Screen printing, separations and the underbase

One screen per colour

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.

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.

Preparing spot separations

For a design in flat spot colours, which is most garment work:

1. Build the artwork in flat colours only: no gradients, no transparency, no soft edges.
2. Put each colour on its own layer, named for the ink: RED, NAVY, WHITE, UNDERBASE.
3. Colour each layer’s artwork with a named spot swatch.
4. 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.
5. Include registration marks on every film, in the same position on each, so the screens can be aligned.

In 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.

The underbase, and the choke

On 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.

That 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.

Make 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.

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.

Halftones on a garment

For tonal areas you need halftones, and the mesh sets the limit.

• Mesh counts of 43 to 61 threads/cm (110 to 156 per inch) are common for garment work.
• A screen can hold roughly a 35 to 65 lpi halftone. Finer than that and the dots fall through the mesh or clog it.
• 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.
• Dot gain is large (over 30% is normal on textile) so a 50% tint prints as a solid-looking mass unless you compensate.

Ask the printer for their mesh count and preferred line screen. These are not numbers you can choose.

Simulated process

For 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.

Free 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.

Trapping on a press with millimetre registration

From 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.

Ask the shop for their trap value and their underbase choke. They have numbers and they will give them to you.

The checklist before films

1. Flat colours only, one layer per ink, named for the ink.
2. Underbase built and choked, thin features checked.
3. Registration marks on every separation, identical position.
4. All text curved, all strokes outlined, all overlaps welded.
5. Films at 1200 dpi, solid black.
6. Colour count confirmed with the shop, because it is the price.