Patent ID: 12242210

DETAILED DESCRIPTION

(1. Overall Configuration of Image Forming Apparatus1)

With reference to the appended drawings, the following describes an embodiment of the present disclosure.FIG.1is a sectional side view showing a schematic configuration of an image forming apparatus1according to the embodiment of the present disclosure. InFIG.1, a right side corresponds to a front side of the image forming apparatus1, and a left side corresponds to a rear side thereof.

The image forming apparatus1(herein, a monochrome printer) includes, in addition to a main body housing10having a housing structure substantially in a rectangular parallelepiped shape, a paper feed section20, an image forming section30, and a fixing portion40, which are housed in the main body housing10. The main body housing10includes a front cover11provided on a front surface side thereof and a rear cover12provided on a rear surface side thereof. When the rear cover12is opened, a unit of the image forming section30can be inserted in or taken out of the main body housing10from the rear surface side thereof. Furthermore, on an upper surface of the main body housing10, there is provided a paper discharge portion13to which a sheet after being subjected to image formation is discharged. In the following description, a term “sheet” refers to a copy sheet, a sheet of coated paper, an OHP sheet, a sheet of cardboard, a postcard, a sheet of tracing paper, or any other sheet material to be subjected to image forming processing.

The paper feed section20includes a paper feed cassette21for housing sheets to be subjected to image forming processing. The paper feed cassette21has a part protruding forward beyond a front surface of the main body housing10. An upper surface of a part of the paper feed cassette21housed in the main body housing10is covered with a paper feed cassette top plate21U. The paper feed cassette21is provided with a sheet housing space for housing a bundle of sheets, a lift plate with which the bundle of sheets is lifted so as to be fed, and so on. A sheet feed-out portion21A is provided above a rear end part of the paper feed cassette21. In the sheet feed-out portion21A, there is disposed a paper feed roller21B for feeding out a topmost sheet one by one from the bundle of sheets in the paper feed cassette21.

The image forming section30performs an image forming operation in which a toner image (a developer image) is formed on a sheet sent out from the paper feed section20. The image forming section30includes, in addition to a photosensitive drum31, a charging portion32, an exposure portion35, a developing portion33, and a transfer roller34, which are disposed around the photosensitive drum31.

The photosensitive drum31(an image carrying member) includes a rotary shaft and an outer circumferential surface (a drum main body) that rotates about the rotary shaft. The photosensitive drum31is formed of, for example, a known organic photoconductor (OPC), and a photosensitive layer composed of an electric charge generation layer, an electric charge transport layer, and so on is formed on the outer circumferential surface thereof. After the photosensitive layer is uniformly charged by the after-mentioned charging portion32, light is applied thereto by the exposure portion35so that an electrostatic latent image with attenuated electrostatic charge is formed thereon, and by the developing portion33, the electrostatic latent image is visualized into a toner image, which is thus carried on the photosensitive layer.

The charging portion32(a charging device) is disposed at a prescribed distance from the outer circumferential surface of the photosensitive drum31and uniformly charges the outer circumferential surface of the photosensitive drum31without contacting it. Specifically, the charging portion32includes a charge wire321and a grid electrode322(both are shown inFIG.2). The charge wire321is a linear electrode extending in a rotation axis direction of the photosensitive drum31and generates corona discharge between itself and the photosensitive drum31. The grid electrode322is a grid-shaped electrode extending in the rotation axis direction of the photosensitive drum31and is placed between the charge wire321and the photosensitive drum31. In the charging portion32, a current having a prescribed current value is passed through the charge wire321so that corona discharge is generated, and a prescribed voltage is applied to the grid electrode322, and thus the outer circumferential surface of the photosensitive drum31opposed to the grid electrode322is uniformly charged to a prescribed surface potential.

The exposure portion35(an exposure device) includes a laser light source and optical system instruments such as a mirror and a lens and applies, to the outer circumferential surface of the photosensitive drum31, light modulated based on image data provided from an external apparatus such as a personal computer. With this configuration, the exposure portion35forms, on the outer circumferential surface of the photosensitive drum31, an electrostatic latent image corresponding to an image based on the image data.

The developing portion33(a developing device) is demountably mounted in the main body housing10and supplies a toner (a non-magnetic one-component developer) to the outer circumferential surface of the photosensitive drum31so as to develop an electrostatic latent image formed on the outer circumferential surface of the photosensitive drum31. To develop an electrostatic latent image means to visualize the electrostatic latent image into a toner image (a developer image). A detailed configuration of the developing portion33will be described later.

The transfer roller34is a roller for transferring, onto a sheet, a toner image formed on the outer circumferential surface of the photosensitive drum31. Specifically, the transfer roller34has an outer circumferential surface that axially rotates and is opposed to the outer circumferential surface of the photosensitive drum31at a position on a downstream side relative to a developing roller331in a rotation direction of the photosensitive drum31. The transfer roller34transfers the toner image carried on the outer circumferential surface of the photosensitive drum31to a sheet passing through a nip between itself and the outer circumferential surface of the photosensitive drum31. During this transfer, a transfer voltage having a polarity opposite to that of the toner is applied to the transfer roller34.

The fixing portion40performs fixing processing in which a toner image transferred to a sheet is fixed on the sheet. The fixing portion40includes a fixing roller41and a pressing roller42. The fixing roller41includes therein a heating source and heats the toner transferred to the sheet at a prescribed temperature. The pressing roller42is brought into pressure contact with the fixing roller41, thus forming a fixing nip between itself and the fixing roller41. When the sheet to which the toner image has been transferred is passed through the fixing nip, the toner image is fixed on the sheet under heat applied by the fixing roller41and a pressure applied by the pressing roller42.

In the main body housing10, there are provided a main conveyance path22F and an inversion conveyance path22B, which are used for sheet conveyance. The main conveyance path22F extends from the sheet feed-out portion21A in the paper feed section20to a paper discharge port14provided to be opposed to the paper discharge portion13on the upper surface of the main body housing10via the image forming section30and the fixing portion40. The inversion conveyance path22B is a conveyance path used in duplex printing on a sheet, along which the sheet with one side thereof having been subjected to printing is conveyed back to an upstream side of the image forming section30in the main conveyance path22F.

