DEVELOPMENT APPARATUS AND AN IMAGE FORMATION APPARATUS

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional drawing of a system used in an experiment regarding the present invention;

FIG. 2 is a cross-sectional drawing showing a flare state of the experimental system;

FIG. 3 is a graph showing relationships between the degree of flare activity and Vmax V/p μm obtained as a result of the experiment;

FIG. 4 is a plot diagram showing relationships between the degree of flare activity and volume specific resistance of a surface layer 3 as a result of the experiment;

FIG. 5 is a cross-sectional drawing of another system used in the experiment regarding the present invention;

FIG. 6 is a graph showing relationships between development gaps and increases in optical concentration on a substrate A as a result of the experiment;

FIG. 7 is a perspective diagram of an example of a toner supporting object according to the embodiment of the present invention;

FIG. 8 is a cross-sectional drawing showing a part of a manufacturing process of the toner supporting object;

FIG. 9 is a cross-sectional drawing showing another part of the manufacturing process of the toner supporting object;

FIG. 10 is a development showing where the toner supporting object is expanded in the shape of a flat surface;

FIG. 11 is a schematic diagram of Example 1 of an image formation apparatus wherein a hopping method is used for development;

FIG. 12 is a schematic diagram of Example 2 of the image formation apparatus wherein the hopping method is used for development;

FIG. 13 is a schematic diagram of Example 3 of the image formation apparatus wherein the hopping method is used for development;

FIG. 14 is a schematic diagram of a variation of the image formation apparatus according to the embodiment;

FIG. 15 is a waveform chart of an A-phase pulse voltage and a B-phase pulse voltage applied to an electrode of the toner supporting object;

FIG. 16 is a waveform chart showing a pulse voltage applied to an electrode of a toner supporting object according to Patent Reference 1; and

FIG. 17 is a schematic diagram of the image formation apparatus according to the embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of the present invention are described with reference to the accompanying drawings.

First, experiments conducted by the Inventors hereto concerning the present invention are described. With reference to FIG. 1, a substrate 4 serving as a toner supporting object was manufactured by first carrying out a vacuum evaporation of aluminum on a glass substrate 1 so that an electrode pattern 2 was formed, wherein two or more electrodes 21, 22, and 23 were arranged at a pitch of p μm in a moving direction. Then, a protection layer 3 was formed on the electrode pattern 2. The protection layer 3 was a resin coat having about 3 μm thickness and a volume specific resistance of about 10¹⁰Ω-cm. On the substrate (toner supporting object) 4, a toner layer 5 that was electrified was formed.

The toner layer 5 was formed on the substrate 4 by developing a solid black image with a 2-component development apparatus that is not illustrated. The toner was of a polyester system, and has a particle diameter of about 6 μm. The amount of electrification of the toner formed into a thin film on the substrate 4 was about −22 μC/g. Then, an AC voltage was applied to the electrodes as shown in FIG. 2. Specifically, to odd-numbered electrodes 21, 23, and so on, the AC voltage in one phase was provided from an AC power supply 6; and to even-numbered electrodes 22, and so on, the AC voltage in the inverse phase was provided. Then, particles of the toner 5 moved from the odd-numbered electrodes to the even-numbered electrodes, and vice versa. This phenomenon is hereinafter called flare (or flare phenomenon). Further, where the flare phenomenon is present is called a flare state.

Four samples of the substrate 4 were manufactured, wherein the electrode pitch p between the electrodes 21, 22, 23, and so on was differentiated, namely, p=50, 100, 200, and 400 μm. Then, the degree of flare activity was observed with a high-speed camera, changing Vmax V that is a voltage difference between a positive peak and a negative peak of the voltage of the AC voltage applied to the electrodes 21, 22, 23, and so on from the AC power supply 6. Results are obtained as shown in FIG. 3. Here, the width of the electrodes 21, 22, 23, and so on, and the distance from the electrodes 21, 22, 23, and so on to the respective adjacent electrodes was equal to one half of the electrode pitch p of the electrodes 21, 22, 23, and so on.

Here, the degree of flare activity was obtained by a sensory evaluation wherein a situation of the toner not moving but sticking to the surface of the substrate 4 was evaluated in 5 levels. As shown in FIG. 3, the degree of flare activity was almost unambiguously determined by a ratio of Vmax V/p μm regardless of values of Vmax and p. As shown in FIG. 3, the flare began to be active at Vmax V/p μm=1, and was completely activated at Vmax V/p μm>=3.

Further, in order to determine the influence of an electrical property of the surface of the substrate 4, a volume specific resistance of the surface layer 3 of the substrate 4 was changed, and the degree of the flare activity was similarly evaluated. The material of the surface layer 3 was a silicone system resin, and the amount of carbon particles contained in the silicone system resin was changed so that the surface layer 3 (thickness of which surface layer 3 is about 5 μm) has a volume specific resistance between 10⁷and 10¹⁴Ω-cm. As a typical example, the electrode pitch was 50 μm. Then, experimental results were obtained as shown in FIG. 4.

From the experimental results, it was determined that an appropriate volume specific resistance of the surface layer 3 ranged between 10⁹and 10¹²Ω-cm. Otherwise, if the volume specific resistance of the surface layer 3 was too high, the surface of the substrate 4 remained being charged by friction of the surface layer 3 and the toner that repeated hopping. The charge causes the surface voltage of the substrate 4 to fluctuate, and a bias voltage for development became unstable. To the contrary, if the conductivity of the surface layer 3 was too high, a charge leakage (short circuit) occurred between the electrodes 21, 22, 23, and so on, and the bias voltage was degraded. That is, the specific resistance of the surface layer 3 had to be appropriate (i.e., between 10⁹and 10¹²Ω-cm) such that the charge accumulated on the surface layer 3 on the substrate 4 could be properly discharged to the electrodes 21, 22, 23, and so on. In addition, the range of the volume specific resistance was determined through experiments using the experimental system shown in FIG. 2. In the case of a development apparatus that includes a developing roller (described below) as shown in FIG. 11, the optimal range may be different, and a suitable range of the volume specific resistance is to be determined through experiments.

