A direct electrostatic printing device wherein charged toner particles are brought proximate to a printhead structure using an electrostatic powder spray gun

Abstract

 A direct electrostatic printing device is described wherein the means for providing a stream of charged toner particles (106) proximate to a printhead structure (102) comprises an electrostatic powder spray gun (105).
Two embodiments are disclosed, one wherein the flow of toner particles towards the image receiving member (109) is directly generated from the spray gun and one wherein the spray gun is used to apply a layer of charged toner particles on a conveyor for charged toner particles.

Description

FIELD OF THE INVENTION

[0001] 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

[0002] 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.

[0003] 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).

[0004] 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.

[0005] Each control electrode is formed around one aperture and is isolated from each other control electrode.

[0006] 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.

[0007] In DEP devices intended for high speed printing, printhead structures with multiple rows of printing apertures are used. Such printhead structures have be disclosed in e.g. US-A-4 860 036 a printhead structure has been described consisting of at least 3 (preferentially 4 or more) rows of apertures which makes it possible to print images with a smooth page-wide density scale without white banding. The main drawback of this kind of printhead structure deals with the toner particle application module, which has to be able to provide charged toner particles in the vicinity of all printing apertures with a nearly equal flux. The problem of equal toner flux has been addressed in several ways (see e.g. US-A-5 040 004, US-A-5 214 451, US-A-5 136 311, EP-A-731 394).

[0008] The printing speed achievable with DEP devices does not only depends on the possibility of using a printhead structure with multiple rows of printing apertures, nor does the printing quality only depend on providing charged toner particles in the vicinity of all printing apertures with a nearly equal flux, but both printing speed and printing quality depend also on the amount of charged toner particles that is presented per unity of time in the vicinity of the printing apertures.

[0009] In EP-A-740 224 it has been disclosed that the relative speed of rotation of the CTC (charged toner conveyor),of the magnetic brush bringing charged toner particles on the surface of the CTC and the image receiving substrate must be related to each other for bringing enough toner particles in the vicinity of the printing apertures. Also in this disclosure it has been disclosed to use a CTC with a radius that is adapted to the extension of the rows of printing apertures in the printing direction. In US-A-5 738 009 and the European equivalent EP-A-736 822 it is disclosed that when extracting the toner particles directly from a magnetic brush, without using a CTC, the speed of rotation of the magnetic brush has to be high.

[0010] The disclosures above do solve the problems above but when very high printing speed is necessary, the speed of rotation the CTC and the magnetic brush have to be extremely high which entail problems of mechanical wear of the bearings of the CTC and the magnetic brush.

[0011] It has been proposed in EP-A-763 785 to use instead of a CTC a fluidized bed of charged toner particles in the vicinity of printing apertures, this approach was very useful when printhead structures with large extension in the printing direction were used (i.e. printhead structures with multiple rows of printing apertures). This approach minimised the moving parts in the device (no rotating CTC and/or rotating magnetic brush) but it proved very difficult to have enough toner particles presented per unity of time in the vicinity of the printing apertures.

[0012] There is thus a need for a DEP device wherein it is possible to provide in a simple and reliable way a large amount of toner particles in the vicinity of the printing apertures for printhead structures having multiple rows of printing apertures.

OBJECTS AND SUMMARY OF THE INVENTION

[0013] It is an object of the invention to provide a device for Direct Electrostatic Printing (DEP) for high speed printing with few moving parts and with a printhead structure with multiple rows of printing apertures wherein charged toner particles are provided in the vicinity of all printing apertures with a nearly equal flux.

[0014] It is an other object of the invention to provide a device for Direct Electrostatic Printing (DEP) for high speed printing wherein clogging of the printing apertures is minimised.

[0015] The object of the invention is realised by providing a direct electrostatic printing device for printing images onto a receiving substrate, comprising

• a means for providing a stream of charged toner particles proximate to a printhead structure,
• means for coupling the receiving substrate to an electric potential so as to create an electric field for attracting said toner particles, as a toner flow, from said stream to the receiving substrate,
• said printhead structure having a first side and a second side located so as to have said first side facing said stream of toner particles and said second side facing the receiving substrate, said printhead structure having an array of printing apertures extending there through from said first side to said second side, said printing apertures being coupled to control electrodes for electrically modulating said toner flow in accordance with image data to cause said toner particles to be deposited onto the receiving substrate to form an image,

characterised in that    said means for providing a stream of charged toner particles proximate to said printhead structure comprises an electrostatic powder spray gun.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Fig. 1 shows schematically a first possible implementation of the direct spray embodiment of this invention, wherein the toner particles for image formation are directly extracted from a stream of toner particles ejected by an electrostatic spray gun.

