by Jackie Johnson Jackie Johnson No Comments

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. 

Critical Factors for Rocket Engine Seals

There are four critical factors for rocket engine seals that apply regardless of the type of propellant used: safety, weight, cost, and cost per kilogram of payload. 

Regardless of whether space flight is privately or federally funded, safety remains the main priority. The seals used in rockets and rocket engines must be highly reliable with predictable behavior for every possible environment in which they will be operational, including the temperatures, pressures, and media involved. Seals in general have been problematic for rockets in the past, and accidents with hypergolic bipropellants are certainly not unheard of, with spills having occurred at Johnson Space Center, White Sands Test Facility, and Edwards Air Force Base. 

Weight has been a major concern in aeronautics and space flight for decades. The weight of the rocket is directly related to the fuel required for launch, and the combined weight and fuel reduces the payload that can be carried.Payload is a determining factor for the commercial viability of the rocket. Lightweight seal materials with high specific stiffness and/or specific strength are in high demand.

Cost is another major factor, and some approaches to sealing within rocket propulsion systems are more expensive with determining factors including the type of seal (e.g., spring energized vs traditional seals) and the material involved (PTFE, Torlon, Silicone) . For custom solutions, the size, manufacturing method, and production run are also key. 

Also, keep in mind that the commercial viability of a rocket is often determined by cost per kilogram of payload. Striking a balance between reliability, weight, and cost to achieve a lost cost per kilogram of payload is a challenging aspect of specifying rocket seals.

Hypergolic Bipropellants

Bipropellants use a mixture of two propellants, a fuel and an oxidizer, and fall into one of two categories: hypergolic and non-hypergolic. Hypergolic bipropellants will  ignite when the oxidizer and fuel come into contact with each other, while non-hypergolic propellants require a separate ignition source.

For example, the last several iterations of Blue Origin (which recently took actor William Shatner into space) rocket engines have been using the follow the following bipropellants:

  • BE – 2: Kerosene + Peroxide
  • BE – 3PM: Hydro-LOX (Liquid Hydrogen + Liquid Oxygen)
  • BE – 3U: Hydro-LOX
  • BE – 4: LNG-LOX (Liquified Natural Gas + Liquid Oxygen)
  • BE – 7: Hydro-LOX

Bipropellants that include liquified oxygen, hydrogen, or natural gas are considered cryogenic because of the extremely low temperatures required to keep these materials in liquid form.

Cryogenic Hypergolic Bipropellant Seal Challenges 

When cryogenic hypergolic bipropellants are used, the cryogenic portion must be stored at extremely low temperatures (e.g., storing LOX before it is fed into the MCC). More conventional sealing materials such as traditional elastomers and uncoated metals do not provide the necessary performance at the cryogenic temperatures involved, in part due to the brittle behavior exhibited at extremely cold temperatures.

To further complicate matters, these cryogenic bipropellants will come into contact with seals as they travel through the various stages of a rocket, including compressors, pumps, ducts, joints, manifolds, and valves. And after ignition, extremely hot temperatures are involved. 

Specifying and Designing Rocket Engine Seals

In the context of cryogenic hypergolic fuels, there are some specifications that must be carefully considered, in addition to typical seal characteristics. These include …

  • Wide Operating Temperature Range
  • Lower Temperature Limit
  • Thermal Cycling
  • Cleanliness
  • Chemical compatibility
  • Wear
  • Low Friction
  • Surface Finish of Glands and Grooves

Seals that come into contact with oxidizers need to be resistant to the aggressive effects of oxidizing liquids, including long-term exposure. In addition, for seals exposed to either extreme heat or cold, their change in dimensions must also be accounted for during the design phase.

Rocket Engine Seal Solutions

One of the most popular options for rocket engine seals are spring-energized seals, which include a spring-energizer that keeps the seal jacket in contact with the sealing surface in a wide range of environments where traditional seals would fail. 

Another potential seal option would be encapsulated o-rings, which have proven themselves in a wide variety of cryogenic applications. These o-rings have a stainless steel, FKM, or silicone energizer encased within a durable, chemically compatible material. 

