across the fixing film 36 via the pressurizing stay 41 and the heater holder 38. According to the present exemplary embodiment, the total contact pressure of the fixing film 36 and the pressure roller 32 is 180 N. Thus, the heater 37 forms the nip portion N having a width of about 6 mm together with the pressure roller 32 against the elasticity of the pressure roller 32 across the fixing film 36.

When the rotational force is transmitted from the driving source (not illustrated) to the drive gear G of the pressure roller 32, the pressure roller 32 is driven to rotate in the clockwise direction in FIG. 2 at a predetermined speed. According to the present exemplary embodiment, the rotational speed of the pressure roller 32 is controlled so that the recording material P is conveyed at a conveyance speed of 100 mm/sec. When the pressure roller 32 is driven to rotate, the fixing film 36 rotates in the counterclockwise direction in FIG. 2 by the frictional force acting between the pressure roller 32 and the fixing film 36 at the nip portion N. Accordingly, the fixing film 36 slides in contact with the heater 37 at the nip portion N, and is driven to rotate in the counterclockwise direction around the heater holder 38 by the rotation of the pressure roller 32.

The fixing film 36 rotates, and power is supplied to the heater 37. When the temperature detected by a thermistor (not illustrated) of the heater 37 reaches a target temperature, the recording material P is conveyed to the nip portion N. A fixing apparatus entry guide 30 guides the recording material P with a non-fixed toner image t carried thereon, toward the nip portion N.

When the recording material P with the non-fixed toner image t carried thereon is conveyed to the nip portion N, the surface of the recording material P carrying the toner image t comes into contact with the fixing film 36 at the nip portion N, and the recording material P is nipped and conveyed at the nip portion N with the rotation of the fixing film 36. In this conveyance process, the non-fixed toner image t on the recording material P is heated and pressurized by the fixing film 36 and the pressure roller 32, respectively, to be fixed to the recording material P. When the recording material P passes through the nip portion N, the recording material P is curvature-separated from the surface of the fixing film 36. Then, the recording material P is discharged out of the fixing apparatus 18 by a discharge roller pair (not illustrated). The maximum paper feedable width of the fixing apparatus 18 according to the present exemplary embodiment is 216 mm, and printing on the recording material P of the LTR size can be performed at a printing speed of 20 prints per minute (PPM).

Contact members 60 and 61 according to the present exemplary embodiment will be described below with reference to FIGS. 5A and 5B. FIG. 5A illustrates arrangements of the contact members 60 and 61 on the fixing film 36 and the pressure roller 32, respectively, in the longitudinal direction. FIG. 5B illustrates contact states of the contact members 60 and 61 on the surfaces of the fixing film 36 and the pressure roller 32, respectively.

Referring to FIG. 5A, the shaded portion indicates the longitudinal region of the nip portion N where the recording material P having the maximum size conveyed at a center reference is fed (hereinafter this region is also referred to as a paper feed region). According to the present exemplary embodiment, the region closer to one end side in the longitudinal direction than the paper feed region is referred to as a region R1, and the region closer to the other end side in the longitudinal direction is referred to as a region R2. The contact member 60 for grounding the fixing film 36 is disposed in the region R1 in the longitudinal direction. The contact member 61 for grounding the pressure roller 32 is disposed in the region R2 in the longitudinal direction. According to present exemplary embodiment, the contact member 60 is disposed at the position by a distance L1 from a conveyance reference, and the contact member 61 is disposed at the position by a distance L2 from the conveyance reference. Although the contact members 60 and 61 are disposed so that the distances L1 and L2 are equal as an example, the present exemplary embodiment is not limited to this example. A contact member needs to be disposed in each of the region R1 on one end side in the longitudinal direction and the region R2 on the other end side in the longitudinal direction.

Although the recording material P is conveyed at the center reference as an example, the present exemplary embodiment is not limited to this example. For example, the recording material P is conveyed with reference to one side, the longitudinal center of the fixing film 36 may be used as a reference. In this case, the contact member 60 is disposed at the position by the distance L1 from the center of the fixing film 36, and the contact member 61 is disposed at the position by the distance L2 from the center of the fixing film 36. Although the contact members 60 and 61 are disposed so that the distances L1 and L2 are equal as an example, the present exemplary embodiment is not limited to this example. A contact member needs to be disposed in each of the region R1 on one end side in the longitudinal direction and the region R2 on the other end side in the longitudinal direction.

The contact member 60 is made of a heat-resistant resin provided with conductivity. According to the present exemplary embodiment, the contact member 60 is a polyimide film containing distributed carbon and having a thickness of 60 μm and a shape of 22 mm by 6.55 mm. Contact states of the contact member 60 and the fixing film 36 are illustrated in FIG. 5B.

The polyimide film as the contact member 60 is disposed on the outer surface (outer circumferential surface) of the fixing film 36 so that the longer side of the contact member 60 is parallel to the rotational direction of the fixing film 36, and the contact member 60 comes into contact with the fixing film 36 with a contact pressure of 0.0148 to 0.0235 N in the forward direction. As a grounding condition, the contact member 60 is connected with a parallel circuit of a capacitor 63 and a diode 64 via a 1.5-MΩ resistor 62 and grounded to a grounding portion. This configuration is intended to prevent an electrostatic offset or banding that occurs when the alternating current (AC) voltage oscillation driving the heater 37 of the fixing apparatus 18 is superimposed on the direct current (DC) voltage at the transfer nip portion across the recording material P.