The main conveyance path22F is provided to extend so as to pass upward from below through a transfer nip formed by the photosensitive drum31and the transfer roller34. Furthermore, a registration roller pair23is disposed on an upstream side relative to the transfer nip in the main conveyance path22F. At the registration roller pair23, conveyance of a sheet is once stopped so that the sheet is subjected to skew correction, and then the sheet is sent out to the transfer nip at a prescribed timing for image transfer. At suitable locations in the main conveyance path22F and the inversion conveyance path22B, there is disposed a plurality of conveyance rollers used for sheet conveyance. A paper discharge roller pair24is disposed in a neighborhood of the paper discharge port14.

The inversion conveyance path22B is formed between an outside surface of an inversion unit25and an inner surface of the rear cover12of the main body housing10. The transfer roller34and one of rollers constituting the registration roller pair23are mounted on an inside surface of the inversion unit25. The rear cover12and the inversion unit25are each axially pivotable about a supporting point121provided at a lower end thereof. Upon occurrence of a jam (a paper jam) in the inversion conveyance path22B, the rear cover12is opened. Upon occurrence of a jam in the main conveyance path22F or in a case where a unit of the photosensitive drum31or the developing portion33is taken outside, not only the rear cover12but also the inversion unit25is opened.

(2. Configuration of Image Forming Section)

FIG.2is a sectional view of the image forming section30in the image forming apparatus1of the present embodiment.FIG.3is a plan view, as seen from above, of a vicinity of a contact part between the photosensitive drum31and the developing roller331of the developing portion33.FIG.4is an enlarged sectional view of a vicinity of a contact part between the developing roller331and a regulation blade334in the developing portion33.FIG.5is an enlarged sectional view of an abutment part between the developing roller331and a supply roller332.

As shown inFIG.2andFIG.3, the developing portion33includes a development housing330(a development container), the developing roller331(a developer carrying member), the supply roller332, an agitation paddle333, and the regulation blade334.

The development housing330contains therein a non-magnetic one-component developer composed only of a toner and houses the developing roller331, the supply roller332, the regulation blade334, and so on. The development housing330includes an agitation chamber335for containing the developer (the toner) in an agitated state. The agitation paddle333is disposed in the agitation chamber335. The agitation paddle333is used to agitate the toner in the agitation chamber335.

The developing roller331includes a rotary shaft331aand a roller portion331b. The rotary shaft331ais rotatably supported to bearings (not shown) provided in the development housing330. The roller portion331bis a cylindrical member stacked on an outer circumferential surface of the rotary shaft331aand is configured by stacking, on a surface of a base rubber (for example, a silicone rubber), a coat layer formed of a coating material having asperities such as urethane. The roller portion331brotates integrally with the rotary shaft331aas the rotary shaft331arotates. A toner layer (a developer layer) having a prescribed thickness is formed on a surface of the roller portion331b. A thickness of the toner layer is regulated (uniformly adjusted to a prescribed thickness) by the after-mentioned regulation blade334. The toner layer becomes charged with static electricity generated by abutment (friction) between the regulation blade334and the roller portion331b.

At a position opposed to the photosensitive drum31, the developing roller331rotates in a direction (a counterclockwise direction inFIG.2) directed from an upstream side to a downstream side in the rotation direction of the photosensitive drum31(a clockwise direction inFIG.2). That is, at the position opposed to the photosensitive drum31, the developing roller331rotates in the same direction as the rotation direction of the photosensitive drum31.

The supply roller332is disposed to be opposed to the developing roller331. The supply roller332retains, on an outer circumferential surface thereof, the developer contained in the agitation chamber335. Furthermore, the supply roller332supplies the developer retained on the outer circumferential surface thereof to the developing roller331.

At a position opposed to the developing roller331, the supply roller332rotates in a direction (the counterclockwise direction inFIG.2) directed from a downstream side to an upstream side in the rotation direction of the developing roller331(the counterclockwise direction inFIG.2). That is, at the position opposed to the developing roller331, the supply roller332rotates in an opposite direction to the rotation direction of the developing roller331. In order to cause the toner to move from the supply roller332to the developing roller331, a prescribed supply voltage (a direct-current voltage) is applied to the supply roller332.

The developing roller331is supplied with the developer from the supply roller332and retains the toner layer on an outer circumferential surface thereof. Further, the developing roller331supplies the developer to the photosensitive drum31. The developing roller331and the supply roller332each have a length in an axial direction thereof (a direction orthogonal to a drawing plane ofFIG.2) substantially equal to a length of the photosensitive drum31in an axial direction thereof. In order to cause the toner to move from the developing roller331to the photosensitive drum31, a prescribed development voltage (a direct-current voltage) is applied to the developing roller331.

In the image forming section30, a pressing mechanism36composed of a pressing member361and a pressing spring362is disposed on an opposite side to the photosensitive drum31via the development housing330(a lower right side inFIG.2, a lower side in FIG.3). The pressing mechanism36is disposed at each of two locations on the development housing330along a longitudinal direction thereof (at positions 85 mm away from a center of the photosensitive drum31in the axial direction). When the developing portion33is mounted in the image forming section30, the development housing330is brought into pressure contact with the pressing member361and thus is pressed in a direction toward the photosensitive drum31(an upper left direction inFIG.2, an upper direction inFIG.3), so that the developing roller331is pressed at a prescribed pressing force to the photosensitive drum31. In the present embodiment, the developing portion33and the photosensitive drum31have no mechanism for regulating a distance between the developing roller331and the photosensitive drum31, namely, no mechanism for regulating a pressing force of the developing roller331with respect to the photosensitive drum31. There, however, may be provided such a mechanism for regulating the pressing force of the developing roller331with respect to the photosensitive drum31.

The regulation blade334is a thin plate-shaped member made of metal. The regulation blade334is configured so that a proximal end334athereof is secured to the development housing330and a distal end334bthereof is a free end. At a position on an upstream side relative to a position at which the photosensitive drum31is opposed to the developing roller331in the rotation direction of the developing roller331, the regulation blade334contacts the outer circumferential surface of the developing roller331.

