Image forming apparatus

Abstract: In an image forming apparatus of the present invention, a resistant layer 13 of each writing electrode 3b is formed substantially in a semi-circular convex shape projecting upwardly. Therefore, the top of the resistant layer 13 is a spherical surface so that the resistant layer 13 is in point contact with the charged layer 2d of the latent image carrier 2. Because of point contacts, foreign matters adhering to the surface of the latent image carrier 2 are easily allowed to pass, thereby preventing the occurrence of filming on the surface of the latent image carrier 2.

Description: BACKGROUND OF THE INVENTION The present invention relates to an image forming apparatus which forms an electrostatic latent image on a latent image carrier by using writing electrodes of a writing device, thereby forming the image. In a conventional image forming apparatus such as an electrostatic copying machine and a printer, the surface of a photoreceptor (photosensitive member) is uniformly charged by a charging device and the charged surface is then exposed to light from an exposure device such as laser beam or LED light, whereby a latent image is written on the surface of the photoreceptor. Then, the latent image on the surface of the photoreceptor is developed by a developing device to form a developing powder image on the surface of the photoreceptor. The developing powder image is transferred to a receiving medium such as a paper, thereby forming the image. In such conventional image forming apparatus, the exposure device as a writing device for electrostatic latent image comprises a laser beam generating device or a LED light generating device. Therefore, the entire image forming apparatus should be large and complex. Therefore, an image forming apparatus has been proposed in Japanese Patent Publication No. S63-45104 (hereinafter, '104B publication) which employs electrodes, as a writing device for forming an electrostatic latent image, to write an electrostatic latent image on a surface of a latent image carrier without using laser beams and LED lights. The image forming apparatus disclosed in the '104B publication is provided with a multistylus having a large number of needle electrodes. The needle electrodes are just arranged in contact with an inorganic glass layer on the surface of the latent image carrier. In accordance with an input signal for image information, voltages are selectively applied to corresponding ones of the needle electrodes of the multistylus, whereby the electrostatic latent image can be formed on the latent image carrier. Since the image forming apparatus according to the '104B publication does not use an exposure device conventionally used as a writing device, the invention of this publication can provide an image forming apparatus which is relatively small in size and relatively simple in structure. In addition, an image forming apparatus has been proposed in Japanese Unexamined Patent Publication No. H06-166206 (hereinafter, '206A publication), comprising ion control electrodes which are disposed on a front end portion of an insulating substrate and are arranged in non-contact with a latent image carrier, wherein the ion control electrodes control ions generated by a corona discharger so as to write an electrostatic latent image on the latent image carrier. Since the image forming apparatus according to the '206A publication also does not use an exposure device as a writing device, the invention of this publication can provide an image forming apparatus which is relatively small in size and relatively simple in structure. However, in the image forming apparatus according to the '104B publication, the large number of needle electrodes of the multistylus are just arranged in contact with the inorganic glass layer on the surface of the latent image carrier. It is difficult to keep the stable contact between the needle electrodes and the inorganic glass layer on the surface of the latent image carrier. Accordingly, it is difficult to stably apply charge to the surface of the latent image carrier. This means that it is hard to obtain a high quality image. Moreover, it is unavoidable to employ an inorganic glass layer on the surface of the latent image carrier for protecting the surface of the latent image carrier from damage due to contacts of a large number of the needle electrodes. This makes the structure of the latent image carrier more complex. In addition, since the inorganic glass layer has quite well physical adsorbed water characteristic, moisture is easily adsorbed by the surface of the inorganic glass layer. Due to the moisture, the electrical conductivity of the glass surface is increased so that electrostatic charge on the latent image carrier should leak. Therefore, the image forming apparatus should be provided with a means for drying the surface of the latent image carrier with adsorbed moisture in order to prevent the apparatus from being affected by absorbed water. This not only makes the apparatus larger but also increases the number of parts, leading to problems of making the structure further complex and increasing the cost. Since the large number of needle electrodes discharge, the apparatus has another problem that there is a high possibility of generation of ozone (O 3 ). The presence of ozone may not only produce rusts on parts in the apparatus but also melt resin parts because ozone reacts with NO x to generate nitrous acid (HNO 3 ). Again ozone may give an offensive smell. Therefore, the image forming apparatus should be provided with an ventilation system including a duct and an ozone filter which sufficiently exhausts ozone from the inside of the apparatus. This also not only makes the apparatus larger but also increases the number of parts, leading to problems of making the structure further complex and increasing the cost. On the other hand, in the image forming apparatus according to the '206A publication, ions produced by the corona discharger are controlled by the ion control electrodes. This means that the apparatus is structured not to directly inject electric charge to the latent image carrier. The invention of the '206A publication has problems of not only making the image forming apparatus larger and but also making the structure complex. Since the application of charge is conducted by ions, it is difficult to stably write a latent image on the latent image carrier. Further, since the generation of ions essentially generates ozone, there are problems similar to those described with regard to the image forming apparatus according to '104B publication. US 5,842,087 discloses an image forming apparatus according to the preamble of claim 1. SUMMARY OF THE INVENTION US 3,624,661 describes an electrographic printing system with plural staggered electrode rows. Another recording head for an electrostatic printing apparatus is known from US 4,233,611. It is an object of the present invention to provide an image forming apparatus capable of more stably writing an electrostatic latent image and yet achieving reduction in size and reduction in the number of parts thereof so as to have more simple and low-priced structure. It is another object of the present invention to provide an image forming apparatus capable of further preventing generation of ozone. In order to achieve these objects, the present invention provides an image forming apparatus which comprises the features of claim 1. Preferably, said each writing electrode is formed in any one of configurations including a portion of sphere, a circular column, a cone, a truncated cone, an elliptic column (column of which cross section is elliptic), an elliptic cone (cone of which cross section is elliptic), a truncated elliptic cone (truncated cone of which cross section is elliptic), an oval column (column of which cross section is oval), an oval cone (cone of which cross section is oval), a truncated oval cone (truncated cone of which cross section is oval), a triangle column, a triangle pyramid, a truncated triangle pyramid, a square column, a square pyramid, a truncated square pyramid, a polygonal column having five corners or more, a polygonal pyramid having five corners or more, and a truncated polygonal pyramid having five corners or more. Preferably, at least said each writing electrode is coated with a protective layer. Preferably, at least a portion of said each writing electrode confronting said latent image carrier is made of a material easily to wear. Preferably, said developing device is a developing device for developing said electrostatic latent image with developing powder consisting of a single component; by further comprising a transferring device for transferring a developing powder image on said latent image carrier, developed by said developing device, to a receiving medium; and in that residual developing powder left on said latent image carrier after transfer is adapted to be charged to have the same polarity as the original polarity of said developing powder consisting of a single component. Preferably, a large number of microscopic particles are interposed at least between said writing electrodes and said latent image carrier to allow free rolling of said microscopic particles, wherein said microscopic particles are adapted to be charged at least to have the same polarity as the original polarity of said developing powder before developing of said electrostatic latent image. Preferably, the apparatus further comprises a charge control device for making said latent image carrier into a uniformly charged state, wherein residual developing powder left on said latent image carrier after transfer is adapted to be charged to have the same polarity as the original polarity of said developing powder consisting of a single component at the same time when said charge control device makes said latent image carrier into the uniformly charged state. Moreover, preferably, said developing is reverse developing. In the image forming apparatus of the present invention having the aforementioned structure, a convexity of each writing electrode is in contact with a latent image carrier so that the surface of the writing electrode is not entirely in contact with the latent image carrier, thereby allowing easy passing of foreign matters adhering to the surface of the latent image carrier and thus preventing the filming of the surface of the latent image carrier. In addition, the writing electrodes are supported by a flexible substrate, thereby stabilizing the positions of the writing electrodes relative to the latent image carrier and thus stably and reliably conducting the application or removal of charge by the writing electrodes relative to the latent image carrier. Therefore, stable writing of an electrostatic latent image onto the latent image carrier is achieved, thus reliably obtaining a high quality image with high precision. Since the writing electrodes can be securely kept in contact with the latent image carrier with a small pressing force by the flexible substrate, the gap (space) between the writing electrodes and the latent image carrier can be eliminated. No gap practically reduces the possibility that air existing in the gap is undesirably ionized, thereby further reducing the generation of ozone and enabling the formation of an electrostatic latent image with low potential. In addition, the latent image carrier can be prevented from being damaged by the writing electrodes, thus improving the durability of the latent image carrier. Further, since the writing device employs only the writing electrodes without using a laser beam generating device or a LED light generating device which is large in size as conventionally used, the apparatus size can be reduced and the number of parts can also be reduced, thereby obtaining an image forming apparatus which is simple and low-price. In the present invention, since the convexity of the writing electrode is allowed to be formed in various configurations, the writing electrode is flexible to be employed in various types of image forming apparatus. In particular, when the convexity of the writing electrode is formed in a portion of sphere, a cone, an elliptic cone, an oval cone, a triangle pyramid, a square pyramid, or a polygonal pyramid having five corners or more, the writing electrode and the latent image carrier are in point contact, thereby further securely allowing foreign matters adhering to the surface of the latent image carrier to pass through. When the convexity of the writing electrode is formed in a circular column, a truncated cone, an elliptic column, a truncated elliptic cone, an oval column, a truncated oval cone, a triangle column, a truncated triangle pyramid, a square column of which cross section is a parallelogram or a trapezoid, a truncated square pyramid of which cross section is a parallelogram or a trapezoid, a polygonal column (having five corners or more), and a truncated polygonal pyramid (having five corners or more), the writing electrode has side faces inclined against the feeding direction, whereby foreign matters adhering to the surface of the latent image carrier can easily pass through because the foreign matters easily slide along the inclined faces. In the present invention, at least the writing electrodes are coated with the protective layers. The protective layers prevent wear of the writing electrodes and prevent foreign matters from adhering to the writing electrodes. In the present invention, since the portion of the writing electrode confronting the latent image carrier is made of material easily to wear, the surface of the writing electrode should wear due to the contact relative to the latent image carrier so as to have a fresh surface so that the surface of the writing electrode can be kept fresh, thus preventing the filming of the writing electrode. In the present invention, residual developing powder which is left on the latent image carrier after the transfer is charged to have the same polarity as the original polarity of the developing powder consisting of a single component. Therefore, the residual developing powder, placed on non-image portions of the latent image carrier and charged as mentioned above, can be moved to a developing roller during the developing, while the residual developing powder, placed on image portions of the latent image carrier and charge as mentioned above, still remains on the latent image carrier as developing powder for subsequent developing . That is, this apparatus can form an image in the cleaner-less cleaning method in which the developing of a latent image and the cleaning of the latent image carrier can be simultaneously conducted. In the present invention, employment of the writing device achieves reduction in size and simplification of the structure of the image forming apparatus. In addition, since it is a cleaner-less image forming apparatus without a cleaning device, further simple structure can be achieved. In the present invention, a large number of microscopic particles are interposed at least between the writing electrodes and the latent image carrier. With the aid of the microscopic particles, foreign matters adhering to the surface of the latent image carrier can easily pass through, thus preventing the filming on the surface of the latent image carrier and on the surfaces of the writing electrodes. In addition, Free rolling of the microscopic particles reduces the friction between the writing electrodes and the latent image carrier, leading to reduction in torque for rotating the latent image carrier. Since the charge of the microscopic particles is adapted to be charged to have the same polarity as the original polarity of the developing power, consisting of a single component, of the developing device, the residual developing powder on non-image portions of the latent image carrier can be further effectively removed or collected by the microscopic particles, placed on the non-image portions of the latent image carrier and charged as mentioned above. Interposing the microscopic particles between the writing electrodes and the latent image carrier enables to eliminate the necessity of the charge control device, thereby further simplifying the structure of the image forming apparatus without cleaning device. In the present invention, since residual developing powder which is left on the latent image carrier after the transfer is charged to have the same polarity as the original polarity thereof at the same time when the latent image carrier is uniformly charged by the charge control device, application of charge to the residual developing powder can be easily conducted. In the present invention, the developing is conducted by the reverse developing method. In this reverse developing method, the residual developing powder can be uniformed to have the same polarity of the developing powder during the process of uniformly charging the latent image carrier, thereby further easily and effectively conducting the cleaning at the same time of developing. Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification. . The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic illustration of the basic structure of an image forming apparatus in accordance with the present invention; Figs. 2(a)-2(h) are views each illustrating an example of the basic process of forming an image in the image forming apparatus of the present invention; Figs. 3(a)-3(f) are views for explaining the principle of writing an electrostatic latent image by writing electrodes of a writing device through application or removal of charge, wherein Fig. 3(a) is an enlarged view of a portion where a writing electrode is in contact with the latent image carrier, Fig. 3(b) is a diagram of an electrical equivalent circuit of the contact portion, and Figs. 3(c)-3(f) are graphs each showing the relation between each parameter and the surface potential of the latent image carrier; Figs. 4(a)-4(c) are views for explaining the application or removal of charge relative to the latent image carrier, wherein Fig. 4(a) is a view for explaining the application or removal of charge relative to the latent image carrier via the charge-transfer, Fig. 4(b) is a view for explaining the - application or removal of charge relative to the latent image carrier via the discharge, and Fig. 4(c) is a graph for explaining Paschen's law; Fig. 5 is a schematic illustration showing an example of the writing device, as seen in an axial direction of the latent image carrier; Fig. 6 is a perspective view partially showing the writing head in the image forming apparatus of the embodiment shown in Fig. 3 through 5; Figs. 7(a) through 7(i) are views for explaining one example of the method for manufacturing the writing head shown in Fig. 6; Fig. 8 is a perspective view similar to Fig. 6, but partially showing another example of the writing head in the image forming apparatus of the embodiment shown in Fig. 3 through Fig. 5; Fig. 9 is a perspective view similar to Fig. 6, but partially showing another example of the writing head in the image forming apparatus of the embodiment shown in Fig. 3 through Fig. 5; Fig. 10 is a perspective view similar to Fig. 6, but partially showing another example of the writing head in the image forming apparatus of the embodiment shown in Fig. 3 through Fig. 5; Fig. 11 is a schematic illustration showing another example of the writing device, as seen in an axial direction of the latent image carrier; Figs. 12(a)-12(c) show array patterns for aligning a plurality of writing electrodes in the axial direction of the latent image carrier, wherein Fig. 12(a) is a view showing the simplest array pattern for writing electrodes and Figs. 12(b) and 12(c) are views showing array patterns for writing electrodes which achieve to solve problems of the array pattern shown in Fig. 12(a); Fig. 13 is a view for explaining the state that adjacent writing electrodes are partially overlapped with each other as seen in the rotational direction of the latent image carrier; Fig. 14 is a view for illustrating the array pattern for the writing electrodes and the wiring pattern for drivers; Fig. 15 is a view showing still another example of the array pattern for the writing electrodes; Figs. 16(a)-16(d) are views showing still another examples of the array pattern for the writing electrodes; Figs. 17(a)-17(d) are sectional views each showing an example of the writing electrodes of the writing device; Fig. 18 is a diagram showing a switching circuit for switching the voltage to be supplied to the writing electrodes between the predetermined voltage V 0 and the ground voltage V 1 ; Figs. 19(a)-19(c) show profiles when the supply voltage for each electrode is selectively controlled into the predetermined voltage V 0 or the ground voltage V 1 by switching operation of the corresponding high voltage switch, wherein Fig. 19(a) is a diagram showing the voltage profiles of the respective electrodes, Fig. 19(b) is a diagram showing a developing powder image obtained by normal developing with the voltage profiles shown in Fig. 19(a), and Fig. 19(c) is a diagram showing a developing powder image obtained by reverse developing with the voltage profiles shown in Fig. 19(a); Fig. 20 is a view similar to Fig. 5 but schematically and partially showing another example of the image forming apparatus according to the present invention; Fig. 21 is a view schematically showing an example of an image forming apparatus employing the writing device of the present invention; Figs. 22(a)-22(c) are views for explaining parts of the cleaner-less cleaning method employing reverse developing; Figs. 23(a)-23(c) are views for explaining the other parts of the cleaner-less cleaning method employing reverse developing; and Fig. 24 is a view showing another embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention will be described hereinafter with reference to the drawings. Fig. 1 is a schematic illustration of the basic structure of an image forming apparatus in accordance with the present invention. As shown in Fig. 1, an image forming apparatus 1 according to the present invention comprises, at least, a latent image carrier 2 on which an electrostatic latent image is formed, a writing device 3 (hereinafter, sometimes referred to as "writing head") which is arranged in contact with the latent image carrier 2 to write the electrostatic latent image on the latent image carrier 2, a developing device 4 which develops the electrostatic latent image on the latent image carrier 2 with developing powder carried and conveyed by a developing powder carrier 4a (developing roller), a transferring device 6 which transfers a developing powder image on the latent image carrier 2, developed by the developing device 4, to a receiving medium 5 such as a paper, and a charge control device 7 which makes the surface of the latent image carrier 2 into the uniformly charged state by removing any residual charge from the latent image carrier 2 after the transfer of the latent image or by charging (i.e. applying charge to) the latent image carrier 2 after the transfer of the electrostatic latent image. Though the following description will be made assuming that the latent image carrier 2 is grounded, this is for the purpose of facilitating the description only and not of limitation. That is, the latent image carrier 2 may not be grounded. The writing head 3 comprises a flexible substrate 3a, having high insulation property and being relatively soft and elastic, such as a FPC (Flexible Print Circuit: hereinafter, referred to as "FPC") or a PET (polyethylene terephthalate: hereinafter, referred to as "PET"), and writing electrodes 3b which are supported by the substrate 3a and which are pressed lightly against the latent image carrier 2 with weak elastic restoring force created by deflection of the substrate 3a so that the writing electrodes 3b are in contact with the latent image carrier 2 so as to write the electrostatic latent image. In the image forming apparatus 1 having a structure as mentioned above, after the surface of the latent image carrier 2 is made into the uniformly charged state by the charge control device 7, an electrostatic latent image is written on the uniformly charged surface of the latent image carrier 2 by the writing head 3 which is in contact with the latent image carrier 2. Then, the electrostatic latent image on the latent image carrier 2 is developed with developing powder of the developing device 4 to form a developing powder image and the developing powder image is transferred to the receiving medium 5 by the transferring device 6. It should be noted that the uniformly charged state includes a state where there is neither positive (+) charge nor negative (-) charge i.e. no charge is uniformly applied to the latent image carrier 2 by removing charge from the latent image carrier 2. Figs. 2(a)-2(h) are views each illustrating an example of the basic process of forming an image in the image forming apparatus 1 of the present invention. As the basic process of forming an image in the image forming apparatus 1 of the present invention, there are four types as follows: (1) making uniformly charged state by removal of charge writing by contact application of charge- normal developing; (2) making uniformly charged state by removal of charge - writing by contact application of charge - reversal developing; (3) making uniformly charged state by application of charge - writing by contact removal of charge - normal developing; and (4) making uniformly charged state by application of charge - writing by contact removal of charge - reversal developing. (1) making uniformly charged state by removal of charge -writing by contact application of charge - normal developing A process illustrated in Fig. 2(a) is an example of this image forming process. As shown in Fig. 2(a), in this example, a photoreceptor 2a is employed as the latent image carrier 2 and a charge removing lump 7a is employed as the charge control device 7. The electrodes 3b of the writing device 3 are in contact with the photoreceptor 2a so that positive (+) charge is mainly transferred (that is, injected) from the writing electrodes 3b to image portions of the photoreceptor 2a, whereby the image portions of the photoreceptor 2a are positively (+) charged. In this way, an electrostatic latent image is written on the photoreceptor 2a. In addition, a bias voltage composed of an alternating current superimposed on a direct current of a negative (-) polarity is applied to the developing powder carrier 4a such as an image developing roll of the developing device 4, as in conventional ones. Accordingly, the developing powder carrier 4a conveys negatively (-) charged developing powder 8 to the photoreceptor 2a. It should be noted that a bias voltage composed only of a direct current of a negative (-) polarity may be applied to the developing powder carrier 4a. In the image forming process of this example, the charge removing lump 7a removes charge from the surface of the photoreceptor 2a to make the surface into the uniformly charged state with nearly 0V (zero volt) and, after that, the image portions of the photoreceptor 2a are positively (+) charged by the writing electrodes 3b of the writing device 3, thereby writing an electrostatic latent image onto the photoreceptor 2a. Then, negatively (-) charged developing powder 8 conveyed by the developing powder carrier 4a of the developing device 4 adheres to the positively (+) charged image portions of the photoreceptor 2a, thereby normally developing the electrostatic latent image. A process illustrated in Fig. 2(b) is another example of this image forming process. As shown in Fig. 2(b), in this example, a dielectric body 2b is employed as the latent image carrier 2 and a charge removing roller 7b is employed as the charge control device 7. As in conventional ones, a bias voltage composed of a direct current of a negative (-) polarity may be applied to the developing powder carrier 4a of the developing device 4. It should be noted that a bias voltage composed of an alternating current superimposed on a direct current of a negative (-) polarity may be applied to the developing powder carrier 4a. On the other hand, a bias voltage composed of an alternating current is applied to the charge removing roller 7b. Other structures of this example are the same as those of the aforementioned example shown in Fig. 2(a). In the image forming process of this example, the charge removing roller 7b is in contact with the dielectric body 2b so as to remove charge from the surface of the dielectric body 2b to make the surface into the uniformly charged state with nearly 0V (zero volt). The image forming actions after that are the same as those of the aforementioned example shown in Fig. 2(a), except that the dielectric body 2b is used instead of the photoreceptor 2a. (2) making uniformly charged state by removal of charge - writing by contact application of charge - reversal developing A process shown in Fig. 2(c) is an example of this image forming process. As shown in Fig. 2(c), in this example, a photoreceptor 2a is employed as the latent image carrier 2 and a charge removing lump 7a is employed as the charge control device 7 just like the example shown in Fig. 2(a). The writing electrodes 3b of the writing device 3 are in contact with the photoreceptor 2a so that negative (-) charge is mainly transferred (that is, injected) from the writing electrodes 3b to non-image portions of the photoreceptor 2a, whereby the non-image portions of the photoreceptor 2a are negatively (-) charged. Other structures of this example are the same as those of the aforementioned example shown in Fig. 2(a). In the image forming process of this example, the charge removing lump 7a removes charge from the surface of the photoreceptor 2a to make the surface into the uniformly charged state with nearly 0V (zero volt) and, after that, the non-image portions of the photoreceptor 2a are negatively (-) charged by the writing electrodes 3b of the writing device 3, thereby writing an electrostatic latent image onto the photoreceptor 2a. Then, negatively (-) charged developing powder 8 conveyed by the developing powder carrier 4a of the developing device 4 adheres to portions, not negatively (-) charged and having nearly 0V (zero volt), of the photoreceptor 2a, thereby reversely developing the electrostatic latent image. A process illustrated in Fig. 2(d) is another example of this image forming process. As shown in Fig. 2(d), in this example, a dielectric body 2b is employed as the latent image carrier 2 and a charge removing roller 7b is employed as the charge control device 7 just like the example shown in Fig. 2(b). The writing electrodes 3b of the writing device 3 are arranged in contact with the dielectric body 2b to negatively (-) charge non-image portions of the dielectric body 2b. Other structures of this example are the same as those of the aforementioned example shown in Fig. 2(b). In the image forming process of this example, the charge removing roller 7b is in contact with the dielectric body 2b so as to remove charge from the surface of the dielectric body 2b to make the surface into the uniformly charged state with nearly 0V (zero volt). The image forming actions after that are the same as those of the aforementioned example shown in Fig. 2(c), except that the dielectric body 2b is used instead of the photoreceptor 2a. (3) making uniformly charged state by application of charge - writing by contact removal of charge - normal developing A process shown in Fig. 2(e) is an example of this image forming process. As shown in Fig. 2(e), in this example, a photoreceptor 2a is employed as the latent image carrier 2 and a charging roller 7c is employed as the charge control device 7. A bias voltage composed of an alternating current superimposed on a direct current of a positive (+) polarity is applied to the charging roller 7c so that the charging roller 7c uniformly positively (+) charges the surface of the photoreceptor 2a. It should be noted that a bias voltage composed only of a direct current of a positive (+) polarity may be applied to the charging roller 7c. In addition, the writing electrodes 3b of the writing device 3 are in contact with the photoreceptor 2a so that positive (+) charge is mainly transferred (that is, extracted) from the non-image portions of the photoreceptor 2a to the writing electrodes 3b, whereby positive (+) charge is removed from the non-image portions of the photoreceptor 2a. Other structures of this example are the same as those of the aforementioned example shown in Fig. 2(a). In the image forming process of this example, the charging roller 7c is arranged in contact with the photoreceptor 2a to positively (+) charge the surface of the photoreceptor 2a to make the surface into the uniformly charged state with a predetermined voltage and, after that, positive (+) charge is removed from the non-image portions of the photoreceptor 2a by the writing electrodes 3b of the writing device 3, thereby writing an electrostatic latent image onto the photoreceptor 2a. Then, negatively (-) charged developing powder 8 conveyed by the developing powder carrier 4a of the developing device 4 adheres to the image portions, positively (+) charged, of the photoreceptor 2a, thereby normally developing the electrostatic latent image. A process illustrated in Fig. 2(f) is another example of this image forming process. As shown in Fig. 2(f), in this example, a dielectric body 2b is employed as the latent image carrier 2 and a corona charging device 7d is employed as the charge control device 7. A bias voltage composed of a direct current of a negative (-) polarity or a bias voltage composed of an alternating current superimposed on a direct current of a negative (-) polarity is applied to the corona charging device 7d, but not illustrated. The writing electrodes 3b of the writing device 3 are arranged in contact with the dielectric body 2b to remove negative (-) charge from the non-image portions of the dielectric body 2b. Moreover, a bias voltage composed of a direct current of a positive (+) polarity is applied to the developing powder carrier 4a so that the developing powder carrier 4a conveys positively (+) charged developing powder 8 to the dielectric body 2b. It should be noted that a bias voltage composed of an alternating current superimposed on a direct current of a positive (+) polarity may be applied to the developing powder carrier 4a. Other structures of this example are the same as those of the aforementioned example shown in Fig. 2(b). In the image forming process of this example, the surface of the dielectric body 2b is negatively (-) charged by the corona charging device 7d to make the surface of the dielectric body 2b into the uniformly charged state with the predetermined voltage and, after that, negative (-) charge is removed from the non-image portions of the dielectric body 2b by the writing electrodes 3b of the writing device 3, thereby writing an electrostatic latent image on the dielectric body 2b. Then, positively (+) charged developing powder 8 conveyed by the developing powder carrier 4a of the developing device 4 adheres to the image portions, negatively (-) charged, of the dielectric body 2b, thereby normally developing the electrostatic latent image. (4) making uniformly charged state by application of charge - writing by contact removal of charge - reversal developing A process shown in Fig. 2(g) is an example of this image forming process. As shown in Fig. 2(g), in this example, a photoreceptor 2a is employed as the latent image carrier 2 and a charging roller 7c is employed as the charge control device 7. A bias voltage composed of an alternating current superimposed on a direct current of a negative (-) polarity is applied to the charging roller 7c so that the charging roller 7c uniformly negatively (-) charges the surface of the photoreceptor 2a. It should be noted that a bias voltage composed only of a direct current of a negative (-) polarity may be applied to the charging roller 7c. The writing electrodes 3b of the writing device 3 are in contact with the photoreceptor 2a so that negative (-) charge is transferred (that is, extracted) from the image portions of the photoreceptor 2a to the writing electrodes 3b, whereby negative (-) charge is removed from the image portions of the photoreceptor 2a. Other structures of this example are the same as those of the aforementioned example shown in Fig. 2(a). In the image forming process of this example, the charging roller 7c is arranged in contact with the photoreceptor 2a to negatively (-) charge the surface of the photoreceptor 2a to make the surface into the uniformly charged state with a predetermined voltage and, after that, negative (-) charge is removed from the image portions of the photoreceptor 2a by the writing electrodes 3b of the writing device 3, thereby writing an electrostatic latent image onto the photoreceptor 2a. Then, negatively (-) charged developing powder 8 conveyed by the developing roller 4a of the developing device 4 adheres to the image portions, not negatively (-) charged, of the photoreceptor 2a, thereby reversely developing the electrostatic latent image. A process illustrated in Fig. 2(h) is another example of this image forming process. As shown in Fig. 2(h), in this example, a dielectric body 2b is employed as the latent image carrier 2 and a corona charging device 7d is employed as the charge control device 7. A bias voltage composed of a direct current of a positive (+) polarity or a bias voltage composed of an alternating current superimposed on a direct current of a positive (+) polarity is applied to the corona charging device 7d, but not illustrated. Other structures of this example are the same as those of the aforementioned example shown in Fig. 2(f). In the image forming process of this example, the surface of the dielectric body 2b is positively (+) charged by the corona charging device 7d to make the surface of the dielectric body 2b into the uniformly charged state with the predetermined voltage and, after that, positive (+) charge is removed from the image portions of the dielectric body 2b by the writing electrodes 3b of the writing device 3, thereby writing an electrostatic latent image onto the dielectric body 2b. Then, positively (+) charged developing powder 8 conveyed by the developing roller 4a of the developing device 4 adheres to the image portions, not positively (+) charged, of the dielectric body 2b, thereby reversely developing the electrostatic latent image. Figs. 3(a)-3(f) are views for explaining the principle of writing an electrostatic latent image by the writing electrodes 3b of the writing device 3 through application or removal of charge, wherein Fig. 3(a) is an enlarged view of a contact portion where a writing electrode 3b is in contact with the latent image carrier 2, Fig. 3(b) is a diagram of an electrical equivalent circuit of the contact portion, and Figs. 3(c)-3(f) are graphs each showing the relation between each parameter and the surface potential of the latent image carrier 2. As shown in Fig. 3(a), the latent image carrier 2 comprises a base member 2c which is made of a conductive material such as aluminum and is grounded and an insulating charged layer 2d formed on the outer periphery of the base member 2c. The writing electrodes 3b supported by the substrate 3a made of FPC or the like of the writing device 3 are in contact with the charged layer 2d with a predetermined small pressing force and the latent image carrier 2 travels (rotates) at a predetermined speed "v". As the aforementioned small pressing force, 10N or less per 300 mm in width, that is, a linear load of 0.03N/mm or less is preferable for stabilizing the contact between the writing electrodes 3b and the latent image carrier 2 and for stabilizing the charge-transfer therebetween. In view of abrasion, it is preferable to achieve the smallest possible linear load while keeping the contact stability. Either of a predetermined high voltage V 0 and a predetermined low voltage V 1 is selectively impressed to the writing electrodes 3b through the substrate 3a (as mentioned, since there are positive and negative charges, the high voltage is a voltage having a high absolute value and the low voltage is a voltage of the same polarity as the high voltage and having a low absolute value or 0V (zero volt). In the description of the present invention in this specification, the low voltage is a ground voltage. In the following description, therefore, the high voltage V 0 is referred to as the predetermined voltage V 0 and the low voltage V 1 is referred to as the ground voltage V 1 . It should be understood that the ground voltage V 1 is 0V (zero volt.) That is, the contact portion (nip) between each writing electrode 3b and the latent image carrier 2 is provided with an electrical equivalent circuit shown in Fig. 3(b). In Fig. 3(b), "R" designates the resistance of the writing electrode 3b and "C" designates the capacity of the latent image carrier 2. The resistance R of the writing electrode 3b is selectively switched to be connected to the A side of the predetermined voltage V 0 of a negative (-) polarity or to the B side of the ground voltage V 1 . Fig. 3(c) shows the relation between the resistance R of the writing electrode 3b and the surface potential of the latent image carrier 2. The aforementioned relation when the writing electrode 3b is connected to the A side in the electrical equivalent circuit to impress the predetermined voltage V 0 of a negative (-) polarity to the writing electrode 3b is represented by a solid line in Fig. 3(c). As shown by the solid line in Fig. 3(c), the surface potential of the latent image carrier 2 is constant at the predetermined voltage V 0 in a region where the resistance R of the writing electrode 3b is small, and the absolute value of the surface potential of the latent image carrier 2 decreases in a region where the resistance R of the writing electrode 3b is greater than a predetermined value. On the other hand the relation between the resistance R of the writing electrode 3b and the surface potential of the latent image carrier 2 when the writing electrode 3b is connected to the B side to ground the electrode 3b is represented by a dotted line in Fig. 3(c). As shown by the dotted line in Fig. 3(c), the surface potential of the latent image carrier 2 is constant at substantially the ground voltage V 1 in a region where the resistance R of the writing electrode 3b is small, and the absolute value of the surface potential of the latent image carrier 2 increases in a region where the resistance R of the writing electrode 3b is greater than the predetermined value. In the region where the resistance R of the writing electrode 3b is small and the surface potential of the latent image carrier 2 is constant at the predetermined voltage V 0 or constant at the ground voltage V 1 , negative (-) charge directly moves from a lower voltage side to a higher voltage side, that is, the charge-transfer is conducted between the writing electrode 3b being in contact with the latent image carrier 2 and the charged layer 2d of the latent image carrier 2, as shown in Fig. 4(a). This means that charge is applied to or removed from the latent image carrier 2 via the charge-transfer. In the region where the resistance R of the writing electrode 3b is great and the surface potential of the latent image carrier 2 starts to vary, the application or removal of charge relative to the latent image carrier 2 via the charge-transfer is gradually reduced and discharge occurs between the substrate 3a and the base member 2c of the latent image carrier 2 as shown in Fig. 4(b) as the resistance R of the writing electrode 3b is increased. The discharge between the substrate 3a and the base member 2c of the latent image carrier 2 occurs when the absolute value of the voltage (the predetermined voltage Vo) between the substrate 3a and the base member 2c of the latent image carrier 2 becomes higher than a discharge starting voltage V th . The relation between the gap, between the substrate 3a and the latent image carrier 2, and the discharge starting voltage V th is just as shown in Fig. 4(c), according to Paschen's law. That is, the discharge starting voltage V th is the lowest when the gap is about 30 µm, so the discharge starting voltage V th should be high when the gap is either larger or smaller than about 30 µm, making the occurrence of discharge difficult. Even via the discharge, charge can be applied to or removed from the surface of the latent image carrier 2. However, when the resistance R of the writing electrode 3b is in this region, the application or removal of charge relative to the latent image carrier 2 via the charge-transfer is greater while the application or removal of charge relative to the latent image carrier 2 via the discharge is smaller. This means that the application or removal of charge relative to the latent image carrier 2 is dominated by the application or removal of charge via the charge-transfer. By the application or removal of charge via the charge-transfer, the surface potential of the latent image carrier 2 becomes to the predetermined voltage V 0 to be impressed to the writing electrode 3d or the ground voltage V 1 . In case of the application of charge via the charge-transfer, the predetermined voltage V 0 to be supplied to the writing electrode 3b is preferably set to a voltage equal to or less than the discharge starting voltage V th at which the discharge occurs between the writing electrode 3b and the base member 2c the latent image carrier 2. When the resistance R of the writing electrode 3b is greater than the region, the application or removal of charge relative to the latent image carrier 2 via the charge-transfer is smaller while the application or removal of charge relative to the latent image carrier 2 via the discharge is greater than that via the charge-transfer. The application or removal of charge relative to the latent image carrier 2 gradually becomes dominated by the application or removal of charge via the discharge. That is, as the resistance R of the writing electrode 3b becomes greater, the application or removal of charge relative to the surface of the latent image carrier 2 is performed mainly via the discharge and rarely via the charge-transfer. By the application or removal of charge via the discharge, the surface potential of the latent image carrier 2 becomes to a voltage obtained by subtracting the discharge starting voltage V th from the predetermined voltage V 0 to be impressed to the writing electrode 3d or the ground voltage V 1 . It should be noted that the same is true when the predetermined voltage V 0 is of a positive (+) polarity. Therefore, the application or removal of charge relative to the latent image carrier 2 via