Additive manufacturing
Powder Characterization using the GranuCharge in AM
The bulk density, the tapped density and the Hausner ratio measurement (commonly named “tap-tap test”) is very popular for powder characterization because of both the simplicity and the rapidity of the measurement...
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 GranuPack and the GranuCharge instruments.
GranuPack
The bulk density, the tapped density and the Hausner ratio measurement (commonly named “tap-tap test”) is very popular for powder characterization because of both the simplicity and the rapidity of the measurement. Moreover, the density and the ability of a powder to increase its density are important parameters for storage, transportation, caking, etc. The recommended procedure is defined in the pharmacopeia. This simple test has three major drawbacks.
First, the result of the measurement depends on the operator. Indeed, the filling method influences the initial powder volume. Secondly, the volume measurements by naked eyes induce strong errors on the results. Finally, with this simple method, we completely miss the compaction dynamics between the initial and the final measurements.
The GranuPack instrument is an automated and improved tapped density measurement method based on recent fundamental research results. The behaviour of the powder submitted to successive taps is analysed with an automatized device. The Hausner ratio Hr, the initial density ρ(0) and the final density after n taps ρ(n) are measured precisely. The tap number is commonly fixed at n=500. Moreover, a dynamical parameter n1/2 and an extrapolation of the maximum density ρ(∞) are extracted from compaction curves. Additional indexes can be used but they are not presented in this report.
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The powder is placed in a metallic tube with a rigorous automated initialization process. Afterwards, a light hollow cylinder is placed on the top of the powder bed to keep the powder/air interface flat during the compaction process. The tube containing the powder sample rose up to a fixed height of ΔZ and performs free falls. The free fall height is generally fixed to ΔZ = 1mm or ΔZ=3mm. The height h of the powder bed is measured automatically after each tap. From the height h, the volume V of the pile is computed.
As the powder mass m is known, the density ρ is evaluated and plotted after each tap. The density is the ratio between the mass m and the powder bed volume V.
With the GranuPack method, the results are reproducible with a small quantity of powder (typically 35 ml). The Hausner ratio Hr is related to the compaction ratio and is calculated by the equation Hr = ρ(500) / ρ(0) , where ρ(0) is the initial bulk density and ρ(500) the tapped density computed obtained after 500 taps.
GranuCharge
Electrostatic charges are created inside a powder during a flow. This apparition of electric charges is due to the triboelectric effect, which is a charge exchange at the contact between two solids.
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During the flow of a powder inside a device (mixer, silo, conveyor, …), the triboelectric effect takes place at the contact between the grains and at the contact between the grains and the device. Therefore, the characteristics of the powder and the nature of the material used to build the device are important parameters.
The GranuCharge instrument measures automatically and precisely the quantity of electrostatic charges created inside a powder during a flow in contact with a selected material. The powder sample flows inside a vibrating V-tube and fall in a Faraday cup connected to an electrometer. The electrometer measures the charge acquired by the powder during the flow inside the V-tube. In order to obtain reproducible results, a rotating or a vibrating device is used to feed the V-tube regularly.
Selected Powders
Five powders were selected during this study:
- Two Duraform PA12 (called DFPA) Powders (Polyamide 12), the first one is virgin and the other recycled;
- Arkema Orgasol Invent Smooth (Polyamide 12);
- Two steel powders, one from Carpenter (1.4404) and the other from LPW (1.4404).
The powders listed above are standard powders used in the Selective Laser Sintering (SLS) and the Selective Laser Melting (SLM) processes. The SLS process is based on polymeric powders while the SLM process uses metal powder as basis material.
The Duraform polyamide 12 is investigated in its virgin and recycled form. The recycling step is needed to ensure a good quality of the built parts because of the post-condensation mechanism leading to an increase in molecular weight of the unused polyamide 12 powder [put Manfred publication]. As the powder bed is heated up just below the melting temperature of the polymer in SLS, reactive end-groups of the polyamide 12 interact with each other to increase the length of their molecular chains and thus the viscosity of the melt. This behavior is not observed with the polyamide 12 powder named Orgasol Invent Smooth, produced by Arkema, its chains’ ends being capped. Finally, the metal powders are both based on the conventional 1.4404 stainless steel also known as the 316L.
Polyamide 12 Powders
 pictures of the SLS polymer powders.png)
Figure 1 displays Scanning Electron Microscope (SEM) pictures of the SLS polymeric powders. These images give a good overview of the extrinsic properties of these powders. The DFPA powders are rather potato shaped while the Orgasol is more spherical. The DFPA are coarser and present disparate sizes while Orgasol particles are smaller and present almost an identical size. These visual observations are correlated with the analytical investigations presented below for the Particle Size Distribution (PSD) performed by laser diffraction on a Beckman-Coulter LS230.
