Not all mechanical seals are made of a single piece of material. Filled PTFE seals, for instance, feature an inner core and an outer jacket. The outer jacket is made of polytetrafluoroethylene (PTFE), but the inner jacket is made of a different material.
PTFE is a strong yet flexible material that’s used in a wide range of applications. In addition to nonstick cookware, tubing and electrical insulation, it’s commonly used in mechanical seals. Pure PTFE seals, however, have their limitations. When exposed to extreme temperatures or pressure, for instance, they may become permanently deformed, thereby losing their ability to create air- and fluid-tight mating surfaces. Filled PTFE seals offer a solution by featuring an inner core made of one or more other filler materials, such as molybdenum disulphide, graphite, or molybdenum disulphide and glass.
Molybdenum Disulphide
Molybdenum disulphide is a common filler material for filled PTFE seals. This inorganic compound is comprised of molybdenum and sulfur. When used for PTFE seals, it allows for a higher temperature range. 5% Moly-filled PTFE seals can handle temperatures of up to 400 degrees Fahrenheit — higher than that of plain PTFE seals.
Moly-filled PTFE seals also offer better extrusion resistance than their plain-PTFE counterparts. They can perform well in cyclic environments while retaining their sealing abilities. As a tradeoff, though, they lose some of their sealing performance in low-temperature or low-pressure environments.
Graphite
Another common filler material for filled PTFE seals is graphite. They are known for their superior strength. 15% graphite-filled PTFE seals can handle temperatures of up to 500 degrees Fahrenheit and pressures of up to 3,000 pounds per square inch (PSI). This makes graphite-filled PTFE seals popular for sealing applications involving liquid and steam.
Graphite-filled PTFE seals are also better protected from friction-related damage than other types of seals. The use of a graphite inner core increases their lubricity, allowing them to operate smoothly with little or no external lubrication.
Glass and Molybdenum Disulphide
You can also find filled PTFE seals with an inner core made of glass and molybdenum disulphide, such as 5% Moly and 15% glass. This unique composition allows for maximum protection against extreme temperatures and pressures. Glass-and-Moly-filled PTFE seals, for example, can withstand pressures of up to 40,000 PSI.
The combination of these two materials allows for a unique balance of stiffness, wear resistance and extrusion control. The glass fibers help to reinforce the PTFE, whereas the Moly helps to maintain lubricity and thermal stability.