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Wing molds out of blue styrofoam?

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karoliina.t.salminen

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I have been experimenting with mold fabrication with CNC. The most inexpensive material for the block to be carved happens to be the blue styrofoam. However its melting properties causes a surface finishing problem.

I have figured out that I can directly paint over the foam using solvent free epoxy primer. However, the problem with that is that the epoxy primer without solvent is very thick. All small details get rounded, and even if I sand and polish this paint, the result is not exactly the shape I was intending.

What would be the best way to cover the foam (plug) to get mirror finish without compromising the shape?

I saw pictures of molding epoxy spread at something like 1 inch thick layer and then machined again directly to mirror finish. Would that be the optimal wy or is there some less material requiring method? I would need quick automated mold fbricatin technique for rapid prototyping of wings in RC scale and later full scale. Typically mold making instructions on web have antiquated hand labor techniques which by definition does not lead to rapid prototyping.

Or should I use urethane instead and finish it with paint that has solvent resulting thinner layer?
 

autoreply

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What kind of CNC-construction, routing, hot wire?

In case of routing with a double curved area, I'd look at something like this for:
https://www.homebuiltairplanes.com/forums/composites/12754-making-re-usable-molds-wtih-cnc-machined-foam-stretchelon-bagging-film.html#post133982

For hot-wired I'd simply use some type of sheeting. I am looking at PVC and other plastic plates, Billski used roof flashing if I'm not mistaken.

For both you have to test with the thickness; too thin and it'll show any imperfections; too think and it will literally cut corners.
 

Jan Carlsson

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Hi

Billski used hotwired foam and glued thin alu "roof panels" I guess they are pre painted, he vacumed them into place, I don't know if we have something similar on this side of Atlantic, but thin alu is to be find. plastic film or bag can be used too, but better on a male plug.
In my boat I have alu inner roof, they are not perfectly mirror like but close to.
 

ultralajt

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Karoliina, I came across very interesting way of CNC made molds, that are "surfaced" by very interesting and easy way!

Creating High Quality, Reusable Molds with CNC Machined Foam and Stretchelon Bagging Film – aka “The Method” « Better Living Through CNC

It is for an RC model molds, but technology, can be used also for bigger parts. It is the fastest and pretty cheap way to make molds for prototyping!

------------------------------------------------
Othervise, I am thinking of creating D torsion box over the styrofoam positive plug, that will be covered by 0.3mm thick plastic sheet. This will smooth the surface and make it acceptable for the mold release agent. At places where some internal structure (ribs, reinforcments, spar caps) will be glued later, some peel ply patches will be placed onto.
Outer surface of the lamiante can be also covered with such plastic, and all thing cured inside a vacuum bag. As my outer surface will be covered by Dacron fabric wing sail, I will use only the peel ply there.


Regards!

Mitja
 

karoliina.t.salminen

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Thanks for the info. I checked this green streching vacuum film is available locally, so I will go to buy some of that.

ultralajt, yes, my video and our machine. Quite modest inexpensive Chinese machine (shoestring budget) but good news is that we are able to do everything from CAD model down to milling ourselves (the workflow is this: XFLR5 (open source, ran on Mac) -> Rhino (Mac version) -> stl export -> refinement + path calculation with Heekscad/cam (open source, ran on Linux machine) -> Linux-CNC (Linux) that controls the CNC-machine, and that everything already works , only the surface finishing was a problem. Using vacuum though might call for stronger foam than the styrofoam, stronger foam is more expensive than I would like for use-once style prototyping. Or maybe I will use MDF next, strong enough for the vacuum. I have some spare MDF sheets lying around, maybe my next step would be gluing them together (that work I do not like, every unnecessary avoidable step is unnecessary when speed and budget is essential, but have to do) and milling new mold out of that material to prepare for this streching technique.

About the video itself: one of my other hobbies is making videos.
 

ultralajt

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About the video itself: one of my other hobbies is making videos.
I believe you, as also this one is awesome!!
Sorry for me going off topic. :emb:

I think that great video presentation of let say brand new aeroplane design can efficiently attract would be buyers or builders or customers.
Video could be an important factor for awerall project succsess.

Mitja
 

wsimpso1

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Blue foam works fine under a vacuum bag. I have experience with blue foam for molds and parts under vacuum.

For single curve molds (straight taper wing skins), I used pre-coated aluminum roof flashing to skin it, vacuum bagged in for the bond job, and a glass tape around the perimeter for the line of mastic.

For more complex molds under vacuum, even epoxy has some solvent effect on it, which rounds over sharp edges and drives the surfaces down slighty. Really fine details can be lost. I ran tests with my foam and epoxy - for my resin, stabilizers and control surfaces it was small enough that I decided not to use an offset for that effect. If you are trying for sharp edge steps and stuff, you have to go with tooling foam, which gives a better surface anyway.

I hope that the film works for a one-off. It looks like a good scheme.

