Snow problem!

Particles as small as 0.1 micron and light oils, hydrocarbons and fingerprints can be removed using liquid "carbon dioxide" released at atmospheric pressure creating "snow". Carbon dioxide is abundant in the earth's atmosphere, making up around 0.04% of the total. It is frequently used in many industrial processes and easily obtained 99.99% to 99.999% pure form from bottle gas suppliers. The gas is inert and humans have a very high tolerance to its exposure. CO2 is considered a "greenhouse" gas, however commercially sold CO2 is recycled CO2 that has been used in some other form of industrial process and the small amount released has no additional net impact on the environment. If liquid CO2 is released to atmospheric pressure through a constriction, it passes through its triple point, the state at which some portion exists in solid, liquid and vapour. A portion of the liquid is converted to dry ice. Snow is formed when these particles of dry ice mass together. The solid phase quickly warms and sublime to a vapour, leaving no residues. Particle contamination affects the quality of products from many different industries. Submicron particles can not only affect the functionality of a product leading to a catastrophic field failure but also detrimentally impair the products appearance. In any given manufacturing situation we can expect to find airborne particles generated by all manner of manufacturing processes and are shed in their millions by operators. It has been estimated that eye shadow from one employee can generate up to 100 million particles of 0.5 micron size (as a perspective, human hair is about 100 microns in diameter). Add to this particulation from all types of cosmetic make up, street clothing, hair loss and human skin cells and the particle count can run into billions. Removal of these contaminants can be attempted using liquid washes, compressed air or dry nitrogen, however particles of 2 microns and smaller are difficult to remove using these conventional methods. Electrostatic attraction bonds products of one electrical polarity to the surface of an opposite polarity and other adhesive forces bond particles to the surface, fixing them firmly. In addition, the smaller the particle, the greater the percentage of its total area is in contact with the surface, leaving less surface area exposed to aerodynamic forces of conventional blow off guns and liquids. Boundary layer phenomenon states that that the closer one gets to the surface the more the velocity of the gas or liquid approaches zero, until at the surface the velocity is zero. Therefore very small particles, which are close to the surface, are not going to be so influenced by the imparted drag, caused through the aerodynamic velocity of the gas. Using CO2 snow, a mass can be introduced to the vapour flow, energy transfer from a collision between this mass and the adhered particle can knock it free. Released particles are carried away in the vapour stream. Utilising other products to generate mass, such as sand or talc, can damage the surface and leave residues, which often outweighs the benefits of their use, CO2 leaves no residues. A Snow-Gun™ has a nozzle which can adjusted to vary the size of the released particles, from <1mm diameter, typical sizes are in the 5 micron range, which have a velocity from 150 fps to 1000 fps. These particles produce an aggressive dry ice storm capable of removing light oils, light greases and hydrocarbons. Adjustments of the dry ice particle size to about 0.5cm, where they produce a very gentle snowfall effect, can be used for cleaning the most delicate products. Fingerprints can be removed from many surfaces. CO2 in its liquid phase acts as a solvent which will dissolve organic particles. The super cold CO2 may also cause a thermal excursion across an organic particle, causing it to fracture and breakaway. Particles can be removed from hybrid circuits, fibre optics, mirrors, silicon wafers, disk drive assemblies, flat panel display substrates, metal and plastic parts, medical instruments, and many other components that require precision cleaning. Care should be exercised when cleaning very heat sensitive low mass parts with CO2, however in most cases the process is so rapid as to impart very little chill to a product. At atmospheric pressure the temperature of dry ice is –78.5?C. The use of this type of gun is safe provided it is not directed at anyone, momentary contact with the skin is also harmless, prolonged contact should be avoided. The amount of CO2 releases is small and work areas should be well ventilated, but if there is a concern regarding oxygen depletion in a working environment, CO2 detection monitors can be employed. During the cleaning process the snow will collect contaminants and carry them along in the vapour flow until the dry ice its self sublimes. Care must be taken to ensure that particles are not taken from one location only to be re-deposited elsewhere. Problems associated with parts being chilled in the cleaning process, dropping below the dew point and attracting moisture, can be eliminated by conducting the cleaning process, inside of a glovebox, where the atmosphere can be controlled. An atmosphere of nitrogen will prevent oxidation or other contamination problems. Complete removal of "blown-off" particles can be guaranteed, if the process is carried out under a laminar flow hood. The cost of pure CO2 can be avoided by using purification equipment. Welding grade CO2 can be purchases and then purified until it is suitable for use as a cleaning medium, significantly reducing cleaning costs.

For more information contact: tel: +44 (0)1823 286688