CMYK separation for screen printing

First, the difference everything else follows from

CMYK uses four *translucent* process inks and builds every colour by overprinting them, so the light has to pass through the ink, bounce off the substrate and come back out — which means the substrate is doing half the work. Simulated process uses six to ten *opaque* inks chosen to suit the specific image, each carrying its own tonal channel, sitting on a white underbase. Translucent versus opaque is the whole decision. It is why CMYK is magnificent on paper and on white cotton and disappointing on navy; it is why simulated process costs you four more screens and gives you a print that reads the same on black as on white.

Is your job a process job? Four conditions

You want most of them true. The garment is white or very light — process inks need a bright substrate to sit on, full stop. The artwork is genuinely photographic, with smooth continuous tone rather than areas of flat saturated colour. You have four heads free and would rather not tie up eight, which on a six-station press with two jobs queued is a real constraint and not a preference. And ink cost matters to the quote: four process inks in standard formulations are cheaper per print than a custom-mixed opaque set plus a white base plus a highlight white, and process is always exactly four screens however complicated the image gets, which makes it the cheapest way to print a complex photograph and the easiest to quote. Add paper, poster and tote work, where CMYK is simply the standard. If your job ticks those boxes, run process and do not overthink it. If it fails the first one — a dark garment — the honest answer is usually simulated process (opaque inks, reads the same on any colour) or index (no angles, far more predictable on press); both have their own pages, and index is free to try here. And if the design carries exact brand colours, note that a process build of a specific PMS number is three or four plates of overlapping dots at the mercy of registration and dot gain, whereas a spot ink mixed to that number simply is the colour.

The three settings that decide the print

These do most of the damage when they are left on a default. Gray component replacement decides how much of a neutral grey is built from equal parts cyan, magenta and yellow versus simply replaced with black ink. Heavy GCR uses less coloured ink, holds neutrals far more stably on press, and is generally what garment printing wants; light GCR keeps richer colour but makes every grey in the image depend on three screens registering perfectly, which on a garment press is optimistic. Undercolour removal trims total ink coverage in the darkest areas so you are not laying four saturated plates on the same spot and fighting to cure it. Dot-gain compensation applies an inverse curve so a 50% dot on your film is closer to 50% on the shirt after the ink spreads — skipping it is the single largest cause of muddy process prints. The strength is a slider rather than a fixed profile, because gain depends on your mesh, emulsion, ink, squeegee and substrate: print a step wedge once on your own setup, read where it lands, and that number is then the same for every job on that press.

Angles, frequency and the mesh ceiling

Screening is at the standard set of mutually non-interfering process angles, with yellow placed where the eye notices its pattern least; you can override any of them if your press has a set it is known to like. Frequency is yours. Garment process work commonly runs 35 to 65 lpi, against far finer on paper, and mesh is the hard ceiling: a screen cannot hold a dot finer than its own openings. The rule of thumb in circulation puts mesh count somewhere around four to five-and-a-half times the lpi — so 45 lpi sits comfortably on 205 mesh, and 55 lpi is a real pairing with 305. Going finer than the mesh supports does not print finer, it produces dots that break up, fill in or wash out of the emulsion. Pick the coarsest frequency that holds the detail the artwork actually has, and set frequency and output DPI explicitly rather than inheriting a default.

How to run one here

Load the artwork at final print size and set the print width, because every choke, spread and trap value is measured in pixels and only means something once the physical size is fixed. Clean the art first — remove a background it arrived on, and be aware that JPEG compression fringe becomes speckle on a plate. Then run a free spot or index separation in the browser: that costs nothing, uploads nothing, and tells you what you actually have. You will see the ink count the design wants, whether flat brand colours are hiding in what looked like a photograph, and how the palette sits over your garment colour in the proof. If that confirms a process job, switch the separation type to CMYK, set your GCR and undercolour removal, your dot-gain strength, frequency, dot shape and output DPI, and render. Read the proof over your substrate colour before you commit to film — that is the composite the customer will see, and it is the only view that catches a decision you got wrong.

What you get out

Four plates — C, M, Y, K — screened at the standard angles, with registration marks if you want them, a manifest recording the angles and frequency actually used, and a proof rendered over your substrate colour. Output is continuous-tone greyscale plates by default, because that is what most RIPs want: a RIP screens the plates itself with curves matched to its output device, and handing it film you have already screened gives it a dot grid to re-screen, which moirés against your own artwork. Switch to pre-halftoned 1-bit film at 600 or 1200 dpi only when there is no RIP in the chain — printing to an inkjet or laser through a plain driver. Separately, and free in your browser with no upload, you can export a multichannel PSD carrying one real Photoshop spot channel per ink with its colour and name attached, so the file behaves like a separation in Photoshop rather than a stack of pictures — one PSD export a day on a free account, uncapped on a monthly plan.

Decide for free, pay only for film

The free half answers the decision, which is the expensive part to get wrong. Spot and index separation run in the browser with no account and no upload, and both PSD exports are free too. CMYK conversion, simulated-process channel construction and halftone screening run on the server, because plate rendering at print resolution is not a browser job — a 14-inch plate at 1200 dpi is a 16,800-pixel image. That render is 10 credits per image, and for 24 hours afterwards you can re-render the same image as often as you like, free, for 24 hours, so changing your mind and running it the other way costs nothing. Credits come with a monthly plan, or as a one-off pack whose credits never expire — see the pricing page for current prices in your currency. No watermarks on any output at any tier.