Image forming apparatus with an improved density adjustment unit

In an image forming apparatus, a density adjustment unit causes to form a cleaner toner image on a surface of an image carrier and remove the cleaner toner image for density adjustment, and causes to form a test pattern after removing the cleaner toner image. The cleaner toner image is formed to cover a whole area on which the test pattern may be formed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application relates to and claims priority rights from Japanese Patent Applications: No. 2011-098982, filed on Apr. 27, 2011 and No. 2011-066766, filed on Mar. 24, 2011, the entire disclosures of which are hereby incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to image forming apparatuses.

2. Description of the Related Art

In an electrophotographic image forming apparatus such as a printer, copier, facsimile machine, or multifunction peripheral thereof, the toner amount (i.e. toner density) of a toner image formed on a photoconductor (e.g. photoconductor drum) directly affects on the quality of a print image. The toner density changes over time according to some conditions such as usage environment and usage time length. For instance, while keeping a developing bias of the photoconductor constant, changing an electrostatic charging characteristic due to its atmosphere may result in reducing toner supplied from a development unit to the photoconductor.

Since a cleaning blade for cleaning the photoconductor and an intermediate transfer member contact the photoconductor and grind its photoreceptor layer, increasing the number of times of image forming results in decreasing the thickness of the photoreceptor layer, and consequently surface electrostatic potential of the photoconductor is difficult to keep constant. Gradually decreasing the surface electrostatic potential results in gradually increasing the toner density of a toner image and degrading image quality.

In order to solve this problem, a toner image for density adjustment (hereinafter called “test pattern”) is carried on a predetermined area of the intermediate transfer member to which a toner image formed on the photoconductor is primary transferred, the density of the test pattern is measured with a sensor, and the toner density is adjusted by controlling process conditions such as developing bias according to the measurement result. For example, the test pattern is formed so as to contain areas with gradually different toner densities.

The reflectance of a surface material of the intermediate transfer member affects on the measurement result of the test pattern. Contacting a cleaning member, a transfer roller for secondarily transferring a toner image on a record medium such as a paper sheet and the like to the surface of the intermediate transfer member results in staining and scratching the surface and adhering a toner external additive to the surface. Therefore, a surface condition of the intermediate transfer member is measured with the sensor before forming the test pattern, and the toner density is adjusted with taking into account the detection value of the sensor.

SUMMARY OF THE INVENTION

A two-component developer used for development of a toner image in the image forming apparatus has toner and carrier, and an external additive such as titanium oxide is attached to the toner.

Due to a developing bias and a primary transfer bias applied when a toner image is developed, the external additive separates and flies from the toner, and adheres on the surface of the intermediate transfer member. Therefore, the external additive adheres on the surface material where the test pattern is formed. As the result, the surface condition of the intermediate transfer member before forming the test pattern may not be measured correctly. When the surface condition of the intermediate transfer member before forming the test pattern is not measured correctly, the toner density is also not measured correctly, and consequently the toner density adjustment is not performed correctly.

Since various shapes of toner images are transferred on the intermediate transfer member in printing before the density adjustment, and residual toner on the intermediate transfer member is removed after transferring each of the toner images to a print paper sheet, the amount of the external additive which adheres on the intermediate transfer member is not uniform. Therefore, it is difficult to correctly determine the surface condition of the intermediate transfer member before forming the test pattern.

FIGS. 6A to 6Fshow an instance of surface condition change of an intermediate transfer belt101.

An external additive adheres on a whole surface of the intermediate transfer belt101as shown inFIG. 6A. A test pattern which contains respective patch images111to113of densities as shown inFIG. 6Bis transferred on the intermediate transfer belt101.

After measuring the toner densities of the patch images111to113with a sensor, when the test pattern is removed, an external additive on the area where the test pattern is formed is removed together with the test pattern. Consequently, as shown inFIG. 6C, areas121appear with a small amount of the external additive.

In the next density adjustment, if a test pattern is transferred when the intermediate transfer belt101is placed as shown inFIG. 6D, then as shown inFIG. 6E, the test pattern is transferred on a position different from the areas121on which the test pattern was transferred in the previous density adjustment.

