How are CDs Replicated?
Replicated CDs are bespoke made for you and the content is moulded into the plastic of the discs. Replication is very suited to large scale production as once the machines are running they are very quick to produce. A CD replication lines which is running well can produce a disc in 4-5 seconds.
Rather than using a laser to burn pits and lands into the surface of the CD the pits and lands are moulded directly into the plastic using a metal stamper when the CDs are presses.
Although it is very quick to produce the discs once the machines are running it takes time to get the machine set up of each job. It’s because of this set up time that CD replication is not suited to small orders.
Stages of production
The first stage in the process is Glass Mastering – When a master is received it is loaded into the mastering system. This system creates an image of the discs (the pits and lands) and re-produces them onto a photoreceptive coating which applied to a thick piece of glass. This is the glass master which is often the name given to the replication process. The coating on the glass is washed off once the metal stamper has been made and a new coating applied. The Glass can then be used again for the next project.
The next stage is to produce a metal stamper. The glass master is placed in a bath of Nickel. An electrical current is passed through the bath and by a process of electrolysis (you may remember doing this in science lessons) Nickel is attracted to the face of the glass producing a metal plate with the impression of the disc on it. This metal plate is known as a father stamper. Fathers can be mounted onto CD replication lines to produce the discs or they can be placed back into the electrolysis baths to produce mothers. Mothers are mirror images of the father stampers and can be used to grow sons (which are the same as fathers). Producing sons is done on very large production runs so that discs can be produced on a number of machines. The glass used to make the stamper is washed and a new coating applied ready for the next job. If a stamper breaks during production then the whole process of creating the glass and growing the father stamper must be repeated.
Replicating the CD’s
The stamper is mounted into the replication line. The replication line presses the stamper against a mould creating a small cavity the size of the disc. Polycarbonate which has been heated to become liquid is then injected into the cavity producing the disc. The mould and stamper are separated and the polycarbonate disc is removed (by the machine).
The next step is to apply a coating of aluminium to the disc (on a CD-R rather than aluminium silver is used as it reflects the laser better which is important when burring the discs) to the top of the CD. The metal is applied through a process called spluttering, a very powerful magnetic pulse transfers a layer of metal from target to the disc. The magnetic pulse is very powerful and will wipe a mobile phone of data is next to the machine when it operation.
Once the metal is applied to the disc a coating of lacquer is added. The metal stops before the edge of the disc allowing the lacquer coat to apply to the clear plastic. The lacquer prevents then metal from peeling off.
Difference between CD-ROM / CD-Audio and CD-R production.
The process used to make a CD-R is very similar to replicating a CD-ROM or Audio CD. There are a few differences though.
Before the metal is applied to the disc a photo-receptive dye is applied to the disc. This dye is what gives a CD-R a slightly green tint. The dye layer is use by a CD burner to record the pits and lands.
Rather than using Aluminium Silver is applied to the CD-R. The silver targets are very well guarded in most CD-R manufacturing plants as they are solid silver discs approximately 20cm in diameter and 5cms deep.
Cheap CD-R may have thinner layers of dye and / or silver to reduce the costs but will also reduce the quality of the discs and its ability to be burnt and read.