Abstract:
A transfer roller is used in an electrophotographic printer. The transfer roller rotates in contact with a rotating photoconductive drum with a print medium travelling therebetween. The transfer roller has a metal shaft around which a rubber roller is formed. The rubber roller is sponge-like and electrically semiconductive. The rubber roller is covered with a resin tube which has a lower layer formed on the rubber roller and an upper layer formed on the lower layer. One of the upper and lower layers has a higher sealing effect than the other of the upper and lower layers, and the other of the upper and lower layers has a higher stability of electrical resistance than the one of the upper and lower layer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a transfer roller. 
     2. Description of the Related Art 
     With conventional electrophotographic printers, a charging device uniformly charges the surface of a photoconductive drum and then an LED head illuminates the charged surface to form an electrostatic latent image. The electrostatic latent is developed by a developing device into a toner image. Then, the toner image is transferred by a transfer device to a print medium. A fixing device subsequently fixes the toner image on the print medium. 
     FIG. 7 is a perspective view of a transfer device of a conventional electrophotographic printer. 
     A transfer roller  11  is in pressure contact with a photoconductive drum  12 . The transfer roller  11  rotates in a direction shown by arrow A and the photoconductive drum  12  rotates in a direction shown by arrow B. The transfer roller  11  receives a transfer voltage or positive high voltage +E. The photoconductive drum  12  carries a toner image  13  formed thereon. When a print medium  14  passes between the transfer roller  11  and the photoconductive drum  12 , a transfer current flows from the transfer roller  11  to the photoconductive drum  12 , thereby causing the toner image  13  to be deposited on the print medium  14 . 
     The transfer roller  11  has a uniform surface resistance, so that a uniform transfer current flows through any areas in contact with the photoconductive drum  12 . The transfer roller  11  is urged by a constant pressure F against the photoconductive drum  12 , thereby creating an appropriate longitudinal contact area between the transfer roller  11  and the photoconductive drum  12 . This ensures that the print medium  14  travels straightforwardly in a direction shown by arrow C as the transfer roller  11  and the photoconductive drum  12  rotate. 
     The transfer roller  11  includes a metal shaft with a rubber material deposited thereon. The rubber material is formed by kneading, for example, a semiconductive urethane rubber, silicone rubber, or the like. The rubber material is foamed, vulcanized into a sponge-like material by adding a vulcanizing agent, and finally formed into a specific shape and dimensions. An electrically conductive material is added to the rubber material, thereby making the rubber material electrically semiconductive. 
     The conventional transfer roller  11  suffers from the following problems. Some rubber materials react with the material of the photoconductive drum  12  when they are left in an environment of high temperature and high humidity for a long time, so that the rubber material welds with the photoconductive drum, or the vulcanizing agent, semiconductive material, or the like is deposited to the photoconductive drum. As a result, the photoconductive drum is contaminated, causing lines to appear in the printed image. This decreases the print quality. 
     In order to solve this problem, the length of time during which vulcanization is performed is extended so that less vulcanizing agent is added to the rubber material, or the surface of the transfer roller  11  is coated with an inert material. However, such treatments not only are insufficient in preventing drum contamination but also lose some degrees of freedom in designing the characteristics of transfer roller  11  such as electrical resistance and mechanical hardness. 
     SUMMARY OF THE INVENTION 
     The present invention was made to solve the aforementioned problems of the conventional transfer rollers. 
     An object of the invention is to provide a transfer roller which prevents drum contamination so as to maintain the quality of printed images. 
     Another object of the invention is to provide a transfer roller, which still maintains the degrees of freedom in designing the characteristics of the transfer roller. 
     A transfer roller is used in an electrophotographic printer. The transfer roller rotates in contact with a rotating photoconductive drum with a print medium travelling therebetween. The transfer roller has a metal shaft around which a rubber roller is formed. The rubber roller is sponge-like and electrically semiconductive. The rubber roller is covered with a resin tube which has a lower layer formed on the rubber roller and an upper layer formed on the lower layer. One of the upper and lower layers has a higher sealing effect than the other of the upper and lower layers, and the other of the upper and lower layers has a higher stability of electrical resistance than the one of the upper and lower layer. 
