JP2002023588A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device which can suppress variation of transfer characteristics due to variation in water content while suppressing the complexity of the constitution and control of the device. SOLUTION: This is an image forming device which transfers toner to a form P and fixes it to form an image on the form P and equipped with a transfer charger 10 which performs transfer processing for transferring the toner to the passing form P and a fixing device 14 which performs fixing processing for fixing the toner on the form by passing the form P where the toner is transferred. This image forming device passes the form P after the transfer processing and fixing processing through the transfer charger 10 and fixing device 14 through the transfer charger 10 and fixing device 14 again to further perform transfer processing and fixing processing for the form P and also performs delay processing for delaying the time when the form P reaches the transfer charger 10 when the form P reaches the transfer charger 10 successively after passing through the fixing device 14.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus capable of performing an image forming process a plurality of times on one sheet, and more particularly, to an image forming apparatus having a plurality of process speeds, This is suitable for an image forming apparatus capable of forming an image on both sides of the image forming apparatus.

[0002]

2. Description of the Related Art Some image forming apparatuses that print (form) an image by transferring toner (developer) onto paper (transfer paper) can perform double-sided printing in which printing is performed on both sides of paper. When printing on the front side of the paper,
When printing on the back side, there is a problem that the image quality does not match between the front side and the back side because the transfer characteristics (transfer performance) of the toner to the paper are different.

[0003] The reason is as follows. In front-side printing in the single-sided printing mode and the double-sided printing mode, the transfer characteristics are determined only by the physical characteristics of the sheet at room temperature. For this reason, good transfer can be performed by taking into account the type of paper and the surrounding environment (temperature, humidity) at the time of transfer as the transfer conditions.

However, in backside printing in the two-sided printing mode, the paper first passes through a fixing unit during frontside printing,
It again passes through the transfer section. In the fixing section, the sheet is heated in order to melt the toner and fix it on the sheet. This heating reduces the surface moisture of the paper. As a result, the surface resistance (surface electric resistance) value of the paper tends to increase as compared with the case of printing on the front side, and good transfer cannot be performed if the printing on the back side is performed under the same conditions as during the printing on the front side.
After printing on one side of the paper,
This is more noticeable in an image forming apparatus of the type in which the paper is switched back and printed on the back side as it is.

[0005] The graph of FIG. 9 shows the water content of the paper surface (hereinafter simply referred to as “paper content”) during double-sided printing in an image forming apparatus of the type in which printing is performed on the front surface of the paper and then switched back and conveyed to perform printing on the back surface. "Moisture content"). The following can be said from FIG. The water content of the paper is greatly reduced by the fixing process in front side printing. Then, the back surface printing is transferred in a state where the water content is reduced. Therefore, the water content of the paper greatly differs between the transfer process at the time of front surface printing and the transfer process at the time of back surface printing.

[0006] The relationship between the water content and the surface resistance of a sheet is as shown in FIG. FIG. 10 is a graph showing the relationship between the water content and the surface resistance value of the paper. In the graph of FIG. 10, a mark “□” indicates an OHP sheet, and a mark “Δ”, a mark “」 ”, and a mark“ × ”indicate sheets made of different types of paper. FIG.
In the graph of (1), the moisture content of the paper in each transfer process at the time of front side printing and the time of back side printing are indicated by a one-dot chain line and a two-dot chain line, respectively.

[0007] From the graph of FIG. 10, in each sheet except OHP, the surface resistance value in the transfer step of the backside printing is increased by about two digits from the surface resistance value in the transfer step of the frontside printing due to a decrease in the water content. You can see that there is. As described above, if the surface resistance value of the paper changes by two digits between the transfer step of the front side printing and the transfer step of the back side printing, it is difficult to realize good transfer in each transfer step. This is for the following reason.

The graph of FIG. 11 shows the transfer voltage (applied between the photosensitive member carrying the toner and the transfer portion provided on the back surface of the transfer surface of the sheet facing the photosensitive member) with respect to the surface resistance value of the sheet. Voltage).

Here, the transfer voltage (the same can be said for the transfer current) is determined by the surface resistance value of the paper, the charge amount of the photosensitive member, and the charge amount of the toner which are usually used. The transfer voltage determined in this way also differs depending on the amount of toner on the photoconductor. That is, the transfer voltage needs to be increased in a portion where the toner amount is large (solid black image), and the transfer voltage needs to be decreased in a portion where the toner amount is small (halftone image). Therefore, the black level transfer voltage at which the transfer of the solid black image is satisfactorily realized is not the same as the halftone level transfer voltage at which the transfer of the halftone image is satisfactorily realized. In FIG. 11, the optimum value of the black level transfer voltage is indicated by a solid line, and the optimum value of the halftone level transfer voltage is indicated by a broken line.

However, it is difficult to switch the transfer voltage for each type of image to be printed or for each part of the same image. Here, each of the black level transfer voltage and the halftone level transfer voltage has a certain allowable range for the resistance value, and there is a portion where both allowable ranges overlap. In FIG. 11, the permissible limit of the black level transfer voltage is indicated by a one-dot chain line, and the permissible limit of the halftone level transfer voltage is indicated by a two-dot chain line. Therefore, for example, when a certain surface resistance value (Y) is assumed, the dashed line (point a) and the dashed line (point b), and the two-dot chain line (point c) and the two-dot chain line at the surface resistance value are assumed. Between (d), the allowable range (allowable width) of each of the black level transfer voltage and the halftone level transfer voltage is shown. Therefore, a range that can cover a case where the toner amount is large to a small amount, that is, an overlapping portion (a hatched portion in FIG. 11) of each allowable range of the black level transfer voltage and the halftone level transfer voltage (the point c in FIG. b
A predetermined value (between the point and the point) is set as the transfer voltage.

However, the overlapping portion is very narrow. Therefore, when the surface resistance value of the paper changes, it deviates from this range. Therefore, as described above, when the water content decreases due to the fixing process in the front surface printing and the surface resistance increases, the transfer of the black solid image is good because the black level transfer voltage is within the allowable range during the back surface printing. Even if there is, the transfer of the halftone image may be deviated from the allowable range of the halftone level transfer voltage, and the image quality may be degraded.

Such a difference in the transfer characteristics between the front and back surfaces caused by the moisture content of the sheet is also affected by the process speed (the speed at which the sheet is conveyed in the image forming process). The graph shown in FIG. 12 shows the relationship between the process speed and the water content of the paper after front side printing. In the graph of FIG.
The broken line indicates the water content of the sheet before fixing in front side printing, and the solid line (dashed line) indicates the water content of the sheet immediately after fixing processing in front side printing.

From the graph of FIG. 12, the lower the process speed is, the more the water content of the paper is significantly reduced. This is because the lower the process speed is, the longer the time required for the paper to pass through the fixing unit is, and the more the paper evaporates.

On the other hand, conventionally, in an image forming apparatus which performs a double-sided printing mode at one process speed, a transfer voltage or a transfer current is changed between when printing on the front side and when printing on the back side printing on the back side. For example, a method of switching the fixing temperature is employed.

