O-rings: moulded O-ring, extruded cord or joined toric cord — material, hardness and selection criteria

The O-ring is one of the most universal sealing elements in mechanical design. But "O-ring" does not describe a single product: it encompasses at least three distinct manufacturing technologies — moulded O-ring, extruded toric cord cut and joined, and toric cord joined by vulcanisation or bonding — with very different applications, tolerances and costs. Add to that the choice of material (VMQ silicone, FKM fluoroelastomer, EPDM, NBR, neoprene, FVMQ fluorosilicone), Shore A hardness and surface finish. The result is that two apparently equivalent seals can have performance and price separated by an order of magnitude.

This article structures the decision on three levels: first the manufacturing technology, then the material, and finally hardness and finish. It includes a fluid selection matrix and a decision tree by volume and tolerance.

Three technologies, three different economies

Moulded O-ring

This is the O-ring par excellence. It is manufactured by injection, compression or transfer moulding in a closed mould with the final seal geometry. The result is a monolithic annular part with no seam, tight tolerances (ISO 3601-3 class A, B or N), fine surface finish and homogeneous properties throughout the circumference.

The moulded O-ring is the reference option when specifying a seal of standardised diameter (AS568, BS1806, DIN 3771), when volume justifies tooling cost, or when sealing performance demands total absence of discontinuity in the cross-section. Its limitation is economic: each diameter requires a mould, and O-ring moulds become expensive rapidly above a certain size. Moreover, very large diameters (above 400–500 mm depending on cross-section) are not physically mouldable with standard tooling.

Extruded toric cord, cut and joined

This is the solution for O-rings of large diameters, non-standardised dimensions or short runs where moulding is not justified. It starts from an extruded cord of silicone, FKM or another elastomer — solid or sponge depending on the application — cut to the length of the seal's developed circumference and the ends are joined. The joint can be made by hot vulcanisation (the technically correct method, which recreates the molecular continuity of the elastomer), by bonding with a specific adhesive (faster and more economical, but with a more visible mechanical and chemical joint line), or by crimping with a metal ring for non-critical applications.

This technology covers virtually unlimited diameters, from 50 mm seals to rings of several metres. The unit cost is low in short runs because it requires no dedicated tooling, but the seal presents a joint line that is the mechanical weak point and, in many cases, the critical sealing point. The quality of the joint makes the difference between a seal that functions for years and one that fails on the first pressure cycle.

Joined toric cord (small-section O-ring cord)

This is the variant of extruded cord for small cross-sections, typically 1 to 10 mm circular section. It is usually sold in coils or rolls and the customer cuts and joins it to the required diameter. It covers repairs, prototypes, pre-series and industrial applications where the exact dimension does not exist as a standardised O-ring.

Market terminology is confusing: "toric cord", "toric thread", "O-ring cord" and "custom seals" frequently refer to the same product with slight variations of cross-section or finish. What matters is that the quality of the final joint depends on the method employed — just as with the larger-section cord — and that the base extruded material has slightly different properties from the same moulded compound (lower moduli, wider dimensional tolerances).

Quick technology comparison

Material selection

The choice of elastomer is independent of the manufacturing technology: a moulded seal and an extruded-joined cord can both be made in silicone, FKM, EPDM or NBR equally. The decision is based on the service fluid, working temperature and applicable regulations.

VMQ Silicone

Wide thermal range (-60 to +200 °C in continuous service, up to +250 °C intermittent), excellent ozone, UV and weathering resistance, biologically inert, suitable for FDA, CE 1935/2004, USP Class VI and ISO 10993 certifications. It is the default choice for food-grade, pharmaceutical, medical and hot water or steam contact O-rings in non-continuous cycles.

Limitations: poor resistance to hydrocarbons, mineral oils, fuels and aromatic solvent vapours. In the presence of these fluids, silicone swells and loses mechanical properties rapidly.

FKM Fluoroelastomer (Viton®, FPM)

Superior chemical resistance. Withstands aliphatic and aromatic hydrocarbons, mineral and synthetic oils, fuels, dilute mineral acids and many solvents. Thermal range -20 to +200 °C continuous (-40 with special grades), up to +230 °C intermittent. It is the reference material for O-rings in contact with hydraulic fluids, lubricants, fuels and the chemical industry.

Limitations: poorer performance at low temperatures (progressively stiffens below -20 °C), not recommended for continuous saturated steam or for amines, esters or ketones.

