JP2012242399A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device that feeds paper having an appropriate property from first print, according to a print attribute, such as color or monochrome mode.SOLUTION: A printer 1 that transfers a toner image carried on photoreceptor drums 31Y-31K to a recording sheet S and thermally fixes it to perform image forming operation includes: a plurality of paper feed trays 41a, 41b and 41c for housing the recording sheets S; resistance detection mechanisms 49a, 49b and 49c arranged in ech of the paper feed trays and detecting electric resistance of the recording sheet in each of the paper feed trays; and a control part 60 that selects any of the paper feed trays 41a, 41b and 41c to perform image forming operation, on the basis of the electric resistance value detected by the resistance detection mechanisms 49a, 49b and 49c, so as to obtain predetermined image quality for transfer.

Description

  The present invention relates to an electrophotographic image forming apparatus, and more particularly to a technique for suppressing a decrease in image quality.

In an electrophotographic image forming apparatus, a toner image formed on a photosensitive drum or an intermediate transfer belt is transferred onto a recording sheet by electrostatic action due to a transfer voltage applied to a transfer member such as a transfer roller. An image is formed.
It has been conventionally known that the superiority or inferiority of the image quality of the toner image transferred to the recording sheet is greatly influenced by the resistance value of the recording sheet to be transferred.

  Therefore, for example, in Patent Document 1, when a voltage is applied between a pair of registration rollers on the sheet conveyance path and a recording sheet fed from a specific sheet feeding tray passes between the registration roller pairs, When a voltage is applied to the pair of registration rollers and an electric current flowing between them is detected to obtain an electrical resistance in the sheet thickness direction, and the detected resistance value deviates from a predetermined range set in advance. A configuration has been proposed in which the recording sheet is discarded and the paper is fed again from a paper feed tray different from the paper feed tray.

JP 2006-91402 A

However, according to the configuration described in Patent Document 1, not only the recording sheet fed first is wasted, but another recording tray is selected until the recording sheet having the optimum resistance value is fed. In this case, the paper must be repeatedly fed again, so that the time until the first page is output (first print time) becomes long, which causes inconvenience to the user.
The present invention has been made in view of the above circumstances, and in order to obtain a good transfer image, it is possible to feed a recording sheet having an appropriate resistance value from the beginning, and to reduce the first print time as much as possible. An object of the present invention is to provide an image forming apparatus that can be shortened.

  In order to achieve the above object, an image forming apparatus according to the present invention is an image forming apparatus that executes an image forming operation by transferring a toner image carried on an image carrier onto a recording sheet and then fixing the image by heating. A plurality of sheet feeding trays for storing recording sheets; a detecting unit arranged for each of the plurality of sheet feeding trays for detecting an electrical resistance of the recording sheet in each sheet feeding tray; and performing the transfer with a predetermined image quality. As described above, the image forming apparatus includes a control unit that selects a paper feed tray based on the electrical resistance value detected by the detection unit and executes the image forming operation.

According to the above configuration, since the electrical resistance of the recording sheet in each paper feed tray can be detected in advance, the target paper feed tray is appropriately selected to obtain a predetermined image quality when executing the image forming operation, A situation in which the first print time is prolonged because an appropriate recording sheet is not selected as in the prior art can be avoided as much as possible.
The detection means detects at least when the apparatus main body is turned on, when returning from the power saving mode to reduce the power consumption of the apparatus main body to the normal mode, and when the paper feed door of the paper feed tray is opened or closed. It is desirable to execute at such timing.

  Further, the detection means is disposed on the paper feed port side of each paper feed tray, and includes a first detection unit that detects an electrical resistance in the thickness direction of the recording sheet, and a side opposite to the paper feed port side and the paper feed direction. And a second detection unit that detects the electrical resistance of the surface of the recording sheet, and the second and subsequent recording sheets stored in the paper feed tray when a plurality of images are continuously formed. It is desirable to obtain the electrical resistance based on the detection result by the second detection unit.

In addition, it is desirable that the first detection unit detects the electric resistance value of the recording sheet by passing an electric current between a pair of the rolling rollers in a state where the recording sheet is sandwiched.
Here, conversion means for converting the electrical resistance value in the thickness direction from the surface resistance value of the recording sheet based on the result detected for the same recording sheet by the first and second detection units in the same paper feed tray. It is desirable to use the converted value in the selection.

In addition, it is preferable that an acquisition unit that acquires an attribute of image data that is an object of image formation to be executed is provided, and the predetermined image quality is determined based on the attribute.
The attribute may be information indicating whether the print is a full color print or a monochrome print.
Alternatively, the attribute may be information related to a ratio between an image and a margin in image data.

In addition, when the electrical resistance value of the recording sheet in any one of the paper feed trays detected by the detection unit is not within a range set in advance to obtain the predetermined image quality, the image quality does not deteriorate. In addition, the transfer voltage may be changed.
Further, each of the plurality of paper feed trays is provided with a heater for heating the recording sheet, and when the electrical resistance of the recording sheet decreases with time and the amount of change becomes a predetermined value or more, The heater of the paper feed tray that stores the recording sheet may be turned on.

1 is a schematic diagram for explaining a configuration of a tandem color printer which is an example of an image forming apparatus according to an embodiment of the present invention. It is a perspective view which shows the structure of the 1st electrical resistance detection part in the said printer. It is a side view which shows the structure of the 2nd electrical resistance detection part in the said printer. (A) And (b) is a figure which shows the electrical resistance detection operation | movement of a recording sheet in a 1st electrical resistance detection part. FIG. 2 is a functional block diagram illustrating a relationship between a control unit of the image forming apparatus and main components that are controlled by the control unit. It is a flowchart which shows the flow until the said control part selects a paper feed tray. It is a flowchart which shows the content of a recording sheet resistance detection process. It is a figure which shows the content of the table provided in EEPROM of the said image forming apparatus. It is a figure which shows the content of the table different from the said table provided in EEPROM of the said image forming apparatus. It is a flowchart which shows the content of the paper feed tray selection process which the said control part implements. It is a figure which shows the content of the table provided in EEPROM of the said image forming apparatus. 10 is a flowchart showing the contents of a paper feed tray selection process in a modified example. It is a figure which shows the content of the modification of the table provided in EEPROM of the said image forming apparatus. It is a figure which shows the content of another modification of the table provided in EEPROM of the said image forming apparatus. (A) And (b) is a figure which shows an example of a structure of the heating part in a modification. It is a flowchart which shows the content of the recording sheet heating process in a modification.