The regulation blade334is flexibly deformable, and there is a contact part (a regulation nip) between the reregulate blade334and the developing roller331in a circumferential direction of the developing roller331. The regulation blade334abuts on the outer circumferential surface of the developing roller331(the roller portion331b) at a prescribed regulation pressure and with a prescribed regulation nip width W. As will be described later, a configuration may be adopted in which a prescribed regulation voltage (a direct-current voltage) is applied to the regulation blade334.

The regulation blade334is made of, for example, stainless steel (SUS304) and has a free length of 10 mm in the present embodiment. The distal end334bof the regulation blade334is bent so that a curved part334cis formed. The curved part334cabuts on the outer circumferential surface of the developing roller331. The curved part334chas a radius of curvature of not less than 0.1 mm.

As shown inFIG.4, the regulation blade334abuts on the developing roller331at a constant regulation pressure (contact linear pressure), and thus the toner layer carried on the outer circumferential surface of the developing roller331is adjusted to be uniform in thickness. With this configuration, the regulation blade334regulates an amount of the toner on the outer circumferential surface of the developing roller331. Furthermore, the regulation blade334rubs against the toner carried on the outer circumferential surface of the developing roller331and thus charges the toner. The contact linear pressure of the regulation blade334with respect to the developing roller331refers to a contact pressure per unit length of the regulation blade334at a contact position between the regulation blade334and the outer circumferential surface of the developing roller331.

As shown inFIG.5, the abutment part (a supply nip N) between the developing roller331and the supply roller332has a configuration in which the developing roller331bites into the supply roller332. Furthermore, a toner pool T is formed on a downstream side (an upper right side inFIG.5) of the supply nip N in the rotation direction of the developing roller331.

It is known that when the developing roller331and the supply roller332are in linear contact with each other at the supply nip N, the toner pool T is not formed, which results in a significant decrease in toner supply property. To avoid this, it is required that an inter-shaft distance between the developing roller331and the supply roller332and respective diameters and hardnesses thereof be designed so that an amount of biting of the developing roller331into the supply roller332is appropriate. The developing roller331contacts the photosensitive drum31, which is a hard member, and thus is designed to have an Asker C hardness of about 50 to 80. In order, therefore, to achieve the configuration in which the developing roller331bites into the supply roller332, it is required that the supply roller332have a hardness lower than that of the developing roller331.

When a potential difference is generated between the supply roller332and the developing roller331, there is generated electric field energy in such a direction as to cause the toner to move from the supply roller332to the developing roller331. Furthermore, a van der Waals force acts between toner particles regardless of the potential difference. By the electric field energy and the van der Waals force, the toner is supplied from the supply roller332to the developing roller331. In order to improve density followability of a solid image (a capability to achieve no density difference between a leading edge and a trailing edge of the image), it is also important that a compression load with which the supply roller332is pressed to the developing roller331be set to be in an optimum range.

(3. Control Paths of Image Forming Apparatus1)

FIG.6is a block diagram showing an example of control paths used in the image forming apparatus1of the present embodiment. In using the image forming apparatus1, the various portions therein are controlled in different ways, and thus the image forming apparatus1as a whole has complicated control paths. Thus, herein, a description of the control paths is made with emphasis on some of the control paths required for implementing the present disclosure.

Based on output signals from a control section90, a main motor50drives to rotate, in addition to the paper feed roller21B and the photosensitive drum31, the developing roller331, the supply roller332, and the agitation paddle333in the developing portion33, the fixing roller41in the fixing portion40, and so on at prescribed respective rotation speeds.

A voltage control circuit51is connected to a charging voltage power supply52, a development voltage power supply53, and a transfer voltage power supply54and, based on output signals from the control section90, operates these power supplies. Based on a control signal from the voltage control circuit51, the charging voltage power supply52applies a charging voltage to the charge wire321in the charging portion32. The development voltage power supply53applies a development voltage to the developing roller331and a supply voltage to the supply roller332in the developing portion33. In a case of applying a regulation voltage to the regulation blade334in the developing portion33, the development voltage power supply53applies the regulation voltage to the regulation blade344. The transfer voltage power supply54applies a transfer voltage to the transfer roller34.

An image input portion60is a reception portion that receives image data transmitted from a personal computer or the like to the image forming apparatus1. An image signal inputted from the image input portion60is converted into a digital signal, which then is sent out to a temporary storage portion94.

An in-apparatus temperature and humidity sensor61detects a temperature and a humidity inside the image forming apparatus1, particularly a temperature and a humidity in a vicinity of the developing portion33, and is disposed in a neighborhood of the image forming section30.

An operation section70is provided with a liquid crystal display portion71and an LED72that indicates various states and thus functions to indicate a status of the image forming apparatus1and to display an image forming situation and the number of printed copies. Various settings for the mage forming apparatus1are made via a printer driver of a personal computer.

The control section90includes at least a CPU (central processing unit)91as a central computation processor, a ROM (read-only memory)92that is a read-only storage portion, a RAM (random-access memory)93that is a readable and writable storage portion, the temporary storage portion94that temporarily stores image data and so on, a counter95, and a plurality of (herein, two) I/Fs (interfaces)96that transmits control signals to the various devices in the image forming apparatus1and receives input signals from the operation section70.

The ROM92contains, for example, data not to be changed during use of the image forming apparatus1, such as control programs for the image forming apparatus1and numerical values required for control. The RAM93stores, for example, data necessitated when control of the image forming apparatus1is in progress and data temporarily required for controlling the image forming apparatus1.

The temporary storage portion94temporarily stores an image signal inputted from the image input portion60, which receives image data transmitted from a personal computer or the like, and converted into a digital signal. The counter95cumulatively counts the number of printed sheets.

Furthermore, the control section90transmits control signals from the CPU91to the various portions and devices in the image forming apparatus1via the I/Fs96. Furthermore, from the various portions and devices, signals indicating respective statuses thereof and input signals are transmitted to the CPU91via the I/Fs96. Examples of the various portions and devices controlled by the control section90include the image forming section30, the fixing portion40, the main motor50, the voltage control circuit51, the image input portion60, and the operation section70.

