Bulk molding compound (BMC) is a composite material made from fiberglass or other reinforcing materials such as carbon fibers combined with a thermosetting polymer matrix. It is widely used in compression and injection molding applications for its strong physical and mechanical properties along with low cost.
Composition of BMC
BMC typically consists of the following main components:
- Fiber reinforcement - Fibers such as glass fibers or carbon fibers provide strength and stiffness to the material. Long or short fibers are used depending on the application requirements. Glass fiber is the most common reinforcing material used due to its low cost.
- Polymer resin matrix - The matrix holds the reinforcing fibers together and transfers stress between them. Thermoplastic or thermoset resins can be used. Thermoplastics such as polyesters and nylons are used for easier recyclability. Thermosets like polyester and epoxy resins are more common due to their better strength and heat resistance properties.
- Fillers - Inert materials like calcium carbonate, talc, or aluminum trihydroxide are added as fillers to improve various properties. They enhance stiffness, fire retardancy, reduce cost, and influence rheological properties.
- Additives - Additives like coupling agents, pigments, flame retardants, etc. are included in small amounts to further enhance the properties of BMC.
Processing of BMC
BMC can be molded using compression or injection molding processes:
- In compression molding, the Bulk Molding Compound (BMC) is first pre-heated and then placed in a pre-heated mold cavity. Pressure is applied to fill the entire mold volume. It solidifies upon cooling. This method is used for large complex parts.
- Injection molding involves melting and injecting the BMC compound into a mold cavity under high pressure. It solidifies quickly due to exothermic reaction of the polymer matrix. This fully automated process suits high production volumes of complex parts.
Properties and Applications of BMC
Some key properties of BMC that make it a popular composite material include:
- Strength and Rigidity - BMC offers high strength-to-weight ratio due to fiber reinforcement. Its modulus can range from 3-20 GPa.
- Dimensional Stability - Reinforced thermoset polymer matrix provides dimensional stability under varying environmental conditions and temperatures.
- corrosion and Chemical resistance - Thermoset polymer matrix imparts good resistance to chemicals, moisture, heat, and various environmental exposures.
- Electric insulation - Glass or carbon fiber reinforced BMC has good electric insulation properties for electronics and switchgear applications.
- Easy processing - BMC can be easily molded into complex shapes using compression or injection molding techniques at ambient pressures.
Owing to the above properties, BMC finds applications in automotive components, equipment panels, electronic housings, appliances, corrosion resistant tanks, etc. Its use is growing in under-the-hood automotive parts due to the demand for lightweight composites. Overall, BMC is a versatile, cost-effective and performance thermoplastic composite material for compression and injection molding applications.
Advantages and Disadvantages of BMC
Advantages:
- Lower cost than sheet molding compound (SMC)
- Higher strength and stiffness than general purpose thermoplastics
- Dimensional stability and durability
- Versatile molding characteristics for complex parts
- Reduced warp tendency compared to SMC
- Good surface finish of molded parts
Disadvantages:
- Limited post-mold recycling ability compared to engineered thermoplastics
- Vulnerable to moisture absorption requiring dry storage
- Lower toughness than metal parts if used without reinforcement
- More brittle than many pure thermoplastics
- Higher cost than inexpensive thermoplastics
Future Perspectives of BMC
With the increasing demand for lightweight composite materials across industries, the global BMC market size is projected to grow at a CAGR of over 5% till 2025. Ongoing research is focused on developing BMC formulations with:
- Higher fiber loadings for increased stiffness, strength and dimensional stability
- Increased versatility to produce thin-walled and elongated parts
- Optimized rheological properties for easy injection molding of complex parts
- Reduced viscosities at lower processing temperatures
- Improved toughness, impact strength and fatigue resistance
- Self-extinguishing fire retardant formulations
The development of post-mold recyclable BMC using thermoplastic matrices can further boost its popularity. Overall, BMC will continue expanding into new applications by delivering enhanced mechanical properties at lower costs compared to metals and other composites.
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