JP2019105810A - Image forming apparatus - Google Patents

Abstract

A white spot phenomenon is prevented in an image forming apparatus in which a charge removal process is omitted. An image forming apparatus includes a photosensitive drum, a charging unit, an exposure unit, a developing unit, and a transfer unit. However, a charge eliminating unit is provided between the transfer unit and the charging unit. Not done. The transfer unit 14 includes a transfer roller 141 that forms a transfer nip portion that allows the recording paper P to pass through the photosensitive drum 10, and a bias power supply unit 142 that applies a transfer voltage to the transfer roller 141. . The image forming apparatus further includes a control unit 30 that controls the transfer voltage and a prediction unit 31 that predicts the timing at which the recording paper P enters the transfer nip. The control unit 30 controls the bias power supply unit 142 at the timing when the recording paper P enters the transfer nip portion based on the prediction of the prediction unit 31 and increases the voltage value of the transfer voltage. [Selection] Figure 1

Description

  The present invention relates to an image forming apparatus using an electrophotographic method.

  In the electrophotographic image forming apparatus, first, an electrostatic latent image is formed on the photosensitive drum by electrostatic charging and exposure processes. Next, in the development process, the electrostatic latent image on the photosensitive drum is developed into a visible image by adsorbing the charged toner (development), and the toner image formed here is electrostatically transferred to the recording paper in the transfer process Do. Then, in the fixing process, the toner image transferred to the recording sheet is heated and melted, fixed to the recording sheet by pressure, and then discharged to the sheet discharge unit of the image forming apparatus.

  In such an electrophotographic image forming apparatus, a cleaning and charge removing process is usually performed between the transfer process and the charging process. That is, in the cleaning process, the transfer residual toner remaining on the surface of the photosensitive drum is directly removed from the surface of the photosensitive drum by the contact means such as a cleaning blade, and then the surface of the photosensitive drum from which the toner has been removed The electrostatic latent image (residual potential) remaining on the surface of the photosensitive drum is removed by irradiating light from a charge removing lamp or the like.

  However, some image forming apparatuses do not have a discharge lamp (for example, Patent Document 1). Such an image forming apparatus from which the discharge lamp is omitted is often employed in a monochrome machine with reduced cost.

Patent No.4939164 specification

  The charge removal in the image forming apparatus from which the charge removal process is omitted is performed using the light source of the exposure unit before the operation for forming the image on the photosensitive drum or after the completion of the image formation and when there is no recording sheet in the transfer unit. . That is, it is performed by irradiating light from the exposure unit while rotating the photosensitive drum. The photosensitive drum changes its characteristics from an insulator to a conductor by being irradiated with light, so the residual potential on the photosensitive drum is not more than a predetermined value (for example, against the electrostatic potential of -400 V or more on the photosensitive drum) Charge of −50 V or less).

  However, the static elimination method using this exposure unit can not be used because the exposure unit performs the operation of forming an electrostatic latent image during the image forming operation. For this reason, there is a problem that the following white spots occur. This is noticeable when the image contains halftones.

  In the image forming apparatus, usually, when the leading edge of the recording sheet approaches the transfer nip portion, a voltage different from the polarity of the toner is applied to the transfer portion to prepare for transferring the toner image formed on the photosensitive drum to the recording sheet. Do. Methods of applying a voltage to the transfer portion at this time include a constant voltage method of applying a predetermined voltage and a constant current method of maintaining a transfer current flowing at the time of voltage application at a predetermined value. The constant current method is generally used without being influenced by the thickness and resistance value of the recording paper, so the constant current method will be described here as an example.

  When the leading edge of the recording sheet rushes into the transfer nip portion ready for transfer, the resistance of the transfer nip portion increases by the amount of the recording sheet, so that the transfer current decreases accordingly. Since insufficient transfer current at the transfer nip causes transfer failure, the power supply that applies the transfer voltage to the transfer member raises the voltage until the transfer current returns to a predetermined value, and the constant current so that the flowing current becomes constant. Control. However, even if such constant current control is performed, there is a slight time lag between the detection of a decrease in transfer current and the return to a predetermined value, but the leading edge of the recording paper rushes into the transfer nip portion. For a while, the sheet is fed with the transfer current lowered.

