Although there are many ways to clean parts (pressure washing, steam cleaning, abrasive cleaning, etc.), there are two main methods. The first, aqueous cleaning (which typically entails 10–20 minutes of processing time) requires a part washer system and uses detergents to lift contaminants from the surface of parts. Heat softens the contaminants, and the force of the flowing aqueous solution dislodges the contaminants from the parts. The contaminants are then collected in a removal system. The downsides of aqueous cleaning include leaving a film on the part surface that can interfere with later processes, difficulties removing fluid from crevices, and excessive energy usage in drying parts.

Solvent cleaning (which typically only entails 5–10 minutes of processing time) requires its own equipment as well and involves the use of heated solvents either in an open tank or in a vacuum environment. Solvent cleaning comprises three main steps: exposure to the solvent, rinse, and dry. Parts are exposed to or immersed in the solvent, which is usually boiling to aid in dislodging contaminants. The solvent is then “rinsed” over the part. Lastly, the parts are dried and cooled, with the rinse solvent evaporates off the parts. Solvent cleaning may present environmental and safety issues. Vacuum vapor degreasing has answered many of these issues; cleaning in a sealed vessel reduces residual contaminants.

Source: Dan Herring, The Heat Treat Doctor®, Atmosphere Heat Treatment, Volume 1, pages 583-857.