EP1111478A1

EP1111478A1 - A device for direct electrostatic printing wherein the distance between the printhead structure and the surface of a charged toner conveyor is kept constant

Info

Publication number: EP1111478A1
Application number: EP99204397A
Authority: EP
Inventors: Guido c/o Agfa-Gevaert N.V. Desie; Jacques c/o Agfa-Gevaert N.V. Leonard; Martin c/o Afva-Gevaert N.V. De Kegelaer
Original assignee: Agfa Gevaert NV
Current assignee: Agfa Gevaert NV
Priority date: 1999-12-20
Filing date: 1999-12-20
Publication date: 2001-06-27

Abstract

A device for direct electrostatic printing wherein the distance between the printhead structure and the surface of the charged toner conveyor (a CTC) is kept constant by contacting the frame carrying the printhead structure with the surface of the CTC. The frame can be kept directly in contact with the surface of the CTC or over spacers inserted between the frame and the surface of the CTC.

Description

FIELD OF THE INVENTION

This invention relates to a recording method and an apparatus for use in the process of Direct Electrostatic Printing (DEP), in which an image is created upon a receiving substrate by creating a flow of toner particles from a toner bearing surface to the image receiving substrate and image-wise modulating the flow of toner particles by means of an electronically addressable printhead structure.

BACKGROUND OF THE INVENTION

In DEP (Direct Electrostatic Printing) toner particles are deposited directly in an image-wise way on a receiving substrate, the latter not bearing any image-wise latent electrostatic image.

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, or from electrophotography in which an additional step and additional member is introduced to create the latent electrostatic image (photoconductor and charging/exposure cycle).

A DEP device is disclosed in e.g. US-A-3 689 935. This document discloses an electrostatic line printer having a multilayered 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 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 perpendicular 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. Such printing devices are quite sensitive to changes in distances from the toner application module towards said printhead structure, more particularly to the row op printing aperture in the printhead structure, small changes in that distance leading to changes in image density.

The problem of keeping this distance constant has been addressed in several ways.

In EP-A 675 417 it is disclosed to use a magnetic brush as toner delivery means, using a two-component developer (comprising toner and carrier particles), and to provide "long hairs" on said brush so that the hairs touch the printing structure. In that case slight deviations in distance between the surface of the toner delivery means and the printhead structure do no longer present problems, while in any case the hairs of the brush, made up by carrier particles and toner particles are in contact with the printhead structure. It was found that such a device could provide very good printing results, but yielded only adequate optical density in the print when the printing speed was not too high. The problem of varying image density, that can remain in a device according to EP-A 675 417, due to a varying distance between the surface of the magnetic brush and the printhead structure can further be decreased by adapting the electrical conductivity of the carrier particles used on the magnetic brush as described in EP-A 836 124.

For devices working at quite high printing speeds, the use of a charged toner conveyer (a CTC), whereon the toner particles can be deposited by a magnetic brush or any other means known in the art, presents advantages. But the problem of uneven density (white banding) in a direction perpendicular to the printing direction has to be solved. This banding is frequently due to the varying distance between the surface carrying charged toner particles and the printhead structure, this distance being in the range of some hundreds of micrometer, slight wobble in the moving surface of the CTC, carrying toner particles, can change that distance for a relatively high percentage. Slight wobble of the surface of a rotating cylinder or of the surface of moving belt is very difficult to avoid and the avoidance thereof puts high demands on the machining of these parts of the DEP device. Thus in DEP devices other ways and means to avoid banding in a direction perpendicular to the printing direction than total avoidance of wobble have been looked for.

In EP-A 740 224 a device is described in which the frequency of said density banding (in a direction perpendicular to the printing direction) due to the variation of the distance from the toner application module towards said printhead structure is diminished. To achieve this better uniformity in printing, it is disclosed to give the toner bearing surfaces of the toner delivery means rather high rotational speeds. Since the surfaces that bear the toner particles rotate very fast and the distance between said toner bearing surfaces and the printhead structure is low, the particles are exposed to quite large shearing force. This high shearing force can give raise to agglomeration and/or deformation of the toner particles (especially when in the toner particles polymeric toner resins with low (< 60 °C) Tg are used. Thus the printing apertures can get clogged by agglomerated or deformed toner particles, leading to images with missing dots and bad image quality.

