Powder flowability is defined as the ease with which a powder will flow under a specified set of conditions. Some of these conditions include: the pressure on the powder, the humidity of the air around the powder and the equipment the powder is flowing through or from. For some applications, ease of flow is simply defined by whether the powder flows or not, the so called “go no-go” approach. The only question is: Will the powder flow through the system or not? For other applications, the rate and consistency of the powder flow is important.

The following are some process examples where the flow rate and consistency is important: a powder die that must be filled with the same amount of powder each and every time or a consumer product that must flow smoothly out of a small container.

Any device used to test powder flowability must take the application problems and processing conditions into account to supply relevant data to the user. The powder in the test device must be in the same state as it is in the process being studied. This ensures that the flow analysis will be applicable to the problem.

The REVOLUTION Powder Analyser is the perfect powder tester for measuring the following:

• the flowability of powders in low stress situations
• to study how the powder behaves once it is flowing in all applications
• to determine the condition of the powder as it moves through a process.

A flowability method is created by selecting the time and rate of rotation based on evaluated process conditions. 10-500cc’s of sample are placed inside of the measurement drum. The instrument is run for a set number of avalanches or data points.

The REVOLUTION Powder Analyser measures many parameters to help determine the difference between powders and establish parameters for predicting powder performance. These measurements include fractal dimension, powder volume and surface linearity. Once the flowability test has been completed, the software will provide the user with the statistical analysis.

A powder is fluidised when a gas is injected into the powder causing the powder particles to separate and enter a fluid like state. The properties of the powder, as well as the pressure and temperature of the gas, determines the degree of fluidisation. For fine powders, the gas pressure required to fluidise the particles is very low. This low pressure can be created by rotating the powder in a drum. Varying the rate of drum rotation results in changes of the fluidisation pressure. The fluidisation of a fine powder can be studied by measuring the volumetric expansion of the powder in a rotating drum as a function of the rotation rate of the drum.

The following are some process examples where the flow rate and consistency is important: a powder die that must be filled with the same amount of powder each and every time or a consumer product that must flow smoothly out of a small container.

In addition, the rate at which a fluidised fine powder settles to its original state can be measured by stopping the rotation and reducing the fluidisation pressure to zero.

The REVOLUTION Fluidisation Test measures a powder’s volume increase at specified angular velocity intervals in a rotating drum to create a fluidisation function for the powder. During the fluidisation test, the rate of decrease in the powder’s volume is measured to create a settling function for the material. Appropriate powders for the REVOLUTION Fluidisation test include: toners, catalysts, powder coatings, and other powders with low pressure fluidisation potential.

There are four optional process steps to the Fluidisation Analysis: Prep, Settling, Analysis, and Settling. These four steps are discussed below.

• Prep Step

  Within the Prep Step, the user can rotate the powder at a chosen rotation rate and length of time to completely fluidise the material. This step will allow the user establish a repeatable initial fluidised state for this analysis. This prep step can be skipped if the user wishes to study the fluidisation of the material in its original state.

• Settling Step

  If the material has been fluidised in the prep step, the material must return to its un-fluidised state to perform the fluidisation analysis. In this step, the rotation of the drum is stopped for a specific length of time or until the volume of the sample reaches its original state. The rate at which the powder’s volume decreases is measured and is used to create a settling function.

• Fluidisation Analysis Step

  During the analysis, the user sets the start and end drum rotation rate with the desired step rate. REVOLUTION will begin the drum rotation at the specified start rotation rate, stepping up the rate at the desired intervals until the end rotation rate is achieved. The user can chose to ramp up the angular speed at any desired intervals to achieve the equilibrium fluidised state.

After a specified equilibration time has elapsed at the end rotation rate, REVOLUTION will begin computing the statistical measurements described below for this fluidisation analysis step. In Figure A, the statistical analysis shows the operator the rate at which the powder fluidised by the increase in the overall volume. In Figure B, the fluidisation test results are shown for two different powder samples: one sample fluidises at a faster rate than the other.

