Patent Grant
8818221
The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2011-051289 filed in Japan on Mar. 9, 2011 and Japanese Patent Application No. 2011-266684 filed in Japan on Dec. 6, 2011.
1. Field of the Invention
The present invention relates a transfer device that transfers a visualized image formed on an image carrier onto a recording medium, and an image forming apparatus including the transfer device.
2. Description of the Related Art
An electrophotographic image forming apparatus forms an image by visualizing a charged latent image obtained by imaging optical image information onto an image carrier that has been evenly charged in advance, using toner supplied from a developing unit, and by transferring and fixing the image which is thus visualized onto a recording sheet (recording medium). In such an image forming apparatus, because a recording sheet has some texture, toner is less easily transferred onto recessed parts than projected parts. In particular, when toner is to be transferred onto a recording sheet with a highly textured surface, the toner might not be transferred well onto recessed parts, and might result in white splotches in the image.
As a countermeasure for this issue, Japanese Patent Application Laid-open No. 2006-267486, Japanese Patent Application Laid-open No. 2008-058585, and Japanese Patent Application Laid-open No. H9-146381, for example, describe technologies for improving a transfer ratio by superimposing an alternating current (AC) voltage on a direct current (DC) voltage.
The technology disclosed in Japanese Patent Application Laid-open No. 2006-267486 performs control using an AC voltage superimposed on a DC voltage as a transfer bias, and charging the surface of a recording sheet to the opposite polarity of that of the toner in a manner suitable for the texture before transferring the image so that toner is to be transferred to recessed parts.
The technology disclosed in Japanese Patent Application Laid-open No. 2008-058585 uses an AC voltage superimposed on a DC voltage as a transfer bias. The AC voltage is superimposed in a manner making the voltage between peaks of the AC voltage equal to or less than twice the DC voltage.
The technology disclosed in Japanese Patent Application Laid-open No. H9-146381 uses fluorine resin on the surface of an intermediate transfer element, and uses an AC voltage superimposed on a DC voltage as a transfer bias. The AC voltage is superimposed in a manner making the voltage between peaks of the AC voltage equal to or more than 2.05 times the DC voltage.
Although all of these technologies attempt to improve transferability by controlling voltages applied from the DC power source and the AC power source to the target values, detailed descriptions in these disclosures merely disclose the relations between the transfer voltage and the transferability.
In a transfer device for improving the toner transferability by superimposing a DC voltage on an AC voltage and applying the resultant voltage to recessed parts of textured paper, depending on the output AC voltage and DC voltage, the density in smooth parts, the transferability in the recessed parts, and abnormalities of images resulting from discharge may vary. Therefore, the AC voltage setting and the DC voltage setting need to be kept within a certain range. However, it is also necessary to change the ranges of the AC voltage setting and the DC voltage setting depending on a change in the resistance in the transfer member caused by environmental changes, e.g., a change in temperature or humidity, or depending on the type of a paper sheet that is a recording medium. In the method in which a DC voltage is superimposed on an AC voltage and the resultant voltage is applied, the acceptable ranges of the voltage settings are more limited for the aforementioned reason than those in a conventional transfer device applying only a DC voltage. Furthermore, the relation between the DC voltage, the AC voltage, and the resultant image is complex. Therefore, it is difficult to cope with resistance changes and different types of paper sheets.
There is a need to address the issue described above in conventional transfer units, and an object of the present invention is to provide a transfer unit and an image forming apparatus that improve the transfer ratio to the recessed parts on a textured surface of a paper sheet, that can transfer toner evenly even to a paper sheet having a highly textured surface, and that can output high quality images in a stable manner even in environmental changes and for different types of paper sheets.
It is an object of the present invention to at least partially solve the problems in the conventional technology.
According to an embodiment, a transfer device includes: an image carrier from which an image is transferred onto a transfer medium using electrostatic toner, the image carrier being applied with a direct current voltage superimposed with an alternating current (AC) voltage as a transfer bias. An output voltage of a power source for applying the voltage is controlled so that a current level of a direct current component output from the power source is kept at a specified current level.
According to another embodiment, an image forming apparatus includes the transfer device mentioned above.
The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings.
An embodiment will now be explained with reference to drawings.
At this time, the voltage being applied is a DC voltage superimposed on an AC voltage. The DC/AC superimposed voltage may be applied from either one of the power source 110 and the power source 111, or the AC and the DC may be applied separately from the power source 110 and the power source 111. The AC/DC superimposed voltage may be applied from one of the power source 110 and the power source 111, and the DC voltage may be applied from the other. By providing an output voltage having only the DC component and an AC/DC superimposed voltage in a selectable manner, these voltages can be switched depending on conditions. For example, if the transfer medium P is a recording medium without any texture, the power source may be switched so as to apply only the DC component.
In this manner, in applications not requiring any AC voltage, the transfer unit may be used with a DC component only, in the same manner as in a conventional transfer unit so that the energy can be saved. In such a case, the power source for applying the AC/DC superimposed voltage may be provided in singularity and is caused to apply only the DC component by causing not to supply the AC component. Alternatively, separate power circuits may be provided for application of the DC voltage and application of the AC/DC superimposed voltage, and be switched when the voltages are to be switched.
