The present invention relates to the timing of applying a developing bias voltage to a component of a xerography image forming apparatus, such as a copying machine, a facsimile device, or a printer, when it is stopped in an emergency.
A copying machine based on xerography repeats a copying process, to thereby successively copy an image on an original document onto a transfer paper. The copying process consists of charging the photoreceptor, exposing the photoreceptor to the document image, developing a latent electrostatic image formed on the photoreceptor, transferring the developed toner image onto a transfer paper, fixing the transferred image, cleaning the photoreceptor. When receiving an emergency stop signal during a copying operation, a conventional image forming apparatus of this type, disclosed in Published Unexamined Japanese Patent Application No. Hei. 4-67160, for example, operates in the following way. In a state when the image forming process is not yet completed, the charger and the photoreceptor are stopped, but the application of the developing bias voltage is continued to prevent toner from being scattered and toner from being mixed (in case of the color development), when not stopped immediately.
In the conventional copying system, in an emergency, it is desirable to quickly stop all the components and the bias power source as well. Immediately after the machine stops in an emergency, a user or a service man may make an access to the inside of the machine for fixing the machine. In this case, it is essential to protect him from electric shock caused by the application of the high developing bias voltage.
In a recent trend in this field, in view of the improvement of the picture quality, carriers used for the two-component developer are reduced in particle diameter or size, the gap between the photoreceptor and the developing roll is also reduced, and an AC bias voltage is applied to the developing unit. This small sized carrier is weak in magnetic holding force. When an intensive electric field is applied to the carrier, the carrier is easily moved to stick to the photoreceptor. If the machine is driven in a state that the carriers are sticking to the photoreceptor, the blade of the cleaning unit accumulatively collects the carrier. These carriers have an adverse influence on the subsequent steps of the image forming process. For example, the carrier may scratch the photoreceptor or enter the transfer unit, possibly causing a leak of the carrier.