How Clean Is "Precision" Clean?

Machining is only one of many production stages prior to assembly for most automotive components. A number of secondary processes, including cleaning, are required before assembly can ensue. Improper part cleaning can adversely affect downstream processes such as heat treating and plating. It can also lead to powertrain failures if debris is released into an engine, transmission, fuel delivery system or steering assembly. It is for these reasons that the automotive companies’ component cleanliness standards have become more stringent over the years.

So how clean is clean enough for the automakers? According to Garth Weston, technical director for Proceco Ltd. (Quebec, Canada) who provided the information for this article, it depends largely on the application. Clearances between mating parts or the size of an orifice that a fluid must flow through are two examples. Automakers generally require that, after final part cleaning, critical components must not release particles larger than 250 microns, or a collection of particles weighing more than 1 milligram.

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These particulate levels are typically determined through a residual particle analysis, which is commonly referred to as a Millipore test. In this test, a cleaned part undergoes a manual washing procedure to collect any remaining debris still attached to the part. The debris released from the part after this test cleaning is weighed and/or examined under a microscope to verify that the initial cleaning process meets particulate specification.

Specially engineered aqueous cleaning equipment is normally necessary to obtain these cleanliness levels. A precision cleaning system may incorporate a combination of power spray, power flush, rotation with immersion, ultrasonics and possibly precisely aimed solid-stream nozzles directed at part orifices or areas where contamination can become trapped. Though such cleaning methods will vary by application, there are certain general system attributes that a precision cleaning system should possess.

Components such as the piping system, cleaning/drying enclosures and workpiece handling systems should be made from stainless steel. Otherwise, corrosion from those components could detach and recontaminate a cleaned workpiece. Basic filtration is necessary to protect and prolong machinery and cleaning solution life, but the full flow of any solution that will contact the workpieces must also be properly filtered. High-efficiency cartridge filters on pump discharge trap fine particles more efficiently than standard bag filters. There should be no drastic bends in the system’s piping because these bends could trap contamination. In addition, the number of threaded fittings should be minimal, as they can introduce contamination over time. Critical spray piping and manifolds should be designed for easy removal for their routine cleaning. Multiple cleaning stages may be required if part contamination levels are high. Other ancillary equipment could also be necessary. This might include a demagnetizer for more efficient particle removal from ferrous materials and special manifolds to remove debris from internal washer surfaces so that they are not deposited on parts. In addition to the cleaning modules, equipment for material handling, cooling/drying, plating, deburring and quality assurance may also be part of a turnkey cleaning system.