Aluminium foam is solid aluminium filled with gas pores, so that 60 – 90 % of its volume is empty space. It weighs a fraction of solid metal, crushes at a steady stress that soaks up impact energy, absorbs sound when its cells are open, and does not burn. This guide explains the main foam types, the properties that matter, where each type is used and how to specify it.
Aluminium foam (also called foamed aluminium, foam aluminium or porous aluminium) is a cellular metal: a network of thin aluminium walls or struts enclosing pores of air. Its bulk density is typically 0.2 – 0.8 g/cm³, against 2.70 g/cm³ for solid aluminium, so a foam panel can be a tenth to a third of the weight of a solid plate of the same thickness.
The pores change how the metal behaves. Solid aluminium is stiff and strong but reflects sound, conducts heat quickly and transmits impact loads almost unchanged. A foam bends and buckles cell by cell when compressed, so the stress stays nearly constant while the material shortens by half or more. That long, flat stress plateau is what makes foam an efficient energy absorber. If the cells are interconnected, sound waves enter the pores and lose energy to air friction, which makes open-cell foam a good sound absorber.
Foam is not a substitute for sheet, plate or extrusions in load-bearing members. It earns its place when combined with them: as the core of a sandwich panel, as a filler inside a thin-walled tube, as an acoustic lining behind a perforated skin, or as a crushable insert that limits the force reaching something fragile. Choosing the right type, density and cell size for that job is most of the specification work.
Foams are grouped first by cell structure, because that decides what they are good at. The three families below cover almost every commercial product; a fourth, very thin "semi-transparent" grade is a sliced open-cell foam used mainly for decoration.
| Type | Typical density | Typical pore size | Cell structure | Strongest at |
|---|---|---|---|---|
| Open-cell (through-pore) foam | 0.2 – 0.5 g/cm³ | 0.5 – 3 mm | Interconnected pores; air and sound pass through | Sound absorption, filtration, heat exchange |
| Closed-cell foam | 0.3 – 0.75 g/cm³ | 2 – 10 mm | Sealed cells separated by thin walls | Energy absorption, stiffness in sandwich panels, buoyancy |
| Spherical-pore (syntactic or space-holder) foam | 0.5 – 1.1 g/cm³ | 3 – 13 mm spheres with small windows | Uniform round pores linked by small openings | Multi-directional impact absorption with some sound absorption |
| Thin sliced (semi-transparent) foam | about 0.25 – 0.35 g/cm³ | Cells cut open on both faces | Pores open right through a 3 – 10 mm slice | Decorative screens, light-diffusing panels, ceilings |
Ranges are indicative and depend on the production route and supplier. Always specify a density band and confirm properties on the test report for the batch.
Made by casting aluminium around a removable salt or sand space-holder that is then leached out, or by infiltrating a pattern. Because the pores connect, it behaves like a rigid, non-combustible acoustic sponge. Strength is lower than closed-cell foam of the same density.
Molten aluminium is thickened (for example with calcium or ceramic particles) and foamed with a gas-releasing agent or injected gas, then solidified as a block and sliced; alternatively a powder precursor is foamed inside a mould. Sealed cells give higher stiffness and plateau stress, so this is the structural and crash grade.
Produced by casting around hollow or removable spheres, giving round pores of very even size. The uniform structure produces a predictable crush response and the small windows between spheres add a measure of sound absorption.
Almost every property of a foam scales with its relative density: the foam density divided by the density of the solid alloy. Doubling density raises compressive strength roughly three- to four-fold, so small changes in density matter. The table summarises what to expect from commercial foams.
| Property | Typical range | What it means in practice |
|---|---|---|
| Density | 0.2 – 0.8 g/cm³ (spherical-pore up to about 1.1) | Specify a band, for example 0.40 ± 0.04 g/cm³ |
| Porosity | about 60 – 92 % | Higher porosity means lighter, weaker and (if open) more sound-absorbing |
| Compressive plateau stress | about 1 – 17 MPa, rising steeply with density | Sets the force at which the foam crushes; choose it below the load the protected structure can take |
| Densification strain | roughly 50 – 70 % | Only this fraction of the thickness is usable crush stroke |
| Tensile and bending strength | a few MPa | Foam alone is weak in tension; use it between skins or inside tubes |
| Sound absorption (open cells) | Peak coefficients near 0.9 or more are reported for thick open-cell panels | Performance improves with an air gap behind the panel, especially at low frequencies |
| Thermal conductivity | A few W/m·K, far below solid aluminium | Slows heat flow, but foam is not an insulation material in the sense of mineral wool |
| Fire behaviour | Metallic and non-combustible | Softens and melts near the melting range of the parent alloy; adhesives and coatings must be assessed separately |
| Corrosion | Similar to the parent alloy, but with a large exposed surface on cut faces | Seal or coat foam in wet or saline service |
Values are drawn from published literature on commercial foams (see references). Foam properties scatter more than those of solid alloys, so design values should come from tests on the specified product, not from a range table.
