WET ELECTROPHOTOGRAPHIC DEVICE

Abstract: A wet type electrophotography apparatus according to this invention employs a non-volatile, high-viscosity, high-concentration liquid toner as a developing solution. The wet type electrophotography apparatus comprises a photoconductive medium 10 on which an electrostatic latent image is formed, a prewetting device 13 for applying a film of a prewetting solution to the surface of the photoconductive medium 10, a developing device 14 including a developing roller, and a developing solution applying means for applying a film of the developing solution on the developing roller. The developing roller feeds the developing solution to the photoconductive medium 10 by making contact with the photoconductive medium 10, and causes toner particles in the developing solution to deposit on an image carrier in accordance with an electrical field formed between the developing roller and the photoconductive medium 10. The developing solution applying means comprises a plurality of interconnecting rotating rollers for transporting the developing solution while spreading and applying it on the surface with the rotating rollers, and applies a film of the developing solution on the surface of the final-stage rotating roller in accordance with an electrical field formed between the final-stage rotating roller and the developing roller by a bias voltage fed to the final-stage rotating roller.

Description: This invention relates to a wet type electrophotography apparatus using a non-volatile, high-viscosity liquid toner. As for electrophotography apparatus in which an electrostatic latent image is formed on a photoconductive medium (photoconductive drum); a toner is caused to adhere to the charged image; and the powder image is then transferred and thermally fixed onto a printing medium, such as paper, the dry type using a powder toner has been widely employed. The powder toner, however, tends to be scattered and often involves the problem of poor resolution due to its particles sizes as large as 7 ∼ 10 µm. In applications requiring high resolution, therefore, the wet type using a liquid toner is usually adopted. The liquid toner is less subject to distortion in toner images and can achieve high resolution because it contains toner particles as small as 1 µm and has a large charging capacity. In the conventional wet type electrophotography apparatus, a low-viscosity liquid toner obtained by mixing 1 ∼ 2% of toner in an organic solvent has been commonly used as the developing solution. This type of developing solution, however, causes environmental concerns because it contains an organic solvent harmful to human bodies and requires a large amount of toner consumption due to low concentration. It was against this background that a wet type electrophotography apparatus using a high-viscosity, high-concentration developing solution obtained by dispersing a high-concentration toner in silicone oil, etc. was disclosed in International Disclosure Number "WO95/08792." The use of this liquid toner eliminates not only the harm to human bodies but also the need for large consumption of developing solution due to high toner concentration. As disclosed in "International Disclosure Number WO95/08792," the wet type electrophotography apparatus using a high-viscosity, high-concentration liquid toner as the developing solution has employed the same construction as with the dry type apparatus using a powder toner, in which a developing solution is applied to a developing roller or belt, which is brought into contact with a photoconductive medium to cause the toner to deposit on the photoconductive medium on which an electrostatic latent image is formed. The same construction is used for the wet type photography apparatus disclosed in JP-A-08328392 which is regarded as the closest prior art. That is, as in the dry type where the powder toner is applied to the developing roller using a purely mechanical contact means, the wet type using a non-volatile, high-viscosity, high-concentration liquid toner also employs a purely mechanical contact means in applying the developing solution to the developing roller. The application of the developing solution is accomplished by finishing the surface which makes mechanical contact with the developing roller to a high-precision finish. That is, development is carried out by applying a liquid toner onto a developing roller 22, causing the developing roller 22 to face a photoconductive medium 10 and applying a voltage to the developing roller 22, as shown in FIG.16. At this time, an image area and a non-image area (background area) are formed randomly in accordance with an image pattern on the toner layer on the developing roller 22 after development. Rollers 23 - 25 in the figure are a series of applicator rollers for feeding the developing solution to the developing roller 22. These rollers are driven by drive motors 31 and 33, and gears 32 and 34. If the image area and the non-image area on the developing roller 22 after development, which have different electrical histories on the toner and different amounts of residual toner, are brought as they are to the contact area at which the developing roller 22 makes contact with the final-stage applicator roller 23, the evenness of the toner layer formed on the developing roller 22 becomes a problem. Even if the toner layer is formed evenly, the image pattern at the previous rotation of the developing roller 22 may appear on the image in synchronism with the period of the roller due to the electrical history experienced by the toner. Such a phenomenon is a trouble often referred to as the development memory. It is necessary therefore to have a reset mechanism for erasing the history left as the result of a development operation. A problem characteristic of the liquid developing process is that when a prewetting solution is used as a release agent to prevent blushing, the prewetting solution deposited on the developing roller 22 causes the dilution of the developing solution. To cope with this problem, a rubber blade 37 has heretofore been used to scrape off the residual toner layer on the developing roller 22. As a means to solve the trouble called the development memory and the dilution of the developing solution, the use of the rubber blade to scrape off the residual toner layer is effective only when the blade is used to wipe off something on a surface of a relatively hard material, as with the automotive windshield wiper. However, when the rubber blade is used to