A seal fails for one of three reasons: the wrong material, the wrong squeeze, or a groove that never gave it a chance. Material is the easy part — most sealing problems we are asked to solve are geometry problems already frozen into someone's housing design.
Quick answer
For static sealing in food, medical, appliance and outdoor applications, use silicone at 50–70 Shore A, designed for 15–30% compression, in a groove filled to 75–85%. Keep silicone away from fuels, oils and abrasive dynamic contact.
1. Is silicone the right elastomer?
| Property | Silicone (VMQ) | EPDM | Nitrile (NBR) |
|---|---|---|---|
| Temperature range | -60 to +230 °C | -45 to +150 °C | -30 to +120 °C |
| Food & medical grades | Excellent | Limited | Limited |
| UV / ozone / weather | Excellent | Excellent | Poor |
| Oil & fuel resistance | Poor | Poor | Excellent |
| Abrasion / tear | Low | Moderate | Good |
2. Compression: the number that decides sealing
Target 15–30% compression of the seal cross-section for static applications. Below about 10% the seal cannot accommodate surface irregularities and tolerance stack-up; above 35% you generate high stress, permanent set and possible groove extrusion. Model the worst-case stack-up of both mating parts, not the nominal dimensions — that is where real assemblies leak.
3. Groove design
- Fill ratio 75–85%. The seal must have somewhere to go when compressed.
- Radii, not sharp corners, at the groove base to avoid stress concentration.
- Surface finish of Ra 0.8–1.6 µm on sealing faces; rougher faces need softer compounds.
- Avoid knife edges and steps across the seal path — every discontinuity is a leak candidate.
- For flat gaskets, keep the width-to-thickness ratio above 3:1 so the gasket does not roll under load.
4. Compression set and long life
Compression set (ASTM D395 Method B) measures how much of the original thickness is permanently lost after prolonged compression at temperature. A properly post-cured silicone typically achieves 15–30% at 175 °C / 22 hours. If your seal must hold for years, put a maximum compression set value in the specification and require it on the material report — otherwise you are buying an untested property.
5. Tolerances and cost
Rubber is not machined metal. Molded tolerances follow ISO 3302-1: class M1 for precision LSR parts, M2 for good general molding, M3 for larger or thicker parts. Demanding tighter than necessary drives up tooling class, scrap and inspection cost without improving sealing performance — the seal's compression range absorbs far more variation than tight tolerances buy you.
6. A checklist before you release the drawing
- Sealing medium, temperature range and pressure stated.
- Hardness with ±5 Shore A tolerance.
- Compression percentage calculated on worst-case stack-up.
- Groove dimensions with fill ratio verified.
- Compression set limit and applicable standard.
- Colour, food/medical compliance where relevant, and marking requirements.
- AQL level and critical dimensions flagged for inspection.
Related reading: Shore A hardness guide and compression vs injection molding.
Working with Tongsheng
Dongguan Tongsheng Electronic Technology Co., Ltd. has manufactured silicone rubber molds and silicone products since 2014. We run in-house CNC tooling, compression and liquid silicone injection molding, and a testing lab, from a 2,000+ sqm ISO 9001:2015 certified factory in Humen Town, Dongguan, with 100+ staff and 15 engineers. Food-grade (FDA / LFGB), medical-grade and industrial-grade compounds, Shore A 10–80, Pantone and RAL color matching.
Send 2D/3D drawings (STP, IGS, PDF) and we quote within 24 hours — request a quote, email coraycn@gmail.com, or message us on WhatsApp.