Electrostatic latent image writing head, method of manufacturing the same and image forming apparatus incorporating the same

In a writing head for forming an electrostatic latent image on a cylindrical image carrier, a plurality of writing electrodes are arranged on a first face of a film substrate in a first direction parallel with an axial direction of the image carrier. The writing electrodes are adapted to be abutted against an outer periphery of the image carrier to provide electric charges thereto. A first wiring member are arranged on the first face of the film substrate to supply signals from a first electrode driver to a first electrode group in the writing electrodes. A second writing member are arranged on a second face of the film substrate to supply signals from a second electrode driver to the second electrode group in the writing electrodes.

BACKGROUND OF THE INVENTION

The present invention relates to a writing head in which a plurality of writing electrodes are arranged on a flexible support substrate and are disposed in contact with or in close proximity to a latent image carrier to supply writing voltages to the latent image carrier to form an electrostatic latent image thereon. The invention also relates to an image forming apparatus incorporating such a writing head.

In conventional image forming apparatus such as electrostatic copiers and printers, in general, the surface of a photosensitive body is charged uniformly by a charging device and the photosensitive body surface is exposed to light of an exposing device such as laser light, light of an LED lamp, or the like to form an electrostatic latent image thereon. The electrostatic latent image is developed by a developing device to form a toner image on the photosensitive body surface. The toner image is transferred to a medium such as a sheet of paper by a transferring device, whereby an image is formed on the transferring member.

In such conventional, general image forming apparatus are large and complex in structure because the exposing device for writing an electrostatic latent image is a device for emitting laser light or LED lamp light or the like. In view of this, an image forming apparatus has been proposed in which an electrostatic latent image is written to the surface of an image carrier by using writing electrodes instead of laser light or LED lamp light.

FIG. 36is a perspective view of part of an example of such a writing head. The writing head3is composed of a flexible support substrate3a,a plurality of wiring portions3c(only two of them are shown inFIG. 36) that are a plurality of strip conductors arrayed on the support substrate3ain the primary scanning direction of a latent image carrier2, and writing electrodes3bas projections that project toward the latent image carrier2from one ends of the respective wiring portions3c.

For example, the writing head3is formed by the following process. A conductor to be electrodes made of copper or the like is joined to an elastic and flexible insulative material to be a support substrate, and the conductor is coated with a photoresist. The photoresist is covered with a mask pattern corresponding to a wiring pattern and then exposure is performed. As a result, a writing head3is formed in which wiring portions3cand writing electrodes3bas rectangular parallelepiped or cubic projections that project from one ends of the respective wiring portions3care arranged on the support substrate3a.

In a writing head disclosed in Japanese Patent Publication No. 2002-172813A, a plurality of writing electrodes3bare arrayed on a flexible support substrate3ain the primary scanning direction in the above-described manner. Two arrays of writing electrodes3bare arranged in a secondary scanning direction. And drivers are disposed on both sides of the writing electrodes3b.

In a latent image writing device disclosed in Japanese Patent Publication No. 2002-113897A, a plurality of writing electrodes are disposed in contact with or in close proximity to a latent image carrier in the above-described manner. A support substrate on which the writing electrodes are formed is pressed against the latent image carrier by a support member, a pressing member, and an urging member. This structure provides a large nip width with weak load.

It is also well-known that writing electrodes are arrayed in the axial direction of an image carrier. It is also well-known that a writing electrode formed on a flexed film-shaped substrate is brought into press contact with an image carrier with the aid of the elastic restoration force of the film-shaped substrate.

However, in the case where the writing electrodes are arrayed in the axial direction of the image carrier, current crosstalk may occur because of a small interval between the wiring portions of adjacent writing electrodes and it is difficult to increase the number of writing electrodes to enhance the resolution.

In the case where the two arrays of the writing electrodes are arranged in the secondary scanning direction, the crosstalk problem can be solved. However, it is difficult to assure high accuracy of positioning among the writing electrodes. Further, it is not suitable for downsizing because the writing electrodes occupy a large space, thereby increasing costs.

In the case where a writing electrode formed on a flexed film-shaped substrate, it is very difficult to stably bring the writing electrodes into contact with the image carrier because the elastic restoration force of the film-shaped substrate is unstable. Further, this method is not suitable for downsizing because the writing head occupies a large space.

In the above-described writing head that is composed of the flexible support substrate and the plural writing electrodes arrayed in the primary scanning direction, the rigidity is much lower in the portions between the electrodes or wiring patterns than in the electrodes or wiring patterns. Therefore, the writing head tends to wave or wrinkle in the primary scanning direction and hence it is difficult to stably bring the writing head into contact with the latent image carrier. As a result, an electrostatic latent image is not formed correctly on the latent image carrier, deteriorating the print quality.

In the writing head in which the two arrays of writing electrodes are arranged in the secondary scanning direction and the drivers are disposed on both sides of the support substrate, no wiring pattern exists in the region between the two arrays of writing electrodes and hence the rigidity is much lower there than in the other portions. Stress is concentrated in the low-rigidity portion and the writing head tends to be bent there: as in the above case, it is difficult to stably bring the two arrays of writing heads into contact with the latent image carrier. As a result, an electrostatic latent image is not formed correctly on the latent image carrier, deteriorating the print quality. If the writing head is bent, the distance between the two arrays of writing electrodes varied, resulting in a problem that disorder in the dot pitch of an electrostatic latent image causes horizontal streaks.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an electrostatic latent image writing head capable of obtaining a high resolution image, capable of solving the current crosstalk problem, and capable of stably bringing the writing electrodes into contact with an image carrier.

It is also an object of the invention to eliminate a local region having low stiffness between the writing electrodes or the wiring patterns, thereby preventing the waving or the wrinkle of the writing head and bringing the writing electrodes into contact with the image carrier stably.

It is also an object of the invention to provide a method of manufacturing such a writing head.

In order to achieve the above object, according to the invention, there is provided a writing head for forming an electrostatic latent image on a cylindrical image carrier, comprising:

a flexible film substrate;

a plurality of writing electrodes, arranged on a first face of the film substrate in a first direction parallel with an axial direction of the image carrier, the writing electrodes adapted to be abutted against an outer periphery of the image carrier to provide electric charges thereto;

a first wiring member, arranged on the first face of the film substrate to supply signals from a first electrode driver to a first electrode group in the writing electrodes; and

a second writing member, arranged on a second face of the film substrate to supply signals from a second electrode driver to the second electrode group in the writing electrodes.

Preferably, the film substrate is formed with at least one through hole through which the second wiring member extends to the second electrode group. Alternatively, the second wiring member may extend to the second electrode group via a side edge of the film substrate.

The first face and the second face of the film substrate may be defined by a single outer face of a folded film member.