The latter configuration can achieve some advantageous effects. For example, a power circuit for applying an AC/DC superimposed voltage can simply be added to an existing transfer unit that applies only the DC voltage, with some upgrade of functions, and development time can be shortened by making some adjustments to the existing system. If the AC power source and the DC power source are arranged separately in the same manner as the power source 110 and the power source 111 illustrated in
When the constant voltage control is performed to the output voltage, high transferability cannot be achieved unless the applied voltage is changed greatly depending on resistance changes in the member caused by humidity, or types of the recording medium. On the contrary, when the constant current control is performed, the transferability varies less in the face of such changes. Detailed data indicating advantages of the constant current control over the constant voltage control will be provided in an embodiment of an image forming apparatus to be explained later.
In such a structure, Ioff is detected by an ammeter arranged internally to the power source 110, and is input to the control circuit 300. A controlling signal is input from the control circuit 300 to the power source 110. The control circuit 300 outputs the controlling signal based on a current setting, and the voltage output from the power source 110 is adjusted so that the output Ioff is kept at the level specified by the current setting. The constant current control may be performed to Ipp in the same manner. According to a research made by the inventors of the embodiment, Ioff represents movements of electrical charges caused by movement of the toner or discharge. Therefore, Ioff setting can be established using the current generated by the toner movement as a guideline. The current Itoner generated by the toner movement can be expressed in a relation represented by Equation (1) below.
Itoner=v*W*Q/M*M/A*10 (1)
where, v represents a velocity [m/s] of the transfer medium P, W represents the width [meters] of the image in the axial direction of the roller, Q/M represents the electric charge [μC/g] of the toner, and M/A represents the amount of attached toner [mg/cm2].
For values of the image width and the amount of toner attached, maximum values that are assumed when a solid black image is transferred onto a recording medium are used so as to allow all toner to be transferred. For example, when v=0.3 [m/s], W=0.3 [meters], Q/M=−30 [μC/g], and M/A=0.5 [mg/cm2], Itoner=−13.50 [μA, or microamperes]. At this time, the absolute value of Ioff is preferably set to a value equal to or more than |Itoner|, for example, Ioff=−20 [microamperes]. The setting for Ioff when a different velocity v of the transfer medium P is used can be obtained by calculating Itoner from Equation (1) above. For example, because Ioff=−6.75 [microamperes] when v=0.15[m/s], Ioff is set to be Ioff=−10 [microamperes].
When the velocity (linear velocity) is to be changed depending on transfer media, different modes for automatically switching Ioff depending on velocities may be configured to achieve the stable image quality for different transfer medium velocities. Furthermore, for a color image having a higher M/A than a monochromatic image, Ioff setting can be estimated from Equation (1) as well. For example, assuming that M/A of a color image is 1.0 [mg/cm2] that is twice that of a monochromatic image, Ioff may be set to −40 [microamperes] that is twice that for a monochromatic image. By providing a color print mode for automatically switching Ioff setting based on image information being output, a stable image can be obtained for both of color images and monochromatic images.
Ipp is required to be at a level that can produce an electrical field that enables the toner to be transferred onto the recessed parts. If Ipp is too low, the toner is not transferred onto the recessed parts. This level differs depending on the resistance in the transfer member and the width of the transfer nip. In this example, Ipp is set to 3.0 [milliamperes] as an example. By setting Ipp to an appropriate value, high transferability to the recessed parts can be maintained for different types of a transfer medium P.
The shape of the transfer member 80 is not especially limited, as long as the AC/DC superimposed electrical field can be applied in the transfer nip. However, the shape of a roller is preferable from the viewpoint of reducing the frictional force. The transfer member 80 may be structured to have a conductive core metal having the shape of a cylinder, and a surface layer made of resin or rubber laid on the outer circumferential surface of the core metal. Various materials may be used for the transfer medium P that is a recording medium, such as paper, resin, and metal. In this embodiment, the waveform of the AC voltage is a sine wave, but may be other waveforms such as a rectangular wave.
The power circuit of the transfer unit will now be explained in detail.
In such a configuration, a signal CLK for setting the frequency of the AC voltage is supplied from the control circuit 300. A signal AC_PWM for setting the current or the voltage of the AC output and a signal AC_FB_I for monitoring the AC output are also connected. A signal DC_PWM for setting the current or the voltage of the DC output that is superimposed on the AC output and a signal DC_FB_I for monitoring the DC output are connected to the DC generating unit as well. Blocks for controlling the AC and the DC (current/voltage) output signals for controlling driving of the high voltage transformers 122 and 126 via the AC driving unit 121 and the DC driving unit 125, respectively, so that a detected signal output from each of the output detecting units 123 and 127 is kept at a predetermined level, based on instructions from the control circuit 300.
In the AC control, to enable both of the constant current control and the constant voltage control, both of the current and the voltage of the AC output are controlled, and the AC output detecting unit 123 detects both of the output current and the output voltage. The same can be said for the DC control. In this embodiment, both of the AC and the DC are usually controlled in a manner prioritizing the detected current so that the constant current control is performed. The detected output voltage is used to suppress the voltage to the upper boundary, and is used for controlling the maximum voltage when no load is applied, for example. The monitoring signals respectively output from the AC output detecting unit 123 and the DC output detecting unit 127 are input to the control circuit 300 as load monitoring information.
