Step 1 Making negatives: The first step in the offset process is to create a negative from the original art. The negative (fig. 2 below) is made using the processes outlined in this tutorial under "What is film?". 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.
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.
3. The idiosyncrasies of disc printing and the surface of the CD that can affect your designs.
Designs with gradual fades from one color to another may reproduce poorly due to dot gain and tonal jump that make colors darker and produce pronounced dots in the image. Anything below 15% of a color tone usually disappears and anything 85% or greater just closes up and becomes 100% tone.
- The thick applications of ink required for screen printing can make fine lines and detail disappear (including very small type and serifs under 6 points).
- Spot color rather than CMYK printing will give you crisper results. Registering a CMYK CD label is more challenging because it is round.
- Because the disc surface is metallic silver, not paper white, you may need to add an additional white flood under the color to allow more accurate tone and detail.
- It is almost impossible to match the colors on your booklet with the CD silk-screen. Use complimentary colors and designs.
- Because the disc's innards switch from aluminum, to a mirror band, to clear plastic, ink color appears differently on the clear inner hub, the mirror band, and the main silver surface of the disc. To eliminate this problem, a white flood is added which equalizes the surface beneath the silk screened inks.