Method of and apparatus for developing a latent magnetic image

Abstract: A method of developing a latent magnetic image in which a layer of magnetically attractable toner powder having a specific resistance less than 10⁹ohms.metre is conveyed by a toner conveyor in a developing zone past a medium carrying the latent image and an AC voltage is applied between the toner conveyor and the medium carrying the latent image. The apparatus comprises an image-recording medium (1), means (2) for recording a latent magnetic image on the medium, a toner powder conveyor (9) and a metering device (10) for metering a layer of toner powder on the toner powder conveyor (9). The shortest distance A (in mm) between the toner powder conveyor (9) and the image-recording medium (1) is between B + 0.6 and B + 1.6 mm, where B is the shortest distance in mm between the metering device (10) and the toner powder conveyor (9).

Description: The invention relates to a method of developing a latent magnetic image, in which a layer of magnetically attractable toner powder is metered on the surface of a toner conveyor by metering means disposed a predetermined distance from the surface of said toner conveyor, the metered layer is fed by said conveyor in a developing zone past a medium carrying the latent image, and an AC voltage is applied between the toner conveyor and the image-carrying medium. The invention also relates to apparatus for performing the method. US Patent 4 368 687 describes a method and apparatus for developing a latent magnetic image, in which a uniform very thin layer of an electrostatically charged insulating and magnetically attractable toner powder is applied to a toner conveyor and the thin layer of toner powder is fed to a developing zone where it is brought to a distance of 200 to 400 micrometres from the image-carrying medium. To carry out the image development, an AC voltage is applied in the developing zone between the toner conveyor and the image-carrying medium. In comparison with other known developing methods in which magnetically attractable toner powder is brought into (frictional) contact with the image-carrying medium in the developing zone, the method according to the said US patent has the advantage that background (deposition of toner particles on the image-free parts of the medium) is prevented. A disadvantage of this method, however, is that apparatus for performing the method must satisfy high accuracy requirements to achieve the required slit width. In addition, thin toner layers have to be used in this method, so that during the development of images with high information density, such as large black areas, the disadvantage may arise that the toner supply in the developing zone may be too low and the developed images consequently have an inadequate density. The object of this invention is to obviate these disadvantages and provide a method with which good-quality images are obtained and which has a wide range of tolerances so that it can be performed with an apparatus which does not have to meet rigorous tolerance requirements. According to the invention, this object is attained in providing a method according to the preamble of claim 1 which is characterised in that a magnetically attractable toner powder is used which has a specific electrical resistance of less than 10⁹ ohms.metre, and in that the shortest distance A ( in mm) between the toner conveyor and the medium which carries the latent magnetic image, in the developing zone, is between B + 0.6 and B + 1.6 wherein B is the distance in mm between the metering means and the surface of the toner conveyor The invention also provides a magnetic printing apparatus for performing the method, comprising a magnetisable image recording medium, means for recording a latent image on the image recording medium, a toner conveyor to convey magnetically attractable toner powder past the image recording medium in a developing zone, a metering device for metering a layer of magnetically attractable toner powder on the toner conveyor, and means for generating an AC voltage in the developing zone between the toner conveyor and the image recording medium. The apparatus is characterised in that the toner conveyor conveys a magnetically attractable toner powder of less than 10⁹ ohms·metre, and in that in the developing zone the shortest distance A in mm between the toner conveyor and the surface of the image-recording medium is between B + 0.6 and B + 1.6 where B is the distance in mm between the metering device and the surface of the toner conveyor. In the method according to the invention, very good quality images are obtained which are free of background. Magnetic images depicted on the image-recording medium with a high resolution of, for example, 400 dpi (dots per inch) are developed free of background, one-pixel lines and loose image pixels also still being distinguishly reproduced. This good image quality is achieved not only at a relatively low speed of advance of the image-recording