Flow rate of metal powders at reduced and elevated air pressure

There are suggestions that the flow rate of dry powder is affected by ambient gas pressure as distinct from pressure differential. This raises the concern that computer controlled powder deposition methods for selective laser sintering in which powder mixtures are delivered to the building platform by acoustic control of flow in capillaries may be affected by partial vacuum applied to reduce oxidation. The flow rate of free flowing powders in the static Hall flow meter was measured at reduced and elevated air pressure. There was no systematic difference in the Hall flow times for these powders over 8 orders of magnitude of pressure in the range from 2.0 × 105 Pa down to 8.1 × 10− 4 Pa. Copper powder as well as a tool steel was investigated because of the suggestion that flow reduction could be due to welding of asperities but flow rate reduction was not observed at reduced pressure down to 10− 4 Pa.