Patent Application
20080095557
The present invention will be explained on the basis of the embodiment, though the present invention is not limited to the concerned embodiment.
The image forming section A1 includes an image writing section 3, a plurality of sets of image preparation sections 4Y (yellow), 4M (magenta), 4C (cyan), and 4K (black), a belt type intermediate transfer medium 42, a sheet feed cassette 5, a sheet feeding section 6, a sheet ejection section 7, a duplex copy feed section 9, and a fixing device 10.
The image preparation sections 4 (4Y, 4M, 4C, 4K) have a developing means and contain respectively a 2-component developer composed of toner of small-diameter particles of each color of yellow (Y), magenta (M), cyan (C), and black (K) and a carrier.
On the upper part of the image forming apparatus A, the automatic document feeder D is loaded. A document loaded on the document table of the automatic document feeder D is conveyed in the direction of the arrow, and an image on one side or images on both sides of the document are read by the optical system of the image reading section 1 and read into a CCD image sensor 1A.
For an analog signal converted photoelectrically by the CCD image sensor 1A, the memory controller performs the analog process, A-D conversion, shading correction, and image compression and then sends a signal to the image writing section 3.
In the image writing section 3, output light from the semiconductor laser is irradiated to photosensitive drums 41 (for M, C, and K, the reference numerals are abbreviated) of the image preparation section 4 and a latent image is formed. In the image preparation section 4, the processes of charging, exposure, development, transfer, separation, and cleaning are performed. Toner images of the respective colors formed by the image preparation section 4 are sequentially transferred onto the rotating intermediate transfer medium 42 by the primary transfer means and a composite color image is formed.
The toner images on the intermediate transfer medium 42, by the transferring means 43, are transferred to a sheet S conveyed by the sheet feeding means 6 from the sheet feed cassette 5. The sheet S carrying the toner images is fixed by the fixing device 10, is ejected outside from the sheet ejection section 7, and is loaded on a sheet ejection tray 8. Or, the sheet S subject to the one-side image processing sent to the double side conveying route 9 by a transfer path switching gate not drawn is ejected again from the sheet ejection section 7 after the double-side image processing in the image preparation section 4 and is loaded on the sheet ejection tray 8.
A fixing belt 102 is composed of a base formed by polyimide with a thickness of about 100 μm and a release layer formed by PFA or PTFE with a thickness of about 25 μm for covering the outer surface of the base and is formed in an endless shape.
A pressing pad 103 is formed from silicone rubber of JIS A hardness of about 10°, is arranged on the inner peripheral side of the fixing belt 102, and is held by a holder 111 made of heat-resistant resin together with a base sheet metal 104 made of stainless steel and a base member 105 made of heat-resistant resin. Further, on the rear of the base member 105, a compression spring 106 (pressing member) is arranged and compresses the pressing pad 103 via the base sheet metal 104 and base member 105.
Here, the pressing pad 103, base sheet metal 104, base member 105, and compression spring 106 including a first sliding member 114a which will be described later are referred to as a pressing member.
A separation member 107 is made of heat-resistant resin or a metal such as aluminum, is arranged on the inner peripheral surface side of the fixing belt 102 and on the downstream side of the pressing pad 103 in the conveying direction of the sheet S, and is held by the holder 111 and a metallic frame 113 arranged at the center. And, with the rear end of the separation member 107, one end of a compression spring 108 (pressing member) which is a different member from the compression spring 106 is in contact and the other end of the compression sprint 108 is in contact with the metallic frame 113.
Here, the separation member 107 and compression spring 108 including a second sliding member 114b which will be described later are referred to as a separating section.
The pressing pad 103 has the first sliding member 114a in a sheet form on the surface thereof and the separation member 107 has the second sliding member 114b in a sheet form on the surface thereof. With respect to the first sliding member 114a, one end thereof is fixed to the frame 113 and the other end is fixed between the pressing pad 103 and the separation member 107. With respect to the second sliding member 114b, one end thereof is fixed between the pressing pad 103 and the separation member 107.
An oil pad 115 is made of sponge, contains a lubricant composed of silicone oil, is held by a holder 112 made of heat-resistant resin, is pressed to the inner peripheral surface of the fixing belt 102, and feeds the lubricant to the boundary surface between the sliding members 114a and 114b and the fixing belt 102.
Here, the frictional coefficient of the sliding members will be explained. Assuming the frictional coefficient between the inner peripheral surface of the fixing belt 102 and the first sliding member 114a which is a sliding surface of the pressing pad as μ1 and the frictional coefficient between the inner peripheral surface of the fixing belt 102 and the second sliding member 114b which is a sliding surface of the separation member 107 as μ2, the kind and material of the sliding members are selected so as to realize μ1>μ2.
