JP5293619B2 - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus for providing an output image of stable density regardless of humidity. <P>SOLUTION: The process speed is reduced according to slowdown of rising of an electric field in a transfer section by low humidity (low humidity measured inside the apparatus), and the process speed is reduced by a value corresponding to the delay, by the slowdown, of the time at which the electric field strength becomes appropriate for transfer. Thus, an image region of paper is conveyed to a nip section after the electric field strength becomes appropriate for transfer. Then, transfer is performed under the electric field strength appropriate for transfer in the image region of paper regardless of the humidity, and reduction in transfer density is prevented. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to an image forming apparatus, and more particularly to transfer of a toner image.

  It has a transfer part that transfers the toner image carried on the image carrier to the paper, and when the paper passes through the nip formed at the opposite part between the image carrier and the transfer roller, the paper is separated at the front edge of the paper. In order to improve toner transferability after that, an image forming apparatus is known in which the voltage applied to the transfer roller for each sheet is changed between the leading edge of the sheet and the subsequent sheet in accordance with the humidity inside the apparatus. (For example, refer to Patent Document 1).

  Further, it is generally known that when the moisture content of the paper decreases due to a decrease in humidity, the impedance of the paper increases, and when the impedance increases, the rise of the electric field generated at the nip portion during transfer becomes dull.

  Then, the electric field rise time from the start of applying an electric field at the transfer portion to the target electric field strength at which transfer can be normally performed is lower than the normal humidity (normal humidity) time, and the low humidity time is lower than the normal humidity. Is known to grow.

  Also, images with high printing ratios such as solid images and pictures tend to be noticeable when there is a change in image density, and images with low printing ratios such as text are less noticeable even when there is a change in image density. It is known that image quality is recognized to be degraded when there is a change in image density in an image having a high printing rate.

JP-A-10-91012

  In the image forming apparatus described in Japanese Patent Application Laid-Open No. H10-228707, the electric field rise time from the start of applying an electric field at normal humidity to the target electric field strength at which transfer can be normally performed (that is, the target at normal humidity). The process speed is set so that the image area of the paper begins to pass through the nip before the electric field is strong enough. For this reason, at normal humidity, when the image area of the paper passes through the nip portion, the electric field of the target intensity is already obtained, and the image area can be transferred with a constant transfer density, and the image quality is not deteriorated. .

  However, if the process speed is set according to the electric field rise time from the start of applying an electric field at normal humidity to the target electric field strength at which transfer can be normally performed, the electric field rise time will be extended. When the leading edge of the image area passes through the nip, the electric field has not yet reached the target strength.

  In such a case, there is a problem in that the transfer density differs before and after the electric field having the intended strength is used, resulting in a reduction in image quality.

  In the following, a detailed explanation will be given of the cause of the problem that the image density at the front end side of the paper cannot be transferred to the normal density, that is, the image density at the front side of the paper is different. To do.

  FIG. 5 is an explanatory diagram of an image transferred onto a sheet.

  P is a sheet, and P1 corresponds to a blank portion in the sheet conveyance direction and indicates a non-image area where an image is not formed on the sheet. P2 indicates an image area where an image in the paper conveyance direction is formed, and P3 indicates a front end portion in the paper conveyance direction. It is assumed that the paper P is conveyed from below to above.

  FIG. 5A shows an image on the paper P when an image is normally formed, and FIG. 5B shows an image on the paper P when transfer to a normal density is impossible in the image area on the leading edge side of the paper. An image is shown.

  In FIG. 5A, an image having a normal density (cross-hatched portion) is formed in the image region P2.

  In FIG. 5B, a low-density image (hatched portion) in which image density does not increase due to insufficient transfer is formed on the paper conveyance direction leading edge P3 side in the image region P2, and a low-density image (hatched portion) is formed. On the downstream side of the hatched portion, a normal density image (cross-hatched portion) having a higher density than the low density image is formed.

  The relationship between the rise of the electric field and the conveyance of the paper will be described below as a cause of the problem that the portion where the image density does not increase due to insufficient transfer as shown in FIG.

  FIG. 6 is an explanatory diagram of the relationship between the rise of the electric field generated at the nip portion in the conventional transfer portion and the conveyance of the paper.

