How Clean is Clean?
The keys to cleaning are knowing the nature of the item to be cleaned, the nature of the soil to be removed, the number of items to be cleaned, and what will be done with the item after it is cleaned. When this information is known, an appropriate cleaning method can be chosen. Sometimes the choice is not to clean.
It is important to consider that a perfectly clean part will cost more to produce, cost more to maintain cleanliness, generate more waste during cleaning, and often functions no better (and sometimes worse) than a part cleaned to a less lofty standard. Perfectly clean is also perfectly impossible to achieve.
The Importance of Specs
Pass/fail criteria are the best way to specify cleanliness. For example, a critical area of the part in question could be examined under magnification with a ten by ten millimeter (mm) field broken into 100, one millimeter squares. A maximum number of machining chips could be specified. For example, the part will be acceptable if no more than three machining chips are visible within the grid, with the largest chip no larger than 0.5 mm in any direction. The presence of oil, dust, or unidentified foreign material could also be specified in a similar manner.
Other pass/fail tests could be devised of varying sophistication. The importance of
specifications is to insure that the part is suitable for use.
Functional specifications can also be applied indirectly. Cleanliness of painted parts can be specified in terms of paint adhesion. If a specific level of adhesion is specified for the part, the level of cleanliness of the part must be such that paint adhesion will not suffer.
It should be noted that manufacturers have found that some items require no cleaning prior to painting. Certain casting techniques, oil-free machining, and proper selection of paint can allow parts to be used without any type of cleaning prior to painting. An important aspect of no-clean methods is prevention; if parts do not get dirty then they do not need to be cleaned.
Cleaning Methods
Common cleaning methods are usually some combination of mechanical, aqueous, or solvent cleaning. When selecting a cleaning method, choose a method that will generate minimal waste. This is especially important if either the cleaning media or the materials cleaned contain hazardous materials.
Blast Cleaning Blast cleaning can be an effective method for removing coatings, scale, oxidation, and other soils. However, blast cleaning can generate a significant amount of potentially hazardous waste, depending on the type of blast media used and the substance being removed. A
blasting medium that generates minimum waste is steel shot. Steel shot can be collected and reused many times, reducing disposal costs. In addition, air knife and carbon dioxide blasting systems also generate little waste. Each system is useful for a particular situation.
Aqueous Cleaning Methods Water is the primary ingredient in aqueous cleaning systems enhanced by various detergents, acids, bases, heat, agitation, and abrasives. Aqueous cleaning can be used on almost any type of material from electronics to precision bearing assemblies.
Spray wash systems range from small, self-contained cabinet systems for single part or batch cleaning, to large continuous processing systems with numerous washing, rinsing, and drying stages. Aqueous dip tanks are sometimes used to clean parts and often use ultrasonic vibrations to improve the solvency of the water and detergent. Dip tanks are also used for hand cleaning of parts. Vibratory and tumble cleaners use abrasive media to remove burrs and radius corners of metal parts while cleaning.
When using an aqueous system, minimize water use and contamination, and choose cleaning chemicals carefully. Many companies have installed expensive aqueous cleaning systems to avoid the use of solvents only to find that wastewater problems are equally as severe.
Semi-Aqueous Cleaning Solvents are sometimes added to water to improve
cleaning or reduce cost in semi-aqueous cleaning. For example, N-methyl pyrrolidone (NMP) can be added to water for cleaning jobs that do not require the full strength solvent. Depending on the particular solvent used, semi-aqueous cleaning can be accomplished using the same methods as aqueous cleaning. Flashpoint, air emissions, worker exposure, waste treatment, and disposal are considerations when choosing a semi-aqueous method.
Solvent Cleaning Solvents are used either in pure form or blended to remove coatings or degrease components. Solvents are used in hand wiping, dip tanks, spray washing, laundry dry cleaning, and vapor degreasers. As in aqueous cleaning, cleaning can be enhanced by agitation, ultrasonic vibration, and heat.
MEK (methyl ethyl ketone), toluene, xylene, Freons, 1,1,1-trichloroethane, and per-chloroethylene are some traditionally used cleaning solvents. They are also either toxic or ozone depleting chemicals and heavily regulated. For these reasons, many companies are seeking other methods of cleaning such as aqueous or blasting methods. Solvents that are very toxic, have a low flashpoint, or are ozone depleting should be avoided.
Once solvents become too laden with soils to clean, they can either be disposed of off-site or recovered by filtration or distillation. Distillation and filtration are good ways to reduce disposal and operating costs.
T-7-97
