Cohesion at 4 speeds

Most powder handling problems only appear at specific operating conditions – at line speed, at the beginning of a run, or after a production pause. A single-speed cohesion measurement captures a snapshot of behaviour; it cannot tell you whether that behaviour changes as throughput increases, or identify the conditions under which a normally acceptable powder starts to fail.

Cohesion at 4 speeds extends the standard Cohesion test by measuring cohesive resistance at multiple, defined flow rates, allowing users to determine whether cohesion is primarily a:

• low-speed, start/stop issue (e.g. hopper initiation, restart), or
• high-speed, dynamic handling issue (e.g. dosing, filling, conveying).

This makes the test particularly valuable for process design, scale-up, and troubleshooting, where changes in throughput often expose problems not seen under a single test condition.

After an initial conditioning cycle to minimise user loading effects, the powder is tested using one flow cycle at each of four defined speeds (typically 10, 20, 50, and 100 mm/s). The same speed is used in both the downward (compaction) and upward (lifting) directions.

As with the standard Cohesion test:

• the downward stroke compacts the powder under controlled conditions
• the upward stroke lifts the powder, allowing cohesive resistance to be quantified

By repeating this process at multiple speeds, the test records Cohesion Index and Compaction behaviour as a function of flow rate, rather than at a single condition.

A final repeat cycle at the initial speed allows assessment of flow stability, highlighting whether powder behaviour changes during handling.

Cohesion at four speeds is most useful when powder behaviour appears to change with process speed, such as during scale-up, high-speed filling, or dosing. This test reveals whether cohesion is primarily a low-speed start/stop problem or a high-speed handling issue, and whether resistance increases or decreases as flow rate changes. It is especially valuable when the standard cohesion test does not fully explain process variability or when speed-dependent effects are suspected.

This test identifies whether cohesive resistance is stable or speed-dependent. The most appropriate follow-up tests depend on how cohesion changes across the tested speeds.

Cross-sample comparison: what this test reveals clearly

Key interpretation takeaway

Cohesion at 4 speeds reveals when and why resistance to flow becomes problematic: baby and cheese powders are limited by high-speed handling, seasoning powder is dominated by start-up and handling history, while cornflour remains robust across throughput changes.

Cohesion at 4 Speeds vs Standard Cohesion

• Standard cohesion provides a single baseline value.
• Cohesion at 4 Speeds reveals how that value evolves with speed.

Why this matters:

A single cohesion value may mask speed-dependent failure mechanisms.

Cohesion at 4 Speeds vs PFSD

• Cohesion at 4 Speeds focuses specifically on cohesive resistance.
• PFSD captures overall speed dependence across multiple flow mechanisms.

Why this matters:

Use Cohesion at 4 Speeds to isolate cohesion-driven speed effects; use PFSD to assess system-level speed sensitivity.

Cohesion at 4 Speeds vs Caking

• Cohesion at 4 Speeds focuses on dynamic behaviour.
• Caking focuses on time-dependent strength development during rest.

Why this matters:

A powder may behave well dynamically but fail after storage – or vice versa.

• Cohesion at 4 speeds is not a replacement for single-speed Cohesion or full PFSD; it bridges the gap between them.
• A powder may appear acceptable at one speed and problematic at another – this test is designed to reveal that risk.
• Results should always be interpreted alongside:
• Cohesion (1 speed) for baseline resistance and failure mode
• PFSD when detailed speed sensitivity and stability are required
• Compressibility and Caking when consolidation or storage effects are suspected

What this test adds vs standard Cohesion or PFSD

Cohesion at 4 speeds sits between single-speed Cohesion and full PFSD:

• It provides cohesion data at multiple speeds, rather than a single snapshot
• It offers clear discrimination for powders whose behaviour only becomes problematic at certain speeds
• It gives speed-related insight without the full complexity of PFSD

For some powders, differences between materials are far more apparent at a non-standard speed, making this test particularly useful for comparative screening.

How Cohesion at 4 Speeds should be used (decision guidance)

Most useful when:

• Investigating speed-related changes in flow behaviour
• Supporting scale-up from R&D to production
• Comparing powders that behave similarly in single-speed tests
• Screening formulations or suppliers for throughput robustness

Should NOT be used alone when:

• Diagnosing severe arching or ratholing – use Cohesion/Bridging Factor
• Investigating long-term storage set-up – use Caking/Consolidation
• Quantifying detailed conveying behaviour – use PFSD
• Measuring product or agglomerate strength – use Texture Analysis