Offset printing

History

Offset printing is a technology that has been around for over 100 years now.
The term "Offset" gets its name from the fact that the plate cylinder which holds the art information does not come in contact with the paper.

Procedure

Step 1: Making negatives:
The first step in the offset process is to create a negative from the original art. From this negative, a printing plate will be made.

Figure 1 below represents a digital file; Figure 2 is created inside the image setter.

Step 2: Printing Plates:
The negative is then placed in contact with a metal printing plate (fig. 3 below), which is coated with a photosensitive, ink-receptive emulsion. It is exposed to light and the plate is developed, leaving the emulsion (illustrated by the magenta book image) only where it was exposed.

back to top

Step 3: Wetting:
As in stone lithography, offset lithography uses the water-versus-oil principle to make a print. The plate is first moistened with water (See main illustration at the top of this section). This keeps the non-image areas damp, and therefore more resistant to the oil-based inks used in this process. The plate is now ready to be inked.

Step 4: Inking / Offsetting:
After the printing plate is developed, it is attached to a cylinder on the printing press. As it turns, the plate offsets this inked image to a rubber blanket, creating a reverse image. The blanket then offsets this image to the paper, reversing it again and making a "right" reading, final print.

Step 5: Multiple Passes To Achieve Full Color:
Multiple colors are accomplished by changing the color of the ink in each station as it passes through the press. Modern presses not only have 4 color stations but also add a varnish coat to help eliminate finger prints on the paper.

back to top