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TEST RESULTS OVERVIEW

  The standard test series on the PTC-04DT 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 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 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 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:

  • 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. To make all graphs representing local parameters geometry independent, an additional variable is introduced: X = 2 Ga - Gamax. In a given compact, the X = -Gamax at the compact end where h=0 and X = +Gamax at the other end of the compact where h=H. X = 0 at a cross section in the compact that has Ga=0.5 Gamax. 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 Gamax.
  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=-Gamax and the other end at X=+Gamax. 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 one-sided pressing in a given direction. The lines are drawn at constant average in-die 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=-Gamax and X=+Gamax
  • identify a density line that corresponds to the average in-die 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=2Ga-Gamax
  • 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 one-sided pressing.

40. Local Radial Dimension Change During Sintering Ld (optional). The radial dimension change represents a ratio between the sintered radial dimension and the green in-die 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:

  • 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

Here H is the axial dimension, d is the radial dimension. The Hgreen and dgreen 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 Lh (optional). The axial dimension change represents a ratio between the sintered axial dimension and the corresponding in-die 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.

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