Full definition
The percentage of permanent deformation remaining in an elastomer after it has been compressed under controlled conditions and then released — the single most critical property for sealing applications. Measured per ASTM D395: Method B (constant deflection, most common — specimen compressed 25% for 22-70 hours at test temperature, then released and measured after 30-minute recovery). Formula: CS = (t₀ - tᵢ) / (t₀ - tₛ) × 100%, where t₀ = original thickness, tᵢ = recovered thickness, tₛ = spacer thickness. Lower values = better recovery = better seal performance. Typical values at 100°C/22h: NR 15-30%, EPDM 15-35%, NBR 20-40%, FKM 15-30%, silicone 20-40%. A seal with high compression set loses its sealing force over time and eventually leaks. Post-cure dramatically reduces compression set in FKM and silicone (by 30-50%). Temperature and time are the main enemies — compression set always increases with higher temperature and longer duration. Per ASTM D395 and ISO 815. This is the first property to specify when selecting O-ring and gasket materials.