HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Sep 07, 2012, 08:57 PM
I hate propellors
emufingers's Avatar
Australia, SA, Normanville
Joined May 2009
375 Posts
Quote:
Originally Posted by acesimmer View Post
I also like the idea of pouring the foam into the mold halves then gluing them together if the closed test fail. This would be very easy to do by just running the hot wire cutter along the tops then glue together or I think that should work?. Anyhow, I will be watching the progress and chipping in with my results as able.

Cheers... Jim
DO NOT use a hot wire on PU foam the fumes are very toxic. I have found the best cutter to be a round diamond covered hack saw blade. Josh use an ordinary blade and scratched his mould even though he was protected it with tape. Ordinary hack saw blades have too much set on the teeth and are hard to control
emufingers is online now Find More Posts by emufingers
Reply With Quote
Sign up now
to remove ads between posts
Old Sep 08, 2012, 05:17 AM
Keep it Simple Stupid
razortoe's Avatar
United States, NC, Raleigh
Joined Jul 2011
244 Posts
I'll bet an electric poultry knife would work well
razortoe is offline Find More Posts by razortoe
Reply With Quote
Old Sep 08, 2012, 05:46 AM
I hate propellors
emufingers's Avatar
Australia, SA, Normanville
Joined May 2009
375 Posts
Sure would. BTW i haven't seen any electic poultry recently.
emufingers is online now Find More Posts by emufingers
Reply With Quote
Old Sep 10, 2012, 10:35 AM
Registered User
United States, CA, Belmont
Joined Jun 2011
16 Posts
Repairing Obechi foam core wing

I need help on figuring out how to repair a wing on a 4 meter scale that met an unusual fate. These are foam core with Obechi. I had a fire in my shop and thankfully this kit just got hit with the heat. The foam melted out from the root for about 30 inches on one wing. The wood is beginning to warp but is all in tact. I would like to refill this with foam. Not sure what is the best method. I don't have a vacuum bag system and the wing is 2 meter long. Any suggestions would be appreciated.

Steve
PrinceLazar is offline Find More Posts by PrinceLazar
Reply With Quote
Old Sep 10, 2012, 04:30 PM
AvB
Wind, hill, ... I'm keen ...
AvB's Avatar
Australia, QLD, Woody Point
Joined Nov 2006
4,470 Posts
Steve, I'd be thinking of hot wire cutting a foam core to fit. Cut out the root, clean out the inside, test fit the new core and trim where required, smear glue inside the skins and slide the new cores in. Hold it all true in the wing beds that you cut the core from, and press the whole shebang flat on the bench while the glue sets.
AvB is offline Find More Posts by AvB
Reply With Quote
Old Sep 10, 2012, 08:40 PM
Registered User
acesimmer's Avatar
United States, NV, Fernley
Joined Oct 2011
1,965 Posts
My A+B sample kit arrived today. Its made by Smooth-On Called foam-it 3 and is 3Lbs per cubic foot. I salvaged an horizontal stab that had gone bad when trying to fill it with the of the shelf spray bomb type expandable foam from the hardware store.

The foamit 3 is a rigid foam designed to expand up to 18 X and is designed for filling models, molds etc. So here goes..... I salvaged the old skins by raking out the garbage stuff. Popped them back in the two part mold and screwed the mold together. I added a strip of balsa for the hinges that I recessed in a 1/4" so my elevators would sit inside them and rotate to scale. Left a tiny gap at each end of the balsa for foam and air expansion. Mixed up two parts of A+B for a total of one fluid ounce. Mixed for thirty seconds then pour a strip down the mold leading edge and swilled it around. Short version...... five minutes later the stab was totally filled and a little foam oozing out the vent sides. Thirty minutes and the heat had dissipated so I unscrewed the mold halves and pulled out the part. Holy crap batman!..... It was already solid with no give and very dense yet still very light. The skin I had used was really light weight that was a little epoxy resin to pick up panel lines rivets etc along with 3/4 oz cloth. Prior to the foam it was really flimsy and gave in at the slightest touch. Now it feels really stiff with zero twisting and very dense. The hinge balsa is locked in really tight and I could not be happier for the first attempt after all those failures using the spray stuff.

