BIW EXTERIOR TRIM : PASSENGER CAR HOOD DESIGN

PASSENGER CAR HOOD DESIGN BIW EXTERIOR MODEL

OBJECTIVE:

The objectives of this project are

• To design Hood Outer and Inner panel along with suitable reinforcements by following the Master Section.
• Hood Styling Surface is used as Input and the model is developed in NX CAD along with suitable reinforcement structures
• Determine the Latch Trajectory Axis and Hinge Axis followed by assembly of the parts

DESIGN METHODOLGY

DESIGN REPORT

MASTER SECTION

The Master Section is the input which is a predefined sketch suitable for various design situations and it used to develop the hood model using the Hood Styling Surface.

Image 1) Master Section Data for Hood Panel Creation

Styling surfaces are aesthetic surfaces obtained from the CLASS A Styling Team and is used by the New Product Design team as a parent model to develop the Passenger Hood Model

Image 2) Styling Surface Hood Panel

DESIGN REQUIREMNTS

The Basic Design requirements for Hood design are

1. Opening Angle Situation
2. Emboss of Inner Panel
3. Hemming and Relief along the outer panel
4. Hinge Axis Definition and Latch and Striker Position
5. Stiffener Pad and Mastic Sealant for NVH Improvement
6. Positive Draft Angle for Deep drawn steel parts

PASSENGER CAR HOOD MODEL

• HOOD or Bonnet is an Exterior Trim that covers the Engine compartment and provides styling at the front of the car.
• It is one of the vital components that requires frontal crash requirements and pedestrian impact safety standards
• After understanding the requirements a top down design approach is followed to prepare the final model

Image 3) Hood Panel

The operations used to develop Hood Panels in NX are

• Surface Offset
• Offset Curve Face
• Trim Sheet
• Through Curves
• Law Extension
• Trim and Extend
• Trim Body
• Extend Sheet
• Edge and Face Blend
• Emboss
• Bridge Curve
• Mirror Geometry
• Sew
• Thicken

INNER PANEL

The inner panel of the hood is more vital which involves various engineering aspects and attachment features The Input Styling surface is validated for Surface discontinuities and using Surface operations in NX the Styling Surfaces is developed along the predefined master section data.

Inner Panel with emboss

Emboss

Emboss is an operation that adds stiffness to the part and are produced through deep drawn process. The Emboss are designed in such a way that it can dissipate the impact forces outward swiftly and also reduces the head impact of the pedestrians.

The requirement and strength of emboss are validated by the CAE Team and suitable cutout are provided to facilitate proper attachment to the outer panel.

For feasible deep drawn process the stamping direction is validated and a suitable draft angle is provided between the tool line and the part

Draft Analysis Along the YC Axis as draw Direction shows emboss have positive draft

OUTER PANEL

The outer panel is a styling surface where the edges of the panel are wrapped around the inner panel using hemming process.

HEMMING is a sheet metal operation where the edges of the part are wrapped around a surface using a roller.

Hemming is basically as an attachment procedure between two working surfaces by reducing the impact of the spot welds on the aesthetic surface.

A Corner relief is provided along the rear and front edges during the hemming to improve the quality of the Outer panel styling surfaces and reduces the intolerances and complexities

Rear Section relief and Front Section Relief

The Gap between Outer and Inner Panel are filled with Mastic Sealant point and holes are plugged of to achieve the water tightness

Section View of Hood Panels with Hemming

REINFORCEMENTS

When adding attachment and weight to the hood the strength of that area is impacted locally and to compensate that reinforcement structures are added.

HINGES

To actuate the movement of opening and closing of hood hinges are provided along with reinforcements

LATCH and STRIKER POINT

The Latch and striker points facilitate the fixation of hood when closed and reinforcement are mounted impart strength on the Latch area

LATCH TRAJECTORY AXIS

For proper opening and closing of hood a trajectory axis is drawn to simulate the axis of movement of latch and striker situation.The design considerations for preparing trajectory axis are

• The latch and striker emboss is provided to accommodate the hook and latch
• The hinges are placed collinearly and a hinge axis is derived
• From the hinge axis and local striker point a trajectory circle is drawn and the striker point should be perpendicular to that axis.
• Upon achieving perpendicularity between the trajectory axis and striker point a local emboss is provided for the Hinge position.

Latch Trajectory Axis

This ensure the closing point of the latch and Striker and also forms the predecessor to provide latch closing point on the Engine Compartment

It also prevents withering of hood and seals the hood mounting perfectly during the closing situation

Thus it is one of the vital design consideration in Hood Design

MASTIC POINTS

Mastic Points are the areas where the mastic adhesive sealant is applied that bonds the Outer and Inner Panel. Mastic sealant provides cover for 80mm locally and provides proper Hem flange bonding and considerably improves the NVH factor

ASSEMBLY

Finally the parts are assembled providing various assembly constraints for the work part and restricting the degrees of freedom.

Front View of Hood Assembly

Hood Upon Closure Condition

TRUE SHADED IMAGES OF INNER PANEL AND OUTER PANEL

EURO NCAP SAFETY STANDARDS
EURO New Car Assessment Programme is a safety assessment center that evaluates the safety performance of the car. They evaluate potential risk of pedestrian impact in the event of collision with the car and provide rating for the new vehicles.

Hood is a part that reduces the risk of potential head injury during pedestrian impact and it is therefore vital to validate the Head Impact Zone for better NCAP assessment rating

Hoods are two types

• When the side reference line drawn from the ground intersects the CAR Hood is called Wrap Around Hood
• When the side reference line drawn from the ground intersects the CAR Fender it is called Inlaid hood

Side Reference Line

Wrap Around Distance determines the impact of Head, Pelvis and Leg of the pedestrian.

Based on these factors IMPACT ZONE are plotted for ADULT and CHILD and while packaging of components like Engine, Fuse Box, Control unit in the engine compartment an offset distance is provided from the Outer Panel and the interference of IMPACT DEFROMATION ZONE should be lower for better NCAP rating

Packaging of Components with clearance from bonnet

Taking this into criteria Hood is designed to have much greener head injury criteria.

Thus for a better Hood Design the following factors are considered

1. Opening Angle Situation
2. Gas Stay Mounting
3. Latch Trajectory Axis
4. Hinge Axis
5. Deformable Brackets
6. Mastic Points for reduced NVH
7. EURO NCAP Assessments

CONCLUSION

Passenger Vehicle Hood is designed in NX by following the master section data within the underlining design parameters by validating the Latch Trajectory and also adhering to the NCAP regulation standards.