Part I: Mold Construction - Part II: Part Production

I've always enjoyed making my own models and designs, so I got to thinking about making my own propellers. I did a lot of "asking around," and the procedure described and illustrated below is what I have come up with so far. By no means do I claim to be an expert, as there is always room for improvement. Hopefully, someone who sees this will be able to make suggestions which I can add to my practice. In the meantime, however, this is the method I've come up with for making homemade molded props.

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Materials needed:


  • A propeller you like (even could be one you made yourself).
  • A good, high-temp tooling epoxy.
  • Metal powder for filler (eg. aluminum, aluminum oxide or steel powder).
  • Plasticine, or some other modelling clay. (The type which does not harden.)
  • Plastic laminated kitchen counter-top material, such as Formica brand. Other materials, such as aluminum plate, may be substituted here.
  • A steel rod which fits snugly in the prop shaft hole in the prop.
  • Mold release wax. A quality automobile wax is generally fine.
  • PVA mold release agent. (Not absolutely essential, but very nice.)
  • Material for making the props. (eg. carbon or glass tow [rovings] and light glass cloth or uni-directional carbon) You'll see that the prop composition is quite flexible, which leads to a lot of fun in experimentation.

    Part I: Mold Construction

    Part II: Part Production - Top of Page

    The first thing you need when starting a molding project is an original prototype part, from which you will make your mold. In this case, I used a commercially available injection molded, carbon reinforced nylon prop. The toughest part of the prop to mold successfully is the hub. This is because the sides are usually vertical and this tends to make parts stick in the mold. What is needed is some "draw" in the sides of the hub. This can be done by carefully sanding the sides of the hub so that there is some taper away from the center line. When you're happy with this, wax the entire prop 3 or 4 times. You'll wax it again later, but you can never have too much wax on your beautiful prop.

    Observe the seam, or part line, on the hub. This is a good indication of where your part line should be at the hub. The part line for the rest of the prop will be around the outside edges of the blades.

    Now cut the plastic laminated material into pieces which come together to make a box with an open top. As an example, for my 5 inch prop mold I used the following dimensions: Base piece, 1 7/8" x 6 3/4"... sides, 3 7/8" x 8 3/8"... end pieces, 1 7/8" x 3 7/8". Note, these dimensions are in standard inch measure and are not critical. I used the same pieces to make two different prop molds, each one a different size. The important thing is that all the sides be square and smooth. Drill a hole in the exact center of the base piece, such that the steel prop shaft pin will fit in and be perpendicular to the base. It's best to ues a drill press for this, obviously.

    Cut the steel rod so that it is about 1/2 the prop diameter. This dimension is not critical. Insert the pin in the hole in the base and lower the prop onto it, with the front side facing up. The prop should be spaced from the base such that the lowest part of the blade is not closer than 1/8" to the base. Make sure that the tips are equidistant from the base (the prop is parallel with the base).

    This next part requires time and patience. Soften the modelling clay by kneading it in your hands. A heat gun may help to accellerate this process. Slowly add clay underneath the blades and hub, extending out and down towards the base. Check frequently to make sure that you aren't stressing the blades, or causing them to lift or twist. When the clay goes all the way around the prop, begin to smooth it all out, making sure that the clay just comes to the sharp point all around the edges of the blades. You don't want the clay to come above the blade, nor do you want it to fall below. Observe the photos below for clarification.


    Note that the clay just comes to the edge of the prop.


    Once you're satisfied with the way everything looks (you won't know for sure until you pour the epoxy ) then you can start waxing the whole thing. I mean everything. Wax, wax, wax and then wax some more. Get at least four coats on every surface which will come in contact with the epoxy. Go ahead and wax anything else, as well, as you might be surprised where the epoxy gets to! The clay is soft, so you won't be able to buff the wax there, but the wax buildup will help you to smooth out any fingerprints, or other imperfections. Now apply your PVA, if you have any. You want this to go on as smoothly as possible, so you can use a brush... a fine cloth (old T-Shirt)... or you can spray it on. In any case, let it dry thoughroughly before proceeding.

    Now you can assemble the waxed / PVA'ed pieces and clamp the whole thing together so you have a nice open box with a prop and some clay at the bottom. Now you can get out your epoxy and filler. In my case, for the 5 inch prop I used 125g total (resin + catalyst) of epoxy and 250g of aluminum oxide powder (for each half of the mold). Smaller props will require less, larger ones more... but you must cover all of the prop and then some, as you wouldn't want your nice new prop mold to crack in half when you clamp it, right?

    Your box will look something like this, but with the fourth side, obviously.


    There are two main schools of thought on mixing epoxy and filler. One says you should mix the filler in with the resin first, because this combination can sit around with indefinite pot life. The othe way to do it is to mix the epoxy resin and catalyst first, and then add the filler. Either way works fine for this application, but you're probably guaranteed a more homogeneous mix if you first mix the resin and catalyst, since the filler will raise the viscosity severely... making stirring difficult. In either case, let the whole thing sit a while after mixing, to get the air bubbles out. You can aid in this by tapping the container on the counter or, even better, seal the container and draw a vaccum. Drawing a vaccuum will force the air bubbles out quite effectively.

