Abstract:
An image forming apparatus includes a movable image carrier, an image forming device for forming an image on the image carrier, and a transferring device for transferring the image from the image carrier onto an image bearing sheet while the image bearing sheet is in contact with the surface of image carrier. The apparatus also includes a separating claw having a tip for contact with the surface of the image carrier for separating the image bearing sheet from the image carrier, and a moving mechanism for moving the separating claw between a first position in which the tip of separating claw is in contact with the surface of the image carrier and a second position in which the separating claw is separated from contact with the surface of the image carrier. The separating claw includes an elastic synthetic resin composition containing a polyurethane resin.

Description:
This application is a continuation of application Ser. No. 110,876, filed October 21, 1987 abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus, and more particularly to separation devices for removing an image bearing sheet from an image carrier in an image forming apparatus. 
     2. Description of the Related Art 
     Conventionally, in a typical image forming apparatus, electrostatic copiers, a transfer charger and a separation charger are arranged around a photosensitive body. After transferring a developed image from the photosensitive body onto a copy sheet while the copy sheet is in contact with the photosensitive body by action of the transfer charger, the copy sheet is separated from the photosensitive body by action of the separation charger. However, occasionally the copy sheet cannot be separated completely from the photosensitive body because an insufficient charge is applied by the separation charger to the copy sheet. In this case, the non-separated copy sheet unfortunately can infiltrate into a cleaning device and cause a jam. For preventing the above-mentioned problem, conventionally, a separating claw is arranged against the photosensitive body to physically remove the copy sheet before the sheet reaches the cleaning device. 
     For the separating claw which constitutes this type of separation device, usually a hard plastic, such as ABS resin or Delrin (a trade name of Du Pont Co. for an acetal resin), and an elastic film, such as Mylar (a trade name of Du Pont Co. for a polyester film) have been used. By causing the sharp tip of the separating claw to make contact with the surface of the photosensitive body at an appropriate time, the copy sheet can be separated from the photosensitive body after passing the transfer charger and separation charger. The separating claw provides an auxiliary mechanism to prevent the copy sheet from entering the cleaning device and causing a jam. 
     However, in the above conventional separating claw, and particularly in one made of a plastic such as ABS resin, the hardness often is too high. For this reason, the surface of the photosensitive body, which normally is formed of a comparatively soft material like selenium (Se), can be easily damaged by this contact with the photosensitive body. Therefore, problems occur in that the picture quality of the developed image may become poor. 
     In order to compensate for the above problems, a soft film such as Mylar has been used for the separating claw. When Mylar is used, there has to be a complex construction in which a harder material is used as a base member, in order to maintain the accuracy and configuration of the Mylar portion, which makes contact with the photosensitive body. Accordingly, construction of the separation claw becomes complex, more expensive and time consuming. In addition, the surface of the photosensitive body may be easily damaged by contact with the harder portions of the separating claw construction. Another problem, which sometimes arises, relates to the electrostatic attraction of the copy sheet to the photosensitive body. If the contact pressure of the claw on the surface of the photosensitive body is too low, the copy sheet may more easily enter the cleaning device without separating from the photosensitive body. Thus a breakdown of the cleaning device or the apparatus may result. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an image forming apparatus in which reliable separation of an image bearing sheet from the surface of an image carrier can be achieved without damaging the surface of the image carrier. 
     Another object of the invention is to better protect the surface of an image carrier in an image forming apparatus during removal of image bearing sheets from the carrier. 
     According to one aspect of the present invention, there is provided an image forming apparatus including a movable image carrier, means for forming an image on the image carrier, and means for transferring the image from the image carrier onto an image bearing sheet while the image bearing sheet is in contact with the surface of image carrier, the apparatus comprising means for separating the image bearing sheet from the image carrier, including a separating claw having a tip for contact with the surface of the image carrier, the separating claw including an elastic synthetic resin composition containing a polyurethane resin; and means for moving the separating claw between a first position in which the tip of the separating claw is in contact with the surface of the image carrier and a second position in which the separating claw is separated from contact with the surface of the image carrier. 
     It is preferred that the elastic synthetic resin composition includes one of a copolymer of the polyurethane resin and a silicon system resin, a copolymer of the polyurethane resin and a fluorocarbon system resin, a dispersed system of the polyurethane resin and a silicon system resin, and a dispersed system of the polyurethane resin and a fluorocarbon system resin. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of this invention will become more apparent and more readily appreciated from the following detailed description of the presently preferred exemplary embodiments, taken in conjunction with the accompanying drawing of which: 
     FIG. 1 is a configurational diagram showing an electrostatic copier including an image forming apparatus according to the present invention; 
     FIGS. 2 and 3 are configurational diagrams showing the operating condition of a separation device; 
     FIG. 4 is a schematic diaagram showing the configuration of the separating claw in the separation device; 
     FIG. 5 is a side view showing the tip of the separating claw; 
     FIG. 6 is a schematic diagram showing the copy sheet separation mechanism; and 
     FIG. 7 is a schematic diagram showing the configuration of a transfer charger and separation charger. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following is a detailed description of an embodiment of the present invention with reference to the drawings. 