As to the most commonly used materials with spring-energized seals and encapsulated o-rings, both FFKM and PTFE provide excellent mechanical and chemical characteristics that suit the harsh operating environment of rocket engine seals in general, and cryogenic hypergolic bipropellant fueled rock engines in particular. Traditional elastomers often do not have the needed performance at cryogenic temperatures, which is why polymer or perfluoroelastomer materials are often preferred. 

FFKM, trade name Kalrez or Viton, is a perfluoroelastomer that offers excellent performance in the extremely harsh environments of rocket engine seals. It is highly resistant to the effects of oxidizers and has very good chemical compatibility. Another commonly used material for hyperbolic bipropellant sealing solutions is PTFE, better known by the trade name Teflon. It provides outstanding chemical compatibility, extremely low friction, and the necessary mechanical and thermal characteristics to provide reliable sealing in extreme environments.

Conclusion

When hypergolic cryogenic bipropellants are used in rocket applications, factors such as safety, weight, cost, and cost per kilogram of payload must be balanced with the various challenges involved with designing and specifying reliable seals. Two potential solutions to the issues related to rocket seal design are spring-energized seals and encapsulated o-rings, both with jackets of FFKM or PTFE.

If you are looking for a rocket sealing solution, let the engineers and experts in the Advanced EMC seal group lend their knowledge and expertise. They will work with you from the early design phase onward to find the seal type, geometry, and materials you need for a successful design. Contact them today!

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!

https://www.dailymotion.com/video/x12638r_teflon-spring-energized-ptfe-seals-by-advanced-emc_tech

Cryogenic Applications of Spring-Energized Seals

There are a host of cryogenic applications that depend on spring-energized seals. In the medical field, they are indispensable for MRI (Magnetic Resonance Imaging) equipment. In space applications, spring-energized seals can be found in equipment for radio astronomy and infrared telescopes as well as rocket propulsion systems. LNG fueling systems and compressors depend on them, as well as speciality gas manufacturing. Spring-energized seals are also needed in both pharmaceutical and medical research and can be found in scientific instrumentation for a wide range of disciplines. They are also critical for many food, dairy, and pharmaceutical applications.

But why do so many cryogenic environments require the use of a spring-energized seal?

Sealing Issues at Cryogenic Temperatures

The temperature range for cryogenic applications ranges from below freezing at -32°F down to absolute zero at -460°F. At these cryogenic temperatures, many seal materials begin to behave unpredictably, often exhibiting stiff or even brittle behavior. And changes in temperatures will cause dimensional changes in the seal, often compromising the integrity of the seal. To complicate things further, media at cryogenic temperatures may be chemically aggressive toward certain seal jacket materials. Finally, lubricants are usually prohibited at cryogenic temperatures because of issues with freezing, which means that a suitable material should be low friction and dry running.

Using the right sealing solution, however, can provide a reliable, gas-tight sealing system. And that, in turn, supports compliance with applicable safety and environmental regulations. 

Spring-Energized Seals

Unlike traditional seals, spring-energized seals include an energizer that applies a near-constant load throughout the circumference of the seal. This allows the lip of the seal to remain in contact with the mating surface in a variety of situations, including …

  • Eccentricity
  • Out of round 
  • Misalignment
  • Wear
  • Pressure fluctuations
  • Temperature fluctuations

In the context of cryogenic applications, spring-energized seals are used to maintain contact with the surface during the dimensional variations that result from temperature changes. In addition, spring-energized seals can be used in both static and dynamic applications, including rotating and/or oscillating movement.

Spring-Energizers Suitable for Cryogenic Temperatures

Spring energizers come in many different geometries, but for cryogenic applications, metal V ribbon springs are typically used. V springs, also known as cantilever springs, are used in extremely harsh operating environments and work extremely well in both cryogenic and vacuum pressure applications. 

A key feature of metal V springs as an energizer is their ability to provide a moderate yet very consistent load over a wide range of deflection. This aids in securing the lip of the seal against the mating surface even during dimensional changes due to wide temperature variations. For cryogenic environments, the spring-energizer is typically manufactured from either stainless steel or Inconel, Elgiloy, or Hastelloy.

