Image forming apparatus

A head device is for writing an electrostatic latent image on an image carrier of an image forming apparatus which is movable in a first direction. In the head device, a plurality of writing electrodes are arrayed on a substrate in a second direction perpendicular to the first direction to form a plurality of electrode arrays which are arranged in the first direction. The writing electrodes are brought into contact with the image carrier with a flexibility of the substrate. A head driver is disposed on the substrate. A plurality of wirings are extending in the first direction to connect the head driver and the respective writing electrodes to supply writing voltages from the head driver to the writing electrodes. At least one of the wirings extended through one of the writing electrodes in one of the electrode arrays is placed between adjacent ones of the writing electrodes in another one of the electrode arrays.

BACKGROUND OF THE INVENTION

The present invention relates to a writing head in which plural arrays of writing electrode are formed on a flexible substrate for forming an electrostatic latent image on a latent the image carrier by applying a writing voltage to the electrode from a head driver. The present invention also relates to an image forming apparatus provided with the writing head.

In an image forming apparatus such as an electrostatic copying machine, a printer, the surface of a photosensitive member is uniformly charged by a charging device, and by exposing light from an exposing device such as a laser or an LED, an electrostatic latent image is formed in the surface of the photosensitive member. Thereafter, through the development of the electrostatic latent image in the surface of the photosensitive member, a developer image is formed on the surface of the photosensitive member. This developer image is transferred to a recording medium such as paper by a transferring device to record an image.

Such an image forming apparatus is likely to be bulky and has a complicated configuration since the exposing device for writing electrostatic latent images must be provided with a light generator.

Hence, there is proposed a head device for writing an electrostatic latent image on an image carrier with voltage applied through electrodes to omit the use of the light generator to downsize or simplify the image forming apparatus.

FIG. 1shows one example of such a writing head. The writing head3is comprised of: a flexible substrate3a; a plurality of strip-shaped wiring patterns3carrayed on the substrate3ain a widthwise direction of a latent the image carrier2(described later); and writing electrodes3bprovided at the respective ends of the wiring patterns3cso as to project toward the latent the image carrier2.

The writing head3is manufactured by the following manner. Conductive members to be electrodes3bsuch as cupper are first bonded on an insulative member to be the substrate3a. Next, photoresist is coated on the conductive members. Next, a mask pattern corresponding to the wiring patterns3cis laminated on the photoresist and an exposing process is performed.

Japanese Patent Publication No. 2002-172813A discloses a writing head (a first related art) in which writing electrodes are arranged in the widthwise direction of the latent the image carrier to form an electrode array, and two electrode arrays are arranged in a rotating direction of the latent the image carrier. Head drivers are disposed at one side or both sides of the two electrode arrays in the rotating direction of the latent the image carrier.

Japanese Patent Publication No. 2002-113897A discloses a writing head (a second related art) in which writing electrodes are pressed against a latent the image carrier with a pressing member or an urging member to establish a large nip width with a weak load.

In the first related art, since there is no wiring pattern at a portion on the substrate between the two electrode arrays, the stiffness of the portion becomes remarkably weaker than any other portions. In a case where stress is concentrated to the weak portion, deformation or creasing of the writing head may be occurred. In such a case, the two electrode arrays are hardly to be abutted against the latent the image carrier uniformly. As a result, electrostatic latent images cannot be correctly formed on the latent the image carrier, causing the deterioration of print quality. Moreover, another problem takes place that the creasing and bending of the writing head act to fluctuate the spacing between the two arrays of writing electrode, and thus giving horizontal streaks in the final image due to pitch fluctuation in the electrostatic latent image.

In the second related art, it cannot sufficiently prevent the bending and creasing of the writing head because the writing head is pressed with an extremely weak load.

Japanese Patent Publication No. 2002-178554A discloses a writing head (a third related art) in which a plurality of writing electrodes are arranged on a flexible base member in the widthwise direction of the latent the image carrier to form an electrode array. An electrostatic latent image is formed by charging the surface of the charge injection layer with predetermined voltage through the writing electrodes, in accordance with an input signal of image information. The writing electrodes are weakly abutted against the surface of a charge injection layer of the latent the image carrier with the flexibility of the base member, so that the abutment condition of the electrodes can be stabilized to securely perform the writing operation of the latent image.

In the third related art, two electrode arrays are arranged in the rotating direction of the latent the image carrier. The writing electrodes of the two electrode arrays are arranged in a zigzag manner such that one electrode in the first array is situated between adjacent electrodes in the second array. Since the electrodes are partially overlapped relative to the rotating direction of the latent the image carrier, non-chargeable region will not be formed on the surface of the latent the image carrier. That is the entire surface of the latent the image carrier is made chargeable.

In this case, the writing electrodes in the array closer to the fixed end of the flexible base member is abutted against the latent the image carrier more strongly than writing electrodes in the array closer to the free end of the flexible base member. Since the stiffness of the portion of the base member between the two electrode arrays is small, an elastic force for separating the electrode array closer to the free end of the base member from the latent the image carrier may act through the electrode army closer to the fixed end of the base member.

In such a case, the contact resistances of the two electrode arrays with respect to the latent the image carrier are made different from each other. In a case where the writing operation is performed by applying the same voltage to the two electrode arrays having different contact resistances, the latent image is deteriorated.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a writing head device capable of eliminating bending and creasing thereof due to the stress concentration, and enabling writing electrodes in individual arrays to stably and evenly contact with a latent the image carrier.

In order to achieve the above object, according to the invention, there is provided a head device, for writing an electrostatic latent image on an image carrier of an image forming apparatus which is movable in a first direction, comprising:

a flexible substrate;

a plurality of writing electrodes, arrayed on the substrate in a second direction perpendicular to the first direction to form a plurality of electrode arrays which are arranged in the first direction, the writing electrodes being brought into contact with the image carrier with a flexibility of the substrate;

a head driver, disposed on the substrate; and

a plurality of wirings, extending in the first direction to connect the head driver and the respective writing electrodes to supply writing voltages from the head driver to the writing electrodes.

wherein at least one of the wirings extended through one of the writing electrodes in one of the electrode arrays is placed between adjacent ones of the writing electrodes in another one of the electrode arrays.

Preferably, the number of wirings situated between a portion between the plural electrode arrays is identical with the number of wirings situated between a portion between the head driver and one of the plural electrode arrays.

According to the invention, there is also provided an image forming apparatus, comprising:

an image carrier on which an electrostatic latent image to be developed as a visible image is formed; and

the above head device, operable to write the electrostatic latent image on the image carrier.

According to the above configurations, the stiffness of the portion between the two electrode arrays can be increased to stabilize the contact condition of the writing electrodes with respect to the image carrier. As a result the fluctuation of the distance between writing electrodes in the individual arrays can be avoided so that the occurrence of horizontal streaks in the final image can be eliminated.

