ELASTOMER MATERIAL / FKM
FKM is screened for elevated temperature and many oils, fuels and aggressive media. Fluorine content, polymer architecture and cure system can change low-temperature flexibility, fluid resistance, compression set and cost substantially.
Upload a drawingFKM is a sensible family to screen when a seal or moulding must combine elevated temperature with exposure to many oils, greases, fuels or hydrocarbons. Its weathering, ozone and ageing resistance can also be valuable. FKM is not a universal chemical material. Amines, strong bases, selected acids, steam and glycol brake fluids can be problematic depending on the type. The exact media list determines the polymer and cure system that should be assessed.
Trelleborg gives FKM a general operating range of approximately −20 °C to +200 °C and short exposure up to +230 °C. Special low-temperature grades can reach about −45 °C. This range reveals an important trade-off: a compound optimised for aggressive fluids or high heat is not automatically flexible at low temperature. Continuous duty, peaks, thermal cycling, media and mechanical load must be assessed together. Do not copy a short-term maximum into a drawing as a continuous approval limit.
FKM is commonly assessed with mineral oils, many greases, aliphatic and aromatic hydrocarbons and fuels. Parker identifies limits for specific FKM compounds with glycol brake fluids, ammonia, amines, alkalis, steam and low-molecular-weight organic acids. Compatibility changes with fluorine content, monomer structure and cure system. The quotation should therefore identify the exact compound and the test evidence behind its compatibility, not merely state “FKM 75”.
The FKM label covers different copolymer and terpolymer families and several cure systems. These choices affect fuel swell, acid and base resistance, low-temperature behaviour, hot-water performance and compression set. Do not qualify a replacement solely by colour and hardness. A second-source specification should define the polymer family, cure basis, density or identification data, relevant ageing and fluid results, and a controlled deviation process.
FKM seals are offered in several Shore A hardnesses; 70 or 75 Shore A are common starting points, not universal defaults for every moulding. Hardness affects installation force, deformation, extrusion and sealing contact. Specify nominal hardness, tolerance and a method such as ISO 48-4. Compression set, temperature, fluid, clearance, surface finish and squeeze remain part of the sealing function. A higher hardness does not replace pressure and extrusion design.
FKM is often selected for thermally loaded static seals, making compression set an important comparison. ISO 815-1 defines test methods, but a result only has meaning with its time, temperature, compression, specimen and recovery condition. Supplier values measured under different conditions cannot be compared directly. A longer ageing programme or functional test in the actual fluid may be more informative than one short standard result for a critical duty.
NBR is economical and mechanically suitable for many mineral-oil duties but normally has a lower upper-temperature capability than FKM. FKM becomes relevant when heat, fuel, aggressive additives or ageing exceed the validated NBR envelope. Its higher material cost is justified only by the real duty. Compare fluid swell, hardness change, compression set, low-temperature performance, target life and total cost including tooling and minimum quantity.
Silicone often provides stronger low-temperature flexibility and a broad thermal span, while FKM typically offers better resistance to many oils and fuels. That shorthand is insufficient for dynamic duty, steam, chemicals or gas permeation. Compare specific FKM and VMQ compounds under identical conditions. Material choice follows the fluid and function, not the highest temperature printed on a generic chart.
ISO 3302-1 provides a dimensional basis for solid moulded rubber parts, while ISO 3302-2 covers selected geometrical tolerances. Tight M1 requirements can add tool iteration, sorting and measurement cost, particularly with expensive FKM compounds and difficult shrink behaviour. Identify functional dimensions individually and use a suitable general class elsewhere. O-rings are governed by their specific standards rather than blanket moulded-part classes. The drawing and inspection plan must use the same datum and conditioning basis.
Material usage can form a larger share of FKM unit cost than it does for standard elastomers. Feed system, flash, scrap, cavity count and process stability therefore matter. Require separate figures for tooling, sampling, compound assumptions, unit price, minimum batch, scrap, secondary work, inspection and packaging. A higher-priced tool may suit stable repeat demand; a simpler tool may suit small batches. No reliable break-even exists without the actual geometry and demand.
Provide the drawing and revision, exact fluids including concentration and additives, continuous and peak temperature, pressure, motion, vacuum or gas, mating surfaces, target hardness, permitted swell, desired life, annual and batch demand, and required evidence. For a replacement compound, document the existing material and reason for change. Require the supplier to disclose deviations, polymer and cure type, and relevant test results before tooling release.
FKM is a capable first choice for elevated temperature and many oil-, fuel- and hydrocarbon-contact duties. It is not a blanket solution for steam, amines, bases or every low-temperature condition. Comparable quotations require a common compound framework, identical tests, one drawing and ISO tolerance basis, and separately stated tooling, material and inspection costs.
The ranges and properties support initial screening. The offered compound, complete specification and customer approval remain decisive.
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