Emulsion systems are best used when rapid superficial cleaning is required and when some protection by light residual oil film is desired. Because the solvent phase of the emulsion is a petroleum derivative, a thin film is left behind when the rest of the emulsion dries. This film protects ferrous parts from rust and can aid lubrication in applications such as gears or bearings.

Emulsions are also used to remove heavy oils, because the solvent can clean with soil loading up to 50%. It is often considered more for gross cleaning than for producing a clean, water-break-free surface. (A water-break-free surface is clean enough that water runs freely off of it. If impurities such as oil or detergent residue are present, water will tend to bead up and stay on the surface.) The solvent phase of the emulsion is very effective in dissolving oils and grease without attacking the base metal. Thus, an emulsion system should be considered when evaluating the most appropriate cleaning method for:

• Delicate parts with tenacious contaminants, such as buffing and polishing compounds that cannot tolerate any mechanical agitation or impingement. The solvent will dissolve the binding agent, allowing the soils to flush away in a basic immersion bath (followed by an alkaline wash to clean off the emulsion).

• Buffed soft metals: Buffed or polished parts typically can be cleaned with an alkaline detergent but may require pH > 12. Brass and bronze tend to tarnish in solutions with pH > 10. Thus, emulsions have been widely used for buffed soft metals. (Detergents have recently been developed that clean buffed soft metals without tarnishing.)

• Intricate internal cavities contaminated only with oils could be cleaned with an immersion emulsion. Care must be taken to ensure that the emulsion can be thoroughly rinsed unless it is compatible with the subsequent process. For example, in one application, an emulsion was chosen for cleaning of aluminum and brass carburetor parts because it did not plug the needle valve holes or interfere with subsequent gaging.

• Parts that cannot be heated may be suitable for cold emulsion if they have light soils. Emulsions work most effectively when heated to 60 to 80 °C (140 to 180 °F), but they will accomplish some cleaning at lower temperatures. This may be needed, for example, in a totally automated machining cell of tight tolerance parts followed by a coordinate measuring machine, where heat from a detergent washing operation may affect part dimensions.

• Delicate parts in small volumes may be suitable for hand wipe. A cold emulsion may be a strong enough cleaning agent.

• Pigmented drawing lubricants

• Residues resulting from magnetic particle inspection

• Adhesives that may need an organic solvent to dissolve the gum binder

• Multiple-soil and multiple-part applications: Emulsions can clean many different soils on ferrous and nonferrous parts that must go through one cleaning stage. The petroleum residue tends to protect ferrous metals from short-term rust, and it protects nonferrous parts from oxidation. Compromises will still be required in deciding what solution to use to clean off the emulsion residue.

• Longevity: Emulsions can be reclaimed and reused for many cleaning charges. Oils separate and can be decanted off, whereas other contaminants would require separate filtration. Emulsions contain other agents that may be removed in a reclamation process. (Emulsion suppliers can provide information about how to ensure proper regeneration.)

The hydrocarbon solvents in emulsion cleaners are generally safe for use on all metals and plastics. However, some rubbers and synthetic materials may absorb the hydrocarbon and become swollen, which can cause problems if they are being used as seals. Also, the solvent may attack and break down some types of rubber.

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