Saturday, October 10, 2009
There are two methods of web offset printing, known as heatset and coldset (or non-heatset). In the heatset process, the ink is dried rapidly by forced-air heating. In the non-heatset or coldset process, the ink dries more slowly by ordinary evaporation and absorption.
Some web offset presses transfer text and images to only one side of the print medium at a time. Others can print on both sides simultaneously. The paper width is usually between 11 and 56 inches (approximately 28 and 142 centimeters). The paper is fed through the system at speeds ranging from 5 to 50 feet per second (approximately 1.5 to 15 meters per second).
Web offset printing differs from sheet-fed offset printing, in which individual pages of paper are fed into the machine. Sheet-fed offset printing is popular for small and medium-sized fixed jobs such as limited-edition books.
Printing is a means of graphic communications. It is the reproduction of quantities of images, which can be seen or perceived visually. Regardless of the great number and variety of printed products they all have one thing in common; each has the same visible image produced in quantity.
Today’s printer owes much to the Age of Science, particularly to electronics, computers, chemistry, optics and mechanics. Modern printing has become highly sophisticated. While the mechanics of printing has not changed greatly, the surrounding other technologies have. As new pre-press systems, plate, electronic controls, and paper have been developed along with other products of modern science and research, printing has gradually been transformed from an art to a science.
An etched plate on the plate cylinder contains the printing pattern. This cylinder is coated with ink by rollers and it transfers the pattern onto an intermediate roller, called the blanket cylinder. Several color stations can be placed around the blanket cylinder for multi-color designs. The blanket cylinder transfers the ink onto the part to be decorated.
The process is characterized by thin, transparent ink films on a white or cream substrate. Fine details are possible and the registration of colors is very good. The different colors are printed wet on wet, and are then dried.
Advantages of offset printing compared to other printing methods include:
* Consistent high image quality. Offset printing produces sharp and clean images and type more easily than letterpress printing because the rubber blanket conforms to the texture of the printing surface.
* Quick and easy production of printing plates.
* Longer printing plate life than on direct litho presses because there is no direct contact between the plate and the printing surface. Properly developed plates running in conjunction with optimized inks and fountain solution may exceed run lengths of a million impressions.
* Cost. Offset printing is the cheapest method to produce high quality printing in commercial printing quantities.
Disadvantages of offset printing compared to other printing methods include:
* Slightly inferior image quality compared to rotogravure or photogravure printing.
* Propensity for anodized aluminum printing plates to become sensitive (due to chemical oxidation) and print in non-image/background areas when developed plates are not cared for properly.
* Time and cost associated with producing plates and printing press setup. As a result, very small quantity printing jobs are now moving to digital offset machines.