Simple Methods for Molding Fiberglass and Carbon Fiber

Step 2: Method #1- Creating a Mold and Pulling a Positve

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I used this method to modify existing bodywork on a friend's Ralt R5 CSR Mazda race car. His new tire/wheel set up was causing the front tires to hit the existing bodywork so he modified it by cutting holes in the fenders and then bolting on some plates with spacers so the front tires would clear when the suspension was fully compressed. The modifications were pretty ugly and un aerodynamic (not to mention the tires would still rub on occasion) so he asked me to make new fender parts that could be grafted onto the existing bodywork so he wouldn't have to buy all new bodywork for the front of his racecar. I made these modifications several years ago so I don't have photos of the entire sequence so I did drawings to fill in the gaps.

The first thing I did was to figure out how much I wanted to raise the fender line and then cut a template to be used to cut the blue foam. The blue foam is then cut using a hot wire cutter and is placed on top of the existing fender to check the fit.

Next the foam is taped down to the fender using clear packing tape- the tape prevents the polyester resin from melting the foam when applying the fiberglass. I then made some vents for the new fender using some wood molding- this is hot glued in place on the taped foam.

The taped foam then gets a coating of mold release wax.

Now the fiberglass cloth is wetted with polyester resin and is applied over the foam. You can use woven fiberglass cloth or chopped strand mat for this. If you use CSM you have to use polyester resin- the polyester resin melts the styrene binder in the CSM. All epoxy resin will do is make a huge mess.

One trick I do when using woven cloth is I pre cut my cloth and stick it down on my pattern with some 3M Super77 spray cement and then wet the woven cloth out with resin using a short bristled paintbrush. If the cloth (or CSM) has to be applied in sections make sure the sections overlap by at least one inch.

Once the resin has cured you can pull the fiberglass mold from the foam pattern.

To mold a carbon fiber positive from the fiberglass mold you first apply a mold release wax to the fiberglass mold.

Then start wetting out the carbon cloth with resin and laying it inside the fiberglass mold. When wetting out the resin use a short bristled paintbrush and a "stippling" action to make sure the resin has fully penetrated the carbon cloth as is placed in the mold. I tend to use epoxy resin when laminating carbon cloth as it produces a much stiffer final product.

I usually apply layers of carbon cloth at alternating 45/90 degree positioning. This is because the woven carbon cloth only has stiffness in the direction of its fiber orientation.

Before the resin has fully cured you can trim the excess material around the edges of the mold with scissors- this is much harder to do later.

Once the resin has fully cured the part can be removed from the mold.

After the molding process was finished all I had to do was bond the new carbon sections onto the existing bodywork and blend it in with some filler.

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