The frequency of the AC voltage is set with reference to the signal CLK output from the control circuit 300. However, the AC voltage generating unit may generate a fixed frequency internally.
In addition, only the power source 110 may be caused to output. In this manner, it is possible to select either a conventional secondary transfer using only the DC component or a secondary transfer using the AC/DC superimposed voltage. The units included in the power sources 110 and 111 have the same functions as those illustrated in
The constant current control is performed in both of the AC voltage generating unit 112 illustrated in the upper half and the DC voltage generating unit 113 illustrated in the lower half. For the voltage of the AC, a coil N3_AC is used to take out a low voltage that approximates the output of the high voltage transformer, and the voltage controlling comparator is used to compare the voltage thus taken out with a reference signal Vref_AC_V. The current of the AC is taken out by an alternating current detector arranged between the ground and a capacitor C_AC_BP for biasing the AC component and connected in parallel with the output of the DC voltage generating unit, and a current controlling comparator is used to compare the alternating current with a reference signal Vref_AC_I. The level of the reference signal Vref_AC_I is set based on the signal AC_PWM for setting the level of the AC output current.
The level of the reference signal Vref_AC_V is set so that the output of the voltage controlling comparator is valid when the output voltage increases to a predetermined level or higher (for example, when no load is applied). The level of the reference signal Vref_AC_I is set so that the output of the current controlling comparator is valid while the load is at a usual level. In this manner, high voltage output currents can be switched correspondingly to conditions of the load (e.g., the facing roller 73, the transfer member 80, and the member between the rollers). The outputs of the voltage controlling comparator and the current controlling comparator are input to the AC driving unit, and the AC high voltage transformer is driven based on the levels of these comparator outputs.
In the DC voltage generating unit as well, both of the output voltage and the output current are detected. The voltage is detected by a DC voltage detector connected in parallel with a rectifying/smoothing circuit arranged at an output coil N2_DC of the high voltage transformer. The current is detected and taken out by a direct current detector connected between the output coil and the ground. The voltage detection signal and the current detection signal are respectively compared with a reference signal Vref_DC_V and a reference signal Vref_DC_I that are weighted in the same manner as for the AC, and used to control the DC component in the high voltage output.
An image forming apparatus according to the embodiment is now to be explained. The effectiveness of the constant current control will be then explained specifically using the results of a research conducted using such an image forming apparatus. The embodiment of the image forming apparatus is merely an example. The effects of the embodiment remain the same even if the configurations or processing conditions are changed, by using different types of image forming apparatuses and various image formation environments.
Only the image forming unit 1Y will now be explained with reference to
The image forming unit 1Y includes a photosensitive element 11 that is an image carrier, a charging unit 21 that charges the surface of the photosensitive element 11 with a charging roller, a developing unit 31 that is an image developing unit that develops an image formed on the photosensitive element 11 into a toner image, a first transfer roller 61 that transfers a latent image carrier onto the intermediate transfer belt 50, and a photosensitive element cleaning unit 41 that cleans the toner remaining on the surface of the photosensitive element 11.
The charging unit 21 has a structure that applies a voltage that is an AC voltage superimposed on a DC voltage to the charging roller that is a roller-shaped elastic conductive element. The photosensitive element 11 is charged to a predetermined polarity, for example, a negative polarity, by inducing direct discharge between the charging roller and the photosensitive element 11. The charged surface of each of the photosensitive elements 11 is irradiated with a laser beam L that is optically modulated and output from an image writing unit not illustrated. In this manner, an electrostatic latent image is formed on the surface of each of the photosensitive elements 11. In other words, an electrostatic latent image is formed as parts where the absolute value of the potential is reduced on the surface of the photosensitive element by being irradiated with the laser beam.
The first transfer roller 61 is a conductive elastic roller, and is arranged in a manner being pressed against the photosensitive element 11 from the rear side of the intermediate transfer belt 50. A bias applied with the constant current control is applied to the elastic roller as a primary transfer bias.
The photosensitive element cleaning unit 41 includes a cleaning blade 41a and a cleaning brush 41b. The cleaning blade 41a cleans the surface of the photosensitive element 11 in a counter direction with respect to the direction of a rotation of the photosensitive element 11 by being kept abutting against the photosensitive element 11, and the cleaning brush 41b cleans the surface of the photosensitive element 11 by being rotated in the counter direction of the rotation of the photosensitive element 11 while being kept in contact with the photosensitive element 11.
The developing unit 31 includes a container 31c filled with two-component developer containing Y toner and carrier, a developing sleeve 31a that is a developer carrier arranged inside of the container 31c in a manner facing the photosensitive element 11 via an opening on the container 31c, and a screw member 31b that is a stirring member arranged inside of the container 31c for stirring and conveying the developer.
The screw member 31b is arranged both on a side where developer is supplied, that is, a side near a developing sleeve, and on a side receiving the supply from a toner supplying unit not illustrated, and is supported rotatably on the container 31c via a shaft bearing member not illustrated.