(4. Settings of Surface Roughness of Developing Roller and Contact Linear Pressure of Regulation Blade in Developing Portion)

The following describes settings of a surface roughness of the developing roller331and the contact linear pressure of the regulation blade334in the developing portion33, which characterize the image forming apparatus1of the present embodiment. In the present embodiment, a ratio Rz1/Rz2 between a ten-point average roughness Rz1 in a circumferential direction and a ten-point average roughness Rz2 in an axial direction (a ratio between a circumferential roughness and an axial roughness) of the roller portion331bof the developing roller331is set to satisfy Rz1/Rz2≤1.5. A ten-point average roughness Rz described herein is stipulated in the Japanese Industrial Standards (JIS B0601: 1944).

Methods for manufacturing the developing roller331are roughly categorized into a polishing method and a non-polishing method. In the polishing method, a base rubber (in the present embodiment, a silicone rubber) forming the roller portion331bis polished by a polishing machine so that an outer diameter thereof is adjusted. In the polishing method, the roller portion331bof the developing roller331, which rotates about the rotary shaft331a, is contacted by a grindstone so that an outer circumferential surface of the roller portion331bis polished. This results in formation of ripple-shaped polishing marks on the roller portion331bin the circumferential direction, thus causing the roller portion331bto vary in surface roughness between in the circumferential direction and in the axial direction. Specifically, the roller portion331bhas a larger surface roughness in the circumferential direction (ten-point average roughness Rz1).

The polishing marks on the roller portion331bin the circumferential direction can be reduced limitlessly (approximated to zero) by decreasing a speed for polishing the roller portion331bor by repolishing, using a finer-grit polishing machine, the roller portion331bthat has been polished once. That is, a case where Rz1/Rz2=1.0 corresponds to a state where there are almost zero polishing marks in the circumferential direction (the same state as in the axial direction). Furthermore, a case where Rz1/Rz2=1.5 corresponds to a state where there remain polishing marks in the circumferential direction, and thus Rz1 is larger than Rz2. In the state where there remain polishing marks in the circumferential direction, i.e., a state where microscopic asperities are largely present in the circumferential direction, it becomes likely that the toner is caught particularly between convex parts of the asperities and the regulation blade334. Conceivably, as a result of this, a strong frictional force is applied to the toner, which then becomes likely to deteriorate, and toner melt adhesion to the regulation blade334also becomes likely to occur.

In short, conceivably, the more the polishing marks on the roller portion331bin the circumferential direction are reduced, the more an effect of suppressing toner melt adhesion by reducing the contact linear pressure of the regulation blade334is improved. In order, however, to reduce the polishing marks on the roller portion331, it is required to decrease the polishing speed or to perform repolishing, resulting in an increase in processing time. Reducing the polishing marks on the roller portion331, therefore, has a contradictory relationship (a tradeoff) with a manufacturing cost of the developing roller331.

As shown by after-mentioned test results, it has been confirmed that when Rz1/Rz2 is not more than 1.5, toner melt adhesion is drastically suppressed to prolong a life of the developing portion33. Since Rz1/Rz2=1.0 corresponds to the state where there are no polishing marks in the circumferential direction as described above, in no case does the developing roller331manufactured by the polishing method satisfy Rz1/Rz2<1.0. In order to effectively reduce toner melt adhesion, preferably, Rz1/Rz2≤1.5 is satisfied, and more preferably, Rz1/Rz2=1.0 is satisfied.

In a case where Rz2 is large, in order to satisfy Rz1/Rz2≤1.5, Rz1 is adjusted to be increased in proportion to Rz2. In a case, however, where Rz1 and Rz2 are both large, the number of toner particles caught between the convex parts of the asperities and the regulation blade334is increased, and thus a frictional force applied to the toner is distributed to decrease an amount of the frictional force applied to each of the toner particles. For this reason, even when Rz2 is large, as long as Rz1 is adjusted to satisfy Rz1/Rz2≤1.5, there is no possibility that toner melt adhesion deteriorates.

While, as described above, there is no particular limitation on a magnitude of Rz2 of the roller portion331bfrom the standpoint of toner melt adhesion, when Rz2 is too small, an amount of the toner conveyed (= an amount of the toner transferred onto a sheet surface) by the roller portion331bis decreased to lower an image density. When, on the other hand, Rz2 is too large, the amount of the toner conveyed by the roller portion331bis excessively increased to decrease a toner charge amount, so that image fogging becomes likely to occur. Furthermore, since the amount of the toner transferred onto a sheet surface is also increased, there occur troubles such as an increase in toner consumption and occurrence of a fixing failure. For this reason, Rz2 is in a range of preferably 2 to 20 μm, more preferably 3 to 15 μm and even more preferably 4 to 10 μm.

In adjusting Rz1 and Rz2 of the roller portion331b, a desired surface roughness may be obtained merely by polishing or by stacking a particle-containing coat layer on the roller portion331bthat has been polished.

Furthermore, the larger the contact linear pressure of the regulation blade334, the more toner deterioration is accelerated, in which case toner sticking to the regulation blade334becomes likely to occur. When, on the other hand, the contact linear pressure of the regulation blade334is too small, the toner layer carried on the outer circumferential surface of the developing roller331can no longer be regulated, and thus an amount of the toner on the developing roller331is drastically increased. In the present embodiment, the contact linear pressure of the regulation blade334is set to not less than 15 [N/m] and not more than 40 [N/m], so that toner deterioration is suppressed to a minimum, and the amount of the toner on the developing roller331is maintained constant.

(5. Setting of Contact Area Ratio of Developing Roller of Developing Portion)

In order to further prolong the life of the developing portion33(in order for a cumulative number of sheets printed until toner melt adhesion occurs to reach 3,000 sheets), an adjustment is performed using a surface profile (the surface roughness) of the developing roller331as a new parameter.

FIG.7is a view schematically showing a method (a measuring instrument) for measuring a surface profile of the roller portion331b. As shown inFIG.7, the measuring instrument for measuring the surface profile of the roller portion331bof the developing roller331includes, as constituent elements thereof, a prism101(a transparent member) and a light source102.

The prism101is made of glass and is a triangular prism having three mutually non-parallel planes. As viewed sideways, the prism101has a shape of an isosceles triangle. Specifically, as viewed sideways, the prism101has a shape of an isosceles right triangle. A transparent member other than the prism100may be used to measure the surface profile of the roller portion331b.

