Image forming apparatus and image forming method using toners with different softening points

An image forming apparatus includes image forming units forming toner images with first toners having different colors and a second toner having a different softening point from the first toners, an image transfer unit superposing the toner images one on top of another and transferring the superposed toner images onto a recording medium at least such that the toner image formed of the second toner is at the top of the superposed toner images, an image fixing unit fixing the transferred toner images, and an image controller that causes the image forming unit that uses the second toner to form the toner image over at least part of an image area and also causes the image transfer unit to perform the transfer such that the toner image formed of the second toner is at the top of the superposed toner images in the at least part of the image area.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2011-201764 filed Sep. 15, 2011.

BACKGROUND

(i) Technical Field

The present invention relates to an image forming apparatus and an image forming method.

(ii) Related Art

Electrophotographic image forming apparatuses are commonly known.

SUMMARY

According to an aspect of the invention, there is provided an image forming apparatus including a plurality of image forming units that form respective toner images by using respective toners including a plurality of first toners having different colors and a second toner having a different softening point from the first toners, an image transfer unit that has a mode in which the toner images formed by the image forming units are superposed one on top of another and are transferred onto a recording medium at least such that one of the toner images that is formed of the second toner is at the top of the superposed toner images, an image fixing unit that fixes the toner images transferred onto the recording medium by the image transfer unit on the recording medium, and an image controller that causes, when the image transfer unit is in the mode, one of the image forming units that uses the second toner to form the toner image over at least part of an image area and also causes the image transfer unit to transfer the toner images onto the recording medium such that the toner image formed of the second toner is at the top of the superposed toner images in the at least part of the image area.

DETAILED DESCRIPTION

Exemplary embodiments of the present invention will now be described.

FIG. 1is a schematic diagram of an image forming apparatus100according to an exemplary embodiment of the present invention.

The image forming apparatus100includes a paper tray10at the bottom thereof. Pieces of papers P are stacked on the paper tray10. An image is formed as follows. A piece of paper P is fed from the paper tray10by a pickup roller11and is transported by transport rollers12along a transport path121in a direction indicated by arrow A up to standby rollers13, where the timing of the subsequent transport operation is adjusted. The transport operation from the standby rollers13will be described separately below.

The image forming apparatus100also includes five image forming engines20CL,20Y,20M,20C, and20K. The image forming engines20CL,20Y,20M,20C, and20K form toner images by using a transparent/clear (CL) toner, a yellow (Y)-colored toner, a magenta (M)-colored toner, a cyan (C)-colored toner, and a black (K)-colored toner, respectively. The softening points of the Y-, M-, C-, and K-colored toners employed in the image forming apparatus100according to the present exemplary embodiment are higher than those of toners employed in related-art image forming apparatuses of the same type as the image forming apparatus100. The softening point of the CL toner of the image forming apparatus100is lower than those of the Y-, M-, C-, and K-colored toners. This feature will be described in detail separately below. The five image forming engines20CL,20Y,20M,20C, and20K all have the same configuration except the toners. Hence, among the five image forming engines20CL,20Y,20M,20C, and20K, the image forming engine20CL will be described hereafter.

The image forming engine20CL includes a photoconductor21CL, and also includes a charger22CL, an exposure unit23CL, a development unit24CL, a first transfer member25CL, and a cleaner26CL that are provided around the photoconductor21CL. The first transfer member25CL is provided across a below-described intermediate transfer belt31from the photoconductor21CL.

The photoconductor21CL has a round cylindrical shape. The photoconductor21CL is electrically charged while rotating in a direction indicated by arrow B. The electric charge generated on the surface of the photoconductor21CL is released when the photoconductor21CL is exposed to light. Thus, the photoconductor21CL carries an electrostatic latent image.

The charger22CL charges the surface of the photoconductor21CL to a certain potential.

