BMC — Bulk Molding Compound Material
BMC (Bulk Molding Compound), also written bulk moulding compound and sometimes called dough molding compound, is a ready-to-mold BMC thermoset plastic material. Unlike SMC's sheet form, BMC bulk molding compound is mixed and supplied as a dough-like mass — a blend of unsaturated polyester resin, chopped glass fiber, mineral filler, initiators and pigments, pre-dosed by weight rather than cut from a roll.
Compared with SMC, this BMC plastic material carries a higher filler content and shorter glass fiber, which trades some mechanical strength for better flow into complex, thick-walled or thin-ribbed cavities. Within the family of SMC BMC composites, BMC is generally the choice when a part's geometry is more intricate than its structural load is demanding.
What Goes Into a BMC Compound
Three main ingredients, blended into a dough-like mass rather than a sheet.
Unsaturated Polyester Resin
Binds the compound together and gives the cured part its rigidity and chemical resistance.
Chopped Glass Fiber
Shorter fibers (roughly 6–13mm) than SMC, chosen for flow into fine, complex cavities.
Mineral Fillers
Calcium carbonate and similar fillers, at a higher loading than SMC, control cost, shrinkage and flame behavior.
The BMC Molding Compound Process
A batch premixing process turns raw resin, filler and fiber into a dough-like BMC compound.
Resin Paste Mixing
Resin, initiator, pigment and mold release agent are blended into a base paste.
Filler Addition
Mineral filler is folded in at high loading to build the bulk of the compound.
Fiber Blending
Short glass fiber is mixed in at reduced speed to avoid breaking the strands.
Thickening & Aging
The dough-like mass thickens to molding viscosity before it's ready for BMC molding.
Why Manufacturers Choose BMC
The higher filler loading and shorter fiber of this BMC composite give it excellent flow, so it fills intricate, thick-section cavities that would be difficult for a sheet-form material.
- Glass fiber content10% – 20%
- Glass fiber length6mm – 13mm
- Molding temperature140°C – 165°C
- Molding processCompression & Injection
- Dielectric propertiesStrong insulator
Industries Built on BMC Compound
Its flow into complex shapes and electrical insulation make BMC a common choice across these fields.
Electrical & Electronics
Breaker housings, switchgear, insulators and terminal blocks that need arc resistance and dielectric strength.
Automotive
Ignition coil covers, small brackets and under-hood parts exposed to heat and vibration.
Home Appliances
Cistern fittings, small housings and handles that need to hold shape over years of use.
Industrial Equipment
Pump housings, valve bodies and small structural brackets facing heat or chemical exposure.
BMC Grades We Supply
The right formulation is matched to your part's electrical, thermal and mechanical requirements.
Standard BMC
Balanced strength, insulation and cost for everyday enclosures and structural parts.
FR-Grade BMC
Meets flammability requirements for switchgear and electrical enclosures.
Low-Shrink BMC
Tighter dimensional control for parts with fine surface finish or fitment requirements.
Reinforced BMC
Higher glass-fiber loading for structural, load-bearing components.
Common Questions
What's the difference between BMC and SMC compound?
BMC is a dough-like bulk material with shorter fiber and more filler, best for complex, thick-section parts; SMC is a sheet material with longer fiber, better suited to large, thin-walled panels.
Is BMC suitable for injection molding?
Yes — BMC bulk molding compound flows well enough to be processed by either compression molding or injection molding, unlike SMC which is compression-only.
How is BMC's flame resistance rated?
Flame-retardant BMC grades are formulated to meet standard flammability classifications for electrical enclosures — the exact rating depends on the grade selected.
Can BMC replace metal parts?
In many electrical and appliance applications, yes — BMC's insulation, corrosion resistance and lighter weight make it a common metal replacement where extreme structural load isn't the main requirement.