Week - 4 - 2D meshing for Plastic components

AIM: 

Extract mid surface for the given plastic bottle cap model and mesh the given plastic bottle cap model by following the quality criteria.

S.NO   Quality Criteria          Value

  1          Target 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

PROCEDURE:

Plastic components are those in which non-uniform thickness is followed throughout the component. So unlike sheet metal component which has uniform thickness, extracting manual mid surface for the plastic component is a time-consuming process.

In order to mesh the given plastic bottle cap model, certain steps should be followed So that better quality mesh is achieved.

First import the cad geometry in the ANSA or any pre-processing software. After importing the geometry there may be a chance of occurring new topological errors, So topo function is to be performed after importing the geometry. This was performed by FACES > TOPO. Next, the cad model orientation issues must be checked by FACES > ORIENT. The entire cad model will be represented in grey and yellow colors in ENT mode.

FIG 1 - Orientation of cad model

After this topological errors must be checked before extracting the mid surface so that accurate results were obtained. This was done by CHECKS MANAGER > GEOMETRY > EXECUTE. The checks manager will show the errors that were present in the cad model. ANSA will provide the option of solving these errors either manually or automatically.  

FIG 2 - Topological errors of cad model

In the fig2 by selecting the fix option, the topological errors were automatically fixed. Some of these errors cannot be automatically fixed, they have to be performed manually.

FIG 3 - Manual fixing of topological errors

Fig 3 shows the triple con error that was formed due to overlapping of the faces. So in order to remove this error delete the  faces present under the thread by FACES > DELETE.

The meshing can be performed in 2D as well as in 3D. 3D meshing is time consuming process as it takes lot of of time to compute the model where as 2D meshing is performed for those models where thickness of components is less than 5mm. For 2D meshing mid surface must be extracted for the cad model and meshing is performed which gives similar result as of 3D meshing.

So in order to mesh the given cad model mid surface extraction must be done. In ANSA for non-uniform thickness components mid surface extraction must be done by FACES > MIDSURFACE > CASTING. But for complicated geometries, ANSA fails to extract the mid surface so manual extraction must be done. 

FIG 4 - MID SURFACE EXTRACTION FOR VARYING THICKNESS

For varying thickness entities, the mid surface extraction must be done by FACES > MIDDLE > MULTI and select the faces and thickness of the entity as shown in Fig 4. For the entities having a uniform thickness, the mid surface extraction can be done by FACES > OFFSET.  For the highlighted inclined faces as shown in fig 5, as the distance between the inner and outer face is uniform, the mid surface is done by offsetting the outer face. 

FIG 5 - INCLINED FACES OF THE CAD MODEL

After extracting the mid surface, as shown in the fig 6 for some parts there is no connectivity between the entities. This can be resolved by extending the feature onto the neighbouring face by FACES > EXTEND and then intersect the two faces by FACES > INTERSECT, after this remove the excess faces that were formed.

FIG 6 - EXTENDING ONE FACE ONTO THE OTHER

For complicated areas like threads and teeth, the mid surface can be extracted by following certain steps. For threads in order to extract the mid surface, capture the middle curves for the top and bottom faces by CURVES > MIDDLE and form a new face between these curves and extend the face onto the base mid surface by FACES > EXTEND. Fig 7 shows the extracted mid surface for the threads. Triple cons are formed as the cons share three neighbouring faces. 

FIG 7 - MID SURFACE FOR THE THREADS

For teeth, mid surface is done by capturing the middle curve for the front and side faces and form a new face with these curves. In this cad model as 25 teeth are present, copy the extracted mid surface to all these teeth by TRANSFORM > COPY > ROTATE.  Fig 8 shows the extracted mid surface for the teeth. Triple cons are formed as the cons share three neighbouring faces.

FIG 8 - MID SURFACE FOR THE TEETH

The midsurfacing should be extracted in such a way that connectedness should be present through out the geometry. As shown in the fig 9, for some features, we can extract more than one mid surface but the midsurface which is having connectedness should be selected. The connectedness is absent in grey colour extracted surface, as yellow colour surface is having connectedness through out the face this midsurface has to be selected. 

FIG 9 - MIDSURFACING THE FEATURE

((PID ASSIGNMENT FOR THE VARIABLE THICKNESS))

Next after extracting mid surfaces, swift to the mesh module from the topo module to create mesh elements. In mesh module all the cons are shown in green colour, hot points in white dots and pink dots indicate perimeter elemental structures. The distance between the perimeter elemental to be assigned to 1mm, this was done by PERIMETERS > LENGTH and select the entire geometry and assign the value of 1mm. Due to the presence of complexities in the geometry, flexibility must be given to the mesh elements which is called criteria range. Next open the quality criteria index by F11 key and assign the criteria ranges to the mesh elements as shown in the fig 10.

FIG 10 - QUALITY CRITERIA INDEX

Next mesh the cad model by MESH GENERATION > BEST and select the surfaces that needs to be meshed. After mesh is generated go to hidden mode to identify the elements that fails to satisfy quality criteria. ANSA will try to minimize the number of elements that fails to satisy the quality criteria by rearranging the mesh by SHELL MESH > RECONSTRUCT.

Some of these types of patterns should be avoided so that better mesh flow is generated and accurate results were obtained-

1) Tria elements on the perimeter of the geometry.

2)Back to back trias.

3)Two trias pointing towards each other.

4)Two trias facing opposite to each other.

The above patterns can be avoided by ELEMENTS > SPLIT, SWAP and JOIN commands.

In some areas, in order to have better mesh flow we have to compromise on one feature. In fig 11 as the distance between the nodes is less quality mesh is not obtained so in order to get good quality mesh defeaturing should be done by making a cut. fig 12 shows the obtained mesh after having a cut.

FIG 11 - BEFORE DEFEATURING

FIG 12 - AFTER DEFEATURING

LEARNING OUTCOMES:

• To extract mid surface for complicated features like threads, teeth.
• To check the extracted mid surface by switching on the transparency.
•  To assign thickness in PID for varying cross-sectional areas.
•  To check the connectedness for the extracted mid surface.

CONCLUSION:

The mid surface is extracted for the plastic component and better mesh flow is obtained. After pre-processing the geometry, next step is to stimulate the geometry by applying suitable boundary conditions.