle. As a result the long-time performance of the DEP-apparatus is improved considerably.

In US-A-5 293 181 the printhead is vibrated in such a way that a mechanical propagating wave is created. The printhead also has a provision in order to prevent reflection of the mechanical propagating wave. Using these implementations a good long-time stability without clogging of the apertures is provided with a good writing characteristic.

In US-A-5 307 092 an antistatic coating is applied to the electrodes in the printhead so that any tribocharge that accumulates during writing can be grounded. As a result the net tribocharge on the printhead (which is unwanted and is responsible for unpredictable results and clogging) is removed and a better longtime performance results.

In WO-A-90 14959 the printhead is treated with pressurised air or vacuum so that the individual toner particles do not adhere to the printhead for such a large amount if compared with a printing engine not using the air treatment. In the same document an additional improvement is described where by the magnetic toner particles are removed from the printhead by using a much stronger magnetic field during the cleaning cycle than during the writing cycle.

In US-A-5 526 029 it is disclosed to use ionised air for blowing over the printhead so that the electrostatic interaction of the toner particles with the printhead is reduced and the toner particles are removed more easily from it than if compared with patent application WO-A 90 14959 where the air used is not pretreated at all.

In EP-A-780 740 a printhead structure, for a DEP (Direct Electrostatic Printing) device is disclosed that comprises an insulating material, a slit, formed by two sides (SA and SB) of said insulating material, as printing apertures and control electrodes characterised in that only one of said two sides forming said slit carries control electrodes. In such a printhead structure the chance of clogging of the printing apertures is lower than in printhead structures wherein fine (maximum dimension around 400 µm) circular, elliptical, rectangular or square printing apertures are used.

In US-A-5 625 392 an edge electrode is described so that instead of individual apertures or a larger slit as described in EP-A-780 740 an even larger free zone between the toner applicator and the receiver exists, resulting in even better properties regarding clogging of the printhead structure.

Said edge electrode system proposed in US-A-5 625 392 suffers however from the drawback that, in order to obtain a good image contrast between image parts of low density and image parts of high density, the overall applied propulsion field between the toner applicator and the receiver on the back electrode must be set to a rather low value, leading to only a moderate printing speed.

The system as described in US-A-5 625 392 operates best when the distance between the edge electrode and the back electrode, i.e. the space wherein an image receiving substrate can be passed, is smaller than 500 µm. These small distances limit the usefulness of the device since printing on thick image receiving substrates as described in EP-A-811 894 is impossible.

Thus there is still a need for further improved DEP devices with enhanced printing speed and less clogging that are stable in time.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the invention to provide a DEP device, i.e. A device for direct electrostatic printing that can print at high speed with low clogging of the control electrodes and with high maximum density and with a high degree of density resolution (i.e. for producing an image comprising a high amount of differentiated density levels) and spatial resolution.

A further object of the invention is to provide a DEP device that can be used with a wide variety of types of toner particles, and that can print at high speed with low clogging of the control electrodes, with high maximum density and with a printing quality that is constant over a long period of time.

It is still a further object of the invention to provide an edge printhead structure making it possible to have a DEP printing with high resolution and no clogging.

It is an other object of the invention to provide an edge printhead structure making it possible to manufacture a DEP printing device capable of printing with high spatial resolution over a large density range and with no clogging.

Further objects and advantages of the invention will become clear from the detailed description herein after.

The objects of the invention are realised by providing a device for direct electrostatic printing comprising

means for creating a flow of charged toner particles from a means for delivering charged toner particles, having a surface bearing toner particles, to an image receiving substrate, applying a DC potential difference between said means for delivering charged toner particles and said image receiving substrate,
a printhead structure having control electrodes, interposed in said flow of toner particles for image wise controlling said flow of toner particles, said printhead structure controlling said flow of toner particles only from one side, wherein
means for applying an AC-field on said surface bearing toner particles, said AC-field having a frequency between 1.5 and 3 kHz.

The objects of the invention are further realised by providing a method for direct electrostatic printing comprising the steps of

providing charged toner particles on a surface of a means for delivering toner particles,
applying a DC potential difference between said surface of said means for delivering charged toner particles and an image receiving substrate for creating a flow of charged toner particles towards said image receiving substrate from said means for delivering toner particles,
interposing an edge printhead structure, carrying control electrodes in said flow of toner particles
applying a DC voltage in accordance with image data to said control electrodes for image-wise controlling said flow of toner particles,
applying an AC voltage to said surface of said means for delivering toner having a frequency between 1.5 and 3 kHz,
depositing said image-wise controlled flow of toner particles on said image receiving substrate and
fixing said toner particles to said substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a DEP device using an edge printhead structure and a magnetic brush as toner source.

