Image forming apparatus having transfer drum with transfer paper charging memberTransfer Paper Abstract: Transfer Paper Claims: 1. An image forming apparatus comprising: an image carrying body on which a toner image is formed; transfer means for transferring the toner image formed on said image carrying body onto a transfer paper by bringing the transfer paper into contact with said image carrying body, said transfer means attracting and holding the transfer paper electrostatically, said transfer means including at least a dielectric layer on an outer surface side and a semi-conductive layer and a conductive layer on an inner surface side; voltage applying means, connected to said conductive layer, for applying a predetermined voltage to said conductive layer; potential difference generating means for pressing the transfer paper against a surface of said transfer means, and for generating a potential difference between said conductive layer to which the voltage is applied and the transfer paper; and transfer paper charging means, provided on an upstream side of said potential difference generating means in a direction in which the transfer paper is transported, for charging the transfer paper to a polarity reversed to that of a polarity of said transfer means. 2. The image forming apparatus as defined in claim 1, wherein said potential difference generating means is a grounded conductive electrode member. 3. The image forming apparatus as defined in claim 1 further comprising pre-curl means for giving a curvature to a transfer paper supplied to a section between said transfer means and said potential difference generating means. 4. The image forming apparatus as defined in claim 1 further comprising charge removing means for removing charges on the surface of said transfer means. 5. The image forming apparatus as defined in claim 1 further comprising cleaning means for cleaning the surface of said transfer means. 6. The image forming apparatus as defined in claim 1, wherein said transfer means is of a layered structure of said dielectric layer, said semi-conductive layer, and said conductive layer, which are laminated in this order from a contact surface side of the transfer paper. 7. The image forming apparatus as defined in claim 6, wherein said dielectric layer and said semi-conductive layer are made into a two-layer one-piece sheet. 8. The image forming apparatus as defined in claim 6, wherein said semi-conductive layer is made of a semi-conductive elastic body. 9. The image forming apparatus as defined in claim 1, wherein said conductive layer and said voltage applying means are connected to each other through an electric resistor. 10. The image forming apparatus as defined in claim 1, wherein said potential difference generating means includes a conductive roller made of a conductive material. 11. The image forming apparatus as defined in claim 1, wherein said potential difference generating means includes a roller type brush made of a conductive material. 12. The image forming apparatus as defined in claim 1, wherein said potential difference generating means includes a comb-shaped brush made of a conductive material. 13. The image forming apparatus as defined in claim 11, wherein an electric resistance value of said roller type brush is set to 36 k.OMEGA. or less. 14. The image forming apparatus as defined in claim 12, wherein an electric resistance value of said comb-shaped brush is set to 36 k.OMEGA. or less. 15. The image forming apparatus as defined in claim 12, wherein said comb-shaped brush has a plurality of groups of brush bristles on a brush supporting member, each-group of brush bristles including a predetermined number of brush bristles, a pitch between said plurality of groups of brush bristles being set to 1.6 mm or less. 16. The image forming apparatus as defined in claim 10, wherein said transfer means is made into a cylinder to serve as a transfer drum, whereby said conductive roller is rotatably driven by said transfer drum. 17. The image-forming apparatus as defined in claim 11, wherein said transfer means is made into a cylinder to serve as a transfer drum, whereby said roller type brush rotates together with said transfer drum. 18. The image forming apparatus as defined in claim 10, wherein said transfer means is made into a cylinder to serve as a transfer drum and an amount of crossover of said conductive roller and said transfer drum is set in a range between 0.5 mm and 3.0 mm inclusive. 19. The image forming apparatus as defined in claim 12, wherein said transfer means is made into a cylinder to serve as a transfer drum and an amount of crossover of said roller type brush and said transfer drum is set in a range between 0.5 mm and 3.0 mm inclusive. 20. The image forming apparatus as defined in claim 12, wherein said transfer means is made into a cylinder to serve as a transfer drum and an amount of crossover of said comb-shaped brush and said transfer drum is set in a range between 0.5 mm and 3.0 mm inclusive. 21. The image forming apparatus as defined in claim 1, wherein said potential difference generating means is movable to touch and separate from the surface of said transfer means so that said potential difference generating means separates from said transfer means after the transfer paper adheres to said transfer means, an amount of spacing between said potential difference generating means and said transfer means being set to 1.0 mm or more. 22. The image forming apparatus as defined in claim 1, wherein said transfer paper charging means is a plate member which charges the transfer paper by friction between the transfer paper and said plate member. 23. The image forming apparatus as defined in claim 22, wherein said plate member is at least 50 mm long in a direction in which the transfer paper is transported. 24. An image forming apparatus comprising: an image carrying body on which a toner image is formed; transfer means for transferring the toner image formed on said image carrying body onto a transfer paper by bringing the transfer paper into contact with said image carrying body, said transfer means attracting and holding the transfer paper electrostatically, said transfer means including at least a dielectric layer on an outer surface side and a semi-conductive layer and a conductive layer on an inner surface side; voltage applying means, connected to said conductive layer, for applying a predetermined voltage to said conductive layer; potential difference generating means for pressing the transfer paper against a surface of said transfer means, and for generating a potential difference between said conductive layer to which the voltage is applied and the transfer paper; and transfer paper charging means for charging the transfer paper in a polarity reversed to a polarity of said transfer means, wherein said potential difference generating means is a grounded conductive electrode member, and said transfer paper charging means is provided on a surface of said electrode member and is a charged layer for charging the transfer paper by friction between the transfer paper and said surface portion. 25. An image forming apparatus comprising: an image carrying body on which a toner image is formed; a transferrer for transferring the toner image formed on said image carrying body onto a transfer paper by bringing the transfer paper into contact with said image carrying body, said transferrer attracting and holding the transfer paper electrostatically and including at least a dielectric layer on an outer surface side and a semi-conductive layer and a conductive layer on an inner surface side; a voltage applier, connected to said conductive layer, for applying a predetermined voltage to said conductive layer; a potential difference generator for pressing the transfer paper against a surface of said transferrer, and for generating a potential difference between said conductive layer to which the voltage is applied and the transfer paper; and a transfer paper charger, provided on an upstream side of said potential difference generator in a direction in which the transfer paper is transported, for charging the transfer paper to a polarity reversed to that of a polarity of said transferrer. 26. The image forming apparatus as defined in claim 24, wherein said charged layer is composed of glass or nylon. 27. The image forming apparatus as defined in claim 24, wherein said charged layer is composed of polytetrafluoroethylene. Transfer Paper Description: The present invention relates to an image forming apparatus employed in a laser printer, a copying machine, a laser facsimile and the like. BACKGROUND OF THE INVENTION An image forming apparatus which develops an electrostatic image formed on a photosensitive drum by adhering toner and transfers the developed image onto a transfer paper wound around a transfer drum is known. Such an image forming apparatus includes, for example, two corona chargers within a cylinder 501 having a dielectric layer 501a as shown in FIG. 69: one is a corona charger 502 for attracting a transfer paper P, and the other is a corona charger 504 for transferring a toner image formed on the surface of a photosensitive drum 503 onto the transfer paper P. Including two corona chargers 502.cndot.504 makes it possible to attract the transfer paper P and transfer the toner image onto the transfer paper P independently. Another image forming apparatus shown in FIG. 70 includes a two-layer structure cylinder 601 made of a semi-conductive layer 601a serving as an outer layer and a base material 601b serving as an inner layer, and a grip mechanism 602 for holding the transported transfer paper P around the cylinder 601. This image forming apparatus grips the end of the transported transfer paper P to hold the same around the surface of the cylinder 601 by means of the grip mechanism 602 first, then charges the surface of the cylinder 601 with electricity either by applying a voltage to the semi-conductive layer 601a serving as the outer layer of the cylinder 601 or triggering a discharge of a charger installed within the cylinder 601, and then transfers a toner image formed on the photosensitive drum 503 onto the transfer paper P. However, the cylinder 501 of the image forming apparatus shown in FIG. 69 must have two corona charges 502.cndot.504 inside thereof, because the cylinder 501, which serves as a transfer roller, is of a single layer structure using the dielectric layer 501a alone. This structure limits the size of the cylinder 501 and presents a problem that the image forming apparatus can not be downsized. In contrast, the cylinder 601 in the image forming apparatus shown in FIG. 70, which serves as the transfer roller, is charged by exploiting its two-layer structure to transfer the toner image onto the transfer paper P, and thus the number of the chargers can be reduced. However, the grip mechanism 602 complicates the entire structure of the image forming apparatus. Moreover, the semi-conductive layer 601a serving as the outer layer and the base material 601b serving as the inner layer must be fixed with mounting hardware and secured to each other by small screws, a double-sided adhesive tape or the like to assemble the cylinder 601. Accordingly, the image forming apparatus requires more components and presents a problem that the manufacturing costs increase. To eliminate these problems, Japanese Laid-open Patent Application No. 2-74975/1990 discloses an image forming apparatus including a corona charger driven by a unipolar power source in the vicinity of a point where a transfer paper separates from a transfer drum made of a lamination of conductive rubber and a dielectric film on a grounded roll of metal. With this image forming apparatus, a transfer paper is attracted to the transfer drum by inducing the charges on the dielectric film by means of the corona charger. Once the transfer paper is attracted, more charges are induced on the dielectric film, thereby enabling the transfer of a toner image onto the transfer paper. Since this image forming apparatus uses a single charger to charge the surface of the transfer drum so as to attract the transfer paper and transfer the toner image onto the transfer paper, the transfer drum can be downsized. Also, the above image forming apparatus omits a mechanism such as the grip mechanism 602, so that the transfer paper can be attracted to the transfer drum by a simple structure. However, since the transfer paper adheres to the transfer drum electrostatically in this image forming apparatus, some charges remain on the transfer drum, which may cause the toner to adhere to the surface of the transfer drum. Thus, these residual charges present problems such as insufficient adhesion of the transfer paper to the transfer drum or back transfer on the transfer paper, thereby degrading the quality of a resulting image. Accordingly, Japanese Laid-open Patent Application No. 6-51645/1994 discloses a transfer device provided in the vicinity of the transfer drum in an image forming apparatus, which includes cleaning means made of a conductive