Thermosets
Thermosets are resins that cross-link once when cured — Epoxy, phenolic (PF), melamine (MF), urea (UF), unsaturated polyester (UP), Vinyl Ester, bismaleimide (BMI) and Cyanate Ester — so they do not soften or remelt with heat. They are stiffer and more heat- and creep-resistant than most thermoplastics, but brittle and unweldable, and they are shaped by casting, compression molding, laminating or machining from cured sheet rather than injection molding from pellets. Cured epoxy runs about 60–90 MPa tensile; glass-epoxy laminates such as G11 reach 300 MPa or more.
Popular thermoset grades
Compare in the explorer| Grade | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Best for |
|---|---|---|---|---|---|
| Epoxy | — | 65 | 1.2 | Good | Potting, adhesives, tooling, laminate matrix |
| PF | — | 52 | 1.4 | Good | Heat-resistant molded handles, insulators, friction parts |
| MF | — | 70 | 1.5 | Fair | Hard, scratch-resistant tableware and switchgear |
| SMC | — | 75 | 1.85 | Poor | Compression-molded panels and housings |
| Vinyl Ester | — | 84 | 1.13 | Good | Chemical-resistant fiberglass tanks and pipe |
| BMC | — | 42 | 1.85 | Fair | Net-shape thermoset insulator |
| BMI | — | 72 | 1.3 | Fair | Between epoxy and polyimide |
Choose thermosets for
Electrical insulators, barriers, terminal boards and standoffs machined from G11, G7 or GPO-3 laminate; potted and encapsulated electronics (Epoxy); compression-molded housings, panels and handles (SMC, BMC, PF, MF); tooling boards and master patterns; friction parts (PF); and composite matrices that must stay stiff above 150 °C (BMI, Cyanate Ester, high-temperature Epoxy).
Look elsewhere when
The part needs impact toughness (thermosets are brittle — PC, PA66 or a TPU-modified grade), it will be injection molded in volume (an engineering thermoplastic runs faster and recycles), it must be welded or re-formed after manufacture (impossible once cured), it needs a clear optical surface (PMMA or PC unless it is a cast epoxy encapsulant), or it is a hot bearing where PEEK or PAI give both heat resistance and toughness.
Loading the library…
Thermoset by application
- Machined electrical insulators
- G11 (glass-epoxy, 180 °C) for the highest strength and dielectric performance; G7 (glass-silicone) for arc resistance and 220 °C; G5 and G9 (glass-melamine) where arc tracking resistance matters most; GPO-3 for polyester-glass barriers and bus supports in switchgear.
- Potting and encapsulation
- Epoxy (E-51 base) for rigid, high-adhesion potting of transformers, sensors and connectors; a flexibilized epoxy or polyurethane when thermal cycling would crack a rigid pot; Cyanate Ester for low-loss, low-outgassing encapsulation in RF and space hardware.
- Molded housings and panels
- SMC and BMC (glass-filled polyester) for large compression-molded panels, enclosures and motor housings with Class A surfaces; PF for small hot parts such as handles, knobs and lamp holders; MF and UF for electrical fittings and tableware.
- Hot composite structures
- BMI for carbon-fiber parts serving at 200–230 °C (engine nacelles, hot ducts); Cyanate Ester for antennas and optical benches needing low moisture uptake and dielectric loss; high-temperature Epoxy to about 150–180 °C where cost matters more.
Working with thermoset
- Casting and molding
- Liquid Epoxy, UP and Vinyl Ester are mixed, degassed and cast or laminated at room temperature, then post-cured at 60–120 °C to reach full properties; PF, MF, UF, SMC and BMC are compression- or transfer-molded at 140–180 °C in heated steel tools. Exotherm in thick epoxy castings can crack the part — pour in layers or use a filled, slow-curing system.
- Machining
- Cured laminates (G11, G7, GPO-3, phenolic) machine cleanly with carbide or PCD tools, but the glass is abrasive and the dust is a respiratory hazard — use dust extraction and expect fast tool wear. Tolerances of ±0.05 mm are routine; drilling leaves fuzz on exit unless backed. Machined thermosets do not move with heat afterwards, which is their main advantage over machined thermoplastics.
- Bonding and assembly
- Thermosets cannot be welded; join them with epoxy or acrylic structural adhesive after abrasion and solvent wipe (bonds are usually stronger than the laminate), or with threaded inserts, rivets and bolts. Do not tap fine threads in laminates — use inserts or through-bolts with washers to spread the load.
- Regulatory and service limits
- Continuous service: UP and Vinyl Ester about 100–120 °C, Epoxy 120–180 °C depending on hardener, PF and MF 150–180 °C, G11 180 °C, BMI 230 °C, Cyanate Ester 200–250 °C. PF, MF and GPO-3 laminates are often UL 94 V-0 and arc-resistant; Epoxy potting grades are available UL-recognized. Most thermosets yellow slowly in UV unless pigmented or coated.
Other polymer series
Thermosets — frequently asked
What is thermoset?
Thermosets are resins that cross-link once when cured — Epoxy, phenolic (PF), melamine (MF), urea (UF), unsaturated polyester (UP), Vinyl Ester, bismaleimide (BMI) and Cyanate Ester — so they do not soften or remelt with heat. They are stiffer and more heat- and creep-resistant than most thermoplastics, but brittle and unweldable, and they are shaped by casting, compression molding, laminating or machining from cured sheet rather than injection molding from pellets. Cured epoxy runs about 60–90 MPa tensile; glass-epoxy laminates such as G11 reach 300 MPa or more.
Which grade of thermoset should I use?
Epoxy for potting, adhesives, tooling and the matrix of structural laminates (E-51 is the standard bisphenol-A liquid resin, E-44 its higher-viscosity sibling). PF (phenolic) for heat-resistant, low-cost molded parts — handles, insulators, brake pads — and the paper or cotton laminates that machine into gears and wear blocks. MF for hard, scratch-resistant, food-safe tableware and switchgear; UF for the cheapest molded electrical fittings. UP and Vinyl Ester for fiberglass laminates and for SMC and BMC compression-molded panels and housings, with Vinyl Ester where chemical resistance matters. G11 and G7 laminates for machined insulators to 180 °C and beyond; GPO-3 for switchgear barriers. BMI and Cyanate Ester when a composite matrix must serve above 200 °C or hold electrical properties in radar and space hardware.
Which grade of thermoset is easiest to machine?
E-44 (machinability good, 60/100), then PF (60/100).
Epoxy or vinyl ester for a fiberglass part?
Epoxy bonds better to the fibers, shrinks less on cure (1–2 % versus 5–8 %) and gives about 20 % higher laminate strength and fatigue life, so it is the choice for structural and carbon-fiber parts. Vinyl Ester is cheaper, cures faster at room temperature and resists acids, chlorine and water better, which is why tanks, pipe, boat hulls and chemical-plant grating are made from it. Polyester (UP) is the lowest-cost option for non-critical glass parts.
Can a thermoset part be repaired or remolded?
No — once cross-linked it cannot be remelted, welded or reshaped, and scrap is not recyclable as resin. Damaged laminates are repaired by grinding back the damage and bonding a new layup or patch with the same resin family; molded phenolic and melamine parts are replaced. Design thermoset parts to be right the first time, and use inserts and adhesives for assembly instead of processes that need rework.