Elastomers
Elastomers are the rubbers: lightly cross-linked or physically networked polymers that stretch 200–800 % and recover. Thermoset rubbers (NBR, EPDM, FKM, Silicone, natural rubber) are vulcanized and cannot be remelted; thermoplastic elastomers (TPU, TPE, TPV) mold and print like a plastic and behave like a rubber. Hardness runs from about 10 Shore A gels to 95 Shore A urethanes, and the choice is made by what the part touches and how hot it gets.
Popular elastomer grades
Compare in the explorer| Grade | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Best for |
|---|---|---|---|---|---|
| TPU | 10 | 40 | 1.18 | Poor | Molded and printed flexible parts; grips, cases |
| NBR | 8 | 16 | 1.25 | Poor | Oil, fuel and hydraulic seals to about 100 °C |
| EPDM | 5 | 12 | 1.15 | Poor | Weather, ozone, steam and coolant seals |
| FKM | 5 | 12 | 1.82 | Poor | Fuel, hot oil and chemicals to 200 °C |
| Silicone | 5 | 9 | 1.15 | Poor | Food, skin, medical; –60 to 200 °C |
| PU Cast | 6 | 12 | 1.1 | Poor | Toughest wheels, bumpers, wear pads |
| TPE | 5 | 8 | 0.96 | Poor | Soft-touch overmolds and low-cost grips |
Choose elastomers for
O-rings, gaskets and lip seals matched to the fluid (NBR, HNBR, FKM, EPDM); grips, bumpers, feet, wheels and straps (TPU, PU Cast, TPE); overmolded soft-touch zones on rigid housings (TPE, SEBS, TPU); vibration mounts and dampers (NR, SBR, Silicone); printed flexible parts (TPU, Agilus); and any part that has to stretch, seal, cushion or grip.
Look elsewhere when
The part must carry load without deflecting (a rigid engineering plastic or a spring), it needs a precise dimension held under compression (elastomers creep and take a compression set — design the seal groove, not the seal, to control it), the service temperature exceeds about 250 °C (FFKM, then metal or graphite seals), or the volume is a few hundred pieces of a simple soft shape, where PU Cast or cast silicone in a silicone mold is faster than a rubber compression tool.
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Elastomer by application
- Seals in oil and fuel
- NBR for mineral oil, grease and hydraulic fluid; HNBR for the same fluids above 100 °C or in sour gas; FKM for fuels, hot engine oil and most chemicals to 200 °C; FFKM for aggressive solvents and semiconductor process chemistries.
- Outdoor and hot-water seals
- EPDM for sunlight, ozone, steam and glycol coolant (it swells in oil); Silicone where the temperature range is wider (–60 to 200 °C) or the part touches food or skin; CR (neoprene) as the general-purpose weather-and-oil compromise.
- Grips, wheels and bumpers
- TPU (85–95 Shore A) for molded and printed grips, cases and wheels; PU Cast for the toughest rollers, bumpers and wear pads; TPE and SEBS overmolds for soft-touch handles that cost less and bond to PP.
- Printed flexible parts
- TPU on FDM, SLS and MJF for functional gaskets, hinges and lattices (about 85–95 Shore A); Agilus on PolyJet for 30–95 Shore A multi-material rubber-like prototypes that look and feel molded.
Working with elastomer
- Molding and casting
- Thermoset rubbers (NBR, EPDM, FKM, Silicone) are compression-, transfer- or injection-molded and vulcanized at 150–200 °C in steel tools; TPU, TPE and TPV injection mold like a plastic, with 1–2 % shrinkage and no cure step, and overmold directly onto PC, ABS or PP. For 10–200 soft parts, casting PU Cast or silicone in a silicone mold avoids tooling.
- Machining and cutting
- Rubber does not machine well: soft grades deflect and tear. Gaskets and pads are die-cut, waterjet-cut or laser-cut from sheet; hard urethane above 90 Shore A and PU Cast turn and grind with sharp tools and coolant, or are frozen first. Design for cut sheet, molded or cast shapes rather than machined ones.
- Bonding and assembly
- TPU and TPE bond to their base plastic by overmolding and take cyanoacrylate and polyurethane adhesives; NBR, EPDM and NR bond with cyanoacrylate or rubber cement after abrasion; FKM needs a fluoroelastomer primer; Silicone bonds only to silicone or a primed surface. Ultrasonic welding works for TPU, not for thermoset rubbers.
- Durometer and compression set
- Specify Shore A hardness (typical tolerance ±5 A) and compression set at temperature, not just tensile strength. Seals usually run 60–90 Shore A; grips 40–70 A; cushioning 10–40 A. Every elastomer hardens with heat and time — EPDM and Silicone the least, NBR and NR the most — so state the service temperature and the fluid on the drawing.
Other polymer series
Elastomers — frequently asked
What is elastomer?
Elastomers are the rubbers: lightly cross-linked or physically networked polymers that stretch 200–800 % and recover. Thermoset rubbers (NBR, EPDM, FKM, Silicone, natural rubber) are vulcanized and cannot be remelted; thermoplastic elastomers (TPU, TPE, TPV) mold and print like a plastic and behave like a rubber. Hardness runs from about 10 Shore A gels to 95 Shore A urethanes, and the choice is made by what the part touches and how hot it gets.
Which grade of elastomer should I use?
TPU for anything molded or printed that must be tough and flexible — grips, wheels, belts, cable jackets, protective cases — with TPU-Ether for hydrolysis resistance and TPU-Ester for oil and abrasion. NBR for oil, fuel and hydraulic seals to about 100 °C; HNBR when the same seal runs hotter. EPDM for weather, ozone, steam and hot water (not oils) — outdoor gaskets, coolant hoses, window seals. FKM for fuel, hot oil and chemicals to 200 °C; FFKM only when nothing else survives the chemistry. Silicone for food, skin, medical and –60 to 200 °C. PU Cast for the toughest wheels, bumpers and wear pads; TPE and SEBS for soft-touch overmolds and low-cost grips.
Which grade of elastomer is the strongest?
TPA (PEBA) has the highest yield strength in the series at 20 MPa; TPC (COPE/TPEE) follows at 18 MPa. Values are for each grade's default condition.
Which grade of elastomer is easiest to machine?
TPO (machinability fair, 50/100), then CSM (35/100).
TPU or cast polyurethane?
TPU is a thermoplastic: it injection molds, extrudes and 3D prints, comes in 60 Shore A to 75 Shore D, and is the choice for volume parts and printed flexible prototypes. PU Cast is a thermoset poured into a mold and cured: it reaches higher tear and abrasion resistance, larger and thicker sections without sink, and any durometer per batch, but each part needs a mold cycle. Low volume, thick, or the toughest wheels and pads: cast. High volume or printed: TPU.
NBR, EPDM or FKM for an O-ring?
Match the fluid: NBR for mineral oils, greases and fuels to about 100 °C; EPDM for water, steam, glycol coolants, brake fluid and outdoor weather (never mineral oil — it swells); FKM for fuels, hot oils, acids and most solvents to 200 °C. If the seal sees both oil and ozone, HNBR or FKM; if it sees ketones, EPDM or FFKM.