Reversal developing system

Abstract: A reversal developing system for developing a charge pattern with electrically charged toner powder, comprising: an image forming medium (10) carrying said charge pattern, a movable developing member (12) transporting said charged toner powder to a developing zone (14) adjacent to said image forming medium (10), means for applying a reversal development voltage to said movable developing member (12) so as to cause said charged toner powder to deposit on virtually uncharged areas of said image forming medium (10), a cleaning electrode (42) disposed at a distance from the surface of the developing member (12) and having an applied voltage of such a magnitude that toner particles which adhere to the surface of the developing member (12) are released therefrom under the influence of the electric field existing in the space between the developing member (12) and the cleaning electrode (42).

Description: The present invention relates to a reversal developing system for developing a charge pattern with electrically charged toner powder, comprising: an image forming medium carrying said charge pattern, a movable developing member transporting said charged toner powder to a developing zone adjacent to said image forming medium, means for applying a reversal development voltage to said developing member so as to cause said charged toner powder to be deposited on virtually uncharged areas of said image forming medium, and a cleaning electrode disposed at a distance from the surface of the developing member. A reversal developing system as indicated above is employed in electrostatic or electrophotographic copiers or printers wherein a charge pattern is to be developed which comprises relatively highly charged background areas (i.e. non-image areas) and substantially lower charged or even uncharged image-areas. These image areas are indicated here and hereinafter as virtually uncharged areas. The polarity of the electrostatic charges in the background and optionally the image areas may be either positive or negative. The reversal development of the charge pattern is effected by using charged toner powders having a charge of the same polarity as the charges in the charge pattern on the image forming medium and applying to the developing member an electric potential of the same polarity as the potential of the charge pattern and of such a magnitude that it prevents toner deposition on the charged (background) areas of the image forming member. Such reversal development process is well-known in the art and is described more in detail for instance in US patent No. 4 200 387. Except by exposing a pre-charged photoconductive image forming member through a negative original, as disclosed in the before mentioned US patent No. 4 200 387, charge patterns for reversal development generally are also formed In modern electrophotographic laser or LED printers in which the laser or LED print head discharges the pre-charged photoconductive image forming member in the image areas according to image information fed thereto from an electronic memory or any other data source. Typically, the charged toner powder forms part of a developer which contains magnetically attractable carrier particles to which toner powder is admixed. Various arrangements such as rollers, doctor blades and the like are known for applying a uniform layer of developer to the surface of the developing member, which is generally in the form of a rotating sleeve. In a commonly used system the developer is contained in a reservoir and is continuously stirred by means of helical screws or the like. The rotatable sleeve is arranged in close proximity to the reservoir such that its surface comes into direct contact with the developer. A magnetic system Is situated within the sleeve and magnetically attracts the carrier particles of the developer to the sleeve. As a result, a layer of the developer will adhere to the surface of the sleeve. The toner particles contained in the developer are charged triboelectrically against the carrier particles, and when the developer carried on the surface of the sleeve reaches the developing zone, the electrically charged toner powder will selectively be attracted by the virtually uncharged portions of the charge pattern, so that a visible image is developed. In the developing zone, the developer is brought into close proximity of the surface of the Image forming medium. Thus, the sleeve and the image forming medium will normally form a very small gap or may even be in direct contact with each other. In order to obtain images of good quality with no stain in the white background areas, It is necessary to make sure that the toner particles will not be deposited onto the charged parts of the Image forming medium which correspond to these white image areas. In a reversal developing system, this is normally achieved by creating an electric field which generally attracts the charged toner particles towards the sleeve in areas where the sleeve in the developing zone faces the charged parts of the image forming member. The electric field attracting the toner particles to the sleeve can easily be created by applying an appropriate bias voltage to the sleeve, as is disclosed already in the afore mentioned US patent No. 4 200 387. In practice, the appropriate bias voltage depends on various parameters such as the properties of the developer, the gap width in the developing zone, the surface voltage of the exposed and unexposed areas of the image forming medium, and the like. In many practical reversal developing systems of this kind it turns out to be difficult to adjust all these parameters including the bias voltage in such a manner that an undesired dark background