The exposure unit23CL receives image data (in the case of the image forming engine20CL according to the present exemplary embodiment that uses the CL toner, image data representing a uniform image, which will be described in detail separately below) that is input thereto from a below-described controller50. The exposure unit23CL emits an exposure light beam231CL modulated in accordance with the image data received. The photoconductor21CL having been charged by the charger22CL is exposed to the exposure light beam231CL emitted from the exposure unit23CL, whereby an electrostatic latent image is formed on the surface of the photoconductor21CL.

The electrostatic latent image formed on the surface of the photoconductor21CL through the exposure with the exposure light beam231CL is developed by the development unit24CL, whereby a toner image (in the case of the image forming engine20CL, a uniform toner image formed of the clear (CL) toner) is formed on the photoconductor21CL.

The development unit24CL includes a casing241containing developer composed of a toner and a carrier, and also includes, in the casing241, two augers242that agitate the developer and a development roller243that transports the developer to a position facing the photoconductor21CL. When the electrostatic latent image formed on the photoconductor21CL is developed, a bias voltage is placed across the development roller243. By the effect of the bias voltage, the toner in the developer adheres to the electrostatic latent image formed on the photoconductor21CL. Thus, a toner image is formed.

The toner image on the photoconductor21CL obtained through the development performed by the development unit24CL is first-transferred onto the intermediate transfer belt31by the first transfer member25CL.

Some toner remaining on the photoconductor21CL after the first transfer is removed from the photoconductor21CL by the cleaner26CL.

The intermediate transfer belt31is an endless belt that is stretched around a driving roller32and other plural rollers33. The intermediate transfer belt31rotates in a direction indicated by arrow C.

Toner images formed by the respective image forming engines20CL,20Y,20M,20C, and20K using the CL, Y, M, C, and K toners are first-transferred onto the intermediate transfer belt31in such a manner as to be sequentially superposed one on top of another, with the toner image formed by the image forming engine20CL using the CL toner lying at the bottom. The superposed toner images are transported to a second transfer position, where a second transfer member34is provided. Simultaneously, the piece of paper P having been transported to the standby rollers13is transported to the second transfer position. Then, the superposed toner images on the intermediate transfer belt31are second-transferred onto the piece of paper P by the second transfer member34. As a result of second-transferring the superposed toner images from the intermediate transfer belt31to the piece of paper P, the order of toner images is reversed. That is, the toner image formed of the CL toner is at the top of the superposed toner images on the piece of paper P. The piece of paper P having the superposed toner images second-transferred thereonto is further transported by a transport belt14in a direction indicated by arrow D to a fixing unit40, where the superposed toner images are fixed on the piece of paper P with pressure and heat applied by the fixing unit40. Thus, an image formed of the superposed toner images thus fixed is provided on the piece of paper P. The piece of paper P having the fixed image is transported by a transport belt15in a direction indicated by arrow E and is then discharged to the outside of the image forming apparatus100.

After the superposed toner images are second-transferred to the piece of paper P by the second transfer member34, the intermediate transfer belt31further rotates, whereby some toner remaining on the surface of the intermediate transfer belt31is removed by a cleaner35.

The image forming apparatus100includes the controller50and an operation/display unit501. Image data Cin is input to the controller50. The controller50processes the image data Cin in accordance with control data that is input thereto together with the image data Cin or in accordance with an instruction made through the operation/display unit501, so that the image data Cin is converted into pieces of image data with which desired exposure light beams231CL,231Y,231M,231C, and231K can be emitted from the respective exposure units23CL,23Y,23M,23C, and23K. The pieces of image data obtained through the above conversion are transmitted to the respective exposure units23CL,23Y,23M,23C, and23K. The exposure units23CL,23Y,23M,23C, and23K apply the exposure light beams231CL,231Y,231M,231C, and231K to the photoconductors21CL,21Y,21M,21C, and21K, respectively, in accordance with the pieces of image data on the CL, Y, M, C, and K colors that are input thereto. The processing operation performed by the controller50will be described in detail separately below.

The operation/display unit501functions as a man-machine interface that displays various messages about the image forming apparatus100to the user. The operation/display unit501also displays various operation buttons and accepts instructions concerning image formation that are made through the buttons by the user.

