1. Technical Field
The present invention relates to an image forming apparatus, such as a printer, a copying machine or a facsimile, an image forming method and a storage medium storing a program for forming an image, and particularly to an image processing in an image forming apparatus, an image forming method and a storage medium storing a program for forming an image.
2. Related Art
In recent years, in an image forming apparatus such as a printer or a copying machine, an intermediate transfer member is often used in a transfer part. As the intermediate transfer member, a belt formed into an endless shape is generally used. Besides, transfer from a photoconductor to an intermediate transfer member is referred to as first transfer, and transfer from the intermediate transfer member to a sheet is referred to as second transfer. As a mechanism used for the second transfer, for example, a bias transfer roll (BTR) system and a bias transfer belt (BTB) system are known.
FIGS. 15A and 15B are views for illustrating a second transfer mechanism, in which FIG. 15A shows the BTR system, and FIG. 15B shows the BTB system. As shown in FIG. 15A, the BTR system is such a mechanism that an intermediate transfer member 201 is supported by a backup roll 202 from its inside, a second transfer roll 203 is brought into contact with the opposite surface, a voltage is applied therebetween, and transfer is performed. On the other hand, as shown in FIG. 15B, the BTB system is such a mechanism that instead of the opposite roll, a second transfer belt 213 made of an elastic endless belt is provided, and transfer is performed, and a poor peeling property for thin paper, which is a defect of the BTR system, is improved by causing the paper to be attracted to the belt.
The intermediate transfer member system as stated above can improve productivity and can expand the versatility of a transfer medium, and is the main stream in the transfer system of a high quality printer. However, in the image forming apparatus using the intermediate transfer member, there are plural factors to degrade the quality of an image.
For example, in the first transfer, according to a stable rotation position of an intermediate transfer belt stretched over plural rolls, the rotation direction of a photoconductor and the traveling direction of the intermediate transfer belt do not coincide with each other, and as shown in FIG. 16A, an intermediate transfer belt 16 sometimes skews with respect to photoconductor drums 152K, 152Y, 152M and 152C. As a result, as shown in FIG. 15B, an image transferred on the belt is sometimes distorted into a parallelogram. Besides, in the second transfer, as shown in FIG. 17, the stable position of the belt is changed by the entry of a sheet and the skewing of the belt sometimes occurs similarly during the transfer. Besides, since the transfer is performed while the sheet itself is rotated, an image is sometimes distorted as shown in FIG. 18.
The distortion of the image as stated above has been conventionally dealt with by providing a mechanical adjustment mechanism. For example, with respect to the skewing of the intermediate transfer belt, a mechanism is provided by which the tension of the roll over which the belt is stretched can be changed between one end and the other end in the lateral width direction of the belt, and the stable position of the belt can be changed. Besides, with respect to the rotation of the sheet at the time of second transfer, an adjustment mechanism of nip pressure is provided in the BTR system, and an adjustment mechanism between shafts on which the second transfer belt is stretched is provided in the BTB system. Both of these adjustment mechanisms require a considerably large space for mounting, and the cost is also relatively expensive.
On the other hand, in the fixing unit, heat is applied, and at the same time, force to draw the sheet is applied so far. That is, in the fixing unit, the roll is formed so that the diameter in both ends is slightly larger than that in the center, and the force to expand the sheet outward is applied. This force serves to smooth the wrinkles of the sheet. The method as stated above is a technique generally used as a countermeasure against paper wrinkles. However, by this, the sheet is slightly deformed as shown in FIG. 19, and an image on the sheet is also deformed as shown in the figure. Besides, to be exact, with respect to the influence of heat given on the sheet, the quantity of the influence is different between the leading end of the sheet and the trailing end, and as shown in FIG. 20, the width of the sheet sometimes becomes different between the leading end and the trailing end depending on moisture content distribution. Besides, by the influence of the eccentricity of the roll, as shown in FIGS. 21A and 21B, the sheet sometimes cockles.
With respect to the deformation caused in the fixing unit, in view of a trade-off with other basic performances, a positive correction has not been preformed so far.
The defect of the image as described above degrades the quality of the image, and particularly influences the registration performance in printing.