However, in some instances, elastomeric o-rings can be used as the energizer as opposed to using a metal spring. O-ring energizers are durable and work well under a wide range of temperatures, but are best used when metal must be avoided in an application. 

Media Involved in Cryogenic Applications

As discussed earlier, there are a wide range of applications that require highly reliable sealing solutions. Spring-energized seals are excellent at maintaining seal integrity under such conditions, but thought must also be given to the seal jacket material, which will be in direct contact with media at cryogenic temperatures.

The most typical media of concern include …

  • LOx (Liquid Oxygen)
  • LHE (Liquid Helium)
  • LH2 (Liquid Hydrogen)
  • LAR (Liquid Argon)
  • LN2 (Liquid Nitrogen)
  • Liquid Xenon
  • LCO2 (Liquid Carbon Dioxide)
  • LNG (Liquid Natural Gas)
  • LPG (Liquid Petroleum Gas)
  • LMG (Liquid Methane Gas)
  • Various refrigerants and coolants

When a spring-energized seal is being specified, it is extremely important to select a material that not only has excellent properties at cryogenic temperatures but is compatible with the chemicals involved.

PTFE Spring-Energized Seals

One of the most widely used seal jackets for cryogenic applications is PTFE, better known by the trade name Teflon. PTFE provides excellent performance at a range of operating temperatures, including cryogenic, as well as pressure fluctuations. Its wide operating temperature range is complemented by a wide operating pressure range that includes vacuum pressures.

Virgin PTFE has the lowest coefficient of friction of any solid material, and even with the addition of filler materials it still remains extremely low. Lubricants will not be needed when a PTFE sealing jacket is used because it is self-lubricating, dry running, and exhibits no start and stop behavior. PTFE is also the most chemically compatible polymer available, solving the problem of chemical resistance issues. And for food, dairy, and pharmaceutical applications, PTFE is available in FDA-approved grades.

Conclusion

Where reliable sealing is critical in the presence of cryogenic temperatures, PTFE spring-energized seals are a proven solution in applications ranging from the rocket propulsion systems to MRIs. If you are looking for the right seal that offers superior performance in a cryogenic operating environment, contact Advanced EMC today. Our team of sealing experts can guide you in the process of specifying the right kind of cryogenic PTFE spring-energized seal.

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?

Industries Where Rotary Shaft Seals Are Used

A good place to start would be looking at some of the industries that depend on the reliable performance of PTFE rotary shaft seals:

Certain characteristics become apparent from this list, such as exposure to corrosive and aggressive chemicals, low friction, reliability, extreme temperatures, and high purity. 

When to Consider a PTFE Rotary Shaft Seal

There are certain circumstances under which PTFE is the material of choice for a rotary shaft seal:

  • When there is aggressive media involved
  • When low friction or dry running is needed
  • When applications involve high speeds
  • When thermal stability is critical
  • When FDA/USDA compliance is necessary
  • When high temperatures are involved
When there is aggressive media involved

PTFE is the most chemically compatible seal lip material on the market, with the main exceptions being rare fluorinated compounds and certain alkali metals. In addition, some halogenated and organic solvents can be absorbed by PTFE and cause temporary (and minor) changes in dimension. On the other hand, it is compatible with chemicals such as acetone, hydrochloric acid, sulfuric acid, citric acid, tallow, and sodium peroxide.

When low friction or dry running is needed

The coefficient of friction for PTFE ranges from 0.04 for virgin PTFE(the lowest for any material currently in existence) to 0.19 for 15% Glass / 5% MoS2. In addition, PTFE is self-lubrication, low coefficient of friction, and lack of stick-slip behavior results in significantly reduced breakout torque.

When applications involve high speeds

PTFE rotary shaft seals perform extremely well in high-speed applications with shaft surface speeds up to 35 m/s. PTFE also has the lowest coefficient of friction of both polymers and elastomers, making it perfect for seals that must have an extremely low coefficient of friction. This can be a critical factor in high-speed rotary shaft seal applications where significant heat can be generated between the rotating shaft and the seal lip. 

