“Pressed at 10 tons for a few minutes” does not let anyone repeat a pellet. The tonnage means nothing without the die diameter, “a few minutes” is not a hold time, and how the load went on and came off is missing altogether. This guide sets out the numbers that define a pellet-press cycle, how to convert between them, what published methods actually use, and how to write the cycle in one line that another laboratory can follow.
In one paragraph: give the die diameter and the pressure in megapascals, then the force on your press. Give the loading time and any intermediate steps, the hold time and whether the force was maintained during it, how the load was released, and how the pellet was ejected. Note when the pellet was measured: in one study of tableting materials, most kept expanding for days after ejection. A large interlaboratory study of solid-state battery cells found pressing times that differed by orders of magnitude between groups. It asked for the pressure and time of each pressing step, and how fast the pressure was applied and released, to be reported. How much each item matters depends on the powder. That is the reason to write all of them down.
The comparison of manual, electric and automatic presses explains why each step of the cycle matters and which press controls which step. This page is about writing the cycle down: which numbers, in which units, and how to check that a press can deliver them.
1. A cycle is a set of numbers
When 21 research groups assembled solid-state battery cells from the same materials, the cells varied widely. So did the way they were pressed. Positive electrodes were compressed at average pressures of 250 to 520 MPa. Separators ranged from “hand-pressing” to 590 MPa. The durations of the compression steps differed by several orders of magnitude. Among the parameters the authors asked to be reported with every result were:
- the pressure in MPa and the time in minutes for each component pressed;
- how fast the pressure was applied and released;
- the pressure during cycling, including how it was controlled.1
An earlier interlaboratory study of solid electrolytes asked for an accurate description of the consolidation.2 A study of a sulfide glass sintered at room temperature followed both pressure and time, and found that lower porosity does not always mean higher conductivity.3 The battery study cites it for the role of compression time in the conductivity of sulfide electrolytes.1
| Item | Write it as | Why |
|---|---|---|
| Die | Bore diameter, material, type | Turns force into pressure; size changed the result at equal pressure in one tablet study |
| Powder | Mass, treatment before pressing | Sets the thickness |
| Target | Pressure in MPa, and the force on your press | The number another laboratory can reproduce |
| Loading | Time to target (s), and any first step with its pressure, hold and release | Rate effects and de-airing are material-specific |
| Hold | Time (s or min), and whether the force was maintained | At constant position the stress relaxes; compensation tops it up |
| Release | Time or manner | A reported variable in its own right |
| Ejection | Push-out, split die or in-ring | A separate step in which pellets can fail |
| Conditions | Temperature, atmosphere, vacuum on the die | Part of the cycle when they change |
| Measurement | When, after pressing | In one study, tablets of most materials kept expanding for days after ejection |
2. Pressure first, then force
The pressure on the sample is the force divided by the area of the bore:
Ten tonnes is about 1250 MPa on a 10 mm bore, far above the rating of a steel die, and about 120 MPa on a 32 mm bore. That is why force alone does not define a pellet. Published methods leave out different parts of the cycle. Three examples:
- A compaction study’s abstract gives 20 kN on an 11 mm flat punch, about 210 MPa, but not the pressure.4
- A government XRF procedure for loose powder packed in a sample cup gives the same packing step once as a torque setting and once as a pressure in pounds per square inch. Neither converts to a force on the sample without more information.5
- A solid-electrolyte method states the die, the mass and 300 MPa precisely, and no hold time.6
Pressure does not make the diameter irrelevant. In a study of tablets from 1 to 11.28 mm pressed at equal pressures of 30 to 160 MPa, the size of the tablet still changed the yield pressure and the compactibility; the mini-tablets were also convex, so their shape differed too.7 Report pressure and diameter together. The same study gave its press speed only as a percentage of the machine setting, which a laboratory with a different machine cannot reproduce. Speeds and times belong in seconds and millimeters per second.
On a manual press the pointer gauge reads oil pressure, converted to force with the factor printed for the frame; the digital gauges read to 0.01 t. On a powered press the display reads force; on the automatic PressPro™ presses the screen reads force to 0.1 t. One step of that reading is a step in pressure too, larger on small dies: 0.1 t on a 10 mm bore is 12.5 MPa. The tonnage-to-MPa calculator converts both ways for every frame.
3. Loading: time to target, and steps
How fast the load goes on matters for some powders and not for others.
- On a compaction simulator at 10, 50 and 100 mm/s, plastic microcrystalline cellulose formed harder tablets at low speed. A brittle, fragmenting excipient was relatively unaffected.4
- Punch velocity raised the yield pressure of plastically deforming powders.8
- For microcrystalline cellulose, only the compaction stress and not the speed determined tensile strength. Starch was strongly speed-dependent.9 This study and the first one came to different results for microcrystalline cellulose. The first varied the punch speed from 10 to 100 mm/s; this one compared dwell times of 10 to 130 ms.
