Electrophotographic recording apparatus having a charging roller with a surface layer comprising a resin and a hardener

An electrophotographic recording apparatus having a charging roller with a surface layer having a mixture of a resin and a hardener. The surface layer includes at least 10%, but less than 30%, by weight of the surface layer, of the hardener. This specific surface layer composition prevents the photosensitive drum, which is in contact with the charging roller, from becoming excessively charged. Furthermore, the amount of toner that adheres to the charging roller is reduced by circumferentially rotating the charging roller at a rate faster than the photosensitive drum.

BACKGROUND OF THE INVENTION
 The present invention relates to an electrophotographic recording apparatus
 including a charging roller for charging the photosensitive drum to a
 desired potential, and more particularly to an electrophotographic
 recording apparatus in which the circumferential speed of the charging
 roller to the circumferential speed of the photosensitive drum is greater
 than 1.
 The conventional electrophotographic recording apparatus, such as a,
 printer or a duplicator, generally includes a photosensitive drum having a
 rotation shaft, and a charging roller having a rotation shaft
 substantially parallel with the rotation shaft of the photosensitive drum
 and also holding its own external circumferential surface in contact with
 the external circumferential surface of the photosensitive drum to thereby
 give a static charge to the external circumferential surface of the
 photosensitive drum.
 The above-mentioned electrophotographic recording apparatus further
 includes a developing section for supplying a toner to the statically
 charged external circumferential surface of the photosensitive drum, and a
 cleaning section for removing the toner remaining on the photosensitive
 drum which toner was not transferred to a recording paper. Among the
 techniques for cleaning the photosensitive drum by the cleaning section,
 there is a recycle system in which the toner recovered from the
 photosensitive drum by the cleaning section is returned to the
 photosensitive drum by the cleaning section, and the toner is carried by
 the rotating photosensitive drum and collected in the developing section.
 In the recycle-system electrophotographic recording apparatus, the greater
 part of the normally charged toner is removed by the cleaning section
 oppositely charged with respect to the polarity of the charge on the
 toner. A small amount of the toner which is opposite in polarity to the
 greater part of the toner remains on the photosensitive drum and passes
 through the cleaning section.
 In the recycle-system electrophotographic recording apparatus, when the
 small amount of the toner that has passed through the cleaning section
 enters between the charging roller and the photosensitive drum, the small
 amount of the toner adheres to the external circumferential surface of the
 charging roller, thus producing a so-called filming phenomenon. When the
 filming phenomenon occurs, the resistance value of the charging roller
 increases due to the toner adhesion, making it difficult for the charging
 roller to give a sufficient amount of charge to the photosensitive drum to
 enable stable printing. To prevent the photosensitive drum from being
 insufficiently charged, the charging roller is rotated faster than the
 photosensitive drum to thereby reduce chances that the small amount of the
 toner enters between the charging roller and the photosensitive drum.
 In the recycle-system electrophotographic recording apparatus, however,
 because the charging roller is rotated faster than the photosensitive
 drum, by triboelectricity produced between the charging roller and the
 photosensitive drum, the photosensitive drum is charged to a potential so
 high as to lead to unstable printing during operation for a long period of
 time. This results in poor quality printing, which has been a problem.
 SUMMARY OF THE INVENTION
 Therefore, the object of the present invention is to provide an
 electrophotographic recording apparatus that can suppress changes in the
 electrical potential of the photosensitive drum to thereby improve
 printing quality.
 To solve the above problem, the present invention has an essential feature
 as follows.
 According to the present invention, an electrophotographic recording
 apparatus comprises:
 a photosensitive drum having a rotation shaft; and
 a charging roller for charging the photosensitive drum to a desired
 potential, the charging roller having a rotation shaft substantially
 parallel with the rotation shaft of the photosensitive drum and holding an
 external circumferential surface thereof in contact with the
 photosensitive drum, the ratio of the circumferential speed of the
 charging roller to the circumferential speed of the photosensitive drum
 being greater than 1, wherein the external circumferential surface of the
 charging roller is defined by a surface layer including a resin and a
 hardener, and wherein the hardener is included in the surface layer at a
 proportion of not less than 10 percent and not greater than 30 percent by
 weight, of the surface layer.
