Image forming device

Abstract

To provide an image forming apparatus that can prevent AC banding and is advantageous for preventing lightning surge without an increase in manufacturing cost.SOLUTION: An image forming apparatus comprises: fixing means having a rotatable fixing belt 50 and a pressure member 51 that is opposite to the fixing belt and forms a fixing nip; and transfer means having a rotatable transfer member 25 and an opposite member 21 that is opposite to the transfer member and forms a transfer nip. The fixing means has, inside the fixing belt, heating means 52 that is in contact with the fixing belt, and the heating means is connected with a power supply that applies AC voltage. A transfer resistance Rt satisfies a predetermined formula 1.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] In electrophotographic image forming devices, there are two methods for powering the resistance heating element of the fixing device: one is to use AC voltage from a commercial power source, and the other is to rectify this AC voltage and convert it to DC voltage. If the AC voltage is not rectified, a rectification circuit is not required, preventing costs from increasing. However, if AC voltage is used without rectification, a problem occurs in which a phenomenon called AC banding occurs, which causes uneven density in the image.

[0003] In conventional image forming devices, a fixing nip is formed between a fixing belt and a pressure roller, and a transfer nip is formed between a transfer roller and a counter roller. In such devices, a heating element is placed in contact with the fixing belt, and an AC voltage is applied to the heating element to heat the fixing belt. In this case, the AC voltage can be transmitted to the fixing nip via the fixing belt. When a recording medium is simultaneously sandwiched between the fixing nip and the transfer nip, the AC voltage at the fixing nip is transmitted to the transfer nip via the recording medium. The AC voltage transmitted to the transfer nip fluctuates the transfer voltage at the transfer nip, causing transfer unevenness and resulting in stripes (density unevenness) in the image. This phenomenon is known as AC banding, and such density unevenness will also be referred to hereinafter as AC banding.

[0004] In contrast, in Patent Document 1, a fixing nip is formed by a pressure roller and a fixing roller, and a heating nip is formed by a fixing roller and a heating roller, and the resistance value between the fixing nip and the heating nip satisfies a predetermined relationship. According to Patent Document 1, it is possible to prevent uneven density of an image caused by the AC component of the AC voltage being superimposed on the transfer voltage of the secondary transfer nip. Summary of the Invention [Problem to be solved by the invention]

[0005] However, in Patent Document 1, it is necessary to use a heating roller, and a heating nip must be formed between the fixing roller and the heating roller, which causes problems such as limitations on the device configuration and makes the device configuration complicated. Another way to prevent AC banding is to reduce the voltage applied to the transfer nip, for example by reducing the fuser ground resistance of the fuser member. However, if the fuser ground resistance is too low, a lightning surge may occur in the event of a lightning strike, in which high voltage is applied to the device from the power supply connected to the resistance heating element. Therefore, there is a need for technology that can suppress AC banding without increasing manufacturing costs and that is also effective against lightning surges.

[0006] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an image forming apparatus that can suppress AC banding without increasing manufacturing costs and that is also advantageous in terms of lightning surges. [Means for solving the problem]

[0007] In order to solve the above problems, the image forming apparatus of the present invention comprises: a fixing unit having a rotatable fixing belt and a pressure member facing the fixing belt to form a fixing nip; An image forming apparatus including a transfer means having a rotatable transfer member and an opposing member that faces the transfer member and forms a transfer nip, the transfer unit is disposed upstream of the fixing unit in the recording medium conveyance direction, the fixing unit has a heating unit that is in contact with the fixing belt on the inner side of the fixing belt, the heating means is connected to a power source that applies an AC voltage; the fixing belt, the pressure member, and the transfer member are connected to different grounds, The resistance value of the protective resistor provided between the fixing belt and the ground is defined as a fixing ground resistance Rf, When the resistance value of the recording medium is a recording medium resistance Rp and the resistance value of the transfer member is a transfer resistance Rt, The transfer resistance Rt satisfies the following formula 1: It is characterized by:

[0008]

