PRINTING APPARATUS OF TONER JET TYPE HAVING AN ELECTRICALLY SCREENED MATRIX UNIT

Abstract: Printing apparatus of the type named "toner jet" printing apparatus, and in which a dry print powder, generally named "toner" is, by a direct process, transferred from a rotating toner feeder roll (1) which is charged with a certain predetermined, relatively low positive potential (for instance +50 V), through toner feeder bores (8) of a fixed toner matrix (3) in the form of a flexible printing circuit and down to an object (5) to be printed, for instance a paper which is conveyed over a support roll (4) which is charged with a certain predetermined, relatively high potential (for instance +1500 V), and in which the toner which has deposited on the paper (5) is finally fixed on the paper by a heating means (12), and in which each individual toner feeder bore (8) of the matrix (3) is surrounded by an electrically conducting control ring (9) which can alternatively be given a certain positive potential (for instance +300 V) which is higher than the potential of the toner feeder roll (1) but which is lower than the potential of the support roll (4), or a potential which is lower than the potential of the toner feeder roll (1), and in which the matrix (3), at the upper surface thereof has a protective layer (15) of an unmagnetic metal and is formed with through bores (16) having a diameter which is substantially the same as the outer diameter of the control rings (9) of the matrix, and in which the inner diameter of each toner control ring (9) is substantially the same as the diameter of the toner feeder bore (8) of the toner matrix base (13).

Description: PRINTING APPARATUS OF TONER JET TYPE HAVING AN ELECTRICALLY SCREENED MATRIX UNIT The present invention generally relates to a printing apparatus of the type which is used in various types of printers, for copying machines, in telefacsimile machines etc., and which operates with a dry print powder which is, in electrical way, applied to the object to be printed, for instance the paper, and which is thereafter fixed to the paper, generally by a heat treatment. The invention is more particularly directed to a printing apparatus of said type which named a "toner jet" printing apparatus, and in which a dry print powder, generally named "toner", is, by a direct method, transferred from a rotating toner feeder roll, through bores of a fixed matrix in the form of a flexible printing circuit and down onto the object to be printed, for instance the paper which is conveyed over a support roll and in which the print powder which has been applied to the paper is finally fixed to the paper by a heat treatment. The basis of said process is that two electrical fields are created for transferring toner from the toner feeder roll to the paper, namely a first electrical field between the toner feeder roll and the toner matrix, which electrical field is brought to invert polarity, and a second, preferably constantly downwards directed positive electrical field between the matrix and the support roll above which the paper is transferred. The toner matrix is formed with a targe number of very small through bores having a diameter of for instance 1 00-300μm, and round each individual such bore an electrically conducting ring of a suitable metal, like copper, in the following referred to as "copper ring". Each copper ring can be charged with a positive potential, for instance + 300V, which is higher than the potential of the toner feeder roll, which potential can for instance be between + 5V and + 1 00V, preferably about + 50V, but which is less than the potential of the support roll for the paper, which potential can for instance be + 1 500V. When the electrically conducting ring is charged with a voltage said ring makes the belonging matrix bore become "opened" for letting toner down. If the matrix bore ring is, on the contrary, charged with a potential which is substantially lower than the potential of the toner feeder roll, for instance in that said ring is connected to ground, the belonging matrix bore becomes "closed" thereby preventing a letting down of toner. The function is as follows: • the toner powder gets a negative potential in that said toner particles rub against each other; • the toner powder is supplied to the toner feeder roll, which is positively charged by a predetermined potential, often a potential which can be controlled between + 0V and + 100V, and the toner powder is distributed in an even, sufficiently thick layer over the toner feeder roll using a doctor blade; • each bore of the matrix which corresponds to a desired toner dot is opened in that the matrix bore ring is charged with a positive potential which is higher than the potential of the toner feeder roll, for instance + 300V; bores corresponding to non toner carrying portions remain connected to the ground, whereby said bores are to be considered "closed", thereby making it impossible to let toner through; the combination of opened matrix bores forms the image