B Pillar RH Cover - Creating CAD Part from Master Sections

Automotive Plastic Component: B Pillar RH Cover - Creating CAD Part from Master Sections and Class A as Input

This report demonstrates the design of B Pillar RH Cover and Automotive Plastic Component. The component has been designed from the Class A and Master Sections as Input. Industry principles such as Draft angles, Draft Analysis, Determining and implementing of Tooling Axis, Side Core or Lifter Axis, Class A, Class B, Class C surfaces and Maintaining the Thickness of the part at 2mm. B side features such as Ribs for strength of the cover, Dog Houses for hosting the locators and 4 Way Locators are also incorporated in the design. The overall design follows the Master Sections to create B surface and B side Features. The component is designed using Generative Shape (Surface) Design and Solid Design techniques in CAD package CATIA V5.

Design Rules for Plastic Components:

• Nominal wall thickness of a plastic component is 3mm. For a strength component such as Instrument Panel, Bumper , door trims the values of thickness are larger than 4mm. For example, Thickness of IP may lie between 5-6mm, for Bumper it may be 6-8mm.
• For smaller components the thickness may be between 2-3mm as in this case of Coin Holder.
• Deviation of `30% from the wall thickness is allowed with gradual increase or decrease to avoid any plastic defects while manufacturing.
• For example: A component having wall thickness 3mm should deviate within the range of 2.1 mm minimum and 3.9 mm maximum.
• Ideal draft angle should be 3º for the overall component along for the faces lying in the tooling direction.

Ribs Design:

• Rib is a B-Side Engineering feature which provides a means to economical Stiffness and Strength in Plastic Molded parts without increase in overall wall thickness.
• Rib also facilitates locating and arresting components of an assembly.
• It provides alignment in Mating Parts.
• It acts as Stop and Guides for Mechanisms.

Guidelines For Rib Design:

• Proper Rib design is necessary to avoid plastic defects which involves the following critical issues that needs to be considered which are:
  • Draft angle
  • Rib Thickness at base
  • Thickness at Top of the Rib
  • Rib Height
  • Location of Ribs and Spacing between 2 Ribs
  • Optional Radius or Fillet Radius
  • Rib Root Diameter
  • Quantity of Ribs
  • Manufacturability
• These guidelines are separate for Show and No – show surfaces.
• The below table gives specific values for both cases.
• Consider ‘t’ as wall thickness of base for component.

Table1: Rib Design Guidelines*

Rib Specification	No-Show Surfaces	Show Surfaces
Draft Angle	Min 0.5º per side, Recommended 1.5º per side	0.5º per side
Rib Thickness at Base (w)	0.5 * t	0.4 * t
Rib Thickness at top	Min 0.75mm	Min 0.75mm
Rib Height (y)	Max 5*t	Max 5*t
Spacing Between 2 Ribs (x)	2*t	2*t
Fillet Radius (r)	0.125*t	Max 0.25mm
Rib Root Dia	± 30 % of t	± 30 % of t
y/x ratio for multiple ribs	3:1 2:1 preferred	3:1 2:1 preferred

*some guidelines may change as per the OEM or the industry requirements.

Figure2: Rib Design Nomenclature

Dog House Design:

• Dog House is a B surface engineering feature used to avoid sink marks primarily.
• Its secondary use is to place an offset B side feature such as boss or locators on it.
• The cross section for dog house near the B surface is less and then increases rapidly.
• It can be extended to have another feature on it.
• Also, if Bosses are perpendicular to the B surface Dog houses are used to place it.

Guidelines and Design rules for Dog House:

• Proper Dog House design is necessary to avoid plastic defects which involves the following critical sections and design rules that needs to be considered:
  • Draft angle
  • Doghouse Walls Section
  • Part Surface
  • Bottom of Doghouse
  • Wall Thickness
  • Height and Width

Figure3: Dog house wall section.

• B = 3 to 6mm, C <= 0.4 x T

Figure4: Dog house width.

• Dog House width (X) = 12mm Minimum
• Height (H) = 6mm Minimum
• X/H = 3:1 or 2:1
• Draft of 5º minimum to draw direction, if slide is considered then this can change to 3º minimum.

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.

Figure5: Draft Angles

Tooling Axis:

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

Figure6: Tooling Axis

Side Core Axis or Lifter:

• 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 Pillar Cover requires a Lifter Axis for the clearinf of the Dog Houses which have internal walls that cannot be cleared along main tooling direction.

Figure7: Lifter Axis for Dog House Clearing

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.

Figure8: Draft Analysis on Class A of B-Pillar RH Cover 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 9 shows class A,B and C surfaces: Purple surface is Class A, Brown surface is Class B and Red surfaces are Class C surfaces.