number

[0009] However, in formula 1, the following is represented. Rf: Resistance value of fixed ground resistance Rt: Resistance value of transfer resistor Rp: resistance value of the recording medium jXs: Impedance of the fixing belt jXh: Impedance of the heating means Also, " / / " in Equation 1 indicates that resistors are connected in parallel in an electric circuit. For example, for resistors Ra, R1, and R2, if Ra = R1 / / R2, then it means Ra = (R1 x R2) / (R1 + R2). Impedance is treated as equivalent to resistance by taking its absolute value. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide an image forming apparatus that can suppress AC banding without increasing manufacturing costs and that is also advantageous in terms of lightning surges. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a schematic diagram illustrating an example of an image forming apparatus according to the present invention. [Figure 2] FIG. 2 is a schematic diagram illustrating a main part of an example of a belt-type secondary transfer unit. [Figure 3] FIG. 2 is a schematic diagram of a fixing unit. [Figure 4] FIG. 2 is a diagram for explaining a fixing unit and a transfer unit in an example of an image forming apparatus according to the present invention, and is a diagram for schematically explaining AC banding. [Figure 5] FIG. 5 is an equivalent circuit diagram of FIG. 4. [Figure 6] FIG. 6 is a circuit diagram in which the arrangement of part of the circuit diagram in FIG. 5 is changed. [Figure 7] FIG. 7 is a circuit diagram in which the circuit diagram of FIG. 6 is expressed in another way. [Figure 8] This is an explanation of Figure 7. DETAILED DESCRIPTION OF THE INVENTION

[0012] The image forming apparatus according to the present invention will be described below with reference to the drawings. The present invention is not limited to the following embodiments, and other modifications, additions, corrections, deletions, and other changes can be made within the scope of what a person skilled in the art can conceive. Any modification that achieves the functions and effects of the present invention is included in the scope of the present invention.

[0013] The image forming apparatus of the present invention comprises: a fixing unit having a rotatable fixing belt and a pressure member facing the fixing belt to form a fixing nip; An image forming apparatus including a transfer means having a rotatable transfer member and an opposing member that faces the transfer member and forms a transfer nip, the transfer unit is disposed upstream of the fixing unit in the recording medium conveyance direction, the fixing unit has a heating unit that is in contact with the fixing belt on the inner side of the fixing belt, the heating means is connected to a power source that applies an AC voltage; the fixing belt, the pressure member, and the transfer member are connected to different grounds, The resistance value of the protective resistor provided between the fixing belt and the ground is defined as a fixing ground resistance Rf, When the resistance value of the recording medium is a recording medium resistance Rp and the resistance value of the transfer member is a transfer resistance Rt, The transfer resistance Rt satisfies the following formula 1: It is characterized by:

[0014]

number

[0015] However, in formula 1, the following is represented. Rf: Resistance value of fixed ground resistance Rt: Resistance value of transfer resistor Rp: resistance value of the recording medium jXs: Impedance of the fixing belt jXh: Impedance of the heating means

[0016] According to the present invention, it is possible to provide an image forming apparatus that can suppress AC banding without increasing manufacturing costs and that is also advantageous in terms of lightning surges.

[0017] FIG. 1 is a schematic diagram of the entire image forming apparatus of this embodiment. The photoconductor 1 is a cylindrical photoconductor drum that rotates in the direction of the arrow. The photoconductor may also be called an electrostatic latent image carrier, an image carrier, or the like. The charger 2 is an example of a charging means, and is, for example, in the form of a roller. The charger 2 is pressed against the surface of the photoreceptor 1 and rotates in response to the rotation of the photoreceptor 1. The charger 2 applies a bias, for example, DC or DC superimposed with AC, to the photoreceptor 1 from a high-voltage power supply, to uniformly charge the photoreceptor 1. The charging means used may be a system using a roller-shaped member or a system using wire discharge.

[0018] After the photoreceptor 1 is charged, image information is exposed to the photoreceptor 1 by an exposure means 3, which is a latent image forming means, to form an electrostatic latent image. The exposure step is carried out using, for example, a laser beam scanner using a laser diode, an LED, or the like.

[0019] The developing device 4 is an example of a developing means, and visualizes the electrostatic latent image on the photosensitive member 1 as a toner image by a predetermined developing bias supplied from a high-voltage power supply. The developing device 4 contains toner.

[0020] The process unit 10 integrates the photoreceptor 1, charger 2, developer 4, and cleaning unit 7. Four process units 10 are arranged in parallel, for example, black, cyan, magenta, and yellow process units. When forming a full-color image, visible images (toner images) of each color (black, cyan, magenta, and yellow) are transferred onto the transfer belt 15 in order, superimposed on top of each other.

[0021] The cleaning unit 8 has therein a cleaning blade 6 for cleaning the photosensitive member.

[0022] The transfer belt 15 is stretched by a secondary transfer opposing roller 21, which also serves as a transfer roller, a cleaning opposing roller 16, a primary transfer roller 5, and a tension roller 20, and is driven to rotate by, for example, a drive motor via the secondary transfer roller 21. In addition, as a stretching mechanism for the transfer belt 15, the tension roller 20 is pressed by springs on both sides of the roller. The secondary transfer opposing roller 21 is an example of an opposing member, and faces a transfer member (for example, a secondary transfer roller 25).

[0023] The process unit 10 and the secondary transfer opposing roller 21 may have separate or common drive sources. Generally, at least the process unit for black and the transfer drive are turned on and off, and from the viewpoint of miniaturization and cost reduction of the device, it is preferable that they share a common drive source.

