There are several fiberglass parts on this scale OV-10 Bronco, including the nose cap, rear cap, and engine cowls.
What You’ll Need
Here are the basic materials needed to make a mold and finished part. Additional tools, like scissors, mixing sticks, and latex gloves, are also required.
The materials you’ll need are readily available online and from local automotive- and marine-supply outlets. There are lots of brands to choose from. First, you’ll need some plain-weave fiberglass cloth. I use 2- and 7-ounce-weight cloth from Thayercraft Industries (thayercraft.com). Everyone is familiar with epoxy resin, and it is a good choice for hobby use as it is odor-free. I use resin with a mix ratio 2:1 and get it from Fiberglass Florida (fiberglassflorida.com). You will also need a PVA (polyvinyl alcohol) release agent, two-part surface-coat epoxy, Rexco Partall #2 molding wax, resin thickener, chopped fiberglass fiber, rubber gloves, and measuring cups. All these supplies are available from Fiberglass Florida.
1 Here is the plug removed from the fuselage after it has been shaped, sanded, and sealed with epoxy resin.
2 Here, body filler has been applied to fill in and finalize its shape.
3 After sanding the plug smooth, it is then primed with filler primer. After it has dried, it can be sanded and reprimed as many times as needed to produce a blemish-free surface.
4 Here is the plug with its final coat of spray paint. The paint should dry for at least 24 hours or longer.
MAKING THE PLUG
Before you can produce fiberglass parts, you have to make a plug. This wood-and-foam structure is usually made on the model so that the finished part will fit properly. For the nose cone, the plug was made by tack-gluing wood plywood cross-sections, both vertically and horizontally, to the front of the fuselage. The plug’s shape was then filled out by using foam blocks cut and sanded to shape. Once the contours were correct, I sealed the plug with epoxy resin. I then applied auto-body filler to fill in any defects and finalize the shape of the part to be molded. Sand and fill your plug as needed until you have a smooth, blemish-free surface. A good coat of filler-primer along with some spot putty help eliminate any imperfections. Sand the surface smooth again with fine sandpaper, and shoot on a light final coat of primer. Once you are satisfied, give your plug a final coat of any good-quality spray paint. Apply several light coats to avoid any runs, then make sure to let the paint dry completely for at least 24 hours (longer is better). This will reduce the potential of the plug sticking to the mold.
5 After attaching the finished plug to a plastic base, apply several coats of wax and PVA and let dry. The actual mold fabrication starts at this point.
6 Mix up the two-part surface-coat resin, and spread evenly over the plug and base. Allow it to cure until it is tacky.
7 Apply a mixture of epoxy resin with Cab-O-Sil thickener and chopped fiberglass fiber. This will build up the mold surface and fill in any voids.
Start by attaching the finished plug to a large plastic base. I find that it’s easier to apply wax and PVA to the plastic base before attaching the plug. A couple of screws will do a good job of holding everything together. Mix up some two-part surface-coat epoxy, and spread it evenly around the plug and onto the base. Allow it to cure until its surface becomes tacky but not soft. This is followed by a layer of epoxy resin mixed with Cab-O-Sil thickener and chopped fiberglass fibers. This application builds up the mold surface and fills in any voids. I use a brush with cut-down bristles to make spreading the mixture easier. Follow this by pouring more of the mixture onto the mold, and spread it around evenly to build up the mold and add strength. After the mixture begins to set up, apply the first layer of precut fiberglass cloth over the plug. Overlap the cloth a little, and force out any air that might get trapped. Keep the cloth evenly distributed around the plug. Add additional layers of cloth, and trim them as needed to fit again, distributing the layers of cloth evenly over the plug. As you build up the surface of the mold, continue to trim the cloth as needed, and use the remnant pieces to build up the flange area on the plastic base. Five or six layers are sufficient for this simple mold. Larger, more complex molds will require more layers.
8 After you have a thin layer brushed onto the plug, you can pour the mixture evenly over the mold to help build up the mold more quickly. Make sure that there are no voids or air pockets.
