a) What are sheet-molding compounds (SMC)? Bulk-molding compounds (BMC)? Sheet-molding compound (SMC) is a fiber glass reinforced thermosetting compound in sheet form, usually rolled into coils interleaved with plastic film to prevent auto adhesion. Made by dispensing mixed resin, fillers, maturation agent, catalyst and mold release agent onto two moving sheets of polyethylene film. The lower one also contains chopped glass roving or glass mat. SMC can be molded into complex shapes with little scrap. Sheet molding compound (SMC) is fiberglass reinforced composite material, produced in a “sheet” format
Bulk-molding compound (BMC) is a combination of chopped glass strands and Resin in the form of a bulk pre-preg. BMC is suitable for either compression or injection molding. Injection molding of BMC is used to produce complex components such as electrical equipment, car components, housings for electrical appliances and tools, in large industrial volumes. Unlike SMC, it is not necessary to include a maturation stage. Consequently, BMC pre-preg formulations contain higher filler contents. The chopped glass strands vary in length depending on the level of performance required. Reinforcement content generally ranges between 15 and 20 percent; however, it may reach 25 percent for the highest performance. BMC uses lower reinforcement content than SMC and permits higher filler loadings with lower costs.
b) List FOUR (4) types of the forms in which reinforcement fibers appear in composite materials?
Based on the form of reinforcement, common composite materials can be classified as follows:
1. Fibers as the reinforcement (Fibrous Composites)
a. Random fiber (short fiber) reinforced composites
b. Continuous fiber (long fiber) reinforced composites
2. Particles as the reinforcement (Particulate composites)
3. Flat flakes as the reinforcement (Flake composites)
4. Fillers as the reinforcement (Filler composites)...