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Limitations of Screen Printing
Some images that work just fine with CMYK offset printing on coated paper may not be as effective when reproduced by forcing ink through a fabric screen onto a mostly silver disc. Screen printing has both charms and limitations, and the trick is to exploit the charms and design around the limitations.
Low line screens
The standard line screen setting for disc film is 85 lpi. That's approximately the same as a newspaper - and nowhere near as fine as the 150 lpi that's standard for jewel case paper products. |
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Offset printing on paper
(8x magnification)
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Screen printing on disc
(8x magnification)
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Fill in/disappearance of fine lines
When ink is forced through a fabric screen with a squeegee, it goes on thick - perhaps 10-30 times as thick as the inks used in the offset printing of the paper products. One side effect of this thickness is that fine lines knocked out through a solid color tend to fill in and disappear, as the ink flows into the trough. This happens to fine rules, thin frames, and most importantly, serif and script text. Brittle serif typefaces such as Bodoni, with it's ultra-fine serifs and thins, script typefaces such as Nuptial Script, or "light" selections from typeface families such as Univers 45 Light, are inappropriate for small knockout text on the disc label. If you're wondering about a font's viability, compare it to 5 point Helvetica bold, generally considered to be the smallest safe size for knocking out through a single ink. If your font has more delicate lines than that, be prepared for parts of the letters to disappear. |
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Four typefaces, knocking out
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Same four typefaces, with simulated fill-in
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Same four typefaces, same amount of simulated fill-in, smaller font size
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| When knocking out through multiple inks, the problem of fill in is compounded by the problem of potential misregistration. We recommend avoiding multi-ink knockouts for disc art in general, but if you must, we strongly recommend that you use robust sans serif typefaces and large point sizes. |
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Good: bold sans serif typeface, knocked out through a single ink.
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Bad: serif type knockout, even though it's only through 1 ink - the serifs close up.
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Bad: knockout through multiple inks, even though the typeface is a sans serif - misregistration compromises the legibility.
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Disappearance of fine positive lines is a related phenomenon. Very small holes in the stencil have a tendency to plug up, preventing the ink from passing through and adhering to the printable surface. For type, this generally occurs at point sizes small enough that they are unlikely to be employed for anything other than "fine print" such as copyright notices. However, disappearance can be a problem with fine positive rules in artwork.
Dot gain, dot disappearance and tonal jump
Dot gain is the name for the phenomenon that occurs when a halftone dot prints larger than was intended. Dot disappearance describes the failure to print of halftone dots under a certain weight. Tonal jump describes a sudden increase in dot gain as a particular dot density is passed. On the disc, you can expect:
- dot gain of 15% or more, starting in the midtones
- solid fill-in of dots over 85%
- disappearance of dots under 10-15%
- at least one significant tonal jump at around 30%, and a few other smaller ones scattered about.
There are a few strategies for dealing with this. First, foremost, don't use smooth gradients for on-disc printing. Tonal jump can wreak havoc on gradients. It can also cause nasty posterization. |
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Offset printing on paper
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Screen printing on disc - note how tonal jump posterizes Elvis Costello's face
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| Next, try to avoid using an image that contains a lot of subtle highlight information in the 0% - 20% range. Some of the dots are likely to get disappeared, and you could end up with a big blown out hole in the middle of your image. |
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Dot disappearance
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| Similarly, if you have a lot of subtle shadow information in the 80-90 % range, you're likely to see some blotchy fill-in. |
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Blotchy fill-in - notice how some of the dots appear "plugged," like a window screen after a rainstorm
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Lousy CMYK color accuracy
The reproduction of color with CMYK process printing depends on delicate, balanced blending of colors. Trying to anticipate the effects of dot gain, dot disappearance and tonal jump on a monotone is hard enough. It's impossible on a CMYK quadtone. CMYK screen printing produces coarse, unpredictable results. That doesn't mean don't use it. But if color accuracy is important, as it might (or might not) be in the reproduction of an existing fine art piece, for example, caution might dictate that the full color image not be employed as a disc design element. |
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Offset printing on paper
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Screen printing on disc - note how the colors differ from the offset version (presumably the goal was to have the two match).
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Obvious rosette patterns
When two or more screen angles are combined, a rosette pattern forms. This is a relatively innocuous pattern, compared to other moire patterns, but at 85 lpi, it's more obvious than most designers would like. The effect is most apparent when two halftone dot patterns are superimposed and both dots are in the midtone range. If one is either solid or absent, voila, no rosette. Any combination of overlapping halftones will form a rosette: including duotones, tritones, or quadtones such as CMYK. Monotones, which consist of only a single halftone, are less noticeable. |
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Monotone (no rosette)
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Duotone (rosette)
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