JP2005043925A - Electrophotographic printer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrophotographic printer which stabilizes a print medium and conveys the same when performing monocolor printing. <P>SOLUTION: The electrophotographic printer has: a transfer belt which is driven by a drive means and conveys the print medium; a plurality of image carriers located along the transfer belt; a transfer means which is arranged oppositely to the respective image carriers across the transfer belt; and an image carrier driving means which brings one or two of the image carriers of corresponding colors and other colors into contact with the transfer belt, when performing the monocolor printing. In such a case, because one or two of the image carriers of corresponding colors and other colors are brought into contact with the transfer belt when performing the monocolor printing, the print medium is stably conveyed to the transfer part of the corresponding color. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an electrophotographic printer.

  2. Description of the Related Art Conventionally, in an electrophotographic printer using a transfer belt, when printing is started, a print medium is fed by a hopping roller or manual feed, and is electrostatically attracted to the transfer belt at an adsorption portion, and then is transferred by the transfer belt. It is conveyed and sent between the photosensitive drum and the transfer roller, that is, to the transfer unit.

  A plurality of photosensitive drums of yellow, magenta, cyan, and black are arranged along the transfer belt so as to be able to contact and separate from the transfer belt so that color printing can be performed. Each transfer roller is disposed with the transfer belt interposed therebetween. In addition, the toner image of each color formed on each photosensitive drum can be transferred to the print medium by transporting the print medium by running the transfer belt and applying a transfer voltage to each transfer roller. (For example, refer to Patent Document 1).

  Each photosensitive drum takes an up state separated from the transfer belt and a down state brought into contact with the transfer belt, and can be placed in the down state only when necessary. Therefore, the durability of the photosensitive drum can be improved. When performing color printing, the yellow, magenta, cyan, and black photosensitive drums are placed in the down state, and when performing black printing, the yellow, magenta, and cyan photosensitive drums are placed in the up state. Then, the black photosensitive drum is placed in the down state.

  When all the toner images have been transferred, the adsorbed print medium is separated from the transfer belt and sent to the fixing device, where the toner image is fixed on the print medium.

  However, in the conventional electrophotographic printer, the transfer voltage is equal when the print medium is present in each transfer portion and when it is not present, so when continuous printing is performed in a low temperature and low humidity environment, the transfer voltage is transferred. The belt is charged, and the control unit malfunctions due to excessive charging, or a conveyance jam occurs.

  FIG. 2 is a conceptual diagram of a conventional electrophotographic printer.

  In the figure, 11 is a photosensitive drum, 16 is a transfer belt, 17 is a transfer roller, 19 is a printing medium, and P is a transfer portion.

  In this case, in order to prevent the photoconductor drum 11 and the transfer belt 16 from being worn, all the photoconductor drums 11 that are not used are placed in an up state. For example, when black printing is performed, yellow, magenta, and cyan photoconductors are used. The drum 11 is placed in an up state. Since the black photosensitive drum 11 is disposed at a position farthest from the suction portion, a sufficient suction force can be maintained up to the black transfer portion depending on the type of the printing medium 19, changes in the environment, and the like. There are cases where it is not possible.

  An object of the present invention is to solve the problems of the conventional electrophotographic printer and to provide an electrophotographic printer capable of stably transporting a print medium when performing monochromatic printing.

  For this purpose, in the electrophotographic printer of the present invention, a transfer belt that is driven by a driving means and conveys a print medium, a plurality of image carriers disposed along the transfer belt, and the transfer belt are sandwiched between the transfer belt and the transfer belt. The transfer means disposed opposite to each of the image carriers, and one or two of the corresponding color image carrier and the other color image carrier when performing monochromatic printing, Image carrier driving means for contacting the belt.

  According to the present invention, in an electrophotographic printer, a transfer belt that is traveled by a driving unit and conveys a print medium, a plurality of image carriers disposed along the transfer belt, and the transfer belt interposed therebetween. The transfer means disposed opposite to each of the image carriers, and one or two of the corresponding color image carrier and the other color image carrier when performing monochromatic printing, Image carrier driving means for contacting the belt.

  In this case, when performing monochromatic printing, one or two of the corresponding color image carrier and the other color image carrier are brought into contact with the transfer belt. Can be conveyed stably.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 3 is a schematic diagram of an image forming unit of the electrophotographic printer according to the first embodiment of the present invention.

  In the figure, 11 is a photosensitive drum as an image carrier, 12 is a charging roller as charging means for charging the photosensitive drum 11, and 13 is not shown by irradiating the surface of the charged photosensitive drum 11. An exposure device 14 serving as an exposure unit for forming an electrostatic latent image is developed by attaching toner (not shown) to the electrostatic latent image, and a developing roller 15 is used as a developing unit for forming a toner image (not shown). A toner supply roller as toner supply means for supplying toner to the developing roller 14. The developing roller 14 and the toner supply roller 15 constitute a developing device.

  Reference numeral 16 denotes a transfer belt that is driven by a driving means (not shown) and conveys a printing medium (not shown) as it travels. Reference numeral 17 denotes a transfer portion P formed between the photosensitive drum 11 and the transfer belt P. A transfer roller 18 serving as a transfer unit that transfers the toner image to a print medium, and a cleaning blade 18 serving as a cleaning unit that removes toner remaining on the photosensitive drum 11 after transfer.

  FIG. 4 is a diagram showing a state during color printing of the electrophotographic printer according to the first embodiment of the present invention, and FIG. 5 is a diagram showing a state during black printing of the electrophotographic printer according to the first embodiment of the present invention. FIG.

  In the drawing, 11Y is a yellow photosensitive drum, 11M is a magenta photosensitive drum, 11C is a cyan photosensitive drum, and 11K is a black photosensitive drum. Any of the photosensitive drums 11Y, 11M, 11C, and 11K Is also opposed to the transfer roller 17 with the transfer belt 16 in between.

  At the time of color printing, image carrier driving means (not shown) is driven, and the photosensitive drums 11Y, 11M, 11C, and 11K for the respective colors are placed in the down state. Reference numeral 21 denotes a first adsorbing roller 21a and a second adsorbing roller 21b. An adsorbing unit 22 adsorbs a print medium (not shown) fed in an arrow direction from a paper supply unit (not shown) to the transfer belt 16, Reference numeral 23 denotes an idle roller, and reference numeral 25 denotes a drive roller that is rotated by a drive means (not shown) to drive the transfer belt 16. The drive roller 25 separates the print medium from the transfer belt 16 when all the transfer is completed. For this purpose, the separated and separated print medium is further conveyed in the direction of the arrow and sent to a fixing device (not shown).

  During black printing, the image carrier driving means is driven, and the yellow, magenta, and cyan photosensitive drums 11Y, 11M, and 11C are placed in the up state, and the black photosensitive drum 11K is placed in the down state.

  FIG. 1 is a control block diagram of the electrophotographic printer according to the first embodiment of the present invention.

  In the figure, 31 is a host computer as a host device, 32 is an operation panel for setting the electrophotographic printer, 33 is a printer that receives print data, setting values, etc. from the host computer 31 and controls the entire electrophotographic printer. A control unit 34 for detecting the temperature of the environment where the electrophotographic printer is placed, and a temperature sensor 35 for sending the sensor output to the control unit 33; 35 for detecting the humidity of the environment where the electrophotographic printer is placed. The humidity sensor 36 sends the sensor output to the control unit 33, and the paper sensor 36 detects the size, conveyance position, conveyance state, etc. of the print medium (not shown) and sends the sensor output to the control unit 33. The temperature sensor 34 and the humidity sensor 35 constitute environmental condition detection means for detecting environmental conditions such as temperature and humidity.

  Reference numeral 37 denotes a high voltage determination unit which is disposed in the control unit 33 and determines a transfer voltage based on the sensor outputs of the temperature sensor 34 and the humidity sensor 35. The high voltage determination unit 37 performs transfer. The transfer voltage value during transfer, that is, the transfer voltage table 37a for determining the transfer voltage value, and the transfer voltage value when the transfer is not performed, that is, the inter-paper voltage value. Is provided with a paper-to-paper voltage table 37b.

  Reference numeral 38 denotes a transfer voltage value and an inter-paper voltage value determined by the high voltage determination unit 37, and a high voltage output unit serving as a transfer voltage application unit that applies a transfer voltage to the transfer roller 17 (FIG. 3) at a predetermined timing. , 39 is a motor driver that generates a timing signal for driving a driving means (not shown).

  FIG. 6 is a diagram showing the inter-paper voltage value sequence in the first embodiment of the present invention, and FIG. 7 is a diagram showing the relationship between the transfer voltage and the moisture content in the air. In FIG. 7, the horizontal axis represents the moisture content in the air, and the vertical axis represents the transfer voltage.

  In FIG. 6, 11 is a photosensitive drum, 16 is a transfer belt, 17 is a transfer roller, 19 is a printing medium, and P is a transfer portion to which transfer voltages of a transfer voltage value a and an inter-paper voltage value b are applied. .

