A semiconductive roller has an electrically conductive shaft, a roller provided on the shaft formed of silicone rubber with carbon contained therein. The silicone rubber is also added olefin oil and zinc oxide. A method is used for manufacturing a semiconductive roller formed of an electrically conductive shaft. The method includes carrying out a first vulcanization where the semiconductive roller is shaped into a roller and carrying out a second vulcanization where the semiconductive roller is heated at 200.degree. C. for substantially four hours.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a semiconductive roller and a method of
 manufacturing the semiconductive roller.
 2. Description of the Related Art
 With a conventional electrophotographic printer, a charging roller charges
 the surface of the photoconductive drum and an exposing unit such as an
 LED head writes an electrostatic latent image on the charged surface of
 the photoconductive drum. The electrostatic latent image is then developed
 with toner into a visible image, i.e., toner image. The toner image is
 subsequently transferred to a print medium when the print medium passes in
 a sandwiched relation between the photoconductive drum and a transfer
 roller. The transfer roller receives a high voltage of about several
 hundred to several thousand volts and of an opposite polarity to the
 toner. An electric field developed between the photoconductive drum and
 the transfer roller causes the toner image to be attracted to the print
 medium. Thus, the toner image is transferred to the print medium.
 Some of the toner fails to be transferred from the photoconductive drum to
 the print medium. Such residual toner is distributed evenly on the surface
 of the photoconductive drum by a cleaning unit. A developing unit
 subsequently collects the evenly distributed residual toner.
 A toner cartridge supplies fresh, unused toner into a toner container,
 which in turn supplies the toner to the developing roller via an agitating
 bar and a semiconductive sponge roller. When the sponge roller transports
 the toner, the toner is charged. The sponge roller is in the form of a
 conductive silicone rubber that contains silicone polymer and carbon as a
 conductive agent. The conductive silicone rubber is foamed to have a large
 number of holes referred to as cells having diameters in the range from
 0.3 to 0.5 mm. The sponge roller serves to ensure required print density
 and prevent variations in print density over time. The sponge roller is
 incorporated in a print process unit, which is a mechanical section of an
 electrophotographic printer.
 The problem with the aforementioned conventional sponge roller is that as
 the cumulative number of printed pages increases, the carbon contained in
 the material tends to clump, increasing conductivity. An increased
 conductivity decreases the electrical resistance of the sponge roller. As
 a result, a larger current flows through the sponge roller. This results
 in a steep increase in toner potential so that more toner than necessary
 is supplied to the developing roller, especially to longitudinal ends of
 the developing roller which have relatively lower electrical resistance.
 Thus, excessive toner falls in the electrophotographic printer so that the
 print medium opposing the longitudinal ends of the developing roller
 becomes black. The toner deposited on the developing roller forms a "ring"
 that surrounds the developing roller, and is referred to as "toner ring"
 in this specification.
 SUMMARY OF THE INVENTION
 An object of the invention is to provide a semiconductive roller in which
 carbon contained in the roller does not clump.
 Another object of the invention is to provide a semiconductive roller in
 which when an external force is applied to the roller, the electrical
 resistance of the roller changes less as compared to a conventional roller
 having olefin oil added thereto.
 A still another object of the invention is to provide a semiconductive
 roller having small manufacturing variation of the electrical resistance
 of the roller.
 A semiconductive roller includes an electrically conductive shaft, a roller
 provided on the shaft formed of silicone rubber with carbon contained
 therein. The silicone rubber preferably contains 100 parts of silicone
 rubber, 0.4 parts of olefin oil, and 30 parts of zinc oxide by weight.
 A method is used for manufacturing a semiconductive roller formed of an
 electrically conductive shaft. The method includes carrying out a first
 vulcanization where the semiconductive roller is shaped into a roller, and
 carrying out a second vulcanization where the semiconductive roller is
 heated at 200.degree. C. for substantially four hours.
 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.

DESCRIPTION OF THE INVENTION
 Preferred embodiments of the present invention will be described in detail
 with reference to the accompanying drawings. Elements of the same
 construction have been given the same reference numerals throughout the
 embodiments and the description thereof is omitted.
 First Embodiment
 &lt;Construction&gt;
 FIG. 1 illustrates a structure of a print process unit according to a first
 embodiment.
 The structure of the print process unit 1 incorporated in an
 electrophotographic printer will be described.
