Molding compound
Sheet molding compound (SMC), bulk molding compound (BMC), or glass mat thermoplastic (GMT).
Composite compression molding uses matched high-strength metal tooling mounted in a hydraulic press to shape and cure composite charges under controlled heat and tonnage. It is the primary process for high-volume automotive, electrical, and infrastructure structural components.
Part quality depends on synchronized press tonnage, uniform tool platen heating, precise charge weight distribution, and controlled venting to eliminate weld lines and internal porosity.
Select and condition thermoset compound (SMC/BMC) or thermoplastic sheet (GMT/organosheet) to target temperature and flow index.
Cut charges according to an engineered nest to cover 30% to 70% of the tool surface area and weigh precisely.
Load metal bushings, threaded studs, and charge stacks onto the lower mold half in calibrated positions.
Close press with fast approach followed by controlled pressing speed (5 to 20 mm/s) under 5 to 20 MPa pressure.
Eject hot part using synchronized hydraulic knockout pins, fixture-cool if needed, and trim peripheral flash.
Sheet molding compound (SMC), bulk molding compound (BMC), or glass mat thermoplastic (GMT).
Chopped strands (25 to 50 mm), continuous directional roving, or woven fabric inserts for local stiffening.
Unsaturated polyester, vinyl ester, epoxy, or phenolic resins formulated with low-profile additives for zero shrinkage.
Preheated brass or steel threaded inserts, grounding plates, and bonded mounting studs.
A supplier claim is useful only when the process window is tied to the offered material, tooling, drawing revision and production batch.
| Stage | What must be controlled | Evidence to request |
|---|---|---|
| Raw material receipt | Viscosity maturation, glass content by burn-off, and volatile matter content. | Material test certificate (CoA) and inbound QC acceptance log. |
| Charge prep | Charge weight, ply shape, orientation, and layout template conformity. | Digital scale logs and charge pattern setup sheet. |
| Press cycle | Upper/lower tool temperatures (130 to 160 deg C), hydraulic pressure, and closing speed profile. | Automated press data log and cycle chart. |
| Part qualification | CMM dimensions, Barcol hardness, fiber distribution, void content, and mechanical load performance. | First article inspection report (FAIR) and mechanical test record. |
Likely cause: Inadequate flow velocity, cold tooling, or converging flow fronts with low local reinforcement.
Detection: Visual inspection under angled lighting and tensile coupon testing across knit zones.
Likely cause: Trapped air, excessive moisture, early resin gelation, or inadequate press breathing/venting.
Detection: Ultrasonic NDT, pycnometer density test, or cross-sectional micrograph.
Likely cause: Differential shrinkage, uneven tool heating between core and cavity, or asymmetric charge stack.
Detection: Coordinate measuring machine (CMM) and go/no-go check fixtures.
This guide uses the JEC process taxonomy, CAMX/ACMA/SAMPE review framework and process-specific material from industry associations, national research centers or government laboratories. Sources explain the process; they do not endorse listed suppliers.
Industry-level process taxonomy covering RTM, infusion, pultrusion, winding, fibre placement and additive manufacturing.
Peer-reviewed conference framework for manufacturing, processing, NDE and testing topics.
Matched-tool SMC/BMC process, charge placement and production characteristics.
Reviewed 16 August 2026. Verify current standards editions, material datasheets and project-specific acceptance requirements before award.
Each row shows the terms its public profile mentions. A match is a discovery signal, not a qualification.
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Compression molding places material directly into an open cavity before closing the press, preserving longer fiber lengths (25 to 50 mm) for higher structural strength; injection molding forces molten material through runners and gates, which shears fibers down to shorter lengths.
Compression molding is preferred for annual volumes above 5,000 to 10,000 units where cycle times under 3 minutes justify steel tooling; RTM has lower tooling costs and suits lower to medium production volumes with complex hollow sections.
Coverage typically ranges from 30% to 70% of the cavity area. Lower coverage increases flow distance which helps vent air but can cause fiber washing and anisotropic properties; higher coverage reduces fiber movement but requires careful vacuum venting to avoid blisters.
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