JP4164304B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to an image forming apparatus that forms an image on a recording material (recording medium such as paper).
[0002]
  More detailsA transfer rotator that electrostatically transfers a toner image formed on the image carrier to a recording material, and a toner image transferred to the recording material, a fixing rotator composed of a heating body and a film, and a pressure rotation And fixing on the recording material at a fixing nip formed by the bodyFixing deviceAnd saidNear the fixing nip,AboveAs the recording material passesOn the recording materialVoltage application means for applying a biasWhenThe present invention relates to an image forming apparatus.
[0003]
[Prior art]
  Conventionally, an electrophotographic method has been widely used as an image forming method for forming an image on a recording material. The general method is to charge and expose a photoconductor using a photoconductive substance to form an electrical latent image, develop the latent image with a colored toner, and record it on a recording material such as paper. Then, the recording material is heated and pressed together to melt and fix the toner on the recording material to obtain a fixed image. Then, the residual toner on the photoconductor is cleaned, and the above process is repeated.
[0004]
  As a fixing device (fixing device) used in such an image forming apparatus, a roller having a heat source inside or a film-shaped heat fixing means, and a pressure nip contact with the heat fixing means to form a fixing nip portion, A pair of elastic rollers for conveying the recording material is used.
[0005]
  Conventionally, as a means for solving the toner offset problem in the fixing device, there is a method of applying a release agent such as silicone oil to the surface of the heat fixing means by a web, a pad or the like, or collecting the offset toner on a cleaning member. It was taken.
[0006]
  Recently, however, a configuration in which a bias is applied to one or both of the heat fixing unit and the elastic roller to prevent toner offset on the heat fixing unit has become widespread. In other words, in the image forming process, the recording material after the toner image transfer has a charge opposite in polarity to the toner, and the toner is electrostatically held on the recording material. In order to prevent the offset to the fixing unit, it is effective to generate a potential difference in the direction in which the toner is pressed against the recording material in the fixing step. As a result, the cleaning member becomes unnecessary, the fixing device can be reduced in size and cost, and the user can save time and effort to replace the cleaning member periodically.
[0007]
  There are mainly the following two methods for applying a bias to the fixing device.
[0008]
  A) One is that the heat fixing means side induces a potential of the same polarity as the toner on the surface, while the elastic roller side that is in pressure contact with the heat fixing means is a roller cored bar using conductive rubber via a diode. This is a method of grounding and removing charges having the same polarity as the toner by the electrode effect.
[0009]
  B) The other is that the conductive substrate on the heat fixing means side is grounded or a weak bias having the same polarity as that of the toner is applied, and the elastic roller side that is in pressure contact with the heat fixing means has a low resistance release layer on the surface. And applying a reverse polarity bias.
[0010]
  The former A) has a merit that an insulating fluororesin can be used as a surface release layer, so that it has high releasability and is difficult to get dirty, but the charge held by the recording material varies depending on the type of recording material and the printing rate, There is a drawback that the potential is difficult to stabilize. Also, if there is a leak site on the surface of the heat fixing unit, in the case of a recording material that has absorbed moisture, a current flows between the recording material and the transfer-fixing process, so the toner holding charge of the recording material disappears and back scattering occurs. There is.
[0011]
  On the other hand, the latter B) has an advantage that the electric potential for holding the toner can be supplied from the back of the recording material, so that the potentials on the surfaces of the recording material and the fixing member are stabilized and offset is hardly generated.
[0012]
  By the way, when a bias is applied to the fixing device as described above, the recording material and the heat fixing unit are likely to be electrostatically adsorbed, and the charge held by the recording material is liable to cause peeling charging at the time of separation from the heat fixing unit. There was a drawback.
[0013]
  In order to prevent the peeling electrification due to the recording material, measures are taken such that the primer for adhering the core metal is made conductive rather than the surface release layer to increase the capacitance of the heat fixing means.
[0014]
  Further, when a film is used as the heat fixing means, as described in Japanese Patent No. 3126552, the resistance of the release layer in contact with the toner image and the primer layer therebelow can be reduced. Proposed.
[0015]
  Further, in addition to the above-described configuration, it is known that it is an effective means to eliminate the charge of the recording material at the time of separation.
[0016]
  As a method therefor, it has been proposed to apply a bias while the recording material is in the fixing nip and to turn off the applied bias in accordance with the timing at which the trailing edge of the recording material reaches the fixing nip.
[0017]
  There is also a method in which a bias is applied to a charging member disposed near the nip of the fixing device to supply toner holding charges from the back of the recording material in accordance with the timing when the recording material comes.
[0018]
  Conventionally, the timing for switching the output of the fixing device or the voltage application means arranged in the vicinity of the nip is determined based on the image writing signal (VSYNC signal).
[0019]
[Problems to be solved by the invention]
  However, there is a cutting variation in the recording material, and the permissible dimension becomes larger especially for long paper. As an example, the tolerance of the paper size determined by ISO 216 is allowed to be ± 2.0 mm in the case of a standard paper having a size exceeding 150 mm and not more than 600 mm.
[0020]
  As described above, when a deviation occurs from the nominal size of the recording material, the following problems occur.
[0021]
  An apparatus that turns off the applied bias in accordance with the timing at which the trailing edge of the recording material reaches the fixing nip will be described as an example.
[0022]
  When the length of the recording material (the length in the conveying direction) is shorter than the nominal size, the timing of discharging the trailing edge of the recording material is delayed, and the surface of the fixing roller as the heat fixing unit is peeled and charged. . As a result, a so-called “peeling offset” occurs in which the image on the recording material after one rotation of the fixing roller is disturbed or the image is peeled off to cause an offset after one rotation of the fixing roller.
