JP6229195B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus capable of adjusting a screen angle of an image.

  In general, in an image forming apparatus, an image is represented by a line matrix. When the screen angle of the image is close to 90 °, the rotational speed unevenness of the photosensitive drum that carries the toner image and transports it to the transfer area, or the transfer medium on which the toner image on the photosensitive drum is transferred (for example, a secondary transfer belt). ), As shown in FIG. 17A, unevenness (hereinafter referred to as pitch unevenness) occurs at intervals between adjacent screen patterns SP (i) and SP (i + 1) (i is a natural number of 1 or more) in the image. As a result, the image density is uneven. On the contrary, when the screen angle is close to 0 °, the image density unevenness is not noticeable as shown in FIG. 17B. Here, in this specification, the screen angle θ is an angle formed by the toner image transport direction α and the screen pattern SP direction β, and is represented by an angle counted counterclockwise from the transport direction α. Is done.

  In a general image forming apparatus, a full color image is expressed by Y (yellow), M (magenta), C (cyan), and Bk (black). Among these four colors, since black has the highest visual sensitivity, black density unevenness is most easily recognized by the viewer. Patent Document 1 describes an image forming apparatus in which the screen angle θ of each color is varied and the black screen angle θBk is minimized.

JP 2000-108412 A

  However, when the screen angle is made small, there is a problem in that transfer loss tends to occur. The term “transfer dropout” means that when a toner image is transferred from a photosensitive drum to a transfer body or the like, a strong pressure is applied to the central portion of the screen pattern, and the central portion of the screen pattern is not transferred to the transfer body. A phenomenon that remains on the body drum.

  As shown in FIG. 18A, when the screen angle θ is as large as 60 °, for example, in the screen pattern of the electrostatic latent image formed on the photosensitive drum 101, the light amount at the peripheral portion is smaller than that at the central portion. Become. Therefore, in the toner image TI obtained by developing the electrostatic latent image, the toner amount is slightly less at the peripheral portion than at the central portion of the screen pattern. Therefore, the width direction area PW of the pressure applied from the transfer medium 103 in the screen pattern of the toner image TI during transfer is slightly smaller than the width direction area TW of the screen pattern. That is, the pressure applied per unit area of the screen pattern of the toner image TI (more specifically, the unit area in a plan view from the normal direction of the circumferential surface of the photosensitive drum 101) does not become extremely large.

  However, as shown in FIG. 18B, when the screen angle θ becomes small, for example, 30 °, the pressure per unit area applied to the central portion of the toner image TI is extremely large, contrary to the case where the screen angle θ is large. Become. As a result, the toner only on the periphery of the screen pattern moves on the surface of the transfer medium 103 by the bias voltage applied to the transfer medium 103. However, the toner at the central portion where the pressure is applied is highly adsorbed and remains on the peripheral surface of the photosensitive drum 101. Thus, at the time of transfer, as shown in FIG. 18C, the peripheral edge portion and the central portion of the screen pattern are separated, and the transfer is lost.

  In the above description, the problem has been described in relation to the photosensitive drum 101 and the secondary transfer belt as the transfer medium 103. However, the problem that the transfer is lost can occur between the secondary transfer belt carrying the toner image and the paper as the transfer medium. Further, in a printer dedicated to monochrome printing, it can also occur between the photosensitive drum and a sheet as a transfer medium.

  SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an image forming apparatus that is less likely to cause transfer loss.

In order to achieve the above object, a first aspect of the present invention is directed to an image forming apparatus. The image forming apparatus includes an image carrier, a charging unit that charges a surface of the image carrier, an exposure unit that irradiates a light beam on the surface of the image carrier to form an electrostatic latent image, and the exposure. Developing means for forming a toner image having a screen pattern on the surface of the image carrier by performing development processing on the electrostatic latent image formed by the means, and disposed opposite to the image carrier, the image carrier A transfer body for transferring a toner image formed and conveyed on the surface of the toner image onto a transfer medium, and a control means for setting a screen angle of the screen pattern according to the degree of adsorption of the toner image to the image carrier; And the control means sets the screen angle to the first angle when the degree of toner image adsorption to the image carrier is the first degree, and the second degree is higher than the first degree. Before The screen angle is set to a second angle close to the conveying direction at 90 ° relative to the toner image than the first angle.

The second aspect of the present invention is also directed to an image forming apparatus. The image forming apparatus includes an image carrier that carries a toner image composed of a plurality of screen patterns on a surface thereof, a transfer member that is disposed opposite to the image carrier, and a space between the image carrier and the transfer member. A transfer medium that is pressed against the surface of the image carrier by the transfer body, on which the toner image on the surface of the image carrier is transferred, and a degree of adsorption of the toner image on the image carrier. And a control unit that sets a screen angle of the screen pattern according to the control unit , wherein the control unit sets the screen angle to the first angle when the degree of adsorption of the toner image to the image carrier is the first level. When the suction degree is a second degree higher than the first degree, the screen angle is set to a second angle that is closer to 90 ° than the first angle with respect to the conveyance direction of the toner image. .