 In the electrophotographic recording apparatus according to the present
 invention, because the surface layer of the charging roller contains 10
 percent or more by weight of the above-mentioned hardener, the frictional
 force produced between the surface layer of the charging roller and the
 photosensitive drum is reduced sharply, and triboelectricity induced
 between the charging roller and the photosensitive drum is inhibited.
 Because the hardener included in the surface layer of the charging roller
 is 30 percent or less by weight, this limited content of the hardener
 inhibits an increase in the amount of the toner adhering to the surface
 la, which would attend on an increasing surface roughness of the surface
 layer due to a large amount of the hardener used. Accordingly, an increase
 in the resistance value of the charging roller due to toner adhesion can
 be inhibited, thus reducing the insufficiency of the static charge on the
 photosensitive drum, which would be caused by an increase in the
 resistance value of the charging roller to inhibit.
 In short, in the electrophotographic recording apparatus according to the
 present invention, it is possible to restrain an excessive static charge
 on the photosensitive drum by triboelectricity generated between the
 charging roller and the photosensitive drum. On the other hand, it is also
 possible to inhibit the insufficiency of the static charge on the
 photosensitive drum due to an increase in the resistance value of the
 charging roller attending on this adhesion of the toner to the surface
 layer. Consequently, the potential of the photosensitive drum is held in a
 desired adequate range, in other words, the photosensitive drum can be
 held at a stable potential.
 To effectively inhibit the adhesion of the toner to the surface layer, the
 surface roughness of the surface layer is preferably more than 0 .mu.m and
 not greater than 15 .mu.m. Under this condition, the resistance value of
 the charging roller can be inhibited from increasing as the result of
 printing for a long period of time, thus reducing the insufficiency of
 charge on the photosensitive drum.
 The charging roller may be fitted with a generally cylindrical member
 disposed coaxially with the rotation shaft of the charging roller with its
 outer surface connected with the surface layer and its inner surface
 connected with the rotation shaft.
 With a charging roller having a higher weight percentage of the hardener
 included in its surface layer, the fusion bonding force between the
 surface layer and the photosensitive drum is lower, and there are less
 chances of the surface layer peeling away from the cylindrical member.
 If the surface layer peels off the cylindrical member, a large amount of
 the toner adheres to the outer surface of the cylindrical member devoid of
 the surface layer, so that the resistance value of the charging roller
 increases so much.
 According to the present invention, because the surface layer is less
 liable to separate from the cylindrical member, chances are more
 effectively reduced that the photosensitive drum is charged insufficiently
 due to an increase in the resistance value of the charging roller.
 To further stabilize the potential of the photosensitive drum, the ratio of
 the circumferential speed of the charging roller to the circumferential
 speed of the photosensitive drum is preferably 1.4 or less.
 The charging roller is arranged slidably in the direction of the rotation
 shaft and on a flat plate. The inclination angle at which the charging
 roller starts to slide when the flat plate is gradually inclined to the
 horizontal plane is in a specified range. The inclination angle when a
 sheet for OverHead Project panel is placed on the flat plate and the
 charging roller starts to slide on the sheet is preferably greater than
 0.degree. and less than or equal to 20.degree..

DETAILED DESCRIPTION OF THE INVENTION
 The present invention will be described by referring to a printer as an
 example of the electrophotographic recording apparatus.
 FIG. 1 is a schematic drawing of a printer according to the present
 invention.
 As shown in FIG. 1, a printer 1 comprises a photosensitive drum 2 capable
 of rotating in the direction of arrow A; a charging section 7 for giving a
 substantially uniform electrostatic charge over the external
 circumferential surface of the photosensitive drum 2; an exposure unit 3
 for forming an electrostatic latent image on the photosensitive drum 2 by
 exposing the charged photosensitive drum 2; a developing section 4 for
 causing a toner 4a to adhere to the electrostatic latent image formed on
 the photosensitive drum 2; an image transfer section 5 for transferring
 the toner 4a on the electrostatic latent image to a recording medium; and
 a cleaning section 6 for removing the toner remaining on the
 photosensitive drum 2 without being transferred to the recording medium.