Figure 2 shows a DEP device using an edge printhead structure and an applicator for non-magnetic mono-component developer as toner source.

Figure 2a shows an enlargement of the portion encompassed by circle X in figure 2 and a specific embodiment of an edge printhead structure of this invention.

Figure 2b shows an enlargement of the portion encompassed by circle X in figure 2 and a further specific embodiment of an edge printhead structure of this invention.

Figure 3 shows various embodiments of an edge printhead structure of this invention.

Figure 4 shows further possible embodiments of an edge printhead structure of this invention.

Figure 5 shows still further possible embodiments of an edge printhead structure of this invention.

DEFINITIONS

"Edge printhead structure" is a printhead structure, comprising an insulating material carrying control electrodes for image-wise modulating the toner flow at the edge of the insulating material, that is interposed in the toner flow in a DEP device on only one side of a toner flow. On the side of the toner flow opposite to the side wherein the edge printhead structure is interposed, NO toner flow influencing members are present. This differentiates the "edge printhead structure" from a printhead structure having a slit wherein the toner flow is image-wise modulated, as disclosed in EP-A- 780 740.

"Toner bearing surface" is the surface of the means for delivering toner particles from where a flow of toner particles to the image receiving substrate originates.

DETAILED DESCRIPTION OF THE INVENTION

It is known in the art of DEP (direct electrostatic printing), as described in the background art section above, that, in a DEP device wherein - in a DC-field - a flow of charge toner particles is created between a means for delivering charged toner particles and an image receiving substrate and a printhead structure - having control electrodes around printing apertures - is interposed in said flow of toner particles for image wise controlling said flow of toner particles, the application of an AC-field to the surface of a means for delivering toner particles could enhance the printing speed by providing a denser cloud of toner particles in the vicinity of the printing apertures. It seems from the prior art that a wide range of frequencies of the AC-field and of peak to peak voltages of the AC-field are useful in that respect. E.g. frequencies between 2 and 5 kHz and voltages between 500 and 2000 V have been disclosed.

It was surprisingly found that the frequency of an AC-field used in a DEP device wherein the printhead structure controls the flow of toner particles only from one side (i.e. a DEP device using an edge printhead structure) has to have a frequency between quite narrow limits. Only when the frequency of the AC-field, applied to the toner bearing surface, is between 1.5 and 3 kHz, good maximum density was obtained. Preferably the frequency of the AC-field is between 1.75 and 2.75 kHz. It was moreover found that a peak tot peak voltage lower than these disclosed in the prior art gave good results. A peak to peak voltage between 400 and 1000 V gave sufficient Dmax. The peak to peak voltage of the AC field (V_AC) to be used was found to be a function of the distance (d) from the toner bearing surface and the control electrodes on the edge printhead structure. It was found that, in a DEP device using an edge printhead structure an acceptable D_max was reached when V_AC/d ≥ 10, an even better D_max was reached when V_AC/d ≥ 15.

The incorporation of means for applying an AC-field to the surface of a means for delivering toner particles in a DEP device wherein the printhead structure controls the flow of toner particles only from one side (i.e. in a DEP device with an edge printhead structure), made it possible to construct a DEP device wherein the printing speed could be more than 100 cm/min, even more than 200 cm/min and wherein D_max is sufficiently high. This fast DEP printing device could be used for a long time without giving problems of printing quality.

It was further found that the device could also be operated when the distance between the control electrodes in the edge printhead structure and the surface of the toner delivery means was such that there was no sliding contact between the edge of the printhead structure and the toner delivery means. In this case there was only sliding contact between a spacer means mounted upon the edge electrode in a zone near to the edge of the edge electrode, and the toner delivery means. In a further embodiment of the present invention said edge electrode could also be mounted on a rigid frame so that there is no sliding contact at all between the edge electrode and the toner delivery means. Also the distance between the edge printhead structure and the back electrode could be raised to 1000 µm or more without loss in printing quality, thus enabling the printing on thick image receiving substrates or on image receiving substrates with large thickness variations.

In fig. 1 a DEP device according to a specific embodiment of the invention is shown.