  According to the above aspect, it is possible to provide an image forming apparatus that is less likely to cause transfer loss.

1 is a diagram illustrating a configuration of an image forming apparatus according to an embodiment. FIG. 6 is a schematic view illustrating a toner image TI (θ = 60 °) during halftone printing. FIG. 6 is a schematic view illustrating a toner image TI (θ = 30 °) during halftone printing. FIG. 6 is a schematic diagram illustrating a relationship between a force applied to toner in a primary transfer region and a transfer dropout. 6 is a graph showing a change in residual toner amount with respect to pressure applied to a primary transfer region at a humidity of 50%. 6 is a graph showing a change in residual toner amount with respect to pressure applied to the primary transfer region at a humidity of 70%. 6 is a graph showing a change in residual toner amount with respect to pressure applied to a primary transfer region in an initial toner state. 6 is a graph showing a change in residual toner amount with respect to pressure applied to a primary transfer region in a toner durability deterioration state. It is a schematic diagram which shows the switching time of the screen angle according to the use period of a transfer roller. 2 is a block diagram showing a control system of the image forming apparatus according to the first embodiment. FIG. It is a flowchart which shows the process of the control circuit of FIG. It is a flowchart which shows the modification of the process of the control circuit of FIG. It is a block diagram which shows the control system of the image forming apparatus which concerns on 2nd embodiment. It is a flowchart which shows the process of the control circuit of FIG. It is a block diagram which shows the control system of the image forming apparatus which concerns on 3rd embodiment. It is a flowchart which shows the process of the control circuit of FIG. It is a schematic diagram which shows a screen pattern ((theta) = 90 degree). It is a schematic diagram which shows a screen pattern ((theta) = 0 degree). FIG. 6 is a schematic diagram illustrating a toner image when θ is 60 °. FIG. 6 is a schematic diagram illustrating a toner image when θ is 30 °. It is a schematic diagram which shows a transfer omission.

(Embodiment)
Hereinafter, an image forming apparatus according to an embodiment of the present invention will be described in detail with reference to the drawings.

(Introduction)
First, the x-axis, y-axis, and z-axis in the figure are defined. In this embodiment, for convenience of explanation, the x-axis, y-axis, and z-axis are the left-right direction (in other words, the horizontal direction), the front-rear direction (in other words, the depth direction), and the up-down direction (in other words, the height) of the image forming apparatus. Direction). In addition, some configurations in the figure have subscripts a, b, c, and d added to the right side of the reference numbers. a, b, c, and d mean yellow (Y), magenta (M), cyan (C), and black (Bk). For example, the image forming means 29a means the yellow image forming means 29. Further, no suffix means each color of Y, M, C, and Bk. For example, the image forming means 29 means image forming means for each color of Y, M, C, and Bk.

(Configuration and operation of image forming apparatus)
In FIG. 1, an image forming apparatus 1 is, for example, an MFP (Multifunction Peripheral), and forms a toner image of each color substantially simultaneously using a photosensitive drum 31 for each color, and then combines the combined toner image. Is printed on the sheet S. For this purpose, the image forming apparatus 1 generally includes a supply unit 3, a timing roller pair 5, a process unit 7, a fixing unit 9, a discharge roller pair 11, and a discharge tray 13.

  The supply unit 3 includes a supply tray 15 and a supply roller 16. A plurality of unprinted sheets S are stacked on the supply tray 15. The supply roller 16 takes out the stacked sheets S one by one from the top, and sends them out to the transport path R. The sheet S is conveyed toward the timing roller pair 5 immediately downstream.

  The timing roller pair 5 includes a pair of rollers that are in contact with each other on the transport path R, and rotates and stops under the control of the control circuit 37. Except when the sheet S is sent, each roller is stopped. Therefore, the conveyed sheet S first hits the contact portion between the rollers and temporarily stops. The timing roller pair 5 starts rotating at a predetermined timing, and sends the sheet S toward a secondary transfer region (details will be described later).

  The process unit 7 includes an exposure unit 17, a transfer roller 19 for each color, an intermediate transfer belt 21, a driving roller 23, a driven roller 25, a secondary transfer roller 27, and an image forming unit 29 for each color. Each image forming unit 29 generally includes a photosensitive drum 31, and a charging unit 33 and a developing unit 35 disposed along the peripheral surface thereof.

  The photosensitive drums 31 for the respective colors are arranged so as to extend in the y-axis direction. Further, the plurality of photosensitive drums 31 are arranged so as to be aligned in the x-axis direction. Each photosensitive drum 31 is rotated clockwise (indicated by arrow CW) in the zx plane about an axis parallel to the y-axis by a driving force from a motor (not shown).

  Each charging means 33 extends in the y-axis direction and charges the peripheral surface of the corresponding photosensitive drum 31.