scrape off the residual toner on a developing roller of a relatively soft developing roller, the contact pressure between both cannot be satisfactorily maintained. The pressure between the blade and the roller can be considerably increased by forcing the edge of the blade onto the roller in the "doctor" direction ("biting" direction in which the blade faces the roller at an acute angle), but when used with a soft roller with a low hardness, the doctor-blade effect is drastically reduced, leading to an increase in the amount of toner left on the roller. As disclosed in International Disclosure Number "WO95/08792," when a developing device is constructed using a flexible developing belt, adjustment of the developing belt is extremely difficult, requiring much labor and time. Too tight a developing belt would reduce the flexibility, making the belt too rigid. This makes it impossible to realize the double-layer construction of the liquid toner applied to the developing belt and the prewetting solution layer applied to the photoconductive medium. Too loose a developing belt, on the other hand, would cause a gap between the liquid toner on the developing belt and the prewetting solution layer applied to the photoconductive medium, preventing the movement of the toner. In this way, a developing device using a developing belt would make the adjustment of the developing belt difficult and time-consuming. As disclosed in JP-A-04247472, the monitoring of the development solution layer guarantees a uniform color strength. When a high-viscosity liquid developing agent is used, the following requirements must be satisfied to form a thin film of the developing agent through the use of a contact roller while maintaining the prewetting solution layer undisturbed. First, the total amount of developing and prewetting agents carried along by the rotation of the photoconductive medium and the developing roller must be passed at a pressure below the contact pressure at the contact area between the developing roller and the photoconductive medium. The lower the hardness of the developing roller the more liquid can be passed. However, the higher the dimensional accuracy of the outside diameter and the wobbling accuracy during rotation of the developing roller, the more stably the pressure on the liquid layer is maintained. In terms of machining, the lower the hardness the more difficult becomes the improvement of machining accuracy. Second, there must be a prewetting oil layer at all times over the entire region between the developing agent and the surface of the photoconductive medium so as to ensure that the high-viscosity developing agent is kept from directing contacting the surface of the photoconductive medium. If the surface of the high-viscosity toner layer is not a uniform plane, having irregularities, projected parts of the toner layer may pierce through the prewetting solution layer, coming in direct contact with the surface of the photoconductive medium, impairing the releasing effects of the prewetting agent. This may lead to blushing (deposition of unnecessary toner) on the non-image area. The uneven layer on the surface of the developing roller appears on the image area, resulting in a poor image. The problem underlying the present invention is to improve the application of a developing solution to a rotatable developing member in a wet type electrophotography apparatus using a non-volatile, high-viscosity, high concentration liquid toner. The problem underlying the present invention is solved by a wet type electrophotography apparatus according to claim 1. According to the invention, the developing solution applying means causes a final-stage rotating roller coming in contact with a rotatable developing member to rotate in the opposite direction to said developing roller. The opposite directions of rotation of the final-stage roller and the developing member result in an even toner layer and color strength by reducing fine streaks. Preferred embodiments of the invention are given in the dependent claims 2 to 15. Said developing member is either formed as a developing roller or a developing belt. In a preferred embodiment, the thickness of the developing solution layer is monitored and controlled. Thus, the color strength of the toner to be applied to the photoconductive medium can be kept constant. In the following, an example of the invention will be described with respect to the following drawings in which: FIG. 1 is a diagram illustrating the overall construction of the wet type electrophotography apparatus according to this invention; FIG. 2 is a diagram of assistance in explaining the operation of the developing roller coming in contact with the photoconductive medium and a series of applicator rollers; FIG. 3 is a diagram of assistance in explaining the state of a liquid toner between two rollers rotating in the same direction; FIG. 4 is a diagram showing measurement results of the relationship between slip speed and the apparent viscosity of the toner; FIG. 5 is a diagram of assistance in explaining the reason why the toner layer can be made uniform by rotating the reverse homogenizing roller while making contact with the developing roller in the opposite direction; FIG. 6 is a diagram of assistance in explaining the operation of the prewetting solution layer on the surface of the photoconductive medium; FIG. 7 is a diagram of assistance in explaining the operation of the prewetting solution layer on the surface of the photoconductive medium; FIG. 8 shows an example of controlling the thickness of the toner layer on the developing roller to a predetermined value; FIG. 9 shows a further example of controlling the thickness of the toner layer on the developing roller to a predetermined value; FIG. 10 shows an example of an elastic roller used as the developing roller; FIG. 11 shows another example of the elastic roller. FIG. 12 shows an example of the developing roller that applies pressure by gravity; FIG. 13 shows an example of the developing roller that applies pressure by a spring; FIG. 14 is a diagram of assistance in explaining why the reverse scraping roller can scrap the residual toner from a soft developing roller; FIG. 15 shows the developing belt used in place of a developing roller; and FIG. 16 shows the developing roller and a series of the applicator rollers for feeding the developing solution to it used in the prior art. FIG. 1 shows the overall construction of a wet type electrophotography apparatus embodying this invention. As shown in the