Image forming method and apparatus therefor

Abstract: An image forming method of the present invention comprises: a first toner image formation process for forming a first toner image by forming, on a latent image carrier, a first latent image which corresponds to a first image and developing the first latent image by a first toner charged to one polarity through a development selected from normal and reverse developments so as to correspond to the polarity of the first toner; a second toner image formation process for forming a second toner image by forming, on the latent image carrier, a second latent image which corresponds to a second image and developing the second latent image by a second toner charged to the other polarity by the other development while applying a developing bias; and a transfer treatment process for simultaneously transferring said first and second toner images to a transfer medium; wherein said developing bias VB2 satisfies the following equations: |VT1 - VB2| > |VH2 - VB2| (1) |VT1 - VB2| > |VT1 - VH2| (2) where a surface potential of said first toner image is VT1, a background potential in said second toner image forming process is VH2, and the developing bias in said second toner image forming process is VB2.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method of and apparatus for forming images of two types by utilizing electrostatic latent images, more particularly, to the improvement of a method of and an apparatus for forming an image in which after the latent images of two types on a latent image holder are developed in a superposed state by adopting so-called superposition development and the developed images are simultaneously transferred to a transfer medium. An example of conventional image forming methods of this type is disclosed in Japanese Patent Laid-Open No. 137538/1980. In this method, the surface of a photosensitive material is uniformly charged; a negative image is first projected to reversely develop the statically eliminated portion of the photosensitive material which has been irradiated with light, with toner having the same polarity as that of the photosensitive material; a positive image is then projected to eliminate the charges at the residual charge portion on the surface of the photosensitive material except the positive-image projected portion; the residual charge portion of the positive-image projected portion is then developed normally with toner having the opposite polarity to that of the photosensitive material, thereby forming negative and positive toner images on the same surface of the photosensitive material; the negative and positive toner images are arranged in the same polarity; and the negative and positive toner images are transferred to a transfer medium simultaneously. According to this type of image forming method, since means for forming toner images of two types are provided around one photosensitive material, and the toner images of the two types are simultaneously transferred to a recording medium by passing the recording medium only once with respect to the transfer position, the apparatus can be made compact and the image formation speed is increased. Furthermore, since negative and positive latent images are developed by toners having different polarities, it may be possible to effectively prevent in the second developing process the second toner from mixing with the first toner image, thereby preventing the blurring of the color of the toner image and preventing the toner of the first toner image from mixing with the second developer. In this type of image forming method, in order to eliminate the charges at the residual charge portion on the surface of the photosensitive material except the positive-image projected portion, the surface potential VT1 of a first toner image T1 substantially coincides with the potential VH2 of the background portion H2 except the second positive-image projected portion Z2. Or, rather, the surface potential VT1 of a first toner image T1 becomes slightly higher in the absolute value than the potential VH2 of the background portion H2 by the charges of the toner, as shown in Fig. 18(a). Therefore, an electrostatic field S0 directing toward the peripheral portion of the first toner image T1 is slightly applied between the peripheral portion of the first toner image T and the surface of the photosensitive material Q, as shown in Fig. 18(b). In this state, if the second developing process is carried out by developing system as described in Japanese Patent Laid-Open No. 137538/1980, the second developer is uniformly sprinkled over the surface portion of the photosensitive material containing the first toner image T1. The second developer therefore impinges on the first toner image T1 frequently and the first toner image is disadvantageously apt to be disturbed by the impact force as well as the action of the field S0. To solve this problem, a method of executing the second developing process by one component non-contacting development may be considered. According to the one component non-contacting development, however, the following problems are occurred. That is, in a non-contacting AC bias development, flying toners collide with the already developed first toners so that the first toners enter into the second developing device, as a result of which the first and second toners are mixed with each other to thereby mix the colors of the first and second toners with each other. On the other hand, in a non-contacting DC bias development, the gradation reproducibility and narrow line reproducibility is deteriorated. Consequently, it is inevitable to adopt a two component development other than cascade development, namely, magnetic brush development for the second developing process. More specifically, if the magnetic brush development is adopted for the second developing process, it is possible to positively attract the second toner to the second positive-image projected portion on the basis of the electrostatic field generated between the second positive-image projected portion and a developing roll by applying an appropriate developing bias VB2 to the developing roll, as indicated by the chain line in Fig. 18(a), and to retain the first toner image T1 by the static attractive force F due to the electrostatic field Sa generated between the developing roll and the first toner image T1, as shown in Fig. 18(c). Accordingly, in comparison with cascade development, the disturbance of the first toner image due to scraping is reduced to a degree corresponding to the existence of the electrostatic attractive force F. However, since the active force F0 caused by the electrostatic field S0 directing toward the peripheral portion of the first toner image T1 is applied, it is impossible to completely prevent the disturbance of the first toner image T1. A two-color electrophotogrgaphic recorder is also described in Patent Abstracts of Japan, Vol. 5, No. 154, (P-80), (826), &JP-A-56 87 060. A latent image carrier is charged to a starting potential and then exposed for forming a latent image with a black toner area potential of about half of the starting potential. This image is reversely developed with black toner having positive charge. Then the latent image carrier is positively exposed so as to render a red toner area potential nearly about the black toner area potential. Afterwards, the latent image carrier is positively developed with red toner having negative charge. Thereby, the developing bias voltage is kept higher than the background potential of the red toner image. In this electrophotographic recorder the black toner area potential is expressly higher in absolute value than the background potential of the positive latent image formed thereafter and the black toner area potential is shifted during exposure for forming the red color image. A further two-color electrophotogrgaphic recorder is known from Patent Abstracts of Japan, Vol. 5, No. 154, (P-80), (826), &JP-A-56 87 059. In this electrophotographic recorder a latent image carrier is charged to a starting potential and then positively exposed for forming a latent image with a black toner area potential of about half of the starting potential. This image is positively developed with black toner having negative charge. Then the latent image carrier is negatively exposed so as to render a red toner area potential nearly zero. Afterwards, the latent image carrier is reversely developed with red toner having positive charge. Thereby, the developing bias voltage is kept somewhat lower than the black toner area potential. In this electrophotographic recorder the red toner might tend to adhere to the fringe field portion of the black toner image in the second development, raising a color mixture problem. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to eliminate the above-described problems in the prior art and to provide a method of and an apparatus for forming an image which are capable of forming a second toner image without disturbing a first image already formed and maintaining a good quality of images of two types. To achieve this object, the present invention provides an image forming method as defined in claims 1 and 3. Furthermore, an apparatus for forming image according to the present invention is defined in claims 4 and 5. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1(a) is an explanatory view of the processes for an image forming method according to the present invention; Fig. 1(b) is an explanatory view of the schematic structure of an image forming apparatus according to the present invention; Fig. 2(a) is an explanatory view of the first toner image formation process in an image forming method according to the present invention which adopts negative-positive development; Fig. 2(b) is an explanatory view of the second toner image formation process therein; Fig. 2(c) is an explanatory view of the state of an electric field acting on the peripheral portion of the first toner image in the second toner image formation process therein; Fig. 3(a) is an explanatory view of the second toner image formation process in an image forming method according to the present invention which adopts positive-negative development; Fig. 3(b) is an explanatory view of the state of an electric field acting on the peripheral portion of the first toner image in the second toner image formation process therein; Fig. 4 is an explanatory view of a two-color printer of the Embodiment 1 of the present invention; Figs. 5(a) to 5(f) are explanatory views of the image forming processes in the Embodiment 1; Figs. 6(a) and 6(b) are explanatory views of potential parameters in the Examples 1 to 6; Fig. 7 is an explanatory view of a standard for grading the image characteristics in the Examples 1 to 6; Fig. 8 is a graph showing the relationship between VT1 - VB2 and the grades; Fig. 9 is an explanatory view of a two-color copying machine of the Embodiment 2 of the present invention; Fig. 10 is an explanatory view of the concrete structure of the developing units in the Embodiment 2; Figs. 11(a) to 11(e) are explanatory views of the image forming processes in the Embodiment 2; Fig. 12(a) is a schematic view of the developing operation of the second developing unit in the Embodiment 2; Fig. 12(b) is a schematic view of the developing operation of a developing unit of another type; Fig. 13 is an explanatory view a two-color printer of the Embodiment 3 of the present invention; Fig. 14 is an explanatory view of the characteristics of the exposing and charging corotron used in Embodiment 3; Fig. 15(a) to 15(f) are explanatory views of the image forming processes in the Embodiment 3; Fig. 16 is an explanatory view a two-color printer of the Embodiment 4 of the present invention; Fig. 17 is an explanatory view of the image forming processes in the Embodiment 4; Fig. 18(a) is an explanatory view of a conventional image forming method; Fig. 18(b) is an explanatory view of the state of an electric field on the peripheral portion of the first toner image in the second toner image formation process in the conventional method; and Fig. 18(c) is an explanatory view of the state of an electric field on the peripheral portion of the first toner image in the second toner image formation process in the conventional method to which magnetic brush development is adapted. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the accompanying drawing, the preferred embodiments of the present invention will be discussed below. As shown in Fig. 1(a), the present invention provides an image forming method comprising: a first toner image formation process A for forming a first toner image by forming a first latent image which corresponds to a first image and which is an object of one of the normal development and reverse development on a latent image carrier and developing the first latent image by a first toner charged to one polarity; a second toner image formation process B for forming a second toner image by forming a second latent image which corresponds to a second image and which is an object of the other of reverse development and normal development on the latent image carrier and developing the second latent image by a second toner charged to the other polarity by magnetic brush development while applying a developing bias; and a transfer treatment process C for simultaneously transferring the first and second toner images to a transfer medium; the developing bias VB2 satisfying the following equations (1) and (2): |VT1 - VB2| > |VH2 - VB2| (1) |VT1 - VB2| > |VT1 - VH2| (2) where the surface potential of the first toner image is VT1, the background potential in the second toner image forming process is VH2, and the developing bias in the second toner image forming process is VB2 In this image forming method, toner images of two types are not necessarily made of different colors and include the toner images composed of the toner of the same color. For the developing steps carried out in the toner image formation processes A and B, either of normal development or reverse development may be adopted, so long as one is adopted in one image formation process and the other in the other image formation process. However, if reverse development is adopted in the first toner formation process A and normal development is adopted in the second toner formation process B, it is possible to secure a sufficiently large contrast between the potential of each image area and the potential of the background, thereby enabling the formation of an image of an adequate density. An apparatus for realizing the above-described image forming method, as shown in Fig. 1(b), for example, comprises: a latent image carrier 1; a first latent image forming means 2 for forming a first latent image which corresponds to a first image and which is an object of either normal development or reverse development on the latent image carrier or holder 1; a first developing means 3 for developing the first latent image by a first toner charged to one polarity so as to form a first toner image; a second latent image forming means 4 for forming a second latent image which corresponds to a second image and which is an object of either reverse development or normal development on the latent image carrier 1 so that the second latent image has a background potential VH2 which is the intermediate potential of the potential of the image area of the second latent image and the surface potential VT1 of the first toner image; a second developing means 5 to which a developing bias VB2 satisfying the relationship of |VT1 - VB2| > |VH2 - VB2| and |VT1 - VB2| > |VT1 - VH2| is applied and which develops the second latent image by a second toner charged to the other polarity by magnetic brush development so as to form a second toner image; and a transfer treatment means 6 for simultaneously transferring the first and second toner images to a transfer medium 7. In these technical means, as the latent image holder 1, any given material such as a photosensitive material and dielectric on which a latent image can be formed by the latent image forming means 2 and 4 may be selected. The latent image carrier may have either a drum-like structure or a belt-like structure. The design of the first and second latent image forming means 2 and 4 may be appropriately changed so long as they are capable of forming latent images having a potential of a predetermined level on the latent image holder 1. For example, the latent image forming means may be designed so as to charge the latent image carrier 1 in advance and to statically eliminate charges at the position corresponding to the image or the non-image area with light or ions to a predetermined level, or so as to form a latent image of a predetermined level with ions without charging the latent image carrier 1 in advance. In the case of forming a latent image with light, an optical write means such as a so-called optical image formation system using mirror and lens system, a laser diode array, a light emitting diode array, liquid crystal shutter array and fluorescent indicator element array is used. In the case of forming a latent image with ions with using a multi-stylus head or ion flow modulation head, a discharge head is appropriately used. For the first and second developing means 3 and 5, a developer to be used and a developing system