Challenge

Design of backdoor

SSandeep M4 Jan 2023

AIM

To design a back door, necessary reinforcements and embosses by using appropriate design methodologies.

THEORY

Back door is the outermost part of the automobile at rear side that give access to boot space or luggage space, spare wheel etc. It is also known as Trunk, Tailgate, Dickey according to the regions. Tail lamps, rear wiper and its motor, rear windshield glass are all assembled on back door.

Types of Opening Based on Mounting

Based on the mounting back door can be mainly classified as:

Top Mounted: Here door is opened upwards which are electrically powered or gas stay supported. These are commonly found in sedan's, Hatchback's and some of Suv's.

Side Mounted: Here the door is opened in any of the sides. These are more convenient for handling goods i.e easy to load and unload. These are common in Van's, SUV's, MPV's.

Bottom Mounted: Here door is opened downwards. These are common in Pickup trucks, some luxury SUV's.

Back Door can be opened and closed by manually (with help of gas stay) and automatically (electrically using remote).

Gas Stay Powered

Electrically Operated

MANUFACTURING AND JOINING PROCESSES

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.

The accuracy of the hemming operation is very important since it affects the appearance of the surface and surface quality. Material deformations, which occur during the hemming process, can lead to dimensional variations and other defects in parts. Typical hemming defects are splits or wrinkles in the flange, material overlaps in the corner areas or material roll-in. This is why it is important to use simulation tools in order to, on the one hand, better understand the hemming process and, on the other hand, significantly reduce the number of “trial and error” loops during production.

TYPES OF HEMMING OPERATIONS

There are various types of hemming operations:

Conventional Die Hemming: Conventional die hemming is suitable for mass production. In die hemming, the flange is folded over the entire length with a hemming tool. Normally the actual hemming is a result of a forming operation in which the flange is formed with a hemming tool after the drawing and trimming operations have been completed. The formed flange is then hemmed in several process steps. These steps include, for example, the pre-hemming and final hemming depending on the respective opening angle of the flange. Production plants for conventional die hemming are very expensive, but the cycle times are very low.

Roll Hemming: Roll hemming is carried out incrementally with a hemming roller. An industrial robot guides the hemming roller and forms the flange. Roll hemming operation can also be divided into several pre-hemming and final hemming process steps. Roll hemming is very flexible to use and tool costs are significantly lower as compared to those of conventional die hemming. However, the cycle times are much higher since the hemming is realized using a hemming roller which follows a defined path.

TYPES OF HEMS

Four most common types of sheet metal hems are:

Closed Hems: Also known as a crushed hem, a closed hem is completely flush. Closed hems are the most popular type of hem because they are so easy to make. The sheet metal is simply flattened onto itself. There are, however, some material limitations. This type of hem works best with material thicknesses of .040”-.125”. The flat length of the hem must also be a minimum of 4x the material thickness so that it doesn’t break.

Teardrop Hem: A tear drop hem is a flange that has been bent more than 180 degrees. Teardrop hems are for materials that lack the flexibility required of closed hems. This type of hem is perfect for aluminum and other fragile metals.

Open Hem: An open hem features a soft, curved outer edge and an air pocket in its bend. It’s ideal for handles and other applications that come into contact with people. Like a closed hem, an open hem works best with material thicknesses of .040”-.125” and a flat length that’s a minimum of 4x the material thickness. Open hems are shaped by forming sheet metal to 145 degrees, then inserting a shim and flattening the material onto itself with the shim in place. Common gap sizes are 060”, .090”, .125”, .187”, and .250”.

Rolled Hem: The purpose of a rolled hem is to eliminate raw sheet metal edges from a part. This type of hem is common for door and figure edges. Rolled hems provide a nice clean look and a safe-to-handle part.

DEEP DRAWING

Deep drawing is one of the most widely used processes in sheet metal forming. Deep drawing is the process of converting blank of sheet metal into a cylindrical containers with a flat or hemispherical base. It is combination of metal forming process & sheet metal process. If the ratio of height to diameter of the product is greater than 0.5 then this drawing operation is known as deep drawing. Apart from its use in many other sectors, it is applied in the automotive industry for the manufacturing of car body parts.

Deep drawing processes are divided into three types:

Deep drawing with tools
Deep drawing with active means
Deep drawing with active energy

In the automotive industry, deep drawing is usually carried out using rigid tools.

The components of deep drawing process are as follows :-

Blank: It is work-piece on which drawing operation is done. It is a blank / Piece of sheet metal which is placed onto the die hole. After application of drawing force, the material of blank form and take shape of punch.

Punch: Punch is used to apply drawing force on blank material. The shape of product depends upon shape of punch. The punch pushes the blank material into the die cavity.

Die: It is generally made of cast iron or tool steel. The blank bend over the circular shape of die. The clearance between the die and punch should be enough to allow drawing and to avoid shearing failure in material. The corners of the die should have fillets to avoid shearing.

