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(11) | EP 1 091 264 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | A device for direct electrostatic printing wherein charged toner particles are applied to a charged toner conveyer that touches the toner dispensing part of a non magnetic mono-component development system |
(57) A DEP device comprising :
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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] One of the problems with this type of printing devices is that charged toner particles can accumulate upon the printhead structure and in the printing apertures. Due to this problem the achievable printing density does not remain constant in the time, while the charged toner particles accumulated on the printhead structure may change the electrical field wherein the charged toner particles are propelled towards the substrate and the toner particles accumulated in the printing apertures can physically block the toner passage.
[0008] This problem of clogging of the printing apertures has been addressed in several ways, there have been disclosed ways and means to avoid the clogging and ways and means to clean the printhead and the printing apertures.
[0009] A first way disclosed to avoid the clogging of printing apertures relies on the design of the printhead structure, the printing apertures or both. In, e.g. US-A-4 876 561 it is disclosed to prevent clogging of the printing apertures by making the apertures large enough and/or the thickness of the isolating layer small enough. In US-A-5 307 092 it is disclosed to apply an antistatic coating to the electrodes in the printhead so that any tribocharge that accumulates during writing can be grounded and the accumulation of toner particles on the printhead avoided. In EP-A-780 740 a printhead structure, for a DEP (Direct Electrostatic Printing) device is disclosed that comprises an insulating material and an oblong slit having control electrodes on the edge of it, instead of having apertures. In, e.g., US-A-5 625 392 an edge electrode is described so that instead of individual apertures or a larger slit, an even larger free zone between the toner source and the receiver exists, resulting in even better properties regarding clogging of the printhead structure
[0010] Another way to avoid the clogging of printing apertures and the smudging of the printhead that was disclosed relies on tuning the charge of the toner particles that are used. In, e.g., US-A-4 755 837 and US-A-4 814 796 it is disclosed that the presence of Wrong Sign Toner (WST) is the main cause of accumulation of toner particles upon said printhead structure and in the printing apertures. Wrong sign toner particles are particles that have a sign different from that of the majority of the particles. Therefore they respond to the applied electrical fields for creating a flow of charged toner particles to the substrate in an opposite way than the majority of the toner particles. In these disclosures it has been described that the problem of wrong signed toner can be solved when in a device for direct electrostatic printing the flow of toner particles towards the substrate originates from the surface of a conveyer for charged toner particles (hereinafter indicated as "charged toner conveyer" or CTC) whereon well behaved (i.e. wherein no wrong sign toner is present) charged toner particles are deposited by using a magnetic brush comprising two-component developer. In US-A-5 337 124 and US-A-5 900 893 a toner application module for electrophotographic and electrographic printing has been described in which two different magnetic brushes are used, one to supply toner particles to a charged toner conveying roller and one to recuperate them from said roller. This is a system wherein a pushing magnetic brush brings the toner particles to the surface of the CTC and after the surface of the CTC has passed near the printing apertures a pulling magnetic brush is used to clean the surface of the CTC and to keep always "well behaved" toner particles on the surface of the CTC. In DE-A-197 45 561 it has been disclosed to bring well behaved toner particles on the surface of the CTC from a non-magnetic mono-component development system by propelling charged toner particles from the non-magnetic mono-component development system to the surface of a CTC-roller over an air-gap with the help of a rather high potential difference.
[0011] Also mechanical ways to prevent clogging or to clean the printing apertures have been disclosed. In, e.g., US-A-5 153 611, US-A-5 202 704, US-A-5 233 392 it is disclosed to prevent clogging of the printing apertures by using an ultrasonic vibration applied to the printhead. In US-A-5 283 594 the level of vibration applied to the printhead is different during writing time and cleaning time. In US-A-5 293 181 the printhead is vibrated in such a way that a mechanical propagating wave is created.
[0012] Electrical means to clean the printhead structure have been disclosed in, e.g., US-A-4 491 855, US-A-4 478 510, US-A-4 903 050, US-A-4 755 837, US-A-5 095 322 etc.
[0013] The ways to prevent clogging as described above, do all more or less solve that problem, but the methods entail their own drawbacks. The electrical means to clean the printhead structure require frequently the use of high voltage and/or electric spark generators, which entails that the DEP apparatus incorporating electric cleaning means are more complicated and/or expensive than DEP device not needing such cleaning provisions. The same goes for the DEP device including ultrasonic vibration as cleaning means for the printhead structure. When the design of the printhead has to be adapted for avoiding clogging, the degrees of freedom in constructing the printhead and the printing apertures become smaller. When thin printhead structures with relatively wide printing apertures are used, the strength of the printhead structures as well as the possible resolution are diminished. Using an edge electrode as printhead structure as described in US-A-5 625 392 solves the problem of clogging, but suffers 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 source and the receiver on the back electrode must be set to a rather low value, so that per unit of time only a moderate amount of toner particles can be attracted so that only a moderate printing speed is possible when high optical density is desired.