[0023]
  On the contrary, when the length of the recording material is longer than the nominal size, the toner holding charge on the back of the recording material disappears before the unfixed toner on the recording material is melted and fixed. There is a problem that "backward scatter" occurs.
[0024]
  Further, in the case of performing double-sided printing, the length of the recording material is reduced once the recording material passes through the fixing device, so that the dimensions of the recording material further change.
[0025]
  In particular, in recent years, in image forming apparatuses that have made it possible to handle irregular-size recording materials, the user inputs the recording material size from the operation panel of the image forming apparatus or a personal computer, and fixing is performed based on the input size. Since the bias timing of the apparatus is determined, a deviation between the input size and the actual size of the recording material is likely to occur, and there is a high possibility that the above problem will occur.
[0026]
  Furthermore, in the device that transfers the toner image on the photosensitive drum onto the recording material by the transfer roller, the peripheral speed of the transfer roller is faster than that of the photosensitive drum in order to prevent missing characters. Deviation occurs at the position of the trailing edge of the recording material sent to the apparatus.
[0027]
  In FIG. 9, the transfer roller 4 is brought into contact with the photosensitive drum 1 that is rotationally driven at the peripheral speed V <b> 1 to form a transfer nip portion T, and the transfer roller 4 is provided with a peripheral speed difference with respect to the photosensitive drum 1. FIG. 2 is a schematic configuration diagram of a system that is driven to rotate and introduces a recording material 19 into a transfer nip T and electrostatically transfers a toner image t on the surface of the photosensitive drum 1 to the surface of the recording material 19.
[0028]
  In this case, the conveyance speed V3 of the recording material 19 is intermediate between the circumferential speed V1 of the photosensitive drum 1 and the circumferential speed V2 of the transfer roller 4, but if there is toner t between the photosensitive drum 1 and the recording material 19, Since the frictional force decreases, the conveyance speed V3 of the recording material 19 approaches the peripheral speed V2 of the transfer roller 4.
[0029]
  As a result, when the printing rate on the photosensitive drum 1 is high, the conveyance speed of the recording material 19 is increased, and the bias timing in the fixing device and the deviation amount of the trailing edge of the recording material increase as the length of the recording material 19 increases. It is.
[0030]
  As described above, in the conventional method, there are a plurality of factors that cause the position of the trailing edge of the recording material sent to the fixing device to fluctuate with respect to the optimum bias timing, and the image defect cannot be completely prevented.
[0031]
  The present invention solves the above-described problems, that is, in the image forming apparatus in which a bias is applied to the fixing member of the fixing device or in the vicinity of the fixing nip portion in accordance with the passage of the recording material. The object is to achieve both tail prevention and always obtain a high-quality image.
[0032]
[Means for Solving the Problems]
  The present invention is an image forming apparatus having the following configuration.
[0033]
  A transfer rotator that electrostatically transfers a toner image formed on the image carrier to a recording material, and a toner image transferred to the recording material, a fixing rotator composed of a heating body and a film, and a pressure rotation A fixing device for fusing and fixing on the recording material at a fixing nip portion formed by the body, and the fixing nipPartIn the vicinityIt is disposed upstream of the fixing nip portion in the conveyance direction of the recording material,In accordance with the passage of the recording material, the recording materialThe toner image has a polarity opposite to the charging polarity of the toner imageVoltage application means for applying a biasA voltage applying means for applying the bias to the recording material from a surface opposite to a surface on which the toner image is transferred to the recording material by the transfer rotator.In an image forming apparatus having
  AboveRecording material transport pathInBetween the transfer nip and the fixing nip formed by the image carrier and the transfer rotatorAnd the position on the transfer nip side of the voltage applying means.A flag type sensor that is arranged on the recording medium and detects a loop of the recording material;
  When transporting the recording material in a state where the recording material is sandwiched between both the transfer nip portion and the fixing nip portion,Flag expressionSensordetectionBased on the results, the conveyance of the recording material is controlled so as to form a loop in the recording material.The trailing edge of the recording material is detected based on the detection result of the flag type sensor after the trailing edge of the recording material passes through the transfer nip portion.Control means for
The control means includesBy the transfer nip portion and the fixing nip portionAboveA state in which the recording material is sandwiched and conveyedAnd the trailing edge of the recording material is detected by the flag type sensor after passing through the transfer nip portion.Until the voltage application meansAboveFor recording materialAboveA bias is applied,Flag expressionDepending on the timing at which the trailing edge of the recording material is detected by a sensor, the voltage applying meansTo the recording materialAn image forming apparatus, wherein application of a bias is stopped.
[0034]
[0035]
[0036]
[0037]
[0038]
[0039]
[0040]
[0041]
[0042]
[0043]
[0044]
DETAILED DESCRIPTION OF THE INVENTION
  Embodiments of the present invention will be described below with reference to the drawings.
[0045]
  <Reference Example 1>
  Figure 1Reference Example 1FIG. 2 is a schematic configuration diagram of the image forming apparatus.Reference Example 1The image forming apparatus is a laser beam printer using a transfer type electrophotographic process equipped with an automatic duplexer. The process speed Vp of this laser beam printer is 200 mm / s.