The contact member 61 is made of metal. According to the present exemplary embodiment, the contact member 61 is stainless steel (SUS) having a thickness of 0.12 mm and a shape of 25 mm by 7.00 mm. Contact states of the contact member 61 and the pressure roller 32 are illustrated in FIG. 5B. The SUS as the contact member 61 is disposed on the outer surface (outer circumferential surface) of the pressure roller 32 so that the longer side of the contact member 61 is parallel to the rotational direction of the pressure roller 32, and the contact member 61 comes into contact with the pressure roller 32 with a contact pressure of 0.245 to 0.343 N in the forward direction. As a grounding condition, the contact member 61 is grounded to the grounding portion via a 1-GΩ resistor 65. This configuration is intended to prevent an offset due to charging, separating discharge, and a transfer current leakage.

FIG. 6 illustrates a comparative example. According to the comparative example, two different contact members are disposed in the region on the same one end side in the longitudinal direction.

FIG. 7 illustrates charts indicating longitudinal surface potentials of the fixing film 36 and the pressure roller 32 according to the present exemplary embodiment and the comparative example. The vertical axis denotes the surface potential, the horizontal axis denotes the longitudinal position of the fixing film 36 and the pressure roller 32, a point A denotes the contact position of the contact member 60, and a point B denotes the contact position of the contact member 61. Referring to FIG. 7, the plots denote the potentials at contact positions A and B, and the solid and broken lines denote approximate lines of these potentials.

The charts in FIG. 7 according to the present exemplary embodiment will be described below. The longitudinal surface potential of the fixing film 36 has a gradient formed between about −80 to −85 V in the vicinity of the contact member 60 and −170 to −175 V on the non-contact side where the contact member 60 is not disposed (plots ●). The longitudinal surface potential of the pressure roller 32 has a gradient formed between about 15 V in the vicinity of the contact member 61 and 110 V on the non-contact side where the contact member 61 is not disposed (plots ○). The potential difference between the fixing film 36 and the pressure roller 32 is about 190 V in the longitudinal direction. This means that variations of the potential difference between the fixing film 36 and the pressure roller 32 are restricted over the entire longitudinal region.

The charts in FIG. 7 according to the comparative example will be described below. The surface potential of the fixing film 36 has a gradient formed between about −80 to −85 V in the vicinity of the contact member 60 and −170 to −175 V on the non-contact side where the contact member 60 is not disposed (plots ▪). The longitudinal surface potential of the pressure roller 32 has a gradient formed between about 10 V in the vicinity of the contact member 61 and 115 V on the non-contact side where the contact member 61 is not disposed (plots □). Therefore, the potential difference between the fixing film 36 and the pressure roller 32 is about 95 V on the contact side (point A) where the contact members 60 and 61 are disposed, and about 285 V on the non-contact side (point B) where the contact members 60 and 61 are not disposed. This means that the potential difference between the fixing film 36 and the pressure roller 32 varies in the longitudinal region.

According to the comparative example, when the non-fixed toner image t is fixed to the recording material P in this state, image scattering occurs in the vicinity of the non-contact side (point B) in a case of forming a halftone image. This is because the potential difference on the non-contact side increases, and the surface potential of the fixing film 36 becomes higher than the surface potential of the pressure roller 32 for retaining the toner image t with the negative polarity on the recording material P to the recording material P. This generates a repulsive force against the toner image t on the recording material P as a cause of image scattering.

The present exemplary embodiment makes it possible to restrain variations of the potential difference between the fixing film 36 and the pressure roller 32 over the entire longitudinal region to a further extent than the comparative example. This also enables preventing the generation of image scattering due to the potential difference. The contact member 60 for grounding the fixing film 36 is disposed in the region R1 on one end side in the longitudinal direction, and the contact member 61 for grounding the pressure roller 32 is disposed in the region R2 on the other end side in the longitudinal direction. This enables equalizing the longitudinal inclinations of the gradient of the surface potential of the fixing film 36 from the contact portion to the non-contact portion and the gradient of the surface potential of the pressure roller 32 from the non-contact portion to the contact portion. This enables providing a uniform surface potential difference between the fixing film 36 and the pressure roller 32.

The contact member 60 for grounding the fixing film 36 is disposed in the region R1 on one end side in the longitudinal direction, and the contact member 61 for grounding the pressure roller 32 is disposed in the region R2 on the other end side in the longitudinal direction. This enables restricting variations of the potential difference between the fixing film 36 and the pressure roller 32 in the longitudinal direction.

A second exemplary embodiment will be described below. The first exemplary embodiment has been described above centering on a configuration where the contact member 60 is brought into contact with the surface of the fixing film 36. The second exemplary embodiment will be described below centering on a configuration where the contact member 60 is brought into contact with the inner circumferential surface of the fixing film 36. Components of the image forming apparatus 100 similar to those according to the first exemplary embodiment are assigned the same reference numerals, and redundant descriptions of the similar components will be omitted.

FIG. 8 illustrates arrangements of a contact member 600 and the contact member 61 according to the present exemplary embodiment. Referring to FIG. 8, like the first exemplary embodiment, the contact member 600 for grounding the fixing film 36 is disposed in the region R1 in the longitudinal direction, and the contact member 61 for grounding the pressure roller 32 is disposed in the region R2 in the longitudinal direction. The contact member 600 is disposed at the position by a distance L1 from a conveyance center, and the contact member 61 is disposed at the position by a distance L2 from the conveyance center. Although the contact members 60 and 61 are disposed so that the distances L1 and L2 are equal as an example, the present exemplary embodiment is not limited to this example. A contact member needs to be disposed in each of the region R1 on one end side in the longitudinal direction and the region R2 on the other end side in the longitudinal direction.