figure, the wet type electrophotography apparatus according to this invention comprises a photoconductive medium 10, an electrostatic charging device 11, a light exposure device 12, a prewetting device 13, developing devices 14, an intermediate transfer medium 15, a blade 16, a static charge eliminator 17, a heating device 18, and a pressure roller 19. The electrostatic charger 11 electrostatically charges the photoconductive medium 10 to approximately 700 V. The light exposure device 12 exposes the photoconductive medium 10 using a laser beam having a wavelength of 780 nm to form on the photoconductive medium 10 a static latent image the potential of whose exposed part is approximately 100 V. The prewetting device 13 applies silicone oil of a viscosity of about 2.5 ∼ 20 cSt on the surface of the photoconductive medium 10 to a thickness of 4 ∼ 10 µ m. The prewetting device 13 may carry out prewetting treatment prior to exposure treatmeat by she light exposure device 12 in some cases, or after the exposure treatment in other cases. This invention uses as the developing solution a non-volatile, high-viscosity liquid toner, which consists of solid particles, such as pigments, dispersed in a liquid carrier (oil). The developing devices 14 are provided to handle four colors of yellow/magenta/cyan/black. Biased at about 400 V ∼ 600 V, the developing device 14 forms a toner layer having a thickness of 2 ∼ 3 µ m on a developing roller (developing solution carrier) 22 by transporting a liquid toner with a toner viscosity of 400 ∼ 4000 mPa · S and a carrier viscosity of 20 cSt while thinly spreading it using interconnecting applicator rollers 23 ∼ 25, as will be described later, referring to FIG. 2. The developing roller 22 deposits the toner on an exposed area of the photoconductive medium 10 that is charged to about 100 V by feeding the toner, which is positively charged by an electrical field formed between the developing roller 22 and the photoconductive medium 10, to the photoconductive medium 10. The toner is prevented from adhering to a non-exposed area of the photoconductive medium 10 by a prewetting solution layer applied by the prewetting device 13, as shown in FIG.7. The toner deposited on the photoconductive medium 10 is transferred on the intermediate transfer medium 15, which is biased at about -800 V, in accordance with an electrical field formed between the intermediate transfer medium 15 and the photoconductive medium 10. The yellow toner, the magenta toner, the cyan toner, and then the black toner deposited on the photoconductive medium 10 are transferred one after another on the intermediate transfer medium 15. The blade 16 removes the toner and the prewetting solution remaining on the photoconductive medium 10. The static charge eliminator 17 removes the charge on the photoconductive medium 10. The heating device 18 fuses the toner deposited on the intermediate transfer medium 15 by heating the surface of the intermediate transfer medium 15. The pressure roller 19 fixes the toner on the intermediate transfer medium 15 that has been fused by the heating device 18. Having the construction where the toner deposited on the intermediate transfer medium 15 is fused and fixed on a printing medium without heating the printing medium, using the heating device 18 and the pressure roller 19, the wet type electrophotography apparatus according to this invention can handle printing media, other than paper. FIG. 2 shows the developing roller 22 coming in contact with the photoconductive medium 10 and a series of applicator rollers 23 ∼ 26 for feeding the developing solution to the developing roller 22. In the next example shown in the figure, the final-stage applicator roller 23 is caused to rotate in the opposite direction to the developing roller 22. This means that both rollers at the contact portion move in opposite directions to each other. Although three applicator rollers are shown in the figure, the applicator rollers are provided in an appropriate number needed to spread the developing solution thinly and uniformly on the developing roller 22. The roller 22 is a developing roller that feeds the developing solution while making contact with the photoconductive medium 10 so that a double-layer construction of the developing solution and prewetting solution films is formed on the photoconductive medium, and deposits on the photoconductive medium 10 the toner particles positively charged in accordance with an electrical field formed between the developing roller 22 and the photoconductive medium 10. Generally speaking, when the developing solution is applied to the developing roller via the applicator roller to form a thin layer, fine streaks of the developing solution may be caused, due to changes the balance among the viscosity, feed rate of the developing solution, roller rotation, etc. These fine streaks can be reduced by causing the contact portion of the final-stage applicator roller 23 to move in the opposite direction to that of the developing roller 22, as described above. In the following, the reason why fine streaks of the developing solution can be reduced by causing the contact portion of the final-stage applicator roller 23 to move in the opposite direction to that of the developing roller 22 will be described. First, the. state of the contact portions of the rollers A and B moving in the same direction, not in the opposite direction, and are going to separate from each other after they make contact with each other is shown in FIG.3. As both rollers separate from each other at the contact portion as the result of rotation of both rollers, the balance between the viscosity of the liquid toner itself and the adhering force thereof to both rollers is disrupted, and the liquid toner is ruptured and separated into two layers at the center. At this time, the carrier oil that tends to concentrate on the separating portion entrains the surrounding liquid toner. This results in local concentrations of the liquid toner, causing streaks in the rotating direction on the developing roller, with almost no liquid toner left in between the streaks. These fine streaks can be prevented by causing the contact portions of the final-stage applicator rollers 23 and the developing roller 22 to move in the opposite direction to each other. FIG.5 is a diagram of assistance in explaining the profile of toner speed between the two rollers that rotate in the same direction, with the contact portions thereof moving in the opposite direction. The liquid toner on the nip