1. Field of the Invention
The present invention relates to an image printing apparatus such as a copying machine, a printer, a facsimile apparatus, and a multifunction peripheral of a copying machine, printer, and facsimile apparatus and, more particularly, to an image printing apparatus and image printing method which can register the positions of images to be printed on the front and back surfaces of a sheet highly accurately.
2. Description of Related Art
An electrophotographic image printing apparatus comprises a photosensitive member, an image write unit, developing portion, feeder, transfer portion, fixing unit, and the like, and can print images on the two surfaces of a sheet.
When printing images on the two surfaces of the sheet, the images on the two surfaces must be registered accurately. This is to prevent the following problems. For example, when a bundle of sheets P are cut or bound, if images printed on the front and back surfaces are misregistered, a blank may be left depending on the images, or the images may partly lack after cutting.
Conventionally, a mark is printed on the front surface of the sheet P, and the position of the mark is detected to correct the image-printing position on the back surface (for example, see Japanese Unexamined Patent Publication No. 10-319674 (patent reference 1)).
In an image printing apparatus which employs thermal fixing, when an image is printed and fixed on the front surface, the sheet P after fixing shrinks to shrink the image simultaneously. If an image is printed and fixed on the back surface in the same manner, the positions of the images printed on the front and back surfaces are misregistered. The method described in patent reference 1 is aimed at correcting the skew of the sheet P caused by a convey error or the like, and shrinkage of the image caused by the shrinkage of the sheet P due to the fixing process is not taken into account. Although the positions of the distal ends of the images on the front and back surfaces may be registered, it is impossible to set the positions and sizes of the images on the front and back surfaces to coincide with each other highly accurately.
Hence, in the image printing apparatus which employs thermal fixing, as the sheet P shrinks as described above, to obtain images on the front and back surfaces that coincide with each other, the position and size of the image to be printed on the back surface must be corrected.
Attempts have been made to print an image on the back surface considering the shrinkage of the sheet caused by the fixing process described above (for example, see Japanese Unexamined Patent Publication No. 2003-156974 (patent reference 2)). In the image printing apparatus described in patent reference 2, marks are printed at four corners on the front surface of a sheet P or at two portions in a direction (to be referred to as the “main scanning direction” hereinafter) perpendicular to the convey direction (to be referred to as the “sub-scanning direction” hereinafter) of the sheet P. The distances between the marks or the like before and after fixing the image on the front surface are obtained. The position and size of the image on the back surface are determined on the basis of the distances or the like.
With the image printing apparatus described in the above patent reference 2, however, the marks are printed at the four corners of the sheet P or at the two portions in the main scanning direction, and the marks serve as cutting marks used as marks in cutting the sheet P, or as color misregistration correction marks used in correction of color misregistration of the images. Thus, the following problems arise.
Since the marks are printed at the four corners of the sheet P or at the two portions in the main scanning direction, a large sensor detection range must be set, or a plurality of sensors must be provided. In order to arrange a one-dimensional line sensor in the main scanning direction to obtain the shrinkage factor in the convey direction of the sheet P, the entire sheet P must pass through the one-dimensional line sensors so the one-dimensional line sensor detects all the marks printed at the four corners or the like. Then, however, the shrinkage factor cannot be obtained until the sheet P has passed through the one-dimensional line sensor, and mark detection is delayed. To feedback the detection result of the one-dimensional line sensor to back surface image printing, the shrinkage factor must be obtained since the marks are detected until back surface image printing to correct the position and size of the image on the back surface. Because mark detection is delayed, the image printing section and the mark detection position, i.e., the position to set the one-dimensional line sensor, must be spaced apart from each other. In this manner, with the prior art, the position to set the sensor is limited.
To detect and recognize a cutting mark or color misregistration correction mark with the line sensor, the line sensor must read the mark quickly and frequently. A large-capacity memory is also necessary to store data read by the line sensor.