Design of backdoor

BACKDOOR

The Back door/Tail Gate of a car is a component which hinges at the back of the car to cover the boot space or luggage area, but there are more functions that come with it which has to be considered while making a design of the same like rear visibility criteria and boot space.

This part is usually designed keeping the master section of the rear roof rail because of the sealant flange and also various other considerations, as it is also in contact with the side body of the car.

It has back lights/tail lamps and braking lights which is termed as "Third brake light" mounts on it, rear windshield fixes between inner and outer panel, in some back door spare wheel can be mounted on it, sometimes car registration number plate mounts on it and it gives a complete closed design to the car.

TYPES OF BACK DOOR

The door or opening of a cargo area may be hinged at the top, side & bottom. If the door hinged in the bottom it is called as a tailgate.

1. Top mounted

2. Side mounted doors

3. Bottom type

• Top mounted

This type of back doors is used in a station wagon, pickup trunk and some of the spot utility vehicles. Today's modern cars use top mounted tailgating system which are both powered and also gas stay supported. These top mounted tailgates provide greater rear visibility compared to side mounted or backdoor which have center pillars in it.

• Side mounted door

As the name signifies the side mounted doors are the one which is used in the van type MPVs, SUVs, pickup truck vehicles. These side mounted doors or barn doors are a lot easier compare to top mounted doors. These doors are generally smaller in size and can be easily accessed. These doors are more convenient for the goods handlers for long pipe and lengthy components where one door is kept open. These door also save accidental head impacts. It is a lot easier to load and unload items in the side mounted doors.

• Bottom type

This bottom type backdoor system are used in the car type such as car, pickup truck and some luxury SUVs. These are the most convenient type of back door in which the bottom split is used as a support or even as a table for the tour going people.

Back door is operated by two types

The main purpose of these mechanisms is to open the back door without heavy efforts, because the backdoor have different parts and the weight of the back door is high, so we are not able to open the back door manually, so we introduced these two types commodities, and these also decides the premium of the car, mostly the electrically operated used in premium cars.

1. Gas Stay Powered

2. Electrically Operated

• Gas Stay Powered

Gas Stay type backdoor 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.

• Electrically Operated

Electrically operated system is another sophisticated way of trunk or lift gate opening mechanism which is to automatically open and close of the trunk and release of the latch by the touch of a bottom. Powered trunk Lift system consists of a power latch, anti pinch switch, PTG assembly, 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.

Design Consideration

• Rear visibilities for safety and functionality
• Support structures like hinges and Gas stay/ Power lifter
• Rear crash regulations
• Boot space
• Mounts for wiper motor and lights
• Strength criteria
• Draft analysis

MATERIAL SELECTION

The major problem in back door is material selection. Mostly in past days steel is used, but it makes a very problems the steel parts have heavy weight, so it reduces the fuel economy, instead of steel presently in high end cars aluminium is used, the main objective of aluminium it have less denser material and its suitable for complex shapes, and it also gives the thickness locally because it done casting, in case of steel we used deep drawing operation. there are some of the advantages and disadvantages too.

SHEET METAL OPERATIONS

• 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.

• 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.

• 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.

• Embossing

Embossing is a metal forming process for producing raised or sunken designs or relief in sheet materials by means of matched male and female roller dies, theoretically with no change in metal thickness or by-passing sheet or a strip of metal between rolls of the desired pattern. Metal sheet is drawn through the male and female roller dies producing a pattern or design on the metal sheet. Depending on the roller dies used, different patterns can be produced on the metal sheet.

Materials commonly used in the metal embossing process include:

• Aluminum alloys
• Steel alloys
• Zinc
• Copper
• Galvanized steel
• Brass
• Cold rolled steel

• Hemming

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:

• Conventional die hemming

In conventional die hemming, the flange is folded over the entire length with a hemming tool.

• 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.

• Blanking

Blanking is a cutting operation of a flat metal sheet and the article punched out is known as blank. Blank is the required product of the operation and the metal left behind is considered as a waste.

• Punching

It is similar to blanking only the main difference is that, the hole is the required product and the material punched out from the hole is considered as a waste.

DESIGN INPUTS

• Outer Panel thickness - 0.75 mm
• Inner Panel thickness - 0.75 mm
• Reinforcements thickness - 1.5 mm

COMPONENTS OF BACKDOOR

The backdoor consists of the following basic components:

• Backdoor Inner Panel.
• Backdoor Outer Panel Upper.
• Backdoor Outer Panel Lower.
• Hinge Reinforcements.
• Gas-Stay Reinforcements.
• Latch and Striker Reinforcement.
• Wiper Motor Reinforcement.

