Open an electrical enclosure and you may see that the neoprene gasket material has taken on the imprint of the door or cover. In technical terms, it’s taken a compression set. In many gasket applications compression set can lead to sealing problems, due to a phenomenon known as creep relaxation.
Introduction to creep
Apply a load to an elastic material and it compresses. This happens because unlike in a metal, the atoms are linked in a way that lets them move. In gasket materials this is good because it lets the gasket deform to take up the irregularities between the two surfaces being sealed. However, there is a downside to this compressibility.
Rubber and rubber-like materials, as used in neoprene gaskets for example, have the ability to spring back. Release the load and the material returns to its original shape, more or less. Some materials do this better than others. The issue is that the material takes on a permanent deformation, or worse still, continues to deform. This behavior is called “creep” or more accurately, “viscoelastic creep.” It’s related to both the strength of the material and the time and temperature to which it’s subjected.
When creep is a problem
In a bolted joint the compressed gasket creates the torque in the securing bolts. But as the gasket material creeps and the gasket thins, the bolts are able to relax. That reduces the torque and the joint begins to loosen.
Polyurethane, silicone and nitrile gaskets tend to have lower creep than some other materials, as quantified by testing to ASTM 38.
The principle is to measure the thickness of a sample of gasket material, subject it to load, temperature and time, then release the load. The recovered thickness is measured and the difference used to calculate a percentage reduction.
Taking creep relaxation numbers into account when choosing gasket material.
As with testing to ASTM 36, the absolute test values are less important than the ability to make comparisons between gasket materials. It’s a parameter of particular importance when lasting bolt tightness is essential.