According to the invention, there is also provided a head device, for writing an electrostatic latent image on an image carrier of an image forming apparatus which is movable in a first direction, comprising:

a flexible substrate, supported in the image forming apparatus in a cantilevered manner;

a plurality of writing electrodes, arrayed on the substrate in a second direction perpendicular to the first direction to form a plurality of electrode arrays which ate arranged in the first direction, the writing electrodes being brought into contact with the image carrier with a flexibility of the substrate; and

a contact force equalizer, provided on the substrate to equalize a contact force acting from one of the electrode arrays closer to a free end of the substrate to the image carrier, with a contact force acting from one of the electrode arrays closer to a fixed end of the substrate to the image carrier.

Preferably, the contact force equalizer is at least one weight member which is configured such that a load imparting to one of the electrode arrays closer to a free end of the substrate is greater than a load imparting to one of the electrode arrays closer to a fixed end of the substrate.

According to the invention, there is also provided an image forming apparatus, comprising:

an image carrier on which an electrostatic latent image to be developed as a visible image is formed; and

the above head device, operable to write the electrostatic latent image on the image carrier.

According to the above configurations, the contact pressure between electrode arrays and the image carrier can be effectively made equal. Therefore, the contact resistance of each electrode array with respect to the image carrier is made equal, enabling the consistent formation of high quality electrostatic latent images on the image carrier.

Further, since the same voltage can be applied to every electrode array, the voltage control can be facilitated and the use of a high-voltage IC can be avoided.

According to the invention, there is also provided a head device, for writing an electrostatic latent image on an image carrier of an image forming apparatus which is movable in a first direction, comprising:

a flexible substrate, supported in the image forming apparatus in a cantilevered manner;

a plurality of writing electrodes, arrayed on the substrate in a second direction perpendicular to the first direction to form a plurality of electrode arrays which are arranged in the first direction, the writing electrodes being brought into contact with the image carrier with a flexibility of the substrate; and

a head driver, operable to apply first writing voltages to the writing electrodes associated with image forming regions, such that an absolute value of a writing voltage applied to one of the electrode arrays closer to a fixed end of the substrate is not greater than an absolute value of a writing voltage applied to one of the electrode arrays closer to a free end of the substrate.

Preferably, the head driver is operable to apply second writing voltages to the writing electrodes associated with non-image forming regions such that an absolute value of a writing voltage applied to one of the electrode arrays closer to a fixed end of the substrate is not greater than an absolute value of a writing voltage applied to one of the electrode arrays closer to a free end of the substrate. Here, a polarity of the second writing voltages is opposite to a polarity of the first writing voltages.

According to the invention, there is also provided an image forming apparatus, comprising:

an image carrier on which an electrostatic latent image to be normally developed as a visible image is formed; and

the above head device, operable to write the electrostatic latent image on the image carrier.

According to the invention, there is also provided a head device, for writing an electrostatic latent image on an image carrier of an image forming apparatus which is movable in a first direction, comprising:

a flexible substrate, supported in the image forming apparatus in a cantilevered manner;

a plurality of writing electrodes, arrayed on the substrate in a second direction perpendicular to the first direction to form a plurality of electrode arrays which are arranged in the first direction, the writing electrodes being brought into contact with the image carrier with a flexibility of the substrate; and

a head driver, operable to apply first writing voltages to the writing electrodes associated with non-image forming regions, such that an absolute value of a writing voltage applied to one of the electrode arrays closer to a fixed end of the substrate is not greater than an absolute value of a writing voltage applied to one of the electrode arrays closer to a free end of the substrate.

Preferably, the head driver is operable to apply second writing voltages to the writing electrodes associated with image forming regions such that an absolute value of a writing voltage applied to one of the electrode arrays closer to a fixed end of the substrate is not greater than an absolute value of a writing voltage applied to one of the electrode arrays closer to a free end of the substrate. Here, a polarity of the second writing voltages is opposite to a polarity of the first writing voltages.

According to the invention, there is also provided an image forming apparatus, comprising:

an image carrier on which an electrostatic latent image to be reversely developed as a visible image is formed; and

the above head device, operable to write the electrostatic latent image on the image carrier.

According to the above configurations, the difference between the contact forces of the writing electrodes with respect to the image carrier is canceled. Since the contact resistances of the writing electrodes with respect to the image carrier are made substantially identical with each other, an uniform electrostatic latent image can be formed. Performing the normal or reversal development with this uniform latent image, a final image with high quality can be stably obtained.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the invention will be described below in detail with reference to the accompanying drawings.

FIG. 2shows an image forming apparatus according to a first embodiment of the invention. The image forming apparatus1comprises: a rotatable the image carrier2on which an electrostatic latent image and a developer image are formed; a writing device3which is brought into contact with the image carrier2to write the latent image thereon; a developing device4which develops the electrostatic latent image on the image carrier2with developer (e.g., toner) held oh and carried by a developing roller4a; a transferring device6which transfers the developer image from the image carrier2onto a recording medium5such as paper; and a cleaner7having a cleaning blade7awhich cleans the surface of the image carrier2by removing the toner remaining on the image carrier2after the transferring operation.

In this case, development of an electrostatic latent image is of the normal type, and the toner used for the development may be either positively or negatively charged.

The explanation hereinafter assumes that the image carrier2is grounded, which however is just for the sake of descriptive convenience. Thus, the invention is not restricted to the case where the image carrier2is grounded.

The image carrier2, which is arranged near the central part of the apparatus and is fabricated into a shape of drum, is composed of a grounded base material2amade of an electro-conductive material such as aluminum, a dielectric layer2bprovided along the outer periphery of this base material2a, and a charge injection layer2cmade of an electro-conductive film formed in the superficial portion of this dielectric layer2b. By way of precaution, the image carrier2may be fabricated in the form of belt.

As shown inFIG. 3, an example of the charge injection layer2chas a large number of charge injection regions2ddistributed in a dispersed manner and electrically independent from each other in the superficial portion of the dielectric layer2b. These many charge injection regions2dconstitute, for example, an island structure in which locally-conductive portions are distributed so as to be electrically independent from one another. The surface of each charge injection region2dis made flush with any other portions of the surface of the superficial part of the dielectric layer2b.

The dielectric layer2b, which serves as the interior of a condenser, is preferably prepared so as to have a predetermined electric resistance (e.g., 1015Ω or less), since it must have a function of holding electrostatic charge on the charge injection region2dof the image carrier2in the form of spot. Dielectric materials used for this dielectric layer2binclude polyester resins, polycarbonate resins, polyethylene resins, fluorinated resins, cellulose, vinyl chloride resins, polyurethane resins, acrylic resins, epoxy resins, silicone resins, alkyl resins, vinyl chloride-vinyl acetate copolymer resins and polyamide resins (nylon).