[0014] Avoidance of wrong sign toner particles is most easily realised by having a large average charge to mass ratio, but toner particles with high charge to mass ratio, although having no wrong sign toner particles, do not solve the problem of clogging. It has been disclosed in US-A-5 337 124 and US-A-5 900 893 to use two different magnetic brushes around a CTC, one to supply toner particles to a charged toner conveying roller (a pushing magnetic brush) and one to recuperate them from said roller (a pulling magnetic brush). By doing so the surface of the CTC is cleaned by the pulling magnetic brush and fresh toner particles are applied to the surface of the CTC during each revolution of the CTC. The pulling magnetic brush was found to be insufficient for removing all toner particles from the CTC so that there have been proposals to use scraper blades to clean the CTC so that the pushing magnetic brush applies fresh toner particles to a virgin surface of the CTC. The implementation of a scraper blade, however, requires that a relatively high amount of toner is presented to said CTC-roller in a single contact: i.e. a typical amount of charged toner applied to said CTC-roller of about 5 to 10 g/m2 has to be deposited in a single pass. If the magnetic brush can not supply enough toner particles to said CTC-roller in a single pass, then "ghost images" will occur in the final printout, because it will take 2 to 5 revolutions of said magnetic brush before the required layer thickness of 5 g/m2 toner is applied upon the surface of said CTC-roller. In DE-A-197 45 561 it has been disclosed to bring well behaved toner particles on the surface of the CTC from a non-magnetic mono-component development system by propelling charged toner particles from the non-magnetic mono-component development system to the surface of a CTC-roller over an air-gap with the help of a rather high potential difference. As a consequence, also in this implementation although clogging is prevented and no wrong sign toner is brought to the CTC, the toner transfer from the non-magnetic mono-component roller to the CTC-roller is not fast enough to prevent "ghost images".
[0015] Thus most of the measures hitherto disclosed for avoiding clogging are or expensive, or give raise to slowdown of the printing speed, or give raise to the appearance of ghost images. Thus there is still a need for further improved DEP devices making it possible to print at elevated speed with no or very low toner accumulation upon said printhead structure and with a reliable and constant flow of well behaved charged toner particles from said toner application module at a moderate to low machine cost.
OBJECTS AND SUMMARY OF THE INVENTION
[0016] 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 combined with low clogging of the printing apertures, with high and constant maximum density and with constant grey level density over a long period of time.
[0017] A further object of the invention is to provide a method for direct electrostatic printing wherein it is possible to combine high speed printing, low clogging of the printing apertures, high maximum density, low or no incidence of ghost images and a printing quality that is constant over a long period of time.
[0018] Further objects and advantages of the invention will become clear from the detailed description herein after.
[0019] The first object of the invention is realised by providing a DEP device comprising :
- a non-magnetic mono-component development system comprising a toner dispensing part (108), with an outer surface carrying charged toner particles and a conveyer for charged toner particles with an outer surface, placed in the DEP device so that both said outer surface touch each other for transferring said charged toner particles from said outer surface of said toner dispensing part to said outer surface of said conveyor,
- one or more voltage sources (V1, V4) for creating an electric field between said outer surface of said conveyor and an image receiving member (109), for forming a flow (111) of charged toner particles from said outer surface of said conveyor to said image receiving member, and
- a printhead structure (106), having printing apertures (107) and control electrodes (106a) coupled to a control voltage (V3), placed in said flow, between said outer surface of said conveyor to said image receiving member, said voltage source, V3, being image wise modulated for image-wise depositing toner particles on said image receiving member.
[0020] The second object of the invention is realised by providing a method for Direct Electrostatic Printing (DEP) comprising the steps of :
- placing a toner dispensing part, with an outer surface, contained in a non-magnetic mono-component development system, in such a way relative to a conveyer for charged toner, with an outer surface, so that both said outer surfaces touch each other and that charged toner particles are brought to said outer surface of said conveyer for charged toner,
- applying an electrical potential difference between said outer surface of said conveyer for charged toner and an image receiving member, for creating a flow of charged toner particles from said outer surface of said conveyer for charged toner to said image receiving member,
- placing a printhead structure having printing apertures and control electrodes in said flow, between said outer surface of said conveyer for charged toner and said image receiving member and
- connecting said control electrodes to a voltage source for image wise opening and closing said apertures for image wise depositing toner particles on said image receiving member.
[0021] Further objects and advantages of the invention will become clear from the detailed description hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] Figure 1 shows schematically a DEP device according to the prior art in which a non-magnetic mono-component development system is used to jump charged toner particles over an air gap upon the surface of a charged toner conveyer roller by applying a voltage difference.
[0023] Figure 2 shows schematically a DEP device according to the present invention in which the toner dispensing part of a non-magnetic mono-component development system is in contact with a charged toner conveyer roller.
[0024] Figure 3 shows schematically a DEP device according to the present invention in which the toner dispensing part of a non-magnetic mono-component development system is in contact with a charged toner conveyer roller, wherein the toner dispensing part is in belt form.
[0025] Figure 4 shows schematically a DEP device according to the present invention in which the toner dispensing part of a non-magnetic mono-component development system is in contact with a charged toner conveyer roller, wherein the toner dispensing part is a roller made of resilient material.
[0026] Figure 5 shows very schematically a DEP device according to the present invention wherein the toner particles are recycled.
DETAILED DESCRIPTION OF THE INVENTION
Definitions
[0027] The term CTC "charged toner conveyer" is used for a conveyer of charged toner particles rotated in one direction, said charged toner particles being applied to the outer surface of it by means of a magnetic brush or a non-magnetic mono-component toner charging member. The CTC can be a roller, a belt, a roller with a core surrounded by a sleeve, etc.
[0028] The term "toner dispensing part" is used to indicate that part of a non-magnetic mono-component development system that has a surface carrying charged toner particles and from where said charged toner particles can be dispensed to a CTC or directly extracted towards the printhead structure. In the figures the part marked (108) is the "toner dispensing part" in the sense of this definition.
[0029] "Ghost images" can be seen in even grey patches that are printed after printing an image : when in the grey level some impression of the previously printed image is left, this image is a "ghost image".
[0030] As can be understood from the background art section above, it is not obvious to develop a DEP method or a DEP apparatus wherein it is possible to combine high speed printing, low clogging of the printing apertures, high maximum density, low or no incidence of ghost images and a printing quality that is constant over a long period of time.
