Patent Application
20060045592
1. Field of the Invention
The present invention relates to an image forming apparatus and image forming method, and more particularly, to an image forming apparatus and image forming method that increase the useful life of the image carrier and improve the image quality by supplying a lubricant to the image carrier.
2. Description of the Related Art
An image forming apparatus of the conventional art using the electrophotographic method, such as a copier or printer, uniformly charges the surface of an image carrier (e.g., a photoreceptor drum) using a charger and forms an electrostatic latent image thereon by exposing the image carrier with light modulated based on the image. The formed electrostatic latent image is developed using toner into a toner image, which is then transferred to a recording medium or to an intermediate transfer unit and then to a recording medium, and the transferred toner image is heated by a fusing device such that it fuses to the recording medium, thereby forming a final image.
Image forming apparatuses of this type include a cleaning device that removes and cleans off the residual toner from the photoreceptor after the toner image formed on the image carrier (e.g., a photoreceptor drum) or the intermediate transfer unit (hereinafter referred to as an ‘image carrier’) is transferred to the recording medium.
The cleaning device is often of the type in which a cleaning blade is brought into contact with the image carrier surface to scrape off the residual toner. However, using this construction, substantial friction between the cleaning blade and the image carrier shaves away the photosensitive film on the image carrier surface and/or wears down the cleaning blade, causing a reduction in the useful lives of both component members.
As a countermeasure against this problem, a technology is known whereby a lubricant is applied to the image carrier surface to reduce the frictional coefficient thereof, thereby preventing wear on the cleaning blade and damage to the photosensitive film on the image carrier surface and extending the useful lives of the component members. However, this technology entails the problem that when the residual toner is removed using a cleaning blade, the toner that has been scraped off adheres to and remains on the blade edge, and the lubricant applied to the image carrier surface is scraped off by the residual toner.
Accordingly, a method has been proposed in which an application brush is used to carry solid lubricant, which is then applied to the image carrier surface by bringing the application brush into contact therewith. For example, according to Japanese Patent Application Laid-Open No. 2002-244486, the degree by which the application brush is pushed into a solid lubricant and the rotation rate of the application brush are controlled based on information regarding the cumulative drive time of the image carrier sought from the cumulative number of prints as well as on the counter value for the number of dots in the image, which indicates the black/white ratio.
Japanese Patent Application Laid-Open No. 7-311531 proposes a technology in which the amount of lubricant applied to the surface of the image carrier is detected, and based on the detection result, the rotation of the application brush is controlled to be ON or OFF, or the rotation rate of the application brush is controlled.
However, using an image forming apparatus of the conventional art described above, such information as the image carrier cumulative drive time and the counter value for the number of effective dots in the image (which indicates the black/white ratio) must be collected and processed, or means to detect the amount of lubricant applied to the image carrier surface and control the rotation of the application brush are needed, which makes the construction of the apparatus complex and increases the manufacturing cost.
An object of the present invention is to provide an image forming apparatus and image forming method that do not require special means such as the lubricant application control means that is required by the image forming apparatus of the conventional art and that can maintain the amount of lubricant on the image carrier surface at an appropriate level.
The image forming apparatus of the present invention comprises an image carrier, a lubricant application device that applies a first lubricant to the image carrier surface, and a developing device that houses a developer to which a second lubricant is added and develops the electrostatic latent image formed on the image carrier. Here, the pure water contact angle θ1 of the first lubricant and the pure water contact angle θ2 of the second lubricant have the relationship expressed by the formula (1):
θ1≦θ2 (1)
In other words, according to the present invention, the surface energy of the developer is reduced by coating the toner particle surfaces with the second lubricant having a smaller surface energy than the first lubricant applied to the image carrier surface.
As a result, the peeling off of the first lubricant applied to the image carrier surface by the toner adhering to the cleaning blade edge can be reduced.
The image forming apparatus of the present invention comprises an image carrier, a lubricant application device that applies a first lubricant to the image carrier surface and a developing device that houses a developer to which a second lubricant is added and that develops the electrostatic latent image formed on the image carrier. Here, the frictional coefficient μ1 of the first lubricant and the frictional coefficient μ2 of the second lubricant have the relationship expressed by the formula (2):
μ1<μ2 (2)
In other words, according to the present invention, by coating the toner particle surfaces with a second lubricant having a frictional coefficient μ2 that is larger than the frictional coefficient μ1 of the first lubricant applied to the image carrier surface, the friction among toner particles in the toner pool formed by removed toner that adheres to and remains on the blade edge is increased, such that it is more difficult for the toner particles to slip through the blade edge, thereby reducing the incidence of cleaning failure.
These and other objects and features of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, in which:
With reference to
The properties of the solid lubricant M1, i.e., the first lubricant, and the solid lubricant M2, i.e., the second lubricant, as well as the lubricant application device 20 that includes an application brush that applies the solid lubricant M1, are described in detail below.
The image formation operation carried out by the image forming apparatus will be described briefly. The surface of the photoreceptor drum 11 is first charged uniformly by the main charger 12. Laser light emitted from the laser device of the exposure device 13 is modulated based on the image signals output from the scanning optical system based on the reading of an original document image placed on a platen not shown or on image signals output from a personal computer or the like not shown, and the modulated light is projected onto the surface of the photoreceptor drum 11, whereby an electrostatic latent image is formed.
The electrostatic latent image formed on the surface of the photoreceptor drum 11 is developed using the developer housed in the developing device 14, whereby a toner image is formed. A recording medium P is conveyed from a paper supply device not shown synchronously with the arrival of the toner image formed on the surface of the photoreceptor drum 11 at the transfer device 15, i.e., the transfer position, based on the rotation of the photoreceptor drum 11 in the direction of the arrow (a). At the transfer position, the toner image formed on the surface of the photoreceptor drum 11 is transferred to the recording medium P based on the operation of the transfer device 15. The recording medium P is then conveyed to the fusing device 16, whereby the toner image on the recording medium P is fused thereto, and the recording medium P is ejected onto an eject tray not shown.
The waste toner remaining on the surface of the photoreceptor drum 11 that was not used for image transfer is cleaned off and removed by the cleaning device 18, and the apparatus moves onto the next image forming operation.
The lubricant application device 20 will now be described. The lubricant application device 20 may comprise one of the two embodiments described below, i.e., a lubricant application device 20A or a lubricant application device 20B. While each of them applies solid lubricant by bringing an application brush into contact with the photoreceptor drum 11, they differ in terms of the construction used to load solid lubricant onto the application brush.
The solid lubricant M1 is held by a holding plate 22, and a compressed spring 23 is disposed between the holding plate 22 and the housing 10A of the image forming apparatus. The solid lubricant M1 held by the holding plate 22 is pressed toward the application brush 21 by the force of the compressed spring 23.
Based on this construction, when the application brush 21 rotates in tandem with the rotation of the photoreceptor drum 11, the bristles of the application brush 21 take up small amounts of the solid lubricant M1, and the solid lubricant M1 adhering to the bristles is applied to the surface of the photoreceptor drum 11.
The solid lubricant M1 is held by a holding plate 22, and a pulled spring 24 is disposed between the holding plate 22 and the bearing 21b of the rotating shaft 21a of the application brush 21. The solid lubricant M1 held by the holding plate 22 is pulled toward the application brush 21.
Based on this construction, when the application brush 21 rotates in tandem with the rotation of the photoreceptor drum 11, the bristles of the application brush 21 take up small amounts of the solid lubricant M1, and the solid lubricant M1 adhering to the bristles is applied to the surface of the photoreceptor drum 11.
In addition, in the second embodiment 20B of the lubricant application device, because, unlike in the first embodiment 20A described above, the housing 10A is not used as a support member for the compressed spring 23, such variables as the variations in the gap between the housing 10A and the holding plate 22 (i.e., the solid lubricant M1) and the slanting of the rotating shaft of the application brush 21 have no bearing on the location of the application 21, and the solid lubricant M and the application brush 21 are maintained parallel to each other at all times. Accordingly, the solid lubricant M1 can be taken up uniformly at either end of the application brush 21, ensuring stable take-up.
The peeling off of the solid lubricant on the photoreceptor surface by toner will now be explained.
When the photoreceptor PH moves in the direction of the arrow (b), the residual toner TN on the photoreceptor PH is stopped and scraped off by the blade edge BE. When this occurs, the toner TN on the photoreceptor PH collides with the edge BE of the cleaning blade with an impact force T1, and force T2 to repel the toner TN is generated on the edge BE. The smaller the frictional coefficient μ1 between the photoreceptor PH and the toner TN, the smaller the impact force T1 of the toner TN becomes, enabling cleaning to be achieved easily.
Scraped off toner TN remains along the very edge of the blade edge BE, creating a toner pool Tm. The toner pool Tm on the blade edge BE tends to become denser and more marked as the toner particle diameter decreases. Previously pooled toner particles and toner particles that currently enter the pool collide in the toner pool Tm, which peels off the solid lubricant on the photoreceptor PH.
Formation of a toner pool Tm is important for cleaning, but where the toner is highly fluid, or when the toner particle diameter decreases, the pool density increases and the amount of toner that escapes through the blade increases, resulting in cleaning failure. In order to effectively scrape off the residual toner via the blade edge BE, it is preferred that the residual toner separate from the photoreceptor easily and that there be little frictional resistance between the toner particles already in the toner pool Tm and the toner particles entering the toner pool Tm.
