10.1: MANUFACTURING CDS

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The Processes

The manufacturing process of optically read discs is a more complex process than vinyl. The cd is a precision moulding carrying detail of truly microscopic size - yet at the same time it is mass produced.

Glass Mastering

Whereas a record is cut by a stylus in the surface of lacquer, a cd is etched by the exposure to laser light of a coating of light sensitive material on a glass base.

A highly polished glass disc is coated with a layer of photoresist 13 thousandths of a millimetre thick. Then laser light is shone on the disc as it spins, this is called encoding. This creates a series of dots and dashes which represent the signal recorded from the master onto the disc. The coated disc, having been exposed to the laser light, is then developed in a process similar to that found in the techniques of developing prints and film in photography. Those areas which were exposed to the laser light drop away to leave pits in the surface, the intervening areas remain unaffected. After rinsing and drying, the developed master is vacuum coated with nickel to make it electrically conductive. It is now ready for plating.

Plating

The nickel coated glass master is then placed in a nickel electroforming bath and a metal master is grown. This metal master is then separated from the glass master, and after cleaning to remove all traces of non-metallic matter it is then passivated and once more placed in the bath to grow a metal positive or mother. The metal master is a negative copy of the developed surface of the glass master, so that the mother is once again a positive copy of it.

After separation the mother is used to grow a nickel stamper, carrying a negative replica (bumps instead of pits) of the original glass master's surface. From the original master several mothers can be grown, each yielding a number of stampers. Each stamper can be used to mould tens of thousands of discs.

Moulding

The favoured method for this in Europe and Japan is called injection moulding. The mould is closed against the stamper and then hot plastic is injected into the cavity very quickly - about one tenth of a second. Once the plastic has cooled, the centre hole is punched and the mould then opens for the finished disc to be extracted. The moulding process must be performed under class 100 clear air conditions because the disc is very vulnerable to dust and dirt until it has been metallized.

Metallizing

Although as the disc leaves the moulding machine it already carries all the information in the form of the pits on the surface, a reflective layer must be applied before it can be read by a cd player. The discs are then coated with atoms of aluminium which are ejected by a low-pressure gas discharge in a vacuum.

Lacquering and Printing

A layer of lacquer is applied over the metallized layer, so that the information pits are completely sandwiched between the plastic of the disc and the lacquer. This is essential to ensure that the microstructure of the pits and the delicate and easily corroded aluminium layer are protected. Once sealed in, the information layer is safe from all but the most careless handling. The label information is then printed over the top of the lacquer.

Discs are then rigorously tested on a variety of parameters, all of which can affect its playability. These include; average error rates, worst case errors, stress, quality of metallizing, centring, and flatness.

Packing

The final stage is to put the disc together with the jewel box, tray, booklet and inlay card. The jewel boxed has been criticised for being hard to open - if anyone can invent a better package which retains the advantage of quality, protection of the disc, and the ability to display information on four of its six sides, cd manufacturers and the public alike will be delighted.

Conclusion

Having CDs manufactured is quite a simple affair, especially compared to vinyl. The unit cost is very low, and getting lower all the time. Plus of course the more you have made, the cheaper they get. This is because there only needs to be one glass master (the most expensive bit), so once its paid for its done - having 2000 or 50, or any other amount pressed doesn't matter. If you look at the difference in cost between 500 and 1000 you can already see the huge effect this has. See the section called 'Costing a release' in the DIY Label section for examples of this.

In addition, some costs can be cut both with PQ encoded mastering and cover design. Its possible to do all the mastering to CD at home or in a local studio for much less than the manufacturers charge. You need a WAV editor, a CD writer, and some patience. Plus, most home printers and PCs are completely capable of being used for design and practice prints of the covers. Some experience will be needed but I learned it for myself so I guess others can!