Cleaning precision parts: from solvents to supercritical CO2
A Swedish-based sewing machine manufacturer faced a problem: the Swedish government banned the chemical solvent it used to clean its sintered parts. The solution? Supercritical carbon dioxide. Steve Minett and Chris Taylor describe the process and the potential.
Prior to 1998, Viking had used trichloroethylene in the cleaning of its sintered components, but that year the chemical solvent was prohibited by Swedish legislation on environmental grounds. Whilst able to negotiate a temporary exemption allowing continued use until a more environmentally friendly alternative could be found, Viking was under pressure to find a less-polluting answer.
A lifetime's lubrication
The process of sintering is based on powder metallurgy. A metallic powder is pressed together in a punch and die tool. The shaped parts are then sintered for three hours at temperatures of 1,120°C. To produce more exact tolerances and shapes of the required components, they are dipped in 'sizing' oil before a second pressing (or sizing). The resultant porosity of components made in this way means that they can be self-lubricating in operation. This means that critical components can be impregnated with a lifetime's lubrication during the manufacturing process - a very real advantage for the customer. To acheive this advantage, a final cleaning phase is needed to remove the sizing oil and any dirt which may have accumulated during the process, and which otherwise would create detrimental effects in the final sewing machine. Components made in this way can be drilled, thread-cut and milled; the manufacturing process creates less waste than with conventional machining; and the finished weight is reduced.
The problem with the process was in getting rid of the sizing oil and other process contaminants - this is where the trichloroethylene came in. But, as Arne Carlsson, manager of Viking's component production, points out: "While this chemical process was effective, it was not very efficient, and not at all environmentally friendly. When we were using the trichloroethylene the process took four hours at a temperature of 140°C. Only 30 minutes of this was the actual cleaning process; the rest of the time was needed to get rid of the chloroethylene gas. The process produced 400 litres of liquid waste per year - waste containing chlorine, which made it very difficult and expensive to dispose of."
Jan-Olof Landén, responsible for mechanical engineering research and development at the Viking plant, explains the various alternatives that were examined in the search for a better - and more environmentally friendly - way to resolve the problem. "First," he says, "we looked at using ultrasound combined with cyclohexane, which is a liquid. But the cleaning was not very efficient, and the gas is highly flammable - risky, bearing in mind that the cleaning takes place immediately next to the oven. Then we looked at petroleum ether, but this is very similar to ordinary vehicle fuel and again too risky so close to the oven. Also, the emissions could not be 100% contained, and so it would have been difficult to get government approval to use it. Next, we looked at water cleaning solutions, but these were not very practical because of the possibility of corrosion; that and the fact that they didn't do the job very well."
The cleaning of parts
Then Viking started to consider the use of supercritical carbon dioxide. The company sent parts to be cleaned by a standard supercritical carbon dioxide cleaning system in America, with somewhat limited success. Viking then made contact with Chematur Engineering AB in Karlskoga in central Sweden and tested their supercritical carbon dioxide cleaning process, which incorporates a unique rotating basket (see picture) to enhance cleaning efficiency. The results were very good. Viking bought a Chematur Rotowasher for their Jönköping plant just over a year ago.
The cleaning process in the Rotowasher is based on dissolving organic contaminants, including oils, in supercritical carbon dioxide at high temperature and pressure. The CO2 is initially pre-cooled, then compressed and heated to the desired level before being fed into the treatment chamber. A rotating cleaning basket maximises the cleaning effect by utilising centrifugal force and enhancing mass transfer.
After dissolving the organic impurities, the CO2 passes through a pressure release valve designed to allow a constant mass flow. The pressure of the loaded CO2 is reduced, and any liquid CO2 is gasified in the evaporator. The oil contaminants are collected in the bottom separator, from where they are removed, and activated carbon filters are used to eliminate any remnants of contaminant from the circulating system. The gaseous CO2 is then liquefied in a condenser and returned to the storage tank ready to be re-used. The only consumables are carbon dioxide - 2-3kg per cleaning operation - and energy. The carbon dioxide can be re-used in the next cleaning operation, and any steel particles from the cleaning process are extracted by a ring-shaped magnet. The cleaning process takes an hour for each batch - compared with four hours using the trichloroethylene process - and the basket can accommodate up to 2,000 components at a time.
The Viking application of Chematur's Rotowasher is the first time supercritical carbon dioxide has been used to clean sintered components. Consequently, IVL (the Swedish Environmental Research Institute in Stockholm) is taking an interest in the process because of the enormous potential in this application. Its use at the Viking plant has been approved by the chemical inspectorate of the Swedish government's environmental agency.
But while the application is new, the process is not, claims Chematur's Lasse Parvinen: "The principle of using this process to replace chemical solvents has existed for many years in America," he says, "but up until now it's been too expensive to mass produce the equipment required." The process itself, however, is well established and has been used since the 1960s to de-caffeinate coffee, and has other applications within the food processing industry. In fact, Viking's Jan-Olof Landén relates that one factor that prompted him to investigate the use of supercritical carbon dioxide was a story he heard about a margarine factory in America. "The plant was using supercritical carbon dioxide to extract fat from its waste product, which was sold to cattle farmers as pressed feed cake. It turned out that the cattle wouldn't eat it because the fat level - and with it any nutritional content - was so low. I realised that this could be a very effective method for cleaning off our sizing oil."
IVL has studied the operating costs of the new method compared with the previous one employed by the company. Arne Carlsson explains: "In terms of energy use in kilowatt-hours per kilogram of parts, the supercritical carbon dioxide used 0.3kWh against 1kWh for the trichloroethylene process. Use of chemicals was higher, 0.15kg for each kilogram of components compared with 0.05 for the trichloroethylene. The cost is slightly higher at SEK1 per kilogram for the supercritical carbon dioxide against SEK0.6 for the trichloroethylene. But there are real savings when we look at the waste produced. From 400 litres a year containing chlorine, which was difficult to dispose of, with the supercritical carbon dioxide method we have only 100 litres of a fairly clean residual which can be used as a fuel. In the future we may even be able to recycle this residue."
Where Viking and Chematur are leading the way, other industries are likely to follow now the effectiveness of the supercritical carbon dioxide cleaning process is proven with sintered components. The use of components manufactured in this way is growing fast in the European and American automotive industry. At present, European motor manufacturers use around 7kg of sintered components per vehicle, with US manufacturers using almost double that figure. Auto industry analysts expect the European figure to rise to around 25kg per vehicle, and the total for US and Japanese cars to be even higher.