9 As the mixture just starts to set up, apply the first layer of precut fiberglass cloth over the plug.
10 Add additional layers of cloth, and trim them as needed to distribute them evenly. Remove any air pockets, and apply more resin to fill any dry spots. While building up the surface of the mold, trim the cloth as needed so that it will lay flat over the last layer, and use the remnant pieces to build up the flange area.
11 Here the final layer of cloth is being applied. Work the cloth over the base to form the flat flange, making sure that there are no trapped air bubbles or dry areas.
12 Here, you see the mold removed from the plug. Note that the interior is clean and smooth. Now, apply more wax and PVA to the inside of the mold.
13 Start the part layup with a layer of 2-ounce cloth, wetting it with resin to smooth it against the inside of the mold.
14 The next two layers of 7-ounce cloth are applied. Make sure that it is applied evenly all around. When the cloth and resin are nearly cured, trim the excess cloth even with the mold’s edge.
15 After the resin has cured, the part is ready to be removed. Use a thin razor blade to break the edge from the mold.
16 Compressed air is sometimes required to release the part from the mold. Here, the new molded nose cone is out of the mold. Remove the wax and PVA by washing the part with soap and water.
This shows one of Rich Uravitch’s Broncos with a fiberglass nose cone installed. Notice that it has a landing light installed. Adding a light is easy using molded parts.
LAYING UP THE PART
After the mold has cured completely (overnight), remove the plug and inspect the inner surface of the mold. The mold should easily pop out. Apply the Partall molding wax, and put on several coats of PVA. Once these dry, apply the first layer of 2-ounce cloth, and work in the resin to wet the cloth and lay it up against the surface of the mold. Overlap the pieces of cloth, and make sure that it lays down smoothly without any wrinkles or air pockets.
Now, apply two layers of 7-ounce cloth in the same fashion. Make sure that the cloth is even all around. Depending on the size of your part, additional layers of the heavier cloth might be required, but for this nose cone, two layers are sufficient. When the epoxy is almost fully cured, use a sharp hobby knife and trim away the excess cloth even with the mold edge.
After the resin has fully cured, the molded part is ready to be removed. Use a razor blade or a Popsicle stick to break the edge of the part from the mold. I sometimes use compressed air applied with an air chuck to help release the part from the mold. Once the part is out of the mold, remove the PVA and wax by washing the part with soap and water.
Making molded fiberglass parts is easier than it might first appear. Once you know the basics, you will find that this technique is a great way to make all sorts of lightweight, strong parts for your model. Save your plugs and molds and you will be able to make new parts if necessary.
Pro Molding Tips
- Before beginning, make sure that your mold-making materials are compatible. Getting them all from the same source is a good idea.
- Do all your mold making and parts layup in a clean, dust-free work area.
- Your molded parts will only be as good as your plug. Make your plug as perfect as you can.
- Make sure that the paint on your plug is completely dry before making the mold.
- To speed up your progress, have all the materials ready to go and precut your cloth.
- Keep the area clean, wipe up spills right away, and change your latex gloves often.
BY LENNY STANKO
This process has gotten a little bit more complex since my days In the USAF. Back then it was meatball fabrication, epoxy, hardener, fiberglass cloth, and PVA film.
That was it, and the process much more streamlined. Great article, and now I know where toget my supplies.
This is a real art! Thanks for sharing. But with 3D printing now-a-days, I think this may become a lost art.
I think it would be harder to ‘get it looking right’ in a computer file vs making a plug and shaping it by hand. Besides…everything is cyclical. There will eventually be a “makers renaissance” and this will come back into favor. I’m not scratch building yet but my interest in the hobby started ONLY in RTF foam planes from a box, then to balsa and composite ARF kits and now I’m getting really interested in scaling these builders kit style models out and possibly glassing over my TF P-40’s sheeting and painting it myself. so I eagerly read this article and thoroughly enjoyed it. Hats off to the author
Im thinking about doing the same with a balsa built p51 1/7th scale
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