Manufacturing Critical Seals via Pulsed Electrochemical Machining
As engines powering the energy, aerospace, and automotive industries are incentivized to increase their work output while simultaneously reducing the required energy input, demand has steadily risen for seals, gaskets, valves and other critical components that can withstand more extreme temperatures, higher pressures, and can insulate more energy, more effectively. To meet these design and manufacturing objectives, engineers must therefore increase usage of temperature-resistant alloys like refractory alloys, improve part features, tolerance, and surface quality, and, crucially, be able to affordably and reliably manufacture parts with these improved materials and features in higher part volumes. Voxel’s pulsed electrochemical machining (PECM) technology can play a role in improving production efficiency of manufacturing high-pressure, temperature-resistant seals for applications requiring low friction coefficients. In this article, we will review how PECM can directly improve the lifespan and efficiency of certain seals via superfinishing, create complex features on special seals, and ultimately repeat this process for high part volumes, allowing manufacturers to improve the capabilities of critical seals –and thereby next-generation engine systems-- powering a variety of critical industries.
If you’re unfamiliar with how our unique technology works, see our introductory article on pulsed electrochemical machining (PECM) technology first!
Surface quality & Seals
A unique advantage of PECM is how it can reduce manufacturing steps by machining and finishing parts in a single process without requiring a secondary step. PECM is capable of achieving surface quality down to .005-.4um Ra on a variety of tough-t
o-machine materials, including some exotic alloys
used in critical seal manufacturing (such as Inconel). PECM also does not produce any burrs or recast layers on a given part, which could otherwise produce unnecessary friction in critical seals. These superfinishing capabilities can be particularly useful for the seal manufacturing industry for a variety of reasons. First, applications requiring airtight, hermetic seals (such as within certain semiconductor electronics, or applications involving toxic substances) can directly benefit from superfinished surfaces made by PECM. Even if a given seal has the ideal proportions, lubrication, material, and compression necessary to fulfill its role, a non-ideal surface finish can drastically affect the part’s performance by allowing leakage.
Adequate surface quality is important for all seal applications, but is especially crucial for seals involving high-pressure (and/or toxic) gases, as opposed to liquids—the decreased viscosity of gases allow them to pass through microscopic indentations easier than liquids. This may be especially important for applications involving low-viscosity gases, such as propane or hexane, as well as gases of smaller molecules, such as hydrogen and helium, the latter of which being often used in leak testing as a measure of a good seal.
Consequently, this leakage can not only affect the seal's own durability and performance, but may also pose a safety or environmental hazard for the parts or processes the seals are serving. While no material, surface finish, or geometry can fully eliminate the possibility of leakage, the potential for leak paths along the groove of critical seals is drastically reduced in some applications with an improved, burr-free surface quality—one that that PECM is capable of producing.
There is also an important distinction regarding surfaces and geometries between seal applications, namely, between static and dynamic seal types. Static seals, a seal which is used between two surfaces with no relative motion between each other, should not have any grooves, and would require tight tolerances and high surface quality.
Dynamic seals, however, are used when there is motion between surfaces. These seals may sometimes want grooves in their surface to capture oils from the lubrication and ultimately reduce friction.
While some dynamic seals may discourage the use of superfinished surfaces (as the lack of microscopic gaps on a smooth surface may work to repel some necessary lubrications), other seal applications directly benefit from superfinishing.
Consider how some temperature-extreme environments affect the viscosity (and therefore the capabilities) of certain lubrications. For example, high temperatures within gas turbines or industrial exhaust systems, low temperatures within industrial cryocooler systems, and some environments that involve both extremes, such as in space, can be too inhospitable for most seal lubrications. As lubrications cannot be used in these environments to reduce friction, seals must instead rely on superfinished surfaces to minimize unnecessary contact.
These burr-free, superfinished surfaces are achievable on many materials via PECM and not only work to prolong the part’s own lifetime, but ultimately reduces replacement costs and also helps prevent problems causing safety or environmental hazards — especially crucial in petrochemical or space-related applications.
