Sealing is a very complex science by itself as it involves many physical aspects, including mechanical design, materials science, surface science and fluid behaviour. Armin Reicharz reports.
For applications requiring hermetically sealed connectors - like vacuum processing equipment, pressure vessels or continually immersed devices - some parameters need to be carefully taken into account to achieve hermeticity.
Different levels of sealing exist. They should be adapted to the operating conditions, in which the equipment will be used. Installations having to withstand dust or water ingresses usually need to be environmentally sealed. Applications requiring gas tightness need a higher degree of protection; they are generally hermetically sealed.
To be referred to as hermetic, a system has to be designed to avoid its content leaking out or gas leaking in over an extended period of time. The effectiveness of a hermetic barrier is calculated in leakage rate values. Leak rates quantify the amount of gas flowing through the barrier every second and are expressed in mbar.l/s or atm.cm3/s. Hermeticity typically concerns leakage rates below 10-6mbar.l/s.
A typical hermetic connector requires several sealing barriers. Some advanced sealing techniques, like the ones developed by Fischer Connectors and described below, enable to exactly adapt the sealing performance of a connector to the level of protection it needs. To achieve such flexibility, each critical area of a connection - the panel interface, the contact block and the connectors interface - is protected by its own independent seal.
The panel seal (A ) is placed at the interface between the receptacle housing and the panel or equipment housing. It plays an important role for hermeticity because it covers a large cross section.
The protection of the contact block is ensured by a combination of two seals (B and C), a gasket sealing the junction between the contact block and the receptacle housing, and an advanced polymer compound covering the rear of the block and sealing the contacts.
Finally, the connectors interface is also protected by a seal (D). Not participating in making the system hermetic, it is dedicated to prevent ingress of water or harmful particles in the connecting area where male and female contacts mate.
Testing enables to guarantee that a connector is hermetic. For instance, all Fischer hermetic connectors are submitted to a 100 per cent quality screening test, using helium as tracer gas.
What are leakage and diffusion?
There are two majors mechanisms by which gas can get through a hermetic barrier: leakage and diffusion. Gas leakages are caused by material defectiveness, such as cracks and defects in the barrier. Even tiny, those imperfections ultimately result into the failure of the device. On the other hand, diffusion also called permeation is a natural process; induced by pressure difference, gas can migrate through solids, even without any defects in the barrier. Diffusion is generally known to occur in most plastics and rubber materials. Being a natural phenomenon, it can be limited but not fully eliminated.
The extent of both leak and diffusion mechanisms depends on the size and mobility of the gas molecules. For example helium molecules are very small and can easily penetrate even the tiniest cracks, while as a rule of thumb the penetration of Nitrogen (or air) is about one third of the helium value.
Material selection is very important in the design of hermetic applications. As explained above, the diffusion effect is correlated to the material used. Additionally, there is also another phenomenon linked to the nature of material: outgassing, also called desorption. Connectors are designed with many materials - such as insulating plastics, sealing resins or elastomeric seals - which naturally absorb a small quantity of water. Over time, or when environmental conditions change, the water molecules and other solution gases contained in the material itself are released. This is called outgassing and, while an environment is transformed in vacuum for example, it increases the gas load during the initial pumping phase. Heating the material can accelerate the outgassing effect.
To minimise both diffusion and outgassing effects, materials used in connector design have to be of high performance. Many materials were tested by Fischer Connectors Engineering team, and some - showing low permeation, outstanding chemical and high temperature resistances as well as low outgassing - proved extremely reliable. To achieve a leakage rate below 10-8mbar.l/s, Fischer hermetically sealed connectors are carefully engineered with state-of-the-art materials. For most receptacle seals, Viton - a fluoropolymer or FPM - is used as standard. The polymer compound used for the contact block sealing is epoxy resin.
Additionally, customised material solutions can also be developed for systems operating in very specific conditions.
The design and installation of the connectors play an important role in hermetic sealing. Indeed, gas can get trapped between assembled components. This gas, when getting released, causes what is called virtual leaks. Such virtual leaks typically result from air caught in bores, screws or between parts having large surfaces in contact.
Moreover, when installing a connector onto a hermetic system, it is important to check that no gas is trapped in the mounting area. Efficient panel sealing can only be achieved if the contact surface is correctly prepared. No excessive torque should be required to mount the connector, which generally implies the surface flatness to be <0.05mm. Following the O-ring shape of the seal, circular machining grooves will contribute to improve the contact between the two surfaces. And finally, the edges of the panel cut should be clean and free from burrs.
In conclusion, to ensure hermeticity and avoid leaking systems, the key parameters to consider are the exploitation of advanced connector sealing technologies, a careful selection of the materials used, optimised connectors designs and a precise installation. Manufacturers specialised in sealed connectors can contribute to the performance of hermetic equipments. Indeed, each project being different, a comprehensive analysis of the specifics of the application is highly recommended.
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Armin Reicharz, Engineering Director at Fischer Connectors, Saint-Prex, Switzerland. www.fischerconnectors.com