VOL. 7 September ISSUE YEAR 2006
in Vol. 7 - September Issue - Year 2006
Blasting with Solid Carbon Dioxide: Dry Ice Blasting - CO2-Snow Blasting
Prof. Eckart Uhlmann
A. Robert Hollan
Figure 1: Functional principle of blasting with solid carbon dioxide
Figure 2: Active principles - thermo, mechanical and sublimation effect
Figure 3: Dry ice pellets
Figure 4: CO2-snow blasting with concentric nozzle and with agglomeration chamber
Table 1: Overview and qualitative rating of process characteristics
Cleaning technology is changing from a mere necessity and cost factor to an integral part of the industrial value creation chain, either as a process step in manufacturing, e.g. surface treatment before coating and joining processes, or in the context of cleaning tools and machine components as part of their maintenance and service.
Today, the main cleaning technologies in production and maintenance are either chemical, mechanical or watery. Partly they may involve ozone-destroying cleaning agents . A lot of these available cleaning technologies do not comply with current or envisaged legal frameworks. In addition to technological and economical requirements in manufacturing and recycling the strain on man and on the environment should be reduced to a minimum.
Blasting with solid carbon dioxide, better known as dry ice blasting or CO2-snow blasting is a highly flexible and environmentally friendly cleaning technology with specific advantages. In principle it can be compared to sandblasting. The blasting medium is accelerated by compressed air and applied onto the part which has to be cleaned or decoated, see figure 1. The distinctive factor is the blasting medium. Solid carbon dioxide with a temperature of -78.5 °C sublimes immediately when impinging onto the surface. Only the removed contamination or coating has to be disposed of. Furthermore the low hardness of solid carbon dioxide allows the cleaning of even sensitive materials .
While the effect of conventional blasting processes is based upon the mechanical impact of the blasting media, blasting with solid carbon dioxide has three collaborating active principles, figure 2. The low temperature of the blasting medium cools the part and coating locally. The coating embrittles and shrinks. Due to different thermal expansion coefficients of coating and substrate shear stresses are generated and cracks within the coating are initiated. The kinetic energy of the particles leads to a mechanical material removal supported by the third active principle, the sudden increase of volume by the factor 600 when the particles sublime .
Carbon dioxide is odourless, colourless, inflammable, electrically non-conducting, and chemically inert. However, it supersedes oxygen and affects heart and breathing rate. Since it is heavier than air, sufficient air supply must be ensured when being used. The earth’s atmosphere contains 0.03 % carbon dioxide, altogether approximately 735 billion tons. The carbon dioxide used for blasting is a waste product of chemical processes.
In contrast to water, CO2 has no liquid phase at ambient pressure. Depending on the temperature, it is either gaseous or solid. Thus, solid carbon dioxide does not melt, but sublimes. It is therefore called dry ice. At an ambient temperature of 20 °C, it liquidizes above a pressure of 57.3 bar. In this state, it is stocked in households in bottles for water-bubbling devices. At 1 bar and -78.5 °C it is solid and can be used as a blasting agent .
The process variants of blasting with solid carbon dioxide are dry ice blasting and CO2-snow blasting. Liquid CO2 can be stored at ambient temperature in gas bottles with a pressure of approx. 57.3 bar. Bigger quantities are stored in low pressure tanks at 20 bar with a regulated temperature of -20 °C. For the production of dry ice pellets the liquid carbon dioxide is expanded to atmospheric pressure. Due to the Joule-Thomson-effect it cools down and approx. 40 % CO2-snow and 60 % CO2-gas are generated. With help of a hydraulic stamp the solid CO2-snow particles are pressed through a mould and cylindrical dry ice pellets are formed, figure 3. Pellet parameters that effect the cleaning process are density, hardness and shape.
Dry ice pellets can only be stored for a limited time. Even in insulated storage containers they sublime continuously and ambient humidity lets them stick together. Therefore the automation of the process is complex. Furthermore the blasting devices contain moving parts which can lead to a reduced lifetime of the device.
For CO2-snow blasting liquid carbon dioxide is used and the solid CO2-particles are generated within the blasting process. The CO2 is stored in its liquid phase and can be fed through hoses directly to the blasting device. There are two types of CO2 snow blasting devices, depending on the principle of CO2 snow generation: CO2 snow Blasting with concentric nozzle and CO2 snow blasting with agglomeration chamber. For CO2-snow blasting with concentric nozzle, figure 4a, liquid CO2 is expanded directly through a concentric blasting nozzle into the ambient air. Solid CO2-snow particles are produced and afterwards accelerated by a supersonic compressed air jet generated by an annular gap.
In CO2 snow blasting devices with agglomeration chamber, figure 4b, the CO2 expansion takes place at blasting pressure. The CO2 is injected into an agglomeration chamber where larger CO2-snow particles are formed. Subsequently they are accelerated by compressed air led through a laval nozzle.
Comparing the different process variants they can be distinguished according to abrasiveness, plane rate, blasting media and compressed air consumption, according to their flexibility regarding contamination and the feasibility of automation. Table 1 gives a brief overview and a qualitative rating of the process characteristics.
Blasting with solid carbon dioxide has different advantages compared to conventional cleaning technologies. Due to the sublimation of the blasting medium there is no additional liquid or solid waste apart from the removed contaminations or coatings . Thus no secondary residues of the blasting medium remain on the cleaned or decoated parts. Blasting with solid carbon dioxide has only a low abrasiveness because of the low hardness of the blasting medium. This allows the treatment of sensitive surfaces and hardly affects the material while the process is flexible with regard to the kind and thickness of the contamination or coating. The non-conductivity as well as the bacteriostatic property of carbon dioxide are other advantages offering further applications.
As any blasting process, blasting with solid carbon dioxide can only be applied to visible surfaces. Blind holes, undercuts or inside contours cannot be cleaned. Furthermore the sublimation of the blasting medium can not only be seen as an advantage. Apart from the high costs for blasting media there are applications where a binding medium for the contamination is necessary. The high compressed air consumption also contributes to the high running costs. Other disadvantages are the electrostatic charging caused by the dry ice particles within hoses and nozzles as well as the high sound level.
In general the application of blasting with solid carbon dioxide should be taken into account when at least one of the two major advantages is needed: no residues of the blasting medium or the low abrasiveness to treat sensitive surfaces. Then blasting with solid carbon dioxide can be an ecological alternative for conventional mechanical, chemical or aqueous cleaning and de-coating methods. Examples of application are the inline cleaning of machinery and equipment or the cleaning of moulds. A proximate article will be focused on specific applications like the pre-treatment of surfaces for varnishing, coating and gluing, cleaning of control and power electronics without downtime, hybrid-laser-dry ice blasting and blasting with spinning wheels.
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Institute for Machine Tools and Factory Management, Technical University Berlin, Germany
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