A method of forming visible images, and toner powder for use in the method

Abstract: A method of forming visible images, in which magnetically attractable toner powder consisting of resin particles containing magnetically attractable material is fed to an image forming zone between an image registration medium and a moving conveying medium exerting magnetic attraction on the toner powder, in which image forming zone the toner powder is deposited on the image registration medium by electrostatic attraction in accordance with the image pattern required. A conveying medium is used which, in the image forming zone, has a magnetic induction of at least 0.2 T at the surface attracting the toner powder and toner powder is used which contains at most 3% by volume of magnetically attractable material. The good results in respect of image quality achieved with the method according to the invention are also obtained if the magnetically attractable material used is a soft-magnetic material, e.g. carbonyl iron. The toner powder used in the above method consists of thermoplastic resin particles containing magnetically attractable material and possibly other additives such as colouring material, charge-control agent or electrically conductive material. These resin particles contain at most 3% by volume of soft-magnetic material and have an effective magnetic susceptibility of at least 2.

Description: A method of forming visible images, in which magnetically attractable toner powder consisting of resin particles containing magnetically attractable material with a particle size not greater than 2 µm is fed to an image forming zone between an image registration medium and a moving conveying medium exerting magnetic attraction on the toner powder, in which image forming zone the toner powder is deposited on the image registration medium by electrostatic attraction in accordance with the image pattern required. A method of this kind is known from US-A-4 154 520. According to this known method, latent electrostatic image patterns on an image registration medium are developed in a copying machine by means of toner powder fed by a magnetic conveying medium in the form of a magnetic brush. The circumferential speed of the magnetic brush according to this method should be at least twice that of the speed of conveyance of the image registration medium and preferably three to five times such speed. The reason for this is that at low circumferential speed of the magnetic brush developing problems occur, such as inadequate gradation, deposition of toner particles outside the image patterns, toner accumulation or over-development. These developing problems ultimately lead to poor image quality of the copy. Although the said developing problems are obviated by use of a much higher circumferential speed, the higher brush speed does entail difficulties due to the increased mechanical load on both the image registration medium and the toner powder. As a result of the high mechanical load, fine abrasive dust forms in the powder brush and is thrown from the latter, thus causing soiling of the copying machine. In order to ensure correct functioning of the copying machine a service engineer then has to regularly clean the machine. Partly the fine abrasive dust is also permanently deposited on the image registration medium, thus reducing its life. An imaging method producing a good image quality which is independent of the circumferential speed of the developing brush formed of the toner powder is taught by EP-A-0 304 983. This method enables the selection of a low circumferential speed of the magnetic brush and thus the elimination of the dust problems mentioned before by providing a toner conveying medium which forms an image forming zone with an image registration medium, such that a magnetic induction (magnetic field strength) of at least 0.3 T is realised, exerting an attracting force on the toner powder within the image forming zone, wherein the toner powder used contains a relatively small quantity of magnetically attractable material. Although toner powders containing a relatively small amount of magnetically attractable material are taught by for instance DE-A-30 08 881 it appears that due to the fact that in the process of preparing the toner powder particles, the magnetically attractable material particles are not perfectly homogeneously distributed through the resin melt,a substantially amount of toner powder particles obtained after grinding are not suitable as magnetically attractable toner powder particles. Especially the smaller size toner particles are either not magnetically enough to be conveyed by the developing brush or they contain so much magnetically attractable material that they cling so strongly to the magnetic developing brush that they can not be deposited on the image registration medium by the electrostatically attraction force normally used in the imaging process, thus introducing image fault. As a consequence, the relation between the particle size of the magnetically attractable material and the minimum particle size of the toner powder itself is important. Therefore, it is an object of the present invention to provide an imaging method which will overcome the above-noted disadvantages. This object is accomplished in accordance with the present invention, in that a conveying medium is used which, in the image forming zone, has a magnetic induction of at least 0.2 T at the surface attracting the toner powder and in that toner powder is used which comprises magnetically attractable material in an amount not greater than 1% by volume of the toner powder, wherein the toner powder has a particle size lower limit of about 8 µm when the toner powder contains 1% by volume of the magnetically attractable material and that the particle