For example, the second sliding member 114b is assumed as a Teflon® coated glass fiber sheet having a low frictional coefficient and the sliding member of the first sliding member 114a is assumed as a crosslinked PTFE sheet having a higher frictional coefficient than it. In this case, when the frictional force at time of sliding with a PTFE sheet with silicone oil coated is measured under the same condition, compared with the frictional force of the Teflon® coated glass fiber sheet aforementioned, the frictional force of the crosslinked PTFE sheet is higher by about two times.
During rotation of the fixing belt 102 shown in
In the fixing device 10 structured like this, the fixing roller 101 heated by the tungsten halogen lamp H and driven by a driving means not drawn rotates clockwise. Further, the pressing pad 103 compressed by the compression spring 106 via the base sheet metal 104 and base member 105 presses the fixing belt 102 to the fixing roller 101 via the sliding member 114a covering the surface thereof. Furthermore, the separation member 107 compressed by the compression spring 108 presses the fixing belt 102 to the fixing roller 101 via the sliding member 114b covering the surface thereof.
Here, the total load for pressing the fixing roller 101 from the pressing member including the pressing pad 103 is referred to as P1 and the total load for pressing the fixing roller 101 from the separating member including the separation member 107 is referred to as P2. The total load P2, as shown below, affects the separability of the sheet S.
Further, the separation member 107 presses the fixing belt 102 to the fixing roller 101, though the fixing roller 101 is softer than the separation member 107, so that the outer peripheral surface of the fixing roller 101 is distorted concavely according to the total load P2 from the end of the separation member 107 (elastic deformation). In this way, between the fixing roller 101 and the fixing belt 102, the second nip portion is formed.
As a result, the peripheral speed V2 of the fixing roller at the second nip portion N2 becomes higher than the peripheral speed 1 of the fixing roller at the first nip portion N1 due to an occurrence of distortion. Depending on the magnitude relationship in the frictional force (conveying force) between the first nip portion N1 and the second nip portion N2, the conveying speed of the sheets S is decided. When Fp2, a frictional force between the sheet S and the fixing roller 101 at the second nip portion N2, is larger than Fp1, a frictional force between the sheet S and the fixing roller 101 at the first nip portion N1, a phenomenon that the conveying speed is changed from V1 to V2 occurs, so that the conveying speed becomes unstable and an image may be displaced in correspondence with speed change.
To make the sheet conveying speed stable so as to prevent an occurrence of an image slipping, it is necessary to make Fp1, the frictional force between the sheet S and the fixing roller 101 at the first nip portion N1, larger than Fp2, the frictional force between the sheet S and the fixing roller 101 at the second nip portion N2.
With respect to the first nip portion N1, the curvature center is positioned on the side of the fixing roller 101, thus the curvature is small, and the nip shape is curved small, while with respect to the second nip portion N2, the curvature center is positioned on the inner peripheral surface side of the fixing belt 102, thus the curvature is large, and the nip shape is curved large.
The second nip portion N2 is used to improve the separability when the sheet S is separated from the fixing roller 101, so that the width thereof is smaller than that of the first nip portion N1 and the inflection point formed by the first nip portion N1 and second nip portion N2 is positioned on the downstream side of the central position of the overall nip portion where the first nip portion N1 and second nip portion N2 are connected in the conveying direction.
To improve the separability of the sheet S, it is desirable to increase the total load P2 at the second nip portion N2. In correspondence with an increase in the total load P2, the concave distortion of the fixing roller 101 at the second nip portion N2, that is, the curve becomes larger, thus the sheet S can be separated easier. To ensure the separation performance, the total load P2 must be increased to a fixed amount or larger.
However, when the total load P2 at the second nip portion is increased, the frictional force Fp2 at the second nip portion increases, so that Fp1<Fp2 becomes satisfied and the aforementioned problem of image slipping arises. To prevent the image slipping, as described previously, it is necessary to make the frictional force Fp1 at the first nip portion N1 between the sheet S and the fixing roller 101 larger than the frictional force Fp2 at the second nip portion N2.
When the sliding portions at the first and second nip portions are the same in the kind, the frictional coefficients at both positions are equal to each other, so that the magnitude relationship of the frictional force between both nip portions is decided by the magnitude relationship of the total load between the nip portions. However, in this embodiment, the sliding member 114a at the first nip portion N1 and the sliding member 114b at the second nip portion N2 are made different in the material, thus between the frictional coefficients at both nip portions, a relation of μ1>μ2 is realized. Therefore, the frictional force F1 (=P1×μ1) at the sliding member 114a (the pressing pad 103) at the first nip portion becomes larger than the frictional force F2 (=P2×μ2) at the sliding member 114b (the separation pad 107) at the second nip portion. Namely, a relation of P1×μ1>P2×μ2 is held, and the difference in the frictional force acts on the frictional forces Fp1 and Fp2 at the nip portions via the fixing belt 102, so that finally, a relation of Fp1>Fp2 is realized. Therefore, an occurrence of an image slipping can be prevented.