  There is a phenomenon that the residual potential on the photosensitive drum changes before and after the leading edge of the recording sheet because the reduction amount of the transfer current generated when the leading edge of the recording sheet enters the transfer nip is large. Occur. That is, at the place where the leading edge of the recording sheet in the photosensitive drum rushes, the residual potential remains large as much as the transfer current is lower than the place before the recording sheet rushes (noise potential). Since the residual potential difference generated does not have a dedicated charge removal portion, it remains charged even when the electrostatic latent image is formed again in the exposed portion because it has no dedicated charge removal portion, and a uniform image of halftone or the like is developed. Occasionally, a white spot phenomenon occurs in which the density of the portion where the residual potential is high is lowered to develop.

  In particular, a margin (a void at the tip) where an image is not formed is set at the leading edge of the recording sheet, and the image writing is not performed at the void at the leading edge. The potential can not be reduced below a predetermined value. Therefore, the white spot phenomenon is likely to occur in the second and subsequent rounds.

  The present invention has been made in view of the above problems, and it is an object of the present invention to prevent the white spot phenomenon in an image forming apparatus in which the charge removal process is omitted.

  In order to solve the above problems, the present invention is an image provided with a photosensitive drum, a charging unit, an exposure unit, a developing unit, and a transfer unit, and an image not having a discharging unit between the transfer unit and the charging unit. The transfer unit includes a transfer member forming a transfer nip portion for passing a recording sheet between the photosensitive drum and the power supply portion for applying a transfer voltage to the transfer member. And a control unit for controlling the transfer voltage, and a prediction unit for predicting the timing at which the recording sheet rushes into the transfer nip unit, the control unit performing the prediction by the prediction unit. Based on the above, it is characterized in that the voltage value of the transfer voltage is increased at the timing when the recording sheet rushes into the transfer nip portion.

  According to the above configuration, the transfer voltage applied to the transfer member is increased by the prediction unit and the control unit at the timing when the leading edge of the recording sheet rushes into the transfer nip region. As a result, when the recording sheet enters the transfer nip portion, it is possible to compensate for the decrease in the transfer current accompanying the increase in the electrical resistance of the recording sheet, and to prevent the drop of the transfer current. Therefore, even in the image forming apparatus having no charge removing unit, the residual potential of the photosensitive drum can be sufficiently and uniformly removed by this transfer current control, so that the white spot phenomenon due to the drop of the transfer current is prevented. be able to.

  In the image forming apparatus, the control unit may maintain the voltage value of the increased transfer voltage until the leading end void portion of the recording sheet passes through the transfer nip portion.

  According to the above configuration, since the voltage value of the increased transfer voltage is maintained in the end void portion where the exposure for image writing is not performed, the above-mentioned blanking phenomenon can be surely prevented.

  Further, the image forming apparatus can change the setting of the width of the leading end void portion of the recording sheet in the sheet conveyance direction, and the control unit increases the transfer voltage according to the set width of the leading end void portion. Can be changed.

  According to the above configuration, even when the width of the front end void portion is changed, it is possible to make the section for increasing the transfer voltage to coincide with the section of the front end void portion of the recording sheet It can be prevented reliably.

  The image forming apparatus further includes a sheet determination unit that determines the sheet type of the recording sheet, and the control unit increases the transfer according to the sheet type of the recording sheet determined by the sheet determination unit. The voltage value of the voltage can be changed.

  According to the above configuration, for example, the voltage value is set to a large value in the case of a sheet having a large electric resistance such as thick paper, and the voltage value is set to a small value in a sheet having a small electric resistance such as thin paper. Thus, the voltage value of the transfer voltage to be increased can be set to an optimum value in accordance with the type of recording paper.

  The image forming apparatus of the present invention can sufficiently discharge the residual potential of the photosensitive drum by suppressing the reduction of the transfer current when the recording sheet enters the transfer nip portion, so that the white spots in the image forming apparatus in which the charge removal process is omitted. The effect is achieved that the phenomenon can be prevented.

FIG. 2 is a diagram showing an embodiment of the present invention, and is a diagram for explaining an image forming process in the image forming apparatus. It is a figure which shows the image formation process in the case of not performing white-out prevention control. It is a figure which shows the image formation process at the time of performing whiteout prevention control. FIG. 13 is a view showing a modified example of the transfer current waveform in the case where the whiteout prevention control is performed. FIG. 7 is a view showing a modified example of the transfer voltage waveform when the whiteout prevention control is performed.

