Battery foam selection begins with the assigned spacer, cushioning or insulation function and its mechanical, electrical and thermal requirements. A short sheet test does not qualify the part for pack life or make ordinary foam a thermal-runaway barrier.
What the Foam Has to Do
Inside a pack, one foam layer usually serves several functions at once:
- Cell spacing and location — holding cells at a defined gap so the module stays dimensionally correct.
- Thermal insulation is different from heat transfer to a cold plate. Define the intended heat path with the pack designer; ordinary low-conductivity foam is not a replacement for a specified thermal-interface material and no thermal-runaway delay is claimed.
- Vibration and impact damping — protecting welds and busbars from road-induced fatigue.
- Dielectric separation — keeping a non-conductive barrier between live parts and the case.
Closed-cell cross-linked polyethylene foam (XPE and IXPE) covers most of this. IXPP, a cross-linked polypropylene foam, is chosen where the operating temperature is higher.
Material Choice: XPE, IXPE or IXPP
- XPE — chemical cross-linking, single layer 3–15 mm, heat-bonded 15–200 mm, ratio 3–40x. Denser, stiffer, the workhorse for module spacers and structural pads.
- IXPE — radiation cross-linking, single layer 0.1–15 mm, laminated 15–200 mm, ratio 3–40x. Thinner and finer-celled, so it conforms to curved cell cans and prismatic cell faces.
- IXPP — cross-linked polypropylene, single layer 1–6 mm, laminated 6–100 mm, ratio 5–25x. Selected for better temperature resistance than PE foam; confirm the service temperature with your supplier for the specific grade.
Identify the grade, layer construction and cut-edge cleanliness. Cutting exposes a different surface; confirm its required particulate and electrical criteria instead of assuming a closed-cell name supplies those results.
Thermal Conductivity and Thickness
Thermal conductivity in the XPE and IXPE ranges improves as the foam gets lighter (higher expansion ratio), because more of the volume is gas rather than polymer. Measured to GB/T 10297:
| Ratio | Density (kg/m³) | Thermal conductivity upper limit (W/m·K) | Compression set upper limit (%) |
|---|---|---|---|
| 3x | 300±40 | ≤0.095 | ≤2 |
| 5x | 200±30 | ≤0.095 | ≤2 |
| 8x | 125±15 | ≤0.084 | ≤3 |
| 10x | 100±10 | ≤0.073 | ≤5 |
| 15x | 66.7±8 | ≤0.065 | ≤7 |
| 20x | 50±6 | ≤0.055 | ≤8 |
| 25x | 40±4 | ≤0.049 | ≤10 |
| 30x | 33.3±3 | ≤0.045 | ≤10 |
| 35x | 28.6±2 | ≤0.040 | ≤11 |
| 40x | 26.8±3 | ≤0.040 | ≤11 |
Specify allowable force across the full displacement range, temperature and intended life cycle. Grade compression-set results, maximum temperature and dimensional change do not replace force-deflection and long-term component evidence.
Moisture, Dimensional Stability and Ageing
- XPE absorption limits under GB/T 1034 method 4 are ≤0.2% at 3x–10x, ≤0.3% at 15x–20x, and ≤0.4% at 25x–40x. IXPE has a different allocation at some ratios; use its own table. None of these values guarantees dry joints or freedom from corrosion.
- Dimensional stability — at 70±2 °C for 22 hours, XPE dimensional change stays within 4% across the sheet (TD) and 6% along it (MD) up to 15x, widening to 6% and 8% from 20x (GB/T 8811).
- Compression-set limits for XPE and IXPE vary by ratio from ≤2% to ≤11% under GB/T 6669, 23±2 °C, 22 h. This is residual deformation in a defined test, not complete recovery, a cushioning service life or proof of performance after a year. Confirm the loaded mat and required ageing or impact tests.
None of these values should be read as a maximum service temperature. Continuous temperature capability depends on grade and on how long the exposure lasts; state your worst-case condition to the supplier and get the material confirmed for it rather than extrapolating from a 22-hour test.
Electrical Behaviour
Polyethylene foam is a non-polar dielectric and is used as a spacer and separation layer inside packs. Two points to be clear about:
- Foam is not solid polymer. Cell structure and thickness drive breakdown behaviour, so validate the finished part at your system voltage and creepage rather than relying on a generic figure.
- If you need a defined resistivity, specify it. Anti-static and conductive foam grades exist for controlled dissipation; standard XPE/IXPE is not a substitute for them.
How to Specify Battery Pack Foam
- Function — spacer, insulator, damper, or a combination.
- Material — XPE, IXPE or IXPP, with the cross-linking process stated.
- Density and tolerance — e.g. 40 ±4 kg/m³.
- Thickness — single layer or heat-bonded, with tolerance.
- Compression force — the requirement that actually locates the cell.
- Electrical requirement — insulating, or a specified resistivity grade.
- Temperature and ageing — worst-case condition, plus flammability and restricted-substance requirements your programme demands, declared by the supplier.
FAQ
Which foam goes between prismatic cells?
Define the allowed force over the full gap range, contact faces, electrical requirements and thermal path first. Evaluate the proposed grade and full component under the required duration and temperature conditions.
Is lighter foam always a better choice?
No. Conductivity limits, force-deflection, residual deformation and the complete thermal design must be assessed together. A lower sheet conductivity does not qualify a cell compression pad or a cooling interface.
Can the same grade be used for the pack lid?
Treat the lid as its own drawing and operating condition. Confirm compression, adhesive, temperature and electrical acceptance rather than reuse a cell-spacer approval automatically.
Application and enquiry
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Related Reading
See also: XPE vs IXPE vs IXPP · Automotive foam solutions · All foam materials