Roll including foam body and method of producing the roll

Abstract: A charging roll is disclosed which includes a center shaft (12), a foam layer (14, 22) formed on an outer circumferential surface of the center shaft, an electrically conductive elastic layer (16) formed on an outer surface of the foam layer, and a resistance adjusting layer (18) and a protective layer (20) formed by coating on an outer surface of the electrically conductive elastic layer. The foam layer consists of an electrically conductive foam body or a foam body a part of which is made electrically conductive. Also is disclosed a method of producing a composite roll, such as the above charging roll, which includes a foam layer formed on a shaft, and a solid elastic layer formed on the foam layer. This method includes the step of forming multiple minute grooves (32, 42) in an outer circumferential surface of a pre-foam tube (34) which gives the foam layer, or an inner circumferential surface of an elastic tube (36) which gives the solid elastic layer.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a roll, such as a charging roll, image developing roll, image transfer roll, cleaning roll, pressure roll, for use in electrophotographic copying machines, printers or the like, and particularly to a method of producing such a charging roll as indicated above. Discussion of the Related Art In conventional electrophotographic copying machines or printers, a corona discharge device has been used for electrostatically charging a surface of a photosensitive body or drum so that the surface is uniformly provided with a predetermined electric potential. However, the use of the corona discharge device results in some problems, such as (1) the necessity of high-voltage power supply, (2) relatively low charging efficiency, (3) the occurrence of a high level of ozone, and (4) local variation in the degree of charging of the photosensitive drum due to contamination of wires. In recent years, therefore, there have been developed contact type charging methods, in particular, a roll charging method which uses an electrically conductive roll as a charging roll. In this roll charging method, the charging roll is rotated with a photosensitive drum which is positively moved or driven by a suitable drive device, with the surface of the roll being in pressed contact with that of the drum. When a given voltage is applied to a center shaft or core of the charging roll, electrical charges are directly given to the surface of the photosensitive drum, whereby the drum surface is charged with a predetermined electrical potential. If the applied voltage consists solely of a DC component, microscopic variation arises in contact between the charging roll and the photosensitive body, which results in spot-like charging variation. As a solution to this problem, a voltage consisting of a DC component and an AC component superimposed on the DC component is applied to the charging roll. The frequency of the AC electric field is determined depending upon a process speed, that is, the frequency increases with an increase in the process speed. In other words, the AC component of the voltage has a low frequency when the processing is effected at a low speed, and has a high frequency when the processing is effected at a high speed. The construction of the charging roll used in the above-described charging method is generally shown in Fig. 3, by way of example. As shown in Fig. 3, an electrically conductive elastic layer 4 formed of a low-hardness electrically conductive rubber composition is formed on an outer circumferential surface of a center shaft or core 2. On an outer circumferential surface of the conductive elastic layer 4, there are formed in lamination a preventive layer 6, a resistance adjusting layer 8 and a protective layer 10 in the order of description. The preventive layer 6 serves to prevent a softening agent from oozing or migrating out of the conductive elastic layer 4. When a voltage is applied to the above-described charging roll with its surface being in pressed contact with the photosensitive drum, the AC component of the voltage causes force to act between the drum and the charging roll, which force depends on the frequency of the AC component. Consequently, the photosensitive drum, which has a thin metallic pipe as a base or core member, vibrates and generates noise. Such vibration and noise are conventionally prevented by various damping methods, such as coating the inner surface of the photosensitive drum with a damping coating, or filling the inside of the drum with a damping material. Otherwise, a noise insulating device is needed for reducing sound or noise which leaks from a housing accommodating the photosensitive drum and charging roll. Since the opposite ends of the center shaft 2 of the charging roll are pressed against the photosensitive drum by means of springs or the like, the width of the nip between the drum and roll is likely to be small at the opposite ends of the roll and to be large at a middle portion of the roll. With this difference in the nip width between the photosensitive drum and charging roll, a clearance tends to be formed between the middle portion of the roll and the drum, thereby causing the photosensitive drum to be non-uniformly or unevenly charged. In view of the above, there is recently proposed a a charging roll which is constructed such that a foam layer consisting of an electrically conductive foam body is formed on the outer circumference of the center shaft, and such that a resistance adjusting layer and a protective layer are formed by coating on the outer circumference of the foam layer. The hardness of the roll can be lowered due to the use of the electrically conductive foam body, in order to effectively reduce the noise as described above. For the above-described charging roll to offer a sufficiently high noise reducing effect, the expansion ratio of the conductive foam body must be increased so as to reduce the hardness of the roll to a sufficiently low level. Consequently, the foam body may likely suffer from cell defects. If the charging roll employs the foam body having such a low hardness, creases are