Specify the loading as a time to target. On a manual press it is the operator’s pace, so record it.
A first step at lower pressure is the other loading variable. In tablet work it serves to let air out and particles rearrange.
- A low precompression force with a longer dwell, followed by the main compression, was the most effective remedy for capping in one rotary-press study.10
- A slow, continuous compacting force was thought to degas granules better than a brief high one.11
- Double compaction generally gave stronger tablets than single compaction. For some powders, lengthening the interval between the two from 30 to 500 ms reduced capping and lamination; for the others, strength did not change significantly.12
If a cycle has a first step, write down its pressure, its hold, and whether the load was released before the main step. The guide to capping and lamination covers why that last point can matter.
4. The hold
A held load is not a constant state.
- At constant position the axial stress decays, whether the cause is viscoelastic or viscoplastic.13 Relaxation experiments followed it for 10 h.14
- Radial stress relaxed during a 4.5 s dwell, and the deformation of the press itself contributed to the axial decay.15
- On a compaction simulator the stress rose unexpectedly during the dwell. The delivered profile can differ from the programmed one.9
Whether the hold changes the pellet depends on the material.
- Longer contact time increased consolidation for several excipients but not for dicalcium phosphate.16
- At matched porosity, shortening the dwell from 150 to 15 ms caused no loss of strength in unlubricated excipients.17
- In kaolinite KBr pellets, the infrared spectrum depended on the pressing time as well as the pressure.18
- Among the battery cells, the best performers were compressed at 300 to 590 MPa for 1 to 5 min.1 This describes what the best groups did; the study did not test the hold time as a variable.
State the hold time, and say whether the force was maintained. On a manual press the operator tops the load up by hand; the specification gives the pressure stability of the manual powder frames as within 1 MPa of gauge pressure in 10 minutes. Electric and automatic presses compensate automatically.
5. Release, ejection and what happens after
- Release. Loading and unloading speeds have been varied separately. Slowing the punch raised strength for some powders, and the authors concluded that changing the rates can minimize capping.19 The way a tablet was unloaded changed whether it capped or laminated.20 The battery study asks for the release rate to be reported.1
- Ejection. Unloading and ejection are distinct, measurable stages in die pressing.21 In an instrumented study of stainless-steel powder, ejection was carried out while a hold-down force of about 100 kN was maintained on the powder column. The paper also gives the die (35 mm), the compaction speed (15 mm/s) and the target density.22
- After ejection. Tablets kept expanding after ejection, for most materials over several days.23 A thickness or density measured right after pressing is a snapshot, so note when it was taken.
6. What published methods actually use
| Material | Die and mass | Pressure and time |
|---|---|---|
| Sulfide electrolyte, battery cells, 21 groups1 | Various | Positive electrode, average 250 to 520 MPa; best cells 300 to 590 MPa for 1 to 5 min |
| Li6PS5Cl24 | 13 mm PEEK die, 200 mg | 370 MPa |
| Li6PS5Cl6 | 10 mm zirconia mold, 55 mg | 300 MPa; hold not stated |
| Li6PS5Cl25 | Die and mass not recorded in the source read | 62.5, 125 or 250 MPa |
| Li2S–P2S5 glass26 | 6 mm steel die | 360 MPa for 10 min (cold) |
| Soil for XRF27 | 23 mm aluminum ring | 300 kgf/cm² (about 29 MPa, if that figure is the pressure on the pellet) |
| Pharmaceutical excipients9 | 9 to 14 mm punches | About 50, 150 and 300 MPa |
| Stainless-steel powder22 | 35 mm die | 15 mm/s to a target density; ejection under a hold-down of about 100 kN |
| KBr pellet, calcite sample28 | 7 mm, about 5 mg sample in 40 mg KBr | Hand press; pressure not stated |
The table shows the spread in published practice, not recommendations. Conversions from other units are calculated here.
7. Standards that bear on the cycle
Some test methods fix the die and the pressing conditions, so that one number can characterize a powder. ASTM B331 determines the compressibility of metal powders by how far a test portion densifies under controlled conditions in a specified die.29 ISO 3927 covers the same measurement.30 Where such a standard applies, its conditions replace your own, and the standard should be cited by number in the method. The values are in the standards themselves and are not reproduced here.
Two other documents bear on pressing without fixing a cycle, as far as their published scope shows.