In US-A 5 552 814 it is disclosed to use a device wherein the CTC and the printhead structure are in close contact. Such a device does indeed decrease the banding in the direction perpendicular to the printing direction, but, as with the fast moving CTC's in EP-A 740 224 referred to above, the particles are exposed to quite large shearing force. This high shearing force can give raise to agglomeration and/or deformation of the toner particles and thus to some clogging of printing apertures. To diminish that problem it has been proposed in US-A 5 497 175 to provide a layer with very low coefficient of friction on the face of the printhead structure contacting the CTC or, in US-A 5 539 438, to provide a layer with low coefficient of friction on the surface of the CTC. These layers may influence the charge or the chargeability of the toner particles and can thus, in some instances, negatively influence the printing quality.

In US-A 5 448 272 an other approach to diminish the shearing forces on the toner particles in a DEP device wherein the CTC contacts the printhead structure has been disclosed. On the face of the printhead structure contacting the CTC a kind of guiding members are provided in the spacing between the printing apertures, and only these guiding members are in contact with the CTC. The guiding members are wedge shaped, with the point of the wedge against the toner feeding direction. In operation the guiding members, that keep the distance between the printhead structure and the CTC constant, "plough" through the layer of toner particles on the CTC and guide the particles to the printing apertures. A drawback of this device is the difficulty of manufacturing such a printhead structure with the desired accuracy for high resolution printers (50 dpi (dot per inch) or 20 dots/cm) or higher. A high resolution printer necessitates a printhead structure with small apertures and small spacing between the printing apertures, necessitating a very accurate positioning of the guiding members.

In JP-A 08/300 715 a printhead structure with "guiding means" is disclosed, wherein the guiding means are placed before the printing apertures and are form an angle with the direction of movement of the toner delivery means. Again at least one guiding means per printing aperture is provided. Thus also in this device a very accurate positioning of the guiding members is required, which complicates the manufacture of the printhead structure.

In WO-A 95/24675 a DEP device is disclosed wherein a spacer means is placed on the printhead structure in a direction parallel to the row of printing apertures and this spacer means is kept in contact with the surface carrying toner particles. In this design, the toner particles are pressed between that surface carrying them and the spacer means before they arrive in the vicinity of the printing apertures. This pose severe mechanical stress on the toner particles and deformation of the particles, partly melting of the particles, etc., may be expected.

In EP-A 816 944 a DEP device is disclosed, wherein a printhead structure is kept at a constant distance D of the first major face by at least two spacing means that are placed at a distance d from each other such that 1 cm ≤ d ≤ 50 cm and preferably outside the array of printing apertures in the printhead structure. In this device the toner particles do not touch the spacers, so that no mechanical stress is applied to the toner.

The measures in the designs wherein either the surface of the printhead structure contacts directly the surface carrying toner particles or spacers are used for keeping the distance between row of printing apertures and toner source constant exert all a pressure on the printhead structure. When this is made of thin flexible material, e.g., polyimide, polyester, etc., this pressure on the printhead structure induces fatigue in the flexible material and/or deformation of it. Thus the distance is constant in the beginning of the printing, but after printing for a longer time, the distance starts to fluctuate again.

In European Application 99202107 filed on June 26, 1999, A DEP device, is disclosed wherein the gap, g, between a conveyor for charged toner particles, i.e. a CTC, and an intermediate image receiving medium is kept constant and wherein also the distance, d, between this intermediate image receiving medium and the printhead structure are kept constant. This results in the fact that the distance, d2, between the conveyor for charged toner particles and the printhead structure is kept constant without having the printhead structure contacting the CTC or having spacer means inserted between the CTC and the printhead structure. The drawback of this solution to the problem is the fact that an intermediate image receiving member and the possibility for transferring an image from this intermediate image receiving member to a final image receiver has to be provided.

Thus there is still a need for a DEP device wherein the distance between the conveyor for charged toner particles and the printhead structure is kept constant without having the printhead structure contacting the CTC or having spacer means inserted between the CTC and the printhead structure and without the risk of deformation of the printhead structure.

OBJECTS 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 high and constant maximum density and wherein even toner particles having low mechanical stress can be used.

