Improved laboratory silo to fulfil Additive manufacturing industries requirements

Application Note using

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Improved laboratory silo to fulfil Additive manufacturing industries requirements

nomenclature of Improved laboratory silo to fulfil Additive manufacturing industries requirements application note

Introduction

Theoretical Framework

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

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.

In a second part, a comparison between Hall Flowmeter and GranuFlow is presented in order to show the advantage of using GranuFlow.

GranuFlow and Hall Flowmeter analysis

Experimental protocol

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.

Experimental results

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):

Figure 1 : Powder flowrate versus aperture area for the SS 316L powders, comparison with the Hall Flowmeter

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.

Conclusions

GranuFlow reproduces Hall Flowmeter results

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).

Bibliography

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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).

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Appendix 1 : The mass flowrate F through a circular orifice of diameter D is given by the product of the mean speed of the grain <V out>, the aperture area and the bulk density p. One has the general expression F=p <v out> π D² / 4

Appendix 2 : Hall flowmeter raw results : Table 1 : Hall flowmeter results for SS 316L Powder (Virgin and Recycled)