  In FIG. 6, the horizontal axis indicates time t, and time tn (t1 to t7) indicates that n seconds have elapsed from time t0. The portion of the paper P that passes through the nip at each of the timings t1 to t7 (whether it is a non-image area or an image area) when the rising state of the electric field E is set on the vertical axis and the process speed is set according to the normal humidity. And.

  For example, in FIG. 6A, the electric field -E for separation is applied to the nip portion during the period t1 to t3, and the electric field + E for transfer is applied to the nip portion during the period t4 to t7. It shows that.

  In addition, the non-image area P1 passes through the nip portion during the time t2 to t5, and the image region P2 passes through the nip portion during the time t5 to t6.

  The low humidity refers to a humidity of less than 20%, the normal humidity refers to a humidity of 20% to less than 80%, and the high humidity refers to a humidity of 80% or more.

  FIG. 6A shows the relationship between the rising of the electric field E generated at the nip portion at normal humidity and the conveyance of the paper P. FIG.

  At time t0, a sheet sensor S2 that detects the leading edge of the sheet in the traveling direction detects the leading edge of the sheet. At time t1, for example, a negative bias is applied to the transfer roller to generate an electric field -E for separation in the nip portion so that the leading edge of the sheet does not wrap around the image carrier (in order to improve separation) in the transfer portion. Let

  At time t2, the leading edge of the paper reaches the nip portion. At this time, the leading edge of the paper advances without being wound around the intermediate transfer belt by the electric field -E for separation. At time t3, a positive bias for transfer is applied to the transfer roller before the non-image area P1 (broken line) on the leading end side of the sheet passes through the nip [before the image area P2 (solid line) reaches the nip). To do. At time t4, an electric field + E for transfer corresponding to the positive bias applied to the nip portion rises.

  At time t5, the image area P2 (solid line) on the leading end side of the sheet starts to pass through the nip portion, and the transfer of the toner image carried on the image carrier by the electric field + E for transfer starts. The region P2 (solid line) passes through the nip portion, and the transfer of the toner image is completed.

  At time t7, 0 bias or minus bias is applied to the transfer roller to prepare for the next transfer. The time t4 when the electric field + E for transfer rises is earlier than the time t5 when the image area P2 (solid line) starts to pass through the nip, and therefore the image area P2 passes through the nip during the generation of the electric field + E for transfer. Therefore, transcription is performed normally.

  FIG. 6B shows the relationship between the rising of the electric field E generated in the nip portion at low humidity and the conveyance of the paper P. The rise of the electric field E at low humidity (θ2) is slower than the rise of the electric field E at normal humidity (θ1) (θ1> θ2).

  At time t1, a negative bias for separation is applied to the transfer roller so that the front end of the sheet does not wrap around the image carrier, for example, an intermediate transfer belt, at the transfer portion (to improve separation), An electric field -E is generated.

  At time t2, the leading edge of the paper reaches the nip portion. At this time, the leading edge of the paper advances without being wound around the intermediate transfer belt by the electric field -E for separation. At time t3, a positive bias for transfer is applied to the transfer roller.

  At time t4 ', an electric field + E for transfer corresponding to the positive bias applied to the nip portion rises. At this time, the timing t4 'is delayed by the period Y from the timing t4 described above due to the slowing of the rising of the electric field due to low humidity.

  In other words, the period between the above-mentioned time t4 and time t4 '(period Y) corresponds to the delay time (rise time increase) of the electric field rise due to low humidity.

  The positive bias for transfer applied to the transfer roller is different in polarity and voltage value from the negative bias for separation, and the electric field for separation -E and the electric field for transfer generated by the application of the bias. The direction of the electric field and the strength of the electric field are different from + E.

  At time t5, the image area P2 (solid line) on the front end side of the sheet starts to pass through the nip portion. However, the image area P2 (solid line) has already started to pass through the nip portion at time t5 due to the delayed time t4 '.

  After time t4 ′, the toner image carried on the intermediate transfer belt is normally transferred by the electric field + E for transferring a value corresponding to the positive bias, and at time t6, the image region P2 (solid line) becomes the nip portion. And the transfer of the toner image is completed.

  At time t7, 0 bias or minus bias is applied to the transfer roller to prepare for the next transfer.