likely to appear on the roll surface due to shrinking of the coating layers (resistance adjusting layer and protective layer), thus making it difficult to provide a smooth or even roll surface. If the smoothness or evenness of the roll surface is unsatisfactory, the use of such charging roll may result in producing a poor or deficient image. Further, since the foam layer has relatively large variation in its dimensions, the outer circumferential surface of the foam layer needs to be ground prior to coating thereof, so that the outside diameter of the charging roll is made unvaried or constant, and is accurately controlled to within a desired range. Moreover, since the inside diameter of the foam body formed in a cylindrical shape varies from portion to portion, that is, the inner circumferential surface of the foam body has poor smoothness, the inner surface needs to be also ground before the shaft is inserted into the cylindrical foam body. Conventional methods of producing a composite roll are known from JP-A-2 202 430, US-A-4 956 211 and EP-A-0 210 871. SUMMARY OF THE INVENTION The present invention has been developed in the light of the above situations. It is therefore an object of the invention to provide a method of producing a charging roll which meets required electrical properties and maintains a smooth surface, and which causes reduced vibrational noise due to an AC component of a voltage applied thereto, and stably assures a constant nip, when the roll is in pressed contact with a photosensitive drum. The above-described object of the present invention may be accomplished according to the invention defined in claim 1. The composite roll having the foam layer and solid elastic layer may suffer from voids which appear between the two layers due to air trapped in the roll upon foaming of the pre-foam tube. The air remains in the roll since the degree of foaming of the pre-foam tube varies from portion to portion, and a portion of the tube which has initially foamed prevents air from escaping when another portion foams. According to the above-described method of the invention, the multiplicity of the minute grooves are formed in the inner circumferential surface of the elastic tube that gives the solid elastic layer, such that the grooves extend in the axial direction of the tube, preferably over the entire axial length thereof, and such that the inner circumferential surface of the elastic tube has a surface roughness Rz of not less than 15µm. Upon foaming of the pre-foam tube, therefore, air can effectively escape through the axial grooves, from the middle portion to the axial end portions of the roll, before the foam body is completely adhered to the inner surface of the elastic tube. Accordingly, the air can be effectively removed from the interface between the foam layer and solid elastic layer of the roll, thus avoiding the occurrence of the voids as described above, and assuring significantly improved surface smoothness of the composite roll. Preferential embodiments of the invention are defined in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings, in which: Fig. 1 is an axial cross sectional view showing a charging roll; Fig. 2 is an axial cross sectional view showing another charging roll; Fig. 3 is an axial cross sectional view showing a conventional charging roll; and Fig. 4 is a view schematically showing the construction of a device for measuring the sound level of each specimen of charging roll. Fig. 5 is an axial cross sectional view showing one process step for forming a composite roll having a foam body and a solid elastic layer, wherein a pre-foam tube giving the foam body and an elastic tube giving the solid elastic layer are positioned in a cylindrical metallic mold; Fig. 6 is an axial cross sectional view showing the composite roll obtained according to a preferred method of the invention which includes the step of Fig. 5; Fig. 7 is an enlarged view showing a part of an end face of the pre-foam tube as shown in Fig. 5; Fig. 8 is a view showing one example of grooves formed in the pre-foam tube as shown in Fig. 5; Fig. 9 is a view showing another example of grooves formed in the pre-foam tube as shown in Fig. 5; and Fig. 10 is a transverse cross sectional view showing the composite roll obtained according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to Fig. 1 showing a charging roll, a foam layer 14 consisting of an electrically conductive foam body is formed on an outer circumferential surface of a center shaft (core) 12, and an electrically conductive elastic layer 16 is formed on an outer circumferential surface of the foam layer 14. Further, a resistance adjusting layer 18 and a protective layer 20 are formed by coating on a base roll body which consists of the foam layer 14 and conductive elastic layer 16. The foam layer 14 is formed of any one of various known foam materials which satisfy required characteristics for use in the charging roll and do not suffer from fatigue or other problems. An electrically conductive foam material generally used for the foamed layer 14 is prepared by mixing an electrically conductive powder or fiber, such as metal powder, carbon black or carbon fiber, into a rubber foam body, such as urethane foam body or hydrin rubber foam body. The foam layer 14 may be formed of an insulating material or a partially conductive material provided the center shaft 12 is electrically connected to the conductive elastic layer 16. Fig. 2 shows an example of a charging roll in which a foam layer 22 is formed of an insulating foam material, and includes axially opposite end portions 24, 24 which are made electrically conductive. For instance, the foam layer 22 is formed by impregnating the end portions 24 with a liquid including an electrically conductive powder or fiber, such as carbon black or carbon fiber, dispersed in water, and then drying the impregnated end portions 24. The lower the hardness (Hs) of the foam body (22) is, the more effectively the foam body reduces noise. Therefore, the expansion ratio and other factors of the foam body are desirably adjusted so that the foam body has a Shore A hardness of not greater than 15, and thus provides a