Powder Testing using the FT4
The FT4 Powder Rheometer is a universal powder tester - three instruments in one, combining the Freeman patented blade methodology for measuring flow energy with a range of shear cells, wall friction modules and other accessories for measuring bulk properties. The methodologies allow measurement of:
- Flow energy in relation to many variables and all packing states
- Shear properties of consolidated and unconsolidated powders
- Bulk properties – precision bulk density, compressibility and permeability
- Process variables such as moisture, attrition and segregation
The System accessories available include:
- Three sizes of testing vessel: 25, 50 and 62mm diameter for different samples sizes.
- Aeration Control module to aerate/fluidise powder samples
- Compaction accessories for consolidating powder
- Shear cell modules – three sizes down to the 1ml cell
- Wall friction modules to measure powder on metal interaction
The above methodologies and system accessories allow powder samples to be comprehensively characterised for the extremes of packing and environmental conditions that occur in everyday processing.
The optimal approach for powder assessment is firstly to use testing methodologies that provide reproducible data and secondly to evaluate flow performance in ways that relate to powder behaviour in real industrial processes. This means quantifying the effects of consolidation, vibration, segregation, moisture uptake, fines, aeration, flow rate, air entrainment and a number of other factors. Such characterisation using complementary methodologies has wide application in the formulation, processing and QC sectors of most powder processing industries.
FT4 User Benefits
- Three instruments in one – rheometer, shear cell and compression tester
- Powder conditioning that allows exceptional reproducibility of measurement
- Automated testing and analysis - independent of the operator
- Quick and straightforward operation- tests completed in minutes
- Ability to evaluate consolidated, conditioned and aerated / fluidised powders
- Unique measurement of aeratability of powders
- Small volume of sample - down to 1ml in the case of the miniature shear cell
- Three test vessel standards – 25, 50 and 62mm diameter for different sample volumes
- Family of three shear cells, 80, 10 and 1ml capacities
- High sensitivity dynamic mode for differentiating between very similar powders
- Can evaluate process variables such as attrition and segregation
Flow Energy measurement using the FT4
It is easy to imagine stirring a powder sample using a spatula or even a finger and feeling the resistance as the powder is made to flow. The FT4 Powder Rheometer operating principle uses a sophisticated finger in the form of a precision twisted blade that is forced through the powder bed. It moves along a predetermined helical path so that a required flow rate and pattern of flow is established. The forces causing the deformation and flow of the powder are those imposed by the moving blade and these are measured continuously and used to calculate the energy needed to make the powder flow. This methodology therefore measures ‘flow energy’ and is described as the ‘Dynamic flow’ methodology because the flow pattern is three dimensional and representative of real flow conditions.
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The helical path along which the blade moves is determined by the combination of axial and rotational speeds. The Dynamic Methodology utilises a single blade operating in different ways. Three important modes of operation are shown in Figs. 1 - 3. Fig. 1 shows the typical downward test mode that applies a bulldozing type of action along the entire length of the blade, producing compaction and high flow. This mode simulates powders flowing in a confined environment where flow is limited by the containing surfaces. It is used to measure the Basic Flow Energy (BFE) and other properties. |
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| Fig. 1: Downward testing mode | ||
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Fig. 2 shows the typical upward testing mode which shears through the powder producing relatively little compaction. It is used to measure the Specific Energy (SE) which is the flow energy per gram of powder tested. This assessment measures the resistance to flow when the powder is unconfined. The Specific Energy usually correlates well with visual assessments of flowability when powders are tumbled in a glass container. |
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| Fig. 2: Upward testing mode | ||
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Fig. 3 shows an important variant of the above used for conditioning which employs a slicing action to gently lift the powder and then deposit it behind the blade. The effect is to loosen the powder and produce a slightly aerated bed ready for testing. This process removes any precompaction or excess of air and makes the result independent of how the operator handles the powder and places it into the testing vessel. |
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| Fig. 3: Conditioning mode | ||
Methodology - how and what is measured
Measured data
Force and torque measurements are sampled continuously whilst the blade traverses downwards and then upwards through the test material. A typical torque profile is shown here for the downward traverse.
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| (click to enlarge) | (click to enlarge) |
The work done is calculated from this data so that an energy gradient profile for each traverse is available as shown as shown here. The total energy consumed during the downward testing traverse is the area beneath this curve.
This graph shows a set of energy gradient profiles for talcum powder. Each curve represents a particular blade speed or flow rate.
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| (click to enlarge) | (click to enlarge) |
The shape and magnitude of these profiles are characteristic of the material. The results of a flow rate test are shown in this graph. Here total energy values from the individual energy gradient profiles (above) are plotted as a function of the blade tip speed or flow rate.
The use of these characterising values to determine a wide range of flowability parameters are described in detail in the methodology section.
Powder Conditioning
It is essential to produce the standardised packing condition mentioned above as a preliminary to each test cycle. This 'conditioning' process involves gentle displacement of the whole powder sample in order to loosen and slightly aerate the powder. The aim is to disturb and gently drop each particle in order to construct a homogenously packed powder bed. This process removes any precompaction or excess of air and makes the result independent of how the operator handles the powder and places it into the testing vessel.
Click here to see the Conditioning Mode video clip
A conditioning cycle comprises a traverse of the blade downward and then a traverse upward. The downward traverse would typically use a 5 degree positive helix in order that the blade action is more slicing than compacting. The upward traverse would typically use a 5 degree negative helix that gently lifts the powder and drops it over the blade, each particle coming to rest behind it as in the above video clip.
