Bulk molding compound, commonly known as BMC, is an engineered thermoplastic composites material predominantly used in compression and injection molding applications. It consists of fiber reinforcement, typically short glass fibers, embedded in a non-crosslinked thermosetting polymer matrix. The polymer matrix is typically made from unsaturated polyester or vinyl ester resin.
Composition and Manufacturing Process of BMC
BMC consists of 30-70% by weight of fiber reinforcement and the remaining polymer resin matrix. The short glass fibers used have a length ranging from 0.25-1 inch. Along with glass fibers, carbon and Kevlar fibers are also sometimes used. The polymer resins used are unsaturated polyester or vinyl ester due to their ability to readily wet out and bond to glass fibers.
The manufacturing process of BMC involves first preparing the compounds by uniformly dispersing the glass fibers in the liquid resin using intensive mixing equipment. Fillers and additives are also added in this stage to impart desired properties to the final BMC material. The thoroughly mixed compound slurry is then sent to a preformer where it is laid on a moving belt in the form of a mat. This mat is then conveyed to a steam chest for preheating. Final curing happens in a compression or injection molding press.
Properties and Applications of BMC
BMC exhibits properties intermediate between those of metal and thermoset plastics. Some key properties of BMC include:
- High strength and stiffness due to fiber reinforcement. Tensile strength ranges from 50-150 MPa.
- Good dimensional stability, creep resistance and heat resistance up to 150°C due to chemically crosslinked polymer matrix.
- Excellent surface finish and detailing capability due to molding process.
- BMC parts are corrosion resistant and electrically non-conductive.
Due to the above properties, BMC finds wide use in automotive, electrical, construction and sanitary applications. Some common applications include:
- Body panels, hoods, doors and trunk lids in cars, vans and trucks.
- Housings for electrical switches, connectors and small appliances.
- Fiberglass fabricated parts for tubs, shower stalls, counters and hot tubs.
- Manhole covers, valve boxes and underground utility lids for municipalities.
Advantages of BMC Over Sheet Molding Compound
While BMC and sheet molding compound, commonly known as SMC, appear similar initially, BMC has certain advantages over SMC:
- Shorter molding cycle times - BMC has a lower viscosity and can be molded faster than more viscous SMC. Typical cycle times are 30-60 seconds for BMC.
- Higher filler loading capability - BMC allows higher filler or fiber loading of up to 70% by weight compared to 45-55% for SMC. This improves mechanical properties.
- Superior surface quality - As BMC flows more readily in the mold, it produces parts with better surface finish and detailing than SMC.
- Low post-mold shrinking and warping - Thanks to closed mold system, BMC parts exhibit less dimensional changes during curing.
Therefore, BMC is preferred over SMC for large or complex parts requiring intricate detailing, very short cycle times or applications needing better dimensional stability.
BMC has proven to be a versatile thermoplastic composites material enabling rapid production of medium to large parts with excellent mechanical and thermal properties. Its adaptability to compression and injection molding along with short cycle times provide significant manufacturing and economic advantages. BMC thus continues to gain increased acceptance across major industry verticals. With ongoing formulation and process advancements, its scope of applications is likely to further expand in the future.
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