On the other hand, as the material for the charge injection region2d, those having an electric resistance lower than that of dielectric layer2b(e.g., 1010Ω or less) are used. In the selection of material, when the electric resistance of charge injection region2dis excessively large, the writing process is influenced by the retardation due to time constant, leading to incorrect latent image writing. Accordingly, the lower electric resistance of charge injection region2dis preferred for the higher process speed.

As the electro-conductive material used for this charge injection region2d, electro-conductive resins or fillers can be used. Materials used to make these electro-conductive resins or fillers include electro-conductive fine particles comprising electro-conductive finely divided polymers such as a polymer complex prepared by doping iodine in polyacetylene, a polymer complex prepared by doping iodine in polythiophine and a polymer complex prepared by doping iodine in polypyrrole, and appropriate combinations of these materials whereby the content of the electro-conductive fine particles or fillers ranges from 10 to 100% by weight to control resistance.

Meanwhile, the charge injection layer2cneed not always be of a island structure. Other various structures may be adopted so long as the structure permits the writing of an electrostatic latent image by the writing electrodes3b.

The image carrier2is driven by a motor (not shown), is constructed so as to rotate clockwise as shown by the arrow inFIG. 2.

As shown inFIG. 2, the writing device3comprises: a substrate3awhich is highly insulating, relatively soft, elastic and flexible made of, for example, FPC (Flexible Print Circuit) or PET (polyethylene terephthalate); plural writing electrodes3bwhich are supported by the substrate3a, brought into light contact with the image carrier2by a weak elastic restoration force due to the flexure of substrate3a; a supporter3cwhich fixes and holds the end of substrate3apositioned at the opposite end of writing electrodes3bto the main body of the image forming apparatus (not shown in the figure), and a driver IC8(hereinafter, simply referred as driver) which drives and controls the writing electrodes3bsupported by the substrate3a.

The substrate3ais fabricated in the form of rectangular plate arranged in the axial (width) direction of the image carrier2with a length roughly equal to the axial direction length of charge injection layer2cof the image carrier2. This substrate3ais configured so as to extend in the same direction as the rotational direction of the image carrier2from left inFIG. 2. By way of precaution, in contrast, the substrate3amay be configured so as to extend in the direction opposite to the advancing direction of the image carrier2from right inFIG. 2.

As shown inFIG. 3, the writing electrodes3bare arranged to form two arrays, and each array has an arrangement pattern for the case where the plural writing electrodes3bare arranged in the axial (width) direction of the image carrier2. A writing head3d1is formed from the plural writing electrodes3b1in the first array arranged close to the free end3a1of the substrate3a(the downstream side of the advancing direction of the image carrier2), and a second writing head3d2is formed from the plural writing electrodes3b2in the second array arranged close to the fixed end3a2of the substrate3a(the upstream side of the advancing direction of the image carrier2). As shown inFIG. 4A, two writing head arrays3d1,3d2are separated with a spacing α, and writing head unit3dis composed of these arrays of writing heads3d1,3d2.

As shown by a dashed chain line inFIG. 2, the wiring electrodes3b1,3b2are arranged in a zigzag manner in the width direction of the image carrier2such that any adjacent electrode3b1,3b2are overlapped when viewed from the advancing direction of the image carrier2. With such an arrangement, uncharged portion that might be formed by the failure in writing by charge injection from writing electrodes3bnever occurs, and thus the entire surface of the charge injection region2dcan be charged or discharged.

In the embodiment, as shown inFIG. 5A, wiring patterns9connected to the writing electrodes3bcloser to the fixed end of the substrate3aare extended through the writing electrodes3bto the free and of the substrate3a, so that the number of wiring patterns9extended from the driver8and the number of wiring patterns9arranged between the two electrode arrays are made identical. Accordingly, the stiffness of the portion between the two electrode arrays can be increased to stabilize the contact condition of the writing electrodes3bwith respect to the image carrier2. As a result, the fluctuation of the distance between writing electrodes3bin the individual arrays can be avoided so that the occurrence of horizontal streaks in the final image can be eliminated.

In other words, the extended portion of each wiring pattern9serves as a reinforcement member that prevents the uneven contact of the wiring electrodes3bdue to the stress concentration onto the portion between the two electrode arrays having less stiffness. Since the reinforcement member can be fabricated simultaneously with the fabrication of the wiring patterns9, the productivity can be improved.

In the situation depicted inFIG. 4A, the substrate3ais elastically bent to some extent and a weak elastic restoration force generates. By this elastic restoration force, the writing electrodes3bare lightly pressed to and brought into contact with the image carrier2. Since the pressing force of writing electrode3bexerted onto the image carrier2is weak, the abrasion of the surface of the image carrier2caused by the writing electrodes3bis suppressed, which improves the durability of the image carrier2. On the other hand, since the writing electrodes3bare in contact with the image carrier2by the elastic force of substrate3a, the contact condition can be stabilized.

FIGS. 4B and 5Bshow a second embodiment of the invention in which driver ICs8are disposed at both sides of the two electrode arrays in the advancing direction of the image carrier2.

In this embodiment, since all the wiring patterns9are extended through the writing electrodes3bfrom one side to another. The number of wiring patterns9arranged between the two electrode arrays is made twice of the number of wiring patterns9extended from the drivers8of one side. Therefore, the stiffness of the portion between the two electrodes arrays are made greater than other portions.

In order to make the stiffness even in the entire region of the wiring device, the extended portions of the wiring patterns9from the drivers8of one side may be omitted so that the number of wiring patterns9arranged between the two electrode arrays is made identical with the number of wiring patterns9extended from the drivers8.

FIG. 6Ashows a third embodiment of the invention, in which the writing electrodes3bare arranged so as to form three electrode arrays.

FIG. 6Bshows a fourth embodiment of the invention, in which the drivers8are disposed at both sides of the three electrode arrays in the advancing direction of the image carrier2.

FIG. 7Ashows a fifth embodiment of the invention, in which the writing electrodes3bare arranged so as to form four electrode arrays.

FIG. 7Bshows a sixth embodiment of the invention, in which the drivers8are disposed at both sides of the four electrode arrays in the advancing direction of the image carrier2.

In any of the above embodiments, the wiring patterns9are extended through the writing electrodes3bso as to reinforce the portion between any adjacent electrode arrays. Therefore, the above described advantages can be attained also in these cases.

Similarly to the second embodiment shown inFIG. 5B, in the configuration shown inFIGS. 6B and 7B, the extended portions of the wiring patterns9from the drivers8of one side may be omitted so that the number of wiring patterns9arranged between the two electrode arrays is made identical with the number of wiring patterns9extended from the drivers8.

