Week 4

Quality checking for a Plastic model

Aim:

To check the quality of the given plastic model by meshing and removing the errors by satisfying the given quality criteria.

Abstract:

The given plastic model is a bottle cap which has few complications such as threads, sealing teeth, fillets etc which should be taken a mid surface depending on their shape and dimensions. The mid surface is taken using different tools, meshing the geometry and maintaining the given quality criteria using the ANSA software.

Methodology:

• A mid surface is to be created for the given plastic model for each and every part individually using different tools suited for particular operation. After creating a mid surface a geometry check should be done so that the errors can be easily identified and fixed.
• While creating the mid surface each component should be given a particular property card mentioning the thickness and a individual colour for differentiating the parts.
• After the geometry clean up, meshing must be done each for every part individually so that the mesh flow can be corrected easily. After meshing the geometry the shell mesh should be converted to solid so that the thickness of the geometry is maintained the same.

Introduction:

A bottle cap should be maintained with a good quality so that there may be no cracks to prevent leakage of any kind liquid and the temperature of the liquid in some cases after production which are not visible to a naked eye. The following challenge is one kind of method to check the quality of the given model.

Procedure:

The following operations must be done for check the quality of a plastic or a casted model.

1. Mid Surfacing:

Creating a mid surface for a plastic or a casted model take more time as compared to a sheet metal mid surfacing. Each and every part is given mid surface using different tools depending on the shape of the object. The following tools are used for creating a mid surface.

1. Offset:

Offset is used to create a mid surface for a plane straight surface which doesn’t have curved, bent and another surfaces attached to it. The function usually identifies the ends by cons in between. The mid surface should be created by mentioning half of the thickness of the original surface. The following steps must be followed using offset to create a mid surface.

Selected mid surface

Steps for giving mid surface for selected object.

2. Middle:

Middle operation is usually used for mid surfacing complicated surfaces which have a bent shape which cannot be done using offset. Mentioning of thickness is not necessary in this operation. There are two types of middle operations used for this particular challenge.

1. Middle single:

The tool is used by selecting the front and back surfaces and accepting the mid surface. The following steps are to be done for this process.

1. Middle multi:

The mid surface is created by selecting the front, back and side faces of the geometry which are represented by three different colours. The following steps are to be done for creating the mid surfaces for a particular object.

3. Mid Surfacing using curves:

This method is usually used for creating mid surface for surfaces such as threading, extinctions on the surface etc. a curve must be created on the surface of the geometry by selecting the respective cons (for the reference creation) . Then a con must be projected on another surface from where the surface must be created. Then create a new surface from the curve to the con.

Selecting coons to creating a curve

Curve created on threading

Projecting con according to the curve on the respective mid surface

Creating a new mid surface for threading

1. Creating mid surfaces for the inner teeth:

The same procedure can be followed to create the mid surface for the teeth. A curve is to be created on the curved region of the geometry. Then project the con on the end part of the teeth to the respective mid surface. Then create a new surface connecting to the curve and the end part of the con maintaining the sufficient gap at the edge of the end and the mid surface of the teeth.

Parent geometry Mid suface

By using these tools the mid surface for the given plastic model is created.

Given Plastic Geometry Created Mid surface

4. Property Cards:

While creating mid surface for each part of the geometry, a respective PSHELL card must be created for mentioning the thickness of the individual part. The PSHELL cards are created in the property manager.

After creating the property card the respective property must be assigned to the card by applying to the particular object.

Appling of the property card

Selecting the parts to apply for the property card

Property card created and applied in property manager

While creating property cards the software assigns different colours for different component so that for a easy identification and selection of the component part.

In ENT mode In PID mode (Property ID)

5. Geometry check:

The created mid surfaces geometry must be checked before proceding for the meshing process so that there may not be any off elements during the meshing process. The software shows the errors if needed.

Errors detected by the software

By selecting a particular error and right clicking on it gives some options to choose. In the options, if selected for ‘show’ then the error will be shown on the geometry. Few errors can be fixed automatically by the software but few error which the software cannot solve should be fixed by the user manually.

Fixing errors automatically

No errors or warning exist in the geometry.

After the check geometry the geometry must be adjusted in a way that no two cons or hot points must be close to each other . the minimum distance should be maintained as mentioned in the quality criteria below so that there will be no mesh off elements formed during mesh process.

6. Quality criteria:

Before proceeding for the mesh operation, some important quality criteria must be mentioned so that the geometry meets the required quality standards. The quality criteria set for the geometry are given below.

