When a part has to be strong, stay stable when hot and insulate electrically, three families of materials usually come up: SMC, BMC and glass-filled engineering thermoplastics such as PA66-GF, PBT-GF or PPS-GF. This guide compares them and gives a practical way to choose.
The short answer
- SMC: larger, stiffer and stronger parts such as enclosures, panels and covers. Compression molded.
- BMC: small to medium parts with fine detail, high heat resistance and very low shrinkage, such as reflectors, breaker housings and motor parts. Injection or compression molded.
- Glass-filled thermoplastics: the fastest cycles, reusable regrind and tough parts, where the temperature and creep requirements are lower.
Comparison
| SMC | BMC | Glass-filled thermoplastic | |
|---|---|---|---|
| Material type | Thermoset sheet | Thermoset dough | Thermoplastic pellets |
| Fiber length (typical) | About 25 mm | About 3–12 mm | Short, well under 1 mm in the part |
| Process | Compression molding | Injection or compression molding | Injection molding |
| Mold | Heated mold (about 140–160 °C) with shear edge | Heated mold (about 150–180 °C) | Temperature-controlled mold, well below the melt temperature |
| Heat resistance | High; does not melt | High; does not melt | Limited by the polymer's softening point |
| Shrinkage | Very low | Very low | Higher and direction-dependent |
| Creep under load at temperature | Low | Low | Higher |
| Part size | Medium to very large | Small to medium | Small to large |
| Fine detail and thin walls | Good | Very good | Very good |
| Scrap reuse | Not remeltable | Not remeltable | Regrind can be reused |
Typical, simplified comparison; properties vary by grade.
Five questions that decide the material
- How big is the part? Large panels and enclosures point to SMC.
- How hot does it get, and for how long? Continuous heat near lamps, motors or electrical contacts favors BMC or SMC over most thermoplastics.
- Does it carry load at temperature? Thermosets creep much less, so bolted joints and mounting points stay tight.
- How accurate must it be? The very low shrinkage of BMC suits reflectors and parts with tight dimensions.
- What volume and budget? Injection molded thermoplastics have the fastest cycles; BMC injection is close; SMC compression has slower cycles but makes large parts that would need very large injection molds.
Electrical and flame requirements
All three families offer flame retardant grades. BMC and SMC are traditional choices for switchgear, breaker housings and enclosures because of their combination of heat resistance, tracking resistance and flame retardance. Always check the rating of the specific grade at your wall thickness.
More detail: BMC vs SMC comparison, what SMC is and BMC injection vs compression molding.
Frequently asked questions
Is BMC the same as DMC?
Yes. BMC (bulk molding compound) and DMC (dough molding compound) are names for the same type of thermoset molding compound.
Can a thermoplastic part be converted to BMC?
Often, but the part usually needs changes: more uniform walls, adjusted ribs and draft, and fastening features suited to a thermoset. A DFM review identifies the changes.
Which is cheaper, BMC or SMC?
It depends on the part. Per kilogram the compounds are in a similar range; the bigger cost drivers are the process, the cycle time, the mold and the finishing each part needs.
Have a BMC or SMC part to discuss? Send your drawing or 3D file through our inquiry form. Our engineers reply with DFM comments and a quotation.