[0017] Fig. 2 shows schematically a second possible implementation of the direct spray embodiment of this invention.

[0018] Fig. 3 shows schematically a third possible implementation of the direct spray embodiment of this invention.

[0019] Fig. 4 shows schematically a fourth possible implementation of the direct spray embodiment of this invention.

[0020] Fig. 5 shows schematically a fifth possible implementation of the direct spray embodiment of this invention.

[0021] Fig. 6 shows schematically a possible implementation of the spray/CTC embodiment of this invention, wherein a layer of charged toner particles is applied to a conveyor for charged toner particles by means of an electrostatic spray gun.

DETAILED DESCRIPTION OF THE INVENTION

[0022] It was now found that the problems associated with high speed direct electrostatic printing could largely be overcome when the device incorporates an electrostatic powder spray gun. This spray gun can be used either to present a stream of charged toner particles directly in the vicinity of the printing apertures or to load, per unit of time, a large amount of charged toner particles on charged toner conveyer (CTC) from where a dense stream of toner particles is presented to the printing apertures. Thus the invention comprises two embodiments, a first one wherein in a DEP device a stream of charged toner particles is presented to the printing apertures direct from an electrostatic spray gun (the direct spray embodiment) and a second one wherein in a DEP device a layer of charged toner particles is applied to a CTC by an electrostatic powder spray gun (the CTC-spray embodiment). Electrostatic powder spray guns are well known in the art of finishing materials by coating the materials with a dry powder instead of by a solvent based paint.

[0023] Electrostatic powder spray guns - herein after indicated a "spray gun" for short - are commercially available and have been disclosed in, e.g., US-A-5 622 313, US-A-5 776 249, US-A-4 653 696, US-A-4 802 625 and US-A-5 482 214.

[0024] A spray gun can easily be adapted to give a flat spray, therefore an even amount of particles over a large area can be provided. A flat spray can be quite fast moving, this presents a disadvantage when a flat spray is used for coating materials since the oncoming particles have a relatively high speed and can easily dislodge the previously deposited particles. When used in a DEP device, this disadvantage turns into an advantage, since the possibility of dislodging previously deposited particles helps to keep the printhead structure clean both at the surface of the printhead, facing the spray gun, and in the printing apertures due to the impact of the particles on the printhead.

[0025] The toner particles are charged in the spray gun in two ways : either the gun has a high voltage charging electrode which produces a corona to charge the powder, or the gun has means to charge the powder by friction, i.e., triboelectrically. Both types of spray gun are useful in a DEP device of this invention.

[0026] The use of a spray gun as toner source has further the advantage that it is easy to construct a DEP device, either of the direct spray type or the CTC-spray type, wherein only right sign toner particles arrive in the vicinity of the printing apertures and wherein the wrong sign toner particles and/or the non-charged toner particles are taken out of the flow of toner particles.

[0027] Although the invention is explained herein below using negatively charged toner particles, it is clear that the invention also works with positively charged toner particles by a simple adaptation of the polarity of the electric potentials.

[0028] In the embodiments, described below, the receiving substrate is coupled to a DC-voltage source by bringing a back electrode kept at a DC-voltage near or in contact with the receiving substrate. It is also possible to operate a DEP device according to this invention when the receiving substrate is coupled to a DC-voltage source by first applying at least one conductive layer on a substrate, and by connecting the conductive layer to a voltage source. Thus in this case the DEP device operates without back electrode. Such a device has been described in EP-A-823 676. Also when using a DEP device according to this invention for printing PCB's (printed circuit boards) the DEP device can be operated without back electrode by coupling the conductive layer of the PCB-precursor to a voltage source. Such a method has been described in European Application 98201302 filed on April 22, 1998.