Two property claims are often overstated in sales literature. First, closed-cell foam on its own is a poor sound absorber: sealed cells reflect sound much like a solid plate, and good absorption needs open cells, cracked cell walls or a perforated face. Second, electromagnetic shielding and thermal values vary widely with thickness and structure, so ask for measured data rather than relying on a headline figure.
Foam is produced as blocks and then sliced, machined or laminated into the form the application needs. Common forms and size ranges are listed below; sizes vary by production route, so treat them as a starting point for an enquiry.
| Form | Typical size range | Notes |
|---|---|---|
| Foam sheet / slab | Thickness about 5 – 100 mm; panels such as 500 × 500, 500 × 1000, 1000 × 1000 and 1000 × 2000 mm | Sliced from cast blocks; larger formats such as 1220 × 2440 mm from some processes |
| Foam block | Up to several hundred millimetres thick | Machined into inserts, crash pads and custom shapes |
| Sandwich panel | Foam core with aluminium sheet faces, typically 0.5 – 2 mm each side | Faces bonded with adhesive or metallurgically joined; high bending stiffness per kilogram |
| Composite panel | Foam combined with stone veneer, wood, laminate or fabric | Used where appearance and low weight both matter |
| Perforated / punched panel | Holes punched through a closed-cell slab | Turns a reflective closed-cell panel into a sound absorber |
| Painted or coated panel | Powder coat, liquid paint or clear protective coating | Seals cut cells and adds colour |
| Machined shapes | Rods, discs, tubes, flanges and threaded parts | CNC or wire-cut to drawing from blocks |
Surface condition matters as much as size. A slab can be supplied with both faces cut (cells open on both sides), one face cut and one with the natural cast skin, or with the closed skin on both faces. Cut faces look more uniform and absorb more sound; skinned faces are smoother, easier to bond and better sealed against moisture.
| Pore size class | Approximate pore size | Typical behaviour and use |
|---|---|---|
| Small pore | 0.5 – 3 mm | High specific surface; best sound absorption without an air gap; fine, even appearance for interiors |
| Medium pore | 3 – 8 mm | Balanced strength, energy absorption and acoustic performance; the most common general-purpose class |
| Large pore | 8 – 12 mm and above | Lowest density and highest permeability; used for filtration, shock absorption and bold decorative surfaces |
Pore size and density are separate choices. Two foams of the same density can have very different pore sizes, and the finer one usually has thinner walls, a smoother cut surface and more uniform crushing. When appearance matters, approve a physical sample, because pore size varies across a cast block.
| Industry | Typical applications | Foam type usually chosen | Property relied on |
|---|---|---|---|
| Road and rail noise control | Highway and railway noise barriers, tunnel and underpass linings, metro station walls and ceilings | Open-cell, perforated closed-cell | Sound absorption that is not degraded by rain, humidity or washing |
| Building and architecture | Acoustic ceilings and wall panels, facade cladding cores, light screens, fire-safe interior linings | Open-cell, semi-transparent, closed-cell sandwich | Appearance, non-combustibility, sound absorption |
| Automotive | Crash-box and pillar fillers, battery underbody protection, floor panels, heat and noise shields | Closed-cell | Energy absorption and stiffness per kilogram |
| Rail vehicles | Floor panels, door cores, interior linings | Closed-cell sandwich, perforated panels | Low weight, fire safety, damping |
| Marine | Deck and bulkhead sandwich panels, buoyancy inserts, engine-room linings | Closed-cell sandwich | Low weight, buoyancy, non-combustibility |
| Aerospace and space | Landing-impact absorbers, equipment mounts, cushioning for airdrop loads | Closed-cell, spherical-pore | Predictable crush at controlled force |
| Defence | Blast-mitigating layers in armour, shelter doors, vehicle floors | Closed-cell, spherical-pore | Energy and blast absorption with limited weight |
| Nuclear and energy | Impact limiters on transport casks, protective components, thermal and shock buffers | Closed-cell | Energy absorption with a non-combustible metal |
| Industrial | Machine-tool damping, filters, flame arrestors, heat exchanger media | Open-cell | Damping, permeability, heat transfer |
| Furniture and interiors | Lightweight panels for cabinets and display systems | Closed-cell sandwich | Flatness, low weight, moisture stability |
Each of the industry pages linked at the end of this guide goes into one sector in more detail. Below are typical configurations, described generically, to show how foam is put to work.
