Fluorosilicone Sealing Rings & Cord

What are fluorosilicone sealing rings and cord?

Fluorosilicone sealing rings are O-rings made from fluorosilicone rubber (FVMQ), and fluorosilicone cord is the same material supplied as a continuous length for making custom rings. Fluorosilicone is a silicone rubber with fluorine added to its structure. This gives it the wide temperature range and flexibility of silicone, together with much better resistance to fuels, oils, and solvents, which ordinary silicone cannot handle.

A standard fluorosilicone sealing ring or cord has these main features:

• A solid, round cross section, measured as its section diameter
• Fluorosilicone rubber (FVMQ), which resists fuels, oils, and many solvents
• Flexibility across a wide temperature range, from very low temperatures up to around +200°C
• A hardness rating on the Shore A scale, commonly 70, and often a blue colour for easy identification.

Fluorosilicone combines the strengths of two materials.

• Like silicone, it stays soft and flexible at very low temperatures, where many oil-resistant rubbers become stiff and brittle.
• Like fluoroelastomers, it resists petrol, jet fuel, mineral oils, and many solvents, which would make ordinary silicone swell and fail.

Fluorosilicone is mainly used for static seals, as its wear and tear resistance are lower than tougher rubbers such as nitrile or FKM.

Why are fluorosilicone sealing rings and cord essential?

Some seals have to cope with fuel or oil and extreme cold at the same time, such as in aircraft, outdoor vehicles, and equipment that runs in freezing conditions. Nitrile and FKM resist fuels but stiffen in deep cold, while silicone stays flexible but is attacked by fuels. Fluorosilicone fills this gap, giving a reliable seal in fuel and oil across a very wide range of temperatures.

It also keeps silicone’s resistance to ozone, weathering, and ageing, so it lasts well in outdoor and long-term static applications.

Typical applications include:

• Aircraft and aerospace fuel systems
• Fuel tanks, pumps, and fuel line connectors
• Automotive fuel and emission control systems
• Equipment operating in very cold climates
• Military and defence vehicles and equipment
• Oil and fuel handling equipment
• Static seals exposed to both fuel and wide temperature changes

The main types of fluorosilicone sealing rings and cord

• Fluorosilicone O-ring cord - Supplied as a continuous length and sold by the metre, for cutting and joining into custom-sized rings and seals.
• Fluorosilicone O-rings - Ready-made rings in set section sizes and bores, for standard grooves.
• 70 Shore fluorosilicone - The most common hardness, which suits most static sealing applications.
• Blue fluorosilicone - Fluorosilicone is often supplied in blue, which helps tell it apart from ordinary silicone, nitrile, and FKM seals.

How to choose the right fluorosilicone sealing ring or cord?

• Fuel and oil plus temperature - Choose fluorosilicone where a seal must resist fuels or oils and also stay flexible at low temperatures. If low temperatures are not a concern, nitrile or FKM may be a lower-cost option.
• Temperature - Fluorosilicone works across a very wide range, from deep cold up to around +200°C. Check the grade suits both the lowest and highest temperatures in use.
• Static or moving - Fluorosilicone is best for static seals. For seals that rub against moving parts, a more wear-resistant material is usually better.
• Fluid compatibility - Fluorosilicone resists petrol, jet fuel, mineral oils, and many solvents. It is not suited to some fluids, such as brake fluids and certain ketones, so always check compatibility.
• Size - Choose a section diameter to suit the groove, and a bore or cord length to suit the seal size.

Essential references and specialised ranges

How the numbering works:

Fluorosilicone cord is referenced by its section diameter, material, and hardness:

3mm FVMQ 70 = 3mm section fluorosilicone cord, 70 Shore hardness

Fluorosilicone O-rings are referenced by their section diameter and bore, in mm:

3x40 FVMQ = 3mm section, 40mm bore

The outside diameter is the bore plus twice the section, so a 3x40 ring has a 46mm outside diameter.

To work out how much cord is needed for a ring, add the bore to the section, then multiply by 3.14. For example, for an 80mm bore ring in 3mm cord: (80 + 3) x 3.14 = about 261mm of cord.

Suffixes:

FVMQ = fluorosilicone rubber

70 = hardness on the Shore A scale

Suffix codes vary between manufacturers, so check the supplier’s catalogue for the exact meaning.

FAQ - All you need to know about fluorosilicone sealing rings and cord

What is the difference between fluorosilicone and silicone?

Both stay flexible over a wide temperature range. Fluorosilicone has fluorine added, which gives it much better resistance to fuels, oils, and solvents. Ordinary silicone swells and weakens in fuel, while fluorosilicone does not.

What is the difference between fluorosilicone and FKM?

Both resist fuels and oils. FKM handles higher temperatures and is tougher, so it suits moving seals. Fluorosilicone stays flexible at much lower temperatures, so it suits seals that must work in extreme cold.

Why is fluorosilicone often blue?

Blue is commonly used to identify fluorosilicone, so it is not confused with ordinary silicone, which looks similar but does not resist fuel. The colour does not change how the seal performs.

How do I join fluorosilicone cord?

Cut both ends squarely with a sharp blade and join them with a suitable adhesive or splicing kit. Check the joint will cope with the fuel or oil and the temperatures it will see in use.

How do I fit a fluorosilicone O-ring?

Clean the groove and remove any burrs or sharp edges, as fluorosilicone can tear more easily than tougher rubbers. Lightly lubricate the ring with a compatible lubricant, then fit it without twisting or overstretching it.

How do I identify the size I need?

Measure the section diameter and bore of the old ring with callipers, or measure the groove. For cord, measure the section and work out the length from the groove size.

How can I tell if a fluorosilicone seal needs replacing?

Warning signs include:

• Fuel or oil leaking around the seal
• A flattened ring that has lost its shape
• Tears, nicks, or splits in the rubber
• Swelling or softening from an incompatible fluid
• Hardened or cracked rubber
• A failed joint on a cord ring