Challenge

2D meshing for Plastic Cap components using ANSA.

SShubham Dabral14 Jul 2023

ANSA

Aim:

To Mesh the given Plastic Cap CAD Model.

Objective:

Check for the Geometry clean-up (if required).

Take the mid surface for all the components after checking the geometrical errors.

Mesh the mid surface with the given element Quality criteria. (Refer the table Below for the Quality criteria).

The features in the model have to be captured properly with good mesh flow.

Assign the thickness to the model surface.

S.No	Quality Criteria	Value
1	Target/Average length	1
2	Minimum Length	0.5
3	Maximum Length	3
4	Aspect	3
5	Warpage	15
6	Skewness	45
7	Jacobian	0.7
8	Minimum Quad Angle	45
9	Maximum Quad Angle	135
10	Minimum Tria Angle	30
11	Maximum Tria Angle	120
10	Tria Percentage	15

Introduction:

Meshing (nodes and elements) is nothing but the discretization of a continuous system (a model or a geometrical shape) with infinite degrees of freedom to a finite degree of freedom.

A mesh is made up of nodes and elements. A Nodes are limited number of points where calculations are made. The entity joining nodes and forming a specific shape such as quadrilateral or triangular is known as an Element.

Basic Terminology:

In the TOPO menu the following entities appear:

The Surface is the underlying mathematical CAD description of the Face where the actual mesh takes place. The CONS (Curves ON Surfaces) are the boundaries of the Faces. According to their connectivity, they are coloured in Red (single edge), Yellow (double connectivity) or Cyan (triple -or more- connectivity). The Hot Points are the end points of CONS. 3D Curves are curves in space that are used for CAD constructions, but they are not connected to the Faces. 3D Points are also used for geometrical constructions.

Correspondingly, in the MESH and DECK menus:

A Macro Area may consist of one or more joined Faces. The Perimeters are the boundaries of the Macro Areas. The Hot Points are the end points of the Perimeters. FE-model mesh is a mesh that is not associated to geometry (dead-mesh). Finally, a yellow Connecting Spot indicates connectivity of a Macro area with FE-model mesh.

Procedure:

1) Importing the CAD model.

To make the coarse edges smooth, they need to be refined. Go to, Mesh -> Perimeters -> Length -> 0.1.

2) Geometric Clean Up of the CAD Model.

2.1) CONES in the geometry

From figure 4, it can be observed that there are Double cones (Yellow Lines) i.e., edges shared with two surfaces.

Now, from figure 4, it can be observed that there are, Single cones (Red Lines) i.e., edges shared with only single surface and Double cones (Yellow Lines) i.e., edges shared with two surfaces and Triple cones (Blue Lines) i.e., edges shared with three or more surfaces.

2.2) Checking the Geometry.

To identify and isolate on the screen the problematic areas that require the user’s attention, checks manager is used.

In the toolbar on the top of the screen, go to, Checks -> Geometry.

This will open ‘Checks Manager’ window.

Here we can select the type of problems that may exist in the geometry of the model to be investigated by activating the corresponding flag in the lower part of the window. Keep the default options and click Execute.

ANSA identifies a total of 15 errors in the geometry of the model.

To auto fix the errors select all the errors, right click and click on fix. This will resolve some or all of the errors.

Auto Fix, has resolved all the errors as shown in Fig 8.

3) Extracting Mid Surface.

Since this is a plastic component, it is having surfaces with varying thickness. And to extract the mid surface one of the following 3 methods could be utilised:

Offset (go to, Faces -> offset)
Midface (go to, Faces -> Middle)
Creating surface via “Curves” (Curves -> Create)

From fig 9, we can see how a mid-surface is formed using a ‘curve’. Firstly, create a mid-curve (go to, -> Curves -> middle). Then project the curve on the bottom surface. Finally, create a new surface (go to, Faces -> New).

Finally, we can see the mid surface of the geometry as shown below.

4) Geometric Clean Up of the Mid-Surface.

Now, again checking the problematic areas in the geometry using Checks manager. As shown in Figure: 10, there are 3 geometrical issues. These errors can be fixed by right clicking the errors and selecting ‘fix’. As shown in Figure: 11, errors are fixed.

5) Assigning mesh element length.

In the Mesh menu, go to, Perimeters -> Length -> Box select the model -> set Perimeter length as 5.

Inspecting the model area by area, to check if the perimeter length is matching our mesh length requirement.

6) Setting up Quality Criterion.

In order to avoid deviation from the quality after meshing, we need set up quality ranges. In Utilities tab, select ‘Quality criterion’.

Setting up mesh parameters. In Utilities tab, select ‘Mesh parameters.

7) Meshing.

To mesh the Model area by area, switch to the mesh module, then go to,

Mesh Generation -> Best. In the ‘Feature section, select “Macro Area”.

Now one by one click on the areas to mesh.

Surfaces which are symmetrical (Figure 15) could be meshed on the half side only and then reflecting the mesh to the other side to get a complete mesh (Figure 16).

Following rules for should be kept in mind to obtain a good mesh:

Features should be captured properly.
Proper Tria Management, i.e., Tria should no touch each other, There should be no opposite trias, Trias near edges and free edges should be avoided.
Proper Mesh Flow.
Check if there is any dysconnectivity in the mesh.
Quality Criteria should be satisfied.

8)Ensuring Quality criteria and Mesh flow.

In some areas we may encounter that the feature lines are very close to each other. Since our min length quality criteria is 0.5, elements forming in between feature line separated by a distance less than 0.5 will fail. To avoid this, we may suppress one of the feature lines, keeping in mind there is no major change in the geometry. Or we may also move the nodes by a small fraction to increase the edge length of the element.

To suppress the feature lines, Activate, Mesh -> Macros -> cut -> Right click a feature line to supress.

To move the node, Activate, Mesh -> Grids -> Move -> Free -> Right click on the node to be moved to show axis then left click to reselect and move.

To keep the mesh flow, we can always rearrange the mesh by using:

“Reconstruct” command from the mesh module.

“Split” or “Collapse” command or the combination of both to join two opposite trias or drag the trias to the edge to eliminate it.

The final meshed model is as shown below:

9) Assigning thickness.

Since the model is a plastic hence its thickness also varies. So, we ne to individually measure the thickness of the individual surfaces and arrange the surfaces having same thickness into a same PID. Thickness can be measured using “Measure” tool.

An example of thickness measurement is given below: