Week - 5 - 2D meshing for Plastic components

AIM :

To extract the Midsurface and generate a Mesh on the given Plastic Bottle Cap model by following the specified quality criteria using ANSA.

OBJECTIVE :

• Import and clean up geometry on the given Plastic Bottle Cap model using ANSA GUI.
• Extract the mid-surface with suitable mid-surfacing techniques.
• Mesh the generated mid-surface and reconstruct the mesh to satisfy the following quality criteria.
• Complete the PID and thickness assignment of the model.

SOFTWARE TOOL USED :

ANSA Version 22.1.0

( NOTE : In this challenge, there will be some off elements found in the chamfer region which cant be removed. It is acceptable as it is been found due to the presence of triple cons on the chamfered surface. Due to those triple cons there will be some quality criteria failure which can be acceptable and those quality fails can be neglected. )

GIVEN DATA :

• CAD model.

• Quality criteria.

THEORY :

1. Quality Criteria :

Every product which is been produced will go through a set of quality checks. If the quality check fails other steps like boundary condition & applying loads will not be feasible, So for this reason the element quality check is performed. To identify the quality failure the quality criteria will be entered as input data. The elements should be maintained within the given quality criteria values. If the given quality value is not maintained there will be a failure in quality.

2. Geometry Clean-up :

Importing the model from other software to ANSA results in some geometrical deviations which need to be cleared before proceeding to other steps. In General chamfer/fillet faces, Unchecked faces, and average length errors occurred which is common while importing the model to ANSA from Modelling software. These errors must be cleared before any further operations. This clean-up will give the proper finish to the component and allow the engineer to work further with ease and accuracy.

3. Topological Activity :

Topology describes how the different faces and features of the geometry get connected with each other by forming CONS. There are 3 types of CONS which are SINGLE CON (Red), DOUBLE CON (yellow), and TRIPLE CON (Cyan Blue). Before mid-surfacing the entire model must be of double cons, the rest all the single and triple cons must be removed for getting better results while doing analysis.

Boundary Cons of the faces and Features were called feature lines other Cons in the Model were called Construction Lines which were generated during the modeling of the component. Construction lines can be altered as one wants to get a better mesh flow. Feature lines should not be disturbed as it will result in a change in geometry however if the cons Length is less than the minimum element size it will result in element failure under such circumstances the feature line can be altered up to some extent without giving up the design feature of the part.

4. Orientation :

Orientation in ANSA is indicated by Grey or Yellow surfaces Grey surface usually acts as a positive direction and Yellow acts as a negative direction. The face is said to be inconsistent if one face has both Grey and Yellow Colours. The faces Orient option will fix this issue by changing one side of the face to be yellow in color and the other side to be Grey in color this will make the model consistent.

5. Mid-surface creation :

Mid-surface is the exact middle layer or surface of the component having uniform thickness over the part. Midsurface is used to do the meshing which will make the operation easy to inspect the errors and perform all the other operations.

Here the component is plastic as the thickness of every face is different (if the thickness of the component is the same in all the faces, the component is metal).

6. Meshing/Mesh generation :

Mesh generation is the process of discretizing the entire model into smaller elements.

2D mesh has three types of elements:

• Quadrilateral or Quad Elements
• Triangular or tria Elements
• Rotating Quadrilateral Elements

Quadrilateral Elements:

Quad elements were the elements having four nodes. The square is considered to be an ideal quad element and the deviations were calculated in comparison with the square element. Quad elements were structured elements. They have short processing time and also have higher accuracy. Therefore, Quad elements were preferred over the tria elements.

Triangular Elements:

Tria elements consist of three nodes. Equilateral Triangle is the Ideal tria element and deviations were compared with the Equilateral Tria Element. Tria element is said to be unstructured mesh. The stiffness of the Tria element is high which results in higher processing time and also has low accurate results. Therefore, every engineer should try to avoid the maximum possible number of triangular elements.

Rotating Quadrilateral Elements:

Rotating quad elements were formed when any one corner of the 5 quad elements meet at a point to form a Star like structure. Even though they were quad elements they should be avoided in the model as they act like the tria elements.

7. Meshing flow and quality improvement :

In ANSA mesh can be generated easily but the mesh will have a lot of elements generated such that they fail to fulfill the Mesh Quality criteria also the mesh will have Triangular elements which need to be eliminated. Mesh Flow and Mesh quality improvement are the most important steps in order to achieve good mesh quality and good Mesh Flow. To improve the mesh Quality there are many options available in ANSA, some of them are :

1. Reconstruct
2. Smooth
3. Split, Swap, Join
4. Changing Nodal numbers (Num+/-)
5. Edge2Perimeter

Mesh is generated and the Mesh quality and the Mesh flow are improved using the above-said options for the entire model.

PROCEDURE :

Step 01 : Importing CAD Model.

• The initial step is to import the geometry into ANSA Software.

File -----> Open -----> Select the file -----> Open (Or we can drag and drop the file into ANSA)

Step 02 : Geometry cleanup

• The resolution of the cons must be done to check the actual shape of the cones in the geometry. There are two tools for that. One is the 'Fine' option in the Topo module under Auxiliaries. The other one is the 'Length' tool from the Mesh module under Perimeters. The 'Length' tool will help to do this in a single step when compared with the fine tool.