medium, but also at a high speed of advance of 30 metres per minute, equivalent to a printing speed of more than 100 pages of A4 per minute. An important advantage of the method according to the invention is that there is a wide working range even when developing high resolution images and at high speeds of advance of the image-recording medium, so that the developing apparatus does not have to satisfy high accuracy requirements. In the method according to the invention, it is possible to form on the toner conveyor a layer of toner powder which is considerably thicker than the layer thicknesses indicated in the above-mentioned US Patent 4 368 687. An additional advantage of the method according to the invention is that the toner powder does not have to be electrostatically charged prior to the image development, thus obviating quality variations resulting from irregularities in the charging of the toner powder. As is already known, the electrostatic and, in particular, the tribo-electric charging of a toner powder is influenced by varying ambient conditions of temperature and humidity. The method according to the invention uses a magnetically attractable toner powder having a specific electric resistance of less than 10⁹ ohms.metre. The toner powder consists of resin particles in which magnetically attractable material is finely distributed. The magnetically attractable material may be soft or remanent magnetic and can be selected from the materials known per se for use in toner powders. The magnetically attractable material content is of the order of magnitude conventional for toner powders and is, for example, 6-20% by volume for soft magnetic material and 1-10% by volume for remanent magnetic material. In addition to magnetically attractable material the resin particles contain electrically conductive material to give the toner powder a specific electric resistance less than 10⁹ ohms.metre. The electrically conductive material which, for example, may consist of fine carbon particles or metal particles, such as silver or copper particles, may be finely distributed in the resin particles in a quantity sufficient to render the toner powder the required specific resistance below 10⁹ ohms.metre, or be deposited on the surface of the resin particles. Preferably, the electrically conductive material is deposited on the surface of the resin particles. Suitable toner powders for use in the method according to the invention are described, inter alia, in Netherlands Patent Application 7203523. The sepcific resistance of the toner powder is measured as follows: A cylindrical container having an inside diameter of 17.2 mm, the base of which consists of brass having a thickness of 1.5 mm, and the wall, which has an internal height of 22.9 mm, consists of Teflon having a thickness of 9 mm, is filled with an excess of powder. The filling is then compressed by crushing it ten times in a crusher made by Engelsmann A.G., of Ludwigshaven, Germany. This filling procedure is repeated twice. Excess powder is then wiped off with a ruler and a brass lid having a diameter of 17.2 mm and a mass of 55 g is placed on the column of powder. The filled container is placed in a Faraday cage and a 10 volt D.C. is applied between the base and the lid. The current intensity is measured after about 20 seconds. The measuring procedure (container filling and current measurement) is repeated three times, whereafter the average current intensity of the three measurements is calculated. The resistance of the powder follows from the formula: ρ= U Ig × A h where: U= the applied voltage (= 10 volts) A= contact area of lid and powder column (= 2.32 x 10⁻⁴m²) h= height of powder column (= 2.29 x 10⁻²m) Ig= average current strenght (in amps). The specific resistance of the toner power should be less than 10⁹ ohms.metre. No critical bottom limit has been found for the resistance. Thus good image development was obtained even with toner powder having a specific resistance of 10³ to 10⁴ ohms.metre, which also was found to give a wide working range. The invention and its advantages are now explained in greater detail with reference to the drawing which diagrammatically illustrates a magnetic printing apparatus in which the method according to the invention is used. The apparatus comprises a cylindrical image recording medium 1 consisting of a drum of copper or copper-plated aluminium, the surface of which is covered with a galvanically applied cobalt-nickel-phosphorus layer about 8 micrometers thick, which has a magnetic coercivity of about 77 kA/m. The image-recording medium 1 can rotate in the direction indicated by the arrow. The following are disposed consecutively along the rotational path of the image recording medium 1, as considered in the direction of rotation: a magnetic head array 2, with which a latent magnetic image with a resolution of 400 dpi can be recorded in the magnetisable layer, a developing device 3, an image transfer device 4, a cleaning device 5, and an erase device 6. The magnetic head array 2 is of the type described in detail in European Patent Application 87200230. The developing device 3 comprises a reservoir 7 for the toner powder, a powder supply roller 8 having a rough surface, a toner conveyor 9 which feeds the toner powder into the developing zone 13, and a metering device 10. The powder supply roller 8 feeds toner powder to the toner conveyor 9. The latter consists of a magnetic roller having a rotatable electrically conductive non-magnetisable sleeve 11 of, for example, copper and a stationary magnet system 12 inside the sleeve 11. In the embodiment illustrated, the magnet system 12 comprises eight magnet poles magnetised as shown in the drawing. The magnet pole situated opposite the image-recording medium 1 generates a magnetic induction of 225 gauss at the surface of sleeve 11 there above, while the other magnet poles generate an induction of 800 gauss at the surface of the sleeve. (The lower magnetic induction in the developing zone 13 is of no essential importance to obtaining a good image development. All that is important is that there should be no magnetic field in the developing zone 13 with an erasing effect on the latent image on image-recording medium 1. The method according to the invention can also be performed using a toner conveyor so constructed that no magnetic field, or only a very weak magnetic field, is present in the developing zone 13. Embodiments of such toner conveyors are indicated in US Patent 4 368 687 mentioned in the introduction to the description). The distance between the sleeve 11 and the image-recording medium 1 can be varied by moving the toner conveyor 9. The distance between sleeve 11 and the metering device 10, which consists of aluminium for example and is in the form of a ruler, is adjustable. The distance "B" denotes the shortest distance between the sleeve 11 and the metering device 10. The electrically conductive sleeves of the toner conveyor 9 and the image-recording medium 1 respectively are connected to an AC supply 14. A powder image developed on the image-recording medium 1 is transferred to an image-receiving material by the transfer device 4. The latter is a two-step transfer device known per se, in which the powder image of the image-recording medium 1 is first transferred, by pressure, to a belt 15 bearing a silicone rubber surface covering. The belt 15 is heated by heating means (not shown) to soften the powder image transferred thereto. In the pressure zone formed between the belt 15 and a biasing roller 16 the softened powder image is then transferred to and fixed on a receiving material fed to the pressure zone from a supply (not shown). The working range of the developing device was determined by using a toner powder having a resistance of 3.5 x 10⁵ ohms.metre, a particle size of between 10 and 20 micrometers, and particles containing 20% by volume of soft magnetic pigment (type Bayferrox B 318 M made by Bayer AG, Germany) and 80% by volume of polyester resin, the surface being covered with carbon particles. Background-free images of good quality were obtained with the following settings: Speed of rotation of sleeve 11: 25 to 45 metres per minute Distance B between sleeve 11 and metering device 10: 0.6-2 mm AC voltage 500-2000V; frequency 900-2400 Hz. Primarily dependent on the value of the applied AC voltage, the optimal value for the distance A (distance between sleeve 11 and image-recording medium 1) was found to be between distance B plus 0.6 to 1.6 mm. In the low area of the AC voltage range (500 to about 900V), the difference between distance A and distance B (hereinafter indicated by delta w) was in the range from 0.6 - 1 mm, and gradually shifted to higher values with increasing AC voltage. The delta w appeared to have a working range of some tenths of a millimetre for each applied AC voltage. With AC voltages of from about 1200 V to about 600 V this working range was even found to be 0.3 to 0.4 mm. With a distance B of about 1.3 mm, an AC voltage of about 1500 V and a frequency of 1800 Hz, the value of delta w, given a speed of rotation of sleeve 11 between 25 and 45 metres per minute and a speed of advance of the image-recording medium 1 of from 15 to 40 metres per minute, was found to be between about 0.9 and 1.3 mm. In a subsequent test series, the resistance of the toner powder used varied between about 10³ and 10⁹ ohms.metre with the above settings (distance B: 1.3 mm; AC voltage 1500 V, 1800 Hz; speed of rotation of sleeve 11: 45 metres per minute; speed of rotation of image- recording medium 1: 15 metres per minute). Good quality prints were obtained in every case with delta w values between 0.9 and 1.3 mm. The quality of the images obtained with toner powder having a specific resistance of more than 10⁸ ohms.metre was a fraction less satisfactory than that of the images obtained with the other toner powders. The toner powders used in these tests consisted of particles containing 20% by volume of soft magnetic pigment (Bayferrox B 318 M) and 80% by volume of polyester resin, the surface being covered with fine carbon particles. Using a toner powder in which the particles had a size of between 10 and 20 micrometers and consisted of 94% by volume polyester resin, 3% by volume remanent magnetic pigment (type Bayferrox 8140 made by Bayer AG, Germany) and 3% by volume carbon, and which were covered with carbon to a specific resistance of 2 x 10⁵ ohms.metre, a same working range was found as described above for toner powder containing 20% by volume of soft magnetic pigment. In the method and apparatus according to the invention, the distance between the toner conveyor and the surface of the image-recording medium in the developing zone can be so widely selected that toner powder is deposited on the image-recording medium only when the AC voltage is applied across the developing zone. If further image development is to be avoided for some reason, e.g. in the event of a malfunction in the image transfer device or in the supply of image-receiving material, this is easy to achieve by switching off the AC supply. Using the method according to the invention, a multi-colour printing apparatus can be embodied in a relatively simple manner, of the type in which a number of developing devices, e.g. 2, 3 or 4, are disposed around the rotational path of an image-recording medium, each such developing device being filled with toner powder of a specific colour and the colour separation images being printed in consecutive rotational cycles of the image-recording medium and the separation images being combined in register on a combining medium, e.g. the image-receiving material or an intermediate. The development of each of the separation images in the associated colour is controlled by applying the AC voltage in the developing device required to be operative. Mechanical means to move developing devices between an operative position and an inoperative position or to cut off the toner supply to developing devices which are not allowed to be operative are thus unnecessary, so that it is possible to embody an apparatus of simpler construction.

Claims: A method of developing a latent magnetic image, in which a layer of magnetically attractable toner powder is metered on the surface of a toner conveyor by a metering means disposed a predetermined distance from the surface of said toner conveyor, the metered layer is fed by said conveyor in a developing zone past a medium carrying the latent magnetic images and an AC voltage is applied between the toner conveyor and the image-carrying medium, characterised in that a magnetically attractable toner powder is used having a specific electrical resistance of less than 10⁹ ohm.metre and in that the shortest distance A (in mm) between the toner conveyor and the medium which carries the latent magnetic image in the developing zone is between B + 0.6 and B + 1.6, wherein B is the distance in mm between the metering means and the surface of the toner conveyor. A method according to claim 2, characterised in that the toner powder consists of resin particles in which magnetically attractable material is finely distributed and the surface of which is covered with electrically conductive material. A method according to claim 1, characterised in that the distance between the toner conveyor and the surface of the image-carrying medium in the developing zone is so widely selected that toner transfer to the image-carrying medium takes place only when the AC voltage is applied. A magnetographic apparatus for carrying out the method of claim 1, the apparatus comprising a magnetisable image recording medium (1), means (2) for recording a latent magnetic image on the image recording medium, a toner conveyor (9) to convey magnetically attractable toner powder past the image recording medium (1) in a developing zone (13), a metering device (10) for metering a layer of magnetically attractable toner powder on the toner conveyor (9), and means (14) for generating an AC-voltage in the developing zone (13) between the toner conveyor (9) and the image recording medium (1), characterised in that the toner conveyor (9) conveys a magnetically attractable toner powder having a specific electrical resistance of less than 10⁹ ohms.metre, and in that the shortest distance A (in mm) between the toner conveyor (9) and the image recording medium (1) in the developing zone (13) is between B + 0.6 and B + 1.6, where B is the distance in mm between the metering device (10) and the surface of the toner conveyor (9). An apparatus according to claim 4, characterised in that the toner conveyor (9) consists of a magnetic roller having a rotatable electrically conductive and non-magnetisable sleeve (11) and a stationary magnet system (12) disposed inside the sleeve (11).

Inventors: KLERKEN PIERRE ANTONIUS MARIE, KLERKEN, PIERRE ANTONIUS MARIE
Applicants: OCE NEDERLAND BV, OCE-NEDERLAND B.V.
Application year: 1991
Main classification: G03G15
IPC codes: G03G9, G03G19, G03G13