Aqueous Cleaning for Aerospace
A turbine manufacturing plant phases out an obsolete vapor degreasing system, making the change to aqueous-based cleaning.
Production Machining, Chris
Click Image to Enlarge The aqueous cleaning line can run as long as 6 months without a cleaning solution change, and part processing has improved by 60 percent versus the previous vapor degreasing system.
These two graphs show the FTIR scan of three cleaned parts. The graph on the left includes an overlay of the dirty sample (the large spectrum) compared with the three cleaned samples. On the right is a close-up without the spectrum of the dirty part. The vapor degreased part shows a more significant signal, indicative of the 1.38-mg/part soil versus 1.02 mg/part for EDM 5 and 0.80 for FPI 5.
Aqueous component cleaning and degreasing systems can be operated manually or can be installed as part of an automated work cell.
With the use of more exotic materials and more complex geometries, shops producing parts for the aerospace industry often have found themselves struggling to find the right cleaning process that can help meet the corresponding critical standards. So when a company has a cleaning system in place that works, it can be painful to make a change. But that change isn't necessarily by choice. New environmental regulations often force the hand of a company that then must find a suitable substitute for proven technology.In the following example, regulatory requirements were the catalyst for change, but the need for better cleaning, improved production and reduced cost should also be considerations for a review of existing cleaning methods at any shop.Replacing the OldA manufacturer of turbine and jet engine blades and vanes was being forced by local regulations to replace a vapor degreaser that had been providing the necessary cleaning results. The company contacted Magnus Equipment to help determine a suitable solution for dealing with soils that included EDM oils, machining oils and non-destructive testing (NDT) dye penetrant oils. Because any residual moisture would be detrimental to secondary operations, parts needed to be 100-percent dry.The existing vapor degreaser was effective and simple to operate, but it required a dedicated operator on every shift to keep up with the cleaning demands. So the sheer number of parts was also a major factor when specifying the proper aqueous cleaning equipment.The first plan of action was to determine the best method of aqueous cleaning and the best cleaning chemistry. The company determined that immersion agitation would be most effective for the variety of parts, given the complex part configurations and the internal cavities that needed to be addressed. Magnus then worked closely with an aerospace-approved chemical supplier to develop the optimum process times, recommended chemistry and chemical concentration.The shop then sent some parts to Magnus to run through 85 individual cleaning tests in its full-scale production-sized cleaning equipment laboratory. Fourier transform infrared (FTIR) analyses were per-formed on the full spectrum of parts with the complete range of soils. Some soils (particularly the dye penetrant oils) were most effectively cleaned from the small crevices with a combination of immersion agitation and ultrasonics. Ultrasonics also aided in the rinsing for these parts and these soils.The submitted parts were washed in a 10-mL volume of tetrahydrofuran (THF, inhibitor-free solvent). A 50-µL aliquot of the extract was then placed on a zinc selenide attenuated total reflectance (ATR) cell, and the solvent was allowed to evaporate and was then analyzed by FTIR. The soil from a dirty part was used as a reference sample (28 mg in 20 mL of THF). Quantification of the residue cast on the ATR cell was done by comparing the intensity of the carbon-hydrogen stretching frequencies in the 2,800 to 3,050 cm-1 region of the mid-IR. The peak height absorbance of each cleaned part was ratioed to the calculated number of micrograms of soil per part.After the cleaning process and parameters were determined and the equipment specified, Magnus proceeded with drying tests. Some parts had the potential to accumulate and pool the cleaner and rinse water. For this reason, drying time would be significantly longer than wet processing time. To avoid a bottleneck in part flow, a multi-stage, variable-speed belt dryer was proposed. With an average washing and rinsing cycle time of 5 minutes, a four-stage dryer would provide 20 minutes of drying while a load would still exit the dryer every 5 minutes, keeping up with the production cycle. The system included a number of features to reduce rinse water usage and chemical consumption and to ensure rinse water purity.Valuing the NewThe resulting cleaning line can run for as long as 6 months without changing the cleaning solution. The rinse water purity in the final rinse is maintained below 20 microsiemens, only requiring make-up water from dragout and evaporation. One operator can process the same number of parts in 3 hours versus the 8 hours that were required with the vapor degreaser. The system has been in full production since 2002 with zero part rejection, and because of its success, the customer has since purchased a second system for another plant location.Another Aqueous OptionAdvances in technology have pushed precision machining to extraordinary ends. The aerospace, defense and medical industries all increasingly rely on machined parts produced to demanding levels of quality and consistency that were never possible before. As a result, all three of these industries are also scrutinizing their cleaning systems more than ever before. Surface preparation has always been an integral part of the manufacturing process, but in the highest-value manufacturing applications, the importance of cleaning has dramatically increased.Yet cleaning technology faces challenges apart from the machining technology, because even though modern cleaning technology is capable of similarly impressive results, meeting the cleanliness standards alone is not the only pressing consideration. Health, safety and environmental policies also present challenges. Particularly in aerospace, defense and medical industries, the demand on the cleaning system is not only for results, but also compliance.The SolutionOne system that was developed specifically for these industries is the MecWash AVD (aqueous vapor degreaser), available in North America through Godfrey & Wing. This cleaning system was designed to remove most contaminants including resins, greases, zinc stearate, lapping pastes and polishing compounds. In place of solvents, the aqueous system does this by harnessing the high thermal energy of boiling water, effectively cleaning and degreasing parts that solvent systems may struggle with.Operating at higher than 212°F in a sealed system, the AVD breaks down and removes heavy contamination in minutes. Subsequent vacuum drying reduces the entire required cycle time typically to less than 5 minutes. The high operating temperature also allows the aqueous cleaning solution to penetrate smaller cavities, pores and other openings that otherwise might be unachievable in lower-temperature aqueous cleaning machines.Environmentally ResponsibleThe MecWash cleaning systems are designed to be "environmentally friendly," but that is a relative term. In the case of the AVD and related systems, here is what that means.The first step toward environmental responsibility in cleaning is to eliminate harsh solvents. Aqueous or water-based cleaning systems have been around for years. The problem, however, has been that they can not achieve the same results as solvent-based systems. But that concern has been addressed. Combining rotational cleaning methods with options for sophisticated ultrasonics, flood wash and rinse, dedicated jetting, vapor degreasing, and vacuum dry technologies, modern cleaning systems allow high level component cleaning with aqueous-based detergents.The second step is to drastically reduce the amount of water used in cleaning through recycling of the wash water. Accomplishing this means installing cleaning machines as closed systems, with 100 percent of the water required for washing the parts remaining within the system. The same water can be reused again and again.But even in closed systems, the contamination of the wash and rinse waters eventually reaches a level where it must be changed. Thus, the operator must shut down the washer, remove all of this contaminated water, have it hauled away, and then refill the closed system with new water from the tap. All of this must occur before restarting the machine and cleaning any additional parts. Of course, this change-over process is time-consuming and the expense can add up.That leads to the third step - zero discharge of the wash or rinse water. By attaching a water purifier to any of the high-performance cleaning systems, all of the wash and rinse can be continuously purified. Hence, these wash and rinse waters can be reused indefinitely.
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