sufficiently high noise reducing effect. The material used for the conductive elastic layer 16 may be optimally selected from electrically conductive rubber compositions or conductive thermoplastic elastomers, which contain conductive powder or fiber, such as metal powder, carbon black or carbon fiber. The elastic layer 16 may also be formed of a hard resin if the layer 16 has such a small thickness as to be easily elastically deformed. In this case, however, it becomes difficult to ensure a sufficiently large contact area or width between the charging roll and the photosensitive drum, since even a relatively thin layer of the hard resin increases the hardness of the roll. The use of the hard resin is also disadvantageous since the thickness of the hard resin layer must be reduced to be as small as about 50µm, making it difficult to form and handle the layer, or causing breakage or other problems. The thickness of the conductive elastic layer 16 as described above is desirably held in a range of 100µm to 1200µm. If the thickness of the elastic layer 16 is smaller than 100µm, the elastic layer 16 is not able to sufficiently cover and eliminate cell defects of the foam layer 14, 22. As a result, recesses and the like tend to appear on the elastic layer 16 at its portions corresponding to the cell defects, resulting in poor smoothness of the surface of the roll. If the thickness of the elastic layer 16 exceeds 1200µm, the effect of the foam layer 14, 22 for reducing noise is deteriorated. To form the foam layer 14, 22 and conductive elastic layer 16 integrally on the center shaft 12, the shaft 12 is disposed at the center of a cylindrical metallic mold, and a relatively thin tube made of an electrically conductive elastic material, which gives the conductive elastic layer 16, is disposed coaxially with the center shaft 12. Then, a molding material which gives the foam body 14, 22 is injected under pressure into a cylindrical space between the shaft 12 and the elastic layer 16, and a foaming operation is effected to form the foam body 14, 22. The foam layer 14, 22 and conductive elastic layer 16, and the foam layer 14, 22 and center shaft 12 are fixed to each other with sufficiently high durability, only due to the pressure of the foam body 14, 22, without using an adhesive. However, the foam layer 14, 22 may be bonded to the elastic layer 14 and the shaft 12 as needed, by means of a suitable adhesive. When the resistance adjusting layer 18 is formed by coating on the outer surface of the conductive elastic layer 16, the elastic layer 16 may be swollen with a solvent of a coating liquid which provides the resistance adjusting layer 18. To avoid this, it is desirable to coat the conductive elastic layer 16 with an electrically conductive resin layer which is not swollen with the solvent, and then coat the resin layer with the resistance adjusting layer 18. The conductive resin layer has a thickness of several microns (1 micron = 10 -6 m), and may be formed of a material having as a major component a nylon-based polymer or modified nylon, such as N-methoxymethylated nylon. The resistance adjusting layer 18 is preferably formed of a rubber containing epichlorohydrin, for example, and is formed by a known coating method, such as dipping. The thickness of this layer 18 is generally held in a range of about 50 ∼ 500µm, preferably, about 80 ∼ 160µm. Further, the protective layer 20 having a thickness of microns is formed by coating on the outer surface of the resistance adjusting layer 18, so as to prevent the layer 18 from adhering to the photosensitive drum. This protective layer 20 may be formed of a material having as a major component a nylon-based polymer or modified nylon, such as N-methoxymethylated nylon. To assure further improved smoothness of the surface of a roll such as the charging roll as described above, the foam layer 14 and electrically conductive elastic layer 16 are advantageously formed on the center shaft 12, in the manner which will be hereinafter described referring to Fig. 5. Initially, the shaft 12 is disposed at the center of a columnar cavity of a cylindrical metallic mold 30, and a pre-foam tube 34 which gives the foam layer 14 upon its foaming is disposed radially outwardly of the shaft 12. Then, an elastic tube 36 which gives the conductive elastic layer 16 is disposed radially outwardly of the pre-foam tube 16. Thereafter, cap members 38, 38 are fitted on the axially opposite end portions of the metallic mold 30. The pre-foam tube 34 giving the foam layer 14 is formed of a suitably selected foam material, such as urethane foam, hydrin rubber chloroprene rubber, EPDM and Cl-EPDM. By using the selected foam material, the pre-foam tube 34 is formed by extrusion, for example, with a thickness which is determined depending upon the expansion ratio of the foam material that is related to the volume of the foam layer 14 to be formed. Generally, the pre-foam tube 34 used for the charging roll has a thickness of about 1.5mm. On the other hand, the elastic tube 36 which gives the conductive elastic layer 16 of the charging roll, for example, is formed of a known elastic material selected from various rubber compositions, such as EPDM or polyamide elastomer, or thermoplastic elastomers. The elastic tube 16 having a suitable thickness is formed by extrusion, by using the selected elastic material. According to the method of the present invention, a multiplicity of minute grooves 42 are formed in an inner circumferential surface of the elastic tube 36, such that the grooves 42 extend in an axial direction of the tube 36, desirably over the entire axial length of the tube 36. These grooves 42 may be formed by a suitable method, such as (1) grinding with sandpaper the inner circumferential surface of the elastic tube 36 which has been extruded, in the axial direction of the tube 36, so as to roughen the inner circumferential surface, or (2) extruding the elastic tube 36 from an extrusion die having grooves, so that the extruded tube 36 is provided at its inner circumferential surface with the minute grooves 42. While the minute grooves 42 extend in parallel with each other in the axial direction of the