 The recycle system is adopted for the printer 1. According to thus recycle
 system, as is generally known, the remaining toner recovered from the
 photosensitive drum 2 by the cleaning section 6 is returned to the
 photosensitive drum 2 by the cleaning section 6 when printing is not
 performed, for example, and the toner is carried by the rotating
 photosensitive drum 2 and collected in the developing section 4.
 FIG. 2 shows the photosensitive drum 2 and the charging section 7 of the
 printer 1 shown in FIG. 1.
 The charging section 7 includes a charging roller 70 having a rotation
 shaft 70a, which is made of a conductive material and substantially
 parallel with the rotation shaft of the photosensitive drum 2; springs 7b
 for contacting under pressure the external circumferential surface of the
 charging roller 70 with the external circumferential surface of the
 photosensitive drum 2; and a power source 7e for applying a DC voltage
 through the rotation shaft 70a to the charging roller 70.
 When the power source 7e of the charging section 7 applies a negative
 voltage, for example, to the charging roller 70, as is conventionally well
 known, the charging roller 70 causes a negative charge to be induced on
 the external circumferential surface of the photosensitive drum 2, which
 is in contact with the charging roller 70.
 The exposure unit 3 exposes the charged surface of the photosensitive drum
 2 according to an image signal. The potential of the exposed portions
 become nearly zero.
 The developing section 4, as is conventionally well known, includes a
 developing roller 4b whose external circumferential surface is in contact
 with the external circumferential surface of the photosensitive drum 2;
 and an auxiliary roller 4c which is arranged in contact with the external
 circumferential surface of the developing roller 4b in order to cause the
 toner 4a to adhere to the external circumferential surface of the
 developing roller 4b.
 In the developing section 4, as is conventionally well known, by applying a
 negative voltage, for example, to the developing roller 4b, the toner 4a
 adhering to the external circumferential surface of the developing roller
 4b is negatively charged. The negatively charged toner 4a adheres to the
 exposed portions of external circumferential surface of the photosensitive
 drum 2.
 The image transfer section 5 includes a transfer roller 5a in contact with
 the external circumferential surface of the photosensitive drum 2. The
 image transfer section 5, as is conventionally well known, if a positive
 voltage, for example, is applied to the transfer roller 5a, the negatively
 charged toner 4a on the photosensitive drum 2 is moved to a recording
 medium that is sent between the photosensitive drum 2 and the transfer
 roller 5a. After the transfer process by movement of the toner 4a at the
 image transfer section 5, as is conventionally well known, the negatively
 charged toner that did not participate in the transfer process and remains
 as it is and the positively charged small amount of the toner both keep on
 adhering to the external circumferential surface of the photosensitive
 drum 2. The positively charged small amount of the toner is likely to
 occur between the developing section 4 and the image transfer section 5 as
 is generally known.
 The cleaning section 6 includes a cleaning roller 6a whose external
 circumferential surface is in contact with the external circumferential
 surface of the photosensitive drum 2. In the cleaning section 6, as has
 been well known, a positive voltage, for example, is applied to the
 cleaning roller 6a. Therefore, because the cleaning roller is positively
 charged, the negatively charged toner on the photosensitive drum 2 is
 attracted to the cleaning roller 6a.
 On the other hand, the positively charged small amount of the toner on the
 photosensitive drum 2 is not attracted to the cleaning roller 6a, and
 passes through the cleaning section 6 as the photosensitive drum 2
 rotates.
 When the positively charged small amount of the toner that has passed the
 cleaning section 6 enters between the charging roller 70 and the
 photosensitive drum 2, the positively charged small amount of the toner
 adheres to the external circumferential surface of the charging roller 70,
 and the increased resistance value of the charging roller 70 caused by the
 adhesion of the small quantity of the toner makes it impossible to hold
 the photosensitive drum 2 at a potential enough for stable printing.
 As a countermeasure, in the photosensitive drum 2, as has been well known,
 the charging roller 70 is rotated faster than the photosensitive drum 2,
 with the result that the ratio of the circumferential speed of the
 charging roller 70 to the circumferential speed of the photosensitive drum
 2 is greater than 1. Because of this circumferential speed ratio exceeding
 1, the above-mentioned small amount of toner is inhibited from entering
 between the charging roller 70 and the photosensitive drum 2. In the
 example shown in FIG. 1, the ratio of the circumferential speed of the
 charging roller 70 to the circumferential speed of the photosensitive drum
 2 is set to be 1.4.