Figure 2 presents the particle size distributions (PSD) of both polymer (left) and metal powders (right). The Orgasol Invent Smooth presents a strongly mono-modal distribution while both DFPA powders are wider, especially in the range 1 to 30 μm. It is a bimodal distribution of fine and coarse particles presenting a potato shape. The difference in size is observed in Table 1 where the volume and number distribution are presented. The number distribution for all materials are strongly skewed to the right because the fine particles weight 1:1 with the coarse while they only weight 1000:1 for the volume distribution. The Carpenter 1.4404 powder presents a PSD with is significantly lower than the LPW powder. For the metal powders in Table 2, it is important to note that the amount of fine particles in the Carpenter is reduced to a minimum compared to the LPW and the polymer powders as well. However, both powders present a rather broad distribution.
| Volume Distribution | Number Distribution | |||||
|---|---|---|---|---|---|---|
| DF PA12 Virgin | DF PA12 Recycled | Orgasol Invent Smooth | DF PA12 Virgin | DF PA12 Recycled | Orgasol Invent Smooth | |
| D10 | 16.17 | 26.20 | 30.56 | 0.06 | 0.06 | 0.05 |
| D50 | 57.43 | 57.75 | 38.39 | 0.09 | 0.08 | 0.09 |
| D90 | 81.78 | 83.95 | 46.72 | 0.21 | 0.17 | 0.2 |
| Aspect Ratio | 1.61 ± 0.38 | 1.76 ± 0.65 | 1.3 ± 0.18 | |||
| Elliptic Smoothness | 1.04 ± 0.05 | 1.23 ± 0.12 | 1.07 ± 0.03 | |||
| Solidity | 0.87 ± 0.05 | 0.81 ± 0.1 | 0.92 ± 0.02 | |||
The form factors presented in the Table 1 were identified as being useful in the prediction of flowing behavior of SLS powders. The aspect ratio, or AR, is the ratio between the major to minor axes of the fitted ellipse around the particle. The elliptic smoothness gives an impression of the roughness of the particle’s surface; it is computed as the ratio between the particle perimeter and its fitted ellipse.
Finally, the solidity quantifies the “compactness” of the particle by the ratio between the particle’s area to the convex area surrounding the particle. Powders that are spherical and smooth will be more likely to flow homogeneously past each other while elongated and rough particles would be hindered and tend to agglomerate.
From the values presented hereabove, the observations on Figure 1 are also confirmed. The Orgasol powders are compact and rather spherical, high solidity and low aspect ratio, while the DFPA are more elongated and convoluted, thus higher AR and a slightly lower solidity.
Metallic Powders
The Carpenter steel particles showed in Figure 3 present not only spherical shapes but also a smooth surface. However, due to the production method, namely gas atomization of molten metal, some particles are welded together and present elongated shapes such as in the right picture. It can also be observed that the fine particles tend to stick to coarser ones. The form factors for the metal powders are not provided but one would expect values approaching the Orgasol powder, i.e. high solidity, low aspect ratio and rather low elliptic smoothness.
| Volume Distribution | Number Distribution | |||
|---|---|---|---|---|
| Carpenter 1.4404 | LPW 1.4404 | Carpenter 1.4404 | LPW 1.4404 | |
| D10 | 10.15 | 18.48 | 7.415 | 0.90 |
| D50 | 17.21 | 29.97 | 10.77 | 1.17 |
| D90 | 33.58 | 45.36 | 17.27 | 2.29 |
Discussion
The five powders presented above are of interest as they all perform well in their respective processes. However, they differ slightly in certain aspects, being the molecular structure for DFPA virgin, recycled and Orgasol or the size distribution for the 1.4404 Carpenter and LPW. The packing behavior of powders for the SLS and SLM processes is crucial as it impacts directly on the final density of the part produced, and subsequently its mechanical properties. Moreover, the packing behavior of a powder is related to its spreading behavior that is equally important for ensuring a homogeneous powder bed and a consistent interaction with the laser source.
The investigation of the packing behavior using the Granupack should enable to get a deeper insight into the way these powders behave under stress and could hopefully help unravelling the complex relationships between size, form and compaction behavior.
GranuPack Analysis
Experimental Protocol
For each experiment with the GranuPack, 500 taps were applied to the sample with a taps frequency of 1Hz and the measurement cell free-fall was 1 mm (∝ tap energy). Before an experiment, air temperature and hygrometry are recorded. Some sample were analyzed two times. The main purpose is to prove the high accuracy of the GranuPack instrument and to highlight the ageing of powders and their flowability.
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Experimental Data
The powder mass is recorded before each experiment. The sample is poured inside the measurement cell by following the software instructions (i.e. without user dependency). Powders bulk densities were investigated under the same moisture conditions (approximately 40% RH and 24°C).
