Bulk Molding Compounds, Thermosets

Bulk molding compounds (BMC) are mixtures usually of short glass fibers, plastic, and additives similar to the SMC compound. This dough mixture, with the consistency of modefing clay, can be produced in bulk form or extruded in rope-like form for easy handling. The extrudate type is called a log that is cut to specific lengths such as 0.3 m (1 ft). 

BMC is commercially available in different combinations of resins, predominantly TS polyesters, additives, and reinforcements. They meet a wide variety of end-use requirements in high-volume applications where fine finish, good dimensional stability, part complexity, and good overall mechanical properties are important. The most popular method of molding BMCs is compression. They can also be injection molded in much the same way as other RTS compoimds using ram, ram-screw, and, certainly BMC mixes, conventional reciprocating screw plasticators. 

Property Unit Sheet molding compound (SMC) Bulk molding compound (BMC) Thick molding compound (TMC) 

Property Unit BMC LP-SMC SMC/C/D/C Spray laminate GF-ABS Steel Aluminum 

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Compression molding involves the use of preforms (bulk molding compounds, sheets of glass fibers impregnated with the thermoset precursors, etc.) that are placed in a heated mold. When the mold is closed, the plastified preform flows to fill the mold and cure takes place in about 1-2 min. Then, the mold is opened and the final part ejected. 

The matrix can be either a thermoset or thermoplastic. The oldest and still widest used material for compression molded products is phenolic. The thermoset materials used to manufacture fiber reinforced compression molded articles is unsaturated polyester sheet or bulk, reinforced with glass fibers, known as sheet molding compound (SMC) or bulk molding compound (BMC). In SMC, the 25 mm long reinforcing fibers are randomly oriented in the plane of the sheet and make up for 20-30% of the molding compound s volume fraction. 

Injection Molding. Injection molding was first developed for metals and then for thermoplastics. With further progress in both materials and equipment, it has been applied successfully to thermosetting plastics. It is used mostly for phenolics and is being used more and more for polyester bulk molding compounds. 

Such glass fiber-reinforced composites based on the unsaturated polyester thermosets are usually fabricated as sheet molding compounds (SMC) and bulk molding compounds (BMC). These are widely used in various metal replacement applications because of their cost-effectiveness, rigidity, light weight, and corrosion resistance properties particularly useful in transportation (cars and trucks), constmction, pipe, and tank applications. Automotive and tmck body panel and strucmral component applications of SMC include doors, hatchbacks, hoods, front grilles, etc. Some nonautomotive applications of SMC and BMC include sanitaryware (bathtubs, shower stalls, sinks), appliances, business machine, and electrical components. 

ATH came into wide use as a flame retardant in the 1960s, primarily as a result of demands of consumer-driven safety legislation for carpet backing and fiberglass-reinforced polyester products. For these applications, ATH imparts flame retardance and smoke suppression. End-use markets for ATH include transportation, construction, cast polymers, electrical/electronic, wire and cable, leisure and appliances. Specific polymer applications range from thermosets such as solid surface, sheet molding compounds (SMCs) and bulk molding compounds (BMCs) to wire and... 

Figure 6.20 Injection machine for thermosetting plastics, available with screw or granular materials or plunger for bulk molding compounds. Machine also has an integral Rollaveyor tumble-blast deflasher inside the housing to deliver deflashed parts on a conveyor out through open end at left. Courtesy of Hull Corporation.)...

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