PURPOSE: To eliminate a tendency to disturb a toner image formed in the previous stage and generate a carrier stick when the toner image of plural colors is formed on a photosensitive body and noncontact development is attained by using a two-component developer.
CONSTITUTION: A unidirectional pulse bias having the same polarity as that of toner is applied to the developing sleeve 2 of a developing machine after the second color. Developing bias conditions are set in a range where the relation of unidirectional pulse bias frequency Fp with a pulse average voltage Y satisfies the following equations (1) and (2), to attain development. Y≤1√ Fp+b, (a=14.0, b=177)...(1). Y≥c√ Fp+d, (c=14.0, d=7.0)...(2). Thus, sharp color printing without irregularities in the toner image and fogging on a background part can be executed.
SHIMAZAKI YUZURU
OKADA HISAO
TOKUYASU NOBORU
SUGAYA TOMIO
HITACHI KOKI KK