  Image data of each color is transmitted to the exposure means 17. The image data is data representing an image to be printed on the sheet S, and is generated by a scanner or a PC (both not shown). The exposure unit 17 generates a light beam B modulated with the image data for each color, and charges and rotates the peripheral surface of each photosensitive drum 31 (more specifically, the charging unit 33 on the peripheral surface). Irradiate the position immediately downstream). Specifically, the exposure unit 17 scans the peripheral surface of the photosensitive drum 31 with the corresponding color light beam B in the main scanning direction (that is, the y-axis direction). The potential of the irradiated portion of the beam B approaches 0V on each peripheral surface. By the exposure described above, an electrostatic latent image of a corresponding color is formed on the peripheral surface of each photosensitive drum 31.

  Each developing means 35 has a developing roller extending in the y-axis direction. The developing roller is disposed opposite to the peripheral surface of the corresponding photosensitive drum 31 immediately downstream of the irradiation position of the light beam B. For example, a two-component developer of a corresponding color is accommodated in the developing unit 35. Each developing unit 35 supplies toner onto the peripheral surface of the photosensitive drum 31 using a built-in developing roller. As a result, the electrostatic latent image is developed on the circumferential surface of the photoconductive drum 31 to form a corresponding color (single color) toner image.

  As a result of the development process, the photosensitive drum 31 carries a toner image on the peripheral surface. Thus, the photosensitive drum 31 is an example of an image carrier. Further, the photosensitive drum 31 conveys the toner image carried by the photosensitive drum 31 downstream in the rotation direction CW.

  Each transfer roller 19 extends in the y-axis direction, and faces the peripheral surface of the corresponding color photosensitive drum 31 immediately downstream of the corresponding color developing means 35 with the following intermediate transfer belt 21 interposed therebetween. Arranged to do.

  The intermediate transfer belt 21 is an endless belt. The intermediate transfer belt 21 is interposed between the transfer roller 19 and the photosensitive drum 31 for each color, and is stretched between the driving roller 23 and the driven roller 25. The intermediate transfer belt 21 is pressed against each photosensitive drum 31 by each transfer roller 19. Hereinafter, the pressure contact portion between each photosensitive drum 31 and the intermediate transfer belt 21 is referred to as a primary transfer region. The driving roller 23 is rotated by a driving force applied from a motor (not shown). The driven roller 25 is driven and rotated by the rotation of the driving roller 23. As a result, the intermediate transfer belt 21 rotates in the direction of the arrow α.

  A primary transfer bias voltage is applied to each transfer roller 19, whereby the periphery of the portion where each transfer roller 19 contacts the intermediate transfer belt 21 is charged with a polarity opposite to that of the toner image. As a result, when the toner image conveyed by the photosensitive drum 31 reaches the primary transfer region, the toner image moves to the outer peripheral surface of the intermediate transfer belt 21. In other words, the toner image formed on each photosensitive drum 31 is transferred to the intermediate transfer belt 21. Hereinafter, the transfer of the toner image to the intermediate transfer belt 21 is referred to as primary transfer.

  Here, the toner images carried on the photosensitive drums 31 of the respective colors are sequentially transferred to the same area on the surface of the intermediate transfer belt 21. Thus, the intermediate transfer belt 21 is an example of a transfer medium. As a result of such primary transfer, a composite toner image in which the toner images of the respective colors overlap is formed on the intermediate transfer belt 21. The intermediate transfer belt 21 conveys the synthetic toner image toward the secondary transfer roller 27 by rotating while carrying the synthetic toner image on its outer peripheral surface. Thus, the intermediate transfer belt 21 is another example of the image carrier.

  The secondary transfer roller 27 is disposed opposite to the driving roller 23 with the intermediate transfer belt 21 interposed therebetween, and is pressed against the intermediate transfer belt 21. A contact portion between the intermediate transfer belt 21 and the secondary transfer roller 27 is hereinafter referred to as a secondary transfer region. As described above, the sheet S is fed into and passed through the secondary transfer area, and the synthetic toner image carried on the rotating intermediate transfer belt 21 is conveyed. In addition, a secondary transfer bias voltage is applied to the secondary transfer roller 27, whereby the secondary transfer roller 27 is charged on the non-transfer surface side of the sheet S with a polarity opposite to that of the composite toner image. As a result, the composite toner image moves from the surface of the intermediate transfer belt 21 to the surface of the sheet S. In other words, the composite toner image carried on the intermediate transfer belt 21 is transferred to the sheet S. Thus, the sheet S is another example of the transfer medium. The transfer to the sheet S is hereinafter referred to as secondary transfer.

  The sheet S on which the toner image is transferred is introduced into the fixing unit 9. The fixing unit 9 fixes the composite toner image on the sheet S by heating and pressurizing the sheet S. The sheet S that has been subjected to the fixing process is discharged as a printed matter P from the discharge roller pair 11 to the discharge tray 13 and placed thereon.

  Each part is controlled by a control circuit 37 built in the main body of the image forming apparatus 1.