to be reproduced; • depending on the difference in potential, for instance + 50V to + 300V = + 250V between the toner feeder roll and the toner matrix negatively charged toner particles are sucked down from the toner feeder roll to the matrix, and depending on the difference in potential between the toner matrix and the support roll mounted underneath same, for instance + 300V to + 1 500V = + 1 200V the toner particles are moved on from the matrix and deposit on the paper above the support roll; • the paper with toner deposited thereon is finally moved through a heat treatment apparatus in which the toner is fixed to the paper. There is an almost linear relationship between the density of the current field and the traction force that said field exerts on the toner particles. The field has its greatest density just above the copper rings, and the density decreases from the ring edges towards the center of the bore. By reducing the potential of the toner feeder roll, which leads to an increasing difference in potential between the toner feeder roll and the matrix, it is possible to increase the amount of toner which is let down. An increase of the potential of the toner feeder roll leads a corresponding reduction of the amount of toner which is let down. By connecting the copper ring of the matrix to the ground the direction of potential between the toner feeder roll is reversed from having been + 250V in the direction downwards to be + 50V in the direction upwards, and this makes negatively charged toner particles stick to the toner feeder roll, or be sucked back thereto, respectively. In a certain embodiment of the printing apparatus the distance between the toner feeder roll and the matrix was about 0.1 mm, and the distance between the matrix and the support roll was about 0.6 mm. At normal printing the toner feeder roll has a voltage of + 50V, and this gives a difference in potential to the matrix, which can have a voltage of + 300V, of + 250V between the toner feeder roll and the matrix, over the above mentioned distance of 0.1 mm this gives a field strength of 2.5V/μm. The distance between the toner feeder roll and the support roll is about 0.7 μm, and the difference in potential is + 1 450V. This gives a field strength of 2 V/μm between the bottom surface of the matrix and the paper. The same electric field is present above the matrix and between the copper rings, and said field acts against the toner on the toner feeder roll, so that toner particles can be released from the toner feeder roll and can fall down on the upper surface of the matrix. As soon as the toner particles reach a copper ring, which is connected to ground (OV), said toner particles jump back to the toner feeder roll, and after having passed the copper ring said particles jump back down to the matrix again. It also can happen that toner which is present above a conduit to a copper ring when the voltage changes from OV to + 300V can be sucked to the upper surface of the matrix and can be kept thereon, and this can prevent other toner particles from being fed into the matrix bore at the centre of the copper ring. Toner which jumps up and down between the toner feeder roll and the upper surface of the matrix obstacles the flow of toner past the printing zone, and the jumping toner particles are often unloaded or may even change charge to the non-desired positive charge. Also, a slight amount of the toner particles normally have a "false" potential, generally 2-4% of the toner particles, and such falsely charged toner particles are often sucked both to the upper surface and to the bottom surface of the matrix. The present invention is intended to solve the problem that toner particles jump between the toner feeder roll and the matrix, and said problem is solved in that a thin, protective metal layer is applied on the upper surface of the matrix. Said protective layer is formed with bores the diameters of which coincide with the outer diameter of the copper rings. The layer is given the same potential as that of the toner feeder roll, for instance + 50V. The protective layer can have a thickness of 20-30 μm, and it is glued onto the upper surface of the matrix. The protective metal layer acts as an electric screen between the toner feeder roll and the matrix with the electric conduits thereof. It is important that the bores of the protective layer has a diameter which is at least the same as the outer diameter of the copper rings, since there would otherwise be a risque that the layer might screen off the field between the toner feeder roll and copper rings. In order to prevent that the material between the bores of the protective layer is too narrow the matrix is preferably formed with the copper rings on the top of the matrix base and with the inner diameter of the copper ring the same size as that of the bores of the matrix, whereby the copper rings may be used to a maximum for feeding toner particles from the toner feeder roll, through the matrix and down to the paper. In a matrix