Patent Publication Number: US-6909873-B2

Title: Image forming apparatus with fixing apparatus and conveying rollers downstream of the fixing apparatus. the conveying rollers having a portion with a releasing layer

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus such as a printer or a copying apparatus utilizing a recording technology such as an electrophotographic process or an electrostatic recording process, and a fixing apparatus adapted to be mounted on such apparatus. 
   2. Description of the Related Art 
   An image forming apparatus such as a printer is usually provided, as described in Japanese Patent Application Laid-open No. 2000-226146, with a pair of rollers for guiding a recording material, having passed a fixing device, to a discharge tray. Such paired rollers may assume a form of “a single roller extended in a direction of a generatrix” provided on a rotary shaft, both in a roller coming into contact with a front surface of the recording material and a roller coming into contact with a rear surface of the recording material, or plural short rollers in the direction of generatrix on a rotary shaft, both in a roller coming into contact with a front surface of the recording material and a roller coming into contact with a rear surface of the recording material. 
   However, in either form, there is encountered a drawback that a conveying operation of the recording material under a pinching pressure uniform in a direction of width (direction perpendicular to a conveying direction) tends to induce a waviness or an undulation of the recording material (FIGS.  8 A and  8 B). 
   Also, a front corner of the recording material may be folded or dog-eared depending on an entry state of the recording material into the paired rollers (FIGS.  8 A and  8 B), and such folding need to be avoided. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in consideration of the aforementioned drawbacks, and has an object of providing an image forming apparatus and a fixing apparatus capable of suppressing a waviness in a recording material. 
   Another object of the present invention is to provide an image forming apparatus and a fixing apparatus capable of suppressing a folding of a corner at a front end of a recording material. 
   Still another object of the present invention is to provide an image forming apparatus including: 
   image forming means which forms an image on a recording material; 
   fixing means which fixes the image formed on the recording material; 
   recording material conveying means provided at a downstream side of the fixing means in a moving direction of the recording material, the conveying means including a first roller having a first shaft and a second roller having a second shaft and maintained in contact with the first roller; 
   wherein the second roller has a first portion provided in a vicinity of a passing reference position of the recording material and maintained in contact with the first roller, and a second portion positioned farther than the first portion from the reference position and having a diameter smaller than in the first portion. 
   Still another object of the present invention is to provide a fixing apparatus including: 
   fixing means which fixes an image formed on a recording material; 
   recording material conveying means provided at a downstream side of the fixing means in a moving direction of the recording material, the conveying means including a first roller having a first shaft and a second roller having a second shaft and maintained in contact with the first roller; 
   wherein the second roller has a first portion provided in a vicinity of a passing reference position of the recording material and maintained in contact with the first roller, and a second portion positioned farther than the first portion from the reference position and having a diameter smaller than in the first portion. 
   Still other objects of the present invention will become fully apparent from the following detailed description which is to be taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic cross-sectional view of an image forming apparatus embodying the present invention; 
       FIG. 2  is a perspective view of paired rollers in a first embodiment immediately after a fixing nip; 
       FIG. 3  is a table showing a relationship between a roller diameter and the behavior of a recording material; 
       FIG. 4  is a perspective view of paired rollers in which, within a lower roller, a width of roller having a surface layer of a fluorinated resin is made larger than a width of a roller without a surface layer of a fluorinated resin; 
       FIG. 5  is a cross-sectional view of an example in which a fluorinated resin tube is employed as a fluorinated resin surface layer; 
       FIG. 6  is a perspective view of paired rollers constituting a second embodiment and employing a lower roller divided in three portions; 
       FIGS. 7A and 7B  are perspective views of paired rollers constituting a third embodiment and employing a lower roller with a crowned shape; 
       FIGS. 8A and 8B  are perspective views showing a wavy state and a corner-folded state of a recording material; and 
       FIG. 9  is a perspective view showing a state in which a recording material is given a rigidity to suppress a waving and a corner folding in a front end. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   (First Embodiment) 
   In the following there will be explained, with reference to accompanying drawings, embodiments of the image forming apparatus and the fixing apparatus of the present invention.  FIG. 1  is a schematic cross-sectional view of an image forming apparatus embodying the present invention. In the following, there will be explained construction, following a sequence of image formation. 