[0031] A prior art DEP device wherein charged toner particles are jumped from a toner dispensing part of a non-magnetic mono-component development system is shown in figure 1. This constellation is equivalent to the one disclosed in DE-A 197 45 561. A non-magnetic mono component development system (101) contains non-magnetic toner particles (102), stirred by stirring means (110), a toner propagating roller (104) rotating in the direction of arrow D, bringing non-magnetic toner particles on the sleeve (108a) placed around a core (108b) of a toner dispensing part (108). The toner dispensing part with toner particles on it rotates in the direction of arrow C and brings the toner particles past a doctor blade (112) and a charging part (113) so that the toner dispensing part carries charged non-magnetic toner particles. The toner dispensing part is connected to a DC-voltage source (V5) and is placed with respect to a charged toner conveyer (103) so that between this toner conveyer and the toner dispensing part a gap, g, is left. The charged toner conveyer is connected to a DC source (V1) and an optional AC-source (AC2). The difference between the DC voltage (V5) on the toner dispensing part and the voltage on the charged toner conveyer (V1, AC2) makes charged toner particles jump from the toner dispensing part (108) to the CTC (103). The CTC (103) is mounted opposite to a back electrode (105) connected to a DC-voltage source V4. The voltage difference |V4-V1| creates an electric field wherein a flow (111) of charged toner particles from the surface of the CTC to the back electrode is generated. An image receiving substrate (109) passes, in the direction of arrow A, adjacent to the back electrode in said flow. Between the image receiving substrate and the surface of the CTC, a printhead structure (106) is placed in the flow of charged toner particles. The printhead structure includes an array of printing apertures (107),shield electrode (106b) common 'to all printing apertures and connected to voltage source V2. The zone between the array of printing apertures and the surface of the CTC forms a development zone in which a toner flow can be initiated(111). Control electrodes (106a) are associated with the printing apertures and are connected to a voltage source, V3, that can apply a voltage that varies in accordance with image data to the control electrode for selectively let charged toner particles pass the printing apertures and stop the charged toner particles from passing the apertures. Optionally a means (114) for recovering an recycling the toner particles is located near the CTC and downstream of the development zone. This prior art device makes it possible to have almost no clogging of the printing apertures and toner adhesion to the printhead structure, but the device does not perform very well in terms of delivering high maximum density, low or no incidence of ghost images and a printing quality that is constant over a long period of time.
[0032] It was now found by the inventors that the combination of the four desired properties mentioned immediately above, i.e. high speed printing and/or high maximum density, low clogging of the printing apertures, , low or no incidence of ghost images, and a printing quality that is constant over a long period of time, could be achieved when the DEP device comprised a "toner dispensing part" that is part of a non-magnetic mono-component development system installed in such a way that the surface of the toner dispensing part and the surface of a CTC (Charged Toner Conveyor) touch each other. "Touch each other" is for the sake of this document to be understood as meaning that there is no air gap present between the two surfaces, but only one or more layers of charged toner particles. Compared to the prior art descriptions in which an air gap is used and charged toner particles are jumped over said air gap from said "toner dispensing part" of said non-magnetic mono component development system towards said CTC-roller, the amount of charged toner particles that can be provided upon said surface of said CTC-roller is high enough so that "ghost images" can be prevented, although the printing speed is measured in several meters/minute.
[0033] In figure 2, a first possible embodiment of a DEP device according to this invention is schematically shown. The numbering of the parts in figure 2 is the same as in figure 1, but the sleeve (108a) of the toner dispensing part is in contact with the CTC. The sleeve (108a) and the core (108b), which is driven by a motor (not shown), of the toner dispensing part (108) are construed so that the sleeve has an inner diameter slightly larger than the outer diameter of the core, so that in the contact point between the toner dispensing part (108) and the CTC (103) a slack (115) is formed. By doing so the surface over which the CTC and the toner dispensing part make contact is enlarged, with the beneficial effect that a large amount of toner particles is brought on the CTC by every revolution of the toner dispensing part. In a typical, but not limitative, design of a dispensing part (108), wherein the sleeve has an inner diameter slightly larger than the outer diameter of the core, the sleeve has a thickness of about 150 µm and the a diameter of about 20 mm. The sleeve is preferably made of a conductive and flexible material, e.g., organic polymers, nylon, nickel, organic polymeric materials filled with carbon black, etc. the core, which is used for driving the sleeve is contained in the sleeve and the outer diameter of it is smaller than the inner diameter of the sleeve, so that a slack is formed. The drive roller is also preferably made from conductive material and is connected to a voltage source or ground potential.
[0034] In an other embodiment of the invention, schematically shown in figure 3, the toner dispensing part is made as an endless belt (108) that is moved around bearings (116) at least one of them being driven by a motor (not shown). The belt is arranged so that again a slack (115) is formed where the toner dispensing part is in contact with the CTC. The belt is preferably made of a conductive and flexible material, e.g., organic polymers as such, e.g., nylon, polystyrene, polyvinylchloride, polyester, polyacrylate, polycarbonate, polyimide, etc or an organic polymeric material filled with carbon black or other conductive particles. The belt can also be made from a metal as nickel, stainless steel, etc.
[0035] In still a further embodiment of the invention, as schematically shown in figure 4, the toner dispensing part is made as a roller having a small rigid core for driving it and wherein said roller is covered with a resilient material, e.g., polyurethane rubber, conductive rubber, etc. It can also be a sponge roller. The roller is arranged so that again the surface over which the toner dispensing part is in contact with the CTC is quite large.