The writing electrodes may be arranged so as to form a plurality of arrays which are arranged in a second direction perpendicular to the first direction.

Here, it is preferable that the writing electrodes are arranged such that writing electrodes in adjacent arrays forms a zigzag arrangement with regard to the first direction. Alternatively, the writing electrodes may be arrayed with regard to both of the first direction and the second direction.

Preferably, the film substrate comprises a first layer forming the first face and a second layer forming the second face. The wiring head further comprises a third wiring member, arranged between the first layer and the second layer to supply signals from a third electrode driver to a third electrode group in the writing electrode.

Preferably, the film substrate is integrally formed with a reinforcement member which provides a reinforcement for the film substrate in a second direction perpendicular to the first direction.

Here, it is preferable that the reinforcement member extends in the first direction so as to support at least a region where the writing electrodes are arranged.

In a case where the writing electrodes are arranged so as to form a plurality of arrays which are arranged in the second direction, it is preferable that the reinforcement member extends in the second direction so as to support at least a region where the arrays of the writing electrodes are arranged.

Alternatively, the reinforcement member extends so as to avoid a portion where each of the writing electrodes is disposed.

According to the invention, a writing head for forming an electrostatic latent image on a cylindrical image carrier, comprising:

a flexible film substrate;

a plurality of writing electrodes, arranged on a first face of the film substrate in a first direction parallel with an axial direction of the image carrier, the writing electrodes adapted to be abutted against an outer periphery of the image carrier to provide electric charges thereto;

a wiring member, arranged on the first face of the film substrate to supply signals from an electrode driver to the writing electrodes; and a reinforcement member, integrally formed with the film substrate to provide a reinforcement for the film substrate in a second direction perpendicular to the first direction.

Preferably, the reinforcement member extends in the first direction so as to support at least a region where the writing electrodes are arranged.

In a case where the writing electrodes are arranged so as to form a plurality of arrays which are arranged in the second direction, it is preferable that the reinforcement member extends in the second direction so as to support at least a region where the arrays of the writing electrodes are arranged.

Alternatively, the reinforcement member may extend so as to avoid a portion where each of the writing electrodes is disposed.

Preferably, the reinforcement member is formed on a second face of the film substrate.

According to the invention, there is also provided an image forming apparatus for forming a visible image from the electrostatic latent image formed by any one of the above wiring heads.

According to the invention, there is also provided a method of manufacturing a writing head for forming an electrostatic latent image on an image carrier, comprising steps of:

providing a flexible film member;

forming a plurality of writing electrodes on a first face of the film member;

forming a first wiring member on the first face of the film member so as to be connected to a first electrode group in the writing electrodes;

forming a second wiring member on the first face of the film member so as to be connected to a second electrode group in the writing electrodes;

defining a folding line on the film member so as to avoid the writing electrodes; and

folding the film member at the folding line to form a film substrate, such that the first wiring member and the second wiring member are arranged on opposite faces of the film substrate.

Preferably, an adhesive agent is applied on at least a part of a second face of the film member which is to be an inner face at the step of folding the film member.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be hereinafter described with reference to the accompanying drawings.

As shown inFIG. 1A, an image forming apparatus1according to the invention is at least provided with the following components. An image carrier2has a base member2athat is made of a conductive material such as aluminum and is grounded and an insulative charge-bearing layer2bthat is provided outside the base member2aand on which an electrostatic latent image is to be formed. A writing head3comprises: a film-shaped substrate3athat is provided as an FPC (flexible printed circuit) and is made of highly insulative, relatively soft, elastic, and flexible material such as PET (polyethylene terephthalate); and writing electrodes3bthat are supported by the film-shaped substrate3ato write an electrostatic latent image on the charge-bearing layer2bof the image carrier2in a state that they are brought into weak contact with the charge-bearing layer2bby weak elastic restoration force that is produced by the flexed film-shaped substrate3a.A developing device4has a developing roller4aserving as a developer carrier. A transferring device6has a transferring roller6aserving as a transferring member.

The charge-bearing layer2bis composed of a dielectric layer (insulating layer)2cand an independent electrode portion2dthat is an image writing portion provided in the surface layer of the dielectric layer2c.As shown inFIG. 1B, the independent electrode portion2dis formed by a large number of independent floating electrodes (hereinafter also referred to simply as “independent electrodes”)2d1arranged in the surface layer of the dielectric layer2c.The independent electrodes2d1have an island structure in which they are electrically independent of each other and are exposed in the surface of the dielectric layer2c.AlthoughFIG. 1Ais drawn in such a manner that the independent electrodes2dare divided from the dielectric layer2c,this is merely for convenience of description. As clearly shown inFIG. 1B, the independent electrodes2dare not clearly divided from the dielectric layer2c:the independent electrode portion2dis a portion where a large number of electrodes2d1exist in the surface layer of the dielectric layer2c.

An image is written to the independent electrode portion2din such a manner that plus voltages, for example, that are supplied via IC drivers11are applied from writing electrodes3bto the independent electrode portion2das a writing voltage V1and image writing portions of the independent electrode portion2dare charged positively.

Examples of the material of the dielectric layer2care a polyester resin, a polycarbonate resin, an acrylic resin, a polystyrene resin, polyallylate, polysulfone, poly(phenylene oxide), a vinyl chloride resin, a polyurethane resin, an epoxy resin, a silicone resin, an alkyd resin, a phenol resin, a polyamide resin, and a vinyl chloride-vinyl acetate copolymer resin and polymer alloys of two or more of them.

In the independent electrode portion2d,a large number of independent electrodes2d1are formed by applying a liquid in which one of the above resins and a large number of conductive fine particles are dispersed in a solvent (diluted mixing dispersion) with adjustment of the mixing ratio (i.e., densities) to the surface of the dielectric layer2cby a common, proper method such as spraying or dipping. The resulting independent electrodes2d1are exposed in the surface. Alternatively, a large number of independent electrodes2d1may be exposed by polishing. This provides advantages that increased surface smoothness decreases the contact resistance with the writing electrodes3band the abrasion of the writing head3and the charge-bearing layer2b.Examples of the material of the conductive fine particles are:i) Metal fine particles of Cu, Al, Ni, Ag, C, Mo, etc.ii) Fine particles produced by making zinc oxide (ZnO), tin oxide, antimony oxide, titanium oxide, or the like conductive (by doping with antimony, indium, or the like).iii) Conductive fine particles as polymer complexes produced by doping polyacetylene, polythiophine, polypyrrole, or the like with iodine.