on the developed images is stably suppressed. When the reversal developing system is operating, part of the toner carried on the surface of the sleeve is transferred to the image forming medium in the developing zone. The rest of the toner which has not been needed for developing the image is supposed to be carried away with the further rotation of the sleeve and then to be dropped back into the developer reservoir along with the carrier particles. However, due to the presence of the electric field in the developing zone, charged toner particles are attracted by the sleeve and are deposited on the surface thereof. As a result, charged toner particles will accumulate on the surface of the sleeve during continuous operation of the developing system. Since the toner has no substantial electric conductivity, the toner particles adhering to the surface of the sleeve constitute an electric surface charge which partly shields the bias voltage of the sleeve and weakens the electric field in the developing zone. As a consequence, the electric field is no longer strong enough to prevent toner particles from being deposited on the white background areas of the developed image. This effect has been identified as a major reason for the undesirable background development in the conventional system. JP-A-58-072 952 discloses an element positioned adjacent to the sleeve for limiting the thickness of the toner layer. This element is kept at the same potential as the sleeve. JP-A-55-018 631 discloses a system according to the preamble of claim 1, wherein a cleaning electrode Is disposed near the sleeve, and the potential of the cleaning electrode is changed over at the time of cleaning and developing, so that the toner concentration in the developer is decreased during cleaning. It is an object of the invention to provide a reversal developing system which can more reliably and stably suppress a dark background on the developed images. According to the invention, this object is achieved by a reversal developing system according to the preamble of claim 1, which is characterized by means applying to said cleaning electrode, during the developing process, a voltage of such a magnitude that toner particles with said predetermined polarity, which adhere to the surface of the developing member, are released therefrom under the influence of the electrostatic field existing in the space between the developing member and the cleaning electrode. According to the invention, the cleaning electrode positively removes the toner particles adhering to the surface of the developing member and hence prevents an excessive surface charge from being built up on the developing member. In a particularly preferred embodiment, the magnetic brush created by the magnet system inside of the developing member is also used for cleaning the cleaning electrode. In this case, the cleaning electrode is disposed in such a positional relationship to one of the stationary magnets in the rotating sleeve, that the bristles of the magnetic brush wipe over the surface of the electrode, thereby returning the toner particles into the magnetic brush. The cleaning effect of the magnetic brush can be improved by utilizing an electrode made of a magnetisable material such as iron or another ferromagnetic material, or even by using an electrode consisting of a permanent magnet. Obivously, the magnetisable material or permanent magnet constituting the cleaning electrode is chosen in such a value that it does not obstruct the passage of developer through the gap between cleaning electrode and developing member. A preferred embodiment of the invention will now be explained in conjunction with the accompanying drawing which shows a schematic cross-sectional view of the developing system. An image forming medium is formed by a photoconductive drum 10, e.g. an organic photoconductor which rotates in the direction of an arrow A. The reversal developing system comprises a thin cylindrical developing member in the form of a sleeve 12 of aluminum which is arranged in parallel with the drum 10 such that a narrow gap defining a developing zone 14 is formed between the surfaces of the sleeve 12 and the drum 10. The sleeve 12 rotates in the direction of an arrow B, i.e. in the same sense as the drum 10, so that the surfaces of the drum and the sleeve move in opposite directions in the developing zone 14. The surface of the drum 10 carries a charge image which has been formed thereon in any known manner and is to be reverse-developed with toner in the developing zone 14. A developer 16 constituted by a mixture of carrier particles (e.g. resin coated iron particles) and a small amount of carbon-containing toner particles is contained in a reservoir 18. A rotating helical screw 20 is provided in the reservoir 18 for continuously stirring the developer 16. The lower part of the circumferential surface of the sleeve 12 projects into the reservoir 18 so that it is brought into intimate contact with the developer. The wall of the reservoir 18 forms a scraper or blade 22 which limits the thickness of a layer of developer adhering to the surface of the sleeve 12. A magnet system 24 is stationarily disposed inside of the sleeve 12 and comprises a cylindrical support body 26 and a number of permanent magnets 28, 30, 32, 34, 36 and 38 extending along the internal circumferential surface of the sleeve 12. The magnets 28, 30 are facing the reservoir 18 and attract the developer 16 contained therein to the surface of the sleeve. The magnet 30 is disposed directly opposite to the blade 22. The magnet 32 holds the developer which has passed the blade 22 on the surface of the sleeve, while the latter moves towards the developing zone 14. The magnet 34 is