fur brush for scraping off the toner adhering to the transfer drum and charge removing means for removing the charges caused by the friction between the conductive fur brush and transfer drum. Note that the charge removing means applies a voltage to the conductive fur brush in a polarity reversed to that of the surface potential of the transfer drum, so that the residual charges on the transfer drum are removed. Since not only the charges remaining on the transfer drum are removed, but also the transfer drum is cleaned, the transfer paper can adhere to the transfer drum satisfactorily and the back transfer on the transfer paper can be eliminated, thereby making it possible to produce a good-quality image. Also, Japanese Laid-open Patent Application No. 3-102385/1991 discloses a cleaning device for an image forming apparatus which attracts a transfer paper to the surface of the transfer drum electrostatically. The cleaning device removes post-transfer residual toner on the surface of a transferring body by applying a bias voltage to a brush cleaner in a polarity reversed to that of the toner. As shown in FIG. 71, the cleaning device includes a conductive brush 702 which makes contact with the inner side of a transfer drum 701 and a cleaning brush 703 which makes contact with the outer surface of the transfer drum 701. According to this structure, the charges remaining on the transfer drum 701 are removed by the conductive brush 702, while the surface of the transfer drum 701 is cleaned by the cleaning brush 703. Thus, the transfer drum can attract the transfer paper satisfactorily and the back transfer on the transfer paper can be eliminated, thereby making it possible to produce a high-quality image. However, the image forming apparatus disclosed in Japanese Laid-open Patent Application No. 2-74975/1990 charges the surface of the transfer drum through an atmospheric discharge by a corona charger. For this reason, if a color image is formed by repeating a transfer process a number of times, the charges are replenished by the corona charger each time a toner image is transferred onto the transfer paper. Thus, the image forming apparatus demands a charging unit comprising a unipolar power source or the like to drive the corona charger under its control. As a result, the number of components of the image forming apparatus increases, thereby presenting a problem that the manufacturing costs increase. In addition, a flaw on the surface of the transfer drum makes an electric field area developed by the atmospheric discharge smaller, and the electric field becomes out of balance over the flaw. Such off-balance of the electric field causes a defect in a transferred image such as a white spot (void), and hence degrades the quality of a resulting image. Also, a considerably high voltage must be applied to charge the surface of the transfer roller through the atmospheric discharge, and the driving energy of the image forming apparatus increases accordingly. Further, since the atmospheric discharge is susceptible to the environments such as the temperature and humidity of air, the surface potential of the transfer roller varies easily, which causes insufficient adhesion of the transfer paper, disordered printing, etc. The transfer device in the image forming apparatus disclosed in Japanese Laid-open Patent Application No. 6-51645/1994 and the cleaning device disclosed in Japanese Laid-open Patent Application No. 3-102385/1991 remove the residual toner and charges on the surface of the transferring body (transfer drum) by bringing the cleaning brush into contact with the surface of the transferring body. Thus, the cleaning brush may cause a flaw on the surface of the transferring body, and the flaw on the transferring body causes a defect in the transferred toner image and degrades the quality of a resulting image. Further, the transfer device in the image forming apparatus disclosed in Japanese Laid-open Patent Application No. 6-51645/1994 employs the conductive fur brush to prevent the transfer drum from being charged with electricity caused by the friction between the transfer drum and the brush portion while the transfer drum is being cleaned, and to remove the charges on the transfer drum. The charges on the transfer drum are removed by applying a voltage to the fur brush in a polarity reversed to that of the surface potential of the transfer drum. However, a structure such that enables satisfactory charge removal is not fully concerned, and the removal of the surface potential is not ensured in this application. Thus, there still occur problems that the residual toner causes a smudge on the back of the transfer paper and the residual charges cause insufficient adhesion of the transfer paper to the transfer drum. Japanese Laid-open Patent Application No. 5-173435/1993 discloses an image forming apparatus which includes a transfer drum having at least an elastic layer made of a foam body and a dielectric layer covering the elastic layer. This image forming apparatus produces a color image on a transfer sheet by sequentially forming a plurality of toner images in their respective colors on a photosensitive drum and superimposing the toner images sequentially on the transfer sheet. The above image forming apparatus applies a voltage to an attracting roller serving as charge giving means as a technique to hold the transfer sheet on the transfer drum, so that the transfer drum attracts the transfer sheet electrostatically. A space is formed between the elastic layer and dielectric layer to enhance an adhesion force, or namely, the adhesion of the transfer sheet to the transfer drum. The image forming apparatus disclosed in Japanese Laid-open Patent Application No. 5-173435/1993 specifies neither the hardness of the elastic layer (foam body layer) nor the contacting pressure between the attracting roller and transfer drum. Further, the application is silent about the width of a close contacting portion between the attracting roller furnished with a power source and transfer drum (known as the nip width), and the time required for an arbitrary point on the transfer sheet to pass by the nip width (known as the nip time). Thus, the nip time is assumed to be constant regardless of the kind of the transfer sheet. However, it is known that the amount of charges given to the transfer sheet during a constant nip time varies depending on the kind of the transfer sheet. Thus, it is assumed that, when the transfer drum attracts the transfer sheet electrostatically, the electrostatic adhesion force differs considerably depending on the kind of the transfer sheet. That is to say, given a constant nip time to all kinds of the transfer sheets, some kinds of the transfer sheets may not adhere to the transfer drum electrostatically in a satisfactory manner, because the amount of the charges given to the transfer sheet during the constant time varies considerably depending on the kind of the transfer sheet. Therefore, as the electrostatic adhesion force decreases over time, there may be a case that the transfer sheet separates from the transfer drum before all of the toner images in their respective colors formed on the photosensitive drum are transferred onto the transfer sheet, thereby presenting a problem that the toner images are not transferred satisfactorily. Further, the above image forming apparatus demands at least two power sources: an attracting roller's power source for enabling the transfer drum to attract the transfer sheet, and a power source for applying a voltage to the transfer sheet in a polarity reversed to that of the toner when transferring a toner image onto the transfer sheet. Accordingly, there occurs a problem that the manufacturing costs increase. In addition, Japanese Laid-open Patent Application No. 4-256977 discloses an image forming apparatus including an attracting roller for giving charges to transfer means to enable the transfer means to attract a transfer paper, and attracting voltage applying means for applying an attracting voltage to the attracting roller. Also, Japanese Laid-open Patent Application No. 4-256978 discloses an image forming apparatus including, in addition to the above-mentioned attracting roller and attracting voltage applying means, transferring voltage applying means for applying a voltage to the transfer means to enable the transfer means to transfer a toner image onto the transfer paper. In the image forming apparatuses disclosed in the above Japanese Laid-open Patent Application Nos. 4-256977 and 4-256978, the transfer paper is attracted to the transfer means in a reliable manner, and thus the toner image is transferred onto the transfer paper satisfactorily, thereby making it possible to produce a high-quality image. However, the above two image forming apparatuses apply a high voltage to the attracting roller in the same polarity as that of the voltage applied to the transfer means. Thus, both the image forming apparatuses demand a high voltage power source, or namely, an attracting bias power source, which not only increases the number of components but also demands a safeguard against the high voltage, such as measures for leakage and insulation. Accordingly, the resulting image forming apparatuses becomes more expensive and has more complicated structure. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an inexpensive image forming apparatus which can attract a transfer paper to the surface of transfer means such as a transfer drum in a stable manner so as to eliminate defects in a transferred toner image and produce a satisfactory image on the transfer paper. To fulfill the above object, an image forming apparatus of the present invention is characterized by comprising: (1) an image carrying body on which a toner image is formed: (2) transfer means for transferring the toner image fomred on the image carrying body onto a transfer paper by bringing the transfer paper into contact with the image carrying body, the transfer means attracting and holding the transfer paper electrostatically, the transfer means including at least a dielectric layer on an outer surface side and a semi-conductive layer and a conductive layer on an inner surface side; (3) voltage applying means, connected to the conductive layer, for applying a predetermined voltage to the conductive layer; (4) potential difference generating means for pressing the transfer paper against a surface of the transfer means, and for generating a potential difference between the conductive layer to which the voltage is applied and the transfer paper; and (5) transfer paper charging means, provided on an upstream side of the potential difference generating means in a direction in which the transfer paper is transported, for charging the transfer paper in a polarity reversed to a polarity of the transfer means. According to the above structure, the transfer paper charging means which charges the transfer paper in a polarity reversed to that of the transfer means is provided on an upstream side of the potential difference generating means in the direction in which the transfer paper is transported. Thus, the transfer paper is charged in a polarity reversed to that of the transfer means before the transfer paper is attracted to the transfer means. Accordingly, the transfer paper can adhere to the transfer means in a stable manner whether the transfer paper was negatively or positively charged before it is attracted to the transfer means. As a result, defects in a transferred toner image caused by insufficient adhesion of the transfer paper can be eliminated, thereby making it possible to transfer a toner image onto the transfer paper satisfactorily. It is preferable that the transfer paper charging means forms a surface portion of the potential difference generating means, so that it charges the transfer-paper by the friction between the transfer paper and the surface portion. This structure makes it unnecessary to provide the transfer paper charging means and the potential difference generating means separately, which can reduce the number of the components and hence save the manufacturing costs. To fulfill the above object, it is preferable that the image forming apparatus of the present invention further comprises: (6) adhesive transporting means, provided on an upstream side of the potential difference generating means in a direction in which the transfer paper is transported, for pressing the transfer paper against the surface of the transfer means, and for transporting the transfer paper to the potential difference generating means while making the transfer paper adhere to the transfer means. According to this structure, the transfer means can attract the transfer paper electrostatically and mechanically, so that the transfer paper can adhere to the transfer means in a stable manner. Thus, defects in a transferred toner image caused by insufficient adhesion of the transfer paper can be eliminated, thereby making it possible to transfer a toner image onto the transfer paper satisfactorily. To fulfill the above object, it is