EPDM Sealing Rings & Cord

What are EPDM sealing rings and cord?

EPDM sealing rings are O-rings made from EPDM rubber (ethylene propylene diene monomer), and EPDM cord is the same material supplied as a continuous length that can be cut and joined into custom rings. EPDM is the go-to rubber for sealing water, steam, and outdoor equipment, because it stands up extremely well to weather, ozone, sunlight, and hot water. Unlike nitrile, it is not designed for oils and fuels.

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

• A solid, round cross section, measured as its section diameter
• EPDM rubber, which resists water, steam, weathering, and ozone
• A hardness rating on the Shore A scale, most commonly 70
• For O-rings, an inside diameter that sets the size of the ring.

EPDM is chosen for water and weather, not oil.

• It keeps its flexibility outdoors and in hot water, where many other rubbers crack, harden, or swell.
• It should not be used with mineral oils, greases, petrol, or diesel, which make EPDM swell and break down.

EPDM cord is supplied as a continuous extruded length, so it can be cut and joined to make a seal of exactly the right size, or fitted into a groove as a gasket or sealing strip.

Why are EPDM sealing rings and cord essential?

Many seals spend their lives in water, steam, or the open air, where oil-resistant rubbers such as nitrile can perish. EPDM is made for these conditions. It resists rain, sunlight, ozone, and temperature changes, and handles hot water and steam well, so it gives a long-lasting seal in plumbing, heating, and outdoor equipment.

EPDM also resists many dilute acids, alkalis, and cleaning chemicals, and is the usual choice for glycol-based brake fluids. Using cord as well as standard rings means large or unusual sizes can be made up quickly, without waiting for a special part.

Typical applications include:

• Plumbing, water supply, and drinking water fittings
• Central heating systems and hot water cylinders
• Steam and hot water equipment
• Outdoor equipment, doors, windows, and enclosures
• Brake systems using glycol-based brake fluids
• Swimming pools, spas, and water treatment
• Washing machines and dishwashers
• Food and beverage processing using water and cleaning chemicals

The main types of EPDM sealing rings and cord

• Metric EPDM O-ring cord - Supplied in metric section sizes and sold by length, for making custom O-rings and seals.
• Imperial EPDM O-ring cord - Supplied in inch section sizes, for imperial grooves and equipment.
• 70 Shore EPDM - The most common hardness, which gives a good balance of flexibility and sealing for most applications.
• Standard size EPDM O-rings - Ready-made rings in standard metric and imperial sizes.
• Peroxide-cured EPDM - A grade with improved heat and steam resistance, used where temperatures are higher.

How to choose the right EPDM sealing ring or cord?

• Fluid compatibility - Use EPDM for water, steam, glycol, brake fluid, and many cleaning chemicals. Do not use it with mineral oils, greases, or fuels; choose nitrile or FKM instead.
• Section size - Choose a section diameter that suits the groove depth and width, so the seal is compressed by the right amount.
• Inside diameter or length - For an O-ring, choose the inside diameter to suit the groove. For cord, work out the length needed to make the ring.
• Temperature - Standard EPDM suits temperatures from around -40°C up to +120°C, and peroxide-cured grades can go higher. Check the grade suits the temperature of the water or steam.
• Hardness - 70 Shore suits most applications. Harder grades may be needed for higher pressures.

Essential references and specialised ranges

How the numbering works:

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

18mm EPDM 70 = 18mm section EPDM cord, 70 Shore hardness

EPDM O-rings are referenced by their inside diameter and section diameter:

50 x 4 EPDM = 50mm inside diameter, 4mm section

To work out how much cord is needed for a ring, add the inside diameter to the section diameter, then multiply by 3.14. For example, for a 200mm inside diameter ring in 8mm cord: (200 + 8) x 3.14 = about 653mm of cord.

Suffixes:

EPDM or EP = ethylene propylene diene monomer 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 EPDM sealing rings and cord

What is the difference between EPDM and nitrile?

EPDM resists water, steam, weathering, ozone, and brake fluid, but is damaged by oils and fuels. Nitrile resists oils, greases, and fuels, but is less resistant to weather and ozone. Choose the material to suit the fluid and environment.

Can EPDM be used with oil or diesel?

No. Mineral oils, greases, petrol, and diesel make EPDM swell and lose its strength, which will cause the seal to fail. Use nitrile or FKM for oil and fuel.

Is EPDM suitable for outdoor use?

Yes. EPDM is one of the best rubbers for outdoor use, as it resists sunlight, ozone, rain, and temperature changes without cracking.

How do I join EPDM cord?

Cut the ends squarely with a sharp blade, then join them with a suitable instant adhesive or a splicing kit. Hold the ends firmly together until the joint has set, and check it is smooth before fitting.

How do I fit an EPDM O-ring?

Clean the groove and surfaces, and remove any burrs that could cut the ring. If a lubricant is needed, use water, silicone grease, or another EPDM-compatible lubricant, never mineral oil or petroleum grease. Fit the ring into the groove without twisting or overstretching it.

How do I identify the size I need?

For an O-ring, measure the inside diameter and section diameter of the old ring, or measure the groove. For cord, measure the section diameter with callipers and work out the length from the groove size.

How can I tell if an EPDM seal needs replacing?

Warning signs include:

• Water or steam leaking around the seal
• A flattened ring that has lost its round shape
• Swollen, soft, or sticky rubber, often a sign of contact with oil
• Cracks, cuts, or nicks in the rubber
• A failed or split joint on a cord ring