Toner set, developer set, image forming apparatus, and image forming method

1. A toner set, which is composed of toners of a plurality of colors that are used in an image forming method for forming an image by forming a toner image using the toners of a plurality of colors and transferring the formed toner image onto a recording medium, wherein

each of the toners of a plurality of colors includes toner base particles that contain a binder resin and a colorant, as well as an external additive that is externally added to the toner base particles,

the external additive includes small-diameter silica particles and large-diameter silica particles larger than the small-diameter silica particles, and wherein

when, of the toners of a plurality of colors, three color toners to be used consecutively for forming the toner image are defined as a first toner, second toner and third toner respectively in order of usage, an average particle diameter of the large-diameter silica particles contained in the second toner is smaller than an average particle diameter of the large-diameter silica particles contained in the first toner and is larger than an average particle diameter of the large-diameter silica particles contained in the third toner.

2. The toner set according to claim 1 , wherein each of the toners satisfies the following expression (1):

30 ≦N 2 −N 1 ≦60  (1)

(where N 1 represents an average primary particle diameter (nm) of the small-diameter silica particles, and N 2 represents an average primary particle diameter (nm) of the large-diameter silica particles).

3. The toner set according to claim 1 , wherein each of the toner satisfies the following expression (2):

0.7 ≦C 1 R/N 1 ≦1  (2)

(where N 1 represents an average primary particle diameter (nm) of the small-diameter silica particles, R represents an average particle diameter (μm) of the toner base particles, and C 1 represents a mass of the small-diameter silica particles with respect to 100 parts by mass of the toner base particles).

4. The toner set according to claim 1 , wherein each of the toners satisfies the following expression (3):

0.05 ≦C 2 R/N 2 ≦0.15  (3)

(where N 2 represents an average primary particle diameter (nm) of the large-diameter silica particles, R represents an average particle diameter (μm) of the toner base particles, and C 2 represents a mass of the large-diameter silica particles with respect to 100 parts by mass of the toner base particles).

5. A developer set, which is composed of developers of a plurality of colors, wherein the developers of a plurality of colors have developer containing the first toner of the toner set described in claim 1 and carrier, developer containing the second toner of the toner set and carrier, and developer containing the third toner of the toner set and carrier.

6. An image forming apparatus, comprising:

a plurality of image bearing members that are arranged in a predetermined direction in order to form a toner image by means of toners of different colors onto surfaces of the plurality of image bearing members; and

a plurality of developing devices that are disposed facing the image bearing members respectively and supply the toners included in developers to the surfaces of the image bearing members, wherein

the developers of the developer set described in claim 5 are employed as the developers to be used in the plurality of developing devices.