“Elastic” Bellows and Diaphragms Made of PTFE

Can PTFE components replace rubber seals?

Bellows and diaphragms are used in engineering as so-called “hermetic seals” that can seal moving components without leakage.
Both technical terms actually originate from names for thin membranes of animal origin.
Historically, these were initially used to make drinking bladders and bellows.
Today, these terms refer to components that are clamped between two spaces to accommodate relative movement.

Various directions of movement are possible:

• Axial / linear compensation
• Radial / lateral compensation
• Radial / lateral compensation

“Diaphragms” primarily refers to flat components that can accommodate only small strokes relative to their diameter.
Bellows, on the other hand, can be described as a series of multiple individual interconnected diaphragms, allowing a greater stroke to be achieved. The term “seal” can refer to either type.

The most important material group for diaphragms and bellows is, of course, elastomers. The most commonly used materials include NBR, HNBR, silicone (VMQ), EPDM, Santoprene® (EPDM-PP copolymer), and Viton® (FKM). A wide variety of molded rubber parts are used in industrial and automotive applications. The development of these custom components is generally only cost-effective for larger production volumes. A major advantage of rubber seals is the material’s exceptional flexibility, which can compensate for minor design errors.

When higher chemical resistance or a higher operating temperature is required, PTFE is often used, or, less commonly, UHMW-PE. Both materials are thermoplastics with little elasticity. Therefore, the design must ensure that the sidewalls of the convolutions are oriented as radially as possible rather than at an angle. This allows the convoluted section to move despite the low elasticity of Teflon® or polyethylene. Bellows are usually machined from solid stock and, less commonly, blow molded.

The same applies to diaphragms: For rubber diaphragms, a flat disk that can easily deform axially is sufficient. Machined Teflon® diaphragms are often designed with a convolution that rolls in either direction during the stroke. The convolution should be designed so that differential pressure acts on the concave (hollow) side. As a rule of thumb, a thickness of 0.5–0.7 times the original thickness of the rubber diaphragm has proven effective for the convolution. Due to the different stress distribution, the wall thickness should decrease toward the outside.

Not all design engineers are aware that hydraulically pressed virgin polytetrafluoroethylene from established manufacturers exhibits exceptionally high flex fatigue resistance. Therefore, if the design guidelines mentioned above are followed, bellows and diaphragms can indeed be designed with a service life comparable to that of rubber seals. This is due to the molecular structure of PTFE, which consists exclusively of one-dimensional chains that can move relative to one another. Another advantage is that the components are manufactured by machining, which requires no tooling. This allows even small quantities to be produced quickly, individually, and cost-effectively.

In summary, even within existing installation spaces, rubber diaphragms and bellows can be replaced with custom-designed PTFE components. The existing assembly can thus benefit from advantages such as universal chemical resistance, a wide operating temperature range, physiological safety, and much more.

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Stefan Bock