The short answer
- Spot colour — one screen per flat colour. The most common method, works on any garment
- Four-colour process — CMYK halftones. Best for photographic work on light garments
- Simulated process — halftones in opaque spot inks. Photorealistic on darks, more vivid than CMYK
- Index — uniform square dots, no halftone angles. Easiest to print, uses more colours
- Garment colour narrows the choice faster than the artwork does
Separation is the step that turns a design into screens. It decides how many you burn, how the press is set up, how forgiving the job is to register, and whether the printed result looks like the file. It is also where a lot of shops accept whatever their software produces without knowing there were four options.
The four methods are genuinely different, not variations on a theme, and choosing correctly is worth more to the final print than almost anything that happens at the press.
What is a separation actually doing?
A screen prints one colour. A design with more than one colour has to be split into layers, one per screen, so that overlaying them in sequence reconstructs the image. That split is the separation, and the four methods differ in how they represent colours that are not flat — gradients, blends, photographs, shadow.
Flat colour is easy: a shape either is that colour or is not. Continuous tone is the hard part, because screen printing cannot lay down half a colour. The solutions all come down to breaking tone into dots small enough that the eye blends them, and the four methods do that differently.
Spot colour
Spot separations split the design into individual layers without gradients or halftones. Each colour gets one screen and one film positive, and prints as a solid. It is the most commonly used method in screen printing and the right default for the majority of decorating work.
It suits designs with a limited number of flat colours — logos, text, club crests, most commercial artwork. It works equally on light and dark garments, because each ink is laid down solid and opacity is a matter of the ink rather than of dot coverage.
Its limitation is exactly its definition: it cannot represent continuous tone. A design with a gradient either gets that gradient converted to halftone within the spot layer, or the gradient has to go. For most jobs, that is not a compromise — it is the correct call, because a clean flat print ages better than a poorly reproduced blend.
Four-colour process
CMYK process uses four transparent inks — cyan, magenta, yellow, black — in halftone dots at different angles. Overlaying them reproduces a full colour range from four screens, which is remarkable and comes with one hard constraint.
Process inks are transparent. They rely on the substrate beneath being white, because the paper or garment is doing the work of the white in the image. On a dark garment there is nothing to reflect back, so CMYK process is a light-garment method and a white-garment method in particular.
It is the right choice for photographic artwork on white or very pale garments where four screens beats the alternative. It is unforgiving of registration — four halftone screens misaligned by a fraction produce visible moiré or colour fringing — so it wants a press and an operator that can hold registration precisely.
Process ink is transparent. On a dark shirt there is nothing underneath it to see.
Simulated process
Simulated process is the answer to that problem, and it is the method most shops reach for on photographic work. It uses halftone dots like CMYK, but in a range of opaque spot inks rather than four transparent ones, usually over a white underbase.
Because the inks are opaque, it prints on dark garments. Because you choose the ink set to suit the image rather than being fixed to four process colours, results tend to be more vivid than four-colour process. It works on light garments too, which makes it the more flexible of the two photographic methods.
The cost is screens. A simulated process separation commonly runs six to eight colours plus an underbase, which means more screens, more setup and more press time. That is a real cost per job and it is the line that does not shrink with quantity, so simulated process work needs either a large run or a price that reflects the setup.
Index
Index separation, done in Photoshop, replaces halftone dots with random square dots all of the same size — a stochastic or diffusion dither. Rather than varying dot size to represent tone, it varies dot density.
That has one large practical advantage: there are no halftone angles, so there is no moiré, and you are printing square dots beside square dots rather than halftone dots on top of one another. Printers consistently describe it as the easiest of the tonal methods to actually run, which matters if your registration is manual or your press is not new.
It works on light and dark garments. Its trade-off is that it typically needs more colours than simulated or process separation to reach the same fidelity, so you are trading screens for press-side simplicity. For a shop where press time is the constraint and screens are cheap, that can be an excellent trade.
Which one for which job?
| Artwork | Garment | Method | Screens |
|---|---|---|---|
| Flat colours, logo, text | Any | Spot | One per colour |
| Photograph | White or very light | Four-colour process | 4 (+ underbase rarely) |
| Photograph | Dark | Simulated process | 6–8 + underbase |
| Photograph, manual press | Light or dark | Index | More, but easier to run |
| Gradient inside a flat design | Any | Spot with halftone in one layer | One per colour |
Two things decide this faster than anything else. Garment colour eliminates four-colour process the moment the shirt is dark. And your press eliminates the tightly registered methods if you are running manual — index exists precisely for that case.
Quantity matters too, in the direction people forget. A photographic design on twelve dark shirts is a simulated process job with eight screens, which is a great deal of setup for a dozen pieces. That is almost always a job for a digital process instead, and recognising it early saves the customer money and saves you a difficult quote.
What should you send the separator?
If you outsource separation — many shops do, and it is skilled work — the brief matters as much as the file. A separator given artwork and nothing else has to guess at half a dozen decisions, and the guesses are where the job goes wrong.
Send all of it
- The highest-resolution original you have, layered if possible
- The finished print size, in millimetres or inches
- The garment colour, and whether the run includes more than one
- Your maximum screen count — this is a commercial constraint, not a technical one
- Whether the press is manual or automatic
- Any brand colours that must match, with their references
- Your mesh counts and typical halftone line count
The screen count line is the one shops leave out and it is the most useful thing on the list. A separator who does not know your limit will produce the best separation the artwork supports, which might be ten colours on a job you quoted for four. Telling them the ceiling turns an impossible file into a good separation within your budget.
The same applies to what arrives from the customer in the first place. A separation is only as good as the artwork under it, and a logo supplied as a compressed screenshot cannot be separated cleanly at any price — which is why the rules about what to accept at upload matter more here than anywhere else. The rest of the artwork archive assumes you are choosing the method rather than accepting a default.
