Challenge

Design of backdoor

SSunil K G6 Jan 2023

BACK DOOR / TAIL GATE DESIGN (BIW) USING CATIA V5

Objective:

The main objective of this project is to learn the basics of the BIW(Body in white) through designing the Back Door also referred as tail gate of a car by considering all the protocols and standards of BIW design.

Introduction:

What is BIW?

BIW is the stage in automobile manufacturing in which a car body's manufacturing in which a car body's frame has been joined together, that is before painting and before the motor, chassis sub-assemby, or trim (glass, door locks/handles, seats, upholstery, electronics, etc.)have been integgrated into the structure. Assembly involves different techniques such as welding (spot, MIG/MAG), riveting, clinching, bonding and laser brazing. Body in white refers to the stage in automotive manufacturing when a cars sheet metal components are welded together which include:

Hood
Trunk id
Roof
Front and rear fender
Side wall
Side Doors
Tail Gate(Back Door)

The term BIW derives from manufacturing practices before steel unibody manocoques- when automobile bodies were made by outside firms on a separate chassis with an engine, suspension, and fenders attached. The manufacturing built or purchased wooden bodies (with thin, nonstructural metal sheets on the outside) to bolt onto the frame. The bodies were painted white prior to the final color.

A folk etymology. for "body in white" is the appearance of a car body after it is dipped into a white bath of primer (undercoat paint) - despite the primer's actual gray color. BIW could also refer to when car bodywork woud be made of timber - all timber products, furniture, etc.,, are considered to be "in the white" when at the stage of raw timber before finishing or varnishing.

In car design, the "body in white" phase is where the final contours of the car body are worked out, in preparation for the ordering of the expensive production stamping die. Extensive computer simulations of crash-worthiness, manufacturing, and automotive aerodynamics are required before a clay model from the design studio can be converted into a body in white ready for production.

What is a Tail gate/Back door?

The tail gate of a car is the door at the rear end of the car, 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 is driven by an electric motor. This kind of system is also called a powered trunk lid system / Power spindle. The back door is also responsilble for dissipating the force created while rear collisions, away from coming inside the car.

Types of backdoor based on working positions:

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 tail-gate.

Top mounted.
Side mounted doors.
Bottom type.

1.Top mounted:

This type of back doors is used in a station wagon, pickp 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.

2.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 vehicle. 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 door are more convenient for the goods handles 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.

3.Bottom Type:

This bottom type backdoor system are used in the car type such as car, pickup truck and some luxuary 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.

Types Based on working mechanisum:

Gas stays type.
Powered lift system.

1.Gas Stays type:

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.

2. Powered lift system:

Powered trunk lift system is another sophisticated way of trunk or liftgate 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 assly, 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.

DESCRIPTION

In this project we are going to create the assembly of a back door using previously given styling surface (outer panel surface) as the reference. The software used here to model the tail gate is CATIA V5

The assembly of back door consist of varios components such as outer panel upper, outer panel lower, inner panel, hinges, gas stay, latch and the reinforcements. Here we are mainly dealing with, the design of inner panel.

For creating a tail gate, there are several factors and standards a design engineer should consider. some of them are below

Major Factors to consider

Material selection.
Stiffness and strength and weight of the part
Rear visibility criteria
Hinge position
Latch position
Distance from the outer panel to mastic points should maintained in between 4-6mm
Drawability (minimum 7 degrees open)
Hemming parameters
Holes(to drain the primer from the cavities)

1.Material selection.

Tail gate BIW is expected to possess a number of significant properties. It must have high tensile strength as well as high stiffness-bending, torsional, static and dynamic. It must also provide good quality safety both to the car body and to its occupants against crashes of all kinds-front, rear ,side, or even rollover, meeting the federal motor vehicle safety standard No.208[44]. It should also be able to protect occupants from noise, vibration, and / or harshness (NVH) by absorbing or reducing these conditions. Also, it sould be easy to weld and form, as well as be highly paintable and easy to design. It should provided a good surface finish that is smooth, and should also be corrosion resistant. Improvement of fuel economy also necessitates the use of lightweight materials in BIW, as it constitutes a significant share of car's weight; in addition, given environmental concerns in recent times, it is also expected to be recyclable. given such extensive requirements, only steel and aluminum have been used for making BIW; the former is preferred for its lower density that helps reduce car weight.

Material	Yeild Strength (MPa)	Ultimate tensile strength (MPa)	Reference
Stell	210-1250	340-1520	46
Aluminium	90	195	47
CFRP		250-585	48

Here we are assuming stainless steel as the material for the designing of back door.