(Details of toner image on photosensitive drum)
FIG. 2 is a schematic view illustrating a toner image TI formed on the photosensitive drum 31 during 128 gradation halftone printing. In FIG. 2, an arrow α is a conveyance direction in the primary transfer region of the toner image TI, which is substantially parallel to the x axis and substantially perpendicular to the y axis. The arrow α is also the rotational direction in the primary transfer region of the intermediate transfer belt 21. The toner image TI is composed of a screen pattern SP (see solid portion) having a screen angle θ of 60 °. Here, as described above, θ is an angle formed by the transport direction α and the direction β of the screen pattern SP, and is represented by an angle counted counterclockwise from the transport direction α. A dotted line frame F indicates a region where the screen pattern SP on the peripheral surface of the photosensitive drum 31 receives pressure from the intermediate transfer belt 21 during primary transfer.

  In the lower part of FIG. 2, the dotted line frame F in the screen pattern SP is enlarged. Since there is little toner at the peripheral edge EG (see solid portion) along the y-axis direction of the screen pattern SP, the pressure from the intermediate transfer belt 21 is not relatively applied. On the other hand, since there is a lot of toner in the central portion C (see the hatched portion) in the y-axis direction, the pressure is concentrated in the central portion C. The pressure is concentrated in an area of about 30% of the length in the y-axis direction in the screen pattern SP for one cycle CY. Assuming that the pressure applied to the toner is 1 Pa in a state where the toner is uniformly present in the primary transfer region, the pressure applied to the central portion C is approximately 1 Pa / 0.3≈3.3 Pa.

  FIG. 3 is a schematic view illustrating a toner image TI formed on the photosensitive drum 31 during 128 gradation halftone printing. However, the toner image TI is composed of a screen pattern SP (solid portion) having θ of 30 °. In the lower part of FIG. 3, the dotted frame F (that is, the area where pressure is applied to the screen pattern SP) is enlarged. The pressure is concentrated in the dotted line frame F in the region of the central portion C (refer to the shaded portion) that is approximately 20% of the length in the y-axis direction for one cycle CY of the screen pattern SP. When calculated under the same assumptions as in the example of FIG. 2, the pressure applied to the central portion C is approximately 1 Pa / 0.2≈5 Pa.

  Compared with the case where θ is 60 °, when θ is 30 °, the length of one cycle CY in the y-axis direction is halved, so the number of screen patterns included in the primary transfer region is doubled. However, the pressure applied to one screen pattern is 1/3. As a result, the pressure applied to the central portion C is about 1.5 times (that is, from 3.3 Pa to 5 Pa) as compared with the case where θ is 60 °. As described above, the increase in the pressure at the central portion C increases the adsorptivity of the toner central portion C, and as a result, it becomes easy to cause a transfer omission.

FIG. 4 is a schematic diagram showing the relationship between the force applied to the toner in the primary transfer region and the transfer dropout. In FIG. 4, the toner particles TP move from the peripheral surface of the photosensitive drum 31 to the outer peripheral surface of the intermediate transfer belt 21 by applying a primary transfer bias voltage to the transfer roller 19 (not shown in FIG. 4). The intermediate transfer belt 21 is in pressure contact with the photosensitive drum 31 at a predetermined pressure (hereinafter referred to as transfer pressure). Thereby, the stability of the width of the primary transfer region (that is, the width in the transport direction α) and the potential of the transfer roller 19 is ensured. The following three types of forces F1 to F3 are applied to the toner particles TP in the primary transfer region.
F1: Adsorption force between the intermediate transfer belt 21 and the toner particles TP F2: Adsorption force between the photosensitive drum 31 and the toner particles TP F3: Coulomb force of the electric field formed in the primary transfer region

  If F3> F2-F1, the force applied in the direction from the photosensitive drum 31 to the intermediate transfer belt 21 is greater. In this case, the toner particles TP move to the outer peripheral surface of the intermediate transfer belt 21 when the photosensitive drum 31 and the intermediate transfer belt 21 are separated after passing through the primary transfer region. Here, roughening the outer peripheral surface of the intermediate transfer belt 21 can increase F1, but as a result, the manufacturing cost of the image forming apparatus increases. Therefore, in an inexpensive image forming apparatus, generally F1 <F2. Further, F1 and F2 are increased and F2 to F1 are also increased depending on conditions such as moisture absorption of the toner particles TP and / or a high transfer pressure.

  Further, if F3 <F2-F1, the force in the direction of the photosensitive drum 31 increases, so that the toner particles TP are applied to the circumferential surface of the photosensitive drum 31 when the photosensitive drum 31 and the intermediate transfer belt 21 are separated. As a result, the transfer is lost. Hereinafter, the amount of toner particles TP remaining on the peripheral surface of the photosensitive drum 31 after the separation is referred to as a residual toner amount.

