JP2008275946A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent bleeding in print and blurring in print in an image forming apparatus which performs constant-voltage control. <P>SOLUTION: A transfer belt 10 carries a toner image. A secondary transfer roller 13 holds a recording medium with the transfer belt 10. A power source 26 applies bias voltage to the secondary transfer roller 13. A control part 18 performs the constant-voltage control to the power source 26. A storage part 25 stores the upper limit value and the lower limit value of a transfer current flowing to the secondary transfer roller 13. An ammeter 27 detects the transfer current. The control part 18 controls the bias voltage so that the ammeter 27 may detect the transfer current at the upper limit value when the transfer current detected by the ammeter 27 is larger than the upper limit value, and controls the bias voltage so that the ammeter 27 may detect the transfer current at the lower limit value when the transfer current detected by the ammeter 27 is smaller than the lower limit value. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus including a secondary transfer roller that secondarily transfers a toner image onto a recording medium.

  In a color image forming apparatus such as a 4-cycle method or a tandem method, a toner image formed on the surface of an electrostatic latent image carrier is a nip portion between the electrostatic latent image carrier and a primary transfer roller. Is transferred to the intermediate transfer belt passing through (primary transfer), and the four-color toner images are superimposed on the intermediate transfer belt by the primary transfer, thereby forming a full-color image. The full-color image is transferred (secondary transfer) to a recording medium such as a sheet passing through the nip portion at the nip portion between the intermediate transfer belt and the secondary transfer roller.

  In the secondary transfer, an electric field for moving the toner from the intermediate transfer belt to the recording medium is formed between the intermediate transfer belt and the secondary transfer roller, so the secondary transfer roller has a polarity opposite to the polarity of the toner. The transfer bias (secondary transfer bias) is applied.

  Therefore, as the secondary transfer roller, a roller having conductivity is used, and in recent years, an ion conductive roller excellent in uniformity of electric resistance is often used. When an ion conductive roller is used as the secondary transfer roller, high transfer performance can be obtained, so that the image quality can be improved.

  The transfer bias control method includes constant voltage control for controlling the transfer bias to a constant voltage and constant current control for controlling the transfer bias to a constant current. The ratio (S ′ / S × 100) of the area S ′ of the toner portion to the area S of the printable portion on the recording medium is generally called coverage, but is transferred well regardless of the coverage of the image to be printed. Therefore, it is preferable to control the transfer bias by constant voltage control. The appropriate value of the current to be applied to the transfer roller (transfer current) varies greatly depending on the image coverage, whereas the appropriate value of the voltage to be applied to the transfer roller (transfer voltage) depends on the image coverage. This is because it does not change greatly.

  By the way, as a recording medium used in a color image forming apparatus, even if the recording medium has the same size and is used for the same purpose, recording media having various resistance values are on the market. Therefore, a recording medium having a resistance value that deviates from the range of resistance values that can be used in the color image forming apparatus may be used in the color image forming apparatus.

  Under such circumstances, when constant voltage control is used as the transfer bias control method, toner scattering, printing bleeding, or printing blurring may occur. Specifically, when a recording medium having a resistance value lower than the range of resistance values that can be used in the color image forming apparatus is used, excessive current flows through the transfer roller, and excessive toner flows on the recording medium. It adheres and causes toner scattering and printing bleeding. On the other hand, when a recording medium having a resistance value higher than the range of resistance values that can be used in the color image forming apparatus is used, the current flowing through the transfer roller is insufficient, resulting in toner transfer failure to the recording medium. And faint printing occurs.

  In order to solve such a problem, Patent Documents 1 and 2 propose an image forming apparatus that sets an upper limit value of a current flowing from a bias power source to a transfer material when performing constant voltage control. According to the image forming apparatus, toner scattering and printing bleeding can be prevented.

However, the image forming apparatuses described in Patent Document 1 and Patent Document 2 still cannot prevent blurring of printing.
Japanese Patent Application Laid-Open No. 11-174871 Japanese Patent Laid-Open No. 11-272095

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to prevent printing bleeding and printing fading in an image forming apparatus that performs constant voltage control.

  The present invention includes an image carrier that carries a toner image, a transfer member that sandwiches a recording medium together with the image carrier, a voltage application unit that applies a bias voltage to the transfer member, and a constant voltage with respect to the voltage application unit. Control means for performing control, storage means for storing an upper limit value and a lower limit value of a transfer current flowing through the transfer member, and detection means for detecting the transfer current, wherein the control means is more than the upper limit value. When the transfer current detected by the detection means is larger, the detection means controls the bias voltage so as to detect the transfer current at the upper limit value, and the detection means detects the transfer current above the lower limit value. When the transfer current is smaller, the detecting means controls the bias voltage so as to detect the lower limit transfer current.

  According to the present invention, when a transfer current deviating from between the upper limit value and the lower limit value of the transfer current flows, constant current control is performed with the transfer current of the upper limit value or the lower limit value. Therefore, the transfer current is prevented from becoming excessive or insufficient, and even if a recording medium of the same type as the recording medium to be originally used and having a different resistance value is used, blurring or blurring of printing is caused. Is prevented from occurring.

