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Density Testing

As a critical property for both raw materials and finished products, density measurements must be conducted with extreme care – from initial process sampling all the way through to calibration of results and statistical analyses of data sets. The density of powder materials affects production operations and, ultimately, product properties. The density of solid product specimens then provides indispensable feedback to production optimization.

NSL Analytical provides powder density testing services to analyze the Apparent Density, Tap Density, Bulk Density, Skeletal Density and Dynamic Density of powder samples, as well as the True Density, Apparent Density, and Specific Density of solid specimens. Our teams of scientists have extensive knowledge of handling, sampling, preparation and analyzing samples to provide accurate, repeatable results.

Types of Density Testing

NSL provides testing services for the determination of materials density, particle density, and bulk powder bed density. The table below shows how various densities are defined by the volumes that are included in density calculations. NSL’s density measurement capabilities are highlighted, along with the corresponding test methods we employ.

Density Testing - Volumes Included in Various Densities Chart

Material Densities

In non-porous materials, the True (or Absolute) Density can be measured using Archimedes’ Principle of fluid displacement. Traditionally, water displacement has been used to provide approximate material density values. However, modern gas pycnometry (often using helium gas) provides much higher precision measurement of True Density, especially for materials that are hydrophobic or have fine surface roughness.

The table below shows the primary differences between the Archimedes’ water displacement and gas pycnometry techniques, along with sample requirements and typical result reporting precision.

Materials Density - Primary Differences b/n Archimedes’ Water Displacement & Gas Pycnometry

Four common test methods for metal AM/PM density measurements by these two techniques are compared in the table below. The first three methods are applied to formed specimens or parts (either green or sintered), while ASTM B923 applies to raw material metal powders. The two Archimedes test methods are distinguished by their application to either non-porous (ASTM B311) or porous (ASTM B962) bodies and associated sample preparation requirements.

Materials Density Table - Differences b/n Archimedes’ Water Displacement & Gas Pycnometry

Specimens produced from processes such as powder metallurgy or additive manufacturing often contain low levels of porosity (less than a few percent). With both water displacement (ASTM B311) and helium pycnometry (MPIF 63) methods, the resulting value represents the specimen’s Apparent Material Density. These density values are often converted into percent density values based on the materials’ specified theoretical maximum density. For plastic specimens (ASTM D792), the material’s Specific Gravity (or Relative Density) is reported.

Conventional gravimetric dimensional measurement of the density of carbon and graphite materials is used to obtain what is referred to as their Bulk Density (ASTM C559).

NSL’s Metallography and Microscopy Laboratory can further characterize the porosity of cross-sectioned specimens using both optical and electron microscopy.

Example: Metal Specimen Density Measurements

The table below (adapted from [1]) compares density measurements on a solid (<2% porosity) specimen of SS 316L produced via selective laser melting using three measurement techniques. The theoretical material density of SS 316L is 7.99 g/cm3. The listed uncertainty is the expanded uncertainty (k=2) from the reported standard deviation.

Materials Density Table Comparing Density Measurements on Solid Specimen of SS 316L

Particle & Powder Densities

When a powder sample is analyzed using helium pycnometry (ASTM B923), the resulting True or Skeletal Density describes the density of the particles that make up the powder. Only non-porous materials are described by the term True Density, with Skeletal Density being applied to materials with any significant porosity. As shown in the figure below, Skeletal Density includes inaccessible (closed) porosity, but not the interparticle void volume, which is easily penetrated by helium gas.

Fig. A:

Particle & Powder Densities Skeletal Density Figure

True/Absolute Density (image #1, Fig. A) – assumes closed porosity is absent or negligible

Skeletal Density (image #2, Fig. A) – includes closed porosity but not interparticle volume

Apparent/Bulk Density (image #3, Fig. A) – external bed volume is negligible for small particles

Apparent (or Bulk) Density refers to the density of a powder bed within a specified measurement volume. By specifying a maximum particle size and minimum cup volume, the techniques employed to measure bed density ensure that the external bed volume (between the bed and the container) is negligible.

The Apparent Density of metal powders is measured using a Hall or Carney funnel in accordance with ASTM B212 and ASTM B417, respectively. We request at least 75g of material to complete this test. For other granular materials, the Loose Bulk Density is measured according to ASTM D7481, which requires at least 50 cm3 of sample.

The Tap Density of metal powders is measured using automated, programmed mechanical tapping, following ASTM B527. A minimum of 100g of material is necessary for this test. For other powder materials, ASTM D7481 is used to measure the Tapped Bulk Density using similar procedures and at least 50 cm3 of sample.

Example: Metal Powder Density Measurements

The table below [2, 3] provides example density values for two 30 μm metal powders. While the Absolute (Theoretical) Density (from crystallographic experiments) of the base material will always be the highest density value, the Skeletal Density of the particles approaches the material density when the particles contain few porosity defects. By comparing these two density values, the relative density of the stainless steel and nickel powders can be expressed as 99% and 99.8%, respectively – and so both powders are capable of producing well-densified final parts. The Apparent Density of the will always be the lowest measured powder density, with the Tap Density increasing according to the powder’s ability to consolidate into a denser bed. In these examples, the Apparent Density of the stainless steel powder is 85% of the Tap Density, whereas the nickel powder is 79% – so the latter undergoes slightly more consolidation by mechanical tapping. By comparing the Tap Densities to their corresponding Skeletal Densities, it can be seen that both powder beds are well-packed, about 60% dense – compared to a theoretical bed density limit of about 64% for randomly-packed spheres of equal-diameter spheres.

Particle & Powder Densities Table - Density Values for Two 30 μm Metal Powders

All of the powder density measurement described above are conducted when the powder is at rest, in a so-called static state. The Dynamic Density measures the density of a powder bed as it flows in an avalanche type powder rheometer. This density value tracks closely with Apparent Density, yet often times exhibits more consistent measurements.

NSL Density Technologies

Our reliable results and quick turnaround times will give your company a competitive advantage to ensure the quality of your product. Our density techniques include:

  • Archimedes water displacement for solid specimens
  • Helium gas pycnometry for both powders and solid specimens
  • Programmed mechanical consolidation for powders and granulated samples
  • Avalanche powder rheometry for dynamic density analysis
  • Optical microscopy and Variable Pressure Scanning Electron Microscopy (VE-SEM) for porosity characterization

NSL conforms to rigorous standardized methodologies, while at the same time supporting our customers with advanced, emerging techniques:

  • ASTM B311 and MPIF 63 for solid specimen densities, such as True/Absolute Density and Apparent Density (equivalent to UOP851)
  • ASTM C559 for the Bulk Density of carbon & graphite specimens
  • ASTM D792 for polymer specimen Specific Gravity
  • ASTM B923 for the True/Skeletal Density of powder samples (equivalent to ISO 12154 and UOP851)
  • ASTM B212, ASTM B417 and ASTM B527 for the Apparent Density and Tap Density of metal powders
  • ASTM D7481 for the Loose and Tapped Bulk Density of granular materials
  • ASTM B09 Metal Powders Committee standardization studies on avalanche (Drum) rheology analysis

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