The photosensitive element 11 in each of the four image forming units is driven in rotation by a photosensitive element driving unit not illustrated in the clockwise direction in
The intermediate transfer belt 50 is an endless belt member having a moderate resistance, for example, and is stretched across the facing roller 73 and a plurality of supporting rollers such as supporting rollers 71 and 72 included in the secondary transfer unit. The intermediate transfer belt 50 can be carried endlessly in the counter clockwise direction in
The supporting roller 72 is grounded, and a surface electrometer 75 is arranged in the manner facing the supporting roller 72. The surface electrometer 75 measures the potential of the surface when the toner image transferred onto the intermediate transfer belt 50 is carried across the supporting roller 72.
The facing roller 73 in the secondary transfer unit is connected to the power source 110 for applying the transfer bias. The power source 110 is capable of superimposing a DC voltage on an AC voltage and applying the resultant voltage, and can perform the constant current control to Ipp and Ioff of the voltage before being applied. By applying the voltage to the facing roller 73 in the secondary transfer unit, a potential difference is generated between the facing roller 73 and the secondary transfer roller 80, thus generating a voltage causing the toner to move from the intermediate transfer element 50 onto the recording sheet. In this manner, the toner image can be transferred onto the recording sheet.
The results of a research conducted using the image forming apparatus according to the embodiment will now be explained with reference to the accompanying drawings.
To begin with, Ipp was fixed to 2.8 [milliamperes], and the direct current that is to be superimposed is fixed to −16 [microamperes]. A solid black image was then output onto a sheet of standard paper at the AC voltage frequency of 282 [mm/s] and the linear velocity of the intermediate transfer belt at 141 [mm/s]. The inventor then checked for the frequencies at which no image unevenness was caused in the granularity of 100 [hertz] from 100 [hertz] to 700 [hertz], to find out that the image unevenness caused by a frequency would not occur when the frequency is at 400 hertz or higher at a linear velocity v of 282 mm/s, and when the frequency is 200 hertz or higher at a linear velocity v of 141 mm/s. The linear velocity of the intermediate transfer belt and the linear velocity of the recording sheet are nearly equal.
The reason why a different frequency is required for a certain linear velocity is related to time for which the transfer voltage is applied. When the nip width between the secondary transfer facing roller 73 and the secondary transfer roller 80 without any paper sheet being conveyed is d [millimeters], the time required for a paper sheet to pass through the nip can be expressed as d/v [seconds] using the linear velocity v and the nip width. When the frequency is at f [hertz], the cycle of the AC voltage will be 1/f [seconds]. Therefore, the number of the AC voltage cycles applied while the paper sheet passes through the nip can be expressed as d*f/v [cycles]. Because the nip width d in this embodiment is approximately 3 millimeters and a frequency of 400 hertz is required when the linear velocity is 282 [mm/s], the number of the AC voltage cycles needs to be applied will be 3*400/282≈4.255. Therefore, when the AC voltage is applied for approximately 4.25 cycles, an image without unevenness can be achieved. When the linear velocity is 141 [mm/s], the number of the AC voltage cycles needs to be applied 3*200/141≈4.255, which gives a good result as well, as the same number of alternative voltage is applied. Because 3*300/282≈3.191 when the frequency is 300 hertz and the linear velocity is 282 [mm/s], high quality images without any unevenness can be achieved if at least four cycles of the AC voltage are applied while the paper sheet passes through the nip. Thus, 4<d*f/v can be defined. Therefore, the frequency of the AC voltage to be applied preferably satisfies the relation represented as Equation (2) below.
f>(4/d)*v (2)
The frequency was then fixed to 500 [hertz], and the linear velocity was fixed to 282 [mm/s]. A solid black image is then output onto Resack 66—260 kg that is paper manufactured by Tokushu Paper Manufacturing Co., Ltd., (paper with a thickness of approximately 320 micrometers and on which the difference between the recessed parts and the projected parts is approximately 130 micrometers at most). In the embodiment, the amount of toner attached to the solid black image on the intermediate transfer belt was 0.55 [mg/cm2], and the electric charge of the toner Q/M was −30 [μC/g]. Because the width of the solid black image in this embodiment along the width direction of the secondary transfer roller is 0.28 meters, Itoner=−13.03 [microamperes] is obtained based on Equation (1). The constant current control was then performed to the output voltage of the power source 110, and the image was output while changing the current settings between −10 [microamperes] and −25 [microamperes] for Ioff, and between 2.0 [milliamperes] to 4.0 [milliamperes] for Ipp. The image is then visually evaluated. Because the paper Resack 66—260 kg has a highly textured surface, evaluations were conducted especially paying attention on the degree how the grooves were filled up and how the white splotches were formed along the grooves. The paper Resack 66—260 kg was denoted as a recording sheet A, and evaluations were made in following three levels:
o: satisfactory, Δ: slightly problematic, and x: problematic
The evaluation results are indicated in
Similar evaluations were then conducted using Resack 66—175 kg paper that is paper manufactured by Tokushu Paper Manufacturing Co., Ltd., (with a thickness of approximately 210 micrometers, and on which the difference between the recessed parts and the projected parts is approximately 120 micrometers at most) as a recording sheet B. Ioff was changed between −11 [microamperes] and −23 [microamperes], and Ipp was changed between 2.2 [milliamperes] and 3.4 [milliamperes]. The results of the evaluations are illustrated in
When the constant voltage control is performed, although the voltage output from the power source is kept constant, the intensity of an electrical field applied to the toner changes because the resistance changes depending on a recording sheet. Therefore, the area representing acceptable densities on the smooth parts illustrated in
In the manner described above, in the constant current control, the dotted line (1) representing the acceptable densities on the smooth parts and the dotted line (3) representing formations of white streaks caused by discharge remain the same across the different types of paper or resistances. Therefore, by setting Ipp and Ioff within the ranges that are effective for a recording sheet with the lowest transferability, high transferability can be achieved on all recording sheets. For example, in this embodiment, by setting Ioff to −18 [microamperes] and setting Ipp to 2.8 [milliamperes] to 3.0 [milliamperes], high transferability can be achieved on a paper sheet with a textured surface.