  In the present invention, the image processing apparatus further includes an acquisition unit that acquires a printing condition, the storage unit stores the upper limit value and the lower limit value for each printing condition, and the control unit acquires the acquisition unit. When the transfer current detected by the detection unit is larger than the upper limit value corresponding to the printing condition, the detection unit controls the bias voltage so as to detect the transfer current of the upper limit value, and When the transfer current detected by the detection unit is smaller than the lower limit value corresponding to the printing condition acquired by the acquisition unit, the bias voltage is set so that the detection unit detects the transfer current of the lower limit value. You may control. Thereby, an upper limit value and a lower limit value suitable for the printing conditions are set.

  In the present invention, the storage means stores a bias voltage for the constant voltage control for each printing condition, and the control means uses the bias voltage according to the printing condition acquired by the acquisition means to Constant voltage control may be performed. Thereby, a bias voltage suitable for the printing condition is set.

  In the present invention, the printing condition may be temperature and / or humidity.

  In the present invention, the printing condition may be the size of the recording medium.

  In the present invention, the printing condition may be a type of the recording medium.

  In the present invention, the printing condition may be an image pattern of a toner image.

  In the present invention, the printing condition may be whether one side of a recording medium on which a toner image is to be formed is a front side or a back side in double-sided printing.

  Hereinafter, a configuration of an image forming apparatus according to an embodiment of the present invention will be described with reference to the drawings.

(Configuration of image forming apparatus)
FIG. 1 is a diagram showing an overall configuration of the image forming apparatus 1. An image forming apparatus 1 shown in FIG. 1 is an electrophotographic color printer, and synthesizes images of four colors (C: cyan, M: magenta, Y: yellow, K: black) in a so-called tandem system. It is composed. The image forming apparatus 1 has a function of receiving a print job and forming an image on a recording medium such as paper. The image forming apparatus 1 includes a printing unit 2, a paper feeding unit 14, a control unit 18 including a CPU, a paper discharge tray 20, A scanner unit 21 and an operation panel 22 are provided.

  The scanner unit 21 is configured to separate and read an original image into three colors of R (red), G (green), and B (blue) by a known mechanism, and to form R, G, and B image data. Has been. The control unit 18 converts the R, G, and B image data into C, M, Y, and K image data.

  The operation panel 22 is a panel for a user to issue an instruction to the image forming apparatus 1 and includes a touch panel 23 and input keys 24. The touch panel 23 serves as an input interface that accepts user input and also serves as display means for displaying an image. The input key 24 is an input interface composed of a plurality of mechanical keys.

  The paper supply unit 14 serves to supply recording media one by one, and includes a paper tray 15 and a paper supply roller 16. On the paper tray 15, a plurality of recording media in a state before printing are stacked and placed. The paper feed roller 16 takes out the paper placed on the paper tray 15 one by one.

  The printing unit 2 serves as an image printing unit that prints an image on a recording medium supplied from the paper feeding unit 14, and includes an image forming station 33 (33 C, 33 M, 33 Y, 33 K), an exposure control unit 3, 1. A secondary transfer roller 8 (8C, 8M, 8Y, 8K), a transfer belt 10, a driving roller 11, a driven roller 12, a secondary transfer roller 13, a cleaning blade 17, and a fixing device 19 are included. The image forming station 33 (33C, 33M, 33Y, 33K) includes the photosensitive drum 4 (4C, 4M, 4Y, 4K), the charger 5 (5C, 5M, 5Y, 5K), and the exposure device 6 (6C, 6M, 6Y, 6K), developing device 7 (7C, 7M, 7Y, 7K) and cleaner 9 (9C, 9M, 9Y, 9K).

  The charger 5 charges the peripheral surface of the photosensitive drum 4. The exposure control unit 3 causes the exposure device 6 to irradiate the laser based on the C, M, Y, and K image data converted by the control unit 18. As a result, an electrostatic latent image is formed on the peripheral surface of the photosensitive drum 4. The developing roller of the developing device 7 forms a toner image on the peripheral surface of the photosensitive drum 4 by supplying toner to the photosensitive drum 4 on which the electrostatic latent image is formed.

  The primary transfer roller 8 transfers the toner image formed on the photosensitive drum 4 to the transfer belt 10 stretched between the driving roller 11 and the driven roller 12. The transfer belt 10 serves as an image carrier that carries the toner image transferred by the primary transfer roller 8, and is made of, for example, polyimide.

  The drive roller 11 serves to drive the transfer belt 10 in the direction of the arrow in FIG. 1 by being rotated by a motor (not shown). Therefore, the surface of the driving roller 11 is preferably made of a material having a large friction coefficient such as rubber or urethane.

  The secondary transfer roller 13 serves as a transfer member that sandwiches the recording medium together with the transfer belt 10 and the driving roller 11 in order to transfer the toner image to the recording medium. As the material of the secondary transfer roller 13, a conductive material is used, for example, an ion conductive material. Specifically, nitrile rubber (NBR) is used as the ion conductive material, but urethane rubber, hydrin rubber, or the like can also be used. The toner image is conveyed to a nip portion N constituted by the driving roller 11 and the secondary transfer roller 13 by driving the transfer belt 10. A bias voltage is applied to the secondary transfer roller 13, and a toner image composed of toner particles having a negative charge is attracted to the secondary transfer roller 13 side by the bias voltage and fed. The image is transferred to the recording medium conveyed from the section 14.

