Why your powder behaves differently at line speed than in the lab

Scale-up failures, fill weight drift, and the measurement that reveals speed sensitivity before it costs you

The scale-up paradox

It is one of the most frustrating experiences in powder processing. You have tested the material in the laboratory. It flows. The cohesion index is acceptable. The caking risk seems manageable. You scale up to production - and the powder that was perfectly well-behaved at bench scale now causes fill weight drift at line speed, surges unpredictably when you increase throughput, or shows erratic behaviour that wasn't visible in any of your development testing.

The testing wasn't wrong. It was incomplete. It tested the powder at one speed - your laboratory speed - and that single condition told you nothing about how the powder would behave at the higher rates, faster cycling, and longer run times of production.

This is the problem that Powder Flow Speed Dependence (PFSD) testing on the Powder Flow Analyser is designed to solve.

What speed dependence actually measures

When the PFA runs a PFSD test, it moves the blade through the powder column at five increasing speeds - typically 10, 20, 50, and 100 mm/s - and measures the work required to move the blade at each speed. It also runs a repeat cycle at the starting speed at the end of the test, creating a before-and-after comparison of behaviour at the same condition.

The test generates three critical outputs:

•     Compaction Coefficient at each speed - the resistance to densification as speed increases. If this value rises with speed, the powder is becoming harder to move at higher throughputs. If it falls, the powder is becoming easier to move.

•     Speed Sensitivity Ratio - the ratio of resistance at the highest test speed to resistance at the lowest (Comp100/Comp10). This single number tells you whether the powder is speed-sensitive and in which direction.

•     Flow Stability - the change in resistance at the same speed between the start and end of the test. This tells you whether the powder's behaviour drifts as it is handled repeatedly.

Together, these three outputs answer the two most important questions for production:

•     Does behaviour change with throughput speed?

•     Does behaviour change over the course of a production run?

The three speed profiles - and what they mean in practice

Profile 1: Resistance increases with speed (SSR above 1.0)

The powder becomes harder to move as speed increases. Granulated sugar is a good example - its Speed Sensitivity Ratio is 1.56, meaning it is 56% harder to move at 100 mm/s than at 10 mm/s. Its Flow Stability is 1.94, meaning it also becomes significantly harder to move over the course of the test.

What this means in practice: at low production speeds, this powder fills acceptably. As line speed increases to meet demand, resistance builds. Fill weights begin to drift downward - the powder is no longer flowing into the filling system as quickly as the machine expects. The problem is not immediately obvious because it develops gradually as speed ramps up.

The scale-up risk is high. The laboratory test at a single slow speed gave no indication that this would happen. Only a multi-speed test reveals it.

Profile 2: Resistance decreases with speed (SSR below 1.0)

The powder becomes easier to move as speed increases. Ethylcellulose is the most striking example - its Speed Sensitivity Ratio is 0.24, meaning it is dramatically easier to move at high speed than at low speed. This sounds beneficial. It is not.

At slow speeds - start-up, low-throughput operation, restart after a brief pause - ethylcellulose offers substantial resistance. It is difficult to initiate flow, and at low line speeds it may not flow reliably. But as speed increases, resistance drops sharply. At high speed it flows much more easily than expected.

What this means in practice: the powder behaves completely differently depending on operating condition. Start-up is difficult, restart after a brief pause is unreliable, but once running at full speed the powder flows freely. This is not a formulation problem - it is a process design problem that a single-speed cohesion test would misdiagnose entirely.

An additional risk for SSR-below-1 powders: if the material becomes dramatically easier to move at speed, there is a flooding or surge risk. At high throughput, the powder may flow more freely than the filling system expects, causing over-filling or loss of dosing control.

Profile 3: Speed-independent (SSR approximately 1.0)

The powder's resistance is largely unchanged across the tested speed range. Sesame seed is a good example - its SSR is 1.01, its Flow Stability is 0.94 - essentially speed-insensitive and stable. This powder can be confidently scaled up because its behaviour is predictable across a wide range of operating conditions.

Speed-independent behaviour is not the same as easy-flowing. A powder can have high absolute resistance at all speeds and still be speed-independent. The SSR tells you about relative change with speed; the absolute compaction coefficient tells you about the magnitude of resistance at any given speed.

Flow Stability: the other half of the story

The Speed Sensitivity Ratio addresses one question: does behaviour change with throughput speed? Flow Stability addresses a different question: does behaviour change during a production run, even at constant speed?

A Flow Stability significantly above 1.0 means the powder becomes harder to move as it is repeatedly cycled through the test - equivalent to a powder that becomes progressively more compacted and resistant during an extended production run. Fill weights drift. Feeder torque increases. Performance that was acceptable at the start of a shift becomes unacceptable by the end.

A Flow Stability significantly below 1.0 means the opposite - the powder becomes easier to move as it is worked. This can indicate particle breakdown, agglomerate disruption, or structural rearrangement during handling. Again, this is information that a single-speed test cannot provide.

Bird sand is an example of a powder with both high speed sensitivity and high flow instability - SSR 1.40 and Flow Stability 1.68. Both resistance and behaviour change as you increase speed and as the powder is repeatedly cycled. This combination represents the highest dynamic handling risk.

The SSR × Flow Stability matrix

Plotting Speed Sensitivity Ratio against Flow Stability gives a clear picture of dynamic risk:

What PFSD testing changes about your scale-up process

Running a PFSD test before scale-up gives you information that transforms the conversation between R&D and production:

•     If SSR is close to 1.0: scale-up is low risk from a speed perspective. The powder will behave at production speed as it did in development.

•     If SSR is significantly above 1.0: warn production that fill performance will degrade at higher speeds. Establish a recommended maximum line speed and test specifically at that speed.

•     If SSR is significantly below 1.0: warn production that start-up and low-speed operation will be more difficult than steady-state. Design start-up procedures accordingly and check flooding risk at full speed.

•     If Flow Stability is far from 1.0: expect performance to drift during a production run. Establish run-in procedures, check fill weights early in each batch, and understand that the laboratory short-run test may not represent end-of-shift behaviour.