CMYK separation for screen printing

First, the difference everything else follows from

CMYK uses four *translucent* process inks and builds colour by overprinting them, so light passes through the ink, reflects from the substrate and returns — the substrate is part of the result. Simulated process uses a larger set of opaque inks chosen for the image, each carrying its own tonal channel, commonly over a white underbase. Translucent versus opaque is the core decision. CMYK is suited to paper and light substrates; simulated process is designed to keep colour visible on dark garments, at the cost of more screens and more manual judgement.

Is your job a process job? Four conditions

You want most of them true. The garment is white or very light, because translucent process inks need a bright substrate. The artwork is genuinely photographic, with smooth continuous tone rather than areas of flat saturated colour. You have four heads free and want to avoid a larger opaque-ink set. Four process-colour screens also make a complex photograph simpler to quote, although an optional white underbase or highlight adds heads. Add paper, poster and tote work, where CMYK is standard. On a dark garment, compare simulated process or index instead; index avoids a halftone angle set but still has a base heuristic and dither pattern to inspect. If the design carries an exact brand colour, note that a process build is several overlapping dot plates at the mercy of registration and gain, whereas a separately mixed spot ink is the direct route to that colour.

Three decisions in the CMYK math — one exposed here

Gray component replacement decides how much neutral grey is built from equal parts cyan, magenta and yellow versus replaced with black ink. Heavy GCR uses less coloured ink and holds neutrals more stably; light GCR keeps richer colour but makes every grey depend on three screens registering. Undercolour removal limits total coverage in dark areas so four saturated plates do not pile onto one spot. The current renderer applies a fixed 80% GCR recipe with black beginning above a 10% neutral level and caps total C+M+Y+K coverage at 320%; the shipped workspace does not expose those three values. Dot-gain compensation is exposed as a slider, because gain depends on your mesh, emulsion, ink, squeegee and substrate. Print a step wedge on your setup and tune that control from what lands on fabric rather than assuming the fixed colour-separation defaults are a press profile.

Angles, frequency and the mesh ceiling

The renderer fixes CMYK screening at C 15°, M 75°, Y 0° and K 45°, with yellow placed where its pattern is less visible. The shipped client does not override those four angles; its custom angle control applies to the other separation types. Frequency is yours. Garment process work commonly runs 35 to 65 lpi, 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 around four to five-and-a-half times the lpi. Pick the coarsest frequency that holds the detail your artwork needs, set frequency, dot shape and output DPI explicitly, and test the fixed angle set against your output device, mesh and substrate.

How to run one here

Load the artwork at final print size and set the print width, because that width determines the source DPI check and the scale of the output plates. 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 no artwork, and gives you a palette to inspect. If that confirms a process job, switch to CMYK, accept the renderer's fixed GCR/UCR and C/M/Y/K angle recipes, set dot-gain strength, frequency, dot shape and output DPI, then render. Per-channel CMYK choke and density controls are not exposed. Inspect the proof and manifest in the downloaded ZIP before committing to film.

What you get out

Four plates — C, M, Y, K — with registration marks if you want them, a manifest recording the angles, frequency and effective output DPI actually used, and a proof rendered over your substrate colour. Selecting CMYK turns halftoning on and initially targets 600 dpi pre-halftoned 1-bit film; the renderer can clamp oversized output to its disclosed 16,800px/200MP limits. Turn halftoning off when a RIP should receive continuous-tone grayscale plates and screen them with its device profile. The paid CMYK deliverable today is a ZIP of plates, proof, README and manifest; it does not contain a multichannel PSD. The local layered and multichannel PSD buttons export the spot or index result currently in the browser, not the paid C/M/Y/K render.

Decide for free, pay for process conversion

A free spot or index pass helps answer whether the art is really photographic or wants discrete inks. It runs in the browser with no account and uploads no artwork; downloads, including its local PSD formats, require free sign-in. CMYK conversion, simulated-process channel construction and high-resolution plate rendering run on the server. That render is 10 credits per image, and for 24 hours afterwards you can re-render the same image as often as you like at no extra cost, including switching between CMYK and simulated process. 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.