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- TEST
RESULTS OVERVIEW
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- The standard
test series on the Powder Testing Center involves three separate test
procedures to determine the following:
- * properties of
the bulk powder
- * compacting &
ejecting characteristics of the powder in a die
- * green compact characteristics.
- The test
requires basic information about the powder and the desired
compaction: the theoretical density of the powder mixture and the
desired parameters (the compact density, length and pressing
pressure). The tests are for dry powders only. The temperature of a
standard cold pressing die can be up to 70oC (160 F). With
hot pressing dies, the temperature can be up to 300oC (570 F).
- PRESSING CONFIGURATION.
The actual test pressing is one-sided to any desired length within
2-16 mm range and to any desired maximum or isostatic pressing
pressure within the available pressure range. All test densities are
referenced to in-die condition which refers to a compact in a die
under load. This is the only fixed density reference since compacts
expand differently after ejection from a die.
- During
compaction, all required measurements are taken at various punch
positions and, for the corresponding actual intermediate compact
densities, the compacting parameters are calculated and stored in the
permanent test data file. These parameters show ACTUAL compaction
conditions at any density up to the actual final test compact
density. A complete data output with plots can be generated (with the
standard driving program) for any desired compact density in that range.
- PRESSING MODES.
There are three different pressing modes selectable by the user: (1)
pressing to a fixed compact density, (2) pressing to a fixed maximum
pressing pressure, (3) pressing to a fixed maximum net (isostatic) pressure.
- TEST NOTES.
Each test may be supplemented with up to 10 lines of notes which
become an integral part of the standard test data file.
- POWDER
COMPACTING CHARACTERISTICS - GIVEN DATA
- 1. Test Die Material.
The die material affects directly the friction between the die and
the powder particles. The test die is replaceable and typically one
has several test dies with different die materials and different die
surface finishes. That allows the user to optimize the compaction
process with respect to the best choice of a die material for a
specific powder type to minimize the die friction and the local
variations of density and shrinkage.
- 2. Theoretical Density
. This is the density of the powder mixture with 100% densification.
The composition of the powder mixture (the density of all components
and their contribution by weight) must be known to calculate the
density properly. The program has an option that allows to store the
densities of all components and can compute the theoretical density
for a specific composition of that powder.
- 3. Estimated
In-Die Average Green Test Compact Density. This density is the
desired density when pressing with the fixed density mode. For other
pressing modes, this density must be estimated to calculate the mass
of a powder necessary to fill the test die. The desired or estimated
compact length or the compact mass must be provided in all cases.
- 4. Die Temperature
and die heating modes: no heating, cold powder pressed in a hot
die, hot powder pressed in a hot die. In some cases, the powder is
heated within the test die for a preset time called temperature
holding time which is set as needed.
- 5. Temperature
Holding Time corresponds to a desired time the powder is heated
within the test die.
- 6. Pressure
Holding Time corresponds to a desired time the compact is kept
under maximum pressure before release and ejection.
- GENERAL POWDER
COMPACTING CHARACTERISTICS - TESTED DATA
- 7. Desired Average
In-Die Green Test Compact Density (user specified). This is the
desired average green compact density measured in die under load and
it should be comparable with the density used in production. In most
cases the density of a free (out-of-die) compact is measured. There
is a simple formula to recalculate the out of die density into the
in-die density: densityin-die =densityout-of-die er2 ea
- Here, er
is the radial green compact expansion (in a direction perpendicular
to pressing direction) and ea is the axial green compact
expansion (in a direction parallel to pressing). That out of die
density is not a good reference since it depends on the green compact
expansions which change significantly with pressing and ejection
conditions and with time. The only reliable reference density is the
in-die density. When calculating the density directly a special care
must be taken in measuring the distance between punches under load to
take into account the elastic deformations of the punches and other components.
- 8. Bulk Density.
This is the density of a loose powder. It is also referred to as the apparent
density.
- 9. Tap Density.
This is the density of a loose powder subjected to a number of
prescribed tappings. The output specifies the number of taps and the
duration of tapping. The tap density provides useful information on
the packing of the loose powder during transportation and when it is
in a container on a press before die filling.
- 10. Hausner Ratio Hr.
This is the ratio of tap density and bulk density.