The DEP device shown comprises means for delivering toner particles with a container (101) for developer (102) wherein a magnetic brush (103) having a core (103a) wherein magnets are present and a sleeve (103b) rotatably mounted around the core is present. The developer (102) can be a mono component developer with magnetic toner particles and then on the surface of the sleeve of the magnetic brush, toner particles are present, i.e. the surface of the sleeve (103b) of the magnetic brush is the toner bearing surface. The developer (102) can as well be a multi-component developer containing magnetic carrier particles and non-magnetic toner particles and then on the sleeve of the magnetic brush carrier and toner particles are present, but the sleeve is still a toner bearing surface. The magnetic brush (103) can have a fixed core (103a) and a sleeve (103b) rotatably mounted around the core equipped with means for rotating the core. In another embodiment, the core (103a) of the magnetic brush is also equipped with means for rotating the core and can thus also be rotated and the sleeve (103b) can be rotated around the core or kept stationary. (The means for rotating the core and/or the sleeve are not shown in the figure). The part of the magnetic brush that rotates, does so in the direction of arrow B. A device for applying a DC voltage is connected to the sleeve of the magnetic brush and applies voltage V1 to said sleeve and a device for applying an AC-field is connected to the sleeve of the magnetic brush and applies AC-field AC1 to said sleeve (the toner bearing surface). The amount of developer on the toner bearing surface is regulated by a doctor blade (113).

The device, as shown, further comprises a back electrode (105) connected to a DC voltage source applying a voltage V4 to the electrode. An image receiving substrate (108) is passed by means for moving the substrate (107) in the direction of arrow A between the surface of the sleeve (103b) and the back electrode by conveying means (107). The difference between V4 and V1 applies a DC propulsion field wherein a flow of toner particles (104) is created from the sleeve of the magnetic brush ( the toner bearing surface) to the image receiving substrate on the back electrode. The AC-field - AC1 - on the sleeve of the magnetic brush makes the flow (104) of toner particles denser than when no AC-field would be present.

On one side of the flow of toner particles, a printhead structure (106), with an insulating material (106c) carrying control electrodes (106a) is interposed in the flow (104) of toner particles. A DC-source (V3) is connected to the control electrodes and the voltage applied by this DC-source is image-wise modulated in order to modulate the toner flow image wise in the vicinity of the control electrodes. The voltage applied by the DC source V3 can be varied between a value totally blocking the passage of the toner particles, and a value leaving the toner flow pass totally unimpeded. The control electrodes in said printhead structure are placed at a distance d from the toner bearing surface, a spacer (110) keeps the distance d constant during operation of the device.

The device comprises further means (109) for fixing the toner particles to the image receiving substrate.

In figure 1, the toner bearing surface is the surface of the sleeve of a magnetic brush, in figure 2 a device according to a further embodiment of the invention is shown, wherein the toner bearing surface is the surface of an applicator carrying toner particles derived from a non-magnetic mono-component developer.

The device, shown in figure 2 is the same as the one shown in figure 1, except for the toner bearing surface, so only the numericals different from those used in figure 1 will be described. In a container (101) for non magnetic mono component developer a roller (112) is present, having a surface On this surface toner particles are applied by means of a feeding roller (111) made of porous foamed polymers. A developer mixing blade (114) mixes and transports said non-magnetic mono-component developer towards said feeding roller. A doctor blade (113) regulates the thickness of the charged toner particles upon the surface said roller (112), i.e. on the toner bearing surface.

In figure 2a an enlarged portion (within circle X) of figure 2 is shown with a specific design of the edge printhead structure. In this figure an edge printhead structure is shown comprising an insulating material (106c) and carrying on the edge control electrodes (106a), isolated from each other and each connected over an integrated circuit with a DC voltage source V3. The voltage applied by this DC-source is image-wise modulated in order to modulate the toner flow image wise in the vicinity of the control electrodes. The voltage applied by the DC source V3 can be varied between a value totally blocking the passage of the toner particles, and a value leaving the toner flow pass totally unimpeded. The face of insulating material (106c) carrying the control electrodes (106a) is covered with an insulating material (110) serving as spacer, keeping the control electrodes at a distance, d, from the toner bearing surface (112). This surface was connected to a DC source (V1) and an AC source (AC1). On the face of the insulating material (106c) opposite to the face carrying the control electrodes a strengthening layer (115) of a plastic (preferably a polyester) is present. The edge printhead structure is attached to a frame (116) in such a way that the printhead structure has a free length (FL). A back electrode (105) is present whereon a DC source applies a voltage V4. Between the back electrode and the printhead structure an image receiving substrate (108) is passed.

