TPU & Elastomers Guide
Thermoplastic polyurethane, silicone/LSR and rubber: hardness, overmolding and printing
Elastomers return to shape after large deformation: Shore A 60–95 hardness covers soft seals to rigid gaskets. Thermoplastic polyurethane (TPU) is injection-moldable and 3D-printable; liquid silicone rubber (LSR) molds to medical- and food-grade parts; neoprene/EPDM/Viton are thermoset rubbers for seals. The choice hinges on temperature, chemicals, hardness and process.
1. Elastomer families
TPU is a thermoplastic: it flows when heated, so it can be injection molded and FDM printed. Shore hardness 60A–75D covers soft gaskets to semi-rigid bumpers; UTS ≈34 MPa at 95A with elongation up to 500%+ and excellent abrasion resistance — the standard for phone cases, wheels and dampers.
Silicone (LSR) resists -60 to +200 °C, is biocompatible and food-safe, but must be molded/cured — it cannot be FDM printed. Thermoset rubbers (EPDM, Nitrile, Viton) are vulcanized; choose by chemical compatibility (oils → NBR/Viton, weather → EPDM).
2. Hardness and design
Shore A is the common hardness scale: 30–40A soft gaskets, 50–70A general seals and bumpers, 80–95A stiff rings and covers. Softer compounds need thicker sections to avoid extrusion under pressure and are harder to demold — draft angles of 2–5° are common.
Overmolding bonds an elastomer to a rigid substrate (TPE over PC/ABS, silicone over metal) in a second shot; bond strength depends on material pairing and surface prep. In printed TPU, infill density tunes stiffness over a wide range.
3. Performance limits
Temperature: TPU continuous use -40 to +80 °C; silicone -60 to +200 °C; Viton (FKM) to +200 °C with chemical resistance; EPDM excellent for ozone/weather but poor with oils. Compression set — permanent deformation under sustained load — favors silicones and some TPU grades for sealing.
Abrasion and tear: TPU is the best abrasion-resistant elastomer; silicone tears more easily, so design thicker sections. UV resistance favors silicones and EPDM; TPU needs UV-stabilized grades outdoors.
4. Selection and quoting
For 3D-printed prototypes use TPU (95A is a good balance); for molded production seals, LSR suits medical/food and extreme temperature, TPU suits mechanical damping and abrasion, EPDM suits weather, NBR/FKM suit oils. State the Shore hardness and any regulatory need (FDA, USP Class VI, food contact) on the drawing.
Elastomer parts are quoted by compound cost and mold/cure cycle; LSR and Viton are premium materials. Tolerances are inherently looser (±0.25–0.5 mm) because elastomers deflect during inspection — use functional gages rather than tight numbers.
Quick reference
| Material | Hardness | Temp range °C | UTS MPa | Elong % | Best for |
|---|---|---|---|---|---|
| TPU 95A | 95A | -40–80 | 34 | 350–500 | Abrasion, damping, print |
| TPU 60A | 60A | -40–80 | 25 | 600+ | Soft bumpers, gaskets |
| LSR silicone | 20–70A | -60–200 | 7–11 | 400–700 | Medical, food, extreme temp |
| EPDM | 40–80A | -45–150 | 8–15 | 300–600 | Weather seals, hoses |
| Nitrile NBR | 40–90A | -30–120 | 12–20 | 300–500 | Oil and fuel seals |
| Viton FKM | 60–90A | -20–200 | 10–16 | 150–300 | Chemicals, fuel, high temp |
Sources & references
- ASTM D2240 — Durometer hardness
- ASTM D412 — Rubber tensile
- ISO 7619 — Durometer hardness
- Xometry Learning Center — TPU & Elastomers Guide (public engineering reference)