- 11. Tapping Compressibility
%. Loose powder packing parameter: 100(Tap Density - Bulk
Density)/Tap Density
- 12. Angle of Repose
ß. This angle represents the actual ability of the powder to
fill uniformly a cavity of a die. For liquids (ideal flowability) the
angle ß = 0, for solids (no flow) ß = 90 degrees. Powders
with high flowability have ß around 30 degrees. The Angle of
Repose could be measured in many ways and is quite independent from a
given measuring technique.
- 13. Slide Coefficient
for the desired density. This is a measure of the frictional
interactions between powder particles and die walls during
compaction. For a given powder and a given die material (with given
surface conditions), it varies slightly with green density. Its
magnitude extends from 0 (infinite friction) to 1 (no friction) and
relates directly the friction forces to the pressing forces. The
value around 0.7 is considered moderate. Values above that indicate
good or very good compaction properties and are desirable. Values
below that suggest difficult compaction process with relatively large
frictional forces which lead to large density variations and,
ultimately, large shrinkage variations. The slide coefficient could
be modified by adding lubricants or changing the die material.
- 14. Compactibility Coefficient
for the desired density. It is a material constant characterizing the
ability of a powder to densify and it represents interactions between
powder particles during compaction. When it decreases the powder
"stiffness" increases, that is, it requires more pressure
to be compacted to a given density. Powders with high compactibility
are "soft" and are compacted to high densities with
relatively little pressure. The coefficient is directly affected by
the type of powder, powder grain sizes, grain distribution, and the like.
- In the absence
of die friction (as is the case in an isostatic compaction) the
compactibility coefficient provides complete information about
compaction pressures. In a rigid die compaction, the compactibility
coefficient and the slide coefficient are needed to determine all
compaction pressures (pressures on punches and friction).
- 15. Net
(Isostatic) Pressure for the desired density. It is a pressure
required in isostatic compaction (no friction between powder and die
walls) for a desired green compact density. For pressing processes in
rigid dies, the net pressure is a pressure equal to the local
pressure at the location in a green compact where the local density
is equal to the average green density of the entire compact. The
cross section in a compact where that occurs is at h = H / 2. In
practical terms, the net pressure is the absolutely minimum
compacting pressure needed for a given compact density.
- 16. Maximum
(Pressing) Pressure for the desired density and the specific
compact length.
- 17. Two-Sided
Maximum Pressing Pressure corresponding to the required pressure
in free floating pressing arrangement resulting in a compact with a
density and length equal to the desired density and the desired length.
- 18. Desired
compact length in-die under load. The length corresponds to a
length of a compact with the average desired density.
- GREEN TEST
COMPACT CHARACTERISTICS - TESTED DATA
- 19. Actual average
test compact density IN-OUT of the die is the actual density in
die under load and that of a free compact.
- 20. Slide Coefficient
for the test compact density. (See 11)
- 21. Compactibility Coefficient
for the test compact density. (See 12)
- 22. Cohesiveness
of a Green Compact C. This cohesiveness represents a ratio
between the green strength of the compact and the maximum friction
forces between the compact and the die walls. It is directly related
to cracking and lamination resistance during compact ejection from a die.
- If the
cohesiveness is less than 1 (friction forces larger than the green
strength), compacts will likely develop cracks during ejection unless
special care is implemented such as slow ejection, hold-down
pressure, or special die exit design. If the cohesiveness is above
one, a compact cracking during removal from dies should not occur
under normal conditions. The cohesiveness can be improved by
increasing the compact green strength (addition of binders), by
lowering the friction forces (addition of lubricants), or both at the
same time. Many additives give both effects at the same time. As is
the case with all additives, there is a problem how to determine the
optimum amount of the additive. Several tests with varying amounts of
a given additive will show their influence on the cohesiveness and
typically it is easy to determine the saturation point after which
adding more additives does not improve the cohesiveness significantly.
- 23. Average
Ejection Pressure for the test compact. This is the compact
ejecting pressure (ejecting force over the cross section area)
averaged over the initial 2.54 mm of compact travel in die after the
starting pressure overshot needed to start the compact to move.
- 24. Stripping Pressure
during ejection start. This is the ratio of the maximum ejection
force and the friction surface of the compact.
- 25. Ejection
(start) Pressure Overshot for the test compact. This is the ratio
of the maximum initial and the average ejection pressure.
