The FT4 Powder Rheometer was developed to measure the flowability and processability aspects of powders. The main benefits were seen as the characterisation and classification of both the materials and the processing machinery so that the consistency and efficiency of powder processing could be improved. The background to this is described in Benefits and Opportunities and The Nature of Powders. |
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The FT4 operating principle and the methodology used, were developed as a new way of evaluating powder flow performance. The approach differs from all existing techniques because it sets out to establish a reproducible dynamic flow condition that more closely resembles the way that powders move during handling and processing. These reproducible flow conditions are used to evaluate how different powder samples compare in relation to factors such as moisture content, level of aeration, particle size range, or merely different batches of the same material.
In terms of a detailed specification, the key benefits are:
- Powder conditioning that allows tests to be repeated and compared (Conditioning of Powders)
- Automated testing and analysis - independent of the operator
- Quick and straightforward operation- tests completed in minutes
- Small volume of sample - down to 10ml
- Repeatability between instruments is high
- Ability to investigate flowability and robustness in relation to any of the key variables
- Dynamic flow modes to simulate gentle to aggressive flow patterns and flow rates
- High sensitivity to detect small rheological changes
- Capable of measuring the rheology of powders and complex semi-solids
- Able to measure the shear strength or cohesion of powders
The FT4 operating principle |
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The FT4 operating principle is to force a twisted blade along a helical path through a powder sample 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 as the basis of the flowability assessment.
The helical path along which the blade moves is determined by the combination of axial and rotational speeds. Thus a downward helical path may be a small or steep helix and it may be left or right handed depending on which direction the blade rotates. Some different modes are shown below.
A blade with a right handed twist, when moved along a right-handed helical path, would produce relatively low compaction and low displacement because of the slicing action as in b). When the helix and blade angles coincide, the action would be a slicing action in the way that a knife slices through a solid.
Movement along a left handed helical path produces more compaction and higher flows, because the angle of approach to the blade face may be close to perpendicular. A bulldozing type of action is then established, c).
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a) typical conditioning upward
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b) slicing downward
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c) typical testing
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| Three of the many displacement modes available | ||
This 'angle of approach' may be pre-set simply by setting the polarity and magnitude of the helix angle. Hence, a 5° negative upward helix, a), would establish a gentle lifting action that would allow the displaced powder to fall to rest behind the moving blade, the normal 'conditioning' mode to be described later. The photo shows the blade leaving the powder after conditioning.
Click here to see video clip of above action. (482 KB)
Conversely a 10° negative downward helix, c), produces high compaction and high flow, commonly used for testing. Hence a wide range of flow modes is available simply by defining the helical path angle. The rate of displacement or flow rate is dependent only upon the blade tipspeed, which is the other set parameter needed to fully define blade movement.
Click here to see video clip of above action. (313 KB)
For most displacement modes the flow is predominantly up and over the blade. Each particle within the powder mass lies at a state of rest until forced to move, coming to rest again as the blade moves on. The pattern of powder displacement is virtually steady state, allowing flow to be observed and generally resulting in smooth, linear or logarithmic profiles of the measured forces. These forces are those required to initiate shearing and breakdown of interparticulate bonding of the powder in the zone immediately around the blade, a process that is continuous.
The versatility of the FT4 allows testing of virtually any material through which a blade may be passed ranging from sub-micron cohesive powders to 5mm granules, dough or other semi-solids.
Measured data |
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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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The work done is calculated from this data so that an energy gradient profile for each traverse is available as shown here. The total energy consumed during the downward testing traverse is the area beneath this curve. |
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This graph shows a set of energy gradient profiles for talcum powder. Each curve represents a particular blade speed or flow rate. The shape and magnitude of these profiles are characteristic of the material. |
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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. |
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The use of these characterising values to determine a wide range of flowability parameters are described in detail in the methodology section.
Determining the energy gradient and total energy consumed |
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The aim is to measure the work done or energy consumed, as the blade moves through the powder. To do this we need to know all the forces acting on the blade. These are: a) The rotational force or torque acting on the blade b) The axial force acting on the blade |
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These two forces are measured using sensitive strain gauged force transducers or load cells that produce a voltage output that is directly proportional to the force applied. |
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The axial force measuring load cell is fitted beneath the table of the FT4, ie the surface to which the testing vessel is attached- see photo. The table and vessel containing the test sample is rigidly attached to and supported by the load cell. |
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The torque is measured using a similar load cell. However in this case we need to measure the blade torque or rotational force and to do this the load cell needs to measure the reaction force on a torque arm fitted to the drive at the top end of the spindle and blade assembly. So it is the torque acting on the blade that is measured. |
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Measurements of torque and force are usually taken approximately every 0.25mm of vertical movement of the blade and these measurements are stored in the test file. |
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The total energy consumed during the traverse is the sum of all the incremental energy values and the total is calculated automatically as part of the automated data analysis. |
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The test file format contains the height, force, torque, energy gradient and energy figures for the entire test. The energy calculations described above are built into the software. However, all the raw data is available within each test file and the energy totals listed may easily be checked using calculations from first principles, if required. |
Operation of the FT4 |
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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 testing programmes
- safety screen to prevent access during operation
- completely automated testing
- completion of the test programme in 1 to 10minutes 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 methodology.
Software |
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The software provided usually includes the following:
- Operating Platform: Microsoft Windows 98, Windows NT4, Windows 2000 or Windows XP
- Control Software: Entirely Freeman Technology designed
- Data Analysis: Entirely Freeman Technology designed - see Data Analysis
- Microsoft Office
System options |
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A full range of system options are provided including:
- 300ml, 200ml and 25ml testing vessels made from precision bore, borosilicate glass
- Blade and spindle assemblies to suit the above vessels
- Vessel accessories including special bases, lids, funnels and thermal jackets
- Compaction accessories including solid and vented pistons
- Aeration modules to suit all vessel sizes
- Shear cell modules
Qualification procedures |
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On installation the FT4 system is subjected to qualification checks and procedures to ensure conformance. These procedures include:
- Design qualification
- Installation qualification
- Operation qualification
- Performance qualification
Calibration |
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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 |
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Version 4 of the Data Analysis software package for the FT4 universal powder tester delivers faster analysis with more flexible reporting options. It incorporates the latest types of test available on the multi-functional FT4, and offers a redesigned and simplified user interface.
Data Analysis is a standalone application that can be installed on any computer, allowing analysis away from the laboratory, as and when required.
The Data Analysis package delivers:
- Graphical display of any combination of force, torque, air velocity or air pressure data captured by the FT4 during testing, as a function of time, height or test number
- Graphical display of all values derived or calculated from the raw data. These include:
- energy /test traverse
- energy /min, energy/sec
- torque energy
- force energy
- bulk density
- permeability or pressure loss across a powder bed
- compressibility
- wall friction
- shear data (see below).
- Rapid interpretation of test results. Data are displayed in a graphical format and can be printed directly or imported into other programmes
- Calculation of key indices such as stability index (SI), basic flowability energy (BFE), specific energy (SE) and flow rate index (FRI), in a matter of seconds using 'Automated Analysis'
- Rapid analysis of multiple files through the creation of macros. Selection of the appropriate macro and test files allows the application of predefined criteria to all files being analysed
- Independent analysis of raw data test files - generated in binary form (.prb) for efficiency and security - in packages such as Excel
Results presentation
Figure 1 shows the results of a stability and flow rate test on two different powders.
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 Figure 1 |
Automated analysis from shear testing
Automated analysis of data from shear testing (Figure 2) includes
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FT4 specifications |
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A detailed specification is available on request.