Shell Parameters

Presentation parameters

The shell parameters mentioned for the quality criteria are

1. Aspect Ratio :

The aspect ratio of 2D element is defined as the ratio of its shortest edge length to its longest edge length. A square has the aspect ratio of 1 since its sides have the same length. As it gets away from the square shape, the aspect ratio becomes smaller than 1.

1. Skew Angle:

An angle is formed by the two lines which pass through the midpoints of the sides of the quadrilateral. The skew angle represents the absolute value of the difference between the angle and 90 degree. A rectangle has the skew angle of 0 since the formed angle between lines is 90 degree. For the triangle and quadrilateral elements, the skew angle is found as shown in the figure. For solid element (Tetra, Hexa, Wedge), it calculates the skew angle for each element faces and assigns with the minimum value.

1. Warpage:

The warpage of the quadrilateral elements is defined as the deviation from a best-fit plane that contains the element. It is not always a case to define a plane with 4 nodes. Therefore, a node of the quadrilateral element may be formed outside of a plane. The warpage is a measurement how this node has been deviated, and as it gets closer to 0, the element become more planar. For the quadrilateral elements, the warpage is found as shown in the figure. For solid element (Hexa, Wedge), it calculates the warpage for each element faces and assigns with the maximum value.

1. Jacobian Ratio:

The Jacobian calculation is done at the integration points of elements commonly known as Gauss Point. At each integration point, Jacobian Determinant is calculated, and the Jacobian ratio is found by the ratio of the maximum and minimum determinant value. The Jacobian Determinant of 2D elements is calculated after it has been projected on to a plane, and the determinant of 3D elements is found by direct calculation.

If the quadrilateral element is not convex, the negative Jacobian ratio will be obtained, and elements with the negative Jacobian Ratio can not be solved with correct result.

The remaining quality parameters are defined according to function and the values given according to the mesh minimum length , maximum length, and the percentage of triangles to be formed in the mesh.

7. Mesh Parameters:

Meh parameters are to mention the average length the minimum length and maximum length for a good mesh flow

Mesh parameter

8. Meshing:

After giving the quality criteria start meshing the mid surface individually for better mesh flow and less triangle formation. During meshing there will be many regions where the off elements are formed. The mesh must be changed by the user to no off elements and less percentage of tria elements.

Off elements formation

To over come this problem a tool known as reconstruct is used for reconstruction of the mesh for better mesh flow in the selected area.

Steps for reconstruction

But only few off elements are removed using the reconstruct option. Some of the off elements and triangle percentage are to be removed by the user manually. To reduce the off elements and the triangle percentage manually the following tools are used

1. Split:

Split option is used for splitting a irregular mesh to a normal mesh by connecting from one element to another element through edges or nodes.

1. Smooth:

Smooth option is used for making good flow of mesh from an irregular flow of mesh. It can also solve the off elements formed due to irregular mesh flow.

1. Swap :

Swap is a tool used for changing one element of the triangle to another element so that to use the swap option effectively or to remove the triangles from the edges.

Before Swaping After swaping

1. Paste:

Paste is an option used to paste to mesh lines into one mesh line. This is mostly used for removing the tria element or moving a tria element from one position to another position.

9. Off elements:

There are few of elements formed in the mesh due to the following reasons:

1. Minimum length between the hot points and cons:

The minimum length errors are formed due to the less distancing between two surfaces hotpoints and coliding cons which cannot be removed and are to be left according to the parent geometry dimensions.

Outside error due to the rib connection to the thread

inside error due to the rib connection to the thread

Outside error due to the less distance between the rib and the inner teeth

inside error due to the less distance between the rib and the inner teeth

Therefore, meshing is done by maintaining less off elements and less percentage of tria elements

10. Shell to solid convention:

After meshing the geometry the mid surface should be given its original thickness so that to resemble the parent geometry after meshing process.

Before Thickness After thickness

Result:

Therefore the topo cleanup and meshing of the given plastic model is completed with less percentage of off elements and tria elements.

Parent Geometry Mid surface

Meshed geometry.

Learning Outcome:

• Learned to create a mid surface for a plastic component.
• Learned new methods and tools for creating the mid surface.
• Learned to create mesh with different tools.

Conclusion:

Present work draws conclusion as given below:

• The mesh consist of 8987 elements in which the quad elements are 97.82% and the tria elements are 2.18%.

https://drive.google.com/file/d/1v3PdIir52J_1o90ilPgQWIzUo4zAZ7xQ/view?usp=sharing