The frictional coefficient μ1 between the photoreceptor PH and the toner TN should be reduced in order to reduce the toner's impact force T1. In order to form a toner pool Tm by increasing the toner's repulsion force T2, the frictional coefficient μ2 between the toner TN particles already in the toner pool Tm and the toner particles entering the toner pool Tm should be increased.
Accordingly, in the present invention, two types of solid lubricants, the relationship of whose pure water contact angles can be expressed by the following formula (1), are selected. A solid lubricant M1 having a pure water contact angle θ1 is used as the solid lubricant applied to the photoreceptor, and a solid lubricant M2 having a pure water contact angle θ2 is used as the solid lubricant added to the developer.
θ1≦θ2 (1)
where, θ1: the pure water contact angle of the solid lubricant M1
θ2: the pure water contact angle of the solid lubricant M2
The frictional coefficient of the solid lubricant M1 and the frictional coefficient of the solid lubricant M2 may also have the relationship expressed by the following formula (2):
θ1<θ2 (2)
where, μ1: the frictional coefficient of the solid lubricant M1
μ2: the frictional coefficient of the solid lubricant M2
As a result, because the photoreceptor surface comes to be coated with the solid lubricant M1 having the frictional coefficient θ1 (or the pure water contact angle θ1) and the friction between the photoreceptor and the toner becomes reduced, the residual toner is easily separated from the photoreceptor. In addition, because the surfaces of the toner particles in the developer come to be coated with the solid lubricant M2 having a frictional coefficient θ2 (or a pure water contact angle θ2), the frictional resistance between the toner particles already in the toner pool and the toner particles entering the toner pool increases. Consequently, a toner pool Tm is appropriately formed and the peeling off of the solid lubricant M1 by the toner on the blade edge BE is reduced.
[Pure Water Contact Angle and Frictional Coefficient]
The relationship between the pure water contract angle and the frictional coefficient of a solid lubricant will be explained. The significance of the contact angle will first be explained. A contact angle is the angle of contact between a liquid surface and a solid surface at the point of contact therebetween when the free surface of the liquid is in equilibrium contact with the solid surface, which could be a solid wall surface or horizontal surface. The angle is taken as the angle between the solid surface and the surface of the liquid that is not in contact with the solid surface. When the contact angle is acute, such state is expressed as ‘wet’, indicating the solid surface to be an easily wetted surface. When the contact angle is obtuse, such state is expressed as ‘not wet’, indicating a solid surface that does not easily become wet.
Therefore, the pure water contact angle of a solid lubricant is a value that indicates the ease with which the photoreceptor surface coated with the solid lubricant becomes wet with pure water. In other words, it can be used as an indicator of the surface state coated with various lubricants.
The method by which to measure the pure water contact angle of a solid lubricant will now be explained. Polycarbonate resin, which is the raw material of the image carrier, and the styrene-acrylic resin mixture, which is the raw material of the toner, were formed into sheets, and measurement samples were created by uniformly coating such sheets with various solid lubricants. The pure water contact angle of these measurement samples was measured using a contact angle measurement device manufactured by Kyowa Kaimen Kagaku Co., Ltd. In this measurement, the raw material resin sheets did not have any effect on the measured contact angle result. Namely, so long as the same solid lubricant was used, the measured contact angle was identical regardless of the type of the raw material resin. In addition, where necessary, the photoreceptor surface coated with a solid lubricant was directly measured, instead of using measurement samples as described above.
The method by which to measure the frictional coefficient of a solid lubricant will now be explained. The frictional coefficient was measured using the Euler belt method.
The digital force gauge was then pulled to the right in
μ=ln (F/W)/(π/2) (3)
where, ln: a natural logarithm symbol
When measuring the frictional coefficient μ1 between the photoreceptor PH and the toner TN, a cylinder comprising polycarbonate resin, which is the raw material of the photoreceptor, was used as the cylindrical unit 56, a belt comprising a mixture of polycarbonate resin and polybutyrate resin and having on the surface thereof a toner resin layer comprising a styrene-acrylic resin mixture was used as the belt 57, and a solid lubricant was applied to the surfaces of the cylindrical unit 56 and the belt 57. Measurement was then taken with the application of weight W of the plumb bob 58=100 g.
When the frictional coefficient μ2 between toner TN particles was measured, a toner resin layer was formed on the surface of the cylindrical unit 56, a belt having a toner resin layer on the surface thereof was used as the belt 57, and a solid lubricant was applied to the surfaces of both toner resin layers. Measurement was then performed with the application of weight W of the plumb bob 58=100 g.
