Application Note using
Improved laboratory silo to fulfil Additive manufacturing industries requirements
Granular materials and fine powders are widely used in industrial applications.
To control and to optimize processing methods, these materials have to be precisely characterized.
The characterization methods are related either to the properties of the grains (granulometry, morphology, chemical composition, …) and to the behaviour of the bulk powder (flowability, density, blend stability, electrostatic properties, …).
However, concerning the physical behaviour of bulk powder, most of the techniques used in R&D or quality control laboratories are based on old measurement techniques.
During the last decade, we have updated these techniques to meet the present requirements of R&D laboratories and production departments.
In particular, the measurement processes have been automatized and rigorous initialization methods have been developed to obtain reproducible and interpretable results.
Moreover, the use of image analysis techniques improves the measurements precision.
A range of measurement methods has been developed to cover all the needs of industries processing powders and granular materials.
However, in this application note, we will be focused on the GranuFlow instrument.
GranuFlow is an improved laboratory silo compared to the ancient Hall Flow Meter (ASTM B213, ISO4490) and compared to the “Flow Through An Orifice” method described in the Pharmacopeia (USP1174).
GranuFlow is a straightforward powder flow-ability measurement device, composed of a silo with different apertures associated with a dedicated electronic balance to measure the flowrate.
This flowrate is computed automatically from the slope of the mass temporal evolution measured with the balance.
The aperture size is modified quickly and easily with an original rotating system.
The measurement and the result analysis are assisted by software.
The flowrate is measured for a set of aperture sizes to obtain a flow curve.
Finally, the whole flow curve is fitted with the well-known Beverloo theoretical model to obtain a flowability index (Cb, related to the powder flowability) and the minimum aperture size to obtain a flow (Dmin) (for theoretical background, user can refer to Appendix 1).
The whole measurement is performed easily, quickly and precisely.
In this paper, we used a complete set of hole diameters: 1, 1.5, 2, 2.5, 3, 3.5 and 4mm.
The main purpose of this application note is to provide information about the measurements reproducibility with the GranuFlow and show some examples about what is it able to offer.
GranuFlow and Hall Flowmeter analysis
A Stainless-Steel 316L powders provided by Höganäs was used for this application note. A comparison between the original powder and a used version (after a SLM process) is also proposed.
All the GranuFlow analysis were performed at 21°C and 34.0%RH. F is the powder flowrate (in g/s) and Cb the Beverloo parameter (in g/cm3) and Dmin is the minimum aperture size to obtain a flow (cf. Appendix 1). All tests were repeated two times.
For the Hall Flowmeter experimental protocol, tests have been repeated five times with approximately the same powder mass (cf. Appendix 2).
All tests were carried out at 36.5%RH and 25°C.
The average mass flowrate is plotted versus the aperture area (this choice is motivated to obtain linear results) The Beverloo law displayed is calculated with the average Cb and Dmin coefficients. Error bars are calculated using the standard deviation (however, sometimes they are too small to be visible):
As we can see, GranuFlow measurements show great accuracy (maximum error close to 1.67%), with this instrument the powders flowability classification can be easily highlighted.
Indeed, due to the curve trends, it is possible to deduce that the virgin powder has a slightly better flowability than the used version.
Moreover, this fact is confirmed by the Beverloo Law, and more precisely with the Cb parameter, which is higher for the virgin sample (2.46g/cm3) than the used material (2.35g/cm3).
Finally, with this equation it is possible to make data interpolation, while it is impossible with the Hall Flowmeter.
Additionally, those results are very interesting, they allow us to see that the GranuFlow and the Hall Flowmeter show similar results.
However, some differences can be highlighted. First, although both instruments results can be compared, the Hall Flowmeter only gave us data for one diameter, while the GranuFlow allows to fully characterize the powder with a lot different holes diameter.
Moreover, the Hall Flowmeter can make no difference between those two samples, while the GranuFlow shows (both with Cb parameter and curves analysis) that SS 316L virgin powder has a slightly better flowability than the recycled one.
✓ GranuFlow allows to plot the full mass flow-rate curve (with 1.67% of average accuracy), while the Hall Flow-meter only gives the mass flow-rate measurement for one diameter.
✓ GranuFlow provides powder flow-ability measurements with the Beverloo Law (i.e. Cb coefficient, with 1.2% of standard deviation for a metallic powder) and an estimation of the Cohesive Index with Dmin parameter (minimum diameter for the powder to flow in silo configuration)
✓ Since Hall Flow-meter only makes one measurement for a small aperture size, it is unable to differentiate one sample to another (especially if they are very similar, cf. Virgin and Recycled versions of the same powder).