It is a further object of the invention to provide a DEP device wherein the distance between the printhead structure and the surface carrying dry toner particles is kept constant over the printing time without imposing stress to the flexible printhead structure.

Further objects and advantages of the present 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

a toner source with a surface carrying charged toner particles placed opposite to an image receiving member

a voltage source applying an electric field between said toner source and said image receiving member said field being adjusted letting toner particles migrate from said toner source towards said receiving member

a framework holding a printhead structure, installed so that said printhead structure is placed between said toner source and said image receiving member, said printhead structure having a flexible insulating base and printing apertures there through and control electrodes associated with said printing apertures, said control electrodes being coupled to a voltage source for selectively opening and closing said printing apertures,

characterised in that

i. said framework and said toner source are mounted such that a distance, d, exists between said printhead structure and said surface carrying charged toner particles,
ii. both said framework and said surface carrying toner particles are kept in contact by an external force, F, so that said distance, d, is kept constant.

Preferably at least one of said framework and said toner source are mounted so as to be movable with respect to each other.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1. shows schematically a prior art DEP device wherein the printhead structure and the surface carrying toner particles are kept at a constant distance by spacers.

Fig 2. shows schematically a DEP device according to the present invention wherein the printhead structure and the surface carrying toner particles are kept at a constant distance by spacers.

Fig 3. shows schematically an other embodiment of a DEP device according to the present invention wherein the printhead structure and the surface carrying toner particles are kept at a constant distance by spacers.

Fig 4. shows schematically a possible implementation of a printhead structure and spacers according to this invention for keeping the printhead structure and the surface carrying toner particles at a constant distance by cylindrical spacers.

Fig 5. shows schematically a possible implementation of a printhead structure and spacers according to this invention for keeping the printhead structure and the surface carrying toner particles at a constant distance by wheels as spacers.

Fig 6. shows schematically a possible implementation of a movable mounting of printhead structure and spacers according to this invention against the surface of the CTC for keeping the printhead structure and the surface carrying toner particles at a constant distance.

DETAILED DESCRIPTION OF THE INVENTION

The problem of keeping, in a DEP device, the distance between the printhead structure and the surface carrying toner particles constant is well known in the art as described in the background art section of this application. In figure 1 a prior art solution for this problem is shown. It shows schematically a DEP device with printing direction perpendicular to the plane of the paper.
Opposite to a conveyor for charged toner particles, a CTC (101), a back electrode (102) is placed. The CTC has a surface carrying toner particles (toner particles not shown) and is coupled to a voltage source V1. The back electrode is coupled to a voltage source V2 placing the back electrode at an electric potential different from the one on the CTC. In this potential difference toner particles (not shown in the figure) can migrate from the surface of the CTC to the back electrode. Between the back electrode and the surface of the CTC an image receiving substrate (103) is passed. Between the image receiving substrate (103) and the CTC (101) a printhead structure (104) with a flexible substrate (104a) is placed. The flexible substrate of the printhead structure is stretched over two bars (105) and attached to a larger framework (not shown) by coil springs (106) and kept at a distance, d, from the CTC over spacers (107). The spacers (107) contact the free hanging portion of the flexible substrate outside the array of printing apertures (108) in the printhead structure. The printhead and the surface of the CTC are kept in contact by an external force, F. When the CTC turns, the surface of it slides over the spacers. The printing apertures are associated with control electrodes coupled to a voltage source (the control electrodes and the voltage source coupled to them are not shown) for selectively opening and closing the printing apertures. This prior art device has been detailed in EP-A-816 944 and does provide even printing, constant distance between printhead structure and CTC and no physical contact with the toner particles so that clogging of the apertures due to deformed toner particles is avoided. However it was observed that after prolonged use, the flexible substrate of the printhead structure became deformed, so that the distance between printhead structure and CTC was no longer kept constant so that fluctuations in density (banding) in the image occurred.

It was now found that it was possible to keep the distance between the printhead structure and the surface of the CTC carrying charged toner particles could be kept constant when the frame over which the flexible substrate of the printhead structure was stretched or the larger framework wherein the printhead structure was mounted was kept at a constant distance of the surface of the CTC, by having a contact between said frame or framework and the surface of the CTC. In this case no pressure, that varies according the wobble of the surface of the CTC, is exerted on flexible substrate of the printhead structure.

