Image-forming apparatus generating electrical discharge upstream of transfer portion

An image-forming apparatus comprising:

• a rotatable photosensitive drum;
• a developer composed of toner particles having protrusion portions formed on surfaces of the toner particles and transfer promoting particles as an external additive adhered to surfaces of the protrusion portions of the toner particles, wherein the protrusion particles are not formed of the transfer promoting particles as the external additive;
• a developing unit storing the developer and including a rotatable development roller, wherein the development roller is configured to
• (i) bear the developer,
• (ii) come into contact with the photosensitive drum and form a developing portion, and
• (iii) supply the developer to a surface of the photosensitive drum in the developing portion;
• an intermediate transfer belt configured to come into contact with the photosensitive drum and form a transfer portion;
• a primary transfer voltage power supply configured to apply a transfer voltage to the intermediate transfer belt to supply a transfer current from the intermediate transfer belt to the photosensitive drum in the transfer portion;
• and
• a controller configured to control the primary transfer voltage power supply, to control rotation of the photosensitive drum, to control rotation of the development roller, and to control contact and separation between the photosensitive drum and the development roller,
• wherein the transfer promoting particles borne on a surface of the development roller can be supplied to the surface of the photosensitive drum in the developing portion while the photosensitive drum rotates,
• wherein, in a state where the toner particles of the developer are interposed between the development roller and the photosensitive drum, when F denotes pressing force for pressing the development roller against the photosensitive drum, and N denotes a total number of the transfer promoting particles interposed between the toner particles and the photosensitive drum, adhesion strength Ft formed between a transfer promoting particle of the transfer promoting particles and the photosensitive drum measured by pressing the transfer promoting particle against the toner particles at a pressing force per unit transfer promoting particle F/N and adhesion strength Fdr formed between the transfer promoting particle and the photosensitive drum measured by pressing the transfer promoting particle against the photosensitive drum at the pressing force per unit transfer promoting particle F/N, satisfy Ft<Fdr,
• wherein, where a closest distance between adjacent two of the protrusion portions is a protrusion interval G, an average protrusion interval of the protrusion interval G is less than or equal to an average particle diameter of the transfer promoting particles,
• wherein, to supply some of the transfer promoting particles to the photosensitive drum from the development roller, the controller is configured to perform control to bring the photosensitive drum and the development roller into contact while the photosensitive drum and the development roller both rotate and while a surface potential of the photosensitive drum is at a non-image-forming voltage, and
• wherein the controller controls the primary transfer voltage power supply to generate an electrical discharge between the photosensitive drum and the intermediate transfer belt in a movement direction of a surface of the intermediate transfer belt upstream of an upstream end portion of the transfer portion and controls a potential difference between the photosensitive drum and the intermediate transfer belt in the transfer portion to be lower than a Paschen discharge threshold.