Powders can behave very differently depending on the amount of energy they are subjected to as they move through handling equipment. One powder may flow more evenly as it is subjected to more mechanical energy while another powder may become more erratic. This behavior can be studied using the REVOLUTION Multi-Flow test method. In the multi-flow method, the sample drum speed is increased gradually over time and the sample powder’s behavior is measured.

The Multi-Flow Analysis studies how a powder or granular material transitions from avalanching to continually flowing as it is subjected to faster speeds. By gradually increasing the rotation speed in the Multi-Flow Analysis, the user can evaluate the speed at which their powder is no longer avalanching in their process but flowing continuously. This data can be used to predict how powders will behave in high speed equipment.

Before analysis, the samples are prepared by rotating the sample drum at a fixed speed for a fixed time. In this case, the preparation consisted of rotating the powder drum at 20 RPM for 30 seconds to aerate the sample. The prep time and rotation speed are user programmable and are selected to best suit the application being studied.

After preparation, the sample drum rotation is started and sample properties are measured. After programmable intervals, the drum speed is increased by fixed intervals and changes in the sample properties are determined.

The Energy Function Graph displays the energy level of the sample powder versus drum rotation rate. This data is used to calculate energy slopes. The gray area presents the standard deviation of the energy level. Figure A shows data for a sample that behaves more erratically as the rotation speed increases while Figure B shows data for a sample that’s behavior improves.

The Packing Analysis studies the powder’s ability to pack or settle after being exposed vibrational energy during transportation and storage.

The following are some process examples where packing and settling are important: a container that must be filled with the same amount of powder each and every time but settles to a different amount during transportation and storage or a powder that packs into a strong cake during handling and storage. An ideal powder has properties that do not change during processing, handling and storage.

Any device used to test the changes in volumetric expansion and compression must take the application problems and processing conditions into account to supply relevant data to the user. The powder in the test device must be in the same state as it is in the process being studied. This ensures that the packing analysis will be applicable to the problem.

The REVOLUTION Packing Test has three process steps: Preparation, Vibration and Analysis.

Sample Preparation

The sample powder is rotated at a chosen rotation rate and length of time to completely aerate the material. This step will allow the user establish a repeatable initial powder state before beginning the packing analysis. After the initial preparation, the RPA measures the powder volume.

Analysis

After the volume measurement, the powder is rotated at a specified speed until the compacted powder breaks (or avalanches). The software calculates the force required to break the powder mass.

Electrical Charge and Material Handling

Powders and granular materials can acquire electrical charge on the surface of their particles due to contact and movement against handling equipment and containers. They can also acquire charge due to contact and movement of particles within the material itself. This process is called tribocharging. Tribocharging is caused by electrons moving from one surface to another when different materials come in contact with each other. One material will become positive and the other will become negative. The amount of charge developed depends on the nature of the materials in contact, the pressure of the contact, the relative velocity of the contact surfaces, and the friction between the contact surfaces.

Measuring the charge acquisition properties of powders and granular materials is important because charge acquisition leads to problems and unstable behavior. Charged materials stick to processing equipment and containers. Charged materials can become airborne more easily. Charge materials flow in different ways than materials with no charge. In fact, many research believe that material electrical properties are the most important contributors to powder flow behavior.

Using the ION Charge Module with the Revolution allows measurement of charge acquisition properties between contact surfaces and test samples while controlling velocity and contact time.

Experimental Data

Charge Versus Particle Size

The data above is for a powder with different particle sizes charged with glass. Typically charge development increases as particle size decreases.

Charge Versus Moisture Content

The data above is for a powder with different moisture content charged with glass. Typically charge development decreases as moisture content increases.

Charge Versus Flow Aid Concentration

The data above is for a powder with different flow aid concentrations charged with polycarbonate. Typically charge development changes as flow aid content changes.

Charge Versus Surface Treatment

The data above is for a powder with different concentrations of surface treatment liquid charged with glass.

The Revolution Temperature Control option can heat samples from room temperature up to 250 degrees Celsius while running flow tests. Samples can be heated moving constantly, moving intermittently or not moving. Tests can be performed before heating and at temperature intervals.