[0024] The transfer belt cleaning unit 32 has a cleaning blade 31 that is brought into counter contact with the belt 15. The transfer belt cleaning unit 32 performs cleaning by scraping off the residual toner on the transfer belt 15 with the cleaning blade 31.

[0025] In addition to the blade cleaning method, electrostatic methods such as an electrostatic brush method and an electrostatic roller method can also be used to clean the transfer belt 15. The blade cleaning method is preferable from the viewpoints of device miniaturization, cost reduction, and ease of cleaning. In the case of an electrostatic method, a brush or roller to which a bias is applied is used, but depending on the usage conditions of the image forming apparatus, pre-charging of the residual toner after transfer may be necessary. Therefore, the electrostatic method may have drawbacks such as a larger cleaning unit, the need for one or two additional high-voltage power supplies, and the need for extra operations for bias cleaning.

[0026] The residual toner scraped off by the cleaning blade 31 passes through a toner transport path and is stored in a waste toner storage unit 33 for the intermediate transfer member.

[0027] The primary transfer rollers 5 (5_1 to 5_4) are, for example, metal rollers or conductive sponge rollers, and are arranged opposite the photosensitive member 1 with the transfer belt 15 interposed therebetween. A primary transfer bias is applied to the primary transfer rollers 5 by an independent high-voltage power supply, and the toner image (visible image) on the photosensitive member 1 is transferred to the transfer belt 15.

[0028] The metal roller used as the primary transfer roller 5 can be made of, for example, aluminum or SUS. The conductive sponge roller used as the primary transfer roller 5 can be made of, for example, an ion-conductive roller, such as urethane with carbon dispersed therein, NBR (acrylonitrile butadiene rubber), or hydrin rubber. In addition to the above, an electronically conductive roller can also be used as the primary transfer roller 5, such as an EPDM (ethylene propylene diene rubber).

[0029] The transfer belt 15 may be an endless resin film made of, for example, PVDF (vinyl fluoride), ETFE (ethylene-tetrafluoroethylene copolymer), PI (polyimide), PC (polycarbonate), TPE (thermoplastic elastomer), etc., in which a conductive material such as carbon black is dispersed. The transfer belt 15 is an example of an intermediate transfer member.

[0030] The image forming apparatus shown in Figure 1 is an example using a secondary transfer unit, and as the secondary transfer unit, for example, a system using a roller (roller system) as shown in Figure 1 can be used, or alternatively, a system using a belt (belt system) can be used.

[0031] In the case of a system using a roller, for example, a sponge roller, an ion-conductive roller, an electronically conductive roller, etc. can be used as the secondary transfer roller 25. For example, urethane with carbon dispersed therein, NBR, hydrin rubber, etc. can be used as the ion-conductive roller. For example, EPDM can be used as the electronically conductive roller.

[0032] The secondary transfer roller 25 is applied with a driving force from, for example, a motor and also with a bias.

[0033] In the case of a system using a roller, it is preferable to use a cleaning unit (not shown in Fig. 1) that cleans the secondary transfer roller 25. This cleaning unit has, for example, a cleaning blade that comes into counter contact with the secondary transfer roller 25, and performs cleaning by scraping off residual toner on the secondary transfer roller 25 with this cleaning blade. For example, cleaning unit 27 shown in Fig. 2 (described later) can be used as such a cleaning unit.

[0034] Fig. 2 is a diagram for explaining a belt-type secondary transfer unit. Fig. 2 is a diagram showing the main part of Fig. 1, in which the roller-type secondary transfer unit has been changed to a belt-type secondary transfer unit.

[0035] In the belt system, the secondary transfer belt 28 is stretched over a secondary transfer roller 25 and a tension roller 29. The secondary transfer belt 28 is driven to rotate via the secondary transfer roller 25 by a drive motor (not shown).

[0036] The secondary transfer belt 28 may be, for example, the same as the transfer belt 15. The secondary transfer belt 28 may be, for example, an endless belt in the form of a resin film, in which a conductive material such as carbon black is dispersed in PVDF, ETFE, PI, PC, TPE, or the like.

[0037] Returning to FIG. 1, the transfer material (also referred to as recording medium or recording material) may be set in paper feed cassette 22 or manual feed slot 42. The transfer material is transported by paper feed transport roller 23, registration roller pair 24, etc. in time with the leading edge of the toner image on transfer belt 15 reaching the secondary transfer position. A predetermined secondary transfer bias is applied to secondary transfer roller 25 by a high-voltage power supply, so that the toner image on transfer belt 15 is transferred to the transfer material.