<Embodiment>
Embodiments of an image forming apparatus according to the present invention will be described below with reference to the drawings.
<Configuration of image forming apparatus>
FIG. 1 is a schematic diagram for explaining a configuration of a tandem color printer (hereinafter simply referred to as “printer”) as an example of an image forming apparatus according to an embodiment of the present invention.

  As shown in the figure, the printer 1 includes an image processing unit 3, paper feeding units 4a, 4b, and 4c, a fixing unit 5, an optical unit 10, and a control unit 60, and is connected to a network (for example, a LAN). When a print job execution instruction is received from an external terminal device (not shown), a toner image composed of yellow, magenta, cyan, and black is formed based on the instruction, and a full color image is formed by multiple transfer of these toner images. Perform formation.

Hereinafter, the reproduction colors of yellow, magenta, cyan, and black are represented as Y, M, C, and K, and Y, M, C, and K are added as subscripts to the numbers of the components related to the reproduction colors.
<Image process part>
The image processing unit 3 includes image forming units 30Y, 30M, 30C, and 30K corresponding to each of Y to K colors, an optical unit 10, an intermediate transfer belt 11, and the like.

The image forming unit 30Y includes a photosensitive drum 31Y, a charger 32Y, a developing unit 33Y, a primary transfer roller 34Y, a cleaner 35Y for cleaning the photosensitive drum 31Y, and the like disposed around the photosensitive drum 31Y. A Y-color toner image is formed on the drum 31Y.
The other image forming units 30M to 30K have the same configuration as the image forming unit 30Y, and the reference numerals are not shown in the figure.

The intermediate transfer belt 11 is an endless belt, is stretched around a driving roller 12 and a driven roller 13 and is rotationally driven in the direction of arrow A.
The optical unit 10 includes a light emitting element such as a laser diode, emits a laser beam L for forming an image of Y to K colors by a drive signal from the control unit 60, and exposes and scans the photosensitive drums 31Y to 31K.

  By this exposure scanning, electrostatic latent images are formed on the photosensitive drums 31Y to 31K charged by the chargers 32Y to 32K. Each electrostatic latent image is formed by superimposing Y to K color toner images on the photosensitive drums 31Y to 31K obtained by developing by the developing units 33Y to 33K at the same position on the intermediate transfer belt 11. The timing is shifted so that the primary transfer is performed.

The toner images of the respective colors are sequentially transferred onto the intermediate transfer belt 11 by the electrostatic force acting on the primary transfer rollers 34Y to 34K to form a full color toner image, and further move toward the secondary transfer position 46.
The sheet feeding units 4a, 4b, and 4c accommodate the recording sheet S, are arranged in three upper and lower stages, and set with different types of sheets.

In the printer 1 according to the present embodiment, for example, it is assumed that color paper is stored in the paper supply unit 4a, and plain paper is stored in the paper supply unit 4b and the paper supply unit 4c.
Here, the color printing paper is paper whose electrical resistance value is managed in a range narrower than that of plain paper so that high-quality color printing can be performed.
More specifically, for the color printing paper, for example, the electrical resistance in the thickness direction is controlled in a range of 1.0 × 10 ¹³ Ω or more and less than 9.9 × 10 ¹⁶ Ω.

On the other hand, the electric resistance of plain paper often increases and decreases within a large range in which the electric resistance in the thickness direction includes the above range.
However, even in the case of plain paper, the recording sheets S enclosed in the same packaging container are considered to have substantially the same manufacturing conditions, and thus the electric resistance tends to vary within a relatively narrow range. .

In other words, with plain paper, the electrical resistance value may change significantly each time new paper is replenished, but once it changes, the electrical resistance in the thickness direction will not change so much until the next replenishment timing. It is considered a thing.
For this reason, if the electrical resistance in the thickness direction of each recording sheet S accommodated in each of the sheet feeding units 4a, 4b, and 4c is detected, the other sheets contained in the same sheet feeding unit are detected. The value of the electrical resistance of the recording sheet S can also be roughly estimated.

Since the paper feed units 4a, 4b, and 4c all have the same structure, the following description will be given mainly by using the paper feed unit 4a as an example. The components of the paper feed unit 4a are as follows. , A are added as subscripts.
Note that when there is a need to describe a sheet feeding unit other than the sheet feeding unit 4a, b or c is added as a subscript to the sheet feeding unit and its components.

The paper feed unit 4a includes a paper feed tray 41a that accommodates the recording sheets S, a pickup roller 40a that feeds the recording sheets S in the paper feed tray 41a one by one onto the conveyance path, and two or more recording sheets S that are fed out simultaneously. In this case, a pair of rolling rollers 47a for conveying only the uppermost sheet, a resistance detection mechanism 49a, and the like are provided.
Here, the resistance detection mechanism 49a is a mechanism for detecting the electrical resistance of the recording sheet S accommodated in the paper feed tray 41a, and details thereof will be described later.

The paper feed unit 4a is provided with an openable / closable door (not shown), and an open / close detection unit 39a for detecting the open / close state.
In addition, a timing roller pair 44 is provided above the paper feed unit 4 a for taking the timing of feeding the recording sheet S sent from the paper feed units 4 a to 4 c to the secondary transfer position 46.

  The control unit 60 selects one of the paper feed trays 41a, 41b, and 41c based on the detection results of the resistance detection mechanisms 49a, 49b, and 49c, and feeds the recording sheet S from the selected paper feed tray. Then, the feeding is temporarily held by the timing roller pair 44 and the recording sheet S is fed to the secondary transfer position 46 by releasing the holding in accordance with the movement timing of the toner image on the intermediate transfer belt 11. To do.

At this transfer position, the toner image on the intermediate transfer belt 11 is secondarily transferred onto the recording sheet S collectively by the action of the secondary transfer roller 45.
The recording sheet S that has passed the secondary transfer position 46 is conveyed to the fixing unit 5, and the toner image (unfixed image) on the recording sheet S is fixed to the recording sheet S by heating and pressurization in the fixing unit 5. Thereafter, the sheet is discharged onto the discharge tray 72 via the discharge roller pair 71.
<Configuration of resistance detection mechanism>
As shown in FIG. 1, the resistance detection mechanism 49 a includes a surface resistance detection unit 50 a that detects an electrical resistance over a predetermined distance on the surface of the recording sheet S, and an electrical resistance in the thickness direction of the recording sheet S (hereinafter “thickness resistance”). The thickness resistance detector 51a detects the thickness resistance detector 51a.

