Polymethacrylimide (PMI) foam is a rigid, high-performance structural core material widely used in aerospace sandwich panels for applications where light weight, thermal stability, and mechanical strength are critical. Its closed-cell structure and ability to withstand high processing temperatures make it ideal for use with carbon fiber composites, particularly in interior and structural aircraft components.

Background and Evolution

PMI foam was developed in the 1970s to meet the growing demand for thermally stable, lightweight core materials in composite sandwich structures. Its inherent heat resistance, compressive strength, and low density quickly made it a favorite in aerospace and spaceflight applications.

Its use in commercial aviation ramped up in the 1990s and 2000s as aircraft manufacturers began favoring carbon fiber reinforced polymers (CFRPs) and needed compatible core materials for radomes, floor panels, bulkheads, and even flap fairings. PMI foam is found in aircraft like the Airbus A350, Boeing 787, and Gulfstream G650.

How PMI Foam Is Used

• Sandwich Panel Cores: Used in aircraft floor panels, ceiling panels, sidewalls, overhead bins, and control surfaces (e.g., flaps, ailerons).
• Structural Parts: In stiffened skins, radomes, spoilers, and nacelles due to its high strength-to-weight ratio.
• Fairings and Covers: PMI core is used with CFRP or GFRP skins for aerodynamic covers and fairings.
• Insulating Panels: Acts as thermal insulation in high-temperature or high-pressure zones (e.g., environmental control systems).

Why PMI Foam Is Used

• High Strength-to-Weight Ratio: Offers excellent compressive and shear strength for minimal mass.
• Thermal Stability: Tolerates processing temperatures up to 180–220°C, suitable for autoclave-cured composites.
• Closed-Cell Structure: Prevents moisture absorption, preserving weight and performance in humid environments.
• Dimensional Stability: Maintains shape and structural integrity under temperature and pressure fluctuations.
• Machinability: Can be milled, shaped, or thermoformed for custom geometries in composite layups.
• Excellent Bonding: Strong adhesion to prepregs and resins without the need for extra surface treatments.

Technical Specifications

• Density: Typically 30–110 kg/m³, depending on grade.
• Compressive Strength: Up to ~1.2 MPa (low-density) to 3.5 MPa (high-density grades).
• Shear Strength: ~0.5–2.5 MPa.
• Thermal Resistance: Continuous use up to 180°C; peak exposure up to ~220°C.
• Closed-Cell Content: >95%, preventing water ingress.
• Flammability: Low flammability; versions are available that meet FAR 25.853 requirements.
• Dimensional Tolerance: Tight tolerances suitable for precision aerospace components.

Comparison to Alternative Core Materials