     FIG. 1 shows an outline of the internal structure of an electrostatic copier which is an image forming apparatus of the present invention. A drum-shaped photosensitive body 12 is rotatably provided approximately in the center of a housing 11. This photosensitive body 12 is designed to be uniformly charged by a main charger 13. An original document table 14 made of transparent glass is provided at upper position of housing 11. An electrostatic latent image of an original document (not shown) placed on original document table 14, is formed on a surface 12a of photosensitive body 12 via an optical device 15. Optical device 15 comprises an exposure lamp 15a, first, second and third reflecting mirrors 15b, 15c and 15d, a l ens 15e and fourth reflecting mirror 15f. The electrostatic latent image is converted into a developed image by a developing agent accommodated in a developing device 16. While a copy sheet P has been transported to a transfer charger 17 from paper supply cassettes 18 via paper supply rollers 19 and aligning rollers 20. At the position of transfer charger 17, the developed image is transferred from photosensitive body 12 onto copy sheet P by the action of transfer charger 17. Copy sheet P is then separated from photosensitive body 12 by the action of a separation charger 21. 
     After developed image transferring and separating, copy sheet P is transported through a conveying belt 22 to a fixing device 23 comprising a pair of heat rollers for fixing the developed image on copy sheet P. Copy sheet P is then dispensed into a receiving tray 25 by exit rollers 24. 
     After the transfer of the developed image onto copy sheet P and separation of copy sheet P from photosensitive body 12, the developing agent remaining on the surface 12a of photosensitive body 12 is cleaned off by a cleaning device 26. The potential on photosensitive body 12 is reduced to a specified level by a discharging lamp 27 and it is returned to a condition in which the next copying operation can be carried out. 
     FIG. 7 shows transfer charger 17 and separation charger 21 in detail. A shielding case 21a has an opening 21d facing the surface of photosensitive body 12. First and second charging wires 21b and 21c are strung inside shielding case 21a along the longitudinal direction of shielding case 21a. Each of first and second charging wires 21b and 21c is made of tungsten wire of diameter 80 μm. Shielding case 21a is constructed so that it is incorporated with a shielding case 17a of transfer charger 17. Separation charger 21 is functionally separated from transfer charger 17 by a dividing plate 28. A guide plate 21e is mounted in shielding case 21 to guide the transportation of copy sheet P. A charging wire 17b is strung in shielding case 17a along longitudinal direction of shielding case 17a. 
     Second charging wire 21c, which is positioned to the rear in relation to the direction of advance of copy sheet P, is strung in a position which is displaced higher than first charging wire 21b in the front position. The distances of these two charging wires 21b and 21c from photosensitive body 12 are almost equal. The discharge field produced by second charging wire 21c (a zone shown by an aperture angle θ2) is set wider than the discharge field produced by first charging wire 21b (a zone shown by an aperture angle θ1). The voltage applied to second charging wire 21c is set lower than the voltage applied to first charging wire 21b. 
     The functions of two charging wires 21b and 21c strung inside shielding case 21a are divided. First charging wire 21b positioned in front in relation to the direction of advance of paper P, acts to separate copy sheet P from photosensitive body 12. Second charging wire 21c positioned farther away, acts to electrostatically discharge copy sheet P. Since the discharge field of charging wire 21c is made wider, it can electrostatically discharge copy sheet P gradually. Thus the electrostatic discharging of copy sheet P is not carried out rapidly as in prior art, and so the problem of the toner on copy sheet P returning to photosensitive body 12 does not arise. 
     As shown in FIG. 1, a separation device 30 is arranged approximately in the middle in the longitudinal direction of photosensitive body 12 between separation charger 21 and cleaning device 26. separation device 30 acts to separate copy sheet P from photosensitive body 12 when copy sheet P cannot be separated from photosensitive body 12 by the action of separation charger 21. 
     As shown in FIG. 2, this separation device 30 is constructed in the following manner. A seesaw lever 31 is provided to swingably rotate through a pivot 31c. A separating claw 31a having a sharp tip 31d, is mounted at one end of seesaw lever 31 so that sharp tip 31d of claw 31a can make contact with a surface 12a of photosensitive body 12. The weight of a rear end portion 31b at the other end of seesaw lever 31, is slightly heavier than the side of separating claw 31a. 