FIG. 2is a schematic diagram of the fixing unit40included in the image forming apparatus100illustrated inFIG. 1.

The fixing unit40includes an endless heat belt41that rotates in a direction indicated by arrow F, a pressure roller42that rotates in a direction indicated by arrow G, a heat roller43provided on the inner side of the heat belt41and across the heat belt41from the pressure roller42, and a stretch roller44provided farthest from the pressure roller42. The fixing unit40also includes a stretch roller45, a position control roller46that prevents the heat belt41from meandering, and a stretch roller47. The stretch rollers45and47and the position control roller46are provided between the heat roller43and the stretch roller44. The stretch roller47is pressed against the heat belt41from the outer side of the heat belt41.

The heat roller43and the two stretch rollers44and47are provided thereinside with respective heaters431,441, and471. The heat belt41is heated by the heat roller43and the stretch rollers44and47. Temperature sensors481and482are provided on the inner and outer sides, respectively, of the heat belt41and measure the temperature of the heat belt41. The heaters431,441, and471are controlled so that the heat belt41is heated to a desired temperature.

A release pad49is provided adjacent to the heat roller43. The release pad49is positioned on the inner side of the heat belt41and on the downstream side, in a paper transport direction, with respect to the heat roller43.

The pressure roller42includes a core421and an elastic layer422made of rubber and provided around the core421. A temperature sensor51that measures the temperature of the pressure roller42is provided near the pressure roller42. A fan52that cools the pressure roller42is provided below the pressure roller42. The rotation of the fan52is controlled such that the temperature of the pressure roller42measured by the temperature sensor51is maintained at a certain level.

The piece of paper P having the superposed toner images second-transferred thereonto from the intermediate transfer belt31by the second transfer member34illustrated inFIG. 1is transported by the transport belt14in the direction of arrow D into the nip between the heat belt41and the pressure roller42, advances through the nip while being pressed against the heat belt41by the pressure roller42, is released from the heat belt41with the aid of the release pad49, and is further transported by the transport belt15in the direction of arrow E. The superposed toner images on the piece of paper P are melted while being transported through the fixing unit40and are then hardened, whereby an image formed of the superposed toner images thus fixed is provided on the piece of paper P. The piece of paper P having passed through the fixing unit40is discharged to the outside of the image forming apparatus100illustrated inFIG. 1, as described above.

The image forming apparatus100illustrated inFIG. 1employs the Y-, M-, C-, and K-colored toners having softening points higher than those of related-art toners and also employs the clear (CL) toner having a softening point as low as those of related-art toners, as described above. The reason for this will now be described.

FIGS. 3A to 3Care graphs illustrating the gloss level of the image formed on paper versus the temperature of the heat belt41of the fixing unit40in respective cases of different softening points of toners.FIG. 4is a graph illustrating the gloss level versus the softening point of toner.

The present exemplary embodiment concerns measurements conducted with Flowtester CFT-500D (manufactured by Shimadzu Corporation), with a die having a 0.5 mm inside diameter, with a pressure load of 0.98 MPa, and at a rate of temperature rise of 1° C./min. In the measurements, the softening point of toner is defined as the temperature corresponding to half the descent speed (the temperature corresponding to half the distance from a point where sample toner that is caused to melt starts to flow to a point where the melted sample toner stops flowing). The gloss level is measured at 60° with respect to the image formed on a piece of paper called “Mirrorkote Platinum” weighing 256 gsm. Referring toFIG. 4, toners whose softening points are about 109° C. are designated as low-softening-point toners, toners whose softening points are about 115° C. are designated as moderate-softening-point toners, and toners whose softening points are about 125° C. are designated as high-softening-point toners.