2.Stiffness, strength and weight of the part

As the back door protects the passengers from rear collisions, it should have enough strength and stiffness. stiffness, strength and weight of part depends up on major two factors. The first is the material selection, which is discussed above and the second one is the design of the part.

A design engineer can easily increase and reduce the stiffness of the part through his or her designs. The methods of increasing stiffness and strength of the parts are adding embosses in the models.

Embossing

Sheet metal embossing is a stamping process for producing raised or sunken designs 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 rools of the desired pattern. Adding of embosses to a sheet metal part, Drastically increase the strength of the part.

3. Rear visibility criteria

This criteria deals with the visibiity of passengers backwards through the rear wind shield glass. it mainly depends up on the wind glass area. Here in this case as we already have a styling surface wind already included the wind shield area we need not to worry about the rear visibility criteria.

4. Hinge position

Hinges are the part which helps to open the back door to the luggage space. One part of the hinge is connected to the inner panel of the back door while the other is connected with the roof.

There are a few things to consider while the positioning of hinges

The surface where the hinges to be mounted must be flat in order to keep the hinge tightly in touch with the inner panel, otherwise it leads to loosening of the nuts and bolts resulting in NVH(Noise, Vibration and Harshness)

Reinforcement must be provided on both hinges. Because in sheet metals adding another solid body on it, will results in decreasing the strength of the part. To overcome this problem reinforcements are used.

The central line or the axis of each hinges must be colinear

5.Latch position

Placing latch/ striker is on of the major factor to take care of. As a latch/striker is separate body and itself a reinforcement, there is no need to add extra reinforcements. But like in hinges its compulsory to have a flat surface to mount it.

The striker in the body and the latch in the back door must be normal to a trajectory path to have a smooth locking and unlocking.

6.Distance from the outer panel to mastic points.

The mastic sealant or the mastic point is an elastic compound that behaves like a rubber. The mastic points are defined on the hood inner panel where the hood can easily deform. They help the hood outer panel to come back to its original shape after it gets deformed. The distance from mastic points to the outer panel must be 2-8 mm

7. Hemming parameters.

Hemming is a sheet metal forming process in which sheets are joined by bending it usually to 180 deg. Automotive body panels and automotive parts such as deck-lids, trunk-lids, doors, hoods and tailgates are formed by hemming process.

While hemming the gap between two sheet metals must be in between 0.2 to 0.5 mm and the distance bend of the outer sheet metal to the edge of the inner sheet metal must be 2mm.

8.Drawability (Draft Analysis)

To understand what is deawability, first we should know what is deep drawing.

Deep Drawing.

Deep draing is a sheet metal forming process in which a sheet metal blank is radially drawn into a forming die by the mechanical action of a punch. It is thus a shape transformation process with material retention. The process is considered "deep" drawing when the depth of the drawn part exceeds its diameter. This is achieved by redrawing the part through a series of dies. The flange region (sheet metal in the die shoulder area) experiences a radial drawing stress and a tangential compressive stress due to the material retention property. These compressive stresses (hoop stresses) result in flange wrinkles (wrinkles of the first order). Wrinkles can be prevented by using a blank holder, the function of which is to facilitate controlled material flow into the die radius. The feasibility of this deep drawing is called drawability.

Draft Analysis in CATIA V5 is one of the tool used to study the deep drawig feasibility easily through checking draft angle in the various faces. The designer should define a tooling direction in-order to check the draft angles.

9.Hole ( To drain the primer from the cavities)

In the inner side of the tail gate we can see several holes in the hood. Every holes in it have its purpose. Apart from the holes for the fasteners, these holes allow e-coating, surface prep, and tail gate skeleton after the chassis is dipped into these during production.

Components to design

Here in this project we are not creating the hinges as well as the latches, only we are creating back door outer panel, Back door inner panel, Reiforcement for hinges, Gas stay and viper mount.

So in this case we need to assume the hinge position and hinge axis appropriately.

The Different panels or regions are as follows:

Inner panel.
Outer Upper panel.
Outer Lower panel.

Backdoor Assembly:

A backdoor is normally manufactured with 1 inner panel and 2 or 3 eterior panel that are hemmed and spot weilded together as needed. Depending on the price range of the car, different materials are utilised for the backdoor. The bilk of inexpensive cars are made of steel since it is less expensive and parts are stamped using moulds. Here, uniform thickness of the panels is maintained throughout.

