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.

Back Door:

               A back door is a BIW component found in all automotive car generally used for opening and closing of the so called car luggage that is usually used for storing items inside a car. In general the opening and closing of the door is coordinated by the Latch and Striker position and the Gas stay position that is mounted on the body and back door of the car. These positions play a major role in ensuring smooth opening and closing of the tail gate.

  Parts of a Tail Gate:

• Outer Panel
• Outer panel Lower
• Inner panel
• Hinge Reinforcements
• Latch and Striker 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.

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 each other because these positions are dependent 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 Striker, 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 determines the overall weight of the back door. The materials generally used are Steel and Aluminum. The Density of Steel is higher than Aluminum 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 Aluminum. If aluminum is used instead of steel there is no need of many parts like Reinforcements for each mounting and the overall weight get reduced by 6-10 kg approximately. But the drawback is Aluminum is much costlier than Steel. Hence based on the budget of the car aluminum is used. In higher end models like Benz, Audi, Porsche and BMW we can find aluminum is used instead of steel resulting in reduced weight and increased performance of the vehicle. 

      Another Factor to make a note is using Casted Aluminum 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 aluminum 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 considerations taken. Since it reduces the space between the inner and the outer panel the rear visibility of the car is also improved 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.

Force Dissipation:

     The major criteria in designing the back door or any BIW part are the force dissipation during an impact or a collision. Hence studies are being done in order to dissipate the force on the sides and avoiding it reach the inner cabin of the car so that possible damage of impact inside the car is reduced. Hence in order 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 criteria’s in back door design.

 

Latch and Striker Positions:

It is important design consideration while creating hood and back door design, because then only opening and closing of the door in easily, if not properly design the latch and striker position then it will create a problem while open and close the door.

Create two points in the center of two hinge emboss. Then create center point between the two hinge emboss by using between two points command. Create a circle it must be passes through inner lower panel because where the latch and striker part going to place according to the design consideration.

Create the tangent line on the circle because the opening and closing of the door follow the path of tangent line.

if the center 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 Striker Positions has to be altered to meet the change.

Hemming and Relief Areas:

         Hemming and Seaming are two similar metal working processes in which sheet metal edge is rolled onto itself. Hemming is the process in which the edge is rolled flush to itself, while a seam joins the edge of two materials.

         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, clearances 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.

The stress relief should give every corners and bends. Those regions have improper surface and stress will presents and during hemming operation roller won’t work well. So we need to provide reliefs.

  

Reinforcement:

    Reinforcements are generally used to increase the strength of the mountings so that there are 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 Striker Reinforcement

            2) Wiper Motor Reinforcement

            3) Hinge Reinforcement

            4) Gas Stay Reinforcements

The Following Image shows the reinforcement that is 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 Offsetting the outer 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 center 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 to show that the Part can be easily drawn out from the mould.

  

So the above design has been analyzed using the tool on NX CAD with having 7degree as the allowance .

Conclusion:

Thus the back door was designed from the given styling surface and Draft analysis is also done on the inner panel to check the manufacturing conditions. In this challenge I learned to design the Inner panel, the Outer panel, Wiper Motor reinforcement, Hinge reinforcement, and Gas stay reinforcements, Latch and Striker reinforcement and its position. Thus the designing of the Back Door or Tail Gate is done as per Industrial standards and Manufacturing Techniques.