When thermal stability is critical

The maximum service temperature for PTFE is around 500°F and it has the highest melting point of all fluoropolymers, which is why it is used extensively in the oil and gas industry. However, it is often a material of choice for cryogenic applications down to -64°F because of its ability to maintain both its strength and elasticity at low temperatures.

PTFE also possesses a low coefficient of thermal expansion, ranging from 3.8×105 for 25% glass-reinforced PTFE to 5.5×105 for virgin PTFE. Because of this, seals made from PTFE are able to maintain their dimensional stability in operating conditions that can involve significant temperature changes.

When FDA/USDA compliance is necessary

PTFE is available in several different grades that are compliant with strict standards related to food, dairy, and water:

  • FDA 21 CFR 177.1550 for fluoropolymers
  • (EU) 1935/2004
  • 3-A sanitary standards 18-03 and 20-27
  • NSF/ANSI standard 61 for drinking water systems

PTFE also holds up extremely well to the intense cleaning and sanitation procedures that such seals may undergo, including hot water, steam, and aggressive cleaning compounds. In addition, PTFE is also hydrophilic, which can prevent water and moisture from being trapped around the seal during cleaning.

PTFE Grades for Rotary Shaft Seals

The most common PTFE grades used for rotary shaft seal applications are:

  • Virgin, which works well for slow rotary light duty
  • Glass-filled, which enhances strength and wear resistance but should only be used on shafts with high hardness 
  • Glass MoS2-filled, which increases wear resistance and strength without the abrasiveness of glass-filled
  • MoS2-filled, which increases wear resistance and life but should not be used on shafts with low hardness
  • Carbon-filled, which will increase wear resistance with less impact on the coefficient of friction
  • Carbon and MoS2-filled, which increases wear resistance, enhances high-temperature performance, and maintains the capability of dry running

Other Benefits of PTFE Rotary Shaft Seals

There are other benefits to using PTFE for rotary shaft seals, such as their wider temperature ranges and longer life span when compared to elastomeric seals. They also have low outgassing, good electrical insulation properties,  They are also inert to most chemicals and not only compatible with most lubricants but also self-lubricating).

If you are considering a PTFE rotary shaft seal, contact Advanced EMC today. Our team of experienced seal experts will work with you to determine is PTFE is the right material for your application.

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.

 

by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

All About Automolding

PTFE is an excellent material for many different applications and operating environments. Its low friction, chemical compatibility, and ability to maintain key properties at extreme temperatures has made it ideal for everything from seals in sterile food handling equipment to sleeve bearings in the harsh world of oil and gas

And there are several different options when it comes to manufacturing PTFE components, but not all methods are the same. In this blog post, we will be discussing auto molding in the context of manufacturing PTFE components.

What is Auto Molding?

Auto molding, as known as compression molding, is a popular manufacturing technique for making thermoset and thermoplastic parts–and one of the oldest plastic forming methods still in use. In short, auto molding uses compression and dies to form a near net shape polymer part. 

Where is Auto Molding Used?

There are numerous industries that depend on auto molded parts, such as aerospace, chemical processing, and the manufacture of semiconductors.

Auto molding is used to manufacture a wide range of parts, as well. These include …

  • Bearings
  • Bushings
  • Piston rings
  • Sleeves
  • Seals
  • Gaskets
  • Valves
  • Valve Seats
  • Diaphragms
  • Bellows
  • Electrical components

When compression molding of a PTFE part is done correctly, then you can depend on key aspects such as specific density, strength, elongation, and flex life as well as permeation resistance.

How Auto Molding Works

In the auto molding process, the raw materials are in the form of molding compounds. These molding compounds may be preforms (which is already shaped somewhat like the final part), granules, or putty-like masses. 

The basic design for the mold is usually generated from a 3D CAD file, and the tool and die maker will then base the mold design on that file. However, the mold designer must account for shrinkage, molding compound flow, size and positioning of channels to carry away excess material, and achieving uniform curing temperatures for the part. It is also important to ensure that the part can be removed from the mold, and there may be a need for ejector pins to achieve this. Needless to say, the mold is the most expensive aspect of auto molding.