  The fixing device 19 serves as a fixing unit that fixes the toner image on the recording medium by performing heat treatment and pressure treatment on the recording medium. A printed recording medium is placed on the paper discharge tray 20.

(About secondary transfer)
Next, secondary transfer will be described with reference to FIG. FIG. 2 is a diagram showing a configuration around the transfer belt 10, the driving roller 11, and the secondary transfer roller 13 where the secondary transfer is performed.

  FIG. 2 shows a transfer belt 10, a driving roller 11, a secondary transfer roller 13, a control unit 18, a storage unit 25, a power source 26, an ammeter 27, a voltmeter 28, a thermometer 29, and a hygrometer 30. . Since the transfer belt 10, the drive roller 11, and the secondary transfer roller 13 have already been described, description thereof will be omitted.

  The thermometer 29 serves to detect the temperature near the secondary transfer roller 13 and outputs the detected temperature to the control unit 18 as a printing condition. The hygrometer 30 serves to detect the humidity near the secondary transfer roller 13 and outputs the detected humidity to the control unit 18 as a printing condition.

  Here, the printing conditions refer to various conditions when the toner image is transferred to the recording medium. In the present embodiment, the printing conditions include absolute humidity, printing surface, paper width, media type, and coverage (image pattern). The absolute humidity is obtained from a temperature detected by the thermometer 29 and a humidity detected by the hygrometer 30 by a well-known mathematical formula. The amount of water vapor contained in 1 cubic meter of air at 1 atm is expressed in grams. It is. The printing surface indicates whether the surface on which the toner image is transferred is the front surface or the back surface when the printing mode is duplex printing. The printing surface is recognized by the number of pages printed by the control unit 18. Specifically, the control unit 18 recognizes that the front side is printed when printing odd pages, and recognizes that the back side is printed when printing even pages. Note that the printing surface in the single-sided printing mode corresponds to the surface in double-sided printing.

  The media type indicates whether the type of the recording medium is plain paper, OHP, or thick paper. The control unit 18 recognizes the media type by selecting which recording medium the user prints using the operation panel 22. The paper width means a width in a direction perpendicular to the paper passing direction of the recording medium, and is classified into four ranges of 209 mm or less, 210 mm to 240 mm, 241 mm to 278 mm, and 279 mm or more. The control unit 18 recognizes the paper width when the user selects the paper width using the operation panel 22.

  The coverage is a ratio of the area S ′ of the toner portion to the area S of the printable portion on the recording medium. In this embodiment, the coverage is 0% to 50%, 51% to 100%, 101% to 200%. Divided into three stages. A state where the coverage is 100% means a state of a solid image of one color, and a state where the coverage is 200% means a state of a solid image of two colors. The coverage is obtained by analyzing the image data read by the scanner unit 21.

  As described above, the thermometer 29, the hygrometer 30, the control unit 18, the operation panel 22, and the scanner unit 21 serve as acquisition means for acquiring printing conditions.

  The ammeter 27 is connected between the control unit 18 and the secondary transfer roller 13 and functions as a detection unit that detects the transfer current flowing through the recording medium by detecting the transfer current flowing through the secondary transfer roller 13. Fulfill. The voltmeter 28 is connected between the driving roller 11 and the secondary transfer roller 13 and plays a role of detecting a bias voltage between the driving roller 11 and the secondary transfer roller 13.

  The power source 26 serves as a voltage applying unit that applies a bias voltage to the secondary transfer roller 13. The control unit 18 is constituted by a CPU, for example, and serves as a control unit that performs constant voltage control on the power supply 26 with a bias voltage corresponding to printing conditions. The storage unit 25 is realized by a nonvolatile memory, a hard disk, or the like, and stores a bias voltage Y for constant voltage control as a first table for each printing condition. The first table shown in FIG. 3 is a table showing the bias voltage Y corresponding to each absolute humidity A under printing conditions where the printing surface is the front side and the media type is plain paper. The first table is prepared for each combination of printing surface and media type.

  Here, a method for determining the bias voltage Y will be described. The bias voltage Y is determined by a known control called ATVC (Auto Transfer Voltage Control) control. The bias voltage Y is a detection voltage X detected by the voltmeter 28 when the control unit 18 performs constant current control so that the ammeter 27 detects a predetermined current (for example, 20 μA). Is obtained by substituting

  Y = aX + b (Formula 1)

Here, the inclination a and the offset b are determined for each printing condition, and are values obtained in advance by experiments. For example, as shown in the first table of FIG. 3, when the printing surface is the front side, the media type is plain paper, and the absolute humidity is 0 g / m ³ or more and less than 3 g / m ³ , The slope a is 1 and the offset b is 1100. The ATVC control is a well-known control, and thus detailed description thereof is omitted.

  Incidentally, as a recording medium used in the image forming apparatus 1, for example, plain paper having various resistance values is sold even if it is plain paper of the same size. Therefore, a recording medium having a resistance value that deviates from the range of resistance values that can be used in the image forming apparatus 1 may be used in the image forming apparatus 1. In such a case, if the control unit 18 performs constant voltage control with the bias voltage Y, printing bleeding or blurring occurs. Below, it demonstrates using FIG.

