How Does the Cleaning and Pretreatment Process Work?
Part cleaning is done by mechanical actions, by application of chemical solutions, or sometimes by a combination of mechanical and chemical processes, like phosphate pre-treatment.
Mechanical cleaning processes include:
- Abrasive blast cleaning, which is used primarily to remove rust, rough surface contaminants, and welding smut. Fine grit (glass, sand, aluminum oxide, or dry ice) is blasted onto parts to remove contaminants and to smooth the surface. Blast cleaning is most commonly done with a compressed air nozzle or a centrifugal wheel. Mechanical cleaning cannot remove oils and greases.
- Mass finishing in a vibratory mill or by part-on-part burnishing
- Grinding or buffing
Chemical cleaning processes generally depend on liquid cleaning solutions (aqueous or solvent). Various processes include:
- Solubilization, in which the contaminants dissolve in the cleaning solutions.
- Emulsification, in which contaminants become uniformly dispersed in the cleaning solution, usually a combination of surfactants, cosolvents, and coupling agents.
- Saponification, in which oils react with alkalis to produce soluble soaps, which can then be rinsed away.
- Wetting, in which contaminants are displaced from the substrate surface by wetting agents that are attracted to the substrate surface and effectively get under the contaminants and break their connection to the surface.
- Complexing, in which cleaning agents called chelators, sequesterants, or complexors form chemical bonds with the contaminants and by so doing detach them from the soiled surface. Complexors also prevent the contaminants from re-attaching to the surface.
Combination cleaning processes combine mechanical and chemical processes. Some of these include:
- Abrasive slurries that include cleaning solutions.
- Abrasive brushing of parts while they are immersed in cleaning solution.
- High- or low-pressure sprays of cleaning solutions that dislodge contaminants by force; these are useful for contaminants that are not easily removed by soaking. Spray washers can deliver varying pressures depending on the type of machine (spiral spray, belt washer, spray strip line, cabinet, or monorail). Some spray machines may also be equipped with rotating brushes.
- Turbo washers or slosh washers in which parts are immersed in turbulent flows of cleaning solutions.
- Defloculation, in which contaminants are broken down into fine particles that can be washed away.
- Ultrasonic cleaning, in which parts are suspended in a cleaning solution or solvent and ultrasonic energy applied to agitate the solution with tiny bubbles that loosen the contaminants.
The type of cleaning and pretreatment solutions, equipment, and processing times chosen for any given situation depend on:
- The size, shape, and material of the parts to be cleaned
- The type of contaminants to be removed
- What sort of cleaning is needed (in-process cleaning and pretreatment or final cleaning after coating)
- The type of final finish that will eventually be applied (painting, plating, anodizing, etc.)
- Volume of work to be processed per hour, and other operational considerations
If necessary, hand tools (brushes, compressed air) and power tools are used to remove loose rust, scale, paint, dust, dirt, and other visible, large-sized loose debris prior to automated cleaning and treatment.
Removal of remaining surface contaminants can be done by wiping with cloths and similar items; dipping into baths or tanks of solvents and other cleaning agents; processing in mechanical washers such as rack lines, barrel lines, spiral washers, or ultrasonic cleaners; steam cleaning; or vapor degreasing. Because simple wiping by hand is labor intensive and is generally not fully effective in preparing the surface for the coating application, most operations use the more efficient and intensive machine methods.
The size and shape of the part will determine how it should be positioned and moved during cleaning and treatment. Parts with holes and irregular shapes may require special handling and treatment to ensure that soil and cleaning solutions are not trapped in cavities and other openings. Small parts can be contained in wire baskets and treated in batches, sometimes in tumbling or rotating machines, immersion barrels, or in ultrasonic cleaners. Larger objects may be sent through a tunnel washer, immersed in a specially-designed tanks or rack immersion devices, or cleaned in place by hand or with mechanical assistance, such as a spray wand.
- Immersion applications are used for parts that have more complex shapes that cannot be entirely covered by spray applications. Immersion can be static (still) or agitated (turbulent).
- Power spray wands are used for large parts that cannot go into immersion vats, cabinet washes, or tunnel washes. Only one chemistry can be used at a time, and the quality of cleaning is dependent on the diligence of the operator. The effectiveness of the spray is influenced by pressure, wand motion, angle, and distance.
- Cabinet or tunnel washes are typically used in mass production operations. They can use immersion or spray processes, or a combination of the two.
Most of these processes require the use of liquid cleaners and are thus known collectively as aqueous cleaning processes.
Heat is an important element in the cleaning process and pre-treatment. Just as washing greasy dishes in hot water is easier than doing so in cold water, heated cleaning solutions clean metal parts produces more quickly and with better results. Heat also helps the chemical processes that occur when the pretreatment solutions are applied, thus forming better bonds for a high-quality finish.
Time is a critical component of every step of the cleaning and pretreatment process, including cleaning, rinse, treatments, and sealing rinses. The chemical solutions have been formulated to be applied for specific times, depending on the characteristics of the parts to be cleaned and the type of soils that need to be removed. The recommended times have been laboratory-tested and must be followed carefully. A cleaning or rinsing stage can be as short as 30 seconds or as long as several minutes or more.
Action or agitation helps the cleaning solutions reach all the soils and dislodge them from the metal surface. Action can be achieved through ultrasound, pressure sprays, agitation of parts (in baskets) within tanks or barrels.
In ultrasonic immersion cleaners, ultrasonic energy is applied to the aqueous cleaning solution, producing bubbles that “scrub” the surface of the immersed part. This method is recommended for precision cleaning of small parts, or those with intricacies such as threads, blind holes, irregular contours, or deep cavities. Effective ultrasonic cleaning requires the right combination of cleaning solution and equipment, matched to the characteristics of the metal and the nature of the soil that is to be removed.