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Building Tips and technical articles. => Building techniques => Topic started by: Tim Wescott on March 01, 2016, 01:23:34 PM
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So, I'm about to go mold balsa for the second time ever, and the first time molding around a complex curve.
In Bob Hunt's method, you use a sheet that's big enough and you slit it as necessary as it shrinks toward the back of the molding.
But I don't have sheets big enough -- I need to splice 3" sheets together to get 4".
So -- should I start by splicing them all together and then try to slit the splice out, or should I start by splicing them just where the curvature is simple, and then counting the rest as "pre split"?
I don't want to wait -- I want to get this done.
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I would make the splices where most convenient before or during the molding. Convenient may involve fill and material savings, and it may involve seeing where the stress is highest and deciding which direction the material should run.
If you are making biased ply then I would make splices as I go since the material would likely need cut in similar places anyways. For single layer it may work easier to mold the center part then mold an edge on of relatively uniform width. ...all depending on the actual shape of the part in question.
I'm no expert at this. I also have not seen the shape you are molding.
Phil
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Shall i mail you some 4" sheets?
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Shall i mail you some 4" sheets?
Thank you Dane -- stand by, I'm going to be trying to mold this thing today some time. Tomorrow I may come begging.
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Hey Tim! Green foam works fine. Here are some bucks that I've made, there are more. From L to R, turtle deck for Chizler, LE for Chipmunk, LE for Fifth Element, turtle deck and canopy for Fifth Element . The canopy has epoxy infested micro balloons on it. Canopy total weight was 4 grams.
I don't have a hot wire set up. This stuff sands easily and is more durable than you might think.
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Hey Tim! Green foam works fine. Here are some bucks that I've made, there are more. From L to R, turtle deck for Chizler, LE for Chipmunk, LE for Fifth Element, turtle deck and canopy for Fifth Element . The canopy has epoxy infested micro balloons on it. Canopy total weight was 4 grams.
I don't have a hot wire set up. This stuff sands easily and is more durable than you might think.
Hey Tim,, once again I must humble myself ,, LOL,, so Mike knows more than I do,, big surprise,, I really did not think it would hold up, shows you what I know,, now I wish I would have tried it, I have two cases of green floral foam that was givent to me,, its smaller chunks but I have glue,, alas, next time
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Hi Tim,
Please give me a call sometime and I can explain it. I have done a few of these molds lately. My cell is 815-557-5375.
Thanks,
Chris
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It turns out that the complex curves weren't a problem -- the wood followed that part just fine. On the other hand, the 3/32" balsa I have isn't terribly light (7lb) and isn't terribly A-grain. My glued-on "wings" broke off, and I got a bit of splitting close to the front of the turtle deck.
I'm going to see how bad things are tomorrow after it dries. If I don't think I can take what I have and patch it together (which is a possibility) then tommorow I'm going to see if National Balsa will hand-pick me some A-grain contest wood, and try again.
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OK Tim, that is why I use a layer of 1/32, a layer of tissue (or silkspan) and the top layer of 1/16. It comes out really rigid and strong. I've also had some problems with 3/32 balsa and the single layer isn't nearly as strong.
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OK Tim, that is why I use a layer of 1/32, a layer of tissue (or silkspan) and the top layer of 1/16. It comes out really rigid and strong. I've also had some problems with 3/32 balsa and the single layer isn't nearly as strong.
I may do that instead -- among other things, I have the stuff.
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OK Tim, that is why I use a layer of 1/32, a layer of tissue (or silkspan) and the top layer of 1/16. It comes out really rigid and strong. I've also had some problems with 3/32 balsa and the single layer isn't nearly as strong.
Mike, did I miss it, what adhesive do you use, and do you apply the adhesive prior to the initial molding stint after wetting the wood, or do you mold the two, together, then seperate, glue and put back on the buck
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Heres a the reara deck , three lam.s of 1 / 32 give it near 1/8 thick . Two would be enough if its not for the rough house .
(http://stunthanger.com/smf/index.php?action=dlattach;topic=40682.0;attach=169305;image)
Damped Sheet under tap for 5 secs, both sides . Wiped with paper towel .
P.V.A. ' aquadere ' to top of one & next , thrown on & coat , so BOTH surfaces ( mated ) are ' pre glued '
and scraped out even & thin , plastic card or a stick used . Coaxed around with Palm .
secued with rubber bands ( from Middle ) or masking tape .
Maybe MARK and PIN at centerline / ends , to maintain symetry .
Rubber Bands are liable to mark it with nasty welts , so cardboard strips underem if there overnight ,
Just Use three & go at it with circumfrential ( ~^ big word ) masking tape , palming lams secure -
rolling on the bench too , thiswhere the pre glued bit gets it adhered without to many ' dry spots ' .
Glued on one side only before fitting you may only get 50% contact , finished . :(
Job as nipper wasd mixing the weeks Aerolite . Gllop on one surface & use roller - brush hardner on the other .
this is gap filling ( GRAIN filling- more . we'll have no bloody gaps. thankyou ) clamp to moulds and have a
beer .
If your Not a Nipper .
>:(
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Mark, I forgot to mention. I use thinned (with water) Tite Bond aliphatic resin smeared on both pieces of soaking wet balsa. The tissue goes on dry. I wrap the whole mess with an Ace bandage and don't pay too much attention to centerlines. I leave enough excess to be trimmed later. The bandage will leave marks that will go away with a light mist of water and sanding. I didn't invent this and can't remember where I saw it, but it works well.
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thanks Mike
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The Thunder Gazer instructions are as Mike has said, but Carbon Vail is used. Works great.
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As things sometimes happen, I just finished a turtle deck and nose piece for my current project. I guess I misspoke about a center line. I put one on for initial layout when trimming the raw stock before molding. It sometimes (most times it seems) moves when wrapping. I let it bother me the first couple of times, then I just started leaving excess to be trimmed later. Enough years in a machine shop teaches one that you can always take material off, but sometimes there are restrictions on putting it back on, (welding). It's just easier for me and seems to work OK.
Yes Tom, CF works great! Tissue is way cheaper and so am I.
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Mike, I like the wax paper over the mold! Great insurance idea. I'll be adding that to my list of things that you guys do.
( And, while you may be thrifty, you certainly are not cheap.)
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I use gray structural foam, but same idea. It's closed foam that sands easily. And I both coat them in epoxy and sometimes use other resins to seal it. I have a ton of bucks on the shelf at this point so I haven't needed to make one in awhile.
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This is what I ended up with. It probably doesn't show in the photo, but there's a crack toward the front, and where the strip just plain fell off I had to patch it back together. I'm declaring it "good enough" and moving on.