In figure 2b an enlarged portion (within circle X) of figure 2 is shown with a specific design of the edge printhead structure. In this figure an edge printhead structure is shown comprising an insulating material (106c) and carrying on one face, at the edge of the face, control electrodes (106a) connected over an integrated circuit with a voltage source V3. The face of the insulating material (106c) opposite to the face carrying the control electrodes (106a) is covered with a single shield electrode (106b) (whereon a single DC voltage is applied (V2). The shield electrode does not extend to the edge of the edge printhead structure. On the shield electrode a spacer (110) is present keeping the control electrodes at a distance, d, from the toner bearing surface (112). This surface was connected to a DC source (V1) and an AC source (AC1). ). A back electrode (105) is present whereon a DC source applies a voltage V4. Between the back electrode and the printhead structure an image receiving substrate (108) is passed.

In the figures 1, 2, 2a and 2b the toner bearing surface is the surface of the sleeve of a magnetic brush (in fig 1), or the surface of an applicator for non-magnetic mono-component developer. A DEP device according to this invention can also be equipped with a charged toner conveyer (CTC) on the surface of which charged toner particles are applied by a magnetic brush or an applicator for non-magnetic mono-component developer. In this case the toner bearing surface is the surface of the CTC and the means for applying the AC-field AC1, are connected to that surface.

The printhead structure, used in a DEP device according to this invention can have any shape and form as described in US-A-5 625 392. The printhead structure (106) used in a DEP device according to the present invention, preferably has the shape and form as shown in figure 3.

In this figure, 106c represents the insulating material, 106a represents a complex addressable electrode structure, hereinafter called "control electrodes" , 106d represent the edge of the printhead structure interposed in the flow of toner particles and arrow TF represents the direction of the toner flow, from the toner bearing surface (not shown) to the image receiving substrate (not shown). In figure 3a, the simplest form of the first embodiment of a printhead structure according to the present invention is shown : on one face of the insulating material (106c) control electrodes (106a) are present. Although in figure 3a the printhead structure is shown with the control electrodes facing in the direction of the toner flow (i.e. facing the image receiving substrate), it is possible to introduce such a printhead structure in a DEP device according to this invention with the control electrodes facing the other way round, i.e. facing the toner bearing surface. In figure 3b a further variant a printhead structure according useful in a DEP device according to the present invention are shown. On both faces of the insulating material (106c) control electrodes (106a) are present. The control electrodes (106a) on both faces of the insulating are located such as to have pairs of control electrodes (106a) (one on every face) exactly in register in pairs. The control electrodes (106a), being present on both faces of the insulating material (106c) can - as shown in figure 3c - , in pairs, be connected to each other via metallisation over edge (106d), forming a single control electrode. Ways and means for connecting electrodes trough printing apertures are known in the art. Examples of such means have been disclosed in EP-A-753 413.

In figure 3d and 3e further variants of a printhead structure useful in a DEP device according to the present invention are shown. In figure 3d and 3e, the control electrodes (106a) on both faces of the insulating material are staggered. In figure 3d the width of the control electrodes parallel to the length of the edge (106d) is selected such as to have some overlap between the control electrodes on one face of the insulating material (106c) and control electrodes present on the other face. In figure 3e, the width of the control electrodes parallel to the length of the edge (106d) is selected such as to have no overlap between the control electrodes on one face of the insulating material (106c) and those on the other face.

In figure 4 an edge electrode according to an other embodiment of the present invention is shown. In said figure 4, 106c represents the insulating material, 106a represents a complex addressable electrode structure, hereinafter called "control electrodes" , 106b represents a common shield electrode located at the other side of said insulating material, 106d represent the edge of the printhead structure interposed in the flow of toner particles and arrow TF represents the direction of the toner flow, from the toner bearing surface means (not shown) to the image receiving substrate (not shown). In figure 4a, the simplest form of a printhead structure according to this embodiment of the present invention is shown : on one face of the insulating material (106c) control electrodes (106a) are present, on the other side the common shield electrode (106b) is present. The edge 106d cuts in a single plane both control electrodes, isolating member and shield electrode. Although in figure 4a the printhead structure is shown with the control electrodes facing in the direction of the toner flow (i.e. facing the image receiving substrate), it is possible to introduce such a printhead structure in a DEP device according to this invention with the control electrodes facing the other way round, i.e. facing the toner bearing surface. In figure 4b a further embodiment of the present invention is shown. On one side of an isolating member control electrodes are present, on the other side a common shield electrode is present. The edge is cutting down both control electrodes and isolating member but the shield electrode does not extent till the edge: i.e. the shield electrode ends at a certain distance from said edge, e.g. 500µ from said edge. The use of a shield electrode on an edge printhead structure has the advantage that a larger tonal scale or larger density range can be printed than by using an edge printhead structure without shield electrode. . It was found that the distance of the shield electrode from the edge was an important parameter for achieving an optimum compromise between printable tonal range and the fog level in the print.

