Design of backdoor

DESIGN OF THE BIW BACK DOOR FOR HATCHBACK CAR 

OBJECTIVE

• To design the back door of a car.
• To design an inner panel for the back door.
• To design the necessary reinforcements like Hinge, Gas-stay, Wiper motor, Latch and Striker on basis of engineering standards.
• The Inner panel is subjected to Draft Analysis for verifying whether it satisfies requirements of Deep Drawing.

INTRODUCTION

A door is a hinged part which is used for entry and exit of the vehicles. Back doors are one of the type doors of the vehicle which are located next to the passenger seat which provides access to the luggage compartment. In most of the cars, these back doors are operated with the help of gas stay which in turn reduces the effort required to lift open the door. Some luxury cars have powered backdoor which in turn is driven by an electric motor. This kind of system is also called a powered trunk lid system. Hatchback or station wagons are sold as 3 door or 5 door models. In this case, the rear hatch is classified as door this is because it enters the passenger compartment. The naming is mainly used in Europe countries. In case of safety, the back door designed should meet the safety requirement for the rear collision.

TYPES BASED ON WORKING MECHANISM

GAS STAYS TYPE

Gas stay types backdoor/liftgate/tailgate is an ideal and most commonly used tailgate mechanism from lower segment car to luxury car segment. The ideal components of tailgate consist of a hinge, door, gas stay and some rubber beadings for sealing of door which is shown in the image below:

POWERED LIFT SYSTEM

Power trunk lid system is another sophisticated way of trunk/liftgate opening mechanism which is to automatically open and close of the trunk/liftgate and release of the latch by the touch of a button. The following image shows the component as used in the powered trunk/back door lift system.

Powered trunk lift system consists of a power latch, anti-pinch switch, PTG assembly’s, PTG main switch. When the switch is pressed in the compartment or outside the power latch releases the latch and the entire backdoor is opened. The anti-pinch switch is located in order to avoid accidental hand insertion during the operation of the door.

MANUFACTURING METHODS

Recently, there have been two approaches to reducing automobile weight. One is by using material lighter than steel and the other is by redesigning the steel structure. Although the former seems very effective, it is very expensive so that it may only be used for an expensive automobile. Therefore, the automobile industry is trying to use steel, which is not costly and recyclable. Lightweight steel can be achieved by improving the performance of the structure or adopting new manufacturing techniques. One of the efforts is the ULSAB (ultra-light steel auto body) concept.

ULSAB suggests three main weight reduction techniques such as hydro-forming and the tailor welded blank (TWB).

Tailor welded blank

A tailor-welded blank is a blank that is comprised of two separate pieces of sheet metal that have been welded together prior to stamping. Tailor welded blanks allow the welding of the different grades, different thickness or different corrosion coatings together in order to give the properties needed in different areas, without increasing the number of tools needed to form the part and eliminating the fit ability concerns. They also allow a high degree of flexibility in designing parts and large blanks can be formed from much smaller sheets. The use of tailor-welded blanks would reduce the weight of the car. Having the ability to selectively place different thickness of material would result in weight reductions. An example of this used in production is the door inner panel. The only strength requirement on a door inner is in the region where the hinges attach to the panel.

BACK DOOR ASSEMBLY

The back door assembly of a hatchback or SUVs has the following components.

• Inner panel
• Outer panel
• Hinge reinforcement
• Gas stay reinforcement
• Wiper motor reinforcement
• Latch/striker reinforcement

DESIGNING AND REINFORCEMENTS

OUTER PANEL

The Outer Panel is the styling surface of the back door. It is the source of design of other parts of the back door, primarily the Inner Panel. The Outer panel of the back door is similar to other BIW components where it is the aesthetic part of a vehicle. The coating provided is of multi stage which has the property of scratch resistance, rush prevention and smooth and attractive finishes. The Outer panel also has the provisions for the tail lamp, rear fog lamp and also has the provision for license plate mountings. Most of the hatch back cars have the provision for wiper motor. Its thickness is in the range of 0.65-0.75 millimetres.

INNER PANEL

• The inner Panel is a component which is used for strengthening the door. Its 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 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 embosses and reinforcements.

REINFORCEMENTS:

The inner panel accommodates the Latch and Striker, Wiper Motor, Trims Mounting, Hinge and Gas Stay, etc and therefore the necessary cut outs 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 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 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.
• Various simulation tests like Bend test, Torsion test, Abuse test performed on the back door as shown above reveals that there a very weak zone developed at the Gas stay region. To compensate that, a reinforcement is added in that region.

3. Wiper Motor Reinforcement:

Wiper Motor Reinforcement is created to mount the Wiper Motor Assembly.

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 hemming process, the sealants were applied along the edge of the flange of outer panel so that water tightness is achieved. The below image shows the cross-section of the inner and outer panel joined by hemming process.

CORNER RELIEFS

Usually, the 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 the 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. 

DRAFT ANALYSIS

• While designing a component a design engineer must make sure that the component can 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 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 place 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.
• In NX draft analysis can be done after modelling so that we can verify whether the product is manufactured by pressing or not. One side of the surface is highlighted by green and other side by red colour means that the model can be manufactured. Result of the draft analysis is shown in the figure below.

CONCLUSION

Thus the parts of back door were designed using top-down approach in NX-cad. All the required reinforcements were designed and assembled.