Most surfaces that we encounter in industrial cleaning are relatively smooth and contiguous. We have talked earlier about the difficulties of cleaning blind holes, threads, capillary spaces and other challenging configurations. The one surface we haven’t yet explored is that of a material that is itself porous. Sintered materials including metal, ceramic, glass, plastic and others immediately come to mind.
Note – Sintering is a process in which small particles of metal or any of many other materials either isolated or in combination with others are compressed, heated or in some other way caused to fuse together to a greater or lesser degree depending on the application to form a solid shape. For some parts, the fusion is complete rendering a part that is essentially solid and without porosity. This process is an economical way to make odd-shaped metal parts that would be difficult to produce using other means (such as machining) as well as to produce a stronger part overall. Partial sintering yields parts that will absorb or actually allow liquids or particles below a particular size pass through them. Sintered parts impregnated with lubricants are used in some bearings to provide a self-lubricating feature as the lubricant leaches out of the sintered material over the life of the bearing. Porous sintered material is also used as a filter in some applications.
Filters of many varieties including those that are intended to be cleaned and reused, as well as those that are not, offer a cleaning challenge because of their porous nature. As discussed below, some filters are, indeed, able to be cleaned while others are difficult to impossible to clean depending on the material and features of construction. For a little more about cleaning filters, check out this previous blog.
Ultrasonic cleaning has proven effective in cleaning sintered parts and is recommended almost universally, especially for cleaning sintered metal and ceramic filters. Successful cleaning, however, may require a little more than simply plunking the part into an ultrasonic cleaner for 30 minutes or so. Cleaning a porous part provides something of a dual challenge. Even if the part only needs to be “superficially clean,” there is always a potential for contamination seeping out from the interior and re-contaminating the surface over time.
When cleaning a porous part, the ideal situation is to use a method that will cause cleaning liquid to penetrate and pass through the porous piece. Without liquid flow, contaminants interior to the piece will not be removed easily if at all. Sprays, turbulation, agitation and even ultrasonics frequently fail to reach the interior of a porous part.
When the contaminant consists either partially or wholly of particles, as in the case of filters, the challenge is further magnified as particles become trapped or “wedged” in the porosity. Here, the solution is to provide flow in a direction opposite to that which caused the particles to become trapped in the first place. In the case of filters, this is not difficult. Simply flow through the filter in the direction opposite to that normally seen. If the direction of flow is indeterminate a flow originating from many directions (as in rotating the part) will probably give the best results.
In cases where it is not possible to force liquid to flow through the porous material, similar results can be achieved by multiple dipping of the part allowing the absorbed liquid to drain out between dips. This can also be assisted by the application of a vacuum (using a vacuum hose or other direct means of connecting to the part) between immersions.
Under no conditions should cleaning a porous part be considered a “no brainer.” And, for the same reasons that these parts are difficult to clean, cleanliness testing is similarly difficult. Remember, in order to find contaminants, one generally needs a cleaning method that is better than that used for production cleaning. In the case of porous parts, there may not be a better means available. Extraction using a vacuum or long periods of immersion offer the most likely candidates.
– FJF –