Create a time-dependent task for a shell geometry.

  Create a new task

Define the mold(s).

  Define molds

1. Create a mold.

     Create a new mold

2. Specify the type of mold. There are two options (note that the Mold with temperature calculation option is not available because it cannot be used with shell elements):

   • To model an adiabatic mold, select Adiabatic mold:

       Adiabatic mold

     If you select this option, there will be no heat exchange between the mold and the fluid in contact.

   • To model a mold that remains at a constant specified temperature throughout, select Mold with constant and uniform temperature:

       Mold with constant and uniform temperature

     If you select this option, the heat transfer between the mold and the fluid in contact will be taken into account in the calculation. The heat transfer is governed by a heat transfer coefficient and a mold temperature (or temperatures, if you assign different temperatures on different parts of the mold).

3. When prompted, specify a name for the mold.

4. Specify the solid region that represents the mold.

     Domain of the mold

   If a mold is composed of multiple subdomains, which are assigned the same motion, define these subdomains as the domain of the mold.

5. Specify the boundary that comes into contact with the fluid (the contact wall), the translation velocity or force, and other mold parameters following the instructions provided for 2D and 3D problems in Inputs for 2D and 3D Contact Detection (step 2).

   If a mold is composed of multiple subdomains, which are assigned the same motion, define these subdomains as the domain of the contact condition.

6. If necessary, return to the Define molds menu and repeat the steps above to define another mold.

Create a sub-task of the appropriate type for the fluid:

  Shell model: Gen Newtonian isothermal

  Shell model: Gen Newtonian non-isothermal

  Shell model: Viscoelastic isothermal

  Shell model: Viscoelastic non-isothermal

For shell models, you have access to Newtonian and integral viscoelastic models (for which isothermal and non-isothermal models are available); you do not have access to differential viscoelastic constitutive models.

Specify the region where the shell sub-task applies.

  Domain of the sub-task

First you will be prompted to select the subdomains on whose boundaries the shell domain lies. After you choose the appropriate subdomains, you will be prompted next to specify which of the boundaries define the shell domain.

Define the flow boundary conditions on the borders of the shell domain, as for any 3D flow problem. Each border is identified as the intersection between the shell domain and an adjacent boundary.

  Flow boundary conditions

See Boundary Conditions for information about setting boundary conditions.

If you want to impose an inflation pressure, turn on the Inflation pressure imposed option in the Flow boundary conditions panel. Ansys Polydata will then prompt you to enter the value of the inflation pressure.

Define the fluid layer(s) representing the polymer in the mold.

  Define layers

For the shell model, define one or more layers, each having its own material properties and initial thickness (which can be defined as a constant, a linear function of X, Y, Z coordinates, or a multi-ramp function of X, Y, or Z, or mapped using a CSV file). Layers can overlap (partly or entirely), but they are not required to do so.

The inputs for a fluid layer are nearly the same as for a film layer in a film casting problem, with the addition of the permeability coefficient. See General Procedure for details. Still, the material description offers more possibilities in the context of blow molding and thermoforming, in particular with the definition of a strain-dependent viscosity. The setup is straightforward, and is performed as follows.

1. Define the physical properties for this layer (viscosity, density, and so on).

     Material data

2. Specify viscosity.

     Shear rate dependent viscosity

3. Specify strain dependence.

     Strain dependence

4. In the main menu for the strain dependence of the viscosity, three models are available. Select the required model and enter the material parameters accordingly.

     Constant function

     "Parabola+Gauss" function

     Smoothed ramp function

Specify the values of the material properties where appropriate.

Specify the contact detection problem.

  Define contacts

1. Create a new contact problem.

     Create a new contact problem

2. Specify where the fluid will contact the mold.

     Select a contact wall

   1. Select the appropriate contact wall (that is, the one you previously defined in the Define molds menu).

   2. Click Select.

3. If you defined the mold as a Mold with constant and uniform temperature, specify the values for and in Equation 17–5.

     Modify alpha

   The default value for is set to 1000. Since it is unit dependent, it is advised to specify a more appropriate value if necessary.

     Modify T_mold

   will be the constant temperature of the mold in this case.

4. Enable contact release and define the adhesion force density ( in Equation 17–3), if necessary.

     Modify adhesion force density

   Contact release is disabled by default, as indicated by the description of the adhesion force density as being undefined in the Modify adhesion force density menu item. When you click this menu item, Ansys Polydata will ask you to confirm the enabling of contact release and then allow you to specify the New value for the adhesion force density. If you do not have test results or specifications to guide your definition of the adhesion force density, it is reasonable to start by replacing the default value of 0 with a small value in your unit system (for example, 10–100 Pa).