This can basically be implemented in two ways :

the frame or framework itself is kept in contact with the surface of the CTC so that no spacers are needed or
the frame or frame work is kept in contact with the surface of the CTC over spacers.

A configuration, wherein no spacers are used, is shown in figure 2. It shows schematically a DEP device with printing direction perpendicular to the plane of the paper. Opposite to a conveyor for charged toner particles, a CTC (101), a back electrode (102) is placed. The CTC has a surface carrying toner particles (toner particles not shown) and is coupled to a voltage source V1. The back electrode is coupled to a voltage source V2 placing the back electrode at an electric potential different from the one on the CTC. In this potential difference toner particles (not shown in the figure) can migrate from the surface of the CTC to the back electrode. Between the back electrode and the surface of the CTC an image receiving substrate (103) is passed. Between the image receiving substrate (103) and the CTC (101) a printhead structure (104) with a flexible substrate (104a) is placed. The flexible substrate of the printhead structure is stretched over two bars (105) and attached to a larger framework (not shown) by coil springs (106). The two bars (105) face the surface of the CTC. The CTC (101) is arranged so that the bars (105) over which the printhead structure is stretched are in kept contact with the surface of the CTC by an external force, F. By doing so the distance, d, is kept constant. Since now no stress is applied on the flexible substrate of the printhead structure by spacers, the flexible substrate gets not deformed over the time and high quality printing (i.e. no banding, high and even density, no clogging due to deformed or partially molten toner particles) remains possible for a long time. The main advantage is that the life-time of the printhead structure is prolonged when compared with DEP devices of the prior art.

In figure 3, a schematic view of a DEP device wherein the distance between the printhead structure and the surface of the CTC is kept constant by keeping the frame or frame work in contact with the CTC over spacers. It shows again schematically a DEP device with printing direction perpendicular to the plane of the paper. Opposite to a conveyor for charged toner particles, a CTC (101), a back electrode (102) is placed. The CTC has a surface carrying toner particles (toner particles not shown) and is coupled to a voltage source V1. The back electrode is coupled to a voltage source V2 placing the back electrode at an electric potential different from the one on the CTC. In this potential difference toner particles (not shown in the figure) can migrate from the surface of the CTC to the back electrode. Between the back electrode and the surface of the CTC an image receiving substrate (103) is passed. Between the image receiving substrate (103) and the CTC (101) a printhead structure (104) with a flexible substrate (104a) is placed. The flexible substrate of the printhead structure is stretched over two bars (105) and attached to a larger framework (not shown) by coil springs (106). The two bars (105) face away from the surface of the CTC. Under the bars (105) and on the substrate of the printhead structure, spacers (107) are provided that contact the surface of the CTC. The bars (105) and the surface of the CTC (101) are kept in contact with each other by external force, F. By doing so the distance, d, is kept constant. While the bars and the spacers press in this system the flexible substrate of the printhead structure between them, no stress that varies according the wobble of the surface of the CTC, is exerted on flexible substrate of the printhead structure by the spacers. Thus, again, the flexible substrate gets not deformed over the time and high quality printing (i.e. no banding, high and even density, no clogging due to deformed or partially molten toner particles) remains possible for a long time. The main advantage is that the life-time of the printhead structure is prolonged when compared with DEP devices of the prior art.

The configuration as schematically shown in figure 3 is the preferred configuration for a DEP-device according to this invention. The frame with the printhead structure faces away from the surface of the CTC carrying charged toner particles and spacers between the frame and that surface are used to keep the distance between the printhead structure and that surface constant. Thus in this invention the force keeping the frame and the surface of the CTC together may be exerted on the flexible substrate of the printhead structure with the proviso that the flexible substrate is sandwiched between the frame and the spacers.
Although the spacers can be located for contacting the surface of the CTC can be located anywhere as long as they keep the distance, d, between the printhead structure and the surface constant, it is preferred that the spacers are positioned so that they contact the surface of the CTC outside the portion of that surface carrying charged toner particles, so that no mechanical stress is applied to the toner particles.

