Automotive Plastic Base Bracket Design using Class A as input

Base Bracket Design – Plastic Component

This report demonstrates the design of a Plastic Component - Bracket using industry principles such as Draft angles, Draft Analysis, Determining and implementing of Tooling Axis, Class A, Class B and Class C surfaces. The component is designed using CAD package CATIA V5.

The terms are briefly explained below:

Draft:

Draft is an angular tapering given at the design stage to every Plastic Component that has to be manufactured. It is given in the direction of the Mold Movement. The Draft allows the parts to release easily from the mold without creating friction between the surface of component and the core and cavity plates.

When a Plastic component cools down inside the mold, the natural tendency of it is to shrink and attach to the core or cavity half. The movement of mold opening causes friction on the walls of the component as well as the core/cavity blocks. This reduces the life of the mold as well as the component itself. The component may also have rough surface finish as well as weak structure due to this. This problem can be solved by giving draft to the component.

Most applications require minimum draft angles of around 0.5º to 1º , however 3º draft angles are widely accepted for cost saving purpose. 7º Draft angles are given to textured surfaces like Instrument Panels. Minimum possible draft angles are given to screw bosses, ribs and stiffeners due to their critical nature.

Figure1: Draft Angles

Tooling Axis:

Tooling Axis is the direction in which the Mold opens and closes. Part Drafts are given along this axis.

Figure2: Tooling Axis

Draft Analysis:

Draft Analysis is a tool in CATIA which identifies the zones which deviates from the specified values of angles along a defined draft direction using colour codes.

It highlights the regions of draft using colours. Draft analysis is the first step performed after receiving the Class A of the component.

Figure3: Draft Analysis showing one of the faces having acceptable 15º draft angle

Class- A Surface:

In automotive design, a class A surface is any of a set of freeform surfaces of high efficiency and quality. It is the visible aesthetic surface of any automotive component. Class A surface refers to those surfaces which are visible and abide to the physical meaning, in a product. This classification is primarily used in the automotive and increasingly in consumer goods industry. It is a requirement where aesthetics has a significant contribution. For this reason, the exterior of automobiles is deemed Class-A.

Class- B Surface:

Class B surfaces are Engineering Surfaces which are invisible to the eyes or often on the back side of the Class A surfaces. It is mainly the thickness of the component or Class A. These surfaces need to be designed as per the Class A surface i.e., the continuity, thickness of the component, functionality etc should be maintained. These surfaces include the main engineering features such as Screw bosses, Ribs, Snaps, Stiffeners and fixations. B surfaces also do not contain any aesthetic styling content and textures since it is not visible to eyes.

Class- C Surface:

Class C surfaces are connectors between Class A and B surfaces. This surface connects both A and B surfaces along the Tooling Axis. It contains the draft as per the design. Its is basically the thickened surface between both A and B surface.

Below Figure 4 shows class A,B and C surfaces: Blue surface is Class A, Green surface is Class B and Red surfaces are Class C surfaces.

Figure4: Depiction of Class A, Class B and Class C surfaces

BASE BRACKET DESIGN:

Procedure to be followed:

Figure5: Procedure in designing Base Bracket

Step 1: Design Check on received Class A:

• After receiving the Class-A for the Base Bracket check for the connectivity of the surfaces on it.
• Use Join operation, boundary operation and disassemble.
• The check is over if any discontinuities do not arise.
• Delete the operations after the check.

Step 2: Determine the Tooling Axis:

• Create a Dummy Tooling axis along the best possible direction as per the Class A as shown in Figure 6.
• In this case we have chosen Z axis as dummy as it doesn’t interfere with any slots or holes

Figure6: Dummy Tooling Axis

• Create an Axis System on this line and the center point as shown in Figure 7.

Figure7: Axis System

• Create an Intersect between the two planes and the Class A surface i.e., the XZ and YZ plane intersection with the Class A of Bracket.
• In this case the XZ intersection is not required as the faces on this plane have acceptable angles for draft and its analysis is not required.
• Intersection with YZ plane is done as one face tends to be problematic in case of draft. It is shown in Figure 8.

Figure8: YZ plane intersection with Class A of Bracket.

• Create extended Lines for the faces which may create problems in terms of Draft angles.
• In this case the lines are for the YZ intersect as shown in Figure 9.

Figure9: Extended Lines for faces on YZ plane intersection with Class A.

