GB2350694A

GB2350694A - Magnetic developer roller

Info

Publication number: GB2350694A
Application number: GB0020334A
Authority: GB
Inventors: Koji Takagi; Yoshio Takizawa; Eiji Sawa; Shigeru Aoki; Hideharu Daifuku; Koutaro Takebe; Taihei Goto
Original assignee: Bridgestone Corp
Current assignee: Bridgestone Corp
Priority date: 1996-09-02
Filing date: 1997-09-02
Publication date: 2000-12-06
Anticipated expiration: 2017-09-02
Also published as: GB0020334D0; GB2350694B

Abstract

A magnet roller comprises a magnet body portion and a shaft portion formed integral with each other. The roller also comprises a plurality of poles in which the magnetic force between a magnetic pole having a maximum magnetic force and another magnetic pole having a minimum magnetic force differs by 100 G or more. Also disclosed is a molding method for manufacturing a magnetic roller. In molding the magnetic mixture, the magnetic body portion (1) of the magnet roller is pushed by ejection pins (4) to thereby remove the magnet roller from a mold (31,32). Alternatively, a gate is provided in the mold at a position corresponding to the outer circumferential surface or the end surface of the magnet body portion or the outer circumferential surface of the shaft portion, and the bonded magnet composition is injected through the gate into the mold cavity. The magnet roller may have an aluminium sleeve in which the inner diameter of the sleeve is X mm and the outer diameter of the magnet roller is (X - 4) to (X - 0.2) mm. Alternatively, the roller may have a resin outer layer whose elongation at break defined by JIS K7113 is 10% or less is formed on the sleeve surface. A developing device is configured by using the developing roller.

Description

MAGNET ROLLER, MANUFACTURING METHOD THEREFOR, DEVELOPING ROLLER AND

DEVELOPING DEVICE USING THE MAGNET ROLLER The present invention relates to a magnet roller for use in a developing roller for supplying a developer to an image forming member such as a photosensitive drum or belt and a sheet of plane paper, OHP, or photographic paper on which an electrostatic latent image is retained, to form a visible image on the surface of the image forming member in an electrophotographic device or an electrostatic recording device such as a copier and a printer, a manufacturing method for the magnet roller, a developing roller consisting of the is magnet roller and a sleeve, and a developing device using the developing roller. The present invention relates more particularly to a magnet roller which can easily obtain a high magnetic force at low costs, a manufacturing method for the magnet roller with reduced man-hours and superior dimensional accuracy, and a developing roller and a developing device both using the magnet roller.

In an electrophotographic device or an electrostatic recording device such as a copier and a printer, a developing roller for making visible an electrostatic latent image retained on a latent image retaining member such as a photosensitive drum is conventionally used. The developing roller is composed generally of a rotating sleeve and a magnet roller located inside of the sleeve. The magnet roller is formed by molding a bonded magnet, for example. As a known developing method, a magnetic developer (toner) carried on the surface of the sleeve is jumped to the latent image retaining member by the magnetic characteristics of the magnet roller, which is called a jumping phenomenon, thereby supplying the toner to the surface of the latent image retaining member to make the electrostatic latent image 2 visible.

The magnet roller is conventionally manufactured by injection molding or extrusion molding a pelletized bonded magnet composition containing a binder of thermoplastic resin, mainly, such as nylon and polypropylene and magnetic powder such as ferrite powder mixed in the resin binder, in a mold to which a magnetic field has been applied, and magnetizing the molded product to desired magnetic characteristics. While the magnet roller is usually provided at its each end with a shaft portion for supporting the roller, it has been proposed that the shaft portion at each end or one end of the roller is formed integrally with a roller body portion by molding of the bonded magnet composition for the purposes of reducing a manufacturing cost and increasing a magnetic force.

However, such a conventional magnet roller obtained by molding a bonded magnet composition to integrally form a roller body portion and a shaft portion projecting from each end or one end of the roller body portion has the following drawbacks.

(1) An image quality becomes higher with recent advances of electrophotographic technology. To obtain a higherquality image, the performance of the magnet roller, especially, a magnetic force of a specific magnetic pole (e.g., developing pole) must be further improved. However, in the conventional magnet roller formed of a bonded magnet composition containing ferrite powder as magnetic powder, a magnetic force of a magnetic pole having a maximum magnetic force is limited up to about 1000 G. Accordingly, a magnetic force generated on the sleeve surface by the magnet roller is limited.

(2) It may be generally considered to increase the content of the magnetic powder in the bonded magnet composition, so as to exhibit a higher magnetic force.

However, the increase in the content of the magnetic powder 3 causes a large reduction in fluidity of the bonded magnet composition in forming the magnet roller. In extreme cases, there is a possibility that the mold cavity may not be fully filled with the bonded magnet composition, and the shape of the mold cavity cannot be accurately transferred to the magnet roller to be formed, causing a reduction in dimensional accuracy. There is also a possibility that pores may be produced in the magnet roller to be formed.

(3) In the case of molding the bonded magnet composition to integrally form the roller body portion and the shaft portion projecting from each end or one end of the roller body portion, it may be difficult to remove the magnet roller from the mold because of an increased contact area therebetween, causing an increase in man-hour or a reduction in dimensional accuracy of the magnet roller upon removal from the mold.

In the conventional developing roller composed of the magnet roller and the sleeve rotatably provided about the outer circumference of the magnet roller, the surface of the developing roller may be worn during long-term use by the friction between -it and a developer, blade, or developer feeding roller provided in the developing device. As a result, there is a possibility of trouble on function of feeding and charging the developer or a possibility of image defects called ghost, depending on the kind of the developer. Further, to improve the function of feeding and charging the developer and thereby obtain a good image, microscopic asperities are formed on the sleeve surface to impart a suitable roughness to the sleeve surface in the prior art.

However, the microscopic asperities may be changed in condition because of wearing of the sleeve surface in long-term use, so that a good image cannot be obtained. Such a problem due to wearing arises regardless of the material for the sleeve. In particular, when the sleeve is formed of aluminum alloy that is a relatively soft metal, the above 4 problem is remarkable.

To cope with this problem, it has recently been proposed to plate the surface of the sleeve of aluminum alloy or stainless steel with a material different from the material of the sleeve, so as to improve the wear resistance of.the sleeve surface (Japanese Patent Laid-open No. 3-41485).

However, in forming such a plating film, it is necessary to form an electroplating film having a thickness of several microns or more, so as to improve the wear resistance of the sleeve surface, and there are variations in thickness of the electroplating film. Accordingly, the dimensional accuracy of the sleeve is reduced to result in a degradation in image quality.

It is accordingly an object of the present invention to provide new and useful magnet rollers, which can preferably obtain a high magnetic force at low costs.

A preferred aim of the present invention to provide a manufacturing method for a magnet roller with reduced man-hours and superior dimensional accuracy.

It is another pref errea atm of the present invention to provide.a developing roller employing the above magnet roller.

it is another preferred aim of the present invention to provide a developing roller which can be improved in wear resistance of its surface and can reliably obtain a high-quality image without the occurrence of ghost.

It is a further aspect of the present invention to provide a developing device employing any one of these developing rollers.

The present inventor has seriously investigated and has found that in obtaining a magnet roller by molding a bonded magnet composition composed of a resin binder and magnetic powder dispersed in the resin binder to integrally form a magnet body portion and a shaft portion projecting from each end or one end of the magnet body portion, a high magnetic force can be obtained without loss of fluidity of the bonded magnet composition upon molding and a magnet roller having a high magnetic force and superior dimensional accuracy can be obtained by using ferrite powder whose strontium/barium element ratio (Sr/Ba ratio) ranges from 1 to 100 as the magnetic powder.

According to the present invention, there is provided a first magnet roller characterized in that in a magnet roller obtained by molding a bonded magnet composition composed of a resin binder and magnetic powder dispersed in said resin binder to integrally form a magnet body portion and a shaft portion projecting from each end or one end of said magnet body portion; said magnetic powder is ferrite powder whose strontium/barium element ratio (Sr/Ba ratio) ranges from 1 to 100.

The present inventor has further found that in obtaining a magnet roller having a plurality of magnetic poles by molding a bonded magnet composition composed of a resin binder and magnetic powder dispersed in said resin binder to integrally form a magnet body portion and a shaft portion projecting from each end or one end of said magnet body portion, the anisotropy of the magnetic poles can be improved and the magnetic force of a specific magnetic pole (e.g., developing pole) can be set relatively high to obtain a high-quality image, by setting the difference in magnetic force between a high magnetic pole having a maximum magnetic force and a low magnetic pole having a minimum magnetic force to 100 G or more.