preferable to enable the voltage applying means to apply an attracting voltage for attracting the transfer paper and a transferring voltage for transferring the toner image onto the transfer paper to the conductive layer of the transfer means while changing the values of these voltages. According to this structure, the value of the attracting voltage and that of the transferring voltage can be changed appropriately depending on the humidity or kind of the transfer paper. Thus, the transfer paper can adhere to the transfer means in a reliable manner, and as a result, a toner image can be transferred onto the transfer paper satisfactorily. The amount of charges given to the transfer paper during a nip time (a time required for an arbitrary point on the transfer paper to pass by a close contacting portion between the transfer means and potential difference generating means) varies depending on the kind of the transfer paper. This means that the amount of charges on the transfer paper can be adjusted by changing the nip time depending on the kind of the transfer paper. Thus, any kind of transfer paper can adhere to the dielectric layer of the transfer means electrostatically in a stable manner. If the relation between the nip time and the amount of charges on each kind of the transfer paper is found in advance, the nip time can be changed to an adequate nip time in which a sufficient amount of charges needed to enable the transfer paper to adhere to the transfer means in a stable manner is given efficiently. Further, it becomes easier to check how to change the current nip time to an adequate nip time for a particular kind of transfer paper to enable the transfer paper to adhere to the dielectric layer of the transfer means in a stable manner. More specifically, physical properties such as resistivity vary in each kind of the transfer paper, and the amount of charges given to the transfer paper during the nip time varies depending on not only the physical properties of the transfer paper, but also the other conditions such as the physical properties (resistivity) of the semi-conductive layer and/or dielectric layer, or an applied voltage. However, even the conditions such as the resistivity of the semi-conductive layer and/or dielectric layer, applied voltage, or the kind of the transfer paper is changed, the relation between the nip time and the amount of charges on the transfer paper is classified into three patterns. Thus, if the relation between the nip time and the amount of charges on the transfer paper is found in advance using an arbitrary semi-conductive layer and an arbitrary dielectric layer for each kind of the transfer paper, the nip time in which a particular kind of transfer paper is charged efficiently can be found easily only by detecting the kind of the transfer paper and the pattern to which the detected kind of transfer paper belongs when the resistivity of the semi-conductive layer and/or dielectric layer, or the kind of the transfer paper is changed. For example, when the amount of charges on the transfer paper reaches its maximal value over the nip time (PATTERN I), the nip time is set in such a manner that the amount of charges will not drop below the initial charge amount, thereby enabling the transfer paper to adhere to the dielectric layer electrostatically in a stable manner. If the nip time is set to a nip time corresponding to the maximal value, the charges are injected effectively, and hence the transfer paper can be charged efficiently. When the amount of charges on the transfer paper increases as the nip time extends (PATTERN II), the nip time is set in such a manner that the potential difference before and after the charge injection will be in a range between 0V and 1000V inclusive in an absolute value. As a result, the transfer paper can adhere to the dielectric layer electrostatically in a stable manner. It is found from experiments that the electrostatic adhesion force of the transfer paper decreases when there is a potential difference exceeding 1000V before and after the charge injection. When the amount of charges of the transfer paper drops below the initial charge amount as the nip time extends (PATTERN III), the nip time is set in such a manner that the amount of charges on the transfer paper will be at least 50% of the initial charge amount. As a result, the transfer paper can adhere to the dielectric layer electrostatically in a stable manner. As has been explained, when the relation between the nip time and amount of charges on the transfer paper is found in advance for each kind of transfer paper, the nip time in which a particular kind of transfer paper is charged efficiently is found based on the kind of the transfer paper using the relation between the nip time and amount of the charges on the transfer paper. Further, when the nip time is changed for a particular kind of transfer paper based on the relation between the nip time and the amount of charges on the transfer paper, a sufficient amount of charges needed to enable that kind of transfer paper to adhere to the dielectric layer of the transfer means can be given. As a result, the transfer paper can adhere to the dielectric layer electrostatically in a stable manner. When the transfer means includes the semi-conductive layer, the nip time can be changed easily by adjusting the hardness of the semi-conductive layer. Also, the nip time can be changed by adjusting a contacting pressure between the transfer means and potential difference generating means. The nip time can be changed by adjusting the rotation speed of the transfer means; however, the rotation speed of the transfer means must be decreased to extend the nip time, and when the rotation speed of the transfer means is decreased, the transfer efficiency per minute decreases. In contrast, the toner-image transfer efficiency is not degraded if the nip time is changed not by the moving speed of the transfer means but by the hardness of the semi-conductive layer and/or the contacting pressure between the transfer means and potential difference generating means as has been explained. Thus, it is preferable to change the nip time by adjusting the hardness of the semi-conductive layer and/or the contacting pressure between the transfer means and potential difference generating means. Also, to fulfill the above object, it is preferable that the image forming apparatus of the present invention further comprises: (7) charge removing means for removing the charges on the surface of the transfer means; and/or (8) cleaning means for cleaning the surface of the transfer means. According to this structure, the residual toner and/or residual charges are removed by the charge removing means and cleaning means, respectively. Thus, not only back transfer on the transfer paper can be eliminated, but also the transfer means can be charged in a stable manner. As a result, defects in a transferred toner image caused by insufficient adhesion of the transfer paper can be eliminated, thereby making it possible to transfer a toner image onto the transfer paper satisfactorily. It is preferable that the charge removing means includes: (a) a conductive member which slides on the transfer means; (b) a charge-removing-use power source unit for applying a voltage to the conductive member; (c) first switching means for switching the connection of the conductive member to the charge-removing-use power source unit from a grounding portion and vice versa; and (d) second switching means for switching the connection of the conductive layer to the voltage applying means from a grounding portion and vice versa. When a roller type brush or comb-shaped brush is used as the conductive member, the charges on the transfer means can be removed while the transfer means is cleaned. When the potential difference generating means comprises a grounded conductive electrode member and electrode member driving means for driving the electrode member to touch and separate from the transfer means, the charge removing means may include: (e) control means for controlling the voltage applying means to apply a voltage to the transfer means in a polarity reversed to a polarity of the transfer means when a toner image has been transferred onto the transfer paper, and for controlling the electrode member driving means to bring the electrode member into contact with the transfer means by pressure. According to the above structure, a voltage is applied to the transfer means in a polarity reversed to that of the transfer means when the toner image has been transferred onto the transfer paper and the electrode member is brought into contact with the transfer means by pressure. Given these conditions, the residual charges on the transfer means are neutralized while they are released through the electrode member. Thus, the residual charges on the transfer means are removed when the toner image has been transferred onto the transfer paper, and the transfer means can be charged in a reliable manner so as to attract the transfer paper in a stable manner. As a result, defects in a transferred toner image caused by insufficient adhesion of the transfer paper can be eliminated, thereby making it possible to transfer a toner image onto the transfer paper satisfactorily. Also, it is preferable that the charge removing means further includes: (f) temperature and humidity measuring means for measuring the temperature and humidity inside of the image forming apparatus; and (g) storage means for storing a value of a charge removing voltage depending on the temperature and humidity inside of the image forming apparatus to remove the charges on the transfer means. According to this structure, the value of a charge removing voltage depending on the temperature and humidity measured by the temperature and humidity measuring means is read out from the storage means, and the voltage applying means is controlled so as to apply a voltage having the same value as the readout value to the transfer means when a toner image has been transferred onto the transfer paper. Accordingly, the transfer means can be charged in a stable manner without being affected by the temperature and humidity. As a result, defects in a transferred toner image caused by insufficient adhesion of the transfer paper can be eliminated, thereby making it possible to transfer a toner image onto the transfer paper satisfactorily. Alternatively, a current flowing through the electrode member may be measured to determine the value of the charge removing voltage for removing the charges on the transfer means, and a voltage having the same value as the determined value is applied to the transfer means to remove the charges on the transfer means. Since the charge removing voltage can be set to an adequate value, the charges can be removed effectively. Further, a surface potential of the transfer means may be measured to determine the value of the charge removing voltage for removing the charges on the transfer means, and a voltage having the same value of the determined value is applied to the transfer means to remove the charges on the transfer means. If the charge removing means includes: (h) a roller type charge removing brush for removing the charges on the dielectric layer of the transfer means as it rotates while making contact with the dielectric layer; and (i) second voltage applying means for applying a voltage, which is of the same polarity as that of a voltage applied to the conductive layer from the voltage applying means and higher than the same, to the charge removing brush. According to this structure, the charges on the surface of the transfer means can be removed in a reliable manner. The principle of the above charge removal will be explained in the following. According to a principle applied to a capacitor (condenser), a current flows when a polarized electrode is energized and the charges on the transfer means are removed as a consequent. However, not all of the charges are removed when the voltages of the same level are applied to the transfer means and charge removing brush, respectively. Thus, when a voltage higher than a voltage applied to the transfer means is applied to the charge removing brush, the polarized charges are attracted to the charge removing brush and removed completely. As a result, back transfer on the transfer paper caused by the toner adhering to the surface of the transfer means or defects in a transferred image caused by insufficient adhesion of the transfer paper to the transfer means due to the residual charges can be eliminated. In addition, the charges needed to attract a following transfer paper to the transfer means can be given to the surface of the transfer means. It is preferable that the transfer means is made into a cylinder to serve as a transfer drum, and the charge removing brush is tilted with respect to a direction in which an axis of the charge removing brush intersects at right angles with a direction in which the surface of the transfer drum moves. According to this structure, the charge removing means makes contact with the transfer