     Separating claw 31a is made of an elastic synthetic resin, such as one of the following materials. These materials include a polyurethane resin, a copolymer of polyurethane resin and silicon system resin, a copolymer of polyurethane resin and fluorocarbon system resin, a dispersed system of polyurethane resin and silicon system resin, or a dispersed system of polyurethane resin and fluorocarbon system resin. Separating claw 31a a s a thickness of 1.5 mm and hardness of 90° in accordance with the JIS (Japan Industrial Standard) for rubber hardness. The hardness of separating claw 31a is measured by a &#34;JIS Hardness Tester&#34; (Shimadzu Co.). 
     One end portion 32a of a rotating link 32 is linked with rear end portion 31b of separating claw 31. Rotating link 32 is pivoted with a pivot 32c to be swingably rotate through pivot 32c. One end portion 33a of a rotating lever 33 butts onto the other end 32b of rotating link 32. A solenoid 34 rotates rotating lever 33. 
     Separation device 30 is designed so that, as shown by the solid line arrow signs in FIG. 2, rotating link 32 is rotated by the switching ON of solenoid 34, which causes rotating lever 33 to perform a pushing action. By this pushing action, one end portion 32a of rotating link 32, which is in contact with rear end portion 31b of seesaw lever 31 permits seesaw lever 31 to rotate under the weight of rear end portion 31b. By this means, sharp tip 31d of separating claw 31a makes contact with the surface 12a of photosensitive body 12. As shown by the solid line arrow signs in FIG. 3, by switching OFF solenoid 34, rotating lever 33 and rotating link 32 are caused to operate in the reverse direction. This causes rear end portion 31b of seesaw lever 31 to be pushed upward against the weight of rear end portion 31b. Thus, sharp tip 31d of separating claw 31a is released, so that it is caused to separate from the surface 12a of photosensitive body 12. 
     The configuration of the contact area at any given time during contact of sharp tip 31d of separating claw 31a with the surface 12a of photosensitive body 12 is as shown in FIG. 4. Tangent line X, which passes through the point of initial contact A with the surface 12a of photosensitive body 12, is taken as reference line. This point of contact A is used as a fulcrum. The two surfaces of separating claw 31a are enveloped by the angle φ1 formed by two lines Y and Z. Two lines Y and Z pass through the fulcrum and form angles φ2 of 220° and φ3 of 60°, respectively, from reference tangent line X in the reverse direction to the direction of rotation of photosensitive body 12. The angle φ1 formed by the two surfaces of separating claw 31a is determined so that it is an acute angel within the region of 20°-60°, for example 35°. The reason for this is that, within the acute angle region of  0°-20° for angle φ2, a separating claw 31a would lie flat against the surface 12a of photosensitive body 12. Thus, copy sheet P would not be separated. On the other hand, if angle φ3 was 60α or more, the claw would damage photosensitive body 12 by bounding when in contact. 
     Moreover, at least the surface of sharp tip 31d of separating claw 31a which comes into contact with the surface 12a, as shown by the diagonal shading in FIG. 5, may be covered with a lubricating layer 40 consisting of polytetrafluoroethylene powder with a particle diameter of 2-3 μm when a polyurthane resin is used as the material of separating claw 31a. By this means, the coefficient of friction when in contact with the surface 12a of photosensitive body 12 can be greatly reduced. 
     The timing of the contact of sharp tip 31d of separating claw 31a with the surface 12a of photosensitive body 12 takes as its reference the timing of the commencement of rotation of aligning rollers 20 which transport copy sheet P to the developed image transferring station. When copy sheet P is transported by the rotation of aligning rollers 20, sharp tip 31d makes contact by the switching ON of solenoid 34. Solenoid 34 is turned ON after the passage of a time when the front edge of copy sheet P has reached a position approximately 10 mm (&#34;&#34; in FIG. 6) before the point of initial contact A on the surface 12a of photosensitive body 12. After the front edge of copy sheet P has passed through the contact point A, and after a time equivalent to a movement of approximately 10 mm has elapsed, sharp tip 31d of separating claw 31a is separated from the surface 12a of switching OFF solenoid 34. By this means, the duration of contact of separating claw 31a with the surface 12a  of photosensitive body 12 is shortened to the minimum time required for separation of the copying paper. Thus the probability of the surface 12a of photosensitive body 12 being damaged by contact with separating claw 31a is even further reduced. 
     As described above, in the present invention, a separation device is formed by a separating claw having a sharp tip 31d made of an elastic synthetic resin which is caused to make contact with the surface of an image carrier at an appropriate time. For this reason, since the material of the separating claw itself is soft, it does not damage the surface of the image carrier. 
     Various other modifications could be made in the present invention without departing from the scope or spirit of the following claims.