Snow white in the cleaning business
Technology used to put powder on the pistes is helping in the rather less glamorous field of abrasive cleaning `Ice-blasting can strip paint from panels without marring the polish'
Tuesday 18 April 1995
In an interesting example of crossover technology, the principle that creates artificial snow is now being put to use in the rather less glamorous field of abrasive cleaning.
The technique of abrasive blast-cleaning has been around for a century or more but has seen few changes since it was first developed. The traditional approach uses high-pressure air to accelerate solid abrasive particles of sand or similar materials to high speeds, which then impact and scour the surface being cleaned.
Unfortunately, this procedure generates large quantities of spent abrasive that contain relatively small quantities of removed coating. It also tends to generate a lot of dust, which can be detrimental to both workers and machinery.
If the coating being removed poses a potential environmental hazard - such as radioactivity - then the blasting site has to be contained, and the residue collected and disposed of at an appointed hazardous waste site. The more waste there is, the more it costs, and industry is now looking for new ways to reduce residual waste.
At Penn State University's Gas Dynamics Laboratory, Dr Gary Settles, professor of mechanical engineering, is looking at new ways of tackling the problem. "In most conventional abrasive blasting operations, about 10 to 15 per cent of the waste is paint or grease and the other 85 to 90 per cent is the abrasive. What we are looking for is an abrasive material which will vaporise or melt."
Water, dry ice, freon, ice and water/ice mixtures have all been tried with varying degrees of success, but, according to Dr Settles, "previous systems were bulky, expensive and didn't work very well. Early attempts at producing an ice-blasting system yielded ice chunks as large as a quarter of an inch."
The problems of trying to clean precision parts with quarter-inch chunks of ice are fairly obvious, and Dr Settles' team has now succeeded in developing a machine in which atomised water is propelled by a stream of compressed air chilled by liquid nitrogen to create a stream of ice particles some three- to four-thousandths of an inch in diameter, moving at up to 500 miles per hour. A nozzle then projects this stream of compressed gas and tiny ice particles against the surface to be cleaned.
All that is required in terms of raw materials are tap water and the addition of a small quantity of Snomax - a cheap and commercially available additive traditionally used in snow-making machines to raise the effective freezing temperature of the airborne water droplets.
"This is rather similar to a snow-making machine, except that ice, rather than snow, is the end product," says Dr Settles.
The beauty of the system is that the ice particles are travelling so fast that they tend simply to sublimate when they hit their target, turning to vapour without going through the liquid phase. The net result is a much smaller amount of solid or liquid waste.
But ice is actually not very hard to begin with, so Dr Settles and his team feel this new technology will be best suited to applications requiring a delicate touch. One example quoted by Dr Settles is that of aircraft "depainting", in which the metallic skin of the aircraft is both thin and expensive to replace. "Conventional cleaning techniques can damage the underlying surface," he says, "but tests have shown that our technology can strip paint from aluminum panels without marring the polish of the surface."
Another area under investigation is that of automotive parts production and reclamation, in which the volatile organic solvents presently used by the industry are coming under increasing scrutiny for their possible effects on the environment during use and disposal.
"Many innocuous materials, including dry ice, starch, baking soda and even ground-up corn cobs have been put forward as candidates to replace the volatile compounds used in degreasing," points out Dr Settles, "but with ice, we have a cheap material that poses little or no disposal problem and no environmental hazard."
At present, there is only a laboratory prototype of this new ice-blasting apparatus, but Dr Settles is confident that this technology will be at least as successful as its "on piste" older sibling.
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