CD Science Part 2 – Replication in Depth
Photo by Robert Chapman: A Singlus CD Replication Line
A little later in its arrival than expected, meet part 2 of our CD Science blog series! Part 2 focusses on the CD Replication process and the CD-ROM’s produced.
The term CD-ROM often causes confusion between customers and manufacturers in the media world, with the common misconception that this is the same as a CD-R.
A CD-R is a recordable CD which may have data burnt onto it via a CD drive or duplicator tower (data is transferred through duplication).
A CD-ROM is a CD produced through replication, with ROM standing for Read Only Memory. Here the data is ‘pressed’ into the disc at the point of disc manufacture.
An Audio CD is a CD- ROM with audio information placed onto it, suitable for playing in any CD player or computer.
A Data CD is a CD-ROM with other kinds of data placed onto, such as files, images, video or games, suitable for accessing in any computer, but not a CD player.
In certain cases, a CD-ROM may be divided between both audio content and other kinds of data, however this is another subject entirely!
Getting Ready to Replicate – Creating a Master
Photo by Robert Chapman: A Metal Stamper for CD Replication
CD Replication uses an injection-moulding process to ‘press’ the content onto the disc from a metal stamper.
To create a metal stamper, the data goes through a process called Glass Mastering, which takes place in a class 100 clean room.
A 180mm glass substrate is cleaned of any imperfection and then placed in a Laser Beam Recorder, which records the content as pits into a photoresist coating on the glass.
The glass master is then developed and metalized with nickel vapour to produce a surface for the stamper formation.
From this glass master a process called electroforming (queue lots of long words) is used to create a ‘Father stamper’ which is the reverse of the glass master (raised bumps instead of pits denoting content).
The father stamper could then be used directly for replication or alternatively, be used to create first a ‘Mother stamper’ and then a series of ‘Son stampers’ which can then be used for replication.
CD Replication Step 1 – Injection Moulding
Phew – the mastering section is a toughie, now replication itself:
Polycarbonate pellets are melted down and injected into a mould, within which the polycarbonate is cooled against the metal stamper to produce the disc substrate.
Photo by Robert Chapman: Polycarbonate Pellets
The mould opens, and the disc is transferred to a conveyor via a robot arm. The whole process, from the mould closing to the mould opening takes just 3-5 seconds.
CD Replication Step 2 - Metallization
The disc is then transferred to a metallizer; a small chamber in which a metal layer coats the disc and pits through a process called ‘sputtering’.
‘Sputtering’ uses an cathode - anode transfer to draw an aluminum alloy from a metal target onto the surface of the disc evenly.
CD Replication Step 3 - Lacquering
This metallized layer is then coated with a lacquer to protect it from corrosion. The lacquer layer is just 70nm thick and is rapidly cured with a UV lamp.
The lacquer also provides a great surface for printing onto the disc later, although it is vital that the printer checks their inks and lacquer are properly compatible.
CD Replication Step 4 – Testing
The final step of CD replication is automatic disc inspection. Every disc produced is scanned by the machine to check for flaws and imperfections in the surface, separating any unacceptable discs as rejects.
Photo by Robert Chapman: Polycarbonate Pellets ready to be pumped into the Replication machines.
So, there we are – one replicated disc produced every 4.5 seconds!
Keep your eyes open for Part 3 of the CD Science blog series – Printing the Replicated Disc.