Performance study on vacuum suction blasting by force and velocity measurements of the abrasive flow

The automated removal of layers by scarfing to repair carbon fibre reinforced plastics (CFRP) has been investigated with vacuum suction blasting (VSB) showing promising results. However, its removal mechanism and particle behaviour were not yet fully understood. The removal leads to cavities on the surface and the central area is less removed after longer static blasting times. Further theoretical considerations and experiments of measured forces and velocities of the VSB particles can explain these effects to optimize the large-scale removal. Former studies on abrasive blasting used simplified approaches with incompressible Bernoulli equations taking the same velocities of particles and carrier medium. For VSB, the air is much faster than the particles and more complex equations are derived in this study. Additionally, it is taken into account, that the impinging forces act against the vacuum forces.
To specify the power and removal mechanism of VSB for large scale removal, forces and velocities of the blasting particles are studied. Calculations by compressible Bernoulli equations, measurements of blasting particle velocities and impact forces give a good approximation. The air velocities and forces are calculated and compared with the measurements. Forces are measured by a piezo-electric sensor. Particle velocities are measured with a Particle Image Velocimetry (PIV) system tracking the abrasive particle movement through a transparent outer nozzle. The influence of different blasting parameters on the forces and velocities are identified. Results correspond to the material removal from former experiments and indicate the favourable process parameter set. Despite the impinging forces are in general low, they lead together with the particle velocity to the large-scale removal rate. The measured particle directions explain the removal behaviour of this VSB process. The measured velocities match to those well-known from grinding with bonded grinding tools.