size lower limit is increased as the percentage by volume of the magnetically attractable material is decreased. As a result it is assured that all toner powder particles do contribute to realize a good image quality in the image forming method. The results of the method according to the present invention are further enhanced if the magnetically attractable material used in the toner powder is a soft-magnetic material, such as carbonyl iron. The present invention, therefore, also relates to a toner powder for use in the above image method, consisting of thermoplastic resin particles containing magnetically attractable material with a particle size not greater than 2 µm and possibly other additions, such as colouring material, a charge-control agent and electrically conductive material. The characterising feature is that the toner particles contain a maximum of 1% by volume of a soft-magnetic material and that the resin particles have an effective magnetic susceptibility of at least 2, wherein the toner powder has a particle size lower limit of about 8 µm when the toner powder contains 1% by volume of the soft-magnetic material, and that the particle size lower limit of the toner powder is raised as the percentage by volume of the soft-magnetic material is decreased. The invention will now be explained with reference to the following description and accompanying drawing, which is a diagrammatic cross-section of a device for developing electrostatic image patterns using the method according to the invention. The developing device comprises a magnetic roller 1 to which magnetically attractable toner powder is applied via the opening 2 of the reservoir 3. The magnetic roller 1 conveys the toner powder to the developing zone 4 where the powder is brought into contact with a photo-conductive material 5 in sheet or web form, which is fed over the backing roller 6 and bears a latent electrostatic charge image on the side facing the layer of powder. The magnetic roller 1 comprises a sleeve 7 of diamagnetic material, e.g. aluminium, brass or stainless steel, which is mounted rotatably on a shaft 8 in known manner. When the device is in operation, the magnetic roller 1 is driven in the direction of arrow A by drive means known but not shown in the drawing. Although not shown in the drawing, the shaft 8 of the magnetic roller 1 is fixed in frame plates of the device. As considered in the axial direction, cylindrical magnets 9 are mounted side by side on the shaft 8. These cylindrical magnets may be magnetised a number of times as considered circumferentially to embody the required number of magnetic poles or be built up from a number of identical cylinder segments magnetised one or more times. Magnets 9 are so disposed around the shaft that like poles are situated in extension of one another and thus together form an axially extending magnetic pole. The outside diameter of the cylindrical magnets 9 is smaller than the inside diameter of the diamagnetic sleeve 7. The result is a homogeneous strong magnetic field at the surface of the diamagnetic sleeve 7. The magnetic induction of at least 0.2 T required at the surface for the method according to the invention can be achieved by making the magnets 9 from highly magnetic materials, such as an alloy of Neodynium-iron-boron or of Samarium-cobalt. The magnetically attractable toner powder is applied to the sleeve 7 of the magnetic roller 1 via the opening 2 of the reservoir 3. The opening 2 extends axially over the sleeve length, a scraper 15 being provided at the opening 2 and also extending axially over the sleeve length in order to spread the applied toner powder over the sleeve 7 in a uniform layer. The layer of toner powder applied to the sleeve 7 is conveyed by the sleeve 7 to the developing zone 4, where the toner powder is directed into a developing brush in the magnetic field operative between one of the magnetic poles of the magnetic roller 1 and the backing roller 6, thus being brought into contact with the electrostatic image to be developed, so that a powder image forms on the photoconductive material 5. The toner powder not transferred to the electrostatic image is then held against the sleeve 7 by the other magnetic poles and thus returned to the powder supply zone by the sleeve 7. According to the method of the invention, the above-described developing device uses a magnetically attractable toner powder consisting of toner particles containing binder and between 0.25 and 3% by volume of magnetically attractable material. If required, the toner particles may also contain colouring constituents, a charge-control material and/or electrically conductive particles. The best results are obtained by using a soft-magnetic material, e.g. carbonyl iron, as magnetically attractable material. The most important properties which make soft-magnetic materials so suitable are the intrinsic magnetic susceptibility X (chi) which is greater than 10 and the form of the magnetisation curve. The greatest effect in respect of magnetisation is obtained if the magnetic material does not become magnetically saturated at the magnetic field strength applied in the developing zone. In the saturation track, the magnetisation is hardly influenced, if at all, on an increase of the field strength. With the magnetic field strength used according to the invention, the magnetic saturation level is sufficiently high for soft-magnetic materials. The magnetic susceptibility X of the material itself is also influenced, when used in toner particles, by the form of the magnetic material and the interaction with other magnetic material particles. A value between 2 and 6 for the effective magnetic susceptibility X eff of the toner particles appears to be a suitable value to give a good result with the method according to