In figure 5 an edge electrode according to a further embodiment of the present invention is shown. In said figure 5, 106c represents the insulating material, 106a represents a complex addressable electrode structure, hereinafter called "control electrodes", 106d represent the edge of the printhead structure interposed in the flow of toner particles and arrow TF represents the direction of the toner flow, from the toner bearing surface means (not shown) to the image receiving substrate (not shown). As shown in figure 5, said edge is not a straight line but is two-level-shaped. In fig 5a and 5 b, the edge looks like a battlement with alternating crenels and merlons. The crenels have a shape making it possible to position control electrodes at the edge of the crenels and the edge of the merlons parallel to the edge of the printhead structure in such a way that neighbouring control electrodes overlap each other to a certain extent. In figure 5c an other way for making an edge printhead structure wherein neighbouring control electrodes overlap each other to a certain extent. In this figure the edge is saw- toothed and each of the teeth carries a control electrode. In the printing direction neighbouring control electrodes overlap each other as in figure 3b but in the embodiments shown in figure 5, both of said neighbouring control electrodes are located on the same face of the insulating material (106c) and are in a single plane. The edge cut (either the saw-toothed shape or the battlement ) can be performed by e.g. an excimer laser.

The insulating material, used for producing printhead structure, useful in a DEP device according to the present invention, can be glass, ceramic, plastic, etc. Preferably said insulating material is a plastic material, and can be a polyimide, a polyester (e.g. polyethylelene terephthalate, polyethylene naphthalate, etc.), polyolefines, an epoxy resin, an organosilicon resin, rubber, etc.

The selection of an insulating material for the production of a printhead structure useful in a DEP device according to the present invention, is governed by the elasticity modulus of the insulating material. Insulating material, useful in the present invention, has a elasticity modulus between 0.1 and 10 Gpa, both limits included, preferably between 2 and 8 GPa and most preferably between 4 and 6 Gpa. The insulating material has a thickness between 25 and 1000 µm, preferably between 50 and 200 µm.

The back electrode (105) of a DEP device according to this invention, can also be made to co-operate with the printhead structure, said back electrode being constructed from different styli or wires that are galvanically isolated and connected to a voltage source as disclosed in e.g. US-A- 4, 568 ,955 and US-A-4, 733, 256. The back electrode, co-operating with the printhead structure, can also comprise one or more flexible PCB's (Printed Circuit Board).

The present invention incorporates the operation of a DEP device according to the present invention in a method for direct electrostatic printing comprising the steps of :

providing charged toner particles on a toner bearing surface of a means for delivering toner particles,
applying a DC potential difference between said surface of said means for delivering charged toner particles and an image receiving substrate for creating a flow of charged toner particles towards said image receiving substrate from said means for delivering toner particles,
interposing an edge printhead structure, carrying control electrodes in said flow of toner particles
applying a DC voltage in accordance with image data to said control electrodes for image-wise controlling said flow of toner particles,
applying an AC voltage to said surface of said means for delivering toner having a frequency between 1.5 and 3 kHz,
depositing said image-wise controlled flow of toner particles on said image receiving substrate and
fixing said toner particles to said substrate.

A DEP device according to the present invention can also be operated without back electrode in a method for DEP printing on an insulating image receiving substrate, having a first and a second face, comprising the steps of :

applying a conductive layer upon said first face of said insulating substrate,
connecting said conductive layer by conductive charge applying device to a voltage source,
providing a DC field between said conductive layer and means for delivering toner particles, creating a flow of charged toner particles from the surface of said means for delivering toner particles to said conductive layer,
applying an AC voltage to said surface of said means for delivering toner having a frequency between 1.5 and 3 kHz,
interposing an edge of a printhead structure, carrying control electrodes in said flow of toner particles
applying a voltage on said control electrodes for image wise controlling said flow of toner particles;
image wise depositing toner particles on said conductive layer on said substrate through said printing apertures and
fixing said toner particles to said substrate.

Such a method has been disclosed in EP-A-823 676.