In the above-configured image forming apparatus1, after the charge-bearing layer2bof the image carrier2is rendered in a uniformly charged state, writing voltages are supplied to writing electrodes3bvia the IC drivers11for the writing electrodes3band an electrostatic latent image is written to the image carrier2in a uniformly charged state mainly through charge transfer (e.g., charge injection) between the image carrier2and the writing electrodes3bof the writing head3that are in surface contact with each other. The electrostatic latent image on the image carrier2is then written to the charge-bearing layer2bof the image carrier2. The electrostatic latent image on the charge-bearing layer2bof the image carrier2is developed with a developer that is transported by the developing roller4aof the developing device4. A resulting developer image is transferred to a medium5such as a sheet of paper by the transferring roller6ato which a transfer voltage is applied.

As shown inFIG. 2A, the image carrier2is composed of the base member2athat is made of a conductive material such as aluminum and is grounded and the insulative charge-bearing layer2bthat is provided outside the base member2a.As described above, the writing electrodes3bof the writing device3that are supported by the film-shaped substrate3asuch as an FPC are brought into contact with the charge-bearing layer2bby a predetermined, weak pressing force. The image carrier2is rotated at a predetermined speed V. To stabilize the contact between the writing electrodes3band the image carrier2and to stabilize the charge injection or discharge, it is preferable that the weak pressing force be 10 N or less for a width 300 mm, that is, the linear pressure be 0.03 N/mm or less. From the viewpoint of abrasion, it is desirable that the linear pressure be made as low as possible while the contact is kept stable.

A predetermined high voltage V0or a predetermined low voltage V1is selectively (with switching) applied to a writing electrode3bvia the film-shaped substrate3a.As described above, the charge has the polarities (plus and minus). The term “high voltage” means a voltage having a large absolute value and the term “low voltage” means a voltage having a smaller absolute value than the high voltage (but the same polarity) or 0 V. In this specification, all low voltages are assumed to be the ground voltage. Therefore, in the following description, the high voltage V0and the low voltage V1will be referred to as “predetermined voltage V1” and “ground voltage V1,” respectively. It goes without saying that the ground voltage V1is 0 V.

That is, an electrical equivalent circuit shown inFIG. 2Bis formed at the contact portion (i.e., nip portion) between a writing electrode3band the image carrier2. InFIG. 2B, character R represents the resistance of the writing electrode3band C represents the capacitance of the image carrier2. The resistance R of the writing electrode3bis selectively connected (with switching) to the A-side predetermined (minus) voltage V0or the B-side ground voltage V1.

In the equivalent circuit, as indicated by a solid line inFIG. 2C, when the writing electrode3bis connected to the A-side and the predetermined minus voltage V0is applied to the writing electrode3b,the resistance R of the writing electrode3band the surface potential of the image carrier2have a relationship that the surface potential of the image carrier2is constant, that is, equal to the predetermined voltage V0, in a range in which the resistance R is small and the absolute value of the surface potential of the image carrier2decreases as the resistance R increases in a range in which the resistance R is larger than a predetermined value.

On the other hand, as indicated by a dashed line inFIG. 2C, when the writing electrode3bis connected to the B-side and hence is grounded, the resistance R of the writing electrode3band the surface potential of the image carrier2have a relationship that the surface potential of the image carrier2is constant, that is, approximately equal to the ground voltage V1, in a range in which the resistance R is small and the absolute value of the surface potential of the image carrier2increases with the resistance R in a range in which the resistance R is larger than a predetermined value.

In the range in which the resistance R of the writing electrode3bis small and the surface potential of the image carrier2is constant and equal to the predetermined voltage V0or the ground voltage V1, as shown inFIG. 3Aminus charge is directly injected from the low-voltage side to the high-voltage side between the writing electrode3bthat is in contact with the image carrier2and the charge-bearing layer2bof the image carrier2. That is, the image carrier2is charged or discharged by the charge injection. In the range in which the resistance R of the writing electrode3bis large and the surface potential of the image carrier2starts to vary, the degree of charging or discharging of the image carrier2by the charge injection decreases as the resistance R increases. As the resistance R increases, as indicated by arrows inFIG. 3Bdischarge comes to occur between a conductive pattern (described later) of the film-shaped substrate3aand the image carrier2.

Charge release occurs between the conductive pattern of the film-shaped substrate3aand the base member2aof the image carrier2when the absolute value of the voltage between the film-shaped substrate3aand the image carrier2(i.e., the predetermined voltage V0) is higher than a threshold voltage Vthfor the charge release.FIG. 3Cshows a relationship between the threshold voltage Vthand the gap G between the film-shaped substrate3aand the image carrier2(Paschen's law). That is, the threshold voltage Vthis minimum when the gap G is equal to about 30 μm and the threshold voltage Vthincreases, that is, the charge release becomes less apt to occur, as the gap G decreases or increases from about 30 μm. The surface of the image carrier2is charged or discharged also by such charge release. However, when the resistance R of the writing electrode3is in this range, the degree of charging or discharging by the charge injection becomes high and that by the charge release becomes low; that is, the charging or discharging of the image carrier2is dominated by the charge injection.

In the case of the charging or discharging by the charge injection, the surface potential of the image carrier2is equal to the predetermined voltage V0or the ground voltage V1that is applied to the writing electrode3b.In the case of the charging or discharging by the charge injection, it is desirable that the predetermined voltage V0applied to the writing electrode3bbe set lower than the threshold voltage Vthabove which the charge release occurs between the writing electrode3band the base member2aof the image carrier2.

In the range in which the resistance R of the writing electrode3bis even larger, the degree of charging or discharging by the charge injection becomes low and that by the charge release becomes high; that is, the charging or discharging of the image carrier2is dominated by the charge release. That is, as the resistance R of the writing electrode3bincreases, the surface of the image carrier2comes to be mainly charged or discharged by the charge release and the contribution of the charge injection becomes negligible. In the case of the charging or discharging by the charge release, the surface potential of the image carrier2is equal to the predetermined voltage V0or the ground potential V1that is applied to the writing electrode3bminus the threshold voltage Vth. The same is true of the case that the predetermined voltage V0is positive.

Therefore, the charging or discharging of the image carrier2can be performed by the charge injection by setting the resistance R of the writing electrode3bsmall in a range in which the surface potential of the image carrier2is constant and equal to the predetermined voltage |V0| (an absolute value is employed because V0may be a plus or minus voltage) or the ground voltage V1and switching-controlling the voltage applied to the writing electrode3bbetween the predetermined voltage V0and the ground voltage V1.

As indicated by a solid line inFIG. 2D, when the writing electrode3bis connected to the A-side and the predetermined minus voltage V0is applied to the writing electrode3b,the capacitance C of the image carrier2and the surface potential of the image carrier2have a relationship that the surface potential of the image carrier2is constant, that is, equal to the predetermined voltage V0, in a range in which the capacitance C is small and the absolute value of the surface potential of the image carrier2decreases as the capacitance C increases in a range in which the capacitance C is larger than a predetermined value. On the other hand, as indicated by a dashed line inFIG. 2D, when the writing electrode3bis connected to the B-side and hence is grounded, the capacitance C of the image carrier2and the surface potential of the image carrier2have a relationship that the surface potential of the image carrier2is constant, that is, approximately equal to the ground voltage V1, in a range in which the capacitance C is small and the absolute value of the surface potential of the image carrier2increases with the capacitance C in a range in which the capacitance C is larger than a predetermined value.

In the range in which the capacitance C of the image carrier2is small and the surface potential of the image carrier2is constant and equal to the predetermined voltage V0or the ground voltage V1, as shown inFIG. 3Aminus charge is directly injected between the writing electrode3bthat is in contact with the image carrier2and the charge-bearing layer2bof the image carrier2. That is, the image carrier2is charged or discharged by the charge injection.

In the range in which the capacitance C of the image carrier2is large and the surface potential of the image carrier2starts to vary, the degree of charging or discharging of the image carrier2by the charge injection decreases as the capacitance D increases. As the capacitance C increases, as indicated by arrows inFIG. 3B, the charge release comes to occur between the film-shaped substrate3aand the image carrier2. The surface of the image carrier2is charged or discharged also by such charge release. However, when the capacitance C of the writing electrode3is in this range, the degree of charging or discharging by the charge injection is high and that by the charge release is low; that is, the charging or discharging of the image carrier2is dominated by the charge injection. In the case of the charging or discharging by the charge injection, the surface potential of the image carrier2is equal to the predetermined voltage V0or the ground voltage V1that is applied to the writing electrode3b.

In the range in which the capacitance C of the image carrier2is even larger, almost no charge injection is performed between the writing electrode3band the image carrier2, that is, the image carrier2is not discharged or discharged by the charge injection. The same is true of the case that the predetermined voltage V0is positive.

Therefore, the charging or discharging of the image carrier2can be performed by the charge injection by setting the capacitance C of the image carrier2small in a range in which the surface potential of the image carrier2is constant and equal to the predetermined voltage |V0| (an absolute value is employed because V0may be a plus or minus voltage) or the ground voltage V1and switching-controlling the voltage applied to the writing electrode3bbetween the predetermined voltage V0and the ground voltage V1.

Further, as indicated by a solid line inFIG. 2E, when the writing electrode3bis connected to the A-side and the predetermined minus voltage V0is applied to the writing electrode3b,the speed (circumferential speed) v of the image carrier2and its surface potential have a relationship that the surface potential of the image carrier2increases with the speed v and the absolute value of surface potential of the image carrier2becomes constant after the speed v of the image carrier2exceeds a predetermined value. The phenomenon that the surface potential of the image carrier2increases with the speed v is considered due to facilitation of the charge injection into the image carrier2by the friction between the writing electrode3band the image carrier2. The degree of facilitation of the charge injection into the image carrier2becomes almost constant after the speed v of the image carrier2exceeds a certain value.

On the other hand, as indicated by a dashed line inFIG. 2E, when the writing electrode3bis connected to the B-side and is hence grounded, the speed v of the image carrier2and its surface potential have a relationship that the surface potential of the image carrier2is constant and equal to the ground voltage V1, that is, it is independent of the speed v of the image carrier2. The same is true of the case that the predetermined voltage V0is positive.

Still further, as indicated by a solid line inFIG. 2F, when the writing electrode3bis connected to the A-side and the predetermined minus voltage V0is applied to the writing electrode3b,the pressing force of the writing electrode3bacting on the image carrier2(hereinafter referred to simply as “pressure of the writing electrode3b”) and the surface potential of the image carrier2have a relationship that the surface potential of the image carrier2increases relatively steeply with the pressure of the writing electrode3band the absolute value of surface potential of the image carrier2becomes constant after the pressure of the writing electrode3bexceeds a predetermined value. The phenomenon that the surface potential of the image carrier2increases steeply with the pressure of the writing electrode3bis considered due to the fact that the contact between the writing electrode3band the image carrier2becomes securer as the pressure of the writing electrode3bincreases. The degree of secureness of the contact between the writing electrode3band the image carrier2becomes almost constant after the pressure of the writing electrode3bexceeds a certain value.

On the other hand, as indicated by a dashed line inFIG. 2F, when the writing electrode3bis connected to the B-side and is hence grounded, the pressure of the writing electrode3band the surface potential of the image carrier2have a relationship that the surface potential of the image carrier2is constant and equal to the ground voltage V1, that is, it is independent of the pressure of the writing electrode3b.The same is true of the case that the predetermined voltage V0is positive.

As described above, the charging or discharging of the image carrier2by the charge injection can be performed reliably and easily by setting the resistance R of the writing electrode3band the capacitance C of the image carrier2so that the surface potential of the image carrier2is kept at a constant, predetermined voltage, controlling the speed v of the image carrier2and the pressure of the writing electrode3bso that the surface potential of the image carrier2is kept at the constant, predetermined voltage, and switching-controlling the voltage applied to the writing electrode3bbetween the predetermined voltage V0and the ground voltage V1.

Although in the above example the predetermined voltage V0that is a DC voltage is applied to the writing electrode3b,the voltage applied to the writing electrode3bmay be such that an AC voltage is superimposed on a DC voltage. In the latter case, it is preferable that the DC component be set to a voltage to be applied to the image carrier2, the amplitude of the AC voltage be set to two or more times the threshold voltage Vth, and the frequency of the AC component be set to about 500 to 1,000 times the rotation frequency of the image carrier2(e.g., of the diameter of the image carrier2is 30 mm and its circumferential speed is 180 mm/s, the rotation frequency of the image carrier2is equal to about 2 Hz and hence the frequency of the AC component should be set to 1,000 to 2,000 Hz).

Superimposing an AC voltage on a DC voltage as described above makes the charging or discharging of the writing electrode3bdue to the charge release more stable. Further, since the writing electrode3bis vibrated by the AC voltage, foreign matter that is attached to the writing electrode3bcan be removed and hence the writing electrode3bis prevented from being stained.

FIG. 4shows a switching circuit for selectively supplying (with switching) the predetermined voltage V0or the ground voltage V1to the writing electrodes3b.The writing electrodes3bthat are arranged in four arrays, for example, are connected to respective high-voltage switches15which supply (with switching) the predetermined voltage V0or the ground voltage V1to the respective writing electrodes3b.An image writing control signal is supplied from a shift register16to each high-voltage switch15. An image signal stored in a buffer17and a clock signal supplied from a clock generator18are input to the shift register16. Each image writing control signal that is output from the shift register16is input, by an associated AND gate19, to the associated high-voltage switch15on the basis of a write timing signal that is supplied from an encoder20. The high-voltage switches15and the AND gates19constitute the above-mentioned driver11for switching-controlling the voltages to be supplied to the respective writing electrodes3b.

