Door Arm Rest Design using Class A as an Input

DOOR ARM REST DESIGN FROM CLASS A AS INPUT - AUTOMOTIVE PLASTIC COMPONENT

This report demonstrates the design of a Door Arm Rest an Automotive Plastic Component using industry principles such as Draft angles, Draft Analysis, Determining and implementing of Tooling Axis, Side Core Axis, Class A, Class B, Class C surfaces and Maintaining the Thickness of the part at 2.5mm. The component is designed using Surface Design and Solid Design techniques in CAD package CATIA V5.

Figure1: Door Arm Rest

Some terms to defined before Designing are the following:

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.

Figure2: Draft Angles

Tooling Axis:

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

Figure3: Tooling Axis

Side Core Axis:

• Some features or walls are difficult to clear by the Main tooling axis itself.
• These features may have negative draft angle or are a part of a sub feature which is an important part and cannot be ignored.
• These features need a different tooling direction for clearing the part in the mold.
• This tooling direction is called Side Core Axis which often tends to clear the sub features or walls at different angle with rwspect to the Main Axis.
• The following part requires 2 Side Core Axis due to clearance requirements as shown in Figure 4.

Figure4: Side Core Axes for Arm Rest

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.
• Draft analysis should be passed in order to make the plastic component manufacturable.
• Since it is the factor which decides perfect the ejection of part from the Mold.
• Acceptable draft angles are used as range for draft analysis.
• Acceptable ranges are decided before planning to manufacture the product and it may vary from product to product.

Figure5: Draft Analysis on Class A of Arm Rest along Main Tooling Axis.

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.
• It is basically the thickened surface between both A and B surface.
• Below Figure 6 shows class A,B and C surfaces: Blue surface is Class A, Purple surface is Class B and Red surfaces are Class C surfaces.

Figure6: Depiction of Class A, Class B and Class C surfaces

DOOR ARM REST DESIGN:

Procedure to be followed:

Figure7: Procedure for designing Arm Rest

Step 1: Design Check (Pre Check) on received Class A:

• After receiving the Class-A for the Coin Holder check for the connectivity of the surfaces on it.
• Use boundary operation and disassemble.
• Check for discontinuous Surfaces.
• The check is over if any discontinuities do not arise.

Step 2: Determine the Tooling Axis:

• Identify faces which can potentially create problems while manufacturing.
• Surfaces at angles along a desired Tooling Axis.
• Extract a plain surface and create a point on its center for Axis system creation.
• In this case Z axis can be a possible Tooling Axis Direction.
• The highlighted Cyan faces can be a problem in terms of draft analysis along this axis Figure 8 demonstrates the step

Figure8: Creation of Dummy Tooling Axis for Reference.

• Create a sketch on the XZ plane of the Axis System and use Intersect 3D to plane option to get the sketch as shown in Figure 9.
• The left Wall is at a negative angle with respect to the proposed Tooling Axis.
• The Right Wall can be Cleared using the Main Axis so Create a Line at an Angle greater than the accepted angle for draft analysis passing through the Origin.
• This Line is the Tool Clearing Axis for X.

Figure9: Tool Clearing for X.

• Do similar steps for the YZ plane shown in Figure 10.
• The tooling created here is Y tooling.

Figure10: Tool Clearing for Y.

• Use Bisecting Line option to Bisect both the tool Clearing Axes and Pass the Bisecting Line through the Origin.
• This Line Created is the Main Tooling Axis as shown in Figure11.

Figure11: Main Tooling Axis.

Step 3: Draft Analysis on Class A and Possible Creation of Side Tooling Axis:

• 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.
• The input Class A also has walls having negative angle to the tooling axis.
• This can be an issue while manufacturing the component as it would be very difficult to draw from the molds.
• So, this design needs to consider Various Side Tooling Options.

Figure12: Draft Analysis along Main Tooling Axis.

Creation of Side Core Axis:

• To Clear the Bin Wall a Side Tooling Aling Y can be considered as a possibility.
• Create a Line Along Y axis through a point on the bin Surface as shown.
• Perform Draft Analysis through this axis.
• If Clearance not achieved, orient the compass as per requirement.
• When Clearance is achieved that is when the green range occurs on the required Bin Wall, Lock the compass orientation.
• Now change the Line direction to the Compass Direction.
• This is the Required Side Tooling Axis for the Bin Wall.

Figure13: Draft Analysis along Side Core Axis 1.

