Full definition
Standard Test Methods for Rubber Property — Compression Set — the critical test for evaluating how well an elastomer recovers its original thickness after being held in compression at elevated temperature, directly predicting sealing performance. Two methods: Method A (constant force — a fixed load compresses the specimen) and Method B (constant deflection — specimen compressed 25% of original thickness, most commonly specified). Procedure: compress specimens between parallel steel plates with spacers, place in oven at test temperature (typically 70, 100, 125, or 150°C per ASTM D2000 line call-out) for 22, 70, or 168 hours, remove from oven, cool in fixture, release, and measure recovered thickness after 30 minutes at room temperature. Calculation: CS(%) = (original - recovered)/(original - spacer) × 100%. Lower compression set = better recovery = better seal performance. Typical values at 100°C/22h: EPDM 15-30%, NBR 20-40%, FKM 15-25%, silicone 25-40%, NR 10-25%. Compression set is the #1 property determining long-term sealing reliability. Per ASTM D395-18. Equivalent: ISO 815. For FKM and silicone seals, post-curing dramatically reduces compression set (by 30-50%).