  In addition, the problem of a decrease in image density due to a decrease in transfer rate is not so noticeable when printing an image with a low print rate such as characters, but in an image with a high print rate such as a solid image or a picture. There is a particularly noticeable feature when it occurs partially.

  As described above, since the electric field + E for transfer is not generated during the period X5 to the period t4 ′ (period X) due to the slowing of the rising of the electric field, the transfer rate is decreased in the period X, and the period t4 After that, the electric field for transfer + E, and a predetermined transfer rate is obtained. For this reason, there arises a problem that the transfer density, that is, the image density is different before and after the timing t4 'and the image quality is lowered.

  As a method for solving these problems, there is a method of increasing the margin portion at the leading end in the paper conveyance direction, but there are cases where the margin portion cannot be increased depending on the customer's request. In addition, there is a method in which the conveyance speed of the paper passing through the nip portion is constantly lowered in accordance with the delay of the rise of the electric field generated in the nip portion. It is not realistic for customer demand.

  In view of the above-described problems, an object of the present invention is to provide an image forming apparatus capable of obtaining an output image having a stable density regardless of humidity and printing rate.

  The above object is achieved by the following configuration.

1. In an image forming apparatus having a transfer unit that changes an electric field applied between a non-image area on a sheet and an image area on a sheet,
A printing rate calculation unit for calculating a printing rate;
A humidity sensor for measuring the humidity inside the device;
The printing rate calculated by the printing rate calculation unit and the humidity value measured by the humidity sensor so that the image area reaches the transfer unit after the electric field strength of the transfer unit reaches a strength suitable for transfer. An image forming apparatus comprising: a control unit that reduces a process speed of the apparatus according to

  2. The control unit reduces the process speed when the humidity value measured by the humidity sensor is less than a preset humidity and the printing rate calculated by the printing rate calculation unit is equal to or higher than a preset printing rate. 2. The image forming apparatus as described in 1 above.

  3. 3. The image forming apparatus according to 1 or 2, wherein the print rate calculation unit calculates a print rate based on digital image data on a front side in a paper conveyance direction among digital image data for one sheet. .

  4). The image forming apparatus according to any one of claims 1 to 3, wherein the control unit changes the process speed in units of jobs.

5. The printing rate calculation unit calculates a printing rate based on the digital image data of the first page of the job,
The control unit changes the process speed for the job according to the printing rate of the first page calculated by the printing rate calculation unit and the humidity value measured by the humidity sensor. 5. The image forming apparatus according to any one of 4 above.

  The above invention can provide an image forming apparatus capable of outputting a high-quality print image without lowering productivity.

FIG. 3 is a cross-sectional view of the image forming apparatus viewed from the side. It is a figure which shows the relationship between the standing-up state of the electric field E which generate | occur | produces in a nip part, and the part of the paper P which passes a nip part. 1 is a block diagram of an image forming apparatus. FIG. 6 is a flowchart according to a toner image transfer method in a secondary transfer unit. It is explanatory drawing of the image transcribe | transferred on the paper. It is explanatory drawing of the relationship between the rise of the electric field which generate | occur | produces in the nip part in the conventional transfer part, and conveyance of a paper.

In the present invention, the process speed is reduced according to the slowing of the rising of the electric field in the transfer portion due to low humidity (low humidity measured inside the apparatus).
The process speed is lowered by an amount delayed by the slowdown in the time when the electric field intensity suitable for transfer is delayed, and the image area of the paper is conveyed to the nip portion after the electric field intensity suitable for transfer is reached. As a result, the transfer is performed under the electric field strength suitable for the transfer within the image area of the paper regardless of the humidity to prevent the transfer density from being lowered, which will be described in detail below.

  FIG. 1 is a cross-sectional view of the image forming apparatus as viewed from the side.

  Explain the wording before entering the concrete explanation. An image area refers to an area in which an image is to be formed (area where a toner image is carried) in one sheet, and a non-image area is an image formation corresponding to a blank portion in one sheet. This refers to a region that is not performed (a region where no toner image is carried).

  Upstream refers to the direction in which one point of the sheet or intermediate transfer belt flows, and downstream refers to the direction in which one point of the sheet or intermediate transfer belt flows.

  The image forming apparatus A is called a tandem color image forming apparatus, and a document image reading apparatus B that reads image information of a document is installed on the upper part of the image forming apparatus A.