The external force, F, in this invention can be the force of gravitation or a resilient force. In both cases either the CTC is fixed and the frame is in contact with the surface of the CTC and is movably mounted, so that the possible wobble of the surface of the CTC is brought on to the frame so that the contact between frame and surface is not broken by the wobble, but that the frame moves with the wobble of the surface. It is of course possible to have the frame with the printhead structure fixed and the CTC movably mounted, with the same result. Thus in a DEP device according to this invention at least one of the CTC and the frame with the printhead structure is movably mounted.

The spacers in a DEP device of this invention can be replaced by air bearings.

In a device according to this invention, the surface of the CTC, carrying charged toner particles, can move over the spacers in a sliding contact. In that case, the spacers can have different shapes, it can be a row of dots, a row of bars, a bar, they can be rectangular, cylindrical, triangular, etc. as long as they perform the effect of keeping the distance between the frame with the printhead structure and the surface of the CTC constant. The spacers can, in the case of sliding contact between the surface of the CTC and the spacers, be made of any material, although spacers made of insulating material, e.g. polymeric material, ceramic material, are largely preferred. The insulating material can preferably be a flexible polymeric material as e.g. a polyester, a polyimide, a polyamide, a polyurethane, a polycarbonate, etc. The face of the spacers contacting the surface of the CTC can be provided with a friction reducing layer for aiding the smooth gliding (sliding) of the face of the spacers over the surface of the surface of the CTC. Such a layer can comprise a solid lubricant dispersed in a binder, e.g. disulfide of molybdenum dispersed in a binder, as disclosed in US 5,497,175, the layer can be made with a perfluorpolymer (e.g. TEFLON (trade name), the friction reducing layer can comprise matting agents protruding above the layer, which diminish the surface of the spacers contacting the surface of the surface of the CTC. Such spacing particles can advantageously comprise a lubricant (e.g. a wax) as described in EP-A 241 600 or comprise fluor-containing compounds as described in EP-A 281 848. The spacers used in a device according to this invention can be permanently attached to the frame with the printhead structure facing the surface of the CTC or can be placed between said frame and the surface of the CTC. In the latter case the spacing particles are kept in place by pressing the surface of the CTC against the spacers that are pressed against the frame with the printhead structure.

The spacers used in this invention can also be wheels or cylinders and the surface of the CTC can then roll over the wheels.

In figure 4 it is schematically shown how spacers in the form of cylinders can be used. In this figure only the printhead structure stretched over a frame and the spacers are shown. A printhead structure (104) with a flexible substrate (104a) and an array of printing apertures (108) is stretched over a frame with 4 bars (105) by coils springs that are attached to a larger framework (not shown). Above two parallel bars spacers (107) in the form of cylinders are present, these cylinders are also sustained by the large framework (not shown) and can rotate over an axis (107a). At the ends of the cylinders a thickened portion (107b) is present and the CTC (not shown for clarity) rests on those thickenings. This configuration is quite well suited for bringing the CTC and the frame with the printhead structure in contact over the spacers by gravitational force. When the larger framework (that is not shown in the figure) is placed so that the frame with the printhead structure is in a horizontal plane, the toner source incorporating the CTC can simply rest on the cylindrical spacers and the surface of the CTC remains in contact with the spacers due to gravitation.

In figure 5, an other configuration wherein the spacers are wheels and the surface of the CTC rotates against those wheels is shown. In this figure only the printhead structure stretched over a frame and the spacers are shown. A printhead structure (104) with a flexible substrate (104a) and an array of printing apertures (108) is stretched over a larger framework by coils springs (106). On the larger framework (109) two bars (110) carrying spacer wheels (107) are mounted. The framework is mounted in the DEP device so that the surface of the CTC touches the wheels and is kept in contact with them.

In figure 6 it is schematically shown how a printhead structure, stretched over a frame with spacers, can movably be mounted against the surface of the CTC. A printhead structure (104) with a flexible substrate (104a) and an array of printing apertures (108) is stretched over an opening (111) in a larger framework (109) by coils springs (106). Spacers (107) are present on the printhead structure where the flexible substrate is supported by the framework (109). The larger framework is mounted with hinges (112) in the frame (113) carrying the CTC (101). The larger framework (109) can be moved toward the frame carrying the CTC so that the spacers (107) contact the surface of the CTC. The framework (109) carrying the printhead structure is kept in resilient contact with the frame (113) carrying the CTC by attaching coil springs (115) to the fastening points (114) in the frame (113) carrying the CTC.