means in a larger area, so that the charge removing effect is upgraded without making the diameter of the charge removing means larger. As a result, the charge removing effect on the transfer means can be upgraded without upsizing the image forming apparatus and increasing the manufacturing costs. To fulfill the above object, it is preferable that the image forming apparatus of the present invention further comprises: (9) charge amount control means, provided on a downstream side of a transfer point between the image carrying body and transfer means in a direction in which the image carrying body moves, for controlling an amount of charges on the surface of the image carrying body. According to this structure, the residual charges on the image carrying body can be removed when a toner image has been transferred onto the transfer paper. Accordingly, the charges on the transfer means will not be affected by the residual charges on the image carrying body. Thus, the transfer means can be charged in a stable manner, and as a result, defects in a transferred toner image caused by insufficient adhesion of the transfer paper can be eliminated, thereby making it possible to transfer a toner image onto the transfer paper satisfactorily. If an erasing lamp is used as the charge amount control means, the structure of the charge amount control means can be simplified while saving the manufacturing costs of the charge amount control means, and thus saving the manufacturing costs of the image forming apparatus as a result. When the transfer means is of a layered structure of the dielectric layer, semi-conductive layer, and conductive layer, which are laminated in this order from a contact surface side of the transfer paper, the charges move to the semi-conductive layer from the conductive layer in a stable manner if the semi-conductive layer and conductive layer are laminated to each other fixedly. Accordingly, the surface of the dielectric layer is charged evenly in a stable manner by the charges moved from the semi-conductive layer. As a result, the charging and discharging characteristics of the dielectric layer can be upgraded. Thus, the transfer means can be charged in a stable manner, and hence defects in a transferred toner image caused by insufficient adhesion of the transfer paper can be eliminated, thereby making it possible to transfer a toner image onto the transfer paper satisfactorily. In particular, when the transfer means comprises a cylinder made of conductive metal, and a one-piece sheet made of at least two layers each having different volume resistivity and layered on the surface of the cylinder, the cylinder can serve as the conductive layer, and the inner layer and the outer-most layer of the one-piece sheet can serve as the semi-conductive layer and dielectric layer, respectively. Accordingly, each layer can adhere to each other fixedly. For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view showing the structure of an image forming apparatus in accordance with the first embodiment of the present embodiment. FIG. 2 is a schematic view showing a copying machine employing the image forming apparatus of FIG. 1. FIG. 3 is a schematic cross sectional view showing a structure of a transfer drum in the image forming apparatus of FIG. 1. FIG. 4 is a view explaining the coupling state of a conductive layer, a semi-conductive layer, and a dielectric layer forming the transfer drum of FIG. 3. FIG. 5 is another view explaining the coupling state of the conductive layer, semi-conductive layer, and dielectric layer forming the transfer drum of FIG. 3. FIG. 6 is a view explaining a charged state of the transfer drum of FIG. 3, and an initial state when a transfer paper is transported to the transfer drum. FIG. 7 is a view explaining a charged state of the transfer drum of FIG. 3, and a state when the transfer paper is transported to a transfer point of the transfer drum. FIG. 8 is a view explaining a comparison between a chargeable width of the transfer drum of FIG. 3 and an effective image width. FIG. 9 is a view explaining the movement of charges between the transfer drum of FIG. 3 and a photosensitive drum when a following relation is established in terms of widths: dielectric layer <semi-conductive layer<conductive layer. FIG. 10 is a view explaining the movement of charges between the transfer drum of FIG. 3 and the photosensitive drum when a following relation is established in terms of widths: semi-conductive layer<dielectric layer=conductive layer. FIG. 11 is a schematic view explaining another structure of the transfer drum of FIG. 3. FIG. 12 is a schematic view explaining still another structure of the transfer drum of FIG. 3. FIG. 13 is a schematic view explaining still another structure of the transfer drum of FIG. 3. FIG. 14 is a block diagram of a control device installed in the above-structured image forming apparatus. FIG. 15(a) is a schematic view showing a structure of an image forming apparatus in accordance with the second embodiment of the present invention, which employs a roller type brush instead of a ground roller shown in FIG. 1. FIG. 15(b) is a schematic view showing a structure of an image forming apparatus in accordance with the third embodiment of the present invention, which employs a comb-shaped brush instead of the ground roller shown in FIG. 1. FIG. 16 is a schematic view showing a structure of an image forming apparatus in accordance with the fourth embodiment of the present invention. FIG. 17 is a schematic view showing a structure of a conductive brush provided around a transfer drum shown in FIG. 16. FIG. 18 is a timing chart showing the timing of operation of each component of the image forming apparatus shown in FIG. 16. FIG. 19 is a flowchart detailing a charge removing job of the image forming apparatus shown in FIG. 16. FIG. 20 is a schematic view showing a structure of an image forming apparatus in accordance with the fifth embodiment of the present invention. FIG. 21 is a schematic view showing another structure around a transfer drum shown in FIG. 20. FIG. 22 is a schematic view showing still another structure around the transfer drum shown in FIG. 20. FIG. 23 is a schematic view showing a structure of a modified image forming apparatus of the fifth embodiment. FIG. 24 is a schematic view showing another structure around a transfer drum shown in FIG. 23. FIG. 25 is a schematic view showing still another structure around the transfer drum shown in FIG. 23. FIG. 26 is a schematic view showing a structure of an image forming apparatus in accordance with the sixth embodiment of the present invention. FIG. 27 is a schematic view showing another structure around a transfer drum shown in FIG. 26. FIG. 28 is a schematic view showing still another structure around the transfer drum shown in FIG. 26. FIG. 29 is a schematic view showing a structure of an image forming apparatus in accordance with the seventh embodiment of the present invention. FIG. 30 is a schematic view showing another structure around a transfer drum shown in FIG. 29. FIG. 31 is a schematic view showing a structure of an image forming apparatus in accordance with the eighth and ninth embodiments of the present invention. FIG. 32 is a block diagram of a control device installed in the above image forming apparatus. FIG. 33 is a flowchart detailing a charge removing job for a transfer drum shown in FIG. 31. FIG. 34 is a schematic view showing another structure of the transfer drum shown in FIG. 31. FIG. 35 is a schematic view showing still another structure of the transfer drum shown in FIG. 31. FIG. 36 is a schematic view showing still another structure of the transfer drum shown in FIG. 31. FIG. 37 is a schematic view showing a structure of an image forming apparatus in accordance with the tenth embodiment of the present invention. FIG. 38 is a schematic view showing a structure of an extruding machine used in a process of manufacturing a transfer drum shown in FIG. 37. FIG. 39 is a view explaining the process of manufacturing the transfer drum shown in FIG. 37. FIG. 40 is a schematic view showing a structure of a receiving machine used in the process of manufacturing the transfer drum shown in FIG. 37. FIG. 41 is a view explaining a degree of adhesion between a dielectric layer and a semi-conductive layer of the transfer drum shown in FIG. 37 when the embossing finish is not applied to the dielectric layer. FIG. 42 is a view explaining a degree of adhesion between the dielectric layer and semi-conductive layer of the transfer drum shown in FIG. 37 when the embossing finish is applied to the dielectric layer. FIG. 43 is a cross sectional view of a metal mold used in another method for manufacturing the transfer drum shown in FIG. 37. FIG. 44 is a schematic view showing a structure of an image forming apparatus in accordance with the eleventh embodiment of the present invention. FIG. 45 is a timing chart showing the timing of operation of each component of the image forming apparatus shown in FIG. 44. FIG. 46 is a view explaining Paschen's discharge occurring at a close contacting portion between the transfer drum and a conductive roller shown in FIG. 1. FIG. 47 is a schematic view showing a structure of an image forming apparatus in accordance with the twelfth embodiment of the present invention. FIG. 48 is a view explaining a structure to change a contacting pressure between a transfer drum and a conductive roller shown in FIG. 47. FIG. 49 is a side view explaining a structure to change the contacting pressure between the transfer drum and conductive roller shown in FIG. 47. FIG. 50 is a schematic circuit diagram showing an equivalent circuit of a charge injecting mechanism between the transfer drum and conductive roller shown in FIG. 47. FIG. 51 is a graph showing a relation between the amount of charges on a transfer sheet and a nip time. FIG. 52 is a graph showing a relation between the amount of charges on the transfer sheet and the nip time under a condition different to that of FIG. 51. FIG. 53 is a graph showing a relation between the amount of charges on the transfer sheet and the nip time under a condition different to those of FIGS. 51 and 52. FIG. 54 is a schematic view showing another structure of the transfer drum shown in FIG. 47. FIG. 55 is a schematic view showing still another structure of the transfer drum shown in FIG. 47. FIG. 56 is a schematic view explaining a structure of an electrode layer of the transfer drum shown in FIG. 55. FIG. 57 is a perspective view showing the structure of the electrode layer of the transfer drum shown in FIG. 55. FIG. 58 is a schematic view showing a structure of an image forming apparatus in accordance with the thirteenth embodiment of the present invention. FIG. 59 is a diagram showing a structure around a transfer drum of the image forming apparatus shown in FIG. 58. FIG. 60 is a block diagram showing a structure of a transfer drum's applied voltage control device of the image forming apparatus shown in FIG. 58. FIG. 61 is a view schematically explaining an operation panel provided on the surface of the image forming apparatus shown in FIG. 58. FIG. 62 is a graph showing a relation between an output value of a humidity sensor used in the image forming apparatus shown in FIG. 58 and relative humidity. FIG. 63 is a schematic view showing a structure of an image forming apparatus in accordance with the fourteenth embodiment of the present invention. FIG. 64 is a diagram schematically showing a charge removing device of the image forming apparatus shown in FIG. 63. FIG. 65 is a view schematically explaining a structure of a rotation driving device of the charge removing device shown in FIG. 63. FIG. 66 is a block diagram schematically showing control means of the rotation driving device shown in FIG. 63. FIG. 67 is a view explaining a position of a roller type conductive brush shown in FIG. 64 with respect'to the transfer drum. FIG. 68(a) is a schematic perspective view explaining effectiveness of the roller type conductive brush shown in FIG. 67 depending on the position and orientation thereof. FIG. 68(b) is a plan view of the roller type conductive brush shown in FIG. 68(a). FIG. 68(c) is a front view of a virtual cross section a of the roller type conductive brush shown in FIG. 68(a). FIG. 68(d) is a front view of another virtual cross section b of the roller type conductive brush shown in FIG. 68(a). FIG. 69 is a schematic view showing a structure of a conventional image forming apparatus. FIG. 70 is a schematic view showing a structure of another conventional image forming apparatus. FIG. 71 is a schematic view showing a structure of still another conventional image forming apparatus. DESCRIPTION OF THE PREFERRED EMBODIMENTS [FIRST EMBODIMENT] An embodiment of the present invention will be explained in the following while referring to FIGS. 1 through 14 and FIG. 46. As shown in FIG. 2, an image forming apparatus of the present embodiment comprises a paper feeding unit 1 for storing transfer papers as recording papers on which toner images are formed and feeding