may appropriately be selected, so long as the first and second electrostatic latent images are reversely or normally developed with toners having opposite polarities, but at least the second developing means 5 should be so designed as to adopt magnetic brush development to which the developing bias VB2 which satisfies the above-described equations is applied in consideration of effective prevention of disturbance of the first toner image. Each of the developing means 3 and 5 is sufficed with one developing function portion, but it may be, for example, so designed to have a multiplicity of developing function portions for different colors and selectively switch the multiplicity of function portions. The second developing means 5 is preferably so designed as to reduce the frictional force with the first toner image. As one measure, a two component developer of a low density consisting of a predetermined color toner and a carrier having a density of not more than 4 g/cm³ may be used, for the reasons set forth in the following: In order to sufficiently reproduce the toner image density, it is generally required to carry a predetermined amount of developing agents to the developing nip portion of the second developing device. Therefore, it is necessary to set the value of TG(Trimming Gap)/DRS(Drum Roll Space) in a range from 0.7 to 1.2. However, in such a case, if the generally-used development agents having carriers with its density not less than 4.0 g/cm³ are used, the force of the second developing agent for scratching off the first developing agent becomes too large. As a result, although the second developing density can be made high, disturbance of the first image is occurred. Therefore, with using such a developing agent as having carrier with its density of not more than 4.0 g/cm³, it becomes possible to make high the second toner image density without any disturbance in the first toner image. In the case where the developing agent having carrier with its density not more than 4.0 g/cm³ is used, the magnetic carrier may be appropriately selected from a porous carrier, ferrite carrier, a carrier consisting of magnetic powder dispersed in a resin binder, etc. Among these, a carrier consisting of magnetic powder dispersed in a resin binder is preferred in the respect that the density can be easily adjusted by the content of the magnetic powder. As another measure, the second developing means 5 may be provided with a developer carrier or holder composed of a magnet roll fixed in a nonmagnetic rotary sleeve. By disposing a repulsion magnetic pole on the magnet roll in correspondence with the developing nip range, it is possible to adjust the magnetic brushing force against the developer in the develop nip range to be soft. As still another measure, the second developing means 5 may be provided with a developer holder composed of a magnet roll rotatably disposed in a nonmagnetic fixed sleeve. The moving speed of the developer on the developer holder is so set as to satisfy the relationship 0.5 ≦ V DEVE /V P ≦ 2.0 where the moving speed of the developer is V DEVE and the rotational speed of the latent image holder 1 is V P , thereby suppressing the impact force of the magnetic brush of the developer within the range which does not impair the developing quality. The transfer treatment means 6 may be so designed as to have an electrostatic transfer system, a heat transfer system and the like as desired, so long as it is capable of simultaneously transferring the first toner image and the second toner image to the transfer means 7. In the respect of maintaining a good transferred state, the electrostatic transfer system may preferably be adopted. When the electrostatic transfer system is adopted, it is necessary to design the transfer treatment means 6 so that after a pretreatment of at least arranging the first and second toner images in the same polarity, the transfer medium 7 is charged to a polarity opposite to that of the toner images, and the toner images are electrostatically attracted to the transfer medium 7. In this case, in order to effectively restrain the toner which has adhered to the background portion on the surface of the latent image holder 1, which is called "fog toner", from being transferred to the transfer medium 7, it is preferable, for example, to charge the fog toner to the polarity opposite to that of the toner at the image area, thereby transferring only the toner at the image area to the transfer medium 7. According to the present invention, as described above, firstly in the first toner image formation process A, a first latent image Z1 which is the object of, for example, reverse development and which corresponds to a first image is formed on the latent image carrier 1 which has, for example, a positive charge characteristic. Then, the first latent image Z1 is reversely developed by a first toner which is charged to a positive polarity so as to form a first toner image T1 having a surface potential of VT1, as shown in Fig. 2(a). Secondary, in the second toner image formation process B, a second latent image Z2 which is the object of normal development and which corresponds to a second image is formed on the latent image carrier 1, and the second latent image Z2 is then normally developed by a second toner which is charged to a negative polarity so as to form a second toner image T2, as shown in Fig. 2(b). At this time, the background potential VH2 of the second latent image Z2 is set at an intermediate potential of the surface potential VT1 of the first toner image T1 and the image area potential of the second latent image Z2. Since the surface potential VT1 of the first toner image T1 is lower than the background potential VH2, the portion of the first toner image T1 constitutes a kind of potential well with respect to the ambience. On the other hand, a magnetic brush holding the second toner brushes against the latent image carrier 1 while a developing bias VB2 is applied. Since the developing bias VB2 is set at a larger value than the background potential VH2 of the second latent image Z2. The second toner is attracted to the second latent image Z2 without adhering to the first toner image portion T1 and the background portion H2 for the second latent image Z2. In this state, since the first toner and the second toner have the opposite polarities to each other, even if the second toner comes into contact with the first toner image T1 or the first toner is about to enter the second developer, both toners repulse each other, thereby effectively avoiding coexistence of both toners. In the state of the developing bias VB2 being applied, since the potential difference ΔVm between the first toner image T1 and the developing bias VB2 becomes larger than that of the ambience, an electrostatic field Sm at the portion corresponding to the first toner image T1 becomes larger than an electrostatic field S at the other portion, and the electrostatic force Fm for pressing the first toner image T1 increases to that degree, as shown in Figs. 2(b) and 2(c). In addition, on the peripheral portion of the first toner image T1, an electrostatic field Sn is formed in the direction indicated by the arrow in Fig. 2(c) on the basis of the potential difference ΔVn between the peripheral portion of the first toner image T1 and the background portion H2, and an electrostatic force Fn which holds and constrains the first toner image T1 in the horizontal direction is generated. As a result, the first toner image T1 is firmly retained on the latent image holder 1 by the electrostatic forces Fm and Fn, and even if the magnetic brush holding the second toner brushes against the first toner image, the disturbance of the first toner image T1 is effectively prevented. Thereafter, in the transfer treatment process C, both toner images T1 and T2 on the latent image holder 1 are simultaneously transferred to the transfer medium 7. In this image forming process, if the potential contrast between the first latent image Z1 and the second latent image Z2 is sufficiently secured, it is possible to obtain a toner image having a sufficient density. Conversely to the above-described image forming processing, the case of normally developing the first latent image Z1 in the first toner image formation process A and reversely developing the second latent image Z2 in the second toner image formation process B will here be explained. In the second toner image formation process, each of the potentials of the first toner image T1 (negative polarity, in this case), the second latent image Z2 and the background portion H2 thereof and the developing bias VB2 are set in the relationship as shown in Fig. 