    Once you've got the bubbles out, begin by carefully brushing a coat of the "mud" over the prop and clay at the bottom. This is to make absolutely sure that there are no air bubbles at the surface of the mold. I learned this the hard way, as my first prop mold was ridden with holes on the surface. Once you have the first layer down, you can gently pour the rest of the metal filled epoxy into the box. Place the whole thing on an absolutely level piece of work bench (How'd I learn this?) where it can sit, undisturbed, for at least 24 hours. The metal filler material will dramatically increase the amount of time required for initial cure, as it acts as a thermal and physical buffer in the epoxy matrix. Go have a beer and come back tomorrow.

    The next day you can remove the clamps and carefully break the side pieces loose. Then carefully remove the base piece. You may be able to tap on / twist the shaft pin to help with this. Now remove all the clay, but leave the prop and shaft pin in place. Use a rag, etc., to get the clay out of all the "nooks and cranneys" and then get out your Dremel tool with a drum sanding attachment and grind some sort of alignment / locator system into the edge of the mold. As an example, this photo shows what I did.

    Here's a closeup of the little nubbin I made for location purposes.


    Now clean everything up and give the whole thing (sides included) at least 4 coats of wax, and then PVA it. Pay special attention to the places you ground away, and the seam line around the prop. We don't want this sucker sticking together!

    Now re-wax and PVA the laminated panels and assemble the box again, this time with your first half of the mold as the base. Clamp it together and check for correct alighnment of the sides. There shouldn't be any gaps around the perimeter of the mold, or the the epoxy will ooze out. Prepare a second batch of metal-filled epoxy in the same proportions and amounts as the first. Again, carefully brush in the first layer, paying close attention to all the corners and crevices. Fill the remainder of the mold, and set it aside for 24 hours, making sure it is level.

    Here comes the scary / exciting part. Remove the laminated panels, and you will see your new prop mold. You'll have to do some cleanup, but what you see now is the real thing. Use a thin putty knife (I use an oil painter's knife) to clean up the seam down the middle of the mold. Run the tool along the seam to open it up a bit. Now you'll probably have to gently slap the thing on a hard, smooth surface. I use my concrete garage floor for this. I slap it down, rotate to the next side, slap it down, etc. This will break the two parts free and enable you to open it up.

    When you're done, you'll see something like this.


    Part II: Part Production

    Part I: Mold Construction - Top of Page

    Once you have the mold opened up and cleaned/trimmed as needed, polish out any imperfections with fine polishing compound and then apply several (~6) coats of wax to all surfaces of the mold, even the outsides. Make sure to be especially careful around the inside corners of the hub area. Once you are satisfied with your waxing job, set the mold aside and collect some large (~3 in) C-clamps. After the wax has set for at least 30 minutes (to allow for the curing/hardening of the wax) then you can apply your PVA if you have any.


    Application of PVA and pre-cut pieces of carbon TOW.

    Set the molds aside again and get out the carbon tow. Cut several lengths to fill the center of the blades, passing straight through the hub area, from tip to tip. Vary the lengths of these pieces to compensate for the varying thickness of the desired finished blades. Also cut short lengths of tow to make the blade faces.

    When the PVA is thoroughly dry, mix up a small batch of epoxy and thicken it with colloidal silica so it is like toothpaste. Add a touch of carbon powder to color it, if desired. If it is not already in place, insert the prop-shaft-hole pin into the bottom half of the mold and run a small bead of PVA around the gap.Using a small scrap of wood, 1/8 in dowel or music wire, etc., apply the thickened epoxy to the hub area. Allow this epoxy to begin to stiffen up before proceeding.


    Application of thickened epoxy to hub area. I used white pigment for clarity.

    Mix up another batch of epoxy and coat the blade faces. Then wet out the carbon tows for the blade faces and lay them in place. Now wet out the full-length tows and gently lay them in position, alternating which side of the hub they pass through. When all the carbon tow is in place, add colloidal silica (and carbon powder) to the remaining epoxy (or make more) and fill up the hub area and blade cores. Make this layer as clean and smooth as possible, taking care to distribute the thickness evenly across the span of the propeller.


    Blade face and core carbon.

    Now carefully assemble the two halves of the mold and clamp securely. Use paper towels to clean away the excess epoxy as it squeezes out of the mold. Try to apply even pressure across the mold, as this will help to ensure a ballanced propeller. Set the mold aside overnight, or longer, depending on the curing requirements of your epoxy.


    Yummy. ;)

    When the epoxy is cured, remove the clamps and slap the side of the mold on a hard, flat, smooth surface (concrete floor) and open up the mold. If your mold surface is of good quality and you have sufficient wax/PVA, then you might not need to slap it on the ground... but merely tap it on your counter. Remove the center pin and propeller from the mold.


    Open it up and find your big surprise...

    Use scissors, X-Acto knife, files, etc. to trim the flashing from around the edges of the propeller. Wash the PVA (if used) from the blades and admire your work! Now balance your new prop as usual (tape/paint/glue/etc. as needed). Then go FLY!!! :)


    Trim, balance...then fly!



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    Please e-mail any comments, questions or suggestions to:
    davidfee@cox.net

    © David Fee 2001
    Last Update May 15, 2002