I have no idea on how much information or how in-depth you wish your
tutorials to go. This attachment may go beyond the scope you wish to
cover. My opinion is that the more information you can give people the
better, but the more information you want to give the more difficult it
becomes to put it into a coherent format, probably made a lot worse by
the way I tend to ramble.
Mostly, this is the information that gets left out of formal educational
programs. It's the stuff that the printer knows but the graphic designer
has to try and pick it up as they go along, usually only learning by
expensive mistakes.
One company I was Production Manager for had a large design house as a
regular client and any time this design house put on a new designer that
new designer would get sent to spend a couple of days with me to find
out why I wanted their art done in particular ways.
None of these kids were dumb, in fact most were highly intelligent and
creative, but they'd never been taught the practical side of their art
as it applies in the real world. Certainly where the print industry is
concerned which can be a very strange world indeed.
Yes, I used to get used for training the apprentices as well.
If you really want to keep it short and sweet the best advice I can give
is "Talk to the printer". Not a sales rep, talk to a real printer. The
best ones all have rather impressive egos, necessary or the dumb
machines will beat you, and most of them will be only too pleased to be
able to show how much they know. (yeh, me too)
Cheers,
Andrew.
Screening your CMYK separation.
Printer's inks are transparent, except for a few specialty colours. This gives the printer control over colour adjustments, the thicker he puts the ink on the darker the colour gets - within reason. This is also why the printer has to put the colours down in the exact reverse order of how your colours "should" have been lifted from the image during separation. Putting the Magenta over the top of the Cyan does not give exactly the same result as putting the Cyan over the top of the Magenta. In a four colour process print job putting the colours down in the wrong order produces an effect similar to a bad white balance shift, then the printer finds out that he can either try to make the skin tones look natural or make your company logo the right colour, not both. (refer to other comment about printers cheating CMKY)
Putting the ink on the paper in different densities all over the page however is physically impossible, in fact the ink roller systems (roller trains) on his press are designed to help prevent this from happening by accident. Instead the image gets broken up into an imaginary grid pattern of squares, where the colour is darkest in the picture each imaginary square is filled with ink but as the colour gets lighter across the picture you get smaller and smaller dots of ink in each imaginary square, making bigger white spaces in between. The graduation of colour is actually an optical illusion.
The less dots per inch you use for your image the less illusionary that graduation is. But if you make your screen too fine, too many dots per inch, you can very easily make it finer than what a simple aluminium plate can hold. The smaller the dot the less deeply the oleic emulsion gets embedded into the surface of the plate and the quicker it gets worn off again.
Printers talk in dots per linear inch, by the way, not dots per square inch. Sometimes to save confusion they'll use the term "lines per inch" instead.
The most commonly used screen size for Offset Printing (lithographic) is 133 dots per inch, this gives a fairly good reproduction and the printer can expect to get between 30000 and 50000 impressions from one plate depending on weather conditions, type of paper, and how good the operator is. This would be for most glossy brochures and fancy business cards.
For a truly high quality image you could use 175 dots per inch but the printer won't get much more than 1000 copies from the plate. Every time the printer has to line up a new plate he basically has to set up the job again and this cost will be passed on to you - the customer. So it's good for short run quality jobs but it also requires a more highly skilled printer, can only be used on small sheet sizes and it will cost more. 225 dots per inch is possible but impractical. If the quality you require is finer than that then you need to talk to a Gravure printer, they can do 2000 dots per inch but you'll probably choke when you hear the cost.
Very large sheets of paper present a new problem - paper stretch. When paper gets wet it expands. As each next colour gets put down the image underneath it gets spread a little bit bigger. Percentage-wise this expansion is tiny but across a large poster sized sheet (A2 or A1) this stretch can be up to a millimetre. A way to help hide this is to use a larger screen size, less dots per inch. And more overlap on your spot colours, the 20 microns I mentioned in the other email is only good up to A4 size. Cheaper papers usually get more stretch as well, and let's be honest, if it's 50000 posters advertising an event and they're going to be useless afterwards then it's cheap paper that you're going to be buying.
Take a close up look at the next truck or bus you see with one of those giant adverts on the side, the bloke that makes the giant stickers for them has the same problem, those screens are huge. Your newspaper also uses a larger screen because it's on cheap paper and it's a large sheet. Those special colour lift-outs you get are actually on expensive paper imported from Finland so that they can make a better quality image, rub it between your fingers and you can feel the difference, the more expensive paper has a much finer texture.
The other thing to consider with your screens is the angles of separation. Most print oriented graphics software will get this right automatically but understanding why won't hurt. If all four of your separated layers get the same screen on the same angle you will get a prominent "moire" effect across the image. Technically this shouldn't happen, until you take paper stretch into account, as each progressive colour gets put down the screen underneath it gets slightly bigger and so none of them can ever fit exactly on top of each other.
How we get around this is by rotating the screen by 15° for each layer, so the Cyan gets a screen square with the edge of the page, Magenta's is rotated to 15°, Yellow's is rotated to 30° and Black's to 45°. When I first started we used to do this in the dark room by putting a special sheet with a freznal screen over the top of the negatives.
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