  In FIG. 7, “a” represents a voltage value during transfer, and “b” represents a voltage value between sheets in the present embodiment. The inter-paper voltage value b is set to a constant value that does not exceed the transfer voltage value a.

  In this case, a transfer voltage suitable for the environment can be applied to the transfer roller 17 based on the moisture content in the air. For this purpose, the high voltage determination unit 37 (FIG. 1) changes the inter-paper voltage value b in a stepwise manner corresponding to the sensor outputs of the temperature sensor 34 and the humidity sensor 35, so that the water content is small and each sensor output is When the value becomes smaller and falls below a predetermined threshold (threshold) value, the paper-to-paper voltage value b is lowered or cut to 0 [V], and the water content is large, and each sensor output is below the threshold value. If it is larger, the inter-paper voltage value b is set to the same value as the transfer voltage value a or a constant value that does not exceed the transfer voltage value a. When the sensor output is medium, the inter-sheet voltage value b is set to about 50% of the transfer voltage value a. Note that the paper-to-paper voltage value b is lowered when each sensor output is equal to or lower than the first threshold value, and the paper-to-paper voltage value b is set to 0 [V] when each sensor output is equal to or lower than the second threshold value. You can also

  Therefore, even if continuous printing is performed in a low-temperature and low-humidity environment, the inter-paper voltage value b is lowered, so that the transfer belt 16 can be prevented from being charged. As a result, the control unit 33 does not malfunction.

  Further, since the transfer belt 16 can be prevented from being charged, when the print medium 19 is lifted by the action of static electricity or when the print medium 19 reaches the transfer portion P, the front end of the print medium 19 is the photosensitive member. The drum 11 or the casing (not shown) is not caught and broken. Therefore, no conveyance jam occurs.

  Further, since the print medium 19 does not float up, the casing and the print surface of the print medium 19 are not rubbed. Therefore, it is possible to prevent the print quality from being deteriorated.

  The transfer voltage value a is stored in the transfer voltage table 37a, and the paper voltage value b is stored in the paper voltage table 37b.

  FIG. 8 is a time chart showing the operation of the high voltage determination unit in the first embodiment of the present invention.

First, when printing is started at timing t0, the high voltage determination unit 37 (FIG. 1) sets the suction voltage F _iX applied to the suction unit 21 (FIG. 5) at the timing t1 to a low level (L) and yellow. Transfer voltage T _r −Y applied to the transfer roller 17, transfer voltage T _r −M applied to the magenta transfer roller 17, transfer voltage T _r −C applied to the cyan transfer roller 17, and black All the transfer voltages T _r -K applied to the transfer roller 17 are set to a low level given by the inter-paper voltage value b (FIG. 7).

Next, at the timing t2, the suction voltage F _{iX is set} to the high level (H), and at the timing t3, the transfer voltage T _r -Y is set to the high level given by the transfer voltage value a.

Subsequently, at timing t4, the transfer voltage T _r -M is set to the high level given by the transfer time voltage value a, at time t5, the adsorption voltage F _{iX is set} to the low level, and the transfer voltage T _r -C is set to the transfer time voltage value a. At a timing t6, the transfer voltage T _r -Y is set to a low level given by the inter-sheet voltage value b, and the transfer voltage T _r -K is set to a high level given by the transfer voltage value a.

Then, the transfer voltage T _r -M at timing t7 to the low level given by the paper between the voltage value b, and the transfer voltage T _r -C timing t8 to the low level given by the paper between the voltage value b, the transfer timing t9 The voltage T _r -K is set to a low level given by the paper-to-paper voltage value b, the suction voltage F _{iX is set} to 0 at the timing t10, and the transfer voltages T _r -Y, T _r -M, T _r -C, T _{r are set.} -K is set to 0 for both.

For convenience of explanation, the attraction voltage F _iX to low level at time t5, and, although a high level given transfer voltage T _r -C by the transfer time of the voltage value a, in fact, the low adsorption voltage F _iX The timing for setting the transfer voltage T _r -C and the timing for setting the transfer voltage T _r -C to a high level are different. For example, when printing on an A4 size printing medium 19 (FIG. 6), the transfer voltage T _r -C is set to the high level given by the transfer voltage value a, and then the suction voltage F _{iX is set} to the low level. .

  Next, the operation of the electrophotographic printer having the above configuration will be described.

  FIG. 9 is a flowchart showing the operation of the electrophotographic printer according to the first embodiment of the present invention.

First, when the control unit 33 (FIG. 1) receives print data from the host computer 31 and starts printing, the voltage value changing means (not shown) of the high voltage determination unit 37 is a sensor of the temperature sensor 34 and the humidity sensor 35. The output is read, the environment is estimated based on the sensor output, and the transfer voltages T _r -Y (FIG. 8), T _r -M, T _r -C, and T _r -K corresponding to the environment are determined. Next, the control unit 33 determines whether the printing is color printing or black printing based on the printing data. If the printing is color printing, all the photosensitive drums 11Y (FIG. 5), 11M, 11C, Place 11K in the down state. On the other hand, in the case of black printing, the photosensitive drums 11Y, 11M, and 11C are placed in the up state, and the black photosensitive drum 11K is placed in the down state.

Subsequently, the control unit 33 adsorbs the print medium 19 (FIG. 6) to the transfer belt 16 and conveys and transfers it. In this case, while the print medium 19 exists in the transfer portion P of each color, the transfer voltages T _r −Y, T _r −M, T _r −C, and T _r −K are set to the transfer voltage value a. Sequentially applied to the transfer roller 17. Actually, the transfer voltages T _r −Y, T _r −M, T _r −C, T _r from the time when the front end of the print medium 19 reaches the transfer portion P of each color until the front end of the printable area arrives. −K is set to a voltage value “a” at the time of transfer and sequentially applied to the transfer roller 17. Then, until the transfer voltages T _r -Y, T _r -M, T _r -C, T _r -K reach the transfer voltage value a and the rear end of the printable area reaches each transfer part P. The transfer voltages T _r −Y, T _r −M, T _r −C, and T _r −K are set to the inter-paper voltage value “b” until the rear end of the print medium 19 reaches the transfer roller 17. Applied. In this case, since the transfer voltages T _r −Y, T _r −M, T _r −C, and T _r −K are not set to the transfer voltage value a when there is no print medium 19 in each transfer portion P, a large amount of transfer is performed. No current flows. Accordingly, it is possible to prevent the transfer belt 16, the photosensitive drums 11Y, 11M, 11C, 11K, and the like from being deteriorated, so that the print quality can be improved.

  Further, when it is determined that the transfer belt 16, the print medium 19 and the like are easily charged based on sensor outputs of the temperature sensor 34 and the humidity sensor 35, for example, a low temperature and low humidity environment, the inter-paper voltage value b. Is set to about 50% of the voltage value a during transfer, or 0 [V].

  In this case, when the inter-paper voltage value b is set to about 50 [%] of the transfer voltage value a or 0 [V], the transfer portion P to which the transfer voltage of the transfer voltage value a is applied. Alternatively, a leakage current may flow from the suction portion 21 toward the transfer portion P to which the transfer voltage of the inter-paper voltage value b is applied. However, in a low-temperature and low-humidity environment, the print medium 19 and the transfer Since the impedance of the belt 16 becomes high, the leakage current is small.

  In this way, when all the transfer in each transfer portion P is completed, the print medium 19 is separated from the transfer belt 16 and sent to a fixing device (not shown), and the toner image is fixed in the fixing device, and print output is performed. Is called.

Next, a flowchart will be described.
Step S1 Printing starts.
Step S2 The sensor outputs of the temperature sensor 34 and the humidity sensor 35 are read.
Step S3: Transfer voltages T _r -Y, T _r -M, T _r -C, T _r -K are determined.
Step S4: It is determined whether or not color printing is performed. If it is color printing, the process proceeds to step S6. If it is not color printing, that is, if it is black printing, the process proceeds to step S5.
Step S5: The black photosensitive drum 11K is placed in the down state.
Step S6: All the photosensitive drums 11Y, 11M, 11C, and 11K are placed in the down state.
Step S7 The printing medium 19 is conveyed.
Step S8: It is determined whether or not the print medium 19 exists between the photosensitive drums 11Y, 11M, 11C, and 11K for each color and the transfer belt 16. If the print medium 19 is present, the process proceeds to step S10, and if not, the process proceeds to step S9.
Step S9: Transfer voltages T _r -Y, T _r -M, T _r -C, T _r -K of the inter-paper voltage value b are applied.
Step S10: Transfer voltages T _r -Y, T _r -M, T _r -C, T _r -K of the transfer voltage value a are applied.
Step S11: It is determined whether all the transfer has been completed. If all the transfer is completed, the process proceeds to step S12. If not completed, the process returns to step S8.
Step S12: Fixing is performed.
Step S13 Printout is performed.

  Thus, even when continuous printing is performed in a low-temperature and low-humidity environment, the inter-paper voltage value b is lowered, so that the transfer belt 16 can be prevented from being charged. As a result, the controller 33 does not malfunction or cause a transport jam.

