{"course":{"slug":"vector-and-print","title":"Vectors, Logos and Things That Get Printed","url":"https://addaly.com/learn/vector-and-print","level":"beginner","track":"create"},"lesson":{"slug":"screen-print-separations","title":"Screen printing, separations and the underbase","url":"https://addaly.com/learn/vector-and-print/screen-print-separations","position":71,"of":73,"minutes":9,"answer":"Screen printing is one screen per colour, so build in flat colours on named layers, and on dark fabric add a white underbase choked inwards by 0.3 to 0.5 mm.","keyIdea":"Screen printing is one screen per colour, so build in flat colours on named layers, and on dark fabric add a white underbase choked inwards by 0.3 to 0.5 mm, with traps around 0.5 mm because manual carousel registration is measured in millimetres, not tenths."},"body":{"format":"markdown","text":"## One screen per colour\n\nA screen-printing frame holds a mesh with a stencil. Ink is pushed through the open areas with a squeegee. One screen prints one colour, so a four-colour design means four screens, four setups, four registration alignments.\n\nThat constraint is why so much good T-shirt and poster art looks the way it does: flat colours, bold shapes, deliberate overprints. It is a design language that came out of a process.\n\n## Preparing spot separations\n\nFor a design in flat spot colours, which is most garment work:\n\n1. **Build the artwork in flat colours only**: no gradients, no transparency, no soft edges.\n2. **Put each colour on its own layer**, named for the ink: `RED`, `NAVY`, `WHITE`, `UNDERBASE`.\n3. **Colour each layer's artwork with a named spot swatch.**\n4. **Export one film per layer**, as pure black on transparent, at **1200 dpi**: films are output as solid black positives, and the stencil is made by exposing emulsion through them.\n5. **Include registration marks on every film**, in the same position on each, so the screens can be aligned.\n\nIn Inkscape: build on layers, then export each layer separately (Layer ▸ Show/Hide, then export PNG at 1200 dpi, or use the *Export layers* extension). In Scribus or Illustrator, exporting separations from a spot-coloured file does the same job more directly.\n\n## The underbase, and the choke\n\nOn a dark garment, a colour printed directly disappears: the fabric shows through and changes the hue. So a layer of **white ink is printed first** as a base, flashed dry, and the colours printed on top of it.\n\nThat underbase is a copy of the whole design's coverage area, and it must be **choked**: shrunk inwards by **0.3 to 0.5 mm**, so that if the colour layers land slightly off, no white peeks out around the edges.\n\nMake it by duplicating the combined artwork, applying an inward offset, and putting it on the `UNDERBASE` layer. Check the thin parts: from the offsets lesson, an inward offset eats thin features, so a fine line may vanish or self-intersect.\n\nThe underbase also costs: an extra screen, an extra pass, and a flash-dry cycle between. It is why printing on white garments is cheaper than on black.\n\n## Halftones on a garment\n\nFor tonal areas you need halftones, and the mesh sets the limit.\n\n- Mesh counts of **43 to 61 threads/cm** (110 to 156 per inch) are common for garment work.\n- A screen can hold roughly a **35 to 65 lpi** halftone. Finer than that and the dots fall through the mesh or clog it.\n- **Angles** matter as much as in litho, to avoid moiré between the halftone and the mesh itself. Many printers use **22.5°** on every colour for garment work rather than the litho angle set, precisely because the mesh is a regular grid too.\n- **Dot gain is large** (over 30% is normal on textile) so a 50% tint prints as a solid-looking mass unless you compensate.\n\nAsk the printer for their mesh count and preferred line screen. These are not numbers you can choose.\n\n## Simulated process\n\nFor a photographic image on a dark shirt, **simulated process** separates the image into a set of flat spot inks (often six to eight) chosen for that image, rather than CMYK. It gives far better results on fabric than true process, and it is done with channels in Photoshop.