New mold construction method?

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Will Aldridge

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Sorry this is somewhat abbreviated from the original as I lost my post by accidentally closing the browser.:mad2:

I started building a 1/4 scale model of the plane I'm designing by slicing the fuselage up into 2" thick sections. Overall it was one of the easier ways I've found to create such a complex shape fairly quickly and required very little sanding and finishing.

My mind started working backwards using that method to make a female mold. I know that molds have to withstand a pretty substantial amount of pressure and from what I've head solid blue foam can take those loads. So if you build yourself a long flat table and if you had a cnc hotwire cutter you could fairly easily make yourself a female mold that would require very little finishing and also be really easy to incorporate the joggles into. Might be somewhat expensive buying enough foam but then again you save money and time not having to build a male mold first.

It would seriously surprise me if no one ever thought of doing this so, has it been done? What are some potential problems that would need to be overcome?
 

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Will Aldridge

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Hughie,

One thing I might not have been clear on was that I am interested in a female mold as opposed to a male mold.

CR,

Can you give some examples?

And again since i haven't seen this process mentioned on this site before I have to assume there are some serious drawbacks to using it? I can see where it might not be up to making more than 1 or 2 parts, but with a 3d CAD and cnc it would be pretty easy to make a mold for a 1 off.
 

Hot Wings

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SchoolStreetDesign

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I'm the guy whose blog that is. Just finished some boat hulls of sorts using this method just blown up much bigger. The mold was about 19' long, 30" wide and 18" deep. It was made up of about a dozen different CNC machined blocks of foam, held in alignment with a jig fixture of CNC water jetted plywood and steel studs. Those parts were certainly the size of fuselage shells. So you can really take this approach to anything homebuilt aircraft size if you want.

-Sky
 

Will Aldridge

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Thanks for the help. I also like your stuff on using Rhino to smooth out airfoils and have been wondering for awhile how to overcome the rebuild commands insistance on equal length segments.
 

SchoolStreetDesign

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Turns out I'm actually allowed to post images of this project. What a rare treat. Here's some shots of the project. The shot of the mold with the green tint is with the stretchelon on it - you can see how well it conforms. 14 pieces of CNC machined foam, held together with CNC waterjet plywood and steel studs. We did the mold design, machining, mold assembly and fabricated 6 shells in about two months.
IMG_1426.jpgIMG_1477.jpgIMG_1497.jpgIMG_1591.jpgIMG_1587.jpg
 

Aircar

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That is a VERY interesting technique --it is similar to the method of prototype wing construction used for the prototype Janus sailplane in 1973 --hot wired Polystyrene blocks between 'inverse' rib templates (the piece from which the airfoil shape was cut ) with a plastic film and wet glass layer vac bagged onto in to form a one off female mold for the first wing . (only in 2D of course ) This "stretchalong" sounds intriguing and the result looks fabulous.

And HUGHIE - can you give any more info about the aircraft (aircar) in the Foreground --the aerobile ? haven't heard anything of it for some years.
 

wizzardworks

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Will, Concerning the rebuild command on an airfoil. Sava a copy of the polyline of the ordinatews. Rebuild to 24 to 36 control points as a degree 3 curve. Line up the unrebuilt copy with the end of the chord at the leading edge. Zoom in to the trailing edge really close and look for a difference at the tip of the airfoil.and trim both curves with the same line keeping the original rear and rebuilt front. Join them and they should loft just fine. If there is any problem loft the two pieces of curve separately and union the surfaces. Both ends of the airfoil must be rebuilt to the same number of control points so pick something like 24 points and use it on all airfoils. You may in future want to morph two defferent sections and they will all have the same control points. If your ordinates keave an open curve at the trailing edge just extend to an intersection and after rebuild cut them back to the original length which will trim off the discontinuity.
How about that stretchalon method. WHAT SANDING SCRATCHES!!

wizzardworks
 

Hugh Lorimer

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That is a VERY interesting technique --it is similar to the method of prototype wing construction used for the prototype Janus sailplane in 1973 --hot wired Polystyrene blocks between 'inverse' rib templates (the piece from which the airfoil shape was cut ) with a plastic film and wet glass layer vac bagged onto in to form a one off female mold for the first wing . (only in 2D of course ) This "stretchalong" sounds intriguing and the result looks fabulous.

And HUGHIE - can you give any more info about the aircraft (aircar) in the Foreground --the aerobile ? haven't heard anything of it for some years.
The flying car was from Moldovia so the university student tried to explain it all in Moldovian ........ Ho Hum. But I was shown a video of it flying and on the road. As far as I could understand it was the brainchild of some professor as a learning project for university students? Hughie.
 
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