Several characteristics of powders make them suitable or unsuitable for uniform feeding. Ideally, powders should be free-flowing, normally distributed, and free of unwanted fine particulates. Fines are often called satellites, because they adhere to larger particles physically or electrostatically. The ideal situation rarely exists, so it is necessary to understand the characteristics of powders that make them usable for feeding and spraying. Among the important powder characteristics are:
- Powder particle size
- Particle size n
- Morphology
- Method of manufacture
- Apparent density
- Moisture content
Apparent density and flowability can be measured quickly and inexpensively. Such tests are illustrated in the figure, which includes a modified flow test that measures powder flow through a metering orifice as a function of time.
Powder that fails to flow through the orifice signifies poor flowability. The angle of repose or slope of the mound of powder formed by powder dropping from a specified height through a flow orifice can also be used to assess relative flowability. Poor flowability would be indicated by steep slopes of the powder pile, while free-flowing powders would have little slope. High angles of repose alone would not exclude a powder from use, although caution in feeding and handling would be suggested.
Image: Powder flowability measurements. (a) Hall flowability test. (b) Powder and angle of repose
This information is from ASM Handbooks Online, Vol. 5A: Thermal Spray Technology, Coating Structures, Properties, and Materials. To find this information (subscription required), click on the link below and scroll down to Figure 7.
https://dl.asminternational.org/handbooks/book/12/chapter/134972/Introduction-to-Coating-Design-and-Processing