The inventors hereto further conducted an observation of the degree of flare activity with two kinds of the surface layer 3; namely, one was made of silicone system resin and the other was made of fluorine system resin. This was to determine the influence of a friction electrification characteristic of the surface of the substrate 4. The volume specific resistance of the surface layer 3 of both the silicone system resin and the fluorine system resin was made to be 10¹¹-10¹²Ω-cm by distributing a small quantity of carbon particles. Then, the degree of flare activity was observed by applying an AC bias to the electrodes 21, 22, 23, and so on from the AC power supply 6. With the surface layer 3 of the silicone system resin, the flare state was maintained for a long time. With the surface layer 3 of the fluorine system resin, the flare quickly disappeared and the toner stuck to the substrate 4.

After the observation, the amount of electrification of the toner on the substrate 4 was measured. With the surface layer 3 of the silicone system resin, the amount of electrification of the toner on the substrate 4 was a little smaller than at the initial stage. With the surface layer 3 of the fluorine system resin, the amount of electrification of the toner on the substrate 4 was almost lost. Next, toner that was not charged was rubbed onto the surface layer 3. In the case of the surface layer 3 of the silicone system resin, the toner acquired a positive polarity charge. In the case of the surface layer 3 of the fluorine system resin, the toner acquired almost no charge, or a slight negative polarity charge. Given that the flare phenomenon is a process wherein the toner collides with the surface of the substrate 4 for a great number of times, the surface layer 3 is desirably made of a material that gives a positive polarity charge to the toner, rather than a material that discharges the toner. That is, a friction electrification series of materials can be referenced, and the surface layer 3 is desirably made of, e.g., a glass system, and a material used in a carrier coat of 2-component developer.

Next, experimental results of another system as shown in FIG. 5 are described. This system includes a substrate A that is formed by a resin layer 8 (assumed as a photo conductor) on a substrate 7 made of aluminum. The resin layer 8 is about 20 μm thick. The substrate 7 is grounded. A toner layer 9 of 0.4 mg/cm²equivalent to a solid black image is formed on the resin layer 8. The toner layer 9 is formed by developing the solid black image on the resin layer 8 with a 2-component development apparatus that is not illustrated.

The system further includes a substrate B that is installed countering the substrate A at a gap d μm. The substrate B is constituted like the substrate 4. The surface layer 3 is made into a white coat layer so that measurement with an optical measurement instrument (a catoptrical light concentration measuring instrument) of an amount of toner to be transferred here by subsequent work is facilitated. Then, dependence of the amount of toner transferred onto the substrate B on the development gap d μm was investigated, wherein four combinations of Vmax V and p μm were used, all of which combinations give Vmax V/p μm=4. Note that the flare was stably formed where Vmax V/p μm is equal to 4 regardless of other conditions as shown in FIG. 3. Results obtained are shown in FIG. 6. The vertical axis of FIG. 6 represents the amount of increase in optical concentration of the surface layer 3 of the substrate B. The horizontal axis of FIG. 6 represents the development gap d μm. Where no toner is adhered to the surface layer 3, the amount of increase in optical concentration is 0. In FIG. 6, there are results wherein the amount of increase in optical concentration is greater rather than 0; this is because some toner of the toner layer 9 adhering to the resin layer 8 of the substrate A was transferred to the surface layer 3 of the substrate B from the toner layer 9 by the influence of the electric field formed on the substrate B. If such transfer occurs in superposition development, toner formed on a latent image supporting object (for example, a photo conductor) for a preceding development is transferred to the development apparatus of a following color at the time of the following development, that is, colors are mixed. Further, an image obtained on the latent image supporting object in the preceding development is disturbed. Accordingly, when the amount of increase in optical concentration is 0, the color mixture and the image disturbance can be avoided. It is determined from FIG. 6 that if the pitch distance p is smaller than the development gap d (p<d), the color mixture and the image disturbance can be avoided.

This can be taken as a condition under which influences of an electric field curtain formed on the toner supporting object (substrate B) do not reach the electrostatic latent image electric field or the toner image on the latent image supporting object (substrate A). Under this condition, not only an isolated dot can be correctly developed at, for example, 1200 dpi and 2400 dpi without scavenging, but also a very high definition toner image superposition can be realized without causing the color mixture of the toner in the development apparatus, and without disturbing the toner image previously formed on the latent image supporting object when forming an image by an image forming process like the toner image superposition on the latent image supporting object (substrate A) as described above.

By the way, conventional development apparatuses used by image formation apparatuses, such as a copying machine, a printer, and a facsimile apparatus, often employ one of a 2-component development method, and a 1-component development method. The 2-component development method is suitable for high-speed development, and currently is a mainstream method for medium to high-speed image formation apparatuses. According to the 2-component development method, in order to obtain high definition, the developer at a position contacting the electrostatic latent image on the latent image supporting object has to be very minute. For this purpose, miniaturization of the diameter of carrier particles is progressing, and a particle diameter of about 30 μm is commercially available.

The 1-component development method, which requires a small and lightweight mechanism, is a mainstream of low speed image formation apparatuses at present. According to the 1-component development method, the toner supported on the surface of a developer supporting object, such as a developing roller, is used for development without hopping. Specifically, in order to form a thin toner layer on the developing roller, a toner regulating member, such as a blade and a roller, is made to contact the toner on the developing roller, and the toner is then electrified by friction with the developing roller and the toner regulating member. The electrified thin toner layer formed on the developing roller is carried to the development position, and the electrostatic latent image on the latent image supporting object is developed. Here, the 1-component development method can be classified into two types. One is a contacting type and the other is a non-contacting type. As for the former, the developing roller and the latent image supporting object make contact; and as for the latter, the developing roller and latent image supporting object do not make contact.

For example, JPA H3-100575 discloses a hybrid method wherein the 2-component development method and the 1-component development method are combined such that shortcomings of one are complemented by the other.

For example, JPA H3-113474 discloses a method of developing minute uniform dots of high resolution. According to the method, a wire, to which a high frequency bias is applied, is provided in the development unit that is based on the hybrid method as described above so that a toner cloud is obtained in the development unit. In this way, high resolution dots can be developed.

Further, JPA H3-21967 (Patent Reference 1) discloses a method of efficiently and stably forming the toner cloud, wherein an electric-field curtain is formed on a rotation roller.