A DEP device according to the present invention can further be operated in a method for direct electrostatic printing with reduced banding comprising the steps of :

creating a DC potential difference between an image receiving substrate and a magnetic brush assembly having a rotatably mounted core and a sleeve rotatably mounted around said core;
rotating said core at a rotational speed equal to or higher than 500 rotations per minute and rotating said sleeve at a rotational speed equal to or lower than 10 rotations per minute;
applying a developer with toner particles and magnetically attractable carrier particles on said magnetic brush assembly;
creating a flow of toner particles directly from said magnetic brush assembly to said image receiving substrate;
applying a voltage on said control electrodes for image wise controlling said flow of toner particles;
applying an AC voltage to said surface of said sleeve of said magnetic brush having a frequency between 1.5 and 3 kHz (and a peak tot peak voltage between 500 and 1000 V,
interposing an edge of a printhead structure, carrying control electrodes in said flow of toner particles
image wise depositing toner particles on said substrate through said printing apertures and
fixing said toner particles to said substrate. In this method the core is preferably kept stationary. Such a method has been described in EP-A-827 046.

In a DEP device, according to of the present invention operate in the methods described above, and wherein the surface of the sleeve of the magnetic brush is used as toner bearing surface, (i.e. the toner flow originates directly from the surface of the sleeve of the magnetic brush), any type of known carrier particles and toner particles can successfully be used. It is however preferred to use "soft" magnetic carrier particles. "Soft" magnetic carrier particles useful in a DEP device according to a preferred embodiment of the present invention are soft ferrite carrier particles. Such soft ferrite particles exhibit only a small amount of remanent behaviour, characterised in coercivity values ranging from about 4 up to 20 kA/m (from 50 up to 250 Oe). Further very useful soft magnetic carrier particles, for use in a DEP device according to a preferred embodiment of the present invention, are composite carrier particles, comprising a resin binder and a mixture of two magnetites having a different particle size as described in EP-B-289 663. The particle size of both magnetites will vary between 0.05 and 3 µm. The carrier particles have preferably an average volume diameter (dv50) between 10 and 300 µm, preferably between 20 and 100 µm. More detailed descriptions of carrier particles, as mentioned above, can be found EP 675 417, that is incorporated herein by reference.

It is preferred to use in a DEP device according to the present invention, toner particles with an absolute average charge (|q|) corresponding to 1 fC < |q| < 20 fC, preferably to 1 fC < |q| < 10 fC. The absolute average charge of the toner particles is measured by an apparatus sold by Dr. R. Epping PES-Laboratorium D-8056 Neufahrn, Germany under the name "q-meter". The q-meter is used to measure the distribution of the toner particle charge (q in fC) with respect to a measured toner diameter (d in 10 µm). From the absolute average charge per 10 µm (|q|/10µm) the absolute average charge |q| is calculated. Moreover it is preferred that the charge distribution, measured with the apparatus cited above, is narrow, i.e. shows a distribution wherein the coefficient of variability (ν), i.e. the ratio of the standard deviation to the average value, is equal to or lower than 0.33. Preferably the toner particles used in a device according to the present invention have an average volume diameter (dv50) between 1 and 20 µm more preferably between 3 and 15 µm. More detailed descriptions of toner particles, as mentioned above, can be found in EP-A-675 417.

A DEP device making use of the above mentioned marking toner particles can be addressed in a way that enables it to give black and white. It can thus be operated in a "binary way", useful for black and white text and graphics and useful for classical bilevel halftoning to render continuous tone images.

A DEP device according to the present invention is especially suited for rendering an image with a plurality of grey levels. Grey level printing can be controlled by either an amplitude modulation of the voltage V2 applied on the control electrodes 106a or by a time modulation of V2. By changing the duty cycle of the time modulation at a specific frequency, it is possible to print accurately fine differences in grey levels. It is also possible to control the grey level printing by a combination of an amplitude modulation and a time modulation of the voltage V3, applied on the control electrode.

The combination of a high spatial resolution and of the multiple grey level capabilities typical for DEP, opens the way for multilevel halftoning techniques, such as e.g. described in EP-A-634 862 with title "Screening method for a rendering device having restricted density resolution". This enables the DEP device, according to the present invention, to render high quality images

EXAMPLES EXAMPLE 1 The printhead structure.

A printhead structure was made from a polyimide film of 50 µm thickness (106c), single sided coated with a 5 µm thick copper film. Along the edge of the front side of the printhead structure, facing the toner delivery means and being interposed in the flow of toner particles from the toner delivery means to the back electrode, rectangular control electrodes (106a) being 220 µm large (measured in the direction parallel with the edge) were arranged at a linear pitch of 300 µm. Each of said control electrodes was connected over 2 MΩ resistors to a HV 507 (trade name) high voltage switching IC, commercially available through Supertex, USA, that was powered from a high voltage power supply. On top of said control electrodes on the front side of said polyimide isolating member a 110 µm thick polyurethane (110) was present, said polyurethane coating making physical frictional contact with the charged toner particles on the sleeve of the toner delivery means. On the back side of the printhead structure, facing the back electrode a 230 µm thick adhesive coating (not shown in the figures) and 175 µm thick polyester sheet was present (115). The printhead structure was mounted on a PVC-frame so that 8 mm (FL) of said edge electrode remained flexible and bendable.