In this embodiment, when printing starts, the sensor outputs of the temperature sensor 34 and the humidity sensor 35 are read, and the transfer voltages T _r −Y, T _r −M, T _r −C, T _r −K are read. Subsequently, whether color printing or black printing is determined. If color printing is performed, all the photosensitive drums 11Y, 11M, 11C, and 11K are placed in the down state, and black printing is performed. In some cases, the photosensitive drums 11Y, 11M, and 11C are in the up state and the black photosensitive drum 11K is in the down state. As described later, immediately after the start of printing, Whether color printing or black printing can also be determined.

In that case, it is determined whether the printing is color printing or black printing. If the printing is color printing, all the photosensitive drums 11Y, 11M, 11C, and 11K are placed in the down state. If the printing is black printing, The photosensitive drums 11Y, 11M, and 11C are placed in the up state, and the black photosensitive drum 11K is placed in the down state. Thereafter, the sensor outputs of the temperature sensor 34 and the humidity sensor 35 are read, and the transfer voltages T _r −Y, T _r −M, T _r −C, and T _r −K are determined.

  In the case of color printing, color printing mode transfer processing is performed, and in the case of black printing, black printing mode transfer processing is performed.

  By the way, in a high temperature and high humidity environment, the printing medium 19 absorbs moisture in the air and the impedance becomes low. Therefore, in the electrophotographic printer according to the present embodiment, the transfer voltage of the inter-paper voltage value b is applied. A leakage current flows from the transferred transfer portion P toward the transfer portion P to which the transfer voltage of the transfer voltage value a is applied. In that case, the potential formed between the photosensitive drums 11Y, 11M, 11C, and 11K and the print medium 19 becomes unstable, the electric field becomes unstable, and transfer defects occur.

  FIG. 10 is a first diagram showing a state of leakage current during black printing of the electrophotographic printer. In addition, about the thing which has the same structure as FIG. 5, the description is abbreviate | omitted by providing the same code | symbol.

  In the figure, 51Y, 51M, 51C, and 51K are yellow, magenta, cyan, and black transfer power sources, 52 is a heat roller, 53 is a backup roller, 54 is a zener diode, and 55 is a transfer belt 16 in the conveyance path of the print medium 19. And a guide sheet metal disposed between the heat roller 52 and the backup roller 53.

  The Zener diode 54 is connected between the guide sheet metal 55 and the heat roller 52 and the ground, and neutralizes the print medium 19 via the guide sheet metal 55 in a low temperature and low humidity environment. Accordingly, since the print medium 19 is not charged, the conveyance failure of the print medium 19 or the print quality is not deteriorated. The Zener diode 54 clamps a constant voltage in a high temperature and high humidity environment. Therefore, the leakage current is prevented from flowing from each transfer portion toward the heat roller 52.

  In this case, the yellow, magenta, and cyan photosensitive drums 11Y, 11M, and 11C are placed in the up state, and only the black photosensitive drum 11K is placed in the down state.

The yellow, magenta, and cyan transfer power supplies 51Y, 51M, and 51C transfer the transfer voltages T _r −Y, T _r −M, and T _r − with the inter-paper voltage value b regardless of whether the print medium 19 is present. C is applied to each transfer roller 17. On the other hand, when the printing medium 19 is present and the transfer is performed, the black transfer power source 51K uses the transfer voltage _Tr- K of the voltage value a at the time of transfer as the printing medium 19 does not exist and the transfer is not performed. In this case, a transfer voltage T _r -K having an inter-paper voltage value b is applied to the black transfer roller 17.

  By the way, in a high temperature and high humidity environment, the print medium 19 absorbs moisture in the air and the impedance becomes low. Leakage current flows from the transfer portion via the print medium 19. For example, as shown in the drawing, in a state where the rear end of the print medium 19 passes through the magenta transfer portion, a leakage current flows in the direction of the arrow from the magenta and cyan transfer portions toward the black transfer portion.

  As a result, the potential between the black photosensitive drum 11K and the print medium 19 becomes high and the electric field becomes strong, so that the transfer becomes stronger than in a low temperature and low humidity environment.

  Note that a part of the transfer current flowing through the black transfer portion flows in the direction of the arrow toward the heat roller 52 via the print medium 19, but as described above, between the black transfer portion and the heat roller 52. A guide metal plate 55 is provided, and a constant voltage is clamped by the Zener diode 54. Therefore, since the amount of current flowing toward the heat roller 52 is extremely small (1 [μA] or less), the current does not affect the transfer.

Next, the transfer voltages _Tr- Y, _Tr- M, _Tr- C, _Tr- K when the leakage current flows will be described.

  FIG. 11 is a first explanatory diagram of the transfer voltage, and FIG. 12 is a second explanatory diagram of the transfer voltage.

  The transfer load when transferring the toner images on the photosensitive drums 11 </ b> Y (FIG. 10), 11 </ b> M, 11 </ b> C, and 11 </ b> K to the print medium 19 varies depending on the print duty of the image formed on one page of the print medium 19. A print pattern with a print duty of 70% to 100% is A, and a print pattern with a print duty of 30% to less than 70% is B, which is less than 30%. Assuming that the printing pattern is C, the range of the transfer voltage at which the transfer can be performed satisfactorily when printing with each of the patterns A to C is as shown by the arrows in FIG. Therefore, as shown by the broken line in FIG. 11, the set value of the transfer voltage is set to a value V1 in the middle of the range AR1 in which the transfer can be satisfactorily performed regardless of the patterns A to C.

For example, when a leakage current flows toward the black transfer portion, the potential difference formed between the front surface and the back surface of the print medium 19 in the transfer portion, that is, the apparent transfer voltage T _r -K is leaked. Increases by the amount of current. However, in a high temperature and high humidity environment, the impedance of the print medium 19 is low, so that the apparent transfer voltage T _r -K is lowered as the impedance is lowered.

Therefore, the apparent transfer voltage T _r -K when a leakage current flows from another transfer portion toward the transfer portion where transfer is being performed in a high temperature and high humidity environment is the transfer power supply 51Y, 51M, 51C, The transfer voltage T _r −K of the set value V1 applied to each transfer roller 17 by 51K is slightly higher, and as shown in FIG. 12, the value V2 is within the range AR1. As a result, transfer at the black transfer portion becomes stronger.

  Next, a case where a leakage current does not flow from a transfer portion where transfer is not performed to a transfer portion where transfer is performed via the print medium 19 will be described.

  FIG. 13 is a second diagram showing the state of leakage current during black printing of the electrophotographic printer, and FIG. 14 is a third explanatory diagram of the transfer voltage. In addition, about the thing which has the same structure as FIG. 10, the description is abbreviate | omitted by providing the same code | symbol.

In this case, as shown in FIG. 13, when the rear end of the print medium 19 is separated from the cyan transfer portion, no leakage current flows from the cyan transfer portion to the black transfer portion, so that the apparent transfer voltage T _r -K does not increase. In a high-temperature and high-humidity environment, the impedance of the print medium 19 is low, so that the apparent transfer voltage T _r -K is lower than the transfer voltage T _r -K of the set value V1, and is outside the range AR1. It becomes the value V3. As a result, transfer at the black transfer portion is weakened.

  Next, a case where color printing is performed will be described.

  FIG. 15 is a diagram showing the state of leakage current during color printing of the electrophotographic printer. In addition, about the thing which has the same structure as FIG. 10, the description is abbreviate | omitted by providing the same code | symbol.

In a high temperature and high humidity environment, as described above, since the impedance of the print medium 19 is low, leakage current tends to flow through the print medium 19, but between adjacent transfer portions via the print medium 19. Each leakage current is offset. However, when the print medium 19 is separated from the cyan transfer portion, the transfer is performed only in the black transfer portion. Therefore, as in the case of black printing, the apparent amount corresponding to the lower impedance of the print medium 19 is apparent. The transfer voltage T _r -K is lowered.

  In color printing, since the yellow, magenta, and cyan toner images are transferred to the print medium 19 before the black toner image is transferred to the print medium 19, a transfer current is printed at the black transfer portion. Flowing in the thickness direction in the medium 19 is suppressed by the yellow, magenta, and cyan toner images.

Therefore, the apparent transfer voltage T _r -K applied to the black transfer roller 17 is further lowered to a value V3 (FIG. 14) outside the range AR1. As a result, transfer at the black transfer portion is weakened.

Note that the transfer voltage T _r −K when performing color printing is equal to the transfer voltage T _r −K when performing black printing.

As described above, in the electrophotographic printer according to the first embodiment, when black printing or color printing is performed in a high-temperature and high-humidity environment, if the rear end of the printing medium 19 moves away from the cyan transfer portion, it appears to be apparent. Since the transfer voltage T _r -K of the toner is lowered and transfer at the black transfer portion is weakened, the print quality is lowered.