     An organic ion electrically conductive material is added to the upper layer for stable electrical resistance while carbon black is added to the lower layer for increased sealing effect. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is a fragmentary perspective view of a transfer roller according to a first embodiment; 
     FIG. 2 is a longitudinal cross-sectional view taken along lines III—III of FIG. 1; 
     FIGS. 3 and 4 show characteristics of a resin tube  27  of FIG. 1; 
     FIG. 5 is a longitudinal cross-sectional view of a transfer roller according to a second embodiment; 
     FIG. 6 is an enlarged view of the transfer roller of FIG. 5; and 
     FIG. 7 is a perspective view of a transfer device of a conventional electrophotographic printer. 
    
    
     DESCRIPTION OF THE INVENTION 
     Preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
     First Embodiment 
     FIG. 1 is a fragmentary perspective view of a transfer roller according to a first embodiment. 
     FIG. 2 is a longitudinal cross-sectional view taken along lines II—II of FIG.  1 . 
     FIGS. 3 and 4 show characteristics of a resin tube  27  of FIG.  1 . 
     FIG. 3 plots the voltage applied to a transfer roller  21  as abscissa and the resistance of the resin tube  27  as ordinate. 
     FIG. 4 plots the time for which the transfer voltage is applied to the transfer roller  21  as abscissa and the resistance of the resin tube  27  as ordinate. 
     In FIGS. 3 and 4, the solid lines indicate the characteristics of the transfer roller  21  when carbon black is added as an electrically conductive material and the dotted lines represent the characteristics of the transfer roller  21  when an organic ion electrically conductive material is added. 
     Referring to FIGS. 1 and 2, the transfer roller  21  includes a metal shaft  26 , a sponge-like electrically semiconductive rubber roller  25  formed around the shaft  26 , and an electrically semiconductive resin tube  27  formed on the rubber roller  25 . The resin tube  27  is of a dual-layer structure having a lower layer  27   a  formed directly on the rubber roller  25  and an upper layer  27   b  formed on the lower layer  27   a . The upper and lower layers  27   b  and  27   a  are formed in an integral construction by simultaneously extruding the resin materials through the extruding die of an extruding machine, not shown. The resin tube  27  is formed in the shape of a tube and fitted over the rubber roller  25 . Then, the resin tube  27  is subjected to heat treatment for shrinkage. A layer of an adhesive, not shown, is applied to the surface of the rubber roller  25 . 
     The lower layer  27   a  is made of a matrix of fluororesin. A material such as carbon black is added as an electrically conductive material to the matrix. The upper layer  27   b  is made of a matrix of fluororesin. An organic ion electrically conductive material such as materials containing, for example, lithium therein is added to the matrix. 
     Since rubber is not used as a matrix, the matrix will not react with the material of the photoconductive drum  12  to weld together even if the transfer roller  21  is left for a long time in an environment of high temperature and high humidity. 
     Fluororesin as a matrix is an excellent binder for the vulcanizing agent and electrically conductive material. Thus, there is no possibility of the vulcanizing agent, electrically conductive material, and the like seeping out of the resin tube  27  and being deposited on the photoconductive drum  12 . Thus, the photoconductive drum  12  is not contaminated. 
     As shown in FIG. 3, if carbon black is added to the matrix, the electrical resistance of the resin tube  27  decreases with increasing applied voltage. If the organic ion electrically conductive material is added, the electrical resistance of the resin tube  27  will not change significantly when the voltage applied changes. 
     As shown in FIG. 4, adding carbon black causes the electrical resistance of the resin tube  27  to increase with increasing accumulated time of application of high voltage. Adding an organic ion electrically conductive causes little or no changes in resistance with increasing accumulated time of application of high voltage. Adding carbon black minimizes changes in the resistance of the resin tube  27  resulting from changes in environmental conditions such as temperature and humidity change. Adding an organic ion electrically conductive material causes changes in the electrical resistance of the resin tube  27  as the environmental conditions change. 
     Adding carbon black increases the density of the resin, more effectively sealing low-molecular materials such as vulcanizing agent added to the rubber roller  25 . Adding organic ion electrically conductive material cannot increase the density of the resin and is therefore less effective in sealing low-molecular materials such vulcanizing agents added to the rubber roller  25 . 
     Adding only carbon black to the resin tube  27  increases the sealing effect and prevents the low-molecular materials such as vulcanizing materials added to the rubber roller  25  from passing through the resin tube  27 . Therefore, drum contamination is sufficiently suppressed. However, when only carbon black is added to the resin tube  27 , if the accumulated time of application of high voltage (transfer voltage) increases, the resistance of the resin tube  27  increases with increasing the accumulated time of application of high voltage. This shortens the lifetime of the transfer roller  21 . 