[0036] In any of the embodiments of the present invention, schematically shown in the figures 2 to 4, it is preferred that the charged toner particles, that are not used in the printing process and remain on the CTC, are further displaced downstream of the printing zone to a cleaning station (114) in which a complete removal of charged (or discharged) toner particles from the surface of said CTC is effected to have a bare surface again. Then the CTC moves further on towards the toner dispensing part of the non-magnetic mono-component development system, located upstream of the development zone where again a fresh population of charged toner particles, wherein no wrong sign toner particles are present, is provided on the surface of the CTC. When the toner particles are removed from the CTC, this proceeds preferably by means of a scraper blade, a rotating brush, a roller with a surface of foamed polymers, a suction device, etc. Preferably, in a method according to this invention, the non-used toner particles are removed from the toner bearing surface of the CTC with a scraper blade, which can be made from a plastic material or of metal. In a method according to this invention it is preferred to use scraper blade made of stainless steel. In a further preferred embodiment of the invention, the non-used toner particles that have been removed from the CTC are recycled in the container and dispenser of non-magnetic mono-component developer.
[0037] A DEP device according to this invention, wherein charged toner particles, that are not used in the printing process and remain on the CTC, are further displaced downstream of the printing zone to a cleaning station in which a complete removal of charged (or discharged) toner particles from the surface of said CTC is effected and from where these toner particles are recycled to the container for non-magnetic mono-component developer is shown in figure 5. In this figure a device for direct electrostatic printing is very schematically shown (from the non-magnetic mono-component development system only the container and the "toner dispensing part" are shown), comprising :
- a non-magnetic mono-component development system comprising a toner dispensing part (108), with an outer surface carrying charged toner particles and a conveyer for charged toner particles with an outer surface, placed in the DEP device so that both said outer surface touch each other for transferring said charged toner particles from said outer surface of said toner dispensing part to said outer surface of said conveyor,
- one or more voltage sources (V1, V4) for applying an electrical potential difference between said toner bearing surface andan image receiving member (109), for creating a flow (111) of charged toner particles from said surface bearing charged toner particles to said image receiving member,
- a printhead structure (106) having printing apertures (107) and control electrodes (106a) placed in said flow, forming a development zone under said printing apertures for image wise depositing toner particles on said image receiving member
- a means (not shown) for moving said conveyer for charged toner particles, in a direction of arrow B, to pass said toner bearing surface repeatedly through said development zone so as to have said means for bringing charged toner particles on said surface upstream of the development zone and
- a means for removing and collecting (117), placed downstream from said development zone, non-used toner particles from said toner bearing surface.
[0038] Optionally the device further comprises means for recycling the collected toenr particles to the container (101) for non-magnetic mono-component developer.
[0039] Altough it is possible in a DEP device according to this invention, wherein the toner dispensing part of the non-magnetic mono component development system touches the outer surface of the conveyer for charged toner particles (CTC) only in a small nip, e.g. when both the toner dispensing part and the CTC are hard rollers, it is pereferred that when the toner dispensing part of the non-magnetic mono component development system touches the outer surface of the conveyer for charged toner particles a slack is formed, so that the surface over which both the toner dispensing part and the CTC touch each other is enlarged.
[0040] In the figures 2 to 5, showing DEP devices according to this invention, the back electrode is a solid electrode, however 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, cooperating with the printhead structure, can also comprise one or more flexible PCB's (Printed Circuit Board).
[0041] A DEP device according to this invention can also function without a back electrode when it is used for printing on an insulating image receiving substrate. In that case, a conductive layer is applied upon said insulating substrate and said conductive layer is connected via a conductive charge applying device to a voltage source, and the substrate whereon the printing proceeds is then its own back electrode. Such device has been described in EP-A-823 676 and EP-A-952 498.
[0042] The location and/or form of the shield electrode (106b) and the control electrode (106a) can, in other embodiments of a device for a DEP method according to the present invention, be different from the location shown in the figures.
[0043] Although in all figures a DEP device using two electrodes (106a and 106b) on printhead 106 is shown, it is possible to implement a DEP device, according to the present invention, incorporating printhead structures with different constructions. It is, e.g. possible to implement a DEP method with a device having a printhead comprising only one electrode structure as well as with a device having a printhead comprising more than two electrode structures. The apertures in these printhead structures can have a constant diameter, or can have a broader entrance or exit diameter. Typical printhead structures useful in DEP devices according to the present invention have been described in, e.g., US-A-5 889 540, US-A-5 714 992, EP-A-753 413, EP-A-780 740, EP-A-812 696, EP-A-816 944, EP-A-924 089, etc.
[0044] When operating DEP devices according to this invention, between said printhead structure (106) and the charged toner conveyer (103) as well as between the control electrode around the apertures (107) and the back electrode (105) behind the toner receiving member (109) as well as on the single electrode surface or between the plural electrode surfaces of said printhead structure (106) different electrical fields are applied. In the specific embodiment of a device, useful for a DEP method, using a printing device with a geometry according to the present invention, shown in fig 2. voltage V1 is applied to the sleeve of the charged toner conveyer 103, voltage V2 to the shield electrode 106b, voltages V30 up to V3n for the control electrode (106a). The value of V3 is selected, according to the modulation of the image forming signals, between the values V30 and V3n, on a time basis or grey-level basis. Voltage V4 is applied to the back electrode behind the toner receiving member. In other embodiments of the present invention multiple voltages V20 to V2n and/or V40 to V4n can be used. Voltage V5 is applied to the surface of the sleeve (108a) of the toner dispensing part of the non-magnetic monocomponent development system. If so desired, an additional AC-source can beneficially be connected to the surface of the sleeve (108a) of the toner dispensing part of said non-magnetic monocomponent development system.