Further, JPA 2003-15419 discloses a development apparatus wherein a developer is conveyed by an electric-field curtain by a progressive-wave electric field. Further, JPA H9-269661 discloses a development apparatus that includes two or more magnetic poles for almost uniformly adsorbing a carrier of about one layer on the circumference of a developing roller. Further, JPA 2003-84560 discloses a development apparatus wherein a periodic conductive electrode pattern is formed on a developer supporting body for supporting non-magnetic toner through an insulation member, and an electric-field gradient is generated near the developer supporting body surface by providing a predetermined bias voltage to the electrode so that non-magnetic toner is adhered to the developer supporting object and conveyed by the developer supporting object.

It is increasingly required that the conventional 2-component development provide high-definition capability. It is required that the dot size of a pixel be equivalent to or even smaller than the present diameter of the carrier particles. From the viewpoint of reproducibility of an isolated dot, the diameter of the carrier particles should be smaller than now. However, the smaller is the diameter, the smaller becomes the permeability of the carrier particles. Small particle carrier tends to be disassociated from a developing roller. If the disassociated carrier particles are adhered to a latent image supporting object, various side effects can occur such as damaging the latent image supporting object in addition to producing a faulty image.

In order to prevent the carrier from disassociating, attempts have been made, e.g., by raising the permeability of the carrier particles by selecting an appropriate material, and by strengthening the magnetism of a magnet included in the developing roller. However, development activities have experienced extreme difficulty in balancing between cost and high definition. Further, requirements for miniaturization of an apparatus lead to reduction of the diameter of the developing roller, which makes it further difficult to design a developing roller with a powerful magnetic field for preventing the carrier disassociation.

First of all, since the 2-component development method is a process of forming a toner image by an ear, called a magnetic brush, of a 2-component developer rubbing an electrostatic latent image, unevenness tends to be produced when developing an isolated dot due to the heterogeneity of the ear. Even though an improvement in quality of the image is possible by forming an alternating current (AC) electric field between the developing roller and the latent image supporting object, it is difficult to completely remove the unevenness of the image due to fundamental unevenness of the ear of the developer.

Further, in order to raise transfer efficiency of a process of transferring the toner image developed on the latent image supporting object, and in order to raise cleaning efficiency of a process of cleaning the toner that remains on the latent image supporting object after transfer, non-electrostatic adhesion of the toner to the latent image supporting object has to be reduced as much as possible. As a method of reducing the non-electrostatic adhesion of the toner to the latent image supporting object, the friction coefficient of the surface of the latent image supporting body may be reduced. However, if the friction coefficient is small, the ear of the 2-component developer may smoothly pass through the development position, which causes the development efficiency and dot reproducibility to be degraded.

According to the 1-component development method, the toner layer on the developing roller is pressed to fully contact the toner regulating member; for this reason, toner response to the electric field at the development position is poor. Accordingly, it is usual practice to form a powerful alternating current (AC) electric field between the developing roller and the latent image supporting object in order to obtain high definition. However, even if the AC electric field is formed, it is difficult to stably develop the toner at a constant rate for the electrostatic latent image, and it is difficult to uniformly develop the minute dot of a high resolution. Further, according to the 1-component development method, great stress is applied to the toner when the toner is formed in lamination on the developing roller, which causes the toner, which is recycled in the development apparatus, to be quickly degraded. With the degradation of the toner, the toner tends to be uneven. For this reason, the 1-component development method is not generally suitable for a high speed/high durability image formation apparatus.

According to the hybrid method (JPA H3-100575), although the size and the number of components of the development apparatus are increased, some problems are overcome. Nevertheless, the same problem as the 1-component development method remains at the development position, that is, it is difficult to develop minute and uniform dots at high resolution.

Although high stability and high definition development may be realized by the method disclosed by JPA H3-113474, the configuration of the development apparatus becomes complicated.

Further, the method disclosed by JPA H3-21967 (Patent Reference 1) may be excellent in obtaining small and high-definition development; however, the Inventors hereto have discovered that, in order to obtain the ideal high definition, conditions concerning such as electric-field curtain forming and development are restrictive. In other words, unless image formation is carried out under the restrictive conditions, merits of the method cannot be enjoyed at all; not only that, the image quality is degraded. Further, according to this method, the toner is conveyed while hopping on the toner supporting object to a development position by movement of the toner supporting object; however, this problem (limited conditions) is common to the method disclosed by JPA 2002-341656, wherein the toner is conveyed to the development position only by a hopping movement, not by a movement of the toner supporting object.

Further, for an image formation process wherein a first toner image is formed on the latent image supporting object, then a second toner image, then a third toner image, and so on, one by one, the development method has to be such that the toner image(s) already formed on the latent image supporting object is not disturbed. According to the non-contacting 1-component development method and the toner cloud development method disclosed by JPA H3-113474, it is possible to form color toner images one by one on the latent image supporting object. However, since an alternating current (AC) electric field is formed between the latent image supporting object and the developing roller, a part of the toner tends to be torn off from the toner image already formed on the latent image supporting object, and the torn-off toner enters the development apparatus. That is, the image on the latent image supporting object is disturbed by this, and further a problem is posed that the toner in the development apparatus is mixed with another color. This is a vital problem for acquiring a high-definition image. A method of generating a toner cloud, wherein no alternating current (AC) electric field is formed between the latent image supporting object and the developing roller, is required.

The toner cloud may be generated by the methods disclosed by JPA H3-21967 (Patent Reference 1), and JPA 2002-41656 described above; however, merits of the methods cannot be enjoyed unless strict conditions are met as described above; otherwise, they are completely ineffective. If the conditions are not appropriate, the toner cloud cannot be generated. Even if the toner cloud is generated, a part of the toner in the toner layer on the latent image supporting object is mixed into the development apparatus of the next color, causing image disturbance and color mixture.

Then, according to the image formation apparatus of the embodiment, the condition of Vmax V/p μm>1 is sufficed in view of the result of the experiment described above. The toner cloud can be surely generated with this configuration. Therefore, according to the embodiment, higher definition and smaller size are realized compared with the conventional techniques.

