Aligning Parts Cleaning with a Metalworking Program
Effective cleaning of machined or formed parts is a key step in virtually all manufacturing processes, but is too often overlooked.
Underperformance of a cleaning program will result in defects and poor quality of subsequent operations. Over cleaning is costly and wasteful. By addressing the substrates, machining operations and the fluids in the front end of the operation (“soils”), a shop can design a robust, but efficient, parts cleaning process that can be monitored effectively, managed proactively, and able to deliver reliable performance, day in and day out.
The chemistry of machining fluids (coolants) and drawing and stamping fluids can vary based on the severity of the operations. Light duty applications such as milling and drilling on cast iron or stamping light-gauge stock can be performed with lower lubricity fluids. However, with more robust operations, such as broaching, threading or gun drilling, or with more challenging substrates such as stainless steels, hardened steels, Inconel or titanium, high lubricity machining fluids are required. To obtain higher lubricity, formulators, where appropriate, will include higher levels of oil and often use extreme pressure additives to meet the performance requirements. Even straight-oil cutting fluids include additives that affect the cleaning of the resulting parts.
Extreme pressure (EP) additives will significantly boost the lubricity characteristics of coolants and drawing and stamping fluids. EP additives, at higher temperatures and pressures, will plate out on the substrate surface in the cutting or forming process, reducing the coefficient of friction, while depositing residues on the parts. Typically, EP additives are based on phosphorous, sulfur and/or chlorine (in some cases in the form of chlorinated paraffin). These compounds, by nature, are more difficult to remove in typical parts washing programs and need to be addressed in the cleaning process.
Additionally, certain operations create soils that are challenging to remove. Typical grinding fluids contain lower oil content with limited additives or are fully synthetic. This type of compound is easily cleaned after light milling or drilling. However, in the grinding process, especially when the parts are cast iron or sintered metals, the resulting “swarf” can pose cleaning challenges. The graphite components and extremely small chips tend to be difficult to remove in typical parts washers, so enhanced cleaning is required.
With any machining program, a key to success in the cleaning process is coolant management. The coolant concentration should always remain in check, and chips need to be continuously removed from the process. Tramp oils (typically hydraulic fluids and way lubes) must be continuously removed by coalescing or skimming or with a centrifuge. Good housekeeping on the front end ensures more consistent and more efficient cleaning downstream.
Aged coolant or stamping fluids that remain on machined parts for extended time are more difficult to remove, and the potential for corrosion is high. Parts should transfer as quickly as possible from the machining tool to the washers. When parts are machined or formed off site, a cleaning process prior to shipment (along with the application of rust preventive) is appropriate.
Many options are available for a parts cleaning program, including the use of solvent. This article, however, focuses on aqueous cleaning, which can be highly effective for use on most machined or stamped parts.
Recirculating spray washers are the most common washers in high production environments. Belt or conveyor washers can be single or multiple stages and allow for the maximum amount of parts to be processed. Spray nozzles should be aligned for the optimum patterns to ensure that the cleaning compound reaches critical areas. Higher pressures will provide improved soil removal (more “action” by the chemistry). Cabinet washers are also effective, especially if the part is rotated to allow for effective contact of the chemistry.
Immersion washers using racks, baskets or barrels are also effective. Any immersion process sees improved cleaning with added mechanical action. This action can be achieved with circulation pumps or by using eductors in the tanks. The part-to-part contact of a barrel or tumbling program is also beneficial, as long as parts are not damaged in the cleaning process.
Ultrasonic energy substantially improves the cleaning process. Ultrasonic washers use high frequency sound waves to agitate the cleaning solution, creating bubbles that rapidly implode on the part’s surface, effectively dislodging surface contaminants. Ultrasonic cleaning is especially effective on challenging inert soils, such as graphite, and allows for better cleaning in deeply recessed areas such as blind holes and lap seams.
Shops have several choices when selecting an aqueous cleaning chemistry type for their parts washers. Most often, an alkaline cleaner is selected. Alkaline cleaners are effective on most soils (especially organic type) and are compatible with ferrous substrates and most washers. Suppliers are able to formulate alkaline cleaners that can be used on aluminum, zinc (and zinc-coated substrates) and yellow metals (copper and brass).
Acid cleaners can be effective when inorganic soils such as oxides, scale or corrosion need to be removed from the part surfaces. Proper equipment selection is required when acidic chemistry is used.
Neutral cleaners typically rely on higher levels of surfactants or water soluble solvents to provide sufficient detergency. These formulations offer the benefit of worker acceptance and environmental favorability, but can be prone to biological degradation.
The four key parameters of any aqueous cleaning program are captured in the term “TACT” (time, action, concentration and temperature). These are the four essential factors that must be controlled to ensure success. If any one of these parameters is compromised, for example, when the time of the parts in the washer is reduced to increase throughput, one or all of the other three factors must be reviewed and adjusted to maintain a consistent cleanliness level.
