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.
Les critères de l'état de surface peuvent varier en fonction de la fabrication du joint en matière élastomère, thermoplastique ou autre. 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. On peut facilement imaginer que s'il y a trop de pics comme celui du côté gauche du diagramme, la matière des joints s'userait et s'abîmerait suite au contact dynamique, sans parler des difficultés à s'adapter à l'irrégularité de la surface. 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.
La sélection de la finition de surface optimale pour la pièce de votre application peut constituer une opération difficile. Generally, we recommend 8 to 12 microinches Ra (Average Roughness) for dynamic seals, and 16 to 32 microinches Ra for static seals. Les experts ingénieurs de Greene Tweed peuvent vous aider à évaluer les paramètres de votre application afin de déterminer la finition de surface optimale.
Additionally, surface finish requirements are different for applications that seal gases or vacuum than those required for sealing liquids. Pour les cas d'étanchéité au gaz ou au vide, une finition de surface plus lisse est recommandée car la structure et la taille de la plupart des gaz leur permet de s'échapper par des espaces restreints, même si ces derniers suffiraient à contenir un liquide.
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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