[0045] The invention encompasses also a method for Direct Electrostatic Printing (DEP) comprising the steps of :
- placing a toner dispensing part, with an outer surface, contained in a non-magnetic mono-component development system, in such a way relative to a conveyer for charged toner, with an outer surface, so that both said outer surfaces touch each other and that charged toner particles are brought to said outer surface of said conveyer for charged toner,
- applying an electrical potential difference between said outer surface of said conveyer for charged toner and an image receiving member, for creating a flow of charged toner particles from said outer surface of said conveyer for charged toner to said image receiving member,
- placing a printhead structure having printing apertures and control electrodes in said flow, between said outer surface of said conveyer for charged toner and said image receiving member and
- connecting said control electrodes to a voltage source for image wise opening and closing said apertures for image wise depositing toner particles on said image receiving member.
[0046] It is preferred to use in a DEP device according to the present invention, toner particles with an absolute average charge over mass ratio (|q/m|) corresponding to 5 µC/g ≤ |q/m| ≤ 15 µC/g, preferably to 8 µC/g ≤ |q/m| ≤ 11 µC/g. The charge to mass ratio of the toner particles is measured by mixing the toner particles with carrier particles, and after 15 min of charging the q/m-ratio is measured as described in US-A-5 880 760. Said toner particles were pulled under vacuum from said CTC-roller to an accurately weighted filter paper (weight was WP in g), which was shielded in a Faraday cage. The amount of charge that arrived, after about 5 minutes vacuum pulling and after an accurate surface area of said CTC-roller was cleaned from said toner particles, at said filter paper was measured with a Coulomb meter in µC. The filter paper with the toner particles was weighted again, giving weight WPT in g. The charge to mass ratio was then determined as µC/(WPT-WP). In this disclosure the charge to mass ratio is taken as the absolute value, as a DEP device according to this invention can function either with negatively charged toner particles or with positively charged toner particles depending on the polarity of the potential difference between V1 and V4. 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 that is incorporated herein by reference.
[0047] It is further preferred in the method and the devices of this invention not-only to prevent to use of toner particles with the wrong sign, but also to use toner particles with a narrow charge distribution, i.e. the charge of the toner particles shows a distribution wherein the coefficient of variability (v), i.e. the ratio of the standard deviation to the average value, is equal to or lower than 0.4 preferably lower than 0.33. The charge distribution 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. In, e.g., US-A-5 569 567, US-A-5 622 803 and US-A-5 532 097 it is disclosed how to prepare both negatively and positively chargeable toner particles with narrow charge distribution. It is a preferred embodiment of the invention to use toner particles prepared according to the method described in these disclosures. Basically negatively chargeable toner particles, according to these disclosures, with both low average charge and narrow charge distribution are provided by adding to toner particles, comprising a negatively chargeable toner resin, at least one resistivity lowering substance having a volume resistivity lower than the volume resistivity of the resin, wherein said substance(s) is fare) capable of lowering the volume resistivity of said resin by a factor of at least 3.3 when present in said binder in a concentration of 5 % by weight relative to the weight of said binder.
[0048] Basically positively charged toner particles, according to these disclosures, with both low average charge and narrow charge distribution are provided by adding to the toner particles, having a triboelectrically positively chargeable thermoplastic resin, at least one substance having a volume resistivity lower than the volume resistivity of the resin, wherein said substance(s) when present in said binder in a concentration of 5 % by weight lower(s) the volume resistivity of said binder by a factor of at least 3.3. Such resisivity decreasing substances are within the following classes of compounds : onium compounds, metal salts containing relatively large (bulky) anionic groups, betaines, amino acids, metal complex compounds, ionically conductive polymers in which the polymer chain carries anionic groups, e.g. sulphonate groups, electronically conductive polymers, e.g. polyanilines, polypyrroles and polythiophenes.
EXAMPLES
The printhead structure (106)
[0049] A printhead structure (106) was made from a polyimide film of 50 µm thickness, double sided coated with a 5 µm thick copper film. The printhead structure (106) had two rows of printing apertures. On the back side of the printhead structure, facing the image receiving member, a rectangular shaped control electrode (106a) was arranged around each aperture. 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 amplifier. The printing apertures were rectangular shaped with dimensions of 360 by 120 µm. The dimension of the central part of the rectangular shaped copper control electrodes was 500 by 260 µm. The apertures were spaced so to obtain a resolution of 33 dots/cm (85 dpi). On the front side of the printhead structure, facing the charged toner conveyer roller, a common shield electrode (106b) was,arranged around the aperture zone leaving a free polyimide zone of 1620 µm. Said printhead structure was fabricated in the following way. First of all the control and shield electrode pattern was etched by conventional copper etching techniques. The apertures were made by a step and repeat focused excimer laser making use of the control electrode patterns as focusing aid. After excimer burning the printhead structure was cleaned by a short isotropic plasma etching cleaning. Finally a thin coating of PLASTIK70, commercially available from Kontakt Chemie, was applied over the control electrode side of said printhead structure.
The charged toner conveyer (CTC)
[0050] The CTC was a cylinder with a sleeve made of aluminium, coated with TEFLON (trade name of Du Pont, Wilmington, USA) with a surface roughness of 2.2 µm (Ra-value) and a diameter of 30 mm. The charged toner conveyer (103) was connected to a DC power supply of 0V.
The printing engine
[0051] The printhead structure, mounted in a PVC-frame, was bent with frictional contact over the surface of the roller of the charged toner conveyer roller. A polyurethane coating was used as self-regulating spacer means.
[0052] 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 was set to 1000 µm and the paper travelled a linear speed (LSM) of 300 cm/min. The back electrode was connected to a high voltage power supply, applying a voltage V4 of + 1000 V to the back electrode.