elastic tube 36 and are arranged over the entire circumference of the tube 34, the grooves 42 may be formed in helical fashion over the inner circumferential surface of the elastic tube 36. To effectively allow air to escape through the grooves 42 during the foaming operation for the pre-foam tube 34, the inner circumferential surface of the elastic tube 36 preferably has a surface roughness Rz of not less than 15µm, where Rz represents the ten-point mean roughness, that is, the average value of the absolute values of the heights of five highest profile peaks and the depths of five deepest profile valleys within a sampling length. Since the surface smoothness of the roll is deteriorated if the surface roughness Rz is too high, the surface roughness Rz is generally controlled to be not greater than 50pm, though the optimum roughness is depending upon the material, thickness and other factors of the elastic tube 36. To enhance the effect of allowing air to escape during the foaming operation, the grooves 42 are arranged in the circumferential direction of the elastic tube 36, preferably at a pitch of about 0.05 mm ~ 1mm. After the center shaft 12, pre-foam tube 34 and elastic tube 36 obtained by the method as described above are disposed coaxially with each other within the cavity of the metallic mold 30, in the manner as described above, the pre-foam tube 34 is subjected to a suitable foaming operation, such as heating, so as to provide a composite roll 40 as shown in Fig. 6. The composite roll 40 has the foam layer 14 and electrically conductive elastic layer 16 which are laminated on each other and formed integrally on the center shaft 12. As described above, air generated during the foaming operation can effectively escape outwards from the axial end portions of the elastic tube 36, through the grooves 42 formed in the inner circumferential surface of the elastic tube 36. Therefore, the air does not remain in the interior of the roll, thereby favorably avoiding voids which may otherwise appear between the foam layer 14 and elastic layer 16. Consequently, the composite roll 40 exhibits excellent surface smoothness. It is to be understood that the above-described method is not only employed for producing a charging roll but may be employed for producing any other kind of roll, such as an image developing roll, image transfer roll, cleaning roll or pressure roll, used in electrophotographic copying machines or printers, for example, provided the roll is a composite roll having a foam layer and a solid elastic layer superposed on the foam layer. EXAMPLES There will be hereinafter described in detail some examples of charging rolls constructed according to the present invention. However, it is to be understood that the invention is by no means limited to the details of the description of these examples, but may be embodied with various other changes, modifications and improvements, which may occur to those skilled in the art, without departing from the scope of the invention as defined in the claims. Initially, various materials for forming charging rolls were prepared in the manners as described below. (1) Urethane foam body A polyol mixture was initially prepared by mixing together the following ingredients: polyether polyol ("FA7030" available from Sanyo Chemical Industries, Ltd., Japan) 100 parts by weight H 2 O 1 part by weight aqueous solution of fatty acid sulfonate ("additive SV" available from Sumitomo Bayer Urethane K.K., Japan) 1 part by weight silicone foaming agent ("SRX274C" available from Toray Silicone K.K., Japan) 2 parts by weight triethanolamine 1.5 parts by weight N,N'-dimethylbenzylamine (catalyst) ("Kaorizer No. 20" available from Kao Corporation, Japan) 2 parts by weight triethylenediamine (catalyst) ("Dabco33 LV" available from Mitsui Air Products K.K., Japan) 1 part by weight The thus prepared polyol mixture was mixed with isocyanate ("Sumidule T-80" available from Sumitomo Bayer Urethane, K.K., Japan), in a proportion of 5.3:1.0, just before the mixture was injected into a mold. (2) Electrically conductive hydrin rubber foam body The ingredients as indicated below were kneaded by an ordinary enclosed-type mixer, and then mixed together by two rolls with a vulcanizing agent and a foaming agent. epichlorohydrin rubber ("Epichlomer C" available from Daiso K.K., Japan) 100 parts by weight Carbon ("Seast SO" available from Tokai Carbon Co., Ltd, Japan) 40 parts by weight precipitated calcium carbonate light 20 parts by weight processing aid 3 parts by weight age resistor 3 parts by weight ethylene thiourea 1.5 parts by weight N,N'-dinitroso pentamethylene tetramine (foaming agent) ("Celmike A" available from Sankyo Kasei K.K., Japan) 5 parts by weight urea and its derivatives (foaming aid) ("Celton N" available from Sankyo Kasei K.K., Japan) 5 parts by weight (3) Electrically conductive rubber (solid) The ingredients as indicated below were kneaded by an ordinary enclosed mixer, and then mixed together by two rolls with a vulcanizing agent and a vulcanization accelerator. polynorbornene rubber 80 parts by weight ethylene-propylene rubber 20 parts by weight ketjenblack 40 parts by weight naphthenic oil 350 parts by weight (4) Electrically conductive EPDM rubber The ingredients as indicated below were kneaded by an ordinary enclosed mixer, and then mixed together by two rolls with a vulcanizing agent and a vulcanization accelerator. ethylene-propylene rubber 100 parts by weight ketjenblack 40 parts by weight naphthenic oil 20 parts by weight (5) Electrically conductive polyester elastomer "Hytrel 4047X08" available from Toray·du Pont K.K., Japan (6) Electrically conductive polyamide elastomer "Pebax" available from Toray Industries Inc., Japan (7) Electrically conductive N-methoxymethylated nylon 100 parts by weight of N-methoxymethylated nylon was dissolved in a mixture of methanol and water, and then 15 parts by weight of ketjenblack was added to the solution thus obtained and dispersed by a beads type disperser. (8) Material for forming Resistance adjusting layer The ingredients as indicated below were kneaded or mixed by two rolls, and dissolved in a mixture of methyl ethyl ketone and