[0038] In this embodiment, the recording material is transported in a vertical path, but this is not limited to this. The transfer material is separated from the transfer belt 15 by the curvature of the secondary transfer opposing roller 21. The transferred toner image is fixed to the transfer material by the fixing means 40, and then the transfer material is discharged from the discharge port 41.

[0039] Next, the fixing means in this embodiment will be described. Figure 3 is a diagram for explaining the fixing means 40 (which may also be called a fixing device) in this embodiment.

[0040] As shown in Figure 3, the fixing means 40 in this embodiment includes a fixing belt 50 consisting of an endless belt member as a fixing member, a pressure roller 51 as a pressure member that contacts the outer surface of the fixing belt 50 to form a fixing nip N1, and a heater 52 as a heating means for heating the fixing belt 50.

[0041] The heater 52 is a planar heater having a longitudinal direction in the direction of the rotation axis of the fixing belt 50. The heater holder 53 serves as a holding member and holds the heater 52. The stay 54 serves as a reinforcing member and reinforces the heater holder 53 in the longitudinal direction.

[0042] The fixing belt 50 has a cylindrical substrate made of polyimide (PI) with an outer diameter of 25 mm and a thickness of 40 to 120 μm. A release layer made of a fluorine-based resin such as PFA or PTFE and having a thickness of 5 to 50 μm is formed on the outermost surface of the fixing belt 50 to enhance durability and ensure releasability. A 50 to 500 μm thick elastic layer made of rubber or the like may be provided between the substrate and the release layer. The substrate of the fixing belt 50 is not limited to polyimide, and may be a heat-resistant resin such as PEEK or a metal substrate such as nickel (Ni) or SUS. The inner peripheral surface of the fixing belt 50 may be coated with a sliding layer made of polyimide, PTFE, or the like.

[0043] The pressure roller 51 has an outer diameter of, for example, 25 mm and is composed of a solid iron core 51a, an elastic layer 51b formed on the surface of the core 51a, and a release layer 51c formed on the outside of the elastic layer 51b. The elastic layer 51b is made of silicone rubber and has a thickness of, for example, 3.5 mm. To improve the release properties of the surface of the elastic layer 51b, it is desirable to form the release layer 51c, which is a fluororesin layer having a thickness of, for example, about 40 μm.

[0044] The heater 52 is provided longitudinally across the width of the fixing belt 50 and is disposed so as to be in contact with the inner circumferential surface of the fixing belt 50. The heater 52 may be in non-contact with the fixing belt 50 or indirect contact with the fixing belt 50 via a low-friction sheet or the like, but direct contact of the heater 52 with the fixing belt 50 improves the efficiency of heat transfer to the fixing belt 50. The heater 52 can also be in contact with the outer circumferential surface of the fixing belt 50, but since there is a risk that the fixing quality will deteriorate if the outer circumferential surface of the fixing belt 50 is damaged by contact with the heater 52, it is preferable that the heater 52 be in contact with the inner circumferential surface of the fixing belt 50.

[0045] The heater 52 has, for example, a resistance heating element, and the resistance heating element is coated with, for example, glass.

[0046] The heater holder 53 and the stay 54 are disposed on the inner circumferential side of the fixing belt 50. The stay 54 is made of a metal channel material, and both ends thereof are supported by both side wall portions of the fixing unit 40. Because the stay 54 supports the surface of the heater holder 53 opposite the heater 52 side, the heater 52 and the heater holder 53 are maintained without being significantly deflected by the pressure force of the pressure roller 51, and a fixing nip portion N1 is formed between the fixing belt 50 and the pressure roller 51. In this embodiment, the heater 52 functions as a nip forming member that sandwiches the fixing belt 50 between the heater 52 and the pressure roller 51 to form the fixing nip portion N1.

[0047] The heater holder 53 is desirably made of a heat-resistant material because it is prone to becoming hot due to the heat from the heater 52. For example, if the heater holder 53 is made of a heat-resistant resin with low thermal conductivity, such as LCP or PEEK, heat transfer from the heater 52 to the heater holder 53 is suppressed, and the fixing belt 50 can be heated efficiently.

[0048] The pressure roller 51 and the fixing belt 50 are pressed against each other by a spring that serves as a biasing member. This forms a fixing nip N1 between the fixing belt 50 and the pressure roller 51. The pressure roller 51 also functions as a drive roller that is driven to rotate by a driving force transmitted from a driving unit provided in the image forming apparatus main body 103. Meanwhile, the fixing belt 50 is configured to rotate in response to the rotation of the pressure roller 51. During rotation, the fixing belt 50 slides against the heater 52. To improve the sliding properties of the fixing belt 50, a lubricant such as oil or grease may be interposed between the heater 52 and the fixing belt 50.