  Therefore, a second embodiment of the present invention will be described which can prevent the printing quality from being lowered when printing is performed in a high temperature and high humidity environment. In addition, about the thing which has the same structure as 1st Embodiment, the description is abbreviate | omitted by providing the same code | symbol.

  FIG. 16 is a main flowchart showing the operation of the electrophotographic printer according to the second embodiment of the present invention, FIG. 17 is a diagram showing a subroutine of black print mode transfer processing according to the second embodiment of the present invention, and FIG. FIG. 19 is a diagram showing a subroutine of color print mode transfer processing according to the second embodiment of the present invention, FIG. 19 is a diagram showing a subroutine of yellow print transfer processing according to the second embodiment of the present invention, and FIG. It is a figure which shows the subroutine of the black printing transfer process in 2nd Embodiment.

  In this case, when receiving a command and print data from the host computer 31, the control unit 33 (FIG. 1) analyzes the command and determines whether or not the instruction from the host computer 31 is color printing. When the instruction by the host computer 31 is color printing, the image carrier driving means (not shown) is driven to place all the photosensitive drums 11Y, 11M, 11C, and 11K as the image carriers in the down state. When the instruction from the computer 31 is black printing, the image carrier driving means is driven to place the photosensitive drums 11Y, 11M, and 11C in the up state and the black photosensitive drum 11K in the down state.

  Subsequently, when the high voltage determination unit 37 reads the sensor outputs of the temperature sensor 34 and the humidity sensor 35 as the environmental condition detection means, and determines the transfer voltage value a and the inter-paper voltage value b corresponding to each sensor output, The control unit 33 sends an instruction to the motor driver 39 to drive a print medium transport unit (not shown) to transport the print medium 19 (FIG. 10).

  Then, the control unit 33 determines whether or not the instruction by the host computer 31 is color printing, and when the instruction by the host computer 31 is color printing, starts a color printing mode transfer process and performs black printing. In this case, the black print mode transfer process is started.

  In this way, when the transfer in each transfer portion is completed, the printing medium 19 is separated from the transfer belt 16 and sent to a fixing device (not shown), where the toner image is fixed and printing output is performed. .

Next, the flowchart of FIG. 16 will be described.
Step S21 Print data is received.
Step S22: It is determined whether or not color printing is performed. If it is color printing, the process proceeds to step S23, and if it is black printing, the process proceeds to step S24.
Step S23 All the photosensitive drums 11Y, 11M, 11C, and 11K are placed in the down state.
Step S24: The black photosensitive drum 11K is placed in the down state.
Step S25 The sensor outputs of the temperature sensor 34 and the humidity sensor 35 are read.
Step S26: The transfer voltage value a and the inter-sheet voltage value b are determined.
Step S27 The printing medium 19 is conveyed.
Step S28: It is determined whether or not color printing is performed. If it is color printing, the process proceeds to step S30. If it is black printing, the process proceeds to step S29.
Step S29 A black print mode transfer process is performed.
Step S30 A color printing mode transfer process is performed.
Step S31 Fixing is performed.
Step S32 Printout is performed.

  Next, the black print mode transfer process will be described.

First, the high-voltage output unit 38 serving as a transfer voltage applying unit applies transfer voltages T _r −Y, T _r −M, T _r −C, and T _r −K of the inter-sheet voltage value b to yellow, magenta, cyan, and black. When applied to each transfer roller 17 as transfer means, the front end of the print medium 19 passes through each yellow, magenta, and cyan transfer portion in order and reaches the black transfer portion, and then the transfer voltage _Tr of the transfer voltage value a. -K is applied to the black transfer roller 17.

Subsequently, when the rear end of the printing medium 19 is separated from the cyan transfer portion, a transfer voltage correction unit (not shown) in the high voltage determination unit 37 corrects the transfer voltage T _r -K, and the high voltage output unit 38 corrects the transfer. A voltage T _r -K is applied to the black transfer roller 17. When the rear end of the print medium 19 is separated from the black transfer portion, the high voltage output portion 38 applies the transfer voltage T _r -K having the inter-paper voltage value b to the black transfer roller 17.

Next, the flowchart of FIG. 17 will be described.
Step S29-1: Transfer voltages T _r −Y, T _r −M, T _r −C, T _r −K of the inter-paper voltage value b are applied to each transfer roller 17.
Step S29-2 The printing medium 19 passes through the yellow transfer portion.
Step S29-3 The print medium 19 passes through the magenta transfer portion.
Step S29-4 The print medium 19 passes through the cyan transfer portion.
Step S29-5 Waits for the front end of the print medium 19 to reach the black transfer portion.
Step S29-6: The transfer voltage _Tr- K of the transfer voltage value a is applied to the black transfer roller 17.
Step S29-7 Wait for the trailing edge of the print medium 19 to move away from the cyan transfer portion.
Step S29-8: The transfer voltage T _r -K is corrected.
Step S29-9 Wait for the rear end of the print medium 19 to move away from the black transfer portion.
Step S29-10: The transfer voltage T _r -K of the inter-paper voltage value b is applied to the black transfer roller 17.

  Next, the color print mode transfer process will be described.

First, the high voltage output unit 38 applies the transfer voltages T _r −Y, T _r −M, T _r −C, and T _r −K of the inter-paper voltage value b to the yellow, magenta, cyan, and black transfer rollers 17. When the front end of the printing medium 19 passes through the yellow, magenta, cyan, and black transfer portions in order, yellow print transfer processing, magenta print transfer processing, cyan print transfer processing, and black print transfer processing are performed, respectively. The toner images of magenta, cyan and black are transferred to the transfer rollers 17.

Next, the flowchart of FIG. 18 will be described.
Step S30-1: Transfer voltages T _r −Y, T _r −M, T _r −C, and T _r −K of the inter-paper voltage value b are applied to each transfer roller 17.
Step S30-2: A yellow print transfer process is performed.
Step S30-3 A magenta print transfer process is performed.
Step S30-4 A cyan printing transfer process is performed.
Step S30-5: Black print transfer processing is performed.

  Next, the yellow print transfer process, the magenta print transfer process, and the cyan print transfer process will be described.

First, the control unit 33 reads the sensor output of the paper sensor 36 serving as a print medium detection unit, and determines whether the front end of the print medium 19 has reached the yellow transfer unit. When the front end of the printing medium 19 reaches the yellow transfer portion, a transfer voltage _Tr- Y having a transfer voltage value a is applied to the yellow transfer roller 17. Subsequently, when the rear end of the print medium 19 is separated from the yellow transfer portion, the high voltage output portion 38 applies the transfer voltage T _r -Y having the inter-paper voltage value b to the yellow transfer roller 17.

Similarly, it is determined whether the front end of the print medium 19 has reached the magenta transfer portion. When the front end of the print medium 19 reaches the magenta transfer portion, the transfer voltage _Tr- M having the transfer voltage value a is applied to the magenta transfer roller 17. Subsequently, when the rear end of the printing medium 19 is separated from the magenta transfer unit, the high voltage output unit 38 applies the transfer voltage T _r -M having the inter-paper voltage value b to the magenta transfer roller 17.

Further, it is determined whether or not the front end of the print medium 19 has reached the cyan transfer portion. When the front end of the printing medium 19 reaches the cyan transfer portion, a transfer voltage T _r -C having a transfer voltage value a is applied to the cyan transfer roller 17. Subsequently, when the rear end of the print medium 19 is separated from the cyan transfer portion, the high voltage output portion 38 applies the transfer voltage T _r -C having the inter-paper voltage value b to the cyan transfer roller 17.

Next, the flowchart of FIG. 19 will be described. Since the magenta print transfer process and the cyan print transfer process are performed in the same procedure as the yellow print transfer process, description of the flowchart is omitted.
Step S30-2-1 Waits for the front end of the print medium 19 to reach the yellow transfer portion.
Step S30-2-2: A transfer voltage _Tr- Y having a transfer voltage value a is applied to the yellow transfer roller 17.
Step S30-2-3 It waits for the rear end of the print medium 19 to move away from the yellow transfer portion.
Step S30-2-4: A transfer voltage T _r -Y having a paper-to-paper voltage value b is applied to the yellow transfer roller 17.

  Next, the black print transfer process will be described.

First, the control unit 33 reads the sensor output of the paper sensor 36 serving as a print medium detection unit, and determines whether the front end of the print medium 19 has reached the black transfer unit. When the front end of the print medium 19 reaches the black transfer portion, a transfer voltage _Tr- K having a transfer voltage value a is applied to the black transfer roller 17.

Subsequently, when the rear end of the print medium 19 is separated from the cyan transfer portion, the high voltage determination unit 37 corrects the transfer voltage T _r -K, and the high voltage output unit 38 sets the corrected transfer voltage T _r -K to black. Applied to the transfer roller 17. When the rear end of the print medium 19 is separated from the black transfer portion, the high voltage output portion 38 applies the transfer voltage T _r -K having the inter-paper voltage value b to the black transfer roller 17.