A conditioning cycle is usually completed before each testing cycle in order to remove any residual compaction from any previous test cycle. The exception to this is where a consolidated sample is being evaluated, in which case conditioning is not used.
Powder Properties that can be measured with the FT4
There are 4 types of parameter derived using different methodologies:
- Parameters derived from Dynamic flow measurements - flow energy, etc
- Bulk Properties derived from compression or other simple methods
- Shear Properties derived from shear cell tests
- Processability properties derived from specific tests
Parameters derived from dynamic flow measurements
The Dynamic methodology described in Flow Energy measurement using the FT4 measures the energy needed to displace the powder under precisely controlled test conditions following Conditioning.
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Basic Flow Energy (BFE) of powder when confined in the testing vessel with blade moving downwards displacing powder and applying some compression. Consequently the BFE is relatively high – especially for non-cohesive powders. Often the most differentiating of all measurements for similar powders. The figures to the right show typical variation of BFE with some important variables. |
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Specific Energy (SE) – measurement of flow energy when un-confined. Blade moves upwards during testing, predominantly shearing with no compression. Correlates well with visual flowability assessments – tumbling in jar for example. |
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Flow Rate Index (FRI) – how flow energy varies in relation to a tenfold change of flow rate. Generally powders require a higher energy to move them slowly. In particular, cohesive powders tend to have a high FRI. |
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Stability Index – how flow energy changes with repeated testing. Can be due to air release, moisture, agglomeration or other reasons. |
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Aeration Ratio – how flow energy reduces with aeration of the powder bed |
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Parameters derived from compression and aeration tests
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Bulk density – measured at various packing conditions. Measuring mass of a precise volume of conditioned powder provides the Conditioned Bulk Density (CBD). The CBD can be greatly increased by consolidation or conversely, reduced by aeration. The extent of this change being dependant upon whether the powder is cohesive or not and also how much consolidation was produced. |
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Compressibility – how bulk density increases on compaction. For cohesive powders most of the 20% to 30% volume reduction results from air removal.
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Permeability – how easily air passes through a powder at increasing bulk stress. Close packed, coarse powders have air voids between the particles and hence are usually highly permeable. Cohesive powders tend to have a large amount of entrained air, but voids are not connected so that permeability is poor. |
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Parameters derived from shear cell measurements
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Shear strength - of already pre-consolidated powder at reducing levels of normal stress. Called Yield Loci.
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Cohesion – shear strength when consolidation stress is zero. Cohesion is the intecept on the Y axis. |
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Internal Angle of Friction – the slope of the yield loci. |
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Unconfined yield strength (UYS) – or compression strength. Stress at which a bridge will collapse in a hopper outlet. Derived from yield loci plot using Mohr Circle analysis.
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Operation of the FT4
Apart from the placement of the powder sample into the test vessel, the entire test procedure is automatic and independent of the operator. The system provides the following:
- Prevention of unauthorised users
- Library of standard test programmes
- Safety screen to prevent access during operation
- Automated testing
- Completion of the test programme in 1 to 20 minutes depending on programme type
- Secure binary files of all test data
- Automated data analysis to derive the key flowability indices
Sampling and testing procedures are described in detail in the User Manual
Software
The software provided usually includes the following:
- Operating Platform: Microsoft Windows XP
- Powder Rheometer Control Software: Freeman Technology designed - bespoke software
- Data Analysis: Freeman Technology designed - bespoke software - see Data Analysis
- USB interface for all automated accessories
- Microsoft Office
Accessories
A full range of accessories are provided including:
- 25mm, 50mm and 62mm testing vessels made from precision bore, borosilicate glass
- Blade and spindle assemblies to suit the above vessels
- Compaction accessories including solid and vented pistons
- Aeration kit to suit all three vessel sizes
- Vessel accessories including special bases, funnels and thermal jackets
Qualification procedures
On installation the FT4 system is subjected to qualification checks and procedures to ensure conformance. These procedures include:
- Design qualification (DQ)
- Installation qualification (IQ)
- Operation qualification (OQ)
- Performance qualification (PQ)
Calibration
All controlled and measured variables are subject to rigorous calibration procedures. The variables are force, torque, height and rotational and linear velocities. The following items facilitate calibration:
- A comprehensive calibration kit of fixtures
- Certified calibration masses
- Software to allow each variable to be calibrated
- Software that permits an audit of each variable to determine if the instrument is still within specification
- A calibration log containing all current and previous calibration data
Data analysis
The FT4 software includes a stand-alone Data Analysis package that enables the measured test data to be quickly interpreted and summarised. The key features are:
- Data Analysis is a stand-alone application that can be installed on any computer, allowing analysis to be performed away from the lab if need be
- Rapid interpretation of test results is achievable. The data is displayed in a graphical format that can be printed directly or copied to the clipboard and imported into Word for example
- Key indices such as Stability Index (SI), Basic Flowability Energy (BFE) and Flow Rate Index (FRI) are calculated automatically within seconds using 'Automated Analysis'
- The raw data test files are in binary form (.prb) for reasons of efficiency and security
- Raw data test files may be easily converted to allow independent analysis using a package, such as Excel, if required
- The graph below shows the results of a stability and flow rate test on two different powders. The addition of labels and titles is straight forward and flexible, as are other functions such as background colour and gridline weight
FT4 specifications
A detailed specification is available on request.