 FIG. 3 is a sectional view of the charging roller 70 taken along the line
 III--III shown in FIG. 2.
 The charging roller 70 includes a cylindrical member 70b enclosing the
 rotation shaft 70a and arranged coaxially with the rotation shaft 70a. The
 cylindrical member 70b is made of a semiconductive polyether-urethane
 rubber with a hardness of 35 (JIS A), for example. Formed around the
 external circumference of the cylindrical member 70b is a surface layer
 70c containing a polymer of a polyether-urethane resin and a polyamide
 resin, for example. This polymer shows a relatively high affinity for the
 above-mentioned polyether-urethane rubber of the cylindrical member 70b.
 In addition, as the hardener for hardening the surface of the surface
 layer 70c to an optional hardness, a melamine resin, for example, is
 included in the surface layer 70c.
 FIG. 4 is a graph showing the relation between the percentage by weight of
 the hardener included in the surface layer of the charging roller and the
 magnitude of the frictional force between the charging roller and the
 photosensitive drum.
 The abscissa of the graph indicates the percentage (%) by weight of the
 hardener, and the ordinate indicates the magnitude of the frictional force
 (N).
 In the graph, the characteristic curve B shows changes in the frictional
 force when the weight percentage of the hardener is varied. As indicated
 by the characteristic curve B, the frictional force produced between the
 surface layer 70c of the charging roller 70 and the external
 circumferential surface of the photosensitive drum 2 during printing on
 the printer 1 decreases as the weight percentage of the hardener included
 in the surface layer is increased. As is apparent from the characteristic
 curve B, the frictional force is sufficiently low over the weight
 percentage of the hardener exceeds 10 percent.
 With the charging roller 70 according to the present invention, as
 mentioned above, because the surface layer 70c contains 10 percent or more
 by weight of the hardener, the frictional force between the surface layer
 70c and the external circumferential surface of the photosensitive drum 2
 is kept to be low, so that triboelectricity generated between the charging
 roller 70c and the photosensitive drum 2 is inhibited by a great measure.
 FIG. 5 is a graph showing the relation between the percentage by weight of
 the hardener included in the surface layer 70c of the charging roller 70
 and the values of the surface roughness of the surface layer 70c.
 The abscissa of the graph indicates the percentage (%) by weight of the
 hardener, and the ordinate indicates the values (.mu.m) of the
 above-mentioned surface roughness.
 The characteristic curve C in FIG. 5 indicates the changes in the values of
 the surface roughness of the surface layer 70c while the weight percentage
 of the hardener is varied. As the weight percentage of the hardener
 included in the surface layer 70c increases, the surface hardening of the
 surface layer 70c progresses more rapidly during the forming process of
 the surface layer 70c. Accordingly, the surface of the surface layer 70c
 is formed roughly as the weight percentage of the hardener is increased as
 shown by the characteristic curve C that rises generally steadily.
 The surface roughness of the surface layer 70c is obtained by the
 arithmetic expression of ten-point-average roughness (JIS B0601) in the
 example shown in FIG. 6. Cross sectional curve 71 shows the magnified
 surface fluctuation of the charging roller 70 along the axis thereof.
 In the arithmetic expression of ten-point-average roughness, as shown in
 FIG. 6, within a cross section 71 as wide as the base length L and
 arbitrarily selected from the surface layer 70c, an optional straight line
 Lb is drawn which is parallel with the direction of the base length L and
 which does not intersect the surface 71a. The heights of the peaks and
 valleys of the surface 71a are measured from the straight line Lb as the
 reference line. This measurement is made without cutting or grinding the
 surface 71a (without cut-off) under the conditions of the base length
 L=2.5 mm, the tracer tip radius=2 .mu.m, the tracer pressure=70 mN and the
 tracer measuring speed of 0.1 mm/s.
