Rubber O-Rings

“Rubber O-ring” is a blanket term that means any elastomer-molded O-ring, including the epitome of all rubber O-rings, nitrile (NBR). EPDM, FKM, neoprene, and others can all be referred to as “rubber”. Silicone O-rings (sometimes abbreviated as VMQ) are one specific elastomer family within that group, differentiated by a few factors: their ability to operate in an even broader temperature range (-85 to +400 F), amazing flexibility at low temperatures (-79 F), being available with FDA and USP Class VI options, and the fact that they can generally only resist liquids (nothing in the way of oils or fuels). So if you want a rubber O-ring to seal fuel or oil, NBR or FKM are your best choices. If you want a food contact or medical grade solution, you’ll want a silicone.

O-rings are specified by 3 measurements: inside diameter (ID), outside diameter (OD), and cross sectional thickness (CS). To measure the ID, use a caliper and measure from the inside edge to the opposing inside edge. To measure the OD, use a caliper and measure from the outside edge to the opposing outside edge. To measure the CS, use a micrometer and measure the thickness from the inside edge to the outside edge. In the US, rubber O-rings number as per AS568F standards (also known as 369 dash numbers, close dimensional approximation on inches). Internally, ISO 3601 (metric with Class A and A/A dash equivalency) standards are used internationally. Japan adheres to JIS B 2401. Our quotation will show which standard your part falls under and the dimensional class.

Whatever your choice, it is primarily driven by three factors, in order of importance: (1) what you are sealing, fuel, oil, water, acid, solvent, food contact, biocompatibility (2) the general temperature range that you require (-40 to +130 F, +35 to 220 F, +9 to +400 F, +15 to +475 F, +50 to +600 F), and if (3) whether it is a static or dynamic seal. If it’s for fuel or oil, go with NBR (Buna-N). For internal brakes or extreme heat, try EPDM. For high temperature fluid or chemical resistance, see FKM (Viton). For aggressive chemistries or semiconductor usage, see FFKM. For food contact or biocompatibility, consider Silicone (domed, color coded for easy identification). The above elatomar matrix spells out exactly what each EPDM, FKM, and FFKM is intended for.

Viton (Chemours-trademarked FKM, ASTM type HK) is a fluorocarbon elastomer rated for chemical and petroleum resistance to approximately 437 F continuous use. Silicone (VMQ, ASTM type FE/FC) is a polysiloxane elastomer rated for dry heat to 400 F, with excellent low temperature flexibility and limited oil/fuel resistance. Viton is resistant to chemical media; silicone is resistant to temperature swings. They are not interchangeable—they can’t be substituted for each other. Designing the wrong family is one of the most common O-ring failure causes we see in incoming prints.

Replacement interval is very service-dependent. Static seals in contamination-free dry service can last through years of operation; dynamic seals under high pressure or temperature cycling or chemical-contact should be inspected at scheduled maintenance intervals—anywhere from 6 to 24 months—and changed at first evidence of compression set or surface cracking or extrusion in our experience. (The same goes for mission-critical medical or aerospace or food contact applications—annual replacement or condition-monitoring as dictated by application environment is, we believe, best practice).

Yes. We vulcanize custom O-rings over a broad cross-section of 0.040 inch (1 mm) to 1.500 inch (38 mm), and inside diameters to 60 inches (via splice-and-vulcanize for the largest IDs). Additional cross-sectional volume for the non-standard diameters is amortized between the approximately 500-piece minimum at a charge of $300-$2,500 for each article cavity. Forward us a drawing via CAD, STEP, PDF, or even a drawing by hand, and we’ll give you a quotation including compound recommendation and tooling fee factors within 48 hours.

Typically 500 pieces is the minimum for a custom O-ring to be compression molded by us. Our standard (AS568 size, not requiring custom tooling) pieces are no-charge unless the part needs to be held to a TIR within 0.010 inch (0.25 mm), in which case we quote to hold to a TIR of 0.005 inch (0.13 mm) based on your drawing. For injection-molded production, the minimum order size is customer-specific, based on the amortization of a tooling investment—generally 5,000+ pieces to amortize at high volume.

Yes. Our materials are also food contact approved (per 21 CFR 177.2600) in nitrile, EPDM, silicone, and FKM. For medical applications, we supply USP Class VI biocompatible platinum-cured silicone and FKM compounds processed in ISO Class 7 or 8 cleanrooms, with packaging accordingly. Drinking water service approved: peroxide cured EPDM, NSF/ANSI 61, WRAS and KTW. Certificates of compliance and test reports can be provided per your PPAP.

Typical lead times from PO for an AS568 standard size in an existing compound are between 14 and 21 days. A custom size requiring an insert mold cavity takes 21-30 days; meanwhile the cavity is being built, the material is being established. In a plan emergency, air freight from our China or Thailand facilities to your plant arrives in 5-7 days to most U.S. and EU ports.

Yes. Engelhardt’s corporate IATF 16949:2016 certificate covers automotive Tier-1 supply, including PPAP Level 3 records, IMDS submissions, SPC process capability data, and surveillance audit reports. Production runs supply O-rings to Tier-1 brake, fuel-system, and engine-component manufacturers in Europe, North America, and Asia. Certificate scope and surveillance audit dates are available on request to qualified buyers.