toluene. epichlorohydrin rubber ("Epichlomer C" available from Daiso K.K., Japan) 100 parts by weight hard clay 40 parts by weight red lead 5 parts by weight processing aid 1 part by weight ethylene thiourea 1.5 parts by weight (9) Material for forming Protective layer 100 parts by weight of N-methoxymethylated nylon was dissolved in a mixture of methanol and water, and then 5 parts by weight of ketjenblack was added to the solution thus obtained and dispersed by a beads type disperser. Examples 1 through 4 and Comparative Example 1 Five elastic tubes having an outside diameter of 12mm were formed of the electrically conductive EPDM rubber (4) as indicated above, by using suitable metallic molds, such that the tubes have respective thicknesses as indicated in TABLE 1 below. The surface of each of the thus formed tubes was ground to remove traces of the divided mold, so as to provide a smooth or even surface. Subsequently, a center shaft (core) having a diameter of 6mm and the above elastic tube were disposed coaxially with each other within a cylindrical metallic mold, and the urethane foam material (1) as indicated above was introduced into a space between the shaft and the tube. Then, the metallic mold was immediately fluid-tightly closed, followed by a foaming operation and heat treatment, so as to form a foam layer and an electrically conductive elastic layer on the outer surface of the shaft. Subsequently, the material (8) for forming the resistance adjusting layer as indicated above was applied by dipping to the outer surface of the conductive elastic layer to form an 80µm-thickness layer, and then dried and subjected to heat treatment. Then, the material (9) for forming the protective layer was applied by dipping to the thus formed resistance adjusting layer, to form a 5µm-thickness layer, and then dried and heat-treated. Thereafter, the axially opposite end portions of the foam layer were impregnated with a liquid including carbon black dispersed in water, and then dried. Thus, the end portions of the foam layer were made electrically conductive, so as to permit the shaft and the conductive elastic layer to be electrically connected to each other. In this manner, there were obtained five specimens of charging roll constructed as shown in Fig. 2. The thus obtained charging rolls were tested by measuring the current value, the hardness of the roll, the noise level (sound level), and the surface roughness, and also evaluating the appearance of the roll. Further, the charging rolls were actually installed on printers, and the image producing capability of each of the printers using the respective charging rolls was evaluated. The results of the tests are indicated in TABLE 1 below. The measurements of the electric current value, roll hardness, sound level, surface roughness and image producing capability of each specimen of charging roll were conducted under the following conditions. [CURRENT VALUE] Under the environment of 23°C and 53%RH, each of the charging rolls to be tested was held in pressed contact with a smooth metallic roll having a diameter of 30mm and rotating at 17rpm, with a 500gf (1gf = 9,80665·10 -3 N) load being applied to each of the opposite end portions of the center shaft of the roll. The current value was measured while a voltage of 500Vp-p (300 Hz) - 200V DC was applied to the shaft of the charging roll. [ROLL HARDNESS] The roll hardness was measured by using a SHORE A hardness meter, with a 1kg load being applied to the charging roll to be tested. [SOUND LEVEL] The sound level was measured by a measuring device constructed as shown in Fig. 4, with a voltage of 2000Vp-p (500Hz) - 550V DC being applied to the shaft of the charging roll to be tested. [SURFACE ROUGHNESS] The surface roughness was measured by the surface roughness meter named "SURFCOM" manufactured by Tokyo Seimitsu Co., Ltd, Japan. [IMAGE PRODUCING CAPABILITY] Under the environment of 10°C and 15%RH, images were continuously printed on 5000 sheets of paper, by using a laser beam printer named "Laser Jet III SI " manufactured by HEWLETT PACKARD. The image producing capability was evaluated with respect to each specimen of charging roll, using the following criteria: ○: no variation in the images produced ▵: slight variation in the images X: variation in the images It will be apparent from the measurement results as indicated in TABLE 1 above that the charging rolls of Examples 1 through 4 made sufficiently small noise and exhibited excellent image producing capability. On the other hand, the charging roll of Comparative Example 1 having a 1400µm-thickness conductive elastic layer showed relatively high roll hardness and suffered from relatively large noise. Examples 5 and 6 and Comparative Example 2 Three elastic tubes made of the electrically conductive polyester elastomer (5) as indicated above were produced by an extruder, such that the tubes had an outside diameter of 12mm, and respective thicknesses as indicated in TABLE 2 below. These elastic tubes were used to provide three specimens of charging rolls as Examples 5, 6 and Comparative Example 2. Each of the specimens were produced in the same manner as Example 1, wherein the foam layer and electrically conductive elastic layer were formed on the outer surface of the center shaft, and the resistance adjusting layer and protective layer were further formed on the conductive elastic layer. The shaft and the conductive elastic layer were electrically connected to each other in the same manner as in Example 1, so as to provide the intended charging roll as shown in Fig. 2. The same measurements and evaluation as employed for Example 1 were effected on the charging rolls thus obtained, and the results thereof are indicated in TABLE 2 below. It will be apparent from the results that the charging rolls of Examples 5 and 6 caused significantly reduced noise, and exhibited excellent image producing capability. On the other hand, the charging roll of Comparative Example 2 having a 80µm-thickness conductive elastic layer was not able to sufficiently cover and eliminate recesses due to cell defects of the foam layer, and the appearance of the roll was accordingly deteriorated, resulting