Patent Grant
6842599
The present invention relates to a developing unit for a liquid-development process in which a developing roller carrying a toner layer of liquid developer formed thereon is disposed so as to face a photosensitive body in such a manner that toner particles of the liquid developer are caused to adhere to the photosensitive body in a selective pattern corresponding to a recorded latent image.
A conventional developing unit using a liquid developer has a structure in which, as shown in
As shown in
As is apparent from its function, the bias blade must exhibit not only flexibility for holding the toner layer in position and allowing the toner layer to pass through, but also electrical conductivity for inducing an electric field. In terms of flexibility in particular, the type of bias blade to be employed must be determined by comparison with the hardness of the developing roller, but in practice the type of bias blade is selected in view of whether the bias blade allows the toner layer to pass through.
Conventionally, a bias blade is manufactured by molding and cutting a relatively thin sheet of rubber or plastic into a desired shape and must be adjusted in electrical resistance with respect to the resistance of the developing unit. Further, a bias blade must allow a very thin toner layer to pass through, depending on the thickness of the layer of a liquid developer to be used. Generally, the amount of liquid that is allowed to pass depends on applied mechanical pressure, viscosity, and speed. For example, when an ordinary elastic rubber is used, the bias blade has the following relation. The amount of liquid allowed to pass is 20 im in the case where roller rubber hardness: 40 (JIS-A); oil viscosity: 20 cSt; and speed: 250 mm/s.
Furthermore, in terms of the function of a bias blade, a bias blade must have a shape designed so as not to rupture or separate toner, in order to prevent formation of any rivulet. A rivulet refers to a stripe-shaped non-uniform layer (irregularity) which is formed as a result of derangement of a layer of a liquid developer attributable to viscosity of toner when the liquid developer layer is broken and separated at the exit of the blade. When such a rivulet is formed, image quality deteriorates or the image encounters fogging. Consequently, instead of a tip-side edge of a blade, a belly portion of the blade adjacent to the tip-side edge must be pressed against the developing roller. For this purpose, the bias blade assumes the shape shown in FIG. 18.
As described above, there has been desired a method for stably applying an electric field to the toner layer so as to maintain passage of a proper amount of toner without breakage and separation of toner, which would result in formation of a rivulet. Further, since the bias blade utilizes an electrical migration phenomenon, the length of contact between the blade and the developing roller (the length of contact in the moving direction: the nip width) must be increased in order to increase a period of time for movement by migration. However, when the manner of contact of the bias blade is determined so as to suppress breakage and separation of toner at the tip end to thereby prevent formation of rivulets, in many cases, it becomes difficult to increase the nip width of the blade.
Conventionally, a blade is positioned in such a manner that the blade comes into contact with a toner layer in an area starting from a belly portion of the blade to a point very close to the tip-side edge thereof. Therefore, adjustment of the position of the blade has been difficult. When the tip-side edge of the blade comes into direct contact with a toner layer, toner is not permitted to pass through such a contact zone and is scraped off by the blade, which results in formation of a stripe in a developed image. In contrast, when the blade is separated from the toner layer at a position spaced too far from the tip-side edge, formation of a rivulet occurs.
Further, conventionally, high voltage is supplied from a power source to the bias blade, and therefore, means for limiting current must be provided in order to protect the developing roller.
Japanese Patent Application Laid-Open (kokai) No. H7-287450 discloses an alternative method for disposing a bias-applied electrode in opposition to a developing roller. In the method disclosed in this publication, a rigid electrode having a cylindrical inner surface is disposed so as to face the developing roller with high precision. However, in the case in which a toner layer has a thickness on the order of 10 im, a very small gap corresponding to such a thin layer is very difficult to form through only mechanical machining.
In addition, as described above, a liquid toner is applied onto a developing roller, and a latent image on a photosensitive body is developed by use of this liquid toner. However, because of its high viscosity and high concentration, the liquid toner often fails to be uniformly applied onto the developing roller unless a sufficient amount of liquid toner is uniformly conveyed and applied to the developing roller.
Consequently, the present applicant previously proposed a liquid toner supply arrangement as shown in
As described above, the patterned roller is disposed in such a positional relation with the toner feed tray as to close its open side. Further, a scraper blade is disposed on the patterned roller at a position downstream of the toner feed tray and is normally pressed against the patterned roller under constant pressure by means of the resilience of a spring, so that the toner can be conveyed and applied onto the developing roller uniformly.