portions of both rollers is entrained by each roller, moving in the opposite direction in different degrees in accordance with its location. The lengths and directions of arrows shown in the figure represent the moving speeds and directions of the liquid toner at the respective locations. The point of zero speed exists in the inner layer between both rollers, at which the liquid toner is sheared, deformed and separated. To enhance the effects of the counteraction of the roller nip portions, the difference in the rotation of both rollers should preferably be at least more than three times. The amount of toner can be adjusted to a level sufficient to obtain image density on the developing roller 22, or preferably to 5 ∼ 10 µ m, by controlling the difference in the rotation of both rollers, or more specifically the peripheral speed of the final-stage applicator roller 23 via independent motors, reduction gears, etc. Thus, the feed of toner can be adjusted in accordance with the types of liquid toners (yellow/magenta/cyan/black) by controlling the difference in roller rotation. In FIG. 5 where the contact portions of both rollers move in the opposite direction to each other, the shearing, deforming and separating force causes the toner viscosity to reduce and the toner fluidity to improve, thereby reducing fine streaks, whereas the liquid toner is ruptured and separated in FIG. 3 where the contact portions of both rollers move in the same direction. FIG. 4 shows the relationship between shearing speed and toner apparent viscosity, as measured using a cyan toner (PFU-C-001) and a magenta toner (PFU-M-002), both obtained by dispersing 20 wt.% of toner particles having Oaverage particle sizes of 0.6 to 0.7 µm comprising resins and pigments in SH-200-20cSt silicone oil manufactured by Toray-Dow Corning as the carrier solution. As is evident from the figure, the toner viscosity sharply falls by imparting shearing speed (shearing force). To ensure an even liquid toner layer by causing the contact portions of both rollers to move in the opposite direction to each other, it is essential to maintain a fine roller gap that is about the same as the thickness of the aimed liquid toner layer, or 5 ∼ 20 µm, for example. It is difficult, however, to guarantee this fine gap in terms of mechanical dimensional accuracy. This can be accomplished by using soft rollers and selecting appropriate values for roller pressure, roller material (surface properties), roller elasticity (hardness), and roller speed to obtain a dynamic gap formed by soft rollers and a viscous fluid. To achieve this, a high-viscosity toner having a viscosity in the high shear region of 5 - 1000mPa · S is used, and the pressure between roller is reduced so that a dynamic gap is produced by the toner between the developing roller and the final-stage applicator roller. To this end, at least one roller whose contact portion is caused to move in the opposite direction to the contact portion of the other roller is made of a roller having a hardness of less than JIS-A 60 degrees. By using such a high-viscosity toner and causing the contact portion of the applicator roller to move in the opposite direction to the developing roller, a zero point appears in the speed profile between the rollers, causing shear, deformation and separation, instead of rupture and separation. The amount of toner on the side of the developing roller from the zero point can be adjusted to a predetermined amount required for development (1 ∼ 20 µ m) by selecting the pressure, material and elasticity, etc. of the roller. When an elastic roller having a relatively low hardness and a coarser surface roughness is used as the applicator roller, the force for holding the toner on the surface of the roller increases, causing the zero point of the speed profile to move toward the side of the developing roller. This increases the blade-like action produced by the zero point on the surface of the fluid, leading to an even layer on the developing roller. A tube roller comprising a sponge roller covered with a film, for example, can be used as an elastic applicator roller as mentioned above. An elastic roller made of a tube roller covered with a film, for example, more preferably having a rubber hardness of less than JIS-A 60 degrees, can be used as the developing roller. Furthermore, belts can be used instead of rollers to construct a developing device. When an elastic roller is used as the applicator roller, the force for holding the toner on the roller surface increases, causing the zero point in the speed profile to move toward the developing roller side, as described above, if a harder roller whose surface has a relatively low affinity to the toner is used as the applicator roller, a state where the roller surface slips on the toner is caused. If a smooth metallic roller having a relatively high hardness of less than Rz4 is used, for example, the smoothness of the roller surface is considered to be transferred onto the developing roller. As a result, the evenness of the toner layer on the developing roller is improved. The final-stage applicator roller can supply a bias voltage. A uniform toner layer can be formed on the developing roller by providing a rigid leveling roller in contact with the developing roller, in addition to the applicator roller, and causing the leveling roller to rotate in the reverse direction. Next, measurement results showing the effects of the reverse leveling roller will be described. The rollers used in measurements were disposed as shown in FIG.2 with only two applicator rollers used, the details of which are as follows: Developing roller 22: A tube roller having a soft and solid surface made of foamed urethane base (hardness: Aska F68 degrees) 32 + electrically conductive PFA (50 µm) Speed of developing roller: 250 mm/s (kept constant throughout all measurements) Applicator roller 23: A hard roller having good surface evenness made of an ebonite roller (hardness: more than JIS-A 95 degrees) Applicator roller 24: A relatively soft roller made of Beet roller (brand name, made of vinyl chloride resin, hardness: JIS-A 20 degrees) Toner: PFU-C-001 Viscosity in high-speed region 50 mPa · S Viscosity in low-speed region 2000 mPa · S The surface of the developing roller 22 was observed by setting the pressing of the final-stage applicator roller 23 and the developing roller 22 to 0.25 mm, rotating the two applicator rollers 23 and 24 at the same speed, and changing the speed of these applicator rollers 23 and 24 with respect