3(a). The portion of the first toner image T1 constitutes a kind of potential hill with respect to the ambience. At this time, since the potential difference ΔVm between the first toner image T1 and the developing bias VB2 becomes larger than that of the ambience, an electrostatic field Sm at the portion corresponding to the first toner image T1 becomes larger than an electrostatic field S at the other portion, and the electrostatic force Fm for pressing the first toner image T1 having the negative polarity increases to that degree, as shown in Fig. 3(b). In addition, on the peripheral portion of the first toner image T1, an electrostatic field Sn is formed in the direction indicated by the arrow on the basis of the potential difference ΔVn between the peripheral portion of the first toner image T1 and the background portion H2, and an electrostatic force Fn which holds and constrains the first toner image T1 having the negative polarity in the horizontal direction is generated. As a result, the first toner image T1 is firmly retained on the latent image carrier 1 by the electrostatic forces Fm and Fn, and even if the magnetic brush holding the second toner brushes against the first toner image T1, the disturbance of the first toner image T1 is effectively prevented. [Description of the Embodiment] The present invention will be explained in detail with reference to embodiments shown in the accompanying drawings. Embodiment 1 Fig. 4 shows a first embodiment of a two-color printer to which an image forming method of the present invention is adapted. In Fig. 4, the reference numeral 10 represents a positive charge type photosensitive drum, 11 a charging corotron for charging the photosensitive drum 11 in advance, 12 a first LED array for forming a first latent image 11, 13 a first magnetic brush type developing unit using black toner which is positively charged, 14 a second LED array for forming a second latent image, 15 a second magnetic brush type developing unit using red toner which is negatively charged, 16 a pre-transfer corotron for arranging the charged toners on the photosensitive drum 10 in the same polarity before a transfer step, 17 a transfer corotron for charging a recording sheet 18 to the opposite polarity to that of the toners adjusted by the pre-transfer corotron 16 and for electro-statically transferring the toner image of each color to the recording sheet 18, 19 a static elimination corotron for separating the recording sheet 18 from the photosensitive drum 10 after the transfer step, 20 a static elimination corotron for eliminating the residual charges on the photosensitive drum 10 and residual toner charges before a cleaning step, 21 a cleaner for removing the residual toner on the photosensitive drum 10, 22 a static eliminating lamp for completely eliminating the residual charges on the photosensitive drum 10 before the next image formation cycle, 23 a sheet supply tray accommodating the recording sheet 18, 24 a stabilizer for stabilizing the toner image on the recording sheet 18 which has passed through the transfer step, and 25 a guide plate for defining the route of travel of the recording sheet 18. The operation of the image formation of the two color printer of this embodiment will be explained hereinunder. As an example of a recording image, an image consisting of a black image area (GB) and a red image area (GR) on a white ground (W) will be cited, as shown in the above of Fig. 5. The photosensitive drum 10 is first uniformly charged positively by the charging corotron 11 (Fig. 5(a)). The portion of the photosensitive drum 10 which corresponds to the black image area GB is exposed by the first LED array 12 to obtain a negative image. At this time, the first latent image Z1 of the photosensitive drum 10 which corresponds to the black image area GB is statically eliminated to a potential of VZ1, while the potentials of the portions of the photosensitive drum 10 which correspond to the white ground W and the red image area GR are maintained at the initial charged potential VH1 (Fig. 5(b)). Thereafter, the developing bias VB1 of the first developing unit 13 is set between the potential VP1 of the first latent image Z1 and the initial charged potential VH1, and the first latent image Z1 is reversely developed by black toner positively charged by the first developing unit 13 to form a first toner image T1 (Fig. 5(c)). The portion of the photosensitive drum 10 which corresponds to the red image area GR is exposed by the second LED array 14 to obtain a positive image. At this time, the potential of the second latent image Z2 of the photosensitive drum 10 which corresponds to the red image area GR is maintained at a potential VZ2 which is substantially equal to the initial charged potential VH1, while the background portion H2 except for the second image Z2 is statically eliminated so as to have a potential of VH2 higher than the surface potential VT1 of the first toner image T1 (Fig. 5(d)). Thereafter, the developing bias VB2 of the second developing unit 15 is set between the potential VZ2 of the second latent image Z2 and the background potential VH2, and the second latent image Z2 is normally developed by red toner negatively charged by the second developing unit 15 to form a second toner image T2 (Fig. 5(e)). At this stage, the toner images T1 and T2 of the two colors have been formed on the photosensitive drum 10. After these toner images T1 and T2 are arranged in the same polarity, e.g., a negative polarity, by the pre-transfer corotron 16 (Fig. 5(f)), they are simultaneously transferred to the recording sheet 18 by the transfer corotron 17. After transfer, the recording sheet 18 is passed through the stabilizer 24 to stabilize the toner image of each color on the recording sheet 18. At this time, almost no disturbance is observed in the images on the recording sheet 18, and the images have a good quality. It was confirmed on the basis of the results of the following experiment that the following equations must be satisfied in the second toner image formation process of the above-described operational process in order to obtain a good two-color image without disturbing the first toner image T1: |VT1 - VB2| > |VH2 - VB2| (1) |VT1 - VB2| > |VT1 - VH2| (2) Experiments Toner images were formed by equalizing the conditions for the first toner image formation process and varying the parameters in the second toner image formation process, and the disturbances of the first toner images T1 and the image densities based on the first and second toner images T1 and T2 were measured. In this case, the toner image to be measured was a line image of 300 µm extending in the axial direction (X) and the circumferential direction (Y) of the photosensitive drum 10. The disturbance was represented by the line width reproducibility which indicates the ratio of the line width of the reproduced toner image T1 on the assumption that the line width of the line image of a monochrome mode is 1 and the coarseness which indicates the degree of disturbance in the dimension at the edge portion of the reproduced toner image T1. The conditions common to the experiments were as follows: * Photosensitive drum Se (selenium) type photosensitive material (positive charge type) Drum diameter 200 mm * Processing speed 160 mm/sec * First Developer Two component type (black toner positively charged) Carrier Ferrite carrier having an average particle diameter of 100 µm Black toner A mixture of 92 parts of a styrene-n-butyl methacrylate copolymer, 8 parts of Carbon Black #4000 (Trade Name, produced by Mitsubishi Chemical Industries, Co., Ltd.) and 2 parts of charging controlling agent (Bontron P-51, Trade Name, produced by Orient Chemical Industries, Co. Ltd.) was melted, kneaded and pulverized to particles having an average particle diameter of 12 µm. Positively charged with respect to the carrier. * Second developer Two component type (red toner negatively charged) Carrier A magnetic particle dispersion type carrier obtained by melting, kneading and pulverizing a mixture of 35 parts of a styrene-n-butyl methacrylate copolymer and 65 parts of magnetite. Average particle diameter: 30 µm, and density: 2.2 g/cm³. Red toner A mixture of 92 parts of a styrene-n-butyl methacrylate copolymer, 8 parts of a red pigment Lithor Scarlet (Trade Name, produced by BASF) and 2 parts of charging controlling agent (E-84, Trade Name, produced by Orient Chemical Industries, Co. Ltd.) was melted, kneaded and pulverized to particles having an average particle diameter of 12 µm. Negatively charged with respect to the carrier. * Parameters in the first developing unit Trimming gas (TG) 0.6 mm Drum Roll Space (DRS) [Space