During measurement of the frictional coefficient μ1 and of the frictional coefficient μ2 between toner TN particles, when the same lubricant was used, the same reading was obtained for both coefficients. In other words, the frictional coefficient μ did not depend on the type of resin on which the solid lubricant was applied, but on the type of solid lubricant used.
[Type of Solid Lubricant and Combination]
The types of solid lubricant M1 that may be used as the first lubricant applied to the photoreceptor surface and the types of solid lubricant M2 that may be used as the second lubricant that are added to the toner, as well as combinations thereof, will now be explained. Both the solid lubricant M1 applied to the photoreceptor surface and the solid lubricant M2 added to the toner must have low surface energy and be chemically inactive and thermally stable.
Specifically, higher fatty acid metallic salts (metallic soaps) such as zinc stearate (ST-Zn), magnesium stearate (ST-Mg) and calcium stearate (ST-Ca) or fluorinated polymers such as PTFE, ETFE and polyvinyliden fluoride are the appropriate substances.
As is clear from
As explained above, it is desirable for the pure water contact angle θ1 of the solid lubricant M1 applied to the photoreceptor and the pure water contact angle θ2 of the solid lubricant M2 added to the toner to have the relationship θ1≦θ2, and that the frictional coefficient μ1 of the solid lubricant M1 and the frictional coefficient μ2 of the solid lubricant M2 have the relationship μ1<μ2.
Therefore, with reference to
In addition, it is preferred that the amount of solid lubricant M2 added to the developer range between 0.01% and 5.0% by weight, and that the solid lubricant M1 be solidified to enable it to be taken up by the application brush and applied to the photoreceptor.
In this measurement, zinc stearate (ST-Zn) was selected as the solid lubricant M1 applied to the photoreceptor surface. For the solid lubricant M2 added to the developer, Sample 1 comprising toner having a 4.5 μm particle diameter to which 0.3% by weight of aluminum stearate (ST-Al) was added, Sample 2 comprising toner having a 4.5 μm particle diameter to which 0.5% by weight of magnesium stearate (ST-Mg) was added, and Sample 3 comprising toner having a 4.5 μm particle diameter to which no solid lubricant was added were prepared.
A test latent image formed on the photoreceptor surface was developed using Samples 1-3, respectively, and the pure water contact angle of the photoreceptor surface was measured. This measurement was repeated while the number of rotations of the photoreceptor was changed.
As is clear from
Zinc stearate (ST-Zn) and calcium stearate (ST-Ca) were then applied respectively to the photoreceptor surface as the solid lubricant M1 and the rate of reduction in the pure water contact angle of the photoreceptor surface was measured when various metallic soaps (stearic salts) or stearic acid were used as the solid lubricant M2 added to the toner. The results of these tests are shown in
The rate of contact angle reduction here is the amount of reduction of the photoreceptor surface contact angle relative to the initial contact angle when development was carried out using a photoreceptor to which a solid lubricant M1 was applied. The measurement was taken when the photoreceptor had rotated ten times.
Where zinc stearate (ST-Zn) or calcium stearate (ST-Ca) was used as the solid lubricant M1, it was observed that there was little reduction in the photoreceptor surface contact angle and very little of the solid lubricant M1 applied to the photoreceptor surface was peeled off if the solid lubricant M1 contact angle θ1 and the solid lubricant M2 contact angle θ2 had the relationship θ1≦θ2.
The minimum contact pressure was then measured when zinc stearate (ST-Zn) was applied to the photoreceptor surface as the solid lubricant M1, and various metallic soaps (stearic salts) were respectively used as the solid lubricant M2 added to the toner. The results of these tests are shown in
In these tests, ‘the minimum contact pressure’ was the smallest pressure that the cleaning blade needed to exert in order to clean off the toner. Specifically, the image forming apparatus shown in
It was shown that where the solid lubricant M2 comprised sodium stearate (ST-Na), stearic acid (ST-H) or aluminum stearate (ST-Al), which have a larger frictional coefficient than zinc stearate (ST-Zn), i.e., where the frictional coefficients had the relationship μ1<μ2, a lower minimum contact pressure was obtained than when no solid lubricant M2 was added to the toner, indicating a better cleaning capability.
The present invention was described using as an example a commonly used image forming apparatus in which the present invention was applied, but needless to say, the present invention can be applied in a monochrome image forming apparatus, a color image forming apparatus, a printer, a facsimile machine or a multifunction peripheral combining the functions of these apparatuses.
The embodiments disclosed herein are examples in every aspect and do not limit the present invention in any respect. The range of the present invention is indicated not by the description provided above but by the claims, and is intended to include constructions equivalent to the claims, as well as all changes and modifications within the scope thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-247695 | Aug 2004 | JP | national |
This disclosure is based upon Japanese Patent Application No. 2004-247695, filed Aug. 27, 2004, the contents of which are incorporated herein by reference.