having a toner feeder bore with a diameter of about 1 90 μm the copper rings can have an outer diameter of for instance 250 μm, and in such case the bores of the protective layer can preferably be given a diameter of 250μm. If the toner feeder roll and the toner is of magnetic type the protective layer has to be of an unmagnetic material like of stainless steel, beryllium copper, hard nickel, brass, aluminum or another hard, unmagnetic material. In order to eliminate the risque of flash over between the toner feeder roll and the matrix and between the copper rings and the support roll it is therefore necessary that the matrix bore ring be insulated. This is done in that the entire matrix is covered, for instance by an evaporation process, with an insulating substance which encloses all free surfaces and edges of the matrix, the matrix bores and the protective layer. An available method is the method named the Parylene ® method (Union Carbide) according to which a polymeric insulation material named poly-para-xylene, using a vacuum apparatus, is applied to the matrix in a very well predetermined thickness. The material has an electric decomposition resistance of about 200 V/μm. This means that it is sufficient to use a layer having a thickness of only 2 μm for insulating an electric field of + 250V between the toner feeder roll and the copper ring of the matrix. Now the invention is to be described more in detail with reference to the accompanying drawings, in which figure 1 diagrammatically and in a perspective view illustrates the basic principle for a printing apparatus of toner j et type, and in which figure 2 shows, in an enlarged scale, a cross section view through a printing apparatus of the toner jet type according to prior art. Figure 3 is a cross section view through a printing apparatus according to the invention, and figure 4 shows, in an enlarged scale, the part of figure 3 which is encircled by a broken circle. Thus, in figure 1 there is diagrammatically shown a printing apparatus of toner jet type comprising a toner feeder roll 1 having an outer layer 2 of a toner powder of known type, a toner matrix 3 mounted underneath the toner feeder roll 1 and a support roll 4 mounted underneath the matrix 3 which support roll is arranged to support an object to be printed which is conveyed between the matrix 3 and the support roll 4, which object is normally a paper 5. In figure 2 is diagrammatically shown that some toner particles can be released from the toner feeder roll 1 and may deposit as waste toner 2a at the upper surface of the matrix 3. Such waste toner obstacles a feeding down of toner particles through the toner feeder bores of the matrix. In some cases waste toner also may deposit on the bottom surface of the matrix, and such toner may smear off on the paper 5 as a non-desired back ground tone. As shown in figure 3 a toner container 6 is mounted above the rotatable toner feeder roll 1 , and from said container 6 toner is let down on the toner feeder roll 1 . A doctor blade 7 spreads and distributes the toner to form an even layer 2 of toner on the toner feeder roll 1 . The toner feeder roll 1 is charged with a certain positive voltage of for instance between + 5 and + 100V, in the illustrated case a voltage of about + 50V. Since the toner particles rub against each other they are charged with a negative polarity, and this makes the toner particles become adhered to the positively charged toner feeder roll. The matπx 3 is formed with a large number of through bores 8 adapted to let toner through when said bores are in open condition. Said bores can have a diameter of 1 00-300 μm, in a certain tested matrix a diameter of 1 90 μm. Round each toner bore 8 there is an electrically conducting ring 9, for instance of copper, for controlling the letting through of toner particles. For enabling a maximum letting down of toner through the toner bores 8 the copper ring is mounted on top of the matrix with its inner diameter flush with the toner bore 8. Each copper ring 9, or control ring, is over conduits 1 0 electrically connected to a control means 1 1 which is diagrammatically shown in figure 3 and which is arranged to alternatively charge to copper ring either with a voltage which is higher than the voltage of the toner feeder roll 1 , for instance a voltage of + 300V, whereby the matrix bore is "opened", or with a voltage which is lower than the voltage of the toner feeder roll, in particular a voltage of ±OV, in that the ring is connected to ground, whereby the matrix bore is "closed". The opening of the toner matrix bore 8 is thus accomplished in that the copper ring 9 is given a potential of for instance + 300V, whereby a difference in potential of + 300 - + 50 = + 250V appears between the toner feeder roll 1 and the matrix 3. Said difference in potential is so great that the negatively charged toner particles are released from the toner feeder roll 1 and are sucked down against the matrix 3 and through the presently opened matrix bores 8. If the