Design of backdoor

BACKDOOR DESIGN

OBJECTIVE:

To develop the car backdoor by using given surface method by automotive sheet metal design.

BACKDOOR:

A backdoor is located at the rear end of the car which gives access for the boot space to store luggage., spare wheel etc. These doors can be operated manually or electric powered. In most of the luxury car is electric powered.

Back Door also called as 'Hatch', or 'Tailgate', 'Trunk-lid', 'Boot-door' is basically the 5 doors in a 4-door car. It's the rearmost part of the vehicle which gives access to the trunk and has mounts for wiper motor, tail lamps, hinges for supporting its weight in opened position, gas stays which help with its opening and keeping it open, radars and cameras etc.

A backdoor is usually made up of 1 inner panel and 2 or 3 outer panels joined together by hemming and spot welding wherever necessary.

The material used for backdoor varies as per the price range of the vehicle. Most of the low-cost vehicles use steel as it is cheaper and use parts that are stamped using moulds. These parts are usually of uniform thickness and reinforcements are added to improve local strength where needed. The premium vehicles use aluminium parts which are usually die casted and hence can have variable thickness. This removes the need of reinforcements hence keeping the weight even lower as aluminium parts are lighter in comparison. On the contrary aluminium is more expensive than steel hence its applications are limited in cheaper vehicle. Currently due to the extensive use of plastics trims and plastic parts the cost of aluminium is affordable for the manufacturers in all price brackets. Use of aluminium reduces the overall weight of the component and hence the vehicle improving its fuel efficiency.

REINFORCEMENTS:

As mentioned for local strengthening reinforcements are usually provided. Instead of making the whole panel off a thicker sheet, a thinner sheet with thicker local reinforcements wherever needed is preferred. The reinforcements are usually 1-1.5mm thick whereas the panel itself is 0.55- 1 mm thick.

Backdoor parts thickness value used in this design:

1. Outer panel         -    0.75 mm
2. Inner panel          -    0.75 mm
3. Reinforcements    -    1 mm

DESIGN CONSIDERATION FOR BACKDOOR INNER PANEL:

1. Rear Visibility.

2. Support structures like hinges and gas stays.

3. Mounts for wiper motor, tail lamps, and other wiring loops.

4. Fit and clearances.

5. Boot space

6.Rear crash regulations and requirements.

7.Sealing and its functions.

8.Joining techniques like hemming, spot welding etc.

TYPES OF DOOR OPENING:

• Top mounted door
• Side mounted door
• Bottom Door

COMPONENTS OF BACKDOOR:

• Inner panel
• Outer panel upper
• Outer panel lower
• Hinge reinforcement LH
• Hinge reinforcement RH
• Gas stay reinforcement LH
• Gas stay reinforcement RH 
• Wiper motor reinforcement
• Latch reinforcement
• Plastic trim

INNER PANEL:

Car backdoor inner panel is made of either steel or aluminium of the luxury car's inner panel are made of aluminium because of is properties. But the cost of aluminium is high.

	STEEL	ALUMINIUM
PARTS	Parts are stamped	Parts are casted
REINFORCEMENTS	Necessary	Can be neglected
THICKNESS	Constant	Can be Varied
COMPONENTS	More	Lessin number
WEIGHT	More	Lighter
COST	less in cost as compared to Aluminium	More as compared to Steel

Cost of aluminium is more as compared to steel. To easily understand, the cost of aluminium for inner panel is higher than the cost of all parts of backdoor together.

The embosses in the inner panel helps stiffening the inner panel and act as a local reinforcement. Another purpose of giving emboss is to distribute impact forces towards hinges.

from the above figure it is seen that the extension is for mastic sealant. In bottom of the panel, there is more gap between the inner panel and outer panel, to give strength and rigidity we should give those extensions and joined using mastic sealant.

And there should be a gap between the extension and the outer panel is minimum 5mm for mastic sealant.

Inner panel draw direction:

The inner panel is produced by either casting or through deep drawn method. Mostly aluminium panel is only casted, steel inner panel is made by deep drawn process only. If it is deep drawn, then there must be a tooling direction through which that direction the panel is deep drawn.

DRAFT ANALYSIS:

Draft analysis of inner panel is carried out through the tooling direction only. Only the green areas are acceptable and i worst cases yellow regions are accepted but the blue and red regions are never accepted.

The draft angle for aluminium casted panel is 2 to 3 degrees is enough but if it is steel then the angle is more than 7 degrees.

In this project i considered it as steel and used 7 degrees as draft angle and direction is chosen as tooling direction which is the deep drawn direction.

From the above fig. it is clear that all areas are in green colour so my design is passed draft analysis.

OUTTER PANEL UPPER AND LOWER:

Outer panel is the styling surface of the backdoor. Glass for rear visibility is mounted on the outer panel upper. Rear lights are also mounted on the outer panel upper.

Number plate is mounted on the outer panel lower. Spot welds and other manufacturing process should be done only on the inner panel because the outer panel is the styling surface which should not have any scratches or welds. Because it affects the style of the car which leads to bad impression on the customers.

HINGE REINFORCEMENT:

Hinges support the weight of the whole backdoor. Hence, the reinforcements must be provided there too. The emboss is designed in such a way that the hinges should operate freely. The hinge is connected to the rear roof of the car.

GAS STAY REINFORCEMENT:

Gas stay is used to lift the backdoor automatically. There are two types of gas stay one is semi-automated and another one is fully automated. In semi-automated type the back boor needs little force to lift after that it lift the backdoor automatically. In fully automated, it has power spindle which lift the backdoor automatically.

WIPER MOTOR REINFORCEMENT:

Wiper is used to wipeout the rain water falls on the backdoor window. It is actuated by the motor which is mounted on the inner panel. Due to the vibration and heat created by the motor. the inner panel where the motor is mounted gets deform and the parts attached to the motor starts to vibrate which leads to poor NVH. So, we have to provide the reinforcement where the wiper motor is placed.

HEMMING:

Hemming is the metalworking process in which a sheet metal edge is rolled into itself. The outer surface of the panel is rolled 2 or 3 times over the inner panel. In first roll the panel is bent slightly, and in the second roll the sheet metal is rolled even more and in the last roll it completely fits or covered the inner panel. There must be a 0.2mm gap between outer and inner panel in both upper and lower side.

Mounting Reinforcements

Final Back Door Design Assembly

CONCLUSION:

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