between the photosensitive drum and the developing roll] 0.8 mm Magnet set angle (MGA) [Deviation angle of the set position of the main magnetic pole from the developing nip range] +5° Diameter and rotational speed of the developing sleeve 50 mm, 480 mm/sec Amount of developer conveyed 120 mg/cm² Type and magnetic force of main pole Propulsion magnetic pole, 750 Gauss * Parameters in the second developing unit TG 0.6 mm DRS 0.8 mm MSA -5° Diameter and rotational speed of the developing sleeve 50 mm, 220 mm/sec Amount of developer conveyed 120 mg/cm³ Type and magnetic force of main pole Repulsion magnetic pole (magnetic poles of the same polarity disposed adjacently to each other), 1220 Gauss * Voltage applied to pre-transfer corotron DC-5.0 KV * Voltage applied to transfer corotron AC 400 Hz, V p-p 8.5 KV, DC +2.5 KV When the first toner image was formed, the potential VZ1 of the first latent image Z1 was fixed at 200 (V), the background potential VH1 of the first latent image Z1 at 800 (V) and the first developing bias VB1 at 650 (V), as shown in Fig. 6(a). When the second toner image was formed 0.7 seconds after the formation of the first toner image, the surface potential VT2 of the second toner image, the second developing bias VB2, the background potential VH2 of the second latent image Z2, and the exposure E2 at the time of forming the second latent image on the assumption that the exposure E1 at the time of forming the first latent image was 1 were varied to select the six Examples 1 to 6 shown in Table 1. When the second tone image was formed, the potentials VZ1 and VZ2 of the first and second latent images Z1 and Z2, respectively, and the surface potential VT1 of the first toner image T1 were fixed at 160 (V), 700 (V) and 190 (V), respectively, with consideration for the darkdecay. The results of the characteristics of Examples 1 to 6 are shown in Table 2. In Table 1, the suitability means whether the conditions (1) and (2) are satisfied or not. If they are satisfied, the mark O is given, while if they are not, the mark X is given. In Table 2, the image characteristics of Examples 1 to 6 were graded in accordance with the standard shown in Fig. 7. It is empirically known that the disturbance of the image is almost insensible, if the reproduced line width is less than 1.30 and the coarseness is less than 15 µm. Therefore, in evaluating the disturbance of the image, this range where the reproduced line width is less than 1.30 and the coarseness is less than 15 µm was assumed to be a good range, wherein grades G = 0 to 1 were set in accordance with the degree of goodness, and if the measured values were out of the good range, grades G = 1.5, 2, 3 and 4 were set in accordance with the degree of badness. According to this grading, it is understood that the images of the Examples 1 to 3 (represented by P1 to P3 in Fig. 8) are in the good range, namely, have the grade of 1 or less and the images of the Examples 4 to 6 (represented by P4 to P6 in Fig. 8) are in the bad range, namely, have the grades exceeding 1. When the degree to which the second toner was mixed with the first toner image was examined, it was confirmed that no phenomenon of the toner mixing was observed in Examples 1 to 3, while it was slightly observed in Example 4, and observed with eye in Examples 5 and 6. Embodiment 2 Fig. 9 shows a second embodiment of a two-color copying machine to which the image forming method of the present invention is adapted. In Fig. 9, the reference numeral 30 represents a negative charge type photosensitive drum serving as a latent image carrier having a photoconductive layer 30a on the periphery therof, 31 a charging corotron for charging the photosensitive drum 30 in advance, 32 an LED array for forming a first latent image, 33 an optical image formation system for forming a second latent image which consists of an exposure lamp 33a for irradiating an original 35 on a platen 34, a group of a plurality of mirrors 33b for introducing the light reflected from the original 35 to a predetermined position of the photosensitive drum 30 and an image formation lens for forming an optical image on the predetermined position of the photosensitive drum, 36 a first magnetic brush type developing unit using black toner which is negatively charged, 37 a second magnetic brush type developing unit using red toner which is positively charged, 38 a pre-transfer corotron for arranging the charged toners on the photosensitive drum 30 in the same polarity before a transfer step, 39 a transfer corotron for transferring the toner image of each color to a copying sheet 40, 41 a static elimination corotron for separating the copying sheet 40 from the photosensitive drum 30 after the transfer step, 42 a static elimination corotron for eliminating residual charges on the photosensitive drum 30 and residual toner charges before a cleaning step, 43 a cleaner for removing the residual toner on the photosensitive drum 30, 44 a static eliminating lamp for completely eliminating the residual charges on the photosensitive drum 30 before the next copying cycle, 45 a sheet supply tray accommodating the copying sheet 40, 46 a stabilizer for stabilizing the toner image on the copying sheet 40 which has passed through the transfer step, 47 a discharge tray for accommodating the discharged copying sheet 40 on which the original image has been transferred and which has passed through the stabilization step, and 48 a sheet conveying system for feeding the copying sheet 40 in the sheet supply tray 45 to a predetermined position for transfer at a predetermined timing and conveying the sheet to the discharge tray 47 through the stabilizer 46. In this embodiment, the second developing unit 37 is composed of a housing 51 which accommodates a developing roll 52, an agitator 53 for agitating a developer, a conveying paddle 54 for supplying the agitated developer g to the developing roll 52, a trimming bar 55 for controlling the trimming gap of the developer g supplied to the periphery of the developing roll 52 and a mixing plate 56 for returning the developer g scraped off by the trimming bar 55 to the side of the agitator 53, as shown in Fig. 10. The developing roll 52 is composed of a fixed sleeve 57 of a nonmagnetic material, and a magnet roll 58 which has a multiplicity of propulsion magnetic poles 58a and 58b mounted therearound and which is disposed in the fixed sleeve 57 so as to be rotatable at a predetermined speed. In this case, if it is assumed that the rotational speed of the photosensitive drum 30 is V P , and the moving speed of the developer g of the developing roll 52 is V DEVE , the condition 0.5 ≦ V DEVE /V p ≦ 2.0 is satisfied on the basis of the results of the later-described experiments. The fundamental structure of the first developing unit 36 is substantially the same as the second developing unit 37. Unlike the second developing unit 37, the developing roll 52 of the first developing unit 36 is composed of a rotary sleeve 59 and a magnet roll 60 which has a multiplicity of propulsion magnetic poles 58a and 58b mounted therearound and which is fixed inside the rotary sleeve 59. The operation of the two-color copying machine of this embodiment will now be explained. The negative charge type photosensitive drum 30 is first uniformly charged by the charging corotron 31 (Fig. 11(a)), and light is then projected by the LED array 32 in accordance with the image information to form the first negative latent image Z1 on the photosensitive drum 30 (Fig. 11(b)). While an appropriate developing bias VB1 is applied to the developing roll 52 of the first developing unit 36, the first negative latent image Z1 is developed by negatively charged black toner to form the first toner image T1 (Fig. 11(c)). After the second positive latent image Z2 (the absolute value of the potential VH2 of the background H2 is larger than the absolute value of the surface potential VT1 of the first toner image T1) corresponding to the image of the original 35 is formed on the photosensitive drum 30 by the optical image forming system 33 (Fig. 11(d)), the second positive latent image Z2 is developed by positively charged red toner to form the second toner image T2 while an appropriate developing bias VB2 is applied to the developing roll 52 of the second developing unit 37 (Fig. 11(e)). Thereafter, the toners T1 and T2 on the photosensitive drum 30 are arranged in the same polarity by the pre-transfer corotron 38 and the toner images T1 and T2 are transferred to the copying sheet 40 by the transfer corotron 39. The toner images T1 and T2 are stabilized through a predetermined stabilization step. In the above-described operational