The toner delivery means

The toner delivery means was a commercially available toner cartridge comprising non magnetic mono component developer, the COLOR LASER TONER CARTRIDGE MAGENTA (M3760GIA), for the COLOR LASER WRITER (Trade names of Apple Computer, USA). The toner bearing surface is the surface of an aluminium roller (112), whereon tone surface was changed as indicated in table 1. The results of the measurement of the printing density is also included in table 1.

EXAMPLES 7-9

The same experimental set-up as described in example 1 was used except for the fact that the printhead structure was changed as follows.

A printhead structure was made from a polyimide film of 50 µm thickness (106c), single sided coated with a 5 µm thick copper film. Along the edge of the back side of the printhead structure, facing the back electrode and being interposed in the flow of toner particles from the toner delivery means to the back electrode, rectangular control electrodes (106a) being 220 µm large (measured in the direction parallel with the edge) were arranged at a linear pitch of 300 µm. Each of said control electrodes was connected over 2 MΩ resistors to a HV 507 (trade name) high voltage switching IC, commercially available through Supertex, USA, that was powered from a high voltage power supply. On top of said control electrodes on the back side of said polyimide isolating member a 110 µm thick polyurethane member was present, a 230 µm thick adhesive layer, and a 175 µm thick polyester sheet. Said edge electrode was mounted on a PVC frame at a distance of 10 mm from said edge. Said edge electrode was bent towards said toner delivery means as described in example 1 but made contact with the charged toner particles upon the sleeve of said toner delivery means over said 50 µm thick polyimide that had the function of a self regulating spacer means. With an AC voltage (AC1) applied upon said toner bearing surface of 1000 V peak to peak and a frequency of 3 kHz, an image density of 1.04 was reached. (compared to 0.54 in example 3). The same experiment was repeated but with only 800 V peak to peak and 500 V peak to peak, and an image density of 0.87 and 0.55 respectively was obtained.

EXAMPLES 10-16

The same experimental set-up as described in example 1 was used except for the fact that the printhead structure was changed as follows. particles are applied by a feeding roller (111) The toner particles carried a negative charge.

The printing engine

The edge of the printhead structure, mounted in a PVC-frame (116), was bent with frictional contact over the surface of the roller of the toner delivery means. The 110 µm thick polyurethane coating was used as self-regulating spacer means (110).

A back electrode was present behind the paper whereon the printing proceeded, the distance between the back electrode (105) and the back side of the printhead structure (i.e. control electrodes (106a)) was set to 1000 µm and the paper travelled at 200 cm/min.

To the individual control electrodes an (image-wise) voltage V3 between 0 V and - 280 V was applied. The back electrode was connected to a high voltage power supply , applying a voltage V4 of + 1000 V to the back electrode. To the toner bearing surface of the toner delivery means an AC voltage (AC1) with 1000 V peak to peak and a frequency of 1 kHz was applied and a DC-offset (V1) of -50 V. The DC propulsion field, i.e. the potential difference between V4 and V1, was 1050 V. Grey-scale images were printed and the density at full image density (D_max) was measured using a MacBeth TR1204 densitometer in reflection mode. A value of 0.58 was measured as indicated in table 1.

COMPARATIVE EXAMPLE

The same experiment was done as described in example 1 except that only a DC voltage (V1) of - 700 V was applied to the toner bearing surface. No image density could be realised under these conditions, although the DC propulsion field, i.e. the potential difference between V4 and V1, was raised to 1700 V.

EXAMPLES 2-6.

The same experiments as described in example 1 were repeated but only the frequency of the AC applied to the toner bearing

A printhead structure was made from a polyimide film of 50 µm thickness, single sided coated with a 5 µm thick copper film. Along the edge of the back side of the printhead structure, facing the back electrode and being interposed in the flow of toner particles from the toner delivery means to the back electrode, rectangular control electrodes being 220 µm large (measured in the direction parallel with the edge) were arranged at a linear pitch of 300 µm. Each of said control electrodes was connected over 2 MΩ resistors to a HV 507 (trade name) high voltage switching IC, commercially available through Supertex, USA, that was powered from a high voltage power supply. On the other side of said polyimide foil a 120 µm thick continuous copper shield electrode (106b) was laminated by a 230 µm thick adhesive. On top of said shield electrode a 800 µm thick polyamide spacer (110) was present, thus the distance d between the toner bearing surface and the control electrodes was 800 µm. The edge had a sharp cutting through all of these layers. The shield electrode was grounded. All other experimental set-ups were identical to those described in example 1 except that the AC voltage (AC1) applied to the toner bearing surface had a peak to peak voltage of 1700 V with a frequency of 3 kHz..

