As powders make up for about 50% of materials used in industry, powders are a significant area of physical testing. Handling and processing particulates, granules, and powders are key to product processing. However, these processes have conventionally been troubled with issues because of their random and irregular behavior, particularly with regards to flowability.
As several raw materials and semi-finished products are in powder form, this sector will surely gain considerable commercial and manufacturing advantages from enhancements in the evaluation of powder flow.
A number of end products that reach consumers need certain consideration of flowability, as well. For instance, while pouring a product like artificial sweetening powder from a jar, lumps and cakes will give the product a low-quality impression.
A Powder Flow Analyzer (PFA) is the most suitable way to evaluate the flow behavior of powder. Stable Micro Systems has replaced conventional manual pouring techniques by patenting a helix blade design that regulates the flow of a powder sample.
The sample movement is caused by a rotor or blade, which is passed via a powder column based on the settings fixed by the user — both rotating and moving vertically. This process conditions the powder and removes any loading variation between operators. It offers a basic quality control test for differences between sources and/or batches. The compacted samples will endure flow and movement.
Exponent software is integral to the Texture Analyzer and Powder Flow Analyzer and performs real-time display of data. Tests are configured rapidly and easily, and data analysis is set up to automatically gather critical parameters from each test in simple or advanced ways.
A wide array of methods can be used to perform many different tests on the Powder Flow Analyzer:
- Caking: This is the propensity of a powder to develop large agglomerates during transportation and storage. A powder’s tendency to form cake is closely associated with its cohesiveness. The cake’s strength will rely on several factors such as particle-to-particle interactions, packing efficiency, and humidity.
The caking test smoothly compacts the powder column to a force preset by the user, and then slices back to the top of the column with the least disturbance, before re-compacting. This is repeated for the set number of cycles, and then, the compacted cake is sliced by the rotor blade.
- Cohesion: Cohesiveness is the propensity of the particles of powder to stick together and agglomerate (develop larger clusters of particles). The PFA quantifies this cohesion feature by operating the blade such that the powder is lifted. A more cohesive powder will stick to itself and to the blade, thereby decreasing the force applied on the base of the vessel.
- Powder Flow Speed Dependence: The flow properties of powder may change with decreasing or increasing flow speeds. For instance, a powder may turn highly resistant to flow as it is pushed to flow faster. Alternatively, it may become freer-flowing as the flow speed intensifies. This issue can result in under- or overfilling due to process changes to match increased output demand.
The PFA quantifies this property by evaluating the force needed to move the blade through the powder at accelerating speeds. An assessment of the flow stability of the powder is performed by comparing the force required to move the blade through the powder at the beginning of the test compared to the work needed to shift the powder at the same speed at the end of the test.
- Bridging: When the flow imposed by the PFA is regulated, powders that flow easily will display minimal resistance transferred through the powder column in an upward or downward direction. By contrast, powders that flow poorly will display a considerable amount of force in both directions.
As the blade works through the powder column in both compression (downward) and lifting (upward) directions, powders resist the advancing blade. Powders that flow freely will easily cascade over and around the blade, thereby producing a clear graph. If the powder briefly arches, bridges, or develops cohesive bonds, further advancement of the blade will break those bonds. In granulated products, the particles can clump up and cause a similar issue.
In general, force increases as the bonds are formed and stressed, immediately followed by a decrease in force as the bonds break. The degree of resistance and failure can be quantified simply by measuring the length of a plotted line (which compares well with the jaggedness of a line). The measurement of bridging is automatically performed during the post-test analysis.
- Bulk Density: A vessel that divides into two with a hinge is available to help remove excess powder (after conditioning) and leaves behind a particular volume. The PFA’s ability to measure weight enables the automatic calculation of bulk density. In the case of solid objects, density is the ratio between volume and mass. The structure of powders is highly variable, however; therefore, their density can be freely modified by alterations to their packing.
The conditioning cycles on the PFA facilitate a reproducible density measurement by treating each powder sample in a repeatable and systematic way prior to each test. At the time of the conditioning process, filling stresses and variances in filling methods are eliminated by directing the blade to slice/aerate (lift) via the sample. This creates an even sample, which supports comparison and repeatability of results.
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Powder Flow Analysis - How to measure powder flow