Further, the example that the sliding members 114a and 114b are changed in the material, thus the frictional coefficients μ1 and μ2 at the pressing member and separating member are changed is explained, though the present invention is not limited to it, and it is possible to change the surface treatment or material of the pressing pad 103 and separation member 107 without installing the sliding members at the pressing member and separating member, thereby change the frictional coefficients μ1 and μ2 with the inner peripheral surface of the fixing belt 102.
Next, the results of the experiment on how the separation performance and image slipping are changed when the total loads P1 and P2 and frictional coefficients μ1 and μ2 are changed in the level will be indicated below.
Fixing roller: Diameter of 40 mm, rubber thickness of 1.0 mm, rubber hardness of 10° (JIS-A)
Fixing belt: Diameter of 35 mm, thickness of 100 μm, material of polyimide
Nip portion (first nip portion) by pressing pad: Nip width of 8 mm
Nip portion (second nip portion) by separation member: Nip width of 2.5 mm
Test sheet: coated paper (art paper), Weight per unit area of 80 g/m2
Fixing temperature: Set at 180° C.
Sheet conveying speed (average): 350 mm/s
Test environment: 30° C. and 80% RH
Evaluation toner image: Solid image
The experimental results are shown in Table 1.
A symbol P given in the table indicates a total load of the nip portion and μ indicates a frictional coefficient between the sliding member 114 and the inner peripheral surface of the fixing belt 102. An accompanying character “1” indicates the first nip portion and “2” indicates the second nip portion.
In Comparative Example 1, the frictional coefficients μ1 and μ2 are the same and a condition of P2>P1 is realized, so that the frictional force Fp2 at the second nip portion is larger than the frictional force Fp1 at the first nip portion. Therefore, a case that the conveying speed is changed to the conveying speed V2 at the second nip portion occurs, thus image slipping is caused.
In Comparative Examples 2 and 3, the total load P2 at the separating member (the second nip portion) is insufficient, so that the separation performance is insufficient. Furthermore, in Comparative Example 3, the frictional coefficients μ1 and μ2 are the same and a condition of P2=P1 is realized, so that with respect to the frictional forces, Fp1=Fp2 is held. Under this condition, an image slipping, though slight, occurs.
In Comparative Example 4, the total load P2 at the separating member is sufficient, so that the separation performance can be ensured sufficiently. Further, the relation of P1>P2 is held, so that with respect to the frictional forces, Fp1>Fp2 is held, thus no image slipping occurs. However, both total loads P1 and P2 are increased under the condition that the frictional coefficients μ1 and μ2 are the same, so that a problem arises that the drive torque of the motor for driving the fixing roller is increased and there are possibilities that the load for the fixing roller is high and the long-term durability is insufficient.
In Comparative Examples 5 and 6, the frictional coefficients μ1 and μ2 are made different from each other. Comparative Example 5 is obtained by changing the frictional coefficient of Comparative Example 2, though the total load P2 at the separating member (the second nip portion) is insufficient, so that the separation performance is insufficient. In Comparative Example 6, the total load P2 at the separating member is increased, so that the separation performance is ensured sufficiently and furthermore, with respect to the frictional coefficients, μ1>p2 is realized. However, the effect is insufficient and with respect to the frictional forces between the inner peripheral surface of the fixing belt and the sliding members, a relation of F1>F2 is not held, so that with respect to the frictional forces between the sheet S and the fixing roller, a relation of Fp1>Fp2 can be realized, so that a slight image slipping (slightly worse than that of Comparative Example 3) occurs.
On the other hand, in Examples 1 and 2, the total load P2 at the separating member is sufficient, so that the separation performance can be ensured sufficiently. Further, although the relation of P1<P2 is held, the relation of μ1>μ2 is held, so that with respect to frictional forces as a relation between the inner peripheral surface of the fixing belt and the sliding members, F1>F2 is realized. Accordingly, with respect to frictional forces as a relation between the sheet S and the fixing roller, the relation of Fp1>Fp2 can be realized, thus an occurrence of an image slipping can be prevented. Further, the sum of the total loads P1 and P2 can be made smaller than that of Comparative Example 4, so that there is an advantage that a problem of an increase in the drive torque and a reduction in the durability arises hardly.
As mentioned above, assuming the total load for the fixing roller by the pressing member at the first nip portion of the fixing device as P1, the total load for the fixing roller by the separating member at the second nip portion as P2, the frictional coefficient between the fixing belt and the pressing member as μ1, and the frictional coefficient between the fixing belt and the separating member as μ2, if the fixing device is set so as to satisfy the conditional expressions of P1×μ1>P2×μ2 and μ1>μ2, even when the total load P2 at the exit of the nip portion is increased in order to enhance the sheet separation performance, a fixing device and an image forming apparatus for preventing an occurrence of an image slipping without increasing the total load P1 at the other nip portion can be obtained.
| Number | Date | Country | Kind |
|---|---|---|---|
| JP2006-287306 | Oct 2006 | JP | national |