In addition, it is conceivable that the toner cloud is surely generated by the method disclosed by JPA 2002-341656, wherein the toner is electrostatically moved by an AC electric field of three or more phases, not by a mechanical movement of the toner supporting object, if the condition as described above is sufficed. However, according to the method, the toner may be deposited on a conveyance substrate triggered by toner that is not electrostatically conveyed by some chance, and as a result, operations stop. In order to solve this problem, JPA 2004-286837 proposes a structure having a combination of a fixed conveyance plate and a toner supporting object that moves on the surface of the fixed conveyance plate; nevertheless, this makes the mechanism highly complicated. Conversely, according to the image formation apparatus of the embodiment of the present invention, wherein the toner is conveyed to the development position by movement of the surface of the toner supporting object, which toner moves to and fro between the electrodes by hopping, the toner is not accumulated, and the mechanism is not complicated.

FIG. 7 is a perspective diagram of a typical example of a toner supporting object 31 for the image formation apparatus according to the embodiment of the present invention. The toner supporting object 31 is formed in the shape of a rotating roller, and includes an electrode pattern that is constituted by two or more electrodes 41, 42, 43, and so on that are spatially-periodically arranged at a pitch of p μm in the rotational direction. There, odd-numbered electrodes are grouped to constitute an electrode shaft 40A, and even-numbered electrodes are grouped to constitute an electrode shaft 40B. The toner supporting object 31 can be rotated with the electrode shafts 40A and 40B serving as a rotation shaft. An AC voltage is provided to each of the electrode shafts 40A and 40B as a bias voltage from an AC power source (not illustrated) through an electrode brush (not illustrated). As shown in FIG. 15, the AC voltage includes a rectangular pulse A (A-phase pulse) that is provided to the electrode shaft 40A that bundles the odd-numbered electrodes, and a rectangular pulse B (B-phase pulse) that is provided to the electrode shaft 40B that bundles the even-numbered electrodes. Voltages of the A-phase pulse and the B-phase pulse have inverse phases as illustrated. The A-phase pulse and the B-phase pulse have the same average voltage in a unit of time. Alternatively, as shown in FIG. 16, a rectangular pulse of a frequency f may be provided to one of the electrode shafts, and a DC voltage that is the same as the average voltage is provided to the other. Either way, the flare phenomenon can take place.

A manufacturing process of the toner supporting object 31 is described with reference to FIGS. 8, 9, and 10. The toner supporting object 31 includes a cylinder 51 made of an acrylic resin that is an insulator, and shaft holes 52 formed in the cylinder 51 as shown at (a) of FIG. 8. Then, the electrode shafts 40A and 40B made of stainless steel are press fit into the corresponding shaft holes 52 as shown at (b) of FIG. 8. The odd-numbered electrodes 41, 43, and so on are connected to the electrode shaft 40A; and the even-numbered electrodes 42, and so on are connected to the electrode shaft 40B. Then, the electrode pattern is formed through processes shown in FIG. 9 at (a) through (e). FIG. 9 shows the surface of the toner supporting object 31 in a direction along the rotation axle. At a process shown at (a) of FIG. 9, the surface of the cylinder 51 is finished flat and smooth by perimeter lathe turning. At a process shown at (b) of FIG. 9, slots 53, each slot being 50 μm wide, are formed at a slot pitch of 100 μm. At the process shown at (c) of FIG. 9, non-electrolyzed nickel 54 is plated on the cylinder 51, in which the slots 53 are formed. At a process shown at (d) of FIG. 9, the perimeter of the cylinder 51, on which the non-electrolyzed nickel 54 is plated, is machined so that an unnecessary conductor film may be removed, and the electrodes 4i−1, 4i, 4i+1, and so on may be formed in the corresponding slots 53. The electrodes 4i−1, 4i, 4i+1, and so on are mutually insulated. Then, the surface of the cylinder 51 is made flat and smooth by coating a silicone system resin on the cylinder 51, and at the same time, a protection layer 55 (about 5 μm thick, having a volume specific resistance about 10¹⁰Ω-cm) is formed on the surface of the cylinder 51. The cylinder 51 manufactured in this way now serves as the toner supporting object 31. FIG. 10 is a development view of the toner supporting object 31 as expanded in the shape of a flat surface.

A thin toner layer is formed on the protection layer 55 of the toner supporting object 31, like the substrate 4. When the AC voltage, serving as a bias voltage, as shown in FIG. 15 is provided to the electrode shafts 40A and 40B from the AC power source through the electrode brush, the toner flares (moves) between the odd-numbered electrodes 41, 43, and so on and the even-numbered electrodes 42, and so on. The flare begins to be activated where Vmax V/p μm>1, and the flare is completely activated where Vmax V/p μm>3, where Vmax V is an absolute value of the difference between a positive peak of the AC voltage and a negative peak of the AC voltage provided to the electrodes 41, 42, 43, and so. Further, an appropriate volume specific resistance of the protection layer 55 of the toner supporting object 31 is in a range of 10⁹-10¹²Ω-cm, like the substrate 4; and the protection layer 55 is made of a silicone system resin. As for the material of the protection layer 55, it is desirable that the material be capable of providing a positive charge to the toner by friction with the toner as described above; for example, it is desirable to use a glass system, and a material that is used by a carrier coat of the 2-component developer. Here, p is made smaller than the development gap d, i.e., p<d.

In addition, according to the example shown in FIG. 7, the electrodes are extended to both edges of the toner supporting object 31, and connected to one of the electrode shafts 40A and 40B at the corresponding edges. According to the example shown in FIG. 10, the electrode shafts 40A and 40B are shaped like flanges, and are formed on both ends in a direction of an axle of the toner supporting object 31, wherein the electrodes are connected to one of the electrode shafts 40A and 40B.

FIG. 11 is a schematic diagram of the image formation apparatus according to Example 1 wherein a hopping method is used for development. The image formation apparatus includes a development apparatus that uses the toner supporting object 31. The development apparatus includes a usual 2-component development unit 56 that provides an ear of the 2-component developer 63 by contacting the toner supporting object 31. The 2-component developer 63 is made of a magnetic carrier powder of particle diameter 50 μm and a polyester toner of particle diameter about 6 μm at 7-8 wt %. The 2-component development unit 56 includes a magnet sleeve 57 that contains a permanent magnet for conveying the 2-component developer to the toner supporting object 31. Then, a part of the toner is transferred to the toner supporting object 31 by a DC bias voltage applied between the magnet sleeve 57 and the toner supporting object 31. The toner transferred to the toner supporting object 31 forms a flare on the toner supporting object 31, is conveyed to a position that counters the latent image supporting object 58 by the toner supporting object 31 that is rotationally driven by a driving unit that is not illustrated, and develops an electrostatic latent image on the latent image supporting object 58 by adhering to the electrostatic latent image according to a difference between an average voltage of the surface of the toner supporting object 31 and a voltage of the latent image supporting object 58. A toner image is formed in this way. In addition, an AC voltage is provided between the electrode shafts 40A and 40B as a bias voltage through an electrode brush, and the like, from an AC power source 59; that is, a temporary periodic voltage difference is formed between the odd-numbered electrodes 41, 43, and so on and the even-numbered electrodes 42, and so on.

