Porous aluminium foam sheet is solid aluminium with 60–90 % of its volume replaced by gas cells, giving a rigid, non-combustible plate that weighs a fraction of solid metal. Open-cell foam absorbs sound and lets air or fluid pass; closed-cell foam insulates, blocks sound and soaks up impact energy. This guide explains how to choose between them, what the property data mean, and how to specify foam panels for supply from Metline.
Aluminium foam is a cellular metal: a continuous aluminium skeleton surrounding a network of pores. It keeps the things that make aluminium useful (stiffness, non-combustibility, corrosion resistance, recyclability) while adding properties solid metal does not have, such as sound absorption, low thermal conductivity and the ability to crush progressively and absorb a large amount of energy at a nearly constant force.
Two numbers describe any foam: porosity (the share of the volume that is empty, usually 60–90 %) and cell size (the average pore diameter, usually 0.5–10 mm). Density follows from porosity. A foam with 90 % porosity has a density of about 0.27 g/cm³, roughly one tenth of solid aluminium (2.70 g/cm³) and light enough to float on water. Most commercial plate falls between 0.2 and 0.7 g/cm³ for closed-cell grades, with denser open-cell grades used where strength or filtration matters.
The other key distinction is whether the cells are connected. In open-cell foam the pores link to each other and to the surface, so sound waves, air and liquids can enter. In closed-cell foam each pore is sealed by thin cell walls, so the plate is airtight, buoyant and a good insulator. The two behave so differently that they should be treated as separate materials when you specify them.
| Characteristic | Open-cell foam | Closed-cell foam |
|---|---|---|
| Pore structure | Interconnected pores open to the surface | Sealed pores separated by thin walls |
| Typical density | About 0.25 – 1.05 g/cm³ | About 0.2 – 0.7 g/cm³ |
| Typical porosity | About 60 – 75 % | About 75 – 90 % |
| Typical cell size | About 0.5 – 10 mm (set by the process) | About 3 – 8 mm |
| Sound | Absorbs sound inside the pores | Reflects and insulates; limited absorption unless perforated or drilled |
| Air and fluid | Permeable: used for filters, silencers, flame arresters and heat exchangers | Impermeable and buoyant |
| Thermal role | Heat dissipation: high surface area for forced-air or liquid cooling | Thermal insulation |
| Mechanical role | Higher-density grades carry load; used in filters and diffusers | Energy absorption, crash and blast protection, sandwich cores |
| Typical uses | Acoustic ceilings, concert halls, gyms, silencers, filtration | Noise barriers, rail and vehicle panels, crash boxes, fire doors, ship interiors |
Open-cell foam is commonly offered in three cell-size classes (large, medium and small), and as panels open on one face or on both faces. A panel open on both faces absorbs sound from both sides and lets air through; a panel open on one face has a closed back skin and is used as a wall or ceiling lining. Closed-cell foam can be supplied with its natural cast skin, with the skin machined off to expose the cells, or perforated to add sound absorption.
Foam properties depend strongly on density, cell size and process, so published figures vary between producers. The ranges below are typical values for commercial plate, used for first selection. Final design values should come from the test report for the specific grade and density you order.
| Property | Open-cell plate | Closed-cell plate | Test basis |
|---|---|---|---|
| Density | 0.25 – 1.05 g/cm³ | 0.25 – 0.70 g/cm³ | Mass ÷ volume |
| Porosity | 60 – 70 % | 75 – 90 % | 1 − (foam density ÷ 2.70) |
| Compressive (plateau) strength | Rises sharply with density; dense grades reach tens of MPa | About 3 – 17 MPa, depending on density | ISO 13314 |
| Bending strength | Higher than closed-cell at equal thickness for dense grades | About 3 – 15 MPa | Three-point bend |
| Sound absorption coefficient | About 0.5 – 0.85 (20 mm plate, 2 kHz) | About 0.4 – 0.8 when perforated or drilled | ISO 354 / ISO 10534-2 |
| Thermal conductivity | Far below solid aluminium | Of the order of a few W/m·K, versus about 200+ W/m·K for solid aluminium | Steady-state method |
| Electromagnetic shielding | Producers report 60 – 90 dB for thick plate | Producers report 60 – 90 dB for thick plate | Frequency-dependent test |
| Reaction to fire | Non-combustible metal | Non-combustible metal | EN 13501-1 classification for coated products |
Plateau strength scales roughly with relative density to the power 1.5, so a small change in density changes strength a lot. Always quote density with any strength figure.
The compressive stress–strain curve of a closed-cell foam is what makes it valuable for protection. After a short elastic region, the cells buckle and collapse one layer after another at a nearly flat "plateau" stress, until the foam is crushed to about 30–40 % of its original thickness and stiffens. The area under that plateau is the energy absorbed. Designers pick the density so that the plateau stress keeps the force on the protected structure, or on a vehicle occupant, below a safe limit while the stroke absorbs the required energy.
A metal hydride (usually titanium hydride, TiH2) is stirred into molten aluminium thickened with calcium. The hydride releases hydrogen and the melt rises like bread. The block is cooled and sliced into plate. Produces fine, uniform closed cells; a slow-release blowing agent gives more time for even foaming.
Air or another gas is blown into an aluminium melt containing ceramic particles (such as SiC or Al2O3) that stabilise the bubbles. The foam is drawn off continuously as a slab. Cost-effective for large closed-cell panels with coarser cells.
Aluminium powder and a blowing agent are compacted into a dense precursor, which expands when heated in a mould. Suited to shaped parts, foam-filled tubes and foam cores bonded metallurgically to aluminium skins.
