A silicone gasket only seals while it pushes back against its mating faces. Compression set measures how much of that push a silicone loses after it has been held squeezed. Get it wrong and a seal that passed on day one leaks a year later.
Defining Compression Set
Compression set is the permanent deformation an elastomer keeps after being held at a fixed compression, at a fixed temperature, for a fixed time, then released. It is reported as a percentage: the fraction of the original deflection that did not recover. A low number means the material springs back and keeps applying sealing force. A high number means it has taken a permanent flat spot and no longer fills the gap.
The common test methods are ASTM D395 and ISO 815. Method B holds the sample at a set deflection, typically 25 percent, for 22 or 70 hours at an elevated temperature, then measures recovery 30 minutes after release. Because temperature and time both drive the result, a compression set figure is meaningless without its test conditions attached.
Why It Matters Across Applications
Static and dynamic seals are the obvious case. Gaskets, O-rings, and face seals in pumps, enclosures, and pipe joints need sustained contact pressure to hold back fluid, gas, or dust. Vibration mounts and bumpers rely on the rubber deforming and rebounding repeatedly; a high set leads to bottoming out and lost isolation.
Electrical connector seals depend on maintaining a squeeze around pins and cable jackets to keep moisture out. In consumer keypads and buttons, the tactile return and the long-term feel both come from low set. Across rail, automotive, marine, and industrial oven hardware, the same principle holds: the seal is only as durable as the material’s ability to keep rebounding.
What Drives the Result
Formulation is the largest lever. Base polymer type, filler system, cure chemistry, and additive package all shape crosslink density and network stability. Peroxide-cured and platinum-addition-cured silicones behave differently, and a proper post-cure bake drives off cure byproducts and completes crosslinking, often cutting set significantly.
Service temperature accelerates set because chain mobility rises and the network relaxes more readily. Longer time under load increases it. Higher compression ratios beyond the design window induce more permanent deformation. Chemical, oil, ozone, and UV exposure age the polymer and raise set over time. Part geometry and section thickness also influence how stress distributes and how the part recovers.
Designing Around It
Specify a maximum acceptable set percentage with its test temperature and duration, chosen to reflect your real service profile rather than a generic 22-hour room-temperature figure. Size the seal so the working squeeze lands in the effective range, commonly 15 to 30 percent for a gasket, enough to seal without overstressing the rubber. Validate with samples aged under conditions that mirror the application, not just the standard test. For help interpreting set data against a service profile, Email Us with your temperature range and expected life.
Related material covers how thermal expansion mismatch causes bonded joints to fail, comparing adhesive strength for heavy-duty repairs, and how cure speed differs between UV and epoxy systems.
Compression Set Versus Stress Relaxation
Compression set and stress relaxation describe the same underlying aging from two directions, and confusing them leads to the wrong test. Compression set holds the deflection fixed and measures how much thickness is permanently lost. Stress relaxation holds the deflection fixed and measures how much sealing force decays over time. A seal can show a modest compression set yet still lose enough contact force to leak, because the two do not track one-for-one. For a gasket whose job is to maintain a minimum closing force, a stress-relaxation or compression-force-deflection measurement over the service temperature and duration tells you more than a set number alone. Ask the supplier which data exists for a candidate grade before committing.
Post-Cure Is Not Optional
For peroxide-cured silicones in particular, the as-molded part carries cure byproducts and an incomplete network. A post-cure bake, often several hours at 200 degrees Celsius or higher per the material guidance, drives off those volatiles and completes crosslinking. Skipping it can leave compression set two to three times higher than the data sheet value, along with higher outgassing and a stronger odor. Build the post-cure into the process routing and the incoming inspection, and treat a missing post-cure as a nonconformance rather than a shortcut.
How Incure Supports Silicone Component Design
Incure develops silicone formulations with controlled compression set, adjusting base polymer, filler loading, and cure system to hit a target balance of recovery, temperature resistance, and chemical compatibility. The technical team advises on grade selection, on cure and post-cure schedules that minimize set for a given chemistry, and on how geometry and squeeze ratio affect long-term behavior. Where a stock grade cannot meet a demanding seal requirement, custom work can target a lower set at the relevant service temperature.
The Practical Takeaway
Compression set is the property that predicts whether a silicone seal still works after years of pressure and heat. Selecting a formulation rated for low set at your service temperature, curing it properly, and sizing the joint to a sensible squeeze are the three decisions that keep the seal alive.
Contact Our Team to discuss your silicone sealing requirements and the grade that fits your service conditions.
Visit www.incurelab.com for more information.