The direct spray embodiment

[0029] In the direct spray embodiment of the invention a DEP device is provided wherein a spray gun forms directly a stream of charged toner particles near the printing apertures.

[0030] This embodiment can be implemented in various ways.

[0031] In a first implementation, the spray gun is mounted so that it directs a stream of toner particles in a direction essentially perpendicular to the plane of the printing apertures and its powder outlets are arranged so that the toner particles are directed from the spray gun direct towards the printing apertures, i.e. the imaginary line drawn from the outlets of the spray gun parallel trough the particle stream, does cross the printhead structure and under a basically right angle. In figure 1, this is schematically shown. A housing (101) with walls has a printhead structure (102) in one of the walls. In the printhead structure, printing apertures (103), coupled to control electrodes (104) are present. In a wall of the housing (101) opposite to the wall containing the printhead structure, a spray gun (105) is mounted on a pivot (105a), with the powder outlets projecting a mixture of negatively charged toner particles (106a), i.e., for sake of the example, "right sign" toner, together with some non-charged toner particles (106b) and some positively charged toner particles (106c), i.e. "wrong sign" toner, directly to the printing apertures. In the housing an electrode (107) is provided that is kept at a DC voltage with a negative polarity, for attracting wrong sign toners (106c), i.e. positively charged toner particles. The housing is kept at a DC voltage (DC1). Outside the housing, opposite to the wall of the housing (101) containing the printhead structure a back electrode (108) is placed so as to define a gap, d, between the printhead structure and the back electrode. An image receiving member (109) is passed through said gap in the direction of arrow A. The back electrode is kept at a second DC voltage (DC2) different from DC1 so a to create an electric potential difference (an electric field) between said housing and said back electrode. During printing a flow of charged toner particles with, for the sake of the example, negative polarity is generated by the spray gun, this stream comes into the electric field created by the electric potential difference |DC1 - DC2| wherein this difference is positive. From the stream of charged toner particles, the negatively charged toner particles (106a) are attracted towards the back electrode and by image wise modulating the voltage applied to the control electrodes (104) by a voltage source (DC3) coupled to said control electrodes the toner particles pass the printing apertures image-wise and form a toner image on the image receiving member. The toner particles that were positively charged (106c), i.e. the wrong sign toner particles, are attracted by the electrode (107) and also collected in the housing. Also the right sign toner that was not used for image formation is collected in the housing. A means for moving toner particles (110) moves the toner from the housing again towards a container (not shown) and from there the toner is circulated back to the spray gun.

[0032] In this case the kinetic energy given to the stream of charged toner particles by the spray gun has to be accurately controlled for avoiding that toner particles pass through printing apertures simply by virtue of the high kinetic energy, although the control electrode around the printing apertures is kept at an electric potential prohibiting the passage of charged toner particles. Also non charged toner particles are projected directly in the direction of the printing apertures and risk to pass the printing apertures in a random way.

[0033] It is preferred that the spray gun is arranged so that the imaginary line drawn from the outlets of the spray gun parallel to the particles stream, does not cross the printhead structure or when it crosses the printhead it does not so under a right angle.

[0034] It is, in such a construction of a DEP device according to the direct spray embodiment of this invention, however preferred to install the spray gun movably so that during operation of the printing device the stream of toner particles is not projected so as to physically contact the printhead structure and the printing apertures but that, during a cleaning cycle, the stream of toner particles can be directed so that the toner particles reach the printhead structure and by the impact of them on the printhead structure clean the printhead by dislodging previously deposited particles.

