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Sealing a solution Part 2
Seals are one of the most important components in many medical devices. While small in cost, seals have a profound effect on the function of a medical device and the outcome of a medical procedure. Engineered sealing solutions have advanced to meet the new medical device designs due to new materials and to new processes for producing these seals.
In the second half of his series, Ted Ahrenholtz discusses the fundamentals of seal design, tools available to assist manufacturing, and common pitfalls in seal design and construction.
Part 2 of 2
Testing the design
Many tests can evaluate a seal design’s performance characteristics before it goes into production. Surface friction is one of the most important variables affecting seal performance. Friction is a very complex subject influenced by many factors including the lubrication state, material modulus, surface finish, temperature, geometry of the part, and amount and direction of relative forces. Seal designers are therefore very focused on friction reduction.
When there is a force pressing two surfaces together, and they are moving past each other as in a seal, it is impossible to calculate or predict the frictional force with accuracy. It can only be measured through experiment and the results are expressed as a coefficient of friction (COF). COF is used for comparison because it is a sealing system measurement rather than a measurement of the property of the material. In order to measure COF accurately, ASTM D1894 is used as a standardized test. (Chart 1)
Dynamic vs static COF
The energy required to start motion (static COF) is different from the energy that it takes to maintain motion (dynamic COF). The difference between static and dynamic COF will vary tremendously by material and application.
Note that the COF and relative difference between static and dynamic COF can be substantially reduced by surface textures, surface coatings, and the presence of fluid. In most dry seals, for example, there is usually a stick/slip action where the seal flexes to accommodate a mating surface’s movement and then pops back to a stable state. If there is fluid present, then the cannula may hydroplane, resulting in a significantly reduced COF.
Surface finishes and coatings of the material can substantially reduce the COF. Intuitively, we think that a rougher surface has greater friction. While this is true for large surfaces, this is not the case on a micro scale. In many applications, a matte finish can greatly reduce the amount of friction. This is because the surfaces begin to ride on top of each other. However, attention must be paid to ensure surface finish does not contribute to leakage around the seal.
The greatest friction reduction is accomplished by surface treatment or coatings applied to the material to reduce the COF. Not only will this reduce the COF in general, but it also reduces the difference between the static and dynamic COF.
Determining what process or coating will be applied is completely dependent upon the materials selected for the seal and mating part. An example can be seen in Chart 2. Note that chlorination of butyl does not result in the friction reduction seen with chlorination of the polyisoprene. This is because the chemical structure of butyl polymer is non-reactive with the chlorination process. Not only will the selection change depending upon the type of elastomer, but it is also important to consider biocompatibility and shelf life. Biocompatible forms of polytetrafluoroethylene (PTFE), Parylene, plasma treatment, chlorination, and other proprietary coating processes can reduce seal friction by up to 90%.
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