Probably the most important question to ask about a polymer seal application is the PV value involved.
While this helps to significantly narrow down the list of viable seal materials, it is not enough information to help you pinpoint the optimum polymer for your application. Lets look at some of questions you need to ask to find the best material for your polymer sealing application.
Applicable Standards
Depending on the application for the seal, there may be standards that the material may have to meet. These could include FDA, USP, 3-A, or Norsok. If a material must meet certain standards, this will significantly narrow the field of acceptable materials very quickly.
Mechanical Factors
The mechanical factors are the starting place for considering the best material for a polymer seal.
Questions to ask may include:
1. Is the application static or dynamic?
2. If dynamic, what is the velocity of the shaft?
3. Is the load known?
4. What is the shaft material/hardware made of?
5. What is the surface finish of the hardware?
Cleaning and Sanitation
Many sealing applications in fields such as food, dairy, and pharmaceutical involve stringent cleaning and sanitation procedures that may expose seals to aggressive chemicals. Other cleaning regimens may involve the use of steam, and not all polymers perform well when regularly exposed to steam.
Know the answers to these questions before selecting a material:
6. Will the seal be exposed to a regular cleaning/sanitation routine?
7. How often does the cleaning take place?
8. What types of chemicals are used in cleaning?
9. Is steam used as part of the cleaning process?
Chemical Compatibility
A major factor influencing the choice of an ideal seal material for an application is chemical compatibility. Some polymers like PTFE are well known for their wide chemical resistivity, while others may be susceptible to certain classes of chemicals (e.g., alcohols, acids, fuels).
These are some key questions to ask about the types of media the seal may be exposed to:
10. What types and classes of chemicals will the seal be exposed to on a regular basis?
11. Are there any chemicals that the seal will be exposed to intermittently?
Temperature
Another key factor in selecting an appropriate material is temperature. For example, PTFE and PEEK work better for cryogenic applications than other polymers. Certain polymers are well adapted to high-temperature environments.
Here are some answers you need before selecting a material for a polymer seal:
12. What is the expected continuous operating temperature?
13. Is this considered an extreme temperature?
14. Is the application cryogenic, meaning below -238°F and down to -460°F?
15. Are there any extreme temperatures that the seal may be exposed to on an intermittent basis, such as during a sanitation process?
Environmental Factors
Another key set of questions to ask about involves the operating environment for the seal. For example, in wind turbine applications some of the seals may be exposed to UV light which can degrade certain polymers.
These are some questions to ask about the seals environment:
16. Will the seal be exposed to UV light?
17. Will the seal be exposed to ozone?
18. Will the seal be exposed to abrasive materials, such as sand or grit?
19. Will the seal be exposed to salt water?
20. Does the seal need to be flame retardant or meet certain requirements regarding smoke if it should be exposed to fire?
Conclusion
If you are looking for more than a seal that will simply do the job, then finding the answers to these questions before selecting a seal material is an excellent start. Factors such as applicable standards, cleaning regimens, continuous and intermittent temperatures, chemical exposure, and environmental factors will aid in selecting the optimal material for your application.
Links to more articles about Polymer Sealing Materials and Applications:
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