(Method of suppressing transfer dropout at high humidity)
FIG. 5 is a graph showing the residual toner amount with respect to the pressure applied to the toner under the condition of 50% humidity. The amount of residual toner on the vertical axis is a value obtained by converting the amount of toner remaining on the peripheral surface of the photosensitive drum 31 into an amount per unit square meter when a 128-tone halftone image is printed on the sheet S of size A3. It is. As can be seen from FIG. 5, when the pressure applied to the toner increases, the toner adsorptivity increases, and therefore the residual toner amount tends to increase.

It has been experimentally confirmed that if the residual toner amount is 1 g / m ² or less, the viewer cannot recognize the image deterioration. From this viewpoint, the upper limit value of the residual toner amount is set to 1 g / m ² . However, since this residual toner amount differs for each toner or for each model, it is preferably set for each model of the image forming apparatus. Here, if θ is 30 °, a pressure of 5 Pa is applied to the central portion C of the toner image TI (see FIG. 3). In this case, the residual toner amount is 0.2 g / m ² and is equal to or less than the upper limit value, so that a good image can be obtained.

  FIG. 6 is a graph showing the amount of residual toner with respect to the pressure applied to the toner under the condition of 70% humidity. The amount of residual toner on the vertical axis is as described above. When the humidity is high, the toner adsorbability increases due to moisture absorption by the toner. As a result, the residual toner amount increases. Therefore, compared with the case where the humidity is 50%, the amount of residual toner at the same pressure increases when the humidity is 70%.

If θ is 30 °, a pressure of 5 Pa is applied to the central portion C of the toner image TI. When the humidity is 70%, the residual toner amount is 1.5 g / m ^{2 at} a pressure of 5 Pa. Since the residual toner amount exceeds the upper limit, a good image cannot be obtained. However, if θ is changed by 60 °, the pressure applied to the toner image TI can be reduced to 0.3 Pa, and the toner adsorptivity can be reduced. As a result, the residual toner amount is reduced to 0.1 g / m ² . With this residual toner amount, the viewer cannot recognize image deterioration and a good image can be obtained.

  As described above, when the humidity is high, the amount of residual toner can be reduced by making the screen angle θ relatively close to 90 °, thereby suppressing the occurrence of transfer dropout.

(Method of suppressing transfer dropout when toner deteriorates)
As described above, there is an upper limit (for example, 1.0 g / m ² ) for the amount of toner that the viewer can recognize image deterioration. As shown in FIG. 7, in the initial state immediately after the start of use of the toner, if θ is 30 ° and the pressure of 5 Pa is applied to the toner, the residual toner amount becomes 0.2 g / m ² . In this case, since the residual toner amount is not more than the upper limit value, the image forming apparatus can provide a good image.

Here, FIG. 8 is a graph showing the amount of residual toner with respect to the pressure applied to the toner when the toner is in a durable / degraded state. When the toner is stirred inside the developing means 35, the external additive is removed. As a result, the adsorption force increases between the toner particles TP, and the residual toner amount increases. For example, if a pressure of 5 Pa is applied to the toner image TI under the condition of θ of 30 °, the residual toner amount becomes 1.5 g / m ² . Since the residual toner amount exceeds the upper limit, a good image cannot be obtained. However, when θ is 60 °, the pressure applied to the toner image TI can be reduced to 3.3 Pa, and the toner adsorptivity can be reduced. As a result, the residual toner amount is reduced to 0.1 g / m ² . With this residual toner amount, the viewer cannot recognize image deterioration and a good image can be obtained.

  As described above, when the toner is durable or deteriorated, it is possible to reduce the amount of residual toner by making θ close to 90 °, thereby suppressing the occurrence of transfer loss.

(Method of suppressing transfer dropout when initial transfer pressure is high)
As described with reference to FIG. 5, there is an upper limit (for example, 1.0 g / m ² ) for the amount of toner that the viewer can recognize image deterioration. In the initial state shortly after the start of use of the transfer roller 19, as in the case described above, when θ is 30 ° and the pressure of 5 Pa is applied to the toner, the residual toner amount becomes 1.5 g / m ² . In this case, since the residual toner amount exceeds the upper limit value, the image forming apparatus cannot provide a good image.

However, if θ is 60 °, the pressure applied to the toner image TI is reduced to 3.3 Pa, and the toner adsorptivity is reduced. As a result, the residual toner amount is reduced to 0.1 g / m ² . With this amount of residual toner, the viewer cannot recognize image deterioration and a high-quality image can be obtained.

  As described above, in the initial state of the transfer roller 19, it is possible to reduce the amount of residual toner by making θ close to 90 °, thereby suppressing the occurrence of transfer dropout.

By the way, when the transfer roller 19 is worn due to durability, the diameter thereof is reduced. As a result, the transfer pressure is reduced. When θ is set to 60 °, the residual toner amount is reduced to a pressure lower than 1.0 g / m ² (4.8 Pa in the example of FIG. 9) even if θ is 30 ° due to a decrease in transfer pressure. There is a timing. At this timing, θ is switched from 60 ° to 30 °. By switching to 30 °, pitch unevenness is suppressed, and the image forming apparatus can provide a high-quality image.

(First embodiment)
FIG. 10 is a block diagram showing a control system of the image forming apparatus according to the first embodiment of the present invention. FIG. 10 shows a humidity sensor 39 in addition to the exposure means 17, the photosensitive drum 31, and the control circuit 37. The humidity sensor 39 is typically attached in the vicinity of the intermediate transfer belt 21, and outputs a signal indicating the surrounding humidity (hereinafter referred to as humidity information) to the control circuit 37. The control circuit 37 includes a CPU, a main memory, and a program memory, and executes a program stored in the program memory using the main memory. In the present embodiment, the processing shown in the flowchart of FIG. 11 is performed to suppress the occurrence of transfer dropout at high humidity.

  In FIG. 11, when receiving a copy instruction from the operation panel of the image forming apparatus 1 or a print job from the PC, the control circuit 37 determines the print mode. Thereafter, the printing process is started in the determined printing mode. Here, the description will be made on the assumption that the monochrome printing mode has been determined.

  After the start of the printing process, the control circuit 37 acquires humidity information detected by the humidity sensor 39 (S01).

  Next, the control circuit 37 determines whether or not the humidity information acquired in S01 is equal to or greater than a predetermined reference value (for example, 60%) (S02).

  If the control circuit 37 determines that the humidity information is greater than or equal to a reference value (hereinafter referred to as first humidity), the control circuit 37 sets the screen angle θd for the Bk toner image to a value relatively close to 90 ° (for example, 60 °). Select (S03).

  On the other hand, if the control circuit 37 determines that the humidity information is less than the reference value (hereinafter referred to as second humidity), the screen angle θd for the Bk toner image is relatively close to 0 ° (for example, 30). °) is selected (S04).

  After S03 or S04, the control circuit 37 controls each unit shown in FIG. 1 to perform image formation (S05). In particular, the photosensitive drum 31d is controlled to be exposed at θd selected in S03 or S04 by adjusting the basic recording frequency of the exposure means 17 or the like.

  When the image formation is completed, the control circuit 37 waits for the next print job to be transmitted.

(Operation and effect of the first embodiment)
According to the processing described above, as described with reference to FIG. 5, in a low humidity environment such as 50%, for example, 30 °, which is relatively close to 0 °, is selected as θd. In this case, a pressure of 5 Pa is applied to the central portion C of the toner image TI, and the residual toner amount is 0.2 g / m ² . Since this value is equal to or less than the upper limit (1 g / m ² ), the image forming apparatus 1 can print a high-quality image.

On the other hand, as described with reference to FIG. 6, 60 °, which is relatively close to 90 °, is selected as θd in a high humidity environment such as 70% humidity. In this case, the pressure applied to the toner image TI is 0.3 Pa, and the toner adsorptivity is reduced. As a result, the residual toner amount is reduced to 0.1 g / m ² . With this residual toner amount, it is difficult for transfer omission to occur, and it is difficult for the viewer to recognize image degradation. As described above, the image forming apparatus 1 can appropriately adjust θd in a high-humidity environment to suppress the occurrence of transfer loss and provide a high-quality image.

(Modification)
Next, a modification of the first embodiment will be described. Since the control system according to the modification is as shown in FIG. 10, FIG. 10 is used in the following description. However, in this modified example, the control circuit 37 performs the processing shown in the flowchart of FIG. 12 to suppress the occurrence of transfer loss during high humidity.

  In FIG. 12, when receiving a print job, the control circuit 37 determines the print mode and starts the printing process. Here, description will be made on the assumption that the color printing mode has been determined.

  After starting the printing process, the control circuit 37 performs the same process as S01 and S02 of the first embodiment (S11 and S12).

If the humidity information is the first humidity equal to or higher than the reference value, the control circuit 37 selects the following angles as the screen angles θa, θb, θc, and θd for each color (S13).
θa: 90 °
θb: 80 °
θc: 70 °
θd: 60 °

In contrast, if the humidity information is the second humidity less than the reference value, the control circuit 37 selects the following angles as θa to θd (S14).
θa: 60 °
θb: 50 °
θc: 40 °
θd: 30 °

  After S13 or S14, the control circuit 37 performs the same process as S05 of the first embodiment (S15). Thereafter, the control circuit 37 waits for the next print job.

(Operation and effect of the modified example)
According to the processing described above, even in the color printing mode, in a low humidity environment, a value close to 0 ° is selected as θ for each color, so the image forming apparatus 1 prints a high-quality image. It becomes possible to do. On the other hand, in a high humidity environment, a value close to 90 ° is selected as θ for each color. Therefore, even in a high humidity environment, θ is appropriately adjusted to suppress the occurrence of transfer dropout. A quality image can be provided.

(Appendix)
In the embodiment or the modification described above, the control circuit 37 has been described as acquiring from the humidity sensor 39 provided in the image forming apparatus 1. However, the present invention is not limited to this, and the control circuit 37 may acquire humidity information by communication from a humidity sensor provided outside the image forming apparatus 1. Further, the user may input humidity information by operating the operation panel, and the control circuit 37 may acquire the humidity information.