For example, Japanese Patent Application Laid-Open No. 5-173384 discloses that in a double-sided printing mode, the first fixing temperature (fixing temperature for front side printing) is lower than the second fixing temperature (fixing temperature for back side printing). Then, assuming that the first fixing is performed (fixing temperature: 90 to 100 ° C.), at the time of the second fixing,
There is disclosed a method in which the first fixed image is also fixed together. According to this method, damage to the photoconductor due to heat can be suppressed, and wrinkles and curls can be suppressed.

Japanese Patent Application Laid-Open No. 5-107945 discloses a method in which the transfer voltage at the time of the second transfer is higher than the transfer voltage at the time of the first transfer in the double-sided printing mode. According to this method, even when the resistance of the paper increases in the toner image formed by the first transfer, the second transfer can be performed satisfactorily.
The transfer characteristics of the front and back surfaces can be made substantially equal.

[0017]

However, in recent years, in an image forming apparatus, the printing speed has been increased and the resolution has been increased. In realizing such a device,
Unlike an apparatus having only one process speed as in the related art, one apparatus can perform image formation at a plurality of process speeds. With such an apparatus, it is possible to switch the resolution or the like according to the user's request.

For such proposals, it is conceivable to employ the techniques described in the above publications as they are, but in that case, the following problems occur.

That is, in a configuration in which the temperature of the fixing unit is switched between front side printing and back side printing as in the technique disclosed in Japanese Patent Application Laid-Open No. Hei 5-173384, when the printing speed is further increased, When backside printing is performed immediately after frontside printing as in switchback conveyance, the temperature adjustment of the fixing unit cannot be performed in time. Even if it is possible to raise the temperature instantaneously, it is difficult to lower the temperature instantly.

In a configuration in which the transfer voltage during backside printing is higher than the transfer voltage during frontside printing as in the technique disclosed in Japanese Patent Application Laid-Open No. 5-107945, the sheet is drawn to the transfer unit side and A gap is easily formed between the sheet and the photoconductor, and the toner is easily separated from the paper to the photoconductor. Therefore, white spots in a black solid image and unevenness in a halftone image occur. Also, paper wrinkles are likely to occur.

Further, in the configuration having a plurality of process speeds, as described above, the change in the water content of the paper differs depending on the process speed. Therefore, when trying to adopt the configuration of each of the above publications, for each process speed, the fixing temperature at the time of front side printing, the fixing temperature at the time of back side printing, or the transfer voltage at the time of front side printing, and the transfer voltage at the time of back side printing are changed. Each must be set, and complicated control is required. As a result, the cost required for the control increases.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and it is possible to reduce the complexity of the configuration and control of the apparatus, and
It is an object of the present invention to provide an image forming apparatus capable of suppressing a change in transfer characteristics caused by a change in water content when a process for forming an image is performed a plurality of times on one sheet.

[0023]

In order to solve the above-mentioned problems, an image forming apparatus according to the present invention is an image forming apparatus that forms an image on a sheet by transferring and fixing a recording agent onto a sheet. A transfer unit for performing a transfer process of transferring a recording agent to a passing sheet; and a fixing unit for performing a fixing process of fixing the recording agent to the sheet by passing the sheet onto which the recording agent has been transferred. The paper having undergone the transfer process and the fixing process after passing through the fixing portion can be passed through the transfer portion and the fixing portion again, and further subjected to the transfer process and the fixing process, and the paper can be transferred to the fixing portion. When the paper subsequently reaches the transfer unit after passing, a delay process for delaying the time for the paper to reach the transfer unit is performed.

According to the above arrangement, an image can be formed by transferring and fixing a recording agent to a sheet to form an image, and then transferring the recording agent to the sheet again and fixing it. . Therefore, for example, double-sided printing in which an image is first formed on one side of a sheet and then an image is formed on the other side becomes possible. In order to perform double-sided printing in this way, the sheet that has passed through the transfer unit and the fixing unit may be turned over and then passed through the transfer unit and the fixing unit again.

When the transfer processing is performed after the fixing processing as described above, a problem may occur in the subsequent transfer processing due to a decrease in the water content of the sheet due to the previous fixing processing. There is.

Therefore, in the above configuration, when the sheet passes through the fixing section and subsequently reaches the transfer section, the time required for the sheet to reach the transfer section is delayed to increase the time, thereby lowering the sheet due to the preceding fixing process. The moisture content can be restored. Thereby, between the previous transfer process and the fixing process and the subsequent transfer process and the fixing process, the paper can be hydrated without changing the process conditions such as changing the transfer voltage or changing the fixing temperature. It is possible to suppress a change in transfer characteristics due to a decrease in the rate.

Further, in the above configuration, since the transfer voltage and the fixing temperature are not changed as described above, even in an image forming apparatus having a plurality of process modes having different process speeds, the transfer voltage and the fixing temperature are not changed. It is possible to prevent the control from becoming complicated with diversification. Therefore, it is possible to suppress an increase in control cost.

As described above, in the image forming apparatus of the present invention, in the above-described image forming apparatus, a plurality of process modes in which the process speed, which is the speed at which the sheet passes through the transfer unit, are set different from each other. When an image is formed, one process mode is selected from among the set process modes. In the delay processing, the time required for the sheet to reach the transfer unit is selected. It is preferable that the time is longer than the required time when the sheet is transported at the process speed in the process mode.

According to the image forming apparatus of the present invention, in the above-described image forming apparatus, the conveying speed of the sheet from when the sheet passes through the fixing section to when the sheet reaches the transfer section is selected in the delay processing. It is preferable that the process speed is lower than the process speed in the process mode.

In the above configuration, the paper transport speed from when the paper passes through the fixing unit to when the paper reaches the transfer unit is made lower than the process speed of the process mode selected in the image formation, so that The time from the fixing process to the subsequent transfer process can be increased, and the water content of the paper can be restored.

Alternatively, in the image forming apparatus according to the present invention, the image forming apparatus further includes:
It is preferable that the sheet be retained between the time when the sheet passes through the fixing section and the time when the sheet reaches the transfer section.

In the above configuration, the paper is retained between the time when the sheet passes through the fixing section and the time when the sheet reaches the transfer section, so that the time from the previous fixing process to the subsequent transfer process is increased, and the water content of the sheet is increased. The rate can be restored. In this configuration, the transport speed in the transport path that transports the sheet from the fixing unit to the transfer unit may be the same as the process speed. Therefore, it is not necessary to provide a mechanism for changing the transport speed, and the moisture content can be restored with a simple device configuration.

The image forming apparatus according to the present invention, wherein the sheet conveying speed is made lower than the process speed at the time of the delay processing, further comprising a reduction in the water content of the paper due to the fixing processing performed earlier. It is preferable that the transport speed of the sheet in the delay processing is determined based on the above.

