Fiberglass molds are commonly used to make multiple copies of a part that may have a complex shape. Some of the advantages of using a fiberglass mold are: the materials are inexpensive, they are easy to make, last many years and can be used to produce hundreds of parts. The process starts with a pattern that you wish to copy. In this case I have started with a wooden pattern of the front portion of a fuselage with an access hatch. This is a common part that a modeler may want to build in fiberglass. At this point the pattern has been sanded to a smooth surface and finished with a primer.
We will start by making a mold for the fuselage hatch, this way we will make a pattern for the hatch and use the hatch pattern to make the fuselage pattern fit the hatch perfectly. Applying mold release to the pattern, it is important to apply both Partall mold release wax and Coverall liquid mold release. Waxing is repeated; a number of times depends upon the surface. If the surface has any potential for porosity, multiple coats are needed. These layers of wax will fill the tiny surface imperfections. If you have any doubt, add more layers of wax. If they are not filled completely, the epoxy will lock into these imperfections. Liquid Coverall mold release is then applied over the wax.
Coverall can be applied with a good quality paint brush, water can be used to clean the paint brush after each coat. Apply a thin even coat being careful to eliminate any excess liquid or runs. Let this coat dry and apply a second coat to assure complete uniform coverage.
Dust and dirt can be trapped by the mold release as it dries and becomes a part of the mold surface. If you have any defects in your mold release remove the layer with water and a paper towel and reapply the Coverall mold release. The surface you see in the mold release will be the surface finish you will see in the mold and the parts. After you mold the part the mold release must be removed from the mold and the part with water and a paper towel and reapplied before the next molding operation.
Now you are ready to start applying the layers of epoxy and fiberglass cloth. Use the following West System Epoxy products:
105 Resin is a low viscosity resin that easily wets out the cloth and allows for minimal air bubbles between the cloth layers.
205 Fast Hardener allows sufficient time to wet out each layer then gels up quickly so the mold building process can be completed in a few hours.
406 Colloidal Silica is used to thicken the epoxy system to aid in applying epoxy to vertical surfaces, filling corners and details in the mold.
423 Graphite Powder is used to give the tooling coat layer of the mold a black color and a harder, more durable surface.
|To make the mold for the fuselage hatch, apply a tooling coat layer of epoxy to the pattern in the area where the hatch will be located . Tooling coat is made by mixing a batch of West Systems 105/205 Epoxy add a teaspoon of graphite powder to the mixture then stir in 406 filler to thicken the mixture to the viscosity of ketchup. Make the mold a little larger than the intended hatch size. Allow the tooling coat to cure to a soft solid; this should take about 30 minutes . Apply two layers of fiberglass cloth and epoxy over the tooling coat layer. On this small mold for the hatch, this lay up should be sufficiently strong.|
|After the hatch mold has cured overnight, remove it from the pattern, and clean off the Coverall with water and a paper towel. Mold release the hatch mold by the method in step 1 and apply a thin coat of epoxy to the inside of the mold. After that cures to a solid, about 30 minutes, lay in a layer of glass cloth and let it cure overnight. Pull the rough hatch from the mold and trim to the finished shape with a Dremel tool and sand paper. Use this hatch pattern to mark the hatch location. Hold it in place and mark the outline with spray paint.|
|With a Dremel tool cut away wood from the pattern. Cut away enough wood so the hatch will fit flush with the fuselage surface.|
|Apply mold release wax and several coats of Coverall to the hatch pattern.|
|Coat the recessed hatch portion of the pattern with West Systems Epoxy and 406 filler mixture. Then press the hatch pattern in place assuring the hatch is flush around the edges. Let the epoxy cure overnight and then sand away all the excess epoxy to achieve a flush smooth edge. Now remove the hatch pattern from the fuselage pattern. You should now have good hatch edge formed into your fuselage pattern.|
|The fuselage will be molded in two half’s then joined. If you have a complex fuselage shape you may need to divide it up into more parts. In order to remove the finished part from the mold, there must be a positive relief angle on all the mold parts. This means that the mold cross section must get wider as you go from the deepest part of the mold to the outer edges. The parting surface of the mold defines how the multiple parts of the mold will fit together. The parting surface is made from a sheet of ½” MDF with the profile of the fuselage cut from the center of the sheet.|
