Cohesion at 4 speeds
How powder flow resistance changes with process speed – and what that means for scale-up, filling and dosing.
What is speed-dependent cohesion?
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.
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Cohesion (4 speeds) testing answers the question: “Does the powder’s resistance to flow change as process speed increases or decreases?” Cohesion at 4 speeds answers a fundamentally different question from single-speed Cohesion: Not just "how cohesive is the powder?" but "when does cohesion become a problem?" |
How the cohesion at 4 speeds test works
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.
Measured parameters
- Cohesion Index (per speed) – resistance to particle separation at each test speed
- Compaction Coefficient (per speed) (g.mm) – degree of densification during dynamic loading
- Flow Stability – change in resistance at the same speed between the start and end of the test
- Conditioned Bulk Density (g/ml) – bulk density after controlled preparation (split vessel)
Interpretation of the graph profile
The test quantifies the dependence of flow characteristics on flow rate of powders. It measures resistance of a powder sample (as Compaction Coefficients) as controlled flow is imposed at 4 different speeds on the downward cycle. At each speed, the force profile reflects how the powder resists movement during both compaction and lifting. Comparing profiles across speeds reveals whether resistance:
- increases smoothly with speed,
- decreases as speed increases, or
- changes during the test due to rearrangement, breakdown, or densification.
Powders that appear similar at one speed can behave very differently at another, particularly when cohesion, aeration, or dynamic packing effects are present.
The Cohesion Coefficient (measured on the upwards direction) is also determined at each of these flow rates to see whether cohesion characteristics are affected by flow rates. Powders that flow freely will transfer very little resistance through the powder column in either a downward or an upward direction. Conversely, poorly flowing powders exhibit substantial amounts of force in either direction.
It also measures the flow stability (how the powder breaks down during testing) and provides an indication of the susceptibility of the product to attrition (breakdown) by comparing the work needed to move the blade though the powder at the start of the test compared to the work required to move the powder at the same speed at the end of the test.
Understanding the measured parameters
Cohesion Index (per speed) – what it means
|
CI behaviour across speeds |
What it indicates |
Likely implications in processing |
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CI roughly constant |
Cohesion largely speed-independent |
Behaviour unlikely to change with throughput; issues are geometry- or consolidation-driven |
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CI highest at low speed |
Resistance dominated by start/stop behaviour |
Hopper initiation problems, sticking at rest, poor restart after pauses |
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CI increases with speed |
Dynamic cohesion or compaction effects |
High-speed dosing variability, poorer performance at increased throughput |
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CI decreases with speed |
Aeration or structural breakdown under motion |
Appears easier to move at speed; may be unstable or flood under certain conditions |
Compaction Coefficient (per speed) – what it means
|
Compaction behaviour |
What it indicates |
Likely implications in processing |
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Low at all speeds |
Powder resists densification |
Stable packing during handling |
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Increases with speed |
Dynamic packing under motion |
Density drift during high-speed filling or conveying |
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High at low speed |
Packing occurs even under gentle motion |
Consolidation sensitivity during storage or slow handling |
Flow Stability – what it means
|
Flow Stability behaviour |
What it indicates |
Likely implications in processing |
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Low change |
Behaviour consistent throughout test |
Predictable performance over long runs |
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Moderate change |
Gradual rearrangement during handling |
Run-in effects, slow drift in filling or dosing |
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High change |
Significant structural change during testing |
Attrition, degradation, or strong handling sensitivity |
Flow Stability highlights whether the powder changes as it is handled, not just how it behaves at a given speed.
Bulk Density – what it means
What this parameter answers
"What density does the powder adopt after controlled, repeatable preparation?"
Bulk density reflects how particles arrange and pack under their own weight and gentle conditioning, not how they flow or fail.
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Bulk Density behaviour |
What it indicates |
Likely implications in processing |
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Low |
Inefficient packing; high voidage |
Larger pack volumes; higher fill variability if packing changes |
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Moderate, consistent |
Repeatable packing under controlled preparation |
Reliable filling and QC trending |
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High |
Efficient packing with little void space |
Smaller packs possible; increased consolidation sensitivity may occur |
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Variable between samples |
Handling-sensitive packing behaviour |
Density drift, feeder instability, batch-to-batch variation |
When is a cohesion-at-multiple-speeds test most useful?
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.
What to test next based on your Cohesion at 4 speed results
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.
Cohesion stable across all speeds
Typical behaviour:
Cohesive resistance remains relatively constant.
Likely risks:
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Flow issues are not driven by speed sensitivity.
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Problems may be related to storage or consolidation instead.
Recommended next tests:
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Caking – to assess storage-induced strength
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Compressibility – to evaluate packing sensitivity
- If CI is also low across all speeds, behaviour is unlikely to be the source of production problems – check bulk density for fill consistency issues and consider whether further testing is necessary
Cohesion increases with speed
Typical behaviour:
Powder becomes more resistant to movement at higher speeds.
Likely risks:
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Poor performance at high throughput
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Overloading of feeders or drives
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Sensitivity during ramp-up
Recommended next tests:
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PFSD – to quantify overall speed dependence
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Compressibility – to assess frictional and packing effects
Cohesion decreases with speed
Why this test matters
Cohesion at 4 Speeds bridges the gap between single-point cohesion testing and full PFSD analysis, helping to pinpoint the operating conditions most likely to cause flow problems.
