Why the Surface Finish of your Seal's Mating Hardware Matters

What would you do if a sealing system on your semiconductor process equipment just failed prematurely or stopped functioning properly? Your first instinct would be to check if the seal is defective. The most often items on a seal failure checklist are chemistry, process pressure, and local temperature.

However, a few important items are missing from that list. These relate to the gland hardware and their corresponding surface roughness. Such subtle aspects of the hardware design could make or break a seal's ability to function properly, and more specifically could impact the dynamic lifetime of a sealing element.

The term "surface finish" usually refers to the level of polishing or texturing applied to the surface of a part or component. Surface finish parameters can be described by three objective measurements of roughness, waviness, and lay. Roughness is usually composed of smaller irregularities, while the waviness consists of larger undulations. Lay refers to the direction of the dominant grain or texture on the surface. For the purposes of elastomer-based sealing products, roughness alone is often used to account for the finish, quantitatively. Various acronyms are used in describing surface roughness, including Ra, Rsk, Rq, Rku, Rz, RMS, etc. Micrometers or even microinches are the most common surface roughness, and smaller the number the finer the finish.

Die Anforderungen an die Oberflächenbeschaffenheit hängen davon ab, ob die Dichtung aus Elastomer, Thermoplast oder einem anderen Material besteht. The overall malleability of elastomers allows for greater variation conformity in the surface finish. Although load (the weight applied to seal from system elements) and chamber pressure differential would often cause the soft, elastomeric material to conform to the gland's textural constraints, too large roughness of static sealing products can create possible leak pathways for the media. Such leakages can, for instance, introduce ambient air into processes that are highly sensitive to water (as air moisture) and oxygen.

Below is a depiction of the localized variations in surface height and roughness that shows how surface roughness measurements are made in a general sense. Es ist leicht vorstellbar, dass bei einer zu großen Anzahl von Spitzen, wie der links im Diagramm, die Dichtungsmaterialien durch den dynamischen Kontakt abgeschliffen oder abgenutzt würden, ganz zu schweigen von den Schwierigkeiten, sich an all die Schwankungen der lokalen Oberflächenhöhe anzupassen. Also, the valley towards the right of the image demonstrates how a pocket is formed, which could allow trapping of the particle contaminants or lubricant for decreasing the friction through continuous dynamic cycles.

Die Wahl der richtigen Oberflächenbeschaffenheit für die Hardwarekomponenten in Ihrer Anwendung kann komplex sein. Generally, we recommend 8 to 12 microinches Ra (Average Roughness) for dynamic seals, and 16 to 32 microinches Ra for static seals. Die erfahrenen Ingenieure von Greene Tweed können Ihnen helfen, indem sie auf Ihre Anwendungsparameter zugreifen, um Sie zur richtigen Oberflächenbeschaffenheit zu führen.

Additionally, surface finish requirements are different for applications that seal gases or vacuum than those required for sealing liquids. Für die Gasabdichtung oder die Abdichtung gegen ein Vakuum wird eine feinere Oberflächenbeschaffenheit empfohlen, da die Struktur und die Größe der meisten Gase es ihnen ermöglichen, durch kleinere Spalten hindurchzugehen, als dies einer Flüssigkeit möglich wäre.

While surface finish of seal glands is largely defined by "as machined" status, end users can and do impact the long-term condition. Specifically, when seals are removed from the gland and when the gland is cleaned prior to a new seal installation, these are opportunities to introduce roughness via hand tools use and cleaning protocols.

 

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