FIG. 4 shows the flow of the secondary transfer roller 13 when the bias voltage Y applied to the secondary transfer roller 13 is changed when the toner image is secondarily transferred to plain paper having a plurality of types of resistance values. 5 is a graph showing changes in transfer current. The vertical axis represents the transfer current, and the horizontal axis represents the bias voltage Y. Three types of plain paper were used as plain paper. The three types of plain paper are standard resistance paper having a standard resistance value, high resistance paper having a resistance value higher than the standard resistance value, and low resistance paper having a resistance value lower than the standard resistance value. It is. As an experimental environment, the experiment was performed in an environment where the absolute humidity was 12 g / m ³ (temperature 23 ° C., humidity 65%).

  For example, when a bias voltage Y of 1.5 kV is used, a toner image can be satisfactorily transferred to standard resistance paper, but a toner image can be satisfactorily transferred to low resistance paper or high resistance paper. Can not do it. Specifically, the transfer current capable of satisfactorily transferring the toner image on low resistance paper is 45 μA to 70 μA. On the other hand, when a bias voltage Y of 1.5 kV is used for low resistance paper, the transfer current is about 80 μA. That is, printing blur occurs. Similarly, the transfer current capable of satisfactorily transferring the toner image on high resistance paper is 25 μA to 45 μA. On the other hand, when a bias voltage Y of 1.5 kV is used for high resistance paper, the transfer current is about 20 μA. That is, printing blur occurs.

  Therefore, in the image forming apparatus 1 according to the present embodiment, the upper limit value of the transfer current is set to 70 μA and the lower limit value is set to 25 μA under the printing conditions as described above. When the transfer current becomes larger than the upper limit of 70 μA, the control unit 18 performs constant current control so that the transfer current becomes 70 μA. When the transfer current becomes smaller than the lower limit value of 25 μA, the control unit 18 performs constant current control so that the transfer current becomes 25 μA.

  Therefore, when the transfer current detected by the ammeter 27 is larger than the predetermined upper limit value, the control unit 18 controls the bias voltage so that the ammeter 27 detects the transfer current of the upper limit value. When the transfer current detected by the ammeter 27 is smaller than the predetermined lower limit value, the ammeter 27 serves as a control means for controlling the bias voltage so as to detect the lower limit transfer current. . And the memory | storage part 25 has memorize | stored the upper limit and lower limit of the transfer current which flows into the secondary transfer roller 13 as a 2nd table shown in FIG. 5 for every printing condition. The second table shown in FIG. 5 includes the upper limit value and the lower limit value corresponding to each absolute humidity A under printing conditions where the printing surface is the front surface, the medium type is plain paper, and the coverage is 51 to 100%. It is a table showing values. The second table is prepared for each combination of print surface, media type, and coverage. Note that the upper limit value and the lower limit value are values obtained by experiments, and mean a range of transfer current in which a toner image can be satisfactorily transferred under each printing condition.

(Operation of image forming apparatus)
The operation of the image forming apparatus 1 configured as described above will be described below with reference to the drawings. The image forming apparatus 1 according to this embodiment performs the following first and second operations.
(First Operation) The control unit 18 performs constant current control periodically to determine the detection voltage X.
(Second Operation) The control unit 18 determines the bias voltage Y based on the detection voltage X and the printing conditions during image printing, and performs constant voltage control based on the bias voltage Y. At this time, in order to prevent the transfer current from becoming insufficient, the control unit 18 performs constant current control so that the transfer current becomes the lower limit value when the transfer current becomes smaller than the lower limit value. Further, in order to prevent the transfer current from becoming excessive, the control unit 18 performs constant current control so that the transfer current becomes the upper limit value when the transfer current becomes larger than the upper limit value.

(About the first operation)
FIG. 6 is a flowchart showing an operation performed by the control unit 18 in the first operation. First, an operation (ATVC control) performed by the image forming apparatus 1 when determining the detection voltage X will be described with reference to FIG. The controller 18 determines whether or not the secondary transfer roller 13 is retracted (step S1). Specifically, the secondary transfer roller 13 is retracted means that the secondary transfer roller 13 is not in pressure contact with the transfer belt 10. If the secondary transfer roller 13 is retracted, the process proceeds to step S2. If the secondary transfer roller 13 is not retracted, the process proceeds to step S3.

  When the secondary transfer roller 13 is retracted, the control unit 18 presses the secondary transfer roller 13 against the transfer belt 10 as shown in FIG. 2 (step S2). Thereafter, the process proceeds to step S3.

  Next, the control unit 18 sets the timer count to T = 0 (step S3), and starts the timer count (step S4).

  The control unit 18 that has started counting the timer performs constant current control (step S5). Specifically, the control unit 18 causes the secondary transfer roller 13 to apply a voltage to the power supply 26 so that the ammeter 27 detects a predetermined current value (for example, 20 μA). Next, the control unit 18 reads the detection voltage X detected by the voltmeter 28 (step S6).

