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 molded.
- 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 millimeters 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.
| Mesh count (threads/in) | Thread diameter | Wet deposit | Typical use |
|---|---|---|---|
| 90–160 | 65–100 µm | 25–30+ µm | Opaque whites, conductive inks |
| 200–260 | 40–55 µm | 15–25 µm | Solvent graphics, solid color |
| 305–355 | 31–35 µm | 10–15 µm | UV inks, fine text, process color |
| 380–420 | 27–31 µm | 8–12 µm | Halftones, 0.1 mm lines |
Ink Systems for Plastics
Ink is specified by series, not color: each series is formulated for a resin family and cure method, and one that passes on ABS can fail on PET.
| Ink system | Cure | Adhesion mechanism | Substrate fit |
|---|---|---|---|
| Solvent-based | Evaporation, air or 60–80 °C dryer, 1–5 min; dry film half of wet | Solvent bite into amorphous resins | PVC, PS, ABS, PMMA, PC; PET and polyolefins with dedicated series and pretreatment |
| UV-curable | Free-radical polymerization in seconds at 200–600 mJ/cm², mercury or 385–395 nm LED | Wetting only; surface energy decides | PC, PMMA, PS, ABS, PET, rigid PVC; pretreated PP and PE with a polyolefin series |
| Water-based | Evaporation, forced air 60–90 °C; slowest | Wetting only; poor spread at 35–45 dyne/cm | Treated PET, some PVC; rarely durable on molded parts |
| Two-component | Isocyanate or epoxy crosslinking; 4–8 h pot life; 30–60 min at 60–80 °C or days at ambient | Crosslinked film on a pretreated or polar surface | Pretreated PP and PE, nylon, PBT, acetal; parts exposed to solvents or sunscreen |
Process Parameters
The settings interact, and every compensation costs edge definition: set tension and off-contact first and touch the squeegee last.
| Parameter | Typical range | Effect |
|---|---|---|
| Squeegee hardness | 60–85 Shore A; 65–75 general, 80–85 fine detail, 60–65 heavy deposit | Harder shears cleaner, deposits less |
| Squeegee angle | 65–80° to the screen | Shallower pushes more ink through, widens lines |
| Squeegee speed | 100–500 mm/s | Faster reduces deposit; too fast skips |
| Squeegee pressure | Lowest that clears the mesh | Excess forces ink under the stencil |
| Off-contact | 2–5 mm flat-bed, scaled to frame size; 1–3 mm cylindrical | Too little smears; too much stretches the image |
| Flood stroke | Flood bar 1–2 mm above the mesh, no pressure, every cycle | Fills the openings; stops drying-in |
| UV dose | 200–600 mJ/cm²; more for opaque white and black | Verified by radiometer, never lamp hours |
Substrate Preparation and Design Rules
Target 38–44 dyne/cm surface energy: ABS, PS, PC, PMMA and PVC reach it as molded; polypropylene and polyethylene, at 29–31 dyne/cm, never do. Flame and atmospheric plasma treat molded 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 mold release defeats every series; specify release-free molding or a validated wash. Ionized 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 multicolor; locate on molded 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
| Symptom | Likely cause | Correction |
|---|---|---|
| Pinholes | Dust on part or screen; stencil pinholes; silicone contamination | Ionized-air clean; block out; trace the release agent |
| Sawtooth edges | EOM too low; stencil following the thread profile | Raise EOM to 15–20%; capillary film; finer mesh |
| Ink bleeding | Excess pressure; thin ink; slack mesh dragging | Less pressure; harder squeegee; re-tension to 20+ N/cm |
| Poor adhesion | Low surface energy; release agent; wrong series; undercure | Measure dyne level, pretreat, wash; confirm the series; test at 24 h |
| Incomplete cure | Lamp aged; thick or dense deposit; LED lamp with mercury ink | Radiometer check; lamps at 1,000 h; thinner deposit or two passes |
| Mesh marks | Ink drying in; coarse mesh; no leveling time | Retarder; finer mesh; flow agent; delay before the lamp |
| Spider webs | Static pulling filaments at peel; dry air | Ionizing bar; 45–55% RH; antistatic additive |
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, sanitizer, IPA — and ASTM G154 for exterior parts.
Pad printing reaches compound curves but deposits 4–8 µm; inkjet prints variable full color at 5–15 µm but needs tight surface-energy control; hot stamping gives specular metallics with no cure; in-mold 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 color, 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 molded: 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.