Cascade of granular flows for characterizing segregation, G. Lumay, F. Boschin, R. Cloots, N. Vandewalle, Powder Technology 234, 32-36 (2013).
Combined effect of moisture and electrostatic charges on powder flow, A. Rescaglio, J. Schockmel, N. Vandewalle and G. Lumay, EPJ Web of Conferences 140, 13009 (2017).
Compaction dynamics of a magnetized powder, G. Lumay, S. Dorbolo and N. Vandewalle, Physical Review E 80, 041302 (2009).
Compaction of anisotropic granular materials: Experiments and simulations, G. Lumay and N. Vandewalle, Physical Review E 70, 051314 (2004).
Compaction Dynamics of Wet Granular Assemblies, J. E. Fiscina, G. Lumay, F. Ludewig and N. Vandewalle, Physical Review Letters 105, 048001 (2010).
Effect of an electric field on an intermittent granular flow, E. Mersch, G. Lumay, F. Boschini, and N. Vandewalle, Physical Review E 81, 041309 (2010).
Effect of relative air humidity on the flowability of lactose powders, G. Lumay, K. Traina, F. Boschini, V. Delaval, A. Rescaglio, R. Cloots and N. Vandewalle, Journal of Drug Delivery Science and Technology 35, 207-212 (2016).
Experimental Study of Granular Compaction Dynamics at Different Scales: Grain Mobility, Hexagonal Domains, and Packing Fraction, G. Lumay and N. Vandewalle, Physical Review Letters 95, 028002 (2005).
Flow abilities of powders and granular materials evidenced from dynamical tap density measurement, K. Traina, R. Cloots, S. Bontempi, G. Lumay, N. Vandewalle and F. Boschini, Powder Technology, 235, 842-852 (2013).
Flow of magnetized grains in a rotating drum, G. Lumay and N. Vandewalle, Physical Review E 82, 040301(R) (2010).
How tribo-electric charges modify powder flowability, A. Rescaglio, J. Schockmel, F. Francqui, N. Vandewalle, and G. Lumay, Annual Transactions of The Nordic Rheology Society 25, 17-21 (2016).
Influence of cohesives forces on the macroscopic properties of granular assemblies, G. Lumay, J. Fiscina, F. Ludewig and N. Vandewalle, AIP Conference Proceedings 1542, 995 (2013).
Linking compaction dynamics to the flow properties of powders, G. Lumay, N. Vandewalle, C. Bodson, L. Delattre and O. Gerasimov, Applied Physics Letters 89, 093505 (2006).
Linking flowability and granulometry of lactose powders, F. Boschini, V. Delaval, K. Traina, N. Vandewalle, and G. Lumay, International Journal of Pharmaceutics 494, 312–320 (2015).
Measuring the flowing properties of powders and grains, G. Lumay, F. Boschini, K. Traina, S. Bontempi, J.-C. Remy, R. Cloots, and N. Vandewall, Powder Technology 224, 19-27 (2012).
Motion of carbon nanotubes in a rotating drum: The dynamic angle of repose and a bed behavior diagram, S. L. Pirard, G. Lumay, N. Vandewalle, J-P. Pirard, Chemical Engineering Journal 146, 143-147 (2009).
Mullite coatings on ceramic substrates: Stabilisation of Al2O3–SiO2 suspensions for spray drying of composite granules suitable for reactive plasma spraying, A. Schrijnemakers, S. André, G. Lumay, N. Vandewalle, F. Boschini, R. Cloots and B. Vertruyen, Journal of the European Ceramic Society 29, 2169–2175 (2009).
Rheological behavior of β-Ti and NiTi powders produced by atomization for SLM production of open porous orthopedic implants, G. Yablokova, M. Speirs, J. Van Humbeeck, J.-P. Kruth, J. Schrooten, R. Cloots, F. Boschini, G. Lumay, J. Luyten, Powder Technology 283, 199–209 (2015).
The flow rate of granular materials through an orifice, C. Mankoc, A. Janda, R. Arévalo, J. M. Pastor, I. Zuriguel, A. Garcimartín and D. Maza, Granular Matter 9, p407–414 (2007).
The influence of grain shape, friction and cohesion on granular compaction dynamics, N. Vandewalle, G. Lumay, O. Gerasimov and F. Ludewig, The European Physical Journal E (2007).