Abstract
Powder processing and manufacturing operations are rate processes for which the bottleneck is cohesive powder flow. Diversity of material properties, particulate form, and sensitivity to environmental conditions, such as humidity and tribo-electric charging, make its prediction very challenging. However, this is highly desirable particularly when addressing a powder material for which only a small quantity is available. Furthermore, in a number of applications powder flow testing at low stress levels is highly desirable. Characterisation of bulk powder failure for flow initiation (quasi-static) is well established. However, bulk flow parameters are all sensitive to strain rate with which the powder is sheared, but in contrast to quasi-static test methods, there is no shear cell for characterisation of the bulk parameters in the dynamic regime. There are only a handful of instruments available for powder rheometry, in which the bulk resistance to motion can be quantified as a function of the shear strain rate, but the challenge is relating the bulk behaviour to the physical and mechanical properties of constituting particles. A critique of the current state of the art in characterisation and analysis of cohesive powder flow is presented, addressing the effects of cohesion, strain rate, fluid medium drag and particle shape.
Highlights
Particulate solids are ubiquitous in many manufacturing industries, ranging from pharmaceuticals, foods, chemicals and minerals to additive manufacturing, which is the fastest growing sector in high value manufacturing and depends critically on powder spreading
Beyond the quasi-static regime the bulk flow parameters are all sensitive to shear strain rate (Tardos et al, 2003), but in contrast to quasi-static test methods, there is no shear cell that can characterise the bulk parameters at high strain rates, i.e. in the dynamic regime
In the work reviewed here, we provide a complementary critique of current trends and challenges in the characterisation methods and instruments, and analysis of dynamics of cohesive powder flow with a focus on the rheological properties and methods for their characterisation
Summary
Particulate solids are ubiquitous in many manufacturing industries, ranging from pharmaceuticals, foods, chemicals and minerals to additive manufacturing, which is the fastest growing sector in high value manufacturing and depends critically on powder spreading. There are a large number of commercial devices as well as some developed inhouse, which can apply a compressive and shearing or a tensile load to a powder bed and provide information on bulk density, bulk cohesion, internal and wall angles of friction, unconfined yield stress and tensile failure stress, all of which can be obtained as functions of applied load and consolidation time There are only a handful of instruments available for powder rheometry, the most prominent ones being the FT4 Rheometer of Freeman Technology, the powder cell of Anton Paar Modular Compact Rheometer and a Couette device, as described later below In these devices the bulk resistance to motion can be quantified as a function of speed of shearing (strain rate) and is reviewed here. The effects of particle size and shape, fluid drag, cohesion and strain rate are covered
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