Photo 1: The sky is the limit.
This rocket is made entirely from molded parts.

Fiberglass Nose Molds

Many individual rocket enthusiasts work with composites regularly, but for the most part they limit themselves to fiberglass reinforcement of other materials. The real potential of composite materials is unlocked when using fiberglass molds to create composite parts. This opens a whole new world of nose cones, boat tails, airframes, fins, and more (see Photo 1).

Most complex castings require a two-part mold to facilitate the removal of the part after the casting is completed. You are probably already familiar with how this looks if you have used a blow-molded nose cone; the seam that runs the length of those parts indicates where the molds separate. This article will describe how to make and use a two-part fiberglass mold that is constructed in much the same way.

Materials

The most important materials used in mold making are the release agents. The release agents form a protective barrier between the mold and the plug or part, and keep them from becoming permanently bonded. Carnauba Wax is a paste sold both as a release agent and as car wax. In mold making it is more valuable than ambergris. PVA (Polyvinyl Alcohol) is kind of like a liquid shrink-wrap and is normally used in conjunction with a top coating of paste wax. The film may be removed with water from the cast part once the part has been removed from the mold. The two agents, properly applied, will guarantee a successful release. Release agents are applied at several steps in this process: Don't shortcut these steps! There are few things more frustrating than having all your work fused together into one useless lump.

Most manufacturers choose to make molds using fiberglass mat and polyester resin, the overriding reason being that fiberglass mat conforms to contours better than woven cloth and builds quickly (see the May/June '04 Construction Zone for more details). Gel coat serves as the surface layer of the mold. It is harder than other resins, and can be tinted to take on a variety of hues - you've probably seen it on the surface of boats and sports cars.

Non-drying modeling clay is the mold-maker's secret weapon. It can be used to make plugs, flanges, or even temporary repairs to your mold. 1/8-inch whiteboard (melamine-covered Masonite) is also a very useful material when forming the mold parts.

Photo 2: Whiteboard Flange

Step One - Plug preparation

The mold will be built up one-half at a time. Determine where the mold will separate - the part should remove easily from each mold half without binding. Build a flange along the separation line. This can be made with modeling clay only, but I prefer to use whiteboard to form a flat, smooth flange. I simply cut the outline of the part out of a piece of whiteboard and use modeling clay to fill all the voids and affix it in place along the separation line (see Photo 2) .

Photo 3: Gelcoat on Plug and Flange

Step Two - Release Agent

Apply a layer of carnauba wax to the first half of the plug and flange. Wipe off after 10 minutes and allow to harden for 30 minutes. Repeat for a total of six coats. Liberally apply PVA - I like to use a spray bottle - and position the part so the excess PVA drips off. Allow to dry.

Step Three - Gel Coat

Mix the gel coat and the hardening catalyst per the instructions that accompanied the product. Apply a thin, even coat to the prepared surface (see Photo 3). Spray application is optimal (using a spray gun or a cartridge aerosol dispenser) but a brush will also work. Allow the gel coat to harden until your fingernail will no longer make an impression in the surface.

Photo 4: Resin and Mat

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Step Four - Cut the mat

I like to use three layers of 1.5-ounce mat to form a rigid, sturdy mold. Even though mat conforms to contours very well, you will still encounter trouble around any sharp angles - like the place where the flange meets the plug or where the nose cone joins to the shoulder. For this reason I try to cut a piece to fit into those sharp corners to prevent a void between the gel coat and the glass. Mat can be overlapped and layered using a lot of small pieces, so it's very easy to work with.

Step Five - first half lay-up

Mix the bonding resin and catalyst per the instructions. Brush resin onto hardened gel coat and lay in the glass (see Photo 4). Use your brush to soak the mat and work out all the bubbles. The glue that holds the mat together is dissolved by chemicals in polyester resin, so it becomes very workable when completely wetted out. Continue until the entire plug and flange are covered, and allow to cure. Carefully separate the whiteboard flange and clay from the mold and discard (see Photo 5), but try to leave the plug in place. Clean off the remaining clay and PVA.

Photo 5: Remove and discard whiteboard flange

Photo 6: Two Mold Halves

Step Five - Second Half

Repeat steps two through four on the second half of the mold. Drill 1/4-inch holes in each flange corner for alignment pins. Separate the two halves, remove the plug, and clean off all residues (see Photo 6). Imperfections in the mold can be spot-fixed with gel coat - apply, cover with cellophane tape until it cures, sand smooth. It's that simple.

Photo 7: Lay-up of casting. Note sanding primer on flanges.

Photo 8: Clamp tightly or it won't fit in the tube.

Casting a Part

To make a part, just start back at Step Two - only this time you will be laying up fiberglass where the plug used to be (see Photo 7). When the resin of your cast part is 'green' (solid and rubbery) trim the excess along the edge of the mold. Align the two halves and clamp them together tightly(see Photo 8). Mix a small amount of resin and pour along the seam. When fully cured, separate the molds, remove the part, and trim the flashing. Use Bondo® for any surface imperfections.

I usually paint the parts after they are cast, so instead of gel coat I substitute two-part polyester sanding primer for the surface coat of the part - that's what I apply first into the mold. It is light gray like other primers (see Photos 7 and 9), sands a little more easily than gel coat, and holds paint well

Closing thoughts

Once again, don't get lazy in the application of release agents. Have good ventilation and use a vapor mask. Wear latex or vinyl gloves. Have plenty of acetone solvent and extra brushes. Get some exercise. Turn your mattress. Call your mom once in a while.

And finally, my apologies for those of you misled by the title who may have been hoping for an article on seasonal sinus conditions.

Photo 9: The finished product next to the plug.