FIG. 2 is a perspective view showing the configuration of the thickness resistance detector 51a.
As shown in the figure, the thickness resistance detector 51a applies a voltage to the pair of rolling rollers 47a sandwiching the recording sheet S to detect the electrical resistance in the thickness direction of the recording sheet S.
In the present embodiment, the roller pair 47a includes a drive roller 42a and a reverse roller 43a.

The drive roller 42a is a roller that is driven to rotate in the direction of the arrow C or the direction of the arrow C ′ by an unillustrated drive source in accordance with an instruction from the control unit 60, and has a columnar shaft 141a having conductivity. The central part of the is covered with an elastic member 142a having conductivity.
Similarly to the drive roller 42a, the reverse rotation roller 43a has a central portion of a cylindrical shaft portion 151a having conductivity covered with an elastic member 152a having conductivity, and is not shown through a torque limiter, for example. By connecting to the drive source, a predetermined rotational torque is given in the direction of arrow D, and when the recording sheet S is double-fed, the lowermost recording sheet S is pushed back.

Further, the power supply member 203a and the power supply member 204a are in sliding contact with one ends of the shaft portion 141a and the shaft portion 151a, respectively.
Here, in this embodiment, the power supply member 203a and the power supply member 204a are block-shaped conductors, and are made of a material such as copper graphite or carbon graphite having slidability and conductivity. Carbon brushes are used, and each is connected to a known electric resistance meter 201a including a power source and an ammeter via a lead wire 202a.

In such a configuration, when the pickup roller 40a is driven in the direction of the arrow B and two or more recording sheets S are conveyed to the nip portion of the roller pair 47a, the lowermost recording sheet S is Then, the sheet is sequentially pushed back in the direction opposite to the normal feeding direction, and as a result, only one recording sheet is rolled and fed forward.
That is, the pair of rolling rollers 47a is in a state in which only one recording sheet S is securely sandwiched by the above-described winding operation (hereinafter referred to as “single sandwiching state”).

At this time, the electric resistance meter 201a passes the current I in the thickness direction of the recording sheet S and measures the electric resistance in the thickness direction.
In addition to the thickness resistance detector 51a that detects the electrical resistance in the thickness direction, a surface resistance detector 50a that detects surface resistance is provided in the paper feed tray 41a.
FIG. 3 is a side view showing the configuration of the surface resistance detector 50a.

The surface resistance detector 50a includes a main body 52a, electrodes 53a and 54a connected to lead wires 57a, a leaf spring 55a, and a wire 56a.
The main body portion 52a is a substantially rectangular parallelepiped insulator, and the cross-sectional shape of the lower corner portion on the pickup roller 40a side is rounded, and the surface thereof is a fluororesin coating that reduces frictional resistance. Etc. are given.

Most of the electrode 53a and the electrode 54a excluding the contact portion with the recording sheet S are embedded in the main body 52a at a predetermined distance.
The lead wire 57a is a conducting wire that connects the electrode 53a and the electrode 54a to an electric resistance meter (not shown).
As shown in the figure, the plate spring 55a is a plate-like spring bent into a U-shape, restricts the position of the main body 52a, and slightly urges the main body 52a toward the recording sheet S. However, the size of the contact pressure between the electrode 54a and the electrode 53a and the recording sheet S is rather determined by the mass of the electrode 54a, the electrode 53a, and the main body 52a. Thereby, even if the height of the bundle of recording sheets S changes, it is possible to stabilize the contact pressure and prevent the contact resistance from fluctuating greatly.

The two wires 56a are respectively inserted into different ones of the two through holes 58a provided in the upper part of the leaf spring 55a, and each end thereof is joined to the upper surface of the main body 52a.
The wire 56a is pulled up in conjunction with the opening operation of the door (not shown) of the paper feeding unit 4a, and is moved up and down by a mechanism (not shown) that is loosened in conjunction with the closing operation of the door. It is configured.
<Detection operation by resistance detection mechanism>
FIGS. 4A and 4B are diagrams for explaining the electric resistance detection operation by the resistance detection mechanism 49a.

As shown in FIG. 4A, a plurality of recording sheets are accommodated in the paper feed tray 41a, and the highest one of these is designated as a recording sheet S1.
The detection operation by the resistance detection mechanism 49a detects only one surface of the recording sheet by the surface resistance detector 50a (hereinafter referred to as “surface resistance detection process”) and the uppermost recording on the paper feed tray. Some sheets may be detected by the thickness resistance detector 51a (hereinafter referred to as “uppermost sheet resistance detection process”) following the detection by the surface resistance detector 50a.

  In the uppermost sheet resistance detection process, as shown in FIG. 4A, the surface resistance detector 50a first detects the surface resistance of the uppermost recording sheet S1, and then rotates the pickup roller 40a in the direction of arrow B. Then, as shown in FIG. 4 (b), the uppermost recording sheet S1 is conveyed to the pair of rolling rollers 47a, and the resistance in the thickness direction of the recording sheet S1 is detected in a single nipped state sandwiched between these rollers. Thereafter, the pickup roller 40a is reversed in the direction of the arrow B ′ to return the recording sheet S1 to the original position.

Thereby, the thickness resistance value is obtained, and the conversion coefficient from the surface resistance value to the thickness resistance value is obtained.
On the other hand, in the surface resistance detection process, as shown in FIG. 4B, only the surface resistance of the second and subsequent recording sheets S exposed at the detection position of the surface resistance detection unit 50a is detected, and the above conversion is performed. The thickness resistance of the recording sheet is calculated from the coefficient.

As a result, for the second and subsequent sheets, it is not necessary to bother to send the recording sheet to the pair of rollers in order to acquire the thickness resistance. The processing speed for continuous printing is improved.
<Configuration of control unit>
FIG. 5 is a block diagram illustrating a configuration of the control unit 60 of the printer 1.

As shown in the figure, the control unit 60 includes, as main components, a CPU 61, a communication interface (I / F) unit 63, a RAM (Random Access Memory) 64, a ROM (Read Only Memory) 65, an EEPROM (Electronically Erasable). and Programmable Read Only Memory) 66, a clock 67, and the like.
The operation panel 73 includes a liquid crystal display, a touch panel stacked on the liquid crystal display, operation buttons for inputting various instructions, and the like, and receives input of various instructions from the user through operations of the touch panel, the operation buttons, and the like.

