Centrifugal wheel blasting with sensitive blasting media as an alternative to compressed air blasting.
Key words: Centrifugal Wheel Blasting, Compressed Air Blasting, Dry Ice, Sensitive Blasting Media
Cleaning technology changes 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 pre-treatment for coating or joining, or as a cleaning tool for service and maintenance of machine components and equipment (Uhlmann et al. (1), 2006).
Conventional cleaning technologies are based either on mechanical and watery or chemical methods and/or substances. Most of them have already known negative environmental effects and do not comply with current or envisaged legal frameworks. Accordingly these effects should be reduced to a minimum while technological and economical requirements have to be met.
Carbon dioxide ([CO.sub.2]) contributes to the greenhouse effect hence. Hence, the usage of this unavoidable carbon dioxide does not add to the global warming additionally! The solid carbon dioxide used for dry ice blasting is a by-product of the chemical industry, e. g. ammoniac synthesis according to the Haber-Bosch process.
1.1 Solid Carbon Dioxide
Carbon dioxide is either stored in fluid form in high pressure tanks at ambient conditions of 20[degrees]C with a pressure of 57 bar or in low pressure tanks at -20[degrees]C with a pressure of 20 bar. At ambient conditions (1 bar) carbon dioxide is either gaseous or solid depending on the temperature. It is non-toxic, non-corrosive and non-abrasive. Furthermore carbon dioxide is non-conducting and chemically inert.
1.2 Dry Ice Blasting
State-of-the-art, dry ice blasting is based on compressed air. After the compressor, a dehumidifier and various filter devices are needed to dry and clean the air used to accelerate the dry ice pellets. While other cleaning processes require complex processing or increase disposal costs, no media remains in the structure of the workpiece (e.g. drillholes and cavities) (Uhlmann et al., 2004). Dry ice blasting allows a flexible soft de-lamination and cleaning even of sensitive or structured surfaces while highly adhering or hard contaminants and protective or functional coatings are difficult to remove. Despite these advantages the maximum allowed work place concentration for gaseous carbon dioxide must not be exceeded.
1.3 Removal Mechanisms
Dry ice blasting is based on a thermal mechanism. This effect occurs as a local cooling down effect at the impact point. It is a mechanical effect caused by the impact of the dry ice pellets and an expansion due to the partial sublimation of the pellet (Uhlmann et al.2, 2006). Owing to this, elasticity is lost and the coating embrittles and shrinks. Different thermal expansion coefficients of substrate and coating produce cracks in the coating. The kinetic energy of the particles and the air stream contribute to the removal. The sublimation of the dry ice leads to a sudden increased volume that supports the process. When the adhesive energy is exceeded by this combined thermo-mechanical effect the coating chips off.
A disadvantage of dry ice blasting is the high sound pressure level of up to 130 dB (A) which occurs because of the high blasting pressure. Owing to the low hardness of the pellets, the mechanical impact has to be enlarged by a higher velocity of the pellets. This can be done by increasing the blasting pressure which results in a higher velocity of the air stream in the blasting nozzle. As a consequence the operator has to wear adequate hearing protection and he has to carry out further safety instructions with regard to the compressed air.
A further disadvantage is the low energy efficiency of compressed air blasting, which is a result of the indirect transformation of electrical energy into kinetic energy of the pellets. Thus, instead of a direct acceleration of the blasting medium, additional devices for the compressed air are needed which in turn lower the efficiency further.
With regard to the mass flow rate the indirect acceleration has another disadvantage. More blasting media accelerated per time demands more of the compressed air or a higher velocity. This results in a higher blasting pressure and/or a higher volume rate per minute of compressed air.
3. ROTATIONAL WHEEL BLASTING WITH SENSITIVE BLASTING MEDIA
3.1 Mechanical Acceleration of Sensitive Blasting Media
For conventional rotational wheel blasting durable blasting media like sand, glass or steel are used. A number of 3,000 rpm at a wheel diameter of ca. 35 cm results in blasting velocities of ca. 70 m/s. This is sufficient for applications such as deburring of metal workpieces, removing of residues of casting sand or surface hardening. A mechanical acceleration of a sensitive blasting media such as dry ice would results in an early sublimation of the pellets within the rotational wheel blasting device. For blasting velocities of ca. 300 m/s, comparable to compressed air blasting, a higher number of revolutions per minute is needed.