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

First Embodiment
FIG. 1 is a view for explaining an image forming process of an image forming apparatus (hereinafter, the present apparatus) according to the first embodiment. The image forming unit of this apparatus has a charging unit 11, an exposure unit 12, a developing unit 13, a transfer unit 14, and a cleaning unit 15 disposed around the photosensitive drum 10.

  The toner image formation on the photosensitive drum 10 is performed in the following procedure. First, the charging unit 11 uniformly charges the surface of the photosensitive drum 10 to a predetermined potential. Next, the surface of the photosensitive drum 10 is exposed by the exposure unit 12 to form an electrostatic latent image on the surface, and the electrostatic latent image is developed by the developing unit 13. Thus, a toner image is formed on the surface of the photosensitive drum 10.

  The transfer unit 14 includes a transfer roller (transfer member) 141 and a bias power supply unit (power supply unit) 142. The transfer roller 141 forms a transfer nip portion with the photosensitive drum 10, and allows the recording sheet P to pass through the transfer nip portion. The bias power supply unit 142 applies, to the transfer roller 141, a transfer voltage (bias voltage) having a polarity opposite to the charge polarity of the toner.

  The recording sheet P is pulled out of the sheet feeding cassette of the apparatus and conveyed to the registration roller 20 through the sheet conveyance path, and then is sent to the transfer nip portion by the registration roller 20 according to the transfer timing of the toner image. When the recording sheet P passes through the transfer nip portion, the toner image on the surface of the photosensitive drum 10 is transferred onto the recording sheet P by the transfer voltage. The recording sheet P on which the toner image has been transferred is conveyed to a sheet discharge tray of the present apparatus after the toner image is fixed by heating and pressure in a fixing unit (not shown).

  The residual toner remaining on the photosensitive drum 10 without being transferred is removed and collected by the cleaning unit 15. The surface of the photosensitive drum 10 from which the residual toner has been removed can be subjected to the image forming process starting from the charging unit 11 again.

  The present apparatus is a monochrome machine, and is configured not to have a discharging process between the transfer process and the charging process. That is, since the present apparatus does not have a discharging means such as a discharging lamp between the transfer unit 14 and the charging unit 11, the image forming process may be performed if the image forming apparatus is powered on or not. In the state where the photosensitive drum 10 is rotated, the light from the light source of the exposure unit 12 is irradiated before performing. As a result, the electrostatic latent image after transfer onto the photosensitive drum 10, that is, the residual potential, can be reduced to a predetermined value (about -50 V) or less. The charge removal by the exposure unit 12 may be performed after the image forming operation.

  Further, in the present apparatus, the bias power supply unit 142 is controlled by the control unit 30, and constant current control is performed so that the transfer current flowing through the transfer nip is constant. Specifically, the control unit 30 monitors the magnitude of the transfer current flowing through the transfer nip portion, and feeds back the transfer voltage applied from the bias power supply unit 142 to the transfer roller 141 so that the transfer current becomes constant. Control. Furthermore, the control unit 30 performs transfer voltage control (whiteout prevention control) to prevent the whiteout phenomenon. The feature of the present invention lies in this whiteout prevention control. The control unit 30 may be part of the function of the main control unit that controls the overall operation of the present apparatus.

  First, an image forming process in the case where whiteout prevention control is not performed (in the case where only constant current control is performed) will be described with reference to FIG. 2A shows the recording sheet P on which an image is formed by this process, FIG. 2B shows the rotation signal of the photosensitive drum 10, and FIG. 2C shows the surface potential of the photosensitive drum 10 immediately after the charging process. 2 (d) is the exposure amount by the exposure unit 12, FIG. 2 (e) is the surface potential of the photosensitive drum 10 immediately after the exposure process, FIG. 2 (f) is the transfer voltage of the transfer unit 14, and FIG. FIG. 2H shows the surface current of the photosensitive drum 10 immediately after the transfer process. The horizontal axis in FIG. 2 represents the circumferential position of the photosensitive drum 10, and the photosensitive drum position in FIGS. 2B to 2H corresponds to the position of the recording paper P shown in FIG. It is described according to the position in the paper conveyance direction.