 The surface roughness of a given cross section is obtained as follows. An
 average value of the five heights from the top to the fifth highest (R1,
 R3, R5, R7 and R9) is obtained. Then, an average value of the five heights
 from the shortest to the fifth shortest (R2, R4, R6, R8 and R10) is
 obtained. A difference in units of micrometer (.mu.m) between the former
 average value and the latter average value represents the surface
 roughness of a given cross section, obtained by the above-mentioned
 arithmetic expression of ten-point average roughness. This
 ten-point-average roughness is obtained at three different locations of
 the surface layer 70c, and an average of those three ten-point averages is
 adopted to indicate the surface roughness of the surface layer 70c of each
 weight percentage of the hardener in FIG. 5.
 The surface roughness of the surface layer 70c of the charging roller 70 is
 greater for a larger quantity of the hardener included in the surface
 layer 70c as has been described with reference to FIG. 5. As the surface
 roughness of the surface layer 70c increases, a larger amount of toner is
 likely to adhere to the surface layer 70c, so that the resistance value of
 the charging roller 70 tends to become greater as the result of printing
 for a long period of time. Before proceeding to detailed description of
 the relation between the surface roughness of the surface layer 70c and
 the resistance value of the charging roller 70, the method of measuring
 the resistance of the charging roller 70 is explained in the following.
 FIG. 7 is a diagram for explaining the method for measuring the resistance
 value of the charging roller 70.
 Contacts each having a bearing 20, arranged equally spaced in the axial
 direction of the charging roller 70 and rotatably contacting the charging
 roller 70 are pressed against the surface layer 70c of the charging roller
 70. While the charging roller 70 is rotated, a voltage of 500V is applied
 across the rotation shaft 70a and the respective contacts. Under this
 condition, the resistance values between the rotation shaft 70a and the
 respective contacts are measured with a resistance meter 21. An average of
 the measured values is used as the resistance value of the charging roller
 70.
 FIG. 8 is a graph showing the relation between the surface roughness and
 the resistance values of charging rollers, measured after 20,000 sheets
 were printed using the charging rollers with different levels of the
 surface roughness of the surface layer at a printing speed of 12 pages per
 minute.
 The abscissa of the graph indicates levels of the surface roughness in
 .mu.m of the surface layer 70c, and the ordinate indicates the resistance
 values in ohm of the charging roller 70, measured after printing was
 finished. The resistance value of the charging roller 70 before printing
 is 1.times.10.sup.8 [.OMEGA.].
 In FIG. 8, the characteristic curve D shows changes in the resistance value
 of the charging roller measured after printing was done with charging
 rollers with different levels of surface roughness. The resistance value
 of the charging roller 70 measured after 20,000 sheets were printed
 becomes greater with the charging roller having greater surface roughness
 of the surface layer 70c as indicated by the characteristic curve D. After
 the resistance value of the charging roller 70 exceeds about
 3.times.10.sup.8 [.OMEGA.], it becomes impossible to charge the
 photosensitive drum 2 to a sufficient potential for stable printing.
 For this reason, the surface roughness of the surface layer 70c is set to a
 level that gives rise to a resistance value of 3.times.10.sup.8 [.OMEGA.]
 of the charging roller 70, in other words, the surface roughness is set to
 15 .mu.m or less; corresponding to a resistance value of 3.times.10.sup.8
 [.OMEGA.]. When the surface roughness is 15 .mu.m or less, the content of
 the hardener included in the surface layer 70c is not greater than 30
 percent by weight as is apparent from the characteristic curve C in FIG. 5
 showing the relation between the quantity of the hardener and the surface
 roughness.
 In the charging roller 70 according to the present invention, as described
 above, because the hardener included in the surface layer 70c is 30
 percent or less by weight, the toner is inhibited from adhering to the
 surface layer 70c, which is caused by an increase in the surface roughness
 of the surface layer 70c attending on an increase in the hardener content.
 Therefore, the resistance value of the charging roller 70 is inhibited
 from increasing due to the toner adhesion to the surface layer 70c.
 Consequently, an insufficient static charge on the photosensitive drum 2
 is inhibited from occurring attending on an increase in the resistance
 value of the charging roller.