     When an organic ion electrically conductive material alone is added to the resin tube  27 , the resistance of the resin tube  27  is stabilized, not changing significantly even if the accumulated time of application of high voltage. Accordingly, the overall resistance of the transfer roller  21  will not change so that the lifetime of the transfer roller  21  is prolonged. However, adding only an organic ion electrically conductive material to the resin tube  27  provides a lower sealing effect and cannot prevent the low-molecular material added to the rubber roller  25  from passing through the resin tube  27 . This causes drum contamination. Therefore, the resin tube  27  needs to be thick, impairing the characteristics such as flexibility of the rubber roller  25 . 
     In the present embodiment, carbon black that carries electrons is added to the lower layer  27   a  to increase the sealing effect and prevent low-molecular material, such as vulcanizing agent added to the rubber roller  25 , from penetrating through the resin tube  27 . Further, an organic ion electrically conductive material is added to the upper layer  27   b  for increased stability of the electrical resistance of the resin tube  27 , thereby prolonging the life of the transfer roller  21 . 
     In the present embodiment, the materials for the upper and lower layers  27   b  and  27   a  are prepared such that the resin tube  27  has, for example, uniform electrical resistance and uniform thickness. The electrical resistance, thickness, and other characteristics of the resin tube  27  can be selected arbitrarily. Thus, these characteristics can be determined independently of each other. Although the lower layer  27   a  is integrally continuous with the upper layer  27   b , the upper and lower layers  27   b  and  27   a  may be formed independently and placed one over the other when actually forming them on the rubber roller  25 . 
     As mentioned above, upper and lower layers  27   b  and  27   a  that have different properties are formed on the rubber roller  25 . The layers  27   a  and  27   b  increases the sealing effect, so that the low-molecular material such as vulcanizing agent added to the rubber roller  25  will not penetrate through the resin tube  27 . Thus, drum contamination can be sufficiently prevented and print quality is improved. 
     The organic ion electrically conductive material added to the upper layer  27   b  maintains the same overall electrical resistance of the transfer roller  21  under different printing conditions. Thus, the life of the transfer roller  21  is prolonged. 
     The fluororesin added to the upper and lower layers  27   b  and  27   a  protects the surface of the rubber roller  25 . Thus, the transfer roller  21  withstands repeated use without being deteriorated. 
     The materials used for the upper and lower layers may be interchanged. In other words, the organic ion electrically conductive material can be added to the lower layer  27   a  to increase stability of electrical resistance and the carbon black maybe added to the upper layer  27   b  to increase sealing effect. 
     Second Embodiment 
     Elements of the same construction as those of the first embodiment have been given the same reference numerals. 
     FIG. 5 is a longitudinal cross-sectional view of a transfer roller according to a second embodiment. 
     FIG. 6 is an enlarged fragmentary view of the transfer roller of FIG.  5 . 
     Referring to FIGS. 5 and 6, a semiconductive resin tube  37  is formed on the rubber roller  25 . The resin tube  37  is of a dual-layer structure having a lower layer  37   a  formed directly on the rubber roller  25  and an upper layer  37   b  formed on the lower layer  37   a . The resin tube  37  is slightly longer than the rubber roller  25 . When the resin tube  37  formed on the rubber roller  25  is subjected to heat shrinkage, the resin tube  37  extending beyond the longitudinal ends of the rubber roller  25  shrinks to wrap around the longitudinal end corners of the rubber roller  25 , leaving a round portion m (FIG. 6) around the longitudinal end corners of the rubber roller  25 . 
     When the transfer roller  31  receives a transfer voltage, a portion (leak current) of the transfer current flows preferentially through the longitudinal ends of the transfer roller  31  where discharge is apt to occur between the photoconductive drum  12  and the transfer roller  31 . As a result, less transfer current flows through the print medium  14  from the transfer roller  31  to the photoconductive drum  12 . Thus, the ratio of the leak current (current not passing through the print medium but passing through the longitudinal ends) to the effective transfer current (current passing through the print medium) increases, decreasing the transfer efficiency at a given transfer voltage. Increasing the transfer voltage in an attempt to increase the transfer current will increase the chance of discharge occurring at the longitudinal ends of the transfer roller  31 . For this reason, the transfer efficiency cannot be improved. 
