Cleaning with CO2
Process offers performance at significant cost, time and space reductions.
Introduced in the 1980s for cleaning precision surfaces in highly controlled environments, carbon dioxide (CO₂) cleaning installations are achieving cleaning performance results in open-cell automotive assembly plants at a fraction of the cost and floor space of traditional aqueous cleaning and drying lines.
Nelson Sorbo, PhD, who heads R&D at Cool Clean Technologies, LLC (coolclean.com), says the patented process starts with liquid CO₂ being injected into a capillary condenser where it is transformed into solid CO₂ particles of variable size and density. These are then mixed with a heated and pressure-regulated inert propellant gas. The resulting CO₂ spray can be adapted to deliver particle impact pressures as high as approaching 10,000 psi without damaging part surfaces. Delivered under pressure, the solid CO₂ “sublimes,” or turns into gas on contact with the part surface, which provides the cleaning energy and allows for a total “dry cleaning” process.
“This cleaning spray can be CO₂ only or can be combined with an integrated additive injection to modify the chemistry, and can further be combined with atmospheric plasma and ionized air de-stat systems to provide additional surface-treatment capability,” Sorbo says. “Because the cleaning spray generated is designed to clean without condensation, no special component heating or cleaning zone segregation is required.”
Sorbo’s company recently installed an integrated CO₂ spray cleaning system at the paint line of a North American supplier that produces various ABS plastic parts. It is configured as a dual robot system in the cleaning room just upstream of the entrance to the paint booth.
- 3-D cleaning of both sides requir
- Cleaning system must accommodate existing fixture on racks on 40 x 60 in.
- Required cleaning time: 45 seconds per rack
- Surface must be clean and dry after cleaning
- Surface must be free of all release agent and remains after cleaning
- Must have no negative impact to visual appearance or function of parts after they are painted
Contaminants of concern:
- hand sweat
- dirt and dust
- mold release agents
- tool oils
The cleaning system selected includes two CO₂ spray modules with four nozzles each; custom-designed nozzle assembly; and two 6-axis robots designed to actuate the nozzle assembly for cleaning both sides of the parts. The system was designed to use the identical part-mounting racks and fixtures used in previous operations with custom cleaning programs developed to accommodate the requirements of this alternative cleaning technology. The fluids for the CO₂ spray process were made up of CO₂ supply (low pressure: 250-350 psig), a pneumatic CO₂ gas booster and two four-nozzle custom CO₂ spray modules. Based on production results, the cleaning system is effective in removing the target contaminants. Further, eliminating the need for hand wiping of plastic parts prior to painting resulted in cost savings of less than 6 months’ return on investment.
Typical aqueous paint line cleaning systems consist of multi-stage conveyor spray washers—typically three to five stages followed by a water rinse. Rinsing is followed by a conveyor tunnel drying system most often laid out in a serpentine manner. Some wash lines are 400 to 500 feet long. Drying time typically takes over 30 minutes. Operational costs to run these lines include:
- Electricity to run blowers for drying and power washers
- Heat for drying ovens, in some cases requiring up to 2 MM BTU/hr
- Inlet and outlet water treatment costs,
- Consumables such as detergents, filters, etc.
Operational costs for these wash/dry lines can range from $50 to $75 per hour. In addition, these systems consume large amounts of factory floor space.
A typical CO₂ spray system consists of one or more cleaning modules, a pneumatic booster, and custom cleaning nozzles. In this application, the process is performed with the 6-axis robot. CO₂ spray cleaning programming is similar to that used for spray paint application. This system can be installed in a conveyor paint prep room, and furthermore, because CO₂ spray cleaning occurs at a single location in less than 1 minute, it is more responsive to possible process variations than a cleaning system that has a cleaning time of 30 to 45 minutes.
Based on comparisons with traditional aqueous-based cleaning systems, CO₂ spray cleaning meets automotive cleaning specifications with substantial cost savings, Sorbo notes. In addition, it is estimated that operating and maintenance costs range from 10 to 20 percent of those for a conventional spray wash system, and take a fraction of the factory floor space—as little as 5 percent of the typical aqueous-based cleaning system.
Sorbo says CO₂ spray cleaning is being widely adopted by German OEMs. In the past few years the United States and Asia have seen fast growth in CO₂ cleaning installations.