Residual toner that has not been used for the development returns from the development position to the magnet sleeve 57. Since the flare is formed, the amount of the toner that remains adhered to the toner supporting object 31 is very small. The toner that has returned from the development position by the toner supporting object 31 is easily scratched by the ear of the 2-component developer that follows the rotation of the magnet sleeve 57, and is homogenized/smoothed. By repeating this, an approximately constant amount of the toner flare is always formed on the toner supporting object 31. The 2-component development unit 56 stirs, conveys, and circulates the 2-component developer 63 in a container 60, while the magnet sleeve 57 conveys a part of the 2-component developer 63 to the toner supporting object 31, and returns the unused toner.

In the following, the case is described wherein the latent image supporting object 58 uses an organic photo conductor that is 13 μm thick, and a latent image is formed using a laser writing unit of 1200 dpi. The latent image supporting object (photo conductor) 58 is rotationally driven by a driving unit that is not illustrated, is uniformly charged by an electrification apparatus, and is exposed by the laser writing unit such that the electrostatic latent image is formed. In this case, the electrostatic latent image is formed under conditions that an electrification voltage of the latent image supporting object (photo conductor) 58 is between −300 and −500 V, and a writing voltage for a solid image part is between 0 and −50 V.

The electrostatic latent image is developed by the toner that forms a flare on the toner supporting object 31, and turns the latent image into a toner image. At this time, conditions for producing good single dots of 1200 dpi with the solid image part being satisfactorily printed without ground dirt for toner made of particles 6 μm in diameter having an amount of electrification about −22 μC/g are as follows:

the gap between the toner supporting object 31 and the latent image supporting object (photo conductor) 58 is about 500 μm; and

the alternating-current (AC) bias is provided from the AC power source 59 to the electrodes of the toner supporting object 31, where the AC bias peaks at −400 V and 0 V, averaging at −200 V, at a frequency of 5 kHz. Further, the phase angle of the AC bias provided to the odd-numbered electrodes is the inverse of the AC bias provided to the even-numbered electrodes.

The toner image on the latent image object 58 is transferred by a transferring unit to a recording medium such as paper fed by a feeding unit, and is fixed to the recording medium by a fixing unit. Then, the recording medium is discharged outside.

If an excessive amount of toner is provided to the toner supporting object 31, the electric-field curtain is shielded by the charge of the toner, which makes it impossible to form the flare. In this view, a direct-current (DC) bias of about 200 V is provided between the magnet sleeve 57 and the toner supporting object 31 from a power source so that the amount of toner on the toner supporting object 31 is 0.2 mg/cm². Incidentally, since the toner is diffused by flaring, some unevenness in toner transferred to the toner supporting object 31 from the magnet sleeve 57 does not cause a problem, a device for superposing an AC bias onto the direct-current bias between the magnet sleeve 57 and the toner supporting object 31 is unnecessary, and a device for strictly homogenizing the ear of the 2-component developer is also unnecessary.

On the other hand, since the amount of the toner necessary for forming a solid image on the latent image supporting object (photo conductor) 58 is 0.4 mg/cm², it is necessary to make the moving speed of the toner supporting object 31 equal to or greater than twice the moving speed of the latent image supporting object (photo conductor) 58 so that a sufficient amount of the toner is provided to the development position. Accordingly, the moving speed of the toner supporting object 31 is made 2.5 times that of the latent image supporting object (photo conductor) 58 in this embodiment. A moving direction of the toner supporting object 31 may be the same as a moving direction of the latent image supporting object (photo conductor) 58 as shown in FIG. 11; nevertheless, the moving direction of one may be opposite to the other. It is preferred that the moving direction of the magnet sleeve 57 be reversed to the moving direction of the toner supporting object 31 as shown in FIG. 11 for enhancing residual toner scratching efficiency.

With the system described above, it was confirmed that high-definition development of 1200 dpi dots was realized with satisfactory solid image quality without ground dirt at a linear speed of 300 mm/s of the latent image supporting object (photo conductor) 58.

FIG. 12 is a schematic diagram of the image formation apparatus that uses the hopping method for development according to Example 2. According to this example, the magnet sleeve 57 provided to the image formation apparatus of FIG. 11 is dispensed with, and the toner is supplied to the toner supporting object 31 by a cascade development phenomenon of the 2-component developer. Because the thin toner layer is formed on the toner supporting object 31 by a simple cascade, the rate of toner transfer to the toner supporting object 31 is small compared with the embodiment shown in FIG. 11. However, by increasing the rotating speed of the toner supporting object 31, the development speed of the latent image supporting object (photo conductor) 58 can be obtained. The development apparatus including the toner supporting object 31 and the 2-component development unit 56 without the magnet sleeve is essentially of the same size as the conventional 2-component development apparatus, that is, it is smaller than the apparatus shown in FIG. 11. Yet the development apparatus shown in FIG. 12 is capable of forming a high-definition image like the development apparatus shown in FIG. 11.

FIG. 13 is a schematic diagram of the image formation apparatus that uses the hopping method for development according to Example 3. The image formation apparatus differs from the image formation apparatuses shown in FIG. 11 and FIG. 12 in that a development unit 64 using a 1-component developer is used instead of the 2-component development unit 56 that uses the 2-component developer. Here, the 1-component developer consists only of toner. According to the development unit 64, the toner is transferred to the toner supporting object 31, and a thin toner layer is formed on the toner supporting object 31. The development unit 64 supplies toner 66 to the toner supporting object 31 while stirring and circulating the toner 66 in a container 65 by a circulation paddle 67. The toner supporting object 31, the surface of which is rotationally driven for endless movement, supports the supplied toner 66 on its surface and conveys the toner to a contact position with a metering blade 68 that serves as a regulating member. In this way, the thickness of the thin toner layer on the surface of the toner supporting object 31 is regulated and made uniform.