• Outer Panel Upper And Lower

Outer panel is the styling surface of the back door. Glass for rear visibility is mounted on the outer panel upper, rear light 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 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 customer.

• Inner Panel

Car backdoor inner panel is made of either steel or aluminium. Most of the luxury car's inner panel are made of aluminium because of its properties. But the cost of aluminium is high. Cost of aluminium is more as compared to steel. To easily understand, the cost of aluminium for inner panel is higher than the cost of all parts of backdoor together.

• Gas-Stay Reinforcement

Gas stay reinforcement is used to lift the backdoor automatically. There are two types of gas stay one is semi-automated type the back door needs little force to lift after that it lift the backdoor automatically. In fully automated, it has power spindle which lift the back door automatically.

• Hinge Reinforcement

• Wiper Motor Reinforcement

Wiper is used to wipe out the rain water falls on the backdoor window. Its actuated by the motor which is mounted on the inner panel. Due to the vibration and heat created by the motor. The inner panel where the motor is mounted gets deform and parts attached to the motor starts to vibrate which leads to poor NVH. So we have to provide the reinforcement where the wiper motor is placed.

• Lach And Striker Reinforcement

At bottom of the panel, reinforcement and emboss are given to accommodate the latch and striker assembly. Generally key operated or automatic remotely controllable mechanism is used to keep the door open or closed. The actuators, receivers and other electronics devices are mounted on the inner panel only.

DESIGN METHODOLOGY

The styling surface is provided to us and considered top-down approach for the assembly. First we create the assembly and the create the components in the assembly it self. Inner and outer panel are the largest parts and all other parts depending on them so we start with them. Also the styling surface is the part of the outer panel so we create it first using various commands and features we design the outer panel from styling surface. We have 2 parts for outer panel namely outer panel upper and outer panel lower. They will be spot welded with each other. Inner and outer panels are connected by hemming process so we can come back to outer panel and do that after completion of inner panel.

Now as we know that the inner panel is at a particular distance from outer panel, we offset the outer panel surface and design inner panel on the same. Inner panel is important as most the mountings will be done on it. We need to define the tooling direction as a reference for manufacturing team. Then we design the inner panel.

While designing the inner panel we need to considered various factors like the mounting point positions, mastic sealant application positions and the cut outs in sheet to make access to tail light assemble easier. As the lower part of the inner panel is not connected to outer panel we need to create a surface that comes out and connects to outer panel as shown in the figure. This part will be connected to outer panel using mastic sealants. Also the hinge embosses must be taken by considering proper directions.

As the inner and outer panel are connected by hamming process we can completed the hemming after completion of the inner panel. Now as the major parts are designed we can move to reinforcements. It is fairly easy to design reinforcements as shown in the figure and we can use the inner panel surfaces using wave geometry linker into the corresponding reinforcement parts.

• Latch and striker trajectory

To define the opening and closing trajectory, the hinge and latch are positioned based on the hinge axis and trajectory. The circle drawn should be perpendicular to the latch striker to satisfy the condition. Based on the rotation trajectory the hinge axis, latch striker and predefined Gas stay are correlated for smooth kinematics of the door opening and closing.

• Hemming

Hemming is a mechanical joints design which is used for joining inner and outer closure panels as found indoors, hoods, bonnets and lift gates. To increase crash resistance the gap between the hemmed panels is filled with a high strength adhesive which also prevents from corrosion.

• Corner Relief

Corner relief are cut out which are provided on the hemmed corner of the panel assembly which in turn to avoid some of the hemming defects such as wave like or wrinkle formed surface. The back door design requires two locations where the roller type of hemming relief is required.

• Mastic region

The mastic points are defined on the backdoor inner panel where the backdoor can easily deform. They help the backdoor outer panel to come back to its original shape after it gets deformed. Mastic sealants are elastic compound that behaves like the rubber. The gap between the inner panel and the outer panel over the embosses is filled with mastic sealants at curtained specified places, this helps to reducing vibrations and overall NVH (Noise, Vibration, Harshness). Then, the sheet will bend with the help of another die to make the flange in "L" shape. Thus, the flange will have somehow same design and the length will completely depend on the Cut-out, if we need more length of flange, then we need to cut more sheet.

DRAFT ANALYSIS

Draft analysis of inner panel is carried out through the tooling direction only. Only the green areas are accepteble and in worst case yellow region are accepteble but the blue and red region are never accepted.

CONCLUSION

Thus the back door was designed and developed from the given styling surfaces with essential reinforcements and emboss using Siemens NX CAD . To satisfy the manufacturing requirements draft analysis was also done on the inner panel.