BMC & SMCMolds · Compression & Injection Molded Parts
Compression molding
How SMC and BMC compression molding works
Compression molding presses a weighed charge of SMC or BMC between the two halves of a heated mold in a vertical hydraulic press. The compound flows to fill the cavity, then cures under heat and pressure. Because it only flows a short distance, the glass fibers stay long and the parts are strong and stiff.
Typical conditions are a mold temperature of about 140–160 °C, a molding pressure of about 5–15 MPa on the projected area of the part (higher for deep or heavily ribbed parts), and a cure time from under a minute to a few minutes depending on wall thickness. The actual settings are fixed during mold trials for each part and compound.
Process steps
- Cut SMC plies to the charge pattern, or weigh the BMC charge
- Lay the charge in the hot, open mold in its set position
- Close the press: fast approach, then slow pressing once the mold meets the charge
- Hold pressure while the part cures
- Open the mold and eject the hot, cured part
- Deflash, and cool the part on a fixture if flatness matters
- Inspect, then machine, fit inserts or paint as required
Compression molding at a glance (typical)
| Materials | SMC, BMC and other thermoset molding compounds |
|---|---|
| Press | Vertical, downstroking hydraulic press |
| Mold temperature | About 140–160 °C |
| Pressure | About 5–15 MPa on the projected part area; higher for deep or heavily ribbed parts |
| Cure time | From under a minute to a few minutes, mainly set by wall thickness |
| Part size | Small to very large, limited by press platen and tonnage |
| Strengths | Long fibers, high strength, little material waste, large parts |
Compression molding FAQ
What decides the cycle time in compression molding?
Mainly the cure time, which depends on wall thickness, mold temperature and the compound. Charge loading, press closing and part removal add to it.
What is vacuum-assisted compression molding?
Air is pulled out of the mold just before it closes fully on the charge. This reduces porosity and blisters in parts with deep ribs or complex shapes.