  The control unit 18 that has read the detection voltage X determines whether or not the timer count is T ≧ T1 (step S7). Here, T1 is set to a time required for the end of the ATVC control. If T ≧ T1, the process proceeds to step S8. If T ≧ T1, the process returns to step S6. In this case, the control unit 18 repeats reading of the detection voltage X until T ≧ T1.

  In the case of T ≧ T1, the control unit 18 determines the value obtained by performing the averaging process on the detection voltage X obtained in step S6 as the detection voltage X (step S8). The detection voltage X is determined by the above processing. Note that the detection voltage X is updated to the latest state by repeatedly executing the first operation every predetermined time.

(About the second operation)
7 to 9 are flowcharts showing operations performed by the control unit 18 in the second operation. This process is started when the user sets a plurality of documents on the scanner unit 21 of the image forming apparatus 1. When the document setting is completed, the user operates the operation panel 22 to input information indicating whether or not double-sided printing is performed, paper size information, and media type information, and issues a print start instruction. In response, the control unit 18 receives an input of printing start from the user (step S10).

  The control unit 18 specifies the paper width on which the image is printed based on the paper size information included in the print start instruction received from the user (step S11).

  Next, the control unit 18 identifies the media type of the recording medium on which the image is printed based on the media type information included in the print start instruction received from the user (step S12). In the present embodiment, the media type is any one of plain paper, OHP, and cardboard.

  The control unit 18 that has specified the media type checks the absolute humidity (step S13). Specifically, since the control unit 18 periodically acquires the temperature and humidity from the thermometer 29 and the hygrometer 30, the controller 18 calculates the absolute humidity based on the acquired temperature and humidity.

  Next, the control unit 18 determines whether the printing mode is double-sided printing or single-sided printing based on the information indicating whether or not double-sided printing is input in step S10 (step S14). If the print mode is duplex printing, the process proceeds to step S15. If the print mode is single-sided printing, the process proceeds to step S18.

  When the print mode is duplex printing, the control unit 18 specifies the first table based on the media type specified in step S12 (step S15). Since the printing mode is double-sided printing, the control unit 18 is the first table corresponding to the media type specified in step S12, and the printing surface is the first table and the printing surface is the back side. 1 table is specified.

Next, the control unit 18 determines the bias voltages Y1 and Y2 based on the absolute humidity confirmed in Step S13 and the detected voltage X determined in Step S8 of FIG. 6 (Step S16). Specifically, for the bias voltage Y1, the detection voltage X is substituted into the formula of the bias voltage corresponding to the absolute humidity in the first table specified by the step S15 and the printing surface is the front table. This is the bias voltage obtained. For example, in the first table shown in FIG. 3, when the detection voltage X is 400V and the absolute humidity is 2 g / m ³ , the bias voltage Y1 is 1500V (400V + 1100V). Also, the bias voltage Y2 is obtained by substituting the detection voltage X into the formula of the bias voltage corresponding to the absolute humidity in the first table whose printing surface is the back surface in the first table specified in step S15. Bias voltage. Thus, when the printing mode is double-sided printing, the control unit 18 determines two bias voltages Y1 and Y2 in advance. Thereafter, the process proceeds to the double-sided printing process in step S17. The double-sided printing process (step S17) will be described below with reference to FIG.

  When the duplex printing process is started, the control unit 18 causes the scanner unit 21 to continuously read a plurality of originals (step S31). Thereby, the control part 18 acquires image data.

  Next, the control unit 18 determines whether or not the secondary transfer roller 13 is retracted (step S32). Specifically, the secondary transfer roller 13 is retracted means that the secondary transfer roller 13 is not in pressure contact with the transfer belt 10. If the secondary transfer roller 13 is retracted, the process proceeds to step S33. If the secondary transfer roller 13 is not retracted, the process proceeds to step S34.

  When the secondary transfer roller 13 is retracted, the control unit 18 presses the secondary transfer roller 13 against the transfer belt 10 as shown in FIG. 2 (step S33). Thereafter, the process proceeds to step S34.

  In step S34, the control unit 18 determines whether an image to be printed is printed on the surface of the recording medium (step S34). If an image is printed on the front surface, the process proceeds to step S35. On the other hand, when the image is not printed on the front surface, the control unit 18 determines that the image is printed on the back surface, and the process proceeds to step S36.

  When an image is printed on the front surface, the control unit 18 determines to perform constant voltage control using the bias voltage Y1 determined in step S16 of FIG. 7 (step S35). Thereafter, the process proceeds to step S37.

  When an image is printed on the back side, the control unit 18 determines to perform constant voltage control using the bias voltage Y2 determined in step S16 of FIG. 7 (step S36). Thereafter, the process proceeds to step S37.

  In step S37, the control unit 18 analyzes the image data of the image to be printed and specifies the coverage (step S37). The control unit 18 specifying the coverage specifies the second table corresponding to the media type and the coverage specified in Step S12 and Step S37 (Step S38). For example, when the printing surface is the front surface and the coverage is 51% to 100%, the second table shown in FIG. 5 is selected.