However, in expensive cars, aluminium is used for the manfacturing of back door which is usually die cast. In such cases, reinforcements are not required and the vehicle is also light in weight. Varying thickness can also be achieved in case of aluminium body back doors.

Inner Panel.
Outer Panel.
Hinge reinforcement.
Gas stay reinforcement.
Wiper motor reinforcement .
Latch and striker reinforcement.

When designing the backoor, we will divide the whole designing process into multiple steps which enables us to seamlessly create different parts of he backdoor assembly

We are provided with the styling surface and we take a top bottom approach to integrate the whole assembly

Back door outer panel

As we have the styling surfce as the reference we need not to work more on the outer panel only need to create the hem flanges after the creation of inner panel.

Here the outer panel of the tailgate consist of two parts which are the outer panel upper and the outer panel lower. These two components are joined using the laser brazing method.

Laser Brazing

Joining methods, i which the brazing solder is melted with the help of a industrial laser, have been used mainly in the mass production of vehicle bodies. There, laser beam brazing is mostly used for joining galvanized metal sheets or lightweight components made of aluminium, but also for other brazing material. Laser automobile brazing is usuall realized with a brazing optic that is integrated in a robot. The laser beam is guided along the joint where it melts the brazing solder- e.g. a copper-silico-wire-and in so doing conects. The success of the brazing process is based on whether the firmness is similar to the welding seams and the high esthetic quality of the joints: brazing seams that were generated with lasers are known for their sealed: smooth and clean structures. This is visually attractive and reduces post-processing to a minimum. car body construction, for example, can often be lacquered immediately after cleaning.

I have created the hem flange appropritely by considering all hemming parameters. I have maintained a 0.2mm gap in between the sheet metals in hemming area and another 2mm gap between the edge of the inner panel and the hem flange of the outer panel.

Draft analysis:

For each upper and lower outer panel i could find positive draft

Back door inner panel

Inner panel is the part which gives enough strength to the outer panel as well as the it is the part where the viper, latch, hinge and the gas stay are mounted along with the reinforcements, Modelling of the inner panel is little risker while comparing all the other components of the back door.

The first thing i have created is the sealing flange of the tail gate.

Sealing Flange

Sealing flange is the part of the tail gate where it touches and presses the rubber gasket on the body side outer in order to avoid the entering of water to the luggage area.

Sealing flange is created by offsetting the outer panel to the required distance and also by creating some studio guided by some bridge curves.

Embosses for Hinge, Viper mount and Gas stays

I have made one emboss for the hinge in the appropriate position and another one emboss for the gas stay mount. Also i have made flat surface on the embosses as the hinge and the gas stay need to be mounted on it.

A 5mm flat space also maintained aroung the holes for the fasteners.

Flanges for the mastic

Apart from the hemming the outer panel of the tail gate must be supported using some flanges from the inner panel and some mastics on it. So i have created some cut outs and created some flanges from the edge of the cut outs for creating the mastic points as shown in the figure. The distance from the mastic points to the outer panel is maintained to 5mm

Embosses for Dissipation of collision force and cut outs for weight reduction.

To dissipate collision force getting inside the car, Some embosses are made and directed towards the sides of the tailgate as shown in the figure.

From the embossed surfaces some materials are removed in order to reduce the weight of the part.

Latch positioning

Latch positioning is one of the critical operation in the designing of the inner panel. For positioning the latch, first a line is drawn from the axis of one hinge to the axis of the other hinge. After that a trajectory circle is drawn from the center of the that line as it crosses the surface where the latch is placed, and then make the latch axis tangent to the trajectory circle.

Here as i doesn't have the hinges as well as the latch, I am drawing a line from the emboss of the hinge to the other from that line i have drawn circle and an emboss is created for the latch where the circle crosses the inner panel.

Inner panel final model.

Draft analysis

I have created a line for the tolling direction and after draft analysis i could find some embosses with negative draft. But in this case its negligible as all the embosses cannot be deep drawn in a sinle stretch.

Reinforcements for hinge, Gas stay and Viper mount

Reinforcements are used in the areas of a sheet metal where another components are attached. The reinforcements compensate reduced strength of the sheet metal while another component added on it.

Reinforcements are the most easier part to create among all other parts in the tail gate. These are made by using the command wave geometry linker. The proper surface from the inner panel is extrated and trim it to desired shape an then thicken it to get the reinforcements.

Hinge Reinforcement: