Caking
What is powder caking?
Caking is what happens when a powder that flowed freely when filled into a hopper, bin, or bag no longer does so after it has sat under its own weight for hours, days, or weeks. Particles bond together under applied stress – driven by moisture, temperature cycling, particle surface chemistry, or simply consolidation pressure – and form a structured mass that resists discharge, clogs outlets, and can require manual intervention to break up.
The Powder Flow Analyser (PFA) Caking test measures this directly: a powder column is subjected to repeated controlled compaction cycles, and the test records how much of the bed transforms into a cake and how strong that cake is. The result is a multi-parameter picture of storage stability – not a single "pass/fail" – which is why the parameters need to be read together rather than in isolation.
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Caking testing answers the question: "After compaction cycles, how much of the bed becomes a cake and how strong is that cake?" |
How the caking test works
The test begins with 2 conditioning cycles to remove any user loading variation and to normalise the powder column after filling. The blade levels the top of the powder column and measures its height. The blade then moves down through the column and compacts the powder to a pre-defined force (usually 750g). When it reaches this force it measures the height of the cake and slices up through the powder with minimum disturbance. This compaction cycle is repeated four more times. The 5th time the target force is reached the blade slices through the compacted cake of powder formed at the bottom of the vessel.
Measured parameters
- Cake Height Ratios (e.g. Ratio 5) – how much of the bed behaves like a cake
- Column Height Ratios – how much the bed consolidated/settled during compaction cycling
- Mean Cake Strength (g) – how hard the cake is to break measured as the average force to cut the cake
- Cake Strength (g.mm) – the total work required to break the cake
- Conditioned Bulk Density (g/ml) – bulk density after controlled preparation (split vessel)
Interpretation of the graph profile
The column height ratio (current cycle column height divided by initial column height) and the cake height ratio (current cycle cake height divided by initial column height) are recorded to give information about the settlement and compaction of the powder column.
A powder that has a high tendency to cake would show a large decrease in column height ratio and a strong increase in cake height ratio.
Once the cake has been formed at the end of the fifth cycle the blade cuts through the cake and measures the force required to do so. This force is recorded as the cake strength and is the work required to cut the cake (g.mm) and the mean cake strength is the average force to cut the cake in grams.
Mean cake strength is a useful measurement as it removes the influence of cake height from the calculation.
Understanding the measured parameters
Cake Height Ratio (Cake Fraction) – what it means
Column Height Ratio – what it means
Mean Cake Strength – what it means
Cake Strength – what it means
Bulk Density – what it means
Which caking parameter answers which question?
Note: No single parameter describes powder behaviour. Different caking parameters describe different aspects of set-up.
Caking behaviour is multi-dimensional. Some powders form large but weak cakes; others form small but extremely persistent ones. Interpreting caking parameters together allows realistic prediction of storage and restart performance.
When is a caking test most useful?
A caking test is most useful when powders form lumps, set up during transport, or fail to discharge cleanly after storage or vibration. It assesses how much of the powder bed becomes a cake under repeated compaction and how strong that cake is. This test is especially relevant for diagnosing storage, transport, and handling issues, as well as understanding how packing and consolidation during movement contribute to post-storage flow problems.
What to test next based on your caking results
The caking test identifies whether a powder is likely to strengthen during storage or rest. The most useful follow-up tests depend on how severe the caking is and how quickly it develops.
Low or negligible caking
Moderate caking
Severe or rapid caking
When caking testing is essential
Why follow-up testing matters
Sample videos
Sample caking test data and its interpretation
Tabulated data and its meaning
Charts
How the caking test compares with other powder flow measurements
Caking vs Cohesion
- Caking measures strength development during storage or rest.
- Cohesion measures resistance to movement during flow.
- A powder may show low cohesion initially but severe caking after storage - leading to restart failures.
Caking vs Compressibility
- Compressibility measures how a powder packs under load.
- Caking measures whether that packed structure becomes mechanically strong.
- High compressibility often increases the risk of caking but does not guarantee it.
Caking vs PFSD
- PFSD focuses on speed-dependent flow behaviour.
- Caking focuses on time-dependent strength development.
- Together, they help distinguish between dynamic flow problems and storage-induced failures.
Additional tests
Caking strength can be a highly desirable property of a finished product and can be assessed easily using the Texture Analyser.
FAQs
What does the caking test actually measure?
The caking test measures a powder’s tendency to consolidate and strengthen over time when subjected to load. It simulates storage conditions where powders experience pressure from their own weight or from stacked containers.
Is caking the same as cohesion?
No. Cohesion describes resistance to movement during flow, while caking describes time-dependent strength development during storage or rest. A powder can be free-flowing initially but still exhibit severe caking after storage.
Does visible caking always mean flow problems?
Not always. Some powders cake during storage but break down easily once disturbed. The caking test helps determine whether the strength developed during storage is likely to cause restart, discharge, or handling issues.
How long should a caking test be run?
The appropriate consolidation time depends on the application. Short consolidation times may simulate brief process stoppages, while longer times represent storage or transport. Comparative testing under consistent conditions is often more informative than absolute values.
Is the caking test suitable for quality control?
Yes. Caking tests are well suited to QC applications where powders must remain free-flowing after defined storage periods or transport conditions.