- 26. Total Ejection Energy
for the test compact.
- 27. Unit Ejection Energy
for the test compact. This is the energy per unit of the compact's
friction surface needed to move the compact in die along a unit
distance. The base unit is MJoule per meter square of area and meter
of travel (MJ/m/m2 or MJ/m3).
- 28. Green Compact
Radial/Axial Expansions er/ea .
These are the relative expansions of a green compact after ejection
from a die. The radial expansion is the ratio between the radial
(perpendicular to pressing) dimension of the compact (out of die) to
the corresponding dimension in die under a full load. Typically, the
dimensions are measured in the mid height of the compact. The axial
expansion is the ratio between the axial (parallel to pressing)
dimension of the compact (out of die) to the corresponding dimension
in the die under a full load. The expansion is typically larger in
the axial direction than in the radial direction. On an industrial
press, it is relatively difficult to determine the true height of the
compact under load unless the elastic characteristic of the pressing
assembly is known. Therefore, most of the time in practice only the
radial expansion is measured. The coefficients are helpful in proper
tool design.
- 29. Compact Height
IN-OUT of a Die. True test compact heights in a test die under
load and out of a die.
- 30. Compact
Diameter IN-OUT of a Die. Actual test compact diameters in a die
under load and out of a die.
- 31. Mass of
the Test Compact.
- 32. Green Axial Strength
of the test compact wsa . It is the maximum crushing pressure
registered during compact crushing along the axial (parallel to
pressing) direction. This is the green strength responsible for
holding the compact together during ejection from a die.
- 33. Energy at
Axial Fracture Eaf . The energy needed to crush the compact.
- 34. Green Radial Strength
of the test compact. It is the maximum crushing pressure registered
during compact crushing along the radial (perpendicular to pressing)
direction. The crushing pressure may be calculated in various ways.
The current choices are a radial strength (a ratio between the
crushing force and the contact area between the crushing punch and
the cylindrical surface of the compact) and a diametral strength (a
ratio between the crushing force and a half of the maximum
cross-section area of the compact perpendicular to crushing
direction.) New interpretations of the radial strength may be added
to the software if needed.
- 35. Maximum
Pressing Pressure for the Test Compact. Maximum compacting
pressure during the test.
- 36. Pressing Speed.
The pressing speed is user selectable in the range 0.5 to 2 mm/s.
- POWDER
CHARACTERISTICS - GENERAL GRAPHS
- The program
provides predefined figures to display the test data. Each figure may
be duplicated and edited to fit a specific need. Any combination of
the allowable parameters may be assigned to the x- or y-axis.
Annotations, texts, fonts, box and line colors and other aspects of a
figure may be modified or changed. The program allows multiple test
display on a single plot. Lines for each test may have assigned a
different color. A selected parameter of interest (test number, test
temperature, density, ...) is printed (in increasing or decreasing
order) below the figure in the same color as the test's lines on the graph.
- 37. Compact
Geometry Non-dimensionalization. The test results show local
characteristics within a compact at any specific cross section. It is
assumed that the parameters are uniform at that cross section. In
actual cases, there is typically a slight variation near the surface
of the compact due to die wall influence.
- The geometry
of a given compact can be represented by a single number Ga (Gasiorek
number) defined as follows: Ga = Sh/4F where S is the total perimeter
of the cross section at h, h is the distance of the cross section
from the pressing punch (or from a selected end face of the compact),
and F is the cross section area of the compact. For compacts with a
constant cross section along the height, the calculation of the Ga at
any height is quite simple. However, for compacts with a varying
cross section along the height the calculations may be simplified by
dividing the compact into slices and considering an average cross
section for a given slice. More accurate relationship can be
developed that involves mathematical integration; however, this may
be needed only in very selective cases.
- Examples:
- Cylindrical compact:
Ga = 3.14 * Dh/(4 * 3.14 * D2/4) = h/D
- Cylindrical bushing:
Ga = 3.14 * (D+d)h/(4 * 3.14 * (D2-d2)/4) = h/(D-d)
- For a given
compact, Ga=0 at h=0 and Ga=Gamax at h=H.
