BMC
BMC is a bulk moulding compound — an unsaturated polyester (or vinyl ester) thermoset dough pre-mixed with short glass fibre and mineral filler, supplied for injection, injection-compression or compression moulding of complex electrical and appliance parts.
Reference datasheet — BMC 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.
BMC grades and fills
BMC is supplied in 8 conditions, soft to hard.
Compare all 8 grades and fillsHide
| Grade · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness |
|---|---|---|---|---|---|
Thermally conductive mineral/ceramic-filled BMC | Use for LED heat-sink housings and encapsulations that need 1–3 W/m·K while staying electrically insulating. | 30 | |||
Low-profile / Class-A surface BMC | Choose for visible pigmented or painted parts (automotive lamp bezels, appliance fronts) needing near-zero shrink and smooth surface. | 35 | 0.5 | 98 HRM | |
Carbon/graphite-filled conductive BMC | Use where ESD dissipation or bulk electrical conduction is required (fuel-cell bipolar plates, static-safe housings) rather than insulation. | 35 | |||
Flame-retardant BMC, UL 94 V-0 / 5VA | Pick when a UL 94 V-0 or 5VA rating and glow-wire performance are mandatory (breakers, contactors, EV connectors). | 38 | 0.4 | 105 HRM | |
High-whiteness reflector BMC | Choose for LED reflectors and lamp cups needing high initial reflectance with minimal yellowing after thermal/UV ageing. | 38 | 0.4 | 105 HRM | |
Heat-resistant / high-CTI arc-resistant BMC | Specify for motor commutators, arc chambers and lamp bases running above 160 °C with arc/track exposure. | 40 | |||
Vinyl ester matrix BMC | Pick when the part sees acids, chlorides, hot water or steam that would hydrolyse standard orthophthalic polyester BMC. | 50 | 0.7 | 105 HRM | |
High-strength BMC, 22–30% longer glass fiber | Use for load-bearing or pressure-containing mouldings (pump housings, structural brackets) where impact and flexural strength dominate. | 60 | 0.8 |
Supply forms, finishes and certifications
Ways BMC is supplied — form, process route, surface or paperwork — without a datasheet of their own. The property values above apply.
| Option | What it is | Type |
|---|---|---|
GPDefault | General-purpose electrical BMC, ~15–20% glass fiber, CaCO₃ filled | Standard supply |
What is BMC?
BMC is a bulk moulding compound — an unsaturated polyester (or vinyl ester) thermoset dough pre-mixed with short glass fibre and mineral filler, supplied for injection, injection-compression or compression moulding of complex electrical and appliance parts. The general-purpose condition gives about 1.85 g/cm³ density, 42 MPa tensile strength and 150 °C maximum service temperature, with FR-V0, high-strength GF25, low-profile, high-temperature, thermally conductive, white LED and vinyl ester variants on file. It is brittle, abrasive to tooling (machinability 35) and, as a thermoset, cannot be welded or reground. Where flat insulating sheet is sufficient, GPO-3 laminate is the cheaper route. BMC has a density of 1.85 g/cm³ (0.0668 lb/in³), heavier than 96% of polymer grades. Its glass-transition temperature is 120°C (248 °F). BMC has a tensile strength of 42 MPa (6.09 ksi), weaker than 55% of polymer grades. Elongation at break is 0.5% and the elastic modulus is 11 GPa (1.6 Msi).
What is BMC used for?
- circuit breaker housings
- appliance components
- automotive under-hood parts
- electrical insulators
Advantages
- Conducts heat well — 0.6 W/m·K (0.347 BTU/hr·ft·°F), better than 99% of polymer grades
- Stiff — 11 GPa (1.6 Msi), better than 97% of polymer grades
- Low, predictable mold shrinkage — 0.15%, better than 97% of polymer grades
- Low embodied carbon — 3 kg CO₂/kg, better than 83% of polymer grades
- Good UV / weathering resistance — 60/100, better than 80% of polymer grades
Limitations
- Limited ductility — 0.5%, worse than 100% of polymer grades
- Limited formability — 3/100, worse than 99% of polymer grades
- Heavy — 1.85 g/cm³ (0.0668 lb/in³), worse than 96% of polymer grades
- Difficult to weld — 3/100, worse than 95% of polymer grades
- Difficult to machine — 35/100, worse than 84% of polymer grades
BMC properties
Typical room-temperature values; bars are the position among polymer grades.
Physical4
BMC has a density of 1.85 g/cm³ (0.0668 lb/in³), heavier than 96% of polymer grades. Its glass-transition temperature is 120°C (248 °F).
Mechanical12
BMC has a tensile strength of 42 MPa (6.09 ksi), weaker than 55% of polymer grades. Elongation at break is 0.5% and the elastic modulus is 11 GPa (1.6 Msi).
