Selecting gasket material requires knowledge of how it’s going to perform in the joint. There are a number of material properties that designers or engineers use to guide their choice for the fabrication of a custom gasket. One of those is compressibility. Essentially a measure of material stiffness, compressibility is defined as the percentage reduction in thickness that occurs under the application of a given load. It’s often presented graphically with thickness reduction along the x-axis and load in pounds per square inch on the Y.
All non-metallic gasket materials compress or densify under load. It’s how they adapt to the mating faces, filling hollows and compensating for poor parallelism. (Metal gaskets are usually designed with compressive features for the same reason.) In general, a softer gasket material is going to deform more easily, so resulting in a leak-tight joint at the lowest possible clamping force.
Complicating the selection process, softer materials often have a tendency to flow or extrude. Bolt loads push material out through the bolt-to-hole clearance and from around the flanges. Internal loads can also lead to the material extruding out, ultimately creating a leak path.
Another issue is relaxation. The compression curve shows the initial load to create a given deflection. However, as with most materials, gasket materials undergo both elastic and plastic deformation. Elastic deformation is temporary: remove the load and the material springs back. But plastic deformation is permanent: the material takes on a ‘set.’ So when the joint is first made the compressive force is high, but over time, (minutes rather than days,) it reduces. This stress relaxation is another important material property for the designer to consider.
Plastic deformation has implications for gasket life too. When a joint is undone some of that initial compressibility has been lost, which is one reason why gaskets shouldn’t be reused.
Gasket compression curves indicate the stiffness of a material. They should be used as an aid to selecting the softest material for an application, having given regards to the other properties needed. If in doubt, it’s always best to consult a specialist! Contact Hennig Gasket & Seals today for fast quotes and accurately cut parts.