process, contrary to the embodiment, if a rotary sleeve 57′ and a fixed magnet roll 58′ are used as the developing roll 53 in the second developing step, as shown in Fig. 12(b), the group of developers g (carrier g c and toner g t ) in the state of erecting on the rotary sleeve 57′, i.e., the state indicated by the solid line falls down to the state indicated by the broken line and rises again to the state indicated by the one-dot chain line. The group of developers g repeat this movement like an inchworm while moving in the direction k of movement of the rotary sleeve 57′. The frictional force between the developers g and the photosensitive drum 30 therefore becomes comparatively large. In this embodiment, however, magnet roll 58 moves in the direction indicated by the arrow U1, as shown in Fig. 12(a), so that the group of the developers g (carrier g c and toner g t ) in the state of erecting on the fixed sleeve 57 revolves in the direction indicated by the arrow U2 at a predetermined speed V DEVE while each developer rotates on its axis. The frictional force between the group of the developers g and the photosensitive drum 30 is restricted to a small force, thereby effectively preventing the disturbance of the first toner image T1. In order to confirm the operational process described above, experiments for measuring the disturbance of the first toner image were carried out by varying the revolution number and the number of the poles of the magnet roll 58 among the parameters of the second developing unit 37, while fixing the parameters of the first developer 36. The conditions common to the experiments were as follows: * Photosensitive drum Negative charge type organic semiconductor Moving speed 100 mm/sec * First Developer Two component type (black toner negatively charged) A mixture of 95 parts by weight of a carrier obtained by coating iron powder with a polymethyl methacrylate copolymer and having an average particle diameter of 100 µm and 5 parts by weight of a toner obtained by dispersing 7 parts by weight of carbon black in 93 parts by weight of a styrene-n-butyl methacrylate copolymer (copolymerization ratio 80 : 20) and having an average particle diameter of 11 µm. * Second developer Two component type (red toner positively charged) A mixture of 90 parts by weight of a carrier obtained by mixing, melting, kneading and pulverizing a styrene-n-butyl methacrylate copolymer (density: 1.1 g/cm³) and cubic type magnetite (density: 4.8 g/cm³) in the ratio of 35/65 and having a density of 2.2 g/cm³ and an average particle diameter of 30 µm, and 10 parts by weight of a toner obtained by melting, kneading and pulverizing 92 parts by weight of a resin obtained by graft polymerization of a styrene butyl methacrylate copolymer with a low-molecular polyolefin and 8 parts by weight of a red pigment "Lithor Scarlet" (Trade Name: produced by BASF) and having an average particle diameter of 9.8 µm. * Potential conditions The first negative latent image Z1 : -60 V The background portion of the first negative latent image Z1 : -600 V The first developing bias VB1 : -400 V The second positive latent image Z2 : -580 V The background portion of the second positive latent image Z2 : -200 V The second developing bias VB2: -300 V * Parameters of the first developing unit Trimming gap: 0.6 mm Drum roll space: 0.8 mm Magnet set angle: +5° Diameter of the developing sleeve: 50 mm Structure of the magnet roll: Asymmetric 6 poles Magnetic force of the main pole: 750 Gauss * Parameters of the second developing unit Trimming gap: 0.6 mm Drum roll space: 1.0 mm Diameter of the developing sleeve: 50 mm Magnetic force of the main pole: 800 Gauss Under these conditions, the number of poles of the second developing unit 37 was charged to 8, 10 and 12 and the revolution number of the magnet roll 58 was varied to 5, 10 15, 25 and 30 (rps). The first toner image was a horizontal line image 250 µm in line width. When the ratio of the width of the line formed after conducting the second development process to the width of the line formed before conducting the second development process was within 1.1, the mark o ○ was given, when it was within 1.2, the mark ○ was given, and in the other cases, the mark x was given. The results are shown in Table 3. On the other hand, the experimental conditions represented by the ratio of the moving speed V DEVE of the developer and the moving speed V P of the photosensitive drum 30 are shown in Table 4. In Table 4, if it is assumed that the diameter of the magnet roll is D (mm), the number of poles N, the revolution number of the magnet roll Rm (rps) and the erection length of the developer ℓ (mm), V DEVE is approximately determined by the equation: V DEVE = (πD x NℓRM)/(πD-Nℓ)[mm/sec]. However, since the effective erection length is about 1 mm, it can be considered that πD » Nℓ , so that V DEVE is approximately determined by the equation: V DEVE = NRm. Revolution Number Number of Poles 8 10 12 5 0.4 0.5 0.6 10 0.8 1.0 1.2 15 1.2 1.5 1.8 20 1.6 2.0 2.4 25 2.0 2.5 3.0 30 2.4 3.0 3.6 In the Tables 3 and 4, assume now that the speed ratio of the moving speed V DEVE of the developer with respect to the rotational speed Vp of the photosensitive drum is m. In order to make the deviation of the line width of the first toner image within a range not more than 40% which is the acceptable deviation of the first toner image, it is required that the m satisfy the equation 0.5 ≦ m ≦ 2.0. Furthermore, in order to make the deviation of the line width of the first toner image within a range not more than 20 %, it is required that the m satisfy the equation 0.8 ≦ m ≦ 1.5. Embodiment 3 Fig. 13 shows a third embodiment of a two-color printer to which the image forming method of the present invention is adapted. In Fig. 13, the reference numeral 70 represents a positive charge type photosensitive drum (Se type in this embodiment) serving as a latent image holder having a photoconductive layer 70a on the periphery thereof, 71 a charging corotron, 72 a first LED array for forming a first latent image, 73 a first magnetic brush type developing unit using black toner which is negatively charged, 74 a recharging scorotron serving as a recharger for recharging the photosensitive drum 70, 75 a second LED array for forming a second latent image, 76 a second magnetic brush type developing unit using red toner which is positively charged, 77 a corotron for exposing and charging the photosensitive drum 70 simultaneously, 78 a transfer corotron, 79 a roll type recording sheet roll, 80 a guide roll for the recording sheet 79, 81 a static elimination corotron, 82 a cleaner and 83 a static eliminating lamp. In this embodiment, the exposing and charging corotron 77 discharges the photoconductive layer 70a of the photosensitive drum 70 by applying to the corotron 77 AC voltage on which a DC voltage having the same polarity as the photosensitive layer 70a is superposed while uniformly exposing the photoconductive layer 70a. Example of the discharging characteristic is shown in Fig. 14. In Fig. 14, the ordinate represents the current I flowing to the surface of the photoconductive layer by the discharging treatment, the abscissa represents the surface potential VPR of the photoconductive layer 70a. V0 represents the surface potential of the photoconductive layer 70a when I = 0. In discharging the photoconductive layer 70a, the potential V0 is set at a higher value in absolute value than the background potential. The operation of the two-color printer of this embodiment will now be explained. The photoconductive layer 70a of the photosensitive drum 70 which was rotating in the direction indicated by the arrow was first uniformly charged to +1300 (V) by the charging corotron 71 (Fig. 15(a)). The portion of the photosensitive drum 70 which corresponded to the first image is exposed by the first LED array 72 to obtain a positive latent image Z1 on the photoconductive layer 70a (Fig. 15(b)). The potential VZ1 of the first latent image Z1 after the exposure was +1200 (V) and the potential VH1 of the background portion H1 is +650 (V). Thereafter, under a developing bias VB1 of +800 (V), the first latent image Z1 is normally developed by black toner negatively charged by the first developing unit 73 to form a first toner image T1 (Fig. 15(b)). The symbol T1′ represents a first fog toner which adheres to the background portion. The photoconductive layer 70a is charged again by the recharging scorotron 74 so that the potential VT1 of the first toner image T1 was +600 (V) and the background potential VH2 is +500 (V) (Fig. 15(c)). The portion of the photosensitive drum 70 which corresponds to the second image was exposed