Next, the control unit 18 determines an upper limit value and a lower limit value of the transfer current corresponding to the paper width and the absolute humidity confirmed in Step S11 and Step S13 of FIG. 7 (Step S39). For example, in the second table shown in FIG. 5, when the absolute humidity is 2 g / m ³ and the paper width is 209 mm or less, the control unit 18 determines the upper limit value as 100 μA and sets the lower limit value as 50 μA. To decide.

  The control unit 18 that has determined the upper limit value and the lower limit value performs constant voltage control using the bias voltage Y1 or the bias voltage Y2 employed in step S35 or step S36 (step S40). Specifically, the control unit 18 applies the voltage generated by the power supply 26 to the secondary transfer roller 13 so that the detection voltage of the voltmeter 28 becomes constant at the bias voltage Y1 or the bias voltage Y2.

  The control unit 18 performing the constant voltage control reads the current I detected by the ammeter 27 (step S41). Then, the control unit 18 determines whether or not the read current I is between the upper limit value and the lower limit value determined in Step S39 (Step S42). When the current I is between the upper limit value and the lower limit value, the process proceeds to step S43. If the current I is larger than the upper limit value, the process proceeds to step S44. If the current I is smaller than the lower limit value, the process proceeds to step S45.

  When the current I is between the upper limit value and the lower limit value, since the transfer current for transferring the toner image satisfactorily flows, the control unit 18 maintains the constant voltage control started in step S40. (Step S43). Thereafter, the process proceeds to step S46.

  When the current I is larger than the upper limit value, an excessive transfer current flows. Therefore, the control unit 18 controls the bias voltage so that the ammeter 27 detects the upper limit transfer current, and the constant current is constant. Current control is performed (step S44). As a result, the transfer current is controlled to a current value at which the toner image is satisfactorily transferred. Thereafter, the process proceeds to step S46.

  When the current I is smaller than the lower limit value, the transfer current is insufficient. Therefore, the control unit 18 controls the bias voltage so that the ammeter 27 detects the transfer current at the lower limit value. Control is performed (step S45). As a result, the transfer current is controlled to a current value at which the toner image is satisfactorily transferred. Thereafter, the process proceeds to step S46.

  In step S46, the control unit 18 determines whether there is image data to be printed on the next page (step S46). If there is image data to be printed on the next page, the process returns to step S34. In this case, the processes in steps S34 to S46 are repeated until printing of all the image data is completed. On the other hand, if there is no image data to be printed on the next page, this processing ends.

  By the way, in step S14 of FIG. 7, when the print mode is single-sided printing, the control unit 18 specifies the first table based on the media type specified in step S12 (step S18). Since the printing mode is single-sided printing, the control unit 18 specifies the first table corresponding to the media type specified in step S12 and the printing surface is the first table.

  Next, the control unit 18 determines the bias voltage Y based on the absolute humidity confirmed in Step S13 and the detected voltage X determined in Step S8 of FIG. 6 (Step S19). Since this process is the same as the process of determining the bias voltage Y1 in step S16, detailed description thereof is omitted. Thereafter, the process proceeds to the single-sided printing process in step S20. The one-side printing process (step S20) will be described below with reference to FIG.

  When the single-sided printing process is started, the control unit 18 causes the scanner unit 21 to continuously read a plurality of originals (step S50). Thereby, the control part 18 acquires image data.

  Next, the control unit 18 determines whether or not the secondary transfer roller 13 is retracted (step S51). Specifically, the secondary transfer roller 13 is retracted means that the secondary transfer roller 13 is not in pressure contact with the transfer belt 10. If the secondary transfer roller 13 is retracted, the process proceeds to step S52. If the secondary transfer roller 13 is not retracted, the process proceeds to step S53.

  When the secondary transfer roller 13 is retracted, the control unit 18 presses the secondary transfer roller 13 against the transfer belt 10 as shown in FIG. 2 (step S52). Thereafter, the process proceeds to step S53.

  Next, the control unit 18 determines to perform constant voltage control using the bias voltage Y determined in step S19 of FIG. 7 (step S53). Thereafter, the process proceeds to step S54.

  Next, the control unit 18 analyzes the image data of the image to be printed and specifies the coverage (step S54). The control unit 18 specifying the coverage specifies the second table corresponding to the media type and the coverage specified in Step S12 and Step S54 (Step S55).

  Next, the control unit 18 determines an upper limit value and a lower limit value of the transfer current corresponding to the sheet width and the absolute humidity confirmed in Step S11 and Step S13 of FIG. 7 (Step S56). The process in step S56 is the same as the process in step S39 in FIG.

  The control unit 18 that has determined the upper limit value and the lower limit value performs constant voltage control with the bias voltage Y employed in step S53 (step S57). Specifically, the control unit 18 applies the voltage generated by the power supply 26 to the secondary transfer roller 13 so that the detection voltage of the voltmeter 28 becomes constant at the bias voltage Y.

  The control unit 18 performing the constant voltage control reads the current I detected by the ammeter 27 (step S58). Then, the control unit 18 determines whether or not the read current I is within the upper limit value to the lower limit value determined in Step S56 (Step S59). If the current I is between the upper limit value and the lower limit value, the process proceeds to step S60. When the current I is larger than the upper limit value, the process proceeds to step S61. If the current I is smaller than the lower limit value, the process proceeds to step S62.