to the developing roller 22. Measurement 1: The contact portions of the final-stage applicator roller 23 and the applicator roller 24 were rotated at the same speed (250 mm/s) in the same forward direction. The toner was deposited on the developing roller 22 in a mottled pattern and more conspicuously than any other measurements 2 ∼ 4 below. Rivulets were 0.2 mm in pitch and 10 µ m in height. Measurement 2: The contact portions of the final-stage applicator roller 23 and the applicator roller 24 were rotated at the same speed (250 mm/s) as that in Measurement 1 with respect to the developing roller 22 in the reverse direction. The pattern of continued streaks was observed though it was finer than that obtained in Measurement 1. Measurement 3: The contact portions of the final-stage applicator roller 23 and the applicator roller 24 were rotated at a speed twice as fast as the speed in Measurement 1 (500 mm/s) in the reverse direction. Several small fragmental mottled patterns were observed. Measurement 4: The contact portions of the final-stage applicator roller 23 and the applicator roller 24 were rotated at a speed three times as fast as the measurement 1 speed (720 mm/s) in the reverse direction. No visible mottled pattern was observed. The thickness of the toner layer was 4 µm, the pitch and height of rivulets were 0.2 mm and 3 µm, respectively. The measurement results revealed that the height of rivulets was substantially reduced, compared with Measurement 1. As indicated in the above measurement results, the effect of the reverse rotation of the contact portion can be obtained by rotating the roller contact portions at least at a speed twice, or more preferably three times as fast as the speed in Measurement 1. FIG. 8 shows an example in which the thickness of the toner layer is controlled to a predetermined value. The figure illustrates a developing roller 22 coming in contact with the photoconductive medium 10 and a series of the applicator rollers 23 ∼ 26 for feeding the developing solution to the developing roller 22. The applicator rollers 24 and 25 are rotated at the same peripheral speed by a motor 2 ○ via gears. The developing roller 22 rotates in the normal direction (the contact portions of the developing roller 22 and the final-stage applicator roller 23 move in the same direction) and at the same peripheral speed as the photoconductive medium 10 rotating at a low speed. The reason why fine streaks can be reduced by causing the contact portion of the final-stage applicator roller 23 to move in the opposite direction to that of the developing roller 22 has been described earlier, referring to the construction shown in FIG.2. The final-stage applicator roller 23 and the developing roller 22 are rotated by a motor 1 ○ via gears. The final-stage applicator roller 23 is normally rotated at higher revolution that the applicator rollers 24 and 25. This is because all the toner on the applicator roller 24 must be transported to the next-stage roller 23 without building up on the roller 24. The final-stage applicator roller 23 is adapted to have a light-reflective surface. As in the previous examples, the roller having a light-reflective surface can be made of a metal, such as aluminum, stainless steel, etc., or a hard resin roller whose surface has been treated with a light-reflective finish by plating metal such as aluminium, stainless steel. The thickness of the toner layer formed on such a light reflective roller can be detected as the color strength of the toner in a toner color strength detection section comprising a light source 54 and a light detection section 50. That is, changes in the thickness of the toner layer that is normally controlled to less than 10 microns are reduced to changes in the color strength of the toner, which can be detected as the strength of the reflected light obtained as the light from the light source 54 passes through the toner layer on the roller 24 and is reflected by the light-reflective surface of the roller 24. A white candescent bulb or LED is used as the light source 54, and a light detection element, such as CCD and photodiode, is used as the light detection section 50. Detection sensitivity can be improved by using a color filter in accordance with the color of the toner on any one or both of the light source 54 and the light detection section 50. The detection signal detected by the light detection section 50 is compared with a predetermined reference value in a comparator 51. A motor 2 ○ is controlled via a revolution control section 55 in accordance with the comparison results to cause the revolution of the applicator rollers 24 and 25, so that the thickness of the toner layer on the final-stage applicator roller 23 is controlled to a predetermined value, and eventually the amount of toner fed to the developing roller 22 is controlled to a desired value. That is, when the color strength of the toner is low, an instruction is given to the revolution control section 55 to increase the revolution of the motor to increase the revolution of the applicator rollers 24 and 25 to increase the feed of the toner, whereas an instruction is given to the revolution control section 55 to carry out the reverse operations when the color strength of the toner is high. With such a construction, the gap or contact pressure between the applicator rollers 24 and 25 is controlled so that the liquid toner is fed between the applicator rollers 24 and 25 to form a toner pool. The liquid toner, controlled to a uniform thickness, is supplied onto the developing roller 22 from this toner pool via the applicator rollers 24 and 23. FIG. 9 shows a further example where the thickness of the toner layer on the developing roller is controlled to a predetermined value according to this invention. In the figure, like numerals denote like elements used in FIG. 8. This further example is different from the third example shown in FIG.8 in that a control unit 53 is provided. In the fourth example, the detection signal detected by the light detection section 50 is compared with a predetermined reference value in a comparator 51. A motor is controlled via a revolution control section 55 in accordance with the comparison results to cause the revolution of the applicator rollers 24 and 25, so that the thickness of the toner.layer on the final-stage applicator roller 23 is controlled to a predetermined value, and eventually the amount of toner fed to the developing roller 22 is controlled to a desired value, as in the third example. In