Should a case gasket material be chosen by temperature or by media first?

Answer: By temperature, which picks the family; media only picks a compound within it. Above 120 °C only silicone qualifies, so oil resistance stops mattering.

Why does a PVC gasket harden after four years outdoors?

Answer: Plasticiser migration. Four years of UV and heat removes 8% to 12% of it, lifting hardness past 80 and set from 46% to 58%.

Can EPDM replace silicone?

Answer: Between -45 and 120 °C yes, at 35% of the annual cost. Above that, 70 h at 150 °C lifts set from 22% to 41%.

What compression ratio should a case gasket material be designed to?

Answer: Silicone 25%, EPDM 30%, PVC 15%, within ±5 points, or a 0.2 mm flatness error on the lip goes unbridged.

Can one groove accept all three sections?

Answer: No. For the same 8.0 mm free height the depths are 6.0, 5.6 and 6.8 mm. Swapping without recutting shifts the ratio over 10 points.

How should the purchase specification be written?

Answer: Cite an ASTM D2000 callout plus an ISO 3302 class. B37, with set at or below 25%, locks the compound; E2 locks the ±0.25 mm.

Case Gasket Material: Silicone, EPDM, PVC

Quick answer: Case gasket material follows the top service temperature first: silicone holds -55 to 200 °C at 25% compression, EPDM -45 to 120 °C at 30%, PVC only below 60 °C in dry, oil-free duty.

Reading ISO 3302 and ASTM D2000
Compression set: where the three split
Temperature range: cold end and hot end
Media resistance: oil, alkali, alcohol
Cost per metre versus cost per year
Groove compression: three materials, three depths
FAQ

1. Reading ISO 3302 and ASTM D2000

Two standards cover one drawing. ISO 3302-1 fixes dimensions: extruded sections fall into classes E1, E2 and E3, so a 6 mm strip varies ±0.25 mm in E2 and ±0.40 mm in E3. ASTM D2000 fixes the compound, locking heat, oil, compression set and low-temperature behaviour into one callout.

That callout reads like M2AA810 A19 B37 C12 F17. M2 is the -40 to 100 °C heat class, A19 caps hardness change at ±15 points after 70 h ageing, B37 limits set to 25% after 22 h at 100 °C, C12 limits swell in IRM 903 oil to 25%, and F17 prevents cracking at -40 °C.

Standards cover the compound, not the fit: a strip that passes B37 still fails in a groove built for 12% compression, so the drawing needs two columns.

2. Compression set: where the three split

ASTM D395 method B compresses a specimen 25%, holds it 22 h at 100 °C and measures it 30 min after release. One batch returned 17% for silicone, 22% for EPDM, 46% for PVC; at 70 °C for 24 h they read 12%, 16% and 38%.

Case gasket material pairing of two red cases, one closed, one open with black egg-crate foam and an orange instrument in a cut cavity

The PVC figure needs unpacking. About 30 points come from plasticiser migration, not chain scission: at six months the strip still springs back, at eighteen the contact stress has fallen to 55%.

Silicone and EPDM sit 5 points apart, worth roughly 0.15 to 0.2 MPa of contact stress, and whether that suffices is settled by the groove.

Property	Silicone	EPDM	PVC
Set, 100 °C for 22 h	17%	22%	46%
Set, 70 °C for 24 h	12%	16%	38%
Shore A hardness	50	65	72
Density, g/cm³	1.15	1.10	1.35
Tensile strength, MPa	8.0	9.5	12.0

3. Temperature range: cold end and hot end

Temperature is judged at two ends, never at one number. ASTM D1329 TR10 puts silicone at -55 °C, EPDM at -45 °C and PVC at -18 °C, measuring where a stretched specimen retracts to 10% elongation.

Continuous service temperature grades the hot end: silicone 200 °C, or 250 °C for 2 h; EPDM 120 °C, or 150 °C briefly; PVC 60 °C, with plasticiser loss doubling above 70 °C. After 50 cycles from -40 to 85 °C, set grows 3 points for silicone, 5 for EPDM and 14 for PVC.