Specifically, in the case ofFIG. 6B, the extended portions of the wiring pattern9from the left drivers8may be provided only for the portion between the center electrode array and the left electrode array, while the extended portions of the wiring patterns9from the right drivers8may be provided only for the portion between the center electrode array and the right electrode array.

In the case ofFIG. 7B, the extended portions of the wiring pattern9from the left drivers8may be provided only for the portion between the left-center electrode array and the left electrode array, while the extended portions of the wiring patterns9from the right drivers8may be provided only for the portion between the right-center electrode array and the right electrode array. With regard to the portion between the two adjacent center arrays, the extended portion may be provided from either the left drivers or the right drivers.

The configuration is not limited to the above configuration. For example, in addition to the reinforcement member, the substrate3amay be reinforced by integrally bonding a reinforcing member made of an elastic material such as PET and polyimide or a metallic material such as SUS and Cu, or by making the region of interest in the substrate thicker than other regions. Further, the substrate may be fabricated with slits to relatively adjust the strength in the advancing direction of the image carrier2, or the substrate may be fabricated so as to have a strength anisotropy by fabricating the substrate itself by stretch forming.

FIG. 8shows a seventh embodiment of the invention. In this embodiment, each the writing electrode3b1,3b2in each writing head array3d1or3d2has a cylindrically shaped convex part protruding from the substrate3atoward the image carrier2. Each writing head3d1or3d2is arranged in the upstream side of the uppermost (the highest) point β of the image carrier2in the advancing direction thereof.

The protruded amount of the writing electrodes3b2from the substrate3ais set larger than protruded amount of the writing electrode3b1from the substrate3a, and thus the tips of the writing electrodes3b1,3b2can be in contact with the surface of the image carrier2simultaneously. In this configuration, the tip of each the writing electrode3b1or3b2is in contact with the image carrier2with a small pressing force exerted by the elasticity of the flexible the substrate3a.

A predetermined number of drivers8are arranged on the upper and lower surfaces of the substrate3bin the axial direction of the image carrier2. As shown inFIG. 2A, the driver8on either one side of the substrate3amay be omitted.

In this embodiment, a plurality of groups, each of which consists of a predetermined number (10 in the illustrated case) of contiguous the writing electrodes3b1in the first array are connected to one driver8provided on the upper surface of the substrate3a, are arranged in the axial direction of the image carrier2. And, a plurality of groups, each of which consists of a predetermined number of contiguous the writing electrodes3b2in the second array are connected to one driver8provided on the lower surface of the substrate3a, are arranged in the axial direction of the image carrier2.

In such configuration, the writing electrodes3bof a group corresponding to each driver8are electrically connected together by a wiring pattern9consisting of a thin plate foil made of cupper or the like having a rectangular cross-section but independently of the other groups of the writing electrodes3b. At the same time, each driver8is electrically connected with each other by an wiring pattern similar to the wiring pattern9and formed on the substrate3a, though the pattern is not illustrated in the figure. These wiring patterns9can be fabricated by any of the conventional thin film forming methods such as etching.

At the time of latent image writing, line data, writing timing signals and a high voltage power are fed via a wiring pattern to each driver8. Moreover, from each driver8to the corresponding the writing electrodes3b1or3b2, predetermined voltages V1and V2are selectively fed via the wiring patterns9.

Since the technique for selective feeding of these predetermined voltages to each the writing electrode3bis disclosed in Japanese Patent Publication No. 2002-178554A, and does not directly relate to the invention, any explanation thereof is omitted in the present specification.

Latent image writing by the writing device3onto the image carrier2is carried out by the charge injection between the charge injection region2dand the writing electrodes3b. This charge injection is conducted through the contact of the writing electrode3bwith a large number of charge injection regions2d. As for the present charge injection, there are cases where charges are injected from the writing electrode3bto the charge injection regions2d, and from the charge injection regions2dto the writing electrode3b. In the former case, the image carrier2is charged while the image carrier2is discharged in the latter case.

The size of the conductive fine particles used for charge injection region2dand the dimension of the writing electrode3bare closely related to the latent image formation. That is, in a case where the contact area of the writing electrode3bwith charge injection layer2cis larger than cross-section of the conductive fine particles, the conductive fine particles in the contact portion of the writing electrode3bare injected with certainty. Thus, the electrostatic latent image to be written in the image carrier2can be reproduced without failure, causing the accuracy of latent image writing to improve.

Further, the shape of the convex part is not restricted to cylindrical form, but can include part of a sphere, cone, truncated cone, elliptic cylinder (a pole with an elliptic horizontal section), elliptic cone (a cone with an elliptic horizontal section), truncated elliptic cone (a truncated cone with an elliptic horizontal section), racetrack pole (a pole with a racetrack horizontal section), racetrack cone (a cone with a racetrack horizontal section), truncated racetrack cone (truncated cone with a racetrack horizontal section), triangular prism, triangular pyramid, truncated triangular pyramid, quadratic prism, quadratic pyramid, truncated quadratic pyramid, pentagonal or more multiangular prisms, pentagonal or more multiangular cones, and truncated pentagonal or more multiangular pyramids.

The essential condition for the material of the writing electrode3bis conductive. Here, the electric resistance is regulated so as to lie in a predetermined resistance region (e.g., 1015Ω or less). When the electric resistance is excessively large, imperfect latent image writing occurs under the retardation effect at the writing electrode3bdue to time constant, similarly to the previously described issue for charge injection region2d. Accordingly, the lower electric resistance is preferred for the higher speed processing.

In this embodiment, a contact force equalizer10which presses the substrate3atoward the image carrier2so as to equalize contact forces of the writing heads3d1,3d2with respect to the image carrier2. This contact force equalizer10consists of weights10aand10bfixed on the substrate3aand each corresponding to each electrode array3d1or3d2. In this embodiment, the two weights10aand10bare fabricated as separate two members.

The weight10afor the first array presses this writing head3dto the surface of the image carrier2by imparting its own weight mainly onto the writing head3d1forming the first array via substrate3a, and the weight10bfor the first array presses the writing head3dto the surface of the image carrier2by imparting its own weight mainly onto the first electrode array3d2via substrate3a. Here, the weight W1imparted by the weight10ais set larger than weight W2imparted by the weight10b(W1<W2). With such a setting, the contact pressures of all the electrode arrays3d1,3d2onto the image carrier2become constant or substantially constant.

As shown inFIG. 9A, one weight10aor10bis provided for each electrode array3d1or3d2. In other words, weight10acorresponds to all the writing electrodes3b1in the first electrode array3d1and weight10bcorresponds to all the writing electrodes3b2in the second electrode array3d2.

FIG. 9Bshows a first modified example of the weight arrangement. In this case, a plurality of weights10aor10bare provided for each electrode array3d1or3d2. Each of the weights10aand10bare aligned when viewed from the advancing direction of the image carrier2. Each weight10aor10bis provided so as to correspond a pre-determined number (five in the illustrated case) of the writing electrodes3b1or3b2in each electrode array3d1or3d2.

FIG. 9Cshows a second modified example of the weight arrangement. In this case, a plurality of weights10aor10bare provided for each electrode array3d1or3d2, and the weights10aand10bare arranged in a zigzag manner.

FIG. 9Dshows a third modified example of the weight arrangement. In this case, weights10aor10bof the same number as that of the writing electrodes3b1or3b2in each electrode array3d1or3d2are provided. In other words, the plural weights10aor10bfor each array are provided so that one weight is allotted to each the writing electrode3b1or3b2.

Arbitrary arrangements other than those shown inFIGS. 9A to 9Dcan be adopted as the arrangement of weight10aand10bso long as the arrangement acts to substantially equalize the contact pressures of electrode arrays3d1,3d2to the image carrier2.

According to the above described configuration, the contact pressures of the two electrode arrays3d1,3d2onto the image carrier2are made substantially constant. Thus, the contact pressure between electrode arrays3d1,3d2and the image carrier2can be effectively made equal. Therefore, the contact resistance of each writing head3d1or3d2with respect to the image carrier2is made equal, enabling the consistent formation of high quality electrostatic latent images on the image carrier2.

As described the above, since the protruded amounts of the writing electrodes3b1,3b2so as to correspond to the outer periphery of the image carrier2, the contact pressure of the respective electrode arrays3d1,3d2with respect to the image carrier2are equalized more effectively.

In such a case, since the same voltage can be applied to every electrode array, the voltage control can be facilitated and the use of a high-voltage IC can be avoided. As a result, production cost can be reduced even if a plurality of electrode arrays are provided.

Furthermore, since the contact force equalizer is realized by the weights10aand10b, the contact pressure between each electrode array and the image carrier2can be equalized with a simple configuration.

Moreover, since each of the weights is fabricated as a separate member for each of the two writing head of each array3d1or3d2, the contact pressure of the writing head of each array3d1or3d2to the image carrier2can be equalized more efficiently.

FIG. 10shows an eighth embodiment of the invention. Hereafter, elements similar to those in the seventh embodiment are designated by the same reference numerals, and repetitive explanations for those will be omitted.

This embodiment is characterized in that the center of the spacing α between writing heads3d1,3d2is positioned at the highest point β of the image carrier2. Accordingly, the first electrode array3d1contacts with the image carrier2at the downstream side of the highest point β in the advancing direction, and the second electrode array3d2contacts with the image carrier2at the upstream side of the highest point β in the advancing direction.

Also in this embodiment, a separate weight10aor10bis individually provided on the substrate3acorresponding to each electrode array3d1or3d2. Accordingly, the weight10aon the substrate3ais arranged at the downstream side of the uppermost point β of the image carrier2in its advancing direction, and the weight10bon the substrate3ais arranged at the upstream side of the uppermost point β of the image carrier2in its advancing direction.

Also in this embodiment, load W1imparted mainly by the weight10aonto the first electrode array3b1is set larger than load W2imparted mainly by the weight10bonto the second electrode array3b1(W2<W1). Due to the circular cross-section of the outer periphery of the image carrier2, at the contact position of the first electrode array3d1with the image carrier2, the outer periphery of the image carrier2moves apart from writing head3d1. Thus, the contact force of the first electrode array3d1to the image carrier2reduces compared with that in the seventh embodiment. Therefore, the load W1imparted to the first electrode array3b1is preferably set larger than the load W1in the seventh embodiment.

In this embodiment, the protruded amounts of the writing electrodes3b1,3b2may be made almost equal to each other. However, as described the above, by using the second electrode array3d2in contact with the image carrier2as a support point, the substrate3aat the gap between electrode arrays3d1,3d2act to separate the first electrode array3d1from the image carrier2. Thus, it is preferable to make the protruded amount of the writing electrodes3b1in the first array slightly longer than protruded amount of the writing electrodes3b2in the second array. By fabricating the protruded amounts in such a manner, the contact pressure between each writing head3d1or3d2and the image carrier2can be made uniform more effectively.

Any other configurations and attained advantages are the same as those described in the seventh embodiment.

FIG. 11shows a ninth embodiment of the invention. This embodiment is characterized in that both of electrode arrays3d1,3d2are in contact with the image carrier2at the downstream side of the uppermost point β of the carrier in the advancing direction. Also in this embodiment, separate weights10aand10bare provided on the substrate3acorresponding to the electrode arrays3d1,3d2in the same manner as in the seventh embodiment.

Also in this embodiment, load W1imparted mainly by the weight10aonto the first electrode array3b1is set larger than load W2imparted mainly by the weight10bonto the second electrode array3b1(W2<W1). Due to the circular cross-section of the outer periphery of the image carrier2, at the contact position of the first electrode array3d1with the image carrier2, the outer periphery of the image carrier2moves apart from writing head3d1. Thus, the contact force of the first electrode array3d1to the image carrier2reduces compared with that in the seventh embodiment. Therefore, the load W1imparted to the first electrode array3b1is preferably set larger than the load W1in the seventh embodiment.

In this embodiment, the protruded amount for the writing electrodes3b1in the first array is preferably made longer than that for the writing electrodes3b2in the second array. By fabricating the protruded amounts in such a manner, the contact pressure between each writing head3d1or3d2and the image carrier2is made uniform more effectively.

Any other configurations and attained advantages are the same as those described in the seventh embodiment.

FIG. 12shows a tenth embodiment of the invention. This embodiment is characterized in that three electrode arrays3d1,3d2and3d3are provided on the substrate3awith a predetermined spacing in the advancing direction of the image carrier2. The center of writing head3din the advancing direction of the image carrier2(i.e., the center of the second electrode array3d2in the advancing direction of the image carrier2) is positioned at the highest point β of the image carrier2. Accordingly, the first electrode array3d1contacts with the image carrier2at the downstream side of the highest point β in the advancing direction, the second electrode array3d2contacts with the image carrier2at the highest point β of the image carrier2, and the third electrode array3d3contacts with the image carrier2at the upstream side of the highest point β in the advancing direction.

Each electrode array3d1,3d2or3d3has plural the writing electrodes3b1,3b2or3b3aligned in the axial direction of the image carrier2. The arrangements shown inFIGS. 13A and 13Bmay be adopted. Any other arrangement may be adopted so long as the adjacent electrodes are partly overlapped when viewed from the advancing direction of the image carrier2.