Once the mold design is complete, it is manufactured out of steel using a CAD/CAM (Computer-Aided Design / Computer Aided Manufacturing) system and a CNC (Computer Numeric Control) milling machine. Additional features and surface finishes may require post-processing of the mold.

Once the mold is ready to go, the amount of compound needed for the part is carefully measured out and placed in the pre-heated open mold cavity. Once in place, the other side of the mold closes over the mold cavity and pressure is applied (most often by a hydraulic ram) in one direction to force the raw materials to fill up all cavities within the mold. Any excess material is carried away from the mold via overflow grooves.

Heat and pressure are both maintained until the polymer has completely cured. Once the part has cured and cooled, it is removed from the mold, and this part of the process may require the use of part ejection pins to completely free the newly cured part from the mold. After the part is removed, any flash can be easily trimmed away and precision machining can be used to ensure the part meets necessary tolerances.

Auto Molding PTFE

There are certain key aspects to auto molding PTFE compounds, including …

  • Pressure (usually between 3,000 and 4,500 psi)
  • Sintering temperature (in the range of 685°F – 720°F)
  • Dwell time (how long the part is held at the sintering temperature)

Pressure and dwell time are dependent on the volume and geometry of the part, as well as the machine being used.

Benefits of Auto Molding 

Auto molding PTFE has numerous benefits:

  • Range of geometries and shapes are possible
  • Can produce larger parts than possible with extrusion
  • Minimal waste material
  • Very cost-effective when compared to injection molding
  • Good surface finish
  • Close tolerances
  • Avoids defects associated with machining a polymer (e.g., internal stresses, warping)

In addition, auto molding can be used with both virgin and filled PTFE. Fillers can include both those that enhance structural and material properties (e.g., carbon fiber, molybdenum disulfide MoS2) and colors. Depending on the size and geometry of the part, it may be possible to compression mold multiple parts simultaneously. In such cases, a multi-cavity die would be used.

Disadvantages of Auto Molding

However, there are pros and cons to every PTFE manufacturing process. In the case of auto molded PTFE, the production speed is slower compared to injection molding (due to longer cycle times). Flash will always form and needs to be removed before the part can be considered finished, and this can also add a bit to the production time. While compression molding can be used to manufacture complex parts, there can be issues, such as underfilling in certain areas and the inability to achieve undercuts.

Auto Molding Costs

The most expensive aspect of auto molding, aside from the machinery needed, is the compression molds. Mold cost depends in part on the size of the component, but is more heavily influenced by the complexity of the die. The more complex the geometry of a part, the more expensive the die will be. However, the cost of a compression molding die is significantly less than that of an injection molding die. This is mostly due to the fact that compression molds do not require a complicated system of gates and runners that are necessary in injection molds.

Conclusion

Auto molding works well for manufacturing PTFE components that are not overly complex, have no undercuts, and involve a medium to large production run. In addition, the auto molding process is generally far more cost effective than injection molding. However, for PTFE parts to be high quality and durable, you need a company that is familiar with the process.

At Advanced EMC, we have the knowledge and experience to assist you with auto molding PTFE parts for your applications. Contact us today if you have any questions or are interested in obtaining a quote.

by Jackie Johnson Jackie Johnson No Comments

The Mexican Plastics Industry During COVID-19

With COVID-19 spreading across the globe, one company in Mexico City has created a rotomolded solution to help curb the infection in one of the world’s largest food markets. In fact, Mexico’s plastics industry has played a large part in COVID-19 relief.

The Mexican plastics industry is a relatively young one. However, it has quickly become one of the fastest developing sectors in the country. In 2016, the country was the tenth largest worldwide plastic producer, with a market of $33 billion.

Because it is so new, the Mexican plastics industry has the unique advantage of having new machinery that is on par with, and even superior to, those in Europe, where equipment could be 50 years old. Which means there are fewer problems to contend with which in turn allows for rapid production and innovation when the country needs it most.

So, in this week’s blog post, we will cover how the Mexican plastics industry has combated COVID-19.

Hospitals and Hand Washing Stations

The Mexico City based company Grupo Rotoplas SAB de CV has teamed up with toilet cleaning brand Harpic as well as the Mexican Red Cross to install a field hospital at the Central de Abastos, an open-air food market in the borough of Iztapalapa in central Mexico City, which was considered in the early days of the pandemic to be one of the main sources of contagion in the city. The hospital has been able to provide tests and care for sufferers of COVID-19.

