Design of backdoor

Aim: To design Back door of car using the surface.

Introduction

It is hinged board or door at the rear the vehicle that can be moved up or during loading and unloading.

Back door is also referred as 'Hatch' or 'Tailgate' or 'Trunk-lid' or 'Boot-Door'. It's the rearmost part of the vehicle which gives access to the trunk and has mounts for tail lamps, hinges, wiper motor, gas stay etc.

Function 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 is mounted on it.

Types of Backdoor

• Top mounted: These types of back doors are used in a station wagon, pickup truck and some other sports utility vehicles. Today's modern cars use top mounted tailgating systems which are both powered and also gas stay supported. These tailgates provide greater rear visibility compare to other types.

• Side mounted: It is generally used in van type MPVs, SUVs, pickup truck vehicles. These side mounted door / barn door are a lot easier compared to top mounted ones. These doors are generally small in size and can be easily accessed. These doors are more convenient for the goods handlers carrying lengthy objects, long pipes where one door is kept open. It also saves accidental head impacts. It is a lot easier to load / unload things in vehicles having side-mounted doors.

• Bottom mounted: Bottom type or split type back door system is used in estate type cars, pickup trucks and some luxury SUVs. These are most convenient type of backdoor in which the bottom split is used as a support or even as a table for the touring people.

Material Selection: The selection of material plays a major role in determining the overall weight of the back door. The common materials used are Steel and Aluminum. The density of Steel is higher than Aluminum thus adding more weight to the door. The material used in the manufacturing of the backdoor depends upon the budget of the car, most low-cost vehicles use steel as it is cheaper compared to Aluminum. Another aspect to be noted is no need of reinforcements and the overall weight of the car is reduced by approximately 5-10kg thus increasing the performance of the vehicle. Also, the draft angle for casted parts is around 3 degrees while for deep drawn parts is 7-10 degrees which helps in increasing the luggage / cabin space.

Fig: Shows Possible number of parts in Aluminium (Backdoor)

Fig: Shows possible number of parts in Steel (Backdoor)

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 folder over another part in order to achieve a tight fit. Usually hemming operation 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 operation to join two sheet metal part (outer and inner) together. Typical part for this type of assembly is hood door trunk lids and fenders.

Corner Relief:

During hemming process multiple rollers are used which causes vibrations and stress concentrations at the same location 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 remove). But most of the time that is not the case at corners so avoid that situation 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 and etc.

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

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. Here embosses (Y/V emboss) are designed in such a way so as to direct them towards the outer end of the backdoor as shown in the image.

Sealing Flange:

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

Reinforcement is generally placed to increase the strength of the panel in localized are as 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 part.

• Hinge reinforcements: Hinge is a pivot mechanism which supports total weigh of backdoor. So, reinforcements are added for strength and rigidity.

• Gas stay reinforcement: Gas stay is used to lift the backdoor automatically and used to support the backdoor in its open position. It can be of two types semi-automated and fully automated.

• Wiper Moter 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 motor, inner panel gets deformed which results in loss of strength and 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 emboss is created for placement 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 and Striker / Latch Trajectory:

Backdoor is the swinging part which rotates about hinge axis. The position orientation of latch striker is important for proper opening and closing of the backdoor. The positioning is done by appropriate with respect to inner panel.

Design considerations:

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

Assembly:

In this assembly Top-down approach was followed where in from given styling surface the inner panel was created using various tools/commanders in NX. After that other parts were modelled as per the requirements one by one.

Analysis: Draft analysis

Every part which is manufactured from press tool or other forming tools require some positive draft on the face which are parallel to the tooling direction. If we don't give draft (Zero draft) or give negative draft 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 gives insights about the critical zones, so that appropriate changes in the design can be made if needed.