   When you enable contact release, you must make sure that the penalty and slip coefficients you define in steps 7.e. and f. are set to the same value, so that slip at contact is not permitted. You should also enter a low value for the penetration accuracy in step 7.g. to allow the material to escape from the contact condition; this is especially important when the time steps are small. The goal is to have the penetration accuracy be smaller than the product of the velocity and the time step, though this can be difficult since low values of penetration accuracy result in small time steps. Finally, it is recommended that you set the convergence test value to 10^-4 when you define the Numerical parameters in step 10. (see Convergence and Divergence for details).

5. Modify the slip coefficient ( in Equation 17–2), if necessary.

     Modify slipping coefficient

   The default value is 10¹². If the slip coefficient and the penalty coefficient have the same value, then it is assumed that the fluid sticks to the mold when it comes into contact. Full slippage at the contact boundary is assumed if the slip coefficient is zero. Actual cases involving partial slippage will require a slip coefficient that typically ranges from 10² to 10⁶, depending on the physics involved as well as on the system of units being used.

6. Modify the penalty coefficient ( in Equation 17–1), if necessary.

     Modify penalty coefficient

   The default value of 10¹² is acceptable for most cases.

7. Modify the penetration accuracy, if necessary.

     Modify penetration accuracy

   If the penetration of a point into the mold is greater than the penetration accuracy, the time step will be rejected. The calculation will then be restarted from the previous time step with a smaller time-step increment. The default value is set according to the dimensions of the mesh, and you will generally not need to modify it.

8. When the mold is a shell, you must define where the mold body (that is, the solid portion of the mold) is and where the cavity is, relative to the shell.

     Specify mold side / cavity side

   Darts (arrows) will be displayed in the graphics window on the surface of the mold shell. Use the dialog box that opens to indicate if they are pointing toward the mold body or toward the mold cavity.

9. As part of the contact detection process, a search is conducted for each node of the fluid shell to establish whether it has come into contact with the mold shell. You have the option of revising the default settings of the search algorithm.

     Advanced options

   Perform the following steps in the menu that opens.

   1. You have the choice of five predefined groups of search settings. The groups can be "faster" (that is, result in faster computation times for the search) or "safer" (that is, conduct searches that are more thorough but require more computation time). By default, a fast group of settings is selected, as this group is appropriate for a relatively broad range of cases.

      Click one of the following menu items to select the settings that best represent how you want the search conducted:

        Reset to very fast settings

        Reset to fast settings

        Reset to safe settings

        Reset to very safe settings

        Reset to safest settings

      The text at the top of the menu displays the details of how the settings are currently defined.

   2. You can revise any of the following settings, in order to customize the predefined group to be more appropriate for your problem. In order to understand these settings, note that the search is conducted within a box-shaped zone that encompasses the mold shell. This zone is divided along the Cartesian axes into overlapping search sectors. For a given node of the fluid domain, the contact search is performed in the sector where that node is located.

      •   Modify expansion of the search zone

        This setting allows you to expand the search zone, by increasing its length along each axis by a specified fraction of the largest mold dimension.

      •   Modify search sector divisions per axis

        This setting defines the number (which must be a positive integer) into which the search zone will be divided along each axis. For example, if you enter 3 for the New value, the search zone will be divided into 27 (that is, 3³) sectors.

      •   Modify overlap of sectors

        This setting defines the amount of overlap between neighboring sectors, and is expressed as a fraction of the largest dimension of each sector.

      •   Modify tolerance on contact

        This tolerance is similar to (and has the same default value as) the penetration accuracy, as defined in a previous step. Round-off errors can cause the fluid node to pass through the mold without registering contact; this typically happens near symmetry planes. The contact tolerance supports contact detection in the vicinity of symmetry planes and helps to maintain contact if the mold velocity changes.

10. Select Upper level menu to return to the Define contacts menu, where you can repeat the steps above to define another contact wall for the free surface or to define distinct parameters (such as slipping, heat transfer coefficient) if the mold is composed of multiple subdomains.

Define the remeshing procedure for the sub-task (after you return to the sub-task menu one level above the Define contacts menu).

  Global remeshing

See Remeshing for details. The Lagrangian method is the only one available for 3D blow molding simulations with shell elements.

If you want to compute the extension components, create a postprocessor sub-task for it, as described in Inputs for Computing the Extension Components.

Define the numerical parameters for the time-dependent task (one level above the sub-task menu).

  Numerical parameters

See User Inputs for Time-Dependent Problems for details about the inputs for time-dependent calculations. If you select Modify the transient iterative parameters, you will see that the velocity field prediction has been disabled for you, as mentioned in Inputs for 2D and 3D Contact Detection. You should not reenable the velocity prediction for a contact detection problem.