Free Square dot test, aka (INDEX or Stochastic) dot test.

[Dottonedan]

I have attached a file that is a square dot test. Referred mostly as INDEX but is also used loosely for a generic form of Stochastic dots.  It tells you what your best resolution is for art files to create your index seps on your specific film output device that works best with the mesh you will be using.
 
 Burn it on a number of meshes like 156, 200, 230, 305 and 350 mesh. That will tell you what your smallest dot is held without filling in...(all the way across. 1% (mid tone and shadow area) are the most important since all the dots are the same size. Then print it on a shirt for best results.


You will see a moire pattern when you have an area like 200 rez (on 200 mesh.  I stay far away from the the same mesh and same resolutions.



Free to download for members of TSB.

[dirkdiggler]

square dots rock!

[ebscreen]

Scared me, I thought for a minute this was that Dr. Squaredot guy.
Another individual seemingly obsessed with screen printing separations.
Index is cool, but it ain't gonna save the world. Screen printing world or
otherwise.

[JBLUE]

square dots rock!

Yes they do. Especially when done right.....

[dirkdiggler]

IMO they cant be beat, BUT, you do a lot of times need a SUPER large press.  Say goodbye to moire, and hello to exact tonal ranges when done right, we love'em at our shop.

[Doug B]

Especially when done right.....

  Or when you don't have a RIP. I use them all the time just so I don't have to fight moire.
Did a 5 color last week that had 9 spot colors with multiple gradients in the original art.

[Sbrem]

I was on about my 10th job using index (probable a good 15 years ago) when I got a moire pattern. I was blown away, because I was sure this couldn't happen. I eventually went to higher mesh counts, as I like 180 to 200 for resolution on these. Great for solid art, not so great for low percentages of stand alone color, which is a good place to introduce halftones and index together...

Steve

[Dottonedan]

I remember being told once, that "technically", the moire we see in stochastic is not really "moire" but "mesh interference". Apparently, there is a slight difference. Visually the same, but caused by two different things.


Moire is caused by the miss alignment of two interacting line screens  (as we can see in 4 color process) using different screen angles). We imagine that we don't get Moire in index using diffusion dither (random pattern) due to the fact that the dots are random (no pattern to them).


The mesh interference comes from having the dots match up to the thread thickness of the mesh. (e.g.) 200 mesh on 200ppi resolution. Now, the size of a 200 rez pixel is thought to be coincidental to the size or near the size of a mesh thread typically found on a 200 mesh screen. These can be thicker or thinner, but still fall near the same size, causing what ends up looking like moire. It's the threads blocking the dots (too much).


Moire and mesh interference can both look like this image below.




You can pre calculate your mesh and square dot size more accurately by using a dot (square dot size) that is at the same size or larger than (2 mesh threads + 1 mesh opening) as far as I can remember. That size will help assure no interference.  You can use my test file above as well, but this would seem to be the more accurate measure. This is the same technique we should use for determining the best (smallest regular halftone dot in a given line screen) to hold on down to the 1% dot.


Many people think that they hold a good 60 line screen, when in fact, they lose those dots under 5-4% and so really should be going back down to a 55 line screen (for example). If they really want to hold as much of a full tonal range as possible.