The toner pattern shown inFIG. 6Eis removed after measuring the toner densities, and therefore, as shown inFIG. 6F, the amount of the external additive on areas131where this toner pattern was removed is different from that on the areas122where the previous toner pattern had been removed.

This invention has been conceived in order to solve this problem, and provide an image forming apparatus which correctly performs toner density adjustment by measuring correct toner density even if an external additive is added to toner.

The Present Invention Solves this Problem as Follows.

An image forming apparatus according to an aspect of the present invention has an image carrier that carries a toner image; a sensor that detects measurement light from a test pattern on a surface of the image carrier for density adjustment; and a density adjustment unit that causes to form a cleaner toner image on the surface of the image carrier and remove the cleaner toner image for the density adjustment, and causes to form the test pattern after removing the cleaner toner image. The cleaner toner image covers a whole area on which the test pattern may be formed.

When the cleaner toner image is removed by a cleaning member, an external additive which adheres on an area where the cleaner toner image is formed is removed together with the cleaner toner image, and therefore, the amount of the external additive on an area which will carry the test pattern is maintained to be uniform. Consequently, correct toner density is measured even if an external additive is attached to toner, and toner density adjustment is performed correctly.

An image forming apparatus according to another aspect of the present invention has a handler belt that conveys a record medium to a photoconductor and carries a test pattern for density adjustment; a sensor that detects measurement light from the test pattern on a surface of the handler belt; and a density adjustment unit that causes to form a cleaner toner image on the surface of the handler belt and remove the cleaner toner image for the density adjustment, and causes to form the test pattern after removing the cleaner toner image. The cleaner toner image covers a whole area on which the test pattern may be formed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, embodiments according to aspects of the present invention will be explained with reference to the drawings.

FIG. 1is a side view that partially shows a mechanical internal configuration of an image forming apparatus in an embodiment according to this invention. The image forming apparatus is an apparatus having a printing function such as a printer, facsimile machine, copier, or multifunction peripheral.

The image forming apparatus in this embodiment has a tandem-type color development device. This color development device has photoconductor drums1ato1d, exposure devices2ato2d, and development units3ato3d. The photoconductor drums1ato1dare four color photoconductors of Cyan, Magenta, Yellow and Black. The exposure devices2ato2dare devices that form electrostatic latent images by irradiating laser light to the photoconductor drums1ato1d. Each of the exposure devices2ato2dhas a laser diode as a light source of the laser light, optical elements (such as lens, mirror and polygon mirror) that guide the laser light to the photoconductor drum1a,1b,1c, or1d.

Further, in the periphery of each of the photoconductor drums1ato1d, a charging unit such as a scorotron, a cleaning device, a static electricity eliminator and the like are disposed. The cleaning device removes residual toner on the photoconductor drum1a,1b,1c, or1dafter primary transfer. The static electricity eliminator eliminates static electricity on the photoconductor drum1a,1b,1c, or1dafter primary transfer.

Toner containers contain toner of four colors: Cyan, Magenta, Yellow and Black, and are attached to the development units3ato3d, respectively. In the development units3ato3d, the toner is supplied from the toner containers, and this toner and carrier compose a developer. An external additive such as titanium oxide is attached to the toner. The development units3ato3dform toner images by attaching the toner to electrostatic latent images on the photoconductor drums1ato1d.

The photoconductor drum1a, the exposure device2aand the development unit3aperform development of Magenta. The photoconductor drum1b, the exposure device2band the development unit3bperform development of Cyan. The photoconductor drum1c, the exposure device2cand the development unit3cperform development of Yellow. The photoconductor drum1d, the exposure device2dand the development unit3dperform development of Black.

An intermediate transfer belt4is a loop-shaped image carrier, and contacts the photoconductor drums1ato1d. Toner images on the photoconductor drums1ato1dare primarily transferred onto the intermediate transfer belt4. The intermediate transfer belt4is an intermediate transfer member. The intermediate transfer belt4is hitched round driving rollers5, and rotates by driving force of the driving rollers5towards the direction from the contact position with the photoconductor drum1dto the contact position with the photoconductor drum1a.

