A glass master cannot stand even one cycle in an injection moulding machine,
where it is subject to a very high pressure and temperatures. Hence, a metal
mirror replica of the glass master is used instead, which can take the heat and
the pressures of injection moulding process. This metal replica is obtained by
the Electroforming process. (Also known as the galvanics)
The electroforming process is just like an electroplating process, where a chemical reaction takes place driven by an electric current. This process takes place in a vessel called a cell. Here, in an electroplating solution, (in this case, nickel sulphamate is used) a current is passed, which causes ions of nickel to get released from the anode and get deposited on the cathode. In electrical terms, a cathode denotes positive voltage potential, and the anode denotes negative voltage potential.
In the CD electroforming process, the anode is usually, a titanium basket containing nickel pellets. The glass master becomes the cathode in this process, and when the current is passed, the anode releases the nickel ions which get deposited onto the glass master. The rate of deposition can be controlled and depends on factors like the voltage difference between the cathode and the anode, temperature of the solution etc.
These nickel ions attach themselves in a matrix form on the surface of the glass master. (See diagram)This is then separated from the glass master and we get a mirror image of the glass master which is called as the "father" . The pits made on the glass master, which were burnt on the glass master appear as "bumps" on the father. This is the mould which is used in the injection moulding machine for replicating the CDs.
Now, if there are more CDs required, due to the replicator having multiple production lines to run the job, or due to large runs requiring additional stampers, then one solution is to make multiple glass masters and make the required fathers from these.
The Glass master can be used only once in the electroforming process, since the electroforming process removes most of the information from the glass master. Making multiple glass masters is costly and time consuming and hence a faster method of dealing with this problem is used, which is to grow the family from the father itself.
What this means, is that we use the father and electroform another nickel deposition on it, to get a negative image deposition. When separated from the father, a negative image plate is obtained which is called the "mother". This mother can be used again in the electroforming process to make a positive image deposition, which is called the son or stamper. This stamper is usable for moulding CDs. Hence it can be seen that the father and the stamper are usable for making CDs, whereas the mother is used only to make multiple stampers, but never for making CDs. It is also possible to make multiple mothers from each father, and multiple stampers from each mother, thus meeting the most replication needs.
Before being sent out of the electroplating room, the father, mother or stamper have a protective layer of lacquer or film applied to them so that the pits are protected against any contamination by dust etc. However, before a father or son can be used for replication, it has to be processed. The stamper (let us call it this, whether a father or son) is then polished on the back side to get a smooth finish and to get it to the desired thickness. A smooth backside on a stamper is desirable as this will avoid problems in replication. The standard stamper thickness is 300 microns (The acceptable range is 290 to 300microns)
Then the stamper is put on a punching machine where the inner diameter hole is punched. This is very important and critical for the resulting CDs to be eccentric. We use an inner diameter of 34 mm. But this size varies depending on the moulding machine used.
After the inner diameter hole is punched, the stamper is then put on another machine where the outer diameter is punched. The stamper is then checked for quality once more in a stamper tester for the signals, and then this finished stamper is sent for replicaiton. The mother if made, is archived for future use for making additional stampers. And the glass master? It is sent for cleaning and coating, to be reused for making another glass master.
