An FKM compound code is read in three layers: the polymer family prefix, the optional cure system, and a trailing durometer indicator (e.g. -70 = 70 ±5 Shore A) that ties the part back to a tested compound on the supplier's data sheet [S4][S1].
FKM, FPM, and Viton™ all name the same fluoroelastomer material class: FKM is the ISO 1629 / ASTM D1418 designation, FPM is the European equivalent, and Viton™ is the Chemours (formerly DuPont) trademark that the industry uses as shorthand for the polymer family [S5][S3]. When a data sheet lists ASTM D2000 / SAE J200 "HK", that two-letter code is the line-callout shorthand for FKM-type fluoroelastomer [S5].
How Parker-Style Compound Codes Carry Polymer, Sequence, and Hardness
Parker O-Ring publishes a two-type numbering system: Type I uses the format "N 0674 -70" (polymer code, four-digit sequence, durometer) while Type II inserts a single-letter special-property descriptor, as in "AA 150 -70" [S4]. The durometer field always carries a Shore A value with an implicit ±5 tolerance, so a "-75" suffix means 75 ±5 Shore A as measured on a finished slab per ISO 7619-1 / ASTM D2240 [S4]. The polymer letter itself names the family, with "V" reserved for Fluorocarbon (FKM, AFLAS®, Hifluor™, Parofluor™), "K" for HNBR, "B" for butyl, and "E" for EPDM [S4]. A specifier who reads the suffix as "just a number" loses the link to a specific tested compound, which is exactly what OEM traceability requires.
Parker further qualifies each grade with a special-property letter, e.g. "F" for fuel-resistant or fully fluorinated, "G" for higher fluorine content, "L" for internally lubricated, and "P" for low-temperature flexible [S4]. That single letter is the difference between a general-purpose FKM and a low-temperature, fuel-rated, or FDA/WRAS-graded variant on the same polymer backbone.
FKM Sub-Classes, Cure System, and What the Hardness Number Doesn't Tell You
FKM is not one polymer: the family breaks into copolymer (VDF/HFP), terpolymer (VDF/HFP/TFE), and peroxide-curable specialty types, and the cure system itself is part of the compound identity [S5]. Standard FKM is bisphenol cured, while peroxide-cured grades offer better resistance to organic acids, amines, and amine-containing lubricants, allowing them to replace legacy litharge-cured materials in acid service [S5]. A "-70" suffix tells you hardness only; the actual fluid compatibility window lives in the polymer sub-class and cure system recorded on the supplier's data sheet.
A worked example: Freudenberg FKM FP605601 is a brown, bisphenol-cured copolymer, nominal 60 ±5 Shore A, density 1.96 ±0.02 g/cm³ (ISO 1183-1), tensile strength 12.5 MPa, elongation at break 305%, compression set not published in this format [S2]. After 70 h / 250 °C air aging per ASTM D573 the hardness drifts from 64 to 67.5 Shore A (+4), tensile drops 9% to 11.4 MPa, and elongation falls 15% to 259.2% [S2]. After 70 h / 23 °C Fuel C immersion per ASTM D471 the hardness drops 4 points to 60 Shore A, tensile falls 21% to 9.9 MPa, and the compound swells 4.5% by volume [S2]. Those are the post-aging values a real "-60" FKM compound has to clear; raw number alone hides them.
Compound Code vs ASTM D2000 Line Callout
Two buyers looking at the same "-75" suffix can end up with different seals if one is reading a proprietary compound number and the other is reading the ASTM D2000 "HK" callout. ASTM D2000 line callouts carry hardness, tensile, elongation, heat-age, and fluid-age requirements as type/ class/ hardness digits, and they assume ±5 Shore A on the hardness cell [S4]. A proprietary code (Parker, ERIKS, Trelleborg, Freudenberg) ties the same hardness to a tested production batch and a specific cure, but does not, on its own, guarantee the broader ASTM D2000 property package [S1][S4]. The Freudenberg FKM FP605601 sheet explicitly states "No ASTM D2000 properties available," which is the supplier admitting the proprietary code does not replace a D2000 callout [S2].
For a plant engineer, the practical move is to keep both on the drawing: the proprietary compound number for batch traceability, and the D2000 callout for qualification. MFG SOLUTION's own guidance to sealing buyers echoes this: "Request the compound code, polymer family, hardness and cure information… preserve the tested compound and manufacturing process in repeat orders" [S1]. Without that combination, a "-75" FKM on paper and a "-75" FKM in the gland can be two different materials.
