JP2014119465A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a reduction in transfer efficiency without the use of a loop sensor and an increase in costs to output good images, even when a peripheral velocity of a pressure roller is changed due to thermal expansion of the pressure roller at a fixing unit.SOLUTION: An image forming apparatus changes control of a transfer power source 5a by transfer control means 5b according to the number of recording materials P to be passed through fixing means 12 and a paper feed history determined from an elapsed time from the completion of the previous image formation.

Description

  The present invention relates to an electrophotographic image forming apparatus such as a copying machine, a laser beam printer, and a facsimile.

  In an image forming apparatus such as a copying machine using an electrophotographic system, an electrostatic latent image formed on an image carrier such as a photosensitive drum is visualized as a toner image by attaching toner with a developing device. A predetermined transfer voltage is applied to the transfer roller at the transfer position (transfer nip portion) formed by the photosensitive drum and the transfer roller, and a toner image is formed on the recording material supplied from the paper feed portion to the transfer nip portion. Transcription. Thereafter, the recording material conveyed to the transfer nip is separated from the photosensitive drum by the curvature, and the toner image on the recording material is fixed by the fixing unit of the fixing device formed by the heating body and the pressure body.

  Here, in a fixing device that uses a roller having an elastic layer, such as a pressure roller, as a pressure member that forms a fixing unit, and the recording material is nipped and conveyed by a fixing portion (fixing nip portion), the heat of the fixing device As a result, the rubber layer, which is the elastic layer of the pressure roller, expands and the roller diameter changes. As the roller diameter changes, the peripheral speed of the pressure roller changes, and the recording material conveyance speed at the fixing nip changes. When the peripheral speed of the pressure roller changes depending on the warming condition of the fixing nip and differs from the peripheral speed of the photosensitive drum or transfer roller in the transfer nip, the distance between the transfer nip and the fixing nip is shorter than the recording material. In this case, the posture of the recording material after transfer varies depending on the conveyance speed of the recording material at the fixing nip portion after the recording material reaches the fixing nip portion. Depending on the orientation of the recording material, the separation point of the recording material from the photosensitive drum changes, and the contact width between the photosensitive drum and the recording material differs. As a result, even with the same transfer voltage, the current value necessary for transfer is insufficient, or the current is excessively supplied, and the transferability of the toner image to the recording material is lowered, so that the optimum transfer efficiency cannot be obtained.

  On the other hand, according to Patent Document 1, a loop sensor is provided between the fixing nip portion and the transfer nip portion, detects the loop amount after the transfer nip, controls the peripheral speed of the roller at the fixing nip portion, and records the recording material. By maintaining a predetermined amount of the loop, the posture of the recording material after transfer was made constant and high transfer efficiency was maintained.

JP 2001-97602 A

  However, in recent years, in order to respond to usage environments and user needs, development of image forming apparatuses requires cost reduction and downsizing. Therefore, the peripheral speed of the roller at the fixing unit and the transfer unit can be sufficiently controlled by reducing the loop sensor by reducing the space in the device and the number of parts, and by using a common drive source for the fixing unit and the transfer unit. The problem was difficult.

  Accordingly, an object of the present invention is to prevent a decrease in transfer efficiency without increasing the cost without using a loop sensor even when the peripheral speed of the fixing unit changes due to thermal expansion of the fixing unit in the fixing unit. An object of the present invention is to provide an image forming apparatus capable of outputting a good image.

    The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention provides an image carrier that carries a toner image, a transfer unit that transfers a toner image on the image carrier to a recording material at a transfer position, and a transfer power source that applies a voltage to the transfer unit. Transfer control means for controlling the transfer power supply, and fixing means for nipping and conveying the toner image transferred to the recording material at a fixing nip portion and fixing the toner image on the recording material, the transfer position The distance between the fixing nip and the fixing nip is shorter than the length in the conveying direction of a recording material of a predetermined size among the recording materials that can be conveyed, and the image carrier and the fixing means are connected to the same drive source. In the image forming apparatus driven by the transfer control unit, the transfer control unit performs the transfer according to the sheet passing history obtained from the number of sheets of the recording material passed through the fixing unit and the elapsed time from the end of the previous image formation. Characterized by changing power supply control An image forming apparatus.

  According to the present invention, even when the peripheral speed of the fixing unit is changed due to thermal expansion of the fixing unit in the fixing unit, a decrease in transfer efficiency is prevented without using a loop sensor and without increasing costs. Can output a simple image.