arranged directly opposite to the developing zone 14 and creates a magnetic brush 40 which sweeps over the surface of the drum 10, so that the toner particles contained in the brush are brought into intimate contact with the surface of the drum 10. The magnets 36, 38 serve to hold the carrier particles and adhering toner particles which have not been used for developing on the surface of the sleeve 12 until they have passed the top of the sleeve and can drop back into the reservoir 18. The drum 10 is uniformly charged to a surface potential of -800 V and is then exposed so as to be discharged locally, thereby creating the charge image. A bias voltage of -700 V is applied to the sleeve 12. Thus, when the photoconductive drum 10 is not exposed and retains its uniform surface potential of, an electric field corresponding to a voltage difference of 100 V is generated across the gap in the developing zone 14. This gap typically has a width in the order of 1.5 mm. Since the toner particles have a negative triboelectric charge, the electric field in the developing zone 14 acts to attract the toner particles toward the sleeve 12 so that they will not be deposited on the unexposed areas of the drum 10. When a charge image has been formed on the drum 10, the exposed surface areas will be discharged to a potential of approximately -300 V. In these areas, the electric field in the developing zone acts in the opposite direction, so that toner particles are deposited on these discharged areas. It will be understood that the shown example is a so-called write-black system, in which the exposed areas of the photoconductive drum 10 are developed with toner. As is shown exaggeratedly in the drawing, each of the magnets creates a magnetic brush similar to the brush 40 created by the magnet 34. The height of the magnetic brush and the orientation of the bristles therein varies with the strength and direction of the magnetic field. The brushes created by the individual magnets are merged with each other and form a continuous layer in which the developer is relatively loosely bound to the surface of the sleeve by magnetic interaction. However, in particular when the image to be developed contains large white areas (as is the case with text or line images), a large amount of the toner particles in the brush 40 is subject to the electric field which attracts them to the surface of the sleeve. Thus, the sleeve 12 is "developed" with toner, i.e. the toner particles adhere relatively firmly to the surface of the sleeve. These firmly adhering toner particles will not drop off from the sleeve when they reach the position above the reservoir 18, and they will not completely be stripped-off, neither, when passing through the developer 16 in the reservoir. In order to avoid that these firmly adhering charged toner particles accumulate on the surface of the drum and decrease the electric field in the developing zone 14 by shielding the bias voltage of the sleeve, a cleaning electrode 42 is provided at the circumference of the sleeve 12. This cleaning electrode 42 is disposed at such a distance from the surface of the sleeve 12 that is does not obstruct the transport of the developer material through the gap formed between the electrode 42 and the sleeve 12. Further the electrode 42 is kept at a voltage which creates in the gap between electrode 42 and sleeve 12 an electric field such that the negative toner particles are drawn off from the surface of the sleeve 12 and are attracted to the electrode 42. The potential to be applied to the electrode 42, obviously, depends on the width of the gap existing between the electrode 40 and the sleeve 12. Preferably, the electrode 42 is disposed at a distance from the sleeve 12 which is about the same or only slightly greater than the distance between the scraper blade 22 and the sleeve 12. Typically, in the embodiment illustrated in the annexed figure, the smallest width of the developing zone 14 is 1.5 mm, of the gap between blade 22 and sleeve 12 is 1.25 mm and the gap between the electrode 42 and the sleeve 12 is 1.3 mm. The potential to be applied to the electrode 42 to obtain an effective cleaning of the surface of sleeve 12 amounts to between about -500 V and -550 V, typically -530 V. If in the embodiment shown in the figure the cleaning electrode 42 is made more positive with respect to the sleeve 12; e.g. is kept at a voltage of -400 to -475 V, a cleaning effect of the sleeve 12 still is obtained. However, now the toner particles tend to adhere firmly to the surface of the electrode and cannot be removed therefrom by the developer flow at the electrode surface, so that gradually a situation is attained in which no cleaning effect of the electrode 42 is observed any more. In general, the electrode 42 could be disposed at any position at the circumference of the sleeve 12, of course with the exception of the developing zone 14. Preferably, the electrode 42 is disposed in the vicinity of one of the magnets, most suitably one of the magnets 30, 32, 36 or 38. This has the advantage that the brush created by this magnet can be used for mechanically cleaning the electrode 42. Thus, the toner particles firmly adhering to the surface of the sleeve 12 are first drawn off from the sleeve by means of the electrode 42 due to the electric field between the sleeve and the electrode, and they are then mechanically wiped off from the electrode 42 by mechanic interaction with the magnetic brush. As a result, the toner is returned into the magnetic brush where it is only loosely bound to the surface of the sleeve, so that it may either be used for developing the charge image or may be dropped back into the reservoir 18. From the viewpoint of suppressing the generation of