Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

PTFE Lip Seals - Applications, Material Choice & Advantages

The effectiveness of PTFE as a sealing material has been explored before. Whether as a gasket material – such as ePTFE – or as a machined component designed for a specific OEM requirement, PTFE combines an ability to self-lubricate with a capacity for higher temperatures, pressures, and corrosive chemicals to be a highly effective sealing element in nearly all conditions.

While traditional lip seals have used either elastomers or even polyurethane (PU), PTFE lip seals are invaluable in areas where harsher environments call for a more robust polymer. Essentially, PTFE lip seals were designed to bridge the gap between conventional elastomer lip seals and mechanical carbon face seals, as they can operate at higher pressures and velocities when compared to most elastomer lip seals, they are an excellent alternative.

That said, there are many similarities between elastomeric rubber lip seals and PTFE lip seals. When we consider basic construction, both seals use metal casing for structure, a press-fit into a stationary housing gland, and a wear lip material to rub on the rotating shaft.  However, when compared with elastomer lip seals, PTFE lip seals use a far wider lip to shaft contact pattern. PTFE lip seals also make use of a lighter unit loading, but a wider footprint. Their design has taken this direction to address the wear rate, and these changes were made to lower the unit loading, which is also known as PV.

In terms of manufacturing, while elastomeric lips seals allow themselves to be directly moulded onto the metal, PTFE – being a non-injection mouldable material – is machined separately and then press-fitted onto the metal housing. This not only makes it critical to get the dimensions spot-on but also means a more expensive end-product, since PTFE is more expensive than most elastomers and because machining is usually far more expensive than moulding.

The specific application for PTFE lip seals is found in sealing rotating shafts, in particular those with high speed. They offer an excellent alternative to elastomer rubber lip seals if the conditions are challenging and exceed their capabilities.

A good example of PTFE’s superior functionality is in industrial air compressors, where they are configured to provide over 40,000 hours of maintenance-free service.

Other benefits of PTFE lip seals include:

  • Tight sealing, even under high pressure in excess of 35 BAR

  • Ability to run at temperatures far above or below elastomer rubber lip seals (with typical temperature ranges from -53 °C to 232 °C)

  • Elastomer coatings on the seal’s outer diameter make for easy installation without damaging mating hardware

  • Available in custom designs and a wide range of sizes and materials

  • Inert to most chemicals

  • Withstands high speed in excess of 35 m/s

  • Low friction and ability to address rotating equipment and vibration for longer life

  • Compatible with most lubricants and able to run in dry or abrasive media

Dynamic rotary sealing applications for PTFE seals include:

  • Industrial applications - PTFE lip seals replace elastomer and carbon face seals is in the air compressor market

  • Screw Compressors - PTFE lip seals offer excellent leak control and the ability to run at 1,000 to 6,000 RPM with a huge range of lubricants and over extended periods (15,000 hours) to reduce warranty claims

  • Aerospace – PTFE lip seals are used in auxiliary power units (APUs), turbine engines, starters, alternators and generators, fuel pumps, Ram Air Turbines (RATs) and flap actuators, which is one of the largest markets for lip seals

  • Automotive – PTFE lip seals are put to work in some of the most challenging applications in crankshafts, distributors, fuel pumps and cam seals, which are used in the racing industry where naturally, engines are frequently pushed to their limits

  • Turbochargers – PTFE lip seals run at high speeds of 4,000 to 36,000 RPM and can cope with extreme temperature ranges from -40 °F to 350 °F (-40 °C to 177 °C), with limited lubrication over an extended seal life

  • Pumps – PTFE lip seals address dynamic sealing environments, extreme speeds, pressures, and temperatures present in vacuum pumps, along with blowers, chemical pumps, encoders, alternators, drilling and tapping spindles, hydraulic motors and pumps, and air conditioning recovery pumps, among many more applications

Enhancing Performance of PTFE with Fillers

While PTFE has been used liberally across this article, the truth is that PTFE is never just one material. The addition of performance-enhancing fillers allows for augmentation in certain base properties. The selection of these fillers is highly dependent on the application in question.

Several other fillers are used in combination with PTFE. 

  • Modified Virgin PTFE – the same basic properties as virgin, but with increased wear and creep resistance and lower gas permeability

  • Carbon-Graphite Filled - Carbon reduces creep, increases hardness, and elevates thermal conductivity of PTFE. Carbon-graphite compounds have good wear resistance and perform well in non-lubricated applications

  • Carbon Fibre Filled - Carbon fibre lowers creep, increases flex and compressive modulus, and raises hardness. The coefficient of thermal expansion is lowered, and thermal conductivity is higher for compounds of carbon fibre filled PTFE. Ideal for automotive applications in shock absorbers and water pumps

  • Aromatic Polyester Filled - Aromatic polyester is excellent for high temperatures and has excellent wear resistance against soft, dynamic surfaces. Not recommended for sealing applications involving steam

  • Molybdenum Disulphide and Fiberglass Filled - Molybdenum disulphide increases the hardness of the sealing surface while decreasing friction. It is normally used in small proportions combined with other fillers such as glass. MoS2 is also inert towards most chemicals

  • Fiberglass Filled – Glass fibre has a positive impact on the creep performance of PTFE. It also adds wear resistance and offers good compression strength

  • Graphite Filled – Since graphite is often used as a lubricant, it does not significantly increase the coefficient of friction of PTFE when used as a filler. The low friction allows the compound to be used when both shaft speed and pressure are high. Graphite also is chemically inert which enables its use in corrosive media

  • Mineral Filled – Mineral is ideal for improved upper temperatures and offers low abrasion to soft surfaces. PTFE with this filler can easily be qualified to FDA and other food-grade specifications

  • Stainless Steel Filled - Although stainless steel filler is very abrasive, this compound has excellent extrusion and high-temperature resistance in static and slow dynamic applications

  • Other fillers – there is virtually no filler that cannot be combined with PTFE to improve or add to the properties of the material. As long as the filler can to withstand the higher temperatures that PTFE needs to be sintered (cured) at, it can be used. 

Features of Other Machinable Plastics

While PTFE is the most commonly used high-performance plastics for lip seals, there remain other polymers that certainly merit further understanding. Some of these include:

UHMW Polyethylene

  • Excellent wear and abrasive resistance

  • Good lubricity in water

  • Excellent sealing of light gases at low pressures

  • Excellent high-pressure extrusion resistance

  • Moderate abrasion to soft hardware

  • Excellent wear resistance in reciprocating applications

Thermoplastic (TPE) Elastomer

  • Excellent wear and extrusion resistance

  • Excellent sealing of light gases at low pressures

  • Excellent high-pressure extrusion resistance

  • Low abrasion to soft dynamic hardware material

  • Minimum dynamic surface hardness 25 Rc

  • Excellent wear resistance in reciprocating applications

  • Good wear resistance in rotary applications

Polychlorotriflouroethylene (PCTFE)

  • Excellent electrical properties

  • Stable for continuous usage until 205°C

  • Low creep at room temperature

Polyetheretherketone (PEEK)

  • Chemically inert

  • Very strong and rigid

  • Temperature range -60 to 300°C

  • Excellent abrasion resistance

Read More

1. Can Indian Manufacturing Survive COVID?

2. The Challenge of Injection Moulded High-Performance Plastics

3. Polymer Prices Go to the Moon

4. Applications and Considerations for PTFE Seals

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