The power source 110 was then changed to a power source that performs the constant current control to the DC component, and that performs the constant voltage control to the AC component, and images were output. The results obtained by setting Ioff to −16 [microamperes] and changing Vpp are provided in Table 1.
As indicated in Table 1, by performing the constant current control in a manner keeping Ioff at an appropriate level, even when the constant voltage control is applied to the AC component, a setting for achieving high quality images on different types of paper can be selected. When the constant voltage control is applied to the AC component, the structures for detecting the alternating current can be omitted. Therefore, the controlling structure can be simplified compared with that when the constant current control is performed.
In this manner, by applying the constant current control to the output voltage using Ioff, or Ioff and Ipp, images can be stably transferred onto recessed parts at high transferability depending on types of transfer media.
Images were then output under different humidity. The results of image outputs described above were obtained in a humidity environment of 40 percent to 50 percent. Explained below are the results of the same evaluation conducted for the recording sheet B in a humidity environment of 55 percent to 65 percent. Used as the power source 110 was a power source outputting a voltage having both of the DC component and the AC component controlled by the constant current control. A relation between the currents and the images is illustrated in
Evaluations were then conducted with different velocities v at which the recording sheet B is conveyed. Used as the power source 110 was a power source outputting a voltage in which both of the DC component and the AC component are controlled by the constant current control. When the velocity v for conveying the recording sheet B is reduced to a half, Itoner will be reduced to a half as well, based on Equation (1) mentioned above. Table 2 indicates the results of image evaluations conducted by setting Ioff to −8 [microamperes] that is half the experiment result mentioned above while changing Ipp. A condition 1 and a condition 2 mentioned in Table 2 are as follows:
Condition 1: conveying velocity v=282 [mm/s] and Ioff=−16 [microamperes]
Condition 2: conveying velocity v=141 [mm/s], and Ioff=−8 [microamperes]
The same tendency can be achieved under different velocities by setting Ioff in a manner being proportional to the transfer medium conveying velocity. Therefore, by changing the DC component used in the constant current control correspondingly to the conveying velocity of the transfer medium, transferability to the recessed parts of the transfer medium can be achieved in a stable manner even in a transfer unit having modes with different velocities.
Evaluations were then conducted using a different amount of toner attached to the transfer belt. Used as the power source 110 was a power source outputting a voltage in which both of the DC component and the AC component are controlled by the constant current control. A paper conveying velocity of 282 mm/s and the recording sheet B were used. For a color image with M/A=0.88 [mg/cm2], the amount of attached toner was M/A=0.55 [mg/cm2] in the evaluations explained above. Therefore, M/A was 1.6 times the result of the previous evaluations. Because Itoner is proportional to M/A based on Equation (1), Ioff was set to −26 [microamperes] that is 1.6 times the value used in the previous evaluations, and image evaluations were conducted using different Ipp. The results are indicated in Table 3. A condition 3 and a condition 4 mentioned in Table 3 are as follows:
Condition 3: M/A=0.55 [mg/cm2], Ioff=−16 [microamperes]
Condition 4: M/A=0.88 [mg/cm2], Ioff=−26 [microamperes]
By setting Ioff proportional to the amount of attached toner, conditions for enabling high quality images to be achieved can be obtained simply by assigning Ipp. Based on researches conducted by the inventors, it has been confirmed that, when the amount of attached toner and the electric charge of the toner increase, optimal Ipp increases along with an increased lower boundary of Ioff. Therefore, optimal Ipp conditions for different amounts of attached toner and different electric charges of the toner may be examined in advance, and a data table based on experiment results may be stored in a memory. A function may then be added so that Ipp is determined when the amount of attached toner and the electric charge of the toner change. In this manner, the settings to be used in the constant current control may be determined automatically depending on the conditions of the amount of attached toner and the electric charge of the toner. Therefore, by changing the settings used in the constant current control applied to the DC component depending on the amount of toner attached on the image carrier, stable transferability to the recessed parts of a transfer medium can be achieved even across images with very different amounts of attached toner, such as a monochromatic image and a color image. It has been confirmed that, when the amount of attached toner and the electric charge of the toner increase, the ranges of currents for enabling high quality images themselves become narrower, and the optimal Ipp increases as Ioff increases, as mentioned earlier. When the amount of toner attached is extremely high, for example, in the graph illustrating the relation between the voltages or the currents and the images, the effective area represented in the shape of a triangle, surrounded by the line (1) indicating the acceptable densities on the smooth parts, the line (2) indicating the acceptable degree of filling of the recessed parts with the toner, and the line (3) indicating the formations of white streaks caused by discharge, might not be formed. However, on a recording sheet with deep recessed parts, although some white splotches may be formed in images, the transferability to the recessed part is higher than that achieved by the conventional transfer in which only the DC voltage is used in transferring. Therefore, the effects of improving the transferability on the recessed parts can be achieved in the ranges other than effective ranges explained above.