• Create a Bisecting Line for the extended lines along the center point which is the Tool clearing axis line for Y.
• Since only one axis line exists in this case, we name it as the Main Tooling Axis. It is shown in Figure10.

Figure10: Bisecting Line renamed as Tooling Axis.

• Publish the Tooling Axis as well as the Class A for easy access using Insert---->Publication as shown in Figure 11.

Figure11: Publication of Tooling Axis and Class A surface.

Step 3: Perform Draft Analysis:

• Change View Mode Customization to Material mode
• Go to INSERT ---> ANALYSIS ---> FEATURE DRAFT ANALYSIS and move the compass to the Tooling Axis.
• Change the range to 3º.
• Apply the settings as Shown in Figure 12
• As shown the target area has 15º draft angle which was the concerned face.
• Thus, it passes the criteria of having suitable draft and the Class A can be further designed into plastic component and manufactured.

Figure12: Draft Analysis on Base Bracket Class A.

Step 4:Class B Surface Creation:

• Extract the relevant surfaces from Class-A surface which excludes the Fillets and holes. One example of extract is shown in the Figure 13

Figure13: Extract of surfaces from Class A.

• Use Untrim Command to untrim the surfaces and create a natural extend. It is shown in Figure 14

Figure14: Untrim of Extracted Surfaces.

• Extrapolate the untrimmed surfaces so that each of them intersects clearly and pass way forward than the Class A which will help in its trimming in further operations.
• The operation is shown in Figure 15.

Figure15: Extrapolated Surfaces created from Untrimmed Surfaces.

• Trim the extra surfaces to obtain similar extended surfaces as in Class A.
• The operation performed on the Ribs of the Bracket are shown in Figure 16.

Figure16: Trimming of Extrapolated Surfaces.

• Offset the obtained Trim to the value of required Thickness of the Part. In this case of Bracket, the thickness value is 2.5 mm.
• The offset faces are the part of B-Surface. The operation is shown in Figure 17.

Figure17: Offset highlighted showing the start of B surface creation.

• Do The Similar operations namely : Extract, Untrim, Extrapolate, Trim, Offset for the other faces from the Class A excluding the Fillets.
• The operation is shown in Figure 18.

Figure18: Offset highlighted showing the start of B surface creation.

• Trim the extra surfaces such that it crates similar surface as Class A. The operation is shown in Figure 19.

Figure19: Output after trimming extra surfaces.

• Create Fillets while maintaining the part thickness between the Class A and Class b surfaces.
• The Value of Fillet is given by the equation (B surface fillet = A surface fillet + - Thickness of the Part).
• The sign + or – must be decided so as to maintain the part thickness which in this case is 2.5mm.
• The operation is shown in Figure 20.

Figure20: Fillets to be created as per the Part Thickness (in this case 2.5mm).

• The fillet values can also be obtained by maintaining the thickness in the intersection sketch of the two surfaces as shown in Figure 21.

Figure21: Fillets to be created as per the Part Thickness using intersect.

• The obtained surface is the required Class B Surface highlighted in Blue as shown in Figure22.

Figure22: Class B surface of Base Bracket highlighted in Blue.

Step 5: Class C Surface Creation:

• Use Boundary option to extract the outer boundary of the Class A surface as shown in Figure 23.

Figure23: Extracted Boundary from Class A.

• Use Sweep Operation to extrude the boundary using the Draft direction option and selecting the Tooling Axis as the direction.
• Use draft angle as 3º and Sweep it well beyond the B surface as shown in Figure 24

Figure24: Sweep operation with draft direction and draft angle.

• Use similar options of Boundary extract and Sweep for the holes on Class A as shown in Figure 25

Figure25: Sweep operation for remaining Class C surfaces .

• Join all the Swept surfaces along with the Class A Surface as shown in Figure 26.

Figure26: Join Operation between Class A and Class C surfaces.

• Trim the obtained Joined surfaces with the Class B surface to obtain the final enclosed Surface for the Bracket shown in Figure 27.

Figure27: Final Closed Surface.

• Switch to Part Design Workbench and use Closed Surface option to create a Solid Part from the final obtained surfaces.
• The part created is the required Base Bracket designed from a Class A.
• The obtained solid Part is shown in Figure 28.

Figure28: Final Closed Surface converted to Solid Part.

Conclusion:

The Design of Plastic Component - Base Bracket from Class A is completed using concepts of Draft angle, Draft Analysis, Tooling Axis, Class B and Class C creation and converting Closed surface to a solid part.