  The document image reading device B scans a document image by the optical system of the document image scanning unit, reads image information corresponding to the document image, and outputs the image information to the image forming device A.

  The image forming apparatus A includes an exposure unit 2 (2Y, 2M, 2C, and 2K) that forms a latent image on the photosensitive member 1 (1Y, 1M, 1C, and 1K) based on the image information, and a periphery of the photosensitive drum. A developing unit 3 (3Y, 3M, 3C, 3K), a charging unit (no figure number), a cleaning unit (no figure number), and a photoconductor 1 (no figure number) are arranged to visualize the latent image formed and formed on the photoconductor 1. 1Y, 1M, 1C, and 1K) are transferred to the intermediate transfer belt 4 and a primary transfer portion 5 (5Y, 5M, 5C, and 5K) that is transferred to the intermediate transfer belt 4, and the intermediate transfer belt 4 that is an image carrier. Secondary transfer unit 6 that transfers the toner image transferred onto the paper P, a fixing unit 7 that fixes the toner image transferred onto the paper P, and a paper feed unit that has a plurality of paper cassettes 81 and supplies the paper from the paper cassette 8 and cleaning for removing the toner remaining on the intermediate transfer belt 4 Has a 9, a.

  Note that Y, M, C, and K attached to each part indicate colors. For example, 1Y, 1M, 1C, and 1K are for yellow (Y), magenta (M), cyan (C), and black (K) colors. Refers to each photoconductor. Hereinafter, for example, the photoconductors (1Y, 1M, 1C, 1K) are simply referred to as photoconductors 1 unless they are related to a specific color.

  Each configuration will be described below.

  Image information (analog signal) read by the document image reading apparatus B is subjected to analog processing, A / D conversion, shading correction, image compression processing, and the like in an image processing unit (not shown), and then converted into digital image data. Input to the exposure unit 2.

  The exposure unit 2 modulates and scans the laser beam based on the digital image data to form a latent image on the surface of the photoreceptor 1.

  The developing unit 3 visualizes the latent image on the surface of the photosensitive member 1 with a two-component developer composed of a small particle size toner of yellow (Y), magenta (M), cyan (C), and black (K) and a carrier. A toner image is formed.

  The primary transfer unit 5 sequentially transfers (primary transfer) the toner image formed on the surface of the photoreceptor 1 to the intermediate transfer belt 4 by the primary transfer roller 51, and synthesizes a color image on the surface of the intermediate transfer belt 4. .

  The intermediate transfer belt 4 is semi-conductive and has an endless belt shape. The intermediate transfer belt 4 is suspended by a driving roller 41 and driven rollers 42, 43, and 44, and is driven by the driving roller 41 in the direction of arrow a.

  The secondary transfer unit 6 is connected to a conveyance belt 61 that conveys paper, a driving roller 62 that suspends and drives one end of the conveyance belt 61, a driving member (for example, a motor) 63 that drives the driving roller 62, and a transfer power source 64. And a driven roller 65 that suspends the other end of the conveyance belt 61, and transfers the toner image carried on the intermediate transfer belt 4 onto the paper P at the nip portion n.

  The driven roller 65 is opposed to the driven roller 44 via the conveyance belt 61 and the intermediate transfer belt 4 and forms a nip portion n for nipping the paper. The transfer power source 64 is a power source having a variable output voltage, and generates an electric field between the driven roller 65 and the intermediate transfer belt 4 via the conveyance belt 61.

  The secondary transfer unit 6 transfers the toner image carried on the intermediate transfer belt 4 to the paper P and prevents the paper nipped at the nip n from being wound around the intermediate transfer belt 4. Combined with separation function.

  The transfer power source 64 of the secondary transfer unit 6 is a power source that can output a plurality of different voltage values. The control unit C outputs a voltage that generates an electric field suitable for transfer during transfer, and an electric field suitable for separation during separation. The voltage that generates is output.

  The paper feed unit 8 has a plurality of paper cassettes 81 and picks up the paper P stored in the paper cassette 81 by the paper feeder 82. The picked-up paper P is transported to the nip portion n in synchronization with the toner image carried on the intermediate transfer belt 4 by transport rollers 83, 84, 85, 86 and resist rollers 87 that transport the paper.