[0046]
  (1) Overall schematic configuration of image forming apparatus
  Reference numeral 1 denotes an electrophotographic photosensitive drum as an image carrier installed in the image forming apparatus. The surface of the photosensitive drum 1 is uniformly charged by a primary charger 29, and image exposure is performed by an exposure device 28. An electrostatic latent image is formed on the surface. This electrostatic latent image is visualized as a toner image by the developing device 30. In many cases, the surface of the photosensitive drum 1 is uniformly charged to a predetermined negative potential by a primary charger 29 and a toner image is formed by combining image exposure and reversal development using a negative toner.
[0047]
  In synchronization with image formation on the photosensitive drum 1, a recording material (paper, OHT, etc.) 19 is taken out from the paper cassette 36 by the paper feed roller 25, and passes through the conveyance guide 34, the conveyance roller 33, and the conveyance guide 32, and the registration roller pair 31 a. -It is conveyed to 13b.
[0048]
  The registration roller pair 31a and 13b are stopped until the recording material 19 arrives, and the skew of the recording material 19 is corrected by the leading end of the recording material 19 abutting against the nip portion R of the registration roller pair 31a and 13b. .
[0049]
  In front of the registration roller pair 31a and 13b, there is a sensor flag 48 arranged in the conveyance path of the recording material 19, and the arrival of the recording material 19 is detected by shielding a photocoupler (not shown). Thereafter, the registration roller pair 31a and 13b starts to rotate in accordance with a writing signal (hereinafter referred to as a VSYNC signal) for starting to form an image on the photosensitive drum 1, and the recording material 19 passes through the transfer guide 14 and the photosensitive drum 1. And a transfer nip T formed by a transfer roller 4 as a transfer rotator. The transfer roller 4 is applied with a positive transfer bias having a polarity opposite to the charging polarity of the toner from a bias power source (not shown) at a predetermined control timing in this example, and thus the toner formed on the photosensitive drum 1. The image is electrostatically transferred onto the recording material 19 at the transfer nip T.
[0050]
  After the transfer, the recording material 19 is separated from the surface of the photosensitive drum 1 by the action of curvature separation by the small-diameter photosensitive drum and electrostatic separation by the static elimination needle 8. The neutralizing needle 8 is applied with a voltage having a polarity opposite to the polarity of the transfer bias by a bias power source (not shown). The recording material 19 separated from the photosensitive drum 1 is conveyed to the fixing device 13 through the conveyance guide 10, where the unfixed toner image is fixed and becomes a fixed image.
[0051]
  In the case of single-sided printing, the recording material 19 is sent to the conveyance guide 38 by the flapper 37 and discharged to the paper discharge tray 46 by the paper discharge roller 45.
[0052]
  On the other hand, in the case of double-sided printing, the recording material 19 that has been fixed is transported to the transport guide 39 by the flapper 37, transported once in the direction of the transport guide 41 by the transport roller 40, and then reversely rotated by the transport roller 40. The recording material 19 is conveyed in the direction of the conveyance guide 42. In this process, the front and back of the recording material 19 are reversed.
[0053]
  The recording material 19 whose front and back are reversed is again subjected to the transfer process through the transport roller 43 and the transport guide 44. The recording material 19 that has completed the second transfer and fixing process is conveyed upward by the flapper 37, and is discharged to the paper discharge tray 46 through the conveyance guide 38 and the paper discharge roller 45. Reference numeral 47 denotes an exterior for protecting and soundproofing the inside of the machine.
[0054]
  (2) Registration roller pair 31a and 13b
  The pair of registration rollers 31a and 13b includes a pair of an upper metal roller 31a (φ14) such as iron or SUS and a lower roller 31b (φ14) in which a solid rubber such as chloroprene rubber or EPDM is wound on a core metal. The recording material 19 is conveyed to the nip portion R formed by the roller pairs 31a and 13b and synchronized with the image writing signal. The peripheral speeds of the registration roller pairs 31 a and 13 b are set to be the same as the peripheral speed of the photosensitive drum 1.
[0055]
  (3) Transfer roller 4
  The transfer roller 4 is a roller φ17 in which a core bar is covered with a rubber such as urethane, EPDM, NBR, or a sponge-like elastic layer obtained by firing them, and is brought into contact with the photosensitive drum 1 with a predetermined pressing force. Thus, a transfer nip T having a predetermined width is formed. A roller having a roller hardness of 40 ° or less (Asker-C hardness, 500 g load) is used for preventing missing characters during transfer and scraping of the photosensitive drum 1. Carbon and metal fine powder are uniformly dispersed in this elastic layer, and the volume resistivity is 10⁶-10⁹By using a medium resistance roller of Ω · cm, good electrostatic transfer can be performed even with a relatively high resistance recording material. Volume resistivity is 10⁶If it is smaller than Ω · cm, an excessive current flows to the photosensitive drum 1 and damages the photosensitive drum. Conversely, the volume resistivity is 10⁹If it is larger than Ω · cm, the current required for transfer is insufficient, resulting in transfer failure.
[0056]
  Therefore, the optimum value exists for the resistance value of the transfer roller 4. At this time, the transfer roller 4 has a resistance of 1 kV to 6 kV depending on the resistance value from the cored bar.AppropriateA large voltage is applied.
[0057]
  The transfer roller 4 is calculated from the peripheral speed (calculated from the outer diameter of the transfer roller 4) using a gear (not shown) in order to prevent “collapse” that easily occurs when the recording material 19 is OHT or thick paper. ) With respect to the photosensitive drum 1 as fast as 105% to 120%. As a result, the actual recording material conveyance speed is about 1% faster than the photosensitive drum 1. This recording material conveyance speed is a speed at which a conveyance defect such as blur does not occur between the pair of registration rollers 31a and 13b even when a long recording material is passed.
