The third step towards the disc
So, how is the disc created? The master template, which was prepared at the mastering stage, before it is put to the mould of the injection moulding machine must be verified at the level of visual assessment. Any mechanical defects such as scratches, corrosion and stains exclude such a master template from further manufacture. Before starting the pressing, in the pressing mould the reflection is checked (the so-called second half of the mould). If everything is normal, the master template is put on the mould. Not everyone is aware that the optical mediums which we often deal with are created from granulated polycarbonate. This powdered mixture, when influenced by high temperature, changes its physical properties and in this way the substance is poured into the mould, where under the pressure of 30 tons it is moulded into a circle, the intended traditional disc shape. At this stage, the material in the form of data is also placed as a spiral, which constitutes the final contents of the disc. The mediums pressed from the granulate are metallized using a vacuum method. The discs can be covered with silver aluminium or so-called golden target. Silicon is also used but the permeable layer has worse properties. The thickness of metallization on discs amounts from 15-20 microns up to about 80 microns. For amateurs, the relevant information can be the fact that the discs can only be read if they are metallized. Before optical scanning, mediums are covered with a lacquered layer using centrifugal power. The lacquer is solidified with UV lamps and at the same time acts as a type of protection against oxidisation.
Before the pressing of appropriate discs, a few diagnostic discs are pressed in order to verify the correctness of the master template installation and also about 100 – 150 discs which undergo the Prometheus test. This aims at checking the physical properties of the CDs and DVDs. The thickness of the discs are measured, the parameters of the laser passing through the medium, the stress created during the injection and the cooling, as well as the uneven surface of the disc are measured. If verification is successful, the discs undergo the Datarius test, where the correctness of material placed on the discs is checked (according to the norms provided for in Phillips Books for each type of disc). During the whole stage of pressing, this test is repeated every 10,000 discs in the case of CDs, and every 5,000 discs for DVDs. Manufacture starts if all the tests are successful.
The pressing of DVDs is different from the replication process for CDs. The additional stage is linking two elements (“disc” – 0.6mm), which creates the DVD medium at a thickness of 1.2mm. Two metallized halves of DVDs are stuck and placed on the centrifuge separator just to spread the glue equally and remove its excess. In the case of DVD5 the lower layer contains data, however the upper one is an extra protective layer. DVD9 is also manufactured with dual-layer technology and read on one side, and one of these layers is permeable, which increases the capacity of discs up to 8.5GB. DVD10 and DVD18 discs are the mediums which are readable on two sides. However, they are pressed more seldom because of the inconvenience of use.
The final stage in the process of disc pressing is optical scanning, which verifies possible errors, scratches and mechanical damage. Defective discs are removed automatically.
Together with GM Record’s development, changes in the international market and the market needs and machines and equipment are systematically supplemented by more and more modern and technologically advanced machines and appliances.
The pressed disc contains only the information which was placed on it in the pressing process. It is not possible to delete or re-record data which takes place in the case of mediums burnt in computer recorders. The unquestionable advantage of the pressed disc is their permanence.