Elastomerics Blog

Sealed electronic enclosures experience temperature changes caused by internal heat generation, solar loading, and ambient conditions. In turn, these temperature changes cause different enclosure materials to expand and contract at different rates. Differential expansion can result in seal failure, though typically not right away. Stockwell Elastomerics helps engineers to avoid these problems.

Thermal Cycling

Thermal cycling refers to repeated temperature fluctuations over time. Each cycle induces expansion and contraction in both the enclosure and the gasket as well in any adhesives or fasteners. When a silicone gasket is compressed between an aluminum lid and a polycarbonate housing, for example, differential movement creates forces that the seal must withstand for many cycles.

Each enclosure material has a coefficient of thermal expansion (CTE), a measure of how much the material expands or contracts when heated or cooled. Since each material has its own CTE, these movements are rarely uniform. Elastomers expand several times more than metals. Plastics also expand more than metals. Specific types of rubber, metal, and plastic have their own CTEs.

Differential Expansion

Differential expansion is a root cause of seal failure that has different failure modes.  Understanding these mechanisms can help engineers to design enclosure seals that maintain their integrity over time and despite repeated temperature changes.

• Compression Set: When an elastomer is constantly compressed and repeatedly heated, it can lose its ability to return to its original size when the compressive force is removed.
• Shear Tearing: If the enclosure walls expand at different rates, the seal may be pulled laterally. Gasket materials with low tear strength can develop micro-tears that propagate.
• Extrusion: During heating, high internal pressure can force a softened elastomer to extrude from the low-pressure side of the gland where it’s seated and compressed.
• Microleaks from Cyclic Pumping: As the enclosure heats, air inside it expands. As the enclosure cools, the air contracts. This “breathing effect” can pull moisture past the seal.

Material Selection

Choosing the right sealing material is one of the most important decisions an enclosure designer can make. This material must tolerate not only the temperature range, but also the mechanical stresses induced by differential expansion. Often, enclosure seals are made of silicone, flurosilicone, EPDM, or polyurethane.

• Silicone is weather-resistant and withstands a wide range of temperatures without losing its properties. It’s used in outdoor electronic enclosures such as telecom equipment.
• Flurosilicone combines the thermal stability of silicone with fuel and solvent resistance. It’s used in aerospace and defense enclosures such as fueling systems.
• EPDM can’t match silicone’s broad temperature resistance, but it’s more mechanically durable and weather resistant. It’s used in some outdoor HVAC enclosures.
• Polyurethane provides sealing and cushioning but can degrade with high temperatures or outdoor conditions. It’s best for indoor electronics or low-temperature environments.

Seal Design

Proper material selection is essential, but the design of a seal also determines its ability to withstand thermal cycling and differential expansion. Use the right compression range, design for movement, avoid over-constraining the seal, and account for housing material mismatches. It’s also important to incorporate pressure equalization.

• Use the Right Compression Range: Too little compression results in leaks and too much compression causes accelerated compression set.
• Design for Movement: Incorporate features that allow the gasket to flex. For example, wider cross-sections improve load distribution and rounded corners reduce stress concentrations.
• Avoid Over-Constraining the Seal: If the enclosure seal is tightly locked in place on all sides, differential expansion can cause it to shear. Allow for some movement of the seal.
• Account for Housings Material Mismatches: Housings that use mixed materials may require a softer elastomer or a thicker seal.
• Incorporate Pressure Equalization: Use membrane vents to prevent pressure buildups that would otherwise mechanically stress the seal during heating and cooling cycles.

Get Expert Assistance from Stockwell Elastomerics

Contact Stockwell Elastomerics for assistance with managing thermal cycling and differential expansion in sealed electronic enclosures