1. Field of the Invention
The present invention relates to image forming apparatuses, such as copying machines, facsimile machines, printers, employing a digital copying method, and more particularly, to a full-color image forming apparatus including an intermediate transfer member.
2. Description of the Related Art
In a conventional image forming apparatus, a toner image formed on an image carrier (hereinafter, “photosensitive member”) is transferred to a transfer medium to form a toner image. A fixing device fixes the toner image on the transfer medium by subjecting it to pressure and heat. In a full color image forming apparatus such as the ones disclosed in Japanese Patent No. 2579137 and Japanese Patent Application Laid-Open Publication No. H08-292655, the transfer medium is borne by a transfer drum serving as a transfer member. Each of the toner images, namely, yellow, magnet, cyan, and black, is transferred sequentially to the transfer medium while being borne by the transfer member. The transfer medium is then lifted off the transfer member and conveyed to the fixing device for fixing.
Alternatively, instead of transferring the toner images to the transfer medium borne by the transfer member, in the full color image forming apparatus disclosed in Japanese Patent Application Laid-Open Publication No. H5-11562, the four toner images may be superposed on an intermediate transfer member, transferred to the transfer medium all at once, and fixed by the fixing device. In the full color image forming apparatus provided with the intermediate transfer member disclosed in Japanese Patent Application Laid-Open Publication No. 2002-229344, as many photosensitive members are arranged in a series as there are toner colors, thereby enabling transfer of all the toner images within the time the intermediate transfer member completes one turn and realizing increased printing speed.
The image forming apparatus provided with the intermediate transfer member obviates the need for the transfer member for bearing the transfer medium. Consequently, a variety of transfer mediums ranging from thin paper sheets (40 g/m2) to thick paper sheets (200 g/m2), to postcards and envelopes can be used. An intermediate transfer drum or an intermediate transfer belt is used as the intermediate transfer member.
Incidentally, even though the full color image forming apparatus provided with the intermediate transfer member allows usage of a wide variety of transfer mediums, the difference in the thickness of the transfer medium leads to a difference in the fixing capacity due to a difference in the amount of heat. This is particularly pronounced in the full color image forming apparatus, which produces full color images by superposing a plurality of toner images. Specifically, if a thin paper sheet is conveyed to the fixing device under a normal process speed, the toner fixing is carried out normally. However, if a thick paper sheet is conveyed to the fixing device under the same conditions, inadequate cohesion of toner results, resulting in the adhesion of the toner to the fixing roller. The toner from the fixing roller is transferred back to transfer medium, resulting in a so-called fixing offset. The lack of close adhesion of the toner to the transfer medium leads to toner peeling. Thus, when two or more toner images are superposed, the desired color is not obtained due to inadequate toner cohesion.
To remedy the problem, an image forming apparatus such as the one disclosed in Japanese Patent Application Laid-Open Publication No. 2002-116643 is proposed in which fixing is carried out at half the normal process speed (hereinafter, “half-speed”) for the transfer medium with a high amount of heat such as, thick paper sheets, OHP sheets, etc. However, the following problems are encountered in the full color image forming apparatus provided with the intermediate transfer member and a mechanism for adjusting the process speed according to the type of transfer medium.
In the image forming apparatus in which a secondary transfer bias value is determined by finding a resistance of a secondary transfer means in a non-image portion, the secondary transfer bias value to be applied at half-speed is estimated based on the resistance determined at normal speed. Consequently, the transferability is adversely affected if the estimated value is off the actual optimum value.
Specifically, even if the resistances were the same at normal speed, at half-speed the resistance values differed.
Upon investigation, the reasons for this difference were found to be as follows:                (1) the resistance of the secondary transfer section is the combined resistances of the intermediate transfer member and the intermediate transfer means, etc. which form the secondary transfer section; and        (2) the ease with which the electric current flows under different processing conditions differs according to the members through which it flows.        