2D Meshing of Bottle cap model in ANSA

AIM:

To do geometrical clean up in the given component and mesh the component with first order 2D elements using the given quality criteria.

 OBJECTIVE:

The main objective of the project is to improvise the methods of mid-surface extraction for sheet metal and plastic components and mesh the component without quality issues.

 PROCESS:

1. Perform geometry clean up and resolve geometry errors.
2. Extract the mid-surface of the component and assign PID for the components.
3. Assign thickness for the mid-surface and assign quality criteria for meshing.
4. Mesh the component and resolve quality issues.

 SOFTWARE USED:

ANSA (Automatic Net generation for Structural Analysis) v19.0.1 64-bit

MODEL:

The given model is a bottle cap model. This is a plastic component, meaning the thickness of the component varies. There are many types of bottle caps ranging from the Crown cork type which is found in beverage bottles to the frequently seen screw cap type that is used in plastic bottles. The given CAD model is of a bottle cap having varying thickness.

 PERFORM GEOMETRY CLEAN UP

The Geometry check can be seen under the TOOLS tab under CHECKS -> GEOMETRY.

The Geometry clean up can also be done through the CHECKS command.

Upon executing , the errors or geometrical issues in the model will pop up.

These errors can be fixed manually one by one or can be fixed automatically. When fixing automatically, the errors are fixed by ANSA.

Once the errors are fixed, the geometry checks should be run again. It should be ensured that there are no geometry issues.

MIDSURFACE EXTRACTION:

Before extracting the mid-surface for the component, the thickness of each of the surfaces needs to be known. The thickness of the surfaces can be measured using the MEASURE command, which is under the UTILITIES tab.

By selecting the CONS/Curves option, the distance between two CONS or Curves can be measured and the thickness value is stored. Similarly, the thickness values are measured at various places in the CAD model.

In places where there are varying thicknesses, the CURVES -> MIDDLE command is used to generate a middle curve between the two curves. This procedure is followed at the top and bottom. Then a new surface is created connecting the two middle curves. This surface serves as the mid-surface.

Upon selecting the curves and accepting the command, a middle curve is generated. Similarly, this process is done in the bottom surface also.

The curve in the bottom surface is generated and can be seen only when the SHADOW mode is turned off. Then, the mid-surface is created by creating a new surface between the two curves. The new surface is created using the FACES -> NEW command.

Upon accepting the command, a new surface is created, which is the mid-surface for that particular part. The mid-surface is generally in red colour as they consist of single cons. The same process can be followed to extract mid-surface for all similar features.

The mid-surface for some surfaces can be done using the FACES -> MIDDLE command. This command generally creates a middle face for the selected faces.

Upon selecting the faces, the confirmation window will pop up. After accepting, the middle face is created between the two selected faces. This will serve as the mid-surface for that part.

The same method is followed for all the similar features. At complex places where we are not able to measure the thickness using the CONS/Curves, WELD SPOTS can be used. Weld spots are basically points which are created on the surfaces. They can be created and projected to the bottom face. Then the distance between the two weld spots is measured. This value is taken as the thickness of that surface. Weld spots command can be found under the HOT POINTS -> WELD SPOTS.   

Weld spot can be created by clicking on any surface. They are in pink colour. After creating the weld spots, they need to be projected to the bottom surface.

The projection of the weld spots to the bottom surface can be done using the HOT POINTS -> PROJECT.

After selecting the command, the weld spot is selected and then the surface to which it needs to be projected, is selected.

In this command, we have a disadvantage. Only one weld spot or hot point can be projected at a time. By using HOT POINTS -> MULTI PROJECT multiple weld spots or hot points can be projected at the same time.

After selecting the weld spots, upon accepting, we need to select the surface to which these weld spots need to be projected.

Upon selecting the surface, the weld spots are projected. The weld spots to be projected can be projected either to CONS/Curves or to Faces.

NOTE: MULTI PROJECT can only be used to project multiple weld spots or hot points to the same surface. Projecting to multiple surfaces is not possible.

Now, to measure the thickness of the surfaces, the distance between the weld spots is measured. This value is the thickness between the surface.

Thus, it is known that the thickness of the surface is 1.53mm. To extract the mid-surface, we can offset the face from the bottom. This can be done using the FACES -> OFFSET.

Upon accepting, the face is offset to the given value. The mid-surface is seen in red colour indicating there are single cons.

This way, the mid-surface is generated for similar surfaces.

Next, the PIDs are assigned to the mid-surfaces. This can be seen under FACES -> SET PID or in the LISTS tab. The property id is created separately for every component.

ASSIGN THICKNESS & QUALITY CRITERIA

The thickness are assigned to the mid-surfaces according to the thickness in the parent surface.

After the thickness is assigned, the quality criteria and the mesh parameters need to be assigned. The given quality criteria is as below.

Before the quality criteria the mesh parameters are assigned. This command is under the UTILITIES tab.

Since this is normal meshing, we choose GENERAL. Then depending on the type of elements, we choose MIXED as there will be a mixture of QUAD and TRIA elements. Then the Target element length, the Minimum element length and the maximum element length are given.

Then the quality criteria is assigned. This command is also under the UTILITIES tab. The quality criteria values should be given under the SHELLS tab as we will be doing 2D meshing. Then APPLY  is given. Next in the PRESENTATION PARAMETERS tab, the ELEMENTS% checkbox is ticked and the slider under the DETAIL ON DEMAND EFFECT is dragged towards ALL DETAILS. This enables us to see the % of quad and tria elements in the mesh.

MESH AND RESOLVE THE QUALITY ISSUES

Meshing is done under the MESH module. The meshing is done for the components one by one. The method used here is BEST. It is under the MESH GENERATION module button. The command is selected and the surface to be meshed is selected and accepted. Then the meshing is done.

This way the meshing is done for all the surfaces. While meshing, the components need to be meshed one by one using their PID.

Here, it can be seen that some of the elements are failing for quality. These elements can be re-structured or the mesh in this surface can be re-meshed to get better mesh flow and to resolve the quality issues. SMOOTH is one such command which can be used to re-structure the mesh in that surface.

After selecting the command, the surface is selected and the command is executed.

Now it can be seen that one of the element failing for minimum element length has been resolved. But there is another element failing for Aspect ratio. This can be resolved using another method. The RECONSTRUCT command can be used in such circumstances where the mesh needs to be re-constructed to resolve quality issues and obtain better mesh flow. It is under the SHELL MESH module button.

The command is selected and the surface is selected. After execution, the mesh in that area is reconstructed.

Now the mesh on that surface is reconstructed. As a result the quality issues are fixed.

At another instance, we can see that some elements are failing for aspect ratio, skewness and minimum element length. Sometimes we need to check in the mid-surface for any unwanted construction lines or unwanted hot points. They make the mesh flow uneven. The unwanted construction lines and hot points need to be removed. They can be re-meshed to resolve the quality issues.

After resolving all the quality issues, we will get a proper meshed model. The quality criteria cannot be met at some places. This is due to reason that the geometry is quite complex. The 2D meshing is not sufficient for meshing the model. In this case, the 3D meshing is done to achieve better results.

LEARNING OUTCOME:

From this project, the procedure for mid-surface extraction for sheet metal & plastic components is learnt. The process of geometry clean up, assigning property IDs and meshing is learnt. The methods to resolve the quality issues in the mesh generated is also learnt.

CONCLUSION:

Thus the given Bottle cap model is meshed with 1^st order elements using 2D meshing with Tria & Quad elements.