Week 4

Aim :- Mid surfacing and mesh generation of bottle cap

Objective :- To create the 2D mesh on bottle cap midsurface using the stated quality critertia and mesh parameters.

S.No	Quality Criteria	Value
1	Target/Average length	1
2	Minimum Length	.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 :- The given components Bottle Cap is a plastic parts. Plastic parts are generally manufactured by

• 3D Printing.
• CNC Machining.
• Polymer Casting.
• Rotational Molding.
• Vacuum Forming.
• Injection Molding.
• Blow Molding.

Saying plastic parts means they are having variable thickness like in his components its has ribs , fillets and certain complex features. So auto-midsurface usually go out of neutral position with many errors. Here we are going extract midsurface manually by each pair of surfaces then use surface-edit panel to combine/extend them.

Methodology :-

1. Importing the model into ANSA software and checking geometrical errors follwed by its solutions.
2. Extracting the midsurface manually .
3. Perform geometrical clean and topology clean up if necessary.
4. Assigning the sperate PID to every features.
5. Assigning the stated quality criteria and mesh parameters before performing meshing.
6. Performi meshing and solving the failed elements.
7. Apply thickness to the final mesh.

Procedure:-

1.Importing the model into ANSA software and checking geometrical errors follwed by its solutions.

First we will import the CAD Bottle cap into the ANSA GUI.As we in the above picture we can see the Bottle cap based model .Here model is shown in 2 colors while we are in entity mode. Entity mode is in which there are no properties applied on the model ex. Thickness ,poision ratio, etc. so part is shown just according to its fits in the assembly.

Here the

Yellow = negative side

Grey = positive side

But the model here is not so fine. We can see features like circle are showing as sharp edges. For that we will use LENGTH option in MESH MODE. This will basically increase the resolution of the component. There also another way to make fine using FINE opition under AUXILLARIES, but the it will take certain good amount of time to fine each and every surface. So the sake of time we will go with the length method.

“1” is the length between 2 points on the perimters. These points cannot be seen here but when in mesh mode. As these is small component with small features we have given its length as 1 to make more fine also these is the length given in quality criteria.

2. Checking for geometrical error :-

Imported CAD model can have certain errors while ANSA platform which can create problem in mesh flow. Errors like here

Unchecked faces : Undefined faces are those where faces are damaged maybe by gaps or there maybe additional surface or presence of multiple cons.

Solution : Isolate the undefined region and release the cons where its showing errors and try to eliminate additional cons or see additional surface created. Other way u can delete the face see for additional cons , release them and again create a fitted surface

Needle faces : Neddle faces needs to be eliminated because those face can create problem in mesh generation as it may fail the mesh elements .

Solution: delete those face , join them with adjacent faces or recreate the face .

3. Extracting the midsurface manually :-

Before going for the midsurface its better to review the model .Reviewing the model means seeing the model thoroughly features by features , region by region , inside to outside to draw the conclusion of best way to proceed with the midsurface. These is necessary so that we can have basic idea like with region needs to excluded , or the some will overlap with the other .

Like here in the above picture we can see that there are two fillets like one outside and the other inside. So its not possible to capture both the fillets in midsurface. Here we can project the line of inner fillet to outside so that will wall then extracting the mid surface of outer.

Measure the thickness of the wall before offset. Select the region for the offset. While giving the offset value the arrow shows the direction of the offset or midsurface. Generally the arrow goes from negative to postive side but we the region here is grey i.e positive so the arrow goes to that side .

The value here of for offset 1.22/2. These value is half of the thickness of the components. Looking at the direction of the offet arrow we will use postive value only. Now make sure not delete the original faces.

In the above picture those gap have to be filled so that fillet can be created in one shot. Here we are using the existing surface option

The one advantage of the existing option is that after the gap filled , the cuts that remained on the surface can be completely removed with no orange marks afterwards.

• Patching of two surface :-

When two surface created have a non considerable amount of gap in between them, it is necessary to patch them up or connect them properly so that there will be no gap. Inorder to fill that gap we two method

1. Create new surface
2. Extend

• Creating a new surface can be lengthy process as some time we have create new curves, new hotpoints etc. These method is generally to fill small gaps, or when there is specific boundary lines.

• Extend method is used to save the time. These method is used when one part is not in the same plane with the other also protruding. So this can be drawback of extend method but the advantage is that it is quick method than in the area if used against creating new surface.

These extend option comes under the faces

Faces > Extend> select surface edge> middle click> select surface ( over which we want to extend).

For confirmation we can see triple cons is created over the surface.

• Surface reconstruction using DACH:

Certain features fillets, beads around the corners cannot be captured in the midsurface or maybe they can be captured in certain area. But the problem may arise in these area while meshing as they can fail quality criteria. To overcome that we have defeature the model in certain way to capture its peak part. Again it can be lengthy process to go.

DACH option provided in the ANSA is simple tool to defeature the fillet area. DACH have two further option

1. Divide the face

These option simply divide the fillets without changing its features

2. DACH

Here we are going to dach method to convert the curved fillets around the corners to somewhat like sharp corners.

DACH option falls under the FACES. We can keep that fillet or we can delete that in DACH, here we going to delete it.

After that we easily extract the mid surface using the offset method.

• Midsurfacing of the threads:-

Since threads are somewhat in different shapes like cycloidal, trapezoidal , square etc. It is quite difficult to capture there shape in mid surface moreover they follow the cyclic shape. Here we are going follow certain method to extract the mid surface of the threads with the help of article found .

Article :- https://skill-lync.freshdesk.com/support/solutions/articles/43000575135

1. MIDDLE curve option to create a single middle curve of the teeth edges

2. FINE :- the curve obtained maybe not particularly cyclic , use FINE to make it look cyclic

3. PROJECT :- project the obtained curve on the surface of the cylinder

4. NEW FACES :- Create the new face using NEW option by selecting the curve and the projected curve to obtain the face.

• Midsurface of the tooth and copying method:-

Following the earlier process of threads we will extract the mid surface of the tooth.

Since here we have numbers of teeth, extracting the mid surface of all will be quite hectic. But we can other option of copying, to copy these identical features on the surface .

Copying method :-

After we obtained the mid surface, we can copy those using TRANSFORM OPTION. For that first create the center point .

1. Points > ON COG> select two different edges of circular to create 2 different center points in different planes

2. Go to TRANSFORM>COPY>select the entities as the tooth> ROTATE

1. Click on the X > select 1 center point
2. Click DX> select second center point
3. Give no. of steps of 35 (no. of teeth are 36 excluding 1 as we created )
4. Angle as 10 ( angle = angle of circle /no. of features )

360/2= 10

Like these using varies function we will create the mid surface of the bottle cap .

4. Performing geometrical and topology clean up :-

It is required check the final mid surface for geometrical errors if in case it may have generated while creating the mid surface .

Here we can individually remove that error but there is another option in ANSA to automatically fix the errors here. We will use that option to save time if not check the remaining error individually.

Topology clean up :

Topology clean is done so that extra cuts , cons , points can be removed from the surfaces where there are chances of failing in mesh also to avoid the generation of tria mesh if necessary .

Another need for topology clean up is to check if there are any mid surface extending beyond the parent geometry. If its extending we will kindly removed that using intersect or cut option and deleting the extra extending region.

5. Assigning the sperate PID to every features :-

PID means property identity. PID caries thickness, density, mass, possion ratio, etc. If the component is having constant thickness throughout , it is alright to have single property card for the mid surface. But when the component is having varying thickness over its surface , it is necessary to assign the separate property card to each and every region.

Here while assigning the property each region will have its own thickness value as we calculated from parent geometry also we will give each region sperate colors for better understanding in visually

Like these we will give each and every feature its separate PID

6. Assigning the stated quality criteria and mesh parameters:

The quality cirteria used here are

1. Average length =1

Average length is the distance between two points that created on the surface of the part. These is to ensure that average size of the element is 1.

2. Minimum length=0.5
3. Maximum length =3

These two value ensures that element defined size. Element outside these value are not created or if created needs to rectify

4. Aspect ratio = 3
5. Skewness = 45
6. Warpage = 15
7. Minimum Quad Angle =45

8. Maximum Quad Angle =135
9. Minimum Tria Angle =30
10. Maximum Tria Angle =120
11. Tria Percentage =15
12. Jacobian = 0.7

Same put the value in the mesh parameters mode

7. Performing the mesh and solving the failed elements:-

Perform the meshing keeping in mind that no rotating quads and no oppposite tria element present near each other also with proper mesh flow:-

Before performing the mesh flow make sure to defined the length of the perimeter and macros as 5. These is to ensure that adjacent nodes have a distance of 5 between them.

Switch to HIDDEN MODE then starting with the mesh generation . here we are using two mostly method of mesh generation as

1. Best
2. Spot mesh

Rectifying the errors of mesh :

Warpage errors: these errors are shown in sky blue color here occurs in quad elements . to rectify these kind of error use the split option and try to split the quad into tria

Min and max length error : these error arrises when element does set into defined value .to rectify these use split option or add number of cons .

Rotating quads : to rectify the rotating quads use the split option , split the quad into trai .

Opposite tria : these can be eliminated by using the split option and also using the paste option to join then as one

Mostly use tool here to remove the errors are :

1. Reconstruct
2. Smooth
3. Split
4. Paste

In certain region like here we see that along edge there is element that is failing in skewness. These kind of errors can be removed by using EDGE2PERMITER option .

Under MACROS> edge2 perimeter> select on the 2 edges of element > confirm

The element edge will form the perimeter over the surface , that we can see in the TOPO mode

Remover that formed face in the TOPO mode using the UNIVERSAL DELETE option. Now the error can be removed like these if any where possible.

But here in certain region there are error that cannot be removed. Like in the below figure we can see that region is failing in like minimum, jacobian, angle errors.

If switch to wire frame mode we can see that there are triple cons that region. So we cannot remove that triple cons even if we try so. Here we cannot solve that error .

Above picture shows Final mesh on the BOTTLECAP mid surface

8. Apply thickness to the final mesh :-

As we already given thickness to the mid surface in the PID manager, we can now see that thickness of the mesh using DRAW SHELL AS SOLID in quality criteria.

To see if that the surface is extracted properly we can check by comparing the these mesh surface with the parent surface. If the glitching happens between two then the mid surface is extracted properly.

Results :-

• Geometrical clean of both mid surface and parent geometery successfully done
• Mid surface of bottle cap created with their PID and assigning thickness
• Mesh flow over the mid surface with uniform flow
• The mid surface is divided into total of 9696 small elements
• Quad elements as 9538(98.29%), Tria element as 166(1.71%)
• There were around 24 off elements that cannot be solved due geometry constrains

Conclusions :-

The bottle cap component mid surface have been extracted successfully through manual mid surfacing. The mesh generation is also performed but the mesh has certain off elements which cannot be solved due constraint that geometry have. It can be solved if there would have been any provision for triple cons elimination in topology clean up.

Learning outcomes :-

• Plastic components mis surface have to extracted by various method such as offset , middle curve , middle faces .
• Comparing the mid surface with the parent geometry to avoid any protruding of the mid surface beyond the parent surface.
• Eliminating those protruding surface by intersection .
• Removing the small surface or defeaturing is acceptable sometime in the region where elements are failing in mesh.
• Certain off elements cannot be solved if geometry has triple cons closer to each other.

https://drive.google.com/drive/folders/1AFMOeRDwrJmU2KPM9JuxHcJC5e7CVnQy?usp=sharing