Binder: Binders/Blank holders are used to hold the blank surface onto the die surface. The blanking force should be optimum. If it is greater than optimum limit then it will results in shearing at die corners. If it is lower than optimum limit then it will results in wrinkling defect at flange surface of product.

The figure illustrates the deep drawing process. The rigid tools consist of a punch, die and binder. In deep drawing, the plate holder closes after the metal sheet blank has been inserted. Next the sheet is clamped between the die and the binder. This process slows down the flow of the sheet while it is being drawn and thereby prevents wrinkles from forming under the binder. The punch stretches the sheet over the die radius and forms it in the die. The amount of punch force necessary for forming is thereby continually increased up to the lower dead center of the punch. Whereas in pure deep drawing there is no reduction of sheet metal thickness, forming is achieved in stretch forming purely as a result of a decrease in sheet metal thickness. Stretch forming is extensively used for the forming of only slightly curved parts with low depth of draw (e.g. roofs, doors). When drawing complex car body parts in practice, there is usually a combination of stretch and deep drawing involved. It is necessary that the sheet metal is stretched as well as possible without reaching the material’s limits

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 rolls of the desired pattern. Sheet metal embossing is generally used to create logos, lettering, small depressions, etc. It is also used to create desired non-slip surfaces as well as other geometric features on the sheet metal. In sheet metal embossing, the deformation of the metal takes place because of stretching of the metal (or tensile force acting on the metal), due to which the sheet metal is subjected to excessive thinning and fracturing. Thus the depth to which the material is depressed depends upon the thickness of the sheet as well as stretchability of the material. Embossing is generally used to make dimples on sheet metal to increase their rigidity as well as it is also used for decorative purposes on panels in houses or other places.

Characterestics Of Embossing

Embossing has an ability to form (change the shape) of ductile metals.
Embossing operations can be used for high production purposes too.
The embossing operation has the ability to maintain the same metal thickness before and after the embossing operation.+
It has the capability to produce longer lengths of embossed sheets.

Factors Effeting During Embossing

Pressure: Pressure applied to the sheet metal should be controlled during embossing.
Heat: Heat level should be maintained consistently during the embossing to get the perfect impressions.
Die depth: The depth of the die should be adjusted such that the extreme thinning or failure of sheet metal does not occur.

PUNCHING

Sheet metal punching is a cutting process that involves applying sufficient shearing force to a sheet of metal to remove material. The most common punched holes are simple geometric shapes (circle, square, rectangle, etc.) or combinations thereof. The die has a cutout in the form of the desired function and is placed underneath the sheet. Punches move through the sheet metal, while dies on the other side of the metal piece act as a support, ensuring that the metal does not crack and that punches create precise and error-free holes. When a punch hits the punching die, a scrap slug is removed from the metal workpiece. The metal workpiece is pierced with a hole result of this operation.

Characteristics

Its ability to create cost-effective holes in strip and sheet metal during medium and high-volume manufacturing processes.
The ability to easily create holes in various shapes.
The punching method involves forcing a hardened steel punch into and through a workpiece. The size of the hole formed in the workpiece is determined by the punch diameter.

BENDING

Bending can be defined simply as a forming operation in which the metal is deformed along a straight axis. Both compression and tension occur when bending sheet metal. The inside radius of the bent metal is in compression or being squeezed together. The outside bend radius is in tension or being stretched. The Bending process involves the shaping of metal plates or profiles to a predetermined shape by applying force to them. This then causes such metal profile to bend at an angle to form a particular shape.

The process of bending steel is called Sheet Metal Bending. The Sheet Metal Bending Process is a metal forming process in which a force is applied to a piece of sheet metal, causing it to bend at an angle and form the desired shape.

MASTIC SEALANT

We cannot join the inner panel and outer panel by welding or riveting as it will make spots or marks on outer panel which will affect the aesthetics. So in order to join these two panels we use mastic sealants. Mastic sealants are hard rubber like material which is applied in between inner and outer panel at specified spots and when it get into hot bath, it will get hardened and join the two panel . The area of influence of each mastic sealants is 80mm diameter. Usage of mastic sealants results in reduction of NVH.

MATERIALS

Material plays a major role in any manufaturing components. Wrong selection of materials results failure of the particular component. The two material used to manufacture back door are Steel and Aluminium

Some of the characterestics of Steel and Aluminium are as follows

STEEL	ALUMINIUM
* Parts are stamped	* Parts are casted
* Reinforcements are necessary	* Reinforcements can be neglected
* Has only uniform thickness	* Local thickness can be varied
* Heavier in weight and more components	* Lighter in weight and less components
* Low Cost	* High Cost

Since the cost of Aluminium is more as compared to Steel, these are used in luxury premium cars.
Aluminium's lighter weight results in reduction of total weight and there by increases the performance.
Unlike the steel with minimum draft angle `7^o`, in Aluminium we can go for `3^o` which will reduce the thickness and there by increases the luggage space.
In Aluminium inner panel will be a casted part and will be single part and outer can be 2 or 3 parts.

DESIGN CONSIDERATION