According to the image forming apparatus of the present invention, in the above-described image forming apparatus in which sheets are retained during delay processing,
It is preferable that the residence time of the sheet in the delay processing is determined based on the amount of decrease in the water content of the sheet due to the fixing processing performed earlier.

With the above arrangement, the time required for the paper to reach the transfer section in the delay processing can be delayed based on the amount of decrease in the water content of the paper in the preceding fixing processing. Therefore, the delay time in the delay processing can be determined based on the time required for the moisture content of the sheet to return. This avoids the disadvantage that the image forming speed is reduced by increasing the delay time more than necessary, or that the transfer processing is performed without sufficiently recovering the water content of the paper due to the delay time being too short. be able to.

[0036] The image forming apparatus according to the present invention is the above-described image forming apparatus in which a sheet is retained during delay processing.
It is preferable that a paper discharge roller for discharging the paper on which the image is formed is provided outside the apparatus, and when the paper is retained in the delay processing, the paper is preferably retained by the paper discharge roller.

For example, when images are continuously formed, the temperature inside the apparatus tends to be high. If the paper stays in such a situation, the time required for the water content of the paper to return may be prolonged. Therefore, in the above configuration, the paper is retained by the discharge roller. This allows the paper to stay in a state where a part of the paper is exposed outside the apparatus. As a result, it is possible to shorten the time until the water content of the paper is restored.

Alternatively, in the image forming apparatus according to the present invention, in the above-described image forming apparatus in which the sheet stays during the delay processing, the image forming apparatus further adjusts the position where the recording agent is transferred onto the sheet in the transfer processing. It is preferable that an idle roller for retaining the sheet is provided, and that the sheet is retained by the idle roller even when the sheet is retained in the delay processing.

In the above configuration, the paper is retained by the idle roller provided on the upstream side of the transfer section. Thus, for example, one moisture content sensor is provided at this position, and the moisture content sensor can be used to measure the moisture content of the sheet immediately before the previous transfer process and immediately before the subsequent transfer process, respectively. Then, the paper can be retained until the moisture content immediately before the subsequent transfer process becomes substantially equal to the moisture content immediately before the previous transfer process.

Further, in the above configuration, since the distance between the staying position and the transfer position is short, the subsequent transfer processing can be performed immediately after the stay is released, so that the time required for image formation can be shortened. Can be.

[0041]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIGS.

FIG. 1 is a schematic configuration diagram showing a schematic configuration of a copying machine as an image forming apparatus according to the present embodiment. The copying machine has a document table 1 made of transparent glass or the like on the upper surface. An automatic document reading device 23 is provided on the document table 1.

A scanner section 2 is arranged below the document table 1. The scanner unit 2 includes an exposure light source 3,
A plurality of reflecting mirrors 6, an imaging lens 4, and a photoelectric conversion element (hereinafter referred to as “CCD”) 5 are provided. here,
The exposure light source 3 irradiates a document placed on the document table 1 with light. The plurality of reflecting mirrors 6 guide the reflected light from the document to the CCD 5 through, for example, an optical path indicated by a chain line in FIG. The imaging lens 4 is arranged immediately before the CCD 5 on the optical path, and forms an image of the reflected light from the document on the CCD 5.

The image data (image information) of the document read by the CCD 5 is subjected to image processing and then processed by a laser scanning unit (hereinafter referred to as "LSU").
(Not shown). The LSU irradiates a laser beam based on the received image data onto the charged surface of the photoconductor 7, thereby forming an electrostatic latent image on the surface.

The photosensitive member 7 has a drum shape and rotates in the direction of arrow A in FIG. Around the photosensitive member 7, the developing device 9 and the transfer charger (from the irradiation position of the laser beam from the LSU (the position indicated by the arrow B in FIG. 1) in the rotating direction of the photosensitive member 7 (the direction of the arrow A). Transfer unit) 10,
Cleaning device (not shown), main charger 8, LS
U and the like are arranged in this order.

Here, the developing device 9 develops the electrostatic latent image formed on the surface of the photoreceptor 7 by the exposure of the laser beam into a visible image (toner image) with toner (developer, recording agent). The transfer charger 10 transfers the toner image on the photoreceptor 7 to paper (transfer paper, recording paper) P (transfer processing).
The cleaning device removes residual toner on the surface of the photoconductor 7 after the transfer. The main charger 8 charges the surface of the photoconductor 7 to a predetermined potential in preparation for the formation of the next electrostatic latent image. The LSU irradiates the laser light toward the irradiation position of the laser light of the photoconductor 7. The photosensitive member 7 and the above-described members arranged around the photosensitive member 7 constitute an image forming unit.

The paper transport section has the following configuration. The paper P is stored in the paper cassette 11. A half-moon-shaped paper feed pickup roller 12 for feeding paper P is arranged at the leading end (paper feed direction side) of the paper feed cassette 11. This feed pickup roller 1
2 to the downstream side, a pre-registration detection switch (not shown), an idle roller 13, and the transfer charger 1
0, a fixing device (fixing unit) 14, a fixing paper detection switch (not shown), a paper discharge detection switch (not shown), and a paper discharge roller 15 are arranged in this order. Note that, for convenience of description, the flow-out side (the paper feed cassette 11 side) of the paper P is “upstream” and the paper discharge side is “downstream”.

Here, the pre-registration detection switch is for detecting the passage of the sheet P. On the basis of the detection signal from the pre-registration detection switch, the idle roller 13 temporarily stays the sheet for the purpose of aligning the toner image on the photoconductor 7 with the sheet P, and releases the stay at a proper timing. The transfer charger 10 transfers a toner image on the photoconductor 7 to a sheet P by generating a potential difference between the transfer charger 10 and the photoconductor 7.

The fixing device 14 includes a fixing roller 14a and a pressure roller 14b. FIG. 2 shows the fixing device 14 in an enlarged manner. FIG. 2 is an enlarged view of the fixing device 14 in the copying machine. The fixing roller 14a includes a halogen lamp 14c as a heat source inside. Further, the pressure roller 14b is arranged to face the fixing roller 14a. The fixing roller 14a and the pressure roller 14b rotate in the directions of arrows C and D in FIG. 2, respectively. Then, the conveyed sheet P is fixed to the fixing roller 14 a and the pressure roller 1.
4b. At this time, the fixing roller 14a heats the sheet P, and the pressure roller 14b presses the sheet P with a predetermined pressure (fixing process). As a result, the paper P receives a predetermined pressure while being heated to a predetermined temperature. Then, the toner image transferred onto the sheet P is melted and fixed (fixed) to the sheet P.

Returning to FIG. 1, the fixing paper detection switch is
It detects that the sheet P has passed through the fixing device 14. In addition, the paper ejection detection switch is provided so that the paper P
It detects that has passed. The paper discharge roller 15 is for discharging the paper P on which image formation has been completed to the outside of the apparatus. The discharged paper P is discharged to a discharge tray 19 provided beside the image forming unit. Here, the paper discharge tray 19 is located above the paper feed cassette 11 and below the scanner unit 2.

