Squeeze is the ratio of the amount of deformation applied to the sealed amount expressed as a percentage of the cross-sectional thickness in the free state. The cross-section of the deformed seal "energizes" the elastomeric matrix much like a compression spring; the inherent elasticity of the rubber material causes it to push back against the mating component. This contact force blocks liquids, gases and dry powders, preventing them from flowing between the rubber seals and mating hardware channels.
The more the O-ring is squeezed, the more force is applied to the hardware and the tighter the seal. However, this doesn't necessarily mean that designers should always specify the most crowded (assuming they know what level that is and why, which is "most"). There are many factors to consider, including:
As just mentioned, tighter seals generally result in higher levels of extrusion. Beyond a certain level, however, other factors intervene that can work on an effective seal, such as stress resulting from the force of the mating hardware.
If squeezing is increased, and its compressive force, too much, it can potentially damage the mating hardware, depending on the material and hardware design.
Having a higher squeeze also comes with more friction and faster wear for dynamic applications. This may be enough to affect the functionality of the device. For example, in medical devices involving manual adjustment, an O-ring that generates excessive friction may prevent the physician from using the device properly.
With higher squeezing comes a higher risk, which occurs when clamping an O-ring is installed - creating a path for fluid or gas flow around the seal. Figure 1 and 2 show the finite element model of what happens when the O-ring is installed with 40% and 25% extrusion, respectively. This model depicts an o-ring pinch damage to the o-ring seal during assembly in a male (piston type). At the 40% level, pinching is difficult to avoid, while pinching is eliminated at the 25% level.
The extruded elastomer is applied against the mating hardware, creating a sealing force that tends to decay over time. When this force is fully attenuated, the O-ring will retain its shape even though it is no longer squeezed. Compression set is a measure of this decay, expressed as a percentage. When the compression set reaches 80%, most O-rings are in danger of losing their sealing ability. O-rings extruded at higher levels generally take longer to reach the 80% compression set level. Based on compression set alone, then, higher levels of compression generally translate to longer working life of the O-ring.
Designers need to carefully consider all of these factors - in addition to other application-specific factors such as temperature and pressure of the material being sealed - before they decide how much O-ring extrusion to apply. The right decision converts to the best seal over the longest O-ring life with the least damage to either the O-ring itself or its mating hardware.