How is it Done?
Silkscreen Presses in Replication Plants
CD (and DVD) silkscreen presses have indentations for the discs set into a round table top. The tables are usually loaded from a spindle full of discs by a robotic arm. The screens are also on robotic arms, and are lowered onto each disc as the table rotates to each position.
Specially formulated UV-curable ink is applied by an automatic dispenser, then the (robotic) squeegee runs it across the screen. After each color is applied, the automated table rotates to the UV-curing station, and some systems may skip another position to allow some air-drying between coats, so you can have two "sets" of discs being printed at once.) Each screening and curing position has a different color and different screen, up to 4, 5 or sometimes 6. Normally to get more than 4 colors the first one is a "donut" or full circle background (which might be applied on a different press), and the 6th color, if the replicator offers it at all, is a spincoated lacquer finish, not a real color, so the tables usually offer only 4 colors per disc.
The ink is applied in a very exactly measured way so there is no dripping or bleeding, and the process is very fast, so there's not much chance of a screen sticking to a disc. The inks are specially formulated to adhere to the CD's lacquered surface, so you don't have to worry about beading up, and they don't have to wait all that long between colors -- a few seconds is all they require. There are stations on the table with UV lamps for curing the ink before they are removed from the press by a robotic arm and stacked on an output spindle.
Disc ink is also especially formulated to avoid causing any damage to the media itself. A funny-sad story about an early experience that demonstrated the necessity for this care is related in Chris Andrews' Education of a CD-ROM Publisher, available in a 2nd edition produced by the publishers of this website. You can buy this classic book in hardback, paperback or CD-ROM versions at our online bookstore.
Manual Silkscreening on CDs
Although it may be more trouble than it's worth (silkscreening is a time consuming, messy job, and many replicators will do it on your CD-Rs for a moderate fee in quantities as small as 100 discs), it is possible to print discs this way without a huge automated press. Care must be taken to use only inks formulated for use on CDs, which generally means UV curing is required, but also that specific pigments must be chosen with care to avoid corrosive effects. Also, the screens used in this process must be very precisely made and attached to their frames evenly and tightly to prevent misregistration. Since many disc designs require very fine detail, extra high-count fabric must be used. If more than one color is to be applied, a method for aligning the screens or discs between colors must be arranged or the results may not be very pleasing. And of course creating the designs on the screens with photoresist and masks can take a steady hand and many hours.