Next, the flowchart of FIG. 20 will be described.
Step S30-5-1 Waits for the front end of the print medium 19 to reach the black transfer portion.
Step S30-5-2: The transfer voltage _Tr- K of the transfer voltage value a is applied to the black transfer roller 17.
Step S30-5-3 Wait for the trailing edge of the print medium 19 to move away from the cyan transfer portion.
Step S30-5-4: The transfer voltage _Tr- K is corrected.
Step S30-5-5 Wait for the trailing edge of the print medium 19 to move away from the black transfer portion.
Step S30-5-6: The transfer voltage T _r -K of the inter-paper voltage value b is applied to the black transfer roller 17.

  Next, the operation of the high pressure determination unit 37 will be described.

  FIG. 21 is a time chart showing the operation of the high voltage determination unit during black printing according to the second embodiment of the present invention, and FIG. 22 shows the operation of the high voltage determination unit during color printing according to the second embodiment of the present invention. FIG. 23 is a first diagram showing an environment variable table according to the second embodiment of the present invention, and FIG. 24 is a second diagram showing an environment variable table according to the second embodiment of the present invention. is there.

As shown in FIG. 21, first, when printing starts at timing t20, the high voltage determination unit 37 (FIG. 1) sets the adsorption voltage F _iX applied to the adsorption unit 21 (FIG. 5) to a low level at timing t21. (L), a transfer voltage T _r −Y applied to the yellow transfer roller 17, a transfer voltage T _r −M applied to the cyan transfer roller 17, and a transfer voltage applied to the cyan transfer roller 17. Both T _r -C and the transfer voltage T _r -K applied to the black transfer roller 17 are set to the low level given by the paper-to-paper voltage value b (FIG. 7).

Next, the suction voltage F _{iX is set} to a high level (H) at timing t22, and the suction voltage F _{iX is set} to a low level at timing t23. Subsequently, at a timing t24, the transfer voltage T _r -K is set to a high level given by the transfer voltage value a.

When the trailing edge of the print medium 19 (FIG. 10) moves away from the cyan transfer portion at timing t25, the transfer voltage T _r -K is corrected, and at timing t26, the transfer voltage T _r -K is changed to the inter-paper voltage value b. The adsorption voltage F _{iX is set} to 0 at the timing t27, and the transfer voltages T _r −Y, T _r −M, T _r −C, and T _r −K are all set to 0.

Incidentally, in order to correct the transfer voltage _Tr- K, a correction voltage is added to the transfer voltage value a. The correction voltage varies depending on the environment, the state of the print medium 19, and the like, but is set to about 10 to 20% of the transfer voltage value a. In the present embodiment, the moisture content in the air is estimated based on the environment, that is, the temperature detected by the temperature sensor 34 and the humidity detected by the humidity sensor 35, and as shown in FIG. The environment is classified into eight stages from the wet environment side to the low temperature and low humidity environment side, environment variables 1 to 8 are set, and a correction voltage is set corresponding to each environment variable 1 to 8. In this case, the correction voltage is increased toward the high temperature and high humidity environment side, and the correction voltage is set to 0 [V] or decreased on the low temperature and low humidity environment side. As shown in FIG. 24, the temperature and the humidity are both divided into nine regions, and are made to correspond to the sensor readings of each sensor output.

  Next, the color printing mode will be described.

As shown in FIG. 22, first, when printing is started at timing t30, the high voltage determination unit 37 sets the adsorption voltage F _iX applied to the adsorption unit 21 to a low level at timing t31 and also transfers the transfer voltage T _r. −Y, T _r −M, T _r −C, and T _r −K are all set to a low level given by the inter-paper voltage value b.

Next, at the timing t32, the adsorption voltage F _{iX is set} to the high level, and at the timing t33, the transfer voltage T _r -Y is set to the high level given by the transfer voltage value a.

Subsequently, the high level given transfer voltage T _r -M by the transfer time of the voltage value a at the timing t34, the transfer voltage T _r -C to high level given by the transfer time of the voltage value a at the timing t35, timing t36 _{Set the} suction voltage F _iX to low level.

At a timing t37, the transfer voltage T _r -K is set to a high level given by the transfer voltage value a, and at a timing t38, the transfer voltage T _r -Y is set to a low level given by the inter-paper voltage value b.

Subsequently, at timing t39, the transfer voltage T _r -M is set to a low level given by the inter-sheet voltage value b, and when the rear end of the print medium 19 is separated from the cyan transfer portion, the transfer voltage T _r -K is set at timing t40. The transfer voltage T _r -C is set to a low level given by the paper-to-paper voltage value b, the transfer voltage T _r -K is made to the low level given by the paper-to-paper voltage value b at timing t41, and the adsorption voltage F is given at timing t42. _{iX is set} to 0, and transfer voltages T _r −Y, T _r −M, T _r −C, and T _r −K are all set to 0.

As described above, in the present embodiment, when printing is performed on the print medium 19 in a high-temperature and high-humidity environment, when the rear end of the print medium 19 is separated from the cyan transfer portion, the black transfer voltage T _By increasing _r− K by 10 to 20%, it is possible to prevent the transfer from becoming weak at the rear end portion of the print medium 19. Therefore, it is possible to prevent the print quality from being deteriorated.

  In the second embodiment, the case where printing is performed on one print medium 19 in a high temperature and high humidity environment has been described. A transfer failure may occur depending on the state.

  FIG. 25 is a relationship diagram between the number of printed sheets and the toner charge amount. In the figure, the horizontal axis represents the number of printed sheets, and the vertical axis represents the toner charge amount.

  In the developing device, the toner is stirred during printing and charged by the rotation of the developing roller 14 (FIG. 3) and the toner supply roller 15. Therefore, the toner charge amount increases as the number of printed sheets increases. Note that ΔT is a pause time, and during the pause time ΔT, the stirring of the toner and the rotation of the developing roller 14 and the toner supply roller 15 are stopped.

  As shown in the figure, when the charge amount of the toner increases as the number of printed sheets increases, stable transfer cannot be performed, and transfer failure occurs.

  Therefore, a third embodiment of the present invention will be described in which transfer failure does not occur when continuous printing is performed.

  FIG. 26 is a control block diagram of the electrophotographic printer according to the third embodiment of the present invention.

  In the figure, reference numeral 61 denotes an environmental condition detection means for detecting environmental conditions such as temperature and humidity when printing is started. The environmental condition detection means 61 is obtained from the temperature sensor 34 (FIG. 1), the humidity sensor 35 and the like. Become. In addition, it can replace with the temperature sensor 34, the humidity sensor 35, etc., and can also use another detection means. Reference numeral 62 denotes manual input from the operation panel 32, reception of the type of print medium 19 (FIG. 10), size data, etc. from the host computer 31, detection by the paper sensor 36 disposed in the conveyance path of the print medium 19, and the like. This is a print medium detection unit that detects the type of print medium 19 to be printed from now on.

  Reference numeral 63 denotes a print number detection unit that counts the number of prints since the start of printing of the electrophotographic printer. Reference numeral 64 denotes a timer unit that detects a time τ until the next print starts after the print is finished or interrupted. It is. The print number detection unit 63 and the timer unit 64 constitute a print condition detection unit that detects print conditions such as the number of prints and time τ.

Information from the environmental condition detection means 61, the print medium detection section 62, the print number detection section 63 and the timer section 64 is sent to the determination section 65, and the transfer voltage correction means (not shown) in the determination section 65 Correction for correcting the transfer voltages T _r -Y, T _r -M, T _r -C and black transfer voltage T _r -K applied to the yellow, magenta and cyan transfer rollers 17 based on the information. Determine the voltage. For this purpose, the determination unit 65 is provided with a table showing the relationship between the temperature, humidity, the type of the printing medium 19, the number of prints, the printing stop time, and the transfer voltage _Tr- K and the correction voltage. The table is created so that optimum transfer control can be performed based on experimental results when the environment, print medium, and the like are changed. Reference numeral 66 denotes a high voltage output unit. The high voltage output unit 66 applies the transfer voltages T _r −Y, T _r −M, T _r −C, and T _r −K determined by the determination unit 65 to each transfer roller 17. Apply to. In this case, when printing is started, the determination unit 65 reads the environmental condition detected by the environmental condition detection unit 61, estimates the environment based on the environmental condition, and transfers the transfer voltage _Tr- Y corresponding to the environment. , T _r -M, T _r -C, T _r -K are determined.

Then, when the determination unit 65 performs initial setting and estimates that the electrophotographic printer is placed in a normal temperature and normal humidity environment or a low temperature and low humidity environment, the print medium 19 is relatively dry and leakage current is generated. The black transfer voltage _Tr- K is set to 0 [V], and the correction voltage level is set to 0. As a result, the high-voltage output unit 66 applies a constant transfer voltage T _r -K to the transfer roller 17 until the rear end of the print medium 19 reaches the black transfer unit and the rear end moves away from the black transfer unit.

