Abstract:
An image forming apparatus includes a movable member carrying a recording sheet bearing a non-fixed image and a separation device for separating the recording sheet from the movable member. The separation device includes a peel member made from resin and having a resistance between 10 3  and 10 11  Ω) and an electricity removing charger. The peel member is supported by a support member and is positioned such that a part of the resin peel member is within the discharge area of the electricity removing charger and the support member is disposed outside the discharge area and is electrically grounded.

Description:
This application is a continuation of application Ser. No. 08/408,280, filed Mar. 22, 1995, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus having a separating means for separating a recording sheet from a movable member, and more particularly, it relates to an image forming apparatus for forming an image on a recording sheet by transferring a toner image formed on an image bearing member onto a recording sheet supported by a movable member. 
     2. Related Background Art 
     There have been proposed color image forming apparatuses wherein a color image can be obtained by successively transferring different color toner images formed on a photosensitive drum onto a recording sheet supported by a transfer drum in a superimposed fashion. In such image forming apparatuses, a separation pawl interposed between the transfer drum and the recording sheet to separate the recording sheet from the transfer drum is made of metal, for example, iron. 
     However, when the separation pawl earthed is made of metal, since ions generated by peeling discharge between the recording sheet and the transfer drum are concentrated into an area where the metal pawl is positioned, there is a danger of distorting the non-fixed toner images transferred to the recording sheet. On the other hand, if the separation pawl is made of insulation resin, there arises risk that the non-fixed toner images transferred to the recording sheet are distorted due to friction charge between the separation pawl and the recording sheet. 
     Further, if the separation pawl is constituted by a metal pawl which is earthed, and a coating layer (coated on the metal pawl) made of insulation resin of fluoro-group, initially, the recording sheets can be separated from the transfer drum. However, in this case, a thickness of the coating layer of the insulation resin of fluoro-group must be reduced to prevent the friction charge between the recording sheet and the resin of fluoro-group (500 μm or less). Therefore, as the number of copies is increased, the coating layer is worn, thereby causing the same problem as the metal pawl. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an image forming apparatus having a separation means for separating a recording sheet from a movable member without distorting non-fixed toner image(s) transferred to a recording sheet. 
     Another object of the present invention is to provide an image forming apparatus including a separation means for effectively separating a recording sheet from a movable member high volume resistance. 
     The other objects and features of the present invention will be apparent from the following detailed description of the present invention explained with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a transfer drum (5a) and therearound for explaining an embodiment of the present invention; 
     FIG. 2 is an enlarged view showing a charged condition in a recording sheet peeled area, generated when a separation pawl is made of metal; 
     FIG. 3 is an enlarged view showing a charged condition in a recording sheet peeled area, generated when a separation pawl is made of insulation resin; 
     FIG. 4 is a table showing image quality, separability and strength regarding various pawl materials; 
     FIG. 5 is an explanatory view for explaining the recording sheet; 
     FIGS. 6A and 6B are views showing a recording sheet separation means; and 
     FIG. 7 is a schematic view showing a means for measuring a resistance value of the separation pawl. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An image forming apparatus according to an embodiment of the present invention will now be explained with reference to the accompanying drawings. 
     After a photosensitive drum (image bearing member) 1 is uniformly charged by a charger, light corresponding to image information is illuminated onto the photosensitive drum, thereby forming an electrostatic latent image on the drum. Then, the latent image is developed by toner from a developing means as a toner image. The toner image formed on the photosensitive drum 1 is transferred onto a recording sheet (for example, paper sheet) P supported by a transfer drum 5a. The above-mentioned image forming process is repeated with respect to magenta toner, cyan toner, yellow toner and black toner, respectively, so that the four different color toner images are successively transferred onto the recording sheet P in a superimposed fashion. Thereafter, the recording sheet P is separated from the transfer drum 5a, and the separated recording sheet is sent to a fixing device, where the four different color toner images are fused and mixed to obtain a full color image. 
     Next, a transfer device having the transfer drum will be fully explained. 
     In FIG. 1, a transfer sheet (recording sheet bearing member) 5f for electrostatically absorbing the recording sheet and for conveying the recording sheet in a direction shown by the arrow C is wound around a peripheral surface of the transfer drum 5a in such a manner that tip and rear ends of the transfer sheet 5f are adhered to a connecting member 5j of the transfer drum 5a. In the illustrated embodiment, the transfer sheet 5f is formed from a flexible PC (polycarbonate) film having a thickness of 150 μm and a volume resistance of 10 15  Ωcm or more, because an absorbing force for absorbing the recording sheet to the transfer sheet 5f is improved by using the dielectric film having high volume resistance as the transfer sheet. Accordingly, it is possible to use a thick sheet as the recording sheet and to increase a process speed by increasing a rotational speed of the transfer drum 5a. 