 Therefore, in the charging roller 70 according to the present invention, as
 mentioned above, because the surface layer 70c contains not less than 10
 percent by weight of the hardener, the photosensitive drum 2 is inhibited
 from being excessively charged by triboelectricity produced between the
 charging roller 70 and the photosensitive drum 2. Because the hardener
 included in the surface layer 70c is not greater than 30 percent by
 weight, the photosensitive drum 2 is inhibited from being charged
 insufficiently due to an increase in the resistance value of the charging
 roller 70 caused by the adhesion of the toner to the surface layer 70c.
 Consequently, according to the printer 1 incorporating the charging roller
 70 according to the present invention, the potential of the photosensitive
 drum 2 is held in a desired adequate range, so that printing quality can
 be improved.
 In the charging roller according to the present invention, because the
 content of the hardener included in the surface layer 70c is not less than
 10 weight percent and not greater than 30 weight percent, the magnitude of
 the fusion strength to be described later, which is produced between the
 surface layer 70c of the charging roller 70 and the external
 circumferential surface of the photosensitive drum 2 can be controlled.
 Before proceeding to description of the function by control of the fusion
 strength, the method for measuring the fusion strength is explained as
 follows.
 The charging roller 70 is contacted under pressure with the photosensitive
 drum 2 and they are left to stand for 24 hours at a high temperature of
 80.degree. C. and a high humidity of 80 percent, for example.
 Subsequently, the photosensitive drum 2 is fused with the surface layer
 70c of the charging roller 70. The charging roller 70 is suspended by a
 non-elastic string and raised vertically by using a tension gauge. When
 the photosensitive drum 2 breaks away from the charging roller 70, the
 value indicated by the tension gauge is read. The value obtained by
 subtracting the dead weight of the charging roller 70 from the indicated
 value represents the fusion strength.
 FIG. 9 is a graph showing the relation between the percentage (%) by weight
 of the hardener included in the surface layer 70c of the charging roller
 70 and the fusion strength measured by pulling in the direction as
 mentioned above.
 The abscissa of the graph indicates percentage (%) by weight of the
 hardener included in the surface layer 70c, and the ordinate indicates the
 magnitude (g) of the fusion strength. The characteristic curve E shows
 changes in the fusion strength when the proportion of the hardener is
 varied.
 The magnitude of the fusion strength between the surface layer 70c of the
 charging roller 70 and the external circumferential surface of the
 photosensitive drum 2 depends on the precipitated amount of low-molecular
 oligomers from the surface layer 70c. The more the hardener is included in
 the surface layer 70c, the more the precipitation of the low-molecular
 oligomers is inhibited. Therefore, the magnitude of the fusion strength
 decreases as the hardener content in the surface layer 70c increases as
 indicated by the characteristic curve E.
 As the magnitude of the fusion strength decreases, the surface layer 70c is
 inhibited from peeling from the cylindrical member 70b due to the fusion
 between the surface layer 70c and the photosensitive drum 2. Consequently,
 the toner is inhibited from adhering to the external surface of the
 cylindrical member 70b due to the peeling of the surface layer 70c from
 the cylindrical member 70b, so that the resistance value of the charging
 roller 70 is inhibited from increasing due to the toner adhesion.
 Therefore, chances are effectively reduced for the photosensitive drum 2
 to be charged insufficiently.
 Description will now be made of the method for inquiring into whether or
 not the potential of the photosensitive drum 2 is held at a value
 sufficient for stable printing.
 FIG. 10 shows a slide angle-measuring device for measuring the angle of
 slide to be described later, which corresponds to a frictional force
 produced between the charging roller 70 and the photosensitive drum 2.
 The slide angle measuring device 10 includes a flat plate 11 on which a
 sheet 12 for OHP (OverHead Projector) panel, for example, formed by a
 polyester film 100 .mu.m thick in the example in FIG. 10.
 The charging roller 70 is placed on the OHP sheet 12 on the flat plate 11
 as indicated by a dotted line. The angle of slide a is an angle at which
 the charging roller 70 starts to slide in the direction of arrow F
 coincident with the direction of the rotation shaft 70a of the charging
 roller 70 when the other end of the flat plate 11 is turned upwardly
 around one end of the flat plate 11 as indicated by the solid line.
 In the example of FIG. 10, the angle of slide a of the charging roller 70
 according to the present invention is greater than 0.degree. and not more
 than 20.degree..