 Referring to FIG. 1, the print process unit 1 includes a photoconductive
 drum 2 that has an electrical conductive base such as aluminum covered
 with a photoconductive layer. The photoconductive layer holds a toner
 image thereon. The photoconductive drum 2 is driven to rotate in a
 direction shown by arrow A at a predetermined process speed. Provided
 around the photoconductive drum are a charging roller 3, an LED head 4,
 developing roller 9, a transfer roller 11, and a cleaning roller 12. The
 charging roller 3 includes a conductive shaft 13 and a roller body 14. The
 conductive shaft 13 receives a negative voltage so that the surface of the
 photoconductive drum 2 is charged via the roller body 14. The LED head 4
 illuminates the charged surface of the photoconductive drum 2 to form an
 electrostatic latent image on the photoconductive drum 2. Toner is
 supplied to a toner container 6 in which the toner is agitated by an
 agitating bar, not shown. A sponge roller 7 supplies the toner stored in
 the toner container 6 to the developing roller 9, which in turn supplies
 the negatively charged toner to the electrostatic latent image on the
 photoconductive drum 2. The transfer roller 11 is in pressure contact with
 the surface of the photoconductive drum 2 and transfers the toner image on
 the photoconductive drum 2 to print paper 10 as a print medium that is
 transported by feed rollers, not shown. The cleaning roller 12 receives a
 positive voltage of 450 V so that the cleaning roller 12 attracts residual
 toner 8a having a polarity opposite to the positive voltage of 450 V to
 clean the surface of the photoconductive drum 2. The residual toner
 attracted to the cleaning roller 12 is again transferred to the
 photoconductive drum 2 during the cleaning operation and collected by the
 developing roller 9.
 A fixing unit, not shown, is provided downstream of the print process unit
 1 in the transport path of the print paper 10, and fixes the toner 8
 transferred to the print paper 10. The fixing unit incorporates a heat
 roller.
 The developing roller 9 and sponge roller 7 form a developing unit. The
 sponge roller 7 includes a metal shaft 15 that receives a voltage and a
 roller 16 that is fixed to the shaft 15 and surrounds the shaft 15. The
 roller 16 is formed of a silicone rubber having olefin oil and zinc oxide
 (ZnO) added thereto. Olefin oil is used to prevent carbon from clumping
 and zinc oxide (ZnO) is used to prevent variation in the internal
 resistance of the roller 16. The roller 16 is foamed to have a large
 number of holes called "cells" having diameters in the range from 0.3
 (fine) to 0.5 (coarse) mm.
 The sponge roller 7 has small variations of resistance and can prevent the
 clumping of the carbon, thus preventing the increase in toner potential
 due to decreased resistance resulting from changes of characteristics of
 the sponge roller over time. Also, an appropriate amount of toner can be
 supplied to the developing roller 9.
 &lt;Test Results&gt;
 The thus manufactured sponge roller 7 is assembled into the print process
 unit 1.
 The following parameters were investigated:
 (1) The fluidity of toner
 (2) Potential of the toner 8 on the developing roller 9
 (3) The weight of toner 8 on the developing roller 9
 (4) The current flowing through the sponge roller 7
 (5) Unwanted deposition of toner on the photoconductive drum
 (6) Electrical resistance of the sponge roller 7
 All of the parameters have values taken immediately after the sponge roller
 9 have been assembled and again after 7200 pages have been printed (i.e.,
 when the toner 8 is nearing exhaustion.)
 Experiments were conducted to evaluate the sponge rollers (i.e., Roller A
 to Roller D) according to the present invention. The experiments revealed
 that the best proportions of silicone, olefin oil, and zinc oxide (ZnO)
 are 100 parts, 0.4 parts, and 30 parts, respectively, by weight. The
 amount of carbon to be added is determined by the desired electrical
 resistance of the sponge roller. In the present embodiment, the carbon in
 the range from 2 to 2.5 parts is added.
 Table I lists the rollers tested.
 TABLE I
 Silicon Zinc Second
 rubber oxide, Olefin vulcani-
 base ZnO oil zation Carbon
 Roller (parts) (parts) (parts) (hour) (parts)
 Roller A1-A4 100 30 0.4 4 2-2.5
 Roller B1 100 30 0.4 2 2-2.5
 Roller C1 100 30 0.4 7 2-2.5
 Roller D1-D2 100 50 0.4 4 2-2.5
 FIG. 2 illustrates experimental results of the first embodiment. Data are
 shown for the first printed pages and for pages after 7200 pages have been
 printed.