According to the present invention, there is provided a second magnet roller characterized in that in a magnet roller obtained by molding a bonded magnet composition composed of a resin binder and magnetic powder dispersed in said resin binder to integrally form a magnet body portion and a shaft portion projecting from each end or one end of said magnet body portion; said magnet roller has a plurality of magnetic poles, and the difference in magnetic force between a high magnetic pole having a maximum magnetic force and a low magnetic pole having a minimum magnetic force is 100 G or more.

Further, the present inventor has seriously investigated improving a manufacturing method for a magnet roller, and has found that in obtaining a magnet roller by injecting a melted bonded magnet composition composed of a resin binder and magnetic powder dispersed in the resin binder into a cavity of a mold to integrally form a magnet body portion and a shaft portion projecting from each end or one end of the magnet body portion from the bonded magnet composition, the magnet roller can be easily and reliably obtained with superior dimensional accuracy, without warpage and damage, and without an increase in man-hour, by opening the mold and next forcing an ejection. pin to push the magnet body portion, thereby removing the magnet roller composed of the magnet body portion and the shaft portion from the mold. Further, the present inventor has found that the flow of the bonded magnet composition in the cavity can be made uniform to thereby reliably prevent a reduction. in dimensional accuracy due to poor charging of the bonded magnet composition into the cavity or the production of defectives due to incomplete charging of the bonded magnet composition into the cavity, by providing a gate in the mold at a position thereof corresponding to the outer circumferential surface or the end surface of the magnet body portion or the outer circumferential surface of the shaft portion, and injecting the bonded magnet composition through the gate into the cavity of the mold.

According to the present invention, there is provided a first manufacturing method for a magnet roller comprising the steps of molding a bonded magnet composition composed of a resin binder and magnetic powder dispersed in said resin binder by using a mold to integrally form a magnet body portion and a shaft portion projecting from each end or one end of said magnet body portion; opening said mold; and forcing an ejection pin to push said magnet body portion, thereby removing said magnet roller composed of said magnet body portion and said shaft portion from said mold. According to the present invention, there is also provided a second manufacturing method for a magnet roller characterized in that in a manufacturing method for a magnet roller comprising the step of injecting a melted bonded magnet composition composed of a resin binder and magnetic powder dispersed in said resin binder into a cavity of a mold to integrally form a magnet body portion and a shaft portion projecting from each end or one end of said magnet body portion from said bonded magnet composition; a gate is provided in said mold at a position thereof corresponding to the outer circumferential surface or the end surface of said magnet body portion or the outer circumferential surface of said shaft portion, and said bonded magnet composition is injected through said gate into said cavity of said mold.

By using any of the first and second magnet rollers and the magnet rollers obtained by the first and second manufacturing methods, and rotatably providing a cylindrical sleeve around the outer circumference of the magnet roller, it is possible to provide a developing roller composed of the cylindrical sleeve rotatably provided and the magnet roller located inside of the sleeve. Further, by carrying a developer on the outer circumferential surface of the sleeve by the magnetic characteristics of the magnet roller to form a thin layer of the developer on the outer circumferential surface of the sleeve, then bringing the thin layer close to an image forming member, and then jumping the developer to the surface of the image forming member by the magnetic characteristics of the magnet roller to thereby form a visible image on the surface of the image forming member, a good image can be reliably obtained by the good magnetic characteristics of the magnet roller.

According to the present invention, there is provided a first developing roller characterized in that in a developing roller comprising a cylindrical sleeve rotatably provided and a magnet roller provided inside of said sleeve, wherein a developer is carried on the outer circumferential surface of said sleeve by the magnetic characteristics of said magnet roller to form a thin layer of said developer on the outer circumferential surface of said sleeve, and said thin layer is then brought close to an image forming member and jumped to be supplied to the surface of said image forming member by is the magnetic characteristics of said magnet roller, thereby forming a visible image on the surface of said image forming member; said magnet roller is any one of the first and second magnet rollers of the present invention and the magnet rollers obtained by the first and second manufacturing methods of the present invention.

The present inventor has further investigated the dimensional relation between a cylindrical sleeve rotatably provided and a magnet roller located inside of the sleeve in a developing roller. As the result, the present inventor has found that a magnetic force owned by the magnet roller can be effectively exhibited as the developing roller by setting the outer diameter of the magnet roller in the range of (X - 4) to (X 0.2) mm with the proviso that the inner diameter of the sleeve is X Mm. Accordingly, a high-quality image can be obtained by carrying a developer on the outer circumferential surface of the sleeve by the magnetic characteristics of the magnet roller to form a thin layer of the developer on the outer circumferential surface of the sleeve, then bringing the thin layer close to an image forming member, and then jumping the developer to the surface of the image forming member by the magnetic characteristics of the magnet roller to thereby form a visible image on the surface of the image forming member.

According to the present invention, there is provided a second developing roller characterized in that in a developing roller comprising a cylindrical sleeve rotatably provided and a magnet roller provided inside of said sleeve, wherein a developer is carried on the outer circumferential surface of said sleeve by the magnetic characteristics of said magnet roller to form a thin layer of said developer on the outer circumferential surface of said sleeve, and said thin layer is then brought close to an image forming member and jumped to be supplied to the surface of said image forming member by the magnetic characteristics of said magnet is roller, thereby forming a visible image on the surface of said image forming member; the outer diameter of said magnet roller is in the range of (X - 4) to (X - 0.2) mm with the proviso that the inner diameter of said sleeve is X mm.

The present inventor has further investigated to obtain a developing roller which can be improved in wear resistance of its surface and can reliably provide a high-quality image wiCh no ghost. As the result, the present inventor has found that in a developing roller having a magnet roller, especially, in a developing roller having a cylindrical sleeve rotatably provided and a magnet roller located inside of the sleeve, wherein a developer is carried on the outer circumferential surface of said developing roller by the magnetic characteristics of said magnet roller to form a thin layer of said developer on the outer circumferential surface of said developing roller, and said thin layer is then brought close to an image forming member and jumped to be supplied to the surface of said image forming member by the magnetic characteristics of said magnet roller, thereby forming a visible image on the surface of said image forming member; the wear resistance of the surface of the developing roller can be improved without a reduction in dimensional accuracy of the surface of the developing roller, and a high-quality image with no image defects such as ghost can be reliably obtained over a long term, by' forming a layer of a resin component that can improve the wear resistance of the surface of the developing roller on the surface of the developing roller and by using a resin component whose elongation at break defined by JIS K7113 is 10% or less as the above resin component.

According to the present invention, there is provided a third developing roller.characterized in that in a developing roller having a magnet roller, wherein a developer is carried on the outer circumferential surface of said developing roller by the magnetic characteristics of said magnet roller is to form a thin layer of said developer on the outer circumferential surface of said developing roller, and said thin layer is then brought Close to an image forming member and jumped to be supplied to the surface of said image forming member by the magnetic characteristics of said magnet roller, thereby forming a visible image on the surface of said image forming member; a surface portion of said developing roller has a layer of a resin component whose elongation at break defined by JIS K7113 is 10% or less.

In the third developing roller, however, the surface roughness of the developing roller changes because of the formation of the resin component layer on the surface of the developing roller. To cope with this, the present inventor has investigated to reliably obtain a proper surface roughness without loss of the effect obtained by the formation of the resin component layer. As the result, the present inventor has found that a proper surface roughness can be maintained by optimizing the relation between the surface roughness of a base member constituting the above surface portion of the developing roller and the thickness of the resin component layer formed on the surface of the base member, especially, by making the thickness of the resin component layer smaller than the surface roughness of the base member. more specifically, by specifying the relation between the surface roughness of the base member and the thickness of the resin component layer to satisfy 0.01 < b/a < 2 where a stands for the JIS ten-point average surface roughness Rz of the surface of the base member and b stands for the thickness of the resin component layer formed on the surface of the base member, a proper surface roughness can be maintained to thereby maintain good developer feedability and the developing characteristics and durability can be improved.

As a preferred embodiment of the third developing roller of the present invention, it is characterized in that the is relation of 0.01 < b/a < 2 is satisfied where a stands for the JIS ten- point average surface roughness Rz of the surface of a base member constituting said surface portion of said developing roller, and b stands for the thickness of said layer of said resin component formed onthe surface of said base member.