- USP chapter 1062 covers the characterization of tablet compression.31
- The ASTM guide to wavelength-dispersive XRF lists pressed briquettes among the specimen forms.32
8. How good is the number on the gauge?
A manual hydraulic press reads oil pressure, not force. A study of hydraulic jacks used for prestressing, calibrated as force machines, makes the point directly. Such machines are calibrated against a reference load cell, and a curve is fitted from gauge reading to force. Friction between piston and cylinder changes that curve, and the curve does not pass through zero, so the lower end of the range has to be defined. The calibration uncertainty was 0.2 to 0.6 % of reading with a spherical cap and 0.5 to 1.2 % without one.33 Testing machines are verified against force standards under ISO 7500-1 and ASTM E4.34,35 At the national-standard level, creep and hysteresis of the force transducer are among the effects calibration has to expose.36
For a pellet method, three practical points follow.
- State how the force was read: a pointer gauge, a digital gauge or a powered display.
- Keep the target well inside the range of the press rather than at its bottom end.
- When results are compared across presses, compare a reference pellet, not just the readings.
On the PressPro™ presses:
- The manual powder frames have pointer gauges with a resolution of 1 MPa of gauge pressure, or digital gauges reading to 0.01 t.
- On the automatic presses the screen reads force to 0.1 t.
- Each automatic model has a printed lower limit, from 0.1 t on the 10 T model upward.
The gauge guide covers resolution, accuracy classes and how to check a press against a reference load cell.
9. Cycle builder
Enter the die and up to five steps, each with its target pressure, the time to reach it and the hold. Choose how the load is released and which kind of press runs the cycle. The tool draws the profile to scale, converts every step to force and to the display or gauge step of the press, checks the steps against the press, and writes the method line.
10. Writing the method line
One sentence can carry the whole cycle:
The numbers in the example only show the format; the values for your material come from your own work or its literature. If the press is manual, say so. Give the gauge reading as well, and say how the hold was kept.
11. What the press does for you
- Manual powder presses (two-column, four-column and protective). The operator does every step. The loading is pumped by hand, the hold is kept by topping up, and the release valve is opened by hand. The specification gives the pressure stability of these frames as within 1 MPa of gauge pressure in 10 minutes. A manual cycle can be written down as exactly as any other, but its repeatability depends on the operator.
- Electric presses. The motor pressurizes and compensates the force automatically. The hold time is under manual control and the release valve is opened by hand.
- Automatic powder presses. These pressurize, hold with compensation and release at the end of a timed hold on their own. They run force programs of 5 segments, upgradeable to 30, each with its own hold time. Cycle data are stored, and the demolding force can be set.
Whether a program can lower the force between segments is not covered here. A step that releases the load before the main step can be run as two cycles. The press comparison lists these functions line by line for every PressPro™ family.
12. Related guides and equipment
- Manual vs Electric vs Automatic Laboratory Presses — why each step matters, and which press controls it.
- Powder Compaction Guide — what happens to powder in a die.
- Pellet Cracking, Capping and Lamination — when the cycle produces defects.
- Tonnage-to-MPa Calculator — force, gauge reading and pressure for every frame.
- Pressure Gauge Resolution, Accuracy and Calibration — how far the reading can be trusted, and how to check it.
- Solid-State Electrolyte Pellet Pressing — fabrication and stack pressure.
- Cloudy or Cracked KBr Pellets — pressing conditions in KBr methods.
- XRF Pellet Defects and Repeatability — pressing conditions and XRF precision.
- How to Choose a Laboratory Hydraulic Press — the whole PressPro™ range by purpose.
- Equipment: two-column, four-column and protective manual presses; electric and automatic powder presses; large-tonnage automatic press.
13. FAQ
How do I convert press tonnage to MPa?
Multiply the force in tonnes by 9800 and divide by the area of the die bore in square millimeters. Five tonnes on a 13 mm die is about 370 MPa; on a 32 mm die it is about 61 MPa.
How long should I hold the pressure?
There is no general answer. The effect of the hold depends on the material, and published methods range from seconds to minutes and more. Choose a hold, keep it the same, and report it with whether the force was maintained.
Does the loading speed matter?
For plastically deforming powders it can. For brittle powders, in the studies cited here, it mattered little. Report the time to reach the target pressure in seconds.
What is a precompression step?
A first, lower load before the main one, used in tablet work to let air escape and particles rearrange. If you use one, report its pressure, its hold and whether it was released before the main load.
Why should I record when the pellet was measured?
Because in one study of tableting materials, most kept expanding for several days after ejection. A thickness or density measured at once may not be the final value.
Is the pressure gauge accurate enough?
A gauge reads oil pressure, and its link to force depends on calibration and friction, especially at the low end of the range. Keep targets inside the range, state how the force was read, and check across presses with a reference pellet.