On the liquid crystal display, an operation screen such as a print setting screen, various display information such as a print result, a message to the user, and the like are displayed.
The communication interface unit 63 is a LAN card, a LAN board, or the like for connecting the CPU 61 to a network such as a LAN. The communication interface unit 63 receives a print job transmitted from the client terminal via the LAN and sends it to the CPU 61.

The RAM 64 is a volatile memory and serves as a work area when the CPU 61 executes a program.
In addition to a program for controlling each unit during image formation, the ROM 65 stores a program, a table, and the like related to selection of a paper feed tray, which will be described later.
The EEPROM 66 is a non-volatile writable memory and serves as a storage area for data and the like when the CPU 61 executes a program.

A CPU (Central Processing Unit) 61 reads a necessary program from the ROM 65 and controls various operations in a unified manner while measuring timing, thereby smoothly executing processing operations.
Furthermore, in the printer 1 according to the embodiment, an image to be printed is displayed based on the electrical resistance value detected by the resistance detection mechanism 49a so that the toner image can be transferred to the recording sheet with a predetermined image quality. By selecting a paper feed tray in which a recording sheet S suitable for secondary transfer is stored, an image forming operation with excellent transfer quality is quickly executed.

Hereinafter, selection of the above-described paper feed tray will be described.
FIG. 6 is a flowchart showing the control contents for selecting the paper feed tray executed by the CPU 61, and is executed as a subroutine of a main flowchart (not shown) for controlling the operation of the entire printer 1.
First, in step S11, the CPU 61 determines whether it is time to detect the electrical resistance of the uppermost recording sheet on each paper feed tray.

The detection timing of such electric resistance includes (1) when the printer 1 is turned on, (2) when returning from the power saving mode (sleep mode) to suppress the power consumption of the printer 1 and (3) supplying power For example, when the paper tray door is opened or closed.
At the timings (1) and (2), it is very likely that a print instruction will be received after that. At the timing (3), a new type of recording sheet is placed on the paper feed trays 41a, 41b, and 41c. This is because there is a possibility that the value of the electrical resistance of the uppermost recording sheet is replenished.

In addition, (4) a certain time interval or (5) the end of one print job may be included in the timing for detecting the electrical resistance.
The reason why the timing of (4) is given is that when the printer 1 is installed in a high humidity environment, it is considered that the recording sheet absorbs moisture and the electrical resistance value changes with time. .

  Further, in the case of (5) above, the operation sound at the time of detecting the thickness resistance can be made inconspicuous. That is, when the resistance value of the uppermost recording sheet is measured as described above, the recording sheet is once transported to the pair of roller rollers for thickness resistance detection (see FIG. 4B), Since the operation of returning to the position is executed, an operation sound for feeding out the recording sheet is generated. If this is executed at the end of the print job, the user will recognize it as an operation sound when the print job is executed, so the user will hardly be bothered, but if the operation sound suddenly occurs separately from the print job, the user will feel uncomfortable or uncomfortable. This is because it may give a pleasant feeling.

In step S11, when it is determined that it is the timing of resistance detection of the uppermost recording sheet based on the determination criterion (step S11: YES), the uppermost recording sheet resistance detection process of step S12 is executed.
FIG. 7 is a flowchart showing the contents of the uppermost recording sheet resistance detection process.
First, in step S121, the CPU 61 detects the surface resistance of the uppermost recording sheet S1 of each paper feed tray by the surface resistance detectors 50a, 50b, and 50c (see FIG. 4A), and is shown in FIG. The value is stored in the surface resistance value column 313 of the conversion coefficient table. In the table, numbers 1 to 3 in the paper feed tray number column 311 correspond to the paper feed trays 41a, 41b, and 41c, respectively.

This conversion coefficient table is formed in the EEPROM 66, for example.
Then, the pickup rollers 40a, 40b, and 40c are driven to rotate in the forward direction to feed out the uppermost recording sheet S1, and the recording sheet S1 is sandwiched between the pair of rolling rollers 47a, 47b, and 47c (see FIG. The thickness resistance value of the recording sheet S1 is detected by the thickness resistance detectors 51a, 51b, and 51c, respectively, and the thickness resistance value is stored in the corresponding column 312 of the conversion coefficient table. Thereafter, the respective pickup rollers are rotationally driven in the reverse direction to return the recording sheet S1 to the original position (step S122).

  Conversion factors (Ra1 / Rb1, Ra2 / Rb2, and Ra3 / Rb3) for converting from the surface resistance value to the thickness resistance value are calculated for each sheet feeding tray from the stored resistance values, and the tray number is calculated. Are stored in the corresponding conversion coefficient column 314 of the conversion coefficient laboratory calculation table (step S123). The conversion coefficient obtained in this way is used to obtain the thickness resistance in the second and subsequent recording sheets. Details will be described later.

Then, the thickness resistance value in the column 312 of the conversion coefficient table in FIG. 8 is read out and stored again in association with the tray number in the uppermost recording sheet resistance table 320 shown in FIG. 9 (step S124).
Thereafter, the process returns to the flowchart of FIG.
Returning to FIG. 6, in the next step S <b> 13, it is determined whether a print job has been received from an external client terminal via the communication I / F unit 63.

In this embodiment, the printer driver is set so that a print job can be issued by designating a print mode for each page.
If it is determined in step S13 that the print job has not yet been received (step S13: NO), the process returns to step S11 to determine again whether it is time to detect the electrical resistance of the uppermost recording sheet. Execute the process.

If it is determined that a print job has been received (step S13: YES), information on the print mode of the page is acquired with reference to the header attached to the image data of the page to be printed from now. (Step S14).
However, even if the print mode designation for each page is not attached to the header, it is possible to determine whether the image is the full color image or the monochrome image by analyzing the image data of the next page by a known technique.

Then, when printing is performed in the acquired print mode, a paper feed tray selection process for selecting a paper feed tray is executed in step S15.
FIG. 10 is a flowchart showing the content of the paper feed tray selection process.
The print mode means a mode in which printing is performed under image forming conditions corresponding to image data attributes such as full color, monochrome, text image, and photographic image. In the present embodiment, two modes, full color print and monochrome print, are used. A case where the print mode can be executed will be described.