[0049] When the printing operation is started, the pressure roller 51 is driven to rotate, and the fixing belt 50 starts to rotate in response. Furthermore, power is supplied to the heater 52, thereby heating the fixing belt 50. Then, when the temperature of the fixing belt 50 reaches a predetermined target temperature (fixing temperature), as shown in FIG. 3, the recording material P carrying an unfixed toner image is conveyed between the fixing belt 50 and the pressure roller 51 (fixing nip N1), whereby the unfixed toner image is heated and pressurized and fixed to the recording material P.

[0050] Next, AC banding will be described with reference to FIG. Figure 4 was created with reference to "The Horizontal Banding Image Related to the Surface Heating Fuser System," by Jun Asami, Yasutaka Yagi, Kenichi Karino, and Ken Oi, Canon, Inc. (Japan) NIP26 and Digital Fabrication 2010 Technical Program and Proceedings, pp. 238-241. In Figure 4, the transfer means and the fixing means are shown in a schematic manner, and the state when the recording material P (recording medium) is simultaneously sandwiched between the fixing nip portion N1 and the transfer nip portion N2 is shown in a schematic manner.

[0051] The image forming apparatus of this embodiment has a fixing unit and a transfer unit. The fixing means 40 includes a fixing belt 50 and a pressure roller 51 (pressure member) that faces the fixing belt 50 and forms a fixing nip portion N1. The transfer means includes a secondary transfer roller 25 (transfer member) and a secondary transfer opposing roller 21 (opposing member) that faces the secondary transfer roller 25 and forms a transfer nip portion N2.

[0052] The transfer means is disposed upstream of the fixing means in the conveying direction of the recording material P (recording medium). The fixing means has a heater 52 (heating means) that is in contact with the fixing belt 50 on the inner side of the fixing belt 50. The heater 52 is connected to a power source (AC power source 60) that applies an AC voltage.

[0053] As shown in the figure, the fixing belt 50, the pressure roller 51, and the secondary transfer roller 25 are each connected to a different ground. More specifically, the conductive portion of the inner surface of the fixing belt 50 is connected to a protective resistor 56 via a contact member 55 (e.g., brass), and the protective resistor 56 is further connected to a ground 62. Therefore, it can be said that the fixing belt 50 is connected to the ground 62 via the contact member 55 and the protective resistor 56. Note that although it is stated here that the fixing belt 50 is connected to the ground, it may also be stated that the fixing belt 50 is grounded.

[0054] In the example shown in FIG. 4, the metal core 51a of the pressure roller 51 is connected to a ground 63, and the metal core 25a of the secondary transfer roller 25 is connected to a ground 64 via a DC voltage source .

[0055] There are two methods for applying the secondary transfer bias: an attractive transfer method and a repulsive transfer method. In the attraction transfer method, a positive (+) bias is applied to the secondary transfer roller 25, and the secondary transfer opposing roller 21 is grounded, thereby forming a secondary transfer electric field. In the repulsive transfer method, a negative (-) bias is applied to the secondary transfer opposing roller 21, and the secondary transfer roller 25 is grounded, thereby forming a secondary transfer electric field.

[0056] 4, an attractive transfer method is used in which a bias is applied to the secondary transfer roller 25, and the bias is applied to the secondary transfer roller 25 by a DC voltage source 70. The DC voltage source 70 is also connected to the ground 64. Therefore, the secondary transfer roller 25 is connected to the ground 64 via the DC voltage source 70.

[0057] In the figure, the white arrow a indicates the conveyance direction of the recording material P. When the recording material P passes through the transfer nip N2 formed by the secondary transfer roller 25 and the secondary transfer opposing roller 21, the toner image on the transfer belt 15 is transferred onto the recording material P. Note that the transfer belt 15 is not shown in FIG.

[0058] The recording material P onto which the toner image has been transferred is transported in the direction of the fixing means (the direction of the white arrow in the figure). When the recording material P passes through the transfer nip N1 formed by the fixing belt 50 and the pressure roller 51, the toner image on the recording material P is fixed.

[0059] At this time, if a recording material P (recording medium) is simultaneously sandwiched between the fixing nip N1 and the transfer nip N2, the AC voltage applied to the heater 52 may be transmitted from the fixing nip N1 to the transfer nip N2 via the recording material P. For example, if the moisture content of the recording material is high, the impedance of the recording material P decreases, making it easier for the AC voltage applied to the heater 52 to be transmitted to the transfer nip N2. In the figure, the transmission of the AC voltage to the transfer nip N2 via the recording material P is schematically shown by a waveform, and the black arrow in the figure indicates the direction of transmission.