1 is a schematic cross-sectional view of an embodiment of an image forming apparatus according to the present invention. It is a figure which shows the change of the temperature of a pressure roller, and an outer diameter. FIG. 3 is a schematic cross-sectional view illustrating a state of a recording material at a transfer nip portion and a fixing nip portion. FIG. 3 is a schematic cross-sectional view illustrating the posture of a recording material at a transfer nip portion. FIG. 6 is a diagram illustrating a relationship between transfer efficiency and transfer voltage depending on a posture of a recording material at a transfer nip portion. It is a figure which shows the relationship between the circumferential speed difference with respect to the photosensitive drum of the number of paper passing and a pressure roller. It is a figure which shows the relationship between the sheet passing number in a prior art example, and transfer efficiency. 6 is a flowchart for explaining counting of the number of sheets to be passed in a printing operation. FIG. 6 is a diagram illustrating a relationship between a sheet passing sheet count and a transfer voltage correction value according to the first exemplary embodiment of the present invention. It is a figure which shows the timing chart which correct | amends the transfer voltage which concerns on Example 1 of this invention. It is the figure which compared the effect of the transfer efficiency which concerns on Example 1 of this invention with the prior art example. It is a figure which shows the relationship between the contact width of a photosensitive drum and a recording material, and time. It is a figure which shows the timing chart which correct | amends the transfer voltage which concerns on Example 2 of this invention. It is the figure which compared the effect of the transfer efficiency in the recording material which concerns on Example 2 of this invention with the prior art example.

  The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

[1. Image forming apparatus M]
FIG. 1 shows an embodiment of an image forming apparatus M according to the present invention. In this embodiment, the image forming apparatus according to the present invention is a laser beam printer.

  First, a schematic configuration of an image forming apparatus that is a laser beam printer will be described with reference to FIG.

  The image forming apparatus shown in FIG. 1 includes a drum-type electrophotographic photosensitive member (hereinafter referred to as “photosensitive drum”) 1 as an image carrier. The photosensitive drum 1 is configured by providing a photosensitive material such as OPC (organic optical semiconductor), amorphous selenium, or amorphous silicon on a drum base on a cylinder formed of aluminum or nickel. The photosensitive drum 1 is rotatably supported and is driven to rotate at a predetermined process speed in the direction of arrow R1 by a drive source m1 which is an image carrier driving means. Here, the driving source m1 is also a fixing unit driving unit that drives a pressure roller 12b that is a pressure rotating body of the fixing unit 12 described later.

  Around the photosensitive drum 1, a charging device 2, an exposure unit 3, a developing device 4, a transfer device 5, and a cleaning device 6 are arranged in this order along the rotation direction.

  A paper feed cassette 7 that stores a recording material P such as paper is disposed below the image forming apparatus M. Further, in the order along the conveyance path of the recording material P, the sheet feeding roller 8, the conveyance roller 9, the top sensor 10, the conveyance guide 11, the fixing device 12, the sheet discharge sensor 13, the sheet conveyance roller 14, the sheet discharge roller 15, and the sheet discharge. A tray 16 is arranged.

  Next, the operation of the image forming apparatus M configured as described above will be described.

  The photosensitive drum 1 rotated and driven in the direction of arrow R1 by the drive source m1 is uniformly charged on the surface of the photosensitive drum 1 to a predetermined polarity and a predetermined potential by a charging device 2 serving as a charging roller. The photosensitive drum 1 after charging is subjected to image exposure L based on the image information by the exposure means 3 such as a laser optical system on the surface, and the electric charge of the exposed portion is removed to form an electrostatic latent image. The formed electrostatic latent image is developed by the developing device 4. The developing device 4 includes a developing roller 4a and a toner storage portion 4b, and a developing bias is applied to the developing roller 4a so that the toner adheres to the electrostatic latent image on the photosensitive drum 1 (on the image carrier). To develop (visualize).

  The toner image is transferred to the recording material P such as paper by the transfer device 5 at the transfer position Nt. In the present embodiment, although not limited, the transfer device 5 is a transfer roller. The transfer roller 5 is pressed against the photosensitive drum 1 by a transfer pressure spring (not shown) so as to be driven and rotated by the photosensitive drum 1, and forms a transfer nip portion (transfer position) Nt between the transfer roller 5 and the photosensitive drum 1. The recording material P is stored in a paper feed cassette 7, fed one by one by a paper feed roller 8, transported by a transport roller 9, and between the photosensitive drum 1 and the transfer roller 5 via a top sensor 10. It is conveyed to the transfer nip portion Nt. At this time, the leading edge of the recording material P is detected by the top sensor 10 and is synchronized with the toner image on the photosensitive drum 1. A transfer voltage having a polarity opposite to the charging polarity of the toner is applied to the transfer roller 5 from the transfer power source 5a, and the toner image on the photosensitive drum 1 is transferred to a predetermined position on the recording material P.

  The transfer voltage applied from the transfer power source 5a is determined by ATVC (Active Transfer Voltage Control) control by the transfer control means 5b. Here, the ATVC control will be briefly described. This is because a voltage is applied from a transfer power source connected to the transfer roller 5 during rotation before printing, the voltage value is controlled so that a predetermined current flows to the photosensitive drum 1 after charging, and the target current value is reached. The voltage value at this stage is recorded as V0. This is a control method for determining the transfer voltage value Vt during the actual image forming operation based on this value. As a result, it is possible to determine a transfer voltage value suitable for variations in resistance value of the transfer roller and resistance variations.

