1. Field of the Invention
The present invention relates to an image forming apparatus such as a xerographic type of copying machine or printer and, more specifically, to an image forming apparatus which has transporting means for transporting a recording sheet between a transfer section for transferring a toner image to the recording sheet and a fixing section for fixing the toner image transferred to the recording sheet and is capable of changing the recording-sheet transporting speed of the transporting means.
2. Description of the Related Art
A conventional type of image forming apparatus such as a xerographic type of copying machine or printer is constructed to transfer a toner image formed on a photoconductor drum to a recording sheet by means of a transfer section, transport the recording sheet to which the toner image was transferred to a fixing section by means of transporting means, and fuse and fix the toner image to the recording sheet by heat and pressure by means of a fixing unit provided in the fixing section, thereby forming an image.
In the fixing unit, since a toner image is fused and fixed to a recording sheet by heat and pressure, it is necessary to set its fixing speed according to a fixing characteristic due to the thickness of the recording sheet or the like or a fixing characteristic due to the kind of toner image. Specifically, in the case of a recording sheet or toner image which is difficult to fix, it is necessary to set the fixing speed to a slow speed, while in the case of a recording sheet or toner image which is easy to fix, it is necessary to set the fixing speed to a fast speed. In this respect, the fixing unit differs from the transfer section which electrostatically transfer a toner image formed on the photoconductor drum, at an approximately constant transfer speed. For this reason, in general, the time required for fixing in the fixing unit, i.e., the recording-sheet transporting speed (fixing speed) in the fixing unit, differs from the recording-sheet transporting speed (transfer speed) in the transfer section. Accordingly, when a recording sheet is to be transported from the transfer section to the fixing section, it becomes necessary to adjust the transporting speed of the recording sheet.
To adjust the transporting speed of the recording sheet, it is more desirable that the transporting path from the transfer section to the fixing section be made longer, because a speed difference between the transfer speed and the fixing speed is easier to be absorbed and a sufficient fixing time can be ensured.
On the other hand, to reduce the size of the image forming apparatus, it is more desirable to make the recording-sheet transporting path shorter.
An art for reducing the size of the apparatus while maintaining optimum fixing speed conditions is disclosed in, for example, Japanese Patent Laid-Open No. 171277/1997. This specification proposes a recording medium transporting system for a xerographic apparatus which defines the relationships between a distance l.sub.1 from a transfer position to the entrance-side axis of a transfer belt, a distance l.sub.2 from the entrance side to the exit side of the transfer belt, a distance l.sub.3 from the exit side of the transfer belt to a speed change point, a distance l.sub.4 from the speed change point to a fixing roller, a maximum length A.sub.max of a usable recording sheet, a minimum length A.sub.min of a usable recording sheet, a transfer speed V.sub.1 and a fixing speed V.sub.2, thereby making it possible to minimize the size of a transporting system which leads to a fixing section from the toner image transfer position, particularly, the length of the transporting belt or the distance to a nip position of the fixing roller.
However, this proposal has the problem that the size, transfer speed, fixing speed and transfer position of a recording medium to be used as well as the length, fixing position and the like of a transporting unit impose restrictions on one another, thereby hindering free layout of individual units.