2D Meshing of Bottle Cap Model

Meshing of Bottle Cap Model

Objective:

Performing Geometry Clean-up to remove any topological error, extracting mid-surface of every sub part and subsequently meshing the bottle cap model using the following quality criteria using ANSA.

Sr 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
12	Tria Percentage	15
13	Tria per node	2

Procedure:

1. Geometry Clean-up

The first step before beginning involves checking that the model is in .ansa or .stp format. ANSA can read file and work on in this format.

After opening the file we need to set the target length in order to get the smooth part as well as achieve target element at an initial stage. We will assign target element length as 1 mm.

The parts are disjointed, in order to join them we will perform TOPO over the entire model.

The next step is crucial step going ahead, that is checking the given file is of good quality. We need to ensure the model is free from any topological error using Geometry Check-Up. We need to use geometry Check-Up to see the numbers of existing errors.

We need to clear all these error before proceeding ahead. Once cleared the check geometry dialogue box will give green signal on execution.

After this stage we will need to check the number of parts, and accordingly extract mid-surface. In the bottle cap model, we have the entire subpart as a whole one single part.

Before we proceed to extracting mid surface we need to make sure that the part is oriented, else the orientation of mid-surface would not be proper. For the given model we will select grey on outside to assign it as a positive side.

Fig: Bottle Cap Model after geometry clean-up

2. Extracting Mid-surface

The given model consist of many intricate portion such as teeth, thread and ribs, parts with variable thickness and multiple ribs, as a result of which we would extract each surface individually.

The first of extracting mid-surface, involves of the outer shell portion with a thickness of 1. 02 mm. We offset the shell at a distance of 0.51 mm.

Following it we will do the adjacent fillet portion with the thickness of 1.80 mm. Here too, we will apply offset command with a offset distance of 0.90 mm.

The portion adjacent to fillet is of 1.22 mm thickness, for which we would use OFFSETcommand again and offset at a distance of 0.61 mm. After which we will TOPO this mid-surface.

As mentioned before, the model consist of many intricate sub parts, one amongst them are the outer ribs.

Here the ribs are of different thickness throughout, but the mid-surface would be connected and be in the same plane. As a result of which we extract mid-surface of the outer portion of ribs.

The following steps involves connecting rib to the main body, we can do these using EXTEND command.

Likewise, we will extend for each and every outer ribs.

Barring the first and last ribs, other ribs are not connected till the end. If not resolve at this stage they create the quality issue at the further stage.

If we look at our parent body we realise that the end portion needs to be extended. We can do this by creating a line on fillet by approximation. Using that line and edge of rib we create NEW surface.

Another intricate portion, albeit and easier one is the thread region. Like outer ribs this too has variable thickness.

Here by using CURVE MIDDLE command we will make curve using the middle portion of thread and end portion of thread.

We will create a new face using this two curve. The resulting will obviously be slanted due to variable thickness. Once they are extracted we will use extend command and PROJECT it on the face of outer shell.

The next intricate member in this model are the inner ribs. For these we will first extract mid-surface of one ribs.

To do that we will use CURVE MIDDLE option to extract the mid of top and bottom region. Once we get it, we will create new face and EXTEND this portion to fillet and lower face adjacent to fillet. Following which we will chop out the extra face.

There are 12 such ribs. To recreate that many ribs, we will first make 2 COG point using two circular inner mid-surface. Under TRANSLATE and COPY, we select the entity. We select rotate in the dialogue box. We will select two points, and for recreating the other steps we need to find number of steps and angle.

Number of step = n – 1 = 12 – 1 = 11

Angle = 360 / n = 360 / 12 = 30⁰

As a result, 12 ribs are created.

The last of the intricate members are the teeth. Here we would simply use CURVE MIDDLE at front and create a middle curve using top and bottom construction line. For the back portion we will simply CREATE a curve using create command under curve.

Using this we will create a surface. From parent body we realise that the back end of teeth is attached to the bottom phase. In order to do that we will project back cons to the lower mid-surface face and PASTE them together.

There are 36 such teeth in total. To recreate them we will use COPY and TRANSFORM command. After selecting entity we would select two point under ROTATE command.

Steps = n – 1 = 36 – 1 = 35

Angle = 360 / n = 360 / 36 = 12⁰

After confirmation 36 teeth are created. After this stage we do not use the ROTATE command, owing to which we will delete the COG point.

The next portion is the top surface, for which we need to take mid-surface till the whole fillet point.