Sample data and its interpretation
Tabulated data and its meaning
|
Sample |
Speed dependence (CI trend) |
Compaction behaviour |
Flow stability |
Overall behaviour type |
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Baby powder |
CI increases with speed |
Strong decrease in compaction with speed |
Stable |
High-speed sensitivity, dynamically loosening |
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Cheese powder |
CI increases at high speed |
High compaction at all speeds |
Stable |
Throughput-sensitive, dense and resistant |
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Cornflour |
CI slightly decreases with speed |
Low compaction overall |
Stable |
Speed-robust, bulk-controlled |
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Seasoning powder |
CI decreases with speed |
Moderate–high compaction |
Unstable |
Start-sensitive, handling-history dependent |
Sample-by-sample interpretation
Baby powder – high-speed sensitive but dynamically stable
What the numbers say
- Cohesion Index increases from ~15.7 at 10 mm/s to 18.3 at 100 mm/s.
- Compaction coefficient drops sharply with speed (9655 to 3446).
- Flow Stability = 0.92, indicating repeatable behaviour during the test.
Interpretation
Baby powder shows increasing resistance to flow as speed increases, even though the powder bed becomes less compacted at higher speeds. This indicates dynamic cohesion effects rather than packing or consolidation.
Expected behaviour in practice
- Performs acceptably at low speeds.
- Increasing line speed may introduce drag, torque increase, or dosing variability.
- Behaviour is repeatable, so problems will be consistent rather than erratic.
Cross-reference
- Matches moderate CI in single-speed cohesion.
- Low consolidation risk unless combined with long storage (check caking separately).
Cheese powder – throughput-sensitive, highly resistant powder
What the numbers say
- High CI at all speeds, increasing to 26.9 at 100 mm/s.
- Very high compaction coefficients, even at high speed.
- Flow Stability ≈ 1.01, indicating stable but consistently resistant behaviour.
Interpretation
Cheese powder is intrinsically resistant to movement, and this resistance worsens at high speed. Unlike baby powder, it also packs efficiently under motion, contributing to sustained resistance.
Expected behaviour in practice
- High risk of under-filling, torque limits, and throughput ceilings.
- Problems become more severe as speed increases.
- Behaviour is predictable but difficult to manage without intervention.
Cross-reference
- High CI aligns with difficult handling.
High compaction suggests density drift and consolidation sensitivity - check Compressibility and Caking.
Cornflour – speed-robust, bulk-controlled powder
What the numbers say
- CI slightly decreases with speed (14.7 to 13.0).
- Lowest compaction coefficients across all speeds.
- Flow Stability ≈ 1.01, indicating highly repeatable behaviour.
Interpretation
Cornflour shows minimal speed dependence and low resistance to densification. The powder’s behaviour is dominated by bulk particle-particle interactions, not speed-driven effects.
Expected behaviour in practice
- Predictable flow across throughput changes.
- Low risk during scale-up.
- Any issues are more likely due to environment (humidity) or storage, not dynamic handling.
Cross-reference
- Moderate CI and low compaction explain why cornflour often appears "well-behaved" dynamically.
- If problems occur, investigate moisture sensitivity or caking, not PFSD.
Seasoning powder – start-sensitive and handling-history dependent
What the numbers say
- High CI at low speed (29.3) that decreases with speed.
- Moderate-high compaction coefficients.
- Flow Stability = 1.11, indicating significant change during handling.
Interpretation
Seasoning powder exhibits strong low-speed cohesion, meaning resistance is greatest during start-up and slow movement. As speed increases, resistance reduces, but the powder changes during the test, indicating sensitivity to handling history.
Expected behaviour in practice
- Poor restart after pauses.
- "Works once running" but unpredictable behaviour over time.
- Dosing drift and run-in effects likely.
Cross-reference
- High low-speed CI explains restart complaints.
- Instability suggests attrition, segregation, or rearrangement – PFSD and Caking highly relevant.
Charts
Cross-sample comparison: what this test reveals clearly
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Behaviour type |
Sample |
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High-speed limited |
Baby powder, Cheese powder |
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Speed-robust |
Cornflour |
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Start-sensitive / handling-dependent |
Seasoning powder |
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Compaction-driven resistance |
Cheese powder |
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Dynamic cohesion without packing |
Baby powder |
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.
How the Cohesion at 4 Speeds test compares with other powder flow tests
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.
Test guidance
- 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
FAQs
What does the Cohesion at 4 Speeds test measure?
This test measures how a powder’s cohesive resistance changes across four defined test speeds. It provides a structured way to assess whether cohesion is stable or sensitive to operating speed.
How is this different from PFSD?
While PFSD quantifies overall speed dependence, Cohesion at 4 Speeds focuses specifically on how cohesion itself changes with speed. It provides more granular insight into speed-dependent cohesive mechanisms.
Why test cohesion at multiple speeds?
Many powders behave acceptably at low speeds but resist movement at higher throughputs – or vice versa. Testing cohesion across multiple speeds helps identify conditions where flow problems are likely to emerge.
Is Cohesion at 4 Speeds a replacement for standard cohesion testing?
No. Standard cohesion testing provides a baseline measure. Cohesion at 4 Speeds builds on that baseline by showing whether cohesion is stable or speed-sensitive.
Is this test suitable for quality control?
It is generally more valuable for process understanding and troubleshooting than routine QC, particularly when operating speed varies or scale-up issues are reported.
