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Choosing an EV Cell Compression Pad: PORON Microcellular Polyurethane vs. BISCO Silicone Foam
A cell-to-cell compression pad is not chosen on a single firmness number; it is chosen by how its force-deflection behavior, its compression set, and its temperature and flame capability line up with the duty of the stack. PORON microcellular polyurethane leads on the mechanical job: the flattest compression-force-deflection (CFD) plateau and the lowest compression set across normal module temperatures. BISCO silicone foam leads when temperature or a stringent UL 94 flame rating governs.
A thermal gap filler, despite living in the same catalog aisle, is built for a different job entirely and should not be put in a compression slot. This guide walks the decision in the order the stack actually imposes it, using the test methods the data sheets cite (ASTM D3574 / ISO 3386 for CFD, ASTM D395 and ASTM D3574 for compression set, ASTM D6147 / ISO 3384 for stress relaxation).
No grade-level force value is quoted here; those come from the manufacturer's current data sheet at your thickness and deflection.
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The application problem: a pad that must push back for the life of the pack
H-O Products is a precision die-cut converter, and every compression pad is made to order from sheet or roll stock cut to a stack drawing. The engineering problem the pad solves is permanent and quietly difficult: every lithium-ion cell breathes reversibly each charge-discharge cycle as lithium moves into and out of the anode, and swells irreversibly over life, commonly running about 10 to 20% thicker at end of life.
The pad between adjacent pouch or prismatic cells has to hold a near-constant push-back force across that whole growing deflection, keeping electrode layers in contact without crushing the cell.
That duty turns material selection into a behavior question, not a property-table question. Two foams with similar firmness at 25% compression can age completely differently: one holds its force for years, the other takes a permanent set, thins out, and lets the preload decay below the window the cell maker specified. The material families on this page are the ones engineered for that duty, and the differences between them are the subject of this guide.
Why the choice is harder than a durometer number
- The CFD curve is the real specification, not one firmness point. Compression force deflection (measured per ASTM D3574 Test C, or ISO 3386) describes the stress a foam pushes back with at each percent compression. Microcellular foams show a roughly linear-elastic region at low compression, a wide plateau in the working zone, and a steep densification region at high compression. The pad must live on the plateau across the full beginning-of-life to end-of-life deflection range, so the curve shape decides the part, and a single catalog number cannot.
- Compression set and stress relaxation decide the pad's old age. Compression set (ASTM D395 Method B for rubber, ASTM D3574 for flexible PU foam) is the permanent thickness the foam never recovers; stress relaxation (ASTM D6147 / ISO 3384) is the slow decay of force at constant deflection. Both are accelerated by heat. A pad that measures perfectly at the build event can still go slack in year five if the family was wrong.
- Temperature and flame duty can override the mechanical ranking. Polyurethane's mechanical behavior is excellent across normal module temperatures, but where the location runs hotter or the design calls for a stringent UL 94 rating, silicone foam's higher continuous-temperature capability and stronger flame performance govern, with V-0 available on specific silicone grades.
- The position in the stack changes the grade. A cell-to-cell pad is a thin, usually softer layer managing the gap between one pair of cells. A cell-to-module or end-plate cushion is thicker and usually firmer, absorbing the accumulated swelling of the whole group. Cell-to-pack and cell-to-chassis designs delete the module frame, so the pad also takes up build tolerance and damps vibration.
The candidate families, by the decision each one wins
PORON microcellular polyurethane (the mechanical lead)
PORON industrial 4701/4790 series microcellular polyurethane is the default starting point for cell-to-cell and cell-to-module compression duty across normal module temperatures.
The family is engineered for the flat CFD plateau and the low compression set the application depends on: as the cell swells and drives the pad deeper into its deflection range, the push-back force stays nearly constant instead of climbing out of the preload window, and the foam resists taking a permanent set under years of sustained load.
Within the family, softer and firmer grades and multiple gauges let the converter place your working window on the plateau. The PORON EVExtend battery grades extend the same microcellular mechanics with formulations developed specifically for cell-stack compression duty; see the PORON EVExtend product page for the current grade options.
BISCO silicone foam (temperature and flame lead)
Closed-cell BISCO HT/BF series silicone foam takes over when the thermal or flame requirement governs. Silicone holds its properties at continuous temperatures that degrade polyurethane, and its flame performance is stronger, with UL 94 ratings available up to V-0 on specific grades.
The trade is that silicone foam's force-deflection behavior differs from microcellular PU, so the CFD curve and the compression-set value for the specific grade must be confirmed at your gauge rather than assumed from the polyurethane experience.
The line spans extra-soft cushioning grades through firm grades, so the same family covers a soft cell-face cushion and a firmer end-plate pad; the medium-firmness BISCO silicone foam product page is a representative anchor point in the range.
