Week 4
AIM:
1. To clear the geometry errors and extract the mid-surface of the given plastic component(BOTTLE CAP).
2. To mesh the given component according to the given quality criteria.
PROCEDURE:
MID-SURFACING:
1. The picture of the given bottle cap model is attached below.
The above picture is the bottle cap plastic component which we have to take mid surface and mesh with respect to the given quality.
2. To extract the mid-surface we must perform the topology and geometry clean-up.
3. Execute the CHECK GEOMETRY option in the CHECKS manager to view the errors in the given model. If there are any errors in the model, use the AUTOFIX option in the CHECKS manager.
4. Refine the model to a length of 1mm. After refining the model perform a complete topo on the model by clicking on the TOPO option in the FACES menu.
5. Now the model is ready for extraction.
MID-SURFACING METHODS USED FOR THE PLASTIC COMPONENT:
1. There are certain rib regions found on the body of the bottle cap. A picture of the rib is attached below.
To extract the mid-surface of these rib regions use the MIDDLE option from the FACES menu in the TOPO module and select both sides of the rib and select the middle mouse button. Now a mid surface will be present exactly in the middle of the rib part. Follow the same method for the remaining rib parts.
2. For the main body part to which the rib is attached, create a WELD SPOT on the body on either sides. This can be performed using the WELD SPOT option found under the HOT POINTS menu. Measure the distance between both weld spots and you will get a thickness value. After arriving at a thickness value, click on the OFFSET option from the FACES menu and enter the measured thickness value divided by 2. Now a mid-surface will be created exactly at the middle of the body.
3. Now let us see on how to extract mid-surfaces for bodies having uneven thickness.
4. The photo of the upper part of the bottle cap (CAP REGION) is attached below.
The cap region is a complex part where there are features of uneven thickness. The parts that are close to the bottle cap have non-uniform measurements. To create a mid surface for those parts we have to create curves on those parts exactly in the middle.
5. To create a middle curve, click on MIDDLE option from the CURVES menu. Select the cons required to create a middle curve and click on the middle mouse button. This is based on group selection where you have to select one group of cons seperately and then select the other group seperately. Create a closed curve effectively to create a new surface. After creating the curve click on NEW option from the FACES menu to create a new face that lies exactly in the middle of the feature area.
6. The cap region's thickness is unknown. Use the WELD SPOT method that we used for the main body part to measure the thickness of the cap portion. Create a weld spot on either side of the cap region to measure the thickness of the region. Use the OFFSET faces option to create a middle surface after determining the thickness.
7. Now let us look at the additional features that are present inside the bottle cap. The inverted image of the bottle cap is attached below.
8. There are thread portions and some additional features present inside the bottle cap.
9. For the thread portion wound around the main body of the bottle cap, use the MIDDLE option from the CURVES menu to create it as a 1-Dimensional feature. Click on the cons that are present on the thread part to create a closed curve so that we can create it as a new face.
10. For the rib features attached at the bottom edge, use the same MIDDLE option from the CURVES menu to create a closed curve for forming a new surface.
11. For the features attached at the bottom part of the cap region, we can use a comination of CONS2CURVE and MIDDLE option from the CURVES menu to create an new surface.
12. After extracting the mid-surface one by one, make sure to extend the features with the main mid-surface. The mid surface of the bottle cap should also look similar to that of the PARENT SURFACE.
13. Extension of the surfaces can be done by clicking on EXTEND option or INTERSECT option from the FACES menu. Click on the edges of the features and select the face with which you want to join. After joining there are TRIPLE CONS created so as to indicate that the features are attached.
14. Below is the attached photo of the complete mid-surface of the bottle cap.
15. We must check whether our mid-surface is correct before proceeding to meshing. TRANSPARENCY option in the PROPERTIES manager allows you to do that. After extracting the mid-surface, store it in a seperate PID using the SET PID option in the FACES menu. Then make sure that you switch on the TRANSPARENCY option in the PROPORTIES manager which can be activated by right clicking on the PID that we have set. Switch the visiblity from ENT to PID mode below to make the mid-surface and the parent surface co-exist with each other. The mid-surface extracted will have a different colour compared to the parent surface. This will show the mid-surface within the parent surface so that we can check whether we have extracted the surfaces properly.
Below is the photo in which the TRANSPARENCY option is switched on. As you can see, the mid-surface is highlightd in a light yellow colour within the parent body surface.
16. It is not necessary to offset every single part in a plastic component. We can capture the main features of the component and just extend and create new faces.
17. Now the mid-surface is finished and we can proceed to meshing.
MESHING:
1. After extracting the mid-surface, we must set our target element length. To do that switch to the MESH module and click on LENGTH option from the PERIMETERS menu. Our target element length is 1mm, so click on the entire mid-surface by box-selection and specify the length value as 1. Now the model is set for a atrget element length of 1mm.
2. Review the entire mid-surface area by area and remove unnecessary construction lines and faces which you may think might be a threat to the quality parameters (eg. angle,warping,jacobian,min and max length,aspect ratio etc.)
3. Before meshing, set the quality parameter values given to us. This can be set in the QUALITY CRITERIA option in the main menu or simply press the F11 shortcut. Below is the attached photo of the quality criteria tab
4. Enter the values given to us and click on apply option. Also switch on the elements percentage in the PRESENTATION PARAMETERS option below to show us the number of quad and tria elements and the percentage of quad and tria elements present in the mesh.
5. Before meshing switch the visiblity from SHADOW mode to HIDDEN mode so that we can see the areas that are failing for quality.
6. Go to MESH PARAMETERS option in the MAIN MENU to set the target element length and the min and max length value given to us in the quality parameters respectively.
7. To get a better mesh flow for plastic component mid-surfaces, we can try different MESH options in the MESH GENERATION menu but it is best that we use the BATCH mesh option to get a better mesh flow entirely. There are other options such as FREE, SPOT, BEST, CFD etc. but batch mesh gives us the best possible mesh flow and also tries to reduce tria elements.
8. Apply the mesh area by area and clear the quality aspects as much as possible. There will be certain areas that fail for our quality criteria but we can ignore them.
9. The final mesh of the bottle cap mid-surface will look like this.
10. After meshing switch on to DRAW SHELL AS SOLID option in the PRESENTATION PARAMETERS under quality criteria to view the model as a 3-D model.
11. Thus the meshing is completed and all the feature areas are captured with a better mesh flow.
CONCLUSION AND LEARNING OUTCOMES:
1. We cannot clear the quality aspects entirely in a plastic component mid-surface since there are many features to capture and extend to the mid surface.
2. We have also learned how to check a mid-surface's accuracy using thr transparency option.
3. We have also learned on how to offset areas and extract the mid-surface having non-uniform thickness.