Powder Morphology Analysis
All OSD manufacturing processes use powders and bulk materials, and these materials are not simple to characterise or understand. A smooth transport and utilisation through production is required for an efficient process.
In parallel with sizing, a powders morphology -the physical structure and shape of a powder - can also affect its flow and compaction characteristics within a manufacturing process. Both optical and Scanning Electron microscopes are used to investigate a powders morphology. Typically, spherical type powders that are not very fine flow well whereas acicular – needle shaped – powders do not flow well. In addition, powders exhibiting a porous structure generally compact well.
A powder or compacts density is another important property to measure. The measurements generally undertaken are True density Bulk (aka apparent / volumetric) density and Tap density. The true density is measured using a Helium Pycnometer. This apparatus uses helium gas to infiltrate porosity within the powder so the volume measured is only that of the actual powder. This gives a real measurement of the powders density which can be used in calculations. Bulk, or volumetric, density along with Tap density are more in-process measurements rather than physical properties of the powder. The density calculated when powder is gently poured into a known volume – without Tapping!- and that powder weighed is the Bulk density. If this weight is then tapped to force the powder to settle, reducing the volume, the resulting calculated density is called the Tap density. All these measurements are important for setting up and controlling a production process.
The last side of this powder characterisation triangle is powder flow behaviour, or Rheology. How a powder flows and moves through a production process is obviously extremely important for a dependable process. Building a link between a powders, or mixture of powders in a formulations, rheology with its other properties is imperative in fully understanding powders in production. For this work we use a Freeman FT4 powder Rheometer and accessories, giving us a universal powder flow tester. This gives us the ability to simulate powder processing conditions, by testing samples in consolidated, moderately stressed, aerated or a fluidised state. Multi-faceted powder characterisation can assess dynamic powder flow, bulk and shear properties to construct a comprehensive understanding of how a powder behaves.
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