Referring toFIG. 5A, assume that the predetermined voltage V0or the ground voltage V1is applied to (n−2)th, (n−1)th, nth, (n+1)th, and (n+2)th electrodes3bby switching control of the high-voltage switches15. If an electrostatic latent image is written to the image carrier2by the electrodes3bbeing in such voltage states and subjected to normal development, a developer is stuck to portions of the image carrier2to which the predetermined voltage V0is applied, whereby a developer image as hatched inFIG. 5Bis obtained. If an electrostatic latent image is written in the same manner and subjected to inverted development, a developer is stuck to portions of the image carrier2to which the ground voltage V1is applied, whereby a developer image as hatched inFIG. 5Cis obtained.

In the image forming apparatus1using the above-configured writing head3, the writing electrodes3bcan be kept in contact with the image carrier2in a stable manner because the writing electrodes3bare brought in contact with the image carrier2by weak pressing force that is weak restoration force of the film-shaped substrate3a.Therefore, the charging of the image carrier2by the writing electrodes3bcan be performed with high accuracy in a more stable manner. Since an electrostatic latent image can be written more stably, a good image can be obtained reliably with high accuracy.

Since the writing electrodes3bare brought in contact with the image carrier2merely by weak pressing force, the image carrier2is prevented from being damaged by the writing electrodes3band hence the durability of the image carrier2can be increased. Further, since the writing device3uses the writing electrodes3band a large-size laser light generation device, LED lamp light generation device, or the like as used conventionally is not employed, the apparatus can further be miniaturized and the number of parts can further be reduced, which makes it possible to provide an image forming apparatus that is simpler and less expensive. Further, the use of the writing electrodes3bis effective in suppressing ozone generation.

As shown inFIG. 6, the drivers11are formed on the film-shaped substrate3aand electrically connected to each other by thin, flat-plate-shaped wiring portions9having a rectangular cross-section and made of copper foil, for example. Likewise, each driver11and a plurality of writing electrodes3bare electrically connected to each other by wiring portions9that are formed on the film-shaped substrate3a.The above wiring portions9can be formed by a conventional thin-film pattern forming method such as etching. Line data, a write timing signal, and a high voltage are supplied to the drivers11from the wiring portions9disposed at the upper side of the drawing.

FIG. 7Ashows a writing head3according to a first embodiment of the invention in which two arrays of writing electrodes3band3b′are formed on a tip end portion of a first face of a film-shaped substrate3aso as to be separated from each other in the secondary scanning direction (i.e., the moving direction of the image carrier2). The writing electrodes3bor3b′of each array are arranged in the primary scanning direction (i.e., parallel with the axial direction of the image carrier2).

Drivers11and11′ are fixed to the two respective faces of the film-shaped substrate3aat positions distant from the image carrier2. The writing electrodes3bthat are more distant from the tip end of the film-shaped substrate3athan the writing electrodes3b′and are connected to the first driver11via wiring portions9that are formed on the first face of the film-shaped substrate3a.The tip-side writing electrodes3b′are electrically connected to the second driver11′ via conductive members in through holes T of the film-shaped substrate3aand wiring portions9′ that are formed on the second face of the film-shaped substrate3a.

As for the arrangement pattern of the writing electrodes3band3b′, in an example ofFIG. 7B, the writing electrodes3bof a first array and the writing electrodes3b′of a second array form a zigzag arrangement with regard to the axial direction of the image carrier2(that is, the first array of the writing electrodes3band the second array of the writing electrodes3b′are separated from each other in the secondary scanning direction and any one of the writing electrodes3band3b′are not aligned in the secondary scanning direction). In an example ofFIG. 7C, the writing electrode3band the writing electrode3b′of each pair are aligned in the moving direction of the image carrier2and gradation control is enabled by turning on one or both of writing voltages for those writing electrodes3band3b′. The shape of writing electrodes3band3b′is not limited to a triangle or a circle and they may assume arbitrary shapes such as a rectangle, a trapezoid, and a trapezium.

In this embodiment, the writing electrodes3band3b′are formed on the first face of the film-shaped substrate3aand the wiring portions9and9′ corresponding to the writing electrodes3band3b′are formed on both faces of the film-shaped substrate3a.Therefore, current crosstalk can be prevented and the wiring portions9and9′ can be arranged densely on both faces of the film-shaped substrate3a,thereby stabilizing the elastic force of the film-shaped substrate3a.

FIGS. 8A and 8Bshow a writing head3according to a second embodiment of the invention. Whereas in the first embodiment the writing electrodes3b′of the second array are formed at the positions of the through holes T, in this embodiment the writing electrodes3b′of the second array are formed at positions distant from the through holes T.

FIGS. 9A and 9Bshow a writing head3according to a third embodiment of the invention. In this embodiment, writing heads3band3b′are alternately arrayed parallel with the axial direction of the image carrier2. The writing electrodes3b′are electrically connected to the second driver11′ via conductive members in through holes T and wiring portions9′ that are formed on the second face of a film-shaped substrate3a.In this embodiment, as in the case of the above embodiments, the writing electrodes3band3b′are formed on the first face of the film-shaped substrate3aand the wiring portions9and9′ corresponding to the writing electrodes3band3b′are formed on both faces of the film-shaped substrate3a.Therefore, current crosstalk can be prevented and the wiring portions9and9′ can be arranged densely on both faces of the film-shaped substrate3a,thereby stabilizing the elastic force of the film-shaped substrate3a.

FIG. 10shows a writing head3according to a fourth embodiment of the invention. In this embodiment, two arrays of writing electrodes3band3b′are formed on a tip end portion of the first face of a film-shaped substrate3aso as to be separated from each other in the secondary scanning direction (i.e., the moving direction of the image carrier2). The writing electrodes3bor3b′of each array are arranged in the primary scanning direction (i.e., parallel with the axial direction of the image carrier2). Drivers11and11′ are fixed to the two respective faces of the film-shaped substrate3aat positions distant from the image carrier2. The writing electrodes3bthat are more distant from the tip end of the film-shaped substrate3athan the writing electrodes3b′and are connected to the first driver11via wiring portions9that are formed on the first face of the film-shaped substrate3a.The tip-side writing electrodes3b′are connected to the second driver11′ via wiring portions9′ that are formed on the tip end face and the second face of the film-shaped substrate3a.This embodiment is effective in cost reduction because no through holes are formed.

