Lesson 11.3CoatChapter 11 · Coatings and surface finishing

Screen printing

Ink pushed through a mesh stencil

Screen printing pushes ink through a fine mesh that is blocked everywhere except where the design should appear. It prints the graphics on many T-shirts, the white lettering on circuit boards and the silver lines on solar cells. It lays down thick, opaque ink on almost any surface, flat or curved.

Photo: Ahmad Ali Karim, CC0, via Wikimedia Commons

Screen printing · cross-section of a print stroke, stencil making and dryingCircled numbers match the steps below. The squeegee travels left to right, so the left side is already printed.

How it works

Make the stencil. A fine mesh of polyester or steel threads is stretched on a frame and coated with emulsion (a light-sensitive liquid). UV light shines through a film printed with the design in black. The light hardens the emulsion everywhere except under the design, which washes out with water and leaves open mesh.
Set up the screen. The screen is clamped a few millimeters above the substrate (the part being printed). This off-contact gap lets the mesh spring away after printing. The part is registered (positioned exactly) against stops or held by a vacuum bed.
Flood the screen. Ink is poured onto the screen and spread over the design with a light pass. This fills the open mesh without printing yet.
Make the print stroke. A squeegee (a flexible rubber blade) presses the mesh down onto the part and sweeps across. Ink is forced through the open mesh, while the stencil blocks it everywhere else.
Snap off. Just behind the blade, the tensioned mesh springs back up and peels away from the wet ink. It leaves a clean, sharp deposit, typically a few to several tens of micrometers thick.
Dry or cure. The part goes through a dryer. Hot air or infrared cures plastisol inks at about 160 °C, while UV inks harden under UV lamps in seconds. Each extra color needs its own screen and its own pass.

Two cousins work the same way. Rotary screen printing uses a cylindrical screen with the squeegee inside, for fast printing of fabric and labels. Stencil printing replaces the mesh with a laser-cut metal foil to put solder paste on circuit boards.

Key facts

Materials: Plastisol, water-based, solvent-based and UV-curing inks, plus functional pastes (silver, carbon, insulators); printed on fabric, paper, plastic, glass, metal and ceramics
Typical parts: T-shirts, posters, bottles, control panels and nameplates, membrane switches, circuit boards, solar cells
Part size: From small labels to sheets over a meter wide; bottles and other curved items on special presses
Precision: Lines down to roughly 0.1 mm in graphics and a few tens of µm on solar-cell printers; color-to-color registration about ±0.1 mm
Surface finish: A raised ink layer, matte to gloss depending on the ink; plastisol on fabric feels slightly rubbery
Batch size it suits: From a few dozen prints to millions; screens are cheap, but each color needs setup
Tooling and machine cost: A screen costs tens of dollars, a manual press hundreds to thousands; automatic and electronics printers tens to hundreds of thousands
Speed: A manual press makes dozens of prints an hour; automatic presses hundreds to over a thousand; solar lines thousands of cells an hour

Strengths and limits

Strengths

Thick, opaque, durable ink that covers even dark surfaces
Prints on almost any material, flat or curved
Cheap tooling and fast once set up
Can print functional pastes: conductors, resistors, solar-cell contacts

Limits

Each color needs its own screen and setup, so many-color designs cost more
Fine detail is limited compared with offset or inkjet printing
Setup and cleanup make very short runs expensive per piece
Some inks and cleaning chemicals need ventilation and careful disposal

Where it's used

Apparel and textilesT-shirts, sports jersey numbers, tote bags, printed fabric on rotary screens
ElectronicsSolder mask and white legend text on circuit boards, membrane switches, sensors
EnergySilver contact lines on silicon solar cells
Automotive and glassBlack border band and heating lines on rear windows, dashboard dials
Packaging and signsGlass bottles, cosmetic tubes, posters, signs and decals
ArtLimited-edition prints and posters

How to recognize it

A slightly raised ink layer you can feel with a fingertip, especially on T-shirts.
Solid, flat colors with sharp edges, each color a separate layer; no dot pattern unless the artwork used halftones (dot patterns that fake shades).
Small misalignments between colors (registration errors), seen as thin gaps or overlaps.
On glass and circuit boards, crisp white or black lettering sitting on top of the surface.

Compare with

Spray painting11.1covers a whole surface with a thin film; screen printing puts ink only where the stencil is open.Chemical machining and photochemical etching6.8uses a similar UV-exposed mask, but to etch metal away instead of adding ink.Photolithography13.2also patterns with light, but at micrometer scale to make computer chips.

Watch it

Taino Ink · The whole process for beginners, from screen to dried print

Minneapolis Institute of Art · An artist's screenprinting process, explained by a museum

PepWear · T-shirt production in a working print shop

ROQ · An automatic carousel press that prints several colors in turn

See real photos

Wikipedia: Screen printing — history, mesh, emulsion and squeegee, plus electronics and solar uses, with photos
Wikimedia Commons photo search

Remember

Screen printing is a stencil on a mesh: ink goes through where the mesh is open and nowhere else.
A squeegee pushes the ink through, and the mesh snaps away to leave a clean, thick deposit.
One screen per color; cheap tooling makes it work for small runs and huge ones.