- 38. Local Pressure
Distribution in One-Sided
and Double-Sided
Pressing. The graphical presentation shows the local pressure at
any cross section along the height of the compact. The plot shows the
local pressure at h/H where H is the final height of the compact (in
die under load) and h represents the distance from the stationary
punch. For the final compact, the h/H changes between 0 and 1. For
intermediate compacts (during compaction), the h/H changes from 0 h/H
>1. The local pressure lines are drawn at constant average in-die
green densities of a compact and are used to relate results on
different graphs.
- Note: The graphs
are based on an assumption that the local pressure is constant across
a given cross section. In reality, there is a pressure increase near
the die walls. The effect may be more significant in very complex
parts with large frictional surfaces. The slope of the curves depends
on the slide coefficient. The higher the slide coefficient (lower
friction), the lower the slope. In a one-sided pressing, the points
on the curves at mid height of the compact represent isostatic
pressing conditions. In pure isostatic compaction the slide
coefficient equals 1 (no friction, the same pressure on both punches)
and the curves will become horizontal lines passing through those points.
- 39. Local Density Distribution.
The local density graph is interpreted in exactly the same way as
the local pressure distribution. The local density distribution is
almost linear in one-sided pressing.
- 40. Local Radial Shrinkage
During Sintering (optional). The local shrinkage graph is
calculate in a similar way as the local density plot with one
difference the H here represents the instantaneous height of the
compact (not the final height). Therefore, all plots are for h/H from
0 to 1.
- The computer
requires an additional information to plot the local shrinkage after sintering:
- - the desired average
density after sintering
- - the total mass loss
during drying and sintering
- - the anisotropic
shrinkage ratio: a=Sh / Sd, Sh=1-Hsintered
/ Hgreen, Sd=1-dsintered / dgreen
- If the above
information is not provided, the computer will not produce the local
shrinkage graphs.
- 41. Local Axial Shrinkage
During Sintering (optional). The axial shrinkage plots are
interpreted in a similar way as discribed above.
- 42. Test
Compaction Pressure: Pressing, Net (Isostatic), Closing. The
figure shows the actual pressure on the pressing and the closing
punches and the corresponding (calculated) net (isostatic) pressure
versus compact density during compaction. The Isostatic Compaction
Characteristic is the middle curve on the graph. In isostatic
compaction (in the absence of die wall friction) the local compacting
pressure and the local density are uniform throughout the compact.
The compactibility coefficient is the only parameter that relates the
compacting pressure and the resulting green compact density.
- This graph is
used frequently in rigid die pressing to determine an optimum average
green compact density for new processes. If the density is too low,
the compaction process is hard to control since small variations in
compacting pressure result in high density variations. Similarly, a
flat end part of the characteristic is not desired since a slight
variation in compact density results in large variation of pressure.
Mechanical presses operating in that region could easily go beyond
the maximum force that the press could deliver if more mass got into
the die cavity.
- Note: For
compacts pressed in a rigid die, the local pressure at the cross
section where local density equals the average compact density is
equal to the isostatic pressure for that density. That cross section
is located at Ga=0.5 and the pressure is called the net pressure.
- 43. Compactibility
& Slide Coefficient Variations with Green Compact Density.
Both coefficients may vary with density. The initial variations at
low densities are primarily due to initial powder grains
reorientation and breakings.
- 44. Green Compact
Ejection Characteristic . The ejection characteristic represents
ejecting pressure on the compact during its removal from the test
die. The characteristic is geometry dependent and be different for
different compacts and die configurations. In the test die, the
compact is initially located approximately 5 mm below the surface of
the die. A typical graph shows an initial spike in ejecting pressure
that is required to overcome the static friction of the compact when
it is forced to move (the initial ejection stage contains more
details due to a high rate of sampling). Next stage involves the
compact before it shows out of the die. The last part of the graph
shows drop in ejecting pressure due to the compact leaving the die.
On the graphs, the 0 position represents the position in the die of
the pressing punch face under full load. When the pressure is
released, the compact expands within the die in the axial direction
and a typical ejection graph shows that the ejection starts before
the 0 position which is the result of that expansion.
- 45. Green Compact
Crushing Characteristics. The graphs show the axial and radial
crushing pressures versus the travel distance of the crushing punch.
- 46. Other Characteristics.
There are predefined figures provided by the program. Each figure
may be duplicated and modified to fit specific needs.
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