Custom EV Battery Module & Frame Seals: Module Perimeter, Coolant-System & Pack-Venting Gaskets

A module or frame seal is the environmental barrier at a joint in the pack, a module-to-frame interface, an enclosure-frame face, or a service joint, that keeps moisture, dust and road fluids out. A compression pad is a different part with a different job: it sits between cells (or cell-to-endplate) and exerts a controlled push-back force against the cells as they breathe and swell.

The seal is not load-bearing; its priority is to fill the joint and stay closed, so it is judged on ingress class, compression set, stress relaxation and seal geometry. The pad is load-bearing; it is judged on its force-deflection curve over the end-of-life deflection. The two are easy to blur and are specified differently, so naming which one you need first prevents the classic mistake of putting a seal where a pad belongs.

The compression pad has its own page in the EV & Battery hub.

Yes, it is the first decision and it drives the rest. A static seal is factory-sealed and opened only for service, which is most module-perimeter and frame seals; conformability and environmental resistance lead, and compression set matters over the long static life but the joint is undisturbed. A dynamic seal is on a service cover or access panel opened repeatedly, so it has to reseal across many open-and-close cycles; that makes low stress relaxation, fatigue resistance and a thicker section to absorb corner tolerance the priorities.

The same material in the same thickness can be right for a static joint and wrong for a dynamic one. Tell us whether the joint is opened in service and how often, and the family, the thickness and the closure-force target can be set accordingly.

IP67 is a common enclosure target (dust-tight plus temporary immersion), and many programs specify IP68 (deeper or longer immersion, defined by the OEM). For underbody exposure to high-pressure washing, the automotive-correct callout is IP6K9K, which combines dust-tight protection (IP6K) with a high-pressure, high-temperature water jet (IP9K).

The reason there is a “K” is that road vehicles use ISO 20653, which adds those high-pressure, high-temperature variants on top of the base IEC 60529 IP code; industrial marketing often writes IP69K, but the automotive-correct form is IP6K9K.

The most important point is that an IP rating is an assembly-level result, not a material property: it depends on the seal, the groove, the closure force, the housing and any vents together. A seal supports the rating; tell us the target class and the groove geometry and the family and compression plan can be set to support it.

IEC 60529 is the base, industrial IP code: a two-digit rating where the first digit is protection against solids and dust (0 to 6, with 6 dust-tight) and the second is protection against water (0 to 9). ISO 20653 is the road-vehicle standard that builds on the same idea but adds the high-pressure, high-temperature “K” variants used in automotive, so a road-vehicle pack is specified as, for example, IP6K (dust-tight, vehicle variant) combined with IP9K (an approximately 80 degree-Celsius, high-pressure steam-jet test), written IP6K9K.

In short, cite IEC 60529 for the IP6X / IPX7 / IPX8 fundamentals and ISO 20653 / IP6K9K for the automotive-correct callout on an underbody pack. Both describe the assembled enclosure, not the raw seal; the seal is one of the things that lets the enclosure reach the class.

Three properties line up. First, silicone cellular grades have a very low compression set, which means the seal stays recoverable and keeps its sealing force over a long life instead of thinning and going slack. Second, silicone holds its sealing properties across a wide temperature span, hotter and colder than EPDM or neoprene, which a pack with continuous temperature exposure and thermal cycling needs.

Third, the closed-cell BISCO HT/BF silicone line is inherently UL 94 V-0, and kSil V-0 is a dedicated V-0 silicone sponge, so the flame requirement common to battery packs is met without a separate flame foam. EPDM and neoprene are cost-down or legacy secondaries with materially higher compression set and narrower temperature ranges. Silicone's one weakness is fuel and oil resistance, where neoprene or fluorosilicone is better.

Confirm the grade-level set per the ASTM D1056 procedure and the V-0 listing on the data sheet.

Use silicone as the primary seal in most cases: closed-cell BISCO HT/BF for a low-set, wide-temperature, V-0 perimeter seal, or kSil V-0 sponge where the closure force must stay low and a V-0 rating is required, with the broad hardness range letting the seal force be tuned. Reach for EPDM (premium closed-cell) when the program needs to cost down and the temperature and flame demands stay inside EPDM's range; accept that its compression set is materially higher than silicone and that it swells in petroleum oil.

Reach for neoprene (SCE-series by density) in legacy platforms and oil-adjacent zones where broad moderate oil resistance helps; it too has higher set and a narrower temperature range and is being displaced by silicone in modern designs. Where fuel or oil contacts the seal directly, fluorosilicone is the move. Send the IP target, the temperature, the chemistry and the cost target and the family follows, confirmed on the TDS.