[0035] An implementation of the direct spay embodiment of this invention, wherein the imaginary line drawn from the outlets of the spray gun parallel to the particle stream, does not cross the printhead structure, is schematically shown in figure 2. A housing (101) with walls is provided comprising a printhead structure (102), with printing apertures (103) and control electrodes (104) arranged in one of said walls and electrically isolated from said wall. A spray gun (105) is arranged in a wall of said housing so that a particle stream containing a mixture of negatively charged toner particles (106a), i.e., for sake of the example, "right sign" toner, together with some non-charged toner particles (106b) and some positively charged toner particles (106c), i.e. "wrong sign" toner is ejected in a direction essentially parallel to the wall containing the printhead structure. The spray gun is movably mounted in said wall around a pivot (105a) so that it is possible, during a cleaning step, the direct the toner particles towards the printhead structure where, by the impact of the toner particles the printhead structure is cleaned. In the housing an electrode (107) is provided that is kept at a DC voltage (DC4) with a polarity equal to said first polarity. The housing is kept at a DC voltage (DC1). Outside the housing and opposite to the wall of the housing (101) containing the printhead structure a back electrode (108) is placed so as to define a gap, d, between the printhead structure and the back electrode. An image receiving member (109) is passed through said gap in the direction of arrow A. The back electrode is kept at a second DC voltage (DC2) different from DC1 so a to create an electric potential difference (an electric field) between said housing and said back electrode. During printing a particle stream containing a mixture of negatively charged toner particles (106a), i.e., for sake of the example, "right sign" toner, together with some non-charged toner particles (106b) and some positively charged toner particles (106c), i.e. "wrong sign" toner is generated by the spray gun, this stream comes into the electric field created by the electric potential difference |DC1 - DC2| wherein this difference is positive. From the stream of toner particles, the negatively charged toner particles (106a) are attracted towards the back electrode by the electric potential difference |DC1 - DC2| and by image wise modulating the voltage applied to the control electrodes (104) by a voltage source (DC3) coupled to said control electrodes the toner particles pass the printing apertures image-wise and form a toner image on the image receiving member. The toner particles that were not charged (106b) in the spray gun are not attracted towards the back electrode and are collected in the housing. The toner particles that were positively charged (106c), i.e. the wrong sign toner particles, are attracted by the electrode (107) and also collected in the housing. Also the right sign toner that was not used for image formation is collected in the housing. A means for moving toner particles (110) moves the toner from the housing again towards a container (not shown) and from there the toner is circulated back to the spray gun. Instead of having the housing (101) coupled to a DC-voltage source (DC1) it is possible to use a housing made from isolating material, and to place electrodes in or on the housing near the printhead structure where then these electrodes are coupled to the DC-voltage source (DC1) for creating together with the DC-voltage on the back electrode an electric field wherein the charged toner particles are attracted towards the receiving substrate.

[0036] A further implementation of the direct spay embodiment of this invention, wherein the imaginary line drawn from the outlets of the spray gun parallel to the particles stream, does not cross the printhead structure, is schematically shown in figure 3. A housing (101) with walls is provided comprising a printhead structure (102), with printing apertures (103) and control electrodes (104) arranged in one of said walls and electrically isolated from said wall. A spray gun (105) is arranged in a wall of said housing so that a particle stream containing a mixture of negatively charged toner particles (106a), i.e., for sake of the example, "right sign" toner, together with some non-charged toner particles (106b) and some positively charged toner particles (106c), i.e. "wrong sign" toner is ejected in a direction basically perpendicular to the printhead structure (102) but the imaginary line from the outlets of the spray gun does not cross the printhead structure. The stream of ejected toner particles bounces on an element (112) arranged for bending the stream of toner particles in a direction essentially parallel to the printhead structure. Basically this implementation works as the implementation shown in figure 2, but it has the advantage that the construction of the housing incorporating the spray gun and the printing apertures can be made more compact. Said bouncing element (112) preferably is made of a plastic material with sufficient elasticity so that charged polymeric particles are repelled without losing kinetic energy after collision upon said bouncing element. More preferably said bouncing element (112) also is provided with a surface coating that helps in tribocharging the impacting toner particles. For that reason the tribological properties of said coating is chosen on the basis of their tribo-position in the tribological range of materials, suitable for the tribobehaviour of the polymeric particles used. Excellent materials useful for coating said bouncing element can be found in the literature with regard to surface coatings used in coating carrier particles for two-component electrophotographic developing systems. The rubbery material of said bouncing element (112) can also be made partially conductive (e.g. by the incorporation of conductive particles such as carbon black) so that excessive tribocharging of said bouncing element is prevented by partially grounding said element to a conductive grounded body, preventing the amount of charged toner particles in the neighbourhood of said printing apertures to diminish or fluctuate as a function of printing time.