I will do one more test layup of the stab and this time just insert 5/8's balsa blocks in three places for the hinges. This way I can easily poor the foam into the mold then sand the remaining off if needed and skin with a touch of five minute epoxy with my fingers. I also came up with a cheap yet amazingly simple way of adding scale panels, rivets and screws to my master plugs that transfer to the molds. I picked up a roll of cheap 3 mil aluminum tape the other day. I laid it out on my test stab to match panels hatches and so fourth. Then used a small and sharpened brass tube and pressed in the rivets. Makes great flush rivets. If you need screws then just press a tiny sharp screw driver over the flush rivet and now you have a perfect screw. By placing the tape quite close to each other then doing the rivet/screw lines it looks the best that I have ever seen and very realistic. I will be doing my entire T-28 this way on the fuse and wings before making the molds. This will save a ton of time over using drafting tape and high build primer etc.
acesimmer is offline Find More Posts by acesimmer
Last edited by acesimmer; Sep 10, 2012 at 08:59 PM.
Reply With Quote
Old Sep 10, 2012, 11:05 PM
AvB
Wind, hill, ... I'm keen ...
AvB's Avatar
Australia, QLD, Woody Point
Joined Nov 2006
4,470 Posts
Brilliant work Acesimmer. That "Holy crap Batman" feeling you described is exactly what hit me when I pulled my fin and stab out of the moulds filled with PU. That foam you're using sounds very good. .... (wait ,,,)

I have tracked it down in Australia - http://www.rowetrading.com.au/smooth_castSeries.html
phoned them straight up, and ordered a starter pack of the Foam-it 3. It sounds good because the pot life is 1 minute, so I'm keen to see if it is easier to work with than the GP330.

They also have the Foam-it 5 which is 80kg/m3 and 10x expansion, and the Foam-it 8 which is 128 kg/m3 and 8x expansion. If it works out OK I'd be keen to try these as different weights for medium slope wings and heavier DS wings.
AvB is offline Find More Posts by AvB
Reply With Quote
Old Sep 10, 2012, 11:27 PM
Registered User
acesimmer's Avatar
United States, NV, Fernley
Joined Oct 2011
1,965 Posts
I also got the starter kit of the foam 5. Once I get my master plugs done in a few days I'll try both with different horiz stabs. Don't want to waste anymore materials on the test mold which is a throw away. I cannot believe how strong this test stab is with only one ply of 3/4 oz cloth. It is also very dense so I expect the 5 lb foam is even denser but a slight weight penalty.
acesimmer is offline Find More Posts by acesimmer
Reply With Quote
Old Sep 10, 2012, 11:58 PM
AvB
Wind, hill, ... I'm keen ...
AvB's Avatar
Australia, QLD, Woody Point
Joined Nov 2006
4,470 Posts
Ace, did you calculate the volume of your part? Do you know what ratio you used, of mixed PU to empty volume??

P.S. I've been thinking ... the light wing I made with GP330 expanded in closed mould ... the leading edge is joined only with the light PU, so it is probably fairly damageable. A simple option would be to cut a slit along the entire LE with a dremel disc, mask either side, and inject the slit with splooge or expanding epoxy. It would bond the 2 skins well, and provide significant ding resistance at the LE. You'd just have to sand it once cured.
AvB is offline Find More Posts by AvB
Last edited by AvB; Sep 11, 2012 at 12:44 AM.
Reply With Quote
Old Sep 11, 2012, 05:44 AM
I hate propellors
emufingers's Avatar
Australia, SA, Normanville
Joined May 2009
375 Posts
Andrew, you could also fill the slit with CF tow. for hardness or Kevlat tow for absolute repairability
emufingers is online now Find More Posts by emufingers
Reply With Quote
Old Sep 11, 2012, 06:54 AM
Registered User
acesimmer's Avatar
United States, NV, Fernley
Joined Oct 2011
1,965 Posts
Quote:
Originally Posted by AvB View Post
Ace, did you calculate the volume of your part? Do you know what ratio you used, of mixed PU to empty volume??

P.S. I've been thinking ... the light wing I made with GP330 expanded in closed mould ... the leading edge is joined only with the light PU, so it is probably fairly damageable. A simple option would be to cut a slit along the entire LE with a dremel disc, mask either side, and inject the slit with splooge or expanding epoxy. It would bond the 2 skins well, and provide significant ding resistance at the LE. You'd just have to sand it once cured.