• Similar Tooling axis is to be created for the arm rest surface wall using similar procedure but in X direction
• The Tooling Axis for this is shown in Fig14

Figure14: Draft Analysis along Side Core Axis 2.

• The Draft Analysis and Tooling Axes Creation stage is over and can be proceeded to B Surface Creation

Step 4: Class B Surface Creation:

• Extract The Class A Surfaces for example the Bin Base.
• Untrim it to obtain a continuous surface.
• Extrapolate the obtained surface to a value such that it passes the class A bin.
• Offset the Surface to value 2.5 on B Side which is downwards in this case as shown in Purple in Figure15.

Figure15: Base Extract and Class B Surface operations.

• Perform Similar Operations for All the Bin Walls and Obtain the Offset and trim it with the Base To obtain Bin on Class B Side Surface as shown in Figure16.

Figure16: B surface for Bin.

• Similar operations to be performed on the Arm rest face and the walls as shown in figure17.

Figure17: B surface for Arm Rest Face Side Wall and Bin.

• Use Similar options for Side Walls and Upper Surface.
• Use Extrapolate option only on required Direction to obtain the Surface trims as shown in Figure 18.
• Use Fillets so as to maintain Wall Thickness to 2.5mm
• Create Fillets while maintaining the part thickness of 2.5mm 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 18.

Figure18: B surface for Arm Rest Face Side Wall with Fillets.

• To create a Connecting Surface for the Class B Create a Axis System on the Extract Point as shown in Figure 19.
• Intersect the Class A with the YZ plane.
• Create a Line Tangent to the Fillets on the Intersection.

Figure19: Sketch for Connecter at B surface for Arm Rest Face and Side Wall.

• Use Sweep operation along the Line for the Smooth Curve.
• The obtained sweep is shown in Figure 20.

Figure20: Connecter Sweep for B surface.

• Offset the Connector to 2.5mm inside
• Now untrim the upper connecter surface and split so as to use Blend option.
• Blend and Join the 2 surfaces to create a full connector as shown in Figure 21.

Figure21: Connecter join for B surface.

• Trim the Connector with other pre created B surfaces.
• Make adjustments as per requirement.
• The obtained Surface should match as per Figure22.

Figure22: B surface Trim.

• Create appropriate Fillets as per the rules.
• Create features aesthetic Ribs on B Surface.
• Create the Hole in Bin offseted to that on the Class A Surface.
• The Final B Surface Obtained is shown in Figure 23.

Figure23: Final Class B surface .

Step 5: Class C Surface Creation:

• Extract Class A boundary and convert to Smooth Curve.
• Sweep this curve at 90 degress to the Class A.
• Sweep value should be greater than the Thickness required for Part.
• The obtained sweep is shown in Figure 24.

Figure23: Class C surface creation .

• Join the Class A Surface with the Class C Surface to create a single surface entity.
• The obtained surface is shown in Figure 24.

Figure24: Class A and C surface Trim.

Step 6: Enclosed Surface Creation:

• Trim the obtained surface with Class B surface.
• Make Slight Adjustments by extrapolating small values and trimming to obtain a final Enclosed Surface
• The Final Enclosed Surface Obtained is shown in Figure 25.

Figure25: Class A,B and C Join making the final Enclosed surface of Arm Rest.

Step 7: Perform Draft Analysis on Final Enclosed Surface:

• Change View Mode Customization to Material mode
• Go to INSERT ---> ANALYSIS ---> FEATURE DRAFT ANALYSIS and move the compass to the Main Tooling Axis.
• Change the range to 2.9º. Apply the settings as Shown in Figure 26.
• Perform Draft Analysis along other Side Tooling Axes.
• Check for Tool Clearing all the required faces on respective Tooling Axis as shown in Figure 27 and 28.

Figure26: Draft Analysis on Final Enclosed surface along Main Tooling Axis.

Figure27: Draft Analysis on Final Enclosed surface along Side Core Tooling Axis1.

Figure28: Draft Analysis on Final Enclosed surface along Side Core Tooling Axis2.

Step 8: Create a Closed Body Part from 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 Arm Rest Component designed from a Class A.
• The obtained solid Part is shown in Figure 29.

Figure29: Door Arm Rest Closed Surface Part- Solid.

Other Views:

Conclusion:

The Design of Automotive Plastic Component – Door Arm Rest from Class A as input is completed using concepts of Draft angles, Draft Analysis, Determining and implementing of Tooling Axis, Side Core Axis, Class A, Class B, Class C surfaces and Maintaining the Thickness of the part at 2.5mm and finally converting the enclosed Surface into solid Part.