PRE-PROCESSING OF INTERIOR PLASTIC COMPONENT OF A SIDE DOOR BY MID-SURFACE EXTRACTION METHOD USING ANSA

INTRODUCTION:

The objective of the project is to extract the mid surface of the passenger car side-door trim model and assigning appropriate thickness for the various regions and to mesh the surfaces with all the mesh elements meeting the quality criteria. Also, the mid surface extraction is done by three methods which are Auto method, Manual method and a combination of auto and manual method together.

GENERAL PROCEDURE:

The following procedure will be followed.

1.Geometry cleanup:

The imported model is checked for the geometry errors using the checks manager. If there is any problem, it should be corrected and the geometry is made clean.

2.Extracting the mid-surface:

Then the mid-surface should be extracted. It can be extracted either manually or automatically using appropriate methods or a combination of both can be used. It is based on the thickness variation and time constraints. In manual method, the faces are selected, and the offset value is given. In automatic method, the mid-surface is extracted in a form FE model straight. For this, the whole part is selected, maximum thickness value, target element size are given in prior.

3.Assigning PID and thickness:

Now, the regions with different thickness are assigned with different PID\'s and the thickness values are assigned accordingly. This process is also done by three methods which are again Auto, Manual and Combination of both the methods.

4.Mesh generation:

Then,if the mid surface is a CAD geometry, then the model is meshed with the target element size defined. Because in auto method, the extracted mid surface is already an FE model itself.

5.Meeting the quality criteria:

Now, the mesh elements are corrected to meet the quality criteria specified below.

Quality criteria to be followed

S.N	Quality Criteria	Value
1	Aspect Ratio	3
2	Skewness	45
3	Warping	30
4	Taper	0.5
5	Min. Length	1mm
6	Max. Length	6mm
7	Min angle Quad	45
8	Max angle Quad	135
9	Min angle Tria	30
10	Max angle Tria	110
11	Tria %	15

TARGET ELEMENT SIZE FOR ALL THE MODELS: 4 MM

ELEMENT TYPE : MIXED

The model to be processed is as follows:

GEOMETRY CLEANUP:

When the model is imported, it has a lot of topological errors. The model is checked by Checks manager. So it is found that there are a lot single cons, triple cons, needle faces and cracks. All these are errors are corrected one by one.

After correction:

MID SURFACE EXTRACTION BY AUTO METHOD:

After correcting all the errors, the model is ready to be mid skinned. The option used is

TOPO ----> FACES ----> MID-SURFACE ----> CASTING

Then the whole model is selected and the target element size is given as 4 mm. Once clicked OK, then the middle surface of the whole model is extracted in the FE model as shown below

Now, the middle surface can be checked using the following algorithm,

TOPO ----> FACES ----> MID-SURFACE ----> CHECK MIDDLE SURFACE

Now, ANSA compares the extracted mid surface with the original model. If there is any deviation, it will automatically correct it.

To automatically assign PID and appropriate thickness,

TOPO ----> FACES ----> MID-SURFACE ----> CALCULATE THICKNESS

By using this algorithm, ANSA will automatically detect the areas with different thickness values and assign those values with seperate PID\'s.

As seen in the above picture, the FE middle surface is assigned with the PID\'s and the thickness values accordingly.

Meeting the quality criteria:

As the mid surface is already in the form of FE model, now quality criteria is specified. After assigning, there will be a number of elements OFF since they do not meet the mesh quality specified as below

Each one of these should be corrected using appropriate methods for each criteria. The most commonly used methods are

MESH ----> GRID ----> MV SURFACE

MESH ----> MACROS ----> CUT

MESH ----> ELEMENTS ----> SPLIT

MESH ----> ELEMENTS ----> SWAP

MESH ----> SHELL MESH ----> RECONSTRUCT

After correction,

As seen in the above snapshot, all the mesh elements are met with quality criteria specified by the user and no elements are OFF. This helps in obtaining the accurate results during analysis.

MID SURFACE EXTRACTION BY MANUAL METHOD:

Unlike the auto method in which the whole model is mid-skinned at a go, in manual manual method each seperate surface is selected and it is been OFFSET depending upon its thickness by the user manually.

TOPO ----> FACES ----> OFFSET

The mid surface can be seen as follows

The red con which is seen in the middle of the surfaces are the mid surfaces which is OFFSET by the user. All these surfaces should be connected in order to form a single model. First the base surfaces are OFFSET and are connected. Then the ribs and other projected surfaces are OFFSET and are extended to meet the original mid surface.

TOPO ----> FACES ----> EXTEND

The OFFSET ribs can be seen in the below snapshot,

After OFFSETTING all the surfaces and connecting together, the whole mid surface is formed as follows

After this, each surface with different thickness is assigned with a different PID and the appropriate thickness values are given by checking the original surface thickness. The model with PID\'s assigned as follows

Mesh generation:

Now the mesh is generated with target element size as 4 mm using the below algorithm

MESH ----> MESH GENERATION ----> BEST ----> MIXED

Once the mesh is generated, quality criteria is specified as mentioned earlier. Then the elements which are not meeting the quality are corrected. And in the end, no elements should be OFF as shown below

MID SURFACE EXTRACTION BY COMBINATION AUTO AND MANUAL METHOD:

Here in this method, a different approach is followed. First the whole model is mid skinned as did in the Auto method. And some of the missing entities and deformed surfaces are OFFSET manually and connected to previously extracted mid surface.

TOPO ----> FACES ----> MID-SURFACE ----> CASTING

Then the whole model is selected and the target element size is given as 4 mm. Once clicked OK, then the middle surface of the whole model is extracted in the FE model as shown below

As the surface extracted is in the form of FE model, it should be converted to a surface geometry for further processes. To do so, the algorithm used is

MESH ----> ELEMENTS ----> TO SURFACE

Once it is converted to surface geometry, entities like the ribs are manually OFFSET by the user according to the thickness values. And then they are connected as shown below

After this, PID's are assigned properly and thickness values are also assigned.

Mesh generation:

Now the mesh is generated with element size as 4 mm. Then all the elements are corrected to meet the quality criteria specified.

CONCLUSION:

Extracting the mid surface by auto method took less than 7 minutes for the whole model. It saves a lot of time. But the accuracy of the surface captured is poor. On the other hand, when doing it manually, the user can produce the mid surface with best finish and dimensional accuracy. But it requires a lot of time. So using the combination of both would be the best choice for mid surface extraction method. It saves time and near accurate finish can be obtained.

Model extracted by auto method:

https://drive.google.com/open?id=1R5Tp7CNbi3UTjb9mkGSGSWa-BAxDNcWT

Model extracted by manual method:

https://drive.google.com/open?id=1Hl2pjB2VzWLT-vHw8HAxTMMpzV_ztKnw

Model extracted by combination of both methods:

https://drive.google.com/open?id=1jyLdPNXWAO142xFlOX_xMZPTYiW_LIje