A barrier module usually combines a structural aluminium frame, a rear reflective panel and a front layer of open-cell or perforated closed-cell foam, 5 – 30 mm thick. Unlike mineral-wool infill, the foam does not sag, rot or lose absorption when wet, and it can be washed.
A common build-up is thin aluminium sheet on both faces of a closed-cell foam core 15 – 30 mm thick. The sandwich gives a stiff, quiet floor that does not absorb moisture and replaces plywood with a non-combustible panel.
A 6xxx extrusion is filled with closed-cell foam. The foam supports the tube walls as they fold, so more energy is absorbed per unit length. Useful where the crash zone is short.
Open-cell or thin sliced foam tiles hung with an air gap behind them. The air gap lowers the frequency at which absorption peaks, improving control of speech and machinery noise.
A machined foam block placed under sensitive equipment crushes at a known stress, limiting deceleration during a drop or landing.
Thin semi-transparent foam is framed in aluminium profiles to make light-diffusing partitions, lamp shades and facade features.
For sandwich panels, the face sheets carry bending stresses and the foam carries shear. Typical face materials are 3003, 5052 and 5754 sheet for general panels and 6061 or 6082 where higher strength is needed. Metline supplies these sheet and coil grades alongside foam, so the core and the faces can be sourced together.
| Criterion | Aluminium foam | Aluminium honeycomb | Mineral wool | Polymer foam (PU, EPS, EPP) |
|---|---|---|---|---|
| Weight | Light | Lightest for its stiffness | Light | Lightest |
| Energy absorption | Good in all directions | Very high along the cell axis only | Negligible | Moderate |
| Sound absorption | Good (open cells or perforated) | Poor unless perforated | Very good | Fair (open-cell types) |
| Fire | Non-combustible | Non-combustible core; adhesive may burn | Non-combustible | Combustible |
| Moisture | Unaffected | Unaffected | Absorbs water, sags | Varies |
| Self-supporting | Yes, rigid | Needs skins | No | Partly |
| Relative cost | High | Medium | Low | Low |
Honeycomb wins on stiffness per kilogram for flat panels loaded in a known direction; see the aluminium honeycomb core guide. Mineral wool is cheaper for indoor acoustic infill that stays dry. Aluminium foam is the choice where a panel must be rigid, absorb sound or impact, survive water and fire, and still look finished.
Prices depend mainly on density, thickness, panel size, surface treatment, quantity and any lamination. Because foam is often used in projects, it helps to share the total area or number of parts, the delivery schedule and any samples or approval steps your client requires.
In open-cell foam the pores connect, so air and sound pass through; it is used for sound absorption, filtration and heat exchange. In closed-cell foam each pore is sealed; it is stiffer and stronger at the same density and is used for energy absorption and sandwich cores.
Most commercial foams weigh 0.2 – 0.8 g/cm³, so a 20 mm panel weighs about 4 – 16 kg per square metre, against 54 kg for a solid 20 mm aluminium plate.
No. The foam is metallic and non-combustible and gives off no smoke of its own. It softens and melts near the melting range of the aluminium alloy. Adhesives, coatings and laminated facings must be assessed separately for fire performance.
It is good for sound absorption (reducing echo and reverberation) when its cells are open or the panel is perforated. Blocking sound transmission through a wall depends mainly on mass and construction, so foam is usually combined with a solid layer for that purpose.
Yes. It is widely used in road and rail noise barriers. It does not absorb water into the metal or lose absorption when wet; coating or sealing cut faces is recommended in coastal or polluted environments.
Not directly in a practical way. Foam is joined by adhesives, fasteners with inserts, or by foaming it in place. In sandwich panels and filled tubes, the solid aluminium skins or tubes are welded instead.
Tell us the foam type, density, thickness, size and quantity, or send a drawing. We will propose options and a delivered price.