[0058]
  (4) Fixing device 13
  Reference Example 1The fixing device 13 is a heat roller type device. That is, a fixing roller 50 as a first fixing member (thermal fixing means, fixing rotating body) in which a core metal such as iron or aluminum is coated with 10 to 50 μm of a fluorine resin having good releasability, iron, A pressure roller 51 as a second fixing member (pressure rotator) in which a silicon rubber or sponge rubber is coated as an elastic layer on a cored bar made of aluminum or the like and a conductive fluororesin 10 to 50 μm is further coated thereon. Have The pressure roller 51 contacts the fixing roller 50 with a predetermined pressure to form a fixing nip portion N having a width of 8 mm. Further, the fixing roller 50 includes a halogen heater H as a heating source therein, and is driven to rotate while the temperature of the roller surface is adjusted to be substantially constant by a temperature detection unit (not shown). The pressure roller 51 is driven rotation.
[0059]
  The outer diameter and gear of the roller are selected so that the peripheral speed of the fixing roller 50 to be driven is approximately the process speed Vp when heated. Accordingly, the recording material 19 is not excessively pulled between the fixing nip portion N and the transfer nip portion T, and conversely, the recording material is not bent and the printing surface does not come into strong contact with other components. .
[0060]
  (5) Voltage on / off control for the fixing device 13
  The voltage on / off control for the fixing device 13 will be described with reference to the schematic diagram of FIG. Here, the recording material conveyance distance from the nip portion R of the registration roller pair 31a and 31b to the fixing nip portion N of the fixing device 13 is L1, and the recording material from the position S of the sensor flag 48 to the fixing nip portion N of the fixing device 13 is recorded. The conveyance distance was L1 ′.
[0061]
  The recording material 19 carrying the unfixed toner image is conveyed to the fixing nip N of the fixing device 13 to form a fixed image, and is separated from the roller 50 by a separation claw (not shown) that contacts the fixing roller 50.
[0062]
  Reference Example 1Then, the fixing roller mandrel is grounded, and the surface resistance of the surface of the pressure roller 51 is 1 × 10.⁶A low-resistance PFA tube of Ω / □ is wound, and a power feeding member is in contact with the surface of the non-sheet passing portion. On the surface of the pressure roller 51, a bias +600 V having a polarity opposite to that of the toner is applied from the high voltage power supply 53 through the power supply member. A bias on / off switch 52 is controlled to be switched on / off by the control circuit 54 in accordance with the timing when the recording material 19 is conveyed to the fixing device 13. The control circuit 54 is a CPU that controls the operation of the image forming apparatus, and a signal is transmitted through the bus line 54 '.
[0063]
  The timing for turning on the bias is the time required from the VSYNC signal until the registration roller pair 31a / 31b starts to move and the time required for the recording material 19 from the nip R of the registration roller pair 31a / 31b to the fixing nip N of the fixing device 13. It is determined by L1 / Vp [seconds].
[0064]
  Next, the timing for turning off the applied bias will be described. FIG. 3 shows how the recording material 19 enters the fixing nip portion N.
[0065]
  (A) shows the state at time T = Ta. As can be seen from the drawing, the print image t at the trailing edge of the recording material has not yet reached the fixing nip N in this state. However, after T = Ta, “backward scattering” does not occur even when the bias is turned off. This is because the recording material 19 is conveyed along the nip tangent line of the fixing roller 50 in the region indicated by A in the figure, and therefore receives heat from the fixing roller 50 and melts strongly. Is not necessary.Reference Example 1In the case of A, the length of the A is 8 mm.
[0066]
  (B) shows the recording material trailing edge position B (4 mm discharge side from the center of the fixing nip N) at the time T = Tb when the time has advanced from T = Ta. The moment of separation from the fixing roller 50 by the illustrated separation claw is shown. At this time, the potential of the recording material 19 needs to be sufficiently lowered so that the fixing roller 50 is not peeled and charged.
[0067]
  According to the study of the present inventor, it has been found that if the potential of the recording material 19 is +200 V or less, it is not peeled and charged.
[0068]
  When the applied bias is turned off, the voltage drop time is delayed by the fall time constant τ determined by the impedance of the fixing device and the high voltage circuit.Reference Example 1The fixing device has a time constant τ of 30 ms.
[0069]
  Therefore, the proper timing for turning off the applied bias is as follows. FIG. 4 illustrates the relationship between the potential of the recording material 19 and the bias-off timing. As described above, when the power is turned off before T = Ta, “backward scattering” occurs. When the power is turned off after T = Tb−τ = Tb ′, the potential of the recording material 19 becomes +200 V or more, and “peeling offset” occurs. Resulting in. That is, there is an appropriate timing for turning off the applied bias between T = Ta and T = Tb ′ (shaded portion in the figure).
[0070]
  Reference Example 1Then, in FIGS. 3A and 3B, the distance from A to B is 12 mm, and within 6 mm obtained by subtracting the length 6 mm (= 200 mm / s × 0.03 s) corresponding to the bias falling time constant. What is necessary is just to turn off the bias.
[0071]
  Specifically, when the rear end of the recording material passes the sensor flag 48 and the light shielding of the photocoupler is released, a signal is sent to the CPU 54, and the switch 52 is turned off at a predetermined timing (L1 ′ / Vp) [seconds]. (B) of FIG. 3 is performed.
