BMC (Bulk Molding Compound)
BMC is a short glass fiber-reinforced unsaturated polyester molding compound, with optional low-shrink formulations. It features good flowability for complex geometries and insert molding. It can be processed by compression molding or injection molding. It has excellent flame retardancy and electrical performance with a high CTI value. Typical applications include electrical enclosures, circuit breaker housings, insulators, motor end covers, charging pile components, and water meter bodies.
SMC (Sheet Molding Compound)
SMC is a long glass fiber-reinforced unsaturated polyester sheet. It delivers higher mechanical strength and is mainly processed via compression molding, suitable for large flat structural parts. Common uses cover PV support parts, coach interior panels, water tank plates, and large electrical boards.
PF (Phenolic Resin)
Phenolic resin is one of the earliest commercial thermosetting materials. It is cost-effective with good wear resistance and inherent flame retardancy, while its main drawback is relative brittleness. Available grades include general grade, glass-filled grade, and cotton fiber filled grade. It is widely used for brake pads, gears, switch bases and cookware handles.
EP (Epoxy Resin)
Epoxy resin provides strong adhesive performance, low curing shrinkage, and superior insulation together with good chemical resistance. It can be processed by casting, impregnation and compression molding. It is widely adopted for electronic potting, transformer components, and insulating sleeves.
DAP (Diallyl Phthalate)
DAP has outstanding resistance to humidity and electric arc, together with ultra-stable dimensions for precision electronic assemblies. Its main limitation is higher material cost.
Amino Plastics (Melamine / UF Urea Formaldehyde)
Melamine material is scratch-resistant and food-contact compliant. Urea formaldehyde is low-cost and mostly used for daily products, tableware and decorative laminates.
Processing Technologies for Thermosets
1. Compression Molding: The most common process for BMC, SMC, and phenolic materials. High molding pressure produces dense finished parts, suitable for mass production.
2. Injection Molding: Mainly applied to flowable BMC compounds. It brings high production efficiency and fits parts with embedded metal inserts and complicated structures.
3. Transfer Molding: Designed for precision electronic components with multiple inserts.
4. Casting & Potting: Suitable for epoxy and neat unsaturated resin under low pressure or pressure-free conditions.
5. Core difference from thermoplastics: Thermoset curing is an irreversible chemical cross-linking reaction in heated molds. Thermoplastics are processed through physical melting and cooling without chemical change.
Advantages and Limitations
Advantages
High continuous heat resistance, reliable flame retardancy, excellent electrical insulation, low creep and stable dimensions, good chemical resistance, high stiffness, and favorable aging resistance.
Limitations
Cannot be remelted and reprocessed after curing. Waste scraps can only be ground and reused as filler instead of being pelletized again. Overall toughness is lower than most engineering thermoplastics. The molding cycle is relatively long. Special VOC control is required for certain formulations.
Thermoset vs Thermoplastic Quick Comparison
| Item | Thermoset (BMC, PF, EP) | Thermoplastic (PA, PBT, PC, etc.) |
|---|---|---|
| Forming Principle | Irreversible chemical cross-linking | Physical melting and cooling, re-meltable |
| Heat Resistance | Resists creep under sustained high temperature | Softening and creep easily at high temperature |
| Insulation & Flame Resistance | Inherent good performance for electrical applications | Flame retardant additives are usually needed |
| Scrap Handling | Not melt-recyclable, reused as filler | Sprues and runners can be recycled and re-pelletized |
| Toughness | Relatively brittle, moderate impact resistance | Better toughness |
| Typical Products | High-voltage insulators, charging components, heat-resistant structural parts | Connectors, flexible housings, moving functional parts |