FRP Composites have their own special set of considerations in design and use. And we will discuss a few here. The traditional fiberglass (unsaturated polyester with glass reinforcement) uses gelcoat on the molded (decorated) side. This is really only one side of the part, as this is not a matched-mold process. So the back side needs to be hidden from view or covered with another material, etc.

A boat, for example has a Deck and a Hull that are mated together so that the back sides of the parts are hidden from view. The inside of the cabin is then upholstered and trimmed out for comfort and decoration.

I have worked with fiberglass tabletops before where the top side is gelcoated in a decorative finish and the edge wraps down and trimmed. All of the unsightly surfaces face the floor where they cannot be viewed.

There are a few closed-mold processes that can be implemented, but these really only achieve a Class B surface out of the mold. With rework and repair, a Class A surface can be accomplished. I know of some folks that make Boat Access Hatches where they repair the backside (bottom) and rework it so it looks pleasant when opened.

Molded-in features must be used with caution. Sharp edges must be avoided because the process hates having to get resin and reinforcement into them, and because they can be stress concentrators.

Parts also need to have a minimum of 2-5 degrees of draft or angle in the pull direction so they can be removed from the mold. Molded in undercuts and reverses with multipiece molds add extreme expense and difficulty. Molded-in holes are not general practice because the process does not allow for crisp edges on these details.