Halftones for screen printing: lpi, angles, dot shape, film output

First: if you want a halftone look, this is not that

Two completely different things share the word "halftone". If you are after a retro comic-book or newsprint dot texture for a poster, a record sleeve, a web hero or an illustration — Ben-Day dots, big visible circles, a risograph vibe — you want a halftone *effect* filter, and there are a dozen free ones a search for "halftone effect generator" will hand you. This is not one of them: the output here is a separation plate, one per ink, at film resolution, screened at a chosen angle. It is intended for transparency film and screen exposure, subject to the same mesh, polarity and press test you would apply to any new output path. If nobody is going to burn a screen with it, use a design-effect filter instead.

The four numbers that decide a halftone

Frequency, in lines per inch, is how many rows of dots per inch — garment work typically runs 35 to 65 lpi. Angle is the rotation of the dot grid, and it matters because two grids at similar angles beat against each other and produce moiré. Dot shape — round, elliptical or square — changes how tones join up as they grow; elliptical dots are popular because they avoid the abrupt tonal jump round dots make at 50%, where round dots all touch their neighbours at once. Output DPI is the resolution of the film itself, and 600 or 1200 dpi is what determines how many distinct dot sizes the device can actually resolve at your chosen frequency. Set all four explicitly. Every one of them is a press decision, and a default that suits somebody else's shop is not a decision.

Mesh count sets your ceiling, not your taste

A screen cannot hold a dot finer than its own openings, so mesh is the hard constraint on frequency and the number to start from. The rule of thumb in general circulation puts mesh count somewhere around four to five-and-a-half times your lpi — so 45 lpi sits comfortably on 205 mesh, and 55 lpi on 305 is a real, commonly-run pairing at the upper end of that band — and going finer than your mesh supports does not produce a finer print, it produces dots that break up, fill in, or wash out of the emulsion. Finer mesh is also more fragile and more of your week spent restretching. And higher is not automatically better even when the mesh can take it: a bold one-colour design that reads clean and solid at 35 lpi can come off faded at 55 because the dots holding the light end have gone too delicate to print reliably. Pick the coarsest frequency that holds the detail the artwork actually has.

Angles spaced to reduce interference

For four-colour process the renderer uses the conventional C 15°, M 75°, Y 0°, K 45° set. For simulated process it selects from a table of eight separated angles; with more than eight colour plates, angles repeat and the manifest explicitly warns that some pairs can moiré. You can enter your own angle list when your shop has a set proven on its mesh, emulsion, substrate and output device. An angle table reduces one source of interference; it cannot guarantee a pattern-free print.

Dot gain, the reason process prints go muddy

Ink spreads. A 50% dot on film can become something well past 50% on fabric, and the effect is worst in the midtones where a print has the most to lose. Dot gain compensation applies an inverse curve before screening to pull the requested tone back toward the intended printed value. The slider is not a measured press profile: gain depends on mesh, emulsion thickness, ink, squeegee and substrate. Print a step wedge on the setup you actually use, read where it lands, and record that value as a starting point while those variables stay the same.

Choose between RIP input and screened film

A RIP wants to do its own screening. Feed it continuous-tone grayscale plates — one greyscale image per ink, in register, at the intended print geometry — and it applies its own frequency, angle set and dot shape with curves tuned for the output device it drives. Screen the plates first and hand it 1-bit film and you get two dot grids fighting: the RIP re-screens your dots and you moiré against your own artwork. In this workspace, Spot and Index begin with halftoning off, while selecting CMYK or Simulated process turns it on; choose the switch deliberately rather than treating either state as a universal default. Turn it off for continuous-tone plates when you print through AccuRIP, Wasatch, FilmMaker or anything similar. Leave it on for pre-halftoned 1-bit film when you print to an inkjet or laser through a plain driver with no RIP in the chain, and set frequency, shape, angle set and target output DPI explicitly. Either way every plate in a render uses the same canvas geometry, so nothing shifts between films; for an oversized halftone request, the manifest records the common effective DPI after the 16,800px/200MP cap. One practical reason the screening is the paid half: a 14-inch plate targeting 1200 dpi is 16,800 pixels along that dimension, which is genuinely painful in a browser tab on a shop PC.

It comes with the separation, not instead of it

Halftoning is applied per plate as part of the print-ready render, so the same 10 credits that separate the image also screen it: choked underbase, highlight white, per-ink choke and spread, dot gain, halftone and registration marks. Re-renders of that image are free for 24 hours, so trying 45 lpi against 55 lpi, or round against elliptical, costs nothing but the time to look — and a day is long enough to burn one and print it before you decide. If all you need is flat spot plates with no tone, you never need halftoning at all — and that path is free and runs in your browser. There is no separate automatic-trapping control in the shipped workspace.

Or skip halftones entirely

Index separation is the escape hatch. Every dot is the same size at 100% ink, tone comes from dot density, and because there are no varying dot sizes there is no angle set to choose and no rotated grid for the next plate to interfere with — moiré stops being the thing you manage. It is coarser than a good halftone and more graphic in character, but it is far more forgiving on press, and on textured garments and older equipment it frequently wins outright. Index separation is free and runs in your browser.