  Further, in the above description, it has been described that a transfer omission occurs when the photosensitive drum 31 as an example of an image carrier and the intermediate transfer belt 21 as an example of a transfer medium are separated. However, transfer skipping can occur at other locations. For example, a monochrome printing printer or the like does not include an intermediate transfer belt, and transfers a toner image carried by a photosensitive drum as an example of an image carrier onto a sheet as another example of a transfer medium. Even during the photosensitive drum and the sheet, transfer loss may occur. In addition, a full color printer or the like transfers a synthetic toner image carried by an intermediate transfer belt as another example of the image carrier onto a sheet as another example of the transfer medium. The intermediate transfer belt and the sheet can also be lost in the transfer. Therefore, θ may be set in order to prevent transfer loss that may occur in these portions. In this regard, the same applies to the following second and third embodiments.

  In the above description, the control circuit 37 selects an appropriate θ based on whether the acquired humidity information is greater than or less than a reference value. However, the present invention is not limited to this, and the control circuit 37 holds, for example, a table in which θ is described for each value represented by the humidity information in the program memory. Here, in the table, θ closer to 0 ° is described as the humidity becomes lower, and θ closer to 90 ° is described as the humidity becomes higher. And the control circuit 37 reads (theta) corresponding to the acquired humidity information from a table, and uses it for an exposure process.

(Second embodiment)
FIG. 13 is a block diagram showing a control system of the image forming apparatus according to the second embodiment of the present invention. FIG. 13 shows the first counter 41 in addition to the exposure means 17, the photosensitive drum 31, and the control circuit 37. The first counter 41 counts, for example, the total driving time as the driving amount of the developing unit 35 and outputs the total driving time to the control circuit 37. The configuration of the control circuit 37 is as described above. The control circuit 37 according to the present embodiment performs the processing shown in the flowchart of FIG. 14 to suppress the occurrence of transfer omission during toner deterioration.

  In FIG. 14, the control circuit 37 determines the print mode in response to the print job, and starts the printing process. Here, the description will be made on the assumption that the monochrome printing mode has been determined.

  After starting the printing process, the control circuit 37 acquires the drive amount counted by the first counter 41 (S21).

  Next, the control circuit 37 determines whether or not the drive amount acquired in S21 is equal to or greater than a predetermined reference value (for example, 100 hours) (S22).

  If the control circuit 37 determines that the drive amount is equal to or greater than the reference value (hereinafter referred to as the first drive amount), the control circuit 37 selects a value relatively close to 90 ° (eg, 60 °) as θd (S23).

  In contrast, if the control circuit 37 determines that the drive amount is less than the reference value (hereinafter referred to as the second drive amount), the control circuit 37 selects a value relatively close to 0 ° (for example, 30 °) as θd (S24). ).

  After S23 or S24, the control circuit 37 performs the same processing as S05 of the first embodiment (S25). Thereafter, the control circuit 37 waits for the next print job.

(Operation / Effect of Second Embodiment)
In the above description, the driving amount of the developing unit 35 represents the degree of toner deterioration. When this drive amount is the second drive amount, the toner has not deteriorated, and a value close to 0 ° is selected as θd as described with reference to FIG. The apparatus 1 can print a high-quality image. On the other hand, in the case of the first drive amount, toner deterioration is progressing, and a value close to 90 ° is selected as θd, and as described with reference to FIG. Can be suppressed, and a high-quality image can be provided.

(Appendix)
In the above description, the control circuit 37 determines the degree of toner deterioration based on the driving amount of the developing unit 35. However, the determination is not limited to this, and the determination may be based on a parameter having a correlation with the degree of toner deterioration such as the value of the current flowing through the developing roller when the developing bias voltage is applied and the number of printed sheets.

  Similarly to the first embodiment, the screen angles θa to θd of the respective colors may be selected according to the degree of toner deterioration in the color printing mode.

  In addition, θd is not necessarily preferably relatively close to 90 ° when the drive amount is the first drive amount, and relatively close to 0 ° when the drive amount is the second drive amount. . In other words, depending on the characteristics of the toner, as the degree of deterioration progresses (that is, as the driving amount of the developing unit 35 increases), the attracting force becomes weaker. In such a case, contrary to the above, θd is set relatively close to 0 ° as the deterioration of the toner progresses (that is, if the drive amount is the first drive amount), and the drive amount is The second driving amount may be relatively close to 90 °.

(Third embodiment)
FIG. 15 is a block diagram showing a control system of the image forming apparatus according to the third embodiment of the present invention. FIG. 15 shows the second counter 43 in addition to the exposure means 17, the photosensitive drum 31, and the control circuit 37. The second counter 43 counts, for example, the total driving time as the driving amount of the transfer roller 19 and outputs it to the control circuit 37. The configuration of the control circuit 37 is as described above. The control circuit 37 of the present embodiment performs the processing shown in the flowchart of FIG. 16 to suppress the occurrence of transfer dropout when the initial transfer pressure is high.

