Injection Molding - CFD Simulation SimFlow Tutorial

This tutorial is a simulation of the injection molding process. It uses the Power Law transport model, which is highly CPU-intensive due to its nonlinearity. The tutorial simulates filling a housing with melted plastic, starting from an empty domain (filled with air). It used time-variable pressure inlet conditions that caused gradual filling of the mold.

SimFlow is a general purpose CFD Software

To follow this tutorial, you will need SimFlow free version, you may download it via the following link:
Download SimFlow

This tutorial will require high CPU usage and might take several hours to finish the calculation depending on the machine used.

Open SimFlow and create a new case named housing_injection

Firstly we need to Download GeometryHousing

Since the imported geometry is in STL format, which does not store unit information, we need to confirm the unit in which the model was created. The Geometry size displays overall size of the model in each direction what should help to decide which unit is the valid one. In this case, the model was created in millimeters.

After importing geometry, it will appear in the 3D view.

The imported geometry is made up of a single surface. We need to split it into separate faces for further processing.

We will add 3 face groups to the original geometry. SimFlow treats each face group as a mesh boundary. By using face groups, we can create separate boundaries for inlet and outlets.

New component should be appeared on the Face Groups list.

Repeat this operation for the outlets. Additionally, we need to add outlet tube walls as separate groups to set the mesh refinement for these regions. Each group should contain a single face.

In order to be able to set different settings per each face, we will convert the single geometry into multiple geometry objects.

Before we start meshing we will specify geometry options used during the meshing. We will also increase the refinement level for better accuracy.

We will use the copy option to apply common settings.

For outlet tubes, we will increase copied refinement levels.

Under the Base tab we will define the background mesh for the housing.

Material Point tells the meshing algorithm on which side of the geometry the mesh is to be retained. Since we are considering flow inside the housing we need to place the material point inside the geometry.

You can specify the point location from the 3D view. Hold the CTRL key and drag the arrows to the destination.

After the meshing process is finished the mesh should appear in the graphics window. In order to check the quality of the mesh, we view the element’s shape using section view.

After the mesh check, we will turn off the section view to return to the full model view.

To be able to assign inlet and outlet boundary conditions we have to adjust the boundary types.

For injection molding, we will use Inter (interFoam) solver. This solver is designed to model two-phase flow with interface capturing capabilities.

In the transport properties panel, we will define fluid parameters. Firstly, we will define High-Density Polyethylene (HDPE) phase using Power Low transport model.

For the second phase, we will use predefined air properties from the material database.

For the purpose of the simulation, we will change the default PIMPLE algorithm parameters.

During computations sometimes velocity can spontaneously approach unphysical values due to numerical errors. In order to prevent computations from divergence, we can limit the velocity. As the result, we should get more stable computations.

In order to define injection, we will use pressure inlet boundary. We will define the pressure by time history table.

Fill in inlet pressure values history.

To assign injected material, the phase fraction parameter \(\alpha_{phase}\) is used. \(\alpha_{HDPE}\) equal 1 will stand for HDPE phase.

Outlets' boundary conditions simulate outflow to the atmosphere. The default phase setup allows both fluids to exit freely while in case of backflow only air can be sucked into the housing.

We will copy the boundary condition settings to the second outlet.

Before we will set the initial condition we need to define the region representing the initial location of HDPE. For this purpose, we will create a box.

By using the previously created geometry we will tell the solver where should be localized HDPE at the initial state.

During calculation, we can observe intermediate results on a section plane or probe plots. Note that runtime post-processing can only be defined before starting calculations and can not be changed later on. Firstly, we need to specify variables that will be sampled. After that, we will define the section plane and probes.

To add sampling data on a plane we need to create a slice. To define a plane we need to set the normal vector and then select the point. In the current step, we will choose the normal vector and in the next step, we will choose a point that belongs to the plane.

Now we will place the slice to the destination by setting the point coordinates. To achieve this we will use arrows in the graphic window. Before that, we will hide unnecessary geometries.

To track sampled parameters in a specific point in space, we can add the probe location monitor.

For any simulation, it is very convenient to let the solver automatically determine the proper time step value. To use this option we need to define time step constraints by providing the initial time step (adjusted by the solver during computations), maximal time step value and Courant number.

This calculation will require high CPU usage and might take several hours to finish the calculation depending on the machine used. If you want to shorten the computation time, you can decrease the Simulation Time to 0.4s - this is enough for purpose of further postprocessing.

We can control how often results should be saved on the hard drive. Only this data will be available for postprocessing.

Due to the non-newtonian fluid and 2 phase model, the calculation is very time-consuming. To speed up the calculation process increase the number of CPUs basing on your PC capability. We recommend using at least 6 cores for this tutorial. If you are using a free version you can use the contact form to Request 30-day Trial

Estimated computation time for 2 processors: over 5 hours

When the calculation is finished we should see a similar residual plot.

Slices tab appears next to Residuals. Under this tab, we can preview results on the defined slice planes. The results preview is available during the calculation and we can track it on a regular basis. The newly calculated time step will be actualized automatically as long as the time selector points to the latest time step.

After computations are finished, we will investigate the mass flow rate during the injection. We will compute it by using already existing data (this feature is not available as a monitor).

We can do the complex visualization of our results with ParaView.

Load the results into the program.

We will focus on the HPDE phase behavior and observe form filling during the injection.

To show the housing together with the fluids we will follow these steps

Now we can see the final results with the original geometry.