As weight-saving assumes greater importance in automobile design, the replacement of steel parts with polymer-composite substitutes becomes increasingly attractive. This case study explores the competition between steel and thermosetting composites for manufacturing a bonnet (hood) of a passenger car (Dieffenbach [1]). The weight of the bonnet depends on the car model: a typical weight is about 8 kg. The shape is a dished-sheet and the requirements on tolerance and roughness are 0.2 mm and 2 μm, respectively. The design requirements for the car bonnet are listed in Table 1.
Figure 1. A Car Bonnet
| Material Class | steel or thermosetting-composite |
| Process Class | primary, discrete |
| Shape Class | sheet-dished-nonaxisymmetric-shallow |
| Mass | 8 kg |
| Surface Finish (Roughness) | 2 μm or better |
| Tolerance | 0.2 mm |
| Batch Size | 200,000 |
Table 1. Car bonnet: design requirements
Figures 2–5 show the selection. Figure 2 shows the first of the selection stages: a bar chart of mass range against material class. 'Ferrous' and 'thermosets' and 'polymer matrix composites' were selected from the material class menu, causing the chart to be divided into three sections, one for each class of material. The selection box for the bonnet is placed at a mass of about 8 kg. Many processes pass this stage.
Figure 2. A chart of mass range against material class. The box isolates processes which can shape both ferrous metals and thermosetting composites and can handle the desired mass range.
The second selection stage is shown in Figure 3, where 'primary shaping processes' is selected from the process class menu for the x-axis. Surface roughness is plotted as the y-axis. The box-selection isolates processes which are capable of achieving roughness levels of 2 μm or better.
Figure 3. A chart of roughness range against process class. The chart identifies primary processes capable of achieving roughness levels of 2 μm or better.
We next seek the subset of processes which can produce the shape of the bonnet. 'Sheet-dished-nonaxisymmetric-shallow' is selected from the shape class menu. Tolerance is chosen as the y-axis. The corresponding chart is presented in Figure 4. The selection box specifies the requirement of a tolerance of 0.2 mm or better. Some of the composite processes (hand lay-up, spray-up, etc.) cannot satisfy the tolerance requirement, and fail. Machining processes fail for a different reason: they are not primary processes for making sheet shapes.
Figure 4. A chart of tolerance against shape class. Processes which can make the desired shape are plotted and the box isolates processes which can produce tolerances of 0.2 mm or better.
The specifications for the car bonnet, so far, have caused many processes to fail the selection stages. Only two processes survived: one for ferrous metals and one for thermosetting-composites. These are press forming for a metal bonnet and SMC molding for a composite bonnet. The results are listed in Table 2.
The choice between these depends on the batch size. In Figure 5, a batch size of 200,000 is represented by the selection box. Both press forming and SMC molding are competitive for the desired batch size, according to our chart.
Figure 5. A chart of economic batch size against process class
| Press forming |
| SMC molding |
Table 2. Processes for making the car bonnet
Two processes (out of 131) are capable of making the bonnet. Most other processes failed because they cannot handle the material or make the shape. A batch size of 200,000 was chosen, at which both press forming of steel and SMC molding for a composite bonnet emerged as the successful processes. More detailed cost analysis is needed before a final selection is made. Commercially, press forming is used for making bonnets out of steel – which is the material commonly used for production volumes of 200,000.