Powder flow can be measured by an MCR rheometer combined with the Powder Cell.
The device can simulate and determine powder flow under different conditions.
An MCR powder rheometer ensures quality control and smooth powder processing, by determining powder flow under various conditions or giving a Cohesion Strength value.
Similar to viscosity in liquids, powder flow values show resistance of a powder to movement if a force is applied – known as Cohesion Strength, said the firm.
Based on Cohesion Strength it is possible to predict whether the powder will flow through a process smoothly and whether quality has changed.
Powder flow behaviour
Patrizia Binder, technical marketing, business unit characterization at Anton Paar, said the human mouth is very sensitive to changes in texture/composition of a well-known product.
“So if for example the salt is not distributed evenly on the potato chips or if the texture of your yoghurt is different than usual the human mouth will detect it," she told FoodQualityNews.
“To avoid these kinds of variations, it is important that the raw materials are stable in quality and the processing of raw materials (often in the form of powder), e.g. the mixing process, has to work properly.
“Characterizing flow behaviour of powder and thus detecting changes of the powder quality (due to changes in particle size, chemical structure, surface morphology etc.) is possible with the new powder rheometer.”
A rheometer is a laboratory device used to measure the way in which a liquid or powder flows in response to applied forces.
The Powder Rheometer was introduced at the 8th International Conference for Conveying and Handling of Particulate Solids (CHoPS) 2015 in Tel Aviv (May 3-7).
An automatic measurement program ensures results within two minutes, automatic calibration and live visualization of the measurements.
It provides the measuring possibilities available from rheology in the field of powder, including rotational and oscillatory modes.
Binder said it is possible to measure powder flow also in the subfluidized and fluidized state.
“This gives you information about powder processability e.g. for pneumatic transport, mixing, spraying, powder coating processes, fluidized bed dryers and drying processes in general,” she said.
“Furthermore the general function of a rheometer - the possibility to preset different shear rates - makes it possible to simulate processes conditions e.g. in nozzles or pipes."
”Powder flow issues
Binder said most common difficulties are related to powder flow.
“Powder can either flow too well, which can cause problems in dosing or flooding funnels or it can flow dull and in this way cause blockages in pipes or nozzles. In some cases that two different batches from the same powder differ in powder flow quality (one runs through the process smoothly and the other one doesn't),” she said.
“This is often related to variations in powder quality caused by the powder supplier (change in composition, humidity, particle size and distribution…) or mistakes during handling or storage.
“The problems when dealing with powders are as versatile as the industries dealing with powder. Some examples from the food industry are e.g. milk powder, starch powder, salt on potato chips, sugar, rice etc.”
Powder flow can be influenced by particle size and shape, chemical structure, humidity, temperature, packaging behaviour, static charge, surface morphology, gas adsorption or distribution of particle size.
Often empirical methods are used for this task, like angle of repose, or measuring time the powder takes for running through a tunnel or other improvised methods. There are also automated instruments that can give an empirical value.
Anton Paar said the most scientific instrument is the shear cell, which was originally developed for silo design and provides values for powder flow under high loads.
There are also methods to characterize single influencing factors on the powder flow like particle sizes analyzers or Carr Ratio.