Replication is the process of actually manufacturing the CDs. In this process, the stamper is put in an
injection moulding machine. The stamper contains the information which has to
be pressed on the CD. The injection moulding machine already has the main mould
which provides the shape and the dimensions of the CD, and the stamper moulds
the information in the form of holes. Hence, when a title changes, all one
needs to do is to change the stamper in the moulding machine.
The main raw material for manufacturing the CD is Polycarbonate granules
(Optical Grade) This is used for their high optical clarity, (needed for the
laser beam to read the pits) and a very high degree of hardness. (so as to
protect the CD from scratches). This Polycarbonate is of a very pure grade, as
any impurities will cause reading errors in the final product. (You know, the
laser beam could misread an impurity as wrong signal) Hence even the
Polycarbonate is manufactured in a Clean Room, and it is always handled in a
such a way that it is never exposed to atmospheric air, where it could be
contaminated. ( Not an easy task, if you understand that this material is
supplied in a sealed three layered bag weighing over 750kgs.)
The Polycarbonate is firstly dried in a material dryer at 120 degrees
Centigrade, so that all moisture is removed. This is important to avoid any
moulding defects on the moulded CDs. Once the Polycarbonate is dried for over
two to four hours, then this is conveyed by pipes to the moulding machines. (so
that there is no exposure to human hands or to the atmosphere) The moulding
machine has the stamper fitted on one side, as explained earlier. Now, the
molten Polycarbonate is injected at high pressure into the mould, so that it
spreads from the centre outwards, to form a clear disc with the information on
it in the form of holes (or "pits").
The mould then opens and the centre part of the moulded CD (called the "sprue")
is punched out and removed leaving behind the moulded CD with a centre hole,
(where the sprue was) and then the CD is taken out and stacked on a spindle.
Thus we get a clear CD with the pits already on it, but this CD cannot be read
or played anywhere. The CD is allowed to cool before being passed to the next
process, viz. Metalizing or Sputtering. Usually the quality of the moulding
determines the quality of the finished CD, as a majority of the errors arising
out of replication can be controlled at this stage. Also, the cycle time of the
moulding machine determines the output of the manufacturing line. Typically
these have come down from about 9 seconds, till a few years ago, to about 3.5
to 4 seconds. ( i.e. cycle time is the time taken to mould one CD.
The cycle time is becoming shorter and shorter as more replicators are moving
towards increasing yields) The moulding machine manufactured the CD with the
information on it. But it is very difficult to read this information as the
player cannot read the pits, on a CD. Hence a very thin layer of very pure
Aluminium is applied on the CD so that the laser beam from the player can
reflect off the CD. What happens here, is that the CD is put in a chamber with
Argon under very high vacuum, and an electric charge is introduced in the
chamber. This causes aluminium ions from a very pure aluminium piece (known as
a target) to deposit as a layer on the CD. The Thickness of the aluminium layer
can be controlled. A mask in this chamber acts like a stencil, ensuring that
the metal deposit is done only on the required areas (and not in the centre or
at the extreme outer edge of the CD).