copper ring 9 is connected to ground the direction of potential is inversed and there appears an upwardly directed difference in potential of + 50V, and toner particles are thereby sucked back towards the toner feeder roll 1 , or are kept thereon, respectively. As mentioned above toner particles may, however, be released from the toner feeder roll 1 and deposit on the matrix, or may jump up and down between the toner feeder roll 1 and the matrix 3. The support roll 4 constantly has a voltage which is higher than the highest voltage, + 300V, of the matrix 3, in the illustrated case a voltage of + 1 500V. In "opened" matrix bores 8 there is consequently a downwards directed difference in potential of + 1 200V, and said difference makes toner particles become sucked down from the matrix 3 towards the support roll 4. Toner particles deposit as dots on the paper 5 which is moved over the support roll 4. A series of such dots from several matrix bores successively form the image or images to be represented on the paper. The paper with the toner particles deposited thereon is thereafter passed through a heat treatment apparatus, for instance between two heater rolls 1 2, in which the toner powder is fixed to the paper. The distances between the different parts marked in the drawings are, for the sake of clearness, strongly exaggerated. The distance between the toner feeder roll 1 and the matrix 3 can, for instance, be 0.1 mm and the distance between the matrix 3 and the support roll 4 can, for instance, be 0.6 mm. As indicated with the dotted lines in figure 3 the matrix 3 may preferably be bowed in a curvature the axis of which coincides with the axis of rotation for the toner feeder roll 1 . For further stabilising the matrix 3 and avoiding vibrations which may bring the bottom surface of the matrix 3 in contact with the paper 5 the bottom surface of the matrix 3 can be laminated with a (not illustrated) metal layer, which is preferably also enclosed in an insulating layer. For avoiding flash over between the toner feeder roll 1 and the matrix 3 and between the matrix 3 and the support roll 4 the copper rings 9 on top of the matrix 3 have to be insulated. The insulation is accomplished in that the electrically conducting copper rings 9 are connected, in a suitable way, to the upper surface of the matrix base 1 1 , for instance by means of glue or tape, so that the copper ring 9 with the inner diameter thereof is flush with matrix bore 8. Thereafter the entire matrix 3 is covered with a thin layer 14 of an insulation material which covers the entire matrix at the top surface and the bottom surface and also extends over the inner edges both of the matrix bores 8 and the copper rings 9. Such covering can be accomplished by an evaporation process with an insulation substance, whereby said substance encloses all free surfaces of the matrix, the matrix bores and the copper rings. ® An available method is named the Parylene method (Union Carbide) according to which process a polymeric insulation material named poly-para- xylene is, in a vacuum apparatus, applied to the matrix in a very accurately controlled layer thickness. The material has an electric decomposition resistance of about 200V/μm. This means that it is sufficient with a thickness of the insulation layer 14 of only 2 μm for insulating an electric field of 250V between the toner feeder roll and the copper ring of the matrix. For the sake of safety the material is generally applied in a layer having a thickness of 5-1 0 μm. Even using such great thickness of the insulating layer as 1 0 μm for a matrix bore 8 having a diameter of 1 70 μm and an inner diameter of the copper ring 9 of 1 90 μm the specific opening area for the matrix bore 8 for letting toner through is as great as 89,9%. This provides a great margin in printing with the printing apparatus in that a more even print quality can be obtained. At the same time problems depending on variations in moisture and temperature are reduced. It is also possible, thanks to the increase in degree of blackness during the printing, to reduce the drive voltage of the control rings 9 and to increase the tolerances of certain parts included in the apparatus. For eliminating the problem that toner particles are released from the toner feeder roll 1 and deposit on the upper surface of the matrix 3, and in some cases also the bottom surface of the matrix, or that toner jumps up and down between the toner feeder roll 1 and the matrix 3 there is provided a protective layer 1 5 of metal on top of the matrix. The protective layer must be made of an unmagnetic metal and can be of stainless steel, beryllium copper, hard nickel, brass, aluminum or another hard, unmagnetic material. The protective layer 1 5 is formed with through bores 1 6 equivalent to the bores 8 of the matrix and the bores of the copper rings 9. For foreseeing that the protective metal layer 1 5 does not provide an electric screen against the copper rings 9 the bores 1 6 of the protective layer 1 5 preferably