Hood design-Week 2

OBJECTIVE

Design hood outer panel, inner panel and reinforcements with the given outer hood skin, master section and design parameters.

INTRODUCTION

The hood or bonnet is the hinged cover over the engine o motor vehicles. Hoods can open to allow access to the engine compartment ( or trunk on rear-engine and some mid-engine vehicles) for maintenance and repair. In design perspective hood should meet some specific conditions like pedestrian safety, hood head zone for the lock striker, force distribution when sbjected to frontal collision (occupant safety), manufacturability etc.

A typical hood assembly consist of hood outer panel, inner panel, latch and striker, hinge, hood stay. Hood stay is an apparatus for supporting the engine hood of a vehicle in its open position. The purpose of opening the engine hood is for inspecting engine room, replenishing cooling water and engine oil, changing parts and making repairs etc. Common types of hood stays are support rod and gas stay. Support rods are manually operated and they are cost effective and simple to design where gas stays require no man power but they are costly. Hoods are typically made out of the same material as the rest of the bodywork. This may include steel and aluminium. Deep drawing, hemming, blanking and piercing are the common sheet metal operations that are used in manufacturing hoods in automotive industries.  

DESIGN PARAMETERS

Hood thickness information

   Hood outer panel thickness = 0.75mm

   Hood inner panel thickness = 0.75mm

       Reinforcement thickness = 1.5mm     

DESIGN OF HOOD 

Outer panel Design : 

• Outer panels are the asthetic elements of a car. It is the styling team who provides the skin for the outer panel. The shape, position and size of the hood should the safety norms. So while designing the hood we should give importance to pedestrian safety. The outer panel is attached to the inner panel by hemming. So, it is necessary to provide required hemming flange and reliefs for the hemming operation. We also provide ,mastic sealants between outer panel and inner panel to reduce vibration. 

        

Inner Panel Design : 

• The hood inner panel is required to have a intial reaction force against the impact exerted on the hood to enhance the energy absorption capacity. The hood is further required to minimize a reduction of a reaction force by deformation in a collision.
• It is an object of the present invention to provide a hood inner panel capable of reduction of a reaction force of the hood inner panel even if a level of deformation of a hood of a vehicle increases, and ensuring a sufficient absorption capacity.
• In the inner panel only we can research and implement the safety of pedestrians and as well as the driver. In this panel only we can attach the reinforcement, embossed, mastic points etc.

        

Embossing in Sheet Metals :

• The metal sheet embossing operation is commonly accomplished with acombination of heat and pressure on the sheet metal, depending on what type of embossing is required. Theoritically, with any of these procedures,the metal thickness is changed in its composition.
• A metal sheet is drawn through the male and female roller dies, producing a pattern on the sheet metal. Depending on the roller dies used, different patterns can be produced on the metal sheet. The pressure and a combination of heat actually "irons" while raising the level of the image higher
• Throughout the inner panel we provide different embosses. We can increase the stiffness of the inner panel through these embosses. The shape and size of these embosses also determines the direction of the force distribution resulted from frontal collision. The force should always be distributed away from the passengers. here in the figure the embosses are designed in such a way that the force is directed to either sides of the car.

Reinforcements :

• Reinforcements are provided according to the design requirements. Mainly it is provided at the hinge and latch regions to improve the strength locally. Usually thickness of reinforcements will be higher than that of the inner panels and the value varies according to cost and requirement. According to the design requirement here it is given 1.5 mm thickness to both hinge and latch reinforcements.

Latch Trajectory :

• Latch trajectory is given by the circle drawnfrom the hinge axis point. Hinge axis point is the centre of the hinge axis. 
• Firstly, we can fix either the latch of the hinge. If we have fixed the hinge position then we should plot the trajectory circle for the latch. For proper mating condition, the striker axis through the tip of the striker should be tangential to the latch trajectory.

       

Mastic Data :

• Hood commonly possess large surface area and hemming can only provided through its boundary. We cannot use joining techniques like welding, riveting etc. because they leave marks on the outer panel which spoil its aesthetic appearance. So, mastic sealants are used to stick the inner surfaces of outer panel and inner panel.
• Mastic sealants are elastic compound that behave like rubber. It is applied between the inner and outer panels at specified spots. Each spot of mastic sealants has its effect around a circular region of 80mm diameter. Thus, the mastic points are arranged in such a way that it covers the areas away from the hemming which are more vulnerable to vibrate.

        

Deep Drawing : 

• Deep drawing is one of the most widely used processes in sheet metal forming. Apart from its use in many other sectors, it is applied in the automotive industry for the manufacturing of car body parts. The deep drawing process is a forming process which occurs under a combination of tensile and a compressive conditions.
• Sheet metal blank is radially drawn in to a forming die by the mechanical action of punch. It is thus a shape transformation process with material retention. The process is considered as "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 can be prevented by using a blank holder, the function of which is to facilitate controlled material flow into the disc radius. 

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 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 its effects 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 provide required reliefs at the corners.
• There are two types of hemming operations, 1) Conventional die hemming   2) Roll hemming
• In conventional die hemming, the flange is folded over the entire length with a hemming tool.
• Roll hemming process is used for the automotive sheet metals. Roll hemming process is carried out incrementally with a hemming roller. An industrial robot guides the hemming roller and forms the flange. Roll hemming process can also be divided in to 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.

        

Forces Distribusion :

•  The embossed channels were designed to direct the impact force towards outside of the structure that is dissipating the forces towards outside of structure.

Cross Section :

DRAFT ANALYSIS :

Draft analysis is carried out to see that the panel can be drawn or not. It also helps to find the negative draft present in the component which can cause difficulty in manufacturing.

Therefore from the above image we can conclude that part can be drawn without any difficulty.

FINAL ASSEMBLY after applying some colors to the parts.

CONCLUSION: 

Therefore we have designed a hood by considering all master sections, design standards and that falls under the rules and regulations of the manufacturer's standards.