Liquid CO2 Immersion Cleaning
Dry cleaning technology provides numerous benefits...
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Elements such as environmental concerns, economic competitiveness and technology have converged, causing industry and government to re-evaluate manufacturing processes. Many companies have changed from traditional solvent cleaning to alternative methods, in a move toward zero discharge. Many manufacturers have been directly affected by the phase-out of chlorinated and fluorinated cleaning solvents. Companies worldwide are faced with new challenges since implementing replacement processes. These challenges include equipment and bath maintenance, rinsing and drying issues, rust and corrosion, chemical disposal and new environmental regulatory issues. Moreover, interim alternative solvents and solvent formulations have uncertain regulatory futures.
Along with the many cleaning alternatives, a dry cleaning technology emerged, liquid carbon dioxide (LCO2) immersion cleaning. It has evolved into an effective and standardized cleaning alternative. Key to its successful development was the invention and optimization of the LCO2 process and recycling technology. This development included both regional and federal environmental agency technology demonstrations. Most importantly, successful early development and introduction of this technology involved considerable financial risk and application risks taken by leading manufacturing companies who put the technology on their production lines.
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The LCO2 immersion system, or SuperFuge™, uses the solvent power of liquefied carbon dioxide in combination with patented bi-directional centrifugal cleaning action to effectively remove contaminants from products. The system has two modules, the cleaning module and the carbon-dioxide-recycling module. The principle of operation is similar to that of closed vapor degreasing and immersion cleaning systems.
The product to be cleaned is placed into the cleaning chamber, and the lid is closed. The operator presses the start button on a graphic-display console, and clean LCO2 is automatically transferred from a supply vessel within the recycling system into the cleaning chamber. During and following the fill cycle, the system cleans the product using bi-directional centrifugal agitation for a predetermined cleaning cycle time.
Following the cleaning cycle, contaminated LCO2 is transferred from the system into the recycling system for separation and recovery operations. Clean dry product is removed from the chamber, and the cleaning process is complete.
The recycling system separates contaminants from the LCO2. The contaminants are captured and filtered before disposal. The recovered CO2 is transferred into the supply tank for reuse. The recycling system is capable of recovering 90 to 95% of the CO2 for reuse. The reclamation system has several features, including additive injection and the capability of recovering CO2 off-line or on-line.
Process parameters for the cleaning system include temperature and pressure, among others. Process temperatures generally range from 50 to 70F. Process pressures range between 750 to 1,200 psi. The cleaning system can be operated in a vertical (centrifugal) or horizontal (centrifugal/tumbling) orientation and can use either liquid or supercritical carbon dioxide.
Liquid CO2 used alone under these conditions is a solvent much like 1,1,1-TCE. As such, it will remove many, but not all, contaminants. Contaminants that are not soluble in LCO2 alone can be solubilized or otherwise separated using proprietary additives, modifiers or mechanical adjuncts in the process.
LCO2 immersion cleaning process meets a variety of cleanliness requirements, ranging from visually clean to more rigorous quality standards requiring sophisticated test methods such as non-volatile residue (NVR) analysis or scanning electron microscopy (SEM). Functional testing such as weld-joint porosity and adhesive strength have also been used to evaluate the technology.
DoverTech-Weldcraft Corporation
"Like many companies, we were faced with the challenge of replacing our conventional vapor degreaser," stated Paul Lien, production supervisor. "Initially we looked at aqueous cleaning thinking it was the obvious solution."
The company tested several aqueous-based systems and found they performed satisfactorily. The company's three main concerns with aqueous cleaning were the waste, since it operates within the Burbank, CA, city limits, and the city demands compliance with air and water disposal and emission guidelines. The second concern was the increase in maintenance costs of both the aqueous equipment and the support equipment. The third concern was bath life and maintenance.
DoverTech-Weldcraft was also aware that current wastewater disposal regulations were subject to change at any time. "We require consistently clean parts all day every day," stated Mr. Lien. "We do not have an in-house chemist checking the pH and concentration of the aqueous solutions."
This is why the dry cleaning process using LCO2 was investigated. At first the company feared it could not afford the process; however, it concluded it could not afford not to have it. After some initial startup glitches, the company has run the system successfully ever since.
Production demands require the company to clean components during a 17-hr day six days a week. "We have had no real downtime," Mr. Lien noted satisfactorily. "Our concerns regarding bath maintenance were resolved since the process does not degrade over time. Also, we do not generate any pollution from the cleaning process."
Accra-Tronics Seals Corporation
This company's cleaning requirements are basic, which means clean enough for inspection and for further processing internally or at various vendors. The company has used TCA; however, environmental issues have made this an expensive cleaning process. Because of this, Accra-Tronics wanted a cleaning system with zero environmental concerns.
Because all existing solvent or aqueous cleaning methods require the use of materials that require permits from water and/or air regulatory agencies, Accra-Tronics chose the LCO2 system. "We have had a comprehensive number of production parts tested prior to purchasing the system and found that it meets all of our needs," stated Bill Fisch, president. "It even eliminated potential etching of aluminum and rust on steel."
The LCO2 immersion cleaning process is used for a variety of applications, including cleaning automotive fuel-injection components, safety restraint systems and turn signal components. Other applications include cleaning aircraft fuel injection components, climate control devices, home appliance switching components and items such as ballpoint pen components.
Like all cleaning technologies, this immersion process is not for every parts cleaning application. The correct application of the technology is based on a combination of factors, including capacity needs, cleaning quality desired, equipment cost, cost of operation and environmental issues.
In applications where both cleanliness and production objectives can be achieved, the technology has proven to be mechanically reliable, provide consistent parts cleaning and quality and perform economically. This technology provides environmental benefits, including elimination of wet chemistries, environmental permits, hazardous waste, bath degradation and maintenance, water and air pollution and toxic chemicals.