In measuring the surface profile of the roller portion331b, any one of the three mutually non-parallel planes of the prism101as a pressing surface101ais pressed against the outer circumferential surface of the roller portion331b. In the present embodiment, one of the three mutually non-parallel planes of the prism101interposed between two mutually perpendicular planes of the prims101as viewed sideways is used as the pressing surface101a.

The light source102makes white light incident on a light incidence surface101bthat is one of the three mutually non-parallel planes of the prism101other than the pressing surface101a. As the light source102, an LED, a semiconductor laser, and so on may be used. In the present embodiment, one (a perpendicular plane inFIG.7) of the two mutually perpendicular planes of the prism101as viewed sideways is used as the light incidence surface101b. Furthermore, inFIG.7, light emitted from the light source102and travelling through the prism101is indicated by broken-line arrows.

In a measurement of the surface profile of the roller portion331b, through an observation surface101cthat is one of the three mutually non-parallel planes of the prism101other than the pressing surface101aand the light incidence surface101b, there is observed a contact state between the outer circumferential surface of the roller portion331band the pressing surface101aof the prism101when light is made incident on the light incidence surface101b. In the present embodiment, one (a horizontal plane inFIG.7) of the two mutually perpendicular planes as viewed sideways other than the light incidence surface101bis used as the observation surface101c.

In order to reproduce a state where the regulation blade334is in contact with the outer circumferential surface of the roller portion331b, the pressing surface101aof the prism101is pressed against the outer circumferential surface of the roller portion331b. Furthermore, the roller portion331bhas microscopic asperities provided on the outer circumferential surface thereof. That is, the developing roller331has concave parts and convex parts provided on the outer circumferential surface thereof. It is, therefore, not that the pressing surface101aof the prism101makes contact in an entire region (100% of an area) thereof with the outer circumferential surface of the roller portion331b.

On the pressing surface101aof the prism101, regions (hereinafter, referred to as convex regions) opposed to the convex parts contact the outer circumferential surface of the roller portion331b, and regions (hereinafter, referred to as concave regions) opposed to the concave parts do not contact the outer circumferential surface of the roller portion331b. Clearance spaces (voids) are generated between the outer circumferential surface of the roller portion331band the concave regions on the pressing surface101aof the prism101.

With this configuration, on the pressing surface101aof the prism101, there occurs a difference in intensity of light depending on whether the light is incident on the concave regions or on the convex regions. In the concave regions on the pressing surface101aof the prism101, white light from the light source102is totally reflected. Meanwhile, in the convex regions on the pressing surface101aof the prism101, no clearance spaces are generated. Thus, in the convex regions on the pressing surface101aof the prism101, white light from the light source102is diffusely reflected or absorbed, so that an intensity of the light is decreased compared with that in the concave regions.

FIG.8is a view schematically showing a result of observing, by using a microscope, the observation surface101cof the prism101in a state shown inFIG.7. By observing the observation surface101cof the prism101, as shown inFIG.8, it is possible to detect a contrast of reflection light (dark regions and bright regions) on the pressing surface101a. The convex regions (regions on the pressing surface101aof the prism101, which are actually in contact with the outer circumferential surface of the developing roller331) manifest themselves as the dark regions. That is, the convex regions can be detected. InFIG.8, the dark regions are indicated by solid black circles.

In the measurement of the surface profile of the roller portion331b, the pressing surface101aof the prism101is pressed at a linear pressure of 50 [N/m] against the outer circumferential surface of the roller portion331b. The regulation blade334makes contact at a linear pressure of not less than 20 N/m and not more than 60 N/m with the outer circumferential surface of the developing roller331. In order, therefore, to reproduce the state where the regulation blade334is in contact with the outer circumferential surface of the roller portion331b, a linear pressure at which the pressing surface101aof the prism101is pressed against the outer circumferential surface of the roller portion331bis set to not less than 20 [N/m] and not more than 60 [N/m].

Further, there is determined a ratio of an actual contact area to an area of a nip between the pressing surface101aof the prism101and the outer circumferential surface of the roller portion331bwhen contacted thereby at a linear pressure of 50 [N/m]. The area of the nip is a product of a nip width and a nip length. The nip width described herein refers to a contact width between the outer circumferential surface of the roller portion331band the pressing surface101aof the prism101in the circumferential direction of the developing roller331. The nip length refers to a contact length between the outer circumferential surface of the roller portion331band the pressing surface101aof the prism101in a rotation axis direction of the developing roller331.

Furthermore, the actual contact area refers to an area of actual contact between the outer circumferential surface of the roller portion331band the pressing surface101aof the prism101. That is, the actual contact area refers to a total area of the convex regions on the pressing surface101aof the prism101. Herein, the ratio of the actual contact area to the area of the nip between the pressing surface101aof the prism101and the outer circumferential surface of the roller portion331bwhen contacted thereby at a linear pressure of 50 [N/m] is used as a contact area ratio.

By observing the observation surface101cof the prism101, it is possible to detect regions on the pressing surface101aof the prism101, which are actually in contact with the outer circumferential surface of the roller portion331b. It is also possible to determine a total area of the regions thus detected. That is, by observing the observation surface101cof the prism101, it is possible to determine the after-mentioned contact area ratio. Specifically, binarization processing of a photographed image is performed based on a set threshold value of luminance information on the photographed image, and there is calculated a ratio of an area occupied by black regions to an entire area of the image after being subjected to the binarization processing (=a total area of the black regions/the area of the image after being subjected to the binarization processing). In this manner, the contact area ratio can be determined in numerical terms.

The smaller the contact area ratio, the more clearance spaces between the developing roller331and each of the photosensitive drum31and the regulation blade334are increased. As a result, physical stress applied to the toner is reduced, and thus it is possible to suppress toner melt adhesion to the regulation blade334. As shown by the after-mentioned test results, when the contact area ratio is not more than 20%, toner melt adhesion to the regulation blade334and resulting occurrence of thin layer streaks in a thin toner layer on the developing roller331are suppressed to prolong the life of the developing portion33.

When, however, the contact area ratio is not more than 5%, thickness unevenness of the thin toner layer on the developing roller331is increased to cause image unevenness. For this reason, preferably, the contact area ratio is in a range of not less than 5% and not more than 20%. The contact area ratio can be adjusted by adjusting a particle size of a roughening agent (particulates of silicone, urethane, or the like) contained in a surface layer (the coat layer) of the roller portion331bor an additive amount of the roughening agent.