   At first there will be given an explanation on image forming means which forms a toner image on a recording material. Laser scanner units ( 1 Y,  1 M,  1 C,  1 K) respectively corresponding to yellow, magenta, cyan and black colors emit laser beams ( 2 Y,  2 M,  2 C,  2 K) according to image data of respective colors to record latent images corresponding to the image-data, on photosensitive drums ( 4 Y,  4 M,  4 C,  4 K) of which surfaces are uniformly charged by charging rollers ( 3 Y,  3 M,  3 C,  3 K). Then, such latent images are developed in an order of yellow, magenta, cyan and black by unrepresented toners of respective colors contained in developing apparatus ( 5 Y,  5 M,  5 C,  5 K) through developing sleeves ( 6 Y,  6 M,  6 C,  6 K) of respective colors. The toner images formed on four photosensitive drums are transferred onto an intermediate transfer belt  8 , rotated by belt driving rollers ( 7   a ,  7   b ,  7   c ) counterclockwise (direction indicated by an arrow) in the drawing, in succession on primary transfer points (T 1 Y, T 1 M, T 1 C, T 1 K) and thus superposed. There are provided back-up rollers  9 Y,  9 M,  9 C,  9 K for transferring the toner images from the photosensitive drums onto the intermediate transfer member. 
   On the other hand, a recording material S fed from a feed cassette  10  by a feed roller  11  is conveyed by pairs of conveying rollers ( 12   a ,  12   b ), ( 13   a ,  13   b ), ( 14   a ,  14   b ) in a direction indicated by an arrow, and is conveyed to a secondary transfer point T 2 . At such point T 2 , a full-color toner image on the intermediate transfer member is collectively transferred (secondary transfer) on the recording material, which is conveyed by pinching between a transfer roller  15 , constituted of a foamed member wound around a metal shaft, and a belt driving roller  7   b.    
   Toner, which is not transferred but remains on the surface of the intermediate transfer belt  8 , is recovered into a cleaner  17  by a cleaning blade  16  positioned in contact with the intermediate transfer belt  8 , and is finally collected in an unrepresented used toner container. 
   The recording material S bearing an image after the secondary transfer passes through a fixing nip portion N formed between a fixing film  19  and a pressure roller  20 . Thus the toner image on the recording material is fixed thereto by heating. In the present embodiment, there is employed a fixing apparatus of electromagnetic induction type in which the fixing film  19  itself generates heat. An exciting coil unit  18  is provided inside the fixing film  19 . 
   After passing the fixing nip portion N of the fixing apparatus  21 , the recording material is conveyed by a pair of conveying rollers (conveying means)  22 ,  23 , driven by an unrepresented drive source through gear G (cf.  FIG. 2 ) and positioned immediately behind the fixing nip. A lower roller  23  (second roller) is a driving roller, and an upper roller (first roller)  22  is an idler roller which is rotated following the rotation of the lower roller  23 . The conveying means  22 ,  23  are provided in the fixing apparatus  21 . Also the fixing apparatus is detachably mounted on a main body of the image forming apparatus. 
   In case of a one-side print mode, the recording material having passed the fixing nip N and the conveying means  22 ,  23  is discharged through a pair of discharge rollers  25   a ,  25   b  onto a face-up tray  60 , or pairs of discharge rollers  26   a ,  26   b ,  27   a ,  27   b  onto a face-down tray  61 . In case an image is formed only on one side of the recording material, the upper roller (first roller)  22  comes into contact with the image. 
   In case of a two-side print mode, a two-side flapper F 1  is so moved by an unrepresented solenoid that the recording material after passing the fixing nip is conveyed to a two-side unit. Then the recording material, bearing an image on a front surface (first surface) and having passed the paired conveying rollers  22 ,  23 , is conveyed after passing a conveying path  24  to a position where a rear end of 20 mm of the recording material remains in a nip portion between a two-side reversing roller  30   a  and a two-side reversing opposed roller  30   b . Thereafter, the paired two-side reversing rollers  30   a ,  30   b  are reversed to convey the recording material in a two-side unit R, and the recording material is re-fed by a pair of two-side conveying rollers  31   a ,  31   b  to a re-feed conveying path K 2 . Thereafter, the recording material is conveyed in a direction indicated by an arrow by pairs of conveying rollers ( 13   a ,  13   b ), ( 14   a,    14   b ), and, after a transfer of an image on a rear surface (second surface) of the recording material S, it again passes the fixing nip N in the fixing apparatus  21  whereby the image is fixed to the recording material S. Subsequently it is conveyed by the lower roller  23  and the upper roller  22 , while a branching flapper F 2  is rotated by an unrepresented solenoid in such a manner that the recording material is discharged to a discharge port  60  or  61  designated by the user, whereby the recording material is conveyed and discharged by respective pairs of the discharge rollers ( 25   a ,  25   b ), ( 26   a ,  26   b ) and ( 27   a,    27   b ). 