The CPU 61 first refers to the uppermost recording sheet resistance table 320 (FIG. 9), and the thickness resistance value of the uppermost recording sheet in each paper feed tray and the thickness resistance value allowed in the print mode to be executed. Are compared (step S151).
FIG. 11 shows a table 330 indicating a range of thickness resistance values of a recording sheet that is desirable for obtaining a constant image quality in this mode in accordance with color printing or monochrome printing. The table 330 is obtained in advance in the ROM 65 or the EEPROM 66. Stored.

In the table 330, the range of the thickness resistance value (hereinafter referred to as “thickness resistance value Rs”) of the recording sheet allowed to ensure the target image quality in monochrome printing and full color printing is indicated by the logRs value. ing.
Here, the log Rs value means the value of the exponent part when the thickness resistance value Rs is expressed as an index (hereinafter, simply referred to as “resistance log value”). For example, the thickness resistance value Rs is 5 .6 × 10 ⁺¹⁴ [Ω], the log value in this case is “14”. For example, when execution of the monochrome mode is designated, the recording sheet having the thickness resistance value Rs is stored. It can be seen that the paper feed tray to be placed is positioned at the second priority order to be selected.

A column 331 indicates a priority order when selecting a paper feed tray. A column 333 indicates a range of thickness resistance values corresponding to each priority order when monochrome printing is performed, and a column 334 indicates a full color. A range of thickness resistance values corresponding to each priority in the case of executing printing is described.
In monochrome printing and full color printing, the condition of the log value of the thickness resistance of the recording sheet that can be desired for secondary transfer with the highest image quality is 13 ≦ log value <16. It is not the priority order but the second priority order.

  This is because, in monochrome printing, even if the thickness resistance log value deviates from this range, the deterioration in quality in secondary transfer is not so noticeable, and the recording sheet satisfies the condition of 13 ≦ log value <16. This is because if S is consumed more and more in monochrome printing, there is a possibility that there is no recording sheet that meets the conditions when full color printing with more severe conditions is executed.

Further, in the column of the full color mode, “x” is entered for the priorities 3 and 4 (columns 337 and 338). For recording sheets corresponding to conditions other than the priorities 1 and 2, full-color prints are included. This means that the target image quality cannot be ensured.
As described above, the thickness resistance value range in the specified print mode of the table 330 is compared with the thickness resistance value of the uppermost recording sheet in each paper feed tray, and the recording sheets satisfying the resistance value are loaded. It is determined whether or not a paper feed tray exists (step S152).

If there is a paper feed tray on which recording sheets satisfying the resistance value are stacked (step S152: YES), the paper feed tray having the highest priority among them (the one paper feed tray satisfying the resistance value is one). In that case, that paper feed tray) is selected (step S153), and the process returns to the flowchart of FIG.
In the case of the full color mode, the thickness resistance value of the uppermost recording sheet corresponds to a range other than the priority conditions 1 and 2 in the full color mode column 334, for example, and the sheet on which recording sheets satisfying the resistance value condition are stacked. If there is no tray (step S152: NO), the target image quality can no longer be ensured, so an appropriate message is displayed on the display unit of the operation panel 73 and the printing operation is not executed. 6 is interrupted and the execution of the received print job is interrupted, the process proceeds to (A) of the flowchart of FIG. 6 and returns to the main flowchart as it is.

An example of the above message is “There is currently no paper suitable for color printing. Please replenish with color paper”.
Returning to FIG. 6, after the paper feed tray selection process, the recording sheet is fed from the paper feed tray selected in step S16, and the printing operation is executed for the recording sheet.
In step S17, the uppermost recording sheet resistance table is updated.

That is, for the paper feed tray that has fed the recording sheet in step S16, since the topmost recording sheet has changed, the thickness resistance value is converted from the surface resistance value of the next recording sheet in the paper feeding tray, The thickness resistance value of the corresponding paper feed tray in the uppermost recording sheet resistance table 320 (FIG. 9) is rewritten.
For example, if a recording sheet is fed from the sheet feeding tray 41a, the surface resistance value of the next recording sheet detected by the surface resistance detecting unit 50a of the sheet feeding tray 41a is used for the conversion coefficient in FIG. The conversion factor (Ra1 / Rb1) in the table 310 is multiplied to convert it into a thickness resistance value, and the thickness resistance value in the paper feed tray number 1 in the uppermost recording sheet resistance table 320 of FIG. 9 is updated by this value.

  In each paper feed tray, the surface resistance of the second and subsequent recording sheets is measured at the surface resistance detector 50a after the upper recording sheet is fed and the lower recording sheet is fed. , 50b, and 50c, it is desirable to execute the measurement promptly after the contact, and it is necessary to complete the measurement at the latest by the time the paper feed tray selection process for printing the next page is executed. . Otherwise, the second and subsequent sheets may be delayed and the processing speed may decrease. The control unit 60 determines the timing of the measurement by measuring the time from the start of feeding the recording sheet.

  Then, it is determined whether or not the print job has been completed (step S18). If not, the process returns to step S14 to execute the print operation for the next page, and the print mode of the page to be printed next. And a paper feed tray is selected based on the topmost recording sheet resistance table 320 updated in step S17 (step S15). Hereinafter, paper feeding / printing from the selected paper feed tray (step S16) is performed. The upper recording sheet resistance table update (step S17) is repeated.

If it is determined in step S18 that the print job has ended, the paper feed tray selection process is ended and the process returns to a main flowchart (not shown).
According to the present embodiment, when the attributes of the image to be printed are different, such as full-color printing and monochrome printing, the degree of influence of the electrical resistance value in the thickness direction of the recording sheet on the image quality at the time of secondary transfer is different. By selecting a paper feed tray that stores a recording sheet having an electrical resistance value suitable for the attribute of each image, and sending out the recording sheet from the paper feed tray during printing, it is possible to suppress deterioration in transfer image quality. it can.

In addition, since the uppermost recording sheet resistance table 320 (FIG. 9) has already been created before receiving a print job, the paper feed tray can be selected immediately, and the first print time is not delayed.
For the second and subsequent recording sheets in each paper feed tray, the surface resistance values detected by the surface resistance detectors 50a, 50b, and 50c are converted into thickness resistance values according to a conversion coefficient determined in advance. Since the uppermost recording sheet resistance table 320 is updated to always obtain the thickness resistance value of the uppermost recording sheet in each paper feed tray, the sheet of the next recording sheet is selected according to the print mode of the next page. Since the selection of the paper feed tray can be quickly determined, an appropriate recording sheet can be fed without reducing productivity even during continuous printing.

