User:Heritage.john/Optical disc production

Optical disc production

Bulk production methods (Pressing)
Still referred to as pressing, this is an example precision injection moulding technology and it is applicable to virtually all standard, reflective optical media.

This method is far faster than writing the data with an optical drive and so more cost effective when the number of discs in an order exceeds approximately one thousand. This figure is set by the capital cost of setting up the tooling to write data by injection moulding compared to the cost created by the extra time it takes to optically write a disc.

Generating the master
Writing data to an optical disc by injection moulding requires a high end and comparatively elaborate production process, and well illustrates the capacity of the moulding process for microscopic feature work; as the pit that represent each bit of data typically have dimensions on the order of a hundred nano meters.

Tooling for injection moulding is typically manufactured using CNC machining. However, in the case of optical discs, even temperature controlled machining is unable to work at the tolerances needed to produce the data pits found on an optical disc.

Instead, the process of media production begins with a disc glass that is made sensitive to light with a primer and photosensitive layer, applied in a spin coater. The disc of glass, now behaving similarly to a piece of photographic film, is exposed using a laser who's beam is modulated to represent the data that will end up on the final product. Washing the disc with developer and deionised water causes the pattern of exposure to appear on the disc. As the photosensitive coating is resistant to etchant, the data can now be permanently etched onto the glass. As with etching printed circuit boards, varying the solution concentration and submersion time controls the depth to which the pits are etched.

Although the glass disc now contains a physical copy of the data, the glass is too fragile to use as a tool in the injection moulder. From here, the glass disc progresses to a plating tank, where nickel and vanadium are chemically deposited on the glass as it spins in a bath of solution to ensure even distribution. Nickel is used as the glass is none conductive and so the process requires a metal that can be deposited by electro forming. Vanadium is added to give the copy better wear resistance. The plating is not like that found on many metallic items in that it is thick enough for a technician to peel it off by hand, as a thin sheet.

The metallic sheet contains a copy of the etched pits that were present on the glass disc, and it is immediately covered with a protective layer of film to prevent the delicate surface being damaged during handling. The original glass disc is no longer needed and the metallic copy, having some excess material around it's rim, is carefully stamped out to the finished size of an optical disc. This is the factory master which will be used to produce the customer's own copies.

Moulding
The protective film is removed from the master and it's loaded into the injection moulder. Liquid polycarbonate plastic is injected into the cavity, as it carries both the optical and mechanical properties desired. This stage is no different to more common injection moulding methods and, once cooled, the 1.2 mm thick transparent plastic discs are ejected, bearing the same surface pits that were etched into the glass and then copied to the metallic master tooling. It is during this stage that dyes can be added to the polycarbonate to create coloured discs. Moulding takes only seconds, as the cavity walls are water cooled, the mass of plastic injected is reasonably small and it is injected at a temperature just high enough to allow it to flow into the data pits correctly.

Note that the data is not present throughout the disc, as it was (and can only be using this technique) moulded onto the surface; it is 2D. The majority of the disc's thickness is purely to give it mechanical strength. The actual depth of plastic required to form the data layer is around twenty thousand times less than that of the finished disc. Meaning there is a potential to store orders of magnitude more information on the disc if it's depth can be exploited; this fact is put to use in increasing disc capacity in the form of dual layer DVDs (which are pseudo 3D, as they contain only a few discrete layers) and Holographic Versatile (being truely 3D)

In this state, the disc is unreadable in an optical drive, as the read beam would simply pass through the disc and out the other side. For the drive to read the data pits, the beam must be reflected back to the read head such that it may determine whether a pit is present or not.

Metallisation
To create the reflective surface needed for read beam reflection, the disc passes through a vacuum sputtering cavity, where a microscopic layer of aluminium, silver or gold is evaporated from what is known as the target (essentially a lump of the metal) by a microwave generator (a magnetron). On contacting the disc, the metal cools and solidifies. As with moulding, this step takes a few seconds. Even the delicate microscopic pits are safe from thermal based distortion as the total amount of metal deposited is too little to significantly heat the polycarbonate up; the process is cold. This has created what is commonly known as a Compact Disc, and the freshly moulded product is now, theoretically, capable of being read in an optical drive.