   In the following, there will be explained a configuration of the upper roller  22  and the lower roller  23  for suppressing a waving and a corner folding of the recording material. Also there will be explained a configuration of the upper roller  22  and the lower roller  23  for suppressing an image deterioration in case of a high coverage print on the recording material. In the image forming apparatus of the present embodiment, a center in the direction of the width (direction perpendicular to the conveying direction) of the recording material is taken as a conveying reference (passing reference). 
   As shown in  FIG. 2 , the upper roller (first roller)  22  is provided, on an external periphery of the roller, with a releasing layer (surface layer) formed by a fluorinated resin, and a diameter of the roller  22  is uniform along a longitudinal direction thereof. The roller  22  of the present embodiment is constituted of an aluminum metal rod (first shaft) and a tube of a fluorinated resin (PFA) provided thereon. The roller of such configuration provides advantages of easier manufacture with a reduced cost. However, there may also be adopted a configuration of forming an elastomer layer such as of silicone rubber on the first shaft and further forming a releasing layer. On the other hand, the lower roller (second roller)  23  is constituted, on a roller rotary shaft, of silicone rubber rollers divided into seven portions and each having a width b in the axial direction. Three central rollers R 2  (first portion) provided in the vicinity of the passing reference of the recording material have a diameter φD 2 , while four end rollers R 1  (second portion) provided distant from the passing reference of the recording material has a diameter φD 1  (φD 1 &lt;φD 2 ). The roller R 2  is provided, on the external periphery, with a releasing layer (surface layer) of fluorinated resin. The roller R 2  of the present embodiment is formed by covering a silicone rubber roller with a tube of fluorinated resin (PFA). The roller R 1  is not provided with a surface layer of fluorinated resin and silicone rubber is exposed. In a state where the recording material is not conveyed, the rollers R 2  (first portion) is in contact with the upper roller (first roller)  22 , but the rollers R 1  (second portion) is not in contact with the upper roller (first roller)  22  and have a gap to the upper roller (first roller)  22 . Also between the upper roller  22  and the lower roller  23 , there is applied a small pressure (30 g in the present embodiment) by unrepresented springs. 
     FIG. 3  is a view showing the relationship between the roller diameter and the behavior of the recording material. As shown in  FIG. 3 , in case of a configuration in which the first portion (R 2 ) and the second portion (R 1 ) of the lower roller  23  have a same diameter and all the first portion and the second portion of the lower roller  23  are contacted with the upper roller  22  with a uniform pressure, the recording material shows a strong waviness W as shown in an upper part of  FIGS. 8A and 8B . Also in case the second portion (R 1 ) of the lower roller  23  has an excessively small diameter to cause an excessively large gap to the upper roller  22 , a corner folding D is generated at the front end of the recording material as shown in a lower part of  FIGS. 8A and 8B . Consequently, a difference between φD 2  and φD 1  is preferably within a range from 0.8 mm to 1.8 mm. In the present embodiment, φD 2  is made larger by 1 mm than φD 1 . Thus, in a state where the recording material is not pinched between the upper roller  22  and the lower roller  23 , the first portion (R 2 ) of the lower roller  23  is in contact with the upper roller  22  while the second portion (R 1 ) has an appropriate gap to the upper roller  22 . In such configuration, while the recording material is conveyed by the upper roller  22  and the lower roller  23  as shown in  FIG. 9 , the recording material is given a rigidity at a boundary between the first portion (R 2 ) and the second portion (R 1 ) of the lower roller  23 , thereby suppressing the waviness in the recording material. Also, the corner folding is not generated at the front end of the recording material since the gap is small between the second portion (R 1 ) of the lower roller  23  and the upper roller  22 . 
