NACA Duct Mold Construction

General Composites Discussions Composites Talk
#1

This is my first personal composite project. I’m wanting to make a NACA duct via resin infusion. Rather than buy a cheap NACA duct and splashing a mold off of it, I’m wanting to make the mold from scratch for the experience.

I planned on using blue styrofoam and body filler to make the plug, but when I cut the styrofoam I couldn’t get a smooth or symmetrical shape. To fix this, I cut the top view outline of the naca duct from cardboard, and used the filler to bond two of the outlines on the top and bottom of the foam piece. I am currently waiting on the filler to finish curing from this last step.

Next I will fill between the cardboard templates with the filler, like how one skins a teaspoon when measuring for cooking. This will help keep the edges perpendicular. After I get the naca shape close, I plan on bonding it to a flat form and final sanding. Because I am making a male plug, the form I splash from it will be the negative of what the end product will be, I will have to make a mold from that mold. I know that is a bit wasteful, but I found it easier to make the initial plug in the male than in the female, and since I want to make multiple parts from this, I would rather end up with a fiberglass mold.

Flat base:

Naca male plug:

I know I’m doing so much wrong, but at least I’m learning.

-Ben

#2

Interesting way of doing it. If it helps, the way I build plugs out of foam is to think about the shape in its 3D form and how that shape is made up of 2D pieces. I normally draw up some profiles of the piece from each view; top, sides, bottom, front, back, etc. Then I can transfer these shapes onto balsa wood or cardboard or whatever suits the best, essentially creating a ‘skeleton’ structure which act as guides. Then fill in the gaps with foam, and cut/sand that down until it meets your guides. It’s very difficult to achieve a symmetrical shape on a 3D part without some sort of guides.

Of course what you’re doing is totally fine and will work well too! Just thought I’d give you another idea for going about it. Interested to see how you go with this.

#3

You could get some inspiration from one of my jobs:

#4

I’ve done several foam base plugs. I paint a thick coat of epoxy resin on the foam. sprinkle a good amount of glass micro balloons on the surface. when it’s cured dump off the excess and lightly sand the surface. fill if needed.
the micro balloons sand very easy, release and start the surface/gel coat

this is a seasons worth of (spares) LMP1 dive planes made from foam molds

#5

Here is a good video showing the technique Hanaldo described.

https://www.youtube.com/watch?v=XeeSsmsm9h0

#6

Hey Ben, how is your duct project going?

There’s always more than one way to do things with composites, just work with the materials you have, learn and have fun!

Tom, so you make a foam plug which is then also the mould? What is the reason for that? Is it just for a short run of parts? How do you find the durability of the foam mould?

#7

On that project I only made the master from foam.
then VE molds.

homebuilt aircraft builders have been doing this for decades