Hybrid development type electrostatographic reproduction machine having a wrong-sign toner purging mode

In an electrophotographic reproduction machine including an image bearing member, latent image devices for forming on the image bearing member electrostatic latent images having a first polarity, and a hybrid development unit for applying to the latent images correct-sign toner particles having a second polarity relatively opposite to the first polarity, a method of automatically purging from the hybrid development unit wrong-sign toner particles having a third polarity relatively the same as the first polarity, the purging method comprising the steps of:

• (a) counting and accumulating a number of image forming events performed by the reproduction machine for comparing to a stored predetermined number of such events;
• (b) changing relative electrical bias values for a magnetic roll and for a donor roll within a housing of the development unit from imaging bias values to wrong-sign toner purging bias values so as to cause wrong-sign toner particles to transfer from the magnetic roll to the donor roll;
• (c) producing an electrostatic field on the image bearing member for causing wrong-sign toner particles to transfer from the donor roll to the image bearing member;
• and
• (d) operating a cleaning device in contact with the image bearing member for removing wrong-sign toner particles from the image bearing member.

In an electrophotographic reproduction machine including an image bearing member, latent image devices for forming on the image bearing member electrostatic latent images having a first polarity, and a hybrid development unit for applying to the latent images correct-sign toner particles having a second polarity relatively opposite to the first polarity, purging apparatus for automatically purging from the hybrid development unit wrong-sign toner particles having a third polarity relatively the same as the first polarity, the purging apparatus comprising:

• (a) first biasing means including a first DC power supply for biasing a magnetic roll mounted within a housing of the hybrid development unit;
• (b) second biasing means including a second DC power supply for biasing a donor roll mounted, and forming a toner transfer nip with the magnetic roll, within the housing of the hybrid development unit;
• (c) a controller having an imaging mode and a purging mode, said controller being connected to said first and said second biasing means for selectively changing biasing values of each of said first and said second DC power supplies so as to control charged toner flow;
• (d) means for producing an electrostatic field having a wrong-sign toner attracting polarity on the image bearing member;
• and
• (e) a cleaning device mounted against said image bearing member for removing wrong-sign toner particles from the image bearing member.