by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

PTFE + Spring-Energized Seals: A Reliable Solution for Food and Pharmaceutical Processing

The primary challenge in specifying a seal is finding a solution that achieves consistent seal integrity for the operating conditions involved. However, when food or pharmaceuticals are involved, additional challenges arise — and can be met using PTFE spring-energized seals.

Design Concerns for Food and Pharmaceutical Seals

There are a number of critical design considerations involved with any type of sealing application, such as operating temperature, pressure, velocity, wear rate, friction, and chemical compatibility.

When food, dairy, or pharmaceuticals are involved, however, there are additional criteria. The first of these is finding a material that is compliant with relevant standards. In the United States, the main standard is the Food and Drug Administration standard FDA 21 CFR Part 177. This standard covers indirect food additives and thus applies to seals. For a material to be considered FDA compliant, it must be safe for human consumption and chemically inert.

Another potential challenge related to food and pharmaceuticals is MRO (Maintenance, Repair, and Operations): 3-A (Dairy and Milk) sanitary standards 18-03 for rubber materials and 20-27 for polymers, as well as NSF (National Sanitation Foundation) sanitary practice standards such as NSF/ANSI 2-2021. Such standards and practices deal with CIP (Clean-In-Place) and SIP (Sanitize-In-Place).  CIP/SIP processes often involve …

  • Extremely high temperatures, which can affect seal integrity and dimensional stability if the right material is not selected
  • Exposure to hot water and steam, which can prove problematic for materials that have a significant water absorption rate
  • Aggressive media, which can permanently compromise seal integrity if the jacket material is not compatible with the cleaning media

Wear resistance is also a critical factor: as the seal begins to wear, its particles become a potential source of contamination. This is particularly problematic when food, dairy, beverage, or pharmaceuticals are involved because those wear particles will likely be ingested. The seal material must, therefore, have a low rate of wear and be safe for occasional ingestion.

Extreme temperatures are often involved and can range from cryogenic (where elastomers and polymers may develop brittle behavior) to extreme heat (where the strength and stiffness of the seal material may be significantly reduced). Seal materials for food and pharmaceutical applications may experience both temperature extremes during regular operation, which may involve the CIP/SIP procedures discussed earlier.

Lubrication is also a major design choice for food and pharmaceutical sealing solutions. But, again, contamination must be considered and the chances are not good when it comes to finding a food-safe lubricant that is compatible with the sealing material and provides the necessary reduction in friction. A better solution would be a material with an extremely low coefficient of friction that is also self-lubricating.

Finally, any application involving food, beverages, or pharmaceuticals must have highly reliable seals. A seal failure can result in ruined products, dangerous contamination, and the potential for lawsuits.

PTFE Spring-Energized Seals

One highly reliable solution for food, dairy, and pharmaceutical applications is spring-energized PTFE seals. Spring-energized seals include an energizer that maintains seal integrity during …

  • Extreme temperatures and temperature variation, including temperatures related to CIP/SIP processes
  • Changes in pressure as well as reliable performance over a range of pressures (including vacuum conditions)
  • Seal or shaft wear
  • Shaft misalignment, eccentricity, or dimensional changes

In addition, spring-energizers add permanent resilience to the seal jacket — and an excellent option for the seal jacket is PTFE.

Virgin PTFE (also referred to as unfilled PTFE) is both FDA and USDA approved. PTFE also provides excellent high-temperature performance, experiences no water absorption, and is extremely chemically inert, all of which combine to give it the ability to maintain seal integrity during the most aggressive CIP/SIP procedures. In addition, PTFE is hydrophobic, thus repelling water and making it even easier to keep clean.

PTFE also exhibits good wear resistance, and what wear the seal jacket does experience will be compensated for by the spring energizer. And virgin PTFE provides excellent performance over a range of temperatures, from cryogenic -450°F to high temperatures up to 450°F. PTFE also has the lowest coefficient of friction of any solid at 0.1. Furthermore, it does not require lubrication because it is self-lubricating.

Conclusion

PTFE spring-energized seals are an excellent solution to the sealing challenges of food, dairy, beverage, and pharmaceutical processing. Combining the outstanding properties of FDA-approved virgin PTFE with the reliability and integrity of spring-energizers leads to high integrity and consistent sealing even in aggressive or extreme operating conditions.