FIG. 11shows a writing head3according to a fifth embodiment of the invention. In this embodiment, an original film-shaped substrate3ais folded and the resulting inside surfaces are bonded to each other. Two arrays of writing electrodes3band3b′are formed on a tip end portion of the first face of the resulting film-shaped substrate3aso as to be separated from each other in the secondary scanning direction (i.e., the moving direction of the image carrier2). The writing electrodes3bor3b′of each array are arranged in the primary scanning direction (i.e., parallel with the axial direction of the image carrier2). Drivers11and11′ are fixed to the two respective faces of the film-shaped substrate3aat positions distant from the image carrier2. The writing electrodes3bthat are more distant from the tip end of the film-shaped substrate3athan the writing electrodes3b′and are connected to the first driver11via wiring portions9that are formed on the first face of the film-shaped substrate3a.The tip-side writing electrodes3b′are connected to the second driver11′ via wiring portions9′ that are formed on the tip end face and the second face of the film-shaped substrate3a.This embodiment is effective in cost reduction because no through holes are formed.

In this embodiment, since the original film-shaped substrate3ais folded and the resulting inside surfaces are bonded to each other, the elastic force of the film-shaped substrate3acan further be stabilized.

A manufacturing method of the writing head according to the fifth embodiment will be described below with reference toFIGS. 12A and 12B.

As shown inFIG. 12A, two arrays of writing electrodes3band3b′are formed on one surface of an original film-shaped substrate3ain such a manner that the writing electrodes3b,3b′are opposed to each other at both sides of a line Y—Y which is parallel with the axial direction of the image carrier2. Wiring portions9and9′ are formed in the direction perpendicular to the line Y—Y so as to be electrically connected to the writing electrodes3band3b′. Drivers11and11′ are disposed on the original film-shaped substrate3aat both longitudinal end positions, and the writing electrodes3band3b′are electrically connected to the drivers11and11′ via the wiring portions9and9′, respectively.

Then, as shown inFIG. 12B, the film-shaped substrate3ais folded along a folding line Y′—Y′ so that the two arrays of writing electrodes3band3b′are located on the first face of a resulting film-shaped substrate3a,whereby the writing head3ofFIG. 11is obtained.

In this embodiment, the writing electrodes3band3b′are formed on the first face of the film-shaped substrate3aand the wiring portions9and9′ corresponding to the writing electrodes3band3b′are formed on both faces of the film-shaped substrate3a.Therefore, current crosstalk can be prevented and the wiring portions9and9′ can be arranged densely on both faces of the film-shaped substrate3a,thereby stabilizing the elastic force of the film-shaped substrate3a.

FIG. 13Ashows a first modification of the fifth embodiment. In this modification, the writing electrodes3band3b′have rectangular shapes and the writing electrode3band the writing electrode3b′of each pair are aligned in the moving direction of the image carrier2. Gradation control is enabled by turning on one or both of writing voltages for those writing electrodes3band3b′.

FIG. 13Bshows a second modification of the fifth embodiment. In this modification, the writing electrodes3band3b′have triangular shapes and the writing electrodes3bof the first array and the writing heads3b′of the second array are alternately arranged.

FIG. 14Ashows a third modification of the fifth embodiment. In this modification, an original film-shaped substrate3is folded after an adhesive is applied to its entire back face and the two parts of the back face are bonded to each other. This makes it possible to stabilize the elastic force when the writing head3is brought into contact with the image carrier2.

FIG. 14Bshows a fourth modification of the fifth embodiment. In this modification, an original film-shaped substrate3is folded after an adhesive is applied to its entire back face excluding a portion opposed to the writing electrodes3band3b′and the two adhesive-applied parts of the back face are bonded to each other. This modification can increase the elasticity of the tip portion of a resulting film-shaped substrate3awhere the writing electrodes3band3b′are formed because the tip end portion of the resulting film-shaped substrate3ais loosely curved.

FIG. 15shows a writing head3according to a sixth embodiment of the invention. In each of the above embodiments, the drivers11and11′ are disposed on both faces of a film-shaped substrate3a.In this embodiment, drivers11and11′ are disposed on the second face of the film-shaped substrate3aand wiring portions9that are formed on the first face are connected to the first driver11via through holes T′.

FIG. 16shows a writing head3according to a seventh embodiment of the invention. In this embodiment, film-shaped substrates3aand3a′are laminated to each other and wiring portions9,9′, and9″ are provided in three layers. Three arrays of writing electrodes3b,3b′and3b″are formed on a tip end portion of the first face of the film-shaped substrate3aso as to be separated from each other in the secondary scanning direction (i.e., the moving direction of the image carrier2). The writing electrodes3b,3b′, or3b″of each array are arranged in the primary scanning direction (i.e., parallel with the axial direction of the image carrier2). As in the embodiment ofFIG. 15, drivers11,11′, and11″ are fixed to the second face of the film-shaped substrate3a′. The writing electrodes3bthat are more distant from the tip end of the film-shaped substrate3athan the writing electrodes3b′and3b″are connected to the first driver11via the wiring portions9that are formed on the first face of the film-shaped substrate3aand through holes T′. The middle writing electrodes3b′are electrically connected to the second driver11′ via conductive members in through holes T of the film-shaped substrate3a,the wiring portions9′ that are formed on the second face of the film-shaped substrate3a,and through holes T′. The tip-side writing electrodes3b″are electrically connected to the third driver11″ via conductive members in through holes T of the film-shaped substrates3aand3a′and wiring portions9′ that are formed on the second face of the film-shaped substrate3a′.

FIG. 17shows a writing head3according to an eighth embodiment of the invention. This embodiment is different from the seventh embodiment in that the tip-side writing electrodes3b″are connected to the third driver11″ via the tip end faces of the film-shaped substrates3aand3a′and the wiring portions9″ that are formed on the second face of the film-shaped substrate3a′.

The invention is not limited to the above embodiments and various modifications are possible. For example, although in the above embodiments the one or two film-shaped substrates are used and the wiring portions are provided in two or three layers, three or more film-shaped substrates may be used and wiring portions may be provided in four or more layers.

FIG. 18shows a second example of an image forming apparatus according to the invention. This image forming apparatus is different from the image forming apparatus ofFIG. 1Ain that the former is equipped with a uniform charge controller7. The other members inFIG. 18are given the same reference symbols as inFIG. 1Aand will not be described in detail. The uniform charge controller7is to perform a control to establish a uniform charge distribution state on the surface of the latent image carrier2by removing charge remaining on the surface of the latent image carrier2after an image transfer or charging the latent image carrier2after an image transfer.

In the writing head3, for example, as shown inFIG. 19A, writing electrodes3bare formed on a tip end portion3a1of a support substrate3aand an end portion3a2of the support substrate3athat is located on the side opposite to the writing electrodes3bis fixed by a proper fixing member. A driver11for controlling the operation of the writing electrodes3bis fixed to the end portion3a2of the support substrate3a.A reinforcing member10for increasing the rigidity in the primary scanning direction (i.e., in the direction parallel with the axial direction of the latent image carrier2) is integral with the flexible support substrate3a.The writing electrodes3bwrite an electrostatic latent image being pressed weakly against the surface of the image carrier2by elastic restoration force that is produced by the flexed support substrate3a.

