screen printing > Separations

Choosing LPI For Output

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drdot:
This is a very, very common question and there's usually some "machoism" associated with it. You know, I can print 85 lpi all day, and so on. Here are some things to consider. Just because you can print a finer dot, doesn't mean you should. This will be the first of several posts on choosing lpi based on production considerations and related factors.

The choice of LPI for your halftone is determined by several factors. Chief among these are:

Substrate Limitations.
Viewing Distance.
Screen stencil resolving capabilities.
Production variables (mainly moire and dot gain.)

The coarser the dot you choose, the easier it's going to be to make an accurate stencil, control moire, dot gain, and tonal range reproduction.

On the flip side, the finer the dot, the more detail you can hold and the less noise the image will have (surface texture that interferes with the image.)

For textiles, the most common ranges fall between 45 lpi and 85 lpi. Some very experienced printers have gone as high as 110. Here are my recommendations to help you choose the best resolution for your purpose.

First and foremost, consider the "dot density". This is how many dots are in a given area.  This is really enlightening and will have the single biggest impact on your choice.

45 lpi      2025 dots/sq inch
55 lpi      3025 dots/sq inch      49.4% increase over 45 lpi
65 lpi      4225 dots/sq inch      39.7% increase over 55 lpi
75 lpi      5625 dots/sq inch      33.1% increase over 65 lpi
85 lpi      7225 dots/sq inch      28.4% increase over 75 lpi
110 lpi    12,100 dots/sq inch   67.5%  increase over 85 lpi

The dot density is the lpi squared.  Since dot gain is uniform around the perimeter of  ANY size dot, having the dots smaller and closer together radically magnifies the gain you will experience on press. For an ideally controlled halftone, normal dot gain(defined and explained in another post) will typically be:

45 lpi       22% - 28%
55 lpi       32% - 45%
65 lpi       38% - 50%
85 lpi       50%+

High dot gain is not necessarily a problem. It only has to be managed effectively. When you consider the relationship between lpi  and dot gain, the most control occurs in the 50 lpi - 60 lpi range. This is why it is so often recommended. 

Add to this the fact that there will be increased tone clipping on both the highlight and shadow end of the tone range as the smaller highlight and shadow dots will be clipped by the thread diameter of the mesh.  Based on 34 micron thread diameter the smallest consistently (meaning all dots print with no eclipsing) printed dot will be:

45 lpi     2%
55 lpi     4%
65 lpi     6%
75 lpi     7%
85 lpi     8%
110 lpi   14%

With todays high resolution dual cure emulsions, you can resolve 1% dots for ALL of these linecounts, you just cannot print them due to the threads of the mesh being thicker than the dot itself thereby blocking the mesh opening.

I'm sure this is going to raise some questions, so let the discussion begin.

Donnie:
Good stuff.

Orion:

--- Quote from: drdot on May 06, 2011, 11:22:32 AM ---On the flip side, the finer the dot, the more detail you can hold and the less noise the image will have (surface texture that interferes with the image.)

--- End quote ---

Great post Mark. If I am reading this right higher line counts help when printing on t-shirt fabric? I was thinking that if the line count is too high that some dots hang in the screen because they don't contact the substrate during the squeegee pass which in turn may cause more dot gain.

Also thanks for providing the data on the dot gain percentages and smallest printable dot.

Northland:
I'm begining to see the value of larger dots as part of the design element.
For example... I really like the way the dots add to the design Artelf2xs posted last week. (see link... REDLINE ROCKETS text)
http://www.norwb.com/index.php?topic=301.0

alan802:
I think I'm going to have to read that a few more times.

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