addition, the detection signal detected by the light detection section 50 is fed to the control unit 53. The control unit 53 comprising an MPU unit, for example, can change the input value of the comparator 51 corresponding to the reference value in accordance with the detected toner color strength signal, the surface irregularities of a printing medium detected by a separate sensor or set manually, and the type of the printing medium (paper or film, for example), thereby intentionally changing the color strength of the toner, or changing the time constant of the control system to any desired value. FIG. 10 shows an example of the developing roller. As shown in the figure, the developing roller has such a construction that a sponge roll is provided around a core metal, with the surface covered with a film tube. When a developing solution containing a liquid toner having a viscosity as high as 400 ∼ 4000 mPa · S, as in this invention, the wet type electrophotography apparatus should employ such a construction that a silicone oil having a lower viscosity than the silicone oil used in the liquid toner is applied as a prewetting solution layer on the surface of the photoconductive medium 10 to impart release properties to the photoconductive medium 10 to prevent the toner from being deposited on the non-exposed area thereof, with the developing roller 22 caused to make contact with the photoconductive medium 10 at such a contact pressure as not to destroy the prewetting solution layer. For this reason, the developing device 14 should have such a construction that the liquid toner and the prewetting solution transported by the rotation of the photoconductive medium 10 and the developing roller 22 be passed through the contact area between the photoconductive drum 10 and the developing roller 22. At the same time, the hardness of the developing roller 22 must not be too high. More specifically, the hardness of the developing roller 22 should preferably be less than 60 degrees in terms of JIS-A hardness measurement. The lower the hardness of the developing roller 22, the more the liquid toner and the prewetting solution can pass through the contact area. In this regard, a sponge-like material is desirable for the purpose. The higher the outside dimensional accuracy and the deflection accuracy during rotation of the developing roller 22, the more favorable because the pressure on the liquid layer can be maintained at a constant level. If the hardness of the developing roller 22 is low, it would become difficult to improve machining accuracy. The hardness and the outside dimensional accuracy of the developing roller 22 should therefore be properly balanced. Furthermore, the higher the revolution of the developing roller 22, the more becomes the amount of liquids to pass the contact area between the developing roller 22 and the photoconductive medium 10, thereby relaxing pressure conditions. Too high revolution of the developing roller 22 would reduce the time for the application of an electrical field to the liquid toner, leading to the shortage of time necessary for the toner to migrate. Increasing the revolution of the developing roller 22 therefore has its limitation. The larger the diameter of the developing roller 22 the more becomes the amount of liquids to pass the contact area, thereby relaxing pressure conditions. But it makes it difficult to maintain outside dimensional accuracy. Furthermore, the surface of the toner layer applied to the developing roller 22 can be irregular because of the high viscosity of the toner. If this occurs, projected parts penetrate the prewetting solution layer to reach the surface of the photoconductive medium 10, causing noise on the non-exposed area, as shown in FIG.6. Even in the exposed area, such an uneven layer may appear on the image area, leading to a poor image. To cope with this, the toner must be applied uniformly on the entire surface of the developing roller 22. This invention uses an elastic soft roller. more preferably a roller having a hardness of less than 60 degrees (JIS-A). The photoconductive medium (drum) 10 and the roller comes in close contact with each other (zero gap) when the drum is at a standstill. As the drum rotates, the drum and the roller receives buoyancy in accordance with the revolution of the drum due to the viscosity of the liquid and the elastic roller is deformed in accordance with the viscosity, with the result that a gap is formed between the drum and the roller, and a toner layer of a predetermined thickness is formed. The elastic developing roller shown in FIG.10 can accomplish this. Furthermore, since an electric field is applied to this elastic developing roller, the sponge roll and the film tube should be made electrically conductive, more preferably should be of a resistivity of 10 4 ∼ 10 9 Ω · cm, respectively. An EPT-51 (Asker F hardness 68 degrees), manufactured by Bridgestone Corp., can be used as the electrically conductive sponge, and an electrically conductive PFA tube (thick.: 30 microns) as the surface film tube. Typical size of the developing roller may be 32 mm in diameter, and 220 mm in barrel length, as shown in the figure. Table 4 below shows the measurement results of the amount of liquid passage using this type of developing roller. In the table, DC344 is a silicone oil produced by Dow Corning. Roller peripheral speed Liquid Amount of passage 100 mm/s DC344 (2.5 cSt) 8 g · m 2 M-toner (400 mPa · S) 20 g/m 2 250 mm/s DC344 (2.5 cSt) 13 g · m 2 M-toner (400 mPa · S) 20 g/m 2 As is evident from the measurement results shown in the table, the amount of liquid passage per unit area is increased with increases in the peripheral speed of t he roller. By making the surface of the surface film tube coarser, to Rz1 ∼ 10, for example, the toner can be transported by holding the toner in the recesses on the coarse roller surface, thereby increasing the adsorption of the toner. Silicone rubber can be used as the elastic roller. Silicone rubber has a volume resistivity of about 104 ∼ 109Ω · cm, a hardness of 60 degrees (JIS-A), and good mechanical repulsion. The elastic roller can of course be made with silicone rubber alone, but should more preferably be made by combining silicone rubber with the aforementioned surface film tube. FIG. 11 shows another example of such a developing roller. As shown in the figure, this developing roller has spacer rollers provided coaxially on both ends. The spacer rollers are provided to maintain the distance between the developing roller and the