Excerpt from out-opts mailing list:
>>I had heard that they can now hold a lot finer screen than that as long as
>>the receiving surface wasn't too absorbent.
>
Well, it is possible in some circumstances to hold finer halftones, but it isn't often necessary because of viewing distance, and it isn't always possible. Screen printing often is called upon to lay down a great deal of ink for various reasons, and this requires you to go toward thicker fabric which means fewer threads per inch requiring a coarser dot.
It's a balance act.
>I had heard something like that too. Even up to 120 - 150 line using
>special frames and screen meshes. Are the angles for color separations
>different than for negative output?
Sure, it can be done. But it isn't often an important issue, because you tend to give up most of the advantages of screen printing when you setup to print those fine dots.
There are a lot of factors at work. Starting with the fact that screen printing is the primary method of decorating absorbent surfaces.
One of the reasons that screen printing is widely used is durability. Part of the way this happens is using thick ink and laying it down thickly. This requires coarser meshes, 200 threads per inch and below. It isn't easy to hold a 60 line halftone on a 200, and it isn't possible to do a good job of it. If we're talking about signs for the side of a semi, durability is important, as is color fidelity. Six line halftones will look far better a year after installation than 60 line halftones.
Another feature is formability, and you can't form thin films without showing the strain. Again, thick ink is more important than small dots.
Solvent evaporative inks tend to "dry in" during printing. The smaller the dot the faster this happens. Smooth long runs are more important than small dots.
There is no good reason for most screen printers to attempt to compete with offset. Particularly if they have to give up the advantages of screen printing to do so, and when they can never get down to the price.
Yes, the angles of the screens are different -- the whole set is rotated 17 to 22 degrees.
vanhorn@whidbey.net (G. Armour Van Horn)
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