the charge-transfer can be achieved by satisfying a condition that the resistance R of the electrode 3b is set in such a small range as to allow the surface potential of the latent image carrier 2 to be constant at the predetermined voltage |V 0 | (this is an absolute value because voltages of opposite (±) polarities are available) or constant at the ground voltage V 1 and by controlling the voltage to be impressed to the writing electrode 3b to be switched between the predetermined voltage V 0 and the ground V 1 . Fig. 3(d) shows the relation between the capacity C of the latent image carrier 2 and the surface potential of the latent image carrier 2. The aforementioned relation when the writing electrode 3b is connected to the A side to impress the predetermined voltage V 0 of a negative (-) polarity to the writing electrode 3b is represented by a solid line in Fig. 3(d). As shown by the solid line in Fig. 3(d), the surface potential of the latent image carrier 2 is constant at the predetermined voltage V 0 in a region where the capacity C of the latent image carrier 2 is small, and the absolute value of the surface potential of the latent image carrier 2 decreases in a region where the capacity C of the latent image carrier 2 is larger than a predetermined value. On the other hand, the relation between the capacity C of the latent image carrier 2 and the surface potential of the latent image carrier 2 when the writing electrode 3b is connected to the B side to ground the writing electrode 3b is represented by a dotted line in Fig. 3(d). As shown by the dotted line in Fig. 3(d), the surface potential of the latent image carrier 2 is constant at substantially the ground voltage V 1 in a region where the capacity C of the latent image carrier 2 is small, and the absolute value of the surface potential of the latent image carrier 2 increases in a region where the capacity C of the latent image carrier 2 is larger than a predetermined value. In the region where the capacity C of the latent image carrier 2 is small and the surface potential of the latent image carrier 2 is constant at the predetermined voltage Vo or constant at the ground voltage V 1 , negative (-) charge is directly transferred between the writing electrode 3b being in contact with the latent image carrier 2 and the charged layer 2d of the latent image carrier 2. That is, charge is applied to or removed from the latent image carrier 2 via the charge-transfer. In the region where the capacity C of the latent image carrier 2 is large and the surface potential of the latent image carrier 2 starts to vary, the application or removal of charge relative to the latent image carrier 2 via the charge-transfer is gradually reduced and discharge is started between the substrate 3a and the latent image carrier 2 as shown in Fig. 4(b) as the capacity C of the latent image carrier 2 is increased. Even via the discharge, charge can be applied to or removed from the surface of the latent image carrier 2. However, when the capacity C of the latent image carrier 2 is in this region, the application or removal of charge relative to the latent image carrier 2 via the charge-transfer is greater while the application or removal of charge relative to the latent image carrier 2 via the discharge is smaller. This means that the application or removal of charge relative to the latent image carrier 2 is dominated by the application or removal of charge via the charge-transfer. By the application or removal of charge via the charge-transfer, the surface potential of the latent image carrier 2 becomes to the predetermined voltage V 0 to be impressed to the writing electrode 3d or the ground voltage V 1 . When the capacity C of the latent image carrier 2 is greater than the region, there is now little charge-transfer between the writing electrode 3b and the charged layer 2d of the latent image carrier 2. This means that little or no charge is applied to or removed from the latent image carrier 2 via the charge-transfer. It should be noted that the same is true when the predetermined voltage V 0 is of a positive (+) polarity. Therefore, the application or removal of charge relative to the latent image carrier 2 via the charge-transfer can be achieved by satisfying a condition that capacity C of the latent image carrier 2 is set in such a small range as to allow the surface potential of the latent image carrier 2 to be constant at the predetermined voltage |V 0 | (this is an absolute value because voltages of opposite (±) polarities are available) or constant at the ground voltage V 1 and by controlling the voltage to be impressed to the writing electrode 3b to be switched between the predetermined voltage V 0 and the ground voltage V 1 . Fig. 3(e) shows the relation between the velocity (peripheral velocity) v of the latent image carrier 2 and the surface potential of the latent image carrier 2. The aforementioned relation when the writing electrode 3b is connected to the A side to impress the predetermined voltage V 0 of a negative (-) polarity to the writing electrode 3b is represented by a solid line in Fig. 3(e). As shown by the solid line in Fig. 3(e), the surface potential of the latent image carrier 2 increases as the velocity v increases in a region where the velocity v of the latent image carrier 2 is relatively low, and the absolute value of the surface potential of the latent image carrier 2 is constant in a region where the velocity v of the latent image carrier 2 is higher than a predetermined value. The reason of increase in the surface potential of the latent image carrier 2 with the increase in the velocity v of the latent image carrier 2 is considered as the charge-transfer to the latent image carrier 2 due to friction between the writing electrode 3b and the latent image carrier 2. The velocity v of the latent image carrier 2 has an extent above which the charge-transfer due to friction is no longer increased and becomes substantially constant. On the other hand, the relation between the velocity v of the latent image carrier 2 and the surface potential of the latent image carrier 2 when the writing electrode 3b is connected to the B side to ground the writing electrode 3b is represented by a dotted line in Fig. 3(e). As shown by the dotted line in Fig. 3(e), the surface potential of the latent image carrier 2 is constant at the ground voltage V 1 regardless of the velocity v of the latent image carrier 2. It should be noted that the same is true when the predetermined voltage V 0 is of a positive (+) polarity. Fig. 3(f) shows the relation between the pressing force applied to the latent image carrier 2 by the writing electrode 3b (hereinafter, just referred to as "the pressure of the writing electrode 3b") and the surface potential of the latent image carrier 2. The aforementioned relation when the writing electrode 3b is connected to the A side to impress the predetermined voltage V 0 of a negative (-) polarity to the writing electrode 3b is represented by a solid line in Fig. 3(f). As shown by the solid line in Fig. 3(f), the surface potential of the latent image carrier 2 relatively rapidly increases as the pressure of the writing electrode 3b increases in a region where the pressure of the writing electrode 3b is very low, and the absolute value of the surface potential of the latent image carrier 2 is constant in a region where the pressure of the writing electrode 3b is higher than a predetermined value. The reason of the rapid increase in the surface potential of the latent image carrier 2 with the increase in the pressure of the writing electrode 3b is considered as that the contact between the writing electrode 3b and the latent image carrier 2 is further ensured by the increase in the pressure of the writing electrode 3b. The pressure of the writing electrode 3b has an extent above which the contact certainty between the writing electrode 3b and the latent image carrier 2 is no longer increased and becomes substantially constant. On the other hand, the relation between the pressure of the writing electrode 3b and the surface potential of the latent image carrier 2 when the writing electrode 3b is connected to the B side to ground the writing electrode 3b is represented by a dotted line in Fig. 3(f). As shown by the dotted line in Fig. 3(f), the surface potential of the latent image carrier 2 is constant at the ground voltage V 1 regardless of the pressure of the writing electrode 3b. It should be noted that the same is true when the predetermined voltage V 0 is of a positive (+) polarity. Therefore, the application or removal of charge relative to the latent image carrier 2 via the charge-transfer can be securely and easily achieved by satisfying conditions that the resistance R of the writing electrode 3b and the capacity C of the latent image carrier 2 are set in such a manner as to allow the surface potential of the latent image carrier 2 to be constant at the predetermined voltage and that the velocity v of the latent image carrier 2 and the pressure of the writing electrode 3b are set in such a manner as to allow the surface potential of the latent image carrier 2 to be constant at the predetermined voltage, and by controlling the voltage to be impressed to the writing electrode 3b to be switched between the predetermined voltage V 0 and the ground voltage V 1 . Though the predetermined voltage V 0 to be impressed to the writing electrode 3b is a direct current voltage in the aforementioned embodiment, an alternating current voltage may be superimposed on a direct current voltage. When an alternating current voltage is superimposed, it is preferable that a DC component is set to be a voltage to be impressed to the latent image carrier 2, the amplitude of AC component is set to be twice or more as large as the discharge starting voltage V th , and the frequency of AC component is set to be higher than the frequency in rotation of the latent image carrier 2 by about 500-1,000 times (for example, assuming that the diameter of the latent image carrier 2 is 30 and the peripheral velocity of the latent image carrier 2 is 180 mm/sec, the frequency in rotation of the latent image carrier 2 is 2Hz so that the frequency of AC component is 1,000-2,000Hz.). By superimposing an alternating current voltage on a direct current voltage as mentioned above, the application or removal of charge via discharge of the writing electrode 3b is further stabilized. In addition, the writing electrode vibrates because of the existence of the alternating current, thereby removing foreign matters adhering to the writing electrode 3b and thus preventing contamination of the writing electrode 3b. Description will now be made as regard to the flexible substrate 3a supporting the writing electrodes 3b of the writing device 3. Fig. 5 is a schematic illustration showing an example of the writing device 3, as seen in an axial direction of the latent image carrier 2. As mentioned, the substrate 3a is made of a flexible material being relatively soft and elastic such as a FPC. The substrate 3a has a plurality of writing electrodes 3b fixed at its end 3a 1 as shown in Fig. 5. The writing electrodes 3b are aligned in a row extending in the axial direction (main scanning direction) of the latent image carrier 2 as will be described later and the substrate 3a is accordingly formed in a rectangular plate shape having a length, along the axial direction of the latent image carrier 2, which is substantially the same as the axial length of the charged layer 2d of the latent image carrier 2. The substrate 3a is fixed by a suitable fixing member at an end 3a 2 opposite to the end 3a 1 where the writing electrodes 3b are fixed. The substrate 3a is disposed to extend from the right side in Fig. 5 to oppose the rotational direction (indicated by an arrow: the clockwise direction) of the latent image carrier 2. It should be noted that the substrate 3a may be disposed to extend from the left side in Fig. 5 in the same direction as the rotational direction of the latent image carrier 2. In this state, the substrate 3a is elastically slightly deflected to produce weak elastic restoring force. By this elastic restoring force, the writing electrodes 3b are lightly pressed against and in contact with the latent image carrier 2 with a small pressing force. The fact that the pressing force of the writing electrodes 3b onto the latent image carrier 2 is small can suppress the wearing of the charged layer 2d of the latent image carrier 2 due to the writing electrodes 3b, thus improving the durability. The fact that the writing electrodes 3b are kept in contact with the charged layer 2d by the elastic force of the substrate 3a achieves stable contact of the writing electrodes 3b to the charged layer 2d. The substrate 3a has drivers 11 fixed to the end 3a 2 for controlling the operation of the writing electrodes 3b. In case where the substrate 3a is disposed to oppose the rotational direction of the latent image carrier 2 as shown in Fig. 5, the substrate 3a can remove foreign matters adhering to the latent image carrier 2, that is, the writing head 3 is provided with a cleaning characteristic. In case where the substrate 3a is disposed to extent in the same direction of the rotational direction of the latent image carrier 2, foreign matters adhering to the latent image carrier 2 are allowed to pass between the substrate 3a and the latent image carrier 2. Fig. 6 is a perspective view partially showing the writing head in the image forming apparatus of this embodiment. The writing head shown in Fig. 3(a) through Fig. 5 comprises a supporting substrate 3a made of a flexible material such as FPC of PET, a plurality of wirings 3c (only two wirings are illustrated in Fig. 6) which are made of a conductive material and are placed on the supporting substrate 3a, each wiring 3c extending in the direction perpendicular to the main scanning direction of the latent image carrier 2, and writing electrodes 3b each of which is formed at one end of each wiring 3c and is composed of a convexity in a rectangular parallelopiped or a cube form to project toward the latent image carrier 2 as shown in Fig. 6. Therefore, the writing electrodes 3b are aligned in the main scanning direction. It should be noted that the other end of each writing 3c is connected to a driver 11 as will be described later. Figs. 7(a) through 7(i) are views for explaining one example of the method for manufacturing the writing head shown in Fig. 6. The method for forming the writing electrodes 3b composed of convexities aligned in the main scanning direction comprises: superposing and bonding a conductive layer 22 such as Cu onto a substrate insulating layer 21 which is elastically flexible as shown in Fig. 7(a); and then coating the conductive layer 22 with a photoresist 23 as shown in Fig. 7(b). The coating of the photoresist 23 may be conducted by laminating a dry film on the conductive layer 22 or by applying liquid photoresist onto the conductive layer 22 using a technique of dip coating. After that, as shown in Fig. 7(c), a mask pattern 24 corresponding to a wiring pattern 9 as will be described later is put on the photoresist 23 and is then exposed to light. As shown in Fig. 7(d), sensitized portions of the photoresist 23 are removed by etching and the mask pattern is then removed so as to expose portions of the conductive layer 22. After that, as shown in Fig. 7(e), the portions of the conductive layer 22 exposed due to the removal of the photoresist 23 are removed by acid (sulfuric acid) etching and residual portions of the photoresist (non-etched portions of the photoresist) 23 are also