Make molded-fiberglass parts

Make molded-fiberglass parts

van Mourik, Dick



Wheel pants and other formed parts made easy

Many modelers reach a point at which they want to do something unique. My particular modeling interest in the Czech-made Zlin aircraft has led me to design two fully composite, 1/4-scale Zlin Z-50s.

A prominent feature of these aircraft is their streamlined wheel pants, and now you can follow these step-by-step instructions to make your own fiberglass wheel pants. Since a wide variety of other aircraft use molded fiberglass parts, you can apply the same basic process to form other parts; it’s amazing what you can make once you know this technique.

1 Several photos of the full-size wheel pant (al different angles) and a sketch provide the basic information for carving the plug. I used a block of hard polyurethane foam, but pine or bass wood could also be used.

2 Here the plug for the wheel pants has been carved and sanded smooth. After a final sanding with fine-grit sandpaper, treat it with a grain sealer and spray-paint it for a very smooth finish. The plug needs to be finished to the highest possible level, as any imperfections will show up in the finished part. Careful sanding and polishing at this stage will pay off later.


So that you can remove the part from the finished mold, you have to split the mold into two separate pieces (in this case, a left and a right half). You’ll need to make a supporting plate on which a flange can be formed. This plate is best made from plastic-coated fiberboard or a similar flat, smooth-surfaced material such as kitchen countertop material available from home improvement stores.

I made my plate with an extra bottom sheet that supports the plug with three wood screws that I can adjust to hold it at the correct vertical position. It’s essential that the plug fit perfectly into the support cutout. A perfect fit can never be achieved just by cutting the plate, so you have to cut the opening 1/8 inch larger than the plug all the way around and fill in the gap later.

4 Next, adjust the screws so the plug sits exactly halfway in the plate. Then, treat the plug with a mold-release agent and apply some blobs of auto-body filler to the screws. After curing, the plug will sit firmly in mace and rest on to he screws

5 Once you’ve cleaned the plug, again apply plenty of the release, agent to the plug and plate. Don’t spare the wax and PVA! Fill in the gap between the plug and plate with auto-body fillet and let it cure.

6 When the filler has hardened, carefully remove the plug and sand the filler flush to the plate. When you put the plug back in position, it will fit perfectly in the supporting plate.

7 Coat the plug and

the surrounding support plate with more release agent, and let them dry completely. Mold compound (gelcoat) is stiff, so stir it carefully to mix the base and hardener thoroughly. Starting in the comers, gently brush on the first layer of mold compound with a soft brush. To avoid air bubbles, use a small brush. You’ll need to apply two layers for a decent result; brush on the second coat after the first layer has hardened but has not yet fully cured (i.e., it’s still sticky). Apply thick layers to avoid retouching the mixture where you’ve already applied it.

To ensure a firm bond between the mold compound and the epoxy resin, apply cotton fibers, chopped glass strands, or a mixture of both over the mold compound after it has hardened slightly. Once it has cured, remove excess material with a vacuum cleaner.

8 Since you won’t be able to force the glass cloth into the mold’s sharp corners, you’ll have to fill them with a mixture of resin, chopped strands and a thickening agent such as Aerosil. A generous layer of this mixture prevents the cloth from showing through. For best results, use only an aircraft-grade epoxy resin.

9 Build up the mold with layers of fiberglass cloth and try to minimize the amount of resin you use. Once the cloth has been soaked, excess resin doesn’t add strength. Add layers of cloth until a rigid mold is formed. For small items such as the plug, the mold wall should be 1/8- to 3/6-inch thick, which requires about five layers of 1-ounce cloth. Be sure to apply the cloth to a fairly wide area (about 1 inch) around the plug and on the plate. This provides a good surface against which you’ll form the mold’s second half.

10 Patience is a keyword in this process! Don’t be tempted to remove the mold from the support plate-or the plug from the mold’s first half-too soon. Only after everything has completely hardened can you form the mold’s second half. Here, the plug and mold have been removed from the support plate. Clean both pieces and liberally coat them with more wax and release agent.





15 11-15. Repeat all the previous steps to form the mold’s second half: first, the gelcoat/mold compound; then, the chopped strands; next, the generous layer of thickened resin; finally, the glass cloth. In most cases, there is plenty of excess resin to soak into the first layer of glass cloth. Note how the multi-directional cloth easily conforms to the plug’s compound curves. Apply the rest ot the cloth layers ana saturate rem witn resin until you build up the desired wall thickness.


Once the mold has fully cured-and is still attached to the plug-trim the excess material from the mold halves and drill through the flat flanges. This ensures accuracy when you reassemble the mold to make parts. Now you can remove the halves from the plug. Use some warm water to dissolve the PVA mold release, or use a plastic credit card to pry the mold flanges apart. Don’t use metal tools or brute force! Here you see the two halves as they came away from the plug. Pretty smooth!


Making the actual fiberglass part is basically the same process that you used to make the mold, except that you apply the materials to the inside of the mold instead of over the plug. Start by waxing and applying PVA to the inside of the two mold halves. Although gelcoat is commonly used for the first layer of the part, it’s also possible to use a good-quality, epoxy-based paint. This has the added advantage of allowing you to match colors exactly. Here, a single layer of yellow Hobby Poxy* has been brushed on.

18 The star

board half of the mold has two layers of paint, followed by thin layers of resin, cotton fibers and thickening agent. These layers prevent the cloth from showing through the final product, but they need to be very thin to minimize weight.

19 Apply one layer of 1/2– ounce and one layer of 1-ounce glass cloth to the mold, and blot up any excess resin with a paper towel.

The cloth should protrude from the mold by about inch so the two halves can be joined.

20 Here are both mold halves just prior to being joined together; at this stage, all the materials are still wet. Cut the cloth on one half of the mold flush with the mold flange, and leave the cloth on the other half protruding.

If you are serious about making scale fiberglass parts, then consider making your own two-piece molds. The final product is well worth the effort.

21 To join both halves successfully, lift the protruding cloth slightly, and apply a mixture of resin and fiber strands underneath (about 1/8 inch wide). The cloth will fold inward slightly, as shown.

22 To avoid distortion of the pants after the cutout for the wheels has been made, put two carbon-fiber strips around the cutout’s perimeter; This stiffens the structure immensely.

23 Here’s the final result as it comes out of the mold, all glossy and shiny. All that’s left is to trim the excess resin and the foot of the pant, and you have a perfect, lightweight product. Making your next wheel pant is straightforward and requires only a fraction of the time you’d need to make a wooden one.

Copyright Air Age Publishing Mar 2001

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