Each of the first to third developing rollers is used in a developing device such that a developer is carried in the form of thin layer.onthe outer cireuxnferential surface of the developing roller and is jumped to be supplied to the surface of the image forming member by the magnetic characteristics of the magnet roller to thereby form a visible image on the surface of the image forming member. In particular, each developing roller is preferably used in a developing device such that a latent image retaining member such as a photosensitive member on which an electrostatic latent image is retained is used as the image forming member, and a developer is supplied to the latent image retaining member to make visible the electrostatic latent image on the surface of the latent image retaining member. Thus, a high-quality image can be reliably reproduced on the surface of the latent image retaining member.

According to the present invention, there is provided a developing device characterized in that in a developing device having a developing roller for carrying a developer on the outer circumferential surface, rotationally feeding said developer to a position close to the surface of a latent image retaining member on which an electrostatic latent image is retained, and depositing said developer to said electrostatic latent image retained on the surface of said latent image retaining member, thereby making said electrostatic latent image visible; said developing roller is any one of the first, second, and third developing rollers of the present invention.

other objects and features of the invention will be more is fully understood from the following detailed description and appended claims when taken with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a preferred embodiment of the magnet roller according to the present invention; FIGS. 2A and 2B are a fragmentary perspective view and a fragmentary sectional view, respectively, showing another preferred embodiment of the magnet roller according to the present invention; FIGS. 2C and 2D are views similar to FIGS. 2A and 2B, respectively, showing a modification; FIGS. 2E and 2F are views similar to FIGS. 2A and 2B, respectively, showing another modification; FIG. 3A is a schematic sectional view illustrating a manufacturing method for a magnet roller according to the present invention in a mold closed condition; FIG. 3B is a view similar to FIG. 3A, showing a mold open condition; FIG. 4 is a schematic view showing a preferred embodiment of the developing roller according to the present invention; and FIG. 5 is a schematic view showing a preferred embodiment of the developing device according to the present invention.

DETAILED DESCRIPTION

The present invention will now be described in more detail.

As shown in FIG. 1, each of the first and second magnet rollers of the present invention is a magnet roller obtained by molding a bonded magnet composition composed of a resin binder and magnetic powder dispersed in the resin binder to integrally form a magnet body portion 1 and a shaft portion 2 projecting from each end or one end of the magnet body portion 1 (the shaft portion 2 projecting from each end is shown by way of example).

The first magnet roller of the present invention is characterized in that in a magnet roller obtained by molding a bonded magnet composition composed of a resin binder and magnetic powder dispersed in the resin binder to integrally form a magnet body portion 1 and a shaft portion 2 projecting from each end or one end of the magnet body portion 1 as shown in FIG. 1, the magnetic powder is ferrite powder whose strontium/barium element ratio (Sr/Ba ratio) ranges from 1 to 100.

Examples of the resin binder include polyamide resin such as nylon 6 and nylon 12, polystyrene resin, polyethylene terephthalate resin (PET), polybutylene terephthalate resin (PBT), polyphenylene sulfide resin (PPS), ethylene-vinyl acetate copolymer resin (EVA), ethylene-ethyl acrylate resin (EEA), epoxy resin, ethylene-vinyl alcohol copolymer resin (EVOH), polypropylene resin, polyolefin such as polyethylene and.polyethylene copolymer, modified polyolefin obtained by introducing a reactive functional group such as a maleic anhydride group, carboxyl group, hydroxyl group, and glycidyl group, into the structure of polyolefin, and mixtures thereof. Although not especially limited, polyamide resin of these resins is preferably used, and nylon 6 having a number-average molecular weight of 8000 to 10500 is more preferably used as the resin binder.

As the magnetic powder dispersed in the resin binder, anisotropic or isotropic ferrite powder is used. In the first magnet roller of the present invention, the ferrite powder is one whose strontium/barium element ratio (Sr/Ba ratio) ranges from 1 to 100.

The determination of the Sr/Ba ratio may be made by any usual elementary analysis methods. Specific examples of the elementary analysis methods include electron probe is microanalysis (EPMA), wavelength dispersive X-ray spectrometry (WDX), energy dispersive X-ray spectrometry (EDX), fluorescent X-ray analysis, and inductively coupled plasma atomic emission spectroscopy (ICP-AES). For example, in making the determination by EDX, the analysis may be performed by using commercially available analyzing equipment such as S-2700 manufactured by Hitachi, Ltd. under suitable conditions such as an acceleration voltage of 25 keV, a magnification of 500 times, and a beam current of 0.1 nA.

Then, the Sr/Ba ratio can be calculated from peak values of strontium (Sr) and barium (Ba) in an EDX scattered X-ray spectrum obtained by the above analysis. The range of the Sr/Ba ratio is more preferably from 1 to 10.

Although not especially limited, the ferrite powder used as the magnetic powder has an average particle size of 1.2 to 1.55 pm, preferably, 1.3 to 1.5p m, and has a BET specific surface area of 1 to 2.5 m/g, preferably, 1.2 to 1.7 e1g.

The average particle size specified above is one measured by a constant-pressure air transmission method, and the BET specific surface area specified above is one measured by an N2 gas adsorption method with a specific surface area - 15 measuring device.

The proportional amount of the ferrite powder as the magnetic powder in the bonded magnet composition is suitably selected according to the strength of a magnetic force required, and not especially limited. Usually, the proportional amount of the ferrite powder is preferably set to about 50 to 95 wt.% of the bonded magnet composition (the density of the ferrite powder in the bonded magnet composition is about 2.5 to 3.5 g/cm').

In addition to the resin binder and the magnetic powder, the bonded magnet composition may further contain a filler having a large reinforcing effect such as mica, whisker, talc, carbon fiber, and glass fiber as required. That is, in the case that a magnetic force required by the molded product is is relatively low and the content of the magnetic powder is therefore low, the rigidity of the molded product tends to become low. In such a case, the rigidity of the molded product can be compensated for by adding a filler such as mica and whisker, thereby reinforcing the molded product. In the present invention, mica or whisker is preferably used as the filler. Examples of the whisker include a nonoxide whisker such as silicon carbide and silicon nitride, a metal oxide whisker such as ZnO, MgO, TiO2, Sn02, and A1203, and a double oxide whisker such as potassium titanate, aluminum borate, and basic magnesium phosphate. of these whiskers, the double oxide whisker is preferably used from the viewpoint of easy compositing with plastics.

The proportional amount of the filler is not especially limited, but it is usually set to about 2 to 32 wt.%, preferably, about 5 to 20 wtA. Further, the bonded magnet composition of the present invention may further containany additives other than the above-mentioned filler within the scope of the present invention.

The first magnet roller is obtained by molding the bonded magnet composition mentioned above to integrally form the magnet body portion 1 and the shaft portion 2 projecting from at least one end of the magnet body portion 1. A molding method for the first magnet roller is not especially limited. For example, an injection molding method or an extrusion molding method well known in the art may be suitably adopted, but it is preferable to adopt a first manufacturing method, a second manufacturing method according to the present invention to be hereinafter described, or the combination thereof.

In forming the magnet roller from the bonded magnet composition as shown in FIG. 1, the magnet roller is magnetized to have desired magnetic characteristics by any suitable method. For example, the magnet roller is magnetized by locating a magnetic field around the cavity of is a mold to orient the magnetic powder upon molding, once demagnetizing and next magnetizing the magnetic powder. As another method, the magnet roller is magnetized by the above magnetic field at the same time the molding is carried out.

The second magnet roller of the present invention is characterized in that in a magnet roller obtained by molding a bonded magnet composition composed of a resin binder and magnetic powder dispersed in the resin binder to integrally form a magnet body portion 1 and a shaft portion 2 projecting from each end or one end of the magnet body portion 1 as shown in FIG. 1, the magnet roller has a plurality of magnetic poles, and the difference in magnetic force between a high magnetic pole having a maximum magnetic force and a low magnetic pole having a minimum magnetic force is 100 G or more. More preferably, the difference in magnetic force between the high magnetic pole and the low magnetic pole is set to 200 to 400 G or more.

The number of the magnetic poles is not especially limited provided that it is two or more, but it is usually set to about 3 to about 9. In this case, any adjacent ones of the magnetic poles may have the same polarity.

The high magnetic pole having the maximum magnetic force is not especially limited, but it is preferably used as a developing pole (a pole for jumping a developer to an opposing member such as an image forming member). In the case of intending to further improve the magnetic force of the developing pole, the magnetic force of another magnetic pole having the same polarity as that of the developing pole may be reduced to thereby relatively improve the magnetic force of the developing pole. As a result, a higher-quality image can be obtained.