Aluminium is used for CDs as is is both cheap and highly reflective at the lasers wavelength.

Finishing of single sided discs
Whilst an optical disc, be it single layer (like a CD) or multilayer (like many DVDs), can be read in an optical drive after metallization, the data pits and metallic layer are still too delicate for everyday handling; a single scuff to the data side could easily render the disc useless.

To protect the disc, it is again put into a spin coater and a resin coating is applied. High intensity UV lamps, again, cure the coating in around a second.

The disc is now ready for handling, but will often go on to screen printing; another process taking only seconds to complete, with the inks being cured under UV light. A final lacquer may be applied to protect the artwork.

Single sided DVDs
Are technically referred to as DVD-5.

Interestingly, unlike CDs, single sided DVDs are form from two separate plastic discs, each 0.6 mm thick. One of these contains the data on it's surface, which is then metallised. The second is glued on top to create the finished 1.2 mm thickness. This method of assembly is used because it makes it virtually impossible to scratch the data pits or the metallic layer. It likely came into being as modern methods of DVD production require manufactures to produce these 0.6 mm thickness discs anyway, as is detailed in the following sections.

Dual sided discs
These discs are like old fashioned vinyl records, in that they can be flipped over to play back different information. They are technically referred to as DVD-10; meaning, two single layer sides.

Mass production of these discs is much the same as detailed above. The only difference being that the finished disc is produced from two separate discs, each 0.6 mm thick. The two transparent discs, bearing differing information to one another, are again metallised to produce the required reflective layer, then bonded (back to back) to create the finished 1.2 mm thickness.

There is, of coarse, no way to label these discs, as the data is read from both sides, albeit one at a time with the disc being flipped over by hand to access the second side.

Multilayer discs
Such a disc is more complex than either of the two previous examples, as it contains two layers of data built up on top of one another. Many high capacity DVDs are dual layer (DVD-9 refers to a dual layer, single sided disc). They may also be dual sided (with a disc having two layers on each side being DVD-18 and an oddball format having one dual layer on one side and a single layer on the other being DVD-14).

To produce these discs en mass, multiple methods of building the disc up have been produced.

The first is similar to producing a dual sided disc. Two separate 0.6 mm discs, containing differing information to each other, are moulded. Rather than metallise them both with aluminium and then glue them back to back, as would be done for a dual sided disc, one disc is metallised with a thin coating of gold. This is semitransparent to a powerful enough laser.

The second layer can be metallised with thicker aluminium, to make it entirely reflective to the laser. As with a dual sided disc, the two are now glue to each other, but with the gold metallised disc on the bottom, the aluminium metallised disc above and both the read sides facing the in the same direction. To read the separate layers, the drives laser refocuses depending on whichever is being accessed.

Another method of producing these stacked layers is to start with an injection moulded, gold metallised disc and to then coat it with a resin. The data can then be stamped into the resin before it hardens and is metallised with aluminium to create the second layer. This is akin to the production of vinyl records; which are also pressed.

Creating dual layer, dual sided discs means following the above procedure but creating two differing dual layer discs that are then glued back to back, resulting in four layers per disc.

Holographic discs
Holographic media relies on the interaction of two laser beams to write data states through the thickness of the disc, with each layer being separated by the wavelength of the depth defining beam used.

Injection moulding or stamping the layers of such discs is unrealistic, they must be written optically.

Colourful discs
Dyes are added to the plastic for a number of reasons, with the first being to make them visually distinctive. This is very common with discs that have been written by injection moulding, as the dye isn't required for anything else. Sony dyed it's PlayStation discs black for this factor alone.

