Records manufactured industrially are hot pressed by compression. The PVC is first transformed from granule to filament, then compressed (with air pressure) inside a container until it becomes a small disc, and subsequently squeezed by the press with a force of 100 tonnes at a temperature of about 180°, until it takes, after cooling, the shape of the record.
The grooves, however, must be impressed on both sides. And it is at this point in the process that the stampers mounted on the press come into play, that is, the negative, metal copies of the two master lacquers. Negative — because they must leave a positive imprint on the surface of the record.
In the photo above, two stampers mounted on the press can be seen. They are already centered and preformed for use, but before their preparation, they were simply nickel films – the metal commonly used for electroforming in the recording industry in general.
The process for their creation is always the same, namely to coat — by electroforming — their metallic layer with another metallic layer, and then separate them, thus obtaining, alternately, positive and negative copies of the grooves.
Electroforming, which uses electrolysis, is also used for the stampers of Compact Discs and DVDs, offering several advantages:
- the same detail of the cutting, therefore a precision to the thousandth of a millimetre (µm);
- negative parts: thickness suitable for subsequent processing (centering, preforming, pressing, etc.); good elasticity (due to the thermal and mechanical stress they undergo during pressing);
- positive parts: suitable thickness to be reproduced on a turntable (for control purposes).
If two electrodes containing a metal are immersed in a solution containing a salt of the same metal, and a difference in electric potential is created between them by using a voltage generator, ions of that metal will be attracted from the positive pole to the negative one with an intensity proportional to the difference in potential.
At the positive pole, there is an agglomerate of metal used to coat the piece, which is placed at the opposite pole: usually pellets or ingots.
It is a process that we also find in many operations that have nothing to do with the records: just think of the nickel plating of objects, or the chrome plating of car exhaust tips.
In the case of records, the part to be coated is flat (like the lacquer) and rotating, to ensure a uniform thickness of the deposited layer.
Furthermore, unlike what happens in other processes, in this case the film that is deposited must then be detached, and therefore – if it is also metallic – the piece on which the deposition is carried out must be chemically treated in advance, to prevent the two parts from sticking together.
A question remains open: since electroforming is based on the difference in electric potential, and therefore on the current, the first piece from which the entire process starts, namely the lacquer, must also be conductive. The answer is that, to achieve this, a thin layer of metallic silver is created on the surface of the lacquer itself, by nebulising two reagents so that their reaction occurs directly on it. This process is called "silvering".
Introduction to electroplating