If you are looking for seals related to food or drug processing, contact Advanced EMC today. Our sealing solution experts will work with you to find the right type of spring-energized seal for your application, including everything from the seal jacket geometry to the spring material and configuration.

by Jackie Johnson Jackie Johnson No Comments

Benefits of PTFE For Sealing Applications

PTFE (Polytetrafluoroethylene), also known by its trade name Teflon, is a polymer material commonly used in sealing applications that offers unparalleled stability and sealing characteristics across an extremely wide range of temperatures, from the extreme heat of a space shuttle engine to the cryogenically cold temperatures used to preserve

In this article, we will discuss how and why PTFE is one of the best materials to use for seals in a wide variety of applications.

Low Friction

PTFE has the highest melting point and lowest friction, and is the most inert of all the fluoropolymers. It has a continuous service temperature rating of 500 degrees Fahrenheit. Molding powders are excellent, fine cut granular resins, well suited for a variety of demanding chemical, mechanical, electrical and non-stick surface applications.

Such applications include:

  • Cookware
  • Outdoor Rain Gear
  • Medical Devices
  • And more!

Cryogenic Applications

Cryogenic seals are used with super-cooled media, like liquid hydrogen or compressed natural gas, at temperatures below -238°F and down to -460°F (absolute zero). Cold temperatures like this are rough on a seal because at these temperatures most materials begin to exhibit highly brittle behavior and lubricants typically cannot be used because they will freeze. PTFE seals, however, can handle temperatures all the way down to -450°F and are capable of dry running because of their extremely low friction. PTFE cryogenic seals are used in industries like oil & gas, pharmaceuticals, and aerospace.

High Temperature Applications

PTFE seals work well at the other end of the spectrum, too. They can continue to function in extreme temperatures up to 600°F, and continuous operating temperatures up to 600°F. Note that a filler may be required to enable the PTFE to dissipate heat more quickly. It’s not uncommon to see PTFE seals in petroleum or steam applications where temperatures greatly exceed 200°F.

PTFE is also non-flammable, making it ideal for use in applications such as jet propulsion engines. Where other materials would simply melt under the pressure of constant exposure to high temperature flames, PTFE is built to withstand even the hottest of environments.

The use of seals for high temperature applications include oil and gas industry and aerospace, to name a few.

Chemical Applications

The chemical resistance of PTFE is some of the best on the market. It is stable in most aggressive and corrosive media, including:

  • Acetone
  • Chloroform
  • Citric Acid
  • Hydrochloric Acid
  • Sulfuric Acid
  • Tallow
  • Sodium Peroxide
  • And more!

However, it should be pointed that that PTFE is not chemically resistive to liquid or dissolved alkali metals, fluorines and other extremely potent oxidizers, as well as fluorine gas and similar compounds. Outside of those, PTFE is an excellent choice for applications involving chemicals.

Oil and Gas Industry

Seals are critical for the safe and reliable operation of oil rigs across the globe. Not only do seals need to be able to withstand a wide variety of extreme temperatures, but they need to be able to handle extreme pressures as well. For well drilling, for example, seals need to handle pressures from 345 to 2070 bar (5000 to 30000 psi).

For those reasons, PTFE is an incredibly popular material to make oil and gas seals out of. Because of it’s resistance to heat, cold and high pressure, PTFE can withstand the rigors of oil and gas unlike any other material.

Spring-energized Seals

In order to retain sealing power under extreme temperatures, many engineers and designers go with spring-energized PTFE seals. The spring provides optimal sealing by forcing the lip of the seal against the mating surface and helps to account for dimensional changes as a result of temperature fluctuations.

A highly efficient seal is created as the system pressure increases enough to take over from the spring and engage the shaft or bore. The spring or energized seal assembly provides permanent resilience to the seal jacket and compensates for jacket wear, hardware misalignment and eccentricity. The jacket material is critical in design to assure proper seal performance.

Rotary Shaft Seals

Using PTFE in rotary shaft seals allows them to be able to run at higher pressures and velocities when compared to other materials. They are also able to have tighter sealing, often exceeding 35 BAR and can run at far more extreme temperatures ranging from -64 degrees Fahrenheit (-53 degrees Celsius) to 450 degrees Fahrenheit (232 degrees Celsius).