In this illustrated conventional construction, the gaps between the patterned roller and nearby parts are tightly covered by means of seals (illustrated seals 1 and 2) from all sides. Practically, a toner vessel is statically sealed by a cylindrical casing, sponge rubber, etc. Although this method enables provision of a structure which can prevent leakage irrespective of attachment angle, a rubber member for sealing comes into contact with portions other than the scraper blade and, therefore, cohesion of toner occurs at these portions. This toner cohesion causes variation in toner concentration, with the result that the image suffers irregularity and stripes, thereby deteriorating image quality.
The developing process will now be described in more detail. The proper amount of toner to be applied onto the developing roller is determined in terms of volume of the engraved cells of the patterned roller. Notably, the amount of toner is determined by the pressure of contact and the shape of cells; however, in general, not all the toner in the cells is fully transferred to the developing roller. Namely, a certain part of toner fails to be transferred and remains in the cells of the patterned roller after these cells have passed the developing roller. Although this does not pose a serious problem, the toner concentration is apt to change or the residual toner is apt to be scraped off subsequently when the toner comes into engagement with the sealing materials, etc. that are provided for tightly closing the above-described gaps. Either problem can be eliminated when the patterned roller passes the standing toner in the toner feed tray. In actuality, this phenomenon does not occur while the patterned roller is rotating at relatively low speed.
Nonetheless, when a sufficiently high printing speed is required, the circumferential speed of the patterned roller also inevitably becomes higher. In such a case, since the period of time for the patterned roller to pass through the standing toner is short, the above-described phenomenon cannot be completely eliminated; consequently, the resulting image still suffers irregularity and stripes.
As described hereinabove, when sponge rubber and sealing rubber used for constituting a closed structure come into engagement with the patterned roller, solid components of tone cohere locally, with the result that intended uniform application of toner cannot be achieved by means of the scraper blade.
Accordingly, an object of the present invention is to provide a developing unit for a high-speed developing process, which unit is easy to assemble and scarcely suffers fogging (dirt in background).
Another object of the present invention is to increase, in a developing roller equipped with a bias blade, the nip width of the bias blade and to stabilize the contact of the bias blade with the developing roller, thereby enhancing the effect of using the bias blade.
Still another object of the present invention is to prevent not only deterioration of image quality but also stagnation and leakage of toner, as well as to simplify the developing unit by shared use of a part or parts thereof.
The present invention provides a developing unit for a liquid-development process in which a developer bearer body carrying a toner layer of liquid developer formed thereon is disposed to face an image bearer body in such a manner that toner particles of the liquid developer are caused to adhere to the image bearer body in a selective pattern corresponding to a recorded latent image. Further, the developing unit includes an electrode module in the form of a flexible sheet which is adapted to contact the toner layer on the developer carrying body before development and to which a bias voltage is applied. The electrode module acts to separate the toner layer into a toner-rich layer and a carrier-rich layer by utilizing electrical migration occurring in the toner layer when an electric field is applied to the toner layer on the basis of the bias voltage. The electrode module is constructed in such a manner that the sheet is pulled toward a surface of the developer bearer body by a force attributable to surface tension and/or wetting characteristic of the liquid developer existing between the sheet and the developer carrying body, whereby the sheet comes into contact with the surface of the developer bearer body.
The developing unit for a liquid-development process according to the present invention further includes a patterned roller serving to apply the liquid developer to the developer bearer body while the patterned roller is rotating in contact therewith; a toner feed tray serving to supply the liquid developer to a surface of the patterned roller; and control means for controlling the rotation of the patterned roller and the supply of the liquid developer. The patterned roller has a structure such that its circumferential surface is free from contact with any components of the developing unit, other than the developer bearer body and a scraper blade. The toner feed tray has a length greater than the transverse length of the patterned roller and a width for covering a portion of the circumference of the patterned roller, is disposed to face the surface of the patterned roller with a gap formed therebetween, and is configured to enable the liquid developer to be fed to the gap and discharged from the gap in a circulating manner. When the liquid developer is supplied to said toner feed tray, the control means effects the supply while rotating the patterned roller, in order to convey the liquid developer onto the patterned roller without any leakage of the liquid developer from the toner feed tray.