[0060] The AC voltage transmitted to the transfer nip N2 fluctuates the transfer voltage at the transfer nip N2, which causes transfer unevenness and results in stripes (uneven density) in the image. This phenomenon is called AC banding. When AC banding occurs, uneven density occurs in the image, making it difficult to obtain a good image. Patent Document 1 and other documents have proposed ways to prevent AC banding, but these have problems such as increased manufacturing costs due to the increased number of components.

[0061] For example, when the alternating current component (e.g., 50 Hz) of the AC power supply is transmitted to the transfer nip N2 via the fixing belt and recording medium and is superimposed on the transfer bias, a banding image with a period of the linear speed of the recording medium x 1 / 50 Hz is produced.

[0062] The present inventors have conducted extensive research into this issue and have arrived at the present invention. One embodiment of the present invention will be described with reference to Figures 5 and 6. Figure 5 is a circuit diagram (equivalent circuit) in the state of Figure 4, and Figure 6 is the same as the equivalent circuit of Figure 5, but with some changes made to the layout for ease of explanation.

[0063] Figures 5 and 6 show the following: Rf: Resistance value of fixed ground resistance Rt: Resistance value of transfer resistor Rp: Resistance value of recording medium Rk: Resistance value of pressure member resistance jXs: Impedance of the fixing belt jXh: Impedance of heating means jXk: Impedance of pressure member

[0064] The fixing ground resistance Rf is the resistance value of the fixing ground resistance, for example, the resistance value of the protective resistor 56 provided between the fixing belt 50 and the ground 62. Transfer resistance Rt is the resistance value of the transfer member, and when, for example, a sponge roller consisting of a core material 25a and a sponge portion is used as secondary transfer roller 25, transfer resistance Rt can be said to be the resistance value of the sponge portion of secondary transfer roller 25. When, for example, an ion-conductive roller or an electronically conductive roller is used as secondary transfer roller 25, transfer resistance Rt can be said to be the resistance value of the ion-conductive roller itself or the resistance value of the electronically conductive roller itself.

[0065] Rk is the resistance value of the resistance of the pressure member, and is, for example, a series resistance component included when the core metal 51a of the pressure roller 51 is grounded. jXs is the impedance of the fixing belt 50; jXh is the impedance of the heater 52. The heater 52 has, for example, a resistance heating element, which is coated with, for example, glass. For this reason, this glass acts as a capacitor in the equivalent circuit, and is therefore expressed as jXh. jXk is the impedance of the elastic layer 51b and the release layer 51c of the pressure roller 51, for example.

[0066] One way to prevent the AC voltage at the fixing nip N1 from being transmitted to the transfer nip N2 via the recording material P is to reduce the resistance (transfer resistance Rt) of the secondary transfer roller 25. In this case, to prevent the AC voltage (AC component) from being transmitted from the AC power source 60 to the fixing nip N1, it is possible to reduce the voltage applied to the transfer means by dividing the voltage. An example of voltage division here is applying a voltage to the fixing ground (ground 62) or the pressure ground (ground 63). To reduce the voltage applied to the transfer means by dividing the voltage in this way, it is sufficient to reduce the total resistance (recording medium resistance Rp + transfer resistance Rt) beyond the fixing nip N1. However, in this case, the resistance of the recording medium cannot be controlled by design, so this can be solved by reducing the transfer resistance Rt.

[0067] One way to reduce the voltage applied to the transfer means is to reduce the fixing ground resistance Rf. When the fixing ground resistance Rf is small, the AC voltage applied to the heater 52 is more easily transmitted to the fixing ground resistance Rf, and the AC voltage is less easily transmitted to the fixing belt 50 and the fixing nip N1. This makes it more difficult for the AC voltage to be transmitted to the transfer means.

[0068] However, when it comes to grounding, there is a trade-off between the issues of lightning surges and AC banding. For AC banding, a smaller fixed ground resistance Rf is more advantageous, but for lightning surges, a larger fixed ground resistance Rf is more advantageous. Therefore, while reducing the fixed ground resistance Rf makes it easier to prevent AC banding, if the fixed ground resistance Rf is too small, a lightning surge occurs in which high voltage is applied to the equipment from the AC power supply 60. If the fixed ground resistance Rf has a certain magnitude, high voltage from the AC power supply 60 can flow to the ground 61 when lightning strikes.

[0069] The inventors have investigated the above and found that if the transfer resistance Rt can be reduced, AC banding can be suppressed and high voltages due to lightning surges can be prevented from being applied to the device, even if the fixing ground resistance Rf is relatively large. In other words, by reducing the transfer resistance Rt, AC banding can be suppressed by suppressing the transmission of AC voltage to the transfer nip N2 via the recording material P, and the device configuration can be made more resistant to lightning surges.

[0070] Furthermore, when the AC partial voltage ratio applied to the secondary transfer roller 25 is less than 30%, AC banding can be suppressed.