  A sheet sensor S2 that detects the leading end of the sheet in the sheet conveyance direction is provided on the downstream side of the registration roller 87 and the upstream side of the nip portion n, and the output of the sheet sensor S2 is read by the control unit C.

  The fixing unit 7 includes a heating roller 72 and a pressure roller 73 in which a heater lamp 71 as a heating source is housed. The paper P on which the toner image is transferred is heated and pressed, and the toner image is fixed on the paper P. Is done.

  Then, the fixed paper P is sandwiched between the paper discharge rollers 88 and discharged to the lift paper discharge tray 89. The toner image remaining on the intermediate transfer belt 4 is removed by the cleaning unit 9 after the toner image is transferred to the paper P by the secondary transfer unit 6.

  Further, the image forming apparatus A includes a control unit C that controls the entire image forming apparatus, and an operation panel SP that allows a user to input job information related to a job, for example. It is read by the control unit C.

  FIG. 2 is a diagram illustrating the relationship between the rising state of the electric field E generated in the nip portion and the portion of the paper P that passes through the nip portion.

  In the following, the relationship between the rising state of the electric field E generated in the nip portion at low humidity in the conventional transfer portion and the conveyance of the paper P [FIG. The difference between the rising state of the electric field E generated in the nip portion and the conveyance of the paper P (FIG. 2B) will be described with reference to FIGS. The non-image area P1 and the image area P2 correspond to the non-image area P1 and the image area P2 in FIG.

  2A and 2B, the horizontal axis indicates the time flow t, and the time tn (t1 to t7) indicates that n hours have elapsed from the time t0. The vertical axis indicates the rising state of the electric field E and the portion of the paper P (non-image area or image area) that passes through the nip portion at each of the times t1 to t7.

  FIG. 2A shows the relationship between the rising state of the electric field E generated in the nip portion at the time of low humidity in the conventional transfer portion and the conveyance of the paper P when the humidity inside the apparatus is reduced to, for example, less than 20%. The relationship between the rising state of the electric field E generated in the nip portion and the portion of the paper P that passes through the nip portion at each of the times t1 to t7 is shown. The relationship between the two is the same as that in FIG.

  Note that during the period from the time t5 to the time t4 ′ (period X), the electric field E does not become the electric field + E for transfer corresponding to the positive bias due to the slowing of the rising of the electric field generated in the nip portion, as described above. The transfer rate is lowered and the image quality is lowered.

  FIG. 2B shows the relationship between the rising state of the electric field E generated in the nip portion at low humidity and the conveyance of the paper P in the transfer portion of the present invention when the humidity inside the apparatus is reduced to less than 20%. A rising state of the electric field E generated in the nip portion, and a portion of the sheet P passing through the nip portion n at each of timings t1 to t7 when the process speed is lowered according to the slowing of the rising of the electric field generated in the nip portion. , Showing the relationship.

  In FIG. 2B, the electric field -E for separation is applied to the nip portion during the period t1 to t3, and the electric field + E for transfer is applied to the nip portion during the period t4 'to t7'. The non-image area P1 passes through the nip portion during the time t2 ′ to t5 ′, and the image area P2 passes through the nip portion during the time t5 ′ to t6 ′. Show.

  The electric field −E for separation is an electric field that imparts a charge having the same polarity as the polarity of the paper to the paper P so that the paper P does not wrap around the intermediate transfer belt 4. For example, the paper has a positive polarity. If so, apply a positive electric field. An electric field + E for transfer is an electric field that imparts a charge having a polarity opposite to the polarity of the toner image to the paper P so that the toner image carried on the intermediate transfer belt 4 is transferred to the paper P. For example, the toner image When has a negative polarity, a positive electric field is applied.

  At time t0, a sheet sensor S2 that detects the leading edge of the sheet in the traveling direction detects the leading edge of the sheet. At time t1, for example, a negative bias is applied to the transfer roller to generate an electric field -E for separation in the nip portion so that the leading edge of the sheet does not wrap around the image carrier (in order to improve separation) in the transfer portion. Let

  At time t2 ', the leading edge of the sheet reaches the nip portion n. At this time, the leading edge of the sheet advances without being wound around the intermediate transfer belt 4 due to the electric field -E for separation.