The following table summarizes experimental data before the experiment (RHpack, Tpack and mpack are respectively the relative humidity, temperature and powder mass measured before samples introduction inside the GranuPack):
| Sample | RHpack (%) | Tpack (°C) | mpack (g) |
|---|---|---|---|
| DFPA Virgin Test n°1 | 41.9 | 23.8 | 16.12 |
| DFPA Virgin Test n°2 | 40.6 | 23.8 | 16.29 |
| DFPA Recycled Test n°1 | 40.3 | 23.8 | 16.06 |
| DFPA Recycled Test n°2 | 40.0 | 23.9 | 16.05 |
| Carpenter 1.4404 | 38.8 | 24.5 | 112.48 |
| LPW 1.4404 Test n°1 | 39.5 | 24.9 | 111.23 |
| LPW 1.4404 Test n°2 | 40.0 | 25.0 | 158.75 |
| Arkema Orgasol IS | 39.7 | 25.0 | 19.23 |
Figure 6 and Figure 7 represent the full compaction curves, which is the bulk density variations (ρ(n) – ρ(0)) versus the number of taps, respectively for Polyamide 12 and Steel powders (error bars have been displayed but they are too small to be visible, in fact bulk density error is close to 0.4%).
Full results are described by the following table. Were ρ(0) is the initial bulk density (in g/mL), ρ(500) is the bulk density after 500 taps (g/mL) and ρ(∞) is the optimal bulk density (in g/mL, calculated by a model available in the GranuPack software) which the minimum density the tapping test can achieved. Hr and Cr are respectively, the Hausner and Carr ratios. n1/2 and τ are two parameters linked to the compaction kinetic:
| Sample Name | p(0) (g/mL) | p(500) (g/mL) | n1/2 | Hr | Cr | p(∞) (g/mL) | τ |
|---|---|---|---|---|---|---|---|
| DFPA Virgin Test n°1 | 0.467 | 0.531 | 8.8 | 1.136 | 12.0 | 0.547 | 10.8 |
| DFPA Virgin Test n°2 | 0.462 | 0.526 | 9.5 | 1.139 | 12.2 | 0.543 | 10.7 |
| DFPA Recycled Test n°1 | 0.453 | 0.539 | 13.6 | 1.191 | 16 | 0.565 | 18.1 |
| DFPA Recycled Test n°2 | 0.456 | 0.543 | 14.9 | 1.192 | 16.1 | 0.569 | 17.7 |
| Carpenter 1.4404 | 4.045 | 4.953 | 14.6 | 1.225 | 18.3 | 5.272 | 30.7 |
| LPW 1.4404 Test n°1 | 4.515 | 5.006 | 30.7 | 1.109 | 9.8 | 5.237 | 67.5 |
| LPW 1.4404 Test n°2 | 4.498 | 4.964 | 30 | 1.103 | 9.4 | 5.176 | 62.6 |
| Arkema Orgasol IS | 0.517 | 0.585 | 5.9 | 1.131 | 11.6 | 0.604 | 13.7 |
Results Interpretation
Figure 6, Figure 7 and Table 4 show that the GranuPack instrument can make differentiations between every powder. Indeed, if we compare DFPA Virgin to Recycled powders, we can see that Virgin powder has a better flowability (with Hausner ratio) and an easy compaction (with n1/2 parameter) than Recycled one. This fact may be due to the aspect ratio of the particles. Virgin powder with a AR close to 1.61 is closer to a spherical shape than the Recycled sample (AR = 1.76). Thus, the compaction kinetics will be slower for Recycled than Virgin powder, because of the higher contact points between particles for the recycled version. This fact is confirmed by the solidity of those particles, close to 0.87 for Virgin powder and to 0.81 for Recycled, which means that the amount of “void” around Recycled polyamide 12 will be larger than Virgin one.
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The flowability and compaction kinetics of the Arkema Orgasol Invented smooth is respectively good and fast due to the powder solidity close to 1 (0.91) and this is confirmed by the Hausner ratio (1.131) and n1/2 parameter (5.9).
Regarding Carpenter and LPW steel powder we can see with GranuPack analysis that the flowability of LPW powder is better than the Carpenter steel. A possible explanation is the particle size distribution, with D50 = 17.21μm for Carpenter and D50 = 29.97μm for LPW. Indeed, the lower the particle size, the lower the flowability (if we consider the same sample material).
GranuCharge Analysis
Experimental Protocol
The triboelectric effect of the powders was investigated with the help of GranuCharge instrument. For each experiment with the GranuCharge instrument, Stainless-steel 316L pipes and rotating feeder were used (cf. Figure 8):
The quantity of product used for each measure was approximately 50mL (exact value describes in the next table) and the powder was not recycled after a measurement. Tests have been repeated two times in order to show GranuCharge accuracy/reproducibility (except for Arkema Orgasol Invent Smooth Powder).