 The major data listed in FIG. 2 are as follows:
 (1) Fluidity of toner 8 in percent after printing 7200 pages
 (2) Potential of toner 8 on the developing roller 9
 (3) Average weight of toner 8 in milligrams per square centimeters
 (mg/cm.sup.2) deposited on the developing roller 9
 (4) Toner-clinging in percent
 (5) Current in micro amperes flowing through the sponge roller
 (6) Resistance in ohms of the sponge roller 7
 "Average toner voltage" is a voltage of charged toner remaining on the
 developing roller when the printer is momentarily turned off in the middle
 of printing a black solid image.
 When a printing is actually, an image printed on a page of printing medium
 tends to have high density for the first one rotation of the sponge roller
 and low density for the second rotations onward. This is due to the
 difference in toner voltage. Thus, FIG. 2 shows both average toner
 voltages for the first one rotation of the sponge roller 7 and for the
 second rotation onward. For example, for Roller A1, the average toner
 voltage is 64.2 V for the first rotation of the sponge roller 7 and 43.4 V
 for the second rotation onward.
 "Average toner weight" is the weight of toner per unit area (mg/cm.sup.2)
 remaining on the developing roller 9 when the printer is turned off
 momentarily.
 In this specification, the term "toner-clinging" is used to cover the
 following phenomenon. An insufficiently small difference in potential
 between the surface of the photoconductive drum 2 and the surface of the
 developing roller 9 causes the developer toner to cling to the background
 of the latent image formed on the photoconductive drum 2, leading to
 soiling of the surface of the photoconductive drum 2.
 The resistance of the sponge roller 7 and variation of the resistance are
 shown for each roller. For example, for Roller A1, the resistance is
 2.33E+09 .OMEGA. (=2.33.times.10.sup.9 .OMEGA.) and the variation of the
 resistance is 3.62. The resistances are measured a plurality of times
 (e.g. 100 times) at each of a plurality of locations on the surface of the
 sponge roller, for example, 6 locations, during one complete rotation of
 the sponge roller. Then, the maximum and minimum values of resistances at
 each location are determined and then an average value of the maximum and
 minimum values is calculated as the resistance of the sponge roller.
 "Variation" is a maximum value divided by a minimum value.
 "Rate of change" is a ratio of the resistance of the sponge roller 7 for
 pages after 7200 pages have been printed to the resistance of the sponge
 roller 7 for first pages. For Roller A1, the rate of change is
 2.78E+09/2.33E+09=1.19.
 Referring to FIG. 2, Rollers A1-A4, B1, C1, and D1 have smaller differences
 in resistance between the initial pages and pages after 7200 pages have
 been printed, as compared to the conventional sponge roller (i.e.,
 Conventional Roller E). For example, the rate of change of the resistance
 of the sponge roller is 1.19 (2.78E+09/2.33E+09=1.19) for Roller A1, and
 0.15 for the Conventional Roller. Therefore, changes in current flowing
 through Rollers A1-A4, B1, C1, and D1 are small after a large number of
 pages have been printed. As a result, the potential of the toner is
 relatively low and stable, so that a sufficient amount of toner 8 is
 supplied to the developing roller 9. For example, the current flowing
 through Roller A1 are 2.9 .mu.A for first pages and 3.7 .mu.A for pages
 after 7200 pages have been printed, respectively. The aforementioned
 Rollers A1-A4, B1, C1, and D1 were manufactured and assembled into the
 print process unit 1 to test for toner ring. Little or no toner ring
 occurred.
 The printing of 7200 pages is a general measure that an electrophotographic
 printer can print before the toner 8 in the toner cartridge 5 is
 exhausted. The electrophotographic printer is assumed to be of a type in
 which the toner cartridge 5 is replaced when the toner 8 is exhausted. If
 the electrophotographic printer can still print normally after having
 printed 7200 pages, then changes in various characteristics will be
 smaller after printing 7200 pages, so that there will be no chance of
 toner ring occurring for the rest of the lifetime of the printer.