In the present copying machine, a sub-transport path 51 is provided in addition to the transport path of the sheet P from the sheet cassette 11 to the discharge roller 15. This sub-transport path 51 is used for printing on the second side in the double-sided printing mode. The sub-transport path 51 branches from the above-described transport path between the fixing device 14 and the paper discharge roller 15. And
By the action of a branching claw (not shown) provided at the branching point and the reverse rotation of the discharge roller 15, the sheet P that has passed through the fixing device 14 is transported in a switchback manner with the front end and the rear end reversed. You. That is, the paper is transported in the direction from the downstream side to the upstream side on the transport path. At this time, the paper P is
1.

The sub-conveying path 51 is connected to the idle roller 13
Between the paper feed pickup roller 12 and the conveyance path. Two transport rollers 21 and 22 are provided in the sub transport path 51, and the sheet P transported in the sub transport path 51 is sent to the idle roller 13 again. At this time, the sheet P has been turned over with respect to the first image formation. Then, the reversed sheet P follows the above-described path from the idle roller 13 to perform printing on the second surface. In the above-mentioned transport path, the transport path between the junction with the sub-transport path 51 and the branch point is referred to as the main transport path 5.
Set to 0.

The copying machine is provided with a manual feed tray 18. The paper P is supplied to the manual tray 18 by a user at the time of manual paper feeding. A half-moon-shaped paper feed pickup roller 17 for feeding paper P is arranged at the tip end (paper feed direction side) of the manual feed tray 18. The paper P fed from the manual feed tray 18 enters the above-described transport path before the idle roller 13.

As described above, in this copying machine, the paper P is
The paper is conveyed from the paper cassette 11 or the manual feed tray 18 to the main conveyance path 50. After the transport of the paper P is confirmed by the pre-registration detection switch, the transported paper P is temporarily stopped by an idle roller 13 that adjusts the timing so that the leading edge of the paper P matches the leading edge of the image printing, and the timing is adjusted. The conveyance is started again in the state where the operation is performed. And the paper P
Is transported between the photoconductor 7 and the transfer charger 10.

On the other hand, the photoreceptor 7 uniformly charged by the main charger 8 is irradiated with a laser beam emitted from the LSU based on image data read by the scanner unit 2 and subjected to image processing. You. The LSU may read image data input to a memory 41a of the control unit 41, which will be described later, and emit a laser beam based on the image data. The irradiation of the laser light forms an electrostatic latent image on the surface of the photoconductor 7. This electrostatic latent image is converted into a toner image by the developing device 9.

The toner image is transferred to the sheet P conveyed between the photosensitive member 7 and the transfer charger 10 by the operation of the transfer charger 10. The sheet P to which the toner image has been transferred is conveyed to the fixing device 14. Fixing device 14
Then, the unfixed toner on the sheet P is thermally fixed by the heat of the fixing roller 14a or the like, and is fixed on the sheet P.

When printing on only one side of the sheet P on which the image is fixed, the sheet P passes through the sheet discharging roller 15 as it is, and
The paper is discharged to the paper discharge tray 19. On the other hand, in the case of double-sided printing, the paper P is switch-back transported by the reverse rotation of the paper discharge roller 15 that can rotate in the reverse direction, enters the sub-transport path 51, and
It reaches the idle roller 13 again. After the printing on the second surface is completed through the same process as described above, the sheet is discharged to the discharge tray 19 by the discharge roller 15.

The operation of each section is controlled by the control section 41. Further, the control unit 41 includes a memory 41a for temporarily storing image data and the like read by the scanner unit 2.
And an image processing unit 41 that performs image processing on image data
b, CPU (Central Processing U
nit: central processing unit) 41c and the like are also provided.

Next, the basic operation of the copying machine will be described.
A description will be given based on FIGS. 3 to 5 with reference to FIGS. 1 and 2. FIG. 3 is a flowchart showing the overall processing flow in the copy mode of the copying machine. FIGS. 4 and 5 are flowcharts showing details of the printing process when performing single-sided printing and double-sided printing, respectively, in the copying machine.

First, based on the flowchart of FIG.
The overall processing of the copying machine will be described.

When a print request for copy processing is made to the copying machine in standby (S401), the number of prints, print magnification, paper size, one-sided printing / two-sided printing set by the user or the like at the time of the printing request. While recognizing the designation of printing, etc.,
The apparatus setting for the original reading mode (single-sided original / double-sided original) and the print mode (single-sided printing / double-sided printing) is performed (S402). Then, the document is transferred to the automatic document reading device 2
3 is placed on the original tray 3 (S403). Note that S4
03 is performed by a user or the like,
1 or before S402.

Next, the original reading processing (S404) and the printing processing (S407) are performed in parallel (parallel processing). Here, in the document reading process, the automatic document reading device 2
The documents are set on the document table 1 one by one from 3 and the document is scanned and read by the scanner unit 2. Then, image data of the original is generated, and the generated image data is stored in the memory 4.
1a or to the image forming unit. In the printing process, printing is performed based on the image data generated in the reading process. This printing process will be further described later. In addition,
The image data is appropriately subjected to image processing.

After the original reading process for one original is completed, it is determined whether all the originals have been read (S405). If there are any documents to be read, the process returns to step S404 and repeats the above processing. If all the originals have been read, it is confirmed that the read original has been discharged to the original discharge tray of the automatic original reading device 23 (S406), and the original is read for one job by this print request. The process ends.

After the printing process for one document is completed, it is determined whether there is an image to be printed next (S408). If there is an image to be printed next, the process returns to step S407 and repeats the above processing. If the printing of the image to be printed has been completed, it is confirmed that the sheet has been discharged to the discharge tray 19 (S409).
The printing process for one job according to the printing request is completed, and the process returns to the standby state.

As described above, by the end of each of the above-described processes in which the parallel processing is performed, one job according to the print request ends.

Next, based on the flowchart of FIG.
A description will be given of a process for performing single-sided printing in the printing process (S407 in FIG. 3). 4 and 5,
In the following description, steps for performing the same processing are denoted by the same reference numerals, and description thereof will be omitted.

In S402 of FIG. 3, single-sided printing is set, and when the printing process starts in S407, it is first determined whether to print one image or a plurality of images for one image of image data in S402. (S701). Here, if it is determined that one sheet is to be printed, the image data read in S404 as a single copy is not input to the memory 41a, but is subjected to image processing after reading, and directly to the image forming unit. Sent to If it is determined that printing of a plurality of sheets is to be performed, the image data read in S404 as multi-copy is input to the memory 41a. At the time of each printing, the image data stored in the memory 41a is read and used for printing.

If it is determined in S701 that a single copy has been made, the following processing is sequentially performed. First, the paper is picked up by the paper pickup roller 12 and transported (S702). Then, the sheet is temporarily stopped by the idle roller 13 (S703).