  The recording material P carrying the unfixed toner image on the surface by the transfer is transported to the fixing device 12 along the transport guide 11, where the unfixed toner image is heated and pressurized and fixed on the surface of the recording material P. .

  The fixing device 12 in this embodiment is a pressure roller driving type fixing device 12 using a flexible endless belt as a fixing film 12a. The fixing device 12 includes a fixing film 12a, a pressure roller 12b, a ceramic heater (hereinafter referred to as “heater”) 12c, and a heater holder 12d as main components. The fixing film 12a is a film-like fixing rotator, and is in contact with a pressure roller 12b as a pressure rotator in contact with the fixing film 12a. A heater 12c that heats the toner through the fixing film 12a is supported by a heater holder 12d that is a heater support member.

  The pressure roller 12b is provided with a heat-resistant elastic layer having elasticity such as silicone rubber on the outer peripheral surface of a metal core, and the outermost layer is made of a material having high releasability such as fluororesin. A layer is provided. The pressure roller 12b presses the fixing film 12a against the heater 12c from below by the outer peripheral surface of the release layer with a pressure spring (not shown) to form a fixing nip portion Nf between the pressure roller 12b and the fixing film 12a. When the pressure roller 12b is rotationally driven by the drive source m1, a rotational force acts on the fixing film 12a due to the pressure frictional force of the fixing nip portion Nf between the pressure roller 12b and the fixing film 12a. As a result, the fixing film 12a is driven to rotate in the direction of the arrow R12a while the inner surface of the fixing film 12a slides in close contact with the downward surface of the heater 12c.

  Fixing of unfixed toner on the surface of the recording material P (on the recording material) at the fixing nip portion Nf is performed as follows.

  The pressure roller 12b is rotationally driven, and the fixing film 12a is driven and rotated accordingly. Further, electric power is supplied to the heater 12c, and the heater 12a is heated up to a predetermined temperature to be in a temperature-controlled state. In this state, the recording material P carrying an unfixed toner image is introduced between the fixing film 12a and the pressure roller 12b in the fixing nip portion Nf. In the fixing nip portion Nf, the toner image carrying surface side of the recording material P is in close contact with the outer surface of the fixing film 12a that is in contact with the pressure roller 12b, and the fixing nip portion Nf is nipped and conveyed together with the fixing film 12a. . In this nipping and conveying process, the heat of the heater 12c is applied to the recording material P through the fixing film 12a, and the unfixed toner image on the recording material P is heated and pressurized on the recording material P and melted and fixed. The recording material P that has passed through the fixing nip portion Nf is separated from the fixing film 12a by curvature.

  The recording material P after the toner image is fixed is conveyed by a conveying roller 14 and discharged by a paper discharge roller 15 onto a paper discharge tray 16 on the upper surface of the image forming apparatus.

  On the other hand, after the toner image has been transferred, the toner (transfer residual toner) that has not been transferred to the recording material P and remains on the surface is removed by the cleaning blade 6a of the cleaning device 6 and used for the next image formation.

  By repeating the above operation, image formation can be performed one after another.

[2. Recording material posture after transfer]
Next, the relationship between the surface temperature of the pressure roller and the change in posture of the recording material P after the toner image transfer at the transfer nip portion Nt and the transfer efficiency will be described with reference to FIGS.

  In the image forming apparatus M to which the present invention is applied, the photosensitive drum 1 and the pressure roller 12b are rotationally driven by a common drive source m1, and the peripheral speed of the pressure roller 12b is adjusted by the drive transmission member (not shown). It is set to be 100% with respect to the peripheral speed.

  Here, the elastic layer of the pressure roller 12b that transports the recording material P by the fixing device 12 causes thermal expansion according to the warming condition of the fixing device 12, and the outer diameter of the pressure roller 12b changes. FIG. 2 shows the relationship between the surface temperature t (° C.) of the pressure roller 12b used in this embodiment and the outer diameter of the pressure roller. In this embodiment, a pressure roller 12b having an outer diameter of 18.00 mm at 25 ° C. is used. According to FIG. 2, the outer diameter increases as the surface temperature t of the pressure roller 12b increases. Therefore, the peripheral speed of the pressure roller 12b that is linearly related to the outer diameter of the pressure roller 12b also increases.

  FIG. 3 is a diagram illustrating a conveyance state Nt−Nf between the transfer nip portion Nt and the fixing nip portion Nf of the recording material P in the image forming apparatus M to which the present invention is applied. 4 is a view showing the posture of the recording material P after the transfer nip Nt and the contact width W between the photosensitive drum 1 and the recording material P, and FIG. 5 is a contact width W (Wa, Wb) shown in FIG. It is a figure which shows the relationship between another transfer voltage and transfer efficiency.

  Here, the transfer efficiency is defined by Equation 1 below.

  In the image forming apparatus of the present invention, the distance between the transfer position (transfer nip portion) Nt and the fixing nip portion Nf is shorter than the length of the recording material P in the transport direction. Here, the recording material P is a recording material of a predetermined size among the recording materials that can be conveyed to the apparatus. For a recording material smaller than the recording material of the predetermined size, the distance is relatively long. Become.