Further, when the determination unit 65 estimates that the electrophotographic printer is placed in a high-temperature and high-humidity environment, the determination unit 65 determines that the print medium 19 absorbs moisture in the air and is likely to generate a leakage current. The correction voltage set corresponding to the high humidity environment is determined, and the correction voltage level is set to 1 to 4. As a result, the high-voltage output unit 66 corrects the black transfer voltage T _r -K when the rear end of the print medium 19 is separated from the cyan transfer unit.

  When continuous printing is performed, the determination unit 65 increases the correction voltage level (up one level) as the number of printed sheets increases.

  When intermittent printing is performed, when the time τ detected by the timer unit 64 elapses, the determination unit 65 temporarily lowers the correction voltage level (down one level) and waits for the next printing activation. When the time 2τ or more elapses, the determination unit 65 further lowers the correction voltage level (2 levels down). If the print activation does not take place even after the time 3τ elapses, the environmental condition is read again, and the initial state is restored. Return to the corrected voltage level.

  Such control is performed when the correction level is set to 1 or more at the initial setting, and when the correction level is set to 0, the correction level remains 0 even if continuous printing is performed. To be.

  Next, the operation of the electrophotographic printer having the above configuration will be described.

  FIG. 27 is a first flowchart showing the operation of the electrophotographic printer according to the third embodiment of the present invention. FIG. 28 is a second flowchart showing the operation of the electrophotographic printer according to the third embodiment of the present invention. FIG. 29 is a diagram showing the relationship between the number of printed sheets and the correction voltage level when continuous printing is performed in a high-temperature and high-humidity environment according to the third embodiment of the present invention.

In this case, the correction voltage level 0 is ± 0 [V], and the correction voltage level is applied in a normal temperature and normal humidity environment and a low temperature and low humidity environment. The correction voltage level 1 is +100 [V], the correction voltage level 2 is +200 [V], the correction voltage level 3 is +300 [V], and the correction voltage level 4 is +400 [V]. The transfer voltage T _r -K (FIG. 22) applied and applied to the black transfer roller 17 (FIG. 10) is increased by adding a correction voltage corresponding to the correction voltage level.

First, when the electrophotographic printer is activated for printing, the determination unit 65 (FIG. 26) estimates the environment, the transfer voltages T _r −Y, T _r −M, T _r −C, T _r −K, and the transfer voltage. A correction voltage level of T _r -K is determined.

For example, when the correction voltage level is 2 in the initial state and 16 sheets of printing (continuous printing 12 times + intermittent printing 4 times) are performed, the first printing is determined based on the environmental conditions. In addition, yellow, magenta, and cyan are printed at the transfer voltages T _r −Y, T _r −M, and T _r −C, and at the same time the rear end of the print medium 19 is separated from the cyan transfer portion, the black transfer roller 17 is applied. The applied transfer voltage _Tr- K is added with a correction voltage corresponding to the correction voltage level 2, that is, +200 [V], and black printing is performed.

  Subsequently, when the second sheet is printed by continuous printing, the correction voltage level is set to 3. When the third and subsequent sheets are printed by continuous printing, the correction voltage level is set to 4. Then, after the printing of the tenth sheet is performed by continuous printing, once the printing is stopped and the time τ elapses, the correction voltage level is set to 3. Thereafter, the printing is started immediately. When the eleventh sheet is printed by intermittent printing, printing is performed at the correction voltage level 3. Subsequently, when the twelfth sheet is printed by continuous printing, the correction voltage level is printed. Is set to 4.

  Further, after the printing of the 12th sheet is performed, when the printing is stopped again and the time τ elapses, the correction voltage level is set to 3, and then the printing is not started immediately, and the time τ elapses. When the time 2τ or more has elapsed since the printing was stopped, the correction voltage level is set to 2.

  When the printing is started again and the 13th printing is performed by intermittent printing, the correction voltage level is set to 2, and when the 14th printing is performed by continuous printing, the correction voltage level is set to 3. When the fifteenth page is printed by continuous printing, the correction voltage level is set to four.

Further, after the 15th printing is performed, when the printing is again stopped and the time 3τ or more has elapsed, the correction voltage level is set to the initial state, and the transfer voltage _Tr determined based on the environmental conditions is set. Printing is performed at -K.

Next, the flowcharts of FIGS. 27 and 28 will be described.
Step S41 Printing starts.
Step S42: Read environmental conditions.
Step S43: Transfer voltages _Tr- Y, _Tr- M, _Tr- C, _Tr- K are determined.
Step S44: Determine a correction voltage level of the transfer voltage T _r -K.
Step S45 The first sheet is printed.
Step S46: It is determined whether continuous printing is performed. If continuous printing is performed, the process proceeds to step S47, and if not, the process proceeds to step S54.
Step S47: The correction voltage level is increased by one.
Step S48 The second sheet is printed.
Step S49: It is determined whether continuous printing is performed. If continuous printing is performed, the process proceeds to step S50, and if not, the process proceeds to step S54.
Step S50: The correction voltage level is increased by one level.
Step S51 Print the nth sheet (n ≧ 3).
Step S52: It is determined whether or not intermittent printing is performed. If intermittent printing is performed, the process proceeds to step S55, and if not, the process proceeds to step S53.
Step S53: to judge whether printing has been completed. If printing has ended, the process ends. If not, the process returns to step S51.
Step S54: It is determined whether or not intermittent printing is performed. If intermittent printing is performed, the process proceeds to step S55. If not, the process ends.
Step S55: It is determined whether the time τ has elapsed. If the time τ has elapsed, the process proceeds to step S58, and if not, the process proceeds to step S56.
Step S56: The correction voltage level is increased by one level.
Step S57 Printing is started, and the process returns to Step S48.
Step S58: It is determined whether or not the time 2τ has elapsed. If the time 2τ has elapsed, the process proceeds to step S59, and if not, the process proceeds to step S60.
Step S59: It is determined whether the time 3τ has elapsed. If the time 3τ has elapsed, the process returns to step S42, and if not, the process proceeds to step S61.
Step S60: The correction voltage level is lowered by 1 and the process returns to Step S57.
Step S61: The correction voltage level is lowered by 2 and the process returns to Step S57.

  In the present embodiment, when the correction voltage level is 4, even when an instruction to increase the correction voltage level is issued, the correction voltage level remains 4 and the correction voltage level is 1. Even if an instruction to lower the correction voltage level is issued, the correction voltage level remains at 1, and when the correction voltage level is 0, the correction voltage level remains at 0 even if continuous printing is performed.

  As described above, in the present embodiment, the correction voltage can be changed depending on the number of printed sheets and the printing state of continuous printing or intermittent printing. Therefore, it is possible to cope with a change in the state of toner in the developing device. Thus, the transfer can be performed, and it is possible to prevent a printing defect from occurring when continuous printing is performed.

By the way, in the second embodiment, when printing is performed on the print medium 19 in a high-temperature and high-humidity environment, the black transfer voltage when the rear end of the print medium 19 is separated from the cyan transfer portion. By increasing T _r -K by 10 to 20%, the transfer at the rear end of the print medium 19 is prevented from being weakened. However, when the leakage current increases, the print medium 19 at the front end is increased. Transcription becomes stronger.

  Therefore, a fourth embodiment of the present invention that can prevent strong transfer at the front end of the print medium 19 will be described. In addition, about the thing which has the same structure as 1st Embodiment, the description is abbreviate | omitted by providing the same code | symbol. The black print mode transfer process will be described.

  FIG. 30 is a diagram showing a subroutine of black print mode transfer processing in the fourth embodiment of the present invention, and FIG. 31 is a time chart showing the operation of the high-pressure determining unit at the time of black printing in the fourth embodiment of the present invention. It is.

As shown in FIG. 31, first, when printing starts at timing t51, the high voltage determination unit 37 (FIG. 1) sets the adsorption voltage F _iX applied to the adsorption unit 21 (FIG. 5) to a low level at timing t52. In addition, the transfer voltages T _r -Y, T _r -M, T _r -C, and T _r -K are all set to a low level given by the inter-sheet voltage value b (FIG. 7).

Next, the suction voltage F _{iX is set} to a high level (H) at timing t53, and the suction voltage F _{iX is set} to a low level at timing t54. Subsequently, at timing t55, the transfer voltage T _r -K is set to a high level given by the transfer voltage value a. At this time, the transfer voltage T _r -K is set to a high level during timings t55 to t58, and the correction voltage of the transfer voltage T _r -K is divided into three stages.

First, at the timing t55, the rear end of the print medium 19 exists in the magenta transfer portion. At this time, since the leakage current flows from the magenta transfer portion and the cyan transfer portion toward the black transfer portion, the current value of the leakage current is the largest. Therefore, the transfer voltage T _r -K is set to a low value at timing t55 so that the transfer does not become strong in the transfer of the black toner image.

Next, at the timing t56, the rear end of the print medium 19 is separated from the magenta transfer portion and exists in the cyan transfer portion. At this time, the leakage current from the magenta transfer portion disappears, and the leakage current flows only from the cyan transfer portion. Therefore, at timing t56, the transfer voltage _Tr- K is set slightly higher than that at timing t55 and lower than the transfer voltage value a.

