Expanded polystyrene (EPS) is made by expanding small polystyrene beads with steam, allowing them to stabilise, then fusing them in a mould. The mould produces either a finished shape or a block that can be cut into sheets and profiles. The result is a lightweight, rigid foam with air held inside its closed cells.
The five stages
1. The raw bead
The process starts with expandable polystyrene beads. Polystyrene is produced by polymerising styrene monomer, and the beads contain pentane as a blowing agent. Pentane helps the beads expand when heated. EPS production does not use CFCs, HFCs or HCFCs.
Raw beads are compact granules. Their cellular foam structure develops during the next stage.
2. Pre-expansion
Steam heats the beads in a pre-expander. As the polystyrene softens and the pentane vaporises, each bead expands to form a network of small, closed cells.
Controlling expansion helps the manufacturer achieve the required density. Greater expansion produces a lower-density material; less expansion produces a denser material. Density strongly influences performance, but the finished grade must also meet its specified physical properties.
3. Maturing
The expanded beads are held in ageing chambers to cool and stabilise. Air diffuses into the cells as internal pressure equalises, preparing the beads for moulding.
Drying and conditioning prepare the beads for consistent moulding.
4. Moulding
The conditioned beads are loaded into a mould and heated with steam again. They expand further and fuse together, taking the shape of the mould.
Block moulding produces large blocks for cutting into sheets, insulation boards and custom profiles. Shape moulding produces items such as waffle pods, protective packaging and boxes directly in a shaped mould.
After moulding, the EPS is cooled and allowed to dry and stabilise before further processing.
5. Cutting and finishing
Hot wires cut blocks into sheets and boards. Profile cutting creates shaped components for specific applications. Depending on the product, further processing may include lamination, coating or assembly. Production offcuts are recovered for recycling.
How EPS grades relate to manufacturing
EPS is specified by grade because different applications require different physical properties. Nominal density is a useful reference, but it does not replace the grade’s performance data.
AS 1366.3-1992 sets minimum properties for six EPS classes: L, SL, S, M, H and VH. Foamex describes Styroboard EPS as manufactured to meet the requirements of this standard. VVH 32, VVH 36 and VVH 38 are additional Foamex grades outside those six classes.
| Grade | Nominal density (kg/m³) | Compressive stress at 10% deformation, min. (kPa) | Thermal resistance at 50 mm, min. (m²K/W) |
|---|---|---|---|
| L | 11 | 50 | R1.0 |
| SL | 13.5 | 70 | R1.20 |
| S | 16 | 85 | R1.22 |
| M | 19 | 105 | R1.25 |
| H | 24 | 135 | R1.40 |
| VH | 28 | 165 | R1.41 |
| VVH 32 | 32 | 250 | R1.43 |
| VVH 36 | 36 | 300 | R1.45 |
| VVH 38 | 38 | 350 | R1.50 |
The table gives Foamex’s published Styroboard EPS figures. Compressive stress at 10% deformation is tested to AS 2498.3. Thermal resistance is reported for a 50 mm sample at a mean temperature of 23°C under AS/NZS 4859.1:2018. These specifications apply to StyroSheet and StyroBloc, not automatically to every moulded EPS product.
A compression-test result is not an allowable design load. The data sheet also lists results at 1% and 5% deformation, but those figures should not be adopted as working loads without an engineering assessment. For load-bearing applications, ask the structural engineer to confirm the grade and request the relevant data from Foamex.
Choosing grade and thickness together
In the table, minimum compressive stress at 10% deformation rises from 50 kPa for L grade to 350 kPa for VVH 38. Minimum R-value at 50 mm also rises, from R1.0 to R1.50. Grade therefore affects both compression performance and thermal resistance.
For an insulation specification, check the required R-value against the selected grade and thickness. For a loaded application, check compression performance as well. Increasing thickness can provide more thermal resistance, but it does not remove the need to select a suitable grade.
Waste and recycled content
Foamex recovers and reprocesses offcuts generated during manufacture into new polystyrene products. Recovered EPS can also be used as a feedstock, with recycled-content options depending on the product. Confirm the recycled content of the product being ordered rather than applying one percentage across the range.
Fire performance
Foamex’s Styroboard EPS building products use fire-retardant raw material. This does not make EPS non-combustible or give a completed wall or roof a fire-resistance rating. The data sheet reports flame-propagation testing to AS 2122.1; the specified building system must meet the fire requirements of its intended use.
What this process produces at Foamex
- polystyrene sheets and boards, cut from moulded blocks
- EPS foam blocks for cutting and custom profiling
- EPS waffle pods, moulded for waffle pod slab construction
- geofoam civil void fill for lightweight fill applications
- EPS boxes for protective and temperature-sensitive transport
Contact Foamex with the application, dimensions and required performance to discuss the appropriate product. Use the product-specific documentation when preparing a specification.
Frequently asked questions
What is EPS made from?
EPS is made from polystyrene beads containing pentane as a blowing agent. Steam expands the beads before they are fused in a mould. The finished foam contains air within its closed cells.
Are ozone-depleting blowing agents used to make EPS?
EPS production does not use CFCs or HCFCs. EPSA also states that the process is HFC-free. Pentane is used to expand the beads.
What does the EPS grade mean?
Grade identifies a set of physical properties, including compression performance. Foamex’s technical data lists nominal densities from 11 kg/m³ for L grade to 38 kg/m³ for VVH 38. Specify the required grade and performance, not density alone.
What is the difference between EPS and XPS?
EPS is made by expanding and moulding beads. XPS is produced by extrusion as a continuous foam board. Both are rigid, closed-cell insulation materials, but their thermal and compression performance must be checked for the specific product and thickness.
Is EPS recyclable?
Yes. Clean EPS can be collected and reprocessed into new products. Keep offcuts separate from other waste and confirm the recycler’s acceptance requirements before arranging collection.
How is EPS density controlled?
Density is controlled through the expansion process. The amount of expansion changes how much polystyrene is present in a given volume of foam. Finished-product performance must still be checked against the specified grade.