  Further, in FIG. 2, the position D1 indicates the position corresponding to the leading edge of the recording sheet P in the sheet conveyance direction, the position D2 indicates the position where the photosensitive drum 10 has just circled from the position D1, and the position D3 indicates the position The position D4 indicates the position where the photosensitive drum 10 has just rotated twice, and the position D4 indicates the position corresponding to the rear end of the recording sheet P in the sheet conveyance direction. In the example shown in FIG. 2, the recording sheet P is A4 (horizontal), and the photosensitive drum 10 has a diameter of 30 mm. That is, in this example, while the recording sheet P passes through the transfer nip portion, the photosensitive drum 10 rotates more than one turn. Further, on the recording paper P, it is assumed that a halftone image portion is formed on the entire paper except for the margins (void portions) provided on the four sides. The void portion is a region in which a range in which image formation is not possible is forcibly set in advance in the image forming apparatus.

  When the photosensitive member rotation signal (FIG. 2B) is turned on to start the rotation of the photosensitive drum 10, the surface of the photosensitive drum 10 is uniformly charged by the charging unit 11 (FIG. 2C) ). The surface potential of the photosensitive drum 10 in this case is -450V. The charging of the photosensitive drum 10 is started from a position slightly before the position D1 corresponding to the leading edge of the recording sheet P.

  In the sheet passing area (D1 to D4) of the recording sheet P, the image writing is performed by the exposure unit 12. The exposure amount in the exposure unit 12 of FIG. 2D is set such that the surface potential on the photosensitive drum 10 after exposure is such that the amount of toner necessary for halftone formation can be adhered. Further, since the void portion where the image is not written is provided at the leading end and the trailing end of the recording sheet P, the exposure starts slightly after the position D1 and ends slightly before the position D4. The surface potential (FIG. 2E) of the photosensitive drum 10 immediately after the exposure process has a lower potential in the exposed image forming area than that immediately after charging. The surface potential of the photosensitive drum 10 in this case is -200V. In the area other than the image forming area, that is, the void portions at the front end and the rear end, since the exposure is not performed, the surface potential during charging is maintained.

  At the transfer nip portion, the transfer voltage (FIG. 2 (f)) is applied to the transfer roller 141 from before the position D1, whereby a transfer current (I0) of a predetermined value (I0) is established between the transfer roller 141 and the photosensitive drum 10. FIG. 2 (g) flows. In the present embodiment, since the toner is negatively charged, the transfer voltage is a voltage of positive polarity. When the leading edge of the recording sheet P rushes into the transfer nip portion in this state, the electrical resistance in the transfer nip portion increases by the amount of the recording sheet P, and the transfer current decreases at the position D1 with the increase in the electrical resistance. However, since constant current control for maintaining the transfer current constant is performed also in the example of FIG. 2, the control unit 30 performs control to increase the transfer voltage when a decrease in the transfer current at the position D1 is detected. As a result, the transfer current is also restored to the level of the original predetermined value (I0). However, since a time lag occurs until the transfer current recovers to the predetermined value (I0), the transfer current becomes lower than the predetermined value (I0) in the section ΔD immediately after the position D1.

  The surface potential (FIG. 2 (h)) of the photosensitive drum 10 that has passed through the transfer portion is lowered by the transfer current flowing between the photosensitive drum 10 and the transfer roller 141. If the transfer current at this time has a predetermined value (I0), the surface potential (residual potential) after passing through the transfer portion also decreases to a predetermined low level reference value (V0). The low level reference value (V0) of the surface potential does not have to be completely lowered to 0 V, and in this case, a potential of -50 V or less is taken as the low level reference value (V0).

  On the other hand, as described above, in the section ΔD immediately after the position D1, the transfer current is lower than the predetermined value (I0). The surface potential (residual potential) on the photosensitive drum 10 to which the transfer current is transferred at a value lower than the predetermined value (I0) is the surface potential on the photosensitive drum 10 (residual current at the predetermined value (I0) (Potential of S1 in FIG. 2). Thus, if a transfer operation with a transfer current lower than a predetermined value (I0) occurs during the transfer operation, the surface potential of the photosensitive drum 10 after transfer can not be made uniform to the low level reference value (V0). A noise potential exceeding the low level reference value (V0) remains.

  In the present apparatus, since the charge removal process is omitted, the processes after the second round are executed on the photosensitive drum 10 while the noise potential remains. That is, in the charging process, the noise potential is added to the charging potential applied by the charging unit 11 (S2 part in FIG. 2). Further, even in the exposure process, the influence of the noise potential can not be removed, and a potential difference remains in the electrostatic latent image (portion S3 in FIG. 2).