Each the writing electrode3b1,3b2or3b3is electrically connected to the driver8by the wiring patterns9.

A separate weight10a,10bor10cis individually provided on the substrate3acorresponding to each electrode array3d1,3d2or3d3. Accordingly, the weight for the first array10ais arranged at the downstream side of the uppermost point β of the image carrier2in its advancing direction, the weight for the second array10bis arranged at the highest point β of the image carrier2, and the weight for the third array10cis arranged at the upstream side of the uppermost point β of the image carrier2in its advancing direction.

Each load W1, W2or W3imparted to each writing head3d1,3d2or3d3by each weight10a,10band10cfor each array is set so as to increase stepwise from the third array to the first array in this order (W3<W2<W1).

Any other configurations and attained advantages are the same as those described in the seventh embodiment.

In the embodiment, the protruded amounts of the writing electrodes3b1,3b2and3b3in electrode arrays3d1,3d2and3d3may be made almost equal to each other. However, it is preferable to make the protruded amounts of the writing electrodes3b1and3b3in the first and third arrays slightly longer than protruded amount of the writing electrodes3b2in the second array, in accordance with the outer peripheral shape of the image carrier2. By fabricating the protruded amounts in such a manner, the contact pressure between each writing head3d1,3d2or3d3and the image carrier2can be equalized more effectively.

Although the drivers8are provided only on the upper surface of the substrate3a, drivers8can be provided only on the lower surface of the substrate3a, and on both surface of the substrate3a.

The center of the writing head may be arranged deviated to the upstream or downstream side of the highest point β of the image carrier2in the advancing direction thereof.

FIG. 14shows an eleventh embodiment of the invention. This embodiment is characterized in that the weights for each array are unified to form a weight10d. Further, the thickness of the weight10dresulting from the unification of the individual arrays is so designed as to continuously become thicker toward the downstream side from the upstream side in the advancing direction of the image carrier2. Accordingly, the center of gravity G of weight10ddeviates from the second electrode array3d2toward the first electrode array3d1. Thus load W1, W2or W3imparted by weight10donto each writing head3d1,3d2or3d3changes so as to continuously increase from the upstream side to the downstream side of the image carrier2(W3<W2<W1).

As the method of arranging weight10dfor this fifth example, for example, those shown inFIGS. 5Aand B may be adopted.

According to the above configuration, since the weight10dis fabricated integrally covering all the plural electrode arrays3b1,3b2and3b3, the fixing operation of weight onto the substrate is simplified. The arrangement of the weight10dmay be the ways shown inFIG. 9Aor9B.

Any other configurations and attained advantages are the same as those described in the seventh embodiment.

FIG. 15shows a twelfth embodiment of the invention. This embodiment is characterized in that the image carrier2is configured in the form of endless belt. This endless belt-type the image carrier2is, for example, suspended between a pair of pulleys arranged with a predetermined spacing (not shown in the figure), and circulates by driving either of the pulleys with a motor. The belt-type the image carrier2shown inFIG. 15is circulated clockwise.

In this embodiment, individual electrode arrays3d1,3d2are in contact with the flat portion of the belt between the pair of pulleys. In this manner, the stability in the contacts of electrode arrays3d1,3d2with the image carrier2improves due to the fact that writing heads3d1,3d2contact with the flat portion of the belt. The weight for the first array10aand the one for the second array10bare fabricated as separate members.

The protruded amount for the writing electrodes3b1arranged close to the free end of the substrate3amay be made equal to that for the writing electrodes3b2arranged close to the fixed end of the substrate3a. However, it is preferable to make the protruded amount of the writing electrodes3b1slightly longer than that for the writing electrodes3b2. By fabricating the protruded amounts in such a manner, the same advantages can be obtained as described previously.

Any other configurations and attained advantages are the same as those described in the seventh embodiment.

Adoption of such an image carrier2in the form of endless belt can be applied to both of the tenth and eleventh embodiments.

FIG. 16shows a thirteenth embodiment of the invention. The embodiment is characterized in that the weights for the individual arrays are unified to a single weight10e. The thickness of weight10echanges so as to increase stepwise toward the downstream side of the advancing direction of the image carrier2. Thus, a large thickness region10fcorresponds to the first electrode array3d1, and a small thickness region10gcorresponds to the second electrode array3d2. Therefore, loads W1and W2imparted to the individual writing heads3d1,3d2, respectively, by weight10eare set so as to satisfy W2<W1.

Any other configurations and attained advantages are the same as those described in the twelfth embodiment.

The configuration of this embodiment be applied to all the seventh to tenth embodiments. Further, the weights for individual arrays may be fabricated in the form of an integral single member the thickness of which changes continuously as in the eleventh embodiment.

FIG. 17shows a fourteenth embodiment of the invention. This embodiment is characterized in that a weight10his provided corresponding only to the first electrode array3d1(i.e., the writing head arranged closest to the free end of the substrate3a). Accordingly, the load of weight10his mainly imparted to the first electrode array3d1.

Since the size of the weight can be reduced along with the decrease of the number of weights, the space needed for fixing weight10honto the substrate3abecomes small resulting in an increased degree of freedom for the fixing of weight10h. At the same time, the fixing operation for weight10hon the substrate3ais simplified.

Any other configurations and attained advantages are the same as those described in the twelfth embodiment.

Provision of weights only for a part of the electrode arrays not for all of the arrays can be applied to any of the preceding embodiments. In a case where three or more electrode arrays are provided as in the tenth and eleventh embodiment, the weights can be provided so as to correspond to arbitrary electrode arrays except the one at the most-upstream side in the advancing direction of the image carrier2.

FIG. 18shows a fifteenth embodiment of the invention. In this embodiment, to perform the normal development, the writing voltage Vim is applied to the respective writing electrodes3b1,3b2associated with an image forming region such that an absolute value of the writing voltage Vim applied to the writing electrodes3b2closer to the fixed end of the substrate3ais not greater than an absolute value of the writing voltage Vim applied to the writing electrodes3b1.

On the other hand, the writing voltage Vnim is applied to the respective writing electrodes3b1,3b2associated with a non-image forming region such that an absolute value of the writing voltage Vnim applied to the writing electrodes3b2closer to the fixed end of the substrate3aIs not greater than an absolute value of the writing voltage Vnim applied to the writing electrodes3b1. Here, the polarity of the writing voltage Vnim is opposite to the writing voltage Vim.

Numerical examples of the writing voltages Vim, Vnim as configured the above will be shown in Table 1.

The above numeric examples may be arbitrarily changed so long as the above relationships in connection with the absolute values are satisfied.

