Design of backdoor

BACK DOOR DESIGN

AIM

The main objective of this project is to learn the basics of the BIW (Body in white) through designing the Back Door also referred to as Tail Gate of a car by considering all the Protocols and Standards of BIW Design.

INTRDUCTION

Body in white (BIW) is the stage in automobile manufacturing in which a car body's frame has been joined together, that is before painting and before the motor, chassis sub-assemblies, or trim (glass, door locks/handles, seats, upholstery, electronics, etc.) have been integrated into the structure. Assembly involves different techniques such as welding (spot, MIG/MAG), riveting, clinching, bonding and laser brazing,

Body in white refers to the stage in automotive manufacturing when a car's sheet metal components are welded together which include

• Hood
• Trunk lid
• Roof
• Front and rear fender
• Sidewall
• Side Doors
• TailGate (Back Door)

The term BIW derives from manufacturing practices before steel unibody monocoques-when automobile bodies were made by outside firms on a separate chassis with an engine, suspension, and fenders attached The manufacturers built or purchased wooden bodies (with thin, nonstructural metal sheets on the outside) to bolt onto the frame. The bodies were painted white prior to the final color.

In-car design, the "body in white" phase is where the final contours of the car body are worked out, in preparation for the ordering of the expensive production stamping die. Extensive computer simulations of crash-worthiness, manufacturability, and automotive aerodynamics are required before a clay model from the design studio can be converted into a body in white ready for production.

BACK DOOR

• The Tailgate or the Back door of a car is the door at the rear end of the car, which provides access to the luggage compartment.
• In most of cars, these back doors are operated with the help of gas stay which in turn reduces the effort required to lift open the door.
• Some luxury cars have a powered backdoor that is driven by an electric motor. This kind of system is also called a powered trunk lid system.
• The Back door is also responsible for dissipating the force created while rear collisions, away from coming inside the car.
• The back door is usually made of an inner panel and 2 or 3 separate inner panels joined together by hemming or spot welding wherever necessary.
• The material used for door manufacturing depends on the budget and performance of the vehicle. Mostly Steel will be used in low-budget vehicle which makes the manufacturing process easy and cost-efficient since it follows the Stamping of Moulds.
• But Aluminium was used for high-budget cars since it increases the manufacturing cost since the Aluminium parts will be mostly die-casted hence it has a variable thickness which reduces the requirement of reinforcements but improves the performance of the car rapidly since Aluminium is not heavy as steel.

The below table gives a comparative information about Steel and Aluminium.

Functions of the Backdoor:

• It protects the occupants and luggage from getting damaged.
• It houses assembly of tail lamps, wiper motor and other components.
• It serves as entry and exit point for luggage/passengers.
• It houses windshield glass which helps in rear visibility.
• Parking sensors, rear view camera are mounted on it.

Design considerations

• Rear visibility
• Boot space & accessibility
• Sealing Flange
• Support structures like hinges and gas stays.
• Mounts for wiper motor, tail lamps and other wiring loops
• Rear crash regulations
• Joining techniques such as hemming, spot welding etc.
• Gap and flush requirements

The back door design has

1. Back door Outer Panel Upper
2. Back door Outer Panel Lower
3. Back door Inner Panel
4. Sealing Flange Development
5. Reinforcement Mountings

Components to be designed:

1) Outer panel

It is the outer styling skin surface of the back door. It is provided with multistage coatings which has property of scratch resistance, rust prevention etc.

It is wrapped around the inner panel through hemming process.

Hemming: Hemming is a forming operation in which the edges of the sheet are folded over another part in order to achieve a tight fit. Usually hemming operations are used to connect parts together, to improve the appearance of a part and to reinforce part edges.

In car part production, hemming is used in assembly as a secondary operation after deep drawing, trimming and flanging operations to join two sheet metal parts (outer and inner) together. Typical parts for this type of assembly are hoods, doors, trunk lids and fenders.

Corner Relief: During hemming process multiple rollers are used which causes vibrations and stress concentrations at the some locations due to which design intent of the panel shouldn’t be changed (i.e the panel should spring back to original formation after roller is removed). But most of the time that is not the case at corners so to avoid that situtaion reliefs are provided at corners.

2) Inner Panel

The inner panel is one of the most important components of backdoor assembly.

It provides strength and helps in dissipation of forces outside the part with the help of embosses made on it. 

It helps in mounting of accessories like hinges, wiper motor, gas stay, latch & striker etc.

The inner panel is produced by either casting or through Deep drawing process.

TOOLING AXIS

Mostly aluminum part is casted, steel inner panel are manufactured by deep drawing process.

If deep drawn, there must be a tooling direction through which that panel must be deep drawn.

Embosses: It is provided to give extra strength to sheet metal panel and to help dissipate forces/energy towards a specific direction.

Sealing Flange: It is that section of the inner panel which enables the backdoor to sit over the rear body side of the car.

The sealing flange will rest over the sealing rubber mounted on the car back side as shown in the above back door image. 

Its design is important as it decides how a backdoor is mounted on the rear body side of the car and also restricts any external matter to enter inside the vehicle.

Mastic Sealants:  As we know the outer panel gives the aesthetic look to the vehicle, so we can’t use bolting or spot welding or any other joining technique which damages the aesthetic looks of the panel.

Thus mastic sealants are used for joining the Inner panel elements with outer panel.

A maximum gap of 5mm is to be maintained in between parts where mastics are to be applied.

Reinforcements

Reinforcements are generally placed to increase the strength of the panel in localized areas which is relatively small compared to sheet metal parts for absorbing loads imposed there on and dissipating such loads throughout the panel assembly to help maintain rigidity of the panel assembly.

The reinforcements are usually 1-1.5mm thick as opposed to 0.75mm thick panel used in this part.

• Hinge reinforcements

Hinge is a pivot mechanism which supports the total weight of the backdoor. So reinforcements are added for strength and rigidity.

• Gas stay reinforcements

Gas stay is used to lift the backdoor automatically and is used to support the backdoor in its open position. It can be of Two types Semi-automated and fully automated.

• Wiper Motor reinforcement

Wiper is used to wipeout the dust particles/water on the windshield glass which affects the rear visibility. It is actuated by the motor which is mounted on the inner panel. Due to vibration and heat of the motor, inner panel gets deformed which results in loss of strength & rigidity of the panel thus leading to poor NVH. So reinforcement is provided to avoid these conditions.

• Latch and Striker reinforcement

In the bottom part of the panel emboss is created for placemnet of latch and striker assembly. There is over slamming and abusing while opening and closing of the door, thus arising the need for reinforcement to be placed.

Positioning of Latch & striker/ Latch Trajectory:

Backdoor is the swinging part which rotates about hinge axis.  

The position and orientation of latch striker is important for proper opening and closing of the backdoor.

The positioning is done by sketching a circle with center as the axis of the hinge which passes through the striker at appropriate position with respect to inner panel. 

FINAL ASSEMBLY

From the given styling surface outer upper panel, outer lower panel and inner panel are designed by providing proper emboss, reinforcement, etc.

Analysis: Draft analysis

• Every part which is manufactured from press tool or other forming tools require some positive draft on the faces which are parallel to the tooling direction.
• If we don’t give draft (Zero Draft) or give negative drafts on these faces it will stick to the mould surface and when the mould is in motion it will destroy surface thus reducing its surface finish.
• Thus Draft Analysis command enables us to detect if the part we have drafted can be easily ejected or not.
• It also give insights about the critical zones, so that appropriate changes in the design can be made if needed.

CONCLUSION:

Thus the Body In the White Back Door of a car was designed following the engineering standards and also the deep drawing requirements were satisfied.