When an AC/DC superimposed bias is applied to the secondary transfer unit, in the structure illustrated in
In response to this issue, to ensure that a potential difference near an ideal is generated in the transfer nip, a constant current may be applied to the transfer unit without any paper (or with paper), and the resistance in the secondary transfer unit (the facing roller 73, the transfer belt 50, and the transfer roller 80) may be measured based on the voltage required at that time, and the AC/DC superimposed voltage may be applied based on the measurement.
To obtain the voltage to be applied to the transfer unit based on the resistance thus measured, the voltage to be applied may be directly obtained from the resistance of the transfer unit, or resistances may be classified using a certain threshold, and the voltage to be applied may be obtained based on the table.
A desired potential difference cannot be generated in the transfer nip when the resistance in the members included in the transfer unit changes not only in the configuration illustrated in
Explained below is an example of a method for correcting the voltage to be applied when the resistance and the like in the secondary transfer unit changes. In the explanation of the correction method below, the constant current control is applied to the DC component, and the constant voltage control is applied to the AC component. However, the embodiment is not limited thereto. Either one of the DC component and the AC component may be controlled by the constant current control or the constant voltage control. In such cases as well, the electrical field to be applied can be obtained from the resistance of the secondary transfer unit, except different correction coefficients are used.
Regardless of how these controls are combined, the DC component and the AC component need to be corrected separately. The reason is as follows. Almost all of the DC component in the applied current flows from the facing roller 73 to the paper and the transfer roller 80. On the contrary, in the AC component, because the polarity changes quickly, almost all of the current is consumed in charging the facing roller 73 or the transfer roller 80, and only a part of the applied current flows from the facing roller 73 to the paper and the transfer roller 80. Specifically, the DC component current applied in this example is −10 microamperes to −100 microamperes, and the AC component current of ±0.5 milliamperes to ±10 milliamperes is applied.
As this exemplary correction method, in “Resistance Correction Coefficient Table” illustrated in Table 4 below, the table is divided into six rows using five resistance thresholds. R−2 to R+3 (R0 as a standard) are set in the ascending order of the resistance, and a correction ratio (correction coefficient) is determined for each.
In Table 4, there is an opposite tendency in an increase and a decrease of the coefficients between the DC component and the AC component. This is because of the difference between the constant voltage control and the constant current control explained earlier.
In the constant current control, because the current passing through the transfer nip is controlled, when the resistance of the facing roller 73 decreases, the potential difference generated in the transfer nip is reduced as well. Therefore, the potential difference generated in the transfer nip will not be constant unless the controlled current is increased.
On the contrary, in the constant voltage control, because the voltage at the core metal in the facing roller 73 is controlled, the potential difference in the transfer nip will have its voltage reduced in the rubber layer of the facing roller 73. Therefore, when the resistance of the facing roller 73 decreases, the potential difference generated in the transfer nip will increase. Hence, the potential difference generated in the transfer nip will not be constant unless the controlled voltage is decreased.
By using the correction coefficients provided in “Resistance Correction Coefficient Table”, the same transferability can be achieved even when the resistance of the secondary transfer unit changes. The correction coefficients provided in the Table 4 are merely examples used in the embodiment, and these correction coefficients are changed when the system is changed.
The electrical field to be applied to the facing roller 73 will also be different depending on the moisture contained in the paper. This is because the electrical resistance of the paper decreases when the moisture in the paper increases. When the electrical resistance of the paper decreases, the potential difference to be generated in the transfer nip is reduced.
For example, in “Humidity Environment Correction Coefficient Table” provided in Table 5, the temperature and humidity in the image forming apparatus are measured, five thresholds are set for the absolute humidity obtained from the measurements, and a table is divided into six rows using these thresholds. LLL, LL, ML, MM, MH, and HH are set in the ascending order of the humidity, and a correction ratio (correction coefficient) is determined for each.
Because the temperature and humidity environment coefficients are intended to correct a resistance change in the paper in the transfer nip, the tendency of a coefficient increase and decrease is the same between the constant voltage control and the constant current control.
As explained above, by controlling the electrical field applied to the facing roller 73, constant transferability can be achieved even when a cause of errors change.
The effects achieved when the constant current control is applied to the AC component in the superimposed transfer bias will now be explained with some comparative examples.
In a transfer performed with the application of an AC/DC superimposed voltage, when the paper becomes thicker, a larger potential difference needs to be generated in the transfer nip.