 FIG. 11 is a graph showing the relation between the angle of slide .alpha.
 (.degree.) of the charging roller 70 according to the present invention
 and the potential of the photosensitive drum 2.
 The abscissa of the graph indicates the angles of slide .alpha. (.degree.)
 of the charging roller 70 and the ordinate indicates the potential (V) of
 the photosensitive drum 2. The characteristic curve H of the graph
 indicates changes in the potential of the photosensitive drum 2 when the
 ratio of the circumferential speed of the charging roller 70 to the
 circumferential speed of the photosensitive drum 2 (hereafter referred to
 simply as the circumferential speed ratio) is 1.4 and the slide angle a is
 greater than 0.degree. and not more than 20.degree.. The characteristic
 curve I in the graph indicates changes in the potential of the
 photosensitive drum 2 when the circumferential speed ratio is 1.4 and the
 slide angle .alpha. is greater than 20.degree.. The characteristic curve J
 indicates changes in the potential of the photosensitive drum 2 when the
 circumferential speed ratio is 1.
 In the printer 1 incorporating the charging roller 70 according to the
 present invention, in other words, the charging roller 70, the slide angle
 .alpha. of which is in a range of greater than 0.degree. and not more than
 20.degree., as indicated by the characteristic curve H, the potential of
 the photosensitive drum 2 is stably held at a setting voltage of -800V
 regardless of the slide angle of the charging roller.
 In a printer incorporating a charging roller with a slide angle .alpha. of
 greater than 20.degree., as indicated by the characteristic curve I, the
 photosensitive drum 2 is charged excessively by triboelectricity produced
 between the surface layer 70c of the charging roller 70 and the external
 circumferential surface of the photosensitive drum. Therefore, the
 potential of the photosensitive drum 2 increases by exceeding the setting
 voltage of -800V and reaches -1000V, for example.
 It has been confirmed that in the charging roller according to the present
 invention with a slide angle a of greater than 0.degree. and not more than
 20.degree., the potential of the photosensitive drum 2 is held at the
 above-mentioned setting voltage without being excessively charged by
 triboelectricity produced between the surface layer 70c of the charging
 roller 70 and the external circumferential surface of the photosensitive
 drum 2 as mentioned above.
 In the above-mentioned example, the ratio of the circumferential speed of
 the charging roller 70 to the circumferential speed of the photosensitive
 drum 2 was set at 1.4, but the ratio may be set at an optional value
 larger than 1 and less than 1.4.
 In the above-mentioned embodiment, description was made of a case in which
 the surface layer 70c of the charging roller 70 contains a polymer of a
 polyether-urethane resin and a polyamide resin. However, instead of this
 polymer, it is possible to use any one of a polymer of a
 polyester-urethane resin and a polyamide resin, a polyamide resin and a
 polyimide resin.
 Further in the above-mentioned embodiment, description was made of the case
 in which the surface layer 70c contains a melamine resin as the hardener.
 However, instead of the melamine resin, it is possible to use as the
 hardener any one of a aniline resin, an alkyd resin, an unsaturated
 polyester resin, an urea resin, an epoxy resin, a xylene-formaldehyde
 resin, a ketone-formaldehyde resin, a furan resin, and a phenol resin.
 Further in the above-mentioned embodiment, description was made of the case
 where the cylindrical member 70b of the charging roller 70 is formed of a
 polyether-urethane rubber. However, instead of the polyether-urethane
 rubber, the cylindrical member 70b may be formed of a polyester-urethane
 rubber.
 In the electrophotographic recording apparatus according to the present
 invention, as mentioned above, because the surface layer of the charging
 roller contains not less than 10 percent by weight of the hardener, the
 photosensitive drum is inhibited from being excessively charged by
 triboelectricity produced between the charging roller and the
 photosensitive drum. Because the hardener included in the surface layer is
 not greater than 30 percent by weight, there are less chances that the
 photosensitive drum is charged insufficiently due to an increase in the
 resistance value of the charging roller caused by the adhesion of the
 toner to the surface layer. Consequently, the potential of the
 photosensitive drum is held in a specified adequate range, so that
 printing quality can be improved.