(6. Setting of Electrostatic Capacity of Developing Roller of Developing Portion)

In order to further prolong the life of the developing portion33, an adjustment is performed using an electrostatic capacity of the developing roller331as a new parameter.

The amount of the toner conveyed can be controlled by adjusting an electrostatic capacity of the roller portion331bof the developing roller331. Specifically, when the electrostatic capacity of the developing roller331is increased, an electric adhesion force (an image force) between the developing roller331and the toner is increased. This makes it possible, without extremely increasing the surface roughness of the developing roller331, to increase an amount of the toner retainable on the developing roller331.

As shown by the after-mentioned test results, the larger the electrostatic capacity of the roller portion331b, the more the amount of the toner retainable on the developing roller331is increased. Furthermore, toner melt adhesion to the regulation blade334becomes unlikely to occur. When, however, the electrostatic capacity is excessively increased, the electric adhesion force between the developing roller331and the toner becomes too large, and thus the toner can no longer respond to an electric field (a development electric field) between the photosensitive drum31and the developing roller331, so that the image density is decreased.

Preferably, the electrostatic capacity of the roller portion331bis not less than 4.0×10−7[F/m2] and not more than 7.0×10−7[F/m2]. The electrostatic capacity can be adjusted by adjusting an additive amount of an electrically conductive material such as carbon black added to the surface layer (the coat layer) of the roller portion331b.

(7. Evaluation of Images Obtained Under Settings Made to Developing Portion)

The following describes results of an evaluation of images obtained in a case where, as in the present embodiment, there are made settings of the surface roughness of the developing roller331and the contact linear pressure of the regulation blade334in the developing portion33. First, an endurance printing test was performed under varying printing conditions (Rz1/Rz2 of the developing roller331and the contact linear pressure of the regulation blade334), and the effect with respect to toner melt adhesion to the regulation blade334was examined. The image forming apparatus1(manufactured by KYOCERA Document Solutions Inc.) shown inFIG.1was used as a test apparatus.

As the developing roller331, there was used a roller including the rotary shaft331aand the roller portion331band having an Asker C hardness of 70° and a roller resistance of 7.1 [log Ω]. The rotary shaft331ahad a shaft diameter of 6 mm. The roller portion331bincluded, as a base material layer, a silicone rubber layer having a thickness of 3.5 mm and coated with a urethane coating and had an outer diameter of 13 mm and a length of 232 mm in an axial direction thereof. A constant pressure load instrument (CL-150 manufactured by Kobunshi Keiki Co., Ltd.) was used to measure the Asker C hardness. For a measurement of the roller resistance, the developing roller331was rotated in contact with a metal roller, and a direct-current voltage of 100 V was applied thereto.

As for the setting of Rz1/Rz2 of the developing roller331, the surface roughness of the roller portion331bwas adjusted by changing polishing conditions so that Rz1/Rz2=1.0 or 1.5. As the regulation blade334, a plate-shaped member made of stainless steel (SUS 304) was used, and the contact linear pressure thereof was adjusted to lie between 15 [N/m] and 50 [N/m] by changing a thickness and a free length of the plate-shaped member.

As the photosensitive drum31, there was used a positively-charged single-layer OPC photosensitive drum (manufactured by KYOCERA Document Solutions Inc.) having an outer diameter of 24 mm and a photosensitive layer thickness of 22 μm.

A toner used was a polyester resin-based toner manufactured by a pulverization method and having a central particle diameter of 8.0 μm and a circularity of 0.96

(Relationship Between Contact Linear Pressure of Regulation Blade and Toner Melt Adhesion)

First, a study was made of a relationship between the contact linear pressure of the regulation blade334and toner melt adhesion. In a test method adopted, an image of standard data stipulated in ISO/IEC 19752 (a character pattern with a printing rate of 3.9%) was outputted in an A4 size as a test image. For evaluation of toner melt adhesion to the regulation blade334, it was visually determined whether or not white streaks had occurred in the outputted test image. The life of the developing portion33was determined based on a cumulative number of printed sheets at a time when the toner in the developing portion33ran out or a time when characters in the outputted test image broke due to increased occurrence of white streaks in the image, whichever was reached first, and a target value of the cumulative number was set to 1,500 sheets. Results are shown inFIG.9.

As shown inFIG.9, in either of a case where Rz1/Rz2 of the developing roller331was set to 1.0 (a data series denoted with solid black circles inFIG.9) and a case where it was set to 1.5 (a data series denoted with asterisks inFIG.9), the number of sheets printed prior to occurrence of toner melt adhesion (a cumulative number of sheets printed until toner melt adhesion occurs) increased with decreasing contact linear pressure of the regulation blade334. Conceivably, this is because a decrease in contact linear pressure of the regulation blade334leads to a decrease in frictional force applied to the toner in a regulation portion. When, however, the contact linear pressure is smaller than 15 [N/m], it is no longer possible to regulate the toner layer carried on the outer circumferential surface of the developing roller331, and thus image stability cannot be maintained (a region shaded with dots inFIG.9). It has thus been confirmed that when the contact linear pressure is not less than 15 [N/m] and not more than 40 [N/m], toner melt adhesion is suppressed, and the image stability can be also maintained.

Furthermore, a comparison between the case where Rz1/Rz2 was set to 1.0 and the case where it was set to 1.5 reveals that the number of sheets printed prior to occurrence of toner melt adhesion was higher in the former case than in the latter case. That is, it has been confirmed that decreasing Rz1/Rz2 further improves an effect of increasing the number of sheets printed prior to occurrence of toner melt adhesion.

(Relationship Between Rz1/Rz2 of Developing Roller and Toner Melt Adhesion)

Next, a study was made of a relationship between Rz1/Rz2 of the developing roller331and toner melt adhesion. In a test method adopted, with the contact linear pressure of the regulation blade334fixed at 20 [N/m] and Rz2 of the developing roller331set to 4.7 μm, Rz1/Rz2 was made to vary in a range of 1.0 to 2.25, and the number of sheets printed prior to occurrence of toner melt adhesion to the regulation blade334was checked. For evaluation of occurrence of toner melt adhesion, it was determined whether or not white streaks had occurred in an outputted test image similar to the above-described test image. Results are shown inFIG.10.

