Media Replication - CD Replication

Construction of a CD:

The compact disc comprises a sandwich of 1.2mm thick optical grade polycarbonate substrate containing the pits which are molded into the upper surface. The optical grade polycarbonate is first "baked" to remove any moisture and is injection molded in a high pressure injection molding machine (commonly called a press) using the stamper mounted in the mold which is fixed to the press. The mold is in two parts and provides a cavity which ensures that perfectly molded discs are produced with the correct dimensions every time. One half of the mold contains the stamper while the other half contains the mirror block to ensure a smooth surface. The hydraulic press applies a force to the two halves of the mold which are closed. Molten optical grade polycarbonate is then injected into the cavity and held in place by the applied pressure while the disc cools and solidifies. Pressed discs, after cooling, are transferred (by robot arms) for the next stage of the process. The polycarbonate discs after molding are transparent. In order that the laser can read the pits they need to be covered by a mirror surface to reflect the laser light.

The next stage is therefore to metallise the active surface of each disc with aluminum by sputtering. Sputtering requires the transparent polycarbonate discs to be transferred to the sputtering changer which is then quickly evacuated or air and filled with argon gas. The argon ions are attracted to the aluminum target by the use of a high voltage source. As the ions strike the target, particles of aluminum are ejected and are deposited onto the CD surface. This aluminum layer is protected by a lacquer which is spread as a liquid evenly across the surface of the disc by spin coating. The centrifugal force created by spinning the disc ensures that the lacquer covers the entire disc in an even layer. It is important that the lacquer overlaps the aluminum therefore sealing it from the elements. If left exposed, aluminum will start to oxidize within a few days. The lacquer is cured by an ultra-violet (UV) light source producing a hard protective surface.

The discs are now ready for printing. The upper surface of a finished disc is printed with up to six colors by a flat silk screen process. Each color requires a different screen created from films produced as color separations from the artwork. Each color is printed using a squeegee which pushes the ink through the mesh of the screen on to the disc surface. Then inks are then cured using UV light to produce a durable surface. Very high quality printing can now be achieved using modern printing machines (offset printers). Automatics checks are carried out during all stages to ensure that all discs being printed are per customer's specifications.

The layout of the CD is shown in the diagram, including the Lead-in, Program & Lead-out areas with their start and end radii and other dimensions. CDs measure 12 cm in diameter with a 15 mm diameter center hole. The audio or computer data is stored from radius 25 mm (after the lead-in) to radius 58 mm maximum where the lead-out starts. All audio CDs are played at a constant linear velocity (CLV) of 1.3 meters per second. The angular velocity (rpm) will reduce from the lead-in to the lead-out by a factor of 2.52. This means that pits retain the same geometry wherever they are on the disc and there will be no change in performance across the disc.