In another writing head3shown inFIG. 19B, writing electrodes3bhave rectangular shapes and two arrays of writing electrodes3bare arranged in the secondary scanning direction (i.e., the circumferential direction of the latent image carrier2). Both end portions of a support substrate3aare fixed to fixing members.

In either case, since a plurality of writing electrodes3bare arranged parallel with the axial direction of the latent image carrier2(i.e., in the primary scanning direction), the support substrate3aassumes a rectangular-plate-shaped shape whose length is approximately equal, in the axial direction of the latent image carrier2, to the length of the independent electrode portion2dof the latent image carrier2. The reinforcing member10prevents local low-rigidity regions from occurring between the writing electrodes3bor wiring patterns and thereby allows the writing electrodes3bto contact the latent image carrier stably. The reinforcing member10also prevents waving or wrinkling of the writing head3. InFIG. 19A, the support substrate3aextends right to left, that is, in the direction opposite to the rotation direction of the latent image carrier2(clockwise; indicated by an arrow).

In the states ofFIGS. 19A and 19B, the support substrate3ais somewhat flexed elastically and thereby produces weak elastic restoration force, whereby the writing electrodes3bare pressed against the latent image carrier2by weak pressing force and thereby brought in contact with the latent image carrier2. Since the force of pressing the writing electrodes3bagainst the latent image carrier2is weak, the abrasion of the independent electrode portion2dof the latent image carrier2by the writing electrodes3bis suppressed and the durability of the independent electrode portion2dis thereby increased. Further, since the writing electrodes3bare brought in contact with the independent electrode portion2dby the elastic force of the support substrate3a,the contact is stable.

FIG. 20Ashows a writing head3according to a ninth embodiment of the invention. In this embodiment, writing electrodes3bare arrayed and a backside reinforcing member10is integral with at least a writing electrode forming portion of a support substrate3athat covers all the writing electrodes3b.The reinforcing member10may be made of either an insulative material or a conductive material, and may be an elastic material such as PET or polyimide or a metal material such as stainless steel or copper. As a further alternative, a tape of conductive foil or metal foil may be stuck to the support substrate3a.In a case where the reinforcing member10has a shape as same as wiring patterns of the writing head3, the reinforcing member10can be formed by using a mask at the same time as the wiring patterns of the writing head3are formed. Therefore, no step of forming the reinforcing member10later is needed and the productivity is improved accordingly.

Instead of arranging the writing electrodes3bin line, plural lines of writing electrodes3bmay be arranged in the secondary scanning direction. For example,FIG. 20Bshows an example in which two arrays of writing electrodes3bare arranged in the secondary scanning direction in such a manner that the writing electrodes3bare staggered and drivers11are disposed on one side of the two arrays of writing electrodes3b.FIG. 20Cshows an example in which two arrays of writing electrodes3bare arranged in the secondary scanning direction in such a manner that the writing electrodes3bare staggered and drivers11are disposed on both sides of the two arrays of writing electrodes3b.

FIG. 21Ashows an example in which three arrays of writing electrodes3bare arranged in the secondary scanning direction in such a manner that the writing electrodes3bof the three arrays are not aligned in the secondary scanning direction, and drivers11are disposed on one side of the three arrays of writing electrodes3b.FIG. 21Bshows an example in which drivers11are disposed on both sides of three arrays of writing electrodes3bin the secondary scanning direction.FIGS. 22A and 22Bshow similar arrangement examples in which four arrays of writing electrodes3bare arranged in the secondary scanning direction. Reinforcing members10are formed for the respective lines of writing electrodes3bin such a manner that each reinforcing member3bcovers all the associated writing electrodes3b.

Since as described above the reinforcing member10or each of the reinforcing members10is integrally formed so as to cover all the writing electrodes3bof each array, the portions between the writing electrodes3band the portions between the wiring patterns where the rigidity is much lower than in the portions of the writing electrodes3band the wiring patterns can be reinforced. Therefore, waving or wrinkling of the writing head3in the primary scanning direction is prevented and hence the writing electrodes3bcan stably be brought in contact with the latent image carrier2. As a result, an electrostatic latent image can be formed correctly on the latent image carrier2and the print quality can thereby be improved.

Where plural lines of writing electrodes3bare arranged in the secondary scanning direction, the reinforcing member10may be formed so as to cover all the arrays of writing electrodes3b.FIGS. 23A and 23Bshow examples in which the reinforcing member10is formed so as to cover both arrays of writing electrodes3barranged in the secondary scanning direction. In the example ofFIG. 23A, the drivers11are disposed on one side of the two arrays of writing electrodes3bin the secondary scanning direction. In the example ofFIG. 23B, the drivers11are disposed on both sides of the two arrays of writing electrodes3bin the secondary scanning direction.

FIGS. 24A and 24Bshow examples in which the reinforcing member10is formed so as to cover all the three arrays of writing electrodes3barranged in the secondary scanning direction. In the example ofFIG. 24A, the drivers11are disposed on one side of the three arrays of writing electrodes3bin the secondary scanning direction. In the example ofFIG. 24B, the drivers11are disposed on both sides of the three arrays of writing electrodes3bin the secondary scanning direction.

FIGS. 25A and 25Bshow examples in which the reinforcing member10is formed so as to cover all the four arrays of writing electrodes3barranged in the secondary scanning direction. In the example ofFIG. 25A, the drivers11are disposed on one side of the four arrays of writing electrodes3bin the secondary scanning direction. In the example ofFIG. 25B, the drivers11are disposed on both sides of the four arrays of writing electrodes3bin the secondary scanning direction.

Since as described above the reinforcing member10is formed so as to be to cover all the arrays of writing electrodes3barranged in the secondary scanning direction, the portions that are located between the arrays of the writing electrodes3barranged in the secondary scanning direction and in which no wiring patterns exist and hence the rigidity is much lower than in the other portions can be reinforced. Therefore, stress concentration and folding of the writing head3is prevented there and hence the lines of writing electrodes3bcan be brought in contact with the latent image carrier2equally and stably. As a result, an electrostatic latent image can be formed correctly on the latent image carrier2and the print quality can thereby be increased. That is, a problem that horizontal streaks appear in an image because of a phenomenon that folding of the writing head3vary the distances between the lines of writing electrodes3bto disorder the dot pitch of an electrostatic latent image can be solved.

