What is Compression Deflection and Compression Set?

When starting a new project, PGC’s engineers look at many variables when considering what materials will work best in a specific application. Typically, solutions will require some type of elastomeric material if the application is sealing. Elastomers behave uniquely in sealing applications because they exhibit unique properties that allow them to be compressed. It is those specific properties of elastomers that are important to know and understand as a gasket or seal design proceeds. Two of those characteristics or properties are compression deflection and compression set.

Compression deflection is a measure of a materials resistance to force applied to a known surface area over a controlled distance. Compression set is the residual amount of deformation of a material after removal of a compression stress. A typical standard used to measure compression deflection or compression set is ASTM D2000 for solid rubber materials and ASTM D1056 for cellular rubber materials.

Why do we care about compression deflection or set?

Understanding how an elastomeric material will behave is key to selecting the proper material for that application problem. Because there are thousands of elastomers in various solid or cellular formats to choose from, one must be able to reduce the choices using appropriate methods and understanding key properties of each material. We then can start the process of developing a solution around that specific application. A typical application involves the mating surfaces that need to be sealed, from air flow, pressurized air, fluids, gases, etc. These surfaces will affect the choices made in determining what material to use and how that material will perform ultimately.

When considering various elastomers (rubber) materials, there are two essential types that consistently surface in a review. Solid rubber and cellular rubber. Solid rubber is used less in sealing applications because while solid rubber will compress, it will not change volume. In many applications, that behavior is not seen as a benefit to long-term performance of that material in a sealing application. In addition, if solid rubber gaskets or seals are over compressed, they will exhibit creep or cold-flow behaviors that essentially, again threaten the materials ability to perform long-term in the application. Cellular materials offer the designer the ability to choose materials that can be compressed more easily (less stress), have less resistance in terms of loading and can be compressed further (strain).

Typically, when selecting materials for gasket and sealing applications, you want to design towards the thinnest material possible that works. Thicker materials tend to introduce other behavioral problems that negatively affect the gasket or seals ability to perform optimally. Thicker materials can create greater chances for leak paths to develop, while typically increase costs (more material, higher prices). Additionally, limits to reducing thickness also come into play. Thin the material out too much and it won’t exert enough energy to seal and resist forces that the gasket or seal experience.

Materials that exhibit higher compression set values tend to perform more poorly over time in a given application. Correspondingly, keeping your compression deflection needs in line with the materials reported results on their TDS (Technical Data Sheet) allow you to optimize the parts ability to exert enough sealing force (energy) in the application while reducing the materials compression set behaviors from negatively affecting part performance over time. All elastomeric materials exhibit some level or degree of compression set over time. Some more than others. In summary, you ideally want to select materials that exert enough force (energy) against your flanges while staying well below that materials compression set limits to ensure that materials ability to meet your applications performance needs over the life of that product.