[0037] An other implementation of the direct spay embodiment of this invention, wherein the imaginary line drawn from the outlets of the spray gun parallel to the particles stream, does not cross the printhead structure, is schematically shown in figure 4. A housing (101) with walls is provided comprising a printhead structure (102), with printing apertures (103) and control electrodes (104) arranged in one of said walls and electrically isolated from said wall. A spray gun (105) is arranged in a wall of said housing so that a particle stream containing a mixture of negatively charged toner particles (106a), i.e., for sake of the example, "right sign" toner, together with some non-charged toner particles (106b) and some positively charged toner particles (106c), i.e. "wrong sign" toner is ejected under a given angle (a) to the printhead structure (102) so the imaginary line from the outlets of the spray gun forms an angle a with the printhead structure and does not cross the printhead structure. The stream of toner particles may reach the wall of the housing wherein the printhead structure is present simply by the kinetic energy given to the particles, but not at the location of the printhead structure. In the housing the same elements as described in figure 2 are present together with an electrode (111) coupled to a voltage source DC5, with polarity opposite to the polarity of the charge on the "right sign" toner particles, that is placed near the printhead structure for aiding the deflection of the "right charge" toner particles, 106a, from the stream of toner particles towards the printhead structure. Basically this implementation works also as the implementation shown in figure 2, but it has the advantage that the construction of the housing incorporating the spray gun and the printing apertures can be made more compact.

[0038] The direct spray embodiment of this invention can also be implemented using more than one spray gun. In figure 5 an implementation with two spray guns is shown. A housing (101) with walls is provided comprising a printhead structure (102), with printing apertures (103) and control electrodes (104) arranged in one of said walls and electrically isolated from said wall. In two opposite walls of the housing ,a spray gun (105) is arranged in a wall of said housing so that a particle stream containing a mixture of negatively charged toner particles (106a), i.e., for sake of the example, "right sign" toner, together with some non-charged toner particles (106b) and some positively charged toner particles (106c), i.e. "wrong sign" toner is ejected in a direction essentially parallel to the wall containing the printhead structure. The spray guns are movably mounted in said wall around a pivot (105a) so that it is possible, during a cleaning step, the direct the toner particles towards the printhead structure where, by the impact of the toner particles the printhead structure is cleaned. In the housing two electrodes (107) are provided that are kept at a DC voltage (DC4) with a polarity equal to said first polarity for taking the wrong sign toner (106c) out of the flow. Also the non charged toner particles are, at least partially, collected on these electrodes (107). These electrodes are rotatably arranged so that they turn in opposite directions and a nip is formed between the two rotating electrodes. The means (110) for moving the non used toner particles can then create a vacuum in the nip so that the wrong sign toners together with the non-charged toner is easily removed from the electrodes. In the housing also two rotating electrodes (111) coupled to a voltage source DC5, with polarity opposite to the polarity of the charge on the toner particles, are placed near the printhead structure for aiding the deflection of the "right charge" toner particles, 106a, from the streams of toner particles towards the printhead structure. Basically this implementation operates as the implementation shown in figure 2. Here also said rotating electrodes (111) can for a nip from which the excessive right-sign toner particles can be removed and recuperated.

The spray/CTC embodiment

[0039] DEP device wherein the charged toner particles are brought near to a printhead structure on a surface of a conveyer for charged toner particles (a Charged Toner Conveyer or CTC) are well known in art. E.g. in US-A-4 814 796, US-A-5 337 124, US-A-5 311 266, US-A-4 491 855, EP-A-740 224 such devices are described. The charged toner particles can be applied on the surface of the CTC, - which is kept at a DC voltage different from the DC voltage coupled to the receiving substrate, - by a magnetic brush. In order to have enough toner particles on the surface of the CTC in the case of high printing speed, the speed of rotation the CTC and the magnetic brush have to be extremely high, which entail problems of mechanical wear of the bearings of the CTC and the magnetic brush. It was now found that an electrostatic spray gun could apply a large amount of charged toner particles to the surface of a CTC in a short time. This makes it possible to have high printing speed and to use a CTC with large diameter (low curvature) while not having problems with toner depletion (i.e. no longer having enough toner particles on the CTC to print the desired density).