When the constant current control is applied to the AC component, the electrical charge supplied to the facing roller 73 will remain constant. Furthermore, if the paper passing through the nip is thicker, the capacity of the transfer unit as a capacitor decreases (because the distance increases). Hence, the potential difference generated in the transfer nip will increase. Therefore, even if the paper thickness is changed, the same transferability can be achieved without changing the target current by a large degree.
An example will now be described on a method of correcting the AC electrical field to be applied when the thickness of the recording sheet is changed. In this example, a correction method used when the constant current control is applied to the AC component is explained as an example of the embodiment, and an example in which the constant voltage control is applied to the AC component is explained as a comparative example. The number of thresholds (the number of rows in the table) or the correction ratios (coefficients) are just examples, and the embodiment is not limited thereto.
For example, in “Paper Thickness Correction Coefficient Table” provided in Table 6, six thresholds are set to the paper thickness to create a table with seven rows, and paper thicknesses 1 to 7 are specified in the ascending order of the paper thickness, and a correction ratio (correction coefficient) is determined for each thickness.
As indicated in “Paper Thickness Correction Coefficient Table”, when the constant current control is applied to the AC component, the correction ratios used for different paper thicknesses are much smaller compared with those used in the comparative example in which the constant voltage control is applied. In this manner, even if the paper thickness changes slightly due to variations in paper, constant transferability can be achieved without changing (correcting) the control value (correction ratio).
Furthermore, in a highly humid environment, because the paper absorbs the moisture and reduces the resistance, the potential difference generated in the transfer nip needs to be reduced.
In this example as well, because the electric permittivity increases due to a moisture increase in the paper passing through the nip, and causes an increase in the capacity of the transfer unit as a capacitor, when the same amount of electrical charge is supplied using the constant current control, the potential difference generated in the transfer nip will be smaller. Therefore, for the humidity changes as well, the same transferability can be achieved without changing the target current by a large degree.
Compared side by side in Table 7 below are the correction coefficients used in different humidity environments when the constant current control is applied to the AC component (the embodiment) and when the constant voltage control is applied (comparative example). The number of thresholds (the number of rows in the table) or the correction ratios (coefficients) are just examples, and the embodiment is not limited thereto.
As indicated in Table 7, when the constant current control is applied to the AC component, the correction ratios used for different environments are smaller compared with those used in the comparative example in which the constant voltage control is applied. In this manner, when the environment changes slightly, constant transferability can be achieved without changing (correcting) the control value (correction ratio).
Finally, an embodiment of an image forming apparatus having a different structure will be explained.
The embodiment is not limited to an intermediate transfer type (indirect transfer type) color printer in which a toner image on the photosensitive element is transferred onto the intermediate transfer belt, and then further transferred onto a recording sheet, but is also applicable to a direct transfer type color printer in which a toner image on the photosensitive element is directly transferred onto a recording sheet, such as one illustrated in
The embodiment is also applicable to a so-called single drum type color image forming apparatus, as illustrated in
In this manner, the transfer unit according to the embodiment can transfer an image onto different types of medium having some texture, regardless of the structures of the image forming apparatus, once a flat image can be formed using electrostatic powder.
As explained so far, the embodiment can achieve stable transferability to the recessed parts of a transfer medium even in environmental changes or differences in the transfer media, in an electrostatic toner transfer unit that applies a voltage having a DC component superimposed on an AC component, by performing the constant current control to the output voltage of the power source using Ioff, or Ioff and Ipp of the output current.
Furthermore, by changing the values used in the constant current control depending on the conveyance velocity of the transfer medium, the stable transferability can be achieved even on the recessed part of a transfer medium in a transfer unit having modes with different velocities.
Furthermore, by changing the values used in the constant current control depending on the amount of toner attached on the image carrier, the stable transferability to the recessed parts of a transfer medium can be achieved even in images in which the amount of attached toner is very different, such as a monochromatic image and a color image.
Furthermore, by allowing the voltage output from the power source to be selected between an output in which the DC is superimposed on the AC and an output having only the DC, the transfer can also be switched to a transfer using a DC voltage (in the same manner as in the conventional transfer).
Furthermore, by configuring a power source for applying only the DC, and a power source for applying a DC/AC superimposed voltage or applying only an AC separately, the latter power source can be easily added to an existing system using only the DC power source, in a switchable manner, so as to improve functions.
Furthermore, by arranging a power source for applying only the DC and a power source for applying a DC/AC superimposed voltage or only an AC separately on the side of the image carrier and on the side of the transfer medium, respectively, the space in a product can be used effectively, and downsizing of the product becomes possible, for example.
By combining the transfer unit according to the embodiment with different types of image forming apparatuses, the transfer unit can be used for different applications in which electrostatic particles are transferred onto a transfer medium having some texture.
The embodiment is explained using the example illustrated in the drawings, however, the embodiment is not limited thereto. For a structure of the transfer unit, an appropriate structure may be used within the scope of the embodiment. For a configuration of the power source for applying the transfer bias, an appropriate configuration may be used as well.
