Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 51 of 707

Additionally, the use of excessive pressure only accelerates the breakdown of the media with minimal decrease in blasting time. For example, blasting at 100 psi may reduce the time cycle by 5% as compared to blasting at 60 psi, but the abra- sive may break down at a 50% higher pace. Pressure selection must also take the type of media into account. For instance, if intricately designed jewelry is to be blasted, a fine abrasive with a soft texture would be used at a pressure of 10 to 15 psi. On the other hand, the removal of scale from steel castings could require a coarse, hard abrasive and an air pressure of 80 to 100 psi. The next variable to be considered is the blast angle vis-a-vis the workpiece. If using aluminum oxide at a 45|Ao angle, maximum scuff, cut, and roughness result. This may be fine if blasting is performed for adhesion or bonding oper- ations; however, if the finest surface finish and the widest possible blast pattern are required, the aluminum oxide should be blasted at a 30o angle. This will pro- duce a smoother scuff pattern. The distance from the nozzle to the part being blasted should remain constant throughout the process, but this distance may vary from project to project. When synthetic abrasives are used, the recommended distance is 6 to 12 in. More distance is required for heavy ferrous metal media. Softer, natural media should be blasted from a distance of 3 to 6 in., depending on the action needed. Lightweight particles are expelled with more momentum from a direct pressure nozzle and retain their energy component over a much greater distance. For blast- ing at distances over 12 in., a direct pressure system is by far the most effective. 50

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