Example Test Data

A polymer was tested at 26C, 110C, and 135C to determine flow changes with temperature between annealed and non-annealed. Increases in temperature caused the powders to flow more poorly and this resulted in a lower powder bed density. In this case the annealed material showed more flow temperature resistance than the non-annealed material.

Each REVOLUTION Powder Analyzer includes:

• One 100mm Large Sample Drum Assembly
• One 100cc Sample Cup
• One Set of Cables
• One Revolution Powder Analyser Software Package

The Following Additional Options are available:

• Ion Static Charge Analysis Module to measure the tribo-charging properties of powders, including charge sensor assembly, polycarbonate drum sides, and de-ionizing blower
• Temperature Control Oven to heat samples to 250 degrees Celsius
• 50mm Small Sample Drum Insert with small sample cup for measuring 25 cm3 of sample.
• An additional 100mm Large Sample Drum Assembly with large sample cup.
• 100m Extra Large Sample Drum Assembly for testing granular material, sample volume 500ccs.
• Packing Analysis with Drum Locking Assembly for high vibrational energy packing test.
• Set of Drum Seals for either small, large or extra large sample drums for making sample drums air and water tight.
• IQ/OQ Certification Package which includes: IQ/OQ Procedure, IQ/OQ Certification Document and Drum Test Standard.

The unconfined yield strength of a material is the force or stress required to deform or break a material when it is not confined by a container (free unstressed surface). From a testing perspective, the unconfined yield strength can be expressed as the stress required to fail or fracture a consolidated mass of material to initialize flow. The force used to consolidate the mass of material is called the Major Consolidation Stress.

The unconfined yield strength is very important in studying the flowability of materials. The reason is that the force required to get a powder or granular material to flow is directly related to the unconfined yield strength. In simple terms, the powder or granular material will flow if the force acting on it is greater than the unconfined yield strength of the material. A flow factor (ff) is calculated by dividing the major consolidation stress by the unconfined yield strength. This flow factor is used to classify materials into categories such as non-flowing (ff < 1), very cohesive (1 < ff < 2), cohesive ( 2 < ff < 4), easy flowing (4 < ff < 10), and free flowing (ff > 10).

The EVOLUTION Powder Tester measures the unconfined yield strength of a material in a two stage process. First, the material is loaded into a sample cell and compressed by vertical pressure.

The EVOLUTION Powder Tester measures the unconfined yield strength of a material by applying pressure to a sample over time. First, the material is loaded into a sample cell.

Then, a predefined pressure is applied to the top of the sample to consolidate it. The pressure can be applied on the instrument or by using weights.

After the material is compressed, the sample is then automatically removed from the sample cell and force is applied to the top of the sample to break or fail the material. The break cap contains the material for easy clean-up. The maximum force recorded when breaking the material is the unconfined yield strength.

Break Stress Versus Break Strain

The unconfined yield strength of a material typically increases as the pressure on the material increases. A plot of the unconfined yield strength versus the major consolidation stress is called a flow function. The flow function presents the powder or granular material’s response to pressure. Flow functions are very useful for predicting flowability because the forces acting on a material change at various points in a typical process. Therefore, it is important to know how the material responds to these forces.

Flow Function

Flow functions are also very useful for comparing the flow behavior of formulations and blends. As can be seen below, at low pressure the two samples are similar but at higher pressures their behavior diverges dramatically.

Flow Function Overlay

In addition, the unconfined yield strength of a powder or granular material typically increases the longer it is under the major consolidation stress. For this reason, it is very important to measure the time unconfined yield strength for materials that will be stored for any length of time. A plot of the time unconfined yield strength versus the major consolidation stress is typically called the time flow function.

The unconfined yield strength of a powder or granular material typically increases the longer it is under the major consolidation stress. For this reason, it is very important to measure the time unconfined yield strength for materials that will be stored for any length of time.

The EVOLUTION Powder Tester measures the time unconfined yield strength of a material by applying pressure to a sample over time. First, the material is loaded into a sample cell.

Then, the sample is compressed by vertical pressure applied from a weight or weights. Each weight delivers 2.5 KPa.

