DESIGN OF BACK DOOR USING NX

AIM : To design a BIW of Back Door from the given Class A Surfacing. The Inner Panel is designed and assembled with Hinge Reinforcements, Gas Stay Reinforcements , Wiper Motor Reinforcements and Latch Reinforcements. 

INTRODUCTION :

• A back door is a BIW component found in all Automotive cars 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.
• Usually Aluminium or Steel is used to manufacture the backdoor. While Aluminium backdoor is lighter in weight and has a lesser number of components (since the part is "cast", it has varying thickness and therefore doesn't require reinforcements). Steel backdoor is "deep drawn" with uniform thickness, added reinforcements and therefore is heavier. But the production cost of the latter is lesser and therefore a Steel backdoor is priced lower.

Components of a Tailgate:

• 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

TYPES OF DOOR OPENING:

• Top Mounted Door
• Side Mounted Door
• Bottom Door

DESIGN CONSIDERATION FOR BACKDOOR INNER PANEL:

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 have to be altered for effective opening and closing of the back door. The Latch and Stricker, Hinge Position and the Gas stay Positions have 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.

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.

MATERIAL SELECTION:

The material selection plays a major role in determining 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 to the door. Basically the material is used based on the budget of the car, because the cost of steel is much cheaper than Aluminium. If aluminium is used instead of steel there is no need for many parts like Reinforcements for each mounting and the overall weight gets reduced by 6-10 kg approximately. But the drawback 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, Porsche and BMW we can find aluminium is used instead of steel resulting in reduced weight and increased performance of the vehicle. 

STYLING CAR SURFACE:

INNER PANEL:

• The inner Panel is a component which is used for strengthening the door. It's thickness is in the range of 0.65-0.75 millimetres. It is one of the most important components on which all accessories are mounted like Hinges, Gas Stay, Wiper Motor , Latch and Striker etc.
• The possible method is the integration method. In the integration method, the part is stamped out of a single blank. This reduces the number of tools needed for the assembly cost and eliminates any fit ability problem.
• There are many cases where the Inner panel is subjected to stress due to frequent opening and slamming. These stress and depression factors are compensated by features like embossed and reinforcements.

EMBOSS:

• Sheet metal embossing is a stamping process for producing raised or sunken design or relief in sheet metal. This process can be made by means of matched male and female roller dies or by passing sheet or a strip of metal between rolls of the desired pattern.
• Embossing is done by pressing a sheet of in to female die, That has a design etched into it. This is usually done with a male counterpart underneath the paper so that the two and the design is transferred to the paper.

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 in order to dissipate the force on the sides and avoid it reaching 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 embossed 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.

Latch and Stricker Positions:

• The Latch and Stricker positions play a major role in determining the opening and closing of the back door. Hence the opening and closing of the had to be smooth and in order to ensure that the the centre point of the latch and striker has to to be at the same point and the trajectory has to be tangent to the open close circle that is drawn. 
• 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.

REINFORCEMENTS:

• The inner panel accommodates the Latch and Striker, Wiper Motor, Trims Mounting, Hinge and Gas Stay, etc and therefore the necessary cutouts are provided. Various reinforcements are also spot welded to the inner panel.

1. Hinge Reinforcement: Hinges enable the opening and closing of the back door. One end of the hinge is fixed on the backdoor and the other end on the Roof, on both sides. Hinges can be placed outside or inside the backdoor. Hinge reinforcements are added in this region to compensate for the weak zone developed due to the load acting while opening and closing the backdoor. Embossing is provided at the Hinge mounting area to compensate for the functional holes punched.

2. Gas Stay Reinforcement:

• Gas Stay is a device used for supporting the back door in its open position. Gas stay requires no man power and is engaged into a support position automatically as the back door opens. One end of the Gas Stay is fixed on the backdoor and the other end on the Body Side or on the Back door stationary area. Embossing is provided at the Gas Stay mounting area to compensate for the functional holes punched compensate for the functional holes punched.
• Various simulation tests like Bend test, Torsion test, Abuse test performed on the back door as shown above reveals that there is a very weak zone developed at the Gas stay region. To compensate for that, reinforcement is added in that region. 

3. Wiper Motor Reinforcement: Wiper Motor Reinforcement is create to mount the Wiper Motor Assembly

4. Latch & Striker Reinforcement: The Latch & Striker forms the locking mechanism of the backdoor with the car. Due to regular opening, closing and over slamming of the backdoor, there is stress developed in the Latch and Striker area. Further, during a rear collision this area will be the first point of contact. To compensate for the same, reinforcements are added in this area. 

5. Reinforcements Assembly: The inner panel accommodates the Latch and Striker, Wiper Motor, Trims Mounting, Hinge and Gas Stay, etc and therefore the necessary cutouts are provided. Various reinforcements are also spot welded to the inner panel.

OUTER 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 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.

HEMMING: Hemming is the process of wrapping the flange of the outer panel to the flange of the inner panel so that they are fitting rigidly. After the hemming process, the sealants were applied along the edge of the flange of the outer panel so that water tightness is achieved. The below image shows the cross-section of the inner and outer panel joined by the hemming process.

CORNER RELIEFS: Reliefs were provided at the corners of the hemming flange to avoid the Hemming relief to avoid stress developed while hemming. stress developed during the hemming process.

MASTIC SEALANTS:

• To improve NVH (Noise, Vibration and Harshness) i.e to reduce these levels. Mastic Sealants are used. They also improve strength. They are made of hard Rubber and are introduced as a paste thereby joining the inner and outer panel with a thickness of 5mm.
• Mastic Sealants are preferred over Spot Welding to join the inner and outer panel due to the fact that the latter reduces the aesthetics of the car, especially on the outer panel which is the styling surface.

TOOLING DIRECTION :

• The Tooling Direction is basically the direction that the mold pulls apart. The mold is built in two separate pieces, basically.
• We have a Cavity side and the Core side. The Core pulls apart from the Cavity and that's the Dieline or the Tooling Direction. 

SPOT WELDING:

• Spot Welding is a form of resistance welding also known as Resistance Spot Welding (RSW).
• Spot Welding involves using heat produced by resistance to an electrical current to join materials together.
• The current heats the materials to a point where they become malleable. The use of strong pressure bonds them together. 

DRAFT ANALYSIS :

• While designing a component a design engineer must make sure that the component can be produced by any means of manufacturing methods with least amount of time, material, manpower and power. So that we can reduce the overall cost of a car.
• When the Back Door is considered it is manufactured by deep drawing. At most of the time Deep drawing is the process where the single sheet of metal is placed over a die which has the desired shape of the final output and the sheet is pressed against it with a tool. For the tool to come out from the part usually pressing relief of 5° to 9° will be provided during modelling itself.

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 shows the assembly model of the Back Door. 

CONCLUSION: Thus the Back door was designed and developed from the given styling surface with essential Reinforcements and Emboss. To satisfy the manufacturing requirements Draft analysis was also done on the Inner Panel.