  Therefore, a noise potential remains on the surface potential of the photosensitive drum 10 at the time of re-exposure at a position where the photosensitive drum 10 makes a full rotation from the leading edge of the recording paper P (immediately after the photosensitive drum 10 enters the second rotation). As a result, white spots occur in the image formed on the recording paper P. Although the noise potential remains at the time of exposure after the second rotation of the photosensitive drum 10, the absolute value of the surface potential is lower than that of the void portion. Therefore, the absolute value of the noise potential is attenuated by the flow of a predetermined transfer current. I will. That is, the white spots on the photosensitive drums 10 are gradually eliminated as the photosensitive drums 10 repeatedly circulate and repeat exposure and charging.

  However, when performing continuous printing, etc., since the new recording sheet P rushes into the transfer nip portion, when the position where the leading end of the new recording sheet P abuts on the photosensitive drum 10 is re-exposed, The phenomenon that the white spot phenomenon occurs again is repeated.

  When the trailing edge of the recording sheet P passes through the transfer nip portion, the electrical resistance in the transfer nip portion decreases by the amount of the recording sheet P, so the transfer current increases as the electrical resistance decreases (position D4). In this case, since the transfer current increases, the residual potential after transfer decreases, so no noise potential remains and no white spots occur. Even when the rear end of the recording sheet P passes through the transfer nip portion, the control unit 30 performs control to decrease the transfer voltage by constant current control of the transfer unit 14.

  In the inter-paper region (region where image formation is not performed) after the trailing edge of the recording paper P passes through the transfer nip, the exposure unit 12 performs maximum exposure over a period of one rotation of the photosensitive drum 10. Before the next recording sheet P is passed, the surface potential of the photosensitive drum 10 may be completely removed.

  Subsequently, an image forming process in the case of performing the whiteout prevention control, which is a feature of the present invention, will be described with reference to FIG. 3A shows the recording sheet P on which an image is formed by this process, FIG. 3B shows the rotation signal of the photosensitive drum 10, and FIG. 3C shows the surface potential of the photosensitive drum 10 immediately after the charging process. 3 (d) is the exposure amount by the exposure unit 12, FIG. 3 (e) is the surface potential of the photosensitive drum 10 immediately after the exposure process, FIG. 3 (f) is the transfer voltage of the transfer unit 14, and FIG. The transfer current of the portion 14 and the surface potential of the photosensitive drum 10 immediately after the transfer process are shown in FIG. The conditions in FIG. 3 are the same as those in FIG. 2 except that the whiteout prevention control is performed.

  When the photosensitive member rotation signal (FIG. 3B) is turned on and rotation of the photosensitive drum 10 is started, the surface of the photosensitive drum 10 is uniformly charged by the charging unit 11 (FIG. 3C) ). Here, the surface potential of the photosensitive drum 10 is -450V.

  In the sheet passing area (D1 to D4) of the recording sheet P, the image writing is performed by the exposure unit 12. In the example of FIG. 3D, the exposure amount at this time is an exposure amount for writing a halftone image. Further, since the void portion is provided at the leading end and the rear end of the recording sheet P, the exposure starts slightly after the position D1 and ends slightly before the position D4. The surface potential of the photosensitive drum 10 immediately after the exposure process (FIG. 3E) is lower in the exposed halftone area than immediately after charging, but the void portions at the front and rear ends are high immediately after charging. The potential is maintained. The surface potential of the halftone region is -200 V in this example.

  In the transfer nip portion, the transfer voltage (FIG. 3F) is applied to the transfer roller 141 from before the position D1, whereby a transfer current (I0) of a predetermined value (I0) is established between the transfer roller 141 and the photosensitive drum 10. FIG. 3 (g) flows.

  In the example of FIG. 3 in which the whiteout prevention control is performed, the transfer voltage is increased at the timing when the leading end of the recording paper P rushes into the transfer nip portion. As described above, the transfer voltage is increased at substantially the same timing as the rushing of the recording paper P into the transfer nip portion, thereby compensating for the decrease in the transfer current accompanying the increase in the electrical resistance of the recording paper and performing the whiteout prevention control. It is possible to prevent the lowering of the transfer current at the position D1 which has been caused when there is no such. That is, also immediately after the position D1, the transfer current can be maintained at the predetermined value (I0). The section in which the transfer voltage is increased is preferably set to coincide with the section of the leading end void portion of the recording sheet P.

