Which case gasket material has the lowest compression set?

Answer: Silicone, at 12% to 18% after 70 °C for 24 h and 20% to 25% at 100 °C. EPDM follows at 15% to 22%; TPE reaches 42% to 50%.

Why can an EPDM gasket not touch machine oil?

Answer: Its chains are non-polar, so mineral oil migrates in. After 70 h at 100 °C it swells more than 60% by volume and the leak test fails.

Can a TPE case gasket be used outdoors?

Answer: Above -35 °C and below 90 °C, yes, at 20% under silicone cost. In a -40 °C cold store, no: 500 h of ageing takes cracking to grade 2.

Does the groove change when the case gasket material changes?

Answer: Yes. Groove width is 1.15 times cord diameter, and 65 Shore A means 3 points less compression.

Protective Case Gasket: Silicone vs EPDM vs TPE

Quick answer: A case gasket is compared on one number, compression set at 70 °C for 24 h: silicone 20% or below, EPDM 25% and TPE 35% under GB/T 7759. At 100 °C the gap widens to 25%, 35% and 50%, so the material follows the hottest service temperature before hardness and compression ratio are set.

One Measure for Case Gasket Comparison
Compression Set at 70 °C and 100 °C
Hardness, Compression Ratio and Cold Rebound
How Each Material Fails
Cross-Section, Groove Width and Ozone Ageing
Replacement Criteria and Selection Table
FAQ

1. One Measure for Case Gasket Comparison

Only compression set puts every case gasket material on one ruler. A B-type specimen is compressed to 75% of its thickness, held 24 h at 70 °C, released for 30 min and measured, and whatever does not return is permanent. Silicone reads 12% to 18%, EPDM 15% to 22%, TPE 25% to 32%, provided all three run in one oven on one batch.

Tensile strength and hardness get quoted too, but their conditions differ and a change of rig changes the number. Compression set locks temperature, time and ratio.

The sealing face is a long groove on the parting line rather than a B-type specimen, so compression ratio bridges the two: 25% on the test piece, 25% to 35% in the groove.

2. Compression Set at 70 °C and 100 °C

Seventy degrees matches a cabin, a trunk or a sunlit case interior, and there the three sit closest. All of them clear the 20%, 25% and 35% lines, and only the margin differs.

Case gasket compression marks on an off-white case seen from above, gloss lid, two colour-matched latches, handle recess, pale grey backdrop

At 100 °C the ranking holds and the gaps open: silicone moves to 20% to 25%, EPDM to 28% to 35%, and TPE to 42% to 50%, where it no longer recovers and leaves a flattened band 30 min after release. Silicone and EPDM only split above that point, silicone holding under 30% at 150 °C while EPDM ages faster from 120 °C.

Material	70 °C × 24 h	100 °C × 24 h	Hardness, Shore A	Cold rebound limit	Relative price
Silicone VMQ	12%–18%	20%–25%	50–60	-55 °C	100
EPDM	15%–22%	28%–35%	55–65	-45 °C	70
TPE (SEBS)	25%–32%	42%–50%	50–60	-35 °C	80

The rebound limit is the temperature at which sealing fails, not cracking, and it decides more applications than the set figures.

3. Hardness, Compression Ratio and Cold Rebound

Hardness sets the force needed to close the lid. A 50 Shore A silicone cord needs about 0.35 MPa of face pressure at 25% compression; a 65 Shore A EPDM cord needs 0.55 MPa, so latch clamp force rises 8% or the lid edge lifts 0.2 mm and the leak test fails.

Compression ratio is a design variable, not a material property. A groove cut to 30% of cord diameter keeps the case gasket at three-tenths compression for years, running 3 to 5 points above the specimen figure; pull it back to 25% and body tolerance has to hold ±0.2 mm.

Cold rebound decides the application. Silicone recovers at -55 °C, EPDM at -45 °C, and TPE stiffens below -35 °C, so in a -40 °C cold store a TPE case gasket stays flat after the first closure.

4. How Each Material Fails

Silicone fails by bleeding, not ageing. Low-molecular siloxane volatilises above 150 °C and condenses as a film on lenses and displays inside the case; a 200 °C post-cure of 4 h pulls volatiles from 1.5% to 0.3% and adds about 0.6 CNY per metre. Tear strength near 8 kN/m is the other weakness, so a groove edge nicks the cord and a 10 mm nick leaks.

Case gasket lip on a blue case viewed from above, smooth lid, colour-matched latches at the front, two hinge ears behind, white surface

EPDM wins on weather and ozone and loses on oil. After 70 h in mineral oil at 100 °C it swells more than 60% by volume, bulges out of the groove and stops sealing, and one wipe with an oily glove halves its life. Bonding to a PP body is poor, so overmoulding needs a primer.

TPE fails on heat. SEBS-based TPE runs to 90 °C continuously and softens after 48 h at 100 °C, where set reaches 50% and never recovers. It costs 20% less and bonds to PP directly, removing adhesive backing and hand laying, but only below 80 °C.

5. Cross-Section, Groove Width and Ozone Ageing

The cross-section sets how much tolerance the seal absorbs. An O-profile needs 25% to 30% compression before it seals, so a 0.2 mm groove error leaks. A D-profile seats a flat face on one side and seals at 20%, doubling the tolerance, while a double-lip profile seals in two stages at 15% more closing force.

Groove width is 1.15 times the cord diameter: a 6 mm cord takes a 6.9 mm groove, and the 0.9 mm of clearance lets rubber displace sideways instead of bulging. Squeeze it to 1.05 times and the cord shears at a corner within three months.

Profile	Sealing compression	Groove width	Application
O-profile	25%–30%	1.1 × cord	Fixed lids, rarely opened
D-profile	20%–25%	1.15 × cord	Standard protective cases
Double lip	20%–25%	1.2 × cord	Frequent opening, dust ingress

After 500 h to GB/T 16422.3, silicone and EPDM show grade 0 surface cracking while TPE reaches grade 2, and all three gain 3 to 5 Shore A, which reads as a firmer seal that seals worse.

6. Replacement Criteria and Selection Table

Three criteria end the life of a case gasket, and any one is enough: compression set above 40%, hardness more than 10 Shore A above the original, or measured groove compression under 15%. Calipers and a durometer settle all three on site.

Application	Material	Hardness	Criterion
Indoor storage	TPE	50–55 Shore A	70 °C set ≤35%
Outdoor at -40 °C	Silicone	55–60 Shore A	Rebound to -55 °C
Frequent opening	Double-lip EPDM	55 Shore A	Set ≤30% after 20,000 cycles
Food-contact requirement	Platinum-cured silicone	50 Shore A	Passes food-contact testing

Selection runs temperature first, compression ratio second and cost last. Below -35 °C, TPE is out; above 90 °C, EPDM gives way to silicone.

Conclusion

The order for a case gasket is temperature, compression ratio, then cost, and compression set is the ruler that lines the three up. At 70 °C for 24 h silicone holds 20% or better, EPDM 25%, TPE 35%; at 100 °C the 25%, 35% and 50% spread splits the uses, silicone for hot cabins and -40 °C sites, EPDM for standard cases, TPE only below 80 °C.

This article discusses only the container (case body) side of specifications and design intent, and does not cover any handling, loading or use of the contents.