Compressibility

Compressibility describes how much a powder bed reduces in height when a normal load is applied, reflecting how readily particles rearrange, deform, or pack more efficiently under stress. Powders that compress easily tend to be packing- and consolidation-sensitive, while powders that resist compression are often already close-packed and mechanically stable.

The Powder Flow Analyser (PFA) measures compressibility under defined, controlled normal stresses, generating a compressibility-versus-stress profile rather than a single-point value. This provides insight into how a powder will behave during filling, handling, storage, and restart, where applied stresses vary.

A powder that is already close-packed will show little change in bed height as stress increases, resulting in low compressibility. A powder that is cohesive, irregular, or loosely packed will typically show a larger reduction in bed height with increasing stress.

Changes in slope across the stress range can indicate stress-dependent rearrangement, where the powder transitions from loose packing to a more mechanically stable structure. These behaviours are often linked to handling sensitivity, density drift, and restart problems after storage. This makes compressibility particularly relevant in processes where applied stresses vary across the powder bed" is already there; adding "including die filling, capsule packing, bin and IBC storage, and transport.

Compressibility is particularly relevant for processes involving:

• filling and dosing
• tablet and capsule filling
• storage in bins, hoppers, or intermediate bulk containers
• transport and vibration

After conditioning the powder column using the PFA blade, the blade is replaced with a compression probe in a split vessel. Using a split vessel excess powder (after conditioning) is discarded and leaves behind a specified volume. The split vessel adds an extra benefit of a flat pre-compression surface once the extra powder has been discarded and the ability of the PFA to measure weight allows the automatic calculation of bulk density.

The probe descends until it contacts the powder surface, at which point the initial column height is recorded and the conditioned bulk density is calculated.

The probe then applies increasing normal force to the powder bed (to 5kg). As load increases, the reduction in column height is measured and expressed as percentage compressibility at defined stresses. This produces a stress-dependent compaction profile, rather than a single bulk/tapped density comparison. At the target force the probe remains at the same distance which is held for 120 seconds. The force will be seen to decrease as the powder relaxes away from the probe. After 120s the probe is unloaded and elastic recovery can be measured.

Holes in the compression probe allow entrained air to escape during compression, ensuring that the measured response reflects particle rearrangement and deformation, not trapped air effects.

A compressibility test is most useful when packing, settling, or density changes are causing filling inconsistency, storage set-up, or formulation challenges. It characterises how a powder bed densifies under applied load and how it relaxes and recovers when the load is removed. This makes it especially valuable for tablet and capsule filling, packing behaviour assessment, and understanding consolidation sensitivity that is not always captured by traditional bulk and tapped density measurements.

The compressibility test identifies how a powder responds to normal stress and consolidation. The most useful follow-up tests depend on how readily the powder densifies and how that structure behaves during rest and motion.

• Compressibility describes how a powder packs under load, not how it flows or discharges.
• High compressibility does not automatically mean poor flow, but it does signal consolidation and density drift risk.
• Compressibility results should be interpreted alongside:
  • Cohesion / Bridging Factor (flow initiation and failure mode)
  • Caking / Consolidation tests (time-dependent set-up)
  • PFSD (handling and speed sensitivity)

No single parameter describes powder behaviour. Compressibility helps explain packing and stress response, not flowability alone.

Compressibility vs Cohesion

• Compressibility measures how structure forms under load.
• Cohesion measures resistance to movement once that structure is disturbed.

Why this matters:

Highly compressible powders often show increased cohesion after rest, but cohesion testing is required to quantify flow resistance.

Compressibility vs Caking

• Compressibility describes packing behaviour.
• Caking describes whether that packed structure becomes mechanically strong over time.

Why this matters:

A powder may compress readily but still break apart easily - or compress and then cake severely. Both tests are needed to assess storage risk.

Compressibility vs PFSD

• Compressibility focuses on static loading.
• PFSD focuses on dynamic response at different speeds.

Why this matters:

Packing behaviour alone cannot predict whether resistance will increase or decrease with speed.