FIG. 17 is a schematic diagram showing the image formation apparatus according to the embodiment of the present invention. The image formation apparatus 64 carries out development using the 1-component developer like the image formation apparatus shown in FIG. 13. Toner 66 is held in a toner container of the development unit 64, and does not touch the toner supporting object 31. Rather, a toner supply roller 90 is provided between the toner container and the toner supporting object 31. The toner supply roller 90, which is rotationally driven by a driving unit that is not illustrated, touches the toner 66 in the toner container.

A roller part of the toner supply roller 90 is made of a porous and elastic material such as foaming urethane formed on a metal core. Accordingly, the periphery of the roller part has hollows of minute holes of the porous material. The toner supply roller 90 is rotated contacting the toner 66 in the toner container and the toner supporting object 31 at a toner contact position, and scoops up the toner 66 with the hollows as the toner supply roller 90 is rotationally driven. The scooped-up toner 66 is supplied to the toner supporting object 31 from the toner supply roller 90 at the toner supply position where the toner supply roller 90 and the toner supporting object 31 touch.

According to this image formation apparatus, an electric field for causing the toner on the surface of the toner supporting object 31 to hop is formed not only at the development position but at the toner supply position where the toner supply roller 90 and the toner supporting object 31 make contact. Although the toner supply roller 90 and the toner supporting object 31 make contact at the toner supply position, minute gaps are formed between the rollers by the hollows on the surface of the toner supply roller 90. The toner that advances to the toner supply position is hopping by the action of the electric field in the minute gaps, and is in sliding contact with the surfaces of the toner supporting object 31 and the toner supply roller 90. Thereby, friction electrification of the toner is desirably promoted at the toner supply position. In this way, generation of the ground dirt is reduced with the promoted friction electrification.

The toner used by the image formation apparatus according to the embodiment is made of a base-material resin (the main material of the toner) that consists of polyester or styrene acrylics that has a negative regular electrification polarity. Further, both a latent image part and a uniformly charged part (background) of the latent image supporting object (photo conductor) 58 are given the same polarity as the regular electrification polarity of the toner (that is, the negative polarity according to this example), and the toner is selectively adhered to the latent image part, a voltage of which is smaller than the background so that the so-called reversal development is performed.

The toner supporting object 31 includes the protection layer 3 as shown in FIG. 1. With this image formation apparatus, the protection layer 3 is made of a material that promotes friction electrification by sliding contact onto a side of a regular electrification polarity of the toner (i.e., negative according to this example). That is, the toner is located on a more negative side on a friction electrification series than the protection layer 3. Materials that can realize such a relationship for the protection layer 3 include an organic material such as silicone, nylon, melamine resin, acrylic resin, PVA, and urethane. Further, the fourth class ammonium salt, a nigrosine system dye, and the like may be used. Furthermore, a combination of two or more of the materials exemplified above may be used.

According to the image formation apparatus including the protection layer 3 as described above, the protection layer 3 promotes friction electrification on to the side of the regular electrification polarity of the toner when the toner is in sliding contact with the protection layer 3. In this way, degradation of image quality due to poor electrification of the toner conveyed to the development position can be reduced.

In addition, toner that has a positive regular electrification polarity may be used. In this case, the protection layer 3 is made of a material that promotes friction electrification on to the side of the positive polarity of the toner by the toner in sliding contact with the layer 3.

The electrification series of toner is that of the toner as a whole including additives such as silica, and titanium oxide to the toner base-material resin (particles). A position in the electrification series of toner may be determined as follows. That is, the toner on a surface-protection layer is in sliding contact with the protection layer 3 for a predetermined time, then the toner is extracted by attraction, then the amount of electrification of the extracted toner is measured by an electro meter. If a measurement result shows that the amount of electrification increased in a direction of the negative polarity of the toner, the toner is determined to have a more negative position in the electrification series than the protection layer 3. Further, if the measurement result shows that the amount of electrification increased in the direction of the positive polarity of the toner, the toner is determined to have a more positive position in the electrification series than the protection layer 3.

Further, a middle layer may be provided between the protection layer 3 and the electrodes. The material of the middle layer may be, e.g., Ti, Sn, Fe, Cu, Cr, Ni, Zn, Mg, Al, TiO₂, SnO₂, Fe₂O₃, Fe₃O₄, CuO, Cr₂O₃, NiO, ZnO, MgO, and Al₂O₃.

According to the image formation apparatus of the embodiment, two or more electrodes of the toner supporting object 31 are in sliding contact for one of the electrode shafts 40A and 40B irrespective of the rotational position of the toner supporting object 31 as shown in FIG. 7 and FIG. 8 at (b) such that the electric field for hopping may be formed all over the periphery of the surface of the toner supporting object 31 in the endless moving direction (rotational direction). With this configuration, the toner on the surface of the toner supporting object 31 continues hopping in a process from the toner supply position until the toner reaches the development position, and the flare state of the toner is maintained.

The inventors hereto prepared a printer testing machine having the same configuration as the image formation apparatus according to the embodiment described above, except that the electrodes of the toner supporting object 31 contact one of the electrode shafts 40A and 40B only when the rotational position of the toner supporting object 31 is at a predetermined position. Experiments were conducted for three different predetermined positions as follows.

(1) Experiment number 1: the rotational position that counters the development position

(2) Experiment number 2: the rotational position that counters the toner supply position, and the rotational position that counters the development position

(3) Experiment number 3: all areas from the rotational position that counters the toner supply position to the rotational position that counters the development position.

In the experiments, a test image was printed with the printer testing machine, and evaluations were made about a distribution of toner electrification amounts, a convergence nature of toner electrification, a presence of ground dirt, and concentration uniformity of half-tone. The distribution of the toner electrification amounts was evaluated as follows. That is, a printing operation (i.e., rotational driving of the toner supporting object 31) was halted during a test printing, and the toner in an area just before the development position of the toner supporting object 31 was extracted. Then, with a commonly known technique, an amount of electrification of each toner particle in the extracted toner was measured, and the distribution of the electrification amounts was analyzed. Then, the distribution was classified into three categories, namely, great, medium, and small.