  Therefore, when the printing is started from the cold state in which the fixing device 12 is sufficiently cooled, the recording material P during the transfer fixing Nt-Nf is conveyed with a loop as shown in FIG. At this time, the posture of the recording material P after passing through the transfer nip Nt is as shown in A of FIG. 4 and contacts the photosensitive drum 1 with a relatively narrow contact width Wa. FIG. 5A shows the relationship between the transfer voltage and the transfer efficiency for the recording material P in this state. Let Va be the transfer voltage value at which the transfer efficiency is highest. Here, when the transfer efficiency is lower than 80%, the toner image on the photosensitive drum 1 is not sufficiently transferred to the recording material P, and density unevenness and line missing occur. For this reason, when the transfer efficiency is lower than 80%, the transfer efficiency is regarded as a defective level, and when the transfer efficiency is higher than that, the transfer voltage is set at a good level in the range of VaL to VaH. Become. When the transfer voltage is lower than VaL, a phenomenon of transfer omission in which the current value necessary for transfer is insufficient occurs, and the transfer efficiency becomes a defective level. On the other hand, when the transfer voltage is higher than VaH, an excessive transfer current flows to the photosensitive drum 1, and the phenomenon of retransfer in which the polarity of the toner is reversed and the toner is not transferred occurs, and the transfer efficiency becomes a defective level. Thus, in the state where the recording material P is in contact with the photosensitive drum 1 with the contact width Wa, good transfer efficiency can be obtained in the range of the transfer voltage from VaL to VaH.

  However, in a hot state in which the fixing device 12 is warm, such as when printing is started immediately after the paper is passed or immediately after the paper is passed, the elastic layer of the pressure roller 12b is thermally expanded, and the outer diameter of the pressure roller 12b is cold. It becomes larger than the outer diameter of the state, and the peripheral speed of the pressure roller 12b increases. When the peripheral speed of the pressure roller 12b increases and exceeds the peripheral speed of the photosensitive drum 1, the recording material P after transfer reaches the fixing device 12 and then expands and is pulled by the pressure roller 12b whose peripheral speed is increased. Will be. Thereby, the posture of the recording material P during the transfer and fixing is in a state of being stretched between the pressure roller 12b and the photosensitive drum 1 as shown in FIG. At this time, the posture of the recording material P after passing through the transfer nip Nt is wound around the photosensitive drum 1 as shown in FIG. 4B, and the photosensitive drum has a contact width Wb wider than the contact width Wa in the cold state. Contact 1 As the contact width W between the recording material P and the photosensitive drum 1 increases, the time during which the recording material P is in contact with the photosensitive drum 1 increases even after passing through the transfer nip Nt. As a result, when the same transfer voltage Va as in the case of the contact width Wa is applied, an excessive transfer current flows to the photosensitive drum 1, retransfer occurs, and transfer efficiency decreases. Therefore, when the recording material P is in contact with the photosensitive drum 1 with the contact width Wb, the transfer efficiency becomes maximum when the transfer voltage is Vb lower than Va as shown in FIG. 5B, and the range is from VbL to VbH. Good transfer efficiency can be obtained. At this time, since the value of VbH is lower than VaL, when the contact width W between the recording material P and the photosensitive drum 1 changes from Wa to Wb, there is no transfer voltage value compatible with both, and the transfer efficiency is poor. Occur.

  As described above, the peripheral speed of the pressure roller 12b is increased due to the warming condition of the fixing device 12, and the contact width W between the recording material P and the photosensitive drum 1 is changed, thereby obtaining good transfer efficiency. The transfer voltage value is different and the transfer efficiency is lowered.

[3. Pressure roller outer diameter and transfer efficiency depending on the number of sheets passed]
Next, changes in the outer diameter of the pressure roller and the transfer efficiency depending on the number of sheets passed will be described with reference to FIGS. The structure of the transfer unit 12 is that the pressure roller hardness is 50 degrees (when Asker C is 1 kg load), the fixing device 12 is 14 kgf of spring force of the pressure spring, the transfer roller hardness is 30 degrees (when Asker C is 1 kg load), and transfer pressure is applied. The spring is 600 g. In this transfer unit 12, when the conveyance speed is set to 150 mm / sec and the temperature of the heater 12 c is set to 200 ° C., the ratio of the number of continuous paper passing and the peripheral speed ratio of the pressure roller outer diameter to the photosensitive drum 1 This is shown in FIG. FIG. 7 shows the relationship between the number of sheets passing and the transfer efficiency. The transfer efficiency is 25% Cotton Content 90g paper, and the transfer efficiency when five strips of 3cm width are printed at regular intervals in the recording material passing direction under a high temperature and high humidity environment of 32.5 ° C / 80%. Was averaged over the printing range. Here, if the transfer efficiency is lower than 80%, the toner image on the photosensitive drum 1 is not sufficiently transferred to the recording material P and density unevenness and line missing occur. A defective transfer efficiency level is set, and when the transfer efficiency is higher, a good level is set. In FIG. 6, for example, when the number of sheets to be passed is 5, the peripheral speed of the pressure roller 12b is slightly increased to 101% with respect to the photosensitive drum 1, whereas the number of sheets to be passed is increased to 70 sheets. It has increased to 105%. This is because the amount of heat stored in the elastic layer of the pressure roller 12b increases as the number of sheets passing increases, and the outer diameter of the pressure roller 12b increases accordingly. As a result, the peripheral speed of the pressure roller 12b exceeds the peripheral speed of the photosensitive drum 1, the posture of the recording material P after transfer approaches the photosensitive drum 1, and the contact width W between the photosensitive drum 1 and the recording material P increases. To do. Therefore, as shown in FIG. 7, the transfer efficiency also decreases according to the number of sheets to be passed.