Another prerequisite for parts operating in these temperature-extreme environments is the use of temperature-resistant materials, including refractory metals and superalloys—which can be more challenging to machine, compared to more conventional materials in seals often used in non-critical applications (such as the food and beverage industry). Fortunately, as PECM does not rely on friction or contact to machine a given material, it is ultimately indifferent to material hardness—allowing PECM to machine temperature-resistant nickel superalloys at a similar speed to copper or aluminum.
Grooves and features on hydrodynamic seals
Aside from its finishing capabilities, PECM has an additional benefit for the critical seal industry—improving part designs.
Hydrodynamic seals, a subset of dynamic seals, are a unique type of seal that is challenging to conventionally machine but may be easier to manufacture with PECM. While the specific design of hydrodynamic seals may vary depending on the application or manufacturer, generally, they have a pattern of grooves on their face that, when rotated at high speeds, creates a small air pocket around the seal that provides both additional insulation capabilities, and works to reduce the seal’s friction coefficient, prolonging its lifetime or reducing energy consumption. These seals are generally used in extreme pressure and temperature flux environments-- ranging from cryogenic temperatures to >1000°F (>537°C) and some able to withstand pressures up to 50psi (compared to more conventional dynamic seals generally operating in pressures between 3-10psi).
Uniform grooves on hydrodynamic seals are generally around ~10um deep. These features become particularly challenging to machine in high volumes on tough materials such as various stainless steel grades and Inconel. Fortunately, PECM has machined somewhat similar features in these tough-to-machine materials. For example, Voxel produced <0.075mm or <0.003" thick walls with a 20:1 aspect ratio on stainless steel for a microchannel heat exchanger application.
PECM could either be used as a primary or secondary machining operation to create any uniform groove design in a hydrodynamic seal, and could repeat this process for thousands, if not tens of thousands, of parts without incurring tool replacement costs. While current conventional methods (including laser ablation, chemical etching, and media blasting) can machine these features adequately, producing uniform, shallow grooves on exotic materials in high part volumes may prove challenging and costly—particularly for industries increasingly using hydrodynamic seal types, such as the pulp and paper industry.
Continued Miniaturization and High Volumes
Aside from the increased usage of temperature-resistant materials and a need for tighter tolerances and improved surface quality, miniaturization of critical components is another obstacle for manufacturing critical seals. Fortunately, PECM may offer a solution to this as well.
The aerospace, automotive, energy, and medical device markets are increasingly adopting smaller parts in critical systems. Some major factors influencing this trend include minimizing weight to optimize fuel efficiency, maximizing available space for other purposes, and (more unique to the medical device industry), improving patient comfort.
An example of medical device miniaturization can be found in many cardiovascular devices, which have dramatically downsized in the past decades. Pacemakers, for instance, can now be manufactured to be the same size as a vitamin tablet, and can last up to a decade implanted. However, many components within these pacemakers are difficult to manufacture, including small glass-to-metal seals used to help power the lithium batteries in pacemakers and other small electronics.
Another example of seal miniaturization can also be found in two-phase heat exchangers cooling power electronics within hybrid electric aviation, electric vehicles, servers, high-power sensors, and other applications. in turn downsizing the various seals necessary in these systems to maintain necessary temperatures, pressures, and insulation.
Fortunately, PECM does not utilize heat or contact to machine parts, therefore it is able to machine sensitive, thin-walled features of small parts that may otherwise be sensitive to thermal distortion common in conventional machining processes. For example, Voxel was able to machine <0.075mm thick walls with a 20:1 aspect ratio on stainless steel. Thin walls, tighter tolerances, smoother surfaces and smaller features are likely to be increasingly prevalent as seals, alongside the larger parts they are serving, downsize.
Interested in learning more about PECM technology, its capabilities, and future potential? See our education portal, or contact us.