removed. Then, as shown in Fig. 7(f), another photoresist 25 is formed on the substrate insulating layer 21 and the residual portions of the conductive layer 22 to coat them by the same coating method as mentioned above. Another mask pattern 26 is prepared which is designed to sensitize portions of the photoresist 25 corresponding to locations, where the electrode convexities should be formed, on the residual portions of the conductive layer 22. The mask pattern 26 is put on the photoresist 25 and is then exposed to light. Sensitized portions of the photoresist 25, i.e. the portions of the photoresist 25 where the electrode convexities should be formed are removed by etching so that the corresponding portions of the conductive layer 22 are exposed as shown in Fig. 7(g). After that, as shown in Fig. 7(h), the exposed portions of the conductive layer 22 are processed by electrolytic plating 27 to form rectangular parallelopiped or cubic convexities. Finally, as shown in Fig. 7(i), the residual photoresist 25, the most front layer, is removed by etching, thereby manufacturing a writing head, as shown in Fig. 6, on which wirings 3c and writing electrodes 3b composed of rectangular parallelopiped or cubic convexities are formed. It should be understood that the method of manufacturing the writing head having writing electrodes 3b composed of convexities is not limited to the method illustrated in Figs. 7(a)-7(i) and any suitable method which can form electrodes composed of convexities and wirings on a flexible substrate 3a may be employed. Fig. 8 through Fig. 10 are perspective views similar to Fig. 6, but partially showing another embodiments of the writing head in the image forming apparatus of this embodiment. In the writing head 3 of the example shown in Fig. 6, each convexity composing each writing electrode 3b is formed in a rectangular parallelopiped or a cube. However, in the writing head 3 of the example shown in Fig. 8, each convexity composing each writing electrode 3b is formed in a truncated square pyramid. In the writing head 3 of the example shown in Fig. 9, each convexity is formed by rounding off the top peripheral edges of a truncated square pyramid of the example shown in Fig. 8. Further, in the writing head 3 of the example shown in Fig. 10, each convexity composing each writing electrode 3b is formed in a square pyramid. Furthermore, as the configuration of the convexity, various configurations are available, including a circular column, a cone, a truncated cone, an elliptic column (column of which cross section is elliptic), an elliptic cone (cone of which cross section is elliptic), a truncated elliptic cone (truncated cone of which cross section is elliptic), an oval column (column of which cross section is oval), an oval cone (cone of which cross section is oval), a truncated oval cone (truncated cone of which cross section is oval), a triangle column, a triangle pyramid, a truncated triangle pyramid, a square column, a polygonal column (having five corners or more), a polygonal pyramid (having five corners or more), and a truncated polygonal pyramid (having five corners or more). The cross section of the square column, the square pyramid, and the truncated square pyramid may be rectangular, quadratic, parallelogramatic, trapezoidal and the like. Fig. 11 is a schematic illustration showing another example of the writing head 3, as seen in an axial direction of the latent image carrier 2. In the former example, the rectangular substrate 3a is fixed at its end 3a 2 and is thus set simply to be elastically slightly deflected. In this example, however, a rectangular substrate 3a which is made of the same material as the substrate 3a of the former example is bent at its center of a direction perpendicular to the axial direction of the latent image carrier 2 into a hair pin curve with a curve top extending along a line of the axial direction of the latent image carrier 2 and the both ends 3a 1 , 3a 2 of the substrate 3a are fixed by a suitable fixing member. In this case, a conductive mounting plate (shield) 10 is interposed between the both ends 3a 1 and 3a 2 of the substrate 3a for preventing the crosstalk between two sections of the substrate 3a about the curve top, i.e. the upper and lower sections in Fig. 11. Also in this example, the length of the substrate 3a in the axial direction of the latent image carrier 2 is set substantially the same as the axial length of the charged layer 2d of the latent image carrier 2 and the substrate 3a is provided at a predetermined location of a hair pin curve portion (a curved portion) 3a 3 with a plurality of writing electrodes 3b aligned and fixed in the axial direction of the latent image carrier 2. In a state where the both ends 3a 1 , 3a 2 of the substrate 3a are fixed as shown in Fig. 11, the hair pin curve portion 3a 3 of the substrate 3a is elastically slightly deflected so that the writing electrodes 3b are lightly pressed against and in contact with the latent image carrier 2 by the weak elastic restoring force of the hair pin curve portion 3a 3 of the substrate 3a. In the writing head 3 of this example, the substrate 3a is supported by the both ends 3a 1 , 3a 2 , thus allowing the writing electrodes 3b to be further securely and stably kept in contact with the latent image carrier 2 as compared to the former example. Though drivers 11 for the electrodes 3b fixed to the both ends 3a 1 , 3a 2 of the substrate 3a, respectively are shown in Fig. 11, this arrangement corresponds to an array pattern of electrodes shown in Fig. 15 as will be described later. Figs. 12(a)-12(c) show array patterns for aligning a plurality of writing electrodes 3b in the axial direction of the latent image carrier 2 wherein Fig. 12(a) is a view showing the simplest array pattern for writing electrodes and Figs. 12(b) and 12(c) are views showing array patterns for writing electrodes which achieve to solve problems of the array pattern shown in Fig. 12(a). In the simplest array pattern (electrode pattern) for the writing electrodes 3b, as shown in Fig. 12(a), a plurality of rectangular writing electrodes 3b are aligned in a row extending in the axial direction of the latent image carrier 2 (main scanning direction) to secure an image formation region. In this case, among the writing electrodes 3b, a predetermined number (eight in the illustrated example) of writing electrodes 3b are connected to and thus united by a driver 11 which controls the corresponding electrodes 3b by switching the supply voltage between the predetermined voltage V 0 or the ground voltage V 1 . Plural units of writing electrodes 3b are put in a plurality of lines along the feeding direction and aligned in the same row extending in the axial direction of the latent image carrier 2. However, when the simple rectangular electrodes 3b are simply put aligned in one row extending in the axial direction of the latent image carrier 2 just like this pattern, there should be clearances between adjacent electrodes 3b. Portions of the surface of the latent image carrier 2 corresponding to the clearances can not be subjected to the application or removal of charge, leading to an image defect due to linear stains. Therefore, in the array pattern (hereinafter, sometimes referred to as "electrode pattern") for the writing electrodes 3b shown in Fig. 12(b), the writing electrodes 3b are each formed in a triangle and are arranged in such a manner that the orientations of the writing electrodes 3b are alternately inverted (that is, one is in the orthographic position while the other one is in the inverted position). In this case, the writing electrodes 3b are arranged such that, as shown in Fig. 13, one end 3b 2 of the triangle base of one writing electrode 3b is overlapped with one end 3b 1 of the triangle base of a next writing electrode 3b on the left of the one writing electrode 3b, as seen in the direction perpendicular to the axial direction of the latent image carrier 2 (the rotational direction of the latent image carrier 2; the feeding direction), while the other end 3b 3 of the triangle base of the one writing electrode 3b is overlapped with one end 3b 4 of the triangle base of the other next writing electrode 3b on the right of the one writing electrode 3b, as seen in the rotational direction of the latent image carrier 2. The design of partially overlapping adjacent writing electrodes 3b in the rotational direction of the latent image carrier 2 can eliminate such portions in the surface of the latent image carrier 2 that are not subjected to the application or removal of charge, thereby achieving application or removal of charge relative to the entire surface of the latent image carrier 2. This design can therefore prevent the occurrence of image defect due to linear stains. Furthermore, foreign matters adhering to the surface of the latent image carrier 2 are allowed to pass through spaces between the adjacent writing electrodes 3b, thereby preventing the occurrence of filming due to foreign matters adhering to the writing electrodes 3b. Also in this example, in the same manner as the example shown in Fig. 12(a), plural units are each formed by connecting a predetermined number of electrodes 3b to one driver 11 and are aligned in one row. It should be noted that, instead of triangle, each electrode 3b may be formed in any configuration that allows adjacent electrodes to be partially overlapped with each other as seen in the direction perpendicular to the axial direction of the latent image carrier 2, for example, a trapezoid, a parallelogram, and a configuration having at least one oblique side among sides opposed to adjacent electrodes 3b. In the array pattern for the writing electrodes 3b shown in Fig. 12(c), the writing electrodes 3b are each formed in circle and are aligned in two parallel rows (first and second rows) extending in the axial direction of the latent image carrier in such a manner that the writing electrodes 3d are arranged in a zigzag fashion. In this case, the electrodes are arranged such that electrodes which are in different rows but adjacent to each other are partially overlapped with each other as seen in the direction perpendicular to the axial direction of the latent image carrier 2. Also this array pattern can eliminate such portions in the surface of the latent image carrier 2 that are not subjected to the application or removal of charge, thereby achieving application or removal of charge relative to the entire surface of the latent image carrier 2. In this example, plural units are each formed of a predetermined number of electrodes 3b some of which are in the first row and the other are in the second row by connecting these electrodes 3b to one driver 11 and are aligned in the axial direction of the latent image carrier 2. The respective drivers 11 are disposed on the same side of the corresponding electrodes 3b. As shown in Fig. 14, the respective drivers 11 are electrically connected by conductive patterns (Cu patterns) 9 made of copper (Cu) foil which is formed on the substrate 3a and each line of which is formed into a thin flat bar-like shape having a rectangular section (sections are shown in Figs. 17(a)-17(d) as will be described later). In the same manner, the drivers 11 are electrically connected to the corresponding electrodes 3b by the conductive patterns 9. In addition, the electrodes 3b and the drivers 11 are connected to a power source (not shown). The conductive patterns 9 can be formed by a conventional known pattern forming method such as etching. Line data signals, writing timing signals, and high voltage power are supplied to the respective drivers 11 from the upper side in Fig. 14 so that the drivers 11 controls the corresponding electrodes 3b by switching the supply voltage between the predetermined voltage |V 0 | and the ground voltage V 1 according to the line data signals and the writing timing signals. Fig. 15 is a view showing still another example of the array pattern for the writing electrodes 3b. As shown in Fig. 15, in this array pattern for the writing electrodes 3b, the writing electrodes 3b are each formed in rectangle. In the same manner as the example shown in Fig. 12(c), the writing electrodes 3b are aligned in two parallel rows (first and second rows) extending in the axial direction of the latent image carrier 2 in such a manner that the writing electrodes 3d are arranged in a zigzag fashion and arranged such that electrodes which are in different rows but adjacent to each other are partially overlapped with each other as seen in the direction perpendicular to the axial direction of the latent image carrier 2. Also this array pattern can eliminate such portions in the surface of the latent image carrier 2 that are not subjected to the application or removal of charge, thereby achieving application or removal of charge relative to the entire surface of the latent image carrier 2. By rounding off the four corners of the rectangle of each writing electrode 3b, sharp angled portions (edges) are eliminated, thereby preventing the discharge between adjacent writing electrodes, but not illustrated. In this example, a predetermined number of electrodes 3b in the first row are connected to and united by one driver 11 and a predetermined number of electrodes 3b in the second row are connected to and united by another driver 11. For each row, plural units are formed and aligned. The drivers 11 for the electrodes 3b in the first row are disposed on the opposite side of the drivers 11 for the electrodes 3b in the second row such that these electrodes 3b are located therebetween and, as shown in Fig. 11, the opposed drivers 11 are fixed to the both ends 3a 1 , 3a 2 , respectively, of the substrate 3a which is bent in a hair pin curve. It should be understood that the rounding off corners of the writing electrodes is not limited to rectangular electrodes and may be applied to triangular electrodes and other polygonal electrodes. Figs. 16(a)-16(d) are views showing still another examples of the array pattern for the writing electrodes 3b. In any of the array patterns for the writing electrodes 3b of the aforementioned examples shown in Fig. 12(c) and Fig. 15, the writing electrodes 3b are aligned in two parallel rows extending in the axial direction of the latent image carrier 2 in such a manner that the writing electrodes 3d are arranged in a zigzag fashion. In the array pattern for the writing electrodes 3b of an example shown in Figs. 16(a) and 16(b), however, writing electrodes 3b are aligned in two rows (first and second rows) which are completely identical to each other and spaced at a predetermined distance in the direction perpendicular to the axial direction of the latent image carrier 2, wherein the first row consists of writing electrodes 3b which are, for example, trapezoidal and the second row consists of writing electrodes 3'b corresponding to the writing electrodes 3b of the first row. That is, two identical writing electrodes 3b, 3'b are arranged in a line along the direction perpendicular to the axial direction of the latent image carrier 2. This design achieves further secured and stable application of charge relative to the charged layer 2d of the latent image carrier 2. It should be noted that, in the same manner as the example shown in Fig. 12(b), opposed oblique sides of adjacent trapezoidal electrodes 3b or 3'b in the same row are partially overlapped with each other as seen in the direction perpendicular to the axial direction of the latent image carrier 2. In the array pattern of an example shown in Fig. 16(c), the trapezoids of the writing electrodes 3b in the first row are