Further, the writing voltage Vnim may be arbitrarily determined so long as the writing voltage Vim satisfies the above relationship. However, in order to make the contact pressure between each writing head3d1or3d2of each array and image carrier2more effectively, it is preferable to configure the writing voltage Vnim so as to satisfy the above relationship.

In other words, in connection with the image forming region, the absolute value of the writing voltage applied to the writing electrodes3b1closer to the free end of the substrate3ais no les than that applied to the writing electrodes3b2closer to the fixed end of the substrate3a. Accordingly, an electrostatic force generated between the writing electrodes3b1and the image carrier2becomes no less than that generated between the writing electrodes3b2and the image carrier2. As a result, the difference between the contact forces of the writing electrodes3b1,3b2with respect to the image carrier2is canceled. Since the contact resistances of the writing electrodes3b1,3b2with respect to the image carrier2are made substantially identical with each other, an uniform electrostatic latent image can be formed. Performing the normal development with this uniform latent image, a final image with high quality can be stably obtained.

Also in the non-image forming region, according to the above described configuration, the difference between the contact forces of the writing electrodes3b1,3b2with respect to the image carrier2is canceled. This also contributes the formation of uniform electrostatic latent image.

Further, since the protruded amount of the writing electrodes3b2from the substrate3ais set larger than protruded amount of the writing electrode3b1from the substrate3a, in accordance with the outer peripheral shape of the image carrier2, the difference between the contact forces of the writing electrodes3b1,3b2with respect to the image carrier2is canceled more effectively.

FIG. 19shows a sixteenth embodiment of the invention. This embodiment is characterized in that the center of the spacing α between writing heads3d1,3d2is positioned at the highest point β of the image carrier2. Accordingly, the first electrode array3d1contacts with the image carrier2at the downstream side of the highest point β in the advancing direction, and the second electrode array12contacts with the image carrier2at the upstream side of the highest point β in the advancing direction.

The writing voltages Vim and Vnim applied to the respective writing electrodes3b1,3b2are configured as well as the fifteenth embodiment.

In this embodiment, the protruded amounts of the writing electrodes3b1,3b2may be made almost equal to each other. However, as described the above, by using the second electrode array3d2in contact with the image carrier2as a support point, the substrate3aat the gap between electrode arrays3d1,3d2act to separate the first electrode array3d1from the image carrier2. Thus, it is preferable to make the protruded amount of the writing electrodes3b1in the first array slightly longer than protruded amount of the writing electrodes3b2in the second array. By fabricating the protruded amounts in such a manner, the contact pressure between each writing head3d1or3d2and the image carrier2can be made uniform more effectively.

Any other configurations and attained advantages are the same as those described in the fifteenth embodiment.

FIG. 20shows a seventeenth embodiment of the invention. This embodiment is characterized in that both of electrode arrays3d1,3d2are in contact with the image carrier2at the downstream side of the uppermost point β of the carrier in the advancing direction.

The writing voltages Vim and Vnim applied to the respective writing electrodes3b1,3b2are configured as well as the fifteenth embodiment.

In this embodiment, the protruded amount for the writing electrodes3b1in the first array is preferably made longer than that for the writing electrodes3b2in the second array. By fabricating the protruded amounts in such a manner, the contact pressure between each writing head3d1or3d2and the image carrier2is made uniform more effectively.

Any other configurations and attained advantages are the same as those described in the fifteenth embodiment.

FIG. 21shows an eighteenth embodiment of the invention. This embodiment is characterized in that the image carrier2is configured in the form of endless belt. This endless belt-type the image carrier2is, for example, suspended between a pair of pulleys arranged with a predetermined spacing (not shown in the figure), and circulates by driving either of the pulleys with a motor. The belt-type the image carrier2shown inFIG. 21is circulated clockwise.

The writing voltages Vim and Vnim applied to the respective writing electrodes3b1,3b2are configured as well as the fifteenth embodiment.

The protruded amount for the writing electrodes3b1arranged close to the free end of the substrate3amay be made equal to that for the writing electrodes3b2arranged dose to the fixed end of the substrate3a. However, it is preferable to make the protruded amount of the writing electrodes3b1slightly longer than that for the writing electrodes3b2. By fabricating the protruded amounts in such a manner, the same advantages can be obtained as described previously.

Any other configurations and attained advantages are the same as those described in the fifteenth embodiment.

This invention is not limited to the adoption of the image forming apparatus shown inFIG. 2, which performs the normal developing. So long as the writing device according to any one of the fifteenth through eighteenth embodiments can be incorporated, any type of image forming apparatus can be adopted.

For example, as a nineteenth embodiment of the invention, an image forming apparatus which performs a reversal development may be adopted. The basic configuration of this apparatus is substantially the same as that shown inFIG. 2. Also in this case, both of positively and negatively charged toners can be used.

In this embodiment, the writing voltage Vnim is applied to the respective writing electrodes3b1,3b2associated with a non-image forming region such that an absolute value of the writing voltage Vnim applied to the writing electrodes3b2closer to the fixed end of the substrate3ais not greater than an absolute value of the writing voltage Vnim applied to the writing electrodes3b1.

On the other hand, the writing voltage Vim is applied to the respective writing electrodes3b1,3b2associated with an image forming region such that an absolute value of the writing voltage Vim applied to the writing electrodes3b2closer to the fixed end of the substrate3ais not greater than an absolute value of the writing voltage Vim applied to the writing electrodes3b1. Here, the polarity of the writing voltage Vim is opposite to the writing voltage Vnim.

Numerical examples of the writing voltages Vnim, Vim as configured the above will be shown in Table 2.

The above numeric examples may be arbitrarily changed so long as the above relationships in connection with the absolute values are satisfied.

Further, the writing voltage Vim may be arbitrarily determined so long as the writing voltage Vnim satisfies the above relationship. However, in order to make the contact pressure between each writing head3d1or3d2of each array and image carrier2more effectively, it is preferable to configure the writing voltage Vim so as to satisfy the above relationship.

In other words, in connection with the non-image forming region, the absolute value of the writing voltage applied to the writing electrodes3b1closer to the free end of the substrate3ais no less than that applied to the writing electrodes3b2closer to the fixed end of the substrate3a. Accordingly, an electrostatic force generated between the writing electrodes3b1and the image carrier2becomes no less than that generated between the writing electrodes3b2and the image carrier2. As a result the difference between the contact forces of the writing electrodes3b1,3b2with respect to the image carrier2is canceled. Since the contact resistances of the writing electrodes3b1,3b2with respect to the image carrier2are made substantially identical with each other, an uniform electrostatic latent image can be formed. Performing the reversal development with this uniform latent image, a final image with high quality can be stably obtained.

Also in the image forming region, according to the above described configuration, the difference between the contact forces of the writing electrodes3b1,3b2with respect to the image carrier2is canceled. This also contributes the formation of uniform electrostatic latent image.

