BMI
BMI (bismaleimide) is a high-temperature addition-cure thermoset used mainly as a composite matrix, supplied as neat and toughened resin, RTM formulations, glass and carbon prepreg (unidirectional, fabric, chopped molding compound) and silica- or graphite/PTFE-filled compounds.
Reference datasheet — BMI is not in the instant-quote catalog. Upload your part and an engineer will quote it in this grade or suggest the closest stocked equivalent.
Values for the NEAT condition
BMI grades and fills
BMI is supplied in 9 conditions, soft to hard. NEAT is the default FabDigit quotes; the values on this page are for it.
Compare all 9 grades and fillsHide
| Grade · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness |
|---|---|---|---|---|---|
Silica / mineral filled BMI | Pick for high-Tg encapsulation, chip-carrier and dimensionally stable insulating parts where CTE must approach that of metal/ceramic. | 60 | 0.8 | ||
Graphite / PTFE lubricated BMI | Self-lubricating high-temperature bushings, wear pads and seal rings running dry to about 230 °C. | 60 | 1 | 88 Shore D | |
NEATDefault Neat cured resin | Baseline cast/cured BMI used for property benchmarking, tooling resin, potting and small high-temperature parts. | 72 | 2 | ||
Low-viscosity RTM / infusion grade | Choose for resin transfer moulding or VARTM of thick high-temperature parts where injection viscosity below ~0.5 Pa·s at 120–140 °C is required. | 75 | 2.2 | ||
Toughened BMI | Pick when damage tolerance / CAI and hot-wet toughness matter (primary aerostructure prepreg) and a 20–40 °C Tg loss is acceptable. | 82 | 3.5 | ||
Chopped carbon-fiber BMI molding compound | For net-shape compression-molded brackets, fasteners, seals and bushing bodies at 200 °C+ where laminate lay-up is uneconomic. | 150 | 1 | ||
Glass-fabric reinforced BMI laminate (~60 vol% E-glass) | Use for high-temperature electrically insulating structures, radomes, motor/generator parts and high-Tg PCB substrates (BT/BMI laminate). | 450 | 1.8 | ||
Woven carbon-fabric/BMI laminate | Choose for drapeable, balanced-property high-temperature panels, ducts and hot tooling where UD tape is impractical. | 800 | 1.2 | ||
Unidirectional carbon-fiber/BMI laminate | The main structural form of BMI: hot engine nacelles, missile/hypersonic skins, supersonic aircraft structure needing 180–230 °C service. | 2,100 | 1.5 |
What is BMI?
BMI (bismaleimide) is a high-temperature addition-cure thermoset used mainly as a composite matrix, supplied as neat and toughened resin, RTM formulations, glass and carbon prepreg (unidirectional, fabric, chopped molding compound) and silica- or graphite/PTFE-filled compounds. Engineers choose it when epoxy runs out of temperature: 230 °C service, 290 °C Tg, 72 MPa tensile strength, 4 GPa modulus, 2% elongation and 1.3 g/cm³ density. A 227–250 °C freestanding postcure is mandatory, prepreg cost and out-time control are real constraints, and machining is abrasive (machinability 50); cyanate ester or polyimide are the neighboring choices. BMI has a density of 1.3 g/cm³ (0.047 lb/in³), heavier than 69% of polymer grades. Its glass-transition temperature is 290°C (554 °F). BMI has a tensile strength of 72 MPa (10.4 ksi), stronger than 78% of polymer grades. Elongation at break is 2% and the elastic modulus is 4 GPa (0.58 Msi).
What is BMI used for?
- Aerospace composite structures
- Engine nacelle components
- High-temp printed circuit substrates
Advantages
- High service temperature — 230°C (446 °F), better than 91% of polymer grades
- Stiff — 4 GPa (0.58 Msi), better than 86% of polymer grades
- Low, predictable mold shrinkage — 0.4%, better than 82% of polymer grades
Limitations
- Difficult to weld — 0/100, worse than 98% of polymer grades
- Low fracture toughness — 0.6 MPa·√m (0.546 ksi·√in), worse than 96% of polymer grades
- Limited ductility — 2%, worse than 95% of polymer grades
- Limited formability — 5/100, worse than 95% of polymer grades
- High embodied carbon — 9.5 kg CO₂/kg, worse than 89% of polymer grades
BMI properties
Typical room-temperature values for BMI-NEAT; bars are the position among polymer grades.
Physical4
BMI has a density of 1.3 g/cm³ (0.047 lb/in³), heavier than 69% of polymer grades. Its glass-transition temperature is 290°C (554 °F).
Mechanical13
BMI has a tensile strength of 72 MPa (10.4 ksi), stronger than 78% of polymer grades. Elongation at break is 2% and the elastic modulus is 4 GPa (0.58 Msi).
Thermal10
BMI is rated for continuous service to 230°C (446 °F). It conducts heat at 0.21 W/m·K (0.121 BTU/hr·ft·°F), better than 55% of polymer grades. Thermal expansion is 40 µm/m·K (22.2 µin/in·°F).
Electrical7
BMI is an electrical insulator with a dielectric strength of 20 kV/mm (508 V/mil) and a resistivity of 1×10¹³ Ω·m.
Chemical & Environmental3
Corrosion resistance is excellent (85/100), better than 65% of polymer grades. UV resistance is fair.
Sustainability4
Producing a kilogram of BMI takes about 190 MJ of energy and emits 9.5 kg of CO₂ — more than 88% of polymer grades. Typical recycled content is 0%.
