Density:
One
of the most common density measurements involves the determination of the
geometric space occupied within the envelope of a solid material... including
any interior voids, cracks or pores. This is called geometric, envelope or bulk
density and only equals true density when there are no internal openings in the
material being measured.
Absolute Density:
1) The ratio of the mass of a volume of solid material to the same volume of
water.
2) The mass per unit volume of a solid material expressed in grams per cubic centimeter.
2) The mass per unit volume of a solid material expressed in grams per cubic centimeter.
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Apparent Density:
The weight of a unit volume of powder, usually expressed as grams per cubic
centimeter, determined by a specific method
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Bulk Density:
Powder in a container or bin expressed in mass unit per volume
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Density Ratio:
The ratio of the determined density of a compact to the absolute density of
metal of the same composition, usually expressed as a percentage. Also referred
to as a percent theoretical density
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Dry Density:
The mass per unit volume of an unimpregnated sintered part
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Green Density:
The density of a green compact
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Packed Density:
Please see preferred term of tap density
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Tap Density:
The density of a powder when the volume receptacle is tapped or vibrated under
specified conditions while being loaded. Each particle of a solid material has
the same true density after grinding, milling or processing, but more geometric
space is occupied by the material. In other words, the geometric density is
less... approaching 50% less than true density if the particles are spherical.
Handling or vibration of powdered material causes the smaller particles to work
their way into the spaces between the larger particles. The geometric space
occupied by the powder decreases and its density increases. Ultimately no
further natural particle packing can be measured without the addition of
pressure. Maximum particle packing is achieved. Under controlled conditions of
tap rate, tap force (fall) and cylinder diameter, the condition of maximum
packing efficiency is highly reproducible. This tap density measurement is
formalized in the British Pharmacopoeia method for Apparent Volume, ISO 787/11
and ASTM standard test methods B527, D1464 and D4781 for tap density.
The true density of powders often differs from that of the bulk material
because the process of comminution, or grinding will change the crystal
structure near the surface of each particle and therefore the density of each
particle in a powder. In addition, voids at the surface of a particle, into
which liquids will not penetrate, can generate apparent volume which will cause
serious errors when density is measured by liquid displacement. The pycnometer G-DenPyc2900 from Gold APP Instruments are specifically designed to measure the true
volume of solid materials by employing Archimedes' principle of fluid (gas)
displacement and the technique of gas expansion. True densities are measured
using helium gas since it will penetrate every surface flaw down to about one
Angstrom, thereby enabling the measurement of powder volumes with great
accuracy. The measurement of density by helium displacement often can reveal
the presence of impurities and occluded pores which cannot be determined by any
other method.
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Wet Density:
The mass per unit of volume of a sintered part impregnated with oil or other
nonmetallic material