Billski
 

autoreply

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Hi

Billski used hotwired foam and glued thin alu "roof panels" I guess they are pre painted, he vacumed them into place, I don't know if we have something similar on this side of Atlantic, but thin alu is to be find. plastic film or bag can be used too, but better on a male plug.
In my boat I have alu inner roof, they are not perfectly mirror like but close to.
The advantage of plastic is two fold compared to alu. First of all, it's relatively hard to scratch the surface (and the glossy finish is next to perfect). Secondly; you can use one piece for a whole wing. That avoids the edges of multiple sheets coming up at the joints. Required thickness is usually in the range of the thickest foils (a few tenth of a millimeter)
 

wsimpso1

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I bought my roof flashing as a coil, 3' by 60'. One piece per mold. It was coated both sides, so I had to sand it on the side that was bonded to the foam. The other side was just waxed and ready to go. A disadvantage was that it would not follow the curve at the leading edge, so the first 35mm or so is not accurate.

Billski
 

karoliina.t.salminen

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Video could be an important factor for awerall project succsess.

Mitja
Little OT reply:
I would be more than happy to do airplane marketing videos for everybody here, but the limitation is that I am in Europe and getting far away for making a video would be expensive due to travel costs. BtW. Not all my videos are online, I have made some internal use corporate videos and also one steadicam video that I like myself a lot, but have not produced soundtrack for it yet and it is with placeholder soundtrack that can not be shared.

Back to topic:
I think I will try out this vacuum bagging technique. Have to go to visit the local composite materials supplier. Hopefully they will sell me less than full 800 meters of it. And this would give a good reason to bag all my parts, been a bit lazy on that as there is lot of things to setup on each session.

Wing mold needs sharp edges basically in the seams. Deformation there would make joining the two halves a mess. Other than that, it could be that the styrofoam can withstand the vacuum. Have to try it. Styrofoam costs almost nothing compared to tooling foam. I think if Dow foam does not work, I could also try higher density Finnfoam.
 

ultralajt

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Some of my friends (RC modellers) uses fine chipboards (MDF?) plates, tailored and glued to the desired thicknes, and then they mill female molds on the CNC router.
Then they lightly sand the inner surface and paint. Then they make glossy finish on the mold surface.
Such molds are not much expencive (remember, you dont need positive plug, you dont need to make "classic" molds from the plug, you dont waste time and epoxy..) and they can whitstand a small batch of products.

Mitja
 

autoreply

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I bought my roof flashing as a coil, 3' by 60'. One piece per mold. It was coated both sides, so I had to sand it on the side that was bonded to the foam. The other side was just waxed and ready to go. A disadvantage was that it would not follow the curve at the leading edge, so the first 35mm or so is not accurate.
Ah. I was never able to find rolls of alu sufficiently wide (and thin). I need just under 2 meters wide (6'), to wrap it around the whole skin; that also avoids the inaccuracies at the leading edge. Yes, this only works with a plug which is more work, but sanding a (laminar in my case) non-precise D-nose is, I think, far more involved.
 

Vigilant1

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I'm guessing the stuff Billski is talking about is probably available almost everywhere. As he mentions, it is called "flashing" and is used by roofers to cover areas around chimneys, in the valleys of the roof, to make custom-sized flashing for pillars (using a metal bending break) etc.

autoreply--if the rolls you can buy are the same size (approx 1 meter wide by 20 meters long), could you just wrap from trailing edge to trailing edge and have chord-wise joints? It wouldn't seem to cause much drag, and the material would be a lot easier to handle than a larger piece.
 

Head in the clouds

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Ah. I was never able to find rolls of alu sufficiently wide (and thin). I need just under 2 meters wide (6'), to wrap it around the whole skin; that also avoids the inaccuracies at the leading edge.
What you are looking for used to be available and for very low cost (scrap price), with a bit of negotiating I'm sure you could find a source. It is the high tensile aly that is used for the lids of aly cans (Coke, Pepsi etc). The can itself is a very soft grade which work hardens as it is broached into the die, but the lids are stamped out of a coil (roll). The rolls used to be (maybe still are) 3ft and 6ft wide, and were about 200m long IIRC and the material is similar tensile to 6061T6 and about 0.016"/0.4mm thick. At the cannery the rolls were replaced at the end of each shift, so that the next shift started with a fresh roll and the production never stopped during a shift. The left over ends of the rolls were often around 30m long and went to the scrap dealers together with the perforated material which was left over from punching the lids.

I don't think they scrap them anymore but a chat with a manager at a canning plant might provide just what you want. You might also ask at the mill, they would be likely to have line breakages during production or whatever or a manager sympathetic to aviation might just 'find' some for you. The point is, the product you want does exist.
 

Detego

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I was never able to find rolls of alu sufficiently wide (and thin).
I need just under 2 meters wide (6'), to wrap it around the whole skin; that also avoids the inaccuracies at the leading edge.
Yes, this only works with a plug which is more work, but sanding a (laminar in my case) non-precise D-nose is, I think, far more involved.

You can use 'rolled Mylar', starting at 2 m x 14 mil thick (see: Thin Films).
 
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