A transfer roller6makes a paper sheet being conveyed contact the transfer belt4, and secondarily transfers the toner image on the intermediate transfer belt4to the paper sheet. The paper sheet on which the toner image has been transferred is conveyed to a fixer9, and consequently, the toner image is fixed on the paper sheet.

A roller7has a cleaning brush, and removes residual toner on the intermediate transfer belt4by contacting the cleaning brush to the intermediate transfer belt4after transferring the toner image to the paper sheet. In density adjustment, the roller7also removes an external additive with toner carried on an area where the external additive adheres on the intermediate transfer belt4.

A sensor8irradiates light to the intermediate transfer belt4and detects its reflection light. In density adjustment, the sensor8irradiates light to a predetermined area on the intermediate transfer belt4, detects its reflection light (measurement light), and outputs an electrical signal corresponding to the detected intensity of the reflection light.

FIG. 2is a block diagram that shows an electronic configuration of the image forming apparatus in this embodiment according to this invention. InFIG. 2, a print engine11is a processing circuit that controls a driving source which drives the aforementioned rollers, a bias induction circuit which induces developing biases and primary transfer biases, and the exposure devices2ato2din order to feed a paper sheet, print an image on the paper sheet, and output the paper sheet. The developing biases are applied between the photoconductor drums1ato1dand the development units3ato3d, respectively. The primary transfer biases are applied between the photoconductor drums1ato1dand the intermediate transfer belt4, respectively.

In this embodiment, the print engine11has a density adjustment unit21. For density adjustment, the density adjustment unit21causes to develop a cleaner toner image on the photoconductor drums1ato1d, transfer the cleaner toner image onto a surface of the intermediate transfer belt and remove the cleaner toner image. The cleaner toner image is formed to cover a whole area on which a test pattern may be transferred. After removing the cleaner toner image, the density adjustment unit21causes to develop the test pattern on the photoconductor drums1ato1dand transfer the test pattern onto the intermediate transfer belt4. In this embodiment, the cleaner toner image is solidly formed along the whole length of the intermediate transfer belt4with a predetermined width. The density adjustment unit21identifies toner density of the test pattern on a measurement area, and performs the density adjustment based on the toner density. The toner density is identified from (a) an output value of the sensor8corresponding to the measurement light from the measurement area before the test pattern is formed after the cleaner toner image is removed, and (b) an output value of the sensor8corresponding to the measurement light from the measurement area which carries the formed test pattern.

FIG. 3is a diagram which explains density measurement with the sensor8inFIG. 1.

As shown inFIG. 3, the sensor8has a light source51which emits a light beam, a beam splitter52on the light emitting side, a light receiving element53on the light emitting side, a beam splitter54on the light receiving side, a first light receiving element55, and a second light receiving element56.

For instance, the light source51is a light emitting diode. The beam splitter52transmits a P-polarized component and reflects an S-polarized component in a beam from the light source51. The light receiving element53on the light emitting side is, for instance, a photodiode, and detects the S-polarized component from the beam splitter52, and outputs an electrical signal corresponding to the detected intensity of the S-polarized component. This signal is used for stabilizing control of the light source51.

The P-polarized component light transmitted through the beam splitter52on the light emitting side is incident to a surface (i.e. either a toner image41or the surface material) of the intermediate transfer belt4and reflects. This reflection light contains a specilar reflection component and a diffuse reflection component. The specilar reflection component is P-polarized.

The beam splitter54transmits a P-polarized component (i.e. the specilar reflection component) and reflects an S-polarized component in the reflection light. The first light receiving element55is, for instance, a photodiode, and detects the P-polarized component from the beam splitter54, and outputs an electrical signal corresponding to the detected intensity of the P-polarized component. The second light receiving element56is, for instance, a photodiode, and detects the S-polarized component from the beam splitter54, and outputs an electrical signal corresponding to the detected intensity of the S-polarized component.