[0053] To the shield electrode 106b a voltage of + 110 V, to the individual control electrodes an (image-wise) voltage V3 between 0 V and + 280 V was applied.
Measurement of printing quality
[0054] Printouts made on paper with a DEP device as described above, were judged for ghost images and variation of the image density after a long printing run (typically 1 hour). The printed image consisted of an ISO original image ('portrait' and 'cafeteria' of the ISO 12640 standard) followed by a 50% grey area and a grey-wedge). If the image quality was very good, then after a continuous printing time of 1 hour the printhead structure was checked for toner adhesion and/or nozzle blocking, and the printouts were analysed for variation in maximum density and appearance of ghost images. The criteria for ghost images were set as follows: in the area of 50% grey level density, the visibility of the previously printed ISO images was checked. If these ISO originals could be hardly recognised in said 50% grey level density area, then an OK was given. For the judgement of the variation of image density the image density was measured using a MacBeth TR1124 densitometer in the area of maximum, half and minimum density of the grey wedge. If the final image density (after printing during 1 hour) was changed with more than 10% compared to the initial image density of the first image in one of these measured density zones, then a "NOT OK" was tabulated. Finally, a criteria for nozzle blocking was also incorporated in the table with printing results: here an "OK" meant that after a printing time of 1 hour less than 1% of the printing apertures were blocked or caused white stripes in the printout. All results are summarised in table 1.
PRINTING EXAMPLE 1 (PE1)
PRIOR ART DEP : Toner particles extracted directly from the toner dispensing part of the non-magnetic mono-component development system
[0055] In this experiment a non-magnetic mono-component development system, commercially available from Apple (Cyan Toner Cartridge M3757 G/A for the APPLE COLOR LASER WRITER 12/600 PS (trade name) printer) was used to jump toner particles directly towards the printhead structure. The front roller of said non-magnetic mono-component development system was placed at 230 µm from the front side of said printhead structure. An AC-voltage of 1600 V peak to peak at 2.9 kHz (sinusoidal) with + 160V DC-offset, was applied to said roller. To the shield electrode a voltage of + 110 V was applied, to said control electrodes image wise modulated voltages of 0 and + 280 V were applied. The back electrode was located at 1000 µm from the back side of said printhead structure, and a voltage of + 1250 V was applied to it. The image receiving paper was travelling at a speed of 3 m/min, while the roller of the non-magnetic mono-component development system was rotating at a linear speed of 6 m/min. Printing was performed during 1 hour. After one hour a severe toner adhesion was observed upon said printhead structure, however, only a few nozzle's were blocked. From the final printouts it could be observed that especially after the periods of Dmin-printing nozzle blocking frequently occurred but the blocked nozzle's could be opened spontaneously during the Dmax-writing conditions. So, since intermittent nozzle blocking (and white stripes in the printouts) can occur a "NOT OK" is tabulated. The variation in density as a function of printing time was considerable, especially in the area's of minimum density, so that here also a "NOT OK" has to be tabulated. Ghost images in the image part of 50% grey density could be hardly detected: leading to an "OK" value.
PRINTING EXAMPLE 2 (PE2) PRIOR ART DEP
PRIOR ART DEP : Toner particles extracted directly from the toner dispensing part of the non-magnetic mono-component development system
[0056] In this experiment a non-magnetic mono-component development system, commercially available from Hewlett Packard (Cyan Toner Cartridge C4192A for the HP COLOR LASERJET 4500 (trade name) printer) was used to jump toner particles directly towards the printhead structure. The front roller of said non-magnetic mono-component development system was placed at 230 µm from the front side of said printhead structure. An AC-voltage of 1600 V peak to peak at 2.9 kHz (sinusoidal) with + 160V DC-offset, was applied to said roller. To the shield electrode a voltage of + 110 V was applied, to said control electrodes image wise modulated voltages of 0 and + 280 V were applied. The back electrode was located at 1000 µm from the back side of said printhead structure, and a voltage of + 1250 V was applied to it. The image receiving paper was travelling at a speed of 3 m/min, while the roller of the non-magnetic mono-component development system was rotating at a linear speed of 6 m/min. Printing could not be performed during 1 hour because severe toner adhesion to said printhead structure led to irreversible nozzle blocking and disappearing image density. Ghost images in the image part of 50% grey density of the first images could not be detected, so that an "OK" value was tabulated for this property.