  When the current I is between the upper limit value and the lower limit value, the control unit 18 maintains the constant voltage control started in step S57 (step S60). Thereafter, the process proceeds to step S63.

  When the current I is larger than the upper limit value, the control unit 18 performs constant current control by controlling the bias voltage so that the ammeter 27 detects the upper limit transfer current (step S61). Thereafter, the process proceeds to step S63.

  When the current I is smaller than the lower limit value, the control unit 18 performs constant current control by controlling the bias voltage so that the ammeter 27 detects the lower limit transfer current (step S62). Thereafter, the process proceeds to step S63.

  In step S63, the control unit 18 determines whether there is image data to be printed on the next page (step S63). If there is image data to be printed on the next page, the process returns to step S53. In this case, the processes in steps S53 to S63 are repeated until printing of all the image data is completed. On the other hand, if there is no image data to be printed on the next page, this processing ends.

(effect)
According to the image forming apparatus 1, when a transfer current deviating from between the upper limit value and the lower limit value of the transfer current determined by the printing conditions flows, the constant current control is performed with the transfer current of the upper limit value or the lower limit value. Is done. Therefore, the transfer current is prevented from becoming excessive or insufficient, and even if a recording medium of the same type as the recording medium to be originally used and having a different resistance value is used, blurring or blurring of printing is caused. Is prevented from occurring.

  Further, according to the image forming apparatus 1, the bias voltage is periodically determined based on the printing condition by ATVC control. The resistance value of the secondary transfer roller 13 varies depending on printing conditions such as absolute humidity, and also varies due to manufacturing variations. Therefore, by determining the bias voltage based on the printing conditions before the constant voltage control, even if the resistance value of the secondary transfer roller 13 fluctuates, the toner image can be transferred with the optimum bias voltage. It becomes like this. Furthermore, in the image forming apparatus 1, since the upper limit value and the lower limit value of the transfer current are changed according to the printing conditions, the upper limit value and the lower limit value can be set to optimum values. As a result, it is possible to more reliably prevent blurring and blurring of printing.

(Test 1)
Test 1 was performed to confirm the relationship between the electrical resistance of the ion conductive roller and the absolute water content around the roller.

  In Test 1, as the ion conductive roller, a roller in which the outer periphery of a core portion made of a pipe was covered with a cylindrical resin portion was used. The length of the roller is 340 mm, the outer diameter is 30 mm, the outer diameter of the core portion is 12 mm, and the thickness of the resin portion is 9 mm. The resin part is made of nitrile rubber having ionic conductivity, and the Asker C hardness of the resin part is 30 degrees.

The results of Test 1 are shown in FIG. In FIG. 10, the vertical axis represents the resistance value (Ω) of the secondary transfer roller 13, and the horizontal axis represents the absolute humidity (g / m ³ ). From the results of Test 1, it can be understood that the electrical resistance of the ion conductive roller tends to fluctuate as the absolute humidity around the roller is lower, and tends to stabilize at a lower value as the absolute humidity is higher.

(Test 2)
Test 2 was performed to confirm the environmental dependence of the ion conductive roller.

In Test 2, the same ion conductive roller as that used in Test 1 was used as the ion conductive roller. Then, LL environment absolute humidity is 2 g / ^{m 3,} NN environment absolute humidity is 12 g / ^{m 3,} and a bias voltage under the three environments of HH environment absolute humidity is 25 g / ^{m 3} transcription The relationship with current was investigated. Standard resistance paper was used as the recording medium.

  The results of Test 2 are shown in FIG. In FIG. 11, the vertical axis represents the transfer current (μA), and the horizontal axis represents the bias voltage (kV). From the results of Test 2, it can be understood that the ion conductive roller is highly dependent on the environment, and it is necessary to set an appropriate bias voltage for each of the HH environment, the NN environment, and the LL environment. Furthermore, since the upper limit value and the lower limit value of the transfer current vary depending on the environment, it can be understood that it is necessary to set appropriate upper limit values and lower limit values.

(Test 3)
Test 3 was performed to confirm the fluctuation of the upper limit value and the lower limit value of the transfer current depending on the paper size.

  In Test 3, the same ion conductive roller as that used in Test 1 was used as the ion conductive roller. Then, the relationship between the bias voltage and the transfer current when the toner image was transferred to the standard resistance paper of A4 vertical size and the standard resistance paper of A3 vertical size was examined. Test 3 was performed in an NN environment.

  The results of Test 3 are shown in FIG. In FIG. 12, the vertical axis represents the transfer current (μA), and the horizontal axis represents the bias voltage (kV). From the result of Test 3, it can be understood that the upper limit value and the lower limit value of the transfer current vary depending on the paper size. That is, it can be understood that it is necessary to set appropriate upper and lower limits according to the paper size.

(Test 4)
In double-sided printing, Test 4 was performed to confirm the fluctuation of the bias voltage and the fluctuation of the upper limit value and the lower limit value of the transfer current when the toner images were transferred to the front surface and the back surface, respectively.

  In Test 4, the same ion conductive roller as that used in Test 1 was used as the ion conductive roller. Then, the relationship between the bias voltage and the transfer voltage when transferring the toner image to the front surface of the standard resistance paper and the back surface of the standard resistance paper was examined. Test 4 was performed in an NN environment.