The image forming apparatus may be configured in any way. For example, the image forming unit in each of the colors in the tandem type image forming apparatus can be arranged in any order. Furthermore, not only the tandem type, but also a structure using a plurality of developing units arranged around a single photosensitive element, or a structure using a revolver type developing unit is also possible. The embodiment may also be applied to a full-color machine using toners in three colors, a multi-color machine using toners in two colors, or to a monochromatic apparatus. The image forming apparatus is obviously not limited to a printer, but may also be a multi-function product (MFP) having a plurality of functions.
In the structure according to another aspect of the present invention, a structure for detecting the alternating current is not required. Therefore, the controlling structure can be simplified.
In the structure according to still another aspect of the present invention, high transferability can be achieved for various types of recording sheets, and stable transfer can be performed even on highly textured paper.
Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-051289 | Mar 2011 | JP | national |
| 2011-266684 | Dec 2011 | JP | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 4962307 | Nakaya | Oct 1990 | A |
| 5132869 | Nakaya | Jul 1992 | A |
| 5142329 | Nakaya | Aug 1992 | A |
| 5595852 | Shimoda et al. | Jan 1997 | A |
| 6284424 | Iimura et al. | Sep 2001 | B1 |
| 6405000 | Nakaya | Jun 2002 | B1 |
| 7127189 | Takamatsu et al. | Oct 2006 | B2 |
| 7236714 | Sato et al. | Jun 2007 | B2 |
| 7269368 | Sato et al. | Sep 2007 | B2 |
| 7561835 | Tanaka et al. | Jul 2009 | B2 |
| 7579716 | Sato et al. | Aug 2009 | B2 |
| 7653339 | Yamashita et al. | Jan 2010 | B2 |
| 7662530 | Iimura | Feb 2010 | B2 |
| 7693440 | Sato et al. | Apr 2010 | B2 |
| 7720418 | Tanaka et al. | May 2010 | B2 |
| 7738819 | Honda et al. | Jun 2010 | B2 |
| 7913098 | Nakaya et al. | Mar 2011 | B2 |
| 7941087 | Tanaka et al. | May 2011 | B2 |
| 7970334 | Yamashita et al. | Jun 2011 | B2 |
| 7979017 | Tanaka et al. | Jul 2011 | B2 |
| 7991342 | Kabata et al. | Aug 2011 | B2 |
| 8041276 | Ogino | Oct 2011 | B2 |
| 8045875 | Aoki et al. | Oct 2011 | B2 |
| 8080096 | Yamashita et al. | Dec 2011 | B2 |
| 8081916 | Tanaka et al. | Dec 2011 | B2 |
| 8081917 | Nakai et al. | Dec 2011 | B2 |
| 8107872 | Ozaki et al. | Jan 2012 | B2 |
| 20050189923 | Ohishi et al. | Sep 2005 | A1 |
| 20070059016 | Sato et al. | Mar 2007 | A1 |
| 20070178398 | Ogino et al. | Aug 2007 | A1 |
| 20080187857 | Ogino et al. | Aug 2008 | A1 |
| 20090142087 | Iimura | Jun 2009 | A1 |
| 20100034560 | Tanaka et al. | Feb 2010 | A1 |
| 20100054829 | Iio et al. | Mar 2010 | A1 |
| 20100098446 | Ishikawa et al. | Apr 2010 | A1 |
| 20100183972 | Hasegawa et al. | Jul 2010 | A1 |
| 20100209143 | Nakai et al. | Aug 2010 | A1 |
| 20100232820 | Usami et al. | Sep 2010 | A1 |
| 20100239309 | Tanaka et al. | Sep 2010 | A1 |
| 20100284705 | Aoki et al. | Nov 2010 | A1 |
| 20100303518 | Aimoto et al. | Dec 2010 | A1 |
| 20100310291 | Tanaka et al. | Dec 2010 | A1 |
| 20100329707 | Yamada | Dec 2010 | A1 |
| 20110008088 | Nakai et al. | Jan 2011 | A1 |
| 20110038656 | Hasegawa et al. | Feb 2011 | A1 |
| 20110052286 | Urayama et al. | Mar 2011 | A1 |
| 20110058831 | Usami et al. | Mar 2011 | A1 |
| 20110131434 | Nakaya | Jun 2011 | A1 |
| 20110181116 | Takeuchi et al. | Jul 2011 | A1 |
| 20110200348 | Aoki | Aug 2011 | A1 |
| 20110206399 | Fujita et al. | Aug 2011 | A1 |
| 20110229168 | Shiraishi et al. | Sep 2011 | A1 |
| 20110255892 | Aoki | Oct 2011 | A1 |
| 20120045231 | Ogino et al. | Feb 2012 | A1 |
| 20120045237 | Aoki et al. | Feb 2012 | A1 |
| 20120045259 | Nakamura et al. | Feb 2012 | A1 |
| Number | Date | Country |
|---|---|---|
| 9-146381 | Jun 1997 | JP |
| 2003-118158 | Apr 2003 | JP |
| 3791386 | Jun 2006 | JP |
| 2006-267486 | Oct 2006 | JP |
| 3918545 | Feb 2007 | JP |
| 2008-58585 | Mar 2008 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20120230715 A1 | Sep 2012 | US |
