Halftone angles and moire in screen printing

What a screen angle is

A halftone is a grid of dots. It has a pitch, set by the line count, and a tilt. The tilt of that grid against the edge of the film is the screen angle.

By itself the angle adds nothing to the image: the same dot at the same size carries the same tone at any tilt. Angles exist for one reason only, to keep one regular grid from lining up with another.

And screen printing has two regular grids. The halftone you calculate, and the woven mesh stretched on the frame. The second one is not going anywhere, and it is the one that decides.

The moire that matters is against the mesh, not between inks

Moire is the pattern you get when two near-identical grids overlap at a small angle. The eye sees broad waves where the physical marks are only small dots.

In offset the two conflicting grids are both halftones, because paper is smooth and has no structure of its own. That is why offset spreads its inks over 15, 45, 75 and 90 degrees, and that is correct there.

On a garment the dominant conflict is different. The halftone lands on the mesh thread, and the pitch of that thread is the same order as the pitch of the dot. A 230 mesh runs around 90 threads to the inch and a 55 line screen puts roughly four threads under every dot. That is exactly the spacing where moire is loudest.

So spreading angles between inks on textile solves a problem you do not have, and creates the one you do: it necessarily puts some ink on a bad angle to the thread.

Why 0, 45 and 90 degrees are the worst three

Mesh is woven vertically and horizontally, which is to say at 0 and 90 degrees. Three angles follow from that, and all three are to be avoided.

• 0 and 90 degrees. A row of dots lies straight along a thread. Those dots either fall into the openings and print, or sit on the thread and vanish, and they do it a whole row at a time. You get banding.
• 45 degrees. The row runs diagonally across the weave, through the knuckles where two threads cross. Ink does not pass at a knuckle, so fine detail crumbles.
• 15 and 75 degrees. Workable, but not symmetric to the weave: one of the two thread directions is always nearer, so highlights behave differently in different areas.

What is left is the angle furthest from all three. That is 22.5 degrees, exactly halfway between 0 and 45.

One angle for every ink, at 22.5 degrees

The trade standard for simulated process on dark garments is blunt: screen every ink at 22.5 degrees and change the mesh, not the angle.

That sounds like throwing away the textbook, but the logic is straight. If 22.5 is the best angle against the thread, then any ink you move off it prints worse than the rest. You have not removed moire, you have chosen which ink suffers.

Will the halftones not beat against each other instead? With opaque plastisol, in practice no. The inks stack rather than mixing optically, and on a dark garment the underbase carries most of the area. The interference that frightens an offset printer barely shows here.

The default shop profile in ColorLayers gives every ink that angle.

When you should spread the angles after all

One angle is a rule about fabric. It stops applying the moment the thread stops being the source of the moire.

• Printing on paper or board. A smooth substrate, a fine mesh, a thin ink film. Offset rules apply and the angles should be spread.
• Transparent inks and overprints. If the inks are deliberately mixing optically, their halftones do start to fight each other.
• Very high line counts. The closer the dot pitch gets to the thread pitch, the harder the one-angle rule binds. The further away, the more freedom you have.

That is why the angle-spacing algorithm lives in the profile rather than in the program. The spread-over-90 rule is still there: a shop printing on paper turns it back on itself.

How to check for moire before you output film

Moire is cheaper to find on screen than on a run.

• Look at the preview at 100 percent. A zoomed-out image produces screen waves of its own that will never reach the garment. Judge it at actual size or not at all.
• Check the midtones. Moire peaks near 50 percent coverage, where dot and gap are the same size. In highlights and shadows it is barely visible.
• Change the line count, not the angle. If a pattern shows, the first move is the neighbouring line count in your profile list. The angle is the last thing to touch.
• Print the test once. A patch running smoothly from 0 to 100 percent on your own mesh. It is a moire test and a printable-range test at the same time.

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