<Modification>
As described above, the present invention has been described based on the above embodiment, but it is needless to say that the present invention is not limited to the above embodiment, and the following modifications can be implemented.
(1) In the above embodiment, in the full-color print mode, if there is no recording sheet that satisfies the resistance values of the priorities 1 and 2 in the table of FIG. 11, the print job is given up and the print job is interrupted. The message is displayed (step S154 in FIG. 10), but the present invention is not limited to this.

  For example, even when the conditions of the priorities 1 and 2 are not met, a paper feed tray in which recording sheets having a thickness resistance value (hereinafter referred to as “recent resistance value”) closest to these conditions is stored. Even when the recording sheet having the latest resistance value is selected, the transfer bias value applied to the secondary transfer roller 45 is controlled to be changed so as to obtain an acceptable transfer image quality in the full-color print mode. Also good.

FIG. 12 shows a modified example of the paper feed tray selection process of FIG. 10 in this case, and steps S155 and S156 are executed instead of step S154 of FIG.
That is, in step S152 of FIG. 12, if there is no sheet feed tray that stores recording sheets corresponding to the resistance value condition in the corresponding print mode (step S152: NO), the resistance closest to these conditions is set. A paper feed tray storing a recording sheet having a value Rs (hereinafter, referred to as “recent resistance value Rs”) is selected (step S155), and the transfer image quality permitted in the print mode is selected even with the latest resistance value Rs. So that the transfer bias value of the secondary transfer roller 45 is changed (step S156).

FIG. 13 shows an example of a table 340 that is referred to when determining the transfer bias.
In the table, only the correction value of the transfer bias when a recording sheet having a resistance value other than priorities 1 and 2 is fed in the full color print mode is described. This is because, in the monochrome print mode, as shown in FIG. 11, it is basically possible to print out in the range of all resistance values that can be taken by the recording sheet.

In the table of FIG. 13, a column 341 indicates the magnitude of the difference between the log value of the thickness resistance value of the selected recording sheet and the upper or lower log value of the resistance value condition closest to the resistance value. In the column 344, a transfer bias value to be increased according to the difference between the log values is obtained and set in advance.
Specifically, when a full color print job execution instruction is received, the log value of the thickness resistance value of the uppermost recording sheet in each paper feed tray is 17, 18 and 19, respectively.

In the table 330, the log value of 17 is the closest value to the upper limit 16 value of “13 ≦ log value <16” of the priority order 1.
As a result, the paper feed tray 41a that accommodates recording sheets corresponding to log values of 17 is selected.
Further, 1 is obtained as the log value difference by subtracting the log value = 16 of the closest condition from the log resistance = 17 of the thickness resistance value of the recording sheet in the paper feed tray 41a.

The CPU 61 refers to the table 340, and when the log value difference is 1, the value of the secondary transfer bias to be increased is + V1, so the secondary transfer bias is increased so that this value becomes the full color. Execute print mode.
As can be seen from the table 340, the value of the secondary transfer bias is set to increase as the log value difference increases.

This is because under a condition where the value of the secondary transfer bias is constant, the transfer current decreases as the electrical resistance in the thickness direction of the recording sheet increases, and the force for electrostatically attracting the toner image to the recording sheet becomes weaker. Therefore, in order to maintain the electrostatic attraction force, the value of the secondary transfer bias must be increased.
As described above, in this modification, the image quality at the time of transfer is maintained in the recording sheets stored in the plurality of paper feed trays according to the designated print mode under the current secondary transfer conditions. Even if there is no electrical resistance that can be as much as possible, among these, the one that is most unlikely to deteriorate the image quality at the time of transfer is targeted for feeding, and by changing the secondary transfer bias value, Since deterioration in image quality during transfer can be suppressed, a situation where a print job is interrupted can be avoided.

  (2) In the above embodiment, as a factor that varies the degree of influence of the electrical resistance value in the thickness direction of the recording sheet S on the image quality at the time of secondary transfer, only the difference in image attributes such as full-color printing and monochrome printing is used. However, in the same full-color print or monochrome print, more detailed case classification may be performed to consider the effect on image quality degradation during transfer.

For example, the degree of influence on the image quality at the time of secondary transfer differs between a case where characters occupy most of the image data for one page and a case where the proportion of photographic images is large.
It is considered that such a difference in image case can be determined by analyzing the image data acquired via the communication I / F unit 63 by the CPU 61 and obtaining the B / W ratio.

Here, the B / W ratio refers to the ratio of the image portion (Black) to the paper surface (White).
The inventor believes that the higher the B / W ratio is, the closer to the photograph, the more accurate secondary transfer is required, and the lower the B / W ratio, the more characters and the less transfer We thought that the image quality degradation at the time was inconspicuous.

  An example of this concept is as follows: (a) 0 [%] ≦ B / W ratio [%] <5 [%], text composed only of characters, (b) 5 [%] ≦ B / If W ratio [%] <20 [%], text with many characters and slightly mixed photos and graphs, (c) 20 [%] ≤ B / W ratio [%] <30 [%] For example, if it is a sentence in which few characters are mixed in a photograph or graph, and (d) 30 [%] ≦ B / W ratio [%], it is estimated that the photograph is a photograph.

The acquired image data includes information indicating whether full-color printing or monochrome printing is to be executed, and is based on the attribute of the image data such as the information and the B / W ratio described above. It is more desirable to select a paper feed tray.
FIG. 14 is a diagram showing the contents of the table 350 defining the relationship between the attribute of the image data classified more finely than the contents of the table 330 and the priority order for selecting the paper feed tray from such a viewpoint. .

A column 351 indicates the priority order from 1 to 5.
In the large item column 353 to be referred to when performing monochrome printing, the range of the use resistance value of the paper feed tray to be selected in the range of priority 1 to 4 is indicated by the log value.
The large item column 353 includes small item columns 353a, 353b, 353c, and 353d, and (a) a condition of 0 [%] ≦ B / W ratio [%] <5 [%], (b ) 5 [%] ≦ B / W ratio [%] <20 [%], (c) 20 [%] ≦ B / W ratio [%] <30 [%], and (d) 30 [%] ] ≦ B / W ratio [%], “x” is shown in the range of log values or message display similar to FIG.