[0072]
  Conventionally, in an apparatus that turns off the bias based on the image writing signal, the timing for turning off the applied bias is selected to be between T = Ta and T = Tb ′ for the above reasons.
[0073]
  Actually, when a character image was printed on a recording material of A4 horizontal size and printed on one side, it could be printed without any problem.
[0074]
  However, when a graphic image was printed in A3 size and double-sided printing was performed continuously, image scattering of the fixing roller period occurred in the graphic image on the first printed surface.
[0075]
  This is because in the conventional method, the following factors that change the length of the recording material are not taken into consideration.
[0076]
  A.Cutting variation of recording material;
      In the case of the present embodiment, the tolerance of the sheet size is ± 2.0 mm.
[0077]
  B.Shrinkage of the recording material by passing once through the fixing device;
      It shrinks the most after passing through a recording material that has been left in a high temperature and high humidity environment.
[0078]
  C.Change in paper transport speed due to fast rotation of transfer roller;
      The transfer roller 4 has manufacturing variations in outer diameter.
[0079]
  Reference Example 1Then, the recording material conveyance speed is increased as the outer diameter of the transfer roller 4 is the upper limit and the use of the roller is advanced. Further, the higher the printing rate, the lower the frictional force between the recording material 19 and the photosensitive drum 1 and the higher the conveyance speed.
[0080]
  Table 1 shows the factors that cause the length of the recording material to vary.Reference Example 1It was measured using the apparatus. In Table 1, the case where the trailing edge of the recording material (paper) is shifted to the transfer side is expressed as “delay”, and the case where it is shifted to the paper discharge side is expressed as “advance”.
[0081]
[Table 1]
[0082]
  The aboveBThe fluctuation amount of the paper is a maximum length 420 mm paper (basis weight 64 g / m) used in this laser beam printer as a recording material.²) Was used.
[0083]
  Also, the aboveCThe amount of fluctuation was determined by printing a graphic image with a transfer roller (new product) at the center of the outer diameter and a transfer roller (used product) with an outer diameter upper limit, and comparing the printing accuracy.
[0084]
  As shown in Table 1, in the conventional method in which the bias is turned off based on the image writing signal, the fluctuation width Δ (advancing side + delaying side) of the recording material rear end position in the fixing device is 7.5 mm when double-sided printing is performed. Yes, it does not fall within the appropriate range of 6 mm for the aforementioned off timing.
[0085]
  That is, “backward scattering” and “peeling offset” are in a trade-off relationship, and there is no solution that satisfies both.
[0086]
  Reference Example 1In this case, an on / off signal is detected when the trailing edge of the recording material passes through the sensor flag 48 provided in front of the registration roller pair 31a and 31b, and control is performed so as to determine the applied bias timing of the fixing device.
[0087]
  As a result, as shown in Table 1, the fluctuation factorsAVariation of recording materials and factors of fluctuationBEven if the recording material shrinks due to passing through the fixing device once, the trailing edge of the recording material is detected by the sensor flag 48, and the bias of the fixing device is turned off based on the timing. As shown in Table 1, the fluctuation factor of the paper length at the apparatus position is 1.5 mm, and it is possible to prevent both “backward scattering” and “peeling offset”.
[0088]
  In the above example, the case where the standard size is used has been described. However, even when the user uses an irregular size recording material and the size input to the image forming apparatus is wrong, the applied bias timing is automatically corrected. Therefore, high-quality images can be provided without bothering the user.
[0089]
  It goes without saying that the same effect can be obtained even in an image forming apparatus that does not include an automatic duplexer and performs manual duplexing.
[0090]
  Further, the means for detecting the trailing edge of the recording material is not limited to the sensor flag 48, and other sensors arranged between the paper feed port and the fixing device can be used. For example, an optical sensor that distinguishes between OHT and paper may be used.
[0091]
  <Reference Example 2>
  Reference Example 2Will be described with reference to FIGS. The aboveReference example 1Parts having the same functions as those in FIG.
[0092]
  Reference Example 2In this case, the trailing edge of the recording material is detected by using the transfer bias applied to the transfer roller 4 during the sheet passing, and the off timing of the applied bias in the fixing device is determined.
[0093]
  First, a power supply system for applying a bias to the transfer roller 4 and its control system will be described. As shown in FIG. 5, the power supply system includes a constant current power supply 59, a constant voltage power supply 58 that can change an output value, and an ammeter 57, which are connected to the core of the transfer roller 4 via a changeover switch 56. It is connected to the. A voltmeter 60 that monitors the voltage when a constant current bias is applied is connected to the constant current power source 59.
[0094]
  The control system is connected to a CPU 54 for controlling the operation of the image forming apparatus and a memory 55 through a bus line 54 ′, and controls the constant current power supply 59, voltmeter 60, constant voltage power supply 58, and changeover switch 56. .
[0095]
  Next, a sequence of bias applied to the transfer roller 4 will be described. FIG.Reference Example 26 is a timing chart showing a transfer bias control sequence in FIG.
[0096]
  As for the transfer bias, first, a constant current power source of 10 μA is applied to the transfer roller 4 by a constant current power supply in the pre-rotation before passing paper, and the voltage Vto at that time is monitored with a voltmeter (section A). Until the image forming apparatus is ready for image formation, the monitored voltage Vto is applied to the transfer roller 4 to wait for rotation (section B). When the image formation preparation is completed, the registration roller pair 31a and 31b are driven in accordance with the image on the photosensitive drum 1, and the recording material 19 is conveyed to the transfer nip T.