photoconductive medium (OPC drum) constant, and should basically have insulating properties. This can be made of an insulating resin, for example. To ensure accuracy, it can also be made of a metallic material, whose surface is coated with an insulating resin layer. The diameter of the spacer roller can be 31.8 mm for a 32-mm developing roller, for example. With this arrangement, the developing roller can press the photoconductive drum with a bite of 0.1 mm, thereby maintaining the pressure on the roller nip at an optimal level. Although an elastic roller is used as the developing roller in the embodiment described above, the embodiments shown in FIG. 12 or 13 use a rigid roller or a roller having a given hardness as the developing roller. The developing roller shown in FIG. 12 applies pressure by the roller's own weight (by gravity). As shown in the figure, a swing arm cantilevered at one end from a support member in an almost horizontal direction has a developing roller at the other end, so that pressure is exerted onto the photoconductive drum by the developing roller's own weight. FIG. 13 shows a developing roller which applies pressure by a spring, rather than the developing roller's own weight. As shown in the figure, the swing arm cantilevered at one end from the support member and having the developing roller at the other end has a spring between the swing arm and the support member in such a manner as to force the developing roller onto the photoconductive medium. In this way, the developing roller forced onto the photoconductive medium by the developing roller's own weight or the spring makes contact with the photoconductive medium when it is at a standstill. As the drum is rotated, both the developing roller and the photoconductive medium receive a buoyancy in accordance with the revolution of the drum due to the viscosity of the liquid, and a gap in accordance with the developing roller's own weight or the force of the spring is formed between the developing roller and the photoconductive medium. Thus, a toner layer of a desired thickness can be formed. FIG. 14 is a diagram of assistance in explaining why the residual toner can be scraped off from a soft developing roller by using the reverse scraping roller. In the lift hand of the figure shown is the prior art where a blade 37 is forced to the developing roller to scrape the residual toner. In this case, the edge surface of the blade has to be forced to the developing roller by exerting a given pressure to scrape off the residual toner from the developing roller. Even with this arrangement, a certain amount of toner cannot be prevented from slipping through the blade. When a reverse scraping roller is used, as shown in the right hand of the figure, on the other hand, the residual toner can be easily prevented from slipping through the nip portion between the developing roller and the reverse scraping roller by exerting a very low pressure. The surface material of the reverse scraping roller, which is required to have good toner removal properties, should have good surface flatness (surface roughness). For this reason, a rigid material, such as a metal or a rubber material having a hardness of more than 40 degrees, is generally preferred not only in toner removal properties but also in machinability. To improve the residual toner removal effect of the reverse scraping roller when used in conjunction with the developing roller, the toner must be prevented from building up at the entrance of the nip portion, thereby increasing the pressure of the toner there, thus allowing the toner to enter and eventually slip through the nip portion. To this end, as much toner as possible has to be carried away in the opposite direction to prevent it from building up in the nip portion. The peripheral speed of the reverse scraping roller should preferably be at least twice the speed of the developing roller. I is difficult, however, to increase the peripheral speed ratio more than ten times because the ceiling of speed is limited by vibration in motor bearings, and the construction of the drive unit. When an elastic roller having a relatively low hardness and a coarse surface roughness is used as the reverse scraping roller, it is possible to more thoroughly scrape off the toner because the toner holding power of the roller surface is increased. This is because the toner is entrapped in recesses on the coarse roller surface and carried away. The surface roughness in this case should preferably be more than Rz10. More preferably, an elastic roller having a rubber hardness of less than JIS-A 60 degrees, or a tube roller obtained by coating a sponge scraping roller with a film can be used. An elastic soft roller, preferably having a hardness of less than 60 degrees (JIS-A), is recommended as a developing roller suitable for use in conjunction with the aforementioned reverse scraping roller. With this arrangement, both the photoconductive medium (drum) and the roller come in contact with each other (zero gap) when they are at a standstill. As the drum is rotated, both the developing roller and the photoconductive medium receive a buoyancy in accordance with the revolution of the drum due to the viscosity of the liquid, and the elastic roller is deformed in accordance with its elasticity. As a result, a gap is formed between the developing roller and the photoconductive medium. Thus, a toner layer of a desired thickness can be formed. A roller having such a construction that a sponge roll is provided around a core metal, with the surface covered with a film tube can be used as such an elastic roller. FIG.15 shows an example where a developing belt is used in place of a developing roller. A developing solution can be fed using a developing belt coming in contact with the photoconductive medium so that a double-layer construction with the prewetting solution film on the photoconductive medium. With this arrangement, positively charged toner particles in the developing solution can be deposited on the surface of the photoconductive medium in accordance with an electric field formed between the developing belt and the photoconductive medium. At this time, a rigid reverse scraping roller 36 is caused to make contact with the developing belt and rotate in the reverse direction, thereby scraping the residual toner from the developing belt, as in the previous example. The toner deposited on the reverse scraping roller 36 is scraped off by a blade 37 coming in contact with the roller 36 and collected in a toner reservoir.