The density adjustment unit21calculates toner density from an output of the first light receiving element55and an output of the second light receiving element56with taking into account a correction amount of the toner density.

Hereinafter, toner density adjustment in this image forming apparatus is explained.FIG. 4is a flowchart that explains toner density adjustment in the image forming apparatus shown inFIGS. 1 and 2.FIGS. 5A to 5Eshow an instance of surface condition change of the intermediate transfer belt4in the image forming apparatus shown inFIGS. 1 and 2.

After starting rotation of the intermediate transfer belt4by the driving rollers5, the density adjustment unit causes to develop a cleaner toner image61on the photoconductor drums1ato1d, and transfer the cleaner toner image61to the intermediate transfer belt4(Step S1, seeFIG. 5A).

The cleaner toner image61transferred on the intermediate transfer belt4is removed by the roller7(Step S2). By removing the cleaner toner image61, the amount of an external additive which adheres on an area71which carried the cleaner toner image61becomes uniform (seeFIG. 5B).

The density adjustment unit21obtains an output value of the sensor8by sampling the output of the sensor8which detects the reflection light from a predetermined area in the area71where the cleaner toner image61was carried and removed on the surface of the intermediate transfer belt4(Step S3). Since a test pattern has not been formed yet, this output value corresponds to the reflectance of a surface material of the intermediate transfer belt4.

The density adjustment unit21causes to form a test pattern on the predetermined area in the area71where the cleaner toner image61was carried (Step S4, seeFIG. 5C). The test pattern contains respective patch images81to83corresponding to different densities. Partial toner images with different colors are formed on the photoconductor drums1ato1d, and transferred onto the intermediate transfer belt4to form the test pattern. For example, the test pattern is formed so as to contain areas with different toner densities of each color.

The density adjustment unit21obtains an output value of the sensor8by sampling the output of the sensor8which detects the reflection light from the test pattern (Step S5).

As mentioned above, after sampling the output of the sensor8which detects the reflection light from the test pattern area (i.e. the area on which the test pattern is transferred) before and after toner development of the test pattern, the density adjustment unit21calculates toner densities (step S6) of the test pattern from the difference between the output values of the sensor8before and after transferring the test pattern.

For instance, the density adjustment unit21calculates a toner density CTD according the following formula.

Here, P is the output value of the first light receiving element55(i.e. the spacilar reflection component) corresponding to the test pattern area after forming the test pattern; S is the output value of the second light receiving element56(i.e. the diffuse reflection component) corresponding to the test pattern area after forming the test pattern; P0 is the output value of the first light receiving element55corresponding to its dark voltage; S0 is the output value of the second light receiving element56corresponding to its dark voltage; Pg is the output value of the first light receiving element55(i.e. the spacilar reflection component) corresponding to the test pattern area before forming the test pattern; and Sg is the output value of the second light receiving element56(i.e. the diffuse reflection component) corresponding to the test pattern area before forming the test pattern.

After measuring the toner densities according to the aforementioned manner, the density adjustment unit21changes process conditions such as developing bias with taking into account the toner densities in order to adjust the toner image density (Step S7).

The test pattern on the intermediate transfer belt4is removed by the roller7(seeFIG. 5D). After removing the test pattern, even if an external additive adheres on the area71on which the cleaner toner image61was carried (seeFIG. 5E), in the next density adjustment, a cleaner toner image61is transferred and removed in the same manner, and consequently at the density adjustment, the amount of the external additive in the area71is maintained to be uniform.

According to the aforementioned embodiment, the density adjustment unit21causes to transfer the cleaner toner image61onto a surface of the intermediate transfer belt4and remove the cleaner toner image61for the density adjustment. The cleaner toner image61is formed to cover a whole area on which a test pattern may be transferred. After removing the cleaner toner image61, the density adjustment unit21causes to transfer the test pattern onto the intermediate transfer belt4.

When the cleaner toner image61is removed by the roller7, an external additive which adheres on an area where the cleaner toner image61is formed is removed together with the cleaner toner image61, and therefore, the amount of the external additive on an area which will carry the test pattern is maintained to be uniform. Consequently, correct toner density is measured even if an external additive is attached to toner, and toner density adjustment is performed correctly.