by the second LED array 75 to form a negative latent image Z2 (Fig. 15(d)). The potential VZ2 of the second latent image Z2 after exposure is +100 (V). Thereafter, under a developing bias VB2 of +350 (V), the second latent image Z2 is reversely developed by the positively charged red toner by the second developing unit 76 to form a second toner image T2 (Fig. 15(d)). The symbol T2′ represents a second fog toner which adheres to the background portion. The photoconductive layer 70a is next subjected to discharging treatment under uniform exposure by the exposing and charging corotron 77. In this case, the background portion of the photoconductive layer 70a having no toner images T1 and T2 thereon is made photoconductive by the uniform exposure. However, at the portions of the toner images T1 and T2, since light is cut off by the toners, the photo-conductive layer 70a at those portions does not become photoconductive, so that the surface potential at the positions of the toner images T1 and T2 is kept higher than the background poten-tial (Fig. 15(e)). The discharging treatment was carried out by applying to the corotron 77 an AC voltage on wihch superposed is a DC voltage having the positive polarity which is the same as that of the photoconductive layer 70a. When V0 is set at a slightly higher value (about 50 (V)) in the absolute value than the background potential, the first and second toner images T1 and T2 at the image area are negatively charged, while the fog toners T1′ and T2′ at the background portion were positively charged (Fig. 15(f)). The toner images T1 and T2 are then transferred by the transfer corotron 78 to which a DC voltage having the opposite polarity to that of the toner at the image area is applied. As a result, the toner images T1 and T2 alone which have been arranged in the negative polarity are transferred to the recording sheet 79, thereby obtaining a good red and black image without fog. Additionally, in this embodiment, if it is so designed that the DC component of the voltage applied to the exposing and charging corotron 77 is variable, it is possible to vary V0 in accordance with a change in potential due to environmental change, so that it is possible to constantly obtain a two-color image having a good quality without being influenced by environmental change. Embodiment 4 Fig. 16 shows a fourth embodiment of a two-color printer to which the present invention is adapted. The fundamental structure thereof is substantially the same as that of the above-described Embodiment 3. Unlike the Embodiment 3, the recharging corotron 74 is not used, and in place of the exposing and discharging corotron 77, a pre-transfer exposure lamp 91 and a pre-transfer charging corotron 92 which are functionally separated from each other are used. The same numerals are provided for the elements which are the same as those in the Embodiment 3, and explanation thereof will be omitted. In this embodiment, in the first latent image formation process, the first LED array 72 exposes to obtain a negative image corresponding to the first image, and in the second latent image formation process, the second LED array 75 exposes to obtain a positive image corresponding to the second image. The first developing unit 73 carries positively charged black toner, while the second developing unit 76 carries negatively charged red toner. The operation of the two-color printer of this embodiment will now be explained. The photoconductive layer 70a of the photosensitive drum 70 is first uniformly charged to +1000 (V) by the charging corotron 71 (Fig. 17(a)). The portion of the photosensitive drum 70 which corresponds to the first image is exposed by the first LED array 72 to obtain a negative latent image Z1 on the photoconductive layer 70a (Fig. 17(b)). The potential VZ1 of the first latent image Z1 after the exposure is +250 (V) and the potential VH1 of the background portion H1 is +900 (V). Thereafter, under a developing bias VB1 of +750 (V) the first latent image Z1 is reversely developed by positively charged black toner by the first developing unit 73 to form a first toner image T1 (Fig. 17(b)). The symbol T1′ represents a first fog toner which adheres to the background portion. The portion of the photosensitive drum 70 which corresponded to the second image is exposed by the second LED array 75 to form a positive latent image Z2 (Fig. 17(c)). The potential VZ2 of the second latent image Z2 after the exposure is +800 (V), the background potential VH2 is 300 (V), and the surface potential VT1 of the first toner image T1 is 200 (V). Thereafter, under a developing bias VB2 of +450 (V), the second latent image Z2 is normally developed by negatively charged red toner by the second developing unit 76 to form a second toner image T2 (Fig. 17(c)). The symbol T2′ represents a second fog toner which adheres to the background portion. The photoconductive layer 70a was next subjected to discharging treatment by the uniform exposure by the pre-transfer exposure lamp 91 (Fig. 17(e)). The photoconductive layer 70a was next subjected to discharging treatment by the pre-transfer charging corotron 92. In this case, by substantially the same action as that in the Embodiment 3, the first and second toner images T1 and T2 at the image area are negatively charged, while the fog toners T1′ and T2′ at the background portion are positively charged (Fig. 17(e)). The toner images T1 and T2 are then transferred by the transfer corotron 78 to which a DC voltage having the opposite polarity to that of the toner at the image area is applied is applied. As a result, the toner images T1 and T2 alone which have been arranged in the negative polarity are transferred to the recording sheet 79, thereby obtaining a good red and black image without fog. As has been explained above, according to a method of and an apparatus for forming an image of the present invention, since toners having the opposite polarities are used to form toner images of two types and a force for preventing the disturbance of the first toner image is provided in the second toner image formation process, it is possible to form a good image based on the toner images of two types while effectively preventing the toners of two types from mixing and the first toner image from being disturbed. According to a method of forming an image in the present invention, it is possible to realize the formation of two types with good efficiency in the case of using a photosensitive material as a latent image holder or carrier. In particular, in the case where the first image is reversely developed and the second image is normally developed, if the photosensitive material is initially charged, it is possible to secure a sufficiently large contrast between the first and second latent images without the need for recharging in the middle course of processing, thereby facilitating the formation of an image having a sufficient density. According to the image forming apparatus of the Embodiment 2, since the constraining force of the magnetic brush with respect to the developer holder in the second developing means is weakened in the developing nip range on the basis of the field of a repulsion magnetic pole, the frictional force between the magnetic brush and the latent image holder in the developing nip range is suppressed, and the disturbance of the first toner image is safely prevented to that degree. Furthermore, according to an image forming apparatus as set forth in the Embodiment 2, since the second developing means is devised so as to suppress the frictional force between the magnetic brush and the latent image holder in the range which keeps the developing capacity, it is possible to safely prevent the disturbance of the first toner image without impairing the state of the formation of the second toner image. According to an image forming apparatus of the present invention, it is possible to transfer toner images having different polarities to a transfer medium with good efficiency by utilizing an electrostatic trans-fer system. In this case, particularly in the Embodiments 3 and 4, it is possible to transfer the toner at the image area alone by making the polarities of the toner at the image area and the toner at the back-ground portion different from each other, thereby enabling the formation of a good image without any fog. In particular, in the case where an AC voltage to which is superposed DC component having the same polarity as the charged polarity of the latent image carrier is applied to the charging means, it is possible to effectively make the polarities of the toner at the image area and the toner at the background portion different from each other.

