Grab a few DVDs and CDs. Not blanks, not ones you've burned, but proper albums and films. Take a look at the images printed on them. Are they different in style? Are some matte? Are others shiny? Do some of them have the silver metal of the disc showing through? Do any have full-colour images on the front? Excellent. Let's begin.
(Used for: Low-resolution images, images with few colours, images where the metal of the CD is left showing)
Screen printing is a lot like stencilling. A screen is created, which consists of a tight mesh pulled over a frame, with parts blocked off. The discs are passed under this mesh, where a squeegee spreads ink through the mesh which oozes through the non-blocked parts creating the required image on the disc. The disc then passes under an ultraviolet light which dries the ink out almost instantly, preventing all sorts of nasty mess and smearing issues. In some cases the disc will carry through to the next printing station, where another colour will be applied, or perhaps even a coat of varnish to give the disc a shiny finish.
Due to the amount of ink applied and alignment issues brought upon by using multiple meshes, it's not particularly desirable to use the screen printing method on a job where non-primary colours needs to be mixed. Screen printing lends itself best to monotone images and images where the different colours don't overlap.
Screen printing produces a fairly low resolution image, due to the relatively large size of the holes in the mesh. It would be too difficult to manufacture a mesh with the extremely small holes needed for high resolution images, and it certainly wouldn't stand up to repeated squeegee-ing and general wear.
Screens do often break, and so they need to be replaced and changed at regular periods. Usually a screen will last long enough to complete a run of 10,000 discs. If the discs begin to show random dots of ink on them, it's likely that one of the holes has become enlarged, and again the screen will need to be replaced.
(Used for: Full-colour images at higher resolutions)
An offset printer consists of 4 or more printing stations which the discs pass through consecutively. At each station a different colour of ink is applied in a thin layer, essentially overlaying different intensities of each colour which will mix to produce the final image. Between each station is an ultraviolet drying light as in the screen printer, as well as screen printing stations at each end of the offsets. One before the offsets, to apply a coat of white ink to the entire disc, and another after the offsets to apply the optional varnish layer.
The individual printing stations consist of an ink well, rollers, a plate or drum, and finally a blanket. The ink well is simply where the ink is provided, constantly rubbed onto the series of rollers which thin out the ink and spread it evenly to be applied to the plate. The plate has been previously prepared (see below, "Plate Imager") with the corresponding ink image, collecting the ink from the rollers as necessary. The plate transfers the image to the soft blanket, which is in turned rubbed onto the discs passing through beneath.
It seems simple enough, but the overhead involved in setting up an offset printing job is quite hefty. After the blankets have been changed, screens prepared, ink levels set, ink transferred to the blankets - the offset stations must be individually aligned. This process is known as aligning the registration, and it basically means getting the separate colours aligned perfectly. This means sending through rejected discs from faulty pressings, examining them under a microscope, and manually adjust the alignment of the stations at a sub-millimetre level. During the course of a job the stations are likely to wander, and so the alignment will have to be constantly monitored and re-adjusted (this time on the fly, as it's usually uneconomical to take all of the loaded discs out from the press and run more rejects through).
Offset printing requires heavy monitoring. The ink levels, offset stations, screen printing stations and output must be constantly monitored. Disasters can happen. In a worst case scenario the white ink screen may split, spreading excess white ink all over the discs, the inside of the €2,000,000 machine, and worst of all, the ink rollers. This transfers the ink to the blankets, and then the plates, the upper rollers, and finally contaminating the ink wells. All then has to be replaced, scraped clean, emptied and refilled. This can take 2-4 hours.
The plates for offset printing are made in a separate machine called the plate imager. Firstly, plates are "made". A special sheet is tightly wound to the plate, and attached so that it is perfectly straight to prevent it from wandering during the printing process. It is this sheet that the image is etched onto. The plate is then inserted into the imager - note that at this point it is especially important to take the cellophane wrapper off the sheet. Accidentally leaving it on tends to cause smoke and anger. The required ink image is loaded onto the imager, and then the laser engraving process is initiated. This can take around 5 minutes per plate, and so is quite a lengthy process when you've got 4 or more plates to prepare.
The printing presses will often be fitted cameras which monitor the output on the discs, to check that the images aren't wandering or changing. After a set number of consecutive errors, the printing will stop and alert you that something isn't quite right. Sometimes the camera is too fussy or picks up dust, causing the machine to stop and ruin any discs currently in the printing process. This can be very irritating.
The monitoring often goes on in an integrated or separate PC. Mine was running Windows 98 with the monitoring screens integrated into the printing machine. Yes sir, I installed Nethack on a printing press.
Do you still have your CDs and DVDs out? Take a look at them again, and see if you can figure out the process used to print them.
DVDs are generally printed in an offset printer, with the exception of those dull silver and black ones. The first thing to look out for is whether they have a layer of white applied. Usually you can see it poking out beyond the edges. With the absence of white, the ink will take on a translucent appearance. Are there any well-defined areas of silver poking through? This is sometimes done by using a screen printer style mesh in the offset printer's white station.
CDs are usually screen printed. Run your finger over them - do you feel how the ink is raised? Can you spot any blobs or splashes where the screen was wearing out? Is the base silver or a solid colour?
Source: My awful, hellish two-months working as an offset printer in a DVD/CD manufacturing plant, in 12 hour shifts. I used a Kammann printing press - its interface was written entirely in German.