Then, image data is written to the photosensitive member 7 by laser light (emitted from the LSU) based on the image data generated in the above-described original reading process and subjected to image processing by the control unit 41. (Electrostatic latent image formation) is started (S704). After the start of writing the image data, the writing process is continuously performed on the image data.

Then, the conveyance of the sheet is restarted at a predetermined timing after the start of the writing of the image data in S704 (S705). Thereby, the timing of the passage of the paper at the transfer position (the position of the transfer charger 10) and the timing of the passage of the toner image on the photoconductor 7 are adjusted so that the toner image is transferred to a predetermined position on the paper. I do.

When the paper and the toner image reach the transfer position, the transfer of the toner image to the paper (transfer processing) is started, and the toner image transferred to the paper is fixed at the fixing position (the position of the fixing device 14). , Fixing of the toner image to the paper (fixing process) is started (S706). Thereafter, the sheet having undergone the fixing process is discharged to the discharge tray 19 (S7).
07). Then, the printing process ends.

On the other hand, if it is determined in step S701 that a multi-copy operation has been performed, the same processing as in the case of a single copy operation (S7)
02 to S707). However, in S704 in the single copy, the image data read by the scanner unit 2 is directly sent to the image forming unit, whereas in S704a in the multicopy,
The difference is that the image data once stored in the memory 41a is read out to the image forming unit.

Then, it is determined whether or not the printing of the requested number of images has been completed for the image data of one image (S708). If it is determined that printing for the requested number of sheets has not been completed, the process returns to step S702 to repeat the above processing. If it is determined that printing for the requested number of sheets has been completed, the printing process is terminated. The single-sided printing is performed by the processing described above.

Next, based on the flowchart of FIG.
In the printing process (S407 in FIG. 3), a process for performing double-sided printing will be described.

Here, the reading of the original in the case of double-sided printing is performed according to the following procedure. For example, the first
It is assumed that the front side (the first page) of the sheet is set down and set on the automatic document reading apparatus 23. In this case, the first page is read first. Then, the read image data of the first page is temporarily stored in the memory 41a. Subsequently, the back side (second page) of the first sheet of the document is read.
The image data of the second page is sent directly to the image forming unit via the image processing unit 41b and the like without being stored in the memory 41a, and is printed on the front surface (first surface) of the sheet (first time). Printing). Subsequently, the image data of the first page previously stored in the memory 41a is sent to the image forming unit, and used for printing on the back surface (second surface) of the sheet (second printing).

As described above, the image data of the front side of each original is temporarily stored in the memory 41a. Then, the image data of the surface read later is directly sent to the image forming unit, and thereafter, the previously read image data stored in the memory 41a is sent to the image forming unit. Thus, in the copying machine having the configuration shown in FIG. 1, the sheets can be discharged to the discharge tray 19 without changing the order of the pages of the document.

First, double-sided printing is set in S402 in FIG. 3, and when the printing process starts in S407, processes similar to the processes from S702 to S706 in the case of single copy of single-sided printing are sequentially performed. However, in S704b corresponding to S704 at the time of single-sided printing, the same processing as S704 is performed on the image on the back side of the document. Also, S7
In S706a corresponding to 06, a toner image corresponding to the image on the back side of the document is transferred to the front side of the paper and fixed.

Then, the sheet on which the image is fixed on the front surface is guided to the sheet discharge tray 19 by the normal rotation of the sheet discharge roller 15 which can be rotated forward and backward. At this time, the sheet on which the image has been formed on the front surface is not completely discharged to the sheet discharge tray 19, but temporarily stops in a state where the rear end in the sheet discharge direction is sandwiched by the sheet discharge rollers 15 (S80).
1).

The main transport path 50 and the sub transport path 51
Switch the branching claw provided at the branching point with
1 and the paper discharge roller 15 are made continuous (S802). Then, the paper discharge roller 15 is rotated in the reverse direction (rotated in the direction opposite to the rotation direction when the paper is discharged), the paper is guided to the sub-conveying path 51, the front and rear surfaces are reversed, and the idle roller 13 is rotated. Main transport path 5 again from upstream
It returns to 0 (S803).

The sheet passes through the sub-transport path 51 and passes through the main transport path 5
After returning to 0, the same processing as the processing from S703 to S706a at the time of the front side printing is sequentially performed on this sheet. However, here, S703 and S704
Steps corresponding to b, S705, and S706a are referred to as S903, S904, S905, and S906, respectively. In addition, S corresponding to S704b at the time of front side printing
In step 904, the same processing as in step S704b is performed on the image of the front side of the document stored in the memory 41a. Also, S7
In S906 corresponding to 06a, the toner image corresponding to the image on the front side of the document is transferred to the back side of the sheet and fixed.

The main transport path 50 and the sub transport path 51
Switch the branching claw provided at the branching point of the main transport path 5
0 and the paper discharge roller 15 are continuous (a state in which the paper can pass through the main transport path 50) (operation opposite to S802) (S804). The switching of the branch claw is not limited to the above, and may be performed after the sheet has passed through the branch claw in S803 and after the sheet reaches the branch claw again through step S906. After that, the sheet that has passed through the branch claws is discharged to the discharge tray 19 (S805). Then, the printing process ends.

Although the basic configuration and operation of the copying machine are as described above, the copying machine further has the configuration described below, and can perform the operation described below.

The copying machine is, for example, 1200 dpi and 60
0 dpi (dot per inch: 1 inch (about 25.4 mm)
(The number of dots per unit) and the resolution (process mode) can be switched. This resolution is 2
Not only the number of stages but also the number of stages can be changed. Then, at the time of image formation, one resolution is selected from a plurality of resolutions.

Such switching of the resolution is realized by switching the process speed.
The process speed is, for example, 60 mm / s at a resolution of 1200 dpi and 120 mm at a resolution of 600 dpi.
/ S (twice that in the case of a resolution of 1200 dpi). The process speed is the peripheral speed of the photoconductor 7, and is also the transport speed of the sheet at the transfer position.

Here, the structure and operation for stabilizing the transfer characteristics on both the front and back sides of the paper are constant. In particular, as described above, one apparatus has a plurality of process speeds. A desirable mode for a copier capable of performing double-sided printing will be described with reference to FIGS.

FIG. 6 is an enlarged view showing the configuration around the main transport path 50 and the sub transport path 51 in FIG.
Some of the components shown in FIG. 1 are omitted. ). Although not shown in FIG. 1, the copier is preferably provided with a moisture content detection sensor. The moisture content detection sensor is located downstream of the position where the main transport path 50 and the sub-transport path 51 join together and upstream of the idle roller 13 in the main transport path 50 (arrow E in FIG. 6). (A position indicated by an arrow F in FIG. 6), and a position (a position indicated by an arrow F in FIG. 6) which is a downstream side of a position where the main transport path 50 and the sub-transport path 51 branch and an upstream side of the paper discharge roller 15. It is desirable to be installed on one or both. Note that the moisture content detection sensor arranged at the position indicated by the arrow E in FIG. 6, particularly the moisture content detection sensor 40 a, the moisture content detection sensor arranged at the position indicated by the arrow F in FIG. 40
b.

