FKM for Custom Molded Components — Properties, Applications & Specifications
Table of Contents
Viton (FKM) resists chemical degradation because its fluorine-carbon backbone has one of the highest bond dissociation energies in polymer chemistry — fluorine atoms shield the carbon chain so aggressively that hydrocarbon fuels, aromatic solvents, and concentrated acids cannot break it down. Without this fluorine shielding, seals made from NBR or neoprene swell, soften, and extrude under sustained hydrocarbon exposure, causing leakpath failures that FKM prevents by maintaining dimensional stability. Fenlora molds custom FKM components across 50–90 Shore A in parts including O-rings, diaphragm seals, and shaft seals for the oil and gas, and chemical processing industries.
Material Properties & Specifications
| Common names / abbreviations | Fluorocarbon Rubber (FKM) |
|---|---|
| Polymer structure / monomer system | Hexafluoropropylene/vinylidene fluoride |
| Tg (°C) / Brittle Point | -17 °C to -26 °C |
| Density (g/cm³) | 1.40 - 1.95 |
| Hardness range (Shore A/D) | 55 - 95 Shore A |
| Tensile strength (MPa) | 13.7 |
| Elongation at break (%) | 150 - 450 |
| 100% / 300% modulus (MPa) | 1.3 - 13.7 at 100% |
| Rebound / tanδ (temp + freq) | 40 - 70 % Rebound |
| Compression set (%) | 12 - 25 % |
| Continuous service temp (°C) | -23°C to 204°C |
| Environmental & Fluid Resistance | Ozone: Outstanding | Oil Swell: Low |
| ASTM D2000 Callouts | ASTM D2000 M1HK 607/707/807 |
| Electrical properties | Vol Res: 2 x 10^13 ohm-cm |
| Thermal properties | Thermal Cond: 0.06 - 0.30 W/mK |
| Tear Resistance | Tier 2 - Limited |
| Abrasion Resistance | Tier 2 - Limited |
| Gas Impermeability Resistance | Tier 4 - Strong |
| Oxygen Resistance | Tier 5 - Excellent |
| Ozone Resistance | Tier 5 - Excellent |
| Weathering Resistance | Tier 5 - Excellent |
| Oil Resistance | Tier 5 - Excellent |
| Acid Resistance | Tier 4 - Strong |
| Alkaline Resistance | Tier 3 - Balanced |
| Water Resistance | Tier 4 - Strong |
| Flame Resistance | Tier 5 - Excellent |
When to Specify FKM
FKM performs where fuel chemistry, heat, and aggressive media make other elastomers a liability.
| Use Viton FKM when… | Avoid Viton FKM when… |
|---|---|
| Continuous exposure to petroleum fuels, aromatic hydrocarbons, or hydraulic fluids above 150°C | Contact with ketones (MEK, acetone) or ethyl acetate — FKM swells significantly in these media |
| Static or dynamic sealing in environments operating between –20°C and +200°C (up to 250°C intermittent) | Cryogenic applications below –40°C where FKM becomes brittle and loses compression set resistance |
| Chemical processing with acids (sulfuric, nitric, hydrochloric at moderate concentrations) or aliphatic solvents | High-pressure steam environments — FKM degrades faster in steam than EPDM at equivalent temperatures |
| Aerospace fuel system seals where swelling tolerance is tightly tolerance-controlled | Cost-sensitive, non-chemical applications where NBR or EPDM would perform adequately |
| Long-term outdoor/ozone exposure combined with elevated temperature, such as under-hood automotive sealing | Applications requiring electrical insulation with water resistance, where silicone is a more appropriate fit |
The most critical “use when” condition is hydrocarbon fuel resistance at elevated temperature — specifically because FKM’s volume swell in ASTM Reference Fuel C (a surrogate for aromatic gasoline blends) remains under 5% even after 168-hour immersion at 150°C. By comparison, NBR compounds formulated for fuel resistance can show 15–25% volume swell under the same conditions, which in a dynamic shaft seal translates directly to lip extrusion, leakpath formation, and accelerated wear against the shaft counterface.