in increased roughness of the roll surface and poor image producing capability. Examples 7, 8 Two pre-foam tubes which give respective foam layers were prepared by an extruder, the tubes being formed of the electrically conductive hydrin rubber (2) as indicated above. Similarly, two elastomer tubes were prepared by extrusion, using the electrically conductive polyester elastomer (5) and electrically conductive polyamide elastomer (6) as indicated above, respectively. These elastomer tubes which give respective conductive elastic layers had an outside diameter of 12mm and the thickness as indicated in TABLE 3 below. Each of the pre-foam conductive hydrin rubber tubes and the corresponding conductive elastomer tube were disposed coaxially with each other within a cylindrical metallic mold, with a 6mm-diameter center shaft (core) being disposed at the center of the mold. The tubes and shaft in the mold were heated by an oven, to achieve foaming and vulcanization of the tubes, so that the foam body and the conductive elastic layer were formed on the outer surface of the shaft. Further, a resistance adjusting layer and a protective layer were formed by dipping on the outer circumferential surface of each conductive elastic layer, in the same manner as Example 1, so as to provide two specimens (Examples 7, 8) of charging rolls as shown in Fig. 1. The same measurements and evaluation as employed for Example 1 were effected on the charging rolls thus obtained, and the results thereof are indicated in TABLE 3 below. It will be apparent from the results that the charging rolls of Examples 7 and 8 caused comparatively small noise, and exhibited excellent image producing capability. Comparative Examples 3, 4 and 5 Three pre-foam tubes were initially prepared by an extruder, using the electrically conductive hydrin rubber (2) as indicated above. Each of the pre-foam tubes and a 6mm-diameter center shaft (core) were disposed coaxially with each other within a cylindrical metallic mold, and heated by an oven, to achieve foaming and vulcanization of the tube, so that a foam layer having a thickness of 3mm was formed on the outer surface of the shaft. Thus, three specimens of charging rolls were produced as Comparative Examples 3, 4 and 5 in which the respective foam layers had different expansion ratios. To prevent the foam layer of each charging roll from being swollen by a solvent of a resistance adjusting layer to be formed, the foam layer was coated by dipping with a 10µm-thickness layer of the electrically conductive N-methoxymethylated nylon (7) as indicated above, which was then dried and heat-treated. Further, the resistance adjusting layer and a protective layer were formed by dipping on the above coating layer, in the same manner as Example 1, so as to provide the intended charging roll. The same measurements and evaluation as employed for Example 1 were conducted on the charging rolls thus obtained, and the results thereof are indicated in TABLE 4 below. In Comparative Examples 3 and 4, the expansion ratio of the foam layer was made relatively high so as to reduce the hardness of the charging roll. In this case, the noise can be effectively reduced, but the appearance of the roll is deteriorated with creases appearing on the coating layer due to shrinking thereof, resulting in increased roughness of the roll surface and poor image producing capability. In Comparative Example 5, the expansion ratio of the foam layer was made relatively low so as to avoid the creases of the coating layer. In this case, however, the roll has a relatively high hardness and the noise made by the roll is undesirably increased. Comparative Example 6 A 6mm-thickness center shaft (core) and the electrically conductive rubber (solid) (3) as indicated above were disposed in a metallic mold, and were heated by press, so that a 3mm-thickness conductive rubber layer was formed on the outer circumferential surface of the shaft. Then, the surface of the conductive rubber layer was made smooth by grinding, and was coated by dipping with a 10µm-thickness layer of the electrically conductive N-methoxymethylated nylon (7) as indicated above, which was then dried and heat-treated. Then, a resistance adjusting layer and a protective layer were further formed by dipping on the coating layer, in the same manner as Example 1, so as to provide an intended charging roll as Comparative Example 6. The same measurements and evaluation as employed for Example 1 were effected on the thus obtained charging roll, and the results thereof are indicated in TABLE 4 above. It will be understood from the results that the Shore A hardness of the charging roll was increased to as high as 64 and the roll made a comparatively large noise. Reference Examples* 9,10 and 11 and Comparative Example 7 Initially, hydrin rubber having a composition as indicated below was prepared, and pre-foam tubes having an inside diameter of 7.5mm and an outside diameter of 10mm were produced by extruding the thus prepared hydrin rubber through grooved extrusion dies, so that a multiplicity of minute grooves were formed in the outer circumferential surface of each pre-foam tube, in the axial direction of the tube. The number and depth of the grooves formed in the pre-foam tube are indicated in TABLE 5 below. Composition of Hydrin Rubber hydrin rubber 100 parts by weight carbon black 40 parts by weight softening agent 10 parts by weight vulcanizing agent 1.5 parts by weight foaming agent 5 parts by weight foaming aid 5 parts by weight By using EPDM (ethylene-propylene copolymer rubber) or polyamide elastomer each having a composition as indicated below, elastic tubes having an inside diameter of 11.4mm and an outside diameter of 11.8mm were formed by extrusion. Composition of EPDM EPDM 100 parts by weight ZnO 5 parts by weight stearic acid 1 part by weight processed oil 30 parts by weight electrically conductive carbon 33 parts by weight peroxide 3.5 parts by weight cross linking agent 3 parts by weight Composition of Polyamide Elastomer polyamide elastomer (available from Toray Industries Inc., Japan Trade name: Pebax) 100 parts by weight electrically conductive carbon 30 parts by weight A cylindrical