The moisture content detection sensor is provided so as to be able to contact the paper on the transport path as needed. Then, the water content detection sensor can detect the water content of the paper by contacting the paper.

The detection result (sensor output) of the moisture content detection sensor is input to the control unit 41 as shown in FIG.
The moisture content of the sheet detected by the moisture content detection sensor 40 is recognized by the control unit 41. Control unit 41
Performs retention and release of paper, which will be described later, and control of paper transport speed (delay processing) based on the recognized moisture content. For this purpose, they are connected as shown in FIG. FIG. 7 is a block diagram showing the relationship between the water content detection sensor 40, the control unit 41, and the idle roller 13.
Although FIG. 7 shows only the idle roller 13 as a control target of the control unit 41, the control target of the control unit 41 may include the paper discharge roller 15, the transport rollers 21 and 22, and the like. In FIG. 7, the memory 41a, the image processing unit 41b, the CPU 41c, and the like included in the control unit 41 are not illustrated.

As shown in FIG. 6, in the configuration provided with the moisture content detection sensor, FIGS. 5 to 7 show the process for stabilizing the transfer characteristics on both the front and back surfaces of the paper by making them constant. This will be described with reference to FIG. FIG. 8 is a flowchart showing a process for stabilizing the transfer characteristics of the front and back surfaces during duplex printing in the copying machine having the configuration shown in FIG.
In the following, unless otherwise specified, it is assumed that the moisture content detection sensor 40a is provided only at the position indicated by the arrow E in FIG.

When a print request for copy processing is made to the copying machine in a standby state (S1), the control unit 41 determines whether the printing is single-sided or double-sided (S2). Note that after S1, processes such as S402 and S403 in FIG. 3 are performed, but are omitted here.

If it is determined in S2 that the printing is single-sided, the printing process shown in FIG. 4 is performed (S3). Then, it is determined whether there is an image to be printed next (S4).
Here, if there is an image to be printed next, the process returns to S2 and the above processing is repeated. If the printing of the image to be printed has been completed, the printing process ends and the process returns to the standby state. After S3, processing such as S409 in FIG. 3 is performed, but is omitted here.

On the other hand, if it is determined in S2 that the printing is not single-sided printing, that is, it is double-sided printing, first, paper is fed (S6). The fed sheet is temporarily stopped by the idle roller 13 in order to align the sheet with the toner image on the photoconductor 7 (S7). Here, while the paper is stopped by the idle roller 13, the water content detection sensor 40a measures the water content of the paper before printing on the surface of the paper. Further, the control unit 41 recognizes and stores the obtained water content of the sheet before printing on the front surface (the value of the water content is "x") (S8).

Then, printing processing (first transfer processing, previous fixing processing) on the surface of the sheet is performed (S9). This printing process corresponds to steps S704b to S706a in FIG. In other words, the toner image is transferred to the surface of the paper,
This toner image is thermally fixed by the fixing device 14.

The paper on which the image is printed on the front side is
The guide is once guided to the paper discharge roller 15 by the guide of the branching pawl, and once stopped with the paper discharge roller 15 sandwiching the rear end in the paper discharge direction between the paper discharge rollers 15 (S10). That is, the paper is discharged onto the paper discharge tray 19 at the front end side with the rear end sandwiched.

Then, the sheet is switched back and the sheet is conveyed to the sub-conveyance path 51 (S11) (corresponding to S802 and S803 in FIG. 5). Then, similarly to S903 in FIG. 5, the sheet conveyed in the sub-conveyance path 51 is stopped again by the idle roller 13 (S12). Also in this case, while the paper is stopped by the idle roller 13, the water content of the paper before printing on the back surface of the paper is measured by the water content detection sensor 40a. The control unit 41 recognizes and stores the obtained water content of the paper before printing on the back surface (this water content is referred to as “y”) (S13).

Then, the water content x of the paper obtained before printing on the front surface of the paper obtained in S8 is compared with the water content y of the paper obtained after printing on the front surface of the paper and obtained before printing on the back surface of the paper obtained in S13. It is determined whether the rate x and the water content y are substantially equal (S14). Here, whether or not the water content x and the water content y are substantially equal is determined based on, for example, a difference between the water content x and the water content y or a ratio between the water content x and the water content y. I just need. Further, the specific reference value may be appropriately set based on the influence of the difference in water content on the transfer characteristics. S
In 14, when it is not determined that the water content x and the water content y are substantially equal, the sheet is retained by the idle roller 13 (S 15), and the determination in S 14 is repeated. That is, the sheet is retained by the idle roller 13 until the water content x and the water content y become substantially equal.

In S14, when it is determined that the water content x and the water content y are substantially equal, that is, the water content has almost returned to the state before the printing on the front surface of the paper, the printing process on the back surface of the paper (post-transfer process, post-fixing process) Process) (S16). This printing process corresponds to steps S904 to S906 in FIG. That is, the toner image is transferred to the back surface of the sheet, and the toner image is thermally fixed in the fixing device 14. Note that S
After step 16, processes such as S409 in FIG. 3 are performed, but are omitted here.

Thereafter, it is determined whether there is an image to be printed next (S17). If there is an image to be printed next, the process returns to S2 and the above processing is repeated. If the printing of the image to be printed has been completed, the printing process ends and the process returns to the standby state.

As described above, the copier as the image forming apparatus according to the present embodiment has a plurality of process modes having different process speeds, and passes through the main transport path 50 to form one surface. The printed paper is transferred to the sub-transport path 51.
Is an image forming apparatus capable of performing double-sided printing by supplying the main surface again to the main conveyance path 50 and performing printing on the other side. So that the time until the supply to the path 50 is delayed from the required time based on the process speed of the corresponding process mode,
A transport control unit (idle roller 13, discharge roller 15, and transport rollers 21 and 22) for controlling transport (delay processing) of the paper when the paper is again supplied to the main transport path 50 using the sub transport path 51 is provided. Is provided. That is, at the time of double-sided printing, the paper having been printed on the one side is again transferred to the main transport path 5
The time required for the sub-transport path 51 to be supplied to 0 is made longer than the time required when the sub-transport path 51 is transported at the process speed of the corresponding process mode.

Thus, the water content reduced during printing on the one side is restored before the sheet printed on one side is again supplied to the main conveyance path 50 via the sub-conveyance path 51. be able to. Therefore, it is possible to suppress a change in transfer characteristics due to a decrease in water content without changing the transfer voltage or the fixing temperature between the one surface and the other surface.

Further, in such a configuration, since the transfer voltage and the fixing temperature are not changed, a complicated control is not required even in an apparatus having a plurality of process modes having different process speeds, and the control cost is not required. Increase can be suppressed.