FIG. 3Aillustrates the 60° gloss level (vertical axis) versus the temperature of the heat belt41of the fixing unit40(horizontal axis) in several cases where images of various colors such as primary colors (Y, M, C, and K) and secondary colors (red (R), green (G), and blue (B)) are formed on the paper by using low-softening-point toners. Similarly toFIG. 3A,FIG. 3Billustrates other cases where moderate-softening-point toners are used, andFIG. 3Cillustrates yet other cases where high-softening-point toners are used. The vertical lines illustrated inFIGS. 3A to 3Ceach represent the lowest point of a temperature range (of the heat belt41) in which satisfactory fixing quality is guaranteed. Low-softening-point toners melt at relatively low temperatures, and the lowest point of the temperature range that guarantees satisfactory fixing quality in the cases of low-softening-point toners is 145° C. as illustrated inFIG. 3A. In the cases of moderate-softening-point toners (FIG. 3B) and high-softening-point toners (FIG. 3C), the lowest point of the temperature range that guarantees satisfactory fixing quality is 150° C.

Comparing the graphs illustrated inFIGS. 3A to 3C, the gloss level of the image is higher in the cases where low-softening-point toners are used and is lower in the cases where high-softening-point toners are used.

The dotted curve of the graph illustrated inFIG. 4represents the average 60° gloss level versus the softening point among several cases where images are formed by using various toners whose softening points are different and at the respective lowest points of the temperature range that guarantee satisfactory fixing quality corresponding to the respective softening points.

The graph inFIG. 4shows that the gloss level becomes lower as the softening point of toner becomes higher within a range in which the toner is practically usable.

FIGS. 5A and 5Bare graphs illustrating the 60° gloss level versus the area coverage (%) of the CL toner in various cases where uniform images having primary colors (Y, M, C, and K), secondary colors (R, G, and B), and the like formed of colored toners are each covered with a uniform image formed of the CL toner.

FIG. 5Aillustrates cases in each of which both the colored toner and the CL toner have low softening points.FIG. 5Billustrates cases in each of which the colored toner has a high softening point whereas the CL toner has a low softening point.

Referring toFIG. 5A, in each case where both the colored toner and the CL toner have low softening points, the gloss level does not change greatly regardless of the area coverage (%) of the CL toner provided at the top of the image, that is, whether or not the CL toner is present at the top of the image.

In contrast, referring toFIG. 5B, in each case where the colored toner (Y, M, C, or K) has a high softening point and the CL toner has a low softening point, the gloss level is low when the CL toner is not present at the top of the image or when the CL toner is provided at the top of the image with a low area coverage (about 0% to 20%), whereas the gloss level is as high as those in the cases illustrated inFIG. 5Awhen the CL toner is provided at the top of the image with a high area coverage (about 70% to 100%). It is also shown that, within a range in which the area coverage of the CL toner provided at the top of the image is moderate (about 30% to 60%), the gloss level becomes higher with the increase in the area coverage of the CL toner.

In view of the above, the gloss level of the image is changeable by employing a high-softening-point colored toner (Y, M, C, or K) and a low-softening-point CL toner and selecting whether or not to provide the low-softening-point CL toner at the top of the image to be formed on the paper. Moreover, the gloss level of the image is changeable among plural levels by changing, within the moderate range, the area coverage of the low-softening-point CL toner to be provided at the top of the image.

FIG. 6is a graph illustrating the 60° gloss level versus the temperature of the pressure roller42of the fixing unit40illustrated inFIG. 2in several cases where uniform images having secondary colors (R, G, and B) are formed by using colored toners and the 60° gloss levels of the respective uniform images are measured. In each of the cases, the temperature of the heat belt41is set to the lowest point of the temperature range that guarantees satisfactory fixing quality in accordance with the softening point of the toner used.

As can be seen fromFIG. 6, in the cases of low-softening-point toners, changes in the gloss level are very small relative to changes in the temperature of the pressure roller42. In contrast, in the cases of high-softening-point toners, changes in the temperature of the pressure roller42tend to affect the gloss level of the image.

In related-art image forming apparatuses in which only low-softening-point toners are employed, the pressure roller only needs to be controlled to be at 70° C. or below so that, in a case where a toner image is to be formed on the back side of the paper already having a fixed toner image on the front side thereof, the fixed toner image on the front side of the paper is prevented from melting again at the contact with the pressure roller and causing an image defect. In contrast, in the image forming apparatus100(seeFIG. 1) according to the present exemplary embodiment in which high-softening-point toners are employed, the pressure roller42is controlled to be at a specific temperature (in the present exemplary embodiment, 70° C.) while being cooled by the fan52. This is because an image defect may occur if the temperature of the pressure roller42exceeds 70° C., as described above, while a problem with gloss may occur if the temperature of the pressure roller42is below 70° C.

