1.  Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus, and more particularly to an image forming apparatus which transfers and fixes a visible image on a recording medium such as paper. 
2.  Discussion of the Background
Conventionally, it has been known that an image forming apparatus such as a copier, a facsimile and a printer forms an image on a recording medium, for example, a recording sheet in a following manner. 
The visible image, for example, a toner image carried on an image carrier such as a photosensitive drum and an intermediate transfer medium is electrostatically transferred to the recording medium by the effect of the transfer electric field. 
Subsequently, the recording sheet is nipped by a fixing nip formed by abutting a heating roller and a pressure roller, for example. 
Accordingly, at least one of the abutting members is heated so that the temperature thereof is relatively high.  The heat and the nip pressure of the abutting members act on the surface of the recording sheet nipped by the fixing nip.  Accordingly, the visible image is fixed thereon. 
With an advancement of a high-quality image in recent years, an enhancement of gloss on the visible image on the recording sheet has been desired. 
Consequently, the fixing temperature tends to be high so that image forming materials are sufficiently fused, and the gloss of the visible image is enhanced. 
In order to realize the fixing process at a high temperature, a fixing power source with a large output capacity is used.  Consequently, an increase in an energy consumption and cost is most likely to occur. 
Furthermore, the recording sheet is overheated at the fixing nip causing recording sheets to easily stick with one another at a sheet stacking portion. 
Conventionally, an image forming apparatus which substantially heats a visible image prior to fixing the visible image on the recording sheet has been proposed. 
In the related art image forming apparatus, the transfer-fixing roller abuts against a front surface of the intermediate transfer belt for transferring a toner image or a visible image formed on the photosensitive drum.  Accordingly a secondary transfer nip is formed. 
Furthermore, a transfer-fixing nip is formed by abutting a pressure roller against the secondary transfer nip.  After the toner image primarily transferred from the photosensitive drum to the intermediate transfer belt is secondarily transferred to the surface of the transfer-fixing roller, the toner image is transported to the transfer-fixing nip in conjunction with rotation of the transfer-fixing roller. 
At this time, a heater in the transfer-fixing roller or a heater disposed opposite to the transfer-fixing roller or the like substantially heats the toner image.  In synchronization with the toner image on the transfer-fixing roller, the toner image is transferred and fixed on the recording sheet nipped by the transfer-fixing nip. 
According to this configuration, the toner is heated separately from the recording sheet, and subsequently adhered to the recording sheet. 
Thus, it may be able to prevent the recording sheet from being excessively heated and from sticking to a stack of recording sheets at the sheet stacking portion. 
Furthermore, when the heater is disposed across from the transfer-fixing roller, it is possible to suppress a heat conduction to the transfer-fixing roller so that an energy consumption is reduced when compared with installation of the heater in the transfer-fixing roller. 
However, according to the image forming apparatus configured in the above manner, when the temperature of the surface of the transfer-fixing roller is increased by the heater, and the surface thereof comes into contact with the intermediate transfer belt at the transfer nip described above, the intermediate transfer belt is heated by a small amount.  Consequently, it is possible to promote deterioration of the intermediate transfer belt. 
Another related art image forming apparatus using a transfer-fixing belt stretchedly arranged between a plurality of rollers instead of the transfer-fixing roller and endlessly travels has been proposed. 
A heater disposed across from the transfer-fixing belt heats a toner image on the belt. 
According to this configuration, after the belt surface heated by the heater advances to the transfer-fixing nip in order to transfer or fix the toner image on the recording sheet, it is possible to cool down the belt surface while traveling to the transfer nip. 
Accordingly, it is possible to suppress deterioration of the intermediate transfer belt due to heat. 
However, according to experiments performed by inventors of the present invention, in the image forming apparatus configured in the above-described manner, a disturbance or a hollow portion may easily be generated in the toner image when secondarily transferring the toner image from the intermediate transfer belt to the transfer-fixing belt.  This phenomena is hereinafter referred to as a hollow defect. 
In the image forming apparatus, the pressure roller in contact with the rear surface of the transfer-fixing belt presses the transfer-fixing belt against intermediate transfer belt so as to form a secondary transfer nip. 
Conventionally, it has been known that when the pressure at the transfer nip, for example, a secondary transfer nip (hereinafter referred to as a fixing nip pressure) is excessively high, the hollow defect is most likely be generated in the toner image. 
According to the above-described related art image forming apparatus, when the recording sheet is nipped by the transfer-fixing nip separately provided from the secondary transfer nip, a rapid stress is applied to the transfer-fixing belt, and thus a speed of the transfer-fixing belt slows down for a brief moment. 
When the recording sheet is ejected from the transfer-fixing nip, the speed of the transfer-fixing belt increases for a brief moment due to the rapid decrease in the stress. 
Consequently, when the speed of the transfer-fixing belt fluctuates, a tension of the transfer-fixing belt in the proximity of the secondary transfer nip temporarily fluctuates by a large amount. 
As a result, with this configuration in which the transfer-fixing belt is stretchedly arranged in a proximity of the secondary transfer nip in a manner as illustrated in FIG.  1, for example, the belt tension in directions shown by arrows A or B temporarily fluctuates by a large amount due to the rapid fluctuation of the speed of the transfer-fixing belt 21. 
When the tension in the directions A and B is loosened, the pressure of a pressure roller 24 in a direction shown by an arrow C may increase.  Consequently, a nip pressure formed by abutting an intermediate transfer belt 11 serving as an image carrier and the transfer-fixing belt 21 increases. 
As a result, the hollow defect is induced in the toner image.