On top of that, they are:

  • Inert to most chemicals
  • Can withstand speeds up to 35 m/s
  • Compatible with most lubricants
  • Come in a wide range of sizes
  • And more!

Conclusion

PTFE is an ideal sealing material for both extremely high temperature applications and demanding cryogenic applications. It retains its key sealing properties: stiffness, strength, dimensional stability (may require spring energizer), low friction, and chemical compatibility- even in the most aggressive operating conditions.

Need PTFE sealing solutions? Advanced EMC Technologies is the leading provider of PTFE spring energized and rotary shaft seals in the US. Contact us today!

by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

FEP Encapsulated O-Rings

FEP encapsulated o-rings can survive corrosive chemicals and retain their sealing power in extreme temperatures, which is the main reason more and more engineers are choosing them for harsh environment applications. But what makes these particular o-rings special and what options are available for them?

What Makes Encapsulated O-Rings Different?

Unlike traditional o-rings, encapsulated o-rings contain a solid or hollow core that is typically made from a very elastomeric material. The exterior of the encapsulated o-ring is able to protect the encased elastomer from corrosive media that would adversely affect its performance. Together, the core and encapsulating polymer are able to provide a highly reliable seal even in extremely harsh conditions that may involve aggressive chemicals, extreme temperatures, and high pressures.

Encapsulated o-rings can be used in a wide variety of applications, including flanges, swivels, joints, valve stems, pumps, and even rocket engines. They serve as an excellent replacement for solid PTFE o-rings that are just not flexible enough for sealing in the long term. 

Characteristics of FEP

One of the most popular materials for the jacket of an encapsulated o-ring is FEP (fluorinated ethylene propylene), which has several trade names including Teflon FEP, Neoflon FEP, and Dyneon FEP. It is well known for its resistance to chemical attack, low friction, and a wide operating temperature range of -420°F through 400°F.  FEP remains flexible even at cryogenic temperatures, as well. One of its key characteristics is a very low compression set, allowing it to return to its original shape after deformation. FEP is also non-flammable and easy to lubricate.

While FEP is often compared to PTFE (Teflon), there are several key differences to keep in mind. For example, it does have a low coefficient of friction but it is higher than PTFE; at the same time, it still possesses very low friction with minimal stick-slip behavior. In addition, FEP does exhibit better vapor and gas permeability, which could be key for some applications. It is also melt processable, which means it can be vacuum formed, injection molded, and extruded. And, like PTFE, it is easy to clean even viscous liquids from.

FEP is available in FDA-approved grades, is considered a high purity material, and is less expensive than PFA, another commonly used jacket material. Note that FEP is commonly used in applications such as pump housings, medical components, food processing, fluid handling, and chemical processing.

Recommended Cores for FEP

FEP encapsulated o-rings work especially well with FKM and silicone cores, but there are other options available. FKM, which is a fluro-elastomer, has rubber-elastic properties which allow it to reassume its original shape and form after deformation. This results in excellent properties related to compression set. Silicone cores are not as stiff or hard as FKM cores and exhibit very good flexibility, even in cold temperatures. When combined with a hollow core geometry, this additional flexibility means that less energy is needed to achieve a tight seal. They work best for applications that involve low compressive forces.

Cores made from stainless steel, such as SS 301 or 302, exhibit excellent performance at both cryogenic and high temperatures, ranging from -420°F to 500°F. These cores usually take the form of a spiral spring (not unlike spring-energized seals) and exhibit minimum compression set and good resilience. They are not commonly used with FEP, however. EPDM, which stands for ethylene propylene diene monomer, is a synthetic rubber that performs well in temperatures ranging from -58°F to 300°F. Again, this particular core material is not recommended for use with FEP.

Selecting an FEP Encapsulated O-Ring

First, there are limitations associated with FEP encapsulated o-rings. They should not be used with liquid alkali metals and some fluorine  compounds, and should not be exposed to abrasive media such as slurries and some powders. 

They are not suitable for applications that involve high pressures and are limited to static or slow moving applications. In addition, they are not recommended for applications where the o-ring will be highly elongated and end-users should be aware that installation forces will be higher for FEP encapsulated o-rings.

However, experts agree that chemical attack and swelling are among the most common causes of o-ring failure, and the use of FEP encapsulated o-rings can solve both of these issues. FEP with an FKM core is a standard solution with a low compression set, recommended for operating temperature ranges not exceeding -4°F to 401°F. 