Flame-rated V-0 silicone sponge (when the flame spec leads outright)
Where the design brief makes the vertical-burn rating the first sort, a dedicated V-0 flame-resistant silicone sponge is the focused option: a closed-cell silicone engineered to the V-0 classification while still compressing and recovering as a cushion. It is commonly the answer where a pad, a seal, and a flame requirement meet on the same part. As with all flame claims, the UL 94 classification is grade- and thickness-specific, so verify the listing on the data sheet for your exact gauge.
What does not belong: thermal gap fillers in a compression slot
A thermal gap filler is engineered for thermal conductance and tolerance take-up. Its stress-strain shape is wrong for holding a controlled spring force: used as a compression pad it may relax and let the preload decay, or stiffen and spike the force out of the window. If one layer genuinely must do both the thermal and the compression job, say so on the drawing, and engineering can evaluate a stack-up that serves both duties instead of a single material that serves neither.
Compression-Pad Families: PORON PU, BISCO Silicone Foam, V-0 Sponge
| Family | CFD plateau behavior | Compression-set resistance | Temperature capability | Flame rating path | Typical position |
|---|---|---|---|---|---|
| PORON microcellular PU | Flattest | Lowest set | Normal module range | HBF / HF-1 classes | Cell-to-cell, cell-to-module |
| BISCO silicone foam | Grade-dependent | Low set | Higher continuous | Up to V-0 (specific grades) | Hotter locations, flame-led duty |
| V-0 silicone sponge | Cushion-grade | Good | Higher continuous | V-0 rated line | Pad-plus-flame combination parts |
| Thermal gap filler | Wrong shape | Not the design intent | Grade-dependent | Grade-dependent | Thermal slots only, not compression |
Illustrative and relative only; confirm every grade against its current technical data sheet and a drawing review. UL 94 foam classes (HBF, HF-1, V-0) are grade- and thickness-specific listings, not family-wide guarantees.
Setting the preload window before picking the foam
The material decision only makes sense inside a force window, and the window comes from the cell, not the foam catalog.
There is no single universal compression number: published research indicates a light-to-moderate preload commonly helps capacity retention and life, while over-compression may accelerate aging, and the beneficial and harmful levels vary widely across chemistries and formats (the stack-pressure literature from Cannarella and Arnold's 2014 work through the 2021 and 2025 Journal of Power Sources studies, and the survey in SAE 2024-01-2430, is consistent on that point).
The practical sequence is the one the parent page walks in detail: take the preload window from your cell supplier's guidance, establish the beginning-of-life gap and the end-of-life swelling allowance, then choose the family, grade, and thickness whose CFD plateau keeps the force inside the window across that whole range. Pouch cells, having no rigid housing, lean on the pad hardest; prismatic cells are stiffer but still benefit from controlled stack pressure; cylindrical cells are generally self-constraining and sit outside this discussion.
Converting considerations that ride along with the material
- Cutting method follows geometry and volume. Die-cutting and kiss-cutting cover most pad footprints; laser and waterjet cutting handle fine features, tight tolerances, and tooling-free prototypes.
- Lamination and stack-to-thickness. Where a single sheet gauge cannot hit the target thickness or the drawing calls for a multi-layer behavior, layers are laminated to a controlled stack height.
- Adhesive, liners, and pull-tabs. A pressure-sensitive adhesive keeps the pad located during module assembly; kiss-cut presentation on a liner with pull-tabs makes line installation repeatable without stretching the foam.
- Tolerances belong on the drawing. Thickness and footprint tolerances interact with the CFD window, so they are part of the engineering review, not an afterthought.
Common selection failure modes
| Failure mode | Root cause | The fix |
|---|---|---|
| Preload decays over years | Family chosen on day-one firmness; compression set and stress relaxation never reviewed | Check set (ASTM D395 / D3574) and relaxation (ASTM D6147 / ISO 3384) for the grade at your temperature |
| Force spikes at end of life | Working deflection runs off the plateau into densification as the cell swells | Size thickness so beginning-of-life and end-of-life deflections both sit on the flat CFD region |
| Gap-fill foam in a compression slot | The two duties blurred at the catalog stage | Specify a true compression material; keep gap fillers on the thermal job they were engineered for |
| Comparing set values across methods | ASTM D395 / D3574 report against original height; ASTM D1056 reports against the compressed height | Compare like-for-like methods and conditions only; note the method on the drawing |
| Flame rating assumed family-wide | UL 94 classes are grade- and thickness-specific listings | Verify the exact class (HBF, HF-1, V-0) on the TDS at your gauge |
How H-O Products helps
H-O Products is a precision die-cut converter, not a raw-material producer. Once the position in the stack, the gap and swell numbers, the target force window, and any temperature or flame duty are on the table, we recommend a family and grade direction across PORON microcellular polyurethane, PORON EVExtend battery grades, BISCO silicone foam, and V-0 silicone sponge, then convert the stock to your drawing: die-cut, kiss-cut, laser or waterjet cut, laminated to thickness, with adhesive, liners, and pull-tabs as the line requires.