Compression set is the permanent thickness a material loses after being held compressed and then released, expressed as a percentage; lower is better. It matters because a seal works by pushing back against the two sealing surfaces, and as it sets it thins, the contact pressure drops, and a leak path can open, especially after thermal cycling or long static compression.

This is the reason silicone is the primary seal material: its cellular grades hold a very low set across a wide temperature span, while EPDM and neoprene set materially more. For the sponge and cellular materials used here, compression set is reported per the ASTM D1056 procedure (the dense-rubber method, ASTM D395, uses different conditions and the values are not directly comparable), so always check the method and conditions on the data sheet.

Call out the service life, the temperature and the duration of compression so the set behavior can be checked against the grade TDS.

Stress relaxation is the loss of sealing force a seal held at a constant compression suffers over time. It is the companion problem to compression set: a seal can pass on the day it is assembled and still leak years later if the force it holds at constant strain decays. Both are accelerated by heat and by vibration, which a battery pack has in abundance, so a seal that is fine on the bench can relax in service.

The defense is to choose a low-stress-relaxation, durable material, silicone and quality polyurethane are specified for this, and to design the joint so the seal stays in its working compression with a controlled closure force and good geometry. It is why a seal cannot be judged only by its day-one fit; the question is whether it keeps its force over the pack's life.

Send the thermal-cycle and vibration environment and the service life so the material and the joint can be set to hold the force.

A few geometry rules keep a perimeter seal sealing. Give the seal a sealing-surface width at least equal to its thickness, and ideally about one and a half times the thickness, so it has a stable land to compress against rather than rolling or extruding. Use radiused corners rather than sharp ones, because sharp corners are stress risers that can tear and are hard to compress evenly.

Keep fasteners outside the seal path so a bolt does not pierce or distort the sealing line, and use an alternating bolt pattern so the seal compresses evenly around the perimeter. Prefer a continuous one-piece gasket where the footprint allows; for a large frame, a jointed “puzzle” or collapsible-joint gasket can be used. Add a pressure-equalizing vent so pressure swings do not deform the seal, and use PSA and kiss-cut presentation so the part places without stretching.

Send the joint footprint, the gap and the closure force and the seal can be sized to these rules.

Because a well-sealed pack has a hidden problem: as it heats and cools, the air inside expands and contracts, which builds a pressure difference across the seals. That pressure swing stresses every perimeter seal, can deform the housing, and over time can pull moisture past a seal.

A pressure-equalization and relief vent, typically an expanded-PTFE membrane such as Gore ePTFE, lets air pass through the wall to balance the pressure while the membrane still blocks liquid water and dust, so the seals are not asked to carry a pressure load they were not designed for.

It is the complement to the perimeter seal: the seal keeps the environment out, and the vent keeps the pack from becoming a pressure vessel. Insufficient venting is one of the recognized seal-failure causes, so the two are designed together. H-O the membrane into vent disks and patches; confirm the membrane grade and airflow against the manufacturer's data.

No. An IP rating, including IP6K9K, is a property of a complete, tested enclosure, not of a raw seal material; the rating is earned when the assembled pack passes the corresponding ingress test, and the seal is one of several things, alongside the groove, the closure force, the housing and any vents, that lets it get there.

UN 38.3 is a transport-safety standard for the lithium battery as a whole; its thermal-cycle test exercises the pack across a temperature range and assesses integrity under thermal stress, and a perimeter seal supports that pack-level robustness, but a seal is never “certified to UN 38.3.” The honest framing is that H-O converts seals that support an enclosure's IP claim and a pack's compliance, using low-set, wide-temperature, flame-rated materials with the right geometry, while the ratings themselves belong to the tested assembly.

Materials are evaluated against the cited test methods through the manufacturer's data sheet; H-O does not independently certify them unless explicitly stated on the quote.

H-O is a precision converter, not a raw-material producer. We do not extrude or mold the silicone, rubber, foam and membrane; we buy sheet, slab and roll stock from the material manufacturers and convert it to your drawing, by die-cutting, kiss-cutting, laser and waterjet cutting, adhesive lamination, slitting and kitting, with material traceability and lot-level data-sheet records.

Every seal is made-to-order; we do not carry finished parts in stock and we do not advertise a no-minimum policy, though prototype quantities through full production runs are equally welcome and the minimum varies by material and part.

Prototype and production timing is summarized in the process strip near the top of the page and on the quote form. Send your drawing or describe the joint, the target IP, the temperature and the chemistry through the form below for a specific quote.