As for the convergence of the toner electrification amounts, an electrification range, which is equal to the standard deviation of the distribution of the electrification amounts, was analyzed. Then, the convergence was classified into three categories, namely, small, medium, and great that are expressed by ◯, Δ, and ×, respectively, in Table 1 that follows. Since the toner electrification was saturated at a certain level, the better was the convergence of the electrification amounts (i.e., the smaller is the electrification range that is equal to the standard deviation), the more sufficiently was the toner electrified.

Further, for evaluating the presence of ground dirt, during test printing, the testing machine was suspended, and an adhesive transparent tape was attached to the background of the latent image supporting object (photo conductor) 58 such that the toner adhering to the background was removed by and transferred to the adhesive transparent tape. Then, the amount of the toner transferred to the tape was classified into four categories; namely, ⊚ (no toner transferred), ◯ (although there is a slight transfer of the toner, it is not visible), Δ (the transferred toner is visible), and × (the transferred toner is conspicuous at a glance).

Further, about the concentration uniformity of the half-tone part, whether there was concentration unevenness in the half-tone part of the printed test image was observed. Evaluation was made into four categories; namely, ⊚ (concentration unevenness was not present), ◯ (slight concentration unevenness was noticeable if an eye was well elaborated), Δ (concentration unevenness was visible if the eyes are elaborated), and × (concentration unevenness is visible at a glance).

Experiment results are shown in the following Table 1.

TABLE 1

Continuous

hopping

formation

from toner
Distribution

Hopping
supply
of
Convergence

formation at
position to
toner
of toner

Halftone

toner supply
development
electrification
electrification
Ground
concentration

Experiment #
position
position
amount
amount
dirt
uniformity

1
None
None
Great
X
Δ
Δ

2
Formed
None
Small
Δ
◯
◯

3
Formed
Formed
Small
◯
⊚
⊚

With the experiment number 1, wherein a hopping electric field was formed only at the development position (not shown in Table 1), ground dirt was visible (Δ). Further, the distribution of toner electrification amounts was relatively great, and the convergence of toner electrification was comparatively small. This was because no friction electrification of the toner was promoted by hopping from the toner supply position to the development position at all. There was concentration unevenness in the half-tone part (Δ), which was considered to be because the toner was not uniformly adhered to the half-tone part due to the great variation in the amount of toner electrification.

According to Experiment 2, the hopping electric field was formed in the development position in addition to the toner supply position. As a result, the ground dirt was improved to an extent that it is practically invisible. Further, the distribution of toner electrification amounts, the convergence of toner electrification, and the concentration uniformity of the half-tone part concentration uniformity are also improved compared with the experiment number 1. This indicates that the toner was hopping at the toner supply position and was in sliding contact with the toner supply roller 90 and the toner supporting object 31, which promoted the friction electrification. That is, it was proved that the toner was able to hop at the toner supply position where both rollers were contacted by using the porous material that has hollows on its surface as the toner supply roller 90.

According to the experiment number 3, the distribution of toner electrification amount, the convergence of toner electrification amounts, ground dirt, and half-tone part concentration uniformity are further improved compared with the experiment number 2. This was because friction electrification of the toner was promoted by hopping between the toner supply position and the development position.

That is, by forming the electric field for hopping all over the periphery of the endless moving direction of the surface of the toner supporting object 31, as performed by the image formation apparatus according to the embodiment, friction electrification of the toner is promoted at the toner supply position, and between the toner supply position and the development position. In this way, generation of ground dirt by poor electrification of the toner can be effectively reduced. In addition, a metering blade 68 as shown in FIG. 13 may be provided, wherein the metering blade 68 contacts the toner supporting object 31 after passing through the toner supply position.

By the way, according to the well-known conventional 1-component development unit, toner is supplied to a developing roller from a toner supply roller made of an elastic material by the rollers rotationally contacting each other. Such contacting position is called a toner supply position. With the 1-component development unit having this configuration, when an image with a high rate of image area such as a solid image is output, the toner of a part of the developing roller, which part corresponds to the solid image, and the like, is almost totally consumed, and the developing roller returns to the toner supply position. At the toner supply position, the toner is supplied; however, since the part carries overwhelmingly less toner than other parts, even if the toner is supplemented at the toner supply position, the toner at the part tends to be still less than at other parts. On the other hand, the toner corresponding to the background of the photo conductor is not consumed at the development position by the developing roller, the development roller returns to the toner supply position. When the toner is supplemented at the toner supply position, since the part corresponding to the background carries an overwhelmingly greater amount of the toner than other parts, the toner amount after toner supplementation tends to be superfluous. That is, the part of the developing roller corresponding to the solid image carries a smaller amount of the toner, and the part of the developing roller corresponding to the background carries a superfluous amount of the toner. This results in toner amount unevenness on the developing roller that follows the shape of the solid image. Under this situation, if the toner amount unevenness synchronizes with a half-tone part of the photo conductor at the next development, an afterimage due to the development concentration unevenness resulting from the toner amount unevenness is generated in the half-tone part.

According to the image formation apparatus of the embodiment, since the configuration is such that the toner is supplied at a position where the toner supply roller 90 contacts the toner supporting object 31, there is a possibility of generating an afterimage at a half-tone part. However, in the experiment numbers 2 and 3, no afterimage was generated at all. The reason for this is considered to be as follows. That is, the toner on the surface of the toner supporting object 31 is made to hop at the toner supply position in the experiment numbers 2 and 3. By hopping, the toner is collected from the surface of the toner supporting object 31 immediately after the toner supporting object 31 enters the toner supply position. After the toner collection, the toner is supplied/supplemented; accordingly, the toner of a proper quantity can be supplied to the surface of the toner supporting object 31. In other words, by collecting the toner from the surface of the toner supporting object 31 immediately after entering the toner supply position, the toner amount before entering the toner supply position does not influence the toner amount after the toner is supplied/supplemented.

In order to prove this, the inventors carried out an experiment number 4 under the same conditions as the experiment number 1, and an experiment number 5 under the same conditions as the experiment number 3. Then, a toner supply amount at the toner supply position, homogeneity of the toner supply amount, a degree of toner degradation, and a presence of an afterimage in a half-tone part. Results are shown in the following Table 2.