Of the four TACT components, shops should focus most heavily on the “action” of their washer programs. Mechanical action aids the cleaning process, which includes spray impingement in spray washers (including nozzle design and alignment), agitation in immersion programs, and ultrasonic action. Regardless of the type of washer, the mechanical action should always be optimized. This is essential when more challenging soils are present.
An aqueous program requires a clean operating environment. While visual inspection is helpful, a test that generates a recordable data point is reassuring from a quality perspective. Care should be taken to ensure that cleanliness tests are performed in a reproducible manner, ideally by the same technician, to ensure valid results. Carbon analyzers, contact angle and residue tests such as Millipore testing can be effective. Simple practices such as checking for a water-break-free surface (using clean water), tape pull testing or white glove testing can also provide valuable data. Cleanliness testing is always relative; benchmark levels need to be established that can be associated with acceptable performance, and the operation should be kept within upper and lower limits at all times.
Washer Control and Maintenance
Aqueous wash systems, like all chemical processes, need to be monitored and maintained. Typically, a titration is performed to determine the chemistry concentration and should be done once per shift. Documenting parameters such as concentration, pH, conductivity, spray pressure and temperature is a good, standard operating practice, and a suitable data logging system must be implemented.
An aqueous cleaner will remove the soils, and in an ideal system, those soils need to be continuously removed from the washer. Filtration can effectively remove insoluble material, such as metal chips or swarf, from the tanks. With ferrous programs, magnetic separators are effective and can be combined with bag filters. Oils can be effectively removed from the washer using skimmers or coalescers. A cleaning compound designed to reject oils aids the soil removal process. The “cleaned” parts must not be removed through a blanket of oil to prevent redeposition of soils back onto the parts.
Even with good housekeeping and maintenance, all chemical programs eventually reach the point when they need to be recharged. Knowing when recharging is required contributes to success. With cleaners, utilizing free to total alkalinity titration can help process efficiency. Many alkaline cleaners are controlled by free alkalinity titrations, so this value will remain constant while the solution is replenished with fresh product. But over time, the total alkalinity will rise in value (typically using indicators with a color change at a pH close to 4, such as methyl orange) as soils build in the solution. The operator can record this ratio value and plan to recharge before it hits an upper limit associated with poor cleaning.
The health of a cleaner bath can also be measured by the amount of emulsified oils in the solution. This “soil load” can be performed by collecting a specific volume in a stoppered graduated cylinder and carefully adding phosphoric or sulfuric acid (for alkaline cleaners) and/or excess table salts. The soils float to the cylinder’s surface, and the volume can be measured. The surfactants in the cleaner may also come out of solution, so the test should be performed when the bath is fresh, comparing the results to future values as they increase.
While many programs use single-stage washers, the addition of a rinse stage improves the cleaning and reduces residues. The rinse stage will flush away the soil-detergent complexes and complete the cleaning process. Rinses should be neutral and overflowed to ensure they remain clean and can be monitored by conductivity.
With any aqueous program, corrosion of cleaned parts is a concern. Corrosion can occur within the washer program, after the cleaning stage is complete, or while parts are in storage. To mitigate corrosion, cleaners can be formulated to include rust preventive agents. These agents can be used within single-stage washers or can be applied after the rinse stage. The rust preventive must be allowed to dry on the parts’ surface, as rinsing will eliminate their effects.
Part drying reduces the potential of surface corrosion. Air blow-off, drying media or ovens are effective in aiding the drying process, as is heating the final stage of the washer.
The potential of corrosion in aqueous washers is increased when “afflictive ions” are allowed to cycle up in the final stage. These are most typically chlorides and sulfates, which are present at various levels in most tap water. As water evaporates from the cleaning stage, these afflictive ions will increase in concentrations. Monitoring the level of chloride and sulfate over time, and establishing upper limits for the process helps ensure corrosion-free production. Use of RO or DI water may be required if levels are significantly high in the tap water. The potential for corrosion is directly related to humidity levels, so the summer months are the most challenging periods. Adjusting the level of rust preventive to compensate for humidity levels is a good idea.
Cleaning and rinsing are the most important steps in any aqueous cleaning or pretreatment program. Having a full understanding of the machining program and the specific challenges that it poses to parts cleaning, a shop can design and operate a parts washing process that ideally fits its requirements. Controlling the chemistry and the filtration process, in both the machining and washing steps, are essential for success. This process includes proactive maintenance (filtration, soil removal and cleanliness testing) and effective troubleshooting. With proper design and operation, a shop can ensure consistent premium quality production at a controlled cost—the foundations for a sustainable and competitive manufacturing program.
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