metallic mold having an inside diameter of 12mm was prepared, and a 6mm-diameter shaft (core) was disposed at the center of a cavity of the mold. Then, the pre-foam tube and elastic tube of each specimen were disposed coaxially with each other, and the opposite axial ends of the metallic mold were closed by respective cap members. Thereafter, the cylindrical metallic mold was put into an oven, and the pre-foam and elastic tubes were heated at 160°C for 40 minutes, so as to foam the pre-foam tube. In this manner, three Reference examples 9, 10 and 11 of composite rolls each having a foam layer formed integrally with a solid elastic layer were prepared. As Comparative Example 7, there was prepared a composite roll formed from a pre-foam tube having no grooves, and the elastic tube as described above. Each of the thus obtained composite rolls was measured by a surface roughness meter, and the surface smoothness of each roll was evaluated. The result of the evaluation is indicated in TABLE 5 below, wherein "O" indicates that no recess appeared on the roll, and "Δ" indicates that recesses having a depth less than 10 µm appeared on the roll, while "X" indicates that recesses having a depth of 10 µm or larger appeared on the roll. The solid elastic layer of each specimen was peeled off from the composite roll, and the surface of the foam layer was observed by human naked eyes to evaluate the condition of the surface. The result of the evaluation is indicated in TABLE 5 below, wherein "○" indicates that no voids appeared on the surface of the foam layer, and "Δ" indicates that relatively small voids (having a diameter of less than 2mm) appeared on the surface, while "X" indicates that relatively large voids (having a diameter of 2mm or larger) appeared on the surface. It will be understood from the above results that the composite rolls of Reference Examples 9-11 having multiple minute grooves formed in the outer surface of the pre-foam tube were free from voids on the surface of the foam layer of each roll. On the other hand, voids appeared in the composite roll of Comparative Example 7, and adversely affected the surface smoothness of the roll. Examples 12-17 and Comparative Examples 8-11 Initially, hydrin rubber having a composition as indicated below was prepared, and pre-foam tubes having an inside diameter of 8 mm and an outside diameter of 10mm were produced by extrusion. Composition of Hydrin Rubber hydrin rubber 100 parts by weight carbon black 40 parts by weight softening agent 10 parts by weight vulcanizing agent 1.5 parts by weight foaming agent 5 parts by weight foaming aid 5 parts by weight By using polyamide elastomer having a composition as indicated below, elastic tubes each having an inside diameter of 11.4mm and an outside diameter of 11.8mm were formed by extrusion. For composite rolls of Examples 16 and 17, a grooved extrusion spindle was used for extruding each of the elastic tubes, so that minute grooves were formed in the inner circumferential surface of the elastic tube, and so that the inner circumferential surface had a surface roughness (Rz) as indicated in TABLE 6 below. For composite rolls of Examples 12-15 and Comparative Examples 9-11, the inner circumferential surface of each of the elastic tubes was ground with various kinds of sand paper, in the axial direction of the tube, after extrusion of these elastic tubes. Thus, the inner circumferential surface of the relevant elastic tube was formed with minute grooves, to provide a surface roughness (Rz) as indicated in TABLE 6 below. Composition of Polyamide Elastomer polyamide elastomer (available from Toray Industries, Ltd., Japan Trade name: Pebax) 100 parts by weight electrically conductive carbon 30 parts by weight A cylindrical metallic mold having an inside diameter of 12mm was prepared, and a 6mm-diameter shaft (core) was disposed at the center of a cavity of the mold. Then, the pre-foam tube and elastic tube of each example were disposed coaxially with each other, and the opposite axial ends of the metallic mold were closed by respective cap members. Thereafter, the cylindrical metallic mold was put into an oven, and the pre-foam and elastic tubes were heated at 160°C for 40 minutes, so as to foam the pre-foam tube. In this manner, ten specimens of composite rolls each having a foam layer formed integrally with a solid elastic layer were prepared. As Comparative Example 8, there was prepared a composite roll formed from the pre-foam tube as described above, and an elastic tube having no grooves. Each of the thus obtained composite rolls was measured by a surface roughness meter, and the surface smoothness of each specimen was evaluated. The result of the evaluation is indicated in TABLE 6 below, wherein "○" indicates that no recess appeared on the roll, and "Δ" indicates that recesses having a depth less than 10 µm appeared on the roll, while "X" indicates that recesses having a depth of 10 µm or larger appeared on the roll. The solid elastic layer of each specimen was peeled off from the composite roll, and the surface of the foam layer was observed by human naked eyes to evaluate the condition of the surface. The result of the evaluation is indicated in TABLE 6 below, wherein "○" indicates that no voids appeared on the surface of the foam layer, and "▵" indicates that relatively small voids (having a diameter of less than 2mm) appeared on the surface, while "X" indicates that relatively large voids (having a diameter of 2mm or larger) appeared on the surface. It will be understood from TABLE 6 that the composite rolls of Examples 12-17 in which the minute grooves were formed in the inner surface of the elastic tube to provide the surface roughness of not less than 15µm were free from voids and exhibited significantly improved surface smoothness. On the other hand, the composite roll of Comparative Examples 8 having no grooves in the elastic tube and the composite rolls of Comparative Examples 2-4 in which the surface roughness of the inner circumferential surface of the elastic tube was less than 15µm suffered from voids, due to air trapped between the foam layer and solid elastic layer, and exhibited poor surface smoothness.

