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Old Mar 06, 2012, 12:28 AM
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United States, AZ, Arizona City
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I have a tail mold, now some questions

I was lucky enough to get a two piece cnc routed tail mold. Lots of curves in the outline. I've tried several things, but not having a cnc router to cut the cores, hand sanding isn't working well. While I can get close, and compress the foam with the molds during the build, it looks to me like the foam is compressing and then expanding a bit. So I'm not happy.

I thought I'd try bagging a thin piece of foam into each half of the mold, hot wiring it after it sets up and then joining them in the mold.

What foam can I hot wire, that also wont compress and expand? Is rohacell the only foam that has that wont expand? I understand if it dents that hot water wont cause it to expand.

Alternate methods would be considered, but I'd like to try this.

Thanks,

Gary
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Old Mar 06, 2012, 03:29 AM
chetosmachine's Avatar
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You can't hotwire rohacell! Or at least that's what I've seen a number of times.
Why don't you do a sandwich layup, with thin 1mm rohacell between light glass layers and apply vacuum?
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Old Mar 06, 2012, 09:10 AM
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Gary, rather than hot wire the excess, try sanding it off. Tail feathers are small, so it won't be too big a job. Just tape off the flange area of the mold, and use a light touch.

Ward
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Old Mar 06, 2012, 01:34 PM
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A sandwich layer would end up with a hollow vertical? Its a dlg. And if you can't hotwire rohacell, how do you get it 1mm thick? I didn't think it came in that thickness.

Ward, are you referring to rohacell for sanding or a foam?

Gary
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Old Mar 06, 2012, 04:39 PM
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Rohacell can be purchased down to fractions of a mm.

I used to hand-shape all cores. If doing it with vacuum, you want to use 6mm lower density XPS like 15psi pink or blue and bag it into each side. Hotwire / sand to the parting plane and add some splooge around the LE/TE then compress. If you sand the cores flush, cover the parting plane with tape or something else thin so it protects from scratching and leaves the foam a couple thousandths proud.

Hotwiring leaves the foam a couple thousandths shallow, which must be filled with a light splooge mix. Kerf is evil in this situation.
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Old Mar 06, 2012, 04:42 PM
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Gary, I was saying you could use a sanding block to sand either a Rohacell or foam core half down to the mold flange.

Here is a thread that Fairlyfaded had where he was selling rohacell in either 1 or 1 1/2 mm thickness.

http://www.rcgroups.com/forums/showthread.php?t=1544867
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Old Mar 06, 2012, 08:38 PM
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Tom,

Did you try a product called cell80 or something like that? Is that a Depron like foam?

Gary
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Old Mar 07, 2012, 12:02 AM
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Fairlyfaded hasn't returned my questions on Rohacell. Besides, you need ~3mm for tails, not 1mm.

Tom is using Cell Foam 88. I am going to give the sanding method a try with 15 psi XPS or regular Depron. That will be in 1 week at the earliest.

-Sam
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Old Mar 07, 2012, 03:01 PM
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Quote:
Originally Posted by tom43004 View Post
If you sand the cores flush, cover the parting plane with tape or something else thin so it protects from scratching and leaves the foam a couple thousandths proud.
I used steel feeler gauge stock for this operation. Its very hard (I think high carbon steel) nominal 1/2" wide. You can get it down to low-thou thicknesses in lengths or even rolls from machining supply places. Just light 3M spray tack it on the flange so it stays put, do the sanding, then pop it off or use a little alcohol on the edge so it doesnt bend or kink. With steel you wont ever have to worry about sanding through it & scuffing up the mold flange.
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Old Mar 07, 2012, 04:02 PM
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Basic question.

I missed something basic about your method here Gary.
You say you have a mould, but then you want to CNC cut foam cores?
Are you describing a hollow moulded flying surface build or a vacuum bagged over foam core surface?
Or some sort of hybridisation of the two techniques?

Maybe the cores you refer to is the core in the composite skin sandwich?

Jim.
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Old Mar 07, 2012, 06:33 PM
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Jim, DLG tails are so thin that hollow construction is impractical. They are typically a solid shaped core.
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Old Mar 07, 2012, 09:56 PM
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Thanks Adam!
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Old Mar 07, 2012, 11:37 PM
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To elaborate: If a full core can be made, either by cnc routing or hand sanding, then the tail is made by putting skins an spars into the mold (epoxy also), joining the mold around the core and applying pressure. Tom Siler has some great video of making tails in molds on the hlg forum. Well worth watching.

Tails can also be made by forming foam in each half and trimming to the surface of the mold. If the skins weren't applied to each side, then the rest of the process could be similar to the above process. I'd guess that applying the skin to each half core , trimming and then joining the halves might work better, but just don't know.
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Old Mar 08, 2012, 03:39 AM
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2 parts then join will be far worse than 1 piece core. Been there, donde that.
Depron, or xx psi foam, wil be far weaker than rohacell (at same density!).
If you want "A class" tails, 1 piece milled cores is the way to go.
If you just want tails, 2 piece core is convenient. But they are heavier and weaker.
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Old Mar 09, 2012, 08:51 AM
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Quote:
Originally Posted by chetosmachine View Post
2 parts then join will be far worse than 1 piece core. Been there, donde that.
Depron, or xx psi foam, wil be far weaker than rohacell (at same density!).
If you want "A class" tails, 1 piece milled cores is the way to go.
If you just want tails, 2 piece core is convenient. But they are heavier and weaker.
+1

Been there, done that too but if you don't have a machine for cores it's do-able. I have also hand shaped rohacell and cellfoam cores for solid core molding. The hand shaping seems like it would be a lot of work, but it really isn't. I can make cores faster by shaping by hand than by running my CNC... but they're a bit more oversized, which means more foam compression, which means more core weight.
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