In an image forming apparatus, a bias controller varies one, while keeping the other fixed, of a charging alternating-current frequency, which is the frequency of the charging alternating-current voltage, and a developing alternating-current frequency, which is the frequency of the developing alternating-current voltage. Specifically, in a case where interference fringes appear in a developed image due to interference between the charging and developing alternating-current frequencies, when the recognizable minimum pitch of the interference fringes is A.sub.1 (mm), the width of the variation region of the other frequency is B.sub.1 (Hz), the rotation speed of an image carrying member is C.sub.1 (mm/sec), and the variation speed of the other frequency in the variation region is D.sub.1 (Hz/sec), the bias controller varies the other frequency at a variation speed D.sub.1 that fulfills |D.sub.1|>B.sub.1/(A.sub.1/C.sub.1).

An image forming system includes first and second apparatuses. The first image forming apparatus includes a first image forming unit and a first transfer unit. The first image forming unit forms an image of at least one color. The first transfer unit transfers the formed image to a recording medium. The second image forming apparatus includes a second image forming unit and a second transfer unit. The second image forming unit forms an image different in at least a part of colors from the first image forming apparatus. The second transfer unit transfers the formed image to the recording medium on which the image has been formed by the first image forming apparatus. A total resistance of the second transfer unit along a direction in which a transfer current flows is larger than that of the first transfer unit.

A charging device includes a charging member and a cleaning member. The charging member electrically charges a surface of a charging target member and is rotatable while being in contact with the surface of the charging target member. The cleaning member is in contact with a surface of the charging member and removes a foreign object attached to the surface of the charging member. Surface free energy of the surface of the charging member is 5.00 dynes per centimeter or more. A dynamic friction coefficient of the surface of the charging member and a surface of the cleaning member falls within a range from 0.48 to 0.88 both inclusive. The surface free energy and the dynamic friction coefficient satisfy a relation represented by the following expression (1).

E63+69(1)

E is the surface free energy in dynes per centimeter, and u is the dynamic friction coefficient.

A charging roller has a roller main body including a cylindrical inner layer having a porous structure formed of a semiconductive rubber composition constituted by ethylene propylene rubber, paraffin oil, and a carbon black having a DBP oil absorption amount of 400 cm.sup.3/100 g or more and an outer layer constituted by a nonporous and seamless semiconductive tube including a polyurethane thermoplastic elastomer having a type A durometer hardness of 93 or less, carbon black, and crosslinked PMMA particles having an average particle size of 5 to 20 m. The production method includes a process in which a semiconductive rubber composition forming the inner layer is foamed with OBSH.

A transfer device includes a transfer unit that transfers an image on an image carrier carrying the image onto a recording material by applying a voltage containing an alternating-current component to the recording material; a humidifying unit that humidifies the recording material transported toward the transfer unit; a transport unit that transports the recording material from the humidifying unit to the transfer unit while guiding the recording material in contact with a guide unit; and a leakage suppressing unit that suppresses leakage of an alternating current of the alternating-current component to the guide unit throughout a maximum length or more of the recording material in a direction in which the recording material is transported.

A charging member including an electro-conductive support; and an elastic layer, the elastic layer is a surface layer thereof and is constituted by a mono layer, the elastic layer includes domains and a matrix, an outer surface of the charging member is constituted by a surface of the matrix and surfaces of the domains, and the charging member has concave portions on the outer surface, the domains exist at bottom portions of the respective concave portions and are exposed to the outer surface of the charging member only at the bottom portions of the respective concave portions, the elastic layer has a volume resistivity of 110.sup.5 cm or more and 110.sup.8 cm or less, and A2 is 20 times or more of A1, wherein A1 and A2 are current values obtained by a prescribed method.

An image forming apparatus includes a controller. The controller controls to perform a first operation in which a first potential difference is formed between a supplying member and a developing member, the first potential difference being a potential difference in a direction in which an electrostatic force in a first direction toward the developing member from the supplying member is acted on toner charged to a normal polarity, and a second operation in which a second potential difference is formed between them and in a second direction opposite to the first direction. When an image forming operation is performed, the controller controls timing for changing from the first operation to the second operation based on information on presence or absence of a recording material in a conveyance passage toward a transferring portion in a non-image forming operation before the recording material is nipped and conveyed at the transferring portion.

Disclosed is a charging member comprising an electro-conductive support; and an elastic layer, the elastic layer is a surface layer thereof and is constituted by a mono layer, the elastic layer includes domains and a matrix, an outer surface of the charging member is constituted by a surface of the matrix and surfaces of the domains, and the charging member has concave portions on the outer surface, the domains exist at bottom portions of the respective concave portions and are exposed to the outer surface of the charging member only at the bottom portions of the respective concave portions, the elastic layer has a volume resistivity of 110.sup.5 cm or more and 110.sup.8 cm or less, and A2 is 20 times or more of A1, wherein A1 and A2 are current values obtained by a prescribed method.

A charging roller has a roller main body including a cylindrical inner layer having a porous structure formed of a semiconductive rubber composition constituted by ethylene propylene rubber, paraffin oil, and a carbon black having a DBP oil absorption amount of 400 cm.sup.3/100 g or more and an outer layer constituted by a nonporous and seamless semiconductive tube including a polyurethane thermoplastic elastomer having a type A durometer hardness of 93 or less, carbon black, and crosslinked PMMA particles having an average particle size of 5 to 20 ?m. The production method includes a process in which a semiconductive rubber composition forming the inner layer is foamed with OBSH.

An image forming apparatus includes an image bearing member, a charging member, a charging voltage source, an electrostatic image forming portion, a developing member, a developing voltage source, a transfer portion, a cleaning member, and a controller. The controller effects control so that a peak-to-peak voltage of an AC component of a charging voltage satisfying the following relationship is applied to the charging member to execute a toner supplying operation: 2Vth (V)Vpp1 (V)(2Vth+200) (V), where a discharge start voltage of a DC component of a charging voltage between the image bearing member and the charging member is Vth (V), and the peak-to-peak voltage of the AC component applied during execution of the toner supplying operation is Vpp1 (V). A peak-to-peak voltage of the AC component applied during image formation is larger than the peak-to-peak voltage Vpp1 (V).