toner dust as far as possible, a position of the electrode 42 near one of the magnets 30 and 32 is particularly preferred. In the shown embodiment, the electrode 42 is disposed between the magnets 30 and 32 and it is itself made of a ferromagnetic material, e.g. of iron, so that it forms part of the magnetic circuit. As a result, the combined effects of the magnets 30, 32 and the magnetized electrode 42 creates an extended brush 44 which efficiently cleans the electrode 42. A particularly intense cleaning effect can be achieved by exploiting the fact that the magnetic bristles flip over from one orientation to another when the sleeve rides over an area where the magnetic field is rather inhomogeneous. If the electric conductivity of the developer 16 is small enough, the whole reservoir 18 can be kept at the potential of the electrode 42 (-530 V), so that it is not necessary to electrically insulate the electrode from the walls of the reservoir 18. In this case, the blade 22 has the same potential as the electrode 42 but would nevertheless not be efficient as a cleaning electrode, because the relatively high density of the developer in the gap between the blade 22 and the sleeve limits the mobility of the toner. The level of the voltage applied to the cleaning electrode 42 should be suitably matched to the bias voltage applied to the sleeve. If the voltage difference between the sleeve and the cleaning electrode is too small, then the electric field will not be strong enough for attracting the toner particles to the electrode. On the other hand, if the potential difference becomes too large, then the toner would be bound to the electrode 42 too strongly, so that the electrode could not be cleaned by the magnetic brush and the toner would accumulate on the electrode. In the shown example, a voltage of about 530 V is optimal. However, satisfactory results can still be achieved when this voltage is varied by 30 V. While a specific embodiment of the invention has been described above, it will occur to a person skilled in the art that various modifications are possible without departing from the scope of the invention as defined in the appended claims. For instance the cleaning electrode 42, whether consisting of magnetisable material or not, could be provided with a thin surface layer of an adhesive material such as silicon rubber or resin or polytetrafluorethylene or like material. Further the electrode 42 could be in the form of a rotating sleeve, if desired with active cleaning means such as a blade or brush-like member, e.g. a fur brush, disposed in the rotational path of the sleeve for removing toner particles adhering thereto. The provision of active cleaning means for removing toner particles adhering to the cleaning electrode, obviously, in most instances will allow the application of a stronger electrical field for cleaning the developing member than would be applicable when no active cleaning means were provided.

Claims: A reversal developing system for developing a charge pattern with electrically charged toner powder having a prededermined polarity, comprising: an image forming medium (10) carrying said charge pattern, a movable developing member (12) transporting said charged toner powder to a developing zone (14) adjacent to said image forming medium (10), means for applying a reversal development voltage to said movable developing member (12) so as to cause said charged toner powder to deposit on virtually uncharged areas of said image forming medium (10), and a cleaning electrode (42) disposed at a distance from the surface of the developing member (12), characterized by means applying to said cleaning electrode (42), during the developing process, a voltage of such a magnitude that toner particles with said predetermined polarity, which adhere to the surface of the developing member (12), are released therefrom under the Influence of the electric field existing in the space between the developing member (12) and the cleaning electrode (42). Reversal developing system according to claim 1, wherein said developing member comprises a sleeve (12) and said means for applying toner powder onto the surface of the sleeve (12) comprise magnets (28, 30) for magnetically attracting a developer (16) containing said toner powder to the surface of said sleeve. Reversal developing system according to claim 2, wherein said electrode (42) is disposed in the vicinity of one of the magnets (30) of the magnet system (24) such that the electrode is mechanically cleaned by a magnetic brush created by the magnet system. Reversal developing system according to claim 2, wherein said electrode (42) includes a magnetizable material. Reversal developing system according to claim 1 or 2, wherein the developer (16) is contained in a reservoir (18) disposed below the developing member (12), which comprises a sleeve, such that the surface of the sleeve comes into contact with the developer, said reservoir (18) forms a blade (22) projecting towards the surface of the developing member (12) for limiting the thickness of the layer of developer on the surface of the developing member, and the electrode (42) is disposed in the circumferential portion of the sleeve between the blade (22) and the developing zone (14). Reversal developing system according to claim 5 in which the smallest distance between the electrode (42) and the surface of the developing member (12) is equal or greater than the distance between the blade (22) and the surface of the developing member.

Inventors: MOOREN GERARDUS JOHANNES CATHA, VALK JOHANNES, MOOREN, GERARDUS JOHANNES CATHARINA, VALK, JOHANNES
Applicants: OCE TECH BV, OCE-TECHNOLOGIES B.V.
Application year: 1997
Main classification: G03G15
IPC codes: G03G15