Claims: An image forming method comprising the steps of: forming a negative latent image (Z1) on a suface of a latent image carrier (1); reversely developing said negative latent image (Z1) with a first toner (T1) while applying a developing bias voltage (VB1) lower in absolute value than a background potential (VH1) of said negative latent image (Z1) to form a first toner image (T1); forming a positive latent image (Z2) having a background potential (VH2) higher in absolute value than a toner layer potential (VT1) of said first toner image (T1) on the surface of the latent image carrier (1, 10); developing said positive latent image (Z2) with a second toner (T2) while applying a developing bias voltage (VB2) higher in absolute value than a background potential (VH2) of said positive latent image (Z2) to form a second toner image (T2); setting the polarities of said first and second toner images (T1, T2) to be the same; and simultaneously transferring said first and second toner images (T1, T2) to a transfer sheet (7). An image forming method according to claim 1, wherein the exposing light when forming said positive latent image is weaker than that when forming said negative latent image. An image forming method comprising the steps of: forming a positive latent image (Z1) on a surface of a latent image carrier (1); normally developing said positive latent image (Z1) with a first toner (T1) while applying a developing bias (VB1) higher in absolute value than a background potential (VH1) of said positive latent image (Z1) to form a first toner image (T1); lowering in absolute value a surface potential (VT1) of said first toner image (T1) without reversing a polarity of said first toner image (T1); forming a negative latent image (Z2) on the surface of the latent image carrier (1); reversely developing said negative latent image (Z2) with a second toner (T2) while applying a developing bias voltage (VB2) lower in absolute value than a background potential (VH2) of said negative latent image (Z2) to form a second toner image (T2); setting polarities of said first and second toner image (T1, T2) to be the same; and simultaneously transferring said first and second toner images (T1, T2) to a transfer sheet (7). An image forming apparatus comprising: a latent image carrier (1); first latent image forming means (2) for forming a negative latent image (Z1) on a surface of said latent image carrier (1); first developing means (3) for developing said negative latent image (Z1) by applying a developing bias voltage lower in absolute value than a background potential of said negative latent image (Z1) to form a first toner image (T1); second latent image forming means (4) for forming a positive latent image (Z2) having a background potential (VH2) higher in absolute value than a surface potential (VT1) of said first toner image (T1) on the surface of said latent image carrier (1); second developing means (5) for developing said positive latent (Z2) image by applying a developing bias voltage (VB2) higher in absolute value than a background potential (VH2) of said positive latent image (Z2) to form a second toner image (T2); pre-transfer teatment means for setting polarities of said first and second toner images (T1, T2) to be the same; and transfer means (6) for simultaneously transferring said first and second toner images (T1, T2). An image forming apparatus comprising: a latent image carrier (1); first latent image forming means (2) for forming a positive latent image (Z1) on a surface of said latent image carrier (1); first developing means (3) for developing said positive latent image (Z1) by applying a developing bias voltage higher in absolute value than a background potential of said positive latent image (Z1) to form a first toner image (T1); charging means for lowering a surface potential of said first toner image (T1) in absolute value without reversing a polarity of said first toner image (T1); second latent image forming means (4) for forming a negative latent image (Z2) on the surface of said latent image carrier (1); second developing means (5) for developing said negative latent image (Z2) by applying a developing bias (VB2) voltage lower in absolute value than a background potential (VH2) of said negative latent image (Z2) to form a second toner image (T2); pre-transfer treatment means (16) for setting polarities of said first and second toner images (T1, T2) to be the same; and transfer means (6) for simultaneously transferring said first and second toner images (T1, T2). An image forming apparatus according to claim 4 or claim 5, wherein said second developing means (5) includes a developer holder comprising a nonmagnetic rotary sleeve (57) with a magnet roll (58) fixed therein, said magnet roll (58) having a repulsion magnetic pole in correspondence with a developing nip range. An image forming apparatus according to claim 4 or claim 5, wherein said second developing means (5) includes a developer holder comprising a nonmagnetic fixed sleeve (57) with a magnetic roll (58) rotatably disposed therein so as to satisfy the equation: 0.5 ≦ V DEVE /V P ≦ 2.0 where the moving speed of said developer is V DEDV and the rotational speed of said latent image carrier (1) is V P .

Inventors: ADACHI KOJI, FURUYA NOBUMASA, MARUYAMA KAZUO, NOAMI TSUNEO, SUMIKAWA TAKESHI, YAMAMOTO TOSHIRO, ADACHI, KOJI, FURUYA, NOBUMASA, MARUYAMA, KAZUO, NOAMI, TSUNEO, SUMIKAWA, TAKESHI, YAMAMOTO, TOSHIRO
Applicants: FUJI XEROX CO LTD, FUJI XEROX CO., LTD.
Application year: 1988
Main classification: G03G15
IPC codes: G03G13, G03G15