  In this example, when the number of sheets passed was 70 or more, the heat storage amount was saturated and the peripheral speed ratio was constant.

[Example 1]
Example 1 will be described with reference to FIGS. In this embodiment, the degree of warming of the pressure roller 12b is estimated using the sheet passing count, and the transfer is performed according to the change in the posture of the recording material due to the change in the peripheral speed of the pressure roller 12b at the fixing nip portion Nf. A control means 100 (see FIG. 1) for changing the transfer control of the power source 5a is provided. As will be described in detail later, the control unit 100 controls the transfer control unit 5b based on the number of sheets of the recording material P to be passed and the sheet passing history obtained from the elapsed time since the end of the previous printing. . As a result, optimal transfer control according to the contact width W between the photosensitive drum 1 and the recording material P is performed, and a decrease in transfer efficiency is prevented.

  First, the sheet passing count used for estimating the temperature of the pressure roller 12b will be described. The sheet passing count, which is the sheet passing history used in this embodiment, is incremented by one according to the number of sheets passed by the control means 100 during the printing operation, and is calculated from the following equation after a series of printings. The count is subtracted according to the elapsed time from the end of printing. In a normal temperature environment, the temperature of the pressure roller 12b returns to the same temperature as the temperature at the start of printing after a predetermined time, in this embodiment, for example, 6 minutes. Further, as described above, the temperature rise of the pressure roller is saturated at the continuous sheet passing number of 70 sheets, and the peripheral speed between the pressure roller and the photosensitive drum is saturated.

  From the above, if Tn is the time that elapses until the count is decremented by 1 from the count number Cnp at the end of printing, the following equation holds.

Tn = 360/70 (seconds)
ΔCn = T / Tn (Expression 2)
Here, T is the elapsed time from the end of the previous printing to the resumption of printing (resumption of paper passing), and ΔCn is the count number that decreases with the elapsed time.

  The relationship between the elapsed time T after the end of printing and the count ΔCn expressed by Equation 2 is the same as the value of the surface temperature of the pressure roller 12b when the sheet passing is resumed after a certain time has elapsed and during continuous printing showing the same temperature. Obtained experimentally from the count.

  By using this equation, the control unit 100 can obtain the sheet passing number count Cn at the time of resuming the sheet passing, based on the elapsed time T and the counter Cnp at the end of the previous printing.

Cn = Cnp−ΔCn (Formula 3)
For example, as described above, Tn = 360 / 70≈5 seconds, and after the image forming apparatus is started and 100 sheets are continuously printed, 5 minutes (T) have elapsed (elapsed time T = 300 seconds). Is ΔCn = 60 from Equation 2. Therefore, the sheet passing number count Cn value at the start of restarting when printing is resumed, that is, when sheet feeding is resumed, is obtained by Equation 3. However, even if the number of continuous prints exceeds 70 sheets (100 sheets in this example), the heat rise is only 70 sheets (FIG. 6).

Therefore,
Cnp = 100⇒Cnp = 70
Cn = Cnp−ΔCn = 70− (300/5) = 10
When T has passed 6 minutes, the heat rise is eliminated, so Cn is set to zero.

  The sheet passing count will be further described with reference to the flowchart of FIG.

  For example, the image forming apparatus is activated for the first time so as to be the first in the morning and the printing operation is started. In the control unit 100, the count Cnp = 0 at the end of printing stored in the storage unit is set. Then, a printing operation is performed (step 1). The number of prints is incremented (incremented) for each print. At the same time, the control unit 100 acts on the transfer control unit 5b to control the transfer power source 5a (step 2). It is determined whether or not the printing is finished (step 3), and when the printing operation is continued, the process returns to step 2. When a predetermined number of printing operations have been completed, the image forming apparatus enters a standby state, and the storage unit stores a count Cnp at the end of printing. At the same time, measurement of the elapsed time T after the end of printing is started by the control means 100 (step 4). Next, it is determined whether the elapsed time T is longer than 360 seconds (step 5).