To delay the time required for the paper to be supplied to the main transport path 50 again by the transport control means,
For example, the paper transport speed by the transport control means may be reduced, or the paper may be retained by the transport control means. Here, the transport speed of the paper or the residence time of the paper may be determined based on the time required for the water content of the paper reduced by the fixing process during the printing on the one side to recover. .

As a result, it is possible to avoid such problems that the printing speed is lowered and the transfer is performed before the water content is sufficiently restored.

In the above description, in order to return the moisture content of the paper before printing on the back surface (second printing) to the moisture content before printing on the front surface (first printing), the idle roller is used. By keeping the paper at 13, the time for the paper to absorb moisture was secured. However, the staying position of the sheet is not limited to this, and may be another position.

For example, the paper may be retained with the paper discharge roller 15 sandwiching the rear end of the paper. In this case, most of the paper is exposed to the outside of the copying machine. Since the inside of the copying machine is likely to be heated to a high temperature by continuous copying, it is preferable to stay in the above state because the time until the water content recovers can be shortened in some cases.

In order to realize the above, in addition to the water content detection sensor 40a at the position indicated by the arrow E in FIG. 6, a water content detection sensor 40b may be provided at the position indicated by the arrow F. Then, the value of the water content obtained by the water content detection sensor 40b at the arrow F may be set as the water content y in S13 in FIG.

However, in this case, the time required for the paper to reach the transfer position after the conveyance of the paper is resumed (after the stagnant state is released) is shorter than that when the paper is stagnated by the idle roller 13. become longer. In consideration of this, it is desirable to adjust the timing of restarting the sheet conveyance.

Further, in the above description, the time for restoring the moisture content of the paper is realized by retaining the paper. However, the present invention is not limited to this, and the time from when the paper passes through the fixing device 14 to when the paper reaches the transfer charger 10 is achieved. This can also be realized by reducing the paper transport speed (that is, lowering the process speed).

For this purpose, for example, in FIG. 6, the transport rollers 21 and 22 for transporting the sheet in the sub-transport path 51 are used.
May be reduced. Then, the transport speed of the sheet in the sub-transport path 51 which is usually set to the same speed as the process speed may be made lower than the process speed. This speed may be any speed that can increase the time required for the water content of the paper to return. FIG.
In the position indicated by the arrow E and the position indicated by the arrow F in
What is necessary is just to provide the water content detection sensors 40a and 40b, respectively.

In order to make the paper transport speed in the sub-transport path 51 different from the process speed, for example, the drive source of the transport rollers 21 and 22 is set to the idle roller 1
3 and the driving source of the fixing device 14.

The procedure of the processing in this configuration may be, for example, the following procedure. At the time of double-sided printing, first, the water content x of the paper before the front surface printing of the paper is measured by the water content detection sensor 40a arranged on the upstream side of the idle roller 13. Thereafter, the moisture content y of the paper having the toner image transferred and fixed on the surface is measured by the moisture content detection sensor 40b disposed between the fixing device 14 and the paper discharge roller 15. Then, the difference between the water content x and the water content y (the detection result of the water content detection sensor 40a and the water content detection sensor 40b
Difference from the detection result) and the paper transport speed.
The time required for the water content of the sheet to return to the original value may be predicted, and the sheet conveying speed in the sub-conveying path 51 may be determined.

In the above, the moisture content detection sensor 40 is used, and based on the measurement result, the time until the paper with the one surface printed is supplied to the main transport path 50 again is delayed. However, the present invention is not limited to this, and the above time may be delayed by another method.

For example, for each process speed, a delay time corresponding to the time when the water content recovers is obtained in advance and the control unit 4
1 is stored in the memory 41a or the like, and a delay time corresponding to the selected process mode is read at the time of image formation, and based on the delay time, the sheet printed on the one surface is returned to the main transport path again. What is necessary is just to delay the time until it is supplied to 50.

Further, in this embodiment, the copying machine for directly transferring the toner image on the photosensitive member 7 to the sheet has been described.
The present invention can also be applied to a copying machine that performs transfer via an intermediate transfer member.

As described above, the copying machine as the image forming apparatus according to the present embodiment reduces the speed of the sheet passing through the sub-conveying path 51 arranged for double-sided printing, or reduces the speed of printing on the back side of the sheet. A wait time. This allows the paper moisture recovery time to be obtained before printing on the back side of the paper, and without changing the transfer voltage or fixing temperature between single-sided printing / front-side printing and back-side printing for each process speed. With simple control, the transfer characteristics of the front and back sides during duplex printing can be made substantially constant.

[0116]

As described above, the image forming apparatus according to the present invention includes the transfer section for performing the transfer processing and the fixing section for performing the fixing processing, and passes through the transfer section and the fixing section to perform the transfer processing and the fixing processing. The paper subjected to is again passed through the transfer unit and the fixing unit, and the paper can be further subjected to a transfer process and a fixing process, and when the paper subsequently reaches the transfer unit after passing through the fixing unit. Is a configuration in which a delay process for delaying the time that the sheet reaches the transfer unit is performed.

In the above configuration, when the sheet passes through the fixing section and subsequently reaches the transfer section, the time required for the sheet to reach the transfer section is delayed to increase the time, thereby reducing the water content reduced by the previous fixing process. The rate can be restored. Thereby, between the preceding transfer processing and fixing processing and the subsequent transfer processing and fixing processing, without changing the process conditions such as changing the transfer voltage and the fixing temperature, for example,
It is possible to suppress a change in transfer characteristics due to a decrease in the water content of the paper.

Further, in the above configuration, since the transfer voltage and the fixing temperature are not changed as described above, even in an image forming apparatus having a plurality of process modes having different process speeds, the transfer voltage and the fixing temperature are not changed. It is possible to prevent the control from becoming complicated with diversification. Therefore, it is possible to suppress an increase in control cost.

As described above, in the image forming apparatus of the present invention, in the above-described image forming apparatus, a plurality of process modes in which the process speed, which is the speed at which the sheet passes through the transfer unit, are set differently. When an image is formed, one process mode is selected from among the set process modes. In the delay processing, the time required for the sheet to reach the transfer unit is selected. It is preferable that the time is longer than the required time when the sheet is transported at the process speed in the process mode.

In the image forming apparatus according to the present invention, in the above-described image forming apparatus, further, in the delay processing, the transport speed of the sheet from when the sheet passes through the fixing section to when the sheet reaches the transfer section is selected. It is preferable that the process speed is lower than the process speed in the process mode.

In the above arrangement, the paper transport speed from the time when the paper passes through the fixing unit to the time when the paper reaches the transfer unit is set to be lower than the process speed of the process mode selected in the image formation. The time from the fixing process to the subsequent transfer process can be increased, and the water content of the paper can be restored.

Alternatively, according to the image forming apparatus of the present invention, the image forming apparatus further comprises:
It is preferable that the sheet be retained between the time when the sheet passes through the fixing section and the time when the sheet reaches the transfer section.