Use of a solid silicone core results in better low temperature performance, with an operating temperature range of -46°F to 401°F. A hollow core, on the other hand, involves lower contact pressures and is ideal for sensitive or fragile equipment. 

Conclusion

FEP encapsulated o-rings involve several key advantages, starting with their excellent chemical resistance, which allows them to be used with corrosive chemicals. These o-rings can handle pressures up to 3,000 psi and provide both an excellent service life and reliable sealing, all at a cost effective price. Their reliability and durability also translate to less downtime and better M&O costs. If corrosive media or extreme temperatures are destroying your o-rings, it may be time to consider an FEP encapsulated solution.

Advanced-EMC will work with you to find the encapsulated o-ring solution your application needs, from FDA-approved solutions for use with food processing equipment or a reliable, cryogenically compatible solution for a rocket. Contact us today to learn more.

by Sara McCaslin, PhD Sara McCaslin, PhD 1 Comment

Rocket Engine Seals For Use With Cryogenic Hypergolic Bipropellants

The use of hypergolic bipropellants such as RP1/LOX have proven to be an efficient approach to seals for rocket engine propellant. However, they require highly reliable, leak-proof seals to keep them separate before actual launch, in part because the bi-propellants will ignite when they come into contact with each other. 

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by Jackie Johnson Jackie Johnson No Comments

Fluoropolymers for Injection Molding: Challenges and Solutions

Fluoropolymers are used in many different industries and applications, ranging from medical devices to oil and gas. Engineers often assume that just because a part is to be manufactured from a fluoropolymer that it cannot be injection molded, but that is not correct. Fluoropolymers are not just limited to manufacturing processes such as machining, compression molding, or sintering. 

What Are Fluoropolymers?

A fluoropolymer, as the name implies, is a fluorocarbon-based polymer with multiple carbon–fluorine bonds. Fluoropolymers are known for certain characteristics such as …

  • Very low friction
  • Non-stick
  • Excellent performance in high temperatures
  • High purity and non-toxic
  • Aging is minimal
  • Easy to sterilize
  • Excellent resistance to acids, bases, and solvents
  • Also resistant to microbiological and enzyme attack
  • Good electrical insulating properties

Commonly Used Fluoropolymers

The most commonly used fluoropolymers include …

  • ECTFE (ethylene chlorotrifluoroethylene), trade name Halar
  • ETFE (ethylene tetrafluoroethylene), trade names FluonETFE, Neoflon, and Tefzel
  • FEP (fluorinated ethylene propylene), trade names Dyneon FEP, Neoflon FEP,  and Teflon FEP
  • FPM/FKM (fluoroelastomer), trade names Viton, Tecnoflon FKM, DAI-EL, and Fluonox
  • PFA (perfluoroalkoxy alkane), trade name Chemfluor, Hostaflon PFA, and Teflon PFA
  • PTFE (polytetrafluoroethylene), trade name Teflon
  • PVDF (polyvinylidene fluoride), trade names Hyldar, KF, Kynar, and Solef
  • PVF (perfluoroalkoxy), trade names Teflon PVF, Fluon PVF, and Dyneon PVF

Only some of these fluoropolymers have the properties that allow them to be injection molded. What makes the difference is whether they are melt processable.

Melt Processable Fluoropolymers

For a fluoropolymer to be injection molded, it must be melt processable. And while some materials like PVF may offer excellent properties, they cannot be injection molded. . However, PFA, FEP, PVDF, ETFE, ECTFE, and PCTFE are melt processable and can be injection molded. PTFE can also be injection molded, but it takes an extremely high level of skill and specialized equipment. In fact, just because a fluoropolymer can be injection molded does not mean there are not major challenges.

Common Issues With Injection Molding Fluoropolymers 

Injection molding offers a host of benefits in manufacturing and is a popular choice for a wide variety of components. However, there are certain problems that must be addressed for fluoropolymer injection molding, including high melt temperature, high melt viscosity, high shear sensitivity, and fluorine outgassing.

High Melt Temperature

A very high melt temperature is required to work with fluoropolymers, and the injection molding equipment and molds may reach temperatures up to 800°F. Hot runner systems are also needed, and careful attention goes into the design of runners and gates to encourage even flow of the material. However, the temperatures must remain controlled to avoid degrading the fluoropolymers.

