1. Field of the Invention
The present invention relates to an image formation device.
2. Description of the Related Art
An electrophotographic image formation device according to the prior art capable of operating in a double side image recording mode, a toner image is transferred to a second side of a sheet after another toner image transferred onto a first side of the sheet is fixed by heating. The sheet is dried as its moisture is evaporated by the image fixation by heat, and the drying shrinks the sheet.
Therefore, when images of the same size have been transferred on both sides of the sheet, the image on the first side becomes smaller than the image on the second side, because the latter is formed in a state in which the former formed on the first side of the sheet has shrunken.
In this case, relative to the original image, the magnification of the image formed on the first side of the sheet and that of the image formed on the second side become different.
The electrophotographic image formation device according to the prior art may be required to split one image into, for instance, two parts and, after recording the split parts of the image on a first side each of two sheets, to stick together the two sheets. In this case, borderlines to mark pasting margins or cut lines to mark margins to be removed may be recorded (or printed) in advance on both sides of the two sheets, and such lines can be aligned when the two sheets are stuck together. In this process, the sheet size may be different between the step of recording (or printing) a partial image on the first side of the sheet and the step of recording (or printing) border lines of the pasting margins (another image) on the second side of the sheet, because the sheet may have shrunken by the second step. In this case, the image on the first side of the sheet and the image (border lines of the pasting margins) on the second side may be misaligned, resulting in an inconsistent image on the first side when the two sheets are put together.
Especially in an electrophotographic image formation device in which sheets on each of which a toner image is transferred and fixed by heating, the sheet before recording on a first side may be relatively expanded by the moisture it contains, but this moisture would evaporate and dry up when the toner image formed on the first side is fixed by heating, and the sheet would shrink, making it liable for the split images to be misaligned more.
A number of techniques to equalize the magnifications of images to be formed on first and second images of a sheet are already known.
For instance, the known techniques include those disclosed in the following references.
(1) Technique described in Patent Reference 1 (Japanese Published Unexamined Patent Application No. 2002-72771)
According to the technique described in this Patent Reference 1, a sheet before and after image fixation by heating is manually set on a platen glass, and the sheet size is measured by a document reader before and after the image fixation by heating. The shrinking rate of the sheet is figured out from the resultant measurements, and the magnification of the image to be formed on the second side is adjusted on that basis.
(2) Technique described in Patent Reference 2 (Japanese Published Unexamined Patent Application No. Hei 4-288560)
According to the technique described in this Patent Reference 2, the vertical and horizontal dimensions of a copying sheet are detected by an optical sensor in a position immediately upstream from the copying position on the sheet-carrying route, and are then detected again by a similar sensor in a position immediately downstream from a thermal fixing device. The vertical dimension of the copying sheet is controlled by figuring out the vertical shrinking rate of the copying sheet from the length of time the copying sheet takes to pass the optical sensors 33 and 35, and switching over the operating speed of the optical system on the basis of the shrinking rate that has been figured out.
(3) Technique described in Patent Reference 3 (Japanese Published Unexamined Patent Application No. Hei 10-149057)
According to the technique described in this Patent Reference 3, the vertical shrinking rate of a first copying sheet is figured out by detecting the vertical dimensions of the first copying sheet before and after fixation from the length of time the copying sheet takes to pass an optical sensor arranged immediately upstream from the toner image transferring position on the sheet carrying route, and control is effected by and switching over the operating speed of the optical system for second and subsequent sheets on the basis of the shrinking rate of the first copying sheet.
[Patent Reference 1] Japanese Published Unexamined Patent Application No. 2002-72771 (lines 11 to 14, paragraph 15 of the pertinent Gazette)
[Patent Reference 2] Japanese Published Unexamined Patent Application No. Hei 4-288560 (paragraphs [0028] and [0030] of the pertinent Gazette)
[Patent Reference 3] Japanese Published Unexamined Patent Application No. Hei 10-149057 (paragraph [0018] of the pertinent Gazette)
(Problem with technique described in Patent Reference 1 (Japanese Published Unexamined Patent Application No. 2002-72771))
The technique described in Patent Reference 1 involves a problem of the extra trouble taken to measure the sheet size because the sheet has to be manually set on the platen glass 22 before and after the image fixation by heating. (Problem with technique described in Patent Reference 2 (Japanese Published Unexamined Patent Application No. Hei 4-288560))
According to the technique described in Patent Reference 2, because the two different optical sensors 33 and 35 are used for detecting the copying sheet size before and after the image fixation by heating, the errors of individual optical sensors in detecting the copying sheet size are added, resulting in a problem that the accuracy of copying sheet size detection deteriorates. Moreover, as the vertical dimension of the copying sheet is detected according to the length time the sheet takes to pass the optical sensors 33 and 35, if the carrying speed of the copying sheet varies, the accuracy of copying sheet size detection will also deteriorate.
(Problem with Technique Described in Patent Reference 3 (Japanese Published Unexamined Patent Application No. Hei 10-149057))
The technique described in Patent Reference 3 involves a problem that a variation in sheet carrying speed would result in a variation in the detected sheet size.