  Then, a positive bias for transfer is applied to the driven roller 65 at time t3 (before the non-image area P1 (broken line) on the front end side of the sheet has passed through the nip n). Then, it rises to an electric field + E for transfer corresponding to the positive bias applied at time t4 '. At this time, the timing t4 'is delayed from the rising timing t4 of the electric field at the normal humidity due to the slowing of the rising of the electric field due to the low humidity.

  Since the process speed is lowered in accordance with the slowdown of the rising of the electric field generated in the nip portion, the image region P2 (solid line) has not yet reached the nip portion at the delayed time t4 ′, and at the time t5 ′, the leading edge of the sheet The image area P2 (solid line) on the side begins to pass through the nip portion.

  Then, the transfer of the toner image carried on the intermediate transfer belt 4 is continued by the electric field + E for transfer, and at time t6 ′, the image region P2 (solid line) passes through the nip portion n and the transfer of the toner image is completed. .

  At time t7 ', 0 bias or minus bias is applied to the driven roller 65 to prepare for the next transfer.

  As described above, since the time t4 ′ when the image area P2 (solid line) starts to pass through the nip portion is preceded by the period Z by the time t4 ′ rising to the electric field + E for transfer, the image area P2 on the front end side of the sheet. When the (solid line) passes through the nip portion, the electric field for transfer is already + E, and the transfer density, that is, the image density does not differ before and after the time t4 ′, and there is no problem of reducing the image quality. .

  The positive bias for transfer applied to the transfer roller is different in polarity and voltage value from the negative bias for separation, and the electric field −E for separation and the electric field + E for transfer are the direction of the electric field. And the electric field strength is different.

  As described above, the rise time until the target electric field strength is reached varies depending on the humidity, but even if the rise of the electric field generated in the nip portion is dull due to low humidity, the process speed is increased in the nip portion. Therefore, the transfer can be performed while the electric field + E for transfer is generated, and an output image having a stable density can be obtained regardless of humidity.

  A configuration and method for performing the above operation will be described below.

  FIG. 3 is a block diagram of the image forming apparatus.

  The control unit C includes a CPU (Central Processing Unit), a storage unit M, and an I / O unit that controls an interface between external devices. The CPU sequentially reads out a program from a ROM (Read Only Memory) of the storage unit M, stores it in a RAM (Random Access Memory), and controls input / output devices via the I / O unit according to the program.

  The input device includes the above-described document image reading device B, the operation panel SP, a humidity sensor S1 that measures the humidity inside the device, and a sheet sensor S2 that detects the leading edge of the sheet in the traveling direction. The read document image information is input from the apparatus B, the job information is input from the operation panel SP, the humidity information in the apparatus is input from the humidity sensor S1, and the front end of the sheet in the sheet conveyance direction is input from the sheet sensor S2. Detection information is input.

  The output device includes a photoreceptor 1, an exposure unit 2, a developing unit 3, an intermediate transfer belt 4, a primary transfer unit 5, a secondary transfer unit 6, a fixing unit 7, a paper feeding unit 8, and the like. For the next transfer unit 6, the output voltage of the transfer power source 64 is controlled.

  The rotation of the photosensitive member 1, the exposure timing of the exposure unit 2, the rotation of the developing roller of the developing unit 3, the rotation of the intermediate transfer belt, the paper feeding timing of the paper feeding unit 8, the paper conveyance speed, etc. are related to each other. Therefore, the control unit C organically controls these components so as to achieve a process speed determined as the image forming apparatus.

  The determined process speed is the highest speed value at which the image density does not decrease on the leading edge side of the sheet due to the slowing of the rising of the electric field, and the first process speed corresponding to the rising of the electric field generated in the nip portion during normal humidity. The process speed information and the second process speed information corresponding to the rising of the electric field generated in the nip portion at low humidity are determined in advance by experiments or the like and stored in the ROM.

  For example, when the process speed at normal humidity is 570 mm / s, and the transfer density at low humidity decreases for 4 mm from the front end in the paper transport direction, the transfer density decreases at 7 mS in terms of paper transport time. Therefore, the reduction of the process speed at the time of low humidity takes the variation of the rising of the electric field into consideration and reduces the speed from 570 mm / s to 440 mm / s (−22%).