  In FIG. 15, the control circuit 37 determines the print mode in response to the print job, and starts the printing process. Here, the description will be made on the assumption that the monochrome printing mode has been determined.

  After the start of the printing process, the control circuit 37 acquires the drive amount counted by the second counter 43 (S31).

  Next, the control circuit 37 determines whether or not the drive amount acquired in S31 is less than a predetermined reference value (for example, 100 hours) (S32).

  If the drive amount is less than the reference value (hereinafter referred to as the first drive amount), the control circuit 37 selects a value relatively close to 90 ° (for example, 60 °) as θd (S33).

  Conversely, if the drive amount is a value less than the reference value (hereinafter referred to as the second drive amount), the control circuit 37 selects a value relatively close to 0 ° (for example, 30 °) as θd (S34). .

  After S33 or S34, the control circuit 37 performs the same process as S05 of the first embodiment (S35). Thereafter, the control circuit 37 waits for the next print job.

(Operation and effect of the third embodiment)
In the above, the driving amount of the transfer roller 19 has a correlation with the usage period of the transfer roller 19. When this drive amount is the first drive amount, the transfer roller has an initial high transfer pressure, and a value close to 90 ° is selected as θd as described with reference to FIG. . As a result, the image forming apparatus 1 can suppress the occurrence of transfer loss due to a high initial transfer pressure, and can print a high-quality image.

(Appendix)
In the above description, the control circuit 37 determines the usage period of the transfer roller 19 based on the driving amount of the transfer roller 19. However, the determination is not limited to this, and the determination may be based on a parameter having a correlation with the usage period of the transfer roller 19.

  Similarly to the first embodiment, θa to θd may be selected in accordance with the usage period of the transfer roller 19 in the color printing mode.

  The image forming apparatus according to the present invention is less likely to cause an intermediate transfer, and is suitable for a printer, a copying machine, a facsimile, a multifunction machine having these functions, and the like.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 17 Exposure means 19 Transfer roller 21 Intermediate transfer belt 27 Secondary transfer roller 29 Image forming means 31 Photosensitive drum 33 Charging means 35 Developing means 37 Control circuit 39 Humidity sensor 41 First counter 43 Second counter

Claims (8)

An image carrier;
Charging means for charging the surface of the image carrier;
Exposure means for irradiating the surface of the image carrier with a light beam to form an electrostatic latent image;
Developing means for developing a latent image formed by the exposure means to form a toner image having a screen pattern on the surface of the image carrier;
A transfer member that is disposed opposite to the image carrier and transfers a toner image formed and conveyed on the surface of the image carrier to a transfer medium;
Control means for setting a screen angle of the screen pattern according to the degree of adsorption of the toner image to the image carrier ,
The control means includes
When the degree of adsorption of the toner image to the image carrier is the first degree, the screen angle is set to the first angle,
When the suction degree is a second degree higher than the first degree, the image angle is set to a second angle that is closer to 90 ° with respect to the toner image conveyance direction than the first angle. . An acquisition means for acquiring humidity information is further provided,
The control means includes
When the humidity information acquired by the acquisition unit represents the first humidity, the screen angle is set to the first angle,
If the humidity information obtained by the obtaining unit represents the second humidity higher than the first humidity, setting the screen angle to the second angle, the image forming apparatus according to claim 1. A first counting means for counting a value correlated with the degree of toner deterioration;
The control means includes
If the value counted by the first counting means represents a first value, the screen angle is set to the first angle,
When it represents a large second value than said first counting means has counted said first value, set the screen angle to the second angle, the image forming apparatus according to claim 1. The image forming apparatus according to claim 3 , wherein the first counting unit counts a driving time of the developing unit. The image forming apparatus according to claim 3 , wherein the first counting unit counts the number of printed sheets. A second counting means for counting a value correlated with a period of use of the transfer body,
The control means includes
If the value counted by the second counting means represents the first value, the screen angle is set to the first angle,
If the value the second counting means has counted represents the small second value than the first value, set the screen angle to the second angle, the image forming apparatus according to claim 1. An image carrier that carries on the surface a toner image composed of a plurality of screen patterns;
A transfer member disposed opposite to the image carrier;
A transfer medium that is provided between the image carrier and the transfer member and is pressed against the surface of the image carrier by the transfer member, on which the toner image on the surface of the image carrier is transferred Medium,
Control means for setting a screen angle of the screen pattern according to the degree of adsorption of the toner image to the image carrier ,
The control means includes
When the degree of adsorption of the toner image to the image carrier is the first degree, the screen angle is set to the first angle,
When the suction degree is a second degree higher than the first degree, the image angle is set to a second angle that is closer to 90 ° with respect to the toner image conveyance direction than the first angle. . The transfer medium is an intermediate transfer belt, an image forming apparatus according to any one of claims 1-7.

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