In view of the above, an operation performed by the controller50of the image forming apparatus100according to the present exemplary embodiment illustrated inFIG. 1will now be described.

FIG. 7is a flowchart illustrating the operation performed by the controller50.

First, in step S01, the controller50receives image data Cin and acquires information on whether the gloss mode is on or off and information on the gloss level (1, 2, or 3). The information on whether the gloss mode is on or off and the information on the gloss level may be input, together with the image data Cin, from an external device or through a user's operation performed on the operation/display unit501.

After the image data Cin is input to the controller50, the operation proceeds to step S02, where a total thickness X of the Y, M, C, and K toners that are to form a toner image based on the image data Cin is calculated. If the total thickness X varies with the position of the expected image, the largest thickness is employed as the total thickness X. The total thickness X is referred to when it is determined whether or not satisfactory fixing will be realized. Even a small part of the image that is not fixed satisfactorily may lead to an image defect. After the total thickness X is calculated, the operation proceeds to step S03, where it is determined whether the gloss mode is on or off.

Although the information on whether the gloss mode is on or off and the information on the gloss level are herein treated as separate pieces of information for ease of understanding, the information on whether the gloss mode is on or off may be integrated into the information on the gloss level. That is, the lowest gloss level may be a level realized when an image is formed with an area coverage of the CL toner of 0% (the level realized when the gloss mode is off).

A case where the gloss mode is off (the gloss level is the lowest) will be described first.

In this case, the operation proceeds to step S04, where it is determined whether or not the total thickness X is 240% or below in dot percentage. In most images, the total thickness X is 240% or below, usually. If the total thickness X of the image exceptionally exceeds 240%, the operation proceeds to step S05, where the amount of toners is adjusted so that the total thickness X becomes 240% or below. The amount of toners is adjusted by related-art methods, for example, by replacing certain amounts of Y, M, and C toners with an amount of K toner that is equivalent thereto or by reducing the amounts of all the Y, M, C, and K toners.

If it has been determined that the total thickness X is 240% or below in step S04or if it has been determined that the total thickness X is over 240% in step S04and the amount of toners is therefore adjusted in step S05so that the total thickness X becomes 240% or below, the operation proceeds to step S06, where the image data Cin is converted into data in a format that is handleable by the exposure units23Y,23M,23C, and23K illustrated inFIG. 1. Then, in step S07, the converted image data is output to the exposure units23Y,23M,23C, and23K. In this case, since the gloss mode is off (the gloss level is the lowest), the image data is not transmitted to the exposure unit23CL for the image forming engine20CL that uses the CL toner.

On the other hand, in step S03, if it has been determined that the gloss mode is on, the operation proceeds to step S08, where it is determined whether or not the total thickness X is 200% or below.

While a threshold of 240% is employed in step S04, another threshold 200% is employed in step S08. This is because the total thickness X of the colored toners needs to be suppressed to a relatively small value, taking into consideration that a thickness of the CL toner is to be added thereto.

In step S08, if it has been determined that the total thickness X exceeds 200%, the operation proceeds to step S09, where the amount of toners is adjusted. The adjustment of the amount of toners performed in step S09is the same as that performed in step S05, except that the upper limit of the total thickness X is 200%.

If it has been determined that the total thickness X is 200% or below in step S08or if it has been determined that the total thickness X is over 200% in step S08and the amount of toners is therefore adjusted in step S09so that the total thickness X becomes 200% or below, the operation proceeds to step S10, where a certain thickness of the CL toner corresponding to the gloss level 1, 2, or 3 is added to the total thickness X. In the present embodiment, for example, the gloss level becomes higher in the order of level 1, level 2, and level 3. Specifically, gloss level 1 corresponds to an area coverage of the CL toner of 40%, gloss level 2 corresponds to an area coverage of the CL toner of 50%, and gloss level 3 corresponds to an area coverage of the CL toner of 100%. Hence, the maximum total thickness of toners including the thickness of the CL toner defined by the area coverage comes to 300%.