Claims: A method of producing a composite roll having a foam layer (14, 22) formed on an outer surface of a shaft (12), and a solid elastic layer (16) formed on an outer surface of the foam layer, comprising the steps of: locating a pre-foam tube (34) which gives said foam layer, radially outwardly of said shaft; forming a multiplicity of minute grooves (42) in an inner circumferential surface of a pre-foam tube (36) which gives said solid elastic layer, such that said minute grooves extend in an axial direction of the elastic tube, and such that said inner circumferential surface has a surface roughness Rz of not less than 15 µm; locating said elastic tube radially outwardly of said pre-foam tube; positioning said shaft, said pre-foam tube and said elastic tube within a cavity of a metallic mold; and effecting a foaming operation for said pre-foam tube, to thereby provide said foam layer, such that said foam layer is formed integrally with said solid elastic layer provided by said elastic tube. A method according to claim 1, wherein said surface roughness Rz of said inner circumferential surface of said elastic tubeis controlled to be not greater than 50 µm. A method according to claim 1 or 2, wherein said grooves are arranged in a circumferential surface of said elastic tube, at a pitch of 0.05 mm to 1 mm. A method according to any of claims 1-3, wherein said minute grooves extend in said axial direction over the entire axial length of said elastic tube. A method according to any one of claims 1 to 4, wherein said composite roll is a charging roll including: a center shaft (12); a foam layer (14, 22) formed on an outer circumferential surface of said center shaft and consisting of an electrically conductive foam body or a foam body a part of which is made electrically conductive; an electrically conductive layer (16) formed on an outer surface of said foam layer; a resistance adjusting layer (18) formed by coating on an outer surface of said electrically conductive elastic layer; and a protective layer (20) formed by coating on an outer surface of said resistance adjusting layer. A method according to claim 5, wherein said electrically conductive elastic layer is formed of one of an electrically conductive rubber composition and an electrically conductive thermoplastic elastomer. A method according to claim 5 or 6, wherein said electrically conductive elastic layer has a thickness of 100 µm to 1200 µm. A method according to any one of claims 5 to 7, wherein said foam layer is formed of an electrically conductive foam material which contains an electrically conductive powder or fiber. A method according to claim 8, wherein said electrically conductive powder or fiber is selected from the group consisting of a metal powder, carbon black and carbon fiber. A method according to any one of claims 5 to 7, wherein said foam layer includes at least one electrically conductive portion (24) through which said center shaft is electrically connected to said electrically conductive elastic layer. A method according to claim 10, wherein said foam layer has axially opposite end portions (24) at said at least one electrically conductive portion. A method according to any one of claims 5 to 11, wherein said foam layer has a Shore A hardness of not greater than 15. A method according to any one of claims 5 to 12, further comprising an electrically conductive resin layer formed by coating on said electrically conductive elastic layer. A method according to any one of claims 5 to 13, wherein said resistance adjusting layer has a thickness of 50 µm to 500 µm.

Inventors: HAYASHI SABURO, KATO HIROYASU, OINUMA SUMIO, SOUMIYA KAZUTOSHI, YAMAMOTO TAKAFUMI, HAYASHI, SABURO, KATO, HIROYASU, OINUMA, SUMIO, SOUMIYA, KAZUTOSHI, YAMAMOTO, TAKAFUMI, Hayashi, Saburo, c/o Tokai Rubber Ind. Ltd., Kato, Hiroyasu, c/o Tokai Rubber Ind. Ltd., Oinuma, Sumio, c/o Tokai Rubber Ind. Ltd., Soumiya, Kazutoshi, c/o Tokai Rubber Ind. Ltd., Yamamoto, Takafumi, c/o Tokai Rubber Ind. Ltd.
Applicants: TOKAI RUBBER IND LTD, TOKAI RUBBER INDUSTRIES, LTD.
Application year: 1993
Main classification: G03G15
IPC codes: F16C13, G03G15, B29C44