Subsequently, at timing t57, the rear end of the print medium 19 is separated from the cyan transfer portion. At this time, the leakage current from the cyan transfer portion is eliminated. Therefore, at the timing t57, the transfer voltage _Tr- K is set to the transfer voltage value a.

Leakage current also flows from the yellow transfer portion, but the current value is small compared to the leakage current flowing from the magenta transfer portion and the cyan transfer portion, and the influence on the transfer of the black toner image is small. If the transfer voltage T _r -K is divided into four stages, the control becomes complicated. Therefore, the transfer voltage T _r -K is divided into three stages.

At a timing t58, the transfer voltage T _r -K is set to a low level given by the inter-sheet voltage value b. At a timing t59, the suction voltage F _{iX is set} to 0, and the transfer voltages T _r -Y, T _r -M, T _{Both r} −C and T _r −K are set to zero.

By the way, when printing is performed in the color printing mode, the photosensitive drums 11Y, 11M, 11C, and 11K of the respective colors are placed in the down state, so that the transfer voltage T _r − applied to each transfer roller 17 in the adjacent transfer unit. Y, T _r -M, T _r -C, and T _r -K are equal, and each leakage current is canceled between adjacent transfer portions via the print medium 19. Therefore, even if the transfer voltage _Tr- K is changed, the above-described effect cannot be obtained.

  Next, the operation of the electrophotographic printer having the above configuration will be described.

First, the print medium 19 adsorbed by the adsorbing unit 21 is sequentially conveyed through the transfer units of yellow, magenta, and cyan as the transfer belt 16 travels. When black printing is performed, the photosensitive drums 11Y, 11M, and 11C for yellow, magenta, and cyan are placed in the up state, so that the front end of the printing medium 19 is transferred to the photosensitive drums 11Y, 11M, and 11C and the respective transfer. It passes through a gap between the rollers 17. At this time, the transfer voltages T _r −Y, T _r −M, T _r −C, and T _r −K applied to the transfer rollers 17 are always between the sheets regardless of whether the print medium 19 exists in each transfer portion. The voltage value is set to b.

Subsequently, when the front end of the print medium 19 reaches the black transfer portion, a transfer voltage T _r -K appropriate for transferring the toner image on the photosensitive drum 11K to the print medium 19 is applied to the black transfer roller 17. Is done. At this time, since the print medium 19 exists in the magenta transfer portion and the cyan transfer portion, the transfer voltage is considered in consideration of leakage current flowing from the magenta transfer portion and the cyan transfer portion to the black transfer portion. T _r −K is corrected and set to be 10 to 20% lower than the voltage value a at the time of transfer.

When the print medium 19 is further conveyed and the rear end of the print medium 19 moves away from the magenta transfer portion, the transfer voltage T _r -K is corrected to be slightly higher and about 10% lower than the transfer voltage value a. Is set.

Next, at the same time when the rear end of the printing medium 19 moves away from the cyan transfer portion, the transfer voltage T _r -K is set to the transfer voltage value a. Subsequently, when the transfer of the black toner image is completed and the rear end of the print medium 19 is separated from the black transfer portion, the print medium 19 is sent to a fixing device to fix the toner image.

Next, a flowchart will be described.
Step S71 Transfer voltages T _r −Y, T _r −M, T _r −C, and T _r −K of the inter-paper voltage value b are applied to each transfer roller 17.
Step S72 The print medium 19 passes through the yellow transfer portion.
Step S73 The print medium 19 passes through the magenta transfer portion.
Step S74 The print medium 19 passes through the cyan transfer portion.
Step S75 Wait for the front end of the print medium 19 to reach the black transfer portion.
Step S76: The transfer voltage _Tr- K is corrected and set lower than the transfer voltage value a.
Step S77 The apparatus waits for the rear end of the print medium 19 to move away from the magenta transfer portion.
Step S78 The transfer voltage T _r -K is corrected and slightly increased.
Step S79 The apparatus waits for the rear end of the print medium 19 to move away from the cyan transfer portion.
Step S80: The transfer voltage _Tr- K having the voltage value a at the time of transfer is applied.
Step S81 The apparatus waits for the rear end of the print medium 19 to move away from the black transfer portion.
Step S82: The transfer voltage _Tr- K having the inter-paper voltage value b is applied.

As described above, in this embodiment, when black printing is performed in a high temperature and high humidity environment, if the current value of the leakage current from the other transfer unit is large, the transfer voltage _Tr- K is set low, When the current value becomes smaller, the transfer voltage _Tr- K is set higher, so that the transfer at the front end of the print medium 19 can be prevented from becoming stronger. Therefore, it is possible to prevent a printing defect from occurring.

  By the way, when the impedance of the transfer belt 16 decreases, a leakage current that flows from the transfer portion to which the transfer voltage having the transfer voltage value a is applied to the transfer portion to which the transfer voltage having the inter-paper voltage value b is applied. It will increase.

  FIG. 32 is a diagram showing a state of leakage current.

  In the figure, 11 is a photosensitive drum, 16 is a transfer belt, 17 is a transfer roller, 19 is a printing medium, P1 and P2 are transfer portions to which a transfer voltage value a is applied, and P3 is an inter-paper voltage value. This is a transfer portion to which a transfer voltage b is applied.

  In this case, if there is a potential difference between the transfer voltage value a and the paper-to-paper voltage value b, as the impedance of the transfer belt 16 decreases, the transfer portion P2 to the transfer portion as shown by the arrows in the figure. The leakage current Ia flowing toward P3 increases. As a result, a sufficient electric field is not generated between the transfer belt 16 and the photosensitive drum 11, so that transfer cannot be performed satisfactorily.

  Therefore, a fifth embodiment of the present invention will be described, in which transfer can be performed satisfactorily even in a high temperature and high humidity environment. In addition, about the thing which has the same structure as 1st Embodiment, the description is abbreviate | omitted by providing the same code | symbol.

  FIG. 33 is a diagram showing a transfer voltage application state in the fifth embodiment of the present invention.

  In this case, if it is determined that the environment is a high temperature and high humidity environment based on the sensor outputs of the temperature sensor 34 (FIG. 1) and the humidity sensor 35 as the environmental condition detection means, the inter-paper voltage applied at the transfer portion P3. The transfer voltage of the value b is set higher than that in the low temperature and low humidity environment, and the potential difference between the transfer voltage value a and the inter-paper voltage value b is reduced.

  The inter-paper voltage value b is set as large as possible within a range in which the photosensitive drum 11 is not damaged, and is set to be equal to or less than the voltage value a during transfer. Accordingly, the potential difference between the transfer voltage value a and the paper-to-paper voltage value b is reduced, so that the leakage current Ib flowing from the transfer part P2 toward the transfer part P3 can be reduced.

By the way, in each of the embodiments, in order to prevent the photosensitive drum 11 and the transfer belt 16 from being worn, all the unused photosensitive drums 11 are placed in an up state. For example, when black printing is performed, yellow, The magenta and cyan photosensitive drums 11Y, 11M, and 11C are placed in the up state. Since the black photosensitive drum 11K is disposed at a position farthest from the suction portion 21 (FIG. 5), a sufficient suction force to the black transfer portion may be obtained depending on the type of the print medium 19, changes in the environment, and the like. May not be able to hold. Therefore, the suction voltage F _iX (FIG. 8) at the suction portion 21 is set high so that the print medium 19 can be stably conveyed to the black transfer portion.

On the other hand, when performing color printing, the yellow, magenta, cyan, and black photosensitive drums 11Y, 11M, 11C, and 11K are placed in the down state, but the yellow photosensitive drum 11Y is the most from the suction unit 21. Since the transfer voltage of the paper-to-paper voltage value b is applied to each transfer roller 17 in addition to being disposed at a close position, the print medium 19 can be transferred to the yellow transfer portion even if the suction voltage F _iX is set low. And can be stably conveyed to the magenta, cyan and black transfer portions.

However, since the suction voltage F _iX is set high as in the case of black printing, not only the power consumption increases, but also the cost of a power supply device (not shown) for applying the suction voltage F _iX is high. turn into.

  Therefore, when performing black printing, the printing medium 19 can be stably conveyed to the black transfer portion, power consumption is not increased, and the cost of the power supply device can be reduced. The sixth embodiment will be described. In addition, about the thing which has the same structure as 1st Embodiment, the description is abbreviate | omitted by providing the same code | symbol.

  FIG. 34 is a diagram showing a state during black printing of the electrophotographic printer according to the sixth embodiment of the present invention, and FIG. 35 is a block diagram of the electrophotographic printer according to the sixth embodiment of the present invention.

  In the figure, reference numeral 41 denotes a print data section for holding the print data of each color, and reference numeral 42 denotes a drum count value as count means for detecting the rotation speed of the photoconductor drums 11Y, 11M, 11C, and 11K for each color as a drum count value in the maintenance mode. A storage unit 43 is a photoconductor down selection unit that selects a photoconductor drum to be placed in a down state according to the color of print data and the number of rotations of the photoconductor drums 11Y, 11M, 11C, and 11K of each color. This is a photoconductor up / down driver unit as image carrier driving means for placing the body drums 11Y, 11M, 11C, and 11K in the up state and the down state.