     In order to prevent discharge from occurring, the transfer roller  31  may be chamfered at the longitudinal ends thereof. However, additional machining time is required, increasing the cost of the transfer roller  31 . 
     In the second embodiment, the round portion m is formed to surround the longitudinal ends of the transfer roller  31 , preventing discharge between the transfer roller and the photoconductive drum. The round portion m also prevents a portion of transfer current from flowing preferentially through the longitudinal ends of the transfer roller  31 . As a result, a larger portion of the transfer current flows through the print medium between the transfer roller  31  and the photoconductive drum  12 , increasing transfer efficiency. Forming the resin tube  37  on the rubber roller  25  and forming the round portion m can be carried out simultaneously without increasing overall machining time. 
     The diameter of the longitudinal ends of the resin tube  37  that extends beyond the longitudinal ends of the rubber roller  25  is smaller than that of the rubber roller  25 . The difference in diameter prevents the resin tube  37  from sliding relative to the rubber roller  25  in a longitudinal direction when a force is exerted on the transfer roller  31 . Less mechanical stress is encountered in printers with relatively low printing speed and printers that can accept only an A4 size print medium. For these printers, there is no need for applying an adhesive between the rubber roller  25  and the resin tube  27 , thus further reducing the manufacturing cost of the transfer roller  31 . 
     While the embodiments have been described with respect to the transfer rollers  21  and  31 , the present invention can be applied to other rollers such as a charging roller and a developing roller, which are required to have similar properties. In the aforementioned embodiments, the resin tubes  27  and  37  are made of fluororesin. Alternatively, the resin tubes  27  may be formed of nylon (trade name) resin. 
     Example 
     The rubber roller  25  is in the form of an electrically semiconductive sponge and the resin tube  27  has a resistance of about 10 8  Ω. The thickness of the resin tube  27  is about 100 μm. The lower layer  27   a  has a thickness of about 20 μm and the upper layer  27   b  has a thickness of about 80 μm. The resistance of the lower layer  27   a  is about ten times as high as that of the upper layer  27   b . The resin tube  27  has substantially the same total resistance as the rubber roller  25 . Thus, the total resistance of the transfer roller  21  is substantially the same before and after the resin tuber  27  is formed over the rubber roller  25 . 
     If only carbon black as an electrically conductive material is added to the resin tube  27 , the electrical resistance increases with increasing the accumulated time of application of high voltage, causing changes in the overall resistance of the transfer roller. Therefore, it is necessary to make the resistance of the resin tube  27  as stable as possible. The resistance changes widely if the volume resistivity ranges from 10 7  to 10 10  Ω·cm. Thus, the volume resistivity of the lower layer  27   a  is selected outside of the aforementioned range from 10 7  to 10 10  Ω·cm. 
     When the volume resistivity of the lower layer  27   a  is above the range of 10 7 -10 10  Ω·cm, the volume resistivity of the upper layer  27   b  is decreased to a value below the range of 10 7 -10 10  Ω·cm and the ratio of the thickness of the lower layer  27   a  to that of the upper layer  27   b  is selected such that the overall electrical resistance of the resin tube  27  is substantially the same as that of the rubber roller  25 . 
     When the volume resistivity of the lower layer  27   a  is below the range of 10 7 -10 10  Ω·cm, the volume resistivity of the upper layer  27   b  is increased to a value above the range of 10 7 -10 10  Ω·cm and the ratio of the thickness of the lower layer  27   a  to that of the upper layer  27   b  is selected such that the overall electrical resistance of the resin tube  27  is substantially the same as that of the rubber roller  25 . 
     Before the aforementioned resin is fitted over the rubber roller  25 , the rubber roller  25  has been machined to desired dimensions and an adhesive has been applied to the surface of the rubber roller  25 . After the resin tube has been fitted over the rubber roller  25 , the resin tube  27  is heated to shrink-fit the rubber roller  25  and firmly bonded to the rubber roller  25 . Thus, the shrinkage of the resin tube  27  and the adhesive prevent the resin tube  27  from sliding relative to the rubber roller  25  in the axial direction when a force is exerted on the transfer roller  21  during its rotation. 
     The upper and lower layers  27   b  and  27   a  of the resin tube  27  are formed of a fluororesin which is not oxidized even if discharge occurs repeatedly between the resin tube  27  and the photoconductive drum  12 . 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art intended to be included within the scope of the following claims.