For writeable discs, the dye plays a much more important role, as it better absorbs the write beam of the drive, converting it to heat which then alters the opacity of that region on the disc; forming a bit of data. This is also one factor in why commercially produced discs tend to be written by injection moulding, even if only being produced in limited quantities; the coloured polycarbonate of writeable discs has become stigmatic and somewhat synonymous with forgery and degraded quality. Whereas the distinctive silver finish on the read side of an injection mould written disc signifies that it was produced at a factory; this finish can't be replicated at home with optical drives.

Note that current leading edge disc technologies, such as those used in holographic storage, have dyes added to the polycarbonate as they rely on florescence to indicate bit states, as opposed to changes in reflection intensity; therefore the dye is important for reading as well as writing, in contrast to reflective media.

Optical writing
When production quantities fall below a thousand units, it becomes prohibitively expensive to set up the tooling for an injection mould written run.

Instead, optical drives take the place of injection moulders, as the data being written can be changed quickly and easily; with the cost being that each disc takes longer to write than it would for writing by moulding.

Optical writing of standard, reflective media is rarely done commercially as order quantities are almost always above the kilo unit switch over point to writing by moulding. It is much more common for home users to make use of this method as the equipment required is exponentially cheaper and the process equally easier to carry out, making it more attractive to those who can not reap the benefits of pressing technologies that rely on producing many millions of discs to be economical.

Writeable disc quality & failures in the moulding and production process
As writeable discs contain no micro features, it is permissible for the tolerances of the tooling and process it's self to fall below the standard required for writing by moulding.

Bad sectors caused by graphics and labelling
It has been found that certain writeable discs have higher corruption rates due to the specific inks used to print graphics (such as a logo) on the label side of the disc. This is due to variations in how the write beam is absorbed by the disc and is worse the less uniform the labelling is; the beam must heat the polycarbonate to a specific temperature to write a bit and differing pigments cause the disc to have differing absorption and radiation patterns. Such discs will show the graphics in the pattern of corruption found when a burnt disc is analysed for errors. This is not a problem for most commercially written discs, where the label is screen printed after the data has been fixed by injection moulding as opposed to burnt with a laser.

Similarly, discs featuring only a lacquer between the metallic layer and label side (which produces a disc with a mirror like label side) can be corrupted by certain marker pen inks when labelling the disc. However, it is still inadvisable to use sticky labels on writeable discs, as these will unbalance the disc, causing vibration and thereby making it difficult for the laser to track and focus correctly. Manufacturers of writeable discs will print the label side with a uniform white ink to avoid these problems; so the write beam may heat the disc uniformly, regardless of any marker pen labelling. The graphical work and any marker pen labelling can often be seen on the read side of poorer quality writeable discs by holding the disc up to a bright light. It is advisable to label writeable discs after burning, as this avoids issues with the inks creating uneven adsorption of the write beam.

Bad sectors caused by vibration
As mentioned above, vibration of the disc and drive mechanism makes it difficult for the optics to focus and track correctly.

Again, this is a problem that is more commonly associated with writeable discs, where the tooling and processing can be of a poorer quality. Inspecting the rim of a poor quality disc, you will often find blobs of surplus polycarbonate that have been left where the tooling of the injection moulder failed to fully mate. As these are not equal in size or spacing, they unbalance the disc, causing vibration.

This is particularly undesirable in today's high rotational speed drives, where the amplitude of the vibration increases with the read/write speed. Thus, decreasing the rotational speed can help alleviate this problem and this is why optical drives can be heard to be scanning down through their speed range when they encounter an read/write error.

When optically writing to poor quality media, decreasing the write speed may allow the disc to complete. But an inevitable side effect of this will be that it will not read at high speed either.

Write failures
Both the ink and vibration induced errors discussed above combine to help explain why some brands of writeable optical media can produce multiple failed write attempts, whilst others may not; despite the drive and data to be written being identical.