Embodiments of the present invention will now be described in detail.
The charger electrically charges the photosensitive body up to approximately 700 V. The exposure device exposes the photosensitive body using laser light or LEDs, to thereby form on the photosensitive body an electrostatic latent image in a pattern of exposed areas whose potential becomes approximately 100 V.
The developing units are usually assigned one to each of yellow, magenta, cyan, and black; each developing unit is biased to approximately 400 V (+E1) and is adapted to form a toner layer having a thickness of 2 to 3 im on a developing roller, from a liquid toner having a toner viscosity of 400 to 4000 mPa.S and a carrier viscosity of 20 cSt. In accordance with an electric field acting between the developing roller and the photosensitive body, the developing roller supplies the positively-charged toner particles to the photosensitive body, whereby the toner particles adhere to the exposed areas (or unexposed areas) of the photosensitive body, the areas having attained a potential of approximately 100 V.
The intermediate transfer body is biased to approximately −800 V (−E2). The toner adhering to the photosensitive body is transferred to the intermediate transfer body in accordance with the electric field acting between the intermediate transfer body and the photosensitive body. For example, yellow toner adhering to the photosensitive body is first transferred to the intermediate transfer body; magenta toner adhering to the photosensitive body is then transferred to the intermediate transfer body; and cyan toner and black toner are successively transferred to the intermediate transfer body.
The color toners adhering to the intermediate transfer body are melted upon heating by an unillustrated heating device. Meanwhile, the backup roller serves to assist in transferring the molten toners from the intermediate transfer body onto a printing medium, and then fixing the transferred toner.
In the present invention, a winding flexible sheet shown in
Notably, the bias blade usually separates a toner layer, before being subjected to development, into a layer characterized by high concentration of toner particles (solid components), and a layer characterized by low concentration of toner solid components and dominated by a carrier solvent, by means of electrical migration. However, the extent of this separation depends on the electrical migrating characteristic (mobility) of toner. The lower the mobility of toner, the smaller the effect that can be achieved. In order to enable obtainment of the effects of suppressing fogging and attaining high-speed response even when toner has low mobility, a period of time for application of an electric field must be increased. That is, the length of contact between the blade and the roller (nip width) must be increased. According to the present invention, the flexible sheet is pressed against the developing roller in order to secure a gap by utilizing the flow pressure of toner. Further, intimate contact is achieved between the sheet and the developing roller by the wetting characteristic of liquid. Therefore, the period of time over which continuous contact is achieved can be increased by a large extent.
Since the blade (sheet) of the present invention exhibits extremely low bending strength, the blade is pulled to the surface of the developing roller by the wettability of the blade with the toner layer on the developing roller, whereby the blade comes into intimate contact with the toner layer on the surface of the developing roller. At this time, the sheet is in intimate contact with the surface of the toner layer up to a tip-side edge of the sheet. Although this winding bias blade (sheet) must be designed to attain uniform contact over its entire area, it is unnecessary to consider (or adjust) the process for the sheet edge.
The bias blade (sheet) may be a thin metal plate whose resistance is approximately zero. For example, the bias blade may be formed of an SUS304 plate according to JIS, the plate having a thickness less than 1 mm, preferably 0.05 mm.
Alternatively, the bias blade (sheet) may assume the form of an electrically conductive low-resistance polymer sheet. For example, the polymer of this alternative sheet may be polyester, polycarbonate, polyimide, polyurethane or any other polymer, so long as electrical conductive properties are imparted. Preferably, a conductive polyethylene sheet, available under the name “Chlopolyfilm” from Achilles Corporation, may be used.
Further, carbon may be added to the polymer in order to impart conductive properties to the polymer. Alternatively, ions of lithium salt, sodium salt, or ammonium salt may be applied the polymer sheet in order to impart electrically conductive properties thereto.
In another alternative form, as shown in
The intermediate- and high-resistance sheet extends beyond the distal edge of the low-resistance layer by a length L (see FIG. 4); apparent resistance of the resulting bias blade can be adjusted by varying the length L.