  Thus, for example, 570 mm / s is stored as the first process speed information, and 440 mm / s is stored as the second process speed information.

  Then, the control unit C selects either the first process speed information or the second process speed information based on the printing rate information described later and the humidity information measured by the humidity sensor S1, and the entire image forming apparatus. Is controlled to achieve the selected process speed.

  Hereinafter, a control method related to the transfer of the secondary transfer unit 6 will be described with reference to FIGS. 1, 3, and 4. Note that the program according to the flowchart is stored in the storage unit M and is sequentially read and executed by the CPU.

  FIG. 4 is a flowchart relating to a toner image transfer method in the secondary transfer unit.

1. Reading job information Step S101
The operation panel SP and the document image reading device B are monitored, and the number of sheets of the job set on the operation panel SP is read. Further, the original image is read by the original image reading apparatus B and the process proceeds to the next step. Note that the read original image may be associated with the job, and the read number may be used as the number of sheets of the job.

  Note that the number of sheets of the job and the document image information may be acquired by a LAN (Local Area Network) not shown.

2. Generation of digital image data Step S102
Based on the document image information read in step S101, digital document image data of each page constituting the job is generated, and the process proceeds to the next step.

3. Determination of printing rate Step S103
Of the digital image data for one sheet, the printing rate is calculated based on a part of the digital image data on the leading side in the sheet conveying direction, and the printing rate is equal to or higher than a preset printing rate (for example, 80%) ( Y) It is determined that the printing ratio is high, and the process proceeds to the next step. If the printing ratio is less than a preset printing ratio (for example, 80%), (N) it is determined that the printing ratio is not high, and the process proceeds to step S110.

  For example, from the digital original image data for one sheet generated in step S102, about 2 to 10 lines of digital original image data from the leading edge in the paper conveyance direction are cut out, and the digital original image data of about 2 to 10 lines is cut out. The printing rate is calculated for.

  The calculation of the printing rate is the ratio of the total number of dots N2 constituting the generated digital original image of about 2 to 10 lines (N2) to the total number of dots N1 when about 2 to 10 lines are solid images. / N1 × 100%).

  In general, high print rate data such as photographs and low print rate data such as documents are often handled as different jobs. For this reason, the print rate is based on the digital image data on the first page of the job. Is calculated.

  Thus, the control unit C has a function as a printing rate calculation unit that calculates the printing rate.

4). Read humidity sensor value Step S104
The humidity sensor S1 is constantly monitored, and for example, the humidity value at the start of the job is read and the process proceeds to the next step.

5. Humidity judgment Step S105
The humidity value read in step S104 is compared with a predetermined humidity value set in advance. If the humidity value read in step S104 is less than the predetermined humidity value (Y), it is determined that the humidity is low, and the process proceeds to the next step. If it is equal to or higher than the predetermined humidity value (N), it is determined that the humidity is not low, and the process proceeds to step S110.

  The predetermined humidity value is the highest limit humidity value at which the image density does not decrease at the leading edge of the sheet due to the slowing of the rising of the electric field. The predetermined humidity value is obtained in advance by experiments and stored in the ROM of the storage unit M or the like.

6). Deceleration of transfer section paper conveyance speed Step S106
The second process speed information is read from the storage unit M. Then, the rotation of the photosensitive member 1, the exposure timing of the exposure unit 2, the rotation of the developing roller of the developing unit 3, the rotation of the intermediate transfer belt, the feeding timing of the paper feeding unit 8, the paper conveyance speed, etc. Deceleration control is performed so that the speed according to the second process speed information is changed from the speed according to the process speed information, and the process proceeds to the next step.

7). Image formation for the job Step S107
Image formation is performed based on the digital image data generated in step S102, and the process proceeds to the next step. Note that the process speed in image formation at this time is a speed according to the second process speed information decelerated in step S106.

8). Job completion confirmation step S108
The number of sheets of the job read in step S101 is compared with the number of sheets for which image formation has been completed. The process proceeds to the next step, and steps S107 and S108 are repeated until the number is the same (N).

9. Returning the transfer section paper conveyance speed Step S109
Since the job is completed, the speed is increased from the speed corresponding to the second process speed information to the speed corresponding to the first process speed information in preparation for the next job (deceleration is released), and the process proceeds to the end.