In the large item column 354 to be referred to when full color printing is executed, the range of the thickness resistance value of the recording sheet in the range from the priority order 1 to the maximum 5 is indicated by the log value.
The large item column 354 includes small item columns 354a, 354b, 354c, and 354d, and (a) a condition of 0 [%] ≦ B / W ratio [%] <5 [%], (b ) 5 [%] ≦ B / W ratio [%] <20 [%], (c) 20 [%] ≦ B / W ratio [%] <30 [%], and (d) 30 [%] ] ≦ B / W ratio [%] The log value range or message display similar to FIG. 11 is shown.

In this way, the image quality at the time of secondary transfer can be optimally maintained by selecting a paper feed tray that stores recording sheets suitable for the attribute of the image data more finely.
(3) In the above embodiment, the difference in electrical resistance mainly due to the type of recording sheet is considered as a factor that causes the electrical resistance of the recording sheet to fluctuate. However, for example, the printer 1 is in a high humidity environment. When used, the recording sheet absorbs moisture with the passage of time and the electrical resistance decreases.

For this reason, even if it is the color printing paper manufactured so that an electrical resistance may fall in the predetermined range initially, an electrical resistance may deviate from the said range with progress of time.
Therefore, the inventor provides a heating unit in each paper feed tray in order to suppress a decrease in electrical resistance of the recording sheet due to such moisture absorption, and heats the heating unit at a predetermined timing to perform recording. The structure which discharge | releases the water | moisture content of a sheet | seat and recovers the moisture content was considered.

The heating unit is a planar heater that is turned ON / OFF by the control unit 60.
FIGS. 15A and 15B are diagrams showing a configuration when such a heating unit is provided in each paper feed tray.
As shown in FIG. 15A, here, a heating unit 80a is provided on a surface of the paper feed tray 41a that faces the rear end of the recording sheet S to be stored.

  Further, as shown in FIG. 15B, heating sections 82a and 83a may be provided on the surfaces of the paper feed tray 41a that face both ends in the direction orthogonal to the conveyance direction of the stored recording sheet S, respectively. . Since the stacked recording sheets absorb water from the end surface exposed to the outside, the water content can be effectively reduced by providing the heating unit on the inner surface of the sheet feeding tray in this way.

It is desirable to apply such a configuration to the paper feed trays 41b and 41c, and to provide heating units in all the paper feed trays provided with the resistance detection units.
Hereinafter, a sheet heating process executed by the control unit 60 in the configuration in which the heating unit as described above is provided in the sheet feeding trays 41a, 41b, and 41c will be described.
The recording sheet heat treatment removes moisture in the recording sheet by heating the recording sheet whose electrical resistance has been reduced with time due to moisture absorption by the heating unit, particularly in the printer 1 placed in a hot and humid environment. This is a process for improving the electrical resistance so as to satisfy the resistance value condition capable of feeding paper.

FIG. 16 is a flowchart showing the content of the sheet heating process executed for one paper feed tray, and is executed independently of the flowchart of FIG.
Hereinafter, the sheet heating process in the paper feed tray 41a will be described, but the same applies to the other paper feed trays 41b and 41c.
First, the CPU 61 determines whether or not a recording sheet has been replenished to the paper feed tray 41a (step S201).

  Usually, the sheet feed tray is provided with a sheet sensor (for example, a reflective photoelectric sensor) for detecting the presence or absence of a recording sheet at the bottom or the like. When the recording sheet is replenished by the user after the (empty) is detected, the sheet sensor detects the recording sheet, so that it can be determined that the recording sheet has been replenished based on this output change.

  Alternatively, a planar member having a surface resistance value greatly different from that of the recording sheet is pasted in a range where the surface of the tray is in contact with the electrode of the surface resistance detection unit 50a, and the resistance value is detected by the surface resistance detection unit 50a. When the recording sheet is detected, the paper sheet is determined to be paper empty. After that, when the recording sheet is replenished and a different surface resistance value (surface resistance value within the range that the recording sheet can take) is detected, it is determined that the recording sheet is replenished. You may make it do.

When it is determined that the recording sheet has been replenished in this manner (step S201: YES), the surface resistance detector 50a detects the surface resistance value R0 of the uppermost recording sheet (step S202). If it is determined in step S201 that the recording sheet is not replenished (step S201: NO), step S202 is skipped.
Then, it is determined whether or not time N (for example, 2 hours) has elapsed since the previous detection of the surface resistance of the recording sheet (hereinafter simply referred to as “resistance detection”) (step S203).

In the present embodiment, the clock IC in the CPU 61 is referred to, and the detected value and time are stored in correspondence in the table in the EEPROM 66 for each resistance detection. This can be easily determined by comparing the latest detection time recorded in the table with the current time.
If the time N has not elapsed since the previous resistance detection (step S203: NO), step S201 and step S202 are repeated.

If it is determined that the time N has elapsed (step S203: YES), the surface resistance value Rn of the uppermost recording sheet at that time is detected (step S204).
When the recording sheet absorbs moisture, its surface resistance value also decreases, so it is determined whether or not the difference (R0−Rn) between the initial surface resistance value R0 and the surface resistance value Rn is greater than or equal to the first threshold value R1 (step). S205).

As described above, since the surface resistance value and the thickness resistance value have a predetermined correlation, the decrease amount of the thickness resistance value can be estimated by the decrease of the surface resistance value. Therefore, in the present embodiment, the amount of change in the surface resistance value when the thickness resistance value exceeds the allowable range is obtained in advance through experiments or the like, and an appropriate value is given to this value in consideration of product variations and safety factors. It is determined as 1 threshold value R1.
However, when detecting the surface resistance value R0 in step S202, the thickness resistance value is also detected in the same manner as the uppermost recording sheet resistance detection process described with reference to FIG. 7, and a conversion coefficient from the surface resistance value to the thickness resistance value is obtained. Each time the surface resistance value is detected, it may be converted into a thickness resistance value, and the difference between the thickness resistance values may be compared with a first threshold value R1 set for the thickness resistance value (also in step S209 described later). the same).

In step S205, when the difference (R0-Rn) in the surface resistance value is equal to or greater than the threshold value R1 (step S205: YES), the process proceeds to step S206, where the sheet heating is started by supplying power to the heating unit. .
When time M (for example, 10 minutes) elapses after the start of heating (step S207: YES), resistance is detected (surface resistance value Rm) (step S208), and the difference (R0-Rm) from the initial surface temperature is the first. It is determined whether it is smaller than 2 threshold value R2 (step S210).