The second magnet roller is obtained by molding the bonded magnet composition containing the magnetic powder dispersed in the resin binder. Examples of the resin binder, the magnetic powder, the filler, and other additives is constituting the bonded magnet composition for the second magnet roller may be similar to those for the first magnet roller. As the magnetic powder, ferrite powder having characteristics similar to those of the ferrite powder used for the first magnet roller is preferably used. However, the material for the magnetic powder is not limited to such specific ferrite powder, but usual ferrite powder or any magnetic powders other than ferrite powder may be used.

Examples of the above-mentioned any magnetic powders other than ferrite powder include strontium-free ferrite powders such as barium ferrite powder, and rare-earth alloy powders such as Sm-Co alloy, Nd-F-B alloy, and Ce-Co alloy powders.

As mentioned above, each of the first and second magnet rollers is obtained by molding the bonded magnet composition to integrally form the magnet body portion 1 and the shaft portion 2 projecting from at least one end of the magnet body portion 1. The shaft portion 2 may be provided with a drive gear for rotating the magnet roller as required. In particular, the drive gear may be formed integrally with the shaft portion 2 from the bonded magnet composition. Further, as shown in FIGS. 2A and 2B, a front end portion of the shaft portion 2 may be formed with a semicylindrical engaging portion 21 to be fixed to a fixing portion of an opposing member (not shown). The shape of the engaging portion 21 may be modified. Further, as shown in FIGS. 2C and 2D, a frustoconical large-diameter portion 22a may be formed between the shaft portion 2 having the engaging portion 21 and one end of the magnet body portion 1. Alternatively, as shown in FIGS. 2E and 2F, a circular cylindrical large-diameter portion 22b may be formed between the shaft portion 2 having the engaging portion 21 and one end of the magnet body portion 1.

The second magnet roller may also be manufactured by an injection molding method or an extrusion molding method known is in the art. However, it is preferable to adopt a first manufacturing method, a second manufacturing method according to the present invention to be hereinafter described, or the combination thereof.

Each of the first and second manufacturing methods includes the step of molding a bonded magnet composition composed of a resin binder and magnetic powder dispersed in the resin binder by using a mold to integrally form a magnet body portion and a shaft portion projecting from each end or one end of the magnet body portion, thereby obtaining a magnet roller.

The first manufacturing method will first be described in detail. As shown in FIG. 3A, a two-piece mold 3 consisting of an upper mold 31 and a lower mold 32 joined separatably from each other is used to inject the bonded magnet composition into a cavity 33 of the mold 3, thereby integrally forming a magnet body portion 1 and a shaft portion 2 projecting from each end or one end of the magnet body portion 1 (the shaft portion 2 projecting from each end is shown by way of example). Thereafter, as shown in FIG.

3B, the upper mold 31 and the lower mold 32 are separated 19 - from each other, and ejection pins 4 provided in the upper mold 31 are forced to push the magnet body portion 1 of the magnet roller, thereby removing the magnet roller from the mold 3. According to the first manufacturing method, the magnet roller can be easily and reliably obtained without warpage and damage, with high dimensional accuracy, and without an increase in man-hour.

It is sufficient that the mold 3 has at least one ejection pin 4, and the other configurations of the mold 3 may be similar to those of any usual mold. For example, a mold separable into three or more mold members may be used. Further, it is sufficient that the ejection pin is to be provided so as to press a part.of the magnet body portion 1 of the magnet roller obtained. In this case, it is preferable to press the magnet roller uniformly along the longitudinal direction thereof. In a preferred embodiment as shown in FIG. 3A, three ejection pins 4 for pressing opposite end portions and an intermediate portion of the magnet body portion 1 along the longitudinal direction thereof are provided in the upper mold 31 of the mold 3. In the case that the magnet roller is sho-r-t in length, two ejection pins may be provided so as to press opposite end portions of the magnet body portion 1, or L single ejection pin may be provided so as to press a longitudinally central portion of the magnet body portion 1. Conversely in the case that the magnet roller is long, four or more ejection pins are preferably provided along the longitudinal direction of the magnet roller. Further, in the case that the magnet roller is large in diameter, a plurality of ejection pins are preferably provided along the circumferential direction of the magnet body portion 1, so as to uniformly press the outer circumferential surface of the magnet body portion 1.

The ejection pin 4 is not especially limited, but the front end portion of the ejection pin 4 is preferably formed as a concave surface having the same curvature as that of the outer circumferential surface of the magnet body portion 1 of the magnet roller to be formed. Accordingly, the outer circumferential surface of the magnet body portion 1 can be formed into a smooth curved surface as designed. The diameter of the ejection pin 4 is not especially limited, but it is usually set in the range of about 2 to 4 mm.

The second manufacturing method will now be described in detail. In injecting a bonded magnet composition composed of a resin binder and magnetic powder dispersed in the resin binder into the cavity of a mold to integrally form a magnet body portion and a shaft portion projecting from each end or one end of the magnet body portion from the bonded magnet composition, thereby obtaining a magnet roller, a gate (not shown) is provided in the mold 3 at a portion thereof is corresponding to an outer circumferential surface 5a or an end surface 5b of the magnet body portion 1 or an outer circumferential surface 5c of the shaft portion 2 as shown in FIG. 3A, and the bonded magnet composition is injected through the gate into the cavity 33 of the mold 3. According to the second manufacturing method, the flow of the bonded magnet composition in the cavity 33 can be made uniform, so that a reduction in dimensional accuracy due to poor charging of the bonded magnet composition into the cavity 33 or the occurrence of defectiveness due to incomplete charging of the bonded magnet composition into the cavity 33 can be reliably prevented to obtain a high-performance magnet roller.

Conventionally, in integrally forming a magnet body portion 1 and a shaft portion 2 from a bonded magnet composition by injection molding, the bonded magnet composition is injected into the cavity 33 from a portion of the mold 3 corresponding to an end surface 5d of the shaft portion 2 shown in FIG. 3A. In such a conventional method, there is a possibility that the cavity 33 cannot be fully filled with the bonded magnet composition, so that the shape of the cavity 33 cannot be accurately transferred to the magnet body portion 1 and the shaft portion 2 to be formed, thus reducing the dimensional accuracy of the molded product. In another case, there is a possibility that pores may remain in the magnet roller formed. To the contrary, by charging the bonded magnet composition from a portion of the mold 3 corresponding to the outer circumferential surface 5a or the end surface 5b of the magnet body portion 1 or the outer circumferential surface 5c of the shaft portion 2 according to the second manufacturing method of the present invention, the flow of the bonded magnet composition in the cavity 33 can be made uniform to thereby eliminate the above conventional problems, thus reliably obtaining a magnet roller with good dimensional accuracy.

The gate may be located at any position corresponding to is the outer circumferential surface 5a or the end surface 5b of the magnet body portion 1 or the outer circumferential surface 5c of the shaft portion 2. Further, the gate may be singular or plural, which is suitably selected according to the size of the magnet roller, for example. Patticularly except in the case of a large-sized magnet roller, the single gate is preferably provided from the viewpoint of prevention of generation of welds, flow marks, etc.

In embodying the first and second manufacturing methods, the molding conditions except the above-mentioned conditions may be made similar to usual conditions. In magnetizing the magnet roller to be formed, the magnetization may be carried out by forming a magnetic field around the cavity 33 of the mold 3 to magnetize the magnet roller according to a desired magnetic force pattern simultaneously with molding, or to orient the magnetic powder in the bonded magnet composition into a desired condition.

The bonded magnet composition used in each of the first and second manufacturing methods is composed of the resin binder and the magnetic powder dispersed therein, and may also contain a filler or other additives as required. In this case, examples of the resin binder, the magnetic powder, the filler, and the other additives may be the same as those mentioned in the description of the first and second magnet rollers. However, any other examples may also be used.

The first and second magnet rollers and the magnet rollers obtained by the first and second manufacturing methods are suitably used as a magnet roller used in a developing mechanism for an electrophotographic device or an electrostatic recording device such as a copier and a printer. In particular, as shown in FIGS. 4 and 5, the magnet roller according to the present invention is suitably used as a magnet roller 7 located inside of a developing sleeve 6 rotating as carrying a toner 10 on its outer surface. By using the magnet roller according to the present invention, a good image can be reliably obtained by the good magnetic characteristics of the magnet roller.