the invention. With the developing devices conventional according to the state of the art, in which a magnetic roller having a magnetic induction of approximately 0.08 T is used in combination with a toner powder containing at least 20% by volume of magnetically attractable material, a good image quality (i.e. no background, high optical density and the like) is obtained only if the circumferential speed of the developing brush is at least twice the speed of conveyance of the image registration medium. With the combination according to the invention, a magnetic roller having a magnetic induction of at least 0.2 T and a toner powder having at most 3% by volume of magnetic material, it has been found that good image quality substantially is independent of the circumferential speed of the developing brush. Thus a low speed can be selected, obviously with reduced dust problems. Also, the strong magnetic field contributes to this reduced dust formation, so that a more compact toner brush is created, from which it is difficult for toner particles to escape. In tests within a range shown in the accompanying drawing, using a magnetic roller 1 having a magnetic induction of 0.32 T in the developing zone 4 with a toner powder containing 2% by volume of magnetically attractable material, excellent image quality was obtained with a circumferential speed of the magnetic roller 1 equal to the speed of conveyance of the photo-conductive material 5. Tests with other combinations of the determining parameters (magnetic field strength and percentage of magnetic material by volume) did not yield results as good as this. Combinations of a strong magnetic brush (> 0.2 T) and a toner powder containing 20% by volume of magnetic pigment, always produced a background irrespective of the circumferential speed of the developing brush, while a magnetic brush of 0.08 T with toner powder containing less than 3% by volume of magnetic pigment did not prove to yield a charge image development at all acceptable at any speed whatever. The more compact toner brush resulting according to the invention necessitates strict requirements in respect of the distance between the photo-conductive material 5 and the surface of the sleeve 7 in the developing zone in order to ensure that the toner powder there comes into contact with the electrostatic charge image. In the above specifications of magnetic field and toner powder material therefore, a maximum value of 0.1 mm must be used for this distance. The toner particles used in the method according to the invention can be produced by a method known per se in which the binder (often a thermoplastic resin) is melted, and the magnetically attractable material, as well as other fine solid particles such as colouring constituents, a charge-control agent and/or electrically conductive material, are distributed in the melt, whereupon after cooling the mass is ground to fine particles. Another method comprises making the toner powder particles by spray-drying a dispersion of the fine solid particles in a solution or a dispersion of the resin. If necessary, the toner particles can also be made electrically conductive by adding conductive particles (e.g. carbon particles) to the melt or dispersion or by afterwards softening the toner particles and causing conductive particles to stick to the surface. By this latter method, the fine conductive particles are anchored in the surface of the softened powder particles. In order to obtain toner powder of which all particles are magnetically attractable sufficiently to be employed in the method according to the invention, the relation between the particle size of the magnetically attractable material and the minimum particle size of the toner powder itself is important. When using the finer fractions of the magnetically attractable material with a particle size of between 1 and 3 µm, there is no need for making special demands upon the minimum particle size of toner powders containing between 1 and 3% by volume of magnetically attractable material. In that case, the particle size of the toner powder may be in the usual range of 5 till 8 up to 30 µm or more. In order that the coarser fractions of the magnetically attractable material from the range between 1 and 10 µm may also be employed in such toner powders, the lower limit for the particle size of the toner powder should be selected higher according as the average particle size of the magnetically attractable material is greater. Roughly speaking, this lower limit of the particle size of the toner powder has to be 8, 10, 15 and 20 µm respectively, when magnetically attractable materials having a particle size of < 2, 2-3, 5 and 10 µm respectively are used. Suitable toner powders containing less than 1% by volume of magnetically attractable material can be obtained by applying magnetically attractable material with a particle size of up to about 2 µm. It than applies that the lower limit for the particle size of the toner powder has to be selected at a higher level, according as the percentage by volume of the magnetically attractable material becomes lower. In case of 0.5% by volume of magnetically attractable material the lower limit is approximately 15 µm, while in case of 0.25% by volume of magnetically attractable material the lower limit is approximately 20 µm. An electrically conductive toner powder containing 2% by volume of magnetically attractable material is prepared, for example, by melting a thermoplastic resin in the form of a polyester resin of the Atlac type made by Imperial Chemical Industries, and distributing fine solid particles of carbon and carbonyl iron homogeneously in the melt, the different materials being used in the proportion of 86.81% by weight of polyester resin, 11.71% by weight of carbonyl iron (particle