Hardness Selection: What the Durometer Suffix Actually Buys You
Shore A hardness correlates with modulus, extrusion resistance, and seal gap tolerance, but the correlation is not linear. Industry data sheets for FKM typically span 50 to 90 Shore A [S5]. Softer grades (50 to 65 Shore A) conform better to rough surfaces, gland imperfections, and thermal cycling, at the cost of higher extrusion risk in high-pressure applications; harder grades (80 to 90 Shore A) resist extrusion and compression set but need tighter gland finishes and higher closure force. A "-70" grade, the most common mid-range spec, is a compromise that suits general hydraulics, hydrocarbon service, and most chemical-process static seals at pressures up to the compound's rated limit.
Comparative positioning against neighbouring elastomers puts the hardness code in context: fluororubber FKM is the all-rounder, NBR is the inexpensive oil/fuel default, and FFKM is the perfluoroelastomer that runs from −40 °C to +340 °C at roughly 100× the FKM raw-material price and ~329× the NBR price [S3]. The hardness code is the same Shore A scale across all three; what changes is the polymer ceiling and the fluid envelope behind it.
Selection Checklist: Reading an FKM Compound Code in a Supplier Quote
A defensible FKM spec line on a datasheet should carry at least six elements: polymer family (FKM / FPM / Viton™), polymer sub-class (copolymer, terpolymer, peroxide-curable specialty), cure system (bisphenol, peroxide, or litharge legacy), nominal hardness with tolerance (-70, -75, etc.), Shore A test method (ISO 7619-1 or ASTM D2240), and a post-aging profile (e.g. 70 h / 250 °C air per ASTM D573) [S2][S4][S5]. A compound code missing any one of those six is a request for trouble, because the missing field is the one that changes when the supplier reformulates.
For comparison across common Shore A grades used in static and dynamic seals, the table below lines up the practical trade-offs, drawing on Parker durometer indicators and Freudenberg FKM FP605601 test data:
Comparison of typical FKM durometer grades:
50–65 Shore A: soft grades, best conformability, lowest extrusion resistance, typical static sealing on rough flanges; example reference 60 ±5 Shore A from FKM FP605601 data sheet [S2].
70–75 Shore A: general-purpose mid range, balanced extrusion and conformability, common in hydraulics and chemical service; matches the "−70" Parker durometer indicator ±5 Shore A [S4].
80–90 Shore A: hard grades, best extrusion resistance, requires tighter gland finishes and higher closure load, typical for high-pressure or high-gap reciprocating service [S4][S5].
Failure Modes When the Code Is Misread
Three failure patterns repeat when the suffix is treated as a number rather than a tested compound. First, hardness drift after thermal aging: the FKM FP605601 sheet records a +4 Shore A shift after 70 h / 250 °C air aging, which would push a "-60" compound above the upper ±5 limit and into the next nominal grade [S2]. Second, fluid-induced softening: immersion in Fuel C 70 h / 23 °C drops the same compound 4 Shore A points and swells it 4.5% by volume, both of which change the squeeze on the gland [S2]. Third, cure-system mismatch: specifying bisphenol-cured FKM for amine- or acid-rich service invites rapid property loss that a peroxide-cured grade would survive, and no amount of durometer tuning will fix it [S5].
The compounding-design side uses checking and testing apparatus such as Shore A durometers (ISO 7619-1 / ASTM D2240), tensile testers (ASTM D412), and compression-set rigs (ASTM D395, Slab B, 22 h / 200 °C, 25%) to verify the published code on every lot, which is why end-users should request the actual data sheet rather than trust the suffix.
Standards and Traceability Anchors
Five published anchors define what an FKM compound code means on paper: ISO 1629 and ASTM D1418 for the FKM polymer designation, ASTM D2000 / SAE J200 for the "HK" line-callout shorthand, ISO 7619-1 and ASTM D2240 for the Shore A hardness test method, and ASTM D573 / ASTM D471 for the heat- and fluid-aging protocols that prove the published numbers [S5][S4][S2]. The Parker Type I / Type II numbering system is the de facto proprietary code structure used across the industry for translating those standards into a single part number [S4].
Trackable signals to watch in the next sourcing cycle: (1) whether suppliers are publishing ISO 23936-2 / NORSOK M-710 sour-service FKM grades with explicit compound codes, since the line between standard FKM and "FKM for oil & gas" is where misreading a suffix is most expensive; (2) whether the shift to peroxide-cured FKM in amine-rich lubricant service, as already noted in industry guidance [S5], continues to displace legacy litharge-cured stock over the next 12 months.
Spec-level background on the components involved: code reader.
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