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Before the experiment, powder mass (mp, in g), air relative humidity (RH, %) and temperature (°C) are recorded. At the beginning of the test, the initial powder charge (Qi, in μC) is measured by introducing powder inside the Faraday cup two times, the average value is given in Table 6. Once these steps are completed, the powder is poured inside the rotating feeder, then experiment is started. Final charge is measured at the end of experiment (Qf, μC). The next table summarize the raw results obtained:
| Sample | RH (%) | T (°C) | mp (g) |
|---|---|---|---|
| DFPA Virgin Test n°1 | 41.4 | 23.6 | 25.15 |
| DFPA Virgin Test n°2 | 42.1 | 23.4 | 25.08 |
| DFPA Recycled Test n°1 | 42.6 | 23.5 | 25.13 |
| DFPA Recycled Test n°1 | 42.0 | 23.7 | 25.04 |
| Carpenter 1.4404 Test n°1 | 41.5 | 22.7 | 158.61 |
| Carpenter 1.4404 Test n°2 | 41.9 | 23.0 | 190.52 |
| LPW 1.4404 Test n°1 | 41.9 | 23.1 | 200.71 |
| LPW 1.4404 Test n°2 | 41.6 | 23.2 | 187.13 |
| Arkema Orgasol IS | 43.0 | 23.2 | 28.22 |
The following figure represents the electrical charge density acquire by every powder during a flow in contact with Stainless-Steel:
Table 6 summarize all the results obtained with the GranuCharge instrument.
Each charge density value corresponds to the average value calculated with all reproducibility tests (in μC/kg):
| Sample | q0Mean (µc/kg) | qf (µc/kg) | qfMean (µc/kg) | τq (µC/kg) |
|---|---|---|---|---|
| DFPA Virgin Test n°1 | 1.40 10-1 | 5.39 10-1 | 5.51 10-1 | 4.12 10-1 |
| DFPA Virgin Test n°2 | 5.63 10-1 | |||
| DFPA Recycled Test n°1 | 4.19 10-2 | 2.97 10-1 | 3.10 10-1 | 2.68 10-1 |
| DFPA Recycled Test n°2 | 3.23 10-1 | |||
| Carpenter 1.4404 Test n°1 | -3.35 10-1 | -4.93 10-2 | -5.38 10-2 | 2.81 10-1 |
| Carpenter 1.4404 Test n°2 | -5.83 10-2 | |||
| LPW 1.4404 Test n°1 | -2.24 | -5.75 10-2 | -5.97 10-2 | 2.18 |
| LPW 1.4404 Test n°2 | -6.15 10-2 | |||
| Arkema Orgasol IS | 8.19 10-1 | -6.74 10-1 | -6.74 10-1 | -1.49 |
Results Interpretation
This table is interesting, because it shows that with the GranuCharge instrument, differentiations between powders can be made with great accuracy. Even with initial charge densities, which are highly different. Indeed, LPW has the highest initial charge, quickly followed by Arkema Orgasol Invent Smooth. Carpenter comes in third position while DFPA virgin and recycled have a similar value, with a slightly lower charge density for the Recycled version.
After a flow, LPW steel is progressively discharged, once this value reach 0 the powder will acquire positive charges on contact with stainless-steel pipes. Arkema Orgasol Invent Smooth have an opposite behaviour, because it will acquire negative charge during the flow. Electrical charges for DFPA Virgin powder is higher than Recycled after a flow inside the Stainless-Steel tubes. The same behaviour is highlighted if we compare LPW to Carpenter steel.
This fact may be due to powders flowability, which is higher for Virgin and LPW than Recycled and Carpenter powders.
Finally, polyamide 12 provided by Arkema is negatively charge at the end of experiment, while DFPA acquire positive charge. One possible explanation may be differences between particles coating, but further testing need to be done to prove this assumption.
Conclusion
- ✓ Ageing of powders is difficult to highlight with classical methods.
- ✓ Hopefully, with its high accuracy measurements, the GranuPack can show differences between the virgin and recycled version of the same powder. However, if one wants to see higher difference between powders, he needs to investigate the triboelectric effect, which is measured by the GranuCharge instrument.
- ✓ Indeed, this analyser is highly sensitive to particles surface state such as oxidation, pollution, rugosity, etc.
- ✓ Ageing of powders is followed by the GranuCharge instrument with great accuracy (cf. reproducibility tests) and it is a great way to show the ageing of powders.
- ✓ The GranuPack instrument is sensitive to particles aspect ratio and solidity.
References
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