10. Image formation for the job Step S110
The first process speed information is read from the storage unit M. Then, the rotation of the photosensitive member 1, the exposure timing of the exposure unit 2, the rotation of the developing roller of the developing unit 3, the rotation of the intermediate transfer belt, the feeding timing of the paper feeding unit 8, the paper conveyance speed, etc. Control the speed according to the process speed information.

  Then, image formation is performed based on the digital image data generated in step S102, and the process proceeds to the next step.

11. Job completion confirmation step S111
Since it is the same as step S108, description is abbreviate | omitted. When the number of sheets on which image formation has been completed becomes the same as the number of sheets in the job (Y), it is determined that the job has been completed, and the process proceeds to the end. (N) Steps S110 and S111 are repeated until the number is the same.

  As described above, the control unit C changes the process speed of the apparatus according to the calculated printing rate and the humidity value measured by the humidity sensor S1.

  That is, by reducing the process speed according to the slowing of the rising of the electric field at high printing rate and low humidity, the image area of the paper is conveyed to the nip portion according to the slowing of the rising of the electric field, and normal within the image area of the paper By performing transfer under a strong electric field, it is possible to provide an image forming apparatus capable of preventing an increase in running cost and outputting a high-quality print image without reducing productivity.

  An experiment was performed in which the humidity, the printing rate, and the process speed were changed, and an image was formed on a sheet using the image forming apparatus described above to check the density state of the output image.

  As for the printing rate, an image having a printing rate of 5% as a low printing rate, 40% as a medium printing rate, and 80% as a high printing rate was formed.

  As for humidity, an image was formed in an environment of 80% as high humidity, 50% as normal humidity, and 19% as low humidity.

  As for the process speed, an image was formed at a speed matched to normal humidity (570 mm / s) and a speed lowered according to the slowing of the rising of the electric field due to low humidity (440 mm / s).

  In the evaluation, the image density of the leading edge of the image-formed paper was visually confirmed. In Table 1, ◯ indicates that there is no decrease in image density, Δ indicates that there is a slight decrease in image density at the leading edge of the paper, but there is no problem with practical skills, and x indicates that there is a problem that the decrease in image density occurs at the leading edge of the sheet. Indicates a level.

  In Table 1, when the process speed is 570 mm / s, it is △ level for normal humidity and high printing rate, and for low humidity and medium printing rate, and × level for low humidity and high printing rate. However, when the process speed was lowered to 440 mm / s, it became a ◯ level under all combinations of conditions.

  As described above, by lowering the process speed in the case of low humidity and high printing rate, it is possible to solve the problem that the image density is lowered and the image quality is deteriorated more than a level where there is no practical problem at the front end in the paper transport direction. It was confirmed that there was.

4 Intermediate transfer belt 6 Secondary transfer portion 61 Conveying belt 62 Drive roller 64 Transfer power supply 65 Driven roller A Image forming apparatus n Nip portion P Paper S1 Humidity sensor

Claims (5)

In an image forming apparatus having a transfer unit that changes an electric field applied between a non-image area on a sheet and an image area on a sheet,
A printing rate calculation unit for calculating a printing rate;
A humidity sensor for measuring the humidity inside the device;
The printing rate calculated by the printing rate calculation unit and the humidity value measured by the humidity sensor so that the image area reaches the transfer unit after the electric field strength of the transfer unit reaches a strength suitable for transfer. An image forming apparatus comprising: a control unit that reduces a process speed of the apparatus according to   The control unit reduces the process speed when the humidity value measured by the humidity sensor is less than a preset humidity and the printing rate calculated by the printing rate calculation unit is equal to or higher than a preset printing rate. The image forming apparatus according to claim 1.   3. The image formation according to claim 1, wherein the print rate calculation unit calculates a print rate based on digital image data on a front end side in a paper transport direction among digital image data for one sheet. apparatus.   The image forming apparatus according to claim 1, wherein the control unit changes the process speed in units of jobs. The printing rate calculation unit calculates a printing rate based on the digital image data of the first page of the job,
The control unit changes a process speed for the job in accordance with a printing ratio of the first page calculated by the printing ratio calculation unit and a humidity value measured by the humidity sensor. The image forming apparatus of any one of -4.

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