\n\nFree equivalent: **Photopea** in a browser has channels, curves and the selective colour tools needed, and runs on a laptop that could never load Photoshop. GIMP can do it with more effort. Either way, this is a specialist skill and it is worth knowing it exists and what it is called when a client asks for a photograph on a black shirt.\n\n## Trapping on a press with millimetre registration\n\nFrom the colour module: trap width is a property of the process. On a manual carousel, registration can be out by a whole millimetre, so traps of **0.5 mm or more** are normal, vastly more than litho's 0.1 mm.\n\nAsk the shop for their trap value and their underbase choke. They have numbers and they will give them to you.\n\n## The checklist before films\n\n1. Flat colours only, one layer per ink, named for the ink.\n2. Underbase built and choked, thin features checked.\n3. Registration marks on every separation, identical position.\n4. All text curved, all strokes outlined, all overlaps welded.\n5. Films at 1200 dpi, solid black.\n6. Colour count confirmed with the shop, because it is the price."},"check":{"question":"A two-colour design on black T-shirts is printed with a white underbase built as an exact copy of the artwork's coverage. The shirts show a thin white halo around the coloured areas. What was omitted?","options":[{"text":"A flash-dry cycle between the underbase and the colours, which let the white bleed outwards under the wet colour layers.","correct":false,"tempting":"Missing flash-drying causes muddy colours and ink pick-up, not a clean white outline sitting proud of the coloured shapes."},{"text":"Trapping between the two colours, which should have overlapped them to hide registration drift.","correct":false,"tempting":"Trapping between the two colours addresses gaps where they meet each other; it does nothing about white showing at the outer edge of both."},{"text":"The underbase should have been printed last, on top, so any misregistration would be hidden beneath the colours.","correct":false,"tempting":"Printing white over the colours would simply obscure them. The underbase must go down first so the colours have something opaque to sit on."},{"text":"The underbase was not choked: shrinking it inwards by 0.3 to 0.5 mm keeps it hidden.","correct":true}]},"prev":{"slug":"cnc-and-built-up-letters","title":"CNC routing, tool radius, and the letters on a shop board","url":"https://addaly.com/learn/vector-and-print/cnc-and-built-up-letters"},"next":{"slug":"garment-printing-processes","title":"DTF, DTG, sublimation and heat transfer","url":"https://addaly.com/learn/vector-and-print/garment-printing-processes"},"authorship":"Founder and series editor: Dr Mustafa Mun. Written by AI models to a written standard. Exam keys checked blind by a second model: 3,665 of 3,666 agreed. Academic review invited.","review":{"status":"Draft, open for academic review"},"cite":"Addaly (2026) 'Screen printing, separations and the underbase', in Vectors, Logos and Things That Get Printed. Edited by M. Mun. Addaly. Available at: https://addaly.com/learn/vector-and-print/screen-print-separations (Accessed: 13 October 2026).","licence":{"name":"CC BY-NC-SA 4.0","url":"https://creativecommons.org/licenses/by-nc-sa/4.0/"},"reader":{"schema":1,"title":"Screen printing, separations and the underbase","courseTitle":"Vectors, Logos and Things That Get Printed","module":{"index":8,"slug":"machines-that-make-things","title":"Machines that cut, stitch and burn"},"answer":null,"keyIdea":"Screen printing is one screen per colour, so build in flat colours on named layers, and on dark fabric add a white underbase choked inwards by 0.3 to 0.5 mm, with traps around 0.5 mm because manual carousel registration is measured in millimetres, not tenths.","hero":{"kind":"plate","image":null,"alt":null,"caption":null},"blocks":[{"t":"h2","id":"one-screen-per-colour","runs":[{"text":"One screen per colour"}]},{"t":"p","runs":[{"text":"A screen-printing frame holds a mesh with a stencil. Ink is pushed through the open areas with a squeegee. One screen prints one colour, so a four-colour design means four screens, four setups, four registration alignments."}]},{"t":"p","runs":[{"text":"That constraint is why so much good T-shirt and poster art looks the way it does: flat colours, bold shapes, deliberate overprints. It is a design language that came out of a process."