What is Foam Core Density & Porosity?

Foam cores are lightweight, closed-cell materials widely used in composite sandwich structures. They provide separation between the face sheets, delivering high bending stiffness and strength at low weight. The two most important physical parameters of a foam core are its apparent density and its porosity.

Apparent density is the mass of the foam divided by its bulk volume (including the gas cells). Porosity is the volume fraction of the gas phase and is calculated from the ratio of apparent density to the density of the solid polymer that forms the cell walls.

These values directly influence:

Structural weight of the sandwich panel
Compressive and shear strength of the core
Thermal and acoustic insulation performance
Resin uptake during infusion or bonding processes
Cost and processability of the finished part

Calculation Formulas

Porosity is obtained from the simple relationship:

Porosity P (%) = (1 − ρ_apparent / ρ_base) × 100

Where:

ρ_apparent = measured (apparent) density of the foam core (g/cm³)
ρ_base = density of the solid base polymer (g/cm³)
P = porosity expressed as a percentage

The solid base density is the density the polymer would have if it contained no voids or cells. Typical values are taken from the polymer data sheet or measured on a solid, non-foamed sample of the same resin.

Enter Values

Foam Base Material Density ρ_base (g/cm³) Foam Core Apparent Density ρ_apparent (g/cm³)

Foam Porosity = ?

Note: The calculation assumes a closed-cell foam with negligible open porosity and no absorbed moisture. For open-cell or highly moisture-sensitive foams the relationship is more complex.

Typical Density Ranges for Common Foam Cores

Foam Type	Typical Apparent Density (g/cm³)	Typical Base Density (g/cm³)	Approx. Porosity
PVC (closed-cell)	0.04 – 0.20	1.35 – 1.45	86 – 97 %
PET (thermoplastic)	0.06 – 0.20	1.35 – 1.40	85 – 96 %
PU (polyurethane)	0.03 – 0.30	1.10 – 1.25	75 – 97 %
PMI (polymethacrylimide)	0.03 – 0.20	1.15 – 1.25	84 – 97 %
SAN (styrene acrylonitrile)	0.05 – 0.20	1.05 – 1.10	82 – 95 %
Honeycomb (for reference)	0.03 – 0.15	—	—

Worked Examples

Example 1 – Standard Aerospace PVC Foam

Base polymer density = 1.40 g/cm³, apparent density = 0.080 g/cm³

Porosity = (1 − 0.080 / 1.40) × 100 = (1 − 0.0571) × 100 = 94.29 %

This is a typical lightweight core used in aircraft secondary structures, UAV wings and high-performance marine panels.

Example 2 – Higher-Density Structural Foam

Base density = 1.40 g/cm³, apparent density = 0.130 g/cm³

Porosity = (1 − 0.130 / 1.40) × 100 = 90.71 %

Higher-density foams provide improved compressive and shear strength at the expense of increased weight, and are often selected for highly loaded regions or impact-critical areas.

Engineering Applications

Sandwich panel design — Select core density to meet stiffness, strength and weight targets.
Weight optimization — Trade core density against face-sheet thickness and overall panel mass.
Process selection — Estimate resin uptake during infusion or prepreg bonding (higher porosity generally absorbs more resin).
Quality control — Verify that incoming foam batches meet the specified apparent density and porosity range.
Thermal & acoustic design — Porosity strongly influences insulation performance.
Cost estimation — Core material cost is closely linked to density; accurate density data improves budgeting.

Important Practical Considerations

Closed-cell foams are preferred for most composite sandwich applications because they limit resin absorption and moisture uptake.
Apparent density should be measured on conditioned samples (temperature and humidity controlled) according to relevant standards (e.g., ASTM D1622).
Very low-density foams (< 0.05 g/cm³) can be fragile and more difficult to machine or handle.
When bonding foam cores, surface preparation and adhesive selection must account for the cell structure and possible residual blowing agents.
In high-temperature applications, the heat resistance of the base polymer (not just the foam density) becomes the limiting factor.

Frequently Asked Questions

What is the difference between apparent density and true density?

Apparent density includes the volume of the gas cells. True (or base) density is the density of the solid polymer that forms the cell walls. Porosity is derived from the ratio of the two.

Why is porosity important in sandwich structures?

Porosity controls weight, insulation properties and the amount of resin that will be absorbed during manufacturing. Excessive resin uptake increases weight and cost without improving structural performance.

Can this calculator be used for open-cell foams?

The basic formula still applies, but open-cell foams absorb significantly more resin and moisture. Additional process-specific corrections are usually required.

How does foam density affect mechanical properties?

Compressive strength, shear strength and modulus of rigid foams generally increase with density, often following power-law relationships. Design data should always be taken from the manufacturer’s technical sheets for the specific grade.

What density range is typical for aerospace sandwich cores?

Most aerospace applications use closed-cell foams between approximately 0.05 and 0.15 g/cm³, balancing weight savings against the required mechanical performance.

Related Calculations

Once foam core density and porosity are known, engineers typically proceed to:

Sandwich panel stiffness and strength calculations
Estimation of resin uptake during infusion
Overall panel areal weight
Thermal conductivity estimates
Cost-per-square-meter comparisons of alternative cores

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