The information on the total thickness of toners including the thickness of the CL toner defined by the area coverage that is calculated as described above is provided to a fixing-unit-temperature controller in step S11. In the present exemplary embodiment, the controller50is also responsible for the operation of controlling the temperature of the fixing unit40. That is, the fixing-unit-temperature controller according to the present exemplary embodiment is a fixing-unit-temperature-controlling processor (not illustrated) included in the controller50. Hence, in the present exemplary embodiment, the information on the total thickness of toners including the thickness of the CL toner defined by the area coverage is provided to the fixing-unit-temperature-controlling processor included in the controller50.

FIG. 8is a graph illustrating the set temperature of the fixing unit40(the heat belt41) versus the total thickness of toners.

In the case where colored toners having high softening points are employed, the toners are relatively difficult to melt quickly. Moreover, if the CL toner is provided over such toners, fixing may not be performed satisfactorily. Therefore, in the present exemplary embodiment, if the total thickness of toners exceeds 240%, the set temperature of the fixing unit40is increased with the increase in the total thickness of toners, the maximum total thickness of toners being 300%. If the temperature of the fixing unit40is set so as to be constant at a level corresponding to the maximum total thickness of toners of 300%, energy is consumed excessively and it becomes difficult to protect peripheral components from being adversely affected by the heat radiated from the fixing unit40. The total thickness of toners seldom exceeds 240% even including the thickness of the CL toner. Therefore, in the present exemplary embodiment, the fixing temperature is not constantly set to a high value but is increased with the total thickness of toners only when the total thickness of toners exceeds 240%.

The description of the operation performed by the controller50will be continued referring to the flowchart illustrated inFIG. 7.

After the information on the total thickness of toners is provided to the fixing-unit-temperature controller in step S11, the operation proceeds to step S12, where the image data Cin is converted into data in a format that is handleable by the exposure units23CL,23Y,23M,23C, and23K, as with step S06. Then, in step S13, the converted image data is output to the exposure units23CL,23Y,23M,23C, and23K. In this case, the image data is transmitted to the exposure unit23CL for the image forming engine20CL that uses the CL toner as well.

In the above exemplary embodiment, the image forming apparatus100employs high-softening-point toners as the Y-, M-, C-, and K-colored toners and a low-softening-point toner as the CL toner that is to be provided at the top of the image, whereby the gloss level is increased. Alternatively, the image forming apparatus100may employ low-softening-point toners as the Y-, M-, C-, and K-colored toners and a high-softening-point toner as the CL toner that is to be provided at the top of the image, whereby the gloss level may be reduced.

The above exemplary embodiment has been described on the premise that the CL toner for changing the gloss level is provided at the top of the image and over the entirety of the image area. Alternatively, the CL toner at the top of the image may be provided only in part of the image area. In that case, the gloss level in that part of the image area changes.

The above exemplary embodiment concerns an image forming apparatus employing a tandem development device, as illustrated inFIG. 1. The present invention is also applicable to, for example, an image forming apparatus employing a rotary development device that includes plural development units whose positions are changeable by the rotation of the development device. In the image forming apparatus employing a tandem development device as illustrated inFIG. 1, to provide the CL toner at the top of the image to be formed on the paper, the position of the image forming engine that uses the CL toner is automatically determined. In contrast, in the case where a rotary development device is employed, the position of the development unit that uses the CL toner is set more flexibly. Moreover, in the case where a rotary development device is employed, the gloss level of the image may be changed without using the CL toner, depending on the kind of the image to be formed. Specifically, one of the colored toners (for example, the Y toner) may have a different softening point from the other colored toners, and the gloss level of the image may be changed by selecting whether to provide the colored toner having a different softening point as the topmost layer or as any of the second-topmost and subsequent layers of the image.