PRINTING EXAMPLE 3 (PE3) PRIOR ART DEP
PRIOR ART DEP : a charged toner conveyer roller was fed from a single magnetic brush carrying magnetic carrier particles and non-magnetic toner particles
[0057] In this experiment a charged toner conveyer roller was fed from a single magnetic brush, wherein the distance between said magnetic brush and said charged toner conveyer roller was 750 µm. The magnetic brush was a stationary core/rotating sleeve type magnetic brush comprising two mixing rods and one metering roller. One rod was used to transport the developer through the unit, the other one to mix toner with developer. The magnetic brush was constituted of the so called magnetic roller, which in this case contained inside the roller assembly a stationary magnetic core, having three magnetic poles with an open position (no magnetic poles present) to enable used developer to fall off from the magnetic roller (open position was one quarter of the perimeter and located at the position opposite to said CTC. The sleeve of said magnetic brush had a diameter of 20 mm and was made of stainless steel roughened with a fine grain to assist in transport (Ra=3 µm) and showed an external magnetic field strength in the zone between said magnetic brush and said CTC of 0.045 T, measured at the outer surface of the sleeve of the magnetic brush. The magnetic brush was connected to 155 V DC-offset. The toner concentration of the developer (commercially available developer for the AGFA CHROMAPRESS (trade name) printer) used was 5 %. To the sleeve of the CTC an AC-potential was applied of 1800 V (peak to peak value) and 2.9 kHz (sinusoidal) with a DC-offset of + 180V. The surface of said CTC-roller was located at 260 µm from the front side of said printhead structure. To the shield electrode a voltage of + 110 V was applied, to said control electrodes image wise modulated voltages of 0 and + 280 V were applied. The back electrode was placed at 1000 µm from the back side of said printhead structure and connected to a DC-potential of + 1250 V. The image receiving paper was travelling at a speed of 3 m/min, while the linear speed of the CTC-roller and magnetic brush roller were 4.5 and 19 m/min, respectively. Both magnetic brush roller and CTC-roller were rotated in an opposite direction. Even after a printing period of 1 hour, no significant toner adhesion to said printhead structure was observed, nor any white stripe in the final image printouts could be found somewhere ("OK"). However, the image density was significantly lower at the end of the printing time compared to the first images ("NOT OK") due to an increase of the charge to mass ratio of the toner particles upon said CTC-roller, and ghost images were somewhat visible ("NOT OK"). They only disappeared completely by enhancing the linear rotation speed of said magnetic brush so that a factor of 5 or higher (preferably 10) if compared with the linear rotation speed of the CTC-roller was obtained: i.e. optimal rotation speed of the magnetic brush for this CTC-roller at 4.5 m/min is 45 m/min. All printing results are also summarised in table 1.
PRINTING EXAMPLE 4 (PE4)
PRIOR ART DEP : Non-magnetic mono-component development system over air gap
[0058] In this experiment a non-magnetic mono-component development system, commercially available from Apple (Cyan Toner Cartridge M3757 G/A for the Apple Color Laser Writer 12/600 PS printer) was used to jump toner particles over a gap to a CTC. The distance between said front-roller of said non-magnetic mono-component development system and said charged toner conveyer roller was 230 µm, the DC-potential applied towards the sleeve of said front-roller of said non-magnetic mono-component development system was + 45 V. To the sleeve of the CTC an AC-potential was applied of 1600 V (peak to peak value) and 2.9 kHz (sinusoidal) with a DC-offset of + 140V. The surface of said CTC-roller was located at 240 µm from the front side of said printhead structure. To the shield electrode a voltage of + 110 V was applied, to said control electrodes image wise modulated voltages of 0 and + 280 V were applied. The back electrode was placed at 1000 µm from the back side of said printhead structure and connected to a DC-potential of + 1250 V. The image receiving paper was travelling at a speed of 3 m/min, while the linear speed of the CTC-roller and front-roller of said non-magnetic mono-component development system were 5 and 10 m/min, respectively. Both front-roller of said non-magnetic mono-component development system and CTC-roller were rotated in an opposite direction. Although the toner adhesion to said printhead structure was greatly improved if compared with the example in which the front-roller of said non-magnetic mono-component development system was used to directly propel charged toner particles to said printhead structure ("OK"), both the image density significantly changed over the printing time ("NOT OK"), and ghost images were highly visible ("NOT OK"). All printing results are also summarised in table 1.
PRINTING EXAMPLE 5 (PE5)
PRIOR ART DEP : Non-magnetic mono-component development system over air gap
[0059] In this experiment a non-magnetic mono-component development system, commercially available from Hewlett Packard (Cyan Toner Cartridge C4192A for the HP COLOR LASERJET 4500 (trade name) printer) was used to jump toner particles over a gap, g, to a CTC (103). The distance between said front-roller ("the toner dispensing part" of said non-magnetic mono-component development system and said charged toner conveyer roller was 230 µm, the DC-potential applied towards the sleeve of said front-roller of said non-magnetic mono-component development system was + 60 V. To the sleeve of the CTC an AC-potential was applied of 1700 V (peak to peak value) and 2.9 kHz (sinusoidal) with a DC-offset of + 150V. The surface of said CTC-roller was located at 240 µm from the front side of said printhead structure. To the shield electrode a voltage of + 110 V was applied, to said control electrodes image wise modulated voltages of 0 and + 280 V were applied. The back electrode was placed at 1000 µm from the back side of said printhead structure and connected to a DC-potential of + 1250 V. The image receiving paper was travelling at a speed of 3 m/min, while the linear speed of the CTC-roller and front-roller of said non-magnetic mono-component development system were 5 and 10 m/min, respectively. Both the front-roller of said non-magnetic mono-component development system and the CTC-roller were rotated in an opposite direction. Although the toner adhesion to said printhead structure was greatly improved if compared with the example in which the front-roller of said non-magnetic mono-component development system was used to directly propel charged toner particles to said printhead structure ("OK"), both the image density significantly changed over the printing time ("NOT OK"), and ghost images were terribly visible ("NOT OK"). All printing results are also summarised in table 1.