the transfer papers sequentially, a transfer unit 2 for transferring a toner image onto a transfer paper, a developing unit 3 for forming a toner image, and a fuser unit 4 for fusing the transferred toner image into place on the transfer paper. The paper feeding unit 1 is attachable to and detachable from the lowest stage of the main body of the image forming apparatus, and includes a paper feeding cassette 5 for storing the transfer papers and feeding the transfer papers sequentially to the transfer unit 2, and a manual paper feeding unit 6, provided on the front side of the main body, for feeding one transfer paper at a time manually. The paper feeding unit 1 further includes a pick up roller 7 for sending the transfer paper on the top in the paper feeding cassette 5, a pre-feed roller (PF roller) 8 for transporting the transfer paper sent from the pick up roller 7, a manual paper feeding roller 9 for transporting the transfer paper from the manual paper feeding unit 6, and a pre-curl roller (PS roller) 10 for curling the transfer paper transported from either the PF roller 8 or manual paper feeding roller 9 before the transfer paper reaches the transfer unit 2. The paper feeding cassette 5 includes a forwarding member 5a energized upward by a spring or the like, on which the transfer papers are piled. According to this structure, the transfer paper on the top of the pile in the paper feeding cassette 5 is brought into contact with the pick up roller 7, so that only the transfer paper on the top is sent to the PF roller 8 as the pick up roller 7 rotates in the direction indicated by an arrow, and further transported to the PS roller 10. The transfer paper fed from the manual paper feeding unit 6 is also transported to the PS roller 10 by the manual paper feeding roller 9. The PS roller 10 curls the transported transfer paper as previously mentioned, so that the transfer paper easily adheres to the surface of a cylindrical transfer drum 11 provided in the transfer unit 2. The transfer unit 2 includes the transfer drum 11 serving as transfer means, and around which a ground roller 12 (potential difference generating means and an electrode member) made of a conductive member serving as a grounded electrode member, a guiding member 13 for guiding the transfer paper so as not to separate from the transfer drum 11, a separating claw 14 for forcefully separating the transfer paper adhering to the transfer drum 11 from the transfer drum 11, etc. are provided. The transfer drum 11 attracts a transfer paper P to the surface thereof electrostatically. For this reason, followings are further provided around the transfer drum 11: a charge removing device 11a serving as charge removing means for removing the charges on the surface of the transfer drum 11, and a cleaning device 11b serving as cleaning means for removing the toner adhering to the surface of the transfer drum 11. Note that the separating claw 14 is movable to touch and separate from the surface of the transfer drum 11, and the structure of the transfer drum 11 will be explained below in detail. The charge removing device 11a, cleaning device 11b, and separating claw 14 are driven by unillustrated driving means so as to be brought into contact with the surface of the transfer drum 11. The developing unit 3 includes a photosensitive drum 15 serving as an image carrying body which is brought into contact with the transfer drum 11 by pressure. The photosensitive drum 15 is made of a grounded conductive aluminium tube 15a, and the surface thereof is covered with an OPC (organic photoconductive conductor) film. Developers 16, 17, 18, and 19, which are filled with toner in yellow, magenta, cyan, and black, respectively, are provided radially around the photosensitive drum 15. Also, provided around the photosensitive drum 15 are: a charger 20 for charging the surface of the photosensitive drum 15, an unillustrated image spacing eraser, and a cleaning blade 21 serving as toner removing means for scraping off residual toner on the surface of the photosensitive drum 15. According to this structure, a toner image is formed on the photosensitive drum 15 for each color. That is to say, a series of charging, exposure, development, and transfer operations is repeated for each color with the photosensitive drum 15. Note that the surface of the photosensitive drum 15 is exposed by being irradiated with a beam of light emanated from an unillustrated optical series through a space between the charger 20 and cleaning blade 21. Thus, when transferring color toner images, one toner image in one color is transferred onto the transfer paper adhering to the transfer drum 11 each time the transfer drum 11 makes a full turn; the transfer drum 11 rotates up to four times to form a color image. Note that the photosensitive drum 15 and transfer drum 11 of the present embodiment press against each other so that a pressure of 2 to 8 kg is applied to a portion where a toner image is transferred onto the transfer paper to enhance transfer efficiency and the image quality. The fuser unit 4 includes a fixing roller 23 for fusing a toner image into place on the transfer paper at a certain temperature and under a certain pressure, and a fixing guide 22 for guiding the transfer paper separated from the transfer drum 11 by the separating claw 14 to the fixing roller 23. A discharging roller 24 is provided on a downstream side of the fuser unit 4 in a direction in which the transfer paper having the toner image fixed thereon is transported, so that the transfer paper is discharged from the main body onto an output tray 25. The structure of the transfer drum 11 will be explained while referring to FIG. 3. As shown in FIG. 3, the transfer drum 11 employs a cylindrical conductive layer 26 made of aluminum serving as a base material, and a semi-conductive layer 27 made of urethane foam is formed on the top surface of the conductive layer 26. Further, a dielectric layer 28 made of polyvinylidene fluoride or PET (polyethylene terephtalate) is formed on the top surface of the semi-conductive layer 27. In addition, the conductive layer 26 is connected to a power source unit 32 serving as voltage applying means, so that a voltage is applied constantly across the conductive layer 26. The above three layers are bonded to each other without using an adhesive agent or the like. For example, they are bonded to each other by a method shown in FIG. 4. To be more specific, a plurality of bosses 30a are formed on a sheet keeping plate 30, and a plurality of through holes 29 are made on the two opposing sides of a sheet made of the semi-conductive layer 27 and dielectric layer 28 so as to pierce through the sheet. Then, the bosses 30a are engaged with the through holes 29 first, and thence with an opening 26a formed on the top surface of the conductive layer 26. As a result, the semi-conductive layer 27 and dielectric layer 28 are fixed to the conductive layer 28. According to the above fixing method, the semi-conductive layer 27 and dielectric layer 28 apply a tension to the inner side of the conductive layer 26 through the sheet keeping plate 30, thereby preventing separation or slack of each layer. In addition, since each layer is fixed by the sheet keeping plate 30 alone, each layer can be replaced easily. Note that methods other than the above fixing method may be applicable. For example, as shown in FIG. 5, a sheet made of the semi-conductive layer 27 and dielectric layer 28 may be fixed to the conductive layer 26 by a sheet keeping member 31. The sheet keeping member 31 has a plurality of bosses 31a on the two opposing sides and a fixing member 31b for fixing the sheet at the center. According to this fixing method, the bosses 31a of the sheet keeping member 31 are engaged with a plurality of engaging holes 26b formed on the two opposing sides of an opening 26a of the conductive layer 26, so that the fixing member 31b of the sheet keeping member 31 is fitted into the opening 26a. Each layer can be also replaced easily when fixed by this method. As shown in FIG. 1, a charging layer 12a, made of a charging member for charging the transfer paper P in a certain polarity before the transfer paper P adheres to the transfer drum 11, is formed on the surface of the ground roller 12 provided below the transfer drum 11. According to this structure, the transfer paper P is charged by friction when the transfer paper P touches the charging layer 12a as the transfer paper P passes through a section between the ground roller 12 and transfer drum 11. Note that the transfer paper P is charged in a polarity reversed to that of a voltage applied to the transfer drum 11. The polarity of the transfer paper P can be changed by the materials forming the charging layer 12a. For example, if a positive voltage is applied to the transfer drum 11, then the charging layer 12a is made of a material such that negatively charges the transfer paper P. Whereas if a negative voltage is applied to the transfer drum 11, then the charging layer 12a is made of a material such that positively charges the transfer paper P. The charging properties of materials available for the charging layer 12a are set forth in TABLE 1 below. The charging properties referred herein are the properties representing the charges induced on each material by the friction between the paper and each material assuming that the initial amount of charges of the paper is nil.
TABLE 1 reveals that it is preferable to make the. charging layer 12a out of glass, nylon, etc. when negatively charging the transfer paper P, and it is preferable to make the charging roller 12a out of polytetrafluoroethylene when positively charging the transfer paper P. Since the transfer paper P is charged by the charging layer 12a in the instant at which the transfer paper P touches the transfer drum 11, the transfer paper P can be charged in a desired polarity regardless of the polarity of the initial charges of the transfer paper P. Thus, if the transfer paper P has the charges of the same polarity as that of the charges of the transfer drum 11 initially and will not adhere to the transfer drum 11 easily, the transfer paper P can be charged in a desired polarity by friction only by being brought into contact with the charging layer 12a, thereby enabling the transfer paper P to adhere to the transfer drum 11 in a stable manner. The ground roller 12 is pressed against the transfer drum 11 with the transfer paper P in between at the moment when the transfer paper P is transported to the section between the transfer drum 11 and ground roller 12. Subsequently, a voltage is applied to the transfer drum 11 to start the charging of the transfer paper P. The amount of thrust of the ground roller 12 into the transfer drum 11, or namely, the amount of crossover of the ground roller 12 and transfer drum 11, and the corresponding charging effect on the transfer paper P are set forth in TABLE 2 below. The amount of crossover referred herein is defined as a balance between a total of a radius of the peripheral circumference of the ground roller 12 and that of the peripheral circumference of the transfer drum 11 and a distance from the center of the one peripheral circumference to that of the other when these two peripheral circumferences are crossed. The charging effect on the transfer paper P referred herein indicates how readily the transfer paper P is charged.
TABLE 2 reveals that the charging effect on the transfer paper P can be realized when the ground roller 12 and transfer roller 11 are brought into contact with each other, and in particular, the charging effect is enhanced when the amount of crossover is in a range between 0.5 mm and 3.0 mm. Since the transfer drum 11 and ground roller 12 are brought into contact with each other when the amount of the crossover of the transfer drum 11 and ground roller 12 is in the above-specified range, not only the transfer paper P can be charged more efficiently, but also the ground roller 12 can be rotatably driven by the transfer drum 11, thereby enabling stable transportation of the transfer paper P. Further, the charging layer 12a of the ground roller 12 may have a slightly irregular surface to enhance the charging and transportation efficiency of the transfer paper P. The charging of the transfer paper P continues until the transfer paper P has made a full turn around the transfer drum 11. When the charging of the transfer paper P ends, the ground roller 12 is separated from the transfer drum 11. Otherwise, the ground roller 12 is brought into contact with the transfer paper P which has made a full turn while adhering to the transfer drum 11 by pressure again, and may touch the toner image attracted to the surface of the transfer paper P electrostatically. The charging effect on the transfer paper P corresponding to the amount of spacing between the transfer drum 11 and ground roller 12 after the transfer paper P has made a full turn is set forth in TABLE 3 below. The charging effect referred herein represents a condition of a toner image formed on the transfer paper P.