|The small opening around the edges of the fuselage pattern must be sealed. This can be done by first mold releasing the pattern. Then place the fuselage pattern into the parting surface with half the pattern exposed. Fill the opening around the edge with a mixture of West Systems Epoxy and 406 filler. After the epoxy has cured remove the pattern from the parting surface and block sand the parting surface flat along the edges. After you have the edge gaps filled build up the areas where openings in the mold are needed with clay. These openings give you access to the inside of the mold so you can join the half’s of the molded part together. I have built up the areas of the hatch opening and tail boom connection opening. It is important to apply both Partall mold release wax and Coverall liquid mold release to the fuselage pattern and the parting surface as you did in step 1.|
|The first layer of the mold is a black tooling coat. The color is important when wetting out fiberglass cloth against the mold surface. Air bubbles and dry places in the fiberglass are white while the properly wet out glass is transparent showing the black surface. The high contrast makes it easy to produce high quality parts. Mix the mold gel coat using West Systems 105 Resin and 205 Hardener. Add a teaspoon of Graphite Powder. Then stir 406 filler, Colloidal Silica to get a mixture that has the viscosity of ketchup. You need to be able to brush on a thin layer (0.01” to 0.02”) and have a mixture that will stay on the vertical surfaces. Begin by brushing a thin layer in all the corners. Be careful to NOT tray any air in the corners as you apply this layer. This is a common problem particularly if you mix the tooling coat too thick.|
Now apply the tooling coat over the remainder of the surface. Mix a little more Colloidal Silica into the mixed epoxy in the pot to increase the viscosity. Apply some additional tooling coat in the corners of the mold to fill in small radii corners. It is difficult to wet out the fiberglass cloth and keep it in tight corners of less than ¼” radius. By filling the corners now, you are making the fiberglass cloth application easier.
|While the tooling gel coat is setting to a solid state, about 30 minutes, cut your fiberglass cloth. Use a coarse weave fiberglass cloth to lay up the mold. CST style G3733 wets out easily and builds up thickness more quickly than lighter materials. If the mold has a complex shape cut the fiberglass cloth on the bias so the fibers run on a diagonal. Strips 2 to 3 inches wide work well for complex shapes as the cloth will stretch and distort as needed to follow the surface.|
When the tooling coat has set to the point where the surface is only a little tacky, mix up another batch of West System 105/205 epoxy (no fillers) and brush a liberal layer on top of the tooling coat layer and begin laying fiberglass on the surface. Whenever possible, apply the epoxy first then lay the dry cloth down. This wets out the fiberglass from the bottom displacing the air in the fabric which reduces the chance of trapping air under the wet fiberglass. Continue wetting the fabric until you have at least two layers of fiberglass over the entire surface.
Let the layers set up for about 30 minutes just as you did with the tooling coat layer. The curing time between layers is not critical. It must be long enough for the epoxy to set and be only slightly tacky as a minimum and short enough that the epoxy is not completely cured. At a 700 room temperature, anything between 30 minutes and 4 hours will work for the West Systems 105/205 mixture we are using.
|Now we will increase the strength of the mold by building up the thickness. An easy way to accomplish this is with a core material such as Coremat, Aeromat or chopped strand mat as oppose to many layers of fiberglass cloth. Cut the mat material into sizes and shapes to fit the mold. Apply this layer with epoxy as you did the fiberglass cloth and apply another layer of fiberglass cloth over the top of the core material creating a sandwich. As you can see, the core material is needed most around the edges of the mold and any large flat areas.|
|Set the mold aside and let it cure overnight, the next day remove the parting board. Use some plastic mixing sticks as wedges to separate the mold and parting surface. Clean up the Coverall mold release on the mold and fuselage pattern.|
|With a Dremel tool and a ¼ inch ball cutter, cut small recesses in the corners of the mold flanges away from the pattern. These will key the second half of the mold into the first half. Now repeat steps 8 through 14 above to build the second half against the first half of the finished mold. Mold releases, tooling coat, fiberglass cloth, core material, fiberglass cloth and let it cure overnight.|
|When completely cured. Use a Dremel toll with a reinforced cut off wheel to trim away all the rough edges on the molds. Carefully separate the mold halves, you may need to use a model knife into one corner working the blade between the molds and gently twisting. When a crack appears, slip a wedge (a mixing stick works great) into the space and run it around the mold to separate the halves.|
|The mold is now complete and you are ready to start molding fiberglass parts.|
Materials used to make the Mold:
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