Design of backdoor

Aim: To design the Back door (Tail gate) of a car from the styling surface provided using NX CAD. The ultimate aim is to design the Back Door by following the Industrial Standards.

Introduction:

Back Door: The trunk (North American English) or boot (British English) of a car is the vehicle's main storage or cargo compartment, often a hatch at the rear of the vehicle. It is also called a tailgate.These doors can be operated manually or electric powered. In most of the luxury cars door is electrically operated. Ii involves a very major role during rear crash cases. so our design must be obey crash requirements. the institute of highway saftey analyse the diffrent accidents and they set a test to rate our car. so these rating depends uopn the design of the trunk and materials used, flanges, embosses, type of joining of inner panel and outer panel etc.

Parts of a Tail Gate:

• Outer Panel
• Outer panel Lower
• Inner panel
• Hinge Reinforcements
• Latch and Stricker Reinforcement
• Wiper Motor Reinforcement
• Gas Stay Reinforcements

All these parts are assembled together to obtain the back door of a car.

Types of Back Doors:

There are three types of back door based on the position of the Hinge axis. They are

• Top Mounted

• Side Mounted

• Bottom Mounted.

In our design we follow the Top Mounted Design where the Hinges are mounted on the top.

In our design we follow the Top Mounted Design where the Hinges are mounted on the top.

Design Considerations In Back Door Design:

The following are the Design considerations taken while designing the Back Door. They are,

• Visibility Criteria
• Force Dissipation
• Latch and Stricker Positions
• Hinge Position
• Gas Stay Position
• Material Selection
• Space Availability
• Hemming and Relief Areas

All these design considerations go hand in hand with eachother because these positions are dependant on each other and if position gets altered all the positions has to be altered for effective opening and closing of the back door. The Latch and Stricker, Hinge Position and the Gas stay Positions has to be maintained in a proper way so that they play a major role in the opening and closing as well as the rear Visibility criteria of the car.

Material Selection:

The material selection play a major role in determing the overall weight of the back door. The materials generally used are Steel and Aluminium. The Density of Steel is higher than Aluminium thus it results in adding more weight of the door. Basically the material is used based on the budget of the car, because the cost of steel very cheap than Aluminium. If aluminium is used instead of steel there is no need of many parts like Reinforcements for each mounting and the overall weight geta reduced by 6-10 kg approximately. But the draw back is Aluminium is much costlier than Steel. Hence based on the budget of the car aluminium is used. In higher end models like Benz, Audi, Porshe and BMW we can find aluminium is used instead of steel resulting in reduced weight and increased performance of the vehicle.

Another Factor to make a note is using Casted Aluminium instead of deep drawn Steel components and adding reinforcements to it is the space of the luggage commodity gets increased by a certain amount because the draft angle for Casted aluminium is much lesser (3 degree) than the deep drawn Steel inner panel (7-9 degree). Thus it reduces the space between the inner and outer panel resulting in much larger cabin space than the deep drawn steel inner panel. Thus the material selection is one of the design consideration taken. Since it reduces the space between the inner and the outer panel the rear visibility of the car is also inproved giving more space for the mirror to view.

Hence in this design the material chosen is steel since it is designed for a medium budget car and hence high cost could not be afforded to design the back door.

Different types of Sheetmetal operations are done in the Back Door

Deep Drawing:

The metal deep drawing process is an effective method of manufacturing of cups, cans and other similar deep drawing products. The deep drawings having two main categories.

1. First- stage drawing: The blank shape of metal is placed on the die and contact with the punch. When the metal specimen is flat piece of sheet metal used in forming and finish the product. Initially the punch moves in downward direction. The outer rim of blank is subjected to pure radial drawing (drawing toward in vertical axes) between die and blank metal. The metal become a bend and sliding over die and also stretched between die and punch. The metal in vicinity of punch head and contact with it bend and slide over radii and stretch over punch head.
2. Redrawing: The components drawn from first stage again it is redrawn convert into more depth to form a cup shape in the die and punch. The second stage of drawing is called as redrawing.