The material is then left for hours or days under controlled conditions to allow the major consolidation stress to act on the material for a specific period of time. These controlled conditions include temperature and humidity. The sample cells are small enough and stable enough to be put in ovens and humidity chambers or simply on laboratory shelves.

After the material is compressed, the sample is then automatically removed from the sample cell and force is applied to the top of the sample to break or fail the material. The maximum force recorded when breaking the material is the unconfined yield strength.

A plot of the time unconfined yield strength versus the major consolidation stress is typically called the time flow function. The time flow function is measured by applying a different number of weights to different sample cells. Each Evolution cell weight corresponds to 5 KPa.

Typically powders and granular materials gain strength as they are exposed to major consolidation stress over time.

Powder flow testers can be difficult to use in quality control or plant settings. The reason is that many testers are difficult to load, time consuming to use, not very precise, and expensive. Not the EVOLUTION Powder Tester. The EPT was designed from the start to be fast, easy to use, precise and inexpensive. In addition, due to its simple design, the EPT requires no routine maintenance. In short, the perfect quality control instrument for measuring flow behavior.

For QC measurements, the EVOLUTION Powder Tester can measure the unconfined yield strength of a sample at one pressure in 3 minutes. Loading the sample into the analysis cell is easy as it is a simple cup. A filling tool is used to overfill the cup and then scrape the top to get the correct amount of sample in the cup. The sample cup it then put on the EPT with the compression top to compress the sample. After compression, the compression top is replaced by the break cap and the unconfined yield strength is measured.

Measuring the unconfined yield strength of a material can provide information as to whether a material is on specification and will handle as expected. Because the test is fast, all shipments or production lots of material can be tested before they are transferred to processes and can create problems.

There are two options for EVOLUTION Powder Tester analysis cells along with time test options for each.

• Small UYS Cell – The patented Small UYS Cell is a test cell for measuring the unconfined yield strength of cohesive or compressible powder samples. The test volume is 5 cm3.

• Standard UYS Cell – The patent pending Large UYS Cell is a test cell for measuring the unconfined yield strength of cohesive granular materials. The test volume is 25 cm3

Time Options

The above cells are sold in sets of five with five weights to allow time tests to be measured.

The Evolution Powder Tester is used to compare the behavior of materials under consolidated load. The only other instruments available for this type of test are powder shear testers. The Evolution was designed specifically as an alternative to shear testers for the following reasons:

1) Shear testers are slow – A typical unconfined yield strength shear test takes 45 minutes. A flow function takes hours. Aside from waiting for data, the slow test time gives the sample material time to changed due to environmental conditions i.e. moisture loss or temperature change. The Evolution requires 3 minutes for an unconfined yield strength test and 15 minutes for a 5 point flow function.

2) Shear testers subject the sample to mechanical stress that causes sample breakdown – The original shear testers used fresh material for every point on the yield locus to ensure that the repeated testing did not change the material. Some instruments use the same material over and over because they are impractical if fresh sample is used each time. This can cause inaccurate strength data due to attrition and prefered particle orientation in the shear zone. This may occur to different degrees in different samples. In addition, a sample should never be exposed to more than one stress level i.e. run a flow function on the same sample. In our experience this causes the flow function to be inaccurate in roughly 80 percent of tested samples. The Evolution uses fresh sample for every test.

3) Shear testers cannot control the stress level on the sample – To compare the unconfined yield strength of samples, it is essential to subject them to exactly the same conditions. This does not happen in shear testers. The major consolidation stress is controlled by the normal load and the shear forces in the sample. The normal load is controlled but the shear stress depends on the sample. This means that flow indexes calculated by shear testers are not performed at the same stress level. This can actually create artificial differences in the measurements between samples. In addition, if one sample tests faster than another, it is exposed to much less mechanical stress. With the Evolution, the stress on the sample is completely controlled and is the same for every sample tested.

