Moiré is a
ratio problem

What moiré actually is

Lay two regular patterns on top of each other, slightly out of step, and a third pattern appears that exists in neither: big, slow ripples, like watered silk. Your halftone is one regular pattern. The mesh is another. Where their spacing or their angle nearly agree, you get ripples across your midtones that have nothing to do with the image.

It isn't the ink. It isn't the squeegee. It isn't the emulsion, so please take the emulsion out of the room. It's arithmetic, and it was already in the film before the screen existed.

Three ways to get it

1. Too many lines for the mesh. You asked a coarse weave to hold a fine halftone. This is the common one, and the one this post is about.
2. The wrong angle. Dot rows at 0°, 45° or 90° line up with the weave. We covered why 22.5° is the only sane answer; PrintFilmReady locks it, so this one is off the table unless you're building halftones by hand.
3. A pattern already in the artwork. A photo of a knit jumper, a herringbone coat, a screenshot, a scan of something that was already printed in dots. Re-screening pixels that already contain a regular pattern gives you two patterns and a fight.

The ratio: about 4½ threads per halftone line

A halftone line needs several threads underneath it before a dot can be a dot instead of a lucky accident. The usual rule of thumb is mesh count ÷ 4 to 5 for the maximum LPI. Push toward 3 and the dots span barely a thread or two, the mesh chops them up, and the beat goes coarse and visible.

These are the pairs PrintFilmReady uses. Mesh counts are threads per inch; a Belgian 61 T/cm mesh is a 156.

Paper prints finer dots than textile on the same mesh, so it is allowed a slightly tighter ratio. Nobody is allowed to go below about 4. Want 55 LPI on a 156 mesh? That is a ratio under 3. The answer is a finer mesh, not a braver film.

Fix it in this order

1. Lower the LPI or go up a mesh. Not the other way round. In PrintFilmReady you pick the mesh you actually print on and the LPI follows. Overriding it “for more detail” is how moiré gets made.
2. Check the angle. 22.5°, round dot, every layer. Automatic here, worth checking anywhere else.
3. Look at the source artwork. Zoom to 100% and look for anything regular: weave, hatching, dots. Start from the clean original if you have one, or blur it a pixel or two before uploading so the pattern breaks up.
4. Check the screen. A slack or unevenly tensioned mesh has threads that wander, and a wavy grid beats against everything. If the moiré is bad in one corner and fine in another, suspect tension, not the film.
5. Test on the real mesh. Print a step wedge on the screen you're going to use, before the job, not during it.

Which one bit you

Will exposure fix it?

No. Bad exposure or bad film contact gives you fat, ragged or lost dots, which is a different problem and a fun one on its own (the file is not the stencil). Moiré is baked into the film-plus-mesh combination. Burn a perfect stencil from a moiré film and you get a perfect moiré.

What this means in PrintFilmReady

The tool derives the LPI from your mesh and locks the angle at 22.5°, so the two big causes are handled before you click anything. It can't clean your source artwork and it can't tension your screen. Those two are yours. It also assumes a square-stretched screen. And as always: these numbers are a place to start testing, not a guarantee. Print a test.

Common questions

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