High Melt Viscosity

Another complication lies in the fact that fluoropolymers such as PFA have a high melt viscosity, which is related to how flexible the polymer chains are as well as their degree of entanglement. Polymers that have a high melt viscosity flow very slowly even in their melted form and the melt can actually fracture if it encounters sharp edges or gates and runners that are too small.

High Shear Sensitivity

Another challenging aspect of injection molding a fluoropolymer is their high shear sensitivity. A material that exhibits shear sensitivity changes its viscosity when subject to stress or pressure, which is a problem. The viscosity of the polymer melt will vary significantly as it goes through the various stages of injection molding.

Fluorine Outgassing

Fluorine outgassing occurs when these polymers are melted, presenting another issue because of the corrosive effects of fluorine gas on barrels, screws, nozzles, runner systems, and molds. To make matters more complicated, fluorine gas is also highly toxic. 

Solutions for Injection Molding Fluoropolymers

Solutions have been developed to address the challenges involved with fluoropolymer injection molding, starting with thermal management. 

Thermal Management

Managing the temperature of the polymer melt as it passes through the various stages in the injection molding is vital to keeping the polymer flowing predictably. It also helps to ensure the integrity of the final part by preventing degradation of the polymer melt. The different zones and points have specific temperatures at which they should be kept and this requires highly precise thermal management. 

Gate, Runner, and Mold Design

While any injection molded part requires careful design of the gate, runner, and mold, this is especially important in the context of fluoropolymers. Depending on the fluoropolymer being processed, there are certain key dimensions for gates, hot runner systems, and the mold cavity. There also exist recommendations for the gating systems, such as whether tunnel, sprue, or fan gating should be used. In addition, careful design is needed to prevent issues with melt fracture.

Corrosion Resistant Materials

Barrels, screws, nozzles, runner systems, and molds must be made from materials that are corrosion resistant. The materials for these components have to possess excellent high-temperature material properties including hardness and wear resistance. Ther barrel, screw, and nozzle must be manufactured from special materials. 

For example, barrel material options include IDM 260, Xaloy 309, and Wexco B022 often work well. Effective screw materials are usually certain grades of  Inconel and Hastelloy as well as Haynes 242 alloy. For the nozzle tip there are Hastelloy grades that provide the needed properties.For the molds, materials such as plated tool steel or nickel alloys possess the needed corrosion resistance, thermal performance, and wear properties. 

Venting

Fluorine outgasses can be managed with proper venting, but the materials for the vent must be extremely corrosion resistant. Furthermore, the vents must be kept very clean. In addition to the danger of outgassing, gas trapped within the mold must also be addressed in order to avoid part defects and reduce the maintenance required to keep the molds ready to use.

Conclusion

There are certain fluoropolymers that can be injection molded, but certain issues must be addressed to successfully manufacture parts of the quality and integrity needed. And while there are many companies that are good at injection molding parts, not all of them have the equipment and experience to injection mold fluoropolymers. 

Here at Advanced EMC, we have the equipment and skill to successfully manufacture fluoropolymer parts through injection molding. In fact, we offer engineering assistance to help you select the best type and grade of material and configure your parts to make them as manufacturable as possible. Our injection molding machines range from 75 to 500 ton and we have a Class 100,000 clean room if needed. Contact us today for all your fluoropolymer needs. 

by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

PTFE Spring Energized Seals for Cryogenic Applications

When cryogenic temperatures are involved, a failed seal can have extremely serious repercussions that can include personal safety, explosions, damage to local ecosystems, and highly expensive downtime. One of the most dependable solutions to date for sealing in cryogenic environments is PTFE spring-energized seals. In this week’s blog post, we will discuss PTFE spring energized seals for cryogenic applications!

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by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

PCTFE Ball Valve Seats for Low Permeation Applications

Ball valve seats that show signs of swelling, blistering, or “popcorning” have been permeated at a molecular level. Needless to say, this can cause some serious issues such as leaks and catastrophic failure. The solution is to find a ball valve seat material that is highly resistant to permeation and an excellent choice would be PCTFE. In this week’s blog post, we will talk about PCTFE Ball Valve Seats and how they are used in Low Permeation Applications.

Introduction

Certain types of media may permeate the ball valve seat, leading to swelling, blistering, and leakage. Applications such as chemical processing and petrochemical transport may require a seat material that is resistant to permeation but still exhibits key properties such as low friction, compressive strength, and resistance to deformation is still needed.

How Permeation Works

Permeation refers to the molecular level penetration of gases, vapors, and liquids through a solid material via diffusion. In diffusion, molecules pass from an area of high concentration to an area of low concentration. This can be extremely problematic when a ball valve is being used because of the potential distortion and leaking of the ball valve seat.

Keep in mind that permeation can take place through a surprising variety of materials, including metals and polymers. In addition, some materials are only semipermeable, which means that only ions or molecules with certain properties can pass through the material. 

The rate of permeation is directly related to crystal structure and porosity, which is why factors such as density and molecular structure are important when selecting materials for applications where low permeation is important. 

Why Permeation is a Problem for Ball Valve Seats

Gas permeation can not only compromise gas stream purity but also result in dimensional changes of the ball valve seat. One form of these dimensional changes is swelling, which can occur if the permeating media becomes a part of the molecular structure of the material. In reinforced polymers, such as glass-reinforced PTFE, swelling can cause separation between the glass fibers and the PTFE matrix. 

Another common manifestation of permeation is referred to as “popcorning” or “popcorn polymerization” which occurs due to a polymeric chemical reaction. And among the most notorious source of problems with popcorning and swelling are monomers with extremely small molecular sizes such as Butadiene and Styrene.

Both popcorning and swelling will lead to leakage, and over time popcorning will completely destroy the ball valve seat. This makes the choice of ball valve seat materials extremely important for applications where this is a problem.

PCTFE for Low Permeability Ball Valve Seat Applications

One of the best materials for a ball valve seat application where permeability is a problem would be PCTFE (Polychlorotrifluoroethylene), a thermoplastic chlorofluoropolymer. PCTFE is sometimes referred to as Modified PTFE or PCTFE, as well as by trade names Kel-F, Voltalef, and Neoflon. PCTFE is often thought of as a second-generation PTFE material that maintains the chemical and thermal resistance of PTFE along with its low friction. It is also similar to other fluoropolymers such as PFA or FEP.

One of the defining characteristics of PCTFE is that it has a much more dense molecular structure and a low void and micro-porosity content when compared to similar ball valve seat materials. This gives it a very low permeability coefficient, which means that the likelihood of it swelling or popcorning is far lower than other materials. For example, its permeability for O2, N2, CO2, and H2 are 1.5 x 10-10, 0.18 x 10-10, 2.9 x 10-10, and 56.4 x 10-10 darcy, respectively.

PCTFE also provides improved toughness and strength along with good deformation recovery and excellent creep and cold-flow resistance. In addition, it has a wide operating temperature range of -100°F to 500°F. In fact, it performs extremely well at cryogenic temperatures. Because of its low friction, it also results in a very low ball valve operating torque. PCTFE also exhibits zero moisture absorption and is non-wetting. 

PCTFE works well in operating environments where other polymers may fail. For example, it is well adapted to nuclear service that may involve high radiation exposure, is non-flammable (D 635), and is resistant to attack by the vast majority of chemicals and oxidizing agents. The only chemicals that might lead to slight swelling are ethers, esters, aromatic solvents, and halocarbon compounds.

In addition to its use in applications requiring low permeability, PCTFE is also considered an excellent choice for applications that need a low-outgassing material and is commonly used in semiconductor applications. Also note that there are PCTFE grades that are FDA approved, such as Fluorolon PCTFE 2800. 

Conclusion

Fuel processing and transport, chemical processing, petrochemical systems, and emissions control are just a few of the applications where low permeation materials may be necessary. For such applications, PCTFE is an excellent option for ball valve seat materials because it combines the basic properties necessary for a seat with an extremely low rate of permeation.

If you need a solution to blistering, swelling, or popcorning of a ball valve seat, contact the experts at Advanced EMC. Our sealing team will work with you to find the right ball valve seat material for your application.

by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

When to Use a PTFE Rotary Shaft Seal

Rotary shaft seals are used in a host of applications, including many that involve harsh environments or strict compliance with FDA standards. But when should PTFE be used?

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by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Underlip Temperatures and Rotary Shaft Seals

Many engineers do not realize the impact that underlip temperatures can have on seal performance and service life, especially when elastomeric materials are used. Learn why underlip temperature is so important and what you can do to reduce its impact.

What is Underlip Temperature?

Before we can define underlip temperature, we should start out with sump temperature, which refers to the temperature of the oil/lubricant. The lip of an oil seal moves across a very thin meniscus of oil. Some friction exists between the seal lip and the shaft which can generate enough heat to increase the temperature under the lip of the seal. This temperature at the point where the seal lip and shaft make contact is referred to as the underlip temperature and it can be higher than the sump temperature, especially for higher shaft speeds.

Why Underlip Temperature is Important

If the seal lip material does not possess good thermal conductivity, the heat generated can raise the underlip temperature high enough to exceed the operating temperature limits of the seal. This will result in accelerated wear and eventual seal failure. Signs of such a problem might be a seal lip that is …

  • Cracked
  • Blistered
  • Hardened

That is why it is important to use the estimated underlip temperature, as opposed to the sump temperature, as the expected operating temperature for a seal.

Estimating the Underlip Temperature

The underlip temperature is related to the friction between the seal and the shaft as well as the shaft speed. One way to estimate the increase in underlip temperature (in degrees Fahrenheit) based on shaft speed is to take the square root of the shaft seal speed in feet per minute.

Change in Underlip Formula | Advanced EMC Technologies

Before we look at an example, remember that …

Example 1

Suppose you need to estimate the increase in underlip temperature for a shaft rotating at 2500 fpm. The associated increase in underlip temperature would be ….

Example 2

If a 3” shaft is rotating at 2500 rpm and the sump temperature is 150°F, what would the underlip temperature be?

If the sump temperature is 150°F, then the underlip temperature is the sump temperature + the change in underlip temperature …

150°F + 44°F = 194°F

Faster Estimate

There is a faster way to estimate the underlip temperature, but it is not nearly as accurate: add 20°F for every 1000 rpm of shaft speed. However, the applicability of this estimate is limited to sump temperatures less than 210°F.

What Influences an Increased Underlip Temperatures

The underlip temperature is a function of several different characteristics. These include …

  • Shaft speed
  • Shaft size
  • Surface condition of the shaft
  • Friction between the shaft and seal
  • Thermal conductivity of the seal lip material
  • Oil level

In the chart below, you can see how the underlip temperature increases as the rotational shaft speed increase for a 1” diameter shaft. Even for a relatively small shaft, the increase in underlip temperature is significant.

Sump Temperatures vs Underlip Temperatures | Advanced EMC Technologies

In this next chart, it is evident how the change in underlip temperatures varies significantly as a function of shaft speed and diameter. In particular, notice that the upper limit for a 5-inch shaft rotating at 5000 rpm can lead to a 90°F increase in underlip temperature.

Change in Underlip Temperatures

When specifying a seal for a specific application, the engineer has control over the surface condition of the shaft, the friction between the shaft and the seal, and the thermal conductivity of the seal lip material. 

Reducing the Change in Underlip Temperatures

A smoother shaft combined with a low-friction seal lip material reduces the amount of heat generated at the point of contact, which can help reduce the increase in underlip temperature. 

Furthermore, a material with high thermal conductivity will be more likely to conduct generated heat away from the seal lip, further reducing the increase in underlip temperature.

Conclusion

Polymer seal lip materials such as PTFE and PEEK provide reduced friction, higher thermal conductivity, and better performance at high temperatures than their elastomeric counterparts when used in rotary shaft seals. When elastomeric seals are exhibiting signs of failure due to high underlip temperatures (i.e., cracking, blistering, hardening), then it may be time to consider a change in seal lip material.

At Advanced EMC, our rotary shaft seal experts can help you troubleshoot the cause of premature seal failure and advise you on the best choice of material for your application. Contact us today for more information.

 

by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Rotary Shaft Seals for Automobiles

Even the simplest automobile requires a wide variety of seals, including rotary shaft seals. In this blog post, you will learn the basics of these seals within the context of the automotive industry, including the materials commonly used and why PTFE is so often recommended.

Rotary Shaft Seals

The goal of a rotary shaft seal is to prevent the leakage of oil, grease, and other fluids (e.g., transmission fluid, brake fluid, air conditioning refrigerant) while also keeping environmental contaminants out. These seals, sometimes called oil or grease seals, are used with bearings to keep lubricants within the bearing and environmental contamination out (i.e., lubrication retention). The term rotary refers to their ability to perform in the presence of both rotary and swiveling movements.