 Adding olefin oil to the sponge roller prevents the clumping of the carbon
 contained in the silicone rubber. Changes in resistance when external
 stresses are exerted to the roller are small as compared to the rollers
 that are not added olefin oil. Adding zinc oxide (preferably 30 parts)
 minimizes variation of electrical resistance.
 This reduces the difference in the resistance of the sponge roller 7
 between when the first few pages are printed and when 7200 pages have been
 printed. Therefore, the current that flows through the sponge roller 7
 does not change significantly over time, so that the potential of the
 toner 8 will not rise steeply and the amount of toner supplied to the
 developing roller 9 remains substantially constant over time. The stable
 supply of toner prevents toner ring that results from excessive toner
 supplied to the developing roller 9, thereby preventing rapid exhaustion
 of toner in the toner cartridge.
 Second Embodiment
 FIG. 3 illustrates experimental results of a second embodiment.
 The base material of the sponge roller 16 according to the second
 embodiment is also semiconductive silicone rubber formed of silicone
 polymer and carbon. The base material also contains olefin oil and zinc
 oxide. Thus, the composition of the sponge roller 7 of the second
 embodiment is the same as the first embodiment. The second embodiment is
 characterized in the method of manufacturing the sponge roller 7.
 Conventionally, the sponge roller is subjected to a first vulcanization and
 subsequently to a second vulcanization. During the first vulcanization,
 the silicone rubber is shaped into a roller body 16 formed around a metal
 shaft. During the second vulcanization, the roller body 16 is given the
 properties of rubber, and the oligomer is removed therefrom. The second
 vulcanization is performed for seven hours at a temperature of 225.degree.
 C.
 Olefin oil added to the roller body 16 causes the roller body 16 to deform
 when the roller body 16 is heated to 225.degree. C. Thus, the second
 embodiment differs from the first embodiment in the conditions of the
 second vulcanization.
 A base material containing olefin oil and zinc oxide was manufactured. This
 composition is the same as in the first embodiment. Then, three types of
 sponge rollers, i.e., Rollers A5-A9, Rollers B2-B3, and Rollers C2-C3 were
 prepared and tested at three different conditions of the second
 vulcanization; heat treatments at 200.degree. C. for about two hours
 (Rollers A5-A9), 200.degree. C. for about fourhours (Rollers B2-B3), and
 200.degree. C. for sevenhours (Rollers C2-C3). In order to prevent the
 olefin oil from denaturing, the second vulcanization is preferably carried
 out at a temperature below 200.degree. C. Three different sponge rollers 7
 (Rollers A5-A9, B2-B3, and C2-C3) were made using the three different
 rollers 16. The three different sponge rollers 7 were assembled into
 corresponding print process units 1.
 Then, the following properties were investigated:
 (1) The fluidity of toner
 (2) The potential of the toner 8 on the developing roller 9
 (3) The weight of toner 8 on the developing roller 9
 (4) Current flowing through the sponge roller 7
 (5) Toner clinging and
 (6) The electrical resistance of the sponge roller 7
 All of these data values were measured immediately after the sponge roller
 7 has been assembled into the printer and after 7200 pages have been
 printed, i.e., when the toner 8 has almost exhausted.
 FIG. 3 shows that Roller B2 (vulcanized at 200.degree. C., 4 hour) provides
 the best fluidity of toner and the least deterioration of toner after 7200
 pages have been printed. Rollers A5-A9 (vulcanized at 200.degree. C., 2
 hour) retain too much oil that impairs the fluidity of the toner. Rollers
 C2-C3 (vulcanized at 200.degree. C., 7 hour) provide poor fluidity of
 toner because the olefin oil has dried up and silica contained in the
 toner is stuck to the silica contained in the roller body 16. Thus, it is
 concluded that the sponge roller 7 formed of a roller body containing
 olefin oil and zinc oxide should preferably be subjected to the second
 vulcanization at 200.degree. C. for four hours.
 In addition to the advantages of the first embodiment, Rollers B2-B3 of the
 second embodiment prevent the fluidity of toner from decreasing when the
 toner 8 in the toner container 6 is nearing exhaustion. Thus, the second
 embodiment reduces the chance of blurred print results occurring.
 While the first and second embodiments have been described with respect to
 a sponge roller 7 made of silicone rubber with olefin oil and zinc oxide
 added thereto, the invention may be applicable to other sponge-like
 rollers.
 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.