Accordingly, a developing roller consisting of the rotating sleeve and the magnet roller located inside of the sleeve according to the present invention is provided as the first developing roller by the present invention. In the first developing roller, a developer is carried on the outer circumferential surface of the sleeve by the magnetic characteristics of the magnet roller, and the sleeve is rotated to form a thin film of the developer. The thin film formed on the sleeve is brought close to an image forming member, and the developer is then jumped to be supplied to the surface of the image forming member by the magnetic characteristics of the magnet roller, thereby forming a visible image on the surface of the image forming member. As shown in FIG. 5 by way of example, the developing roller consisting of the sleeve 6 and the magnet roller 7 according to the present invention is provided between a toner applying roller 8 for supplying the toner 10 and a photosensitive drum 9 (image forming member) retaining an electrostatic latent image thereon in such a manner that the outer circumferential surface of the sleeve 6 is close to the surface of the photosensitive drum 9. The sleeve 6, the photosensitive drum 9, and the toner applying roller 8 are rotated in the directions of the respective arrows shown in FIG. 5 to thereby supply the toner 10 from the toner applying roller 8 to the surface of the sleeve 6 and feed the toner 10 carried on the surface of the sleeve 6 by the magnetic characteristics of the magnet roller 7. The toner 10 carried on the surface of the sleeve 6 is formed into a uniform thin layer by a layer forming blade 11. Then, the thin layer of the toner 10 is fed to a position close to the surface of the photosensitive drum 9 by the rotation of the sleeve 6. At this time, the toner 10 in the form of thin layer is jumped from the surface of the sleeve 6 to the surface of the is photosensitive drum 9 by the magnetic characteristics of the magnet roller 7 and deposited to the electrostatic latent image formed on the surface of the photosensitive drum 9, thereby making the electrostatic latent image visible. In FIG. 5, reference numeral 12 denotes a transfer unit for transferring a toner image onto a recording medium such as a sheet of paper. Reference numeral 13 denotes a cleaning unit having a cleaning blade 14 for removing a residual toner present on the surface of the photosensitive drum 9 after transferring.

The second developing roller according to the present invention is characterized in that in a developing roller like that shown in FIGS. 4 and 5, which comprises a cylindrical sleeve 6 rotatably provided and a magnet roller 7 provided inside of the sleeve 6, wherein a developer (toner) 10 is carried on the outer circumferential surface of the sleeve 6 by the magnetic characteristics of the magnet roller 7 to form a thin layer of the toner 10 on the outer circumferential surface of the sleeve 6, and the thin layer is then brought close to an image forming member (photosensitive drum) 9 and jumped to be supplied to the surface of the image forming member 9 by the magnetic characteristics of the magnet roller 7, thereby forming a visible image on the surface of the image forming member 9; the outer outer diameter D2 of the magnet roller is in the range of (X - 4) to (X - 0.2) mm with the proviso that the inner diameter Dl of the sleeve 6 is X mm.

In this developing roller, the positional relation based on dimensions between the sleeve 6 and the magnet roller 7 located inside of the sleeve 6 is optimized to thereby effectively exhibit the magnetic force of the magnet roller 7 as the developing roller, thus obtaining a high-quality image.

The sleeve 6 constituting the developing roller is not especially limited, but any usual sleeve formed of resins or metal materials such as aluminum alloys, stainless steel, and copper alloys. In particular, however, it is preferable to use a sleeve whose surface is provided with a resin component layer having a specific elongation to be hereinafter described. Further, the magnet roller 7 constituting the developing roller is also not especially limited, but any usual magnet roller formed of a bonded magnet composition or a sintered magnet may be used. In particular, however, it is preferable to use any.one of the first and second magnet rollers of the present invention and the magnet rollers manufactured by the first and second manufacturing methods of the present invention.

The third developing roller according to the present invention is characterized in that in a developing roller like that shown in FIGS. 4 and 5, which comprises a cylindrical sleeve 6 rotatably provided and a magnet roller 7 provided inside of the sleeve 6, wherein a developer (toner) 10 is carried on the outer circumferential surface of the sleeve 6 by the magnetic characteristics of the magnet roller 7 to form a thin layer of the toner 10 on the outer circumferential surface of the sleeve 6, and the thin layer is then brought close to an image forming member (photosensitive drum) 9 and jumped to be supplied to the surface of the image forming member 9 by the magnetic characteristics of the magnet roller 7, thereby forming a visible image on the surface of the image forming member 9; a surface portion of the developing roller, specifically, the outer circumferential surface of the sleeve 6, has a layer of a resin component whose elongation at break defined by JIS K7113 is 10% or less.

In this developing roller, the layer of the resin component having the above-specified elongation is formed on the surface of the developing roller, e.g., the surface of the sleeve 6, thereby improving the wear resistance of the surface of the developing roller, e. g., the surface of the is sleeve 6, without a reduction in dimensional accuracy and reliably obtaining a high-quality image with no image degradation such as ghost over a long term.

While there is no clear knowledge on a mechanism for preventing such a reduction in image density due to long-term use by forming the resin component layer on the surface of the developing roller, e.g., the sleeve 6, this mechanism may be inferred in the following manner. Owing to the presence of the resin component layer having a very small elongation at break of 10% or less on the surface of the developing roller, e.g., the sleeve 6, wearing of the surface of the developing roller upon development can be effectively prevented. Further, a change in characteristics of the resin component is greatly small even after long-term use, and a good surface condition can be maintained. As a result, it is inferred that the above-mentioned effect can be obtained. Further, by suitably selecting the resin component according to the applications of the developing roller or the kinds of the developer, the charging performance of the developer can be properly controlled to thereby reliably obtain a good image.

The material for the sleeve 6 used in the third developing roller is not especially limited, but any usual materials including resins and metal materials such as aluminum alloys, stainless steel, and copper alloys may be used. Usually, a sleeve formed of aluminum alloys is preferably used from the viewpoints of material cost and workability.

The sleeve 6 of the third developing roller consists of a cylindrical base member formed of a metal material or the like and the resin component layer formed on the surface of the base member. Although not especially limited, microscopic asperities may be formed on the surface of the base member of the sleeve 6 by blasting or the like prior to formation of the resin component layer, so as to improve an image quality. Examples of the blasting include a pressure type or suction type air blasting method, vacuum blasting method, water blasting method, and centrifugal blasting method. An abrasive used in the blasting may be selected from cast iron grit, steel grit, copper slag, nickel slag, fused alumina, and silicon carbide, for example. In particular, for the purpose of improving adhesion of the resin component layer to the surface of the base member of the sleeve 6, glass beads, plastic beads, sand, walnut shell, etc. may also be used to polish or clean the surface of the base member of the sleeve 6. Blasting conditions including pressure, distance, and angle are not especially limited, and they may be suitably set according to the shape of the microscopic asperities to be formed, the degree of surface polishing, etc.

Further, to further improve the adhesion of the resin component layer to the surface of the base member of the sleeve 6, the surface of the base member of the sleeve 6 may be subjected to suitable pretreatment in addition to the blasting prior to the formation of the resin component layer.

Examples of the pretreatment include solvent cleaning, acid and alkali cleaning, water cleaning, flame treatment, corona discharge treatment, and plasma treatment. In particular, water cleaning and plasma treatment are preferable from the viewpoints of high effectiveness of treatment and no need for liquid waste disposal.

In the third developing roller of the present invention, the resin component.layer whose elongation at break defined by JIS K7113 is 10% or less is formed on the surface of the developing roller, specifically, on the surface of the sleeve 6 of the developing roller shown in FIG. 4. Alternatively, a developing roller of the type such that no sleeve is employed and a developer is carried on the magnet roller and supplied to the image forming member may be used as the third developing roller of the present invention. In this case, is the resin component layer is formed on the surface of the magnet roller.

The resin component is not especially limited, but various resins having the above-mentioned elongation may be adopted. Examples of such resins include urea resin, melamine resin, alkyd resin, modified alkyd resin such as phenol-modified alkyd resin and silicone-modified alkyd resin, oil-free alkyd resin, acrylic resin, silicone resin, fluororesin, phenolic resin, polyamide resin, epoxy resin, polyester resin, maleic acid resin, and urethane resin. Of these resinst urea resin, melamine resin, silicone resin, phenolic resin, modified alkyd resin, oil-free alkyd resin, acrylic resin, moisture curing urethane resin, and mixtures thereof are preferably used from the viewpoints of film formation and adhesion. In particular, phenolic resin and the mixture of modified alkyd resin or oil-free alkyd resin and melamine resin are more preferably used because of their low elongation and high mechanical strength. Further, melamine resin is also preferably used because it has an electron donating property due to an unpaired electron present at a nitrogen moiety in the resin, and phenolic resin is preferably used because it has an electronic polarizing property due to high permittivity.

