Developing roller, electrophotographic process cartridge and electrophotographic image forming apparatus

1. A developing roller, comprising:

a substrate and an electro-conductive layer on the substrate;

an outer surface of the developing roller having an electrical insulating first region and an electro-conductive second region adjacent to the first region; and

the first region having a plurality of domains independent from each other, wherein

at least 50% of a total number of domains included in a square region 300 μm on each side placed on the outer surface of the developing roller have a circle equivalent diameter of 3 to 15 μm, and

at least one of the domains having a circle diameter of 3 to 15 μm has a plurality of grooves.

2. The developing roller according to claim 1 , wherein each of the grooves extends in a direction crossing a direction orthogonal to a circumferential direction of the developing roller with an angle.

3. The developing roller according to claim 2 , wherein a narrow angle of each of the grooves with respect to the circumferential direction of the developing roller is 0 to 45°.

4. The developing roller according to claim 1 , wherein a first surface of the electro-conductive layer on a side opposite to a side facing the substrate constitutes the second region.

5. The developing roller according to claim 1 , wherein the electro-conductive layer retains an electrical insulating part so as to be exposed on a side of the electro-conductive layer opposite to a side facing the substrate, and

an exposed portion of the electrical insulating part constitutes the first region.

6. The developing roller according to claim 1 , wherein at least one of the domains having a circle equivalent diameter of 3 to 15 μm has four or more grooves.

7. The developing roller according to claim 1 , wherein the grooves have an average pitch of 0.5 to 5.0 μm.

8. The developing roller according to claim 1 , wherein the grooves of each of the domains having a circle equivalent diameter of 3 to 15 μm have an average depth of 0.5 to 5.0 μm.

9. The developing roller according to claim 1 , wherein a percentage of an area of the domains included in the square region 300 μm on each side placed on the outer surface of the developing roller is 10 to 60%.

10. The developing roller according to claim 1 , wherein a potential damping time constant defined as a time required for damping a potential of the surface to V 0 ×(1/e) (V) is 60.0 seconds or more when a surface of the first region constituting the outer surface of the developing roller is charged to a potential Vo (V).

11. The developing roller according to claim 1 , wherein a potential damping time constant defined as a time required for damping a potential of the surface to V 0 ×(1/e) (V) is less than 6.0 seconds when a surface of the second region constituting the outer surface of the developing roller is charged to a potential Vo (V).

12. The developing roller according to claim 1 , wherein the electro-conductive layer has a stacked structure of two more layers and an electro-conductive elastic layer as a first electro-conductive layer has thereon, as a component constituting a surface layer as a second electro-conductive layer, the first region having an electrical insulating surface and the second region having an electro-conductive surface.

13. The developing roller according to claim 1 , wherein the first region has an uppermost surface having a height equal to or higher than a height of an uppermost surface of the second region.

14. An electrophotographic process cartridge detachably mounted on a main body of an electrophotographic image forming apparatus, comprising a developing roller;

the developing roller having a substrate and an electro-conductive layer on the substrate,

an outer region of the developing roller having an electrical insulating first region and an electro-conductive second region adjacent thereto; and

the first region having a plurality of domains independent from each other, wherein

at least 50% of a total number of domains included in a square region 300 μm on each side placed on the outer surface of the developing roller have a circle equivalent diameter of 3 to 15 μm, and

at least one of the domains having a circle diameter of 3 to 15 μm has a plurality of grooves.

15. An electrophotographic image forming apparatus, comprising an image carrier for carrying an electrostatic latent image, a charging device for primarily charging the image carrier, an exposure device for forming the electrostatic latent image on the primarily charged image carrier, a developing device for developing the electrostatic latent image with a toner to form a toner image and a transfer device for transferring the toner image to a transfer material,

the developing device having a developing roller, the developing roller having a substrate and an electro-conductive layer on the substrate;

an outer surface of the developing roller having an electrical insulating first region and an electro-conductive second region adjacent thereto; and

the first region having a plurality of domains independent from each other, wherein

at least 50% of a total number of domains included in a square region 300 μm on each side placed on the outer surface of the developing roller have a circle equivalent diameter of 3 to 15 μm, and

at least one of the domains having a circle diameter of 3 to 15 μm has a plurality of grooves.