Developed in the 1970s, Iztapalapa is one of Mexico City’s sixteen municipalities, and with a population of 1.8 million it is the most populous. The borough is heavily working class, and the people of Iztapalapa have been still working on the streets even while the threat of the Corona virus looms. When COVID hit the Central de Abastos, it spread rapidly, with one testing center testing 15,000 people with 1,347 testing positive.

In response, in addition to the field hospital, Rotoplas has installed multiple hand washing stations across the market, which covers 800 acres, as well as hanging banners offering advice on preventative action. Because of this it is estimated that 12,000 people have benefited from the new sanitary measures.

Rotoplas is best known for manufacturing large water tanks. They have a 27 product lines operate a score of manufacturing facilities across North and Central America and employs about 3,000 people.

From Coca-Cola to Face Masks

Like most of the world, Mexico has had a hard time procuring masks for hospitals and other medical facilities. One company, food-grade PET recycler PetStar SAPI de CV, stepped up to the plate by donating around 212,000 20-calibre face shields made from 1-million plastic bottles, which in turn were donated by Arca Continental, a company that is part of the Mexican Coca-Cola industry.

The process of recycling plastics is a labor intensive one. The plastic must first be washed to remove impurities that could impede operation. Then the plastic is fed into shredders which break down the plastic into much smaller pieces. These smaller pieces can be processed into the next stages for reuse. Before that, they must be checked again for any remaining impurities and given a second wash. After the plastic is further tested and identified by class and quality, they are melted down and crushed together to form pellets. These pellets can then be molded to form items such as face shields.

Recycling plastic bottles not only helped provide more masks to frontline workers, it also kept plastic bottles out of landfills and the ocean, where they could linger for generations. Not only did PetStar provide life-saving face masks, they also helped reduce plastic waste by recycling bottles.

Much Needed Protection

Dow Inc. contributed 25,000 protective gowns to the health sector of Mexico. For healthcare professionals battling COVID-19, isolation gowns are among the most used personal protective equipment right behind masks. And like masks there has been a shortage. In response, DOW, Inc. collaborated with nine key partners across a multitude of industries to develop donate 100,000 isolation gowns to frontline workers in Texas, Louisiana, and of course, Mexico.

Michelle Boven of Dow, Inc. had this to say on the subject:

“Many companies have shown tremendous ingenuity and speed in changing over production to meet the needs for respirators, masks, face shields, hand sanitizer and other products critical to fighting this pandemic,” said Boven from Dow. “With the accelerated product development, testing and certification of these medical gowns, Dow is proud to be among these innovators and we will continue to look for ways to use our vast material science expertise to address the needs of frontline workers around the world.”

Other Examples

  • Chemical manufacturer Alpek SAB de CV donating 500 gallons of hand sanitizing gel to public hospitals.
  • Polyethylene maker Braskem Idesa S.A.P.I. donated 12 metric tons of PE for the production of one million bottles of disinfectant.

In Conclusion

Mexico’s plastic industry is relatively young, but still booming despite the pandemic. It has expanded rapidly to become a $33 billion industry. With the increase in growth the Mexican plastic industry has been in the unique position to help the people of Mexico withstand the deadly pandemic.

Whether it’s providing lifesaving protective equipment or simply placing banners with tips on how to stay safe, the plastics industry has stepped up to help during these uncertain times.

by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

The Basics of Ball Valve Seat Materials

Ball valves play a critical role in controlling the flow of fluid and pressure within a pipeline, but their effectiveness and safety is only as good as the seat material used. In this blog post, we are going to review the basics of five commonly used ball valve seat materials.

Ball Valves

Whether found in a petrochemical application where a leak could be environmentally devastating, or in a pharmaceutical laboratory where cleanliness and sanitation are critical, ball valve seats must be reliable and robust. A ball valve consists of the body of the valve, the body cap, the stem, the hollow ball, and the round ball valve seat. 