AS mentioned above, the elongation at break of the resin component as defined by JIS K7113 is 10% or less, preferably, 5% or less. If the elongation at break is higher than 10%, deformation of the surface of the developing roller becomes large, and control of the toner becomes difficult, causing a reduction in image density.

The resin component may or may not contain a conductive material, which may be suitably determined according to adjustment of the resistance of the surface portion of the developing roller, e.g., the sleeve 6. In the case of adding the conductive material, conductive powder is preferably used. In particular, carbon black is more preferably used is although not especially limited.

As means for forming the resin component layer on the surface of the developing roller such as the sleeve 6, a method of surface treating the developing roller with a resin solution containing the resin component may be preferably adopted. The surface treatment may be carried out by a spraying method, roll coater method, dipping method, etc.

after preparing the resin solution. As the surface treatment by the dipping method, for example, a method of dipping the developing roller in the resin solution at room temperature for 5 seconds to 5 minutes, preferably, 10 seconds to 1 minute, next lifting the roller from the resin solution, and finally drying the roller may be usually adopted. In the case of adopting the spraying method, the concentration of the resin in the resin solution can be set higher than that in the dipping method. In this case, the resin concentration may be set to 10 to 60%, for example. A solvent for preparation of the resin solution is not especially limited as far as it can dissolve the resin component. Usually, lower alcohols such as methanol, ethanol, and isopropanol, ketones such as acetone, methyl ethyl ketone, and cyclohexanone, toluene, and xylene, for example, are preferably used.

By the surface treatment with the resin component as mentioned above, the friction of the surface of the developing roller such as the sleeve 6 can be reduced to some extent. TO further reduce the friction, various additives may be used. Preferable examples of additives capable of reducing the friction without reducing the uniformity of the surface treatment include silicone resin, silicone resin powder, fluorocarbon surface active agent, silicone surface active agent, silicon coupling agent, and silica powder.

Examples of the silicone resin having solvent solubility include methyl silicone polymer, methyl phenyl silicone polymer, their modified polymers, and silicone-epoxy block is copolymer.

Examples of the silicone resin powder include fine powders of methyl silicone polymer, methyl phenyl silicone polymer, and amino-modified silicone polymer. These fine powders having an average particle size of 0. 1 to 100pm with a spherical or unfixed particle shape are preferably used.

Examples of the fluorocarbon surface active agent include an ionic type such that alkyl fluorides are combined with carboxylic acids, carbonates, sulphonates, etc., a nonionic type such that alkyl fluorides are combined with alcohols, ethers, etc., and a polymeric type such as polymers and copolymers each containing alkyl fluorides in the side chain or main chain.

Examples of the silicone surface active agent include combined substances between methyl silicone such as siloxane oxyethylene and a hydrophilic or lipophilic segment, and a polymeric type such as copolymers of methyl silicone and an acrylic segment.

Examples of the silicon coupling agent include not only usual silane coupling agents but also silanes having an amino group, isocyanate group, vinyl group, etc. at the terminal.

These kinds of additives may be used individually or in combination. Further, fluororesin also effectively acts as a friction reducing agent. The proportional amount of the friction reducing agent is 1 to 100 parts by weight, preferably, 10 to 75 parts by weight with respect to 100 parts by weight of the resin component. In the case of using an ionic conductive substance as the conductive material, the proportional amount of the ionic conductive substance to be added to the resin component is set to 0.001 to 1 part by weight with respect to 100 parts by weight of the resin component.

The base member constituting the surface portion of the developing rolleraccording to the present invention, specifically, the sleeve 6 in the developing roller shown in is FIG. 4 by way of example is not especially limited, but the surface roughness of the base member of the sleeve 6, i.e., the surface roughness of the sleeve 6 prior to formation of the resin component layer thereon and the thickness of the resin component layer are preferably adjusted so that the relation of 0.01 < b/a < 2 is satisfied where a stands for the surface roughness (JIS ten-point average surface roughness Rz (p m)) of the base member of the sleeve 6 and b stands for the thickness ( gm) of the resin component layer formed on the surface of the base member of the sleeve 6.

With this adjustment, a proper surface roughness can be maintained to obtain satisfactory feedability of the developer, and the chargeability and durability of the sleeve 6 can be improved. If b/a:-tt 2, the surface roughness of the sleeve 6 becomes low, causing a lack of the amount of the toner to be fed, whereas if b/a _:5 0.01, the durability is reduced to cause a possible problem that the effect of the present invention cannot be sufficiently obtained.

The surface roughness a of the base member of the sleeve 6 and the thickness b of the resin component layer are not especially limited as far as the above-specified relation is satisfied. Usually, the surface roughness a of the base member, i.e., the JIS ten-point average surface roughness Rz of the base member is set to preferably 30 pm or less, more preferably about 0.1 to 30 pm, most preferably about 1 to 15 pm. On the other hand, the thickness b of the resin component layer is only required to have the above-specified relation to such a preferable surface roughness Rz (a). More specifically, the thickness b is set to preferably about 50 to 0.5g m, more preferably about 15 to 1pm. Although not especially limited, the surface roughness (JIS ten-point average surface roughness Rz) of the sleeve 6 after forming the resin component layer is set to preferably 30pm or less, more preferably 10 to lp m.

The resin component layer formed on the surface of the is base member of the sleeve 6 by the surface treatment mentioned above may take various forms including a film covering the whole surface of the base member, a dispersive form such that the resin component is dispersed into the pits of the microscopic asperities formed on the surface of the base member, and a partial covering form such that a large part of the surface of the base member is covered with the resin component in such a manner that some of the projections of the microscopic asperities on the surface of the base member are exposed. The thickness b of the resin component layer means the average thickness of the resin component layer present in any of the above-mentioned various forms on the surface of the base member of the sleeve 6, and it can be measured by the following method. For example, the average thickness of the resin component layer can be obtained by first cutting the surface of the sleeve 6 having the resin component layer into a plurality of parts each having a size of 1 cm x 1 cm, next observing the cross section of each part to measure the thickness of each part, and finally dividing the sum of the thicknesses of all the parts by the number of the parts.Means for observing the cross section of each part of the resin component layer is not especially limited, but any known means such as an electron microscope and an optical microscope may be used.

The magnet roller 7 constituting the third developing roller is not especially limited, but any magnet roller formed from a usual bonded magnet composition or a sintered magnet may be used. However, it is preferable to use any of the first and second magnet rollers of the present invention and the magnet rollers manufactured by the first and second manufacturing methods of the present invention. Further, the dimensional and positional relations between the magnet roller 7 and the sleeve 6 are also not especially limited, but the magnet roller 7 and the sleeve 6 may be located in usual conditions. However, it is preferable to locate these members so that the above-mentioned dimensional relation between the magnet roller 7 and the sleeve 6 in the second developing roller is satisfied.

Each of the first to third developing rollers according to the present invention may be incorporated into a usual, developing device using a developer including a toner and a carrIer. In particular, each developing roller may be preferably used in a developing device such that a developer is supplied to a latent image retaining member such as a photosensitive drum on which an electrostatic latent image is retained, thereby making the electrostatic latent image visible. For example, each developing roller may be used in the developing device shown in FIG. 5.

As shown in FIG. 5 by way of example, each of the first to third developing rollers of the present invention is preferably used in making visible the electrostatic latent image retained on the surface of the photosensitive drum 9 as the latent image retaining member by means of the developer. The latent image retaining member, or the image forming member for forming a visible image by means of the developer supplied by the developing roller is not limited to the photosensitive drum, but other latent image retaining members having any shapes other than the drum shape, such as a belt shape may be adopted. Further, a recording medium such as a sheet of plane paper, OHP, and photographic paper may be adopted as the latent image retaining member, whereby a developer is supplied directly to the recording medium to form a visible image on the recording medium. For example, as in a mechanism disclosed in Japanese Patent Laid-open No.

8-129293, a back electrode roller is provided on the back side of a recording medium, and a developing roller carrying a developer is provided on the front side of the recording medium in the vicinity thereof. The developer on the developing roller is controlled by an aperture electrode and jumped toward the back electrode roller, thereby supplying the developer to the recording medium present between the back electrode roller and the developing roller to form a visible image on the recording medium.