}]},{"t":"h2","id":"preparing-spot-separations","runs":[{"text":"Preparing spot separations"}]},{"t":"p","runs":[{"text":"For a design in flat spot colours, which is most garment work:"}]},{"t":"ol","start":1,"items":[[{"text":"Build the artwork in flat colours only","b":true},{"text":": no gradients, no transparency, no soft edges."}],[{"text":"Put each colour on its own layer","b":true},{"text":", named for the ink: "},{"text":"RED","code":true},{"text":", "},{"text":"NAVY","code":true},{"text":", "},{"text":"WHITE","code":true},{"text":", "},{"text":"UNDERBASE","code":true},{"text":"."}],[{"text":"Colour each layer’s artwork with a named spot swatch.","b":true}],[{"text":"Export one film per layer","b":true},{"text":", as pure black on transparent, at "},{"text":"1200 dpi","b":true},{"text":": films are output as solid black positives, and the stencil is made by exposing emulsion through them."}],[{"text":"Include registration marks on every film","b":true},{"text":", in the same position on each, so the screens can be aligned."}]]},{"t":"p","runs":[{"text":"In Inkscape: build on layers, then export each layer separately (Layer ▸ Show/Hide, then export PNG at 1200 dpi, or use the "},{"text":"Export layers","i":true},{"text":" extension). 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Check the thin parts: from the offsets lesson, an inward offset eats thin features, so a fine line may vanish or self-intersect."}]},{"t":"p","runs":[{"text":"The underbase also costs: an extra screen, an extra pass, and a flash-dry cycle between. It is why printing on white garments is cheaper than on black."}]},{"t":"h2","id":"halftones-on-a-garment","runs":[{"text":"Halftones on a garment"}]},{"t":"p","runs":[{"text":"For tonal areas you need halftones, and the mesh sets the limit."}]},{"t":"ul","items":[[{"text":"Mesh counts of "},{"text":"43 to 61 threads/cm","b":true},{"text":" (110 to 156 per inch) are common for garment work."}],[{"text":"A screen can hold roughly a "},{"text":"35 to 65 lpi","b":true},{"text":" halftone. Finer than that and the dots fall through the mesh or clog it."}],[{"text":"Angles","b":true},{"text":" matter as much as in litho, to avoid moiré between the halftone and the mesh itself. Many printers use "},{"text":"22.5°","b":true},{"text":" on every colour for garment work rather than the litho angle set, precisely because the mesh is a regular grid too."}],[{"text":"Dot gain is large","b":true},{"text":" (over 30% is normal on textile) so a 50% tint prints as a solid-looking mass unless you compensate."}]]},{"t":"p","runs":[{"text":"Ask the printer for their mesh count and preferred line screen. These are not numbers you can choose."}]},{"t":"h2","id":"simulated-process","runs":[{"text":"Simulated process"}]},{"t":"p","runs":[{"text":"For a photographic image on a dark shirt, "},{"text":"simulated process","b":true},{"text":" separates the image into a set of flat spot inks (often six to eight) chosen for that image, rather than CMYK. 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What was omitted?"}],"options":[{"text":[{"text":"A flash-dry cycle between the underbase and the colours, which let the white bleed outwards under the wet colour layers."}],"correct":false,"tempting":[{"text":"Missing flash-drying causes muddy colours and ink pick-up, not a clean white outline sitting proud of the coloured shapes."}]},{"text":[{"text":"Trapping between the two colours, which should have overlapped them to hide registration drift."}],"correct":false,"tempting":[{"text":"Trapping between the two colours addresses gaps where they meet each other; it does nothing about white showing at the outer edge of both."}]},{"text":[{"text":"The underbase should have been printed last, on top, so any misregistration would be hidden beneath the colours."}],"correct":false,"tempting":[{"text":"Printing white over the colours would simply obscure them. The underbase must go down first so the colours have something opaque to sit on."}]},{"text":[{"text":"The underbase was not choked: shrinking it inwards by 0.3 to 0.5 mm keeps it hidden."}],"correct":true,"tempting":null}],"order":[2,1,0,3],"why":null},"prev":{"slug":"cnc-and-built-up-letters","title":"CNC routing, tool radius, and the letters on a shop board","minutes":8},"next":{"slug":"garment-printing-processes","title":"DTF, DTG, sublimation and heat transfer","minutes":8}}}