As shown inFIG.10, when Rz1/Rz2 of the developing roller331was smaller than 1.5, the number of sheets printed prior to occurrence of toner melt adhesion was drastically increased. That is, it has been confirmed that setting Rz1/Rz2 to not more than 1.5, more preferably, to 1.0 further improves the effect of increasing the number of sheets printed prior to occurrence of toner melt adhesion.

(Relationship Between Contact Area Ratio of Developing Roller and Toner Melt Adhesion)

Next, a study was made of a relationship between the contact area ratio of the developing roller331and toner melt adhesion. In a test method adopted, an endurance test was carried out (3,000 sheets of test images were continuously printed) in which the contact linear pressure of the regulation blade334was made to vary in a range of 15 to 50 [N/m], Rz1/Rz2 of the developing roller331was made to vary in a rage of 1.0 to 2.0, and the contact area ratio was made to vary in a range of 5 to 30%, and an image and the thin toner layer on the developing roller331at each of an initial stage and a time when a target life was reached (after the printing of 3,000 sheets) were visually checked.

Based on a state resulting from the printing of 3,000 sheets, evaluation criteria were defined as follows. That is, a case where the thin toner layer was uniformly formed on the developing roller331and the image was also properly formed with no occurrence of thin layer streaks was indicated as “G (good),” a case where minor occurrence of thin layer streaks in the thin toner layer on the developing roller331or thickness unevenness thereof was observed but was not manifested in the image and thus presented no problem in practical use was indicated as “F (fair),” a case where an abnormality was observed on the developing roller331and on the image and thus led to unacceptable image quality was indicated as “P (poor),” and a case where thickness unevenness of the thin toner layer on the developing roller331and density unevenness on the image occurred and thus led to unacceptable image quality was indicated as “U (unacceptable).” Results are shown in Tables 1 to 5.

TABLE 1CONTACT AREA RATIORz1/Rz25%1.01.251.52.0CONTACT LINEAR15UUUUPRESSURE OF20GGGGREGULATION BLADE30GGGF[N/m]40GGGF50GGGP

TABLE 2CONTACT AREA RATIORz1/Rz210%1.01.251.52.0CONTACT LINEAR15GGGGPRESSURE OF20GGGGREGULATION BLADE30GGGF[N/m]40GGGP50GGGP

TABLE 3CONTACT AREA RATIORz1/Rz215%1.01.251.52.0CONTACT LINEAR15GGGGPRESSURE OF20GGGFREGULATION BLADE30GGGF[N/m]40GGFP50GFFP

TABLE 4CONTACT AREA RATIORz1/Rz220%1.01.251.52.0CONTACT LINEAR15GGGGPRESSURE OF20GGGFREGULATION BLADE30GGFF[N/m]40GFFP50GFFP

TABLE 5CONTACT AREA RATIORz1/Rz230%1.01.251.52.0CONTACT LINEAR15PPPPPRESSURE OF20PPPPREGULATION BLADE30PPPP[N/m]40PPPP50PPPP

As shown in Table 5, in a case where the developing roller331had a contact area ratio of 30%, no matter how the contact linear pressure of the regulation blade334and Rz1/Rz2 of the developing roller331were adjusted, there had occurred thin layer streaks in the thin toner layer on the developing roller331before the number of printed sheets reached 3,000 sheets as the target life. In contrast, as shown in Tables 1 to 4, in a case where the developing roller331had a contact area ratio of not more than 20%, when the contact linear pressure of the regulation blade334was adjusted to be in the range of 15 to 50 [N/m] and Rz1/Rz2 of the developing roller331was adjusted to be in the range of 1.0 to 2.0, the occurrence of thin layer streaks in the thin toner layer and thickness unevenness thereof after the number of printed sheets reached 3,000 sheets could be reduced to such a range as to present no problem in practical use. In a case where the contact area ratio was 5%, when the regulation blade334had a contact linear pressure of 15 [N/m], there occurred thickness unevenness of the thin toner layer on the developing roller331and density unevenness on the image.

The foregoing results have confirmed that decreasing the contact area ratio of the developing roller331from a conventional value of about 30% to not more than 20% can suppress the occurrence of thin layer streaks in the thin toner layer and thickness unevenness thereof. It has been also confirmed that, while respective adjustment ranges of the contact linear pressure of the regulation blade334and Rz1/Rz2 of the developing roller331for reaching the target life (3,000 sheets) expand with decreasing contact area ratio, when the contact area ratio is not more than 5%, thickness unevenness of the thin toner layer on the developing roller331occurs to cause image unevenness.

(Relationship Between Electrostatic Capacity of Developing Roller and Toner Melt Adhesion)

Next, a study was made of a relationship between the electrostatic capacity of the developing roller331and toner melt adhesion. In a test method adopted, as the developing roller331, rollers were manufactured so as to have Rz1/Rz2 fixed at 1.5 and vary in electrostatic capacity in six levels. The electrostatic capacity was adjusted by changing the additive amount of the electrically conductive material (carbon black) added to the surface layer of the roller portion331b. Table 6 shows electrostatic capacity values of the rollers each manufactured as the developing roller331.

TABLE 6ELECTROSTATICELECTRICALLYCAPACITYCONDUCTIVEROLLER NO.[10−7F/m2]MATERIAL13.2Not Added24.3Added34.845.757.067.7

With the contact linear pressure of the regulation blade334fixed at 20 [N/m], an endurance test was carried out (1,000 sheets of test images were continuously printed), and an amount of the toner conveyed on the developing roller331, the number of sheets printed prior to occurrence of toner melt adhesion to the regulation blade334, and the image density (a reflection density) at the end of the endurance test were measured. For evaluation of occurrence of toner melt adhesion, it was determined whether or not white streaks had occurred in an outputted test image. Results are shown inFIG.11andFIG.12.

As shown inFIG.11, it is understood that, in a case where the electrically conductive material is added (a data series denoted with solid black circles inFIG.11), the amount of the toner conveyed is larger than in a case where the electrically conductive material is not added (a data series denoted with a solid black triangle inFIG.11), and that the larger the electrostatic capacity of the developing roller331, the more the amount of the toner conveyed is increased.

