Week 9 - Project - Master Section Development

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

Aim:

To design plastic B pillar along with B side features using provided class A surface and master sections.

Objectives:

• To create tooling axis for the component.

• To design class B and C surface from given class A.

• Add B side features like Dog house, 4-way locator and Ribs to component.

• Perform draft analysis on the component.

Introduction:

The B pillar is the vertical support in the car located between front door and rear door. It is a sheet metal part attached to the BIW of the car. B pillar trim is the plastic component, which acts as a cover for the B pillar to make it look aesthetically good. B pillar trim is manufactured using injection molding techniques and attached to the BIW with plastic attachment features.

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 – In this design we maintained 50mm space between two ribs as per the Master section given.

  • 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*

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

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

• B = 3 to 6mm, C <= 0.4 x T

• 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.

• At a time we have maintain the given master section for matching.

Tooling Axis:

• Tooling Axis is the direction in which the Mold opens and closes.

• Part Drafts are generally given along this 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 respect to the Main Axis.

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

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 color codes.

• It highlights the regions of draft using colors.

• 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.

Class- A Surface:

• In automotive design, a class A surface is any of a set of free-form 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 shows class A, B and C surfaces: Purple surface is Class A, Brown surface is Class B and Red surfaces are 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 Flashiness

  • 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 decide the Fixation Strategy, Feasibility, Clearance and the Packaging of the component before working on 3D.

• Figure shows the Master Sections provided as input along with Class A for the B pillar Cover.

B-PILLAR COVER RH DESIGN:

Procedure to be followed:

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.

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.

• The Figure 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.

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 shows the Class B surface.

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

Step 5: Flange creation using master section

To create flanges on the edge of the class A surface, area near the edge is extracted and its boundary is extracted.

• This extracted edge is then swept using sweep command. In sweep command, with reference surface option is used and extracted surface is given as support.

• The intersection of swept surface and plane at master section is taken to observe if the flange is proper or not. After some changes in the sweep angle, the sweep surface is offset to match with the master section.

• Similarly flange at the other edge is created and the both the flanges are trimmed with the class A surface to get final output of class A surface with flanges. Fillet is applied at the sharp intersection of class A and flanges.

  Maintain the length and sweep angles as per the Master Section.

Step 5: Class C Surface Creation:

• Extract Class A boundary and convert to Smooth Curve.

• Sweep this curve at 3 degrees 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.

• Trim the above product with Class B Surface.

• The obtained product is the Enclosed Surface for the B Pillar Cover as shown.

•  Switch to Part Design workbench.

• Use Closed Surface Command to Create a Solid Part for the RH B Pillar Cover. 

Step 6: B side Feature Creation:

• B side Features such 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 do 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.

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.

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

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

Step 8- Draft Analysis on Class A surface and B Pillar Solid body:

Screenshots of the class A surface model along tooling axis with the proper color code of the draft angle in various orientations (Isometric, Front, Side, and Top View)

Screenshots of the solid body model along tooling axis with the proper color code of the draft angle in various orientations (Isometric, Front, Side, and Top View)

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 strengthening and Locating features on B side.