Subaru 2.5rs Hood

The following project is the construction of a carbon fiber hood for a Subaru Impreza 2.5rs. Due to its size and geometric complexities, this part is not a recommended for inexperienced molders. To those looking to begin making their own composite parts, starting with small and simple projects to gain experience before undertaking a larger project is recommended. The materials are expensive and the process is time consuming so a devotion to the project is a necessity.

Building the Plug

This is a hood off of a 2001 Subaru Impreza 2.5rs. It will be used as the plug for this project. The hood be used, is off an operational vehicle. The downside of this, is that the hood needs to be repeatedly removed and replaced to use as a plug. Additionally it needs to remain in good condition which means additional prep work. When preparing a plug for a mold it is much easier to use a destructible part as we will see later on. The ability to permanently alter a part, allows for much simpler plug preparation.

One of the most important features to include when building a plug, is the draft. A draft is a slight angle on a vertical or horizontal surface. A draft on plug allows for easy separation of the plug and mold after the layup. A plug that has no draft, or a negative draft, will be impossible to remove from the mold and it would be nessecary to the mold to free the plug. For this plug, the hood was placed on single piece of MDF (Medium Density Fiberboard). The hood does not lay flush against the MDF so foam, drywall compound, and Bondo was used to fill in the gaps between the hood and the MDF. The foam used was insulation board which had been thrown out from a construction site. It is important to know the composition of any materials will come into contact with the materials the mold will be made from. For example, some body filler will dissolve plastics

The foam came in sheet form so it needed to be shaped to fill in the gaps between the hood and the MDF. A hacksaw blade and box cutter are good tools to get the basic shape. Then 60 grit sandpaper was used to smooth out the foam. If multiple pieces of foam are needed to fill a larger gap, use a compatible adhesive to hold the pieces of foam together. This is one area where having a disposable plug is helpful. If the plug is disposable, expanding foam would be a good material to fill in gaps between the plug and baseboard.

Plug Finishing

The next step in the process is sealing the foam and MDF. Due to its chemical makeup, the foam will dissolve if put in contact with the polyester gel coat and resin we will be using to make the mold. Wood, specifically MDF, is a porus material. Therefore, it is necessary seal the MDF. A smooth surface is needed so that a mold release agent can be applied to the plug which will in turn, allow the gel coat to pull away from the plug. To seal the foam a layer of epoxy resin was used. Epoxy does not react with the foam and it provides a good base to build off of. To get the correct draft I was looking for all around the hood, I used with a combination of drywall compound and Bondo. The drywall compound takes a long time to dry but it sands easily and gives a nice finish. It must be applied in small layers to prevent cracking. Bondo cures very quickly but is more difficult to sand. This process is very time consuming. 200 grit sandpaper will povide a sufficiently smooth surface. The final part will have a surface finish that is only as good as the plug

When the bondo has a sufficent surface finish, the small gap that is left between the hood and the foam can be filled with clay. You have to use an oil based clay or else it will dry out and crack. This is what the final product looked like.

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