[0097]
  In accordance with the conveyance timing of the recording material 19, the constant voltage power supply 58 is selected by the switch 56 and switched to the constant voltage bias for printing (section C). If the transfer voltage in section C is Vt, Vt is
    Vt = Vt0 × a + b [kV]
It is determined according to the following formula. Here, a = 1.0 and b = 1.1.
[0098]
  At this time, the ammeter 57 connected to the constant voltage power source 58 monitors the current during paper passing, and the current value is stored in the memory 55 via the bus line. This is I1.
[0099]
  The transfer voltage Vt is applied to the transfer roller 4 longer than the size of the recording material, and the time point when the current I2 that flows instantaneously through the transfer roller 4 greatly exceeds the value I1 stored in the memory 55 is detected as the trailing edge of the recording material. . In this embodiment, the setting is made so that the rear end of the recording material is determined when I2 exceeds 150% of I1.
[0100]
  The reason why the detection threshold is set to 150% is that the resistance unevenness in the circumferential direction of the transfer roller 4 is sufficiently small, there is a sufficient margin with respect to the current fluctuation during paper feeding, and the recording material having the lowest resistance is passed. This is because the change in current after passing through the recording material was 170% during paper passing. Actually, basis weight 64g / m²When I paper was passed, I1 = 10 μA and I2 = 20 μA.
[0101]
  If the transfer voltage Vt is applied for 10 mm longer than the recording material size, the trailing edge of the recording material 19 can be reliably detected even if the recording material size varies.
[0102]
  The timing at which the bias applied to the fixing device is turned off is L2 / Vp from the time when the transfer roller 4 determines that it is the trailing edge of the recording material, where L2 is the distance from the center of the transfer nip T to the center of the fixing nip N. Set to turn off in seconds.
[0103]
  Reference example 2Has detected the trailing edge of the recording material at the transfer nip portion T.Reference example 1The deviation of the rear end position of the recording material due to the rapid rotation of the transfer roller described in the above does not occur. ThereforeReference example 1The bias can be turned off more accurately in the fixing device.
[0104]
  As a result, it is possible to provide the user with a high-quality image without back scattering even when the trailing edge zero edge (edgeless) printing, which has been difficult in the past, is performed.
[0105]
  The above describes the case where transfer to a recording material is performed using a constant voltage power source as a transfer bias during printing. However, constant current control is performed as the transfer bias to detect a voltage change at the trailing edge of the recording material. Also good.
[0106]
  <Example>
  Next, the present inventionExamples ofWill be described with reference to FIGS. The aboveReference example 1WhenReference example 2The same components as those in FIG.
[0107]
  In this embodiment, a case where the present invention is applied to a laser beam printer having a process speed of 100 mm / s that enables on-demand fixing will be described.
[0108]
  FIG. 7 is a schematic configuration model diagram of the fixing device 13 used in this embodiment. The fixing device 13 used in the present embodiment is a film heating method using a cylindrical film and a rotating body driving method for pressurization (tension) disclosed in JP-A-4-44075 to 44083 and 4-204800 to 204984. Less type) on-demand fixing device.
[0109]
  Reference numeral 80 is a film assembly as a first fixing member (thermal fixing means), and 81 is an elastic pressure roller as a second fixing member (pressure rotating member). Is formed.
[0110]
  The film assembly 80 includes a heat resistant and rigid film guide member (stay) 83 having a substantially semicircular arc shape in cross section, and a groove portion provided on the lower surface of the film guide member 83 along the length of the member 83. An endless belt-like (cylindrical) shape as a fixing rotating body loosely fitted around a ceramic heater 84 as a heating body and a film guide member 83 to which the heater 84 is attached. A heat-resistant fixing film 82 and a reinforcing stay 86 inserted into the film guide member 83 are included.
[0111]
  The fixing film 82 has a film thickness of 100 μm or less, preferably 20 to 60 μm in order to reduce the heat capacity and improve the quick start property, and has excellent heat resistance, strength, durability, polyimide, polyamide, PEEK, PES, etc. Or a composite layer film in which a release layer such as PTFE, PFA, or FEP is coated on the above-described single layer film. The surface of the fixing film has a low resistance in order to reduce peeling offset.
[0112]
  The ceramic heater 84 as a heating element includes an energization heating element 84a as a heating source that generates heat by power supply, and has a low heat capacity as a whole, and quickly rises in temperature by power supply.
[0113]
  The pressure roller 81 has a diameter of 25, and a rubber elastic layer 81b made of silicone rubber (thickness 3 mm) is formed on a core metal pipe 81a made of iron or aluminum, and a release layer 81c made of an insulating PFA tube is formed on the surface layer. It is.
[0114]
  The pressure roller 81 is disposed such that both ends of the cored bar pipe 81a are rotatably supported by bearings between side plates on the front side and the back side of the apparatus chassis (not shown) via a bearing member.
[0115]
  The film assembly 80 is disposed on the upper side of the pressure roller 81 and parallel to the pressure roller 81 with the ceramic heater 84 facing downward, and is pressed in the axial direction of the pressure roller 81 by a biasing spring (not shown). Thus, the downward surface of the ceramic heater 84 is brought into pressure contact with the elastic layer of the pressure roller 81 through the fixing film 82 against the elasticity of the elastic layer, and a fixing nip portion N having a predetermined width necessary for heat fixing is formed. It is formed.
