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 Heidelberg Offset Printing Press  Plate and blanket cylinders What is Offset Printing?Offset lithography operates on a simple principle: ink and water don't mix. Image information (art and text) is put on thin metal plates which are dampened by water and ink by rollers on the press. The oil-based ink adheres to the image area, the water to the non-image area. The inked area is then transferred to a rubber cylinder or "blanket" and then onto the paper as it passes around the blanket. The process is called "offset" since the image doesn't go directly from the plates to the paper, but is offset or transferred to another surface as the intermediary.Why is it called 4-Color Printing?Offset commercial printing presses and inkjet desktop printers both use four basic ink colors: CMYK. Where inkjet printing puts all the different ink colors on the paper in one pass through the printer, in offset printing each color of ink is applied separately - one plate per color. Small dots of the four inks - cyan (blue), magenta, yellow, and black (K) - are deposited in specific patterns that make our eyes believe we are seeing a wide range of colors. That's why the standard offset printing process is often called 4-color process lithography or 4-color printing. Offset printing can also use premixed inks in specific colors including metallic and fluorescent colors, called spot colors.Why Use Offset Printing?The advantages of traditional commercial offset printing are higher quality and the best cost-effectiveness for volume quantities.
 Plate being mounted What's involved in setting up an offset printing run?To start a press run, the imaged plates are clamped onto the cylinders and the ink settings are adjusted for the density of ink needed for the particular run. The press operator then runs a batch of initial sheets through the press at low speed and carefully checks registration (alignment) of the colors and the ink/water balance to ensure full and steady ink flow. The operator can adjust the plate positions while the press is running at low speed. This set up process is called a "make ready," where highly-skilled operators prepare the press for perfect offset printing.Are there different types of offset printing Presses?Yes. "Sheet-fed" presses run individual sheets, ranging from "full size" sheet-fed presses running sheets that are about 28 x40 inches, to "half size" and "quarter size" which use proportionately smaller sheets. "Web-offset" presses are fed from a large roll of paper, which goes through the press in a continuous length of paper, called a "web." Sheet-fed presses are generally used for business marketing printing, while web presses are more cost-effective for very high volume printing (around 50,000 quantity or more depending on the size and type of piece). Commercial offset printing presses can also vary in size, ranging from "full size" sheet fed using sheets that are about 28 inches x 40 inches, to "half size" and "quarter size" which use proportionately smaller sheets. |
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