Once we take out mid-surface we will find out certain section are disjointed. We will join them by using new face option and once it is done, we will TOPO this portion. It should be duly noted that the construction line which are not needed or does not define a feature should be removed to avoid quality issue.

The mid-surface of outer inside circular portion is taken by OFFSET. But for inner portion a curve is generated and we create a circular face.

The following supporting rib has the thickness of 0.61 mm. We will use OFFSET command for this and offset distance of 0.305 mm.

After offsetting we will connect this rib by creating HOT POINTS followed by a new face.

For the following supporting rib the process remains the same, the only difference is the thickness of ribs. It is of 1.02 mm, so offset distance is 0.51 mm.

For this portion too, we will connect making HOT POINTS and create a NEW face.

The extended rib support is off uniform thickness. Here we can use MIDDLE under face to create a mid-surface. There are four extended ribs, likewise we will extract mid-surface for them. We will extend them to their following adjacent surface following that.

For the following support rib with variable thickness, we need to use MIDDLE under faces to create mid-surface. After that we need to EXTEND it to the adjacent faces.

For the support portion with a variable thickness we will create a MIDDLE CURVE at the top and PROJECT that curve at the bottom.

After projecting this curve at the bottom we will create a NEW surface. Now we have fully extracted the mid-surface of all individual portion.

After extracting the mid-surface, we will give different portion its different properties ID using Set PID. Under thickness we will assign it actual thickness. Once this is done, we will put mid-surface in different lock view.

Fig: Mid-surface of Bottle Cap Model

3. Meshing

Before we begin meshing, we need to set the target element length. For the given model it is 1 mm. We need to set it in the mesh parameter.

After setting the target element length we need to set the mesh parameter as given. We will assign tria per node as 2 in order to get to know the places with back to back trias. By that we can eliminate them to avoid any failure.

Once it is set, in presentation parameter we will select all details and element % in graphics.

At first, we will do BEST mesh for all off the parts. By applying Best mesh we get too many quality issue. We will use Reconstruct command to get the best mesh result ANSA can give, while also at the same time reducing the off elements. The following is the result of BEST mesh without reconstruction.

We will extend the triple cone construction using CUT option, to reduce any undesirable tria. With this we get quad elements with failing quality. But they can be arranged. The failing quads elements are rearranged using MOVE command and the entire surface is smoothened. Likewise, we will repeat the same over entire area.

For portion failing for aspect ratio, we notice they are already a quad element. For that we will use MOVE option and smooth the region individually.

One of the most important thing we need to make sure that there should be no trias at the perimeter and edge. But in the following case, the trias have to be compromised to one place. We will opt for the edge rather than the perimeter for better results. We will SWAP the trias away from perimeter of fillets.

We need to make sure we remove construction line from the edge and fillet region to avoid failure. We will use SPLIT command to move it further and SMOOTH command to better the existing mesh pattern.

There are areas that cannot be resolved perhaps rather should not be. We will allow such area to fail for quality as it should not be resolve. We will use ORIGIN command to bring it back to its actual point. We will observe that the element fail for aspect ratio, but we will keep it that way.

The same applies to smaller support rib. Here we will first use ORIGIN command to bring it back to its actual point. Once done, we will realise it fails for maximum length, something we cannot do much about.

In the following situation we can bring tria back using PASTE command. This will make sure that the tria is off the edge region.

Once tria is brought back, we will smoothen the region.

For trias together we can use split command to separate them and for opposite tria we will split to a point that two opposite trias are adjacent to each other. The by using SPLIT we will make it a quad and smoothen the region.

For the following area we will reconstruct the region.

First we will go to topo mode. We will realise the reason for failure of quality were triple con closer to edge and adjacent triple con. We will sort this by reconstructing the region.

At first we will chose one of the part of the region.

We will provide HOT POINTS such as this. Create a construction line using CUT command.

DELETE the region by deleting FACE. Delete the HOT POINTS.

We will see that there is some loss of material, but we have compromised it to avoid failure for quality.

We will do these for rest of the region on both sides.

After this step, we will apply BEST mesh and RECONSTRUCT the region, which will give significantly good result.

For the following consecutive tria, little can be done, as they exist on triple cons. Doing anything will only bring more quality issues.

Fig: Meshed Bottle Cap Model

Result:

Quads = 9596 (98.02 %)

Trias = 194 (1.98%)

Total = 9790

Conclusion:

The meshing of Bottle Cap model after clean-up of topological error is done.

Link of Meshed Bottle Cap Model:

https://drive.google.com/file/d/1TckGkb9HrsNQ6z--PchLQAl6KDyaAOZO/view?usp=sharing