  As described above, since the reduction of the transfer current at the position D1 can be prevented, the noise potential at the end void portion is prevented from remaining on the surface potential of the photosensitive drum 10 after transfer (FIG. 3H). it can.

  In this apparatus, although the charge removal process is omitted, since the residual potential can be sufficiently removed in the transfer process, each process of the second and subsequent rounds is executed on the photosensitive drum 10 while the noise potential remains. There is nothing to do. As a result, it is possible to prevent the white spot phenomenon caused by the noise potential of the end void portion.

  In the example of FIG. 3, the transfer voltage is lowered at the timing when the trailing edge of the recording sheet P passes through the transfer nip portion. This is to prevent the electrical resistance in the transfer nip portion from being reduced by the amount of the recording sheet when the recording sheet P passes through the transfer nip portion, and to prevent the transfer current from increasing at the position D4 with the reduction in electrical resistance. Control of Thus, the transfer current can be maintained at the predetermined value (I0) immediately after the position D4. However, the control for reducing the transfer voltage at the position D4 has nothing to do with the white spot prevention, which is the object of the present invention, and this control is not essential in the present invention.

  Incidentally, the transfer current in FIG. 3 (g) is maintained at a predetermined value (I0) without changing before and after the position D1, but this shows an ideal current waveform, and in practice These may show some variations as shown in FIGS. 4 (a) to 4 (c). The white spot prevention control according to the present invention only needs to suppress the noise potential of the end void portion to such an extent that the white spot region is not noticeable (at least as compared with the case where the white spot prevention control is not performed). It does not have to be.

  Further, in the transfer voltage in FIG. 3F, the transfer voltage is increased to a rectangular pulse shape in the area where the transfer voltage immediately after the position D1 is increased, but the present invention is not limited thereto. Alternatively, voltage waveforms as shown in FIGS. 5 (a) and 5 (b) may be used. Further, the drop of the transfer current may be suppressed by applying a voltage so as to gradually increase the voltage immediately before the position D1 at which the leading edge of the recording sheet P reaches the transfer nip portion. That is, the change of the transfer voltage may be started before the leading edge of the recording sheet P reaches the transfer nip portion.

  In the above-mentioned whiteout prevention control, it is necessary to increase the transfer voltage at the timing when the leading edge of the recording paper P rushes into the transfer nip portion. In order to perform such transfer voltage control, it is necessary to predict the timing at which the leading edge of the recording paper P rushes into the transfer nip portion, and to increase the transfer voltage according to the prediction result.

  In order to carry out the above-mentioned timing control, the present apparatus is provided with a sheet sensor 21 for detecting the leading edge of the recording sheet P to be conveyed on the downstream side of the registration roller 20 in the sheet conveyance direction and on the upstream side of the transfer nip portion. There is. Although the type of the paper sensor 21 is not particularly limited, it is desirable to use optically detected ones with less erroneous detection.

  Furthermore, the present apparatus has a prediction unit 31 that receives the detection signal from the paper sensor 21 and predicts the timing when the leading edge of the recording paper P rushes into the transfer nip portion. The prediction unit 31 may be part of the function of the control unit 30.

  The conveying speed of the recording sheet P by the registration roller 20 is a constant predetermined speed, and the distance from the sheet sensor 21 to the transfer nip portion is a fixed value. Therefore, the prediction unit 31 can predict that the leading end of the recording sheet P rushes into the transfer nip portion with the lapse of a predetermined time after the sheet sensor 21 detects the leading end of the recording sheet P. The control unit 30 controls the bias power supply unit 142 to increase the transfer voltage at the prediction timing of the prediction unit 31.

  Further, the prediction unit 31 may predict not the detection signal from the sheet sensor 21 but the drive start signal of the registration roller 20 to predict the timing when the leading end of the recording sheet P rushes into the transfer nip portion. In this case, the prediction unit 31 can predict that the leading end of the recording sheet P rushes into the transfer nip portion with a lapse of a predetermined time after the registration roller 20 starts conveying the recording sheet P.

Second Embodiment
In the second embodiment, a specific example in the case of changing the increasing section of the transfer voltage will be described.

  In the present apparatus, when the width (length) of the leading end void portion of the recording sheet P in the sheet conveyance direction can be changed, it is preferable to change the increase section of the transfer voltage accordingly. In this case, the increase section of the transfer voltage can be controlled as follows.