mirror images of those of the writing electrodes 3'b in the second row in the example shown in Fig. 16(b). The array pattern of an example shown in Fig. 16(d) is similar to that shown in Fig. 15, but additional writing electrodes 3'b are aligned in two additional rows each of which is arranged adjacent to each of the original rows, of which writing electrodes 3b are arranged in zigzag fashion shown in Fig. 15, wherein the original and additional rows are parallel and extend in the axial direction of the latent image carrier 2 and writing electrodes 3'b in each additional row are identical and correspond to those in the adjacent original row, so that two identical writing electrodes 3b, 3'b are arranged in a line along the direction perpendicular to the axial direction of the latent image carrier 2. The actions and effects of these examples are equal to those of the example shown in Fig. 16(a). Figs. 17(a)-17(d) are sectional views each showing an example of the writing electrodes 3b of the writing head 3. In the drawings for the aforementioned examples, the writing electrodes 3b of the writing head 3 are illustrated with their contact portions to the latent image carrier 2 facing downward. In Figs. 17(a)-17(d), however, the writing electrodes 3b are illustrated with their contact portions to the latent image carrier 2 facing upward. In the writing head 3 of an example shown in Fig. 17(a), a resistant layer 13 having a rectangular section is formed on each electrode forming portion of the surface of the conductive pattern (Cu pattern) 9 formed on the substrate 3a so as to form each writing electrode 3b having double layered structure. The resistant layer 13 can be formed by a conventional known coating method, for example by using an inkjet printer. Another known coating means may be employed instead of the inkjet printer. In case of using an inkjet printer, the thickness of the resistant layer 13 can be controlled with high precision, thereby achieving further accurate control of charge on the latent image carrier 2. When the resistance of the resistant layer 13 is relatively small, the application or removal of charge is dominated by the charge-transfer between the writing electrodes 3b and the latent image carrier 2. On the other hand, when the resistance of the resistant layer 13 is relatively large, the application or removal of charge is dominated by the discharge between the writing electrodes 3b and the latent image carrier 2. When the resistance value of the writing electrode 3b is set at 10 8 Ω cm or less, a predetermined time constant can be ensured, thus achieving uniform charge. On the other hand, when the resistance value of the writing electrode 3b is set at 10 6 Ω cm or more, the electrostatic breakdown due to pin holes of the charged layer 2d of the latent image carrier 2 can be prevented. Therefore, it is preferable that the resistance value of the resistant layer 13 of the writing electrode 3b is set in a range from 10 6 Ω cm to 10 8 Ω cm. The writing electrode 3b of this example is designed such that the surface of the resistant layer 13 is in plane contact with the charged layer 2d of the latent image carrier 2. The function of the resistant layer 13 of the writing electrode 3b provided on the conductive pattern 9 prevents the broadening of the charge-transfer in the lateral direction. This achieves effective charge-transfer between the writing electrode 3b and the latent image carrier 2. It should be noted that the resistant layer 13 is not limited to be formed to have a rectangular section as shown in Fig. 17(a) and thus may be formed in a half-cylindrical configuration having a semi-circular section which projects upwardly in Fig. 17(a) and of which axial direction is parallel to the direction perpendicular to the axial direction of the latent image carrier 2. In case of the resistant layer 13 having this half-cylindrical configuration, the resistant layer 13 should be in line contact with the charged layer 2d of the latent image carrier 2 along the direction perpendicular to the axial direction of the latent image carrier 2. It should be noted that this line contact may be inclined against the direction perpendicular to the axial direction of the latent image carrier 2. In the writing head 3 of an example shown in Fig. 17(b), the resistant layer 13 of the electrode 3b is formed substantially in a semi-circular convex shape projecting upwardly, instead of the shape having a rectangular section of the aforementioned example shown in Fig. 17(a). That is, the writing electrode 3b has a convexity projecting toward the latent image carrier 2. Therefore, the top of the resistant layer 13 is a spherical surface so that the resistant layer 13 is in point contact with the charged layer 2d of the latent image carrier 2 so that the surface of the writing electrode 3b is not entirely in contact with the latent image carrier 2. According to this structure, charge-transfer is conducted at the point contact portion between the resistant layer 13 and the charged layer 2d and charge-transfer due to charge leak is also conducted around the point contact portion, whereby application or removal of charge relative to the charged layer 2d can be conducted via the charge-transfer. Since the surface of the resistant layer 13 is spherical, discharge is conducted at location around and near the point contact portion between the resistant layer 13 and the charged layer 2d. Therefore, application or removal of charge relative to the charged layer 2d can be conducted also via the discharge. Further, this discharge can achieve application or removal of charge relative to the charged layer 2d without formation of portions, as mentioned above, in the charged layer 2d which are not subjected to the application or removal of charge. Furthermore, because of point contacts, foreign matters adhering to the surface of the latent image carrier 2 are allowed to pass, thereby preventing the occurrence of filming on the surface of the latent image carrier 2. Still further, since the resistant layer 13 is made of material easily to wear, the resistant layer 13 is shaved by contact of the surface of the resistant layer 13 of the writing electrodes 3b relative to the latent image carrier 2, whereby the resistant layer 13 of the writing electrode 3b can have a fresh surface. In this manner, by making the portion of the writing electrode 3b confronting the latent image carrier 2 from material easily to wear, the surface of the writing electrode 3b can be kept fresh, thus preventing the filming. In the writing head 3 of an example shown in Fig. 17(c), a protective layer 14 is formed as an overcoat on the spherical tops of the resistant layers 13 as the example shown in Fig. 17(b) and the surface of the substrate 3a. This protective layer 14 makes the surfaces of the resistant layers 13 hard to wear and hard to be adhered with foreign matters. In the writing head 3 of an example shown in Fig. 17(d), a large number of microscopic spherical particles 12 are arranged to be freely roll on the surface of the substrate 3a supporting the writing electrodes 3b with the resistant layers 13 each having a spherical top as the example shown in Fig. 17(b), facilitating passing of foreign matters. With the aid of the microscopic particles 12, foreign matters can easily pass between the writing electrodes 3b and the latent image carrier 2 and improved lubrication can be obtained between the writing electrodes 3b and the foreign matters, thereby preventing adhering of foreign matters to the writing electrodes 3b. The particle size of the microscopic particles 12 is normally set to have a diameter widely smaller than the particle diameter of toner (developing powder). Because the particle diameter of toner is normally about 10 µm, the microscopic particles 12 are set to have a very small diameter of 1 µm or less. The microscopic particles 12 are made of transparent resin such as acrylic resin. Since the microscopic particles 12 are made of transparent resin, the microscopic particles 12 never affect the image portions even if the particles 12 move to the image portions. The microscopic particles 12 are supplied to both the substrate 3a and the writing electrodes 3b, only to the writing electrodes 3b, or to other locations than the writing electrodes 3b. When the microscopic particles 12 are supplied to the both, lubricity between the substrate 3a, the writing electrodes 3b and the latent image carrier 2 is improved. When the microscopic particles 12 are supplied only to the writing electrodes 3b, the gap between the writing electrodes 3b and the latent image carrier 2 can be kept constant so as to improve the discharge. When the microscopic particles 12 are supplied to other locations than the writing electrodes 3b, the charge-transfer is conducted by the writing electrodes 3b and lubricity at the locations supplied with the microscopic particles 12 is improved. Fig. 18 is a diagram showing a switching circuit for switching the voltage to be connected to the writing electrodes 3b between the predetermined voltage V 0 and the ground voltage V 1 . As shown in Fig. 18, the writing electrodes 3b which is arranged, for example, in four lines are connected to corresponding high voltage switches (H.V.S.W.) 15, respectively. Each of the high voltage switches 15 can switch the voltage to be supplied to the corresponding electrode 3b between the predetermined voltage V 0 and the ground voltage V 1 . An image writing control signal is inputted into each high voltage switch 15 from a shift resistor (S.R.) 16, to which an image signal stored in a buffer 17 and a clock signal from a clock 18 are inputted. The image writing control signal from the shift resistor is inputted into each high voltage switch 15 through each AND circuit 19 in accordance with a writing timing signal from an encoder 20. The high voltage switches 15 and the AND circuits 19 cooperate together to form the aforementioned driver 11 which controls the supply voltage for the corresponding electrodes 3b. Figs. 19(a)-19(c) show profiles when the supply voltage for each electrode 3b is selectively controlled into the predetermined voltage V 0 or the ground voltage V 1 by switching operation of the corresponding high voltage switch 15, wherein Fig. 19(a) is a diagram showing the voltage profiles of the respective electrodes, Fig. 19(b) is a diagram showing a developing powder image obtained by normal developing with the voltage profiles shown in Fig. 19(a), and Fig. 19(c) is a diagram showing a developing powder image obtained by reverse developing with the voltage profiles shown in Fig. 19(a). Assuming that the electrodes 3b, for example as shown in Figs. 19(a)-19(c), five electrodes indicated by n-2, n-1, n, n+1, and n+2, respectively, are controlled to be into the voltage profiles shown in Fig. 19(a) by switching operation of the respective high voltage switches 15. When an electrostatic latent image is written on the latent image carrier 2 with the electrodes 3b having the aforementioned voltage profiles and is then developed normally, the developing powder 8 adheres to portions at the predetermined voltage V 0 of the latent image carrier 2, thereby obtaining a developing powder image as shown by hatched portions in Fig. 19(b). When an electrostatic latent image is written in the same manner and is then developed reversely, the developing powder 8 adheres to portions at the ground voltage V 1 of the latent image carrier 2, thereby obtaining a developing powder image as shown by hatched portions in Fig. 19(c). According to the image forming apparatus 1 employing the writing head 3 having the aforementioned structure, since the convexities of the writing electrodes 3d are in contact with the latent image carrier 2 so that the surface of the writing electrode 3b is not entirely in contact with the latent image carrier 2, foreign matters adhering to the surface of the latent image carrier 2 are allowed to pass, thereby preventing the occurrence of filming on the surface of the latent image carrier 2. In addition, the writing electrodes 3b are supported by the flexible substrate 3a, thereby stabilizing the positions of the writing electrodes 3b relative to the latent image carrier 2 and thus stably and reliably conducting the application or removal of charge by the writing electrodes 3b relative to the latent image carrier 2. Therefore, stable writing of an electrostatic latent image onto the latent image carrier 2 is achieved, thus reliably obtaining a high quality image with high precision. Since the writing electrodes 3b can be kept in contact with the latent image carrier 2 with a small pressing force by the flexible substrate 3a, the gap (space) between the writing electrodes 3b and the latent image carrier 2 can be eliminated. No gap practically reduces the possibility that air existing in the gap is undesirably ionized, thereby further reducing the generation of ozone and enabling the formation of an electrostatic latent image with low potential. Since the convexity of the writing electrode is allowed to be formed in various configurations, the writing electrode is flexible to be employed in various types of image forming apparatus. In particular, when the convexity of the writing electrode 3b is formed in a portion of sphere, a cone, an elliptic cone, an oval cone, a triangle pyramid, a square pyramid, or a polygonal pyramid having five corners or more, the writing electrode 3b and the latent image carrier 2 are in point contact, thereby further securely allowing foreign matters adhering to the surface of the latent image carrier 2 to pass through. When the convexity of the writing electrode 3b is formed in a circular column, a truncated cone, an elliptic column, a truncated elliptic cone, an oval column, a truncated oval cone, a triangle column, a truncated triangle pyramid, a square column of which cross section is a parallelogram or a trapezoid, a truncated square pyramid of which cross section is a parallelogram or a trapezoid, a polygonal column (having five corners or more), a polygonal pyramid (having five corners or more), and a truncated polygonal pyramid (having five comers or more), the writing electrode 3b has side faces inclined against the feeding direction, whereby foreign matters adhering to the surface of the latent image carrier 2 can easily pass through because the foreign matters easily slide along the inclined faces. Since the writing electrodes 3b are in contact with the latent image carrier 2 by a small pressing force, the latent image carrier 2 can be prevented from being damaged by the writing electrodes 3b, thus improving the durability of the latent image carrier 2. Further, since the writing head 3 employs only the writing electrodes 3b without using a laser beam generating device or a LED light generating device which is large in size as conventionally used, the apparatus size can be reduced and the number of parts can also be reduced, thereby obtaining an image forming apparatus which is simple and low-price. Furthermore, since the resistant layer 13 of the writing electrode 3b is made of material easily to wear, the surface of the resistant layer 13 of the writing electrode 3b should wear to have a fresh surface so that the surface of the writing electrode can be kept fresh, thus preventing the filming of the writing electrode 3b. Every pair of writing electrodes 3b which are next to each other are partially overlapped with each other as seen in the feeding direction of the latent image carrier 2, thereby eliminating such portions in the surface of the latent image carrier that are not subjected to the application