Process selection
Compression Molding vs Injection Molding
Compression molding places a measured charge in a heated open mold and squeezes it to shape while it cures. Injection molding forces melted resin into a closed mold under high pressure on fast repeatable cycles. The trade is tooling cost and large-part fiber strength against cycle speed and geometric complexity.
| Compression Molding | Injection Molding | |
|---|---|---|
| Tooling cost | $10,000 to $80,000 | $25,000 to $250,000+ |
| Cycle time | 60 s to 5 min thermoset cure | 15 to 60 s |
| Materials | SMC, BMC, rubber, phenolics, some thermoplastics | Thermoplastics mainly; LSR and thermoset variants |
| Part complexity | Limited undercuts; generous ribs and bosses | Cores, threads, living hinges, overmolding |
| Cavity pressure | 300 to 2,000 psi | 5,000 to 20,000 psi |
| Fiber retention | 25 mm fibers survive; higher structural strength | Screw shears fibers below 1 mm |
| Volume sweet spot | 1,000 to 100,000 parts per year | 50,000 to millions per year |
Choose Compression Molding when
- Large flat or structural panels in SMC or BMC
- Long-fiber reinforcement drives the strength spec
- Annual volumes under about 50,000 where tooling dominates
Choose Injection Molding when
- Volumes above 100,000 parts per year
- Small complex geometry: threads, snaps, thin walls
- Cycle time economics set the piece price
The verdict
Below roughly 50,000 parts per year, or for large fiber-reinforced panels, compression molding wins on tooling at a third the cost with better fiber strength. Above 100,000 parts of complex thermoplastic geometry, injection molding's 15 to 60 s cycles bury the bigger mold invoice. Between those volumes, quote both.
Cost comparison
A single-cavity compression tool for an SMC panel runs $15,000 to $80,000; the injection equivalent with cooling, ejection, and a hot runner starts near $50,000 and reaches $250,000 in multi-cavity steel. Injection claws it back at $0.10 to $0.50 lower cost per part from 15 to 60 s cycles against 1 to 5 min cures. Break-even typically lands between 30,000 and 75,000 parts: below it compression's cheap tooling wins, above it injection's cycle time takes over.
Common questions
Can compression molding hold injection-level tolerances?
Not quite. Expect +/-0.1 to 0.25 mm on compression-molded features versus +/-0.05 mm on a good injection tool, and wall thickness floats with charge weight control across the flash plane. Critical bores and mating faces on compression parts are usually machined after molding.