Figure9: Depiction of Class A, Class B and Class C surfaces

Master Sections:

• During the period of Vehicle Concept Design, the evaluation of master section performance mostly depends upon reference vehicle data and experience of structural design engineers.
• Master Sections are created at the Concept Validation and Feasibility Check Stages.
• Each Sub Assembly is provided with a section view of the member which it is going to assemble into.
• These Sections help create a part from Class A of the sub assembly components.
• It helps decide the thickness of the Component, the location and thickness of B side features such as Dog House, Locators.
• Things decided while creating master sections are:
  • Gaps and Flsuhness
  • The Feasibility of all components and its environment.
  • Feasibility such as Clearances with Driver/Passenger, Clearances with Mechanisms and Clearance with the environment.
  • Fixation Strategy of components.
• Once the Master Section is developed it is forwarded to the NPD team to work on 3D Part development.
• In short the Master Sections helps to dedcide the Fixation Strategy, Feasibility, Clearance and the Packagong of the component before working on 3D.
• Figure 10 shows the Master Sections provided as input along with Class A for the B pillar Cover.

Figure10: Master Sections highlighted in Yellow and Green along with Class A

B-PILLAR COVER RH DESIGN:

Procedure to be followed:

Figure11: Procedure for Design

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

• After receiving the Class-A for the Pillar Cover 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 Y axis can be a possible Tooling Axis Direction.
• The tooling Axis is created from the Master Sections references.
• Extract the Master Section B features on a plane.
• Use Bisecting Lines and Choose the Main tooling Axis which clears the Draft Analysis.
• In this case the Tool Clearing for X passes the Draft Analysis.

Figure12: Extracts from the Master Sections for the Tooling Axis Creation

Figure13: Main Tooling Axis

Step 3: Draft Analysis for Class A:

• 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 2.9º.
• Apply the settings as Shown in Figure 14.

Figure14: Draft Analysis along Main Tooling Axis.

• The Figure15 shows areas which are having around 1º of drafts along the Main tooling Direction
• This is not much of a problem as minimum required draft is 0.5º and the area of consideration is too small.

Figure15: Analysis along Main Tooling Axis showing walls having less draft anle.

Step 4: Class B Surface Creation:

• Offset the Class A Surface with the Thickness value of 2mm as per the Master Section.
• For faces which do not offset use Multi Section Surfaces and Splines to fill those gaps.
• Figure 16 shows the Class B surfce.

Figure16: Class B Surface in Beige colour.

• Create Flanges as per the Master Sections on Both Class A and Class B surfaces using Sweep.

• Step 4: Class B Surface Creation:

  Maitain the length and sweep angles as per the Master Section

Figure17: Class A surface with Flange.

Step 5: Class C Surface Creation:

• Extract Class A boundary and convert to Smooth Curve.
• Sweep this curve at 3 degress with respect to the Draft Direction.
• Sweep value should be greater than the Thickness required for Part.
• Join the Class A and C.
• The obtained product is shown in Figure 18.

Figure18: Class C surface with Class A.

• Trim the above product with Class B Surface.
• The obtained product is the Enclosed Surface for the B Pillar Cover as shown in figure19
• Switch to Part Design workbench.
• Use Closed Surface Command to Create a Solid Part for the RH B Pillar Cover.

Figure19: Enclosed Surface for B Pillar Cover.

Step 6: B side Feature Creation:

• B side Features suc as Dog House and the 4 way locators to be designed following the Master Section.
• Maintain the Wall thickness with the reference as the Master Section.
• Make sure the Features are drafted along the Main tooling axis.
• Creation for Side /Lifter axis comes into play here.
• Create a Lifter Axis along the X direction and dro draft analysis on the Features.
• Create Ribs 50mm apart from each other following the Rib Design rules.
• Apply appropriate Fillets and Chamfers.
• The Final Component obtained is shown in Figure 20.

Figure20: Final Component with all the B side Features.

Step 7: Draft Analysis for Final Component:

• 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 0.5º.
• Apply the settings as Shown in Figure 21.

Figure21: Draft Analysis on Final component along Main Tooling Axis.

• Reverse the draft analysis direction for the Clearing of Class A Surface and the C surface walls as shown in Figure 22.

Figure22: Draft Analysis on Final component along Main Tooling Axis for Class A and C surfaces.

• Perform Draft Analysis along the Lifter Axis to check clearance for the inner Walls of the Dog House as shown in Figure 23.

Figure23: Draft Analysis on Doghouse inner Walls.

Figure24: Draft Analysis on Doghouse 2 inner Walls.

Conclusion:

The Design of Automotive Plastic Component – B Pillar Cover Class A and Master Sections 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 2mm and finally converting the enclosed Surface into solid Part along with strenthening and Locating features on B side.