size 2-3 µm) and 1.48% by weight of carbon. After cooling of the melt, the mass is ground into powder particles having a particle size of between 5 and 40 µm. Another toner powder according to the invention containing 0.5% by volume of magnetically attractable material is prepared by melting a polyester resin of the type described hereinbefore and distributing carbon and carbonyl iron particles in the melt, the materials being used in the proportion of 95.4% by weight of polyester resin, 3.13% by weight of carbonyl iron (particle size 1-2 µm) and 1.47% by weight of carbon. The cooled mass is ground to give powder particles having a particle size between 15 and 40 µm. The powder particles produced by one of the above-described methods are rendered electrically conductive by softening the powder particles and causing conductive particles to stick to their surface in the manner described in example II of NL-A 7203523. This produces a magnetically attractable, electrically conductive developing powder having a specific resistance below 10¹² ohm.cm and preferably between 10⁷ and 10⁸ ohm.cm, measured by means of the cell resistance measurement as described in the same Netherlands patent application. Although the method according to the invention has been described hereinbefore for use in a magnetic brush developing device for electrostatic charge patterns, the invention is not limited thereto. On the one hand it is not necessary to convey the toner powder particles to the developing zone by means of a magnetic brush, it naturally being possible to use any desired conveying means for this purpose. On the other hand, the method according to the invention can also be used in an electrostatic printer of the type described in US-A-4 704 621. A printer of this kind comprises a movable image forming element having a dielectric surface, an image forming station disposed along the trajectory of the image forming element, said station comprising a magnetic roller having an electrically conductive sleeve near the surface of the image forming element and means for generating an electric field corresponding to an information pattern between the image forming element and the magnetic roller. By supplying electrically conductive magnetically attractable toner powder having the specifications described hereinbefore to the zone between the image forming element and the magnetic roller and applying a magnetic field there in accordance with the invention, an excellent image quality is also produced with this printer.

Claims: A method of forming visible images, in which magnetically attractable toner powder consisting of resin particles containing magnetically attractable material with a particle size not greater than 2 µm is fed to an image forming zone between an image registration medium and a moving conveying medium exerting magnetic attraction on the toner powder, in which image forming zone the toner powder is deposited on the image registration medium by electrostatic attraction in accordance with the image pattern required, characterised in that a conveying medium is used which, in the image forming zone, has a magnetic induction of at least 0.2 T at the surface attracting the toner powder and in that toner powder is used which comprises magnetically attractable material in an amount not greater than 1% by volume of the toner powder, wherein the toner powder has a particle size lower limit of about 8 µm when the toner powder contains 1% by volume of the magnetically attractable material, and that the particle size lower limit is increased as the percentage by volume of the magnetically attractable material is decreased. A method according to claim 1, characterised in that the magnetically attractable material in the toner particles is soft-magnetic. A method according to claim 2, characterised in that the magnetically attractable material in the toner particles consists of carbonyl iron. A method according to any one of the preceding claims, characterised in that toner powder is used in which the powder particles, at least at their surface, contain a quantity of electrically conductive material such that the powder particles have a specific resistance below 10¹² Ω.cm. A toner powder for use in the method according to claim 1, consisting of thermoplastic resin particles containing magnetically attractable material with a particle size not greater than 2 µm and possibly other additions, such as colouring material, a charge-control agent or electrically conductive material, characterised in that the resin particles contain at most 1% by volume of soft-magnetic material and that the resin particles have an effective magnetic susceptibility of at least 2, wherein the toner powder has a particle size lower limit of about 8 µm when the toner powder contains 1% by volume of the soft-magnetic material, and that the particle size lower limit of the toner powder is raised as the percentage by volume of the soft-magnetic material is decreased. Toner powder according to claim 5, characterised in that the magnetically attractable material consists of carbonyl iron. Toner powder according to claim 5, characterised in that the resin particles are coated at their surface with electrically conductive material and in that the specific resistance of the toner powder is less that 10¹² Ω.cm.

Inventors: HOEP ANTOON LEENDERT, VAN DER MEY ARNOLD BARBARA MAR, VAN GAGELDONK JOHANNES FRANSIS, VAN SAS LAMBERTUS MARIA LEONAR, VERCOULEN GERARDUS CORNELIS PE, HOEP, ANTOON LEENDERT, VAN DER MEY, ARNOLD BARBARA MARIA HENRI, VAN GAGELDONK, JOHANNES FRANSISCUS JOZEF, VAN SAS, LAMBERTUS MARIA LEONARDUS ARNOLDUS, VERCOULEN, GERARDUS CORNELIS PETER
Applicants: OCE NEDERLAND BV, OCE-NEDERLAND B.V.
Application year: 1991
Main classification: G03G13
IPC codes: G03G9, G03G15, G03G13