4) Time tests are expensive and difficult if impossible with shear testers – Shear test cells are complex which makes them expensive. In addition, they usually have a large lid area which means large forces are needed to keep the sample under pressure for any length of time. These two factors typically preclude time measurements. Some manufacturers claim to run time test by leaving the sample in the instrument for long periods of time. However, this is not practical for two reasons: 1) the instrument cannot be used for other tests during this period; and 2) the sample is not under controlled conditions (unless the whole instrument is put in a glove box – but then temperature and humidity conditions are severely limited). The Evolution was designed for time tests with inexpensive test cells, small lid areas requiring lower forces, and standard weights included. Test time after removal from ovens or humidity chambers is 20 seconds giving the sample no time to change.

The only claim shear tester manufacturers can make against the Evolution is that they are following a standard test for powder strength measurements. However, this is not really true. There are no universally accepted methods or shear cell designs for measuring the true strength of materials. The only real claim shear tester manufacturers can make is that their instruments get the correct strength for the only recognized powder flow standard. This flow standard is BCR limestone. This limestone was a sample that was tested in a round robin method at several European powder flow laboratories using the linear shear cell. The average results of all of the laboratories has become the “standard” value. Therefore, the thinking goes, if an instrument measures the correct value for the limestone then it is accurate for every other sample. The good news is that the Evolution measures the correct values for the limestone standard under all test conditions. We are happy to provide potential customers with a complete report with this data. More good news is that it makes these measurements faster, easier, and less expensively than shear testers.

Power flowability can be measured using the Volution Powder Flow Tester.

Powder flowability is defined as the ease with which a powder will flow for a specified set of conditions. Powder is generally defined as a collection of individual solid particles surrounded by gas phases. This includes granular materials, bulk solids, pelletised materials, etc. An accepted method for quantifying powder flowability is the Mohr-Coulomb Model. The Mohr-Coulomb Model is a limit state or “Go/ No Go” model and can be used to accurately predict flow behavior. This model quantifies powder flowability with two measurable parameters, Cohesion and Angle of Internal Friction, and two derived parameters, Unconfined Yield Strength and Major Consolidation Stress.

Cohesion is a measure of particle to particle bonding strength. This bonding strength results from various inter-particle forces generated by electrical charges, van der Waals forces, moisture, etc. The Angle of Internal Friction is a measure of the force required to cause particles to move or slide on each other. Internal friction is influenced by many parameters including particle surface friction, particle shape, hardness, particle size, etc. distribution, etc. Cohesion and Angle of Internal friction are determined by measuring a powder’s yield locus. The Yield Locus is a graph of the shear force require to cause a powder to yield or fail relative to compressive load. Cohesion is the intercept of the yield locus and the angle of internal friction is the slope.

Yield Locus
Shear Stress versus Normal Stress

The Unconfined Yield Strength is the shear stress needed to fail or fracture a consolidated powder mass to initialize flow. The force used to consolidate the powder mass is called the Major Consolidation Stress. In other words, the unconfined yield strength is a measure of the strength of a powder mass when the powder experiences major consolidation stress. The Unconfined Yield Strength is calculated using the below formula:

A Flow Function Plot can be generated by plotting a powder’s Unconfined Yield Strength versus Major Consolidation Stress. The flow function plot is a quantitative measure of the flowability of the powder. The inverse of the slope of the flow function plot can be used as a flow index. Generally, the closer a powder’s flow function is to the x-axis, the more easily the powder will flow. The Volution is used to measure a powder’s cohesion and angle of internal friction at various loads to generate its flow function and thus quantify its flow behavior.

The yield locus analysis is designed to determine the angle of internal friction and cohesion for a sample material and then calculate its overall strength under compressive load.This is achieved by measuring the failure strength of a sample under various loads after consolidation under a preset pre-shear load. Plotting the failure strength of the material under different loads generates a yield locus for the sample under the pre-shear load.

The test consists of three parts for every point on the yield locus: consolidation, steady state and failure analysis. Depending on the type of cell used, failure points can be generated on the same sample or fresh sample can be used for each failure point. Generally 3 to 5 points are used to generate the yield locus due to the time required for each point as well as the wear on the sample. If time consolidation is used, a delay time occurs after the steady state step.