  In step 5, it is determined whether printing is resumed when the elapsed time T is 360 seconds or less (step 6). If printing is not resumed in step 6, the Cnp value is returned to 0 and the operation is terminated. In the case of resuming printing, a sheet passing count Cn at the time of resuming printing is calculated (step 7). In step 8, a printing operation is started, transfer control is performed based on Cn calculated in step 7, the Cn value is read as a Cnp value, and the process returns to step 2.

  If the elapsed time T is longer than 360 seconds in step 5 and printing is resumed, the process returns to step 1. If printing is not resumed, the Cnp value is returned to 0 and the operation is terminated.

As described above, in order to prevent a decrease in transfer efficiency, the value of the transfer voltage after the recording material P reaches the fixing device 12 is corrected according to the number counter (steps 2 and 8). Here, the correction values shown in FIG. 9 were obtained as follows. In this embodiment, when the above conditions are studied, as shown in FIG. 6, when the sheet passing count reaches a hot state of 70 or more, the peripheral speed ratio of the pressure roller 12b to the photosensitive drum 1 is saturated at 105%. . Further, the difference ΔV between the transfer voltage value at which the transfer efficiency becomes maximum in the cold state where the sheet passing number count Cn is 0 and the transfer voltage value at which the transfer efficiency becomes maximum in the hot state where the sheet passing number count Cn is 70 is 200V. Met. Therefore, the maximum value of the correction value is set to −200 V, and the correction value is determined for each sheet passing sheet count in accordance with the relationship between the sheet passing number and the peripheral speed ratio in FIG.

  The transfer voltage Vt obtained by ATVC is corrected according to the correction value of FIG. FIG. 10 shows a timing chart for performing transfer voltage correction. This correction is performed when the leading edge of the recording material P reaches the fixing nip Nf after passing through the transfer nip Nt. The timing is calculated from the distance between the process speed and the transfer fixing Nt−Nf by detecting the leading edge position of the recording material P by the top sensor 10. In FIG. 9, for example, when the sheet passing count Cn is 5, since the heat storage amount of the pressure roller 12b is small and the outer diameter change slightly increases, -50V is set as a correction voltage with respect to Vt obtained by ATVC. To do. When the sheet passing count Cn is 70, the heat storage amount of the pressure roller 12b is large and the change in the outer diameter is large, so −200 V is set as the correction voltage. In this way, according to the correction voltage value set according to the Cn value of each sheet passing count, the transfer voltage Vt obtained by ATVC is corrected when the leading edge of the recording material P reaches the fixing nip Nf.

  As described above, in order to examine the effect of this embodiment for correcting the target voltage value of the transfer voltage according to the Cn value of the sheet passing count, the conveyance speed is set to 150 mm / sec and the heater temperature is set to 200 ° C. A comparative experiment was conducted with respect to the relationship between the conventional example in which the transfer voltage was not corrected and the sheet passing count and the transfer efficiency.

  The results are shown in FIG. Here, a transfer efficiency of 80% or more is a good level, and a transfer efficiency lower than that is a defective level.

  First, in the conventional example in which the fixing voltage Vt is not corrected, the amount of heat stored in the pressure roller 12b increases and the outer diameter of the pressure roller 12b increases as the sheet passing count value Cn advances. The separation speed of the recording material P from the photosensitive drum 1 after the recording material P reaches the fixing nip portion Nf differs from the peripheral speed of the roller 12b. For this reason, the contact width W between the photosensitive drum 1 and the recording material P is different, the optimum transfer voltage value deviates from the value of Vt obtained from ATVC, and excessive current flows to the photosensitive drum 1, thereby reducing transfer efficiency. A defect level has occurred.

  On the other hand, in this embodiment, since the transfer voltage is corrected based on the sheet passing number count value Cn correlated with the change in the outer diameter of the pressure roller 12b, for example, when the sheet passing sheet count Cn value is 5. When the amount of heat stored in the pressure roller 12b is relatively small and the change in outer diameter is small, the correction value is set to -50V. Further, when the heat storage amount of the pressure roller 12b is large and the change in the outer diameter is large as in the case where the Cn value of the sheet passing count is 70, the correction value is set to −200V. Thus, the transfer voltage was appropriately supplied to the recording material P without excessive current flowing, and good transfer efficiency could be obtained.

  As described above, in this embodiment, the correction value of the transfer voltage is set according to the Cn value of the sheet passing number count, so that the number of passing sheets increases and the outer diameter of the pressure roller 12b changes. However, an appropriate transfer voltage can be set, and good transfer efficiency can be obtained.

  Note that the correction value of the transfer voltage corresponding to the Cn value of the sheet passing number count is as shown in FIG. 9, but this is only an application example to the Cn value of the sheet passing number count in the present embodiment. Different values may be set according to the sheet passing count.

