I am progressing forwards from concentrating on aerodynamics to also fabrication and optimizing the fabrication process. Been doing hand layups for a quite long time now, but I need to start doing shape accurate parts. Testing aerodynamics requires very high accuracy.
I have been doing several molds lately. One was almost successful, but it was a lot of work and it still had non-sharp edges.
So what I am trying to do is a wing mold with CNC fabrication. I have a CNC mill that can carve blue styrofoam, wood, MDF, Corecell etc. and is large enough for producing a half (lower or upper) of a wing section (with the limitation of the length, have to glue a big mold from ~1.2 meter pieces together).
I would need a rapid prototyping technique to produce shape accurate molds with glossy or at least almost glossy surface. So that manual work would be minimal. This is especially pronounced in case of making wing molds.
The problem I am facing is this:
- if I carve the plug to blue styrofoam, and then paint it and polish it, there is the downside:
- the styrofoam can be only painted with a paint that does not have solvent in it, and the only paint that will fill the surface is solvent free epoxy primer. The problem with that is that it also makes sharp edges round, and especially in a small scale (RC scale) the roundness becomes way too big to be acceptable. The edges where the lower and upper half meet, should be also absolutely accurate and sharp. Doesn't happen with this technique.
Has any reader used molding epoxy? I saw some picture of a mold being filled with a molding epoxy and then milled with CNC again to shiny surface directly (?). Would that be viable option for my use? As it would be for rapid prototyping and for fabricating many wings (and not just one pair), it should be somewhat reasonably cost effective. Making the mold from wood is not completely inexpensive either - requires a very thick perfect wood block (or MDF block). I could not afford consider replacing the styrofoam with a huge solid mold plastic block (that is used in industry for prototyping shapes with CNC), because the same volume is much more expensive, would be possibly fine for a CNC model of a small device, but for making a mold for large wing the cost hikes out of the roof very quickly. Styrofoam is cheap and very easy for the machine to carve, but that's the best part of it, otherwise it is really poor material.
Any first hand experiences on this?
13 comments:
Hi Karoliina,
If you're looking for information on molding wings for composite layup as well as painting in the mold, then the RCgroups.com forums just might hold a lot of the information you need.
Personally I have assembled, repaired and currently fly a DLG (Discus Launch Glider), where airfoil accuracy is critical to performance. The market is very tight, and so a lot of people build their own planes, especially since laying up a glider wing is not all that costly in terms of tooling investment. Time is another matter, of course.
I would personally recommend that you search the hand-launch forum for the Fr3ak Zone glider, I know the builder personally, and everyone who bought one of his planes has said nothing but good things about the quality of his manufacture. If you tell him you're trying to up-scale the building process to a full-size or powered-plane, I'm sure he'll give you all the help he can give you.
Most DLGs now use a hollow-molded wing, where if I understand it correctly, a PVC "balloon" is inserted in between the foam/fiber layers, and then inflated after the epoxy has been added and the mold closed, to push the foam and fiber into the shape.
A common approach to avoid the problem of your paint eating away your foam is to apply the paint to the mold (search for gelcoat techniques), and when the paint has dried is when you place the fibers, foam, spars and bladder in place, soak it in epoxy and close the mold. When the epoxy cures, it'll bind the gelcoat directly to the top surface of your layup, so you're guaranteed that the paint will have the shape of your airfoil.
You might also try other types of foam. Polyurethane foam might be resistant to the paint solvents. If you can't find blocks of it, you can make blocks using the spray foam that is common in house insulation. BASF is considered by some here to be the best brand (and the least toxic as well). PU foam can also be purchased as two liquids that you mix and pour into a form.
Another idea is to make a slurry of glass microspheres and epoxy and use it to fill a block form.
Either of these would produce a machinable block for a reasonable price.
This company has a lot of videos demonstrating various systems: http://www.freemansupply.com/video.htm
It might help to stimulate an idea that works for you.
Will you let us know what you end up using?
Have you considered doing a negative mold? This hold sharp edges. The epoxy is used to coat the mold.
Then you put in the interior of the wing. And mate the two wing halves.
Karolina, how to contact with you? I have some questions about you post for sailplane airfoil usage...
I was wondering if you could tell us more about the wind tunnel you are using and the size of the wing you are using for testing?
Thanks for sharing nice information!! Great work..
rapid prototyping
cover the foam with vinyl tape.
the brown tape use to close cardboard packages for instance.
Very thin tape.
Tape folowing the air movement.
cover the foam with vinyl brown tape
The useful and helpful stuff for me.
3D printing Blog
I have 3 ideas to suggest:
1) do 2 passes on the CNC: Do a rough cut, then cover the foam in micro-epoxy slurry, then do a fine pass. I *think* the micro should be more machineable, so that you can get a better finish.
2) How about heat-shrink finishes? Its been years since I looked at RC planes, but I recall there being products that are heat-shrink plastics meant for covering rib-and-spar wings. I don't see why you couldn't cover the foam in such a product to give you a smooth finish.
3) You could machine the object out of machinist's wax, and then do investment casting. I don't know much about the properties of 2-part foams, but it sounds good to me! you should be able to recover the wax, melt it down, and make more parts.
I was just quoted $7,000 USD to CNC a foam plug with tooling paste.
For around $10,000 I can get an 8ft x 4ft CNC router and make my own plug from MDF. I'm laminating 3/4" sheets of MDF to the required thickness. After cutting the plug, I'm going to spray Duratec surfacing primer and sand/polish to finish.
Check out the old news archives on the Perlan Project page. They use very similar methods but they finish their plugs with epoxy and paint.
Hi Karoliina,
unfortunately accuracy require a lot of work. I encounter the same problem like you did. The most satisfying approach was to create the wing (or other part) from foam, covering it with fiber glass and 2 thick layers of primer. I sanded carefully between primer layers to eliminate irregularities. The next step was to use that part as a master for mold creation.
At the end the mold itself was sanded with a soft sand paper (i.e. 2000 roughness).
The result is quite professional; very glossy pieces with no defects.
Sometimes depend the foam type (some are not heat resistant) I had to cover the master foam with wood veneer instead of glass fiber.
Hope that help you a bit.
Arthur
Hi Karoliina,
unfortunately accuracy require a lot of work. I encounter the same problem like you did. The most satisfying approach was to create the wing (or other part) from foam, covering it with fiber glass and 2 thick layers of primer. I sanded carefully between primer layers to eliminate irregularities. The next step was to use that part as a master for mold creation.
At the end the mold itself was sanded with a soft sand paper (i.e. 2000 roughness).
The result is quite professional; very glossy pieces with no defects.
Sometimes depend the foam type (some are not heat resistant) I had to cover the master foam with wood veneer instead of glass fiber.
Hope that help you a bit.
Arthur
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