A plain guide to screen print colour separations: what a film is, how many inks you need, halftone vs spot colour, and how to get print-ready films from a photo or drawing without Photoshop.
If you've ever taken a design to a screen printer, you've probably heard the question: "Is it separated?" Most people aren't sure what that means. This guide explains it in plain words, then shows how to make separations yourself from a photo, a drawing or a poster you already have.
Screen printing lays down one ink at a time. Each ink needs its own screen, and each screen is made from a film: a black and clear transparency that shows exactly where that one ink goes.
So a three colour shirt needs three films. A separation is just the job of splitting your design into those films.
A good set of films has:
If your file has those four things, any shop can work with it.
Every extra ink is another screen, another setup and usually a higher price per shirt. That's why most shirt designs use one to three inks.
If you're printing a small run, start with one or two. You'll be surprised how good a photo looks in two inks.
There are two ways to turn a photo into a few flat inks.
Tone bands (spot colour). The image is split into a handful of tonal steps, and each step becomes one ink. Shapes are solid, edges are crisp. It looks like a stencil or a screen printed gig poster.
Halftone. Tones are made from dots of different sizes, like a newspaper. One ink can show a whole range from light to dark. This is how you get smooth photographic shading out of a single colour.
A quick rule: if your design is graphic, use bands. If it's a photo and you want the shading, use halftone. Keep halftone dots large enough for your printer's mesh. Very fine dots fill in on fabric, so ask your shop what they can hold.
The usual route is a mix of channels, posterize, threshold, bitmap mode and a lot of manual cleanup. For a halftone you switch to greyscale, then to bitmap mode with a halftone screen, once per ink. It works, but it's slow, and you need to know the steps well. Many people end up buying separation plug-ins.
Bitgrain's shirt print tool does the separation for you and shows it on a shirt while you choose.
A few details that save back and forth with your printer:
Send the film PDF and the print notes together. Tell them the shirt colour and quantities per size. That's everything they need to quote and burn screens, and it's a lot less likely to come back with questions.
Separations sound technical, but the idea is simple: one film per ink, at real size, lined up. Once you have that, screen printing gets a lot cheaper and a lot less confusing.
Try it with an image you already like at bitgrain.app/apparel and see how it looks in two inks.