Molten aluminium is pressed into a bed of removable particles, typically salt grains, which are dissolved out afterwards. Gives open-cell foam with a cell size set by the particle size, from fine to coarse.
A polymer foam pattern is replicated in aluminium, or aluminium powder or fibres are sintered. Produces very regular open-cell structures for heat exchangers and filters.
Aluminium or ceramic hollow spheres are bonded or cast into a matrix to make a syntactic or sphere foam with very uniform cells.
Melt-foamed material shows a slight density gradient from top to bottom, because gravity drains liquid metal down the cell walls before the foam solidifies. Producers control this with melt viscosity and foaming time, and by slicing plate from the uniform middle of the block. If density uniformity matters for your design, ask for density measured on samples from several positions.
Foam panels are made to order. State the property you need (sound absorption, insulation, energy absorption) as well as the size, so the right type and density can be matched.
Band saws, circular saws with fine-tooth carbide blades, and waterjet all cut foam cleanly. Support the panel close to the cut so thin cell walls are not torn.
Drill with sharp bits at moderate feed. Use large washers, inserts or bonded bushes, because cell walls cannot take high point loads from screws.
Bonding aluminium sheet to both faces of a closed-cell core produces a very stiff, light panel. Structural adhesives are common; powder-metallurgy foams can be bonded metallurgically to skins.
Spray paint and powder coating are both used. On acoustic panels, use a thin coat that does not close the open pores, or the absorption falls.
Aluminium foam resists normal indoor and outdoor atmospheres. In marine or polluted environments, choose a coated finish and avoid trapping water in open cells.
Foam is aluminium plus a small amount of additive and can be remelted with aluminium scrap at end of life.
| Sector | Typical foam parts | Foam type |
|---|---|---|
| Building and acoustics | Ceiling and wall absorbers for halls, gyms, stations and plant rooms; fire doors and fire partitions; elevator cab panels | Open-cell for absorption, closed-cell for doors and partitions |
| Road and rail noise | Noise barrier panels along highways and urban rail lines; under-floor and wall panels in rail cars | Closed-cell, often perforated |
| Automotive | Crash boxes, foam-filled tubes and beams, engine-bay and floor noise insulation, battery protection | Closed-cell |
| Marine | Cabin linings and engine-room absorbers that do not absorb moisture or release fibres; buoyant structural cores | Open- and closed-cell |
| Aerospace and space | Impact-protection and buffer structures, thermal-control parts | Closed-cell and open-cell |
| Electronics | EMI shielding enclosures and partitions, heat-dissipating open-cell inserts | Both |
| Industrial and environmental | Silencers, flame arresters, filters, catalyst supports and diffusers | Open-cell |
| Defence and protection | Blast- and impact-absorbing layers in protective panels and transport cases | Closed-cell |
In most of these uses aluminium foam is chosen because it combines several functions in one panel. A rail car floor panel, for example, can be stiff, light, sound-insulating and non-combustible at the same time, where a conventional design might need a structural panel plus a separate mineral-wool layer. That combination, rather than any single property, is usually what justifies foam over cheaper single-purpose materials.
| Factor | Aluminium foam | Aluminium honeycomb | Mineral wool / polymer foam |
|---|---|---|---|
| Stiffness-to-weight as a core | Good, isotropic | Excellent in the through-thickness direction | Low |
| Energy absorption | Excellent, in any direction | Excellent along the cell axis only | Poor |
| Sound absorption | Good (open-cell) | Low unless perforated | Good |
| Moisture and fibres | None | None | Absorbs moisture / sheds fibres (wool) |
| Fire | Non-combustible | Non-combustible core; depends on adhesive | Wool non-combustible; polymer foams combustible |
| Machinability and shaping | Easy to cut and shape in 3D | Easy to cut; limited 3D shaping | Easy |
| Relative cost | Higher | Medium | Low |
For flat panels where stiffness per kilogram is the main goal, aluminium honeycomb is usually the more efficient core. Foam earns its place when loads come from several directions, when the panel must absorb sound or impact, or when the core must be shaped or fill a closed section such as a crash tube.
A short sample trial avoids most of these problems. Ask for a sample panel of the proposed density and finish, check it with your fixing method and, for acoustic or impact work, test it in the real mounting arrangement before committing to the full quantity.
Open-cell foam has interconnected pores, so it absorbs sound and lets air or liquid through. Closed-cell foam has sealed pores, so it is airtight and buoyant, insulates against heat and sound, and absorbs impact energy. Choose by function, not by appearance.
Most closed-cell plate weighs 0.2–0.7 g/cm³, about one quarter to one tenth of solid aluminium. A 1 m² panel 20 mm thick at 0.4 g/cm³ weighs about 8 kg.
Aluminium foam is a non-combustible metal and does not give off toxic smoke. Like all aluminium it softens and melts at high temperature (around 600–660 °C), so for fire doors and partitions it is used within a tested system. Coated panels need their own fire classification.
With ordinary saws, waterjet or CNC routers. Panels are fixed with bonded inserts, large washers, clips or adhesives, because thin cell walls cannot take concentrated screw loads.
Yes, if the coating is thin enough not to close the open pores. Heavy paint layers seal the surface and reduce absorption, so acoustic panels use light spray coats or factory finishes.
Start from the job. For impact absorption, pick the density whose plateau stress keeps transmitted force below the limit while absorbing the required energy. For acoustics, cell size and thickness matter more than density. For sandwich cores, higher density gives more shear strength.
Send the application, foam type, density, thickness and panel size. We will match a grade, provide test data and quote delivered to your site.