  The results of Test 4 are shown in FIG. In FIG. 13, the vertical axis represents the transfer current (μA), and the horizontal axis represents the bias voltage (kV). From the result of Test 4, in double-sided printing, the appropriate bias voltage differs between the transfer of the toner image to the front surface and the transfer of the toner image to the back surface, and the appropriate upper and lower limits of the transfer current vary. I understand that. That is, in double-sided printing, it can be understood that it is necessary to set an appropriate bias voltage, upper limit value, and lower limit value according to the printing surface.

(Test 5)
Test 5 was performed to confirm that the bias voltage and the upper limit value and the lower limit value of the transfer current fluctuate according to the media type.

  In Test 5, the same ion conductive roller as that used in Test 1 was used as the ion conductive roller. Then, the relationship between the bias voltage and the transfer current when the toner image was transferred to each of the standard resistance paper plain paper and the standard resistance paper thick paper was examined. Test 5 was performed in an NN environment.

  The results of Test 5 are shown in FIG. In FIG. 14, the vertical axis represents the transfer current (μA), and the horizontal axis represents the bias voltage (kV). From the results of Test 5, it can be understood that the appropriate bias voltage differs depending on the media type, and the appropriate upper and lower limits of the transfer current vary. That is, it can be understood that it is necessary to set an appropriate bias voltage, upper limit value, and lower limit value according to the media type.

1 is a diagram illustrating an overall configuration of an image forming apparatus. FIG. 4 is a diagram illustrating a configuration around a transfer belt, a driving roller, and a secondary transfer roller on which secondary transfer is performed. It is the figure which showed the structure of the 1st table. In the case of secondary transfer of a toner image to plain paper having a plurality of types of resistance values, a change in transfer current flowing through the secondary transfer roller was shown when the bias voltage applied to the secondary transfer roller was changed. It is a graph. It is the figure which showed the structure of the 2nd table. It is the flowchart which showed the operation | movement which a control part performs in the case of a 1st operation | movement. It is the flowchart which showed the operation | movement which a control part performs in the case of 2nd operation | movement. It is the flowchart which showed the operation | movement which a control part performs in the case of 2nd operation | movement. It is the flowchart which showed the operation | movement which a control part performs in the case of 2nd operation | movement. 3 is a graph showing the results of Test 1. 6 is a graph showing the results of Test 2. 6 is a graph showing the results of Test 3. 6 is a graph showing the results of Test 4. 10 is a graph showing the results of Test 5.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 4 Photosensitive drum 5 Charger 6 Exposure apparatus 7 Developing apparatus 8 Primary transfer roller 10 Transfer belt 11 Drive roller 12 Driven roller 13 Secondary transfer roller 18 Control part 19 Fixing apparatus 21 Scanner part 22 Operation panel 23 Touch panel 24 Input Keys 25 Storage Unit 26 Power Supply 27 Ammeter 28 Voltmeter 29 Thermometer 30 Hygrometer

Claims (8)

An image carrier for carrying a toner image;
A transfer member sandwiching a recording medium together with the image carrier;
Voltage applying means for applying a bias voltage to the transfer member;
Control means for performing constant voltage control on the voltage application means;
Storage means for storing an upper limit value and a lower limit value of a transfer current flowing through the transfer member;
Detecting means for detecting the transfer current;
With
The control means includes
When the transfer current detected by the detection unit is larger than the upper limit value, the detection unit controls the bias voltage so as to detect the transfer current of the upper limit value, and
If the transfer current detected by the detection means is smaller than the lower limit value, the detection means controls the bias voltage so as to detect the transfer current of the lower limit value;
An image forming apparatus. An acquisition means for acquiring print conditions
In addition,
The storage means stores the upper limit value and the lower limit value for each printing condition,
The control means includes
When the transfer current detected by the detection unit is larger than the upper limit value corresponding to the printing condition acquired by the acquisition unit, the bias voltage is set so that the detection unit detects the transfer current of the upper limit value. And controlling
When the transfer current detected by the detection unit is smaller than the lower limit value corresponding to the printing condition acquired by the acquisition unit, the bias voltage is set so that the detection unit detects the transfer current of the lower limit value. To control,
The image forming apparatus according to claim 1. The storage means stores a bias voltage for the constant voltage control for each printing condition,
The control means performs the constant voltage control with a bias voltage corresponding to the printing condition acquired by the acquisition means;
The image forming apparatus according to claim 2. The printing conditions are temperature and / or humidity;
The image forming apparatus according to claim 2, wherein the image forming apparatus is an image forming apparatus. The printing condition is the size of the recording medium;
The image forming apparatus according to claim 2, wherein the image forming apparatus is an image forming apparatus. The printing condition is the type of the recording medium;
The image forming apparatus according to claim 2, wherein the image forming apparatus is an image forming apparatus. The printing condition is an image pattern of a toner image;
The image forming apparatus according to claim 2, wherein: The printing condition is that in double-sided printing, one side of a recording medium on which a toner image is to be formed is the front side or the back side.
The image forming apparatus according to claim 2, wherein the image forming apparatus is an image forming apparatus.

Images