EPDM

Excellent resistance to hot water, saturated steam, ozone, UV, weathering, dilute acids and bases. Thermal range -50 to +150 °C continuous. It is the standard rubber for O-rings in potable water, heating, domestic hot water systems and contact with polar chemical products.

Limitations: incompatible with mineral oils, hydrocarbons and petroleum-derived greases.

NBR (Nitrile) and HNBR

NBR is the reference elastomer for oils and fuels in the moderate temperature range (-30 to +100 °C). HNBR (hydrogenated nitrile) extends the range to +150 °C and improves ozone resistance. Both are incompatible with continuous hot water, steam and aggressive polar fluids.

FVMQ Fluorosilicone

Combines silicone's thermal range with improved hydrocarbon resistance. Range -60 to +200 °C with partial compatibility with fuels and oils. It is specified when extreme low temperature and intermittent hydrocarbon exposure coexist, typically in aerospace and mobile systems.

Neoprene (CR)

General-purpose elastomer with good ozone, weathering and light mineral oil resistance. Range -40 to +110 °C. Used for industrial O-rings where neither the chemical inertness of FKM nor the biocompatibility of silicone is required.

Fluid selection matrix

For critical applications or combined aggressive fluids (oil and hot water mixtures, steam with chemical additives, aggressive CIP at temperature), the matrix is indicative only: it is advisable to validate with chemical compatibility testing on the specific compound before finalising the specification.

Shore A hardness selection

The correct hardness is neither the highest nor the lowest: it is the one that allows the seal to deform sufficiently to seal in the groove without extruding through the gap or losing elasticity over time. Standard O-ring ranges are 50 to 90 Shore A, with 70 Shore A as the default value.

Temperature also plays a role: all silicone and all FKM lose hardness when heated and gain it when cooled. If a seal operates at -40 °C in standard silicone, the effective in-service hardness may rise 8–10 Shore A points above the value specified at 23 °C. At +200 °C it may drop 5–8 points. In extreme ranges, hardness should be specified considering the actual working temperature, not just the nominal value.

Surface finish and special versions

The surface finish of an O-ring does not usually appear on the data sheet, but it is decisive in dynamic applications. A seal with too smooth a surface may suffer from adhesion at the start of movement (stick-slip). One that is too rough introduces microscopic leaks. Standard moulding produces finishes of Ra approximately 0.8–1.6 µm, suitable for most applications. For demanding dynamic or gas sealing applications, specific machined or polished finishes are specified.

Metal-detectable food-grade versions. For packaging machinery where accidental seal fragmentation must not reach the product, silicone formulations with ferromagnetic fillers (metal-detectable) and coloured in intense blue for visual contrast are available. They are a requirement in BRC, IFS certifications and in many food industry client audits. The trade-off is a slightly reduced thermal range and somewhat lower mechanical properties compared with the equivalent unfilled compound.

Lubrication and surface treatments. For dynamic applications, O-rings can be supplied with PTFE surface treatment or lubricated with specific silicones that reduce the initial friction coefficient. For biocompatible applications, lubricant compatibility with the end use must be validated.

Functional colouring. Beyond the food-grade blue, colour coding (red, green, yellow) is used to differentiate batches, materials or applications in installations with multiple references. This is mass pigmentation, not a functional alteration.

Quick decision tree

For a specifier who arrives with a new requirement and needs to finalise the configuration in a few steps:

1. Is the dimension standardised (AS568 / BS1806 / DIN 3771) and does the series justify tooling or do we use existing moulds? → Moulded O-ring.
2. Is the dimension non-standard or the diameter > 400 mm and the runs are short? → Extruded cord cut and joined (vulcanised if sealing is critical, bonded if not).
3. Does the repair or prototype require an immediate solution without tooling? → Joined toric cord, with the joint made in the workshop.
4. Material: apply the fluid matrix above.
5. Hardness: 70 Shore A by default; increase if there is significant gap, decrease if the groove geometry requires high adaptability.
6. Finish / special version: detectable if certified food industry, polished surface if dynamic, colour if plant coding applies.

Minimum specification for tender

The minimum data to include in a quotation request or tender are: inner diameter and cross-section (or standardised reference AS568/BS1806/DIN), material and specific compound, Shore A hardness ± tolerance, minimum and maximum working temperature, fluid or fluids in contact, applicable regulations (FDA, USP VI, ISO 10993, EC 1935/2004, other sector-specific), type of service (static, reciprocating dynamic, rotary dynamic), maximum working pressure, required manufacturing technology (moulded vs extruded-joined) and estimated quantity.