or removal of charge and thus achieving application or removal of charge relative to the entire surface of the latent image carrier 2. Therefore, the occurrence of image defect of linear stains due to spaces between the adjacent electrodes 3b can be prevented. By rounding off the corners of a polygon of each writing electrode 3b, sharp angled portions (edges) are eliminated, thereby preventing the discharge between adjacent writing electrodes. Moreover, the substrate 3a and the writing electrodes 3b are coated with the protective layers 14, 29. The protective layers 14, 29 prevent wear of the writing electrodes 3b and prevent foreign matters from adhering to the writing electrodes 3b. It should be noted that, according to the present invention, it is not necessary to coat both of the substrate 3a and the writing electrodes 3b with the protective layers 14, 29 and it is enough to coat at least the writing electrodes 3b with the protective layers 14, 29. In addition, since the resistant layer 13 is made of material easily to wear, the resistant layer 13 is shaved by contact the surface of the resistant layer 13 of the writing electrodes 3b relative to the latent image carrier 2, whereby the resistant layer 13 of the writing electrode 3b can have a fresh surface. In this manner, by making the portion of the writing electrode confronting the latent image carrier from material easily to wear, the surface of the writing electrode can be kept fresh, thus preventing the filming. Fig. 20 is a view similar to Fig. 5 but schematically and partially showing another example of the image forming apparatus according to the present invention. In any of the aforementioned examples, the charge control device 7 for uniformly charging the latent image carrier 2 is provided separately from the writing head 3. In the image forming apparatus 1 of this example, the charge control device 7 is disposed on the substrate 3a of the writing head 3 as well as the writing electrodes 3a as shown in Fig. 20. That is, a uniformly charging electrode 7e of the charge control device 7 is disposed on the end 3a 1 of the substrate 3a of the writing head 3 in such a manner that the writing electrodes 3b are spaced apart from the uniformly charging electrode 7e at a predetermined gap. In this case, the uniformly charging electrode 7e is formed into a thin plate-like shape having a rectangular section. The uniformly charging electrode 7e is continuously disposed to extend in the axial direction of the latent image carrier 2 along the same length as the axial length of the charged layer 2d of the latent image carrier 2. The writing electrodes 3b and the uniformly charging electrode 7 are kept in contact with the surface of the latent image carrier 2 with a small pressing force by weak elastic restoring force created by deflection of the substrate 3a. In the image forming apparatus 1 of this example having the aforementioned structure, after the surface of the latent image carrier 2 is uniformly charged by the uniformly charging electrode 7e on the end 3a 1 of the substrate 3a, the writing electrodes 3b write an electrostatic latent image on the surface of the latent image carrier 2 by applying charge to or removing charge from selected areas of the surface of the latent image carrier 2 through charge-transfer of the writing electrodes 3b. In the image forming apparatus of this example, the uniformly charging electrode 7e and the writing electrodes 3b are disposed together, thereby allowing the manufacture of an image forming apparatus 1 which is smaller in size and simpler in structure. The other structures, actions, and effects of the image forming apparatus 1 of this example are the same as those of the example shown in Fig. 5. It should be understood that the design of providing the uniformly charging electrode 7e and the writing electrodes 3b as one unit is not limited to the illustrated example shown in Fig. 20, may be applied to any of the image forming apparatuses of the aforementioned examples and, in addition, any case applied with this design can exhibit the same works and effects. A suitable insulator may be arranged in the gap between the writing electrodes 3b and the uniformly charging electrode 7e. Fig. 21 is a view schematically showing a concrete example of an image forming apparatus employing the writing device of the present invention. As shown in Fig. 21, an image forming apparatus 1 as a concrete example of which a writing head 3 comprising a substrate 3a extending from the upstream toward the downstream in the rotational direction of a latent image carrier 2, and writing electrodes 3b which are fixed to the end of the substrate 3a and are arranged in contact with the latent image carrier 2. In the image forming apparatus 1 of this example, a developing roller 4a of a developing device 4 is in contact with the latent image carrier 2 to perform contact developing. Disposed on the downstream side of a transferring device 6 in the rotational direction of the latent image carrier 2 is a brush 29. Residual developing powder 8' on the latent image carrier 2 after the former transfer of a latent image is dispersed to be homogenized by the brush 29. In the image forming apparatus 1 of this concrete example having the aforementioned structure, after the surface of the latent image carrier 2 is made into the uniformly charged state by a charge control device 7 (not shown), the writing head 3 writes an electrostatic latent image on the surface of the latent image carrier 2 by applying charge to or removing charge from the surface of the latent image carrier 2 through the writing electrodes 3b of the writing head 3. The developing device 4 develops the latent image on the latent image carrier 2 to form a developing powder image by bringing developing powder to adhere to the wrote latent image through the developing roller 4a of the developing device 4. Then, the transferring device 6 transfers the developing powder image on the latent image carrier 2 to a receiving medium 5. Residual developing powder 8' on the latent image carrier 2 after the transfer is dispersed to be homogenized on the latent image carrier 2 by the brush 29. In the next uniformly charging process, the surface of the latent image carrier 2 and the residual developing powder 8' are made into the uniformly charged state by the charge control device 7. Then, the writing electrodes 3b of the writing head 3 write an electrostatic latent image on the surface of the latent image carrier 2 and on the residual developing powder 8'. By the developing device 4, the electrostatic latent image is developed. During this, by selectively charging the writing electrodes 3b to have the same polarity as the original polarity of the residual developing powder 8', residual developing powder 8' on non-image portions of the latent image carrier 2 is charged into the polarity by the writing electrodes 3b so as to move toward the developing roller 4a of the developing device 4, while residual developing powder 8' on image portions of the latent image carrier 2 still remains on the latent image carrier 2 as developing powder for subsequent developing. By transferring the residual developing powder on the non-image portions toward the developing roller 4a, the surface of the latent image carrier 2 can be cleaned even without a device for cleaning the latent image carrier 2. That is, the image forming apparatus 1 of this example is designed to form an image in the cleaner-less cleaning method in which the developing process and the cleaning process are simultaneously conducted. Description will now be made as regard to as the cleaner-less cleaning method. Figs. 22(a)-22(c) and Figs. 23(a)-23(c) are views for explaining the cleaner-less cleaning method employing reverse developing. This cleaning method will be described with reference to a case of the image forming process shown in Fig. 2(g). It should be understood that this description is for illustrative purpose and this cleaner-less cleaning method may be applied to other image forming processes employing the reverse developing. As shown in Fig. 22(a), residual toner (the aforementioned residual developing powder 8') are adhering to the photoreceptor 2a after transfer for the former image forming process. Generally, the potentials Ver of the residual toner particles (hereinafter, residual potential Ver) are not in the same polarity. That is, there are positively charged particles and negatively charged particles (only negatively charged particles are shown). In this state, as shown in Fig. 22(b), the photoreceptor 2a is uniformly charged into a negatively charged potential V 0 by the charge control roller 7c so that the photoreceptor 2a is homogenized to have charge of a negative polarity. At the same time of negatively charging the photoreceptor 2a, all of particles of the residual toner are also uniformed into charge potential V 0 of the negative polarity. The negatively charged particles of the residual toner have the same polarity as that of the developing toner carried by the developing roller 4a. In other words, the residual toner is charged to have the negative polarity which is equal to the original polarity thereof. As shown in Fig. 22(c), an electrostatic latent image is written on the latent image carrier 2 by removing charge from image portions of the latent image carrier 2 through the writing electrodes 3b (that is, changing the image portions from the residual potential Ver to a potential nearly 0V (zero volt)). At this point, the residual toner particles exist both on the image portions and on the non-image portions. As shown in Fig. 23(a), developing toner is supplied by the developing device 4 and adhere to portions of the latent image carrier 2 of which surface potential is attenuated by the writing of the writing electrodes 3b, whereby the electrostatic latent image on the latent image carrier 2 is developed by reverse developing method. During this, the negatively charged toner particles move to the image portions, of which potential is higher than the developing bias, for the purpose of developing. At the same time, residual toner particles adhering to non-image portions of which potential is lower than the developing bias move to the surface of the developing roller 4a of which potential is at the developing bias, thereby cleaning the residual toner. Then, as shown in Fig. 23(b), the toner on the photoreceptor 2a is transferred to a paper as a receiving medium. The distribution of toner left on the image portions of the photoreceptor 2a after the transfer is uniformed by the brash 29. The removal of charge during the writing process can be easily and efficiently conducted. In addition, residual toner particles are reliably moved to the developing roller 4a, thereby facilitating the cleaning. Since the surface of the latent image carrier 2 is cleaned, thereby preventing the filming of the latent image carrier 2 and thus reducing image defects. According to the image forming apparatus 1 of this example, residual developing powder 8' which is left on the latent image carrier 2 after the transfer is charged to have the same polarity as the original polarity thereof at the same time when the latent image carrier is uniformly charged by the charge control device 7, whereby the residual developing powder 8' on non-image portions of the latent image carrier 2 can be moved to the developing roller 4a during the developing by the developing device. That is, this apparatus can form an image in the cleaner-less cleaning method in which the developing of a latent image and the cleaning of the latent image carrier 2 can be simultaneously conducted. As mentioned above, employment of the writing head 3 achieves reduction in size and simplification of the structure of the image forming apparatus 1 of this example. Particularly, since it is a cleaner-less image forming apparatus without a cleaning device, further simple structure can be achieved. Because of employing the reverse developing, the residual developing powder 8' can be uniformed to have the same polarity of the developing powder 8 of the developing device 4, thereby further easily and effectively conducting the cleaning at the same time of developing. Fig. 24 is a view showing another embodiment of the present invention. In the drawings for the aforementioned examples, the writing electrodes 3b of the writing head 3 are illustrated with their contact portions to the latent image carrier 2 facing downward. In Fig. 24, however, the writing electrodes are illustrated with their contact portions to the latent image carrier 2 facing upward. An image forming apparatus 1 of the example shown in Fig. 24 is different from the image forming apparatus 1 of the example shown in Fig. 21 by microscopic particles which are interposed between the writing head 3 and the latent image carrier 2. In the writing head 3 of an example shown in Fig. 24, a resistant layer 13 having a rectangular section is formed on each electrode forming portion of the surface of the conductive pattern (Cu pattern) 9 formed on the substrate 3a so as to form each writing electrode 3b having a convexity of double layered structure. The resistant layer 13 can be formed by a conventional known coating method, for example by using an inkjet printer. Another known coating means may be employed instead of the inkjet printer. In case of using an inkjet printer, the thickness of the resistant layer 13 can be controlled with high precision, thereby achieving further accurate control of charge on the latent image carrier 2. When the resistance of the resistant layer 13 is relatively small, the application or removal of charge is dominated by the charge-transfer between the writing electrodes 3b and the latent image carrier 2. On the other hand, when the resistance of the resistant layer 13 is relatively large, the application or removal of charge is dominated by the discharge between the writing electrodes 3b and the latent image carrier 2. In the writing electrode 3b of this example, the resistant layer 13 of the electrode 3b is formed substantially in a semi-circular convex shape projecting upwardly so that the top of the resistant layer 13 is a spherical surface. In addition, a large number of microscopic spherical particles 12 are arranged to be freely roll on or adhere to the substrate 3a and the entire surfaces of the writing electrodes 3b for the purpose of facilitating passing of foreign matters. The tops of the resistant layers 13 of the writing electrodes 3b touch the charged layer 2d of the latent image carrier 2 via the microscopic particles 12. That is, the writing electrodes 3b and the latent image carrier 2 are in non-contact state where they do not directly touch each other. In other words, the writing electrodes 3b are in proximity to the latent image carrier 2. Foreign matters adhering to the surface of the latent image carrier 2 can easily pass not only because of the non-contact between the tops of the resistant layers 13 of the writing electrodes 3b and the latent image carrier 2 via the microscopic particles 12 but also with the aid of the microscopic particles 12 interposed therebetween. Therefore, filming of the surface of the latent image carrier 2 and also filming of the surfaces of the writing electrodes 3b can be effectively prevented. Free rolling of the microscopic particles 12 reduces the friction between the writing electrodes 3b and the latent image carrier 2, leading to reduction in torque for rotating the latent image carrier 2. The particle size of the microscopic particles 12 is normally set to have a diameter widely smaller than the particle