TABLE 3 reveals that it is necessary to have the amount of spacing of at least 0.5 mm, and more preferably, 1.0 mm or more, between the ground roller 12 and transfer drum 11 to obtain the charging effect on the transfer paper P. Thus, when the ground roller 12 and transfer drum 11 are spaced apart 1.0 mm or more, a toner image is formed satisfactorily on the transfer paper P, thereby producing a satisfactory image. In contrast, when the ground roller 12 and transfer drum 11 is spaced apart 0.5 mm or less, an unsatisfactory toner image is formed on the transfer paper P. Solenoids 12b (shown in FIG. 14) serving as electrode member driving means are provided on the two opposing sides of the center of rotation of the ground roller 12, so that the ground roller 12 moves mechanically to touch and separate from the transfer drum 11. This structure enables the ground roller 12 to have a constant nip width and a constant spacing amount. In the following, the paper attracting operation and transferring operation by the transfer drum 11 will be explained while referring to FIGS. 6, 7, and 46. Assume that a positive voltage is applied to the conductive layer 26 of the transfer drum 11 from the power source unit 32. First, a process of attracting the transfer paper P will be explained. As shown in FIG. 6, the transfer paper P transported to the transfer drum 11 is transported further while being pressed against the surface of the dielectric layer 28 by the ground roller 12. At this point, the transfer paper P is negatively charged by the friction between the charging layer 12a formed on the surface of the ground roller 12 and the transfer paper P. Also, charges accumulated on the semi-conductive layer 27 move to the dielectric layer 28, thereby inducting the positive charges on the surface of the dielectric layer 28. The dielectric layer 28 is charged by the conductive ground roller 12 mainly through Paschen's discharge and a charge injection. More specifically, when the positive charges are induced on the surface of the dielectric layer 28 as has been explained, an electric field develops from the transfer drum 11 side to the ground roller 12 side as shown in FIG. 46. Here, the surface of the transfer drum 11 is charged uniformly as the ground roller 12 and transfer drum 11 rotate. In the meantime, an atmospheric dielectric breakdown occurs when the electric field strength on a close contacting portion between the dielectric layer 28 and ground roller 12 known as the nip increases as the ground roller 12 approaches to the dielectric layer 28 of the transfer drum 11. Accordingly, a discharge, or namely, Paschen's discharge, is triggered from the transfer drum 11 side to the ground roller 12 side in a domain (I). Further, when the discharge ends, the charge injection from the ground roller 12 side to the transfer drum 11 side occurs in the nip between the ground roller 12 and transfer drum 11 indicated as a domain (II), and the negative charges are accumulated on the surface of the transfer drum 11. In short, the negative charges are accumulated on the transfer paper P on the inner side making contact with the dielectric layer 28 by Paschen's discharge and the following charge injection. As a result, the transfer paper P adheres to the transfer drum 11 electrostatically. Since the adhesion force of the transfer paper P does not vary if a voltage is supplied constantly, the transfer drum 11 can attract the transfer paper P in a stable manner. As has been explained, since the transfer paper P is not charged through the atmospheric discharge but by contact electrification, a voltage applied to the conductive layer 26 can be lowered. Various experiments show that it is adequate to apply a voltage of +3 kV or less, and more preferably, a voltage of +2 kV, to charge the transfer paper P and transfer a toner image onto the transfer paper P satisfactorily. The transfer paper P attracted to the transfer drum 11 is transported as far as a toner-image transfer point X as the transfer drum 11 rotates in the direction indicated by an arrow with its outer surface being positively charged. Next, a process of toner-image transfer onto the transfer paper P will be explained. As shown in FIG. 7, the photosensitive drum 15 attracts the negatively charged toner on the surface thereof. Thus, when the transfer paper P whose surface is positively charged is transported to the transfer point X, the toner is attracted to the surface of the transfer paper P due to the potential difference between the positive charges on the surface of the transfer paper P and the negative charges of the toner, thereby transferring a toner image onto the surface of the transfer paper P. The ground roller 12 separates from the transfer drum 11 to keep the above-specified amount of spacing when a first toner image has been transferred onto the transfer paper P as the transfer drum 11 makes a full turn. Note that the transfer drum 11 and photosensitive drum 15 are pressed against each other in such a manner that they have a certain nip width at the transfer point X. This means that this nip width affects the transfer efficiency, or namely, the image quality. The nip width referred herein is a width of a close contacting portion between the transfer drum 11 and the photosensitive drum 15 in a circumferential direction. The relation between the nip width and image quality is set forth in TABLE 4 below.
TABLE 4 reveals that it is preferable to have the nip width of 2 mm to 7 mm, and more preferably, 3 mm to 6 mm, to produce an image satisfactorily on the transfer paper P. The semi-conductive layer 27 has a volume resistivity of 10.sup.8 .OMEGA..multidot.cm, a thickness of 2 mm to 5 mm, and a hardness of 25 to 50 in ASKER C, because the transfer drum 11 and photosensitive drum 15 are pressed against each other under a pressure of 2 to 8 kg in the present embodiment. Note that it is preferable that the transfer drum 11 and photosensitive drum 15 are pressed against each other under a pressure of 6 kg. ASKER C indicates the hardness of a sample which is measured by a hardness measuring device produced in accordance with the standard of Japanese Rubber Association. Specifically, the hardness measuring device indicates the hardness of a sample by pressing a ball-point needle designed for hardness measurement against a surface of the sample using a force of a spring and measuring the depth of indentation produced by the needle when the resistive force of the sample and the force of spring balance. With the standard of ASKER C, when the depth of the indentation produced by the needle with the application of load of 55 g on the spring becomes equal to the maximum displacement of the needle, the hardness of the sample is indicated as zero degree. Also, when the depth of indentation produced by the application of load of 855 g is zero, the hardness of the sample is indicated as 100 degree. The relation among ASKER C, the quality of post-transfer toner image, and the adhesion of the transfer paper P is set forth in Table 5.
TABLE 5 reveals that a satisfactory image can be produced and the transfer paper P can adhere to the transfer drum 11 satisfactorily when the hardness is in a range between 25 and 50 in ASKER C. In other words, since the pressing pressure between the transfer drum 11 and photosensitive drum 15 varies depending on the material of the semi-conductive layer 27, the thickness, hardness, etc. of the semi-conductive layer 27 are adjusted for each material to obtained a desired image quality. Thus, using the semi-conductive layer 27 having the above-specified thickness and hardness limits the nip width between the transfer drum 11 and photosensitive drum 15 within the above-specified range. If the semi-conductive layer 27 has no volume resistivity (0 .OMEGA..multidot.cm), the voltage drops before the transfer paper P reaches the transfer point X due to the ground roller 12 placed where the adhesion of the transfer paper starts. To eliminate such a drop in voltage, the semi-conductive layer 27 must have a certain volume resistivity so as to play a role of a capacitor (condenser). The relation between the volume resistivity and image quality is set forth in TABLE 6 below.
TABLE 6 reveals that a toner image is transferred onto the transfer paper P efficiently without causing re-transfer or defects when the volume resistivity of the semi-conductive layer 27 is in a range between 10.sup.5 .OMEGA..multidot.cm and 10.sup.8 .OMEGA..multidot.cm, and in particular, the toner image is transferred onto the transfer paper P more efficiently when the volume resistivity of the semi-conductive layer 27 is in a range between 10.sup.6 .OMEGA..multidot.cm and 10.sup.7 .OMEGA..multidot.cm. Since the semi-conductive layer 27 of the present embodiment has the volume resistivity of 10.sup.8 .OMEGA..multidot.cm, the toner image can be transferred onto the transfer paper P satisfactorily, and hence a good-quality image can be produced. In general, the dielectric layer 28 must have a high dielectric constant and a charge maintaining force. This is the reason why the dielectric layer 28 is made of polyvinylidene fluoride, and the dielectric constant thereof is set in a range between 8 and 12. Thus, a charge capacity c of the dielectric layer 28 is found by an equation: c=.epsilon..multidot.s/l , where c is a charge capacity, .epsilon. is a dielectric constant, s is an area, and l is a thickness of the dielectric layer 28. It is understood from the above equation that the smaller the dielectric constant .epsilon., the smaller the charge capacity c and the better the transfer efficiency. However, since the charge capacity c is small, the adhesion force becomes weaker. It is also understood from the above equation that the thinner the dielectric layer 28 becomes, the larger the capacity c and the worse the transfer efficiency. However, since the capacity c is large, the adhesion force becomes stronger. Therefore, the dielectric constant .epsilon. and the thickness l of the dielectric layer 28 must be set appropriately. That is to say, adequate adhesive force and transfer efficiency can be obtained with the transfer paper P when the dielectric layer 28 has the dielectric constant in a range between 8 and 12 and the thickness of 100 .mu.m to 300 .mu.m. As shown in FIG. 8, the dielectric layer 28 of the transfer drum 11 is wider than a photosensitive body tube (aluminum tube 15a) forming the photosensitive drum 15. The photosensitive body element is wider than an effective transfer width, and the effective transfer width is wider than an effective image width (OPC applied width). As shown in FIG. 9, if the transfer drum 11 is assembled in such a manner that the following relation is established among the above-mentioned three layers in terms of widths: conductive layer 26>semi-conductive layer 27>dielectric layer 28, then the semi-conductive layer 27 may touch the grounded aluminum tube 15a of the photosensitive drum 15. To be more specific, when a positive voltage is applied to the conductive layer 26 from the power sour unit 32, the positive charges are induced on the conductive layer 26, and the induced positive charges move to the surface of the semi-conductive layer 27. If the grounded aluminum tube 15a of the photosensitive drum 15 touches the semi-conductive layer 27 under these conditions, all of the charges on the semi-conductive layer 27 move to the aluminum tube 15a, thereby making it impossible to induce the positive charges on the surface of the dielectric layer 28. As a result, the transfer drum 11 can not attract the negatively charged toner adhering to the OPC film 15b, and thus causes defective transfer. Thus, the conductive layer 26 and dielectric layer 28 are made into the same width, and the semi-conductive layer 27 is made narrower than the other two layers as shown in FIG. 10, so that the semi-conductive layer 27 will not touch the grounded aluminum tube 15a to prevent leakage of the charges. As a result, the transfer drum 11 can attract the negative charges adhering to the OPC film 15b, thereby eliminating defects in a transferred toner image. The transfer drum 11 is of a diameter such that prevents an overlap of the transfer paper P when it is wound around the transfer drum 11. To be more specific, the transfer drum 11 is designed to have a diameter corresponding to the width or length of a transfer paper of a maximum size used in the image forming apparatus of the present embodiment. Accordingly, the transfer paper P is wound around the transfer drum 11 in a stable manner, which enhances the transfer efficiency and the image quality as a result. A process of image formation by the above-structured image forming apparatus will be explained while referring to FIGS. 2, 6, and 7. As shown in FIG. 2, in case of the automatic paper feeding, the pick up roller 7 steadily sends the transfer papers P per sheet from the top of the pile in the paper feeding cassette 5 provided in the lowest stage of the main body to the PF roller 8. The transfer paper P having passed through the PF roller 8 is curled by the PS roller 10 substantially in the same shape as the transfer drum 11. Whereas in the case of the manual paper feeding, the transfer papers P are sent to the manual paper feeding roller 9 from the manual paper feeding unit 6 provided on the front surface of the main body per sheet, and transported further to the PS roller 10 by the manual paper feeding roller 9. Subsequently, the transfer paper P is curled by the PS roller 10 substantially in the same shape as the transfer drum 11. Next, as shown in FIG. 6, the transfer paper P curled by the PS roller 10 is transported to the section between the transfer drum 11 and ground roller 12. Accordingly, the charges accumulated on the semi-conductive layer 27 of the transfer drum 11 induce the charges on the surface of the transfer paper P through the surface of the semi-conductive layer 27 and the inner surface of the transfer paper P, thereby allowing the transfer paper P to adhere to the surface of the transfer drum 11 electrostatically. Subsequently, as shown in FIG. 7, the transfer paper P thus attracted to the transfer drum 11 is transported further to the transfer point X where the transfer drum 11 and photosensitive drum 15 are brought into contact with each other by pressure. Then, a toner image is transferred onto the transfer paper P due to the potential difference between the charges of the toner on the photosensitive drum 15 and the charges on the surface of the transfer paper P. Here, a series of charging, exposure, development, and transfer operations is performed for each color with the photosensitive drum 15. Thus, an image of one color has been transferred onto the transfer paper P when the transfer paper P makes a full turn while adhering to the transfer drum 11, and the transfer paper P rotates up to four times to make a full-color image. Note that the transfer drum 11 rotates only once when making a black-and-white or monochrome image. When the toner images of all colors are transferred onto the transfer paper P, the transfer paper P is forcefully separated from the surface of the transfer drum 11 by the separating claw 14 provided in the circumference of the transfer drum 11 so as to move to touch and separate from the transfer drum 11, and the transfer paper P is further guided to the fixing guide 22. Subsequently, the transfer paper P is guided to the fixing roller 23 by the fixing guide 22, and the toner image on the transfer paper P is fused into place at a certain temperature and under a certain pressure. The transfer paper P with the image thus fixed thereon is discharged onto the output tray 25 by the discharging roller 24. As has been explained, the transfer drum 11 comprises the conductive layer 26 made of aluminum, semi-conductive layer 27 made of urethane foam, and dielectric layer 28 made of polyvinylidene fluoride or PET (polyethylene terephtalate), which are placed from inward to outward in this order. According to this structure, the charges are induced in the above order when a voltage is applied to the conductive layer 26 and the charges are accumulated on the semi-conductive layer 27. When the transfer paper P is transported to the section between the transfer drum 11 and ground roller 12 under these condition, the accumulated charges on the semi-conductive layer 27 move to the transfer paper P, thereby allowing the transfer paper P to adhere to the transfer drum 11 electrostatically. As has been explained, the transfer paper adhesion and toner-image transfer of the present embodiment are performed not by the charge injection through a conventional atmospheric discharge, but the charge induction. Thus, the method of the present embodiment demands a relatively low voltage and makes it easy to control the voltage. In addition, this method prevents the voltage from varying due to an external pressure. Accordingly, a constant voltage can be applied to the transfer drum 11 independently of the environments including humidity and temperature, thereby making it possible to enhance the transfer efficiency and image quality. Unlike the conventional method where the surface of the transfer drum 11 is charged through the atmospheric discharge, the method of the present embodiment makes it possible to charge the surface of the transfer drum 11 reliably, thereby enabling the adhesion of the transfer paper P and toner-image transfer in a stable manner. Moreover, the charges are induced on the semi-conductive layer 27 and dielectric layer 28 in this order to charge the surface of the transfer drum 11 only by applying a voltage to the conductive layer 26. Thus, unlike the conventional method where the surface of the transfer drum 11 is charged through the atmospheric discharge, the method of the present embodiment demands a low voltage, which makes it easy to control the voltage and saves the driving energy. In addition, unlike the conventional method where the voltage is applied to each charger, the voltage is applied to only one point. Thus, the method of the present embodiment not only simplifies the structure of the image forming apparatus, but also saves the manufacturing costs. Since the transfer drum 11 is charged through contact electrification, the electric field domain does not vary if there is a flaw on the surface of the transfer drum 11. Thus, the electric field does not become out of balance over the flaw on the surface of the transfer drum 11. This prevents defects in a transferred toner image such as a white spot (void), thereby enhancing the transfer efficiency. Further, unlike the atmospheric discharge, the affects resulted from the environments such as the temperature and humidity of air are almost negligible to the method of the present embodiment. Therefore, the surface potential of the transfer drum 11 does not vary, which makes it possible to prevent insufficient adhesion of the transfer paper P and disordered printing. This also enhances the transfer efficiency and image quality. Since the transfer paper P is charged in a polarity reversed to that of the transfer drum 11, the initial charges on the transfer paper P are removed. Accordingly, the adhesion degree of the transfer paper P to the transfer drum 11 is enhanced, which enables the transfer drum 11 to steadily attract the transfer papers P when a number of copies are made, thereby making it possible to produce a good-quality image on each copy. Note that the conductive layer 26 of the present embodiment is cylindrical aluminum; however, the other conductors may be used as well. Likewise, although the semi-conductive layer 27 of the present embodiment is made of urethane foam, other semi-conductors such as elastic bodies including silicon may be used, and although the dielectric layer 28 of the present embodiment is made of polyvinylidene fluoride, however, other dielectric bodies such as resins including PET (polyethylene terephtalate) may be used. As shown in FIG. 3, the transfer drum 11 of the present embodiment is of a three-layer structure made of the conductive layer 26, semi-conductive layer 27, and dielectric layer 28. However, the transfer drum 11 is not limited to the above structure; the transfer drum 11 may be of any structure as long as the conductive layer 26 and dielectric layer 28 are used as the inner most layer and outer most layer, respectively. For example, the transfer drum 11 may be replaced with a transfer drum 36 shown in FIG. 11, which comprises the conductive layer 26 serving as the inner most layer and the dielectric layer 28 serving as the outer most layer. A voltage is applied to the conductive layer 26 from the power source unit 32 in this case also. Besides the transfer drum 36, a transfer drum 37 shown in FIG. 12 may be used, which comprises the conductive layer 26 serving as the inner most layer and the dielectric layer 28 serving as the outer most layer. The conductive layer 26 of the transfer drum 37 is connected to the power source unit 32 through a resistor 33 whose resistance value is the same as that of the semi-conductive layer 27 of the transfer drum 11. A voltage is applied to the conductive layer 26 from the power source unit 32 in this case also. Further, other than the above alternatives, a transfer drum 38 shown in FIG. 13 may be used. The transfer drum 38 comprises the conductive layer 26 serving as the inner most layer, and a two-layer film made of a semi-conductive film 34 (placed inner side of the transfer drum 38) having substantially the same dielectric constant and resistance value as those of the semi-conductive layer 27 of the transfer drum 11 and a dielectric film 35 (placed outer side of the transfer drum 38) having substantially the same dielectric constant and resistance value as those of the dielectric layer 28 of the transfer drum 11; the conductive layer 26 and semi-conductive film 34 are layered from inward to outward in this order. A voltage is applied to the conductive layer 26 from the power source unit 32 in this case also. Note that the transfer drums 36, 37, and 38 respectively shown in FIGS. 11 through 13 are also applicable to each of the following embodiments. Also, note that each member used in the present embodiment is driven under the control of a control device 148 shown in FIG. 14, and each member used in the following embodiments is also driven under the control of the control device 148 unless specified otherwise. In the following, the second through fourteenth embodiments of the present invention will be explained. The major structure of an image forming apparatus in each of the following embodiments is identical with that of the counterpart in the first embodiment, and only the difference will be explained. In the following embodiments, like numerals are labeled with like numeral references with respect to the first embodiment and the description of these components is not repeated for the explanation's convenience. [SECOND EMBODIMENT] Another embodiment of the present invention will be explained in the following while referring to FIG. 15(a). Compared with the counterpart in the first embodiment, an image forming apparatus of the present embodiment includes a roller type brush 101 shown in FIG. 15(a) instead of the ground roller 12. The roller type brush 101 is substantially as wide as the transfer drum 11, so that the roller type brush 101 presses the transfer paper P against the transfer drum 11 when the transfer paper P passes through a section between the transfer drum 11 and roller type brush 101. The roller type brush 101 is driven by the same driving mechanism as that of the ground roller 12 of the first embodiment. Also, the roller type brush 101 is grounded through a grounding conductor 101a. A charging member 102 is provided on an upstream side of the roller type brush 101 in a direction in which the transfer paper P is transported. The charging member 102 charges the transfer paper P in a certain polarity, or namely, a polarity reversed to that of the transfer drum 11. The charging member 102 comprises a plate member as long as the width of the transfer drum 11 so as to charge the transfer paper P in the above-mentioned polarity by the friction between the transfer paper P and plate member. The charging member 102 is also grounded through the grounding conductor 101a of the roller type brush 101. Further, the charging member 102 is made of any of the materials set forth in TABLE 1 in the first embodiment. For example, in a case where a positive voltage is applied to the transfer drum 11, a charging member 102 made of a material which negatively charges the transfer paper P is adopted. Whereas in a case where a negative voltage is applied to the transfer drum 11, a charging member 102 made of a material which positively charges the transfer paper P is adopted. Note that the charging member 102 can be of any shape as long as it charges the transfer paper P in a desired polarity. Since the transfer paper P is forcefully charged in a polarity reversed to that of the transfer drum 11 before the transfer paper P adheres to the transfer drum 11, unwanted charges on the transfer paper P, or namely, the charges of the same polarity as that of the transfer drum 11, can be =removed. As a result, the adhesion of the transfer paper P to the transfer drum 11 can be upgraded. The relation between the length of the charging member 102 in a direction in which the transfer paper P is transported when the charging member 102 is a plate member and the charging effect is set forth in TABLE 7 below.