Furthermore, as shown inFIG.12, the larger the electrostatic capacity, the more the number of sheets printed prior to occurrence of toner melt adhesion (shown by a solid line inFIG.11) was increased (the occurrence was delayed), and it has thus been confirmed that an effect of prolonging the life of the developing portion33is achieved. On the other hand, the larger the electrostatic capacity, the more the image density (shown by a broken line inFIG.11) was decreased. Conceivably, this is because of the following reason. That is, when the electrostatic capacity is excessively increased, the electric adhesion force between the developing roller331and the toner becomes too large, and thus the toner can no longer respond to the development electric field, so that the image density is decreased. It is understood, from the results shown inFIG.12, that when the electrostatic capacity is in a range of not less than 4.0×10−7[F/m2] and not more than 7.0×10−7[F/m2], it is possible to achieve the effect of prolonging the life of the developing roller331with respect to toner melt adhesion and also to maintain the image density.

(Relationship Between Electrostatic Capacity and Rz1/Rz2 of Developing Roller and Toner Melt Adhesion)

With the contact linear pressure of the regulation blade334fixed at 20 [N/m], using, as the developing roller331, the rollers (No. 1 and No. 4 in Table 6) that varied in electrostatic capacity in two levels, an endurance test was carried out (1,000 sheets of test images were continuously printed) in which Rz1/Rz2 was made to vary, and the number of sheets printed prior to occurrence of toner melt adhesion to the regulation blade334was checked. Results are shown inFIG.13.

As shown inFIG.13, in a case of one of the rollers as the developing roller331having an electrostatic capacity of 3.2×10−7[F/m2] (shown by a solid line inFIG.13), when Rz1/Rz2 was 1.0, the number of sheets printed prior to occurrence of toner melt adhesion was 2,200 sheets, whereas when Rz1/Rz2 was 1.5, the number of sheets printed prior to occurrence of toner melt adhesion was 1,900 sheets. On the other hand, in a case of the other roller as the developing roller331having an electrostatic capacity of 5.7×10−7[F/m2] (shown by a broken line inFIG.13), when Rz1/Rz2 was 1.0, the number of sheets printed prior to occurrence of toner melt adhesion was 2,700 sheets, whereas when Rz1/Rz2 was 1.5, the number of sheets printed prior to occurrence of toner melt adhesion was 2,400 sheets. These results have confirmed that a combination of decreased Rz1/Rz2 and an increased electrostatic capacity makes it possible to further enhance the effect of prolonging the life of the developing portion33with respect to toner melt adhesion.

(8. Other Configurations)

In the present embodiment, both of a toner manufactured by the pulverization method (a pulverized toner) and a toner manufactured by a polymerization method (a polymerized toner) can be used. Due to its truly spherical shape having a high circularity, the polymerized toner is low in adhesion force to provide good development performance and thus has a broader usable range. The present disclosure is particularly useful in the non-magnetic one-component development type using the pulverized toner that, while being less costly than the polymerized toner, has a narrower usable range.

Furthermore, in the present embodiment, it has been confirmed that the use of a toner having a central particle diameter of 6.0 to 8.0 μm provides excellent results. The reason for selecting a central particle diameter in this range is as follows. That is, a central particle diameter outside this range is not preferable in that a central particle diameter smaller than 6.0 μm leads to an increase in manufacturing cost of the toner, and a central particle diameter larger than 8.0 μm leads to an increase in toner consumption and thus to deterioration in fixability.

Furthermore, in the present embodiment, it has been confirmed that the use of a toner having a circularity of 0.93 to 0.97 provides excellent results. A circularity outside this range is not preferable for the following reason. That is, a circularity of not more than 0.93 tends to decrease image quality. A circularity of not less than 0.97 leads to a substantial increase in manufacturing cost.

Furthermore, in the present embodiment, it has been confirmed that the use of a toner having a melt viscosity of not more than 100,000 Pa·s at 90° C. provides excellent results. A melt viscosity exceeding 100,000 Pa·s at 90° C. leads to deterioration in fixability of the toner and thus is not preferable from the standpoint of energy saving.

It has been confirmed that a linear speed difference between the photosensitive drum31and the developing roller331in a range of 1.1 to 1.6 times (a linear speed of the developing roller331is higher than that of the photosensitive drum31) provides similar results. A linear speed difference smaller than 1.1 times makes it likely that there occurs image fogging in which the toner adheres to a blank part of a sheet and thus is not preferable. A linear speed difference larger than 1.6 times leads to an increase in driving torque or vibrations of the developing portion33or an increase in physical stress on the toner and thus is not preferable from the standpoint of the life of the developing portion33.

Furthermore, it has been confirmed that a surface potential VO in a range of 500 to 800 V and a post-exposure potential VL in a range of 70 to 200 V of the photosensitive drum31provide similar effects.

Other than the above, the present disclosure is not limited to the foregoing embodiment and can be variously modified without departing from the spirit of the present disclosure. For example, while the foregoing embodiment describes a monochrome printer as an example of the image forming apparatus1, the present disclosure is applicable also to, for example, a color printer of a tandem type or a rotary type. Furthermore, the present disclosure is applicable also to an image forming apparatus such as a copy machine, a facsimile, or a multi-functional peripheral equipped with functions thereof. It is required, however, to include the photosensitive drum31and the developing portion33of the non-magnetic one-component development type. Furthermore, while the foregoing embodiment describes the configuration in which a non-magnetic toner is stored in the development housing330of the developing portion33, a toner container or a toner cartridge for containing a non-magnetic toner may be provided separately from the development housing330.

Furthermore, while the photosensitive drum31in the foregoing embodiment uses a cylindrical raw tube as a support, a support having any other shape may also be used. Examples of the other shape may include a plate shape and an endless belt shape. Furthermore, while the photosensitive drum31in the foregoing embodiment uses an amorphous silicon photoconductive layer, there may be provided, for example, an electric charge injection blocking layer that blocks injection of electric charge from the support.

The present disclosure is usable in a developing device of the non-magnetic one-component development type using a non-magnetic toner. Through the use of the present disclosure, it is possible to provide a developing device capable of, in a configuration using a non-magnetic one-component development method, effectively suppressing toner melt adhesion to a regulation blade by using a simple configuration and an image forming apparatus including the same.