Especially the accelerating components have to be adapted to sensitive blasting media. Otherwise the mechanical stress that affects the dry ice pellets leads to early sublimation. The application of conventional wheel blasting devices for dry ice blasting is impossible because of the pellet impact inside the acceleration bushing and onto the blades.
3.2 Rotational Wheel Blasting Prototype
At the Institute for Machine Tools and Factory Management IWF of the Technical University of Berlin together with the Fraunhofer IPK a first prototype for rotational wheel blasting with sensitive blasting media has been developed (Uhlmann et al.3, 2006). Compared with conventional wheel blasting devices the main improvement is a pre-acceleration chamber.
This device will be patented in due course. The components are manufactured by selective laser sintering (SLS), which is usually used for rapid prototyping. This manufacturing process makes it possible that complex three dimensional geometries can be produced which are necessary to reduce the mechanical stresses on the blasting medium.
The device consists of a storage and supply unit, an acceleration unit and a clamping device for the actuation shaft. The screw conveyer is state-of-the-art regarding dry ice blasting devices. The acceleration bushing and the blades are of conventional wheel blasting devices. The acceleration bushing is used to speed up the blasting media to the speed of the inner blade edge to reduce mechanical loads feeding the blades.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
To reduce the mechanical stress during the rotational acceleration of the pellets after feeding, a rotational pre-acceleration bushing was constructed. Furthermore, the inner contour of the acceleration bushing is adapted and chamfered.
3.3 High Speed Camera Investigations
Owing to this pre-acceleration bushing almost unbroken pellets were observable at 2400 rpm using a high speed camera. A blasting velocity of 62 m/s was measured which correlates to the calculated velocity of the diameter and the number of revolutions per minute. The results of the high speed camera investigations of the tests with different numbers of revolutions are shown in fig. 2.
4. HIGH SPEED ROTATIONAL WHEEL BLASTING WITH DRY ICE
In further prototype studies in co-operation with IWM Strahltechnik, Metzingen/Germany new components will be developed to achieve a higher blasting velocities of the pellets. In general, this can be done by increasing the number of revolutions per minute as well as by increasing the blasting wheel's diameter, e. g. by constructing longer blades. To realize blasting velocities comparable to dry ice blasting based on compressed air about 12,000 rpm will be needed. Therefore, the prototype's construction has to be revised again. Critical points regarding the actual prototype construction are the connection of the screw conveyer and the pre-acceleration bushing, the connections of the pre-acceleration and acceleration bushing, the rotation in the pockets of the acceleration bushing and finally the impact and downforce on the blades.
5. RESULTS AND CONCLUSION
As an alternative to compressed air based dry ice blasting a prototype for rotational wheel blasting with sensitive blasting media has been developed. The stress caused by mechanical acceleration on the sensitive blasting media has been reduced. This device will be patented in due course for blasting with sensitive blasting media. The blasting velocities of almost unbroken pellets are comparable to the velocities of conventional blasting wheel devices.
For both conventional dry ice blasting and conventional rotational wheel blasting applications a higher final blasting velocity is needed. Therefore, a modification of the accelerating components' geometry, of the material and of the surface roughness is also planed. Based on these results comparative studies of an improved prototype with compressed air dry ice blasting will be carried out.
Uhlmann, E.1, Veit, R., Hollan, R., El Mernissi, A. Blasting with Solid Carbon Dioxide: Dry Ice Blasting--[CO.sub.2]-Snow Blasting. MFN--Metal Finishing News, Nov. 2006, pp. 48-49.
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
Uhlmann, E.3, El Mernissi, A., Krieg, M., Gottheil, I. (2006). Schleuderradstrahlen mit Trockeneis, JOT--Journal fur Oberflachentechnik, No. 8 2006, pp. 60--62.
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|Author:||Uhlmann, Eckart; Hollan, Robert|
|Publication:||Annals of DAAAM & Proceedings|
|Date:||Jan 1, 2007|
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