  First, the width of the leading end void portion of the recording sheet P in the sheet conveyance direction can be changed by the user, and the set value is set to w1 (mm). Further, the conveyance speed of the recording sheet P is v1 (mm / s). In this case, the time t1 (s) required for the leading end void portion of the recording paper P to pass through the transfer nip portion is t1 = w1 / v1.

  The control unit 30 may maintain the increased transfer voltage until time t1 (s) elapses after the transfer voltage is increased according to the prediction timing by the prediction unit 31. That is, when the width of the end void portion is changed, the application time of the transfer voltage to be increased may be changed accordingly. As a result, even when the width of the leading end void portion is changed, it is possible to make the section in which the transfer voltage is increased coincide with the section of the leading end void portion of the recording paper P.

Third Embodiment
In the third embodiment, when increasing the transfer voltage, a specific example of changing the voltage value of the transfer voltage to be increased will be described.

  The increase in the transfer voltage performed in the whiteout prevention control compensates for the decrease in the transfer current accompanying the increase in the electrical resistance of the recording sheet when the leading edge of the recording sheet P rushes into the transfer nip portion. It is desirable that the voltage value of the transfer voltage to be increased at this time be changed to an optimal value according to the sheet type of the recording sheet P. For example, when the recording sheet P is a thick sheet, the electric resistance is larger than that of the thin sheet, and therefore, it is desirable to set the voltage value of the transfer voltage to be increased.

  Specifically, the present apparatus includes a sheet determination unit that determines the sheet type of the recording sheet P, and the control unit 30 determines the voltage value of the transfer voltage to be increased in the whiteout prevention control by the sheet determination unit. Change according to the sheet type of the recording sheet P. That is, in the case of a sheet having a large electric resistance such as thick paper, the voltage value may be set to a large value, and in the case of a sheet having a small electric resistance such as thin paper, the voltage value may be set to a small value. The sheet determination unit can cause the control unit 30 to share the function. In this case, the sheet type of the recording sheet P stored in advance is set in each of the sheet feeding cassettes storing the recording sheet P, and the control unit 30 receives the setting information and inputs the sheet of the recording sheet P. The type can be determined.

  The white spot prevention control performed in the present apparatus can prevent the white spot phenomenon caused by the noise potential of the end void portion. On the other hand, this white spot phenomenon is noticeable when it occurs in the halftone region, but it is not so noticeable in the character region, the black solid region, and the like. Therefore, the present apparatus has an image determination unit that determines the presence or absence of a halftone area in a print image, performs whiteout prevention control only when printing an image having a halftone area, and does not have an halftone area. The whiteout prevention control may not be performed at the time of printing.

  The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims, and embodiments obtained by appropriately combining the technical means disclosed in the different embodiments. Is also included in the technical scope of the present invention.

DESCRIPTION OF SYMBOLS 10 Photosensitive drum 11 Charge part 12 Exposure part 13 Development part 14 Transfer part 141 Transfer roller (transfer member)
142 Bias power supply (power supply)
15 cleaning unit 20 registration roller 21 sheet sensor 30 control unit 31 prediction unit

Claims (4)

An image forming apparatus comprising a photosensitive drum, a charging unit, an exposure unit, a developing unit, and a transfer unit, and having no discharging unit between the transfer unit and the charging unit.
The transfer unit has a transfer member that forms a transfer nip portion for passing a recording sheet between the photosensitive drum and the photosensitive drum, and a power supply unit that applies a transfer voltage to the transfer member.
Furthermore, a control unit that controls the transfer voltage;
And a prediction unit that predicts the timing at which the recording sheet rushes into the transfer nip portion.
The image forming apparatus, wherein the control unit increases a voltage value of the transfer voltage at a timing when the recording sheet rushes into the transfer nip portion based on the prediction of the prediction unit. The image forming apparatus according to claim 1,
The image forming apparatus, wherein the control unit maintains the increased transfer voltage value until the leading end void portion of the recording sheet passes through the transfer nip portion. An image forming apparatus according to claim 1 or 2,
The width of the leading end void portion of the recording sheet in the sheet conveyance direction can be set and changed.
The image forming apparatus, wherein the control unit changes the application time of the transfer voltage to be increased according to the set width of the end void portion. The image forming apparatus according to any one of claims 1 to 3, wherein
A sheet determination unit that determines the sheet type of the recording sheet;
The image forming apparatus, wherein the control unit changes the voltage value of the transfer voltage to be increased according to the sheet type of the recording sheet determined by the sheet determination unit.

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