Where Rotary Shaft Seals Are Used in the Automotive Industry

Rotary shaft seals are a necessary part of many components and systems within cars, trucks, buses, high performance vehicles, and motorsports. And seals are needed for EVs (electric vehicles) and HEV (Hybrid Electric Vehicles) as well. These seals are also used with ATV (All Terrain Vehicles).  Some of the most common areas of application in automotive transportation are:

  • Air conditioning compressors
  • Braking systems
  • Pumps
  • Gearboxes
  • Power transmissions
  • Steering wheels

In most of these applications, the failure of a seal can lead to serious repercussions that include bodily injury, damage to the vehicle, and danger to those around the vehicle. Because of this, finding the right high quality seal for your application is extremely important.

Automotive Seal Operating Conditions

While the conditions for automotive rotary shaft seals do vary depending on their specific applications, the most common operating environments include …

  • Extreme temperatures
  • Environmental elements
  • Vibration and shock loadings
  • High contamination exposure
  • Chemical compatibility
  • Low friction
  • Wear resistance
  • Compliance with automotive standards

Environmental elements include exposure to sunlight, ozone, UV, and oxidation, all of which can accelerate the degradation of a seal. Contamination can include water, dirt, grease, and other debris, while the seals are likely to be exposed to materials such as diesel, hydraulic fluid, brake fluids, coolants, and chemical solvents. 

Materials Used in Rotary Shaft Seals

The basic components of a spring energized seal include a flexible inner seal lip that is bonded to a rigid outer component. In addition, some of these seals may include a spring energizer to keep the lip in contact with the sealing surface (note that spring-energized seals are most commonly used for oil retention as opposed to grease retention). Furthermore, some applications may require a seal with two lips where one serves as a wiper seal or dust lip to further prevent the ingression of contaminants.

The outer material for a rotary shaft seal is responsible for seal positioning and retention in the seal housing. This part of the seal is typically made from stainless steel, aluminum, or a rigid non-metallic composite material. 

The seal lip itself is made from either an elastomer or a polymer, with high performance PTFE being one of the most commonly used polymers. PTFE meets all the requirements for an effective, dependable seal lip, including the ability to handle high pressures, wide ranging chemical compatibility, extremely low friction, and excellent wear resistance. PTFE can also include additives such as carbon or MoS2 that can enhance properties such as strength, stiffness, wear resistance, and low friction.

Nitrile rubber (NBR, Buna-N) and polyacrylate rubber (ACM) are both widely used elastomeric materials for automotive rotary shaft seals. Another often used elastomer category is fluoroelastomers commonly referred to as FKM, Viton, and FPM. These offer superior performance compared to nitrile and polyacrylate, but they do cost more. 

PTFE Rotary Shaft Seals

At Advanced-EMC, we highly recommend the use of PTFE rotary shaft seals in the automotive industry where possible. They provide excellent performance in the harsh conditions often involved and are the most chemically compatible and low friction polymer on the market today. They can outperform elastomeric materials and are quickly replacing their use in many applications.

There are several grades of PTFE to choose from, including …

  • Virgin PTFE: light-duty service with slow speeds
  • 25% Glass-filled PTFE: wear and extrusion resistant but abrasive to shafts with a hardness less than 62C
  • 23% Carbon / 2% Graphite-filled PTFE: general purpose service where extrusion and deformation resistance are necessary
  • 15% Glass / 5% MoS2 filled PTFE: excellent wear resistance makes it well adapted to higher speed applications
  • Polyimide-filled PTFE: because of its low abrasion, works well with soft materials such as 300 SS and Aluminum
  • Modified PTFE: higher mechanical strength and better wear resistance

Conclusion

Rotary shaft seals can be found everywhere from the air conditioning to the power transmission system on a vehicle. Finding the right sealing solution that can handle the extremely harsh operating conditions and high temperature environments can be challenging, but PTFE has become one of the most popular choices for the seal lip material. 

If you are interested in an effective, reliable sealing solution for your automotive application, contact the knowledgeable team at Advanced-EMC today. Our engineers can work with you to find the right seal, made from the right materials, for even your most challenging designs.