FIG. 26Ashows a writing head3according to a tenth embodiment of the invention. As shown inFIG. 26B, a reinforcing member10in this embodiment is a frame-shaped which surrounds a region where the writing electrodes3bare formed in both of the primary and secondary scanning directions. In addition, patterns extending in the secondary scanning direction are arrayed in the intermediate portions of the frame in the primary scanning direction. Specifically, the patterns extending in the primary scanning direction prevent waving and wrinkling of the writing head3and the patterns extending in the secondary scanning direction reinforce the portions between the four arrays of writing electrodes3b.

FIGS. 26C and 26Dshow an example in which a reinforcing member10is composed of a pattern disposed at a center portion in the secondary scanning direction of the region where the writing electrodes3bare formed and extending in the primary scanning direction, and a plurality of patterns extending from the central pattern to both ends of the region in the secondary scanning direction. The central pattern extending in the primary scanning direction attains reinforcement for preventing waving and wrinkling of the writing head3.

In the writing head3, the support substrate3ais somewhat flexed elastically to produce weak elastic restoration force, whereby the writing electrodes3bare brought into contact with the latent image carrier2by weak pressing force. Since the pressing force is weak, the abrasion of the charge-bearing layer2bof the latent image carrier2by the writing electrodes3bis suppressed and the durability of the charge-bearing layer2bis thereby enhanced. Further, the writing electrodes3bare brought in contact with the charge-bearing layer2bstably by the elastic force of the support substrate3a.However, since the reinforcing member10is formed on the back face that is opposite to the surface where the writing electrodes3bare formed, the writing electrodes3bmay lower the elasticity to thereby increase the pressing force and hence the abrasion or to lower the stability of their contact to the charge-bearing layer2b.To avoid this problem, the reinforcing member10may be formed in such a manner that the reinforcing member10is not opposed to the writing electrodes3b.

FIGS. 27A and 27Bshow a writing head3having such a reinforcing member10according to an eleventh embodiment of the invention.FIGS. 27C and 27Dshow writing heads3that correspond to the writing heads3ofFIGS. 20B and 20C, respectively, and in which reinforcing member10are formed so as not to oppose to the writing electrodes3b.

FIGS. 28A and 28Bshow writing heads3that correspond to the writing heads3ofFIGS. 21A and 21B, respectively, and in which reinforcing member10are formed so as not to oppose to the writing electrodes3b.

FIGS. 29A and 29Bshow writing heads3that correspond to the writing heads3ofFIGS. 22A and 22B, respectively, and in which reinforcing member10are formed so as not to oppose to the writing electrodes3b.

FIGS. 30A and 30Bshow writing heads3that correspond to the writing heads3ofFIGS. 23A and 23B, respectively, and in which reinforcing member10are formed so as not to oppose to the writing electrodes3b.

FIGS. 31A and 31Bshow writing heads3that correspond to the writing heads3ofFIGS. 24A and 24B, respectively, and in which reinforcing member10are formed so as not to oppose to the writing electrodes3b.

FIGS. 32A and 32Bshow writing heads3that correspond to the writing heads3ofFIGS. 25A and 25B, respectively, and in which reinforcing member10are formed so as not to oppose to the writing electrodes3b.

The invention is not limited to the above embodiments and various modifications are possible. For example, although in the above embodiments the reinforcing member10made of an elastic such as PET or polyimide or a metal material such as stainless steel or copper is integral with the support substrate3aor the corresponding portion of the support substrate3ais made thicker than in the other portions, the strength of the support substrate3ain the primary scanning direction (i.e., the direction parallel with the axial direction of the image carrier2) may be made relatively higher by forming, in the support substrate3a,slits extending in the secondary scanning direction, or strength anisotropy may be imparted to the support substrate3aitself by draw molding. Although the above embodiments are directed to the writing heads3in which the reinforcing member10is formed on the surface of the support substrate3athat is opposite to its surface on which the writing electrodes3bare formed, the reinforcing member10may be formed on the surface on which the writing electrodes3bare formed. In the latter case, naturally the reinforcing member10should be formed so as not to interfere with the writing electrodes3b.

FIGS. 33A–33Jillustrate a writing electrode manufacturing method according to the invention. First, metal foil Cu is laid on one surface of an insulative member (corresponds to a film-shaped substrate3a) PI and a photoresist PR is applied to the top surface of the metal foil Cu (seeFIG. 33A). The photoresist PR is covered with a mask M that is formed with wiring patterns, and is then exposed to light (seeFIG. 33B). Light-exposed portions of the photoresist PR are etched away (seeFIG. 33C). Then, wiring portions9are formed by etching away the exposed portions of the metal foil Cu (seeFIG. 33D). After another photoresist PR is applied to the entire surface (seeFIG. 33E), holes are formed through the photoresist PR by laser light illumination in regions where to form writing electrodes (seeFIGS. 33F and 33G). Then, metal layers PL (i.e., projections corresponding to writing electrodes) having a necessary thickness are formed by plating in the holes of the photoresist PR (seeFIG. 33H). By removing the photoresist PR, a writing head3having, on a film-shaped substrate3a,the wiring portions9and writing electrodes3bthat project from the respective wiring portions9is obtained (seeFIGS. 33I and 33J).

FIGS. 34A–34Iillustrate a first modification of the above manufacturing method. Steps ofFIGS. 34A–34Eare the same as in the above manufacturing method. After the formation of the wiring portions9, photoresist layers PR are formed in regions where to form projections corresponding to writing electrodes (seeFIG. 34F). Portions of the metal foil Cu that are not covered with the resist layers PR and have a predetermined thickness are etched away to form projections, that is, steps (seeFIG. 34G). By removing the photoresist layers PR that are located on the projections, a writing head3having, on a film-shaped substrate3a,the wiring portions9and writing electrodes3bthat project from the respective wiring portions9is obtained (seeFIGS. 34H and 34I).

FIGS. 35A–35Jillustrate a second modification of the manufacturing method ofFIGS. 33A–33J. First, metal foil Cu is laid on one surface of an insulative member (corresponds to a film-shaped substrate3a) PI and a photoresist PR is applied to the top surface of the metal foil Cu (seeFIG. 35A). The photoresist PR is covered with a mask M that is formed with writing electrode patterns, and is then exposed to light (seeFIG. 35B). Light-exposed portions of the photoresist PR are etched away to form holes (seeFIGS. 35C and 35D), the exposed portions of the metal foil Cu is plated with copper (seeFIG. 33E), and the photoresist layers PR are removed (seeFIG. 35F). Then, another photoresist PR is applied to the entire surface (seeFIG. 35G). The photoresist PR is covered with a mask that is formed with wiring patterns, and is then exposed to light (seeFIG. 35H). By etching away unnecessary portions of the wiring portions and removing the photoresist PR, a writing head3having, on a film-shaped substrate3a,wiring portions9and writing electrodes3bthat project from the respective wiring portions9is obtained (seeFIGS. 35I and 35J).