Next, in relation to the method for application of voltage to the sheet, when a proper sheet resistance value is selected and a voltage is applied to the tip edge side of the sheet, an electric field acting on the sheet can be stronger toward its tip edge side and weaker toward its toner inlet side, depending on a voltage drop attributable to a current flowing into the sheet.
For the purpose of applying voltage to the tip edge side of the sheet, as shown in
As shown in
The winding sheet of the present invention can have a sufficient length of contact (nip width) and, therefore, a voltage to be applied can be reduced. Accordingly, a circuit for limiting current becomes unnecessary, and thus the cost of a power source, etc. can be lowered. In an alternative case shown in
The patterned roller (e.g., Anilox roller of Asahi Roll) is a roller having, for example, a striped pattern of 100/inch to 350/inch grooves inclined with respect to the circumferential direction; or a fine latticework pattern which has additional grooves crossing the above-described grooves. The transfer of toner by use of these grooves of the patterned roller enables supply of toner at a constant rate, which is limited only by the number and size (cross-sectional area) of the grooves. Notably, an individual area surrounded by the grooves is called a cell. The toner to be supplied to the patterned roller originally comes from the toner feed tray.
By virtue of the thus-constructed patterned roller, when the patterned roller is rotated, toner can be supplied to the entire circumferential area of the patterned roller without leaking from the toner feed tray. Further, in an alternative form having an toner outlet as shown in
Liquid toner supplied to the feed tray flows in the channel between the patterned roller and the feed tray, and the discharged liquid toner returns to a toner tank for subsequent use; this supplying and discharging procedure is repeated for the sake of recycling. As a result, liquid toner can be speedily conveyed onto the patterned roller uniformly without any leakage from the feed tray.
In practice, however, since the toner feed tray is not completely sealed, leakage of toner could occur, depending on the supplied amount and viscosity of toner, the circumferential speed of the patterned roller, and the shape of the feed tray.
The method of supplying toner while rotating the patterned roller is very effective method, because the method provide a great margin in relation to toner leakage. Our experiments indicate that because toner tends to follow the circumferential movement of the patterned roller even when the toner feed tray assumes a slightly tilted posture, leakage can scarcely occur.
Generally, in a color electrophotographic apparatus, one developing unit must be provided for each color. Assume that these developing units dedicated one to each color are arranged along the circumference of, for example, a photosensitive drum or a transfer drum. Such an arrangement would result in a complex apparatus structure, because the individual developing units must have different structures, depending on their installation angles; i.e., due to the restriction imposed on the installation angles of respective toner feed trays. However, the method of supplying toner while rotating the patterned roller mitigates the restriction imposed on the installation angles of respective toner feed trays. Therefore, even when developing units for respective colors are installed at different angles, toner can be supplied by use of a structure common among the colors.
Needless to say, since the feed tray is not closed by means of sealing, when the pattern roller is stopped, toner leaks from the feed tray by means of free fall, depending on the installation angle of the feed tray. This toner leakage occurring when the patterned roller is stopped can be prevented by controlling the supply of toner in accordance with a pump control timing shown in FIG. 16.
1. Start of Toner Supply:
2. Stop of Toner Supply:
Determination as to whether or not the toner feed tray is empty can be achieved by monitoring the toner supply flow rate and the rpm of the patterned roller. The amount of toner in the toner feed tray remains constant by virtue of the toner circulating structure of the toner feed tray as described above. Further, the patterned roller supplies toner to the developing roller at a constant rate. Therefore, determination as to whether or not toner remains in the toner feed tray can be performed by monitoring the toner supply flow rate and the rpm of the patterned roller. This method enables obtainment of a structure which prevents leakage attributable to inclination of the toner feed tray, without sealing the toner flow channel of the toner feed tray.
The scraper blade contacting the patterned roller functions to scrape off excessive toner bulging from the cells of the patterned roller. In the absence of this scraper blade, supply of a constant amount of toner cannot be guaranteed even if the patterned roller has engraved cells. In order to realize stable and reliable contact between the scraper blade and the patterned roller, a pushing force is applied to the toner feed tray by use of the resilience of a spring, and the scraper blade is brought into contact with the patterned roller via the toner feed tray. Alternatively, as shown in
Further, for the purpose of preventing leakage of toner from the contact zone between the scraper blade and the toner feed tray or the opposite end sides of the patterned roller, an elastic material, such as urethane rubber or closed-cell foamed sponge, is employed so as to enhance liquid-tightness. However, in this case, for the above-mentioned reasons the elastic material cannot be brought into contact with the circumferential surface of the patterned roller, at least within the range of the effective image bearing area.