In the above configuration, the transport speed in the transport path for transporting the sheet from the fixing unit to the transfer unit may be the same as the process speed. Therefore, it is not necessary to provide a mechanism for changing the transport speed, and the moisture content can be restored with a simple device configuration.

The image forming apparatus according to the present invention, in which the sheet conveying speed is made lower than the process speed during the delay processing, further comprises a reduction in the water content of the paper due to the previously performed fixing processing. It is preferable that the transport speed of the sheet in the delay processing is determined based on the above.

[0125] The image forming apparatus of the present invention is characterized in that, in the above-described image forming apparatus in which paper is retained during delay processing,
It is preferable that the residence time of the sheet in the delay processing is determined based on the amount of decrease in the water content of the sheet due to the fixing processing performed earlier.

In the above arrangement, the image forming speed is reduced by increasing the delay time more than necessary, or the transfer processing is performed without sufficiently restoring the water content of the paper due to the delay time being too short. Such a problem can be avoided.

According to the image forming apparatus of the present invention, in the above-described image forming apparatus in which sheets are retained during delay processing,
When the sheet is retained in the delay processing, it is preferable that the sheet is retained by the discharge roller.

In the above arrangement, the paper is retained by the discharge roller. This allows the paper to stay in a state where a part of the paper is exposed outside the apparatus. As a result, it is possible to shorten the time until the water content of the paper is restored.

Alternatively, the image forming apparatus according to the present invention may be arranged such that, in the image forming apparatus for retaining the sheet during the delay processing, the sheet is retained by the idle roller when the sheet is retained in the delay processing. Is preferred.

In the above arrangement, for example, one moisture content sensor is provided at this position, and the moisture content sensor is used to measure the moisture content of the sheet immediately before the previous transfer process and immediately before the subsequent transfer process, respectively. be able to. Then, the paper can be retained until the moisture content immediately before the subsequent transfer process becomes substantially equal to the moisture content immediately before the previous transfer process.

Further, in the above configuration, since the distance between the staying position and the transfer position is short, the subsequent transfer processing can be performed immediately after the stay is released, so that the time required for image formation can be shortened. Can be.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram illustrating a schematic configuration of a copier as an image forming apparatus according to an embodiment of the present invention.

FIG. 2 is an enlarged view of a fixing device in the copying machine of FIG.

FIG. 3 is a flowchart showing an overall processing flow in a copy mode of the copying machine of FIG. 1;

FIG. 4 is a flowchart showing details of a printing process when performing one-sided printing in the copying machine of FIG. 1;

FIG. 5 is a flowchart showing details of a printing process when performing double-sided printing in the copying machine of FIG. 1;

FIG. 6 is an enlarged view showing a configuration around a main transport path and a sub transport path in the copying machine of FIG. 1;

FIG. 7 is a block diagram illustrating a relationship among a moisture content detection sensor, a control unit, and an idle roller in the copying machine of FIG. 1;

8 is a flowchart showing a process for stabilizing the transfer characteristics of the front and back surfaces during duplex printing in the copying machine having the configuration shown in FIG.

FIG. 9 is a graph showing a change in the water content of the paper surface during double-sided printing in an image forming apparatus of a type in which printing is performed on the front surface of a paper and then printed on the back surface by switchback conveyance.

FIG. 10 is a graph showing the relationship between the water content and the surface resistance value of paper.

FIG. 11 is a graph showing a relationship between a transfer voltage and a surface resistance value of a sheet.

FIG. 12 is a graph showing the relationship between the process speed and the water content of the paper after front side printing.

[Explanation of symbols]

 P paper (transfer paper, recording paper) 10 transfer charger (transfer section) 12 paper feed pickup roller 13 idle roller 14 fixing device (fixing section) 14a fixing roller 14b pressure roller 15 discharge roller 21 transport roller 22 transport roller 41 control Unit 50 Main transport path 51 Secondary transport path 40 Moisture content detection sensor 40a Moisture content detection sensor 40b Moisture content detection sensor

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference) G03G 15/00 518 G03G 21/00 384 3F102 21/00 384 372 (72) Inventor Eiji Izumi Osaka-shi, Osaka 22-22 Nagaike-cho, Abeno-ku, Sharp Corporation (72) Inventor Susumu Murakami, 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka F-term (reference) 2H027 DC02 ED17 EE03 EF10 FA13 FB07 2H028 BA06 BA09 BB02 2H072 AA13 AA24 AA32 AB07 AB15 BA01 BA06 CA01 CA02 3F049 DA12 LA01 LB03 3F100 AA02 BA17 CA13 CA15 EA03 EA05 3F102 AA01 AB02 BA02 BB02 CA00 CB00 DA08 EA03

Claims (8)

[Claims] An image forming apparatus for forming an image on a sheet by transferring and fixing a recording agent on a sheet, a transfer unit for performing a transfer process for transferring the recording agent to a sheet passing therethrough, A fixing section for performing a fixing process for fixing the recording agent to the sheet by passing the transferred sheet, and transferring the sheet subjected to the transfer process and the fixing process through the transfer section and the fixing section again to the transfer section and The sheet can be passed through the fixing section and subjected to further transfer processing and fixing processing. When the sheet subsequently reaches the transfer section after passing through the fixing section, the time for the sheet to reach the transfer section is delayed. An image forming apparatus, wherein delay processing is performed. 2. The image forming apparatus according to claim 1, wherein a plurality of process modes are set such that a process speed, which is a speed at which a sheet passes through a transfer unit, is different from each other. One of the set process modes is selected from among the set process modes. In the delay processing, the time required for the paper to reach the transfer section is determined by the process speed of the selected process mode. An image forming apparatus, wherein the time is longer than a required time when transported. 3. The image forming apparatus according to claim 2, wherein, in the delay processing, the transport speed of the sheet from the time when the sheet passes through the fixing unit to the time when the sheet reaches the transfer unit is set in the process of the selected process mode. An image forming apparatus wherein the speed is lower than the speed. 4. The image forming apparatus according to claim 1, wherein, in the delay processing, the paper stays between a time when the paper passes through the fixing unit and a time when the paper reaches the transfer unit. Image forming device. 5. The image forming apparatus according to claim 3, wherein the transport speed of the sheet in the delay processing is determined based on the amount of decrease in the water content of the sheet due to the fixing processing performed earlier. Image forming apparatus. 6. The image forming apparatus according to claim 4, wherein the residence time of the sheet in the delay processing is determined based on the amount of decrease in the water content of the sheet due to the fixing processing performed earlier. Image forming apparatus. 7. The image forming apparatus according to claim 4, further comprising a paper discharge roller for discharging the paper on which the image is formed out of the apparatus, wherein the paper is retained in the delay processing. An image forming apparatus, wherein the sheet is retained by the sheet discharging roller. 8. The image forming apparatus according to claim 4, further comprising: an idle roller for holding the sheet in order to adjust a position where the recording material is transferred to the sheet in the transfer process. An image forming apparatus according to any one of claims 1 to 3, wherein the sheet is retained by the idle roller even when the sheet is retained.