[0040] In figure 6 a possible implementation of the spray/CTC embodiment of this invention is shown. A housing (101) with walls has a conveyer for charged toner particles, i.e. a CTC (117) in one of the walls, the CTC is movably mounted so as to turn in the direction of arrow B. The CTC is coupled to a DC-voltage source (DC1) and to an AC-voltage source (AC1). The latter voltage source is optional. Said CTC-roller is preferably a metallic roller, e.g. a roller of aluminium with a surface coating. Said surface coating can have properties tuned for optimal tribologic and electric characteristics, e.g. it can comprise carbon black and tribo-particles in a polymeric rubbery matrix. In an other wall of the housing (101), a spray gun (105) is mounted, with the powder outlets (105b) projecting, for sake of the example, negatively charged toner particles (106) to the CTC. Near the CTC (117) a corona (113) is provided for further equalising the charge distribution of the toner particles on the CTC. An electrode (115) is installed for attracting the "wrong sign" toner that would have reached the CTC, despite of the presence in the housing of an electrode (107) that is kept at a DC voltage with a negative polarity, for attracting wrong sign toners (106c), i.e. positively charged toner particles, the non-charged toner particles are projected in the housing. Outside the housing, opposite to the wall of the housing (101) containing the CTC (117), a back electrode (108) kept at a second DC voltage (DC2) different from DC1 so a to create an electric potential difference (an electric field) between said CTC and said back electrode. During printing charged toner particles that are present on the surface of the CTC are attracted in the electric field created by the electric potential difference |DC1 - DC2| wherein this difference is positive, forming a stream of toner particles. A printhead structure (102) having printing apertures (103) coupled to control electrodes (104) is placed in said stream of toner particles from the CTC to the back electrode. The printhead structure is arranged in such a way as to define a gap, d, between the printhead structure and the back electrode. An image receiving substrate is moved through said gap, d, in the direction of arrow A. By image wise modulating the voltage applied to the control electrodes (104) by a voltage source (DC3) coupled to said control electrodes the toner particles pass the printing apertures image-wise and form a toner image on the image receiving member. The toner particles that were positively charged (106c), i.e. the wrong sign toner particles, are attracted by the electrode (107) and collected in the housing. Also the right sign toner that was not used for image formation is collected in the housing. Near the CTC, downstream from the printhead structure a corona (114) is placed and kept at a DC voltage suited to detach the toner particles, that were not used in the process from the CTC. A cleaning member (116) is placed further away from the corona (114) so as to further clean the surface of the CTC. By doing so the spray gun brings charged toner particles (106a) always to a clean CTC surface so that this surface provides always fresh toner particles to the surface of the CTC. This ensures that the charge distribution of the toner particles on the CTC is kept constant during printing, which results in increased reproducibility of the printing result over the time. All toner particles (non-charged particles, "wrong sign" toner and "right sign" that was removed from the CTC) that were collected in the housing are moved by a means for moving toner particles (110) again towards a container (not shown), wherein the particles can be charged again and from there the toner is circulated back to the spray gun.

[0041] The spray guns used in this invention can be guns charging the particles with a high voltage charging electrode as well as spray guns charging the particles triboelectrically, .

[0042] Using a spray gun equipped for charging the particles with a high voltage charging electrode e.g. a GEMA MPS-1L gun (trade name of Ransburg-Gema SA, St- Gall, Switserland), said high voltage charging electrode is set to a value yielding charged toner particles with sufficient but not to high charge over mass ratio. By using said gun for charging toner particles commercially available from Agfa-Gevaert N.V., Mortsel Belgium for use in CHROMPRESS (trade name Agfa-Gevaert N.V., Mortsel Belgium)and setting the high voltage charging electrode to a voltage exceeding 20,000 V a q/m-ratio of -30 µC/g and higher was obtained, resulting in blurry images and high toner sticking upon said CTC-surface. By lowering the voltage applied to said high voltage charging electrode to about 5,000 to 10,000 V a q/m-ratio of about -10 to -30 µC/g could be obtained. For the embodiment 6 in which said charged toner particles are propelled towards said CTC-roller, it was found that a charging voltage of about 3,000 to 7,000 V led to the best printing results. However, it must be clear for those skilled in the art that for the direct spray embodiment said charging voltage can be set to a much higher value because toner adhesion is less important if compared to the spray/CTC-embodiment. In this case the air pressure has to be fine-tuned so that said highly charged toner particles are not blown through said printing apertures due to their acquired kinetic energy. Otherwise, this could only be prevented by using higher image-wise modulated voltages that would lead to a much too expensive apparatus. Therefore it is preferred in both embodiments of this invention to use a spray gun operated at an air pressure below 10 bar (i.e. below 10⁶ Pa).

