Week 4

PRE-PROCESSING OF A PLASTIC BOTTLE CAP USING ANSA

• Aim

Extract mid-surface of a Plastic component ( Bottle cap ) and mesh the model with required element quality criteria. and assign thickness to the component.

Model is a plastic bottle cap, with many minute features like ribs, threads, wedges, etc. and surfaces having varying thickness, our aim is to capture every feature in the model and extract mid-surface then prepare a good meshed model.

Steps involved in the process are,

• Geometry cleanup
• Mid-surface extraction
• 2D Meshing of the model
• Element quality checkup
• Remesh- Quality Improvement.

• PROCEDURE

Step 1- GEOMETRY CLEANUP

Import the cad model and do a topo operation initially to clean up the geometry by Topo module-> Faces-> Topo, then refine the perimeter of the model to 1 mm as our element target length is 1 mm, go to Mesh module-> Perimeters-> Length. select the entire model as input perimeter length as 1 mm. it's shown in below images

Now check for geometric errors, Tools-> checks-> Geometry. several geometric errors appear.

Most of the errors can be fixed automatically, select all the errors in the checks box, select fix by right mouse click.

Step 2- MID-SURFACE EXTRACTION

After geometric cleanup, we can move to mid-surface extraction of the model. since our model is a plastic component it's having varying thickness throughout and has many features like ribs, threads, wedge, etc. first we have to measure the thickness of different parts and decide on which method we extract mid-surface.

For parts having uniform thickness, we can opt for offset method to extract mid-surface. go to Topo module-> Faces -> Offest, select the face and input offset value.

for example, bottom wall of the model have uniform thickness of 1.224, so we can use offset method to extract the mid, as shown in fig. below

For features like ribs , fins having both faces identical we can use middle option, go to Topo module-> Faces-> Middle, we can choose single or multi option. in middle-> single option we have to select two faces and middle face is created in between them. In middle -> multi option we have to select outer faces indicated by red , green colours and select thickness indicated by blue coilur.

To extract mid-surface for threads, we have to create a curve in the middle on top surface of thread using, Topo module-> Curves-> middle option, then project that curve into the mid-surface of wall using Topo module-> cons-> Project. then create a new face between projected edge and the middle curve using, Topo module-> Faces-> New

To extract mid-surface of 36 threads present inside the bottle cap, we can extract mid surface for one teeth and copy that mid-surface to all other 35 teeth using, Transform-> copy -> rotate option.

when we extract mid-surfaces of some faces which are having some features ( for eg. face on which threads are attached) there will be holes formed on the mid-surface, we can fill the holes using Topo module-> cons -> Fill holes. or create a new face using Topo module-> Faces-> New

• Mid-surface of Bottle cap

• Step 3- Meshing of the mid-surface extracted

After creating mid-surface we can move on to mesh the model, under the given element quality criteria (as shown in fig. below) by pressing F11. also enter mesh parameters

.

For meshing switch to Mesh module, go to Mesh generation-> spot/ best mesh, select the face and middle click. mesh is generated on the face.

• Step 4- checking element quality criteria and remesh

To check if all the elements are passing element quality criteria switch to hidden mode in drawing styles. In order to clear the OFF elements, we can reconstruct the mesh using

Mesh module -> Shell Mesh -> Reconstruct. Also we can eliminate opposite tria elemts formed, rotating quad elements using Mesh module-> Elements-> Split option and

Grids-> paste oprin etc.

• Meshed bottle cap model

• Showing element quality of meshed model

• Step 5- Assigning thickness

After successfully meshed the mid-surface, we can assign thickness to the model by switch on "draw shell as solid " option in Presentation parameter tab under Quality criteria.

• LEARNING OUTCOMES
• Learned how to clear geometric errors in the cad model
• Learned to extract mid-surface of complex plastic components having minute features like threads, ribs, fins, wedges,L etc.
• Learned to create a good mesh on the mid-surface and how to clear element quality.
• Learned about how to assign thickness to the meshed model

• CONCLUSION

Successfully extracted mid-surface of plastic bottle cap model capturing all features like threads, ribs, fins, etc. and created a good mesh on the mid-surface with no OFF elements then assigned thickness.