Hood design-Week 2
Hood Design NX CAD
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 of 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. From a design perspective, the hood should meet some specific conditions like pedestrian safety, a hood head zone for the lock striker, force distribution when subjected to frontal collision (occupant safety), manufacturability, etc.
A typical hood assembly consists of a hood outer panel, inner panel, latch and striker, hinge, and hood stay. Hood stay is an apparatus for supporting the engine hood of a motor vehicle in its open position. The purpose of opening the engine hood is for inspecting the 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 whereas gas stays require no manpower but they are costly. The Hood is typically made out of the same material as the rest of the bodywork. This may include steel and aluminum. 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
Outer Panel Thickness = 0.75mm
Inner Panel Thickness = 0.75mm
Reinforcement Thickness = 1.5mm
Master Section
Components
1. Hood outer panel
Outer panels are the aesthetic 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 satisfy the safety norms. So while designing the hood we should give more importance to pedestrian safety. The outer panel is attached to the inner panel by hemming. So, it is necessary to provide the required hemming flange and reliefs for the hemming operation. We also provide mastic sealants between the outer panel and inner panel to reduce vibration. 
2. Hood inner panel
The inner panel is the part that gives strength and rigidity to the outer panel. The master section represents the initial emboss shape and size. Then we provide the emboss for the latch and hinge elements after fixing the location.
Figure 3: Hood inner panel
3. Reinforcement
Reinforcements are provided according to the design requirements. Mainly it is provided at the hinge and latch regions to improve the strength locally. Usually, the 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 1.5mm thickness to both hinge and latch reinforcements.
Figure 4: Hinge and latch reinforcement
Latch Trajectory
Latch trajectory is given by the circle drawn from the hinge axis point. The hinge axis point is the center point of the hinge axis. First, we can fix either the latch or the hinge. If we have fixed the hinge position then we should plot the Trajectory circle for the latch. For proper mating conditions, the striker axis through the tip of the striker should be tangential to the latch trajectory.
Emboss definitions
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 the emboss also determine the direction of the force distribution resulting from a frontal collision. The force should be always distributed away from the passengers. Here in the figure the embosses are designed in such a way that the force is directed to either side of the car.
Figure 6: Inner panel embosses
Mastic Data
Hood commonly possesses a large surface area and hemming can only be provided through its boundary. We cannot use other joining techniques like welding, riveting, etc because they leave marks on the outer panel which spoils its aesthetic appearance. So, mastic sealants are used to stick to the inner surfaces of the outer panel and inner panel. Mastic sealants are elastic compounds 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 is more vulnerable to vibrating.
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 that occurs under a combination of tensile and compressive conditions.
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 a "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 radial drawing stress and 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 die 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, improve the appearance of a part, and 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 provide the required relief at the corners.
There are two 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
It is the roll hemming process that is used for automotive sheet metals. 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 that follows a defined path.