1. Field of the Invention
The present invention relates to a printing apparatus, and more particularly to a printing apparatus for forming images by an electrophotographic process such as a copier or printer.
2. Description of the Related Art
A printing apparatus that makes prints on both sides of a recording medium has been commercialized conventionally from a resource/environmental protection viewpoint. The double-sided printing is implemented using a paper reverse mechanism for inverting a paper after printing its first side, and a paper refeed mechanism for feeding the paper again.
In such a type of printing apparatus, a contrivance is made to effectively carry out double-sided printing by determining the number of sheets of paper waiting on the passage of the paper reverse mechanism and paper refeed mechanism in accordance with the paper size, and by exchanging the printing sequence (refer to Japanese Patent Application Laid-open No. 2002-091102, for example). When the number of sheets of the double-sided printing is large, the printing sequence is changed such that the number of sheets of paper waiting on the passage of the paper reverse mechanism and paper refeed mechanism becomes maximum depending on the paper size. The change of the printing sequence is made by storing print information on a plurality of pages received from a PC (personal computer) into a memory of a printer, and by changing sequences of the pages.
However, when the printer has a small memory capacity, it can store only a small number of pages of the print information, thereby being unable to change page sequence. Accordingly, the small capacity of memory forces to apply a method of printing the first side, followed by reversing the sheet of paper to refeed it and printing the second side corresponding to the back of the sheet, and to repeat the method to conduct the double-sided printing of a plurality of sheets of paper.
In other words, the method keeps only a sheet of paper waiting on the passage of the paper reverse mechanism and paper refeed mechanism. In addition, regardless of the memory size, to carry out the double-sided printing of a sheet of paper, the method is applied of printing the first side, followed by reversing the sheet of paper to refeed it and printing the second side corresponding to the back of the sheet. Furthermore, as for the double-sided printing of reading data by a document reader to make double-sided copy, since it is performed with reading the data by the document reader, it cannot change the order of pages. Accordingly, the method of printing the first side, reversing the sheet of paper to refeed it and printing its second side must be repeated to carry out the double-sided copy of a plurality of sheets of the original document.
The method of printing the first side, reversing the sheet of paper to refeed it and printing its second side, and hence leaving only a sheet of paper on the passage of the paper reverse mechanism and paper refeed mechanism has a problem of prolonging the paper transport duration because of reversing the paper and refeeding it. Thus, a contrivance is made to prevent scraping of an electrophotographic photoconductive body and needless heater driving by halting the charge generation in the electrophotographic process or the heater driving for fixing (refer to Japanese Patent Application Laid-open No. 8-320642 (1996), for example).
In addition, the following contrivance is made of the method of printing the first side, reversing the sheet of paper to refeed it and printing its second side, and hence leaving only a sheet of paper on the passage of the paper reverse mechanism and paper refeed mechanism. When no printing instruction as to the second side is given after completing the printing of the first side, a contrivance is made to continue the printing preparation for a predetermined time period. Thus, even when the printing instruction is delayed a little to develop the printing image of the second side to laser-dot information, the efficiency of the double-sided printing is maintained at a certain level (refer to Japanese Patent Application Laid-open No. 6-019255 (1994), for example).
However, the transport speed of the recording medium is rapidly increasing with the speedup of the printer, thereby sharply shortening the time period of printing the first side, and reversing the sheet of paper to refeed it. Accordingly, the method of printing the first side, reversing the sheet of paper to refeed it and printing its second side, and hence leaving only one sheet of paper on the passage of the paper reverse mechanism and paper refeed mechanism comes to have the following problems. It cannot halt the electrophotographic charge generation during the time from the paper reversal to the refeed as the conventional system does. It cannot halt the rotational driving of the electrophotographic photoconductive body and fixing pressurizing rollers, or can stop them only a very short time even if it can stop them.
In addition, to ease user maintenance of the printer, the life has been prolonged of the electrophotographic photoconductive body and fixing pressurizing rollers, which are replacement parts. The electrophotographic photoconductive body is worn by rotation, and reaches the end of its life by scraping by the charge generation. The fixing pressurizing rollers reaches the end of their life by the worn out because of rotation. Consequently, if the time period is shortened or eliminated of halting the electrophotographic charge generation or halting the rotational driving of the electrophotographic photoconductive body and fixing pressurizing rollers, a problem arises of shortening the life of the electrophotographic photoconductive body and fixing pressurizing rollers.
Furthermore, the method of printing the first side, reversing the sheet of paper to refeed it and printing its second side, and hence leaving only the sheet of paper on the passage of the paper reverse mechanism and paper refeed mechanism has the following problem. Since it makes the contrivance to continue the printing preparation for the predetermined time period when no printing instruction as to the second side is given after completing the printing of the first side, it continues to prepare for the printing of the second side, even when it is not printed. This offers a problem of shortening the life of the electrophotographic photoconductive body and fixing pressurizing rollers, and wasting power.