Force Dissipation:

The major criteria in designing the back door or any BIW part is the force dissipation during an impact or a collision. Hence studies are being done inorder to dissipate the force on the sides and avoiding it reach the inner cabin of the car so that possible damage of imapct inside the car is reduced. Hence inorder to do that the V or Y embosses are created on the inner panel so that the force gets transmitted on to the sides instead reaching it on the cabin. This is one of the main criterias in back door design.

Thus from the above image it is clear that the forces are dissipated on the sides instead of reaching inside. This is the reason to have Y or V embosses on the panel to increase the stiffness and to dissipate forces onto the sides.

Load Cases Analysis: Generally, we consider following tests for Load Analysis on Back Door -

Figure: Different load case analysis on back door

CAE team members will test these cases on car back door because of forces or loads at different areas and different ways effected to back door.

Inner panel:

The inner panel is developed with the help of styling surface of outer panel upper and lower and created necessary embosses to give strength to the inner panel and also added reinforcement like hinge reinforcement, gas stay reinforcement and latch striker reinforcement at edges. Supports between inner panel and outer panel by mastic reinforcement point to avoid non vibrations.

Outer Panel:

Outer panel is a styling portion to look good design the surface is created different curvatures. It should be designed according to customers point view and main reason to attract the customers.

Latch and Striker Positions:

The Latch and Stricker positions play a major role in determing the opening and closing of the back door. Hence the opening and closing of the had to smooth and inorder to ensure that the the centre point of the latch and stricker has to to be at the same point and the trajectory has to be tangent to the open close circle that is drawn. This is shown in the below image. Only if the centre points and the tangent line are perfect the opening and closing of the back door is smooth and clear. Or else the Hinge positions, Gas stay positions and Latch and Stricker Positions has to be altered to meet the change.

Here the hinge centre and the striker centres are in line with each other and the trajectory of the movement is clearly obtained.

Hemming and Relief Areas:

Hemming is a forming operation in which the edges of the sheet are folded or folded over another part in order to achieve a tight fit. Normally 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.

There are various types of hemming operations:

1. Conventional die hemming: In conventional die hemming, the flange is folded over the entire length with a hemming tool.
2. Roll hemming: In roll hemming, the hemming roller is guided by an industrial robot to form the flange.

Open Hem: A Hem has a full round feature and a return flange. In open hem bend angle is equal to 180 degrees.

Tear drop Hem: In tear drop hem the bend angle is greater than 180 degrees.

Closed Hem: Hems without any inside radius are called as closed hem. Closed hems are not recommended if the hem geometry is to be painted or if the material used is SST or Aluminium.

The Hemming is done on the outer and the inner panel with an clearance of 0.2mm on the either sides. Since at the corners the hemming is difficult to perform, clerances has been given at the ends of panels so that it would for the hemming operation is performed. Here is the image of the hemming relief at the corners.

Reinforcements:

Reinforcements are generally used to increase the strength of the mountings so that there is no unnecessary dents or noise occurs at the time of motion. This not only add strength to the part but also increase the overall stiffness of the part so that the part is completely stable. The Reinforcements used are

1) Latch and Stricket Reinforcement

2) Wiper Motor Reinforcement

3) Hinge Reinforcement

4) Gas Stay Reinforcements

The Following Image shows the reinforcement that are used on different areas of the inner panel. They are usually spot welded on the inner panel.

Back Door Assembly:

Thus the top down approach is followed to design the back door assembly. At first from the given styling surface the inner panel was created by Offseting the outrer Panel and then individual parts were designed accordingly by using the Inner panel Surfaces. The Reinforcement and other commodities were designed by using the inner surface as the reference thus following the top down approach. The following image show the assembly model of the Back Door.

Mastic Sealants:

The highlighted region in the image is the region where both the inner and outer panel are joined at the centre with the help of Mastic Sealants. There is a gap of 5mm for the mastic sealants to melt and gets fixed at the surface of the panels.

Draft Analysis:

Draft analysis is done w.r.t to tooling direction and draft angle maintained 7 degrees.

Conclusion:

The car back door is developed with the help of styling surface and developed essential embosses, reinforcement to give strength to back door. Draft analysis done for inner panel w.r.t to tooling direction.

Views:

Front View-

Back -

Right -

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