The ball valve seat is responsible for sealing the fluid inside and uniformly distributing the seating stress. In soft seat ball valve designs, either an elastomer or polymer is used as the seal and are inserted into a metallic seat ring. This approach, as opposed to hard seat ball valves, is popular because it provides good sealing action, is lighter weight, and more cost effective. 

Key Properties of Ball Valve Seat Materials

When choosing a polymer material for a ball valve seat, there are numerous factors that are involved. Key material properties include …

  • Sufficient ductility to provide a reliable seal
  • Dimensional stability to ensure the ball valve seat retains its shape for reliable sealing and performance
  • Very low friction to keep stem torque at a minimum
  • Low coefficient of thermal expansion so that the ball valve seat retains its shape when temperature changes occur
  • Excellent wear resistance for a long service life
  • Chemical compatibility with all media involved 

In some operating environments, it is also important that ball valve seat materials exhibit these properties:

  • Low moisture absorption to prevent dimensional changes in the presence of water or high humidity
  • Maintain performance with repeated sterilization that can include hot water, steam, and harsh cleaning chemicals
  • Good performance in the presence of sudden decompression (i.e., pressure drops over 650 psi)

Recommended Materials for Ball Valve Seats

There are several materials that work well as ball valve seats, including acetal, PEEK, PTFE, TFM, and UHMW-PE.

Acetal Ball Valve Seats

When aggressive environments are involved, Acetal (also known as Delrin) is often used. Acetal provides excellent wear resistance, is very rigid, has good toughness, and is resistant to cold flow. Although its operating temperature range is not very wide (-70°F to 180°F), it can handle pressures up to 5,000 psi. Acetal also works well in radioactive environments  but should not be used with oxygen flow.

PEEK Ball Valve Seats

PEEK offers excellent chemical resistance, very low friction, self-lubrication, and is flame retardant while also possessing a wide operating temperature range (from -70°F to 550°F). It can handle very aggressive applications and works well when there is a need for hot water and steam exposure–but does not do well in the presence of sulfuric acid.

In addition, PEEK is very well adapted to nuclear applications and is available in FDA-approved grades as well as filled grades with improved wear properties and better thermal conductivity. Note that PEEK is usually chosen for ball valve seats when the operating temperature range is outside that of PTFE.

PTFE Ball Valve Seats

PTFE (also known by its trade name, Teflon) has many of the same properties as PEEK, but involves even lower friction, dry running capabilities, and more extensive chemical compatibility. Like PEEK, it is available in FDA-approved grades and can handle cryogenic temperatures down to -50°F and high temperatures up to 550°F as well as pressures up to 5,000 psi.  \

Also like PEEK, PTFE can continue to perform even when repeatedly exposed to hot water and steam. Keep in mind, however, that PTFE does not perform well in the presence of fluorine or alkalies. PTFE is also very easy to clean and available in glass or carbon-reinforced grades that can provide improved wear characteristics, less propensity to cold creep, and lower thermal conductivity. 

TFM Ball Valve Seats

TFM (sometimes referred to by the brand name Dyneon) is a second-generation PTFE material that combines the best properties of PTFE (low friction, chemical resistance, high-temperature performance) with better stress recovery and the ability to handle higher pressures. It is also more elastic and resilient than PTFE. The operating temperature of TFM ranges from -100°F to 450°F and it is well adapted to applications involving steam and thermal fluids.

UHMW-PE Ball Valve Seals

UHMW-PE, which stands for Ultra-High Molecular Weight Polyethylene, has a low coefficient of friction, an operating temperature ranging from -70° F to 200°F, good chemical resistance, good dimensional stability, and good abrasion resistance. In general ball valve seats made from UHMW-PE can handle pressures up to 1,500 psi and can handle low to medium levels of radiation exposure.

Conclusion

Ball valve seals are used in many different industries, including chemical processing plants, oil and gas operations, manufacturing facilities, food preparation, and even residential use. As a leak-proof means of pressure and flow control, their smooth and reliable operation is critical–which is why polymer materials work extremely well for ball valve seats. If you are in the market for a ball valve seat material, contact the experts at Advanced EMC. We can put our years of experience to work for you, helping you select the right material for your project.