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Vol. 9
November Issue
Year 2008
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Science Update


in Vol. 9 - November Issue - Year 2008
Sustainability within Manufacturing



Fig. 1 Process model: Combination of a mechanical and a thermal mechanism as well as a sublimation effect


Fig. 2 Carbon dioxide snow generated by the acceleration process of conventional equipment


Fig. 3 Wheel blasting device with pre-acceleration for the non-durable blasting media, lateral cut


Fig 4 Correlation of calculated and measured velocities of blasting particle


Fig. 5 Mechanical acceleration device with rotational pre-acceleration


Fig. 6 Blasting wheel for up to 6000 rpm


Fig. 7 Wheel blasting device, safety equipment enables high speed camera Investigations

Introduction

Today sustainability becomes more important. With regard to rising or fluctuating prices energy and resource efficiency are key competencies of a successful market positioning of any company. Most companies are focused on primary factors e.g. resources, tools and machine tools or semi-finished products of sub-suppliers, which can easily be calculated by the purchasing department. In details, the interdependencies of influencing factors of energy and resource consumption within secondary processes are complex. Even the determination of cleaning costs within the manufacturing chain of ball bearings in the automotive industry is difficult. The result, 25% of the total costs, shows the importance [1].
Cleaning technologies has become an integral part of the industrial value creation chain, either as a process step in manufacturing, e.g. surface pre-treatment for coating or joining, or as a cleaning tool for service and maintenance of machine components and equipment. Conventional technologies are mostly based on mechanical or chemical methods with water or chemical substances. These will be reduced to a minimum to meet current or envisaged legal frameworks as well as technological and economical requirements.
Dry ice blasting and CO2-snow blasting are highly flexible and environmentally friendly alternatives. The blasting process does not contribute additionally to the greenhouse effect because the carbon dioxide is amongst others a by-product of the chemical industry. The conventional acceleration of blasting media by compressed air has different disadvantages, e.g. low energy efficiency. Up to now the mechanical acceleration by wheel blasting wasn’t suitable for non durable blasting media. This problem has been solved with specific pre-acceleration and blades.

Solid carbon dioxide

Carbon dioxide (CO2) is stored in fluid form either at 20°C in high or at -20°C in low pressure tanks [2]. At ambient conditions (1 bar) carbon dioxide is either gaseous or solid depending on the temperature, therefore no blasting residues remain [3]. It is non-toxic, non-corrosive and non-abrasive. Furthermore carbon dioxide is non-conducting and chemically inert. Due to a sudden expansion the liquid stored carbon dioxide it is cooled down to -78.5°C because of the Joule-Thomson-Effect and solid carbon dioxide is generated [4]. This snow is pressed through a mould and finally forms the cylindrical dry ice pellets used for blasting.

Dry ice blasting

State-of-the-art, dry ice blasting is based on compressed air, that has to be pre-processed by a dehumidifier and various filter devices. A flexible cleaning even of sensitive or structured surfaces is possible while highly adhering or hard contaminants are difficult to remove. Dry ice blasting is based on a combination of a thermal mechanism, a mechanical effect and the sublimation of the pellet [5]. Besides cleaning tasks the pre-treatment of surfaces, e. g. before painting, coating or adhesive bonding, is a new field of application. Figure 1, process model
Wheel blasting with sensitive blasting media

Wheel blasting has a higher energy efficiency (80% - 90%) compared to compressed air blasting (up to 5%). Furthermore, the high sound pressure level of compressed air blasting is another disadvantage. Typical conventional wheel blasting with durable blasting media, e. g. sand, glass or steel, is done at 3,000 rpm and a wheel diameter of approx. 35 cm. Resulting blasting velocities of up to 70 m/s are sufficient for deburring of metal workpieces, removing of casting residues or surface hardening. Non-durable blasting media like dry ice sublime due to strain and stresses of this mechanical acceleration by conventional equipment. Figure 2, CO2-snow in the acceleration chamber.

The problem of early sublimation was solved by a pre-acceleration within a promising first prototype developed by the Institute for Machine tools and Factory Management (IWF). First results showed a good correlation between the theoretical calculated velocities and the velocity measurements with dry ice pellets by high speed camera investigations. Figure 3, lateral cut of first prototype; Figure 4, correlation of calculated and measured velocities.

Further improvements

To meet particle velocities comparable to dry ice blasting with compressed air, up to 300 m/s, further improvements of the acceleration devices were necessary. A critical point is the rotational acceleration solved by the patented pre-acceleration chamber (Fig. 5, B) of IWF and the Fraunhofer Institute for Production Systems and Design Technology (IPK). Another critical moment is the release of the pellets by the notch (E) of the focusing device (D) from the acceleration chamber (C) onto the blades because of the different circumferential velocity. Figure 5, mechanical acceleration device.

Results and outlook

Because of this a second prototype was developed by the IWF to enable a higher number of revolutions per minute as well as a larger diameter. The construction shown in Figure 6 realized up to 6000 rpm while still pellets have been observed by high speed camera investigations. The next step will be to upscale the diameter of these first tests of approx. 35 cm up to 58 cm shown in Figure 7. Taking further improvements into consideration, e. g. geometry and surface of the blades, up to 250 m/s are expected. Besides dry ice blasting, these results might be of significance for conventional wheel blasting, e. g. with regard to wear of the blades, too.

References

[1] Krieg, M.: Markt und Trendanalyse in der industriellen Reinigung. In: Vortragssammlung des 11. IAK Trockeneisstrahlen, Fraunhofer IPK, Berlin, 2007

[2] Air Liquide, 2002, Safety Data Sheet Carbon Dioxide, Version 1.01.

[3] Uhlmann, E., El Mernissi, A., Dittberner, J.: Blasting Techniques for Disassembly and Remanufacturing, Proc. Global Conference on Sustainable Product Development and Life Cycle Engineering, Berlin 2004, p. 217 - 223.

[4] Elbing, F., Möller, D., Ulbricht, M.: Fog Dissipation by Dry Ice Blasting: Technology and Applications, Proc. 2nd International Conference on Fog Collection, St. John’s, 2001.

[5] Uhlmann, E.2, Hollan, R., Veit, R., El Mernissi, A. (2006). A Laser Assisted Dry Ice Blasting Approach for Surface Cleaning, Proceedings of 13th CIRP International Conference on Life Cycle Engineering, pp. 471 - 475, Leuven/Belgium, 2006




Author: Robert Hollan
For Information:
Institute for Machine Tools and Factory Management
Technical University Berlin, Germany
Tel. +49.30.314-22413, Fax +49.30.314-25895
E-mail: hollan@iwf.tu-berlin.de