In example 10, the shield electrode reached to the very edge of the printhead structure, the paper travelled at 200 cm/min, i.e. the printing speed is 200 cm/min.

In example 11 the same printing device was used except that the continuous copper shield electrode with polyamide spacer means was located at 500 µm away from the edge of the control electrodes. The printing speed was 100 cm/min.

In example 12 the same printing device was used except that the continuous copper shield electrode with polyamide spacer means was located at 1000 µm away from the edge of the control electrodes. The printing speed was 40 cm/min

In Example 13, example 12 was repeated except for the printing speed, which was now set at 100 cm/min.

In Example 14, example 12 was repeated except for the printing speed, which was now set at 200 cm/min.

In Example 15, example 13 was repeated except for the thickness of the spacing means, which was now 200 µm instead of 800 µm.

In example 16, example 14 was repeated except for the thickness of the spacing means, which was now 200 µm instead of 800 µm.

The examples 12 to 16, showed not only a low background density (D_min), but also a wide density range. When comparing the density range that was printed in examples 12 to 16 with the density range printed in example 3, it was found that the density range printed in examples 12 to 16 was larger.

The printing conditions, the maximum density and the minimum density of the examples 1 to 16 are summarised in table 1 below.

It must be clear for those skilled in the art that many deviations to this concept can be realised without departing from the scope of this invention. It is e.g. possible to fix the edge electrode on a rigid frame without spacer means towards the toner delivery means, it is possible to enhance the resolution of the device by making an edge electrode having separate sets of control electrodes as depicted in figure 3e, or it is possible to enhance the effect of the control electrodes over a larger area by staggering and overlapping said sets of control electrodes either in different planes as depicted in figure 3d or in the same plane as depicted in figure 4.

Claims

A device for direct electrostatic printing comprising

means for creating a flow of charged toner particles from a surface bearing charged toner particles to an image receiving substrate, said means applying a DC potential difference between said surface bearing charged toner particles and said image receiving substrate,

a printhead structure with an insulating material having a first and a second major face, one of said faces carrying control electrodes, interposed with an edge in said flow of toner particles for image wise controlling said flow of toner particles, said printhead structure controlling said flow of toner particles from one side only, wherein

means for applying an AC-field on said surface bearing toner particles, said AC-field having a frequency between 1.5 and 3 kHz.
A device according to claim 1 wherein said AC-field has a frequency between 1.75 and 2.75 kHz.
A device according to claim 1, wherein said toner bearing surface is placed at a distance > 100 µm from said control electrodes in said printhead structure.
A device according to any of claims 1 to 3, wherein said control electrodes are present on both said major faces of said insulating material of said printhead structure.
A device according to claim 4, wherein said control electrodes on both said major faces are staggered.
A device according to claim 5, wherein neighbouring electrode from said staggered control electrodes overlap with each other.
A device according to any of claims 1 to 3, wherein said control electrodes are present only on one of said major faces of said insulating material and said printhead structure has an edge with a shape selected from the group of a saw and a battlement.
A device according to any of claims 1 to 3, wherein said control electrodes are present on said first major face of said insulating material and a single shield electrode is present on said second major face, said control electrodes extending to said edge of said insulating material and said shield electrode being at least 200 µm away from said edge.
A method for direct electrostatic printing comprising the steps of

providing charged toner particles on a surface of a means for delivering toner particles,

applying a DC potential difference between said surface of said means for delivering charged toner particles and an image receiving substrate for creating a flow of charged toner particles towards said image receiving substrate from said means for delivering toner particles,

interposing an edge of a printhead structure, carrying control electrodes in said flow of toner particles, from one side only,

applying a DC voltage in accordance with image data to said control electrodes for image-wise controlling said flow of toner particles,

applying an AC voltage to said surface of said means for delivering toner having a frequency between 1.5 and 3 kHz,

depositing said image-wise controlled flow of toner particles on said image receiving substrate and

fixing said toner particles to said substrate.
An edge printhead structure, for use in a DEP device for controlling a flow of charged toenr particles from one side only, with an insulating material having a first and a second major face and an edge wherein both said faces carry control electrodes, said control electrodes being staggered and neighbouring electrode from said staggered control electrodes overlapping with each other.