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Moulding CD & DVD Discs
Stampers are used to create replicas by
moulding, which is one of the most critical processes in
replication. |
The first stage in producing CD and DVD discs is injection moulding,
which uses the stamper to create the plastic discs or substrates
containing the pits.
Optical grade polycarbonate is
first 'baked' to remove any moisture and is
injection moulded in a high pressure moulding machine (press) using the stamper mounted in
the mould fixed to the press. This mould is in two parts and provides a cavity which
ensures that perfectly moulded discs are produced with the correct dimensions every time.
One half of the mould contains the stamper (to form the pits) 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 mould which are closed. Molten
polycarbonate is then injected into the cavity and held in place by the applied pressure
while the disc cools and solidifies. During cooling the centre hole is
punched. After cooling, the press opens and the pressed disc is transferred by
robot arms to a conveyor to allow the disc to cool before the next
stage. The centre hole with sprue (formed by the injected
polycarbonate) is discarded or re-cycled.
Successful moulding of CDs which meet the CD specification,
requires stable processes with the machines setup correctly. Moulding parameters
which can affect the resultant CD include stamper geometry, mould temperature,
polycarbonate temperature, compression force and cycle time.
Cycle times for injection moulding of CDs have decreased
substantially and are now around 3 seconds for the latest
presses and moulds.
DVD vs CD Moulding
DVD moulding
is similar to CD moulding but with some important differences.
- Two pressings are needed for each final DVD disc
- Each half disc (substrate) is 0.6mm thick instead of 1.2mm
- The thinner disc also requires different moulding
parameters, such as a shorter injection time and higher mould temperature.
- Redesigned or new moulding machines are needed with
injection compression where the mould is kept slightly apart until most of the
polycarbonate has been injected.
- Cycle times tend to be longer than for CDs.
The quality of the final disc, including tilt and jitter,
is critically dependent on the moulding process.
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