[0043] A direct electrostatic printing device according to this invention wherein the means for providing a stream of charged toner particles proximate to a printhead structure comprises an electrostatic powder spray gun, can be used with any printhead structure known in the art, it can be used e.g. with a printhead structure as described in US-A-5 889 540, US-A-5 714 992, EP-A-812 696, EP-A-895 867, European Application 99200479 filed on February 18, 1999, European Application 99200478 filed on February 18, 1999 and European Application 99200480 filed on February 18, 1999. It can also be used with a printhead structure in mesh form as described in US-A-5 036 341.

[0044] A DEP-device according to this invention wherein the means for providing a stream of charged toner particles proximate to a printhead structure comprises an electrostatic powder spray gun, can be used in large format printers as described in e.g. EP-A-849 645, EP-A-849 087 and European Application 98203008 filed on September 8, 1998.

[0045] A DEP device according to this invention implemented in the CTC/spray embodiment can incorporate a CTC wherein the dimensions are adapted to the extension of the array of printing apertures in the printhead structure as described in, e.g., EP-A-740 224.

Claims

1. A direct electrostatic printing device for printing images onto a receiving substrate, comprising

- a means for providing a stream of charged toner particles proximate to a printhead structure,

- means for coupling the receiving substrate to an electric potential so as to create an electric field for attracting said toner particles, as a toner flow, from said stream to the receiving substrate,

- said printhead structure having a first side and a second side located so as to have said first side facing said stream of toner particles and said second side facing the receiving substrate, said printhead structure having an array of printing apertures extending there through from said first side to said second side, said printing apertures being coupled to control electrodes for electrically modulating said toner flow in accordance with image data to cause said toner particles to be deposited onto the receiving substrate to form an image,

characterised in that
   said means for providing a stream of charged toner particles proximate to said printhead structure comprises an electrostatic powder spray gun.

2. A direct electrostatic printing device according to claim 1, wherein said stream of charged toner particles is directly provided proximate to said printhead structure by said spray gun.

3. A direct electrostatic printing device according to claim 2, wherein said spray gun provides said stream of toner particles in a direction essentially parallel to said printing apertures in said printhead structure.

4. A direct electrostatic printing device according to claim 2, wherein said spray gun provides said stream of toner particles in a direction essentially perpendicular to said printhead structure on a bouncing element arranged for bending said stream of toner particles in a direction essentially parallel to the printhead structure.

5. A direct electrostatic printing device according to any of claims 1 to 4, wherein proximate to said printhead structure an electrode is present for attracting right-sign-toner particles towards said printhead structure.

6. A direct electrostatic printing device according to any of claims 1 to 5, wherein an electrode is present for attracting wrong-sign-toner particles and preventing said wrong-sign-toner particles of coming proximate to said printhead structure.

7. A direct electrostatic printing device according to any of claims 1 to 6, wherein at least two spray guns are used.

8. A direct electrostatic printing device according to any of claims 4 to 7, wherein said bouncing plate has a surface coating for for giving toner particles bouncing on it a higher charge to mass ratio.

9. A direct electrostatic printing device according to claim 1, wherein said stream of charged toner particles is provided proximate to said printhead structure from a charged toner conveyer whereon charged toner particles are deposited by an electrostatic spray gun.

10. A direct electrostatic printing device according to claim 9, wherein proximate to said charged toner conveyer an electrode is present for attracting right-sign-toner particles towards said charged toner conveyer.

11. A direct electrostatic printing device according to claim 9 or 10, wherein proximate to said charged toner conveyor a corona device is present for levelling the charge on the toner particles on said charged toner conveyor to a constant level.

12. A direct electrostatic printing device according to any of the previous claims, wherein a spray gun having a high voltage charging electrode is used and operated at a voltage below 30,000 V.

13. A direct electrostatic printing device according to any of the previous claims, wherein a spray gun is operated at an air pressure below 10⁶ Pa.

Drawing