EXAMPLE

The present invention will now be more particularly described by means of the following examples in conjunction with comparisons. The examples are not construed to limit the scope of the present invention.

(Example 1)

A bonded magnet composition having the composition specified below was injected into the cavity 33 of the mold 3 shown in FIG. 3A from a position corresponding to the outer circumferential surface 5a of the magnet body portion 1 to form a magnet roller consisting of the magnet body portion 1 and the shaft portions 2 as similar to that shown in FIG. 1.

At this time, a magnetic field was formed around the cavity

33 of the mold 3 to magnetize the magnet roller upon molding, so as to form a magnetic force pattern having two N poles and two S poles. After forming the magnet roller, the mold 3 was opened and the three ejection pins 4 were then forced to push the outer circumferential surface of the magnet body portion 1 as shown in FIG. 3B, thereby removing the magnet roller from the mold 3. In the magnet roller thus obtained, the length and the diameter of the magnet body portion 1 were 310 mm and 16 mm, respectively, and the diameter of each shaft portion 2 was 6 mm. The front end surface of each ejection pin 4 was previously worked to become a concave surface having the same curvature as that of the outer circumferential surface of the magnet body portion 1.

Bonded Magnet Composition (in the form of pellets) Resin Binder:

nylon 6 (number-average molecular weight = 10000) 10 wt.% Magnetic Powder:

Sr ferrite (Sr/Ba ratio = 3.4; average particle size 1. 43 p m; BET specif ic surf ace area = 1. 7 e1g) 90 wt. % injection Molding Conditions Cylinder Temperature: 290 - 3000C Mold Temperature: 110 - 120C Injection Pressure: 700 kg /CM2 The magnet roller obtained was evaluated on the following items.

Surface Magnetic Force A probe was located in the vicinity of the outer circumferential surface of the magnet roller to measure a surface magnetic force with a gaussmeter as rotating the magnet roller. As the result, the magnetic force of the magnetic pole having a maximum magnetic force was 900 G, and the magnetic force of the magnetic pole having a minimum magnetic force was 600 G. That is, the difference in magnetic force between the magnetic force having a maximum magnetic force and the magnetic pole having a minimum magnetic force was 300 G.

Turbulence of Magnetic Force near Ejection Pin Marks and Warpage of Magnet Roller No turbulence of a magnetic force near the marks of the ejection pins was observed. Further, no warpage of the magnet roller was also.observed and good dimensional accuracy was maintained.

Appearance The appearance of the magnet roller was good, and no turbulence of a magnetic force in the longitudinal direction of the magnet roller was observed.

is (Comparison 1) The procedure of Example 1 was followed except that a bonded magnet composition having the following composition was used to obtain a magnet roller.

Bonded Magnet Composition (in the form of pellets) Resin Binder:

nylon 6 (number-average molecular weight = 10000) 10 wtA Magnetic Powder:

Ba ferrite (Sr/Ba ratio = 0.5; average particle size 1.44 p m; BET specific surface area = 1.6 m/g) 90 wt.% The magnet roller obtained was evaluated on the following item.

Surface Magnetic Force A surface magnetic force was measured as similarly to Example 1. As the result, the magnetic force of the magnetic pole having a maximum magnetic force was 500 G, and the magnetic force of the magnetic pole having a minimum magnetic force was 250 G. That is, the difference in magnetic force between the magnetic pole having a maximum magnetic force and the magnetic pole having a minimum magnetic force was 250 G.

(Example 2 and Comparison 2) A pipe having an inner diameter of 18 mm and a pipe having an inner diameter of 27 mm were prepared by using an aluminum alloy (6063]. The outer circumferential surface of each pipe was treated with Alundum #46 by a pressure air blasting process to adjust the surface roughness defined as JIS ten-point average surface roughness Rz to 7.0 pm. Thus, two kinds of sleeves were prepared from these two pipes.

Both sleeves thus obtained were rotatably located and the magnet roller obtained in Example 1 was located inside of each sleeve, thus preparing two kinds of developing rollers.

A surface magnetic force was measured on each developing roller by the following method.

Surface Magnetic Force A probe was located in the vicinity of the outer circumferential surface of the sleeve of each developing roller to measure a surface magnetic force with a gaussmeter as rotating each sleeve and obtain a maximum magnetic force.

The result of measurement is shown in Table 1.

Table 1

Example 2 Comparison 2 Inner Diameter of Sleeve (mm) 18 27 Surface Magnetic Force (G) 900 400 (Example 3)

The procedure of Example 1 was followed except that a bonded magnet composition having the following composition - 37 was used and the following conditions were adopted to obtain a magnet roller.

Bonded Magnet Composition (in the form of pellets) Resin Binder:

ethylene-ethyl acrylate (EEA) 10 wt.% Magnetic Powder:

Sr ferrite 90 wt.% Injection Molding Conditions Cylinder Temperature: 230 - 250C Mold Temperature: 50 - 70C Injection Pressure: 700 kg/cm2 is A pipe having an inner diameter of 18 mm was prepared by using an aluminum alloy [6063], and the outer circumferential surface of the pipe was treated with Alundum #46 by a pressure air blasting process to adjust the surface roughness defined as JIS ten-point average surface roughness Rz to 7.Og m. Thus, a sleeve was prepared from this pipe.

Then, a mixture of oil-free alkyd resin and melamine resin (Beckolite M-6402 and Superbeckamine L110, respectively, both manufactured by DAINIPPON INK AND CHEMICALS, INCORPORATED) in a mixing ratio of 4:1 was dissolved in toluene at a concentration of 10 wt.% to prepare a resin solution. The sleeve obtained above was dipped into this resin solution, next lifted from the resin solution, and finally dried with heat. Thus, the sleeve was surface-treated. A cross section of the sleeve thus surface-treated was observed. As the result, the thickness of a layer of the above mixed resin formed on the surface of the sleeve by the.surface treatment was 5p m (average thickness; the same applies to the following).

By using the above mixture of oil-free alkyd resin and melamine resin, a first type test piece defined by JIS K7113 was prepared. Then, the test piece was submitted to a tensile test defined by JIS K7113. AS the result, the elongation at break of this test piece was less than 3%.

The above magnet roller was located inside of the above sleeve to prepare a developing roller similar in configuration to the roller shown in FIG. 4.

(Example 4)

The procedure of Example 3 was followed except that the surface treatment of the sleeve was carried out by using a 10 wt.% resin solution of a resol type phenolic resin PR50232 (manufactured by Sumitomo Dules Inc.) in methyl ethyl ketone.

A cross section of the sleeve thus surface-treated was observed. As the result, the thickness of a layer of the is phenolic resin formed on the surface of the sleeve by the surface treatment was 2pm.

On the other hand, a first type test piece defined by JIS K7113 was prepared by using the above phenolic resin, and submitted to a tensile test defined by JIS K7113. As the result, the elongation at break of this test piece was less than 3%.

(Example 5)

The procedure of Example 4 was followed except that the surface treatment of the sleeve was carried out by adding 20 phr of carbon black Printix 35 (manufactured by DEGUSSA) as a conductive material to the above phenolic resin. A cross section of the sleeve thus surface-treated was observed.. As the result, the thickness of a layer of the phenolic resin containing the carbon black formed on the surface of the sleeve by the surface treatment was 3p m.

on the other hand, a first type test piece defined by J1S K7113 was prepared by using the phenolic resin containing the carbon black and submitted to a tensile test defined by ilS K7113.As the result, the elongation at break of this test piece was less than 3%. Further, the resistivity of this resin containing the carbon black was measured to obtain 104 9 cm, which shows that the resin has been made conductive by the addition of the carbon black.

(Example 6)

The procedure of Example 3 was followed except that the surface treatment of the sleeve by dipping was repeated several times to increase the thickness of the resin layer formed on the surface of the sleeve. A cross section of the sleeve thus surface-treated was observed. As the result, the thickness of the resin layer formed on the sleeve surface by the surface treatment was 20 pm.

is (Comparison 3) The procedure of Example 3 was followed except that the surface treatment of the sleeve was carried out by using a 15 wt.% resin solution of soluble copolymerized nylon CM8000 (manufactured by Toray Industries Inc.) in methanol. A cross section of the sleeve thus surface-treated was observed. As the result, the thickness of the nylon layer formed on the sleeve surface by the surface treatment was 5p m.

On the other hand, a first type test piece defined by JIS K7113 was prepared by using the above soluble copolymerized nylon, and submitted to a tensile test defined by JIS K7113. As the result, the elongation at break was 100% or more.