     The recording sheet P is supplied by a sheet supply means (not shown) from a direction shown by the arrow B. An absorb roller 5g is contacted with the transfer sheet 5f by a drive source (not shown) and absorb current (charge) is applied by an absorb charge brush 5c. Since the absorb roller 5g is electrically earthed, current (charge) having polarity opposite to that of the current (charge) applied by the absorb charge brush 5c is induced on the absorb roller, thereby electrostatically absorbing the recording sheet P to the transfer sheet 5f. 
     Then, current (charge) is applied from a positive high voltage source to a transfer brush 5b so that the toner image charge negatively is transferred from the photosensitive drum 1 onto the recording sheet as mentioned above. In the multi-transferring, the above transferring operation is repeated by desired times. 
     After the transferring operations are finished, the recording sheet is moved in a direction shown by the arrow A. When the recording sheet approaches an urging roller 8b, the urging roller 8b is driven by a drive means, thereby pushing up the transfer sheet 5f toward a separation pawl (separation member) 8a as shown in FIG. 5. In synchronous with this operation, the separation pawl 8a is operated by a drive means to be lowered toward the transfer sheet 5f as shown in FIG. 5. In this case, a separation pawl roller 8c is contacted with the transfer sheet 5f to prevent the transfer sheet from being damaged by a tip end of the separation pawl 8a. That is, as shown in FIG. 5, the transfer sheet 5f is deformed by the urging roller 8b so that a tip end of the recording sheet is peeled from the transfer sheet 5f. Then, when the recording sheet is shifted toward the separation pawl 8a, the recording sheet is separated from the transfer sheet by the separation pawl. 
     On the other hand, a separation electricity removal charger 5h shown in FIGS. 1 and 5 performs discharging operation during the recording sheet separating operation to assist the separation of the recording sheet and to neutralize ions generated due to peel discharge between the recording sheet P and the transfer sheet 5f, thereby preventing occurrence of uneven discharge. 
     After the separating operation is finished, a transfer sheet cleaner 16 is rotated by a drive motor (not shown) and is urged against the transfer sheet 5f by a drive means, so that the cleaner cooperates with a back-up brush 17 opposed to the cleaner 16 with the interposition of the transfer sheet 5f to remove the residual toner from the transfer sheet 5f. Further, an inner and outer electricity removal chargers 5d, 5e are operated before and after the series of absorbing, transferring and separation operations to electrically initialize the transfer sheet 5f. 
     Next, concrete values of various elements are shown. 
     A diameter of the transfer drum 5a is 180 mm and a moving speed of the transfer drum is 130 mm/sec. Further, 
     (i) Current of absorb brush I Q  =15 μA 
     (ii) Current of transfer brush I T  =10 μA 
     (iii) Output of separation electricity removal charger 
     AC V=12 kvpp 
     DC I s  =300 μA 
     (if necessary) 
     (iv) Output of inner electricity removal charger 
     AC V=12 kVpp . . . (a) 
     DC I i  =-200 μA 
     (v) Output of outer electricity removal charger 
     AC V=12 kVpp . . . (b) 
     DC I o  =200 μA 
     Incidentally, the above (a) and (b) are sine waves having opposite phases. 
     The separation pawl 8a is made of polyester including dispersed carbon and having resistance of about 10 7  Ω. The resistance value of the separation pawl 8a was measured by applying voltage of 1 KV to the pawl having a thickness (d) of 6 mm, as shown in FIG. 7. 
     Further, as shown in FIGS. 6A and 6B, the separation pawl 8a is supported by a separation assist wing 8d for supporting the separation pawl 8a and for directing the recording sheet separated by the separation pawl toward the fixing device. The separation assist wing 8d is formed from iron coated for UV protection, if possible, grounded and is electrically. Incidentally, the separation pawl 8a may be made of polyamide having resistance of 10 3  to 10 11  Ω. 
     (Test 1) 
     The function of the separation pawl is to stably separate the recording sheet from the transfer drum 5a. Strictly speaking, the following three factors must be ensured; that is, (i) predetermined recording sheets should be stably separated (separability), (ii) the toner images on the recording sheet should not be distorted during the separating operation (image quality), and (iii) the separation pawl should not be damaged if the sheet jam occurs during the separating operation (strength). 