In the consolidation step, the sample in the measurement cell is compressed to the preset normal load.With linear cells, this step includes twisting of the lid to help pack the material in the cell to what is called its “critical consolidation”.Critical consolidation is defined as the sample density at which it will reach a steady shear with minimal shear travel.This state in indicated by constant sample density or by a leveling off of the drop in normal load after each twist of the cell lid.For rotational cells, the consolidation step simply consists of compressing the sample until the normal load is reached.

Sample Consolidation

Normal Load versus Time

In the steady state step, shear stress is applied to the sample until the measured shear force and sample volume become stable. With linear cells, the shear stress is applied by moving pushing the lower ring of the cell at a fixed rate relative to the upper ring.For rotational cells, the lid is rotated a fixed rate. The steady state point is the point at which the shear force becomes stable.At the steady state point, the sample has reached a repeatable, stable density relative to the applied compressive load.

Steady State

Shear Force versus Time

In the analysis step, the shear stress is reduced to zero by reversing the shear stress mechanism.The normal load is then reduced to a predetermined level called the shear load and the shear stress is again applied.The shear forces rises as the sample resists shearing until a maximum shear force is reached.At this point the sample fails and the shear force drops rapidly.The generate yield point consists of the maximum shear force and the shear load.

Static Failure Analysis

Shear Force versus Time

By repeating the above sequence 3 to 5 times, a series of yield points are generated from which a yield locus can be plotted.The yield points are selected so that they are in the linear portion of the yield locus.

Static Failure Points

Shear Force versus Time

A least squares regression is performed to calculate a linear function for the yield locus.The slope of the calculated line is the angle of internal friction.The intercept of the line is the cohesion.From the cohesion, angle of internal friction and steady state point, the unconfined yield strength and major consolidation stress are calculated using Mohr Coulomb equations.

Static Yield Locus

Shear Force versus Normal Load

Because the yield points are generated by measuring several steady states for the same sample, the steady state point used for the strength calculation is the average of all the steady state points.In addition, to account for the effect of the steady state on the measured shear force during failure analysis, the measured shear force can be adjusted based on whether its steady state was higher or lower than the average.This is called prorating and can correct for variations in sample density for each yield point measurement.

Compressibility is calculated using the sample’s initial density and density after the consolidation step.

Static yield analysis generates the strength of a static or not-moving sample.This would be the condition in a silo or chute when the sample is at rest.Therefore, to get the sample to flow, the force used to move the sample must be greater than the static yield strength.

The wall friction analysis is designed to determine the kinematic angle of surface friction for a sample material against a container material. This is achieved by measuring the friction force between the container material and the sample material under different loads to generate a wall yield locus. The analysis consists of three parts: consolidation, steady state and analysis. All parts are automatic.

In the consolidation step, the sample in the measurement cell is compressed to the preset starting load. With linear cells, this step includes twisting of the lid to help pack the material in the cell to what is called its “critical consolidation”. Critical consolidation is defined as the sample density at which it will reach a steady friction with minimal shear travel. This state in indicated by constant sample density or by a leveling off of the drop in normal load after each twist of the cell lid. For rotational cells, the consolidation step simply consists of compressing the sample until the normal load is reached.

Sample Consolidation

Normal Load versus Time

In the steady state step, shear stress is applied to the sample until the measured friction force and sample volume become stable. With linear cells, the shear stress is applied by moving pushing the container material at a fixed rate relative to the upper ring. For rotational cells, the lid is rotated a fixed rate. The steady state point is the point at which the shear force becomes stable. At the steady state point, the sample has reached a repeatable, stable density relative to the applied compressive load.

Steady State

Shear Force versus Time

In the analysis step, the friction force under the starting load is maintained until it is stable. The load on the sample is then reduced to a preset level and the friction force is again maintained until it is stable. This is repeated several times to produce a friction value for several applied loads.

Friction Points

Shear Force and Load vs Time

The shear versus load data is then plotted to create a wall yield locus. A least squares regression is performed to calculate a linear function for the yield locus. The slope of the calculated line is the kinematic angle of surface friction.