(Comparison 4) The procedure of Example 3 was followed except that the sleeve was not subjected to surface treatment.

The developing rollers obtained in Example 3 to 6 and Comparison 3 and 4 were submitted to the following durability test. By using each of the developing rollers of Examples 3 to 6 and Comparison 3 and 4, a developing device as shown in FIG. 5 was constructed. The developing device was built in a laser beam printer. Then, a test pattern including five pitch patterns at pitches of 0.1 mm to 3 mm was continuously printed by the laser beam printer. At the time the number of prints has reached 5000, 15000, and 40000, each developing roller was taken out of the printer to measure the surface roughness (JIS ten-point average surface roughness Rz) of the sleeve. Further, a change in printed image quality with an increase in the number of prints was also checked. The results are shown in Table 2.

is Table 2

Surface Thickness Surface Roughness of Sleeve Resin Elongation Roughness of Resin ( g m) at break of b/a Layer Base Layer(b) Image Quality Member(a) 11 m) 000 15000 40000 ( pm) 3 Alkyd/ < 3 7 5 0.7 5.0 5.0 4.9 4.8 Good up to Melamine 40000 prints 4 Phenolic < 3 7 2 0.3 4.8 4.8 4.8 4.7 Good up to 40000 prints Conductive < 3 7 3 0.4 5.2 5.2 5.1 5.0 Good up to Phenolic 40000 prints Example

Density was somewhat lower than that in Alkyd/ Examples 3 to 6 Melamine < 3 7 20 2.8 6.0 5.9 5.7 5.5 5, but no degradation of image occurred up to 40000 prints Degradation of 3 Nylon g; 100 7 5 0.7 5.0 15. 0 unmeasurable unmeasurable image started to occur near 10000 prints Comparison Degradation of 4 None 7 7.0 5.0 4.0 3.0 image started to occur near 15000 prints is As apparent from Table 2, according to the developing rollers of Examples 3 to 6 according to the present invention, the surface roughness of the sleeve was not almost changed even after long-term use, and the degradation of the image due to poor developer feedability was not observed even at the time of 40000 prints. In particular, it was confirmed that each developing roller of Examples 3 to 5 optimizing the surface roughness (a) of the base member of the sleeve and the thickness (b) of the resin layer formed on the base member of the sleeve is superior in developer feedability and can reliably provide an image with a satisfactory density. To the contrary, in the developing rollers of Comparisons 3 and 4, the surface roughness was largely changed with an increase in the number of prints. In Comparison 3, the degradation of the image started to occur near 10000 prints, and in Comparison 4, the degradation of the image started to occur near 15000 prints. At this time, the occurrence of ghost was remarkable. While the invention has been described with reference to specific embodiments, the description is illustrative and is not to be construed as limiting the scope of the invention.

- 43

Claims

CLAIMS:

10, is 1. A magnet roller obtained by molding a bonded magnet composition composed of a resin binder and magnetic powder dispersed in said resin binder to integrally form a magnet body portion and a shaft portion projecting from each end or one end of said magnet body portion; characterized in that said magnet roller has a plurality of magnetic poles, and the difference in magnetic force between a high magnetic pole having a maximum magnetic force and a low magnetic pole having a minimum magnetic force is 100 G or more.
2. A magnet roller according to claim 1, wherein said high magnetic pole having said maximum magnetic force is a developing pole.
3. A manufacturing method for a magnet roller comprising the steps of: molding a bonded magnet composition composed of a resin binder and magnetic powder dispersed in said resin binder by using a mold to integrally form a magnet body portion and a shaft portion projecting from each end or one end of said magnet body portion; opening said mold; and forcing an ejection pin to push said magnet body portion, thereby removing said magnet roller composed of said magnet body portion and said shaft portion from said mold.
4. A manufacturing method for a magnet roller according to claim 3, wherein the front end surface of said ejection pin is formed as a curved surface having substantially the same curvature as that of the outer circumferential surface of said magnet body portion.
S. In a manufacturing method for a magnet roller comprising the step of injecting a melted bonded magnet composition composed of a resin binder and magnetic powder dispersed in said resin binder into a cavity of a mold to integrally form a magnet body portion and a shaft portion projecting from each end or one end of said magnet body portion from said bonded magnet composition; the improvement characterized in that a gate is provided in said mold at a position thereof corresponding to.the outer circumferential surface or the end surface of said magnet body portion or the outer circumferential surface of said shaft portion,, and said bonded magnet composition is injected through said gate into said cavity of said mold.
6. A developing roller comprising a cylindrical sleeve rotatably provided and a magnet roller provided inside of said sleeve, wherein a developer is carried on the outer circumferential surface of said sleeve by the magnetic characteristics of said magnet roller to form a thin layer of said developer on the outer circumferential surface of said sleeve, and said thin layer is then brought close to an image forming member in order to jump onto the surface of said image forming member by the magnetic characteristics of said magnet roller, thereby forming a visible image on the surface of said image forming member; characterized in that said magnet roller is a magnet roller according to claim 1 or 2.
7. A developing roller comprising a cylindrical sleeve rotatably provided and a magnet roller provided inside of said sleeve, wherein a developer is carried on the outer circumferential surface of said sleeve by the magnetic characteristics of said magnet roller to form a thin layer of said developer on the outer circumferential surface of said sleeve, and said thin layer is then brought close to an image forming member in order to jump onto the surface of said image forming member by the magnetic characteristics of said magnet roller, thereby forming a visible image on the surface of said image forming member; characterized in that said magnet roller is a magnet roller manufactured by a manufacturing method according to claim 3 or 4.
8. A developing roller comprising a cylindrical sleeve rotatably provided and a magnet roller provided inside of said sleeve, wherein a developer is carried on the outer circumferential surface of said sleeve by the magnetic characteristics of said magnet roller to form a thin layer of said developer on the outer circumferential surface of said sleeve, and said thin layer is then brought close to an image forming member in order to jump onto the surface of said image forming member by the magnetic characteristics of is said magnet roller, thereby forming a visible image on the surface of said image forming member; characterized in that said magnet roller is a magnet roller manufactured by a manufacturing method according to claim 5.
9. A developing roller comprising a cylindrical sleeve rotatably provided and a magnet roller provided inside of said sleeve, wherein a developer is carried on the outer circumferential surface of said sleeve by the magnetic characteristics of said magnet roller to form a thin layer of said developer on the outer circumferential surface of said sleeve, and said thin layer is then brought close to an image forming member in order to jump onto the surface of said image forming member by the magnetic characteristics of said magnet roller, thereby forming a visible image on the surface of said image forming member; the improvement characterized in that the outer diameter of said magnet roller is in the range of (X - 4) to (X - 0.2) MM, Xmm being the inner diameter of said sleeve.
10. A developing roller having a magnet roller, wherein a developer is carried on the outer circumferential surface of said developing roller by the magnetic characteristics of said magnet roller to form a thin layer of said developer on the outer circumferential surface of said developing roller, and said thin layer is then brought close to an image forming member in order to jump onto the surface of said image forming member by the magnetic characteristics of said magnet roller, thereby forming a visible image on the surface of said image forming member; characterized in that a surface portion of said developing roller has a layer of a resin component whose elongation at break defined by JIS K7113 is 10% or less.
A developing roller according to claim 10, wherein the relation of 0.01 < b/a < 2 is satisfied where a stands for the JIS ten-point average surface roughness Rz of the surface of a base member constituting said surface portion of said developing roller, and b stands for the thickness of said layer of said resin component formed on the surface of said base member.
!2. A developing roller according to claim 10, wherein said resin component contains a resin selected from the group consisting of urea resin, melamine resin, alkyd resin, modified alkyd resin, oil-free alkyd resin, acrylic resin, silicone resin, phenolic resin, moisture curing urethane resin, and mixtures thereof.
13. A-developing roller according to claim 10, wherein the developing roller comprises a cylindrical sleeve rotatably provided and a magnet roller provided inside of said sleeve, and said layer of said resin component is formed on the surface of said sleeve.
14. A developing device having a developing roller for carrying a developer on the outer circumferential surface, rotationally feeding said developer to a position close to the surface of a latent image retaining member on which an electrostatic latent image is retained, and depositing said developer to said electrostatic latent image retained on the surface of said latent image retaining member, thereby making said electrostatic latent image visible; characterized in that said developing roller is a developing roller according to any one of claims 6 to 13.

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