     In this test 1, in addition to the earthed separation pawl 8a made of resin having resistance of 10 7  Ω, as comparison examples, earthed metal Iron, earthed resin and earthed metal with fluoride coat were also used as separation pawls, respectively, to determine good (O) or bad (x) regarding the above three factors. The results are shown in FIG. 4. 
     Regarding the separability (i), recording sheets P having weights of 64, 80, 105, 128 and 157 gr/m 2  were used, respectively (in this case, if the poor separation occurred regarding even one sheet, the result was judged as bad (x)). In FIG. 4, regarding the recording sheet having weight of 64 gr/m 2 , the poor separation occurred. It was found that this phenomenon occurs because (although the tip end of the recording sheet could be separated) the separation pawl was penetrated into the recording sheet during the movement of the separated sheet (particularly, recording sheet having a large size, for example, A3 size). 
     Regarding the image quality (ii), toner scattering was checked at a position where the separation pawl 8a is disposed. When the separation pawl is formed from the earthed metal, a white stripe was generated at that position. The reason is that, as shown in FIG. 2, the ions generated due to the peel discharge between the recording sheet P and the transfer sheet 5f are concentrated onto the separation pawl 8a and the toner image on the sheet P is distorted by the ions. Further, since the charges generated due to the corona discharge from the separation electricity removal charger 5h are concentrated into the position where the separation pawl 8a is disposed, the image at that position is distorted. On the other hand, when the separation pawl is formed from the insulation resin, the arrangement of toner at a position where the pawl is disposed was delicately changed to generate a stripe pattern. The reason is that, as shown in FIG. 3, the non-fixed toner image on the recording sheet P is distorted by the friction charge between the pawl 8a and the recording sheet P. When the separation pawl is formed from metal with insulation fluoride coat, although the good result was obtained in the initial period, after about 500 sheets were copied, the fluoride coat was worn by the recording sheets P, the same problem as the metal separation pawl occurred. 
     The thickness of the fluoride coat must be reduced (less than 500 μm) to prevent the distortion of the non-fixed image due to the friction charge between the separation pawl 8a and the recording sheet P. When the transfer sheet 5f having high volume resistance (more than 10 15  Ωcm) is used as is in the illustrated embodiment, since the absorbing force between the transfer sheet 5f and the recording sheet P is great, the wear of the coating due to the contact between the coating and the recording sheet is also great. 
     However, according to the illustrated embodiment, since the thickness P (in FIG. 6) of the separation pawl 8a made of resin having resistance of 10 7  Ωis 3 mm or more and the entire separation pawl is formed from the resin having the resistance of 10 7  Ω, even when a large number of recording sheets are treated, the non-fixed toner image on the recording sheet P is not distorted. 
     Regarding the strength (ii), the poor separation of the thick sheet (having a weight of 157 gr/m 2 ) was forcibly occurred at the separation pawl 8a and damage of the pawl 8a was checked. When a width of the tip end of the separation pawl 8a in a direction (axial direction of the transfer drum 5a) perpendicular to the moving direction of the recording sheet P was 1 mm and the separation pawl was made of insulation resin, the separation pawl 8a was damaged by the resiliency of the recording sheet P. Similarly, when a separation pawl having a width of 1 mm and made of resin having resistance of 10 7  Ω was used, this pawl was also damaged by the resiliency of the recording sheet P. 
     On the other hand, when the separation pawl was made of metal, the transfer sheet 5f was damaged when the transfer sheet is contacted with the separation pawl 8a. 
     (Test 2) 
     The image quality was checked by changing the resistance value of the earthed separation pawl by varying an amount of carbon dispersed in the polyester resin. To this end, five kinds of separation pawls having resistance values of 10 3 , 10 6 , 10 9 , 10 11  and 10 13  Ωwere prepared. 
     Each separation pawl had a configuration as shown in FIGS. 6A and 6B and the resistance values were measured at a position where the thickness P is 6 mm in the manner as shown in FIG. 7. 
     As a result, although the separation pawls having resistance values of 10 3 , 10 6 , 10 9 , and 10 11  Ωachieved good result as is in the separation pawl having resistance of 10 7  Ω(test 1), the separation pawl having resistance of 10 13  Ω created a stripe pattern as is in the insulation resin pawl (test 1). 
     (Conclusion) 
     From the test result, the separation pawl 8a formed from earthed resin having resistance value of 10 3  to 10 11  Ω is optimum for the image quality. However, in order to obtain this resistance value, since conductive filler such as carbon is normally dispersed in the plastic, it is preferable that a width of the separation pawl 8a in a direction perpendicular to the moving direction of the recording sheet is 2 mm or more to ensure the strength and separability of the separation pawl.