FIG. 22shows a twentieth embodiment of the invention. This embodiment is characterized in that three electrode arrays3d1,3d2and3d3are provided on the substrate3awith a predetermined spacing in the advancing direction of the image carrier2. The center of writing head3din the advancing direction of the image carrier2(i.e., the center of the second electrode array3d2in the advancing direction of the image carrier2) is positioned at the highest point β of the image carrier2. Accordingly, the first electrode array3d1contacts with the image carrier2at the downstream side of the highest point β in the advancing direction, the second electrode array3d2contacts with the image carrier2at the highest point β of the image carrier2, and the third electrode array3d3contacts with the image carrier2at the upstream side of the highest point β in the advancing direction.

Each electrode array3d1,3d2or3d3has plural the writing electrodes3b1,3b2or3b3aligned in the axial direction of the image carrier2. The arrangements shown inFIGS. 13A and 13Bmay be adopted. Any other arrangement may be adopted so long as the adjacent electrodes are partly overlapped when viewed from the advancing direction of the image carrier2.

Each the writing electrode3b1,3b2or3b3is electrically connected to the driver8by the wiring patterns9.

In the embodiment, the protruded amounts of the writing electrodes3b1,3b2and3b3in electrode arrays3d1,3d2and3d3may be made almost equal to each other. However, it is preferable to make the protruded amounts of the writing electrodes3b1and3b3in the first and third arrays slightly longer than protruded amount of the writing electrodes3b2in the second array, in accordance with the outer peripheral shape of the image carrier2. By fabricating the protruded amounts in such a manner, the contact pressure between each writing head3d1,3d2or3d3and the image carrier2can be equalized more effectively.

In this embodiment, to perform the normal development, the writing voltage Vim is applied to the respective writing electrodes3b1,3b2and3b3associated with an image forming region such that an absolute value of the writing voltage Vim applied to the writing electrodes3b3closest to the fixed end of the substrate3ais not greater than an absolute value of the writing voltage Vim applied to the writing electrodes3b1,3b2.

On the other hand, the writing voltage Vnim is applied to the respective writing electrodes3b1,3b2and3b3associated with a non-image forming region such that an absolute value of the writing voltage Vnim applied to the writing electrodes3b3closest to the fixed end of the substrate3ais not greater than an absolute value of the writing voltage Vnim applied to the writing electrodes3b1,3b2. Here, the polarity of the writing voltage Vnim is opposite to the writing voltage Vim.

Numerical examples of the writing voltages Vim, Vnim as configured the above will be shown in Table 3.

The above numeric examples may be arbitrarily changed so long as the above relationships in connection with the absolute values are satisfied.

Further, the writing voltages V1 and V2 may be different from each other. In this case, it is preferable that the absolute value of the writing voltage V1 is made greater than the absolute value of the writing voltage V2.

Further, the writing voltage Vnim may be arbitrarily determined so long as the writing voltage Vim satisfies the above relationship. However, in order to make the contact pressure between each writing head3d1,3d2or3d3of each array and image carrier2more effectively, it is preferable to configure the writing voltage Vnim so as to satisfy the above relationship.

In other words, in connection with the image forming region, the absolute value of the writing voltage applied to the writing electrodes3b1,3b2closer to the free end of the substrate3ais no less than that applied to the writing electrodes3b3closest to the fixed end of the substrate3a. Accordingly, electrostatic forces generated between the writing electrodes3b1,3b2and the image carrier2becomes no less than that generated between the writing electrodes3b3and the image carrier2. As a result, the difference between the contact forces of the writing electrodes3b1,3b2and3b3with respect to the image carrier2is canceled. Since the contact resistances of the writing electrodes3b1,3b2and3b3with respect to the image carrier2are made substantially identical with each other, an uniform electrostatic latent image can be formed. Performing the normal development with this uniform latent image, a final image with high quality can be stably obtained.

Also in the non-image forming region, according to the above described configuration, the difference between the contact forces of the writing electrodes3b1,3b2and3b3with respect to the image carrier2is canceled. This also contributes the formation of uniform electrostatic latent image.

Further, since the protruded amount of the writing electrodes3b1and3b3from the substrate3ais set larger than protruded amount of the writing electrode3b2from the substrate3a, in accordance with the outer peripheral shape of the image carrier2, the difference between the contact forces of the writing electrodes3b1,3b2and3b3with respect to the image carrier2is canceled more effectively.

As a twenty-first embodiment of the invention, an image forming apparatus which performs a reversal development may be adopted. The basic configuration of this apparatus is substantially the same as that shown inFIG. 2. Also in this case, both of positively and negatively charged toners can be used.

In this embodiment, to perform the reversal development, the writing voltage Vnim is applied to the respective writing electrodes3b1,3b2and3b3associated with a non-image forming region such that an absolute value of the writing voltage Vnim applied to the writing electrodes3b3closest to the fixed end of the substrate3ais not greater than an absolute value of the writing voltage Vnim applied to the writing electrodes3b1,3b2.

On the other hand, the writing voltage Vim is applied to the respective writing electrodes3b1,3b2and3b3associated with an image forming region such that an absolute value of the writing voltage Vim applied to the writing electrodes3b3closest to the fixed end of the substrate3ais not greater than an absolute value of the writing voltage Vim applied to the writing electrodes3b1,3b2. Here, the polarity of the writing voltage Vim is opposite to the writing voltage Vnim.

Numerical examples of the writing voltages Vnim, Vim as configured the above will be shown in Table 4.

Further, the writing voltage Vim may be arbitrarily determined so long as the writing voltage Vnim satisfies the above relationship. However, in order to make the contact pressure between each writing head3d1,3d2or3d3of each array and image carrier2more effectively, it is preferable to configure the writing voltage Vim so as to satisfy the above relationship.

In other words, in connection with the image forming region, the absolute value of the writing voltage applied to the writing electrodes3b1,3b2closer to the free end of the substrate3ais no less than that applied to the writing electrodes3b3closest to the fixed end of the substrate3a. Accordingly, electrostatic forces generated between the writing electrodes3b1,3b2and the image carrier2becomes no less than that generated between the writing electrodes3b3and the image carrier2. As a result, the difference between the contact forces of the writing electrodes3b1,3b2and3b3with respect to the image carrier2is canceled. Since the contact resistances of the writing electrodes3b1,3b2and3b3with respect to the image carrier2are made substantially identical with each other, an uniform electrostatic latent image can be formed. Performing the reversal development with this uniform latent image, a final image with high quality can be stably obtained.

Also in the image forming region, according to the above described configuration, the difference between the contact forces of the writing electrodes3b1,3b2and3b3with respect to the image carrier2is canceled. This also contributes the formation of uniform electrostatic latent image.