PRINTING EXAMPLE 6 (PE6) DEP ACCORDING TO THIS INVENTION
[0060] In this experiment the charged toner conveyer roller was fed from a non-magnetic mono-component development system, commercially available from Lexmark (Cyan Toner Cartridge 1361752 for the LEXMARK OPTRA SC1275 (trade name) printer). The front roller, i.e. the "toner dispensing part" of said non-magnetic mono-component development system, was in direct contact over the charged toner particles with said CTC-roller. A DC-potential of + 170 V was applied towards the sleeve of said front-roller of said non-magnetic mono-component development system. The doctor blade in said non-magnetic mono-component development system cartridge was connected to a voltage of -20 V, and the conductive strip in said cartridge was connected to a voltage source of + 135 V. To the sleeve of the CTC an AC-potential was applied of 1800 V (peak to peak value) and 2.9 kHz (sinusoidal) with a DC-offset of + 200V. The surface of said CTC-roller was located at 260 µm from the front side of said printhead structure. To the shield electrode a voltage of + 110 V was applied, to said control electrodes image wise modulated voltages of 0 and + 280 V were applied. The back electrode was placed at 1000 µm from the back side of said printhead structure and connected to a DC-potential of + 1250 V. The image receiving paper was travelling at a speed of 3 m/min, while the linear speed of the CTC-roller and front-roller of said non-magnetic mono-component development system were 5 and 10 m/min, respectively. The front-roller ("toner dispensing part") of said non-magnetic mono-component development system was rotated in a direction opposite to the direction wherein the CTC-roller was rotated. Even after a printing period of 1 hour, no significant toner adhesion to said printhead structure was observed, nor any white stripe in the final image printouts could be found somewhere ("OK"). The image density in all parts of the grey-wedge was constant over the total printing period ("OK"), and ghost images were never visible ("OK"). All printing results are also summarised in table 1. So, compared to the other printing examples it was surprisingly found that not only ghost images could be prevented, and constant image density could be obtained, but that even with this system in which the front roller ("toner dispensing part") of said mono-component non-magnetic development system is in direct contact over the charged toner particles to said CTC-roller, no significant toner-adhesion towards said printhead structure occurs.
Table 1 :
| Results of the printing | ||||
| # | CONFIGURATION | A | D | G |
| PE1 | DIRECT-MOC | NOT-OK | NOT-OK | OK |
| PE2 | DIRECT-MOC | NOT-OK | NOT-OK | OK |
| PE3 | CTC-MB | OK | NOT-OK | NOT-OK |
| PE4 | CTC-MOC | OK | NOT-OK | NOT-OK |
| PE5 | CTC-MOC | OK | NOT-OK | NOT-OK |
| PE6 | TOUCH-CTC-MOC | OK | OK | OK |
Abbreviations
[0061]
DIRECT-MOC : Prior art DEP : Toner particles extracted directly from the toner dispensing part of the non-magnetic mono-component development system
CTC-MB : Prior art DEP : a CTC loaded with toner particles from a magentic brush with two component developer.
CTC-MOC : Prior art DEP": Non-magnetic mono-component development system over air gap
TOUCH-CTC-MOC : This invention
A = Adhesion of toner particles to printhead structure.
D = Density fluctuation from first to last image
G = Ghost images in image zone of 50% grey density
[0062] The invention has been explained with negatively charged toner particles, but the devices and the method of this invention can also function with positively charged toner particles, for the person skilled in the art it is obvious to adapt the polarity of the applied voltages to the use of positively charged toner particles.
1. A DEP device comprising :
- a non-magnetic mono-component development system comprising a toner dispensing part (108), with an outer surface carrying charged toner particles and a conveyer for charged toner particles with an outer surface, placed in the DEP device so that both said outer surface touch each other for transferring said charged toner particles from said outer surface of said toner dispensing part to said outer surface of said conveyor,
- one or more voltage sources (V1, V4) for creating an electric field between said outer surface of said conveyor and an image receiving member (109), for forming a flow (111) of charged toner particles from said outer surface of said conveyor to said image receiving member, and
- a printhead structure (106), placed in said flow, between said outer surface of said conveyor to said image receiving member and having printing apertures (107) coupled to control electrodes (106a), coupled to a control voltage (V3), being image wise modulated for image-wise depositing toner particles on said image receiving member.
2. A device according to claim 1, wherein said toner dispensing part is construed so
has to have a slack formed when it touches said conveyer for charged toner.
3. A device according to claim 1 wherein said toner dispensing part comprises a sleeve
(108a) with an inner diameter and the core (108b) with an outer diameter, said inner
diameter of said sleeve being larger than said outer diameter of said core.
4. A device according to claim 1 wherein said toner dispensing part is an endless belt.
5. A device according to claim 4, wherein said endless belt is made of a material selected
from the group consisting of metals, organic polymers as such and an organic polymeric
material filled with conductive particles.
6. A device according to claim 1 wherein said toner dispensing part is a roller comprising
a core surrounded with resilient material.
7. A device according to any of the preceding claims further comprising means for collecting
and recycling non used toner particles.
8. A method for Direct Electrostatic Printing (DEP) comprising the steps of :
- placing a toner dispensing part, with an outer surface, contained in a non-magnetic mono-component development system, in such a way relative to of a conveyer for charged toner, with an outer surface, so that both said outer surfaces touch each other
- bringing charged toner particles from said outer surface of said toner dispensing part to said outer surface of said conveyer for charged toner,
- applying an electrical potential difference between said outer surface of said conveyer for charged toner and an image receiving member, for creating a flow of charged toner particles from said outer surface of said conveyer for charged toner to said image receiving member,
- placing a printhead structure having printing apertures and control electrodes in said flow, between said outer surface of said conveyer for charged toner and said image receiving member and
- connecting said control electrodes to a voltage source for image wise opening and closing said apertures for image wise depositing toner particles on said image receiving member.
9. A method according to claim 8, wherein in said step of bringing charged toner particles
from said outer surface of said toner dispensing part to said outer surface of said
conveyer for charged toner, toner particles with an average charge over mass ratio
(in asolute value, |q/m|) such 5 µC/g ≤ |q/m| ≤ 15 µC/g are used.
10. A method according to claim 8 or 9, wherein in said step of bringing charged toner
particles from said outer surface of said toner dispensing part to said outer surface
of said conveyer for charged toner, toner particles,with a charge distribution having
a coefficient of variability (v) equal to or lower than 0.4 preferably lower than
0.33 are used.