[0116]
  The pressure roller 80 is rotationally driven in a counterclockwise direction indicated by an arrow at a predetermined peripheral speed when a rotational force is transmitted from the drive system 90. A rotational force acts on the cylindrical fixing film 82 by the pressure frictional force at the fixing nip N between the outer surface of the pressure roller 81 and the fixing film 82 by the rotational driving of the pressure roller 81, and the fixing film 82 While the inner surface is in close contact with the downward surface of the ceramic heater 84 and slides, the outer periphery of the film guide member 83 is rotated in the clockwise direction indicated by the arrow. Silicon grease is applied to the surface of the ceramic heater 84 and the film guide member 83 to improve slidability with the inner surface of the fixing film 82.
[0117]
  Then, power is supplied from the heater drive circuit (not shown) to the energizing heating element 84a of the ceramic heater 84, so that the energizing heating element 84a generates heat and the ceramic heater 84 is quickly heated. The heater drive circuit controls energization to the energization heating element 84a so that the heater temperature detected by the temperature detection means 85 such as a thermistor provided in contact with the heater back surface is maintained at a predetermined substantially constant temperature (fixing temperature). To do. That is, the ceramic heater 84 is heated and adjusted to a predetermined fixing temperature.
[0118]
  The pressure roller 81 is driven to rotate, and accordingly, the cylindrical fixing film 82 is driven and rotated, and the ceramic heater 84 is energized. The heater 84 is heated to rise to a predetermined temperature, and the temperature is adjusted. In this state, the recording material 19 carrying the unfixed toner image t is introduced between the fixing film 82 and the pressure roller 81 in the fixing nip N, and the toner image carrying surface side of the recording material 19 is located in the fixing nip N. The fixing nip portion N is brought into close contact with the outer surface of the fixing film 82 and conveyed together with the film 82. Reference numeral 87 denotes a guide member for smoothly guiding the recording material 19 to the fixing nip portion N. In this nipping and conveying process, the heat of the ceramic heater 84 is applied to the recording material 19 through the fixing film 82, and the unfixed toner image t on the recording material 19 is heated and pressurized on the recording material 19 to be melted and fixed. The
[0119]
  The recording material 19 that has passed through the fixing nip N is separated from the fixing film 82 by the curvature, and is discharged and conveyed.
[0120]
  Next, an outline of a part that characterizes the present invention will be described for a laser beam printer including the fixing device 13 described above. FIG. 8 is a schematic diagram of the printer main body showing between the transfer process and the fixing process, which are the main components of the present embodiment.
[0121]
  Reference numeral 90 denotes a motor for controlling the rotational speed of the pressure roller 81 of the fixing device 13, and a sensor flag (in the recording material conveyance path between the transfer nip T and the fixing nip N) (Flag expressionA sensor 49 detects a loop amount formed during the conveyance of the recording material between the transfer nip portion T and the fixing nip portion N, and feedback-controls the motor 90.The sensor flag 49 for detecting the loop of the recording material is a position on the fixing nip portion side between the transfer nip portion and the fixing nip portion as shown in FIG. 8, and the fixing nip portion N as will be described later. Is disposed at a position closer to the transfer nip portion than the charging needle 77 as a voltage applying means disposed near the fixing nip portion N on the inlet side of the toner.
  That is, when the recording material 19 is conveyed while the recording material 19 is sandwiched between both the transfer nip portion T and the fixing nip portion N, the CPU 54 serving as the control means performs the sensor 49.detectionBased on the result, the motor 90 is controlled so as to form a loop in the recording material to control the conveyance of the recording material.
[0122]
  P1 is a state where a loop is formed, and P2 is a state where the speed of the recording material 19 is increased and the loop disappears.
[0123]
  This is because, in a system in which the recording material 19 is conveyed by the pressure roller 81 and on-demand fixing is performed, the pressure roller 81 is heated and expands while the paper is being passed, so that the speed of the recording material increases at the rear end of the recording material. When the loop disappears, the recording material 19 is stretched and the unfixed toner image on the recording material is blurred. For this reason, a means for controlling the loop is effective.
[0124]
  77 is an entrance side of the fixing nip portion N and is near the fixing nip portion N.That is, upstream of the fixing nip in the recording material conveyance directionIs a charging needle as a voltage applying means, connected to a constant current power supply 78 for applying a constant current of 3 μA and a switch 79 for switching on and off, and the recording material 19 passes over the charging needle 77. Reverse polarity with toner only duringThat is, the polarity opposite to the charging polarity of the toner imageThe bias is controlled to be applied.As shown in FIG. 8, the charging needle 77 applies the bias to the recording material 19 from the surface opposite to the surface on which the toner image is transferred to the recording material 19 by the transfer roller.
[0125]
  As described above, the reason why the charging needle 77 is provided in the vicinity of the fixing nip portion N is that it is difficult to apply a bias to the fixing device because of the use of the fixing film 82, and the toner is held from the back side of the recording material. This is because a low-cost member for supplying electric charge is required separately.
[0126]
  The reason why the bias is applied only while the recording material 19 passes over the charging needle 77 is as follows.
[0127]
  The charging needle 77 is installed as close to the recording material 19 as possible, and while the recording material 19 is held in the fixing nip portion N, the charge is uniformly supplied from the front end to the rear end of the recording material 19. Since it is a place that can be formed, a place near the fixing nip N is preferable. However, if the charging needle 77 is brought close to the fixing nip portion N and a bias is applied when the recording material 19 is not present, the surface of the fixing film 82 near the charging needle 77 is positively charged and an offset occurs. . Therefore, it is necessary to turn off at the trailing edge of the recording material so that a bias is not applied between the sheets.