Friction Yield Locus

Shear Force vs Normal Load

If you need a shear tester, the Volution Powder Flow Tester (VFT) is the one to get. The VFT offers the following advantages over other other powder shear testers on the market:

Low Cost: The VFT is very affordable compared to other shear testers. The reason is that we designed the instrument ourselves. We do not pay university licensing fees or royalties because we designed it using our 20 years of experience in the powder flow business. We also did not use external engineering companies which further reduces our costs. These savings are passed on to users.

Large Pressure Range – Due to our heavy duty frame and drive system, the VFT can deliver up to 50 kg of vertical force. That’s about 6 times more than competing instruments.

Automatic Sample Weighing: The VFT weighs the sample automatically during the measurement eliminating the need for an external balance and the time required to weight the sample.

Normal Load Correction Due To Sample Density: The VFT automatically adjusts the normal force applied to the sample lid to correct for the force from the powder mass above the shear zone. This is very important for dense powders. Other systems do not make this adjustment resulting in shear force that are artificially high.

True Time Testing: The analysis cells of the VFT can be removed and kept under load off of the instrument. This means time tests can be performed while other samples are being run on the instrument. Other shear testers have no capability to run time tests or you must leave the sample on the instrument for hours and hours so no other testing can be done.

Can Test Powders and Granular Materials : Due to the geometry of the test cell, the Volution can test both powders and granular materials. Other shear testers cannot. The reason is that the dimensions of the test cells for other instruments are too small to allow large particles to be measured. It is generally recommended that a layer of a minimum of 20 particles separate shear planes from cell edges. Some cells are not deep enough. Other cells have vanes will not allow large particles to enter or will only a thin layer.

The SpreadStation powder spreadability analyser can be equipped with up to four powder spreading assemblies. Each spreading assembly is set up with a powder feeding system and a powder spreading plate. The feeders and spreading plates can be changed quickly to study different printer parameters and simulate different printer feeding and spreading systems. The spreadering assemblies are removed from the SpreadStation for cleaning between samples. This requires approximately 30 seconds.

The powder feeders deliver sample powder to the spreading zone of the spreading assembly. The feeder can be removed without tools for cleaning and quick changes. The gap that the powder must flow through to reach the spreading zone can be adjusted for all feeders. The height of the exit of the feeder can also be adjusted.

The powder spreaders determine how the powder layer is formed when the powder is being spread. The spreading gap (leveling height) is set when installing the spreader on the spreading assembly.

Layers created by the SpreadStation are analysed using three independent measuring systems and produce three independent sets of data. The measuring systems are: 1) weighting system; 2) laser triangulation distance system; and imaging system.

Weighting System for the SpreadStation Powder Spreadability Analyser

The weighting system measures the mass of powder being spread by the SpreadStation over time. The weighting system uses load cells to measure the powder mass and 24 bit A to D converters to digitize the load cell readings.

Weighting System Data:

Spreading Efficiency:

The spreading efficiency is the ratio of the spreading density to the material density. A spreading efficiency of 100% means the spread layer is equivalent to a solid layer of material while a spreading efficiency of 0% means there is no powder in the layer.

Spreading Density:

The density of the layer of the powder, units grams/cm3

Spreading Rate:

The mass of powder exiting the spreader over time, units grams/cm

Spreading Uniformity:

The uniformity of the layer density from the start of the test to the end of the test, units %

Laser Triangulation System

The laser triangulation system measures the thickness of the powder layer created by the SpreadStation.

Laser Triangulation System Data:

Layer thickness:

The thickness of the powder layer measured over time, units micrometres

Layer Thickness Uniformity:

The uniformity of the powder layer thickness, units %

Imaging System:

The imaging system consists of digital cameras and LED lighting that collect images of the powder layers created by the SpreadStation. Image analysis software is then used to extract information about the layer quality.

Imaging System Data:

Area Coverage:

The area coverage is the ratio of the area in the image covered by powder to the total area of the image, units %

Channel Detection:

The image analysis software determines if there are any channels is the powder layer and if so the width of the channel, units % channels, width millimetres

Wave Detection:

The image analysis software determines if there are any waves in the powder layer and their widths, units % waves, width millimetres