'A DVD' is the new optical disc format that is proving popular throughout the world and in fact is already beating the CD as the most successfully launched consumer product ever.

A DVD is essentially a higher capacity CD, that comes in a number of forms depending on the number of information layers moulded into the disc. A DVD always consists of two discs bonded together, each disc being 12 cm in diameter and 0.6 mm thick, so once bonded they are exactly the same dimensions as a CD. Each disc can have information moulded into it, therefore varying the amount of data each DVD can contain.
  

Currently DVD has found its niche in the 'pre-recorded video¹ market, where it is supplementing sales of VHS tapes. There are some DVD audio titles available, but the number of players in the market is very limited, whereas DVD ROM players are now much more widely available, although few DVD ROMs have yet been released. 

The DVD has also found an outlet in the game console market. The new Sony PlayStation. 2 uses DVD game discs and will actually play DVD Video titles and the much publicised Microsoft X-Box will feature DVD based games, although an extra controller will be required if DVD Video is to be watched.

It would appear the DVD is here to stay, so what challenges does manufacturing and decorating a DVD pose for the ink manufacturer? 

When DVD5 and DVD9 discs are being made, the top, non-read surface can be decorated in much the same way as a CD, by using screen, offset, flexo or tampo, printing. There are however some issues to consider when decorating DVDs. 

The 'pits' in a DVD are much smaller than those in a CD, therefore when being read, the players are much less tolerant of any imperfections in the disc. Furthermore, printing the top surface of a DVD may actually impart some imperfections that cause the discs to be outside the industry standard specifications, especially after ageing. 

Notwithstanding this, it is still true that many DVDs are made every day across the world using 'standard¹ CD inks, the specifications are being met and discs supplied to consumers with confidence. The constant striving to improve the process and productivity of manufacture has driven many of the observations, investigations and subsequent changes to the products and processes to be made.

The first point to consider is the design of the image to be printed on the disc. A very asymmetrical design (such as a solid covering of half the disc) may cause the disc to 'wobble¹ during spinning in the player due to its asymmetry, which could cause the read laser to fail. Most DVDs in fact have a 'picture print' (four colour process print) applied and these inherently tend to be quite symmetrical, so this image induced wobble is not likely to be too prevalent unless artwork designs change.

Another cause of read errors can be heat absorption during the UV curing process, which can make the discs warp, especially in combination with the shrinkage of the surface applied ink. This effect is known as 'dishing¹ and is generally considered to be caused by a combination of factors, one of which is the heat from UV curing lamps. To reduce this heat induced warping, many machinery manufacturers now fit cooled UV lamps to new printing machines and in some cases a retro fit option is available.

However, the major cause of warping in DVDs may be shrinkage of the UV inks applied to the surface during the curing process. This can be especially prevalent when multicolour bui

lds are used. The warping of the discs may also not always manifest itself immediately after curing, as UV inks tend to post-cure over a number of days after initial exposure, so the discs tend to worsen on ageing.

A number of solutions to this problem have been offered. It has been suggested that since screen printing applies the highest filmweight of ink, it will become obsolete in DVD decoration, with offset printing being preferred (despite the fact that offset prints generally need to be made onto a screen printed white base coat), but with so much screen printing equipment already installed, this would seem unlikely.

It is true that an offset printed DVD looks great. However, the printing machines are expensive, the set up times tend to be longer than for screen printing, and as run lengths are getting shorter, for many the offset route is not necessarily the most economical. So assuming that most DVDs will continue to have at least some screen printing on them (for instance the under print white for subsequently offset printed discs), the ink manufacturer has had to look for a solution to this disc deformation problem, before and after disc ageing.

The challenge for Coates Screen, was to develop a fully functional screen printing ink for CDs and DVDs, that maintained all the properties of the well known CD1 ink range, whilst reducing the shrinkage of the ink during curing and the subsequent ageing process, so as to minimise any possible warping or dishing.

White tends to be the most used colour on both CDs and DVDs. It is generally used to cover the base silver layer for subsequent overprinting, and since it is in direct surface contact with the polycarbonate, is likely to cause the highest degree of deformation, although subsequent colours to play their part too. 

It was therefore priority to develop a white formulation that would have absolute minimum shrinkage after printing, curing and ageing; this white had to maintain the fast curing, high opacity and printability of the previous CD whites, and also provide an excellent base coat for subsequent offset printing. The result was WE R White, DVD-S102, a white ink for CDs and DVDs that performs as well as any of the other CD whites and yet exhibits almost zero shrinkage after curing and then ageing (under various environmental test conditions).