O-ring extrusion and blow-out under high pressure: physics, back-up rings, and ISO 3601-2 groove design

O-ring extrusion is one of the most dangerous failure modes at high pressure. Unlike gradual wear or chemical degradation, extrusion is an acute process: the elastomer is forced into the gap between components under pressure and progressively torn away, until the ring is partially lost. The result is fluid leakage and potential loss of sealing. However, the physics of this phenomenon is understandable and can be eliminated through proper groove design and material selection.

The physics of extrusion: pressure, gap and hardness

Extrusion occurs when three factors combine: high pressure, a large gap between components, and a soft (low hardness) elastomer. An elastomer is a highly elastic material that fills available space under pressure. Ideally, the groove is designed so the gap is minimal and controlled. If the gap exceeds what the standard allows, high pressure forces elastomer into that gap and progressively tears it away — lower hardness increases susceptibility to this extrusion. ISO 3601-2 limits the maximum diametral clearance in the groove. For static applications, the recommended groove depth is approximately 0.78 times the cross-section diameter (0.78 × CS), yielding a target radial squeeze of around 22%. For dynamic applications, the groove is deeper (~0.90 × CS, ~10% squeeze) to reduce friction and wear. Static groove width is 1.3 × CS. These dimensions directly control the gap size and thus the extrusion risk.

Back-up rings — the PTFE solution

The classic solution to extrusion is a back-up ring, typically made from polytetrafluoroethylene (PTFE, trade name Teflon®). PTFE is a rigid polymer (not an elastomer), with Shore hardness 55–65 per standard. It cannot seal on its own, but it is resistant to high pressure and is not elastic — thus it excellently protects the elastomer from extrusion into the gap. The back-up ring sits between the primary O-ring and the groove wall (on the high-pressure side). As pressure increases, the elastomer sits firmly against the PTFE, which bears all the load. PTFE does not extrude — its rigidity eliminates the failure mechanism. Typical ISO 3601-2 high-pressure systems use PTFE back-up rings with O-rings of NBR, FKM, or PU depending on application. PTFE rings are usually manufactured in the same nominal size as the O-ring (slightly smaller in cross-section to not interfere with sealing) and must be precisely positioned. Their cost is higher than the O-ring alone, but at high pressures (>20 MPa) and wide gaps they are practically essential.

Material choice: higher hardness and extrusion-resistant elastomers

An alternative to PTFE is to change the elastomer itself. FKM (Viton®) has a hardness range of 60–90 Shore A. For high-pressure applications, FKM is chosen at the upper end of the range (80–90 Shore A), which is less elastic and more extrusion-resistant. Similarly, PU (polyurethane) spans 70–95 Shore A and is known for extrusion resistance — historically, PU was the first choice for hydraulic rod seals without a back-up ring. NBR (nitrile-butadiene rubber) with hardness range 40–90 Shore A is often chosen as the 70 Shore A standard at low pressures (< 10 MPa). At higher pressures, 80–85 Shore A NBR is preferred, although FKM or PU offer longer-term performance. ISO 3601-2 (annex table) recommends minimum hardness as a function of pressure — for pressures above 20 MPa, 80–90 Shore A is recommended regardless of material.

Groove design per ISO 3601-2: clearance and squeeze as protection

ISO 3601-2 is the standard devoted to O-ring groove geometry. Its tables define: • Radial squeeze percentage: 20–25% for static, 10–15% for dynamic. Squeeze is computed as (CS − groove depth) / CS × 100%. At CS = 3.0 mm and static depth 2.34 mm, 22% squeeze is achieved — precisely in the middle of ISO's recommended band. • Chamfer on groove edges: 15–20° angle to prevent ring damage during installation. • Corner radius: approximately 0.10 × CS to avoid sharp edges that might crack the elastomer under pressure. • Clearance control: The size of the gap depends on the O-ring diameter and cross-section (CS) and is calculated per ISO 3601-2 tables, not as a fixed mm value. At high pressures (>20 MPa), the gap must be minimized per the applicable size and pressure class. When these parameters are met, the gap is small enough to prevent elastomer extrusion. Problems arise when the groove is machined with excess clearance (manufacturing error) or when component wear enlarges the gap over time — so regular groove inspection and maintenance are critical in ageing systems.

Rapid gas decompression (RGD) — a related but distinct phenomenon

Extrusion is sometimes confused with rapid gas decompression (RGD or AED — Rapid Gas Decompression failure). These are distinct phenomena. RGD occurs in gas and oil-and-gas applications when gas is absorbed into the elastomer at high pressure and rapidly decompresses, causing gas bubbles and cracks to form inside the material. This is a chemico-physical phenomenon requiring AED-certified materials (FKM or FFKM in special formulations). Extrusion, by contrast, is purely mechanical — elastomer is forced into the gap under pressure without gas involvement. The fixes differ: extrusion is solved by PTFE back-up rings or higher hardness, while RGD is solved by materials with denser cross-linking and gas-absorption capability.

Practical steps to eliminate extrusion

If you are experiencing O-ring extrusion: 1. Inspect the groove: Measure the actual clearance (ideally under magnification). Per ISO 3601-2 tables for your ring size, verify it is within tolerance. If not, the groove must be reworked or a back-up ring must be added. 2. Increase hardness: If the ring is NBR 70 Shore A, upgrade to 80–85 Shore A or try FKM 80 Shore A. Higher hardness reduces elasticity and thus the tendency to extrude into the gap. 3. Add a PTFE back-up ring: This is the most effective short-term fix at high pressure (> 20 MPa). Place the PTFE ring on the high-pressure side of the groove. 4. Consider PU: Polyurethane 85–90 Shore A is traditionally extrusion-resistant. It suits hydraulic and pneumatic applications at medium to high pressure (5–25 MPa). 5. Verify assembly and lubrication: Insufficient lubricant can increase friction and hasten degradation. Use the recommended assembly lubricant. 6. Lower operating pressure if possible: If the design is too close to the pressure ceiling, reducing pressure is the simplest fix.