In all figures wherein a CTC (Charged Toner Conveyor) is shown, the CTC is shown as a cylinder or roll, the present invention can nevertheless also be implemented with a CTC that is an endless belt moving past the printing apertures over, e.g. rollers.

The toner particles on the surface of the CTC that is mounted as per this invention can originate from different sources. The CTC can be the exit roller of a cartridge containing non-magnetic mono-component developer, it can be the roller of a magnetic brush carrying magnetic toner particles. The CTC, in a device of this invention, can be also be a separate roller or belt whereon charged toner particles are applied from a magnetic brush carrying a multicomponent developer (as disclosed in, e.g., EP-A-740 224). It can also be roller or belt whereon charged toner particles are applied from the exit roller of a cartridge containing non-magnetic mono-component developer. In this case the exit roller can be kept at a certain distance from the surface of the CTC as disclosed in, e.g., DE-A-197 45 561 or the exit roller can be in contact with the surface of the CTC as disclosed in, e.g., European Application 99203242 filed on October 4, 1999.

The printhead structure, used in a device of the present invention, can have any configuration of printing apertures known in the art. It can carry control electrodes on one side of the flexible substrate and a shield electrode on the other side of the substrate, as disclosed in, e.g., US-A-3 689 935. It can be a printhead structure having only control electrodes on one side of the substrate and no electrodes on the other side. It can be, a printhead structure as disclosed in, e.g., EP-A-812 696, EP-A-753 413, EP-A-754 557, etc..

A DEP device according to this invention can be operated by applying a DC-voltage to the control electrodes in order to selectively open and close the printing apertures, it can also be operated by applying an AC-voltage to the control electrodes and image wise modifying the strength of the AC-voltage in order to selectively let toner particles pass the apertures and stop them. Such devices are described in, e.g., EP-A-911 706 and European Application 99203305, filed on October 8, 1999.

Parts list

101. Conveyor for charge toner particles (CTC)

102. Back electrode

103. Image receiving substrate

104. Printhead structure

104a. Flexible substrate of the printhead structure

105. Bar of a frame over which the printhead structure is stretched

106. Coil spring for stretching the printhead structure

107. Spacers

107a. Axis

107b. Thickening

108. Array of printing apertures

109. Larger framework

110. Bars carrying spacers

111. Opening in the larger framework

112. Hinges

113. Frame carrying a CTC

114. Fastening points o, the frame carrying the CTC

115. Coil springs for fastening the larger frame work to the frame carrying the CTC.

Claims

A device for direct electrostatic printing comprising

a toner source including a surface carrying charged toner particles placed opposite to an image receiving member

a voltage source applying an electric field between said toner source and said image receiving member said field being adjusted letting toner particles migrate from said toner source towards said receiving member

a framework holding a printhead structure, installed so that said printhead structure is placed between said toner source and said image receiving member, said printhead structure having a flexible insulating base and printing apertures there through and control electrode associated with said printing apertures, said control electrodes being coupled to a voltage source for selectively opening and closing said printing apertures, characterised in that

i) said framework and said toner source are mounted such that a distance, d, exists between said printhead structure and said surface carrying charged toner particles, and

ii) both said framework and said surface carrying toner particles are kept in contact by an external force, F, so that said distance, d, is kept constant.
A device according to claim 1, wherein at least one of said framework and said toner source are mounted so as to be movable with respect to each other.
A device according to claim 1 or 2, wherein said external force is the gravitational force.
A device according to claim 1 or 2, wherein said external force is a resilient force.
A device according to claim 4, wherein said resilient force is exerted by springs.
A device according to any of the preceding claims , wherein between said framework and said surface carrying toner particles spacers are present.
A device according to claim 6, wherein said surface carrying toner particles is in sliding contact with said spacers.
A device according to claim 6 or 7, wherein said spacers are made of insulating material selected from the group consisting of ceramic materials, polyesters, polyimides, polyamides, polyurethanes and polycarbonates.
A device according to claim 6, wherein said surface carrying toner particles is in rolling contact with said spacers.
A device according to claim 1 or 2, wherein said distance, d, is kept constant by means of air bearings placed between said frame and said toner source.