Tablet press

In this tutorial we model a tablet pressing process in four steps: first, granular material is inserted in a cylinder, then the powder is compacted to a tablet. In the last two steps the pressure onto the tablet is release and the tablet is ejected in two steps. A specific powder model is used which allows for the compaction of the material and which provides the required cohesion for creating a stable tablet.

Name of the case: Project_Tablet_Press

Case 1: Filling

As a first step, the particle shape, the used materials command and the simulation domain are defined in the General Settings tab.

General settings

In this simulation we set particles of particle shape sphere:

Two materials command “powder” and “wall_material” are used:

The simulation domain ranging from -0.025 to 0.025 in x, from -0.01 to 0.08 in y and from -0.02 to 0.055 in z direction is created:

Meshes and regions

Three meshes are inserted, one for the side walls of the cylinder, one for the bottom shape of the tablet and a floor. Please note that the floor exceeds the simulation domain. By setting the keyword “move_protruding_mesh_elements” to “yes” we enforce Aspherix to ignore all mesh elements that are outside the calculation domain. All meshes are of “wall_material”:

The floor mesh is scaled by a factor of 0.5 in x direction and by 0.1 in y and z direction:

Physics models

The contact models are now defined. For the particles we use the normal contact model hertz and the tangential model history and the cohesion model sjkr/powder are used:

_images/01_03_particle_contact_model12.png

Since we added meshes in the previous section, a wall contact model is also added automatically to this section. We use the default models normal hertz and tangential history:

The powder compression model is activated by adding the powder/update command:

The gravity is enabled automtially:

Per default the gravity has a magnitude of 9.807 and acts into negative z-direction. These defaults can be changed by either setting the magnitude or the direction manually.

Particles

In the general settings tab the material properties for the powder and the wall_material are specified:

	powder	wall_material
Density	2000	7000
Young’s modulus	1e9	1e9
Poisson’s ratio	0.3	0.3
Coefficient of restitution	0.5	1
Coefficient of friction	0.5	1
Cohesion energy density	1e6	0
Cohesion energy density variation	100	0
Cohesion yield factor	2	0
alphaI_min	0.65	1
Compressibility	10000	0
Dilatability	0	0
Hardening	0.95	0

_images/01_04_material_properties_powder.png

_images/01_04_material_properties_wall2.png

Since two materials are present, also the material interaction properties must be set:

	wall_material - powder
Coefficient of friction	0.5
Coefficient of restitution	0.5
Cohesion energy density	0
Cohesion energy density variation	0
Cohesion yield factor	0

_images/01_04_material_interaction_properties4.png

A particle template of shape sphere with radius 0.00025 is added and used for a particle distribution:

_images/01_04_particle_distribution11.png

0.0003 kg of particles with a velocity of 0.2 m/s in the negative z-direction is now inserted. We use an insertion command command with mode stream, the massrate is set to 0.003 kg/s. The stream command requires a face for the insertion which can either consist of a mesh or a primitive insertion shape. In our case we are using a circle with a radius of 0.003 m and a center of (0,0,0.015):

Simulation control

In this tab, we first define the time-step settings such as the desired simulation timestep of 1e-7 seconds and the total simulation time of 0.2 seconds. Per default also the check_timestep command command is enabled, which throws a warning when the time step exceeds 10 % of the Rayleigh or the Hertz time. These thresholds can be changed by setting an alternative value for hertz_fraction or rayleigh_fraction.

Loadbalancing is enabled per default:

Output settings

In this tab we first define the timestep width for the file and the terminal output:

_images/01_07_output_timestep_settings10.png

The output_settings command command the takes care that all particle information and the meshes are written to files accordingly. Furthermore it triggers the output of global quantities such as the kinetic energy of the particles or total normal and shear stresses onto meshes to the terminal. Please note that if the output of per-cell information on meshes such as wear, velocity of stress are desired, one has to add the keyword mesh_properties and select the required output. If one or more meshes are added to the meshes list but no mesh properties are selected, all available properties are added to the output.

Running the simulation

After completing the setup and saving it, the simulation can be launched. Change to the Simulation view and set the number of cores to use to 2. Please note that using more cores would rather increase the simulation times instead of decreasing them due to the low number of particles in the simulation. First hit the “Check settings” button, and if the check returns “Check settings successfully completed” the simulation can be run by hitting “Start”.

The graph below shows the kinetic energy as a function of time:

Postprocessing with Paraview

After the simulation is completed, all data written with the output_settings command can be visualized automatically by going to the Visualization view and hitting the “Start Paraview” button.

The image below shows the final state of the system:

Case 2: Compression

In this case the lose particle bed is compressed with the help of a servo wall. We start by creating a derived simulation that depends on the filling case.

Meshes and Regions

In the meshes and regions tab we first define a mesh module servo:

_images/02_02_mesh_module_compression.png

The servo wall will be force controlled with a target value of 1 Newton. We set the maximal velocity of the mesh to 0.1 m/s and for obtaining compression with respect to gravity we set the axis to (0,0,-1). The center of mass is set to (0,0,0.007), the mode is auto and the ratio is set to 0.01.

This mesh module is applied to a mesh:

This mesh is based on the same mesh file as the bottom used in the filling simulation, only rotated and translated.