TEST RESULTS
OVERVIEW
The
standard test series on the PTC04DT involves three separate test
procedures to determine the following:
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 70^{o}C (160 F). With
hot pressing dies, the temperature can be up to 300^{o}C (570 F).
PRESSING
CONFIGURATION. The actual test pressing is onesided to any
desired length within 216 mm range and to any desired maximum or
isostatic pressing pressure within the available pressure range. All
test densities are referenced to indie 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
InDie 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 InDie 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 (outofdie) compact is measured.
There is a simple formula to recalculate the out of die density into
the indie density: density_{indie} =density_{outofdie} e_{r}^{2} e_{a}
Here, e_{r}
is the radial green compact expansion (in a direction perpendicular
to pressing direction) and e_{a} 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
indie 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. TwoSided
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 indie 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 INOUT 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, holddown
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/m^{2} or MJ/m^{3}).
28. Green
Compact Radial/Axial Expansions e_{r}/e_{a} .
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 INOUT of a Die. True test compact heights in a test die
under load and out of a die.
30. Compact
Diameter INOUT 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 E_{af} . 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 crosssection 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 24 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
yaxis. 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 Nondimensionalization. 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 is represented by a single number Ga
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. Example:
For
a given compact, Ga=0 at h=0 and Ga=Ga_{max} at h=H. To make
all graphs representing local parameters geometry independent, an
additional variable is introduced: X = 2 Ga  Ga_{max}. In a
given compact, the X = Ga_{max} at the compact end where h=0
and X = +Ga_{max} at the other end of the compact where h=H.
X = 0 at a cross section in the compact that has Ga=0.5 Ga_{max}.
That location has a special significance in that the local compact
density in that cross section is equal to the average density of the
entire compact and the local pressure (called the net pressure) is
equal to the isostatic pressure necessary to result in a density
equal to the average density of the compact (see below).
When a local characteristic for a desired compact with a constant
cross section along its height is presented on a plot, the plot is in
terms of h/H where h is the position of a cross section and H is the
height of the desired compact. Such plots are specific (only for such
compacts) and the Gamax that corresponds to the H of the desired
compact is always provided in the plot description. Those plots are
true for any compact that has at H the given Ga_{max}.
The
local characteristics (for pressure, density and dimensional changes
after sintering) always show compaction curves (dashed lines) which
indicate the positions of given cross sections in a compact during compaction.
38. Local
Pressure Distribution. The graphical presentation shows the
local pressure at any cross section along the height of the compact.
The graphs are based on the geometrical nondimensionalization where
the compact height is expressed in terms of the X parameter. For
compacts with a constant cross section along the compact height, one
end appears at X=Ga_{max} and the other end at X=+Ga_{max}.
The middle of the compact is at X=0. That cross section has a local
density equal to the average density of the entire compact and the
pressure there is equal to the net pressure for that density. The
solid lines represent onesided pressing in a given direction. The
lines are drawn at constant average indie green densities of a
compact and are used to relate results on different graphs.
To
find the local density distribution in a given compact, follow the
steps below:

calculate Gamax=SH/4F
 on the
graph, identify the coordinates that correspond to compact ends: X=Ga_{max}
and X=+Ga_{max}
 identify
a density line that corresponds to the average indie density of the
green compact
 identify
(interpolate if necessary) the compaction curves (dashed) that go
through the end points on the density line (the compaction proceeds
along those lines)
 for a
given h, calculate Ga=Sh/4F and X=2GaGa_{max}
 on the
graph, find the point on the characteristic that corresponds to the X
and read the local pressing pressure.
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. The points
on the curves at X=0 are stationary and for slide coefficient of 1
(no friction) 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 onesided pressing.
40. Local
Radial Dimension Change During Sintering L_{d}
(optional). The radial dimension change represents a ratio between
the sintered radial dimension and the green indie radial dimension
at the same corresponding location on the compact. The graphs are
plotted for specific compacts only. Subtracting the local dimension
change from 1 gives the local shrinkage.
The computer requires an additional information to plot the local
dimension changes after sintering:
Here H is
the axial dimension, d is the radial dimension. The H_{green}
and d_{green} are measured in die under load. If the above
information is not provided, the computer will not produce the local
dimension change graphs.
41. Local
Axial Dimension Change During Sintering L_{h}
(optional). The axial dimension change represents a ratio between the
sintered axial dimension and the corresponding indie green axial
dimension on the compact. The interpretation of the graph is the same
as 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 X=0 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 24 predefined figures provided by the
program. Each figure may be duplicated and modified to fit specific needs. 