Hood design-Week 2

HOOD DESIGN

• Introduction

• Hoods are the sheet metal compartment that covers the engine compartment and are located at the front of the vehicle.
• Hoods are made up of an outer panel as cover and an inner panel for strength and durability
• The major components of a hood assembly are: Hood outer and Inner panel, Latch and striker, Hinges, stiffeners and mastiff points

• Design criteria

Hoods are designed keeping various aspects in mind. Most of the important ones are as listed bellow:

• The location at which the vehicle is used. This is used to determine the angle at which the hinge has to be in the open condition. The average height of a person is taken and the appropriate changes are made
• Type of hood stay. It could be a cheap and effective support rod or a more robust and gas stay. The selection depends weather the company targets cost cutting or adding value to the vehicle.
• The embosses on the hood are deep drawn keeping in mind in case of a head on impact, most of the forces gets dissipated to the sides and thereby reducing the impact acting on the passenger side
• The position of gas stay or support rod will always be on the other half were the battery is kept. This helps in easy assembly
• To get a higher NVH score, various methods are used. To reduce noise and vibrations, mastiff points are given between outer and inner panel and stiffeners are used in between the hemming flange
• Adhesive sealants are applied at the mastiff points which provide reinforcement to a radius of 80mm. The adhesives are activated while hot dipping process.
• To reduce the hardness levels, some material is removed from the inner panel, so that in case of an impact, the pedestrian would not have a very high impact load and the material is removed from such location the pedestrian could land on in case of an impact
• The maximum depth of the emboss will be less than 25mm as the engine comes directly below this
• The position of the latch and striker plays a very important role in hood design. Any changes in alignment of either the hinge or the striker will result in problems related to closing and opening of the hood
• The striker should be placed in such a position which is perpendicular to the hinge axis.
• The distance between striker and hinge axis determines the radius at which the hood is opened and closed.

• Manufacturing Process

The hood is mostly made using alloys of steel, aluminum, and magnesium. They are also made using carbon fibers depending on the requirements. Carbon fiber and Magnesium alloys are not usually used in consumer car and are mostly used in racing cars for significant weight reduction and improved strength. . The hood is manufactured by using various practices. The most important ones are the forming processes such as deep drawing and blanking process. Spot welding, grinding and deburring processes are also commonly done to manufacture the hood.

The following are the steps taken while manufacturing the hood

• The sheet metal is laser cut to the required dimensions. The outer most and the spaces for flanges at the inner panel is given at this stage
• The sheet metal will be deep drawn by using various forming die. The angle of the emboss will always be in an open manner such that the die will be able to drawn out once the forming is done
• All the reinforcements will be created at this stage and the metal that is unwanted will be removed by blanking process
• The sheet metal is later passed on to a hydraulic press were the metal is bended to the shape
• Some of the reinforcement that cannot be deep drawn will be welded into the inner panel and the deformities and the burrs will be removed by grinding process
• A coat of paint will be applied over the inner panel by dipping the inner panel into a liquid bath of paint
• Pores are given such that all the liquid will be washed away at the time of drying
• It is to be noted that there will be no spot welds on the outer panel of the car as it affects the aesthetics of the vehicle
• The adhesives are then applied over the mastic points in the inner panel which gets activated only after the application of hot liquid bath
• Striker is placed over the emboss inside the inner panel
• The outer panel is then placed above the inner panel and hemming is done at the edges around the panel. Hemming is a process of bending the outer panel with a very sharp radius over the inner panel.
• Stiffeners are placed in between the outer and inner panels to deduce the noise and vibration. It also helps to prevent the water from entering through the hemming region
• Stiffeners are sponge like material that becomes rigid and takes the shape it is placed in
• A total of 15mm will be overlapped over the inner panel after the hemming is done and a gap of 2mm will be there between the inner panel and outer panel at the hemming region.
• Hemming process can be imagined as a process of trapping the inner panel by sandwiching in between the outer panel.