Manufacturability7
BMI's machinability is fair (50/100, worse than 73% of polymer grades); weldability not recommended (0/100); formability not recommended (5/100).
Common Calculations4
Values are nominal handbook figures for design screening. Certified mill or lot data ships with every FabDigit order on request.
Strength versus weight in the polymer family
Yield strength against density for every polymer grade FabDigit runs. BMI is highlighted; hover a dot for its name, or open the interactive family chart.
Working with BMI
Is BMI easy to machine?
Machine fully cured and postcured stock with PCD or diamond-coated carbide, high speed / low feed, sharp tools and vacuum dust extraction; fiber-reinforced laminates are highly abrasive and prone to delamination on drill exit — use backing plates and dagger/brad drills, coolant mist to keep the surface below Tg.
Can BMI be welded?
Not weldable — thermoset network; join by high-temperature adhesive bonding (BMI or polyimide film adhesive), co-cure/co-bond, or mechanical fasteners with titanium/A286 hardware to match CTE.
Can BMI be formed, bent or molded?
Processed as prepreg (autoclave 177–191 °C / 0.6–0.7 MPa), RTM/VARTM, or compression molding; ALWAYS requires a freestanding postcure at 227–250 °C to develop full Tg; exotherm and volatile control matter in thick laminates, and low-humidity prepreg storage/out-time control is required.
BMI chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| 4,4'-bismaleimidodiphenylmethane (BDM/MDmain maleimide monomer; other backbones (BPA-BMI, m-phenylene-BMI, eutectic monomer blends) used to lower melt point | 50 – 75 | 65 |
| o,o'-diallylbisphenol A (DABA) co-reactaEne/Diels-Alder co-monomer; also aromatic diamine, bisallyl or vinyl co-reactants | 20 – 45 | 33 |
| toughener (thermoplastic PES/PEI, CTBN, only in toughened prepreg grades; lowers Tg | 0 – 15 | 0 |
| catalyst / inhibitor / processing aidsfree-radical initiator, hydroquinone-type inhibitor, release/flow aids | 0.1 – 2 | 0.5 |
| carbon fiber (structural laminate gradesvol% in UD prepreg laminate; not in neat resin | 0 – 68 | 60 |
| glass fiber (electrical / radome grades)vol% in woven-glass laminate | 0 – 65 | 60 |
What surface finishes work on BMI?
Sand/abrade and solvent-wipe or peel-ply then prime with a high-temperature epoxy/BMI primer; polyimide or silicone-modified topcoats for 200 °C+ service; UV protection needed outdoors; electroless/vapor-deposited metallization possible for EMI shielding.
How much does BMI cost?
BMI is a high-cost polymer ($$$$), pricier than about 78% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
BMI — frequently asked
What is BMI used for?
BMI is typically used for aerospace composite structures, engine nacelle components and high-temp printed circuit substrates. In short: between epoxy and polyimide; 230 °C service matrix; postcure is mandatory.
What is the tensile strength of BMI?
BMI has a typical tensile strength of 72 MPa (10.4 ksi) — stronger than 78% of polymer grades. Strength varies by condition: see the 9 listed tempers.
Is BMI easy to machine?
Reasonably — machinability is rated fair (50/100, worse than 73% of polymer grades). Machine fully cured and postcured stock with PCD or diamond-coated carbide, high speed / low feed, sharp tools and vacuum dust extraction; fiber-reinforced laminates are highly abrasive and prone to delamination on drill exit — use backing plates and dagger/brad drills, coolant mist to keep the surface below Tg.
Can BMI be welded?
Not readily — weldability is rated not recommended (0/100). Not weldable — thermoset network; join by high-temperature adhesive bonding (BMI or polyimide film adhesive), co-cure/co-bond, or mechanical fasteners with titanium/A286 hardware to match CTE.
What surface finishes work on BMI?
Sand/abrade and solvent-wipe or peel-ply then prime with a high-temperature epoxy/BMI primer; polyimide or silicone-modified topcoats for 200 °C+ service; UV protection needed outdoors; electroless/vapor-deposited metallization possible for EMI shielding.
Can BMI be formed, bent or molded?
Processed as prepreg (autoclave 177–191 °C / 0.6–0.7 MPa), RTM/VARTM, or compression molding; ALWAYS requires a freestanding postcure at 227–250 °C to develop full Tg; exotherm and volatile control matter in thick laminates, and low-humidity prepreg storage/out-time control is required.
What is the maximum service temperature of BMI?
BMI is rated for continuous use to about 230°C (446 °F), and briefly to 300°C. Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
How much does BMI cost?
BMI is a high-cost polymer ($$$$), pricier than about 78% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
Sources
- CYCOM 5250-4 / 5250-4RTM BMI prepreg and resin data sheets — Solvay Composite Materials (formerly Cytec) (2022)
- MATRIMID / bismaleimide resin technical data — Huntsman Advanced Materials (2019)
- ASM Handbook Vol. 21: Composites — Bismaleimide resins — ASM International (2001)
- Engineered Materials Handbook Vol. 2: Engineering Plastics — ASM International (1988)
- CAMPUS / high-temperature thermoset property compilations — CAMPUS Plastics (2023)
Property data is compiled from published supplier and standards handbooks and normalised for comparison; hardness values on non-Brinell scales are converted approximately for charting only. Manufacturability limits are FabDigit quote-engine defaults and may be relaxed by engineering review. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.