If (R0−Rm) <R2 (step S209: YES), it is considered that the moisture content is sufficiently lowered by heating and is recovered to near the resistance value at the time of replenishment. Stop (step S210), return to step S203, and wait for the next resistance detection timing.
If (R0−Rm) <R2 is not satisfied (step S209: NO), the heating is not sufficient, and resistance is detected again after the lapse of time N to determine the difference (steps S207, S208, and S209).

By executing the sheet heating process as described above for each sheet feeding tray, the moisture content of the recording sheet is prevented from changing significantly, and the recording sheet satisfying the resistance value condition for maintaining a predetermined image quality is satisfied. Paper can be fed.
(4) In the above embodiment, three paper feed trays 41a, 41b and 41c are provided. However, it is sufficient if at least two paper feed trays are provided, and four or more paper feed trays are provided. It may be provided.

(5) In the above embodiment, the resistance detection mechanism includes the surface resistance detection unit and the thickness resistance detection unit. However, the resistance detection unit may not necessarily include two types of resistance detection units, and only one of them may be provided. I do not care.
If there is only a thickness resistance detection part, at least the first print start time can be shortened compared to the prior art. In addition, there is a correlation between the surface resistance value and the thickness resistance value, and the thickness resistance value within a certain error range based only on the surface resistance value without obtaining an accurate conversion coefficient each time as in the above embodiment. It is because it can guess.

(6) In the above embodiment, each time a recording sheet is fed, a paper feed tray is selected according to the print mode of the next page.
However, in the case of a model that can only specify the print mode for the entire job and cannot determine the print mode for each page, the print job can be executed without changing the first selected paper feed tray. . This is because there is a high probability that the same type of recording sheet is usually stored in one sheet feeding tray.

(7) In the above embodiment, an example in which the image forming apparatus according to the present invention is applied to a tandem color digital printer has been described. However, the present invention is not limited to this, and may be applied to, for example, a monochrome printer. In short, any image forming apparatus that performs an image forming operation by transferring a toner image carried on an image carrier onto a recording sheet and then fixing it by heating may be used.
This is because the monochrome printer is configured to directly transfer from the photosensitive drum to the recording sheet S. Even in such a situation, the value of the electrical resistance in the thickness direction of the recording sheet S is the image quality at the time of the transfer. It is because it greatly affects.

  Further, the contents of the above embodiment and the above modification may be combined as much as possible.

  The image forming apparatus according to the present invention can be widely applied to an image forming apparatus including a plurality of paper feed trays.

DESCRIPTION OF SYMBOLS 1 Printer 3 Image process part 4a, 4b, 4c Paper feed part 5 Fixing part 10 Optical part 11 Intermediate transfer belt 12 Drive roller 13 Driven roller 30Y, 30M, 30C, 30K Image forming part 31 Photosensitive drum 32 Charger 33 Developer 34 Primary transfer roller 35 Cleaner 39 Open / close detection unit 40a Pickup roller 41a, 41b, 41c Paper feed tray 42a Drive roller 43a Reverse roller 44 Timing roller pair 45 Secondary transfer roller 46 Secondary transfer position 47a Rolling roller pair 49 Resistance detection mechanism 50a , 50b, 50c Surface resistance detector 51a, 51b, 51c Thickness resistance detector 52a Main body 53a, 54a Electrode 55a Leaf spring 56a Wire 56a Through hole 57a Lead wire 60 Controller 61 CPU
63 Communication I / F section 64 RAM
65 ROM
66 EEPROM
67 Clock 71 Discharge roller pair 72 Discharge tray 73 Operation panel 80a Heating portion 141a, 151a Shaft portion 142a, 152a Elastic member 201a Electrical resistance meter 202a Lead wire 203a, 204a Feed member 310 Conversion coefficient table 320 Top recording sheet resistance table 330 , 340,350 tables

Claims (10)

An image forming apparatus that performs image forming operation by transferring a toner image carried on an image carrier to a recording sheet and then fixing by heating.
A plurality of paper feed trays for storing recording sheets;
A detecting means arranged for each of the plurality of paper feed trays for detecting an electrical resistance of a recording sheet in each paper feed tray;
Control means for selecting a paper feed tray and executing the image forming operation based on an electrical resistance value detected by the detection means so that the transfer can be performed with a predetermined image quality. Forming equipment.   The detection by the detection means is at least when the apparatus main body is turned on, at the time of returning from the power saving mode for suppressing the power consumption of the apparatus main body to the normal mode, or at the time of opening / closing the paper feed tray door The image forming apparatus according to claim 1, wherein the image forming apparatus is executed at a timing. The detection means is disposed on the sheet feed port side of each sheet feed tray, and is disposed on the first detection unit that detects electrical resistance in the thickness direction of the recording sheet, and on the side opposite to the sheet feed port side in the sheet feed direction. The second detection unit for detecting the electrical resistance of the recording sheet surface,
In the case where a plurality of images are continuously formed, the electrical resistance of the second and subsequent recording sheets stored in the paper feed tray is obtained based on the detection result of the second detection unit. The image forming apparatus according to claim 1 or 2.   4. The image according to claim 3, wherein the first detection unit detects an electric resistance value of the recording sheet by causing a current to flow between the pair of the rolling rollers in a state where the recording sheet is sandwiched. 5. Forming equipment. Conversion means for converting the electrical resistance value in the thickness direction from the surface resistance value of the recording sheet based on the result detected for the same recording sheet by the first and second detection units in the same paper feed tray ,
The image forming apparatus according to claim 3, wherein the converted value is used in the selection. Comprising an acquisition means for acquiring an attribute of image data to be subjected to image formation;
The image forming apparatus according to claim 1, wherein the predetermined image quality is determined based on the attribute.   The image forming apparatus according to claim 6, wherein the attribute is information indicating whether the print is a full color print or a monochrome print.   The image forming apparatus according to claim 6, wherein the attribute is information relating to a ratio between an image and a margin in image data. In the case where the electrical resistance value of the recording sheet in any one of the paper feed trays detected by the detection means is not within a range set in advance to obtain the predetermined image quality,
9. The image forming apparatus according to claim 1, wherein a transfer voltage is changed so that the image quality does not deteriorate. Furthermore, each of the plurality of paper feed trays is provided with a heater for heating the recording sheet,
2. The heater of a paper feed tray that accommodates the recording sheet is turned on when the electrical resistance of the recording sheet decreases with time and the amount of change exceeds a predetermined value. 10. The image forming apparatus according to any one of items 9 to 9.

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