Color image forming apparatus and control method thereof

A color image forming apparatus, comprising:

• a plurality of image forming devices placed serially in the direction of conveyance of recording sheets;
• a transfer support which is arranged in abutment with image supports provided in the image forming devices and is separable therefrom, for transferring images from the image supports to a recording sheet;
• a mode switching means for making a changeover between a monochrome image forming mode for forming images by abutting the transfer support with only an image support of one image forming device of the plural image forming devices and a multi-color image forming mode for forming images by abutting the transfer support with the image supports of the plural image forming devices;
• and
• a control means for governing the image forming devices, the image supports, the transfer support and the mode switching means, the control means making a control so that the transfer support can abut only the image forming devices which are actually engaged in image forming, among the plural image forming devices,
• wherein when the images are formed in the monochrome image forming mode, the transfer support abuts only the image support of the most upstream image forming device in the direction of the conveyance of the recording sheet,
• wherein the mode switching means starts the changeover from the monochrome image forming mode to the multi-color image forming mode while the recording sheet remains on the transfer support, and
• wherein the transfer support conveys the recording sheet at a speed higher than the speed of conveyance of the recording sheet at the image forming device used in the monochrome image forming mode, during the period from when the rear end of the recording sheet has passed through the image forming device used for the monocrome image forming mode until it passes through at least the most upstream image forming device among those used in the multi-color mode.

A color image forming apparatus comprising:

• a plurality of image forming devices placed serially in the direction of conveyance of recording sheets;
• a transfer support which is arranged in abutment with image supports provided in the image forming devices and is separable therefrom, for transferring images from the image supports to a recording sheet;
• a mode switching means for making a changeover between a monochrome image forming mode for forming images using only one image forming device of the plural image forming devices and a multi-color image forming mode for forming images using the plural image forming devices;
• and
• a control means for governing the image forming devices, the image supports, the transfer support and the mode switching means, the control means making a control so that the transfer support can abut only the image forming devices which are actually engaged in image forming, among the plural image forming devices,
• wherein the mode switching means starts the changeover from the monochrome image forming mode to the multi-color image forming mode while the recording sheet remains on the transfer support,
• wherein the mode switching means starts the mode change immediately after the image has been transferred from the image support of the most upstream image forming device to the recording sheet, and
• wherein, upon a mode change from the monochrome image forming mode to the multi-color image forming mode, the speed of conveyance of the recording sheet on the transfer support is increased to be greater than the speed of conveyance of the recording sheet passing at the image forming device used in the monochrome image forming mode, and during this period, the image forming devices for the monochrome image forming mode and the multi-color image forming mode, abutting the transfer support, are rotated at a peripheral speed equal to or greater than the speed at which the recording sheet is conveyed by the transfer support.

A color image forming apparatus, comprising:

• a plurality of image forming devices placed serially in the direction of conveyance of recording sheets;
• a transfer support which is arranged in abutment with image supports provided in the image forming devices and is separable therefrom, for transferring images from the image supports to a recording sheet;
• a mode switching means for making a changeover between a monochrome image forming mode for forming images using only one image forming device of the plural image forming devices and a multi-color image forming mode for forming images using the plural image forming devices;
• and
• a control means for governing the image forming devices, the image supports, the transfer support and the mode switching means, the control means making a control so that the transfer support can abut only the image forming devices which are actually engaged in the image forming, among the plural image forming devices,
• wherein the mode switching means starts the changeover from the monochrome image forming mode to the mult-color image forming mode while the recording sheet remains on the transfer support,
• wherein the mode switching means starts the mode change immediately after the image has been transferred from the image support of the most upstream image forming device to the recording sheet, and
• wherein the rotational speed of the plural image forming devices is kept to be equal to or greater than the speed of conveyance of the recording sheet on the transfer support until the leading end of the recording sheet is conveyed to the fixing mechanism which is located downstream of the transfer support.

A control method of a color image forming apparatus which comprises:

• a plurality of image forming devices placed serially in the direction of conveyance of recording sheets;
• and a transfer support arranged in abutment with each image support provided in each image forming device, in a separable manner therefrom, and is controlled so that the transfer support is separated from the image supports when a plurality of image forming devices are not used for image forming, wherein an operating mode can be changed over between a monochrome image forming mode for forming images using only one image forming device of the plural image forming devices and a multi-color image forming mode for forming images using the plurality of image forming devices,
• wherein the mode change from the monochrome image forming mode to the multi-color image forming mode is started while the recording sheet remains on the transfer support,
• wherein the mode switching means starts the mode change immediately after the image has been transferred from the image support of the most upstream image forming device to the recording sheet, and
• wherein the transfer support conveys the recording sheet at a speed higher than the speed of conveyance of the recording sheet at the image forming device used in the monochrome image forming mode, during the period from when the rear end of the recording sheet has passed through the image forming device used for the monochrome image forming mode until it passes through at least the most upstream image forming device among those used in the multi-color mode.