A range is an interval plus a margin. Deducting 20 °C from the peak before selecting is steadier than selecting at the peak.

4. Media resistance: oil, alkali, alcohol

After 70 h in IRM 903 oil at 100 °C, silicone swells 32%, EPDM swells 88% and PVC shrinks 10%. All three are unusable: silicone grows, EPDM grows until it will not seat, and PVC loses plasticiser and hardens.

Case gasket material dimension view of red cases marked 345 by 290 by 135 mm, with a 315 by 235 by 85 mm cavity

Alkali separates them far less: seven days in 10% sodium hydroxide at 23 °C moves each material under 3%, and 30 days in 5% brine stays under 2%. Alcohol shows the spread, as 24 h in isopropanol swells PVC 18%, EPDM 10% and silicone 5%. Cleaning is the item most often omitted: 200 isopropanol wipes leave PVC 8 points harder.

Media and condition	Silicone	EPDM	PVC
IRM 903 oil, 100 °C, 70 h	+32%	+88%	-10%
10% NaOH, 23 °C, 7 d	+2%	+3%	+1%
5% brine, 30 d	+1%	+2%	+1%
Isopropanol, 23 °C, 24 h	+5%	+10%	+18%

5. Cost per metre versus cost per year

For an 8 by 6 mm solid strip, silicone costs 8.0 per metre, EPDM 3.4 and PVC 2.0. On a 2.6 m perimeter that is 12 more per case than EPDM, only 4% to 7% of case cost, so price alone rarely rejects silicone outright.

Cost per year reverses it: with outdoor lives of 10, 12 and 4 years, one metre runs 0.80, 0.28 and 0.50 per year. EPDM wins on price and life across -45 to 120 °C, while above that the 8.0 against 3.4 comparison stops existing.

Item	Silicone	EPDM	PVC
Cost per metre, 8×6 mm	8.0	3.4	2.0
Cost per kilogram	32	15	11
Outdoor design life	10 years	12 years	4 years
Cost per metre per year	0.80	0.28	0.50

6. Groove compression: three materials, three depths

Compression ratio has one formula: free height minus groove depth, divided by free height. Silicone is designed at 20% to 30%, EPDM at 25% to 35% and PVC at 12% to 18%, so an 8.0 mm strip pairs with grooves of 6.0, 5.6 and 6.8 mm at mid-values.

Two numbers must be fixed beside the depth. Fill ratio, strip section over groove section, stays at or below 0.80 so the remaining 20% absorbs expansion and tolerance stack-up. Groove width runs 0.3 to 0.5 mm wider than the strip, letting material displace sideways instead of being sheared. At ISO 3302-1 class E2 the ±0.25 mm tolerance moves the ratio ±3.1 points, which the 25% mid-value absorbs. EPDM is 15 points harder and needs 5 points more compression to follow an uneven lip. PVC at Shore A 72 keeps the lid from closing when over-compressed, so it takes 15% with a wider section.

One counter-example: an 8.0 mm silicone strip in the 5.6 mm EPDM groove runs at 30%, seals hard for a quarter and relaxes faster, ending 6 points above design after 12 months.

Conclusion

Selecting a case gasket material ends with three temperature bands on one chart: silicone -55 to 200 °C, EPDM -45 to 120 °C, PVC -20 to 60 °C. Whichever band encloses the highest and lowest working temperature settles most of the decision; compression ratio, at 25%, 30% and 15%, settles the rest. Media only refines the compound inside a family, since oil rules out the 88% swell of EPDM. Below 60 °C, EPDM at 0.28 per metre per year needs no margin bought at 8.0; above 120 °C only silicone survives, and the work shifts from comparing prices to tightening groove depth and tolerance together.

This article discusses container-side specifications and design implications only, and does not cover any handling, loading or use of the contents.