The right side of
When the patterned roller is in rotation, toner assumes a toner speed profile as shown in the right side in FIG. 15. Namely, this speed profile indicates that the toner speed increases linearly within the range of zero at the surface of the fixed toner feed tray to the circumferential speed of the patterned roller. So long as this speed profile is maintained, toner does not leak. However, when the pressure for feeding toner into the toner feed tray becomes high, this speed profile collapses, resulting in leakage of toner. Generally, the higher the viscosity, the greater the flow resistance of the discharge pipe; consequently, internal pressure of the toner feed tray becomes high, so that toner is prone to leak.
As described hereinabove, in the electrophotographic apparatus of the present invention, when a toner applicator roller assumes the form of a patterned roller having a pattern of cells engraved in its circumferential surface, nothing that can come in contact with the circumferential surface is present thereon, except for a developing roller and a scraper blade, so that toner concentration cannot change. Therefore, toner can be supplied to a toner supply section of a developing unit at a sufficient rate, while preventing deterioration of image quality, stagnation of toner, and leakage of toner.
Further, the present invention enables supply of toner by use of a structure common among required colors while maintaining the structure simple, even in the case in which developing units are arranged in number equal to the required colors, and restriction is imposed on the installation angle of each toner supply tray in accordance with the installation angle of the corresponding developing unit.
The configuration of the bias blade (sheet) according to the present invention increases the contact area of the bias blade (sheet), to thereby enable the process to be performed at a higher speed. It was confirmed experimentally that the blade can cope with a process speed of 500 mm/s. Moreover, conceivably, the blade can cope with a process speed of 1000 mm/s upon suppression of internal mechanical vibration of developing units attributable to operation at increased speed, proper selection of toner mobility, and adjustment of bias voltage to be applied. For example, when a process speed over 500 mm/s is realized, the printing speed can exceed 100 PPM, which should enable penetration of the professional offset printing market, which is a very lucrative market.
Further, since the level of electric charge of toner particles directly influences the image quality of development, stabilized voltage supply from the bias blade is required in order to make the level of electric charge of toner particles apparently stable. In this regard, the present invention enhance the effect of injecting electric charge into toner particles by increasing the nip width, to thereby increase the allowable range of toner.
Still further, in the present invention, since the bias blade can have an increased contact area, toner particles can be cohered electrically into a film shape. So long as the film-like toner layer offers a proper resistance against rupture and separation, an improved solution for rivulets can be achieved. Conventionally, the bias voltage must be increased in order to render the effect of an electrical field apparently large in order to minimize rivulets; in practice, however, increasing the bias voltage is difficult to attain within the range over 1000 V, partially because of problems such as undue discharging, and consequently only an electric field of insufficient strength can be applied. In contrast, the present invention enables achievement of a desired effect with a lower voltage by means of increasing the contact area or nip width and hence minimizing rivulets.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2001-260946 | Aug 2001 | JP | national |
| 2001-388658 | Dec 2001 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCTJP02/07948 | 8/5/2002 | WO | 00 | 7/16/2003 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO0302136 | 3/13/2003 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 3917880 | Wells et al. | Nov 1975 | A |
| 5826148 | Iino et al. | Oct 1998 | A |
| 6038421 | Yoshino et al. | Mar 2000 | A |
| 6072972 | Obu et al. | Jun 2000 | A |
| 6256468 | Liu | Jul 2001 | B1 |
| 6311034 | Nakashima et al. | Oct 2001 | B1 |
| Number | Date | Country |
|---|---|---|
| 03-279986 | Dec 1991 | JP |
| 7-287450 | Oct 1995 | JP |
| 11-194622 | Jul 1999 | JP |
| 11-194623 | Jul 1999 | JP |
| 2000-056576 | Feb 2000 | JP |
| 2000-250321 | Sep 2000 | JP |
| 2002-278300 | Sep 2002 | JP |
| WO 0188630 | Nov 2001 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20040057754 A1 | Mar 2004 | US |