Thermal10
BMC is rated for continuous service to 150°C (302 °F). It conducts heat at 0.6 W/m·K (0.347 BTU/hr·ft·°F), better than 99% of polymer grades. Thermal expansion is 25 µm/m·K (13.9 µin/in·°F).
Electrical7
BMC is an electrical insulator with a dielectric strength of 14 kV/mm (355.6 V/mil) and a resistivity of 1×10¹² Ω·m.
Chemical & Environmental3
Corrosion resistance is very good (70/100), worse than 78% of polymer grades. UV resistance is good.
Sustainability4
Producing a kilogram of BMC takes about 55 MJ of energy and emits 3 kg of CO₂ — less than 98% of polymer grades. Typical recycled content is 0%.
Manufacturability7
BMC's machinability is fair (35/100, worse than 84% of polymer grades); weldability not recommended (3/100); formability not recommended (3/100).
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. BMC is highlighted; hover a dot for its name, or open the interactive family chart.
Working with BMC
Is BMC easy to machine?
Cured BMC is abrasive and brittle — use PCD or diamond-coated tooling, high speed / low feed, support the part and vacuum the dust; drill from the moulded side to avoid chip-out.
Can BMC be welded?
Thermoset: cannot be welded; join by mechanical fasteners, moulded-in inserts, or epoxy/MMA adhesives after light abrasion and solvent wipe.
Can BMC be formed, bent or molded?
Injection, injection-compression or compression mould with stuffer-feed screws at 20–45 °C barrel and 140–180 °C tool, ~30–60 s/mm cure; vent well for styrene and avoid fibre-degrading high shear gates.
BMC 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 |
|---|---|---|
| unsaturated polyester resin (in styrene)ortho/iso-phthalic UP; vinyl ester in VE grades | 18 – 32 | 25 |
| glass fiber (chopped, 3–12 mm)higher in HS-GF25 structural grades | 10 – 30 | 18 |
| low-profile thermoplastic additivePS/PVAc shrink-control additive | 0 – 6 | 3 |
| internal mould release (zinc stearate) | 1 – 3 | 2 |
| peroxide initiator (TBPB/BPO) | 0.5 – 1.5 | 1 |
| thickener (MgO/Ca(OH)2), pigment, additi | 0.5 – 3 | 1 |
| calcium carbonate / mineral fillerreplaced partly by ATH in FR grades, by Al2O3/BN in TC grades | ≥ 35 (balance) |
What surface finishes work on BMC?
As-moulded surfaces take flash removal by tumbling or media blast; low-profile grades accept primer and 2K polyurethane paint, and vacuum-metallizing after a UV base coat for reflectors.
How much does BMC cost?
BMC is a standard-cost polymer ($$), cheaper than about 71% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
BMC — frequently asked
What is BMC used for?
BMC is typically used for circuit breaker housings, appliance components, automotive under-hood parts and electrical insulators. In short: net-shape thermoset insulator; stable to 150 °C; V-0 flame-retardant grades.
What is the tensile strength of BMC?
BMC has a typical tensile strength of 42 MPa (6.09 ksi) — weaker than 55% of polymer grades. Strength varies by condition: see the 8 listed tempers.
Is BMC easy to machine?
Not especially — machinability is rated fair (35/100, worse than 84% of polymer grades). Cured BMC is abrasive and brittle — use PCD or diamond-coated tooling, high speed / low feed, support the part and vacuum the dust; drill from the moulded side to avoid chip-out.
Can BMC be welded?
Not readily — weldability is rated not recommended (3/100). Thermoset: cannot be welded; join by mechanical fasteners, moulded-in inserts, or epoxy/MMA adhesives after light abrasion and solvent wipe.
What surface finishes work on BMC?
As-moulded surfaces take flash removal by tumbling or media blast; low-profile grades accept primer and 2K polyurethane paint, and vacuum-metallizing after a UV base coat for reflectors.
Can BMC be formed, bent or molded?
Injection, injection-compression or compression mould with stuffer-feed screws at 20–45 °C barrel and 140–180 °C tool, ~30–60 s/mm cure; vent well for styrene and avoid fibre-degrading high shear gates.
What is the maximum service temperature of BMC?
BMC is rated for continuous use to about 150°C (302 °F), and briefly to 200°C. Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
How much does BMC cost?
BMC is a standard-cost polymer ($$), cheaper than about 71% 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
- ASTM D1201 — Thermosetting Polyester Molding Compounds — ASTM International (2019)
- ISO 14526 / ISO 12114 — SMC and BMC compounds, cure and test methods — ISO (2018)
- Engineered Materials Handbook Vol.2: Engineering Plastics (thermoset molding compounds) — ASM International (1988)
- BMC / TMC compound datasheets (IDI Composites, Menzolit, Polynt-Reichhold, Bulk Molding Compounds Inc.) — various (2023)
- UL Yellow Card / Prospector data for thermoset polyester molding compounds — UL Solutions (2024)
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.