Screen Printing on Plastics

Screen Printing on Plastics

Key Takeaways

• The thickest deposit of any plastics printing method, 8–30 µm, which is why it owns opaque white on dark resins.
• Mesh count and stencil thickness set the deposit; tension and off-contact set the edge.
• Polyolefins need pretreatment to 38–44 dyne/cm. ABS, PS, PC, PMMA and PVC print as moulded.
• Fixed artwork, flat or single-curvature parts. Variable data belongs to inkjet or laser.

Screen Printing on Plastics

Screen printing is a decorating process in which a squeegee forces ink through the open areas of a stencil carried on a tensioned mesh, depositing an ink film 8–30 µm thick onto the plastic part held beneath the screen. The mesh sits a few millimetres above the part, peeling away behind the squeegee to shear the ink out cleanly.

Flat-bed machines hold the part parallel to the screen; cylindrical machines traverse the screen under a fixed squeegee while the part rotates at matched surface speed (bottles, tubes, cups, syringes); conical machines distort the artwork onto an arc and tilt the part so a taper up to about 15° stays tangent to the screen.

Mesh, Stencil and Ink Deposit

The mesh is plain-woven monofilament polyester: thread count sets open area and deposit, thread diameter sets resolution. It is bonded to the frame at 18–30 N/cm and retired below about 15 N/cm, because a slack screen smears the edge at peel. The stencil, a photo-emulsion or capillary film, adds an emulsion-over-mesh (EOM) of 10–20% of mesh thickness, with a face below Rz 5 µm so the edge gaskets against the part instead of tracing the threads.

Ink Systems for Plastics

Ink is specified by series, not colour: each series is formulated for a resin family and cure method, and one that passes on ABS can fail on PET.

Process Parameters

The settings interact, and every compensation costs edge definition: set tension and off-contact first and touch the squeegee last.

Substrate Preparation and Design Rules

Target 38–44 dyne/cm surface energy: ABS, PS, PC, PMMA and PVC reach it as moulded; polypropylene and polyethylene, at 29–31 dyne/cm, never do. Flame and atmospheric plasma treat moulded parts and bottles; corona treats sheet and film. Treatment decays: print inside a window proven by trial, confirm with a dyne test at the press, and size the margin with the surface energy calculator.

Silicone mould release defeats every series; specify release-free moulding or a validated wash. Ionised air and 45–55% RH control the static that pulls filaments at peel and attracts dust.

• Minimum line width: 0.1–0.15 mm positive with 355–420 mesh and UV ink; 0.2–0.25 mm reversed-out, which floods inward from both sides.
• Registration: ±0.1–0.2 mm on a pinned fixture, 0.2–0.3 mm trap for multicolour; locate on moulded features, never the parting line.
• Halftones: 45–85 lpi, mesh at least four times the ruling, tones 10–90%, angles offset 22.5° from the mesh.
• Curvature: constant radius or single taper; compound curves and recesses go to pad printing. Keep the image off gates, parting lines and sinks.

Defects and Troubleshooting

Specifying and Verifying a Screen-Printed Decoration

“Screen print, white” specifies nothing. The drawing should carry ink series and part number, resin grade, pretreatment and dyne target, mesh count, dry film thickness, datums and acceptance tests with their timing; solvent and two-component inks keep curing for days, so a test at the press proves little.

• Adhesion: ASTM D3359 Method B at 24 h for UV and 72 h for solvent inks; state tape and minimum class, usually 4B.
• Abrasion: Taber (ASTM D4060, CS-10) or crockmeter to a stated cycle count; pencil hardness (ASTM D3363) screens cure.
• Chemical and weathering: ASTM D1308 spot tests with the fluids the product meets — sunscreen, DEET, sanitiser, IPA — and ASTM G154 for exterior parts.

Pad printing reaches compound curves but deposits 4–8 µm; inkjet prints variable full colour at 5–15 µm but needs tight surface-energy control; hot stamping gives specular metallics with no cure; in-mould decorating is durable but costliest to tool; laser marking is permanent and variable but has no white. Screen printing wins on opacity, deposit and cost at volume, and loses when artwork changes part to part (see plastics decorating methods compared).

Related Terms and Reading

• Decorating and finishing
• Surface pretreatment methods
• Inkjet printing on plastics
• Laser marking vs inkjet printing
• Abrasion and chemical resistance testing
• ASTM D3359 tape test

Applying this in production

The Sabreen Group provides independent engineering support for screen printing qualification, ink and pretreatment selection and decoration adhesion troubleshooting. If you are specifying a process, qualifying a material or troubleshooting a production problem, our engineering services team can help. Contact us to discuss your application.

Frequently Asked Questions

What mesh count should I use for screen printing plastics?

A 305–355 threads/inch mesh with UV ink gives 10–15 µm and handles most text and graphics. Drop to 200–260 for solvent inks and solid colour, 110–160 for opaque white or conductive inks, and go to 380–420 for halftones and 0.1 mm lines at the cost of opacity.

Can polypropylene be screen printed?

Yes, but not as moulded: at 29–31 dyne/cm no standard ink wets it. Flame or atmospheric plasma treat the part to 42–44 dyne/cm, print inside the treatment window, and use a polyolefin-rated UV or two-component series. Verify every resin lot; migrating slip additives shorten the window.

Why does a UV screen print pass the tape test at the press but fail later?

Usually undercure inside a thick or opaque deposit that a tack check misses; marginal surface energy, lost to hydrophobic recovery and additive migration; or sunscreen and hand oils attacking a film with no chemical resistance. Measure dose with a radiometer, test at 24 h and run ASTM D1308 with the real fluids.

How fine a line can screen printing hold on a plastic part?

About 0.1 mm positive on gloss with a 420 mesh, capillary-film stencil and hard squeegee; 0.15 mm is the comfortable production limit. Reversed-out lines need 0.2–0.25 mm because ink floods inward from both sides. Texture, curvature and solvent inks each add width; prove the limit on the production part.

---