Compact Disc

WHAT IS COMPACT DISC?

A compact disc, or CD, is an optical storage medium with digital data recorded on it. The digital data can be in the form of audio, video, or computer information.

MANUFACTURING PROCESS:

Preparing the disc master

The original music is first recorded onto a digital audio tape. Next, the audio program is transferred to a 3/4-inch (1.9 centimeters) video tape, and then data (called subcodes) used for indexing and tracking the music is added to the audio data on the tape. At this point, the tape is called a pre-master. The pre-master tape will be used to create the disc master, which is a disc made from specially prepared glass. The glass is polished to a smooth finish and coated with a layer of adhesive and a layer of photoresist material. After the adhesive and photoresist are applied, the disc is cured in an oven. Next, both the pre-master tape and the disc master are put into a complex laser cutting machine. The machine plays back the audio program on the pre-master tape. As it does so, the program is transferred to a device called a CD encoder, which in turn generates an electrical signal. This signal powers a laser beam, which exposes or "cuts" grooves into the photoresist coating on the glass disc (the disc master). The grooves that have been exposed are then etched away by chemicals; these etched grooves will form the pits of the CD's surface. A metal coating, usually silver, is then applied to the disc.

Electroforming

After etching, the disc master undergoes a process called electroforming, The disc is bathed in an electrolytic solution, such as nickel sulfamate, and as the electric current is applied, a layer of metal forms on the disc master. Next, the newly applied metal layer is pulled apart from the disc master, which is put aside. The metal layer, or father, contains a negative impression of the disc master track. The metal father then undergoes further electroforming to produce one or more mothers, each mother then produces a son. It is the son that is then used to create the actual CD. After being separated from the mother, the metal son is rinsed, dried, polished and put in a punching machine that cuts out the center hole and forms the desired outside diameter.

Replication

The metal son is then put into a hollow cavity—a die—of the proper disc shape in an injection molding machine. Molten polycarbonate plastic is then poured into this die to form around the metal son. Once cooled, the plastic is shaped like the son, with the pits and grooves—once again in a positive impression of the original disc master track—formed into one side. The center hole is then punched out of plastic disc, which is transparent at this stage. Finally, a clear acrylic plastic is applied to the disc to help protect the underlying layers from physical damage such as scratches. After the label is printed, generally using a silk-screening process, the compact disc is complete and ready for packaging and shipment.

Quality Control

The disc master, for instance, is inspected for smoothness and its photoresist surface for proper thickness by means of laser equipment. At later stages in the process, such as before and after the aluminum coating is deposited and after the protective acrylic coating is applied, the disc is checked automatically for warps, bubbles, dust particles, and encoding errors on the spiral track. This mechanical checking is combined with human inspection using polarized light, which allows the human eye to spot defective pits in the track.

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