diameter of developing powder. Because the particle diameter of toner is normally about 10 µm, the microscopic particles 12 are set to have a very small diameter of 1 µm or less. The microscopic particles 12 are made of transparent resin such as acrylic resin. Since the microscopic particles 12 are made of transparent resin, the microscopic particles 12 never affect the image portions even if the particles 12 move to the image portions of the latent image carrier 2. Because of the non-contact between the writing electrodes 3b and the latent image carrier 2 via the microscopic particles 12, discharge occurs at and around the contact portions between the microscopic particles 12 and the latent image carrier 2. During this, the gap between the writing electrodes 3b and the latent image carrier 2 can be kept constant because of the existence of the microscopic particles 12, thus improving the discharge. The discharge applies charge to or remove charge from the charged layer 2d so as to write an electrostatic latent image on the latent image carrier 2. In this case, such portions, not subjected to the application or removal of charge, as mentioned above are not formed in the charged layer 2d. When the microscopic particles 12 are supplied to both of the substrate 3a and the writing electrodes 3b, lubricity between the substrate 3a, the writing electrodes 3b and the latent image carrier 2 is improved. The microscopic particles 12 may be supplied only to the writing electrodes 3b or other locations than the writing electrodes 3b. When the microscopic particles 12 are supplied only to the writing electrodes 3b, the gap between the writing electrodes 3b and the latent image carrier 2 can be kept constant because of the existence of the microscopic particles 12 so as to improve the discharge, just like the above case that the microscopic particles 12 are supplied to the both. When the microscopic particles 12 are supplied to other locations than the writing electrodes 3b, lubricity between the substrate 3a of the writing head 3 and the latent image carrier 2 is improved, just like the above case that the microscopic particles 12 are supplied to the both. As a method of supplying (bonding) the microscopic particles 12 to at least one of the substrate 3a and the writing electrodes 3b, there is a method of supplying the microscopic particles 12 to the substrate 3a and/or writing electrodes 3b by providing a storage tank of microscopic particles 12 to the writing head 3 and gradually delivering the microscopic particles 12 from the storage tank through pores or brush slits. There is another method including: disposing an applying means such as a brush on a face of the writing head 3 confronting the latent image carrier 2, and applying the microscopic particles 12 to at least one of the substrate 2 and the writing electrodes 3b by using the applying means. There is still another method of previously applying the microscopic particles 12 onto at least one of the substrate 3a and the writing electrodes 3b by a brush or the like; and using the wring device previously applied with the microscopic particles 12. The microscopic particles 12 may be supplied (or bonded) to the latent image carrier 2 other than the substrate 3a and the writing electrodes 3b of the writing head 3. As a method of supplying the microscopic particles 12 to the latent image carrier 2, there is a method of arranging a applying means such as a brush which is filled with the microscopic particles 12 at a position around the periphery of the latent image carrier 2; and applying the microscopic particles to the latent image carrier 2 by using the applying means. There is another method of previously applying the microscopic particles 12 to the entire peripheral surface of the latent image carrier 2 by using a brush or the like and using this latent image carrier 2 which is previously applied with the microscopic particles 12. The charge of the microscopic particles 12 is selectively set to be charged to have the same polarity as the original polarity of the developing power 8 of the developing device 4. Therefore, the residual developing powder 8' on non-image portions of the latent image carrier 2 can be further effectively removed or collected. Interposing the microscopic particles 12 between the writing electrodes 3b and the latent image carrier 2 is extremely advantage for the structure of the image forming apparatus 1 without cleaning device, eliminating the necessity of the charge control device 7. As apparent from the aforementioned description, in the image forming apparatus of the present invention, a convexity of each writing electrode is in contact with a latent image carrier so that the surface of the writing electrode is not entirely in contact with the latent image carrier, thereby allowing easy passing of foreign matters adhering to the surface of the latent image carrier and thus preventing the filming of the surface of the latent image carrier. In addition, the writing electrodes are supported by a flexible substrate, thereby stabilizing the positions of the writing electrodes relative to the latent image carrier and thus stably and reliably conducting the application or removal of charge by the writing electrodes relative to the latent image carrier. Therefore, stable writing of an electrostatic latent image onto the latent image carrier is achieved, thus reliably obtaining a high quality image with high precision. Since the writing electrodes can be securely kept in contact with the latent image carrier with a small pressing force by the flexible substrate, the gap (space) between the writing electrodes and the latent image carrier can be eliminated. No gap practically reduces the possibility that air existing in the gap is undesirably ionized, thereby further reducing the generation of ozone and enabling the formation of an electrostatic latent image with low potential. In addition, the latent image carrier can be prevented from being damaged by the writing electrodes, thus improving the durability of the latent image carrier. Further, since the writing device employs only the writing electrodes without using a laser beam generating device or a LED light generating device which is large in size as conventionally used, the apparatus size can be reduced and the number of parts can also be reduced, thereby obtaining an image forming apparatus which is simple and low-price. According to the present invention, since the convexity of the writing electrode is allowed to be formed in various configurations, the writing electrode is flexible to be employed in various types of image forming apparatus. In particular, when the convexity of the writing electrode is formed in a portion of sphere, a cone, an elliptic cone, an oval cone, a triangle pyramid, a square pyramid, or a polygonal pyramid having five corners or more, the writing electrode and the latent image carrier are in point contact, thereby further securely allowing foreign matters adhering to the surface of the latent image carrier to pass through. When the convexity of the writing electrode is formed in a circular column, a truncated cone, an elliptic column, a truncated elliptic cone, an oval column, a truncated oval cone, a triangle column, a truncated triangle pyramid, a square column of which cross section is a parallelogram or a trapezoid, a truncated square pyramid of which cross section is a parallelogram or a trapezoid, a polygonal column (having five corners or more), a polygonal pyramid (having five corners or more), and a truncated polygonal pyramid (having five corners or more), the writing electrode has side faces inclined against the feeding direction, whereby foreign matters adhering to the surface of the latent image carrier can easily pass through because the foreign matters easily slide along the inclined faces. According to the present invention, at least the writing electrodes are coated with the protective layers. The protective layers prevent wear of the writing electrodes and prevent foreign matters from adhering to the writing electrodes. According to the present invention, since the portion of the writing electrode confronting the latent image carrier is made of material easily to wear, the surface of the writing electrode should wear due to the contact relative to the latent image carrier so as to have a fresh surface so that the surface of the writing electrode can be kept fresh, thus preventing the filming of the writing electrode. According to the present invention, residual developing powder which is left on the latent image carrier after the transfer is charged to have the same polarity as the original polarity of the developing powder before developing an electrostatic latent image, whereby the residual developing powder on non-image portions of the latent image carrier can be moved to a developing roller during the developing by a developing device. That is, this apparatus can form an image in the cleaner-less cleaning method in which the developing of a latent image and the cleaning of the latent image carrier can be simultaneously conducted. According to the present invention, employment of the writing device achieves reduction in size and simplification of the structure of the image forming apparatus. In addition, since it is a cleaner-less image forming apparatus without a cleaning device, further simple structure can be achieved. According to the present invention, a large number of microscopic particles are interposed at least between the writing electrodes and the latent image carrier. With the aid of the microscopic particles, foreign matters adhering to the surface of the latent image carrier can easily pass through, thus preventing the filming on the surface of the latent image carrier and on the surfaces of the writing electrodes. In addition, Free rolling of the microscopic particles 12 reduces the friction between the writing electrodes 3b and the latent image carrier 2, leading to reduction in torque for rotating the latent image carrier 2. Since the charge of the microscopic particles is adapted to be charged to have the same polarity as the original polarity of the developing power, consisting of a single component, of the developing device, the residual developing powder on non-image portions of the latent image carrier can be further effectively removed or collected by the microscopic particles, placed on the non-image portions of the latent image carrier and charged as mentioned above. Interposing the microscopic particles between the writing electrodes and the latent image carrier enables to eliminate the necessity of the charge control device, thereby further simplifying the structure of the image forming apparatus without cleaning device. According to the present invention, since residual developing powder which is left on the latent image carrier after the transfer is charged to have the same polarity as the original polarity thereof at the same time when the latent image carrier is uniformly charged, application of charge to the residual developing powder can be easily conducted. According to the present invention, because of employing the reverse developing, the residual developing powder can be uniformed to have the same polarity of the developing powder during the process of uniformly charging the latent image carrier, thereby further easily and effectively conducting the cleaning at the same time of developing.

Claims: An image forming apparatus comprising a latent image carrier (2) on which an electrostatic latent image is formed, a writing device (3) for writing said electrostatic latent image on said latent image carrier (2), and a developing device (4) for developing said electrostatic latent image on the latent image carrier (2), wherein said electrostatic latent image, written on said latent image carrier (2) by said writing device (3), is developed by said developing device (4), thereby forming an image, said writing device (3) having a plurality of wirings (3c) each having a writing electrode (3b) at one end for writing said electrostatic latent image on said latent image carrier (2), and a flexible substrate (3a) for supporting said wirings (3c) and writing electrodes (3b), characterized in that said writing electrodes (3b) are in contact with said latent image carrier (2) with a small pressing force due to elasticity of said flexible substrate (3a), and each of said writing electrodes (3b) at the end of said wirings (3c) comprises a portion projecting from said substrate (3a) toward said latent image carrier (2). An image forming apparatus as claimed in claim 1, characterized in that each writing electrode (3b) is formed in a portion of sphere, a circular column, a cone, a truncated cone, a column with an elliptic cross section, a cone with an elliptic cross section, a truncated cone with an elliptic cross section, a column with an oval cross section, a cone with an oval cross section, a truncated cone with an oval cross section, a triangle column, a triangle pyramid, a truncated triangle pyramid, a square column, a square pyramid, a truncated square pyramid, a polygonal column having five corners or more, a polygonal pyramid having five corners or more, or a truncated polygonal pyramid having five corners or more. An image forming apparatus as claimed in claim 1 or 2, in which at least each writing electrode(3b) is coated with a protective layer. An image forming apparatus as claimed in claim 1 or 2, in which at least a portion of each writing electrode (3b) confronting said latent image carrier (2) is made of a material easily to wear. An image forming apparatus as claimed in any one of claims 1 to 3, in which said developing device (4) is a developing device (4) for developing said electrostatic latent image with developing powder consisting of a single component; the apparatus further comprises a transferring device (6) for transferring a developing powder image on said latent image carrier (2), developed by said developing device (4), to a receiving medium (5); and residual developing powder left on said latent image carrier (2) after transfer is adapted to be charged to have the same polarity as the original polarity of said developing powder consisting of a single component before developing said electrostatic latent image. An image forming apparatus as claimed in claim 5, in which a large number of microscopic particles are interposed at least between said writing electrodes (3b) and said latent image carrier (2) to allow free rolling of said microscopic particles, wherein said microscopic particles are adapted to be charged at least to have the same polarity as the original polarity of said developing powder before developing of said electrostatic latent image. An image forming apparatus as claimed in claim 5 or 6, further comprising a charge control device for making said latent image carrier (2) into a uniformly charged state, wherein residual developing powder left on said latent image carrier (2) after transfer is adapted to be charged to have the same polarity as the original polarity of said developing powder consisting of a single component at the same time when said charge control device makes said latent image carrier (2) into the uniformly charged state. An image forming apparatus as claimed in any one of claims 5 to 7, in which said developing is reverse developing.

Inventors: ABE NOBUMASA, KAMOSHIDA SHINICHI, KOGA YOSHIRO, NOMURA YUJIRO, YODA KANEO, ABE, NOBUMASA, KAMOSHIDA, SHINICHI, KOGA, YOSHIRO, NOMURA, YUJIRO, YODA, KANEO
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 2001
Main classification: B41J2
IPC codes: G03G15, B41J2, H04N1