  When performing monochrome printing, the photosensitive drum of that color is unconditionally placed in the down state, and the photosensitive drum having the smallest drum count value is placed in the down state among the photosensitive drums of other colors. When there are a plurality of photosensitive drums having the smallest drum count value, the photosensitive drum that is farthest from the photosensitive drum that is unconditionally placed in the down state is preferentially placed in the down state.

  For example, when black printing is performed as monochromatic printing, the black photosensitive drum 11K is unconditionally placed in the down state, and the drum count value is the smallest among the yellow, magenta, and cyan photosensitive drums 11Y, 11M, and 11C. The photosensitive drum, for example, the photosensitive drum 11M is placed in the down state. For example, when the photosensitive drums having the smallest drum count value are the photosensitive drum 11M and the photosensitive drum 11C, the photosensitive drum 11M is preferentially placed in the down state.

  Next, the operation of the electrophotographic printer having the above configuration will be described.

  FIG. 36 is a flowchart showing the operation of the electrophotographic printer according to the sixth embodiment of the present invention.

  First, when printing starts, the photoreceptor down selection unit 43 (FIG. 35) determines whether color printing or black printing is performed based on the print data in the print data unit 41. When black printing is performed, the photoconductor up / down driver unit 44 unconditionally places the black photoconductor drum 11K (FIG. 34) in the down state.

  Next, the photoconductor down selection unit 43 reads the drum count values of the yellow, magenta, and cyan photoconductor drums 11Y, 11M, and 11C in the drum count value storage unit 42, detects the smallest drum count value, and detects it. The photosensitive drum corresponding to the drum count value thus set is placed in the down state for auxiliary suction. When there are two or more smallest drum count values, or when the drum count values of the yellow, magenta, and cyan photosensitive drums 11Y, 11M, and 11C are all the same, they are located at the position farthest from the black photosensitive drum 11K. A photoconductor drum is preferentially placed in the down state.

  When the selection of the up and down states of the photosensitive drums 11Y, 11M, 11C, and 11K for each color is completed, the print medium 19 (FIG. 33) is attracted to the transfer belt 16 and sent to the black transfer unit. Transfer is performed at the black transfer portion. Subsequently, the printing medium 19 is separated from the transfer belt 16 and sent to a fixing device (not shown), where the toner image is fixed and printing output is performed.

As described above, when black printing is performed, one or two of the photosensitive drums 11Y, 11M, and 11C of yellow, magenta, and cyan are placed in the down state, thereby configuring the auxiliary suction unit. Therefore, the suction force can be maintained between the suction portion 21 and the auxiliary suction portion and between the auxiliary suction portion and the black transfer portion. Therefore, the print medium 19 can be stably conveyed to the black transfer portion. Further, since the adsorption voltage F _iX (FIG. 8) of the adsorption unit 21 can be lowered, not only the power consumption can be reduced, but also the cost of a power supply device (not shown) for applying the adsorption voltage F _iX can be reduced. can do.

Next, a flowchart will be described.
Step S91 Printing starts.
Step S92: It is determined whether or not black printing is performed. If it is black printing, the process proceeds to step S94. If it is color printing, the process proceeds to step S93.
Step S93 All the photosensitive drums 11Y, 11M, 11C, and 11K are placed in the down state.
Step S94: The black photosensitive drum 11K is placed in the down state.
Step S95 The drum count values of the photosensitive drums 11Y, 11M, and 11C are read, and the smallest drum count value is detected.
Step S96: It is determined whether there are two or more smallest drum count values. When there are two or more smallest drum count values, the process proceeds to step S97, and when there is one, the process proceeds to step S98.
Step S97: The photosensitive drum located farthest from the photosensitive drum 11K is placed in the down state.
Step S98: The photosensitive drum corresponding to the smallest drum count value is placed in the down state.
Step S99 The printing medium 19 is conveyed.
Step S100 Transfer is performed.
Step S101 Fixing is performed.
Step S102 Print output is performed.

  In addition, this invention is not limited to the said embodiment, It can change variously based on the meaning of this invention, and does not exclude them from the scope of the present invention.

FIG. 2 is a control block diagram of the electrophotographic printer according to the first embodiment of the present invention. It is a conceptual diagram of the conventional electrophotographic printer. 1 is a schematic diagram of an image forming unit of an electrophotographic printer according to a first embodiment of the present invention. It is a figure which shows the state at the time of color printing of the electrophotographic printer in the 1st Embodiment of this invention. It is a figure which shows the state at the time of black printing of the electrophotographic printer in the 1st Embodiment of this invention. It is a figure which shows the sheet voltage value sequence in the 1st Embodiment of this invention. It is a figure which shows the relationship between a transfer voltage and the moisture content in air. It is a time chart which shows operation | movement of the high voltage | pressure determination part in the 1st Embodiment of this invention. 3 is a flowchart showing an operation of the electrophotographic printer according to the first embodiment of the present invention. It is a 1st figure which shows the state of the leakage current at the time of black printing of an electrophotographic printer. It is the 1st explanatory view of transfer voltage. It is the 2nd explanatory view of transfer voltage. It is a 2nd figure which shows the state of the leakage current at the time of black printing of an electrophotographic printer. It is the 3rd explanatory view of transfer voltage. It is a figure which shows the state of the leakage current at the time of color printing of an electrophotographic printer. It is a main flowchart which shows operation | movement of the electrophotographic printer in the 2nd Embodiment of this invention. It is a figure which shows the subroutine of the black printing mode transfer process in the 2nd Embodiment of this invention. It is a figure which shows the subroutine of the color printing mode transfer process in the 2nd Embodiment of this invention. It is a figure which shows the subroutine of the yellow printing transfer process in the 2nd Embodiment of this invention. It is a figure which shows the subroutine of the black printing transfer process in the 2nd Embodiment of this invention. It is a time chart which shows operation | movement of the high voltage | pressure determination part at the time of black printing in the 2nd Embodiment of this invention. It is a time chart which shows operation | movement of the high voltage | pressure determination part at the time of color printing in the 2nd Embodiment of this invention. It is a 1st figure which shows the environment variable table in the 2nd Embodiment of this invention. It is a 2nd figure which shows the environment variable table in the 2nd Embodiment of this invention. FIG. 6 is a relationship diagram between the number of printed sheets and the charge amount of toner. It is a control block diagram of the electrophotographic printer in the third embodiment of the present invention. It is a 1st flowchart which shows the operation | movement of the electrophotographic printer in the 3rd Embodiment of this invention. It is a 2nd flowchart which shows operation | movement of the electrophotographic printer in the 3rd Embodiment of this invention. It is a figure which shows the relationship between the number of printed sheets and a correction voltage level when performing continuous printing in the high temperature, high humidity environment in the 3rd Embodiment of this invention. It is a figure which shows the subroutine of the black printing mode transfer process in the 4th Embodiment of this invention. It is a time chart which shows operation | movement of the high voltage | pressure determination part at the time of black printing in the 4th Embodiment of this invention. It is a figure which shows the state of leakage current. It is a figure which shows the application state of the transfer voltage in the 5th Embodiment of this invention. It is a figure which shows the state at the time of black printing of the electrophotographic printer in the 6th Embodiment of this invention. It is a block diagram of the electrophotographic printer in the 6th Embodiment of this invention. It is a flowchart which shows operation | movement of the electrophotographic printer in the 6th Embodiment of this invention.

Explanation of symbols

11, 11Y, 11M, 11C, 11K Photosensitive drum 16 Transfer belt 17 Transfer roller 19 Print medium 34 Temperature sensor 35 Humidity sensor 36 Paper sensor 37 High pressure determination unit 38 High voltage output unit 42 Drum count value storage unit 44 Photoconductor up / down driver Section 61 Environmental condition detection means 63 Number of printed sheets detection section 64 Timer section 65 Discrimination section a Transfer voltage value b Paper-to-paper voltage values _Tr- Y, _Tr- M, _Tr- C, _Tr- K Transfer voltage

Claims (3)

(A) a transfer belt that is driven by a driving unit and conveys a print medium;
(B) a plurality of image carriers disposed along the transfer belt;
(C) transfer means arranged to face each of the image carriers with the transfer belt interposed therebetween;
(D) In the case of performing monochromatic printing, it has an image carrier for the corresponding color and an image carrier driving means for bringing one or two of the other color image carriers into contact with the transfer belt. And an electrophotographic printer. (A) having means for detecting the used state of the image carrier;
(B) The electrophotographic printer according to claim 1, wherein the image carrier driving means brings an image carrier of another color into contact with the transfer belt based on a situation in which the image carrier is used. The electrophotographic printer according to claim 1, wherein the image carrier driving unit causes the image carrier located at a position farthest from the corresponding color image carrier to contact the transfer belt.

Images