Flowability
Flowability is not a constant material property – it arises from the interaction between the powder, the additive and the mixing process. Particularly with poorly flowing powders, the type of mixing determines whether minimal quantities of a flow aid can have any effect at all. This requires a mixing process that is efficient, brief and gentle. Only in this way are additives distributed precisely at the contact points between the particles, without causing unnecessary compaction, segregation or agglomeration.
Various parameters are used to assess flowability. A simple, practical parameter is the angle of repose (also known as the angle of repose). It describes the angle of inclination of a freely poured powder cone and is derived from the geometry:
tan(α) = H/R
- • α is the angle of repose
- • H is the height of the powder cone
- • R is the radius of the base
Small angles of repose indicate good flowability, whilst large angles indicate cohesive or poor-flowing behaviour. The angle of repose is sensitive to the use of flow aids – though this effect is only reproducible if these are distributed homogeneously. This is precisely where the importance of a suitable mixing concept becomes apparent.
Andrew Jenike shear tests are frequently used to provide a robust, quantitative description. They provide the so-called flow function FF, which describes the relationship between the uniaxial compressive strength σc and the principal normal stress σ1:
FF = σ1/σc
Based on these parameters, powders can be classified and flow characteristics for silos determined – for example, for the design of discharge openings, wall angles and discharge aids. The higher the flow function, the better the flowability of the material. Powders with FF<4 are, for example, considered to flow poorly, whilst materials with FF>10 are free-flowing.
The key principle remains: flow aids do not work through quantity, but through distribution. An efficient, gentle mixing process can specifically improve flowability, reduce angle of repose and shift flow functions – thereby stabilising the entire process chain from conveying through storage to dosing.