should be at least as large as the outer diameter of the copper rings 9. The protective layer 1 5 is, via a conduit, charged with the same voltage as that of the toner feeder roll, in the illustrated case a voltage of + 50V. Since the toner feeder roll 1 and the protective metal layer 1 5 has the same voltage and polarity there is no electric field between said parts, and there is consequently no force tending to tear off toner particles from the toner feeder roll. For the same reason it is also not necessary to provide any insulation of the protective metal layer 1 5. REFERENCE NUMERALS 1 toner feeder roll 1 1 control means 2 toner layer 1 2 heater rolls 3 toner matrix 13 matrix base 4 support roll 1 4 insulation layer 5 paper 1 5 protection layer 6 toner container 1 6 bore 7 doctor blade 17 conduit 8 toner feeder bore 9 copper ring 1 0 conduit (for 9)

Claims: C L A I M S 1 . Printing apparatus of the type named "toner jet" printing apparatus, and in which a dry print powder, generally named "toner" is, by a direct process, transferred from a rotating toner feeder roll ( 1 ) which is charged with a certain predetermined, relatively low positive potential (for instance + 50V) , through toner feeder bores (8) of a fixed toner matrix (3) in the form of a flexible printing circuit and down to an object (5) to be printed, for instance a paper which is conveyed over a support roll (4) which is charged with a certain predetermined, relatively high potential (for instance + 1 500V), and in which the toner which has deposited on the paper (5) is finally fixed by a heating means ( 1 2) , and in which each individual toner feeder bore (8) of the matrix (3) is surrounded by an electrically conducting control ring (9) which can alternatively be given either a certain positive potential (for instance + 300V) which is higher than the potential of the toner feeder roll ( 1 ), whereby a corresponding bore (8) of the matrix (3) is opened for letting toner down, but which is lower than the potential of the support roll (4) or a potential which is lower (for instance grounded ring 9) than the potential of the toner feeder roll ( 1 ), whereby a corresponding bore (8) of the matrix is closed preventing a letting down of toner, characterized in - that the matrix (3), at the upper surface thereof, has a protective layer ( 1 5) formed with through bores ( 1 6), - which bores ( 1 6) have a diameter which is at least of the same size as the outer diamater (8) of the control rings (9) - in that the protective layer ( 1 5) is of an unmagnetic metal, - and in that the entire toner matrix (3) including the electrically conducting control rings (9) are coved, both at the upper surfaces and at the bore edges thereof, with an electrically insulating layer ( 14) . 2. Printing apparatus according to claim 1 , characterized in that the metallic protective layer ( 1 5) is of a hard metal and can for instance be stainless steel, beryllium copper, hard nickel, brass, aluminum. 3. Printing apparatus according to claim 1 or 2, characterized in that the protective layer ( 1 5) is charged with a voltage which is substantially the same as the voltage of the toner feeder roll ( 1 ) . 4. Printing apparatus according to any of the preceding claims, characterized in that the inner diameter of each toner control ring (9) of the matrix (3) is substantially the same as the diameter of the toner feeder bore (8) of the matrix base ( 1 3) . 5. Printing apparatus according to claim 4, characterized in that each electrically conducting toner control ring (9) is secured directly on top of the toner matrix base ( 1 3) with the inner diameter of the toner control ring (9) flush with the toner bore (8) of the matrix (3). 6. Printing apparatus according to claim 5, characterized in that the electrically insulating layer is a layer of a polymeric material, like poly-para- xylene, which layer is applied in a very accurately controlled thickness. 7. Printing apparatus according to claim 5 or 6, characterized in that the insulating material of the matrix (3) is applied by an evaporation method, ® for instance the method named Parylene method (Union Carbide). 8. Printing apparatus according to claim 5, 6 or 7, characterized in that the electrically insulating layer has an electrical decomposition resistance of about 200V/μm, and that said layer is applied in a thickness of more than 2 μ , preferably 5-10 μm for insulating an electric field of + 250V between the toner feeder roll-( 1 ) and the control ring (9) of the matrix (3). 9. Printing apparatus according to any of the preceding claims, characterized in that the matrix (3) is bent in a curvature the axis of which coincides with the axis of rotation of the toner feeder roll ( 1 ), and in that the matrix (3) at the surface thereof facing the paper (5) has a stabilising metal layer ( 1 5).

Inventors: SUNDSTROEM PER, SUNDSTROEM, PER
Applicants: ITO ENGINEERING AB, SUNDSTROEM PER, SUNDSTROEM, PER
Application year: 1997
Main classification: G03G15
IPC codes: B41J2, G03G15