TABLE 2

Formation

of hopping

electric

Homo-

field at

geneity

After-

toner
Toner
of toner

image

Experiment
supply
supply
supply
Toner
in half-

#
position
amount
amount
degradation
tone part

4
Not formed
Great
Δ
X
Δ

5
Formed
Small
◯
◯
⊚

As shown in Table 2, an afterimage was visible (Δ) in the half-tone part with the experiment number 4 wherein a hopping electric field was not formed at the toner supply position, and no afterimage was generated (⊚) with the experiment number 5 wherein a hopping electric field was formed at the toner supply position. Accordingly, the image formation apparatus of the embodiment can be configured such that an afterimage in the half-tone part may be reduced.

The toner supply amount was greater with the experiment number 5 than the experiment number 4. This was considered to be because transfer of the toner from the toner supply roller 90 to the toner supporting object 31 was promoted by hopping. Further, the homogeneity of the toner amount was better with the experiment number 5 than the experiment number 4. This was because the toner amount after toner supply was not easily influenced by the toner amount before the toner supply as described above. Further, the toner degradation was better with the experiment number 5 than the experiment number 4. This was considered to be because the pressure applied to the toner at the toner supply position was reduced by hopping.

FIG. 14 shows the outline of the image formation apparatus according a variation of the embodiment. This image formation apparatus includes development apparatuses 73K, 73Y, 73C, and 73M. Each of the development apparatuses includes a development unit that is the same as the development unit 64 as shown in FIG. 17, and a toner supporting object that is the same as the toner supporting object 31 as shown in FIG. 17.

The development apparatuses 73K, 73Y, 73C, and 73M are for developing latent images in predetermined colors with toners of the corresponding colors, wherein the toner images of the colors are superposed on a belt-like organic photo conductor 69. The belt-like organic photo conductor 69 is wound around two rollers that are not illustrated, and rotationally driven by a driving unit that is not illustrated.

The image formation apparatus according to the variation includes image formation units 70K, 70Y, 70C, and 70M for forming images in two or more colors, for example, black, yellow, cyan, and magenta, respectively, that are shown on the left-hand side of the organic photo conductor 69 in FIG. 14. The photo conductor 69 is uniformly charged by an electrification unit 71K, and exposed by an optical beam 72K modulated by image data for black color and provided by a writing unit (not illustrated) of the imaging formation unit 70K. In this way, an electrostatic latent image is formed. The electrostatic latent image is developed by the development apparatus 73K, which is configured the same as the development apparatus that includes the development unit 64 and the toner supporting object 31 shown in FIG. 17, and turns into a toner image in black. Then, electrification of the organic photo conductor 69 is removed by an electrification removing unit 74K; then the following image formation is processed.

Then, the organic photo conductor 69 is uniformly charged by an electrification unit 71Y, and exposed by an optical beam 72Y modulated by image data for yellow color and provided by a writing unit (not illustrated) of the image formation unit 70Y such that an electrostatic latent image is formed. The electrostatic latent image turns into a toner image in yellow, and the toner image is developed by the development apparatus 73Y, which is configured the same as the development apparatus including the development unit 64 and the toner supporting object 31 shown in FIG. 17. The toner image in yellow is superposed on the toner image in black. Then, the electrification of the organic photo conductor 69 is removed by an electrification removing unit 74Y, then, the following image formation is processed.

Next, the organic photo conductor 69 is uniformly charged by an electrification unit 71C, and is exposed by an optical beam 72C modulated by image data for cyan color and provided by a writing unit that is not illustrated of the image formation unit 70C such that an electrostatic latent image in cyan is formed. The electrostatic latent image turns into a toner image in cyan color, which is then developed by the development apparatus 73C having the same configuration as the development apparatus including the development unit 64 and the toner supporting object 31 shown in FIG. 17. The toner image in cyan is superposed on the toner image in black and the toner image in yellow. Then, the electrification of the organic photo conductor 69 is removed by an electrification removing unit 74C, then the following image formation is processed.

The organic photo conductor 69 is uniformly charged by an electrification unit 71M, and is exposed by an optical beam 72M modulated by image data for magenta color and provided by a writing unit (not illustrated) of the image formation unit 70M such that an electrostatic latent image in magenta is formed. The latent image is developed by the development apparatus 73M having the same configuration as the development apparatus including the development unit 64 and the toner supporting object 31 shown in FIG. 17. The toner image in magenta is superposed on the toner image in black, the toner image in yellow, and the toner image in cyan so that a full color image is formed.

On the other hand, a recording medium such as paper is provided by a feeding unit that is not illustrated, and the full color image on the photo conductor 69 is transferred to the recording medium by a transfer roller 75, to which a transfer bias is provided by a power source. The full color image on the recording medium is fixed by a fixing unit 76, and the recording medium is discharged outside. After full color image transfer, residual toner on the photo conductor 69 is removed by a cleaner 77.

According to the image formation apparatus, since the toner images in four colors are formed on the same organic photo conductor 69, compared with the usual 4-color tandem system, position gap hardly occurs theoretically, and a high-definition full color image can be acquired.

In addition, according to the image formation apparatus as described above, both conditions of Vmax V/p μm>1 and p μm<d μm are sufficed in view of the results of the experiments described above. That is, no influences are affected in the toner image formed on the organic photo conductor 69, and no toner is transferred from a foregoing toner image formed on the organic photo conductor 69 to the development apparatus of the next color. In this way, high quality images are stably formed for a long period without problems such as scavenging and color mixture.

Although the embodiments are described about the image formation apparatus wherein the toner is conveyed to the development position by the surface movement of the toner supporting object while the toner is flared and hopping between two adjacent electrodes, the present invention can be applied to an image formation apparatus wherein toner is conveyed to a development position of a toner supporting object by repeatedly hopping from a certain electrode upward to an adjacent electrode in a direction from one end to the other of the toner supporting object like the method disclosed by JPA 2002-341656. Further, the present invention is also applicable to an image formation apparatus wherein the toner is conveyed to the development position by both hopping the toner and by movement of the surface of the toner supporting object.

Further, the present invention is not limited to these embodiments, but variations and modifications may be made without departing from the scope of the present invention.

The present application is based on Japanese Priority Application No. 2006-279539 filed on Oct. 13, 2006 with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.

DEVELOPMENT APPARATUS AND AN IMAGE FORMATION APPARATUS

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CPC

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