Mesh module -----> Perimeters -----> Length -----> Perimeter/macros (do this for both) -----> Select the whole surface -----> Middle mouse click -----> Give the value as 1 -----> Apply.

• After the resolution of the con, the TOPO operation is carried out to convert all single cons into double cons.

Topo module -----> Faces -----> Topo -----> Select the whole surface -----> Middle mouse click.

• It is observed that the given surface is with mixed yellow and grey color. This shows a mixed normal direction on the surface. So, to correct it as either yellow or grey on the outer surfaces to either gray(+ve) or yellow(-ve), preferably gray the tool, 'Orient' is used for this purpose.

Topo module -----> Faces -----> Orient -----> Choose select, Visible, and Auto appropriately.

If cons correction is done properly, choosing the visible option will change the color of entire surfaces in one go.

• After all the above operations the last step for the cleanup will be checking any geometrical errors and cleaning it.

Tool tab -----> Check -----> Geometry -----> Execute -----> If any errors pop up in red color select them at once -----> Right mouse click -----> Fix -----> If not fixed, try manually -----> Then execute the geometry check again, it will show No errors or warning exist.

Step 03 : Mid-surface generation

• The mid surface is generated to reduce the complexity as by simplifying the cad model we get more accurate simulation results in a shorter time.

• Taking measurements of thickness in random parts of the model gives random values of thickness, which concludes that it is a plastic model. So manual mid-surface generation techniques for the model must be used. Based on the shape and conveniences, different techniques for the mid-surface generation will be used. Each part needs a different kind of operation for the extraction of the mid-surface.

• For creating the mid surface different approaches must be used and they are

Topo module -----> Faces -----> Middle -----> Single/Multi -----> Selecting faces according to the order gives the mid-surface of that part

Topo module -----> Faces -----> Offset -----> Selecting the faces and the offset value

• These are the main tools used in this model for the extraction of mid-surface. Also Cut, Project, Curve -----> Middle, Points -----> ON COG, Delete, New, Topo, and other tools are used in the process. By combining and using these tools the mid-surface of the model gets extracted.
• With the above approach mid surface can be created piece by piece and properties must be assigned after creating them.

• After creating the midsurface the faces will not be joined. The faces must be joined for proper meshing. For joining extend, intersect, and delete options are used.

Topo module -----> Faces -----> Extend -----> Selecting CONS and faces will extend the CONS to that face.

Topo module -----> Faces -----> Intersect -----> Select one face that must intersect -----> Mouse middle click -----> Select another face -----> Mouse middle click.

Topo module -----> Faces -----> Delete -----> Select the face which must be deleted -----> Mouse middle click.

Step 04 : Setting up the Quality Criteria and Mesh parameter

Setting up the quality criteria as well as the mesh parameters is very important in every pre-processing process. This will make the mesh proper as per the requirements. Here the parameters are given, as per the given data fill the dialogue box, and apply to the component.

Utilities tab -----> Mesh Parameter -----> Set Element type: Mixed, target length, minimum target length -----> Ok

Utilities tab -----> Quality Criteria -----> From the Shell tab set 2D element qualities as per the given values -----> Ok

Step 05 : Meshing

Generating the 2D mesh on the mid surface can be done with two approaches, the first is meshing the whole component at once by selecting the whole surface and the second is meshing small macro-regions for better mesh. For this, select the 'Best' option from the Mesh generation group in the mesh module. Select the surfaces to mesh, and the middle mouse click.

Classic Mesh module -----> Mesh Generation -----> Best -----> Select the surfaces -----> Apply (Middle mouse click).

After Meshing there will be some errors that will not follow the quality criteria and will be seen in different colors. To make it proper and better mesh some commands must be used. Other commands like split, smooth, Num+/-, Swap, Join etc can be used to correct mesh flow.

Smooth : Mesh module -----> Shell Mesh -----> Smooth.

Reconstruct : Mesh module -----> Shell Mesh -----> Reconstruct.

Split : Mesh module -----> Elements -----> Split -----> Left click splits quads into trias and Right click acts as Swap edge and combines trias.

Num+/- : Mesh module -----> Perimeters -----> Num+/-.

Swap : Mesh module -----> Elements -----> Swap.

Join : Mesh module -----> Elements -----> Join.

Step 06 : Assigning the Thickness

Here a properly meshed (as per the given quality criteria and meshing parameters) Symmetry Bottle Cap plastic component Mid surface is created. Now thickness must be assigned to the mid-surface. To see the elements with respect to the thickness, an option named "Draw shells as solid" in the quality criteria panel must be enabled as shown in fig.

After enabling it, the elements with the assigned thickness appear as shown in fig below.

CONCLUSION :

The geometrical errors and mesh, with the given element Quality criteria, has been performed successfully for the given Plastic Bottle Cap Model. Also after meshing the component thickness has been assigned.

LEARNING OUTCOMES :

By doing this challenge, the following aspects can be learned:

• Perform geometry cleanup for any complex plastic components.
• Generate a mid surface with a flow for any complex plastic components.
• Generate shell mesh meeting the given quality criteria for any complex plastic components.
• Identify the quality failure and resolve the failure & improve the mesh quality.
• Work on ANSA with a flow.