Thermoset
- No melting point, so it does not creep or sag as it nears its heat limit
- Very good arc and tracking resistance around live conductors
- Runners and rejects cannot be reground
Process guide
Thermoset Injection Molding
A thermoset part is chemically set in the mold. Heat it again and it chars before it softens, which is exactly why it gets picked for breaker housings, motor parts, and lamp reflectors that sit next to heat for years.
The cycle
One shot, from the hopper to the deflash drum
The press looks familiar. The temperatures are upside down.
- 1
Feed warm, not hot
Granules, or a doughy compound in the case of BMC, go into a screw kept just warm enough to soften them. Too much heat here and the material cures in the barrel, which means pulling the screw to clean it.
- 2
Inject into hot steel
The mold runs much hotter than the barrel. The compound thins out as it picks up heat, fills the cavity, and starts to cross link.
- 3
Hold for cure
The part stays closed in the mold until it is rigid enough to push out. Cure time, not cooling time, sets the cycle.
- 4
Eject hot
Parts leave the mold hot and already stable. Nothing has to cool into shape on a fixture.
- 5
Deflash
Thin fins at parting lines and around pins are expected. Tumbling, media blasting, or hand trimming takes them off.
Families
Which thermosets get injection molded, and where they end up
The names on old drawings are usually one of these.
| Family | What it is good at | Typical parts |
|---|---|---|
| Phenolic | Heat, flame, and dimensional stability without a premium price | Pot handles, commutators, brake pistons, electrical bases |
| BMC (polyester) | Stiffness, arc resistance, and a smooth surface that takes metallizing | Breaker housings, headlamp reflectors, motor end caps |
| Epoxy | Low shrink and strong adhesion to metal | Encapsulated coils, sensors, insulators |
| DAP | Electrical properties that hold up in damp heat | Connectors in military and aerospace hardware |
| Melamine | Hard, scratch resistant surface | Dinnerware, wiring devices |
Tooling
What a thermoset does to a mold
Filled compounds are hard on steel, and runny at exactly the wrong moment.
- Glass and mineral fillers are abrasive, so cavities, gates, and screws wear quickly unless the steel is hardened or coated
- The compound is thinnest at the moment of fill, so shut offs and ejector pin fits are held tight to keep flash down
- Mold temperature has to be even across the cavity. A hot spot cures ahead of the rest and the part warps or blisters
- Curing releases gas, and a blocked vent shows up as a burn or a porous patch
Other routes
Injection, transfer, and pressing a charge
Thermosets were pressed long before they were injected, and many still are.
Compared with injection, compression molding keeps long fibers intact and suits large, flat panels, while injection wins on intricate shapes, molded in metal parts, and cycle time. The trade is mostly about fiber length and part size.
Transfer molding sits between the two. A measured charge is pushed from a pot through a short runner into a closed mold. It is common for encapsulating coils and lead frames, where full injection pressure would shift the part being covered.
Alternative
Thermoset or high heat thermoplastic
Many parts that were phenolic a generation ago are now PPS or PPA. A few should have stayed thermoset.
PPS, PPA, or PEI
- Molds on standard thermoplastic presses, with regrind possible
- Tougher, so thin snaps and clips survive assembly
- Loses stiffness closer to its heat limit than a thermoset does
FAQ
Questions about thermoset molding
Why can't thermoset scrap be reground?
Curing links the polymer chains permanently. Heating the material again burns it instead of melting it, so runners and rejects are ground into filler at best and usually thrown away.
Is thermoset molding slower than thermoplastic molding?
Often not by much. Cure time replaces cooling time, and a thick thermoset part can come out sooner than a thick thermoplastic one because it does not have to cool to hold its shape.
Why do thermoset parts come out with flash?
The compound gets very thin as it heats in the cavity, thinner than most plastic melts. It slips into gaps a thermoplastic would bridge, so a light fin is normal and deflashing is part of the process.
Does NetProto mold thermosets?
No. NetProto's production path is thermoplastic injection molding. This page is a reference for engineers weighing a thermoset against high heat thermoplastics like PPS, PPA, and PEI, which can be quoted from CAD.
Keep reading
Related guides
The thermoplastics that replace thermosets, and the other way thermosets are shaped.
PPS
Heat and chemical resistance in a thermoplastic.
PPSPPA
Stiff, hot, and common in under hood parts.
PPAPEI (Ultem)
Flame resistance without additives.
PEI (Ultem)Glass filled PBT
Electrical housings and connector bodies.
Glass filled PBTPressing versus injecting
How pressing a charge compares with injection.
Pressing versus injectingConnector housings
Where arc resistance and heat meet tight fits.
Connector housingsProcesses hub
Every molding process and secondary operation.
Processes hubNext step
Test the thermoset part against a high heat thermoplastic
Upload CAD with the temperature, load, and electrical requirements. The review looks at whether PPS, PPA, or PEI meets them on a standard injection mold. NetProto manufacturing is ISO 9001 certified.