[0128]
  In this embodiment, the trailing edge of the recording material is detected by the sensor flag 49 used for loop control, and the bias at the trailing edge of the recording material of the charging needle 77 is switched.The detection of the trailing edge of the recording material by the sensor flag 49 is performed based on the return operation of the sensor flag 49 (returning operation from the broken line posture to the solid line posture in FIG. 8) accompanying the passage of the sensor flag 49 at the trailing edge of the recording material. The
  That is, in a state where the recording material 19 is nipped by the transfer nip portion T and the fixing nip portion N and the recording material is conveyed, a bias is applied (turned on) to the recording material by the charging needle 77 which is a voltage applying means. Thus, the application of the bias by the charging needle 77 is stopped (turned off) in accordance with the timing at which the rear end of the recording material is detected by the sensor 49.
[0129]
  The method and control for detecting the trailing edge of the recording material are described above.Reference example 1The description is omitted.
[0130]
  As described above, even in the case of the present embodiment in which the recording material speed after the transfer nip T changes, the distance from the sensor flag 49 to the fixing nip N is short, so that the bias is accurately applied at the trailing edge of the recording material. The charge can be supplied to the recording material 19 in accordance with the rear end of the recording material.
[0131]
  In addition, this invention is not limited to the above-mentioned embodiment at all, and various deformation | transformation are possible within the technical thought of this invention.
[0132]
[0133]
AdditionThe heating element 84 may be a flat plate heater or an irregular curve heater using a SUS material or a metal material as a heater substrate. The heating element 84 is not limited to a ceramic heater. For example, an electromagnetic induction heating member can be used.
[0134]
AdditionThe pressure roller as the pressure rotator may be in the form of a rotating belt.
[0135]
[0136]
[0137]
PictureThe image forming principle and process of the image forming apparatus are not limited to the electrophotographic system of the embodiment, but may be an apparatus such as an electrostatic recording system, a magnetic recording system, and an intermediate transfer system.
[0138]
[0139]
【The invention's effect】
  As explained above, according to the present invention,,RecordThe rear end of the recording material is detected and the applied bias is switched based on the detection timing, so that it is possible to prevent trade-off between the peeling charging and back scattering of the fixing device due to the recording material, and a high-quality image at all times. Is obtained.
[0140]
  In addition, even if the user uses an irregular size recording material and the size input to the image forming apparatus is wrong, the applied bias timing at the trailing edge of the recording material is automatically corrected, so that the user's hand is not troubled. High quality images can be provided.
[0141]
  Further, even when zero edge (edgeless) printing, which has been difficult in the past, is performed, a high-quality image can be obtained over the entire area of the recording material.
[Brief description of the drawings]
[Figure 1]Reference example 1Schematic configuration model diagram of image forming apparatus
[Figure 2]Reference example 1Schematic diagram of the control system
[Fig. 3]Reference example 1Showing the positional relationship of the recording material in the fixing device
[Fig. 4]Reference example 1Pressure bias timing chart
[Figure 5]Reference example 2Schematic diagram of the control system
[Fig. 6]Reference example 2Transfer bias timing chart
[Fig. 7]FruitSchematic schematic diagram of film heating and fixing device in examples
[Fig. 8]FruitSchematic diagram of the control system in the example
FIG. 9 is an explanatory diagram of a conventional image forming apparatus that performs fast rotation of a transfer roller.
[Explanation of symbols]
  DESCRIPTION OF SYMBOLS 1 ... Photosensitive drum, 4 ... Transfer roller, 19 ... Recording material (paper), t ... Toner
48 · 49 Sensor flag 50 Fixing roller 51 Pressure roller 5
2 ... changeover switch, 53 ... high voltage power supply, 54 ... CPU

Claims (1)

A transfer rotator that electrostatically transfers a toner image formed on the image carrier to a recording material, and a toner image transferred to the recording material, a fixing rotator composed of a heating body and a film, and a pressure rotation A fixing device that melts and fixes the recording material on the recording material at a fixing nip portion formed by a body, and is disposed upstream of the fixing nip portion in the conveyance direction of the recording material in the vicinity of the fixing nip portion. Voltage applying means for applying a bias having a polarity opposite to the charging polarity of the toner image to the recording material in accordance with the passage of the toner image, and a surface on which the toner image is transferred to the recording material by the transfer rotator. In an image forming apparatus having a voltage applying means for applying the bias to the recording material from the opposite surface ,
In the transport path of the recording medium, the position of the fixing nip portion between the image bearing member and the transferring member transferring nip portion formed by the said fixing nip, and said voltage applying means A flag type sensor that is arranged at a position closer to the transfer nip than the recording material and detects a loop of the recording material;
When the recording material is conveyed in a state where the recording material is sandwiched between the transfer nip portion and the fixing nip portion, a loop is formed on the recording material based on the detection result of the flag type sensor. Control means for controlling conveyance of the recording material, and detecting the trailing edge of the recording material based on the detection result of the flag type sensor after the trailing edge of the recording material has passed through the transfer nip portion ,
Wherein, the state in which the recording material is nipped and conveyed to the recording material, and the rear end of the recording material passes through the transfer nip and the transfer nip portion by said fixing nip from until detected by the flag sensor is so as to apply the bias to the recording medium by said voltage applying means, said voltage application in accordance with the timing of detecting the trailing edge of the recording material by the flag sensor image forming apparatus characterized by stopping the application of the bias to that by the means the recording material.