Custom Parts We Make in FKM
Fenlora produces precision-molded FKM components where dimensional repeatability and compound selection directly determine seal life.
→ O-rings (static and dynamic face seals): FKM’s low compression set — typically under 20% after 70 hours at 200°C — makes it reliable in flanged face seal assemblies where NBR or neoprene would permanently deform and lose sealing force over time in hot fuel or oil environments.
→ Diaphragm seals: In chemical dosing pumps and fuel regulators, FKM diaphragms maintain flexural integrity across hundreds of thousands of cycles in contact with aliphatic and aromatic hydrocarbon media where other elastomers hydrolyze or stiffen.
→ Shaft seals / lip seals: The combination of FKM’s low friction coefficient and chemical stability makes it the specified choice for rotating shaft seals in gearboxes and axle assemblies running synthetic gear oil at sustained temperatures above 120°C.
→ Custom gaskets (cut or compression-molded): FKM flat gaskets in chemical flange assemblies tolerate aggressive fluid media and bolt load relaxation at high temperature — both conditions that degrade compressed fiber or NBR gaskets within months.
→ Grommets and bushings for fuel system routing: In diesel and aviation fuel line routing, FKM grommets prevent jacket degradation from fuel vapor permeation that would cause NBR to crack and lose the protective vibration isolation function.
If your drawing specifies FKM or Viton, or you’re working from functional requirements around fuel chemistry, temperature, or solvent exposure, contact our engineering team at contact us to discuss compound selection and hardness range.
Industries That Use FKM
Aerospace
FKM is used in hydraulic system seals, fuel cell bladder interfaces, and engine firewall grommets where MIL-spec fluid resistance (Skydrol, Jet-A, MIL-H-5606) and broad operating temperature range are both mandatory. Aerospace qualification demands that seals meet AS568 dimensional standards with low extractables — FKM’s chemical inertness means it doesn’t leach plasticizers into hydraulic fluid, which protects servo valve clearances.
Automotive
Fuel injector O-rings, crankshaft seals, and transmission fluid seals in modern gasoline direct injection (GDI) and diesel platforms are predominantly FKM because fuel blends with ethanol and aromatic content cause rapid NBR degradation at under-hood temperatures. The engineering requirement is swell control: FKM must maintain dimensional tolerances within ±0.05 mm in contact with E85 or B20 biodiesel blends at 120–140°C sustained. (Link to automotive industry)
Chemical Processing
Pump diaphragms, agitator shaft seals, and inline valve seats in reactors handling chlorinated solvents, aromatic hydrocarbons, or concentrated mineral acids rely on FKM because alternative elastomers either swell or harden within weeks of continuous immersion. The specific performance requirement is long-term volume stability — FKM must hold dimensional tolerances in continuous-duty service with minimal maintenance cycles to reduce process downtime.
Oil & Gas
FKM is specified for downhole tool seals, wellhead O-rings, and valve stem packing where exposure to sour gas (H₂S), crude oil fractions, and elevated wellbore temperatures makes NBR or neoprene unsuitable. The critical performance requirement is retained sealing force — FKM compounds formulated with peroxide cure systems maintain compression set below 25% after sustained exposure at 175°C, where sulfur-cured nitrile would set permanently and allow bypass.
Compare Materials
FKM is not the right answer for every high-performance application — here’s where other materials outperform it.
| If your application involves… | Consider instead |
|---|---|
| Outdoor UV or ozone weathering | EPDM |
| Budget-sensitive oil resistance only | Nitrile (NBR) |
| Continuous temps above 150°C | Silicone (VMQ) |
| High dynamic flex and tear resistance | Natural Rubber (NR) |
| High abrasion and mechanical wear | Polyurethane (AU/EU) |
| Moderate oil + weather resistance | Neoprene (CR) |
| Air/gas impermeability | Butyl (IIR) |
| Cost-sensitive general-purpose use | SBR |
Not sure which material fits your application? Send us your requirements and we’ll recommend the right compound.