  Further, in this embodiment, the case where the contact width W between the photosensitive drum 1 and the recording material P increases as the number of passing sheets advances and the outer diameter of the pressure roller 12b changes is described. Depending on the configuration of the apparatus, when the outer diameter of the pressure roller 12b changes, the posture of the recording material P after passing through the transfer nip Nt is wound around the transfer roller 5, and the contact width W between the photosensitive drum 1 and the recording material P is increased. It may decrease. In this state, the time during which the recording material P is in contact with the photosensitive drum 1 is reduced, a current necessary for transfer cannot be obtained, transfer loss is likely to occur, and the transfer efficiency of the toner image onto the recording material P is increased. Falls. In such a case, it is necessary to increase the correction value in accordance with the Cn value of the sheet passing number count. In this case also, as in the present embodiment, the transfer voltage depends on the Cn value of the sheet passing number count. The idea of correcting this remains unchanged.

[Example 2]
Next, Example 2 will be described with reference to FIGS. In this embodiment, the correction value of the transfer voltage is set in accordance with the Cn value of the sheet passing count, and the recording material P is conveyed in the conveyance direction after the leading edge of the recording material P reaches the fixing nip Nf. The transfer voltage is continuously corrected until the rear end passes through the transfer nip Nt. Further, since the second embodiment is different from the first embodiment only in the portion where the transfer voltage is continuously corrected in the recording material P, the description of the portions overlapping with the first embodiment is omitted.

  In Example 1, the transfer voltage correction value was set according to the Cn value of the sheet passing count, and the transfer voltage correction was performed when the leading edge of the recording material P reached the fixing nip Nf. At this time, the transfer efficiency was determined as an average of the printing range of the recording material P. However, since the recording material P is gradually pulled by the pressure roller 5 after reaching the fixing nip Nf, the posture of the recording material P after the transfer also gradually changes, and the photosensitive drum 1 and the recording material P are changed. The contact width W gradually changes as shown in FIG. Therefore, if the same transfer voltage is applied to one sheet of recording material P through the transfer nip Nt, the current flowing to the photosensitive drum 1 increases toward the rear end of the recording material P, and retransfer occurs. As a result, the transfer efficiency gradually decreases toward the rear end of the recording material P. Further, as in the first embodiment, when the transfer voltage is corrected in accordance with the average value of the transfer efficiency of one recording material P1, the transfer efficiency can be improved on average. Immediately after the leading edge of the material P reaches the fixing nip Nf, the transfer voltage is corrected despite the fact that the contact width W between the recording material P and the photosensitive drum 1 has not changed so much. It happens and transfer efficiency decreases. Further, immediately before the trailing edge of the recording material P passes through the transfer nip Nt, the change in the contact width W between the recording material P and the photosensitive drum 1 is large, so that the transfer voltage is not sufficiently corrected, and the current flows excessively. Retransfer occurs and transfer efficiency decreases.

  Therefore, in this embodiment, the correction value of the transfer voltage is set in accordance with the Cn value of the sheet passing count, and the trailing edge of the recording material P is reached after the leading edge of the recording material P reaches the fixing nip Nf. The transfer voltage correction is performed by continuously changing until the toner passes through the transfer nip Nt.

  FIG. 13 shows a timing chart for performing transfer voltage correction in this embodiment. This correction is continuously performed from the stage when the leading edge of the recording material P reaches the fixing nip Nf until the trailing edge of the recording material P passes through the transfer nip Nt. The timing for starting the correction is calculated from the process speed and the distance between the transfer and fixing Nt-Nf by the top sensor 10 detecting the leading edge position of the recording material P.

  As described above, the effect of this embodiment in which the correction of the transfer voltage is continuously changed after the leading edge of the recording material P reaches the fixing nip Nt until the trailing edge of the recording material P passes through the transfer nip Nf. In order to investigate, an experiment was conducted with the following settings. The conveyance speed of the recording material P is set to 150 mm / sec, the heater temperature is set to 200 ° C., and the leading end of the recording material P is compared with the conventional example without the correction of the transfer voltage and the transfer voltage correction under the same conditions as in the first embodiment. A comparative experiment was performed on the transfer efficiency with Example 1 performed when the transfer nip Nf was reached. FIG. 14 shows the result of the transfer efficiency in the conveyance direction of the recording material P when the Cn value of the sheet passing count is 70. Here, a transfer efficiency of 80% or more is a good level, and a transfer efficiency lower than that is a defective level.

  In FIG. 14, in the conventional example in which no correction is made to the fixing voltage Vt, as the contact width W between the photosensitive drum 1 and the recording material P gradually increases after the leading edge of the recording material P reaches the fixing nip Nf. The current flowing through the photosensitive drum 1 becomes excessive, and retransfer occurs. For this reason, the transfer efficiency gradually decreases, and a defect level occurs. Further, in Example 1, in which the correction of the fixing voltage is executed when the leading edge of the recording material P reaches the fixing nip Nf, the transfer efficiency of a satisfactory level is obtained as a whole. However, when viewed partially, immediately after the recording material P reaches the fixing nip Nf, the contact width W between the photosensitive drum 1 and the recording material P does not change so much, and is necessary for transfer by correcting the transfer voltage. The transfer efficiency becomes 80% or less of the target due to insufficient current and transfer loss. Also, just before the trailing edge of the recording material P passes through the transfer nip Nt, the transfer voltage is not sufficiently corrected and the current flows excessively, so that retransfer occurs and the transfer efficiency is maintained at the target 80%. It is difficult.

