At present, most high volume full color printing is performed by web fed and sheet fed offset color presses. These machines print color pages using four printing plates, one for each of the four color components used in process printing; cyan, magenta, yellow, and black (CMYK). While these machines are highly efficient in printing large volumes of color pages, it is difficult, time consuming, and expensive to change the image being printed. When a new image is to be printed, color separations of the image must be created. Then proof sheets are created, to verify the quality and color of the printed image. These are usually created by a photographic process using the color separations created for the printing press. When the proof sheets are approved, four printing plates must be etched with the color separation images. Offset presses are also large and expensive and required extensive technical knowledge to operate effectively. Many technical parameters, such as dot gain, registration, and screen angles must be carefully controlled to obtain acceptable results. If the print run is greater than 10,000 copies, the set-up costs of the press can be effectively amortized over the volume printed. However, the cost and time required to set up a color press mean that only rarely is fewer than 500 copies of a page printed. If fewer than one hundred copies of a page are to be printed, then color copiers are generally used.
There is increasing recognition in the industry of the need for digital color printing presses, which are capable of printing high quality color pages directly from computer data, without requiring photographic and platemaking processes. These are considered to be most cost effective for print runs of between 100 copies and 10,000 copies.
A digital color printing press accepts a digital version of the page from a computer system, and directly prints the color images. Many technologies have been developed to directly print color pages from digital information, but none yet are cost effective for medium or high volume color printing.
One such technology presently on the market is digital laser electrophotographic color printing. However, the throughput and image quality of this system is inadequate for medium volume printing. As the system uses a single scanned laser beam to generate the image, the throughput is inherently limited by the modulation rate, intensity, and scanning rate of the laser. Other electrophotographic based approaches have been developed and marketed with success in some lower throughput regions of the 100 to 10,000 copies range.
While such machines are viable for short run printing, they are not suitable as replacements for offset presses for medium or large run printing. The throughput is substantially lower, and cost per page substantially higher, than offset printing for print runs in excess of a few thousand copies. Although these machines can be used in parallel to increase the overall printing throughput, the cost of these systems is quite high. The capital cost combined with the high cost per page makes parallel systems not cost competitive with traditional offset printing for medium or large print runs.
Thus, there is a widely recognized need for a high speed digitally controlled printing system able to produce high quality images using standard paper and low cost inks, that is able to compete effectively against mechanical technologies for medium and high volume printing.