The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art.

For example, although the image forming apparatus of the aforementioned embodiment is an indirect-transfer image forming apparatus (i.e. having an intermediate transfer member), this invention can also be applied to a direct-transfer image forming apparatus. In the direct-transfer image forming apparatus, photoconductor drums carry the cleaner toner image and the external additive is removed with the cleaner toner image.

Toner density adjustment of the direct-transfer image forming apparatus is specifically explained with reference toFIG. 7.FIG. 7is a side view that partially shows a mechanical internal configuration of a direct-transfer image forming apparatus in an embodiment according to this invention. In this image forming apparatus, a development device has a photoconductor drum201, an exposure device202, and a development unit203. The exposure device202forms an electrostatic latent image by irradiating laser light to the photoconductor drum201. A toner container which contains black toner is attached to the development unit203. In the development unit203, the toner is supplied from the toner container and this toner and carrier compose a developer. An external additive such as titanium oxide is attached to the toner. The development unit203forms a toner image by attaching the toner to an electrostatic latent image on the photoconductor drum201. In the direct-transfer image forming apparatus, a handler belt204rotates by driving force from driving rollers205, and conveys a paper sheet (i.e. a record medium) to the photoconductor drum201. A transfer roller206makes the paper sheet contact the photoconductor drum201, and transfers a toner image on the photoconductor drum201to the paper sheet. The paper sheet on which the toner image has been transferred is conveyed to a fixer209, and consequently, the toner image is fixed on the paper sheet. As mentioned above, printing is performed by a direct-transfer system.

In the direct-transfer image forming apparatus, for toner density adjustment, a test pattern is transferred from the photoconductor drum201to the handler belt204. This image forming apparatus has a printer engine similar to the print engine11. A sensor208is the same as the sensor8, and irradiates light to a predetermined area on the handler belt204, detects its reflection light (measurement light), and outputs an electrical signal corresponding to the detected intensity of the reflection light. This signal is output to a density adjustment unit in the printer engine the same as the density adjustment unit21. After measuring toner density, the test pattern which has been transferred on the handler belt204is collected by the photoconductor drum201, and removed from the photoconductor drum201by a cleaning device207.

In the direct-transfer image forming apparatus, as well as the indirect-transfer image forming apparatus, the density adjustment unit causes to transfer the cleaner toner image onto a surface of the handler belt204. This cleaner toner image is formed to cover a whole area on which the test pattern may be transferred. The density adjustment unit causes the photoconductor drum201and the cleaning device207to remove the cleaner toner image on the handler belt204. After removing the cleaner toner image, the density adjustment unit causes the handler belt204to carry the test pattern. The density adjustment unit identifies toner density of the test pattern on a measurement area, and performs the density adjustment based on the toner density. The toner density is identified from (a) an output value of the sensor208corresponding to the measurement light from the measurement area before the test pattern is formed after removing the cleaner toner image, and (b) an output value of the sensor208corresponding to the measurement light from the measurement area which carries the formed test pattern. Therefore, the direct-transfer image forming apparatus carries the cleaner toner image on the handler belt204, and collects the cleaner toner image from the handler belt204to the photoconductor drum201. An external additive is removed with the cleaner toner image.

Furthermore, in the aforementioned embodiment, the cleaner toner image may be developed with toner of one color. Furthermore, in the aforementioned embodiment, the cleaner toner image may be developed with toner of different colors at different times of density adjustment.

Furthermore, this invention can be applied to an indirect-transfer monochrome image forming apparatus and a direct-transfer color image forming apparatus.

Furthermore, toner on the intermediate transfer belt4is removed by the roller7in the aforementioned embodiment. Alternatively, the toner may be removed by the photoconductor drums1ato1d.

Furthermore, instead of the output of the sensor8before forming the test pattern, the output of the sensor8corresponding to one or two areas before and/or after the formed test pattern (i.e. the output of the sensor8corresponding to one or two areas on which the test pattern is not formed) may be used.