  On the other hand, in this embodiment, the transfer voltage from the time when the leading edge of the recording material P reaches the fixing nip Nt until the trailing edge of the recording material P passes through the transfer nip Nf according to the Cn value of the sheet passing count. The correction value is continuously changed. For this reason, when the contact width W between the photosensitive drum 1 and the recording material P is not so large immediately after the recording material P reaches the fixing nip Nf, the transfer voltage is suppressed to prevent transfer omission and to prevent a decrease in transfer efficiency. be able to. Further, when the contact width W between the photosensitive drum 1 and the recording material P immediately before the rear end of the recording material P passes through the transfer nip Nt, retransfer is prevented by largely correcting the transfer voltage, thereby reducing the transfer efficiency. Can be prevented.

  As described above, in this embodiment, the correction value of the transfer voltage is set according to the Cn value of the sheet passing count, and after the leading edge of the recording material P reaches the fixing nip Nf, Correction is performed by continuously changing the correction value of the transfer voltage until the rear end passes through the transfer nip Nt. Therefore, even when the contact width W between the photosensitive drum 1 and the recording material P changes in one sheet of the recording material P, an appropriate transfer bias can be set, and good transfer can be performed over the entire recording material P. Efficiency can be obtained.

[Example 3]
In the first and second embodiments, the case where so-called constant voltage control for controlling the transfer voltage is used for controlling the transfer means. However, the present invention employs so-called constant current control for controlling the transfer current with the constant current control power source. It can be applied to the case of using. When the constant current control is used, the transfer current value is controlled when the peripheral speed of the pressure roller 12b increases due to the warming condition of the fixing device 12 and the contact width W between the recording material P and the photosensitive drum 1 changes. As a result, the transfer voltage value changes. When the contact width W between the recording material P and the photosensitive drum 1 increases in the hot state, the transfer voltage value decreases if the current value flowing to the photosensitive drum 1 is controlled to the same current value as in the cold state. In this state, the holding power of the unfixed toner image transferred onto the recording material P is reduced, and an image defect occurs. In such a case, it is necessary to correct the transfer current in accordance with the Cn value of the sheet passing number count, but in this case as well, in this case, the transfer current for transfer control according to the sheet passing number count is also provided. The idea of correcting the value does not change.

1 Photosensitive drum (image carrier)
5 Transfer roller (transfer means)
5a Transfer power source 5b Transfer control means 12 Fixing means 12a Fixing film (fixing rotator)
12b Pressure roller (Pressure rotating body)
12c Ceramic heater 12d Heater holder P Recording material Nt Transfer nip (transfer position)
Nf fixing nip m1 photosensitive drum and pressure roller drive source

Claims (7)

An image carrier for carrying a toner image;
Transfer means for transferring a toner image on the image carrier to a recording material at a transfer position;
A transfer power supply for applying a voltage to the transfer means;
Transfer control means for controlling the transfer power supply;
Fixing means for nipping and conveying the toner image transferred to the recording material at a fixing nip portion and fixing the toner image on the recording material,
A distance between the transfer position and the fixing nip portion is configured to be shorter than a length in a conveyance direction of a recording material of a predetermined size among the recording materials that can be conveyed, and the image carrier and the fixing unit are In an image forming apparatus driven by the same drive source,
The control of the transfer power source by the transfer control unit is changed according to the sheet passing history obtained from the number of sheets of the recording material passed through the fixing unit and the elapsed time from the end of the previous image formation. An image forming apparatus.   When the sheet passing history increases to a predetermined value, the control of the transfer power source by the transfer control means increases the voltage, or when the sheet passing history decreases from a predetermined value, the transfer by the transfer control means. The image forming apparatus according to claim 1, wherein the voltage of the power supply is decreased.   The transfer power source is controlled by the transfer control unit from when the leading end of the recording material in the transport direction reaches the fixing nip portion of the fixing unit until the trailing end of the recording material in the transport direction exits the transfer unit. The image formation according to claim 1, wherein the transfer voltage value or the current value is changed to a constant transfer voltage value or a constant current value, or is changed continuously. apparatus.   The image forming apparatus according to claim 3, wherein the transfer power source is a constant voltage control unit, and the transfer control unit corrects a transfer voltage of the transfer power source.   The image forming apparatus according to claim 3, wherein the transfer power source is a constant current control power source, and the transfer control unit corrects a transfer current of the transfer power source. The fixing unit includes a fixing rotator and a pressure roller having an elastic layer,
The fixing rotator includes a fixing film, a heater, a heater holder,
The image forming apparatus according to claim 1, comprising:   The image forming apparatus according to claim 1, wherein the transfer unit is in pressure contact with the image carrier at the transfer position.

Images