Monday, 31 December 2007 09:00

From plug to prototype: Making a Mold For Custom Parts

Written by Rich Evans

Last month’s article discussed creating plugs. You should have a polished and finished prototype plug, ready to make molds.

This process is similar to the plug process, since we use the same materials, steps, and techniques. The mold is an exact mirror image of the plug and is what will be used to reproduce new prototype parts over and over again.


Start by looking at the plug to decide how many pieces will be necessary to make an efficiant mold. Depending on the plug, one mold may suffice or it may be necessary to split it into two halves. The goal is to have mold pieces open in a wide angle, starting with the deepest point at the center of the plug and spreading out towards the edges of the mold.

The more complicated the shape of the plug, the more pieces will be necessary.

    Rich tip: Make sure to plan this step out carefully. As a rule, we only used one or two molds per part.

The next step is to install the flanges to the centerline of the plug. We used 2-inch and 3-inch wide flanges to split the plug from the mold and separate multiple pieces of the mold, if necessary.

At this point, it is important to have a clean, designated workspace with the necessary materials on it — 3 ounce sheets of fiberglass matting, cloth, resin, catalyst, wax, and gel coat.

    Rich tip: Be sure your plug is smooth and polished so each copy will be a perfect replica of the prototype.

Four coats of wax are applied to the plug. Special mold release products are available for this process. Apply wax to the plug, let it dry, and then buff it to a shine. Repeat until there are four coats of wax on the plug.

Not only does this help the mold release from the plug, but it also acts as a filler for any imperfections that might remain on your plug. There will be imperfections as the mold is being made. If you have any doubt, put on some extra layers of wax. Remember to let each layer dry completely.


To help seal up the plug during the mold-making process, we spray the plug with a bottom-feed spray gun.

Go to the worktable and start soaking the 3-ounce fiberglass matting and cloth sheets with resin. Begin applying them to the plug.  Smooth out the matting as much as possible and remove every air bubble you can.

    Rich tip: Don’t freak out about the bubbles. There will always be some. Just get rid of as many as possible.

Repeat the process over and over again, until there is a total of 12 ounces of fiberglass making up the mold.

Now, using the flanges that we have placed, separate the mold as wide as possible. Next, trim off any excess flash from the mold and clean it up by wet sanding with 800-grit sand paper. Finish up by polishing it with a medium compound.


That’s about it. You have a finished mold and are ready to begin producing your own line of aftermarket fiberglass parts.

As always, we have a lot of projects in the works at Huntington Beach Bodyworks and through their production, there will be some jobs receiving these techniques that I have explained in detail over these last few articles. They will be more specific and I think you will be interested in them. Not to mention, we have some very interesting vehicles in the shop right now. I will shine some light on those in the future, so come back and check us out regularly.