Documentation available on request includes the material technical data sheet, a certificate of conformance, lot traceability, and first-article inspection, all under our ISO 9001:2015 certified quality management system. We hold converting tolerances to the drawing; we do not produce the raw foam, and grade-level CFD, set, and flame values are confirmed against the manufacturer's current data sheet. Pack-level results such as UN 38.3 and UL 9540A belong to the tested pack, not to any pad material.
Frequently asked questions
When should I move from PORON polyurethane to BISCO silicone foam?
When temperature or flame rating governs the location. PORON microcellular polyurethane delivers the flattest CFD plateau and the lowest compression set across normal module temperatures, which is exactly what the mechanical job wants.
BISCO silicone foam offers higher continuous-temperature capability and stronger flame performance, with UL 94 V-0 available on specific grades, so it takes over where the pad runs hot or the design brief demands a stringent vertical-burn class. Confirm the silicone grade's CFD curve and compression set on its data sheet, because its force-deflection behavior differs from polyurethane.
Can I use a thermal gap filler as a compression pad if it feels similar?
No. A gap filler is engineered for thermal conductance and tolerance take-up, and its stress-strain shape is wrong for holding a controlled spring force. In a compression slot it may relax and let the preload decay, or stiffen and push the force out of the window. Specify a true compression material (PORON microcellular PU, PORON EVExtend, or BISCO silicone foam), and if one part genuinely must serve both duties, flag it so engineering can review a stack-up that does both.
How do I pick thickness once the family is chosen?
From three numbers: the beginning-of-life gap the pad fills, the end-of-life swelling allowance (cells commonly run about 10 to 20% thicker at end of life), and the target push-back force window from your cell supplier. The grade and thickness are then set so both the beginning-of-life and end-of-life deflections land on the flat part of the CFD curve, neither below the window early nor into the densification knee late. A slightly thicker pad on a flatter grade often holds the window better than a thin pad that runs out of plateau.
Are compression pads certified to UN 38.3 or UL 9540A?
No. UN 38.3 is a transport-safety standard for the lithium battery, and UL 9540A is a test method for thermal-runaway propagation in energy-storage systems; both apply at the pack and system level. A flame-rated pad material can support a pack's compliance program, but the demonstrated result belongs to the tested pack. At the material level the relevant citation is the UL 94 classification on the specific grade and gauge, confirmed on the manufacturer's data sheet.
Get a compression-pad material recommendation
Tell us the cell format, the beginning-of-life gap, the expected breathing and end-of-life swelling, and the target pressure window, and we will recommend a family and grade direction and quote die-cut pads on your footprint.
Samples typically ship in 3 to 5 business days for common die-cut configurations; standard production runs about two weeks; special orders are quoted to the job. MOQ varies by material and part.
Family-owned in Winsted, Connecticut since 1971 · ISO 9001:2015 certified · OEMs worldwide send us a drawing — and get back their part.
Related reading
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Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- ASTM D3574 (flexible cellular PU foam test methods, incl. Test C compression force deflection) and ISO 3386 (CFD / CLD).
- ASTM D395 (rubber compression set; Method B constant deflection) and ASTM D1056 (sponge and cellular rubber; note the different set-reporting basis).
- ASTM D6147 / ISO 3384 (force decay under constant deflection: stress relaxation).
- UL 94 (flammability classes for foams: HBF, HF-1, V-0); UN 38.3 and UL 9540A (pack- and system-level context).
- SAE Technical Paper 2024-01-2430 (compression-pad selection and optimization); Cannarella & Arnold (2014) on stack pressure and capacity fade; module-compression aging studies, Journal of Power Sources (2021, 2025).
- Manufacturer technical data sheets: PORON industrial 4701/4790 series and PORON EVExtend grades; BISCO HT/BF closed-cell silicone foam grades; V-0 silicone sponge grades.
This article summarizes selection principles for EV battery compression pads using published test methods and the material families named above. Levels and comparisons are qualitative; no force, temperature, set, or flame value is stated for any specific grade, and grade-level properties must be confirmed on the manufacturer's current technical data sheet at your gauge.
All parts are custom manufactured to order and minimum production run quantities vary by material and part. H-O is a die-cut converter, not a raw-material producer, and does not certify cells, modules, or packs.