[0071] On the other hand, if the transfer resistance Rt is too small, abnormal images such as white spots and uneven images will occur. Also, if the transfer resistance Rt is too small, it may become impossible to transfer a multi-color image portion (e.g., a three-color superimposed image) and a single-color image portion that exist on the same image, and the transfer efficiency may decrease.

[0072] As a result of investigations taking these factors into consideration, it was found that the desired effect can be obtained by making the transfer resistance Rt satisfy the following formula 1.

[0073]

number

[0074] However, in formula 1, the following is represented. Rf: Resistance value of fixed ground resistance Rt: Resistance value of transfer resistor Rp: Resistance value of recording medium jXs: Impedance of the fixing belt jXh: Impedance of heating means

[0075] Note that the above formula 1 does not include the resistance (Rk) or impedance (jXk) resulting from the pressure member (pressure roller 51). The pressure roller 51 typically has a thick elastic layer 51b, which results in a much larger impedance than other paths. Therefore, no or almost no AC current flows in the direction of the pressure member, and the impedance and resistance of the path toward the pressure member can be omitted.

[0076] 7 and 8 are diagrams for explaining the derivation of the above formula 1. Fig. 7 is a circuit diagram in which the circuit diagram of Fig. 6 is expressed in another way, and Fig. 8 is an explanation of Fig. 7.

[0077] Equation 1 calculates the voltage division ratio, but because it contains the complex component of the capacitor, the voltage division ratio is also a complex number. Therefore, taking the absolute value of the complex number gives the amplitude voltage division ratio. For this reason, the left side of Equation 1 is an absolute value.

[0078] The transfer resistance Rt is measured as follows. The secondary transfer roller 25 is placed on a conductive metal plate, and a load of 4.9 N is applied to each end of the core metal 25a of the secondary transfer roller 25 (a total of 9.8 N on both sides). A voltage of 1000 V is applied between the core metal 25a and the metal plate, and the measurement is calculated from the current value that flows. The measurement environment is high temperature and high humidity (27°C, 80%).

[0079] The fixed earth resistance Rf is measured by measuring both ends of the resistor with a tester. The resistance value of the recording medium (recording medium resistance Rp) is measured as follows. The paper is left in the measurement environment for 24 hours or more, and the surface resistivity is measured using a Type HA probe with a Hiresta IP (MCP-HT450) measuring instrument manufactured by Mitsubishi Chemical Analytech.

[0080] The impedance jXs of the fixing belt 50 and the impedance jXh of the heating means (heater 52) are measured as follows. Attach the electrodes of an LCR meter to the inner and outer circumferences of the fixing belt, set the measurement frequency to the commercial frequency of 50Hz / 60Hz, which is a problem in AC banding, and measure the resistance / capacitance (impedance) (see

[0065] of Patent No. 6961123).

[0081] When the transfer resistance Rt satisfies Equation 1, the AC voltage division ratio becomes smaller than 30%, and AC banding can be suppressed. Furthermore, satisfying Equation 1 results in an apparatus configuration that is effective against lightning surges. If AC banding can be prevented by adjusting the transfer resistance Rt, as in the present invention, the fuser ground resistance Rf can be increased. Furthermore, if the fuser ground resistance Rf is increased, the voltage applied to Xh in Equation 1 (for example, the heater's insulating layer) can be reduced through voltage division. This is therefore advantageous for preventing heater insulation breakdown due to lightning surges. Note that when considering lightning surges, the equivalent circuit does not include the path from the recording medium to the transfer, so the resistance value Rp of the recording medium and the transfer resistance Rt are excluded from the consideration.

[0082] In order to satisfy the formula 1, for example, the following method can be considered. (1) Increase the thickness of the insulating layer of the heater and fixing belt (increase jXh and jXs). (2) Increase the distance between the fixing point and the transfer point to increase the resistance value Rp of the recording medium. (3) Decrease the value of the fixed ground resistance Rf (4) Decrease the resistance value (transfer resistance Rt) of the secondary transfer roller 25 Among these, the method (4) of reducing the resistance value (transfer resistance Rt) of the secondary transfer roller 25 is preferable.

[0083] When reducing the value of transfer resistance Rt, it is preferable to consider its relationship with the fuser ground resistance Rf. The range of transfer resistance Rt required to reduce the AC voltage division ratio to less than 30% varies depending on, for example, the value of fuser ground resistance Rf, the thickness of the insulator of secondary transfer roller 25, and the distance from the fixing location to the transfer location. For example, when fuser ground resistance Rf is 2 MΩ, if transfer resistance Rt is 1250 MΩ or less, the AC voltage division ratio can be reduced to less than 30%, and AC banding can be suppressed.

