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Sealing A Transit Case

Cases can be water tight and air tight with O-ring seal.An IP-rated transit case has a water and air-tight seal between lid or cover and body. That seal must be maintained in the face of relative movement caused by rough handling, pressure differentials between interior and exterior, and temperature extremes. That seal is formed by an O-ring.

The O-ring

Circular in cross-section, an O-ring is a one-piece ring of molded rubber. It is placed into a groove running around the periphery of the case opening. When the lid is closed and latched down, the O-ring is compressed between the two surfaces. This creates a seal that’s water and air-tight. Because the O-ring is made of soft Ethylene Propylene Diene Monomer (EPDM) rubber, the seal is maintained as the case lid moves and the walls flex.

Benefits of EPDM

EPDM makes a great sealing material. It’s compressible and remains soft. It’s effective at temperatures as low as -25°C (-13°F) and endures prolonged temperatures up to 125°C (257°F). Some formulations will go as low as -50°C and as high as 150°C. In extreme cold, it will lose elasticity. But, the effect reverses as the temperature rises. However, exceeding the upper temperature limit can cause permanent damage.

EPDM also demonstrates good resistance to heat, ozone and steam. That makes it a good material to use in hot, humid, and polluted environments.


EPDM will take a compression set, meaning it retains its compressed form. Once a seal has been made, the case may be opened and closed hundreds of times and remain air and water tight. Extreme high temperature and pressure deteriorates rubber. In this case, the O-ring may soften and flow through the parting line. A replacement O-ring will be needed. In very low temperatures though, when the O-ring shrinks, the seal may fail.

Another problem is EPDM swells when exposed to hydrocarbons like oil and gasoline. In this situation, the rubber expands to take up the space available in the groove. It may expand past the parting line and interfere with closing the case. The effect is not reversible. The O-ring must be replaced.

EPDM is also vulnerable to UV degradation. So, prolonged exposure to bright sunlight should be avoided.

Maintaining an IP-rating

An O-ring is essential for creating an air and water-tight seal. If it appears damaged, the case may not provide the expected level of protection. If in doubt, consider replacement.

COG O-ring Basics
ERIKS Technical Documentation O-rings
EPDM Properties

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