   Also since the upper roller  22  is provided with the releasing surface layer, the toner fixed but not completely solidified on the first recording surface of the recording material S in a one-side recording is prevented from sticking to the upper roller  22 , whereby the image is prevented from being deteriorated, while, in a two-side recording, the toner not completely fixed does not stick to the lower roller  23  in contact with the first recording surface of the recording material nor to the upper roller  22  in contact with the second recording surface of the recording material, whereby the image can be prevented from deterioration. 
   Also, among seven rubber rollers of the lower roller  23 , use of different diameters of R 1 : φD 1  and R 2 : φD 2  allows to control the behavior of the recording material, and a releasing layer is not provided on all the rollers but only in the rollers R 2  coming in contact with the upper roller, thereby realizing an effect of cost reduction. Also a relationship φD 1 &lt;φD 2  allows to suppress the waving of the recording material on both end portions thereof in the direction of the roller shaft. In particular, a configuration in which the rollers R 2  are maintained in contact with the upper roller  22  while the rollers R 1  are not in contact with the upper roller  22  enhances the effect of suppressing the waving on both ends of the recording material. 
   Furthermore, in the lower roller  23 , the rollers R 2  may have a width b 2  satisfying a relationship b 2 &gt;b 1  where b 1  is a width of the rollers R 1 , as shown in FIG.  4 . In this manner, it is rendered possible to increase the conveying power without changing the total load to the pair of the upper roller  22  and the lower roller  23 , and to improve a conveying margin for a thin paper or the like. In such situation, a conveying operation is sufficiently possible in case a maximum width of contact between the lower roller  23  and the upper roller  22  (namely a width of the first portion R 2  of the lower roller) is equal to or larger than the width of a maximum passable sheet. 
   The configuration explained above allows to provide a fixing apparatus and an image forming apparatus of a low running cost, capable of preventing a waving or a corner folding of the recording material or an image deterioration. 
   In the following there will be explained a method of forming a releasing layer to the lower roller  23 . In the present embodiment, for forming a releasing layer on a silicone rubber roller, there is employed a method of covering with a PFA tube. As shown in  FIG. 5 , a face C of a width of 2 mm is provided on each end face of the roller, thereby allowing a PFA tube P to shrink thermally and to cover the roller in a more wrap-around manner. Also, since no adhesive is employed, a disassembling is made easier to improve a recycling property when the apparatus is recovered at the end of the service life. 
   The above-described configuration allows to produce a lower roller of a lower cost, an improved matching to the environment and a longer service life without employing an adhesive material, thus enabling stable image formation to the end of a service life of the fixing apparatus. 
   (Second Embodiment) 
   In the following, a second embodiment of the present invention will be explained with reference to FIG.  6 . 
   In contrast to the first embodiment in which the lower roller  23  is divided into seven portions, it is divided into three portions in the present embodiment. As in the first embodiment, a second portion (R 2 ) of the lower roller  23  is smaller in diameter than in a first portion (R 2 ). Also a surface releasing layer is provided only in the first portion, while an elastic layer is exposed externally in the second portion. A width a of the first portion (R 2 ) is made equal to or larger than ⅓ of a recording material passing area. 
   Such configuration also allows to obtain effects similar to those in the first embodiment. 
   (Third Embodiment) 
   In the following, a third embodiment of the present invention will be explained with reference to  FIGS. 7A and 7B . 
   In the present embodiment, the lower roller  23  is not divided but has a crowned shape having a smaller diameter in the vicinity of both ends. A first portion (R 2 ) of the lower roller  23  has a uniform diameter over a range B, while second portions (R 1 ) is so shaped that the diameter becomes gradually smaller toward the end. A width of the first portion (R 2 ) is made equal to or larger than ⅓ of a recording material passing area. In  FIGS. 7A and 7B , an upper portion shows a configuration in which a surface releasing layer is provided over the entire lower roller  23 , while a lower portion shows a configuration in which a surface releasing layer is provided only in the first portion (R 2 ). F indicates a conveying reference for the recording material. 
   Such configuration also allows to obtain effects similar to those in the first embodiment. 
   The present invention is not limited to the foregoing embodiments but includes any and all modifications within the technical concept.