[0084] When AC banding is taken into consideration, the AC voltage division ratio of the AC component may be referred to as the AC leakage rate. An AC voltage division ratio of less than 30% corresponds to an AC leakage rate of less than 30%.

[0085] The transfer resistance Rt is preferably 35 MΩ or less. When the transfer resistance Rt is 35 MΩ or less, the AC division ratio can be made smaller than 30% regardless of the value of the fixing ground resistance Rf, and AC banding can be suppressed.

[0086] The transfer resistance Rt is preferably 5 MΩ or more. When the transfer resistance Rt is 5 MΩ or higher, abnormal images such as white spots and grainy images can be suppressed. Furthermore, if the transfer resistance Rt is too low, it may be impossible to transfer both a multi-color image area (e.g., a three-color overlapping image) and a single-color image area on the same image. A single-color area (also called a single-color image area) of an image requires a relatively low voltage to transfer a sufficient current. On the other hand, a multi-color area (also called a multi-color image area) of an image requires a voltage higher than the optimal voltage for the single-color image area. Therefore, if a voltage is set that allows the multi-color image area to be transferred, the transfer current in the single-color image area will be excessive, reducing transfer efficiency.

[0087] In this embodiment, based on the idea of ​​reducing the transfer resistance Rt, the relationship between the fixing ground resistance Rf and other factors is set as shown in Equation 1. This eliminates the need to provide new components to prevent AC banding, preventing increases in manufacturing costs. Furthermore, the device configuration is simple, making it effective against AC banding and lightning surges.

[0088] The heater 52 is a planar heater having a longitudinal direction in the direction of the rotation axis of the fixing belt 50. By using such a planar heater, the fixing belt 50 can be heated efficiently.

[0089] 4 to 6, a repulsive transfer method is used, in which a negative (-) bias is applied to the secondary transfer opposing roller 21, and the secondary transfer roller 25 is grounded. In this embodiment, an attractive transfer method can also be used, in which a positive (+) bias is applied to the secondary transfer roller 25, and the secondary transfer opposing roller 21 is grounded. Even in the attractive transfer method, the relationship of Equation 1 remains unchanged.

[0090] In addition, in the examples shown in Figures 4 to 6, a secondary transfer roller is used as the transfer member, but in this embodiment, as described above, the secondary transfer unit is not limited to the roller type, and a belt type (Figure 2) can also be used.

[0091] In the case of the belt system, the transfer resistance Rt is the same as in the roller system because the secondary transfer roller 25 is also used. The relationship in Equation 1 remains the same even in the case of the belt system. [Explanation of symbols]

[0092] 21 Secondary transfer opposing roller 25 Secondary transfer roller 50 Fixing belt 51 Pressure roller 52 Heater 55 Contact member 56 Protective Resistor 60 AC power supply 70 DC voltage source N1 Fixing nip N2 Transfer nip [Prior art documents] [Patent documents]

[0093] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-113910

Claims

1. a fixing unit having a rotatable fixing belt and a pressure member facing the fixing belt to form a fixing nip; An image forming apparatus including a transfer means having a rotatable transfer member and an opposing member that faces the transfer member and forms a transfer nip, the transfer unit is disposed upstream of the fixing unit in the recording medium conveyance direction, the fixing unit has a heating unit that is in contact with the fixing belt on the inner side of the fixing belt, the heating means is connected to a power source that applies an AC voltage; the fixing belt, the pressure member, and the transfer member are connected to different grounds, The resistance value of the protective resistor provided between the fixing belt and the ground is defined as a fixing ground resistance Rf, When the resistance value of the recording medium is a recording medium resistance Rp and the resistance value of the transfer member is a transfer resistance Rt, The transfer resistance Rt satisfies the following formula 1: An image forming apparatus characterized by: [Equation 1] However, in formula 1, the following is represented. Rf: Resistance value of fixed ground resistance Rt: Resistance value of transfer resistor Rp: resistance value of the recording medium jXs: Impedance of the fixing belt jXh: Impedance of the heating means In addition, " / / " in Equation 1 indicates that resistors are connected in parallel in an electric circuit. For example, for resistors Ra, R1, and R2, if Ra = R1 / / R2, then Ra = (R1 x R2) / (R1 + R2). Impedance is treated as equivalent to resistance by taking its absolute value.

2. The transfer resistance Rt is 35 MΩ or less.

2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

3. The transfer resistance Rt is 5 MΩ or more.

3. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

4. The heating means is a planar heater having a longitudinal direction in the direction of the rotation axis of the fixing belt.

4. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

5. The transfer member is a secondary transfer roller, which transfers the toner image transferred from the image carrier to the intermediate transfer member onto the recording medium.

5. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

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