Cleaning finished parts is an important final step in the finishing process. To ensure the consistent performance of your shop's aqueous cleaning system, the proper equipment is essential. Specifying an optimal system involves interaction between the end user and equipment suppliers. At the outset, however, it's essential for both parties to understand the necessary or desired level of part cleanliness.
The end user must establish some rules that will guide equipment selection and system design. Several baseline factors must be defined by the end user before the cleaning system's parameters can be specified, including the following:
Parts. The end user should define the type of parts to be cleaned, including their minimum and maximum size, overall geometry and material composition. Another important determination is whether, during the cleaning cycle, it's permissible for parts to come in contact with each other, or whether parts must be maintained in specific orientations—or perhaps be fixtured. It must be taken into account if the parts incorporate blind and threaded holes or are chip-laden and incorporate critical surfaces.
Productivity. The user must determine the shop's normal or projected production level that the cleaning system is expected to accommodate. If a large number of parts are processed simultaneously, the equipment should be capable of ensuring that the product flow does not result in part mixing.
Material Handling. How parts will be delivered to the aqueous system is another important consideration. The means of delivery may determine the equipment configuration, as well as the degree of automation required. Parts can feed into cleaning systems in bulk (directly), in baskets or in-line. Material handling also dictates whether the overall equipment concept will be continuous flow, centralized or cellular type.
Acceptable Cleanliness Standard. An acceptable cleanliness specification must be defined by the user. Some aqueous systems are geared only for removal of large particulates and oil, while other systems are capable of achieving high-precision or low-molecular-weight ("low millipore") cleaning criteria. Specifying the cleaning standard and the acceptable cleanliness level in advance is important in determining which aqueous system to use.
Once the end user has defined part geometry, productivity, product delivery and cleanliness specifications, a suitable type of aqueous cleaning system can be specified. Additionally, some factors have important impacts on system selection.
Mechanical Interaction. The parameters defined by the end user are important to determine the type of mechanical interaction that the aqueous system uses. Mechanical interaction is the most important factor in system design and can dictate success or failure of the system. Several forms of mechanical interaction can be employed either separately or simultaneously. These run the gamut from static soak to full agitation with ultrasonics.
By understanding the relative cleaning capacity of each type of mechanical interaction, the user can understand the degree of cleanliness or consistency that a proposed system will be able to achieve.
Chemistry. The specific type of mechanical interaction determines which chemistry will be employed in the aqueous solution. This includes factors such as pH level, concentration/ratio and the ability of this chemistry to interact with and remove particular soils and oils from the parts. The majority of aqueous cleaning chemistries are alkaline, and the percentage of specific chemicals must be carefully controlled to maintain a uniform process. Mechanical interaction will dictate whether this chemistry must be a non-foaming type or whether soil splitting or soil emulsification will occur.
Temperature. Proper temperature of the cleaning solution ensures that the chemical solution is reaching its maximum capability. This temperature must be consistent, and the heat source must be able to compensate for heat lost due to spraying or pumping. Sufficient heat recovery is necessary to allow introduction of cold parts to the solution while maintaining its temperature for proper chemical interaction and drying. The particular heat source used must also be specified.
Time. The time during which components are exposed to mechanical interaction and the chemistry of the solution is also important to achieve acceptable production levels and cleanliness specifications.
Drying. This includes considerations such as how dry components must be after cleaning, whether parts must be dried spot free or whether they need to be dried at all.
Understanding Cleaning Parameters
While part geometry, productivity, part delivery and cleanliness specifications are normally defined by the user, the type of mechanical interaction, the chemistry, its concentration, pH, temperature, time and drying are usually derived through product testing and interaction between suppliers and end users. Satisfying all these requirements is essential for the success of an aqueous cleaning system.
The Aqueous Process
Most aqueous processes require, at minimum, one wash, one rinse and one drying cycle. Depending upon the cleanliness level desired, multiple wash and rinse cycles may be necessary. Wash cycles typically require a heated cleaning solution. This cleaner must be compatible with what's being cleaned and be able to emulsify, or split oils and remove other contaminants.
Rinses remove residual chemicals remaining on parts. If only one rinse is employed, rinse clarity must be maintained by providing a continuous flow of clean water into the rinse tank. A system employing two rinses should consume half the water that a single rinse system will require. Depending upon cleanliness specification and whether spot-free drying is necessary, RO or DI water may be necessary for final rinses. If it is, consideration should be made to prevent flash rusting on ferrous components. Rust inhibitors or wetting agents may be necessary to prevent flash rusting when ferrous parts are rinsed.
All tanks and other surfaces exposed to cleaning solutions should be constructed with stainless steel to resist corrosion. Although mild steel was acceptable in the past, the cost of materials to construct the system is less important than the labor factor. Stainless steel, although three times the cost of mild steel, will have an overall cost impact of less than 20% on the system. Mild steel always oxidizes in time and degrades parts baths. This degrades the system's cleaning ability and contaminates rinse water with ferrous oxides.
While costs for aqueous systems are normally higher than solvent systems, operating and chemical purchasing costs are much lower. This very well could lead these systems to be more widespread.