Claims: A wet type electrophotography apparatus using a non-volatile, high-viscosity, high-concentration liquid toner as a developing solution comprising an image carrier (10) on which an electrostatic latent image is formed, a rotatable developing member (22) for feeding said developing solution by making contact with said image carrier (10) and causing toner particles in said developing solution to deposit on said image carrier (10), and a developing solution applying means (14) comprising a plurality of interconnecting rotating rollers (23-26) for transporting said developing solution while spreading and applying to the surface of said rotatable developing member (22) via said rotating rollers (23-26), and applying a film of said developing solution on the surface of a final-stage rotating roller (23) to said rotatable developing member (22) making contact with said final-stage rotating roller (23) characterized in that said developing solution applying means (14) causes said final-stage rotating roller (23) coming in contact with said rotatable developing member (22) to rotate in the opposite direction to said rotatable developing member (22). A wet type electrophotography apparatus as set forth in Claim 1, wherein said final-stage rotating roller (23) is a rubber roller having a low hardness and a coarse surface roughness. A wet type electrophotography apparatus as set forth in Claim 1 wherein said final-stage rotating roller (23) is an elastic tube roller comprising a sponge roller covered with a film. A wet type electrophotography apparatus as set forth in Claim 1, wherein said final-stage rotating roller (23) is a metallic roller having a high hardness and a smooth surface. A wet type electrophotography apparatus as set forth in any of the Claims 1 to 4, wherein a bias voltage is fed to said final-stage rotating roller (23). A wet type electrophotography apparatus as set forth in any of the preceding Claims, wherein a roller is brought in contact with said rotatable developing member (22), independently of said final-stage rotating roller (23), and is caused to rotate in the opposite direction to said rotatable developing member (22). A wet type electrophotography apparatus as set forth in any of the preceding Claims, wherein said rotatable developing member (22) is a developing roller. A wet type electrophotography apparatus as set forth in any of the preceding Claims, wherein said rotatable developing member is a developing belt. A wet type electrophotography apparatus as set forth in any preceding Claim, wherein said wet type electrophotography apparatus comprises a means for monitoring (50, 54) the thickness of said developing solution layer applied to the surface of said final-stage rotating roller (23) and controlling said layer thickness to a predetermined value. A wet type electrophotography apparatus as set forth in Claim 9, wherein said layer thickness is controlled by controlling the peripheral speed of rotating rollers (23-26) of said developing solution applying means so as to make the feed of said developing solution variable. A wet type electrophotography apparatus as set forth in Claim 9 or Claim 10, wherein said layer thickness is controlled by applying an electrical field between a first-stage rotating roller (26) and a next-stage rotating roller (25) of said developing solution applying means and controlling the intensity and direction of said electrical field. A wet type electrophotography apparatus as set forth in any of the Claims 9 to 11, wherein said layer thickness is controlled by controlling a gap or contact pressure between said rotating rollers (23-26) of said developing solution applying means. A wet type electrophotography apparatus as set forth in Claim 12, wherein said gap or contact pressure between said rotating rollers (23-26) is controlled by causing roller shaft positions to move via a piezoelectric element (52) provided on roller bearings. A wet type electrophotography apparatus as set forth in Claim 12 or Claim 13, wherein said gap or contact pressure between said rotating rollers (23-26) is controlled independently on right and left sides so that the thicknesses of said developing solution on the right and left sides of said final-stage roller (23) in the longitudinal direction become substantially equal. A wet type electrophotography apparatus as set forth in any of the Claims 9 to 14, wherein said layer thickness is monitored by making the surface of said final-stage rotating roller (23) light-reflective, and detecting the color strength of toner on said light-reflective surface based on the reflection of light irradiated on said light-reflective surface by a light source.

Inventors: ICHIDA MOTOHARU, INAMOTO AKIHIKO, MOTO SATORU, NAKASHIMA YUTAKA, OKANO SHIGEHARU, TAKABATAKE MASANARI, TAKEDA SEIICHI, UESUGI SHIGEKI, ICHIDA, MOTOHARU, INAMOTO, AKIHIKO, MOTO, SATORU, NAKASHIMA, YUTAKA, OKANO, SHIGEHARU, TAKABATAKE, MASANARI, TAKEDA, SEIICHI, Ichida, Motoharu, PFU Limited, Inamoto, Akihiko, PFU Limited, Moto, Satoru, PFU Limited, Nakashima, Yutaka, PFU Limited, Okano, Shigeharu, PFU Limited, Takabatake, Masanari, PFU Limited, Takeda, Seiichi, PFU Limited, Uesugi, Shigeki, PFU Limited
Applicants: PFU LTD, PFU LIMITED
Application year: 1998
Main classification: G03G15
IPC codes: G03G15