A silicone gasket that seals perfectly on day one can leak two years later without any visible damage. The usual culprit is compression set: the permanent deformation a rubber retains after a sustained compressive load is removed. When a gasket takes a set, it stops pushing back against its sealing surfaces, and the joint opens a path for fluid, gas, or contamination. What Compression Set Measures Compression set is expressed as the percentage of original deflection that a specimen fails to recover after a defined time, temperature, and compression, per test methods such as ASTM D395. A result of 0 percent means full recovery; 100 percent means the material stays exactly as compressed. Well-formulated silicones typically fall in the low tens of percent after 22 hours at elevated temperature, and lower is better for long-service seals. The number matters because sealing force is what keeps a joint tight. As set accumulates, the gasket's residual contact stress decays. Once that stress drops below the pressure the joint must contain, the seal fails, often intermittently as temperature cycles change the geometry. What Drives Compression Set in Silicone Crosslink density and network structure. A more completely cured, tightly crosslinked network recovers better. Incomplete cure, whether from a short bake, low catalyst level, or cure inhibition, leaves dangling chain ends that flow under load and never spring back. Cure system. Peroxide-cured silicones generally show higher set than addition (platinum) cured systems, partly because of residual cure byproducts. A post-cure bake drives off volatiles and completes the network, and it is one of the most effective single steps for lowering set. Filler type and loading. Reinforcing fumed silica improves strength but interacts with the polymer network; excessive or poorly dispersed filler can raise set. Extending fillers added only to reduce cost usually make set worse. Temperature. Set rises sharply with service temperature. A gasket rated comfortably at 100°C may take an unacceptable set at 200°C. Always qualify at the real maximum continuous temperature, not an average. Time and strain level. Set accumulates with dwell under load, and running a gasket at high compression accelerates it. Designing for moderate compression extends sealing life. Design and Process Strategies Specify the set limit. Put a maximum compression set value, with the test temperature and duration, directly in the material specification. A generic "silicone rubber" callout does not control this property. Design compression into the mid range. Target roughly 15–30 percent compression on the cured gasket cross-section. Enough to develop sealing force, not so much that the material is overstrained. Use a groove that limits over-compression. A defined groove depth and land width caps deflection during assembly and stops installers from crushing the seal. Complete the cure. For molded parts, hold the specified post-cure. For dispensed form-in-place silicones, confirm the cure mechanism has finished throughout the section before the part enters service. Control temperature exposure. If the seal sees a hot spot, either move the material selection up a temperature class or shield the gasket. Related Failure Modes Compression…