Each seal (or gasket) can be classified into one of two types with regard to the motion acting relatively upon it.

Static Seals
When there is very little or no motion of the mating assembly parts upon the seal, a static condition exists. Usually just an initial compression or periodic (open and close) compressions typify this type of seal; i.e. a liquid seal in a hose joint, a door seal on a home. Design Considerations are rather straightforward.

• When compressed, as with an O-ring or face gasket, pressure acts in essentially a vertical direction relative to the face, squeezing between two opposing surfaces to eliminate gaps.
• Outward expansion results in the horizontal direction as a pressure relief, and this profile change needs to be accommodated in the mounting surface design.
• Compression forces need to be assessed. Various design elements can be introduced if too high; i.e., lower durometer, hollow shapes, sponge/foam and/or thin walls.
• Recommended compression range is 10% - 30%, and no more than 50%.
  

Dynamic Seals
Where repeated motions act upon the seal as in a circumferential, axial or angular direction, a dynamic condition exists. Repeated actions upon the seal, either regularly or periodic, typify this type of seal; i.e. a piston ring seal or shock absorption mechanism.

Design Considerations are more complex than static seals, primarily resulting from the changing stress levels. In addition to the static seal considerations, the environment may require:

• Abrasion resistance becomes important and is affected by the mating surface finishes.
• Recovery speed after compression to the original released height.
• Compression set, or the percentage of relaxation of the dimensions versus the original uncompressed dimensions
• Temperature changes, thermal cycling.
• Distortion: squeeze, stretch - ability to recover.
• Physical strength: tensile strength, tear resistance.
  

Tubing
Tubing shapes can be considered a form of seal, either static or dynamic, particularly where they are terminated. Design considerations should be similar to the seal recommendations as above. Uses are primarily the transportation of liquids where the following characteristics should be considered: temperature resistance, flexibility, chemical resistance, food compatibility, taste, bacteria growth/mold.