TABLE 7 reveals that it is possible to charge the transfer paper P when the charging member 102 is at least 10 mm long in the direction in which the transfer paper P is transported, and in particular, the charging effect is improved when the charging member 102 is not less than 50 mm long. The transfer paper P is charged when a voltage is applied to the transfer drum 11 in the instant at which the transfer paper P having passed by the charging member 102 reaches a point where the roller type brush 101 is brought into contact with the transfer drum 11. The amount of thrust of the brush portion of the roller type brush 101 into the transfer drum 11 at this point, or namely, the amount of the crossover of the roller type brush 101 and transfer drum 11, and the corresponding charging effect on the transfer paper P are set forth in TABLE 8 below. The amount of crossover referred herein is defined as a balance between a total of a radius of the peripheral circumference of the roller type brush 101 and that of the peripheral circumference of the transfer drum 11 and a distance from the center of the one peripheral circumference to that of the other when these two peripheral circumferences are crossed. The charging effect on the transfer paper P referred herein indicates how readily the transfer paper P is charged.
TABLE 8 reveals that the charging effect on the transfer paper P can be obtained when the roller type brush 101 and the transfer drum 11 are brought into contact with each other, and in particular, the charging effect is improved when the amount of the crossover is in a range between 0.5 mm and 3.0 mm. Since the transfer drum 11 and roller type brush 101 are brought into contact with each other when the amount of the crossover of the transfer drum 11 and roller type brush 101 is in the above-specified range, not only the transfer paper P can be charged more efficiently, but also the roller type brush 101 can be rotatably driven by the transfer drum 11, thereby enabling stable transportation of the transfer paper P. The charging effect on the transfer paper P corresponding to the amount of the spacing between the transfer drum 11 and roller type brush 101 when the transfer paper P has made a full turn is set forth in TABLE 9 below. The charging effect on the transfer paper P referred herein represents a condition of a toner image formed on the transfer paper P.
TABLE 9 reveals that it is necessary to have the amount of spacing of at least 0.5 mm, and more preferably 1.0 mm or more, between the roller type brush 101 and transfer drum 11 to obtain the charging effect on'the transfer paper P. Accordingly, when the roller type brush 101 and transfer drum 11 are spaced apart 1.0 mm or more, the toner image is formed satisfactorily on the transfer paper P, thereby upgrading the quality of a resulting image. In contrast, if the roller type brush 101 and transfer drum 11 are spaced apart 0.5 mm or less, an unsatisfactory toner image is formed on the transfer paper P. The relation between the resistance of the brush portion of the roller type brush 101 and the charging effect on the transfer paper P is set forth in TABLE 10 below. Also, the relation between a brush density of the roller type brush 101 and the charging effect on the transfer paper P is set forth in TABLE 11 below.
TABLE 10 reveals that the charging effect on the transfer paper P can be realized when the value of the brush resistance is 60 k.OMEGA. or less, and in particular, the charging effect is enhanced when the value of the brush resistance is 36 k.OMEGA. or less. Also, TABLE 11 reveals that the charging effect on the transfer paper P can be realized when the brush density is 5000 pieces/cm.sup.2 or more, and in particular, the charging effect is enhanced when the brush density is 20000 pieces/cm.sup.2 or more. According to the above structure, the transfer paper P is charged in a polarity reversed to that of the transfer drum 11, and thus the charges on the pre-charge transfer paper P can be removed. Accordingly, a degree of adhesion (hereinafter referred to as adhesion degree) of the transfer paper P to the transfer drum 11 can be upgraded. As a result, a plurality of the transfer papers P can steadily adhere to the transfer drum 11 when a plurality of copies are made, thereby producing a high-quality image on each copy. [THIRD EMBODIMENT] A further embodiment of the present invention will be explained in the following while referring to FIG. 15(b). As shown in FIG. 15(b), an image forming apparatus of the present embodiment includes a comb-shaped brush 103 instead of the ground roller 12 of the first embodiment shown in FIG. 1. The come-shaped brush 103 is formed in such a manner that the brush surface thereof is substantially as wide as the transfer drum 11, so that the comb-shaped brush 103 presses the transfer paper P against the transfer drum 11 when the transfer paper P passes through a section between the transfer drum 11 and comb-shaped brush 103. The comb-shaped brush 103 is driven by the same driving mechanism as that of the ground roller 12 of the first embodiment. Also, the comb-shaped brush 103 is grounded through a grounding conductor 103a. A charging member 104 is provided on an upstream side of the comb-shaped brush 103 in a direction in which the transfer paper P is transported. The charging member 104 charges the transfer paper P in a certain polarity, or namely, a polarity reversed to that of the transfer drum 11. The charging member 104 comprises a plate member as long as the width of the transfer drum 11 so as to charge the transfer paper P in the above-mentioned polarity by the friction between the transfer paper P and plate member. The charging member 104 is also grounded through the grounding conductor 103a of the comb-shaped brush 103. Further, the charging member 104 is made of any of the materials set forth in TABLE 1 in the first embodiment. For example, in a case where a positive voltage is applied to the transfer drum 11, a charging member 104 made of a material which negatively charges the transfer paper P is adopted. In contrast, in a case where a negative voltage is applied to the transfer drum 11, a charging member 104 made of a material which positively charges the transfer paper P is adopted. Note that the charging member 104 can be of any shape as long as it charges the transfer paper P in a desired polarity. As has been explained, by charging the transfer paper P in a polarity reversed to that of the transfer drum 11 before it adheres to the transfer drum 11, unwanted charges on the transfer paper P, or namely, the charges of the same polarity as that of the transfer drum 11, can be removed, thereby upgrading the adhesion of the transfer paper P to the transfer drum 11. The relation between the length of the charging member 104 in a direction in which the transfer paper P is transported when the charging member 104 is made of a plate member and the charging effect on the transfer paper P is forth in TABLE 12.
TABLE 12 reveals that the transfer paper P can be charged when the charging member 104 is at least 10 mm long in the direction in which the transfer paper P is transported, and in particular, the charging effect is enhanced when the charging member 104 is not less than 50 mm long. The transfer paper P is charged when a voltage is applied to the transfer drum 11 at the same time when the transfer paper P having passed the charging member 104 reaches a point where the comb-shaped brush 103 is brought into contact with the transfer drum 11. The amount of thrust of the comb-shaped brush 103 into the transfer drum 11, or namely, the amount of crossover of the comb-shaped brush 103 and transfer drum 11, and the corresponding charging effect on the transfer paper P are set forth in TABLE 13 below. The amount of crossover referred herein is defined as the length of the comb-shaped brush 103 within the peripheral circumference of the transfer drum 11 when the comb-shaped brush 103 in a natural state and the peripheral circumference of the transfer drum 11 are crossed. The charging effect on the transfer paper P referred herein indicates how readily the transfer paper P is charged.
TABLE 13 reveals that the charging effect on the transfer paper P can be realized when the comb-shaped brush 103 and transfer drum 11 are brought into contact with each other, and in particular, the charging effect is enhanced when the amount of the crossover of the comb-shaped brush 103 and transfer drum 11 is in a range between 0.5 mm and 3.0 mm. Since the transfer drum 11 and comb-shaped brush 103 are brought into contact with each other when the amount of the crossover of the transfer drum 11 and comb-shaped brush 103 is in the above-specified range, not only the transfer paper P can be charged more efficiently, but also the comb-shaped brush 103 can move together with the transfer drum 11, thereby enabling stable transportation of the transfer paper P. The charging effect on the transfer paper P corresponding to the amount of the spacing between the transfer drum 11 and comb-shaped brush 103 when the transfer paper P has made a full turn is set forth in TABLE 14 below. The charging effect on the transfer paper P referred herein represents a condition of a toner image formed on the transfer paper P.
TABLE 14 reveals that it is necessary to have the amount of the spacing of at least 0.5 mm, and more preferably 1.0 mm or more, between the comb-shaped brush 103 and transfer drum 11 to obtain the charging effect on the transfer paper P. Accordingly, when the comb-shaped brush 103 and transfer drum 11 are spaced apart 1.0 mm or more, the toner image is formed satisfactorily on the transfer paper P, thereby producing a good-quality image. In contrast, if the comb-shaped brush 103 and transfer drum 11 are spaced apart 0.5 mm or less, a toner image is formed unsatisfactorily on the transfer paper P. The relation between the resistance of the brush portion of the comb-shaped brush 103 and the charging effect on the transfer paper P is set forth in TABLE 15 below. Also, the relation between a pitch (fur pitch) between the groups of bristles of the comb-shaped brush 103 and the charging effect on the transfer paper P is set forth in TABLE 16 below.
TABLE 15 reveals that the charging effect on the transfer paper P can be realized when the value of the brush resistance is 60 k.OMEGA. or less, and in particular, the charging effect is enhanced when the value of the brush resistance is 36 k.OMEGA. or less. Also, TABLE 16 reveals that the charging effect on the transfer paper P can be realized when the fur pitch is 3.0 mm or less, and in particular, the charging effect is enhanced when the fur pitch is 1.6 mm or less. According to the above structure, the transfer paper P is charged in a polarity reversed to that of the transfer drum 11, and |