I did not do calculations, I just wanted to try it after all those other failures. I was only doing one side of an horizontal stab. I left the trailing hinge edge open in the mold for pouring. I pre cut a strip of balsa to drop in the trailing edge after pouring in the foam. Had lots of time to get it inserted after the pour. For the stab I used one ounce total. 1/2 oz of each part. It rises slowly not crazy fast like many other foams. I would suggest your first test be calculated for an 8X expansion until you get used to it. I should have scraped the mixing cup sides a little better when pouring. The cup expanded to 3/4 full from what was left in after the pour. Next time I know not to panic and that I have a bit more pot and working time. I mixed in th ecup for 30 seconds which seems to work well.
acesimmer is offline Find More Posts by acesimmer
Reply With Quote
Old Sep 11, 2012, 04:05 PM
Arrarrar!
josh18's Avatar
Australia, NSW, Wagga Wagga
Joined Jan 2010
3,272 Posts
Andrew could you just roun a wet bead of sploge into the leading edge when closing? I think this is what 2stroke does too.
josh18 is offline Find More Posts by josh18
Reply With Quote
Old Sep 11, 2012, 04:33 PM
AvB
Wind, hill, ... I'm keen ...
AvB's Avatar
Australia, QLD, Woody Point
Joined Nov 2006
4,470 Posts
I'd like to use Ampreg in the LE, but I suspect the PU would push it out. But a thick bead of icing-thick splooge might work. I also tack a kite spar to the skin, tightly behind the LE of the top half, which is a huge help in LE ding proofing. I'm going to test out this new foam and the next wing will be a DS/ heavy sloper, with a socked spar and aileron stiffeners wetted out with epoxy before putting the PU in. I am worried by the problems Tim's been having with full Ampreg wings and keen to keep working on PU as an option for the stronger wings. The Ampreg always seems to result in deep print pinholing or worse, but not the PU.
AvB is offline Find More Posts by AvB
Reply With Quote
Old Sep 11, 2012, 06:44 PM
Registered User
acesimmer's Avatar
United States, NV, Fernley
Joined Oct 2011
1,965 Posts
Today I got to try out the 5 Lb/cubic foot foam. Alas not in a wing but to repair a friends foam Sensei that he took off the front end . When it arrived the foam was no good to repair and he said he would just chuck it and buy a new one. So nothing lost I said lets try something new! I cut off what was left of the existing nose and made a cardboard dam around the fuse. I then drilled a few 3/8" holes in the surround foam that was very uneven. We put the plane vertical and strapped it to my bench. I mixed up 2oz of the foam 5 and dripped some into the drilled holes and the rest around the jagged inner damage. It expanded very slowly so gives quite a bit of working time to spread it around. I let it over flow the dam and fifteen minutes later it rock rock hard and I mean hard!!. So what the hell I cut into it with a saw. Cut really nice and then I rasped it to shape and blended with 240 grit sand paper. Total job from pour to finish was around thirty minutes. Amazing how fast it set up. Outside temp was around 85 deg F with humidity around 25%. So it may react differently for other regions. One thing I would say is do not believe the claimed expansion adds. The foam 3 is around 8-10 x not 18 the foam 5 is a round 4-5 x. I will be using the foam 5 inside my layups around wing and tail supports. It seems perfect for laying a thin bead at the wing root corners and let it do its thing then cut away any excess foam to make nice and tidy. I will use foam 3 in the wing and tail cores with the molds acting as the clamps. So far so good.
acesimmer is offline Find More Posts by acesimmer
Reply With Quote
Old Sep 11, 2012, 07:14 PM
AvB
Wind, hill, ... I'm keen ...
AvB's Avatar
Australia, QLD, Woody Point
Joined Nov 2006
4,470 Posts
That sounds great Ace. Lateral thinking ... a different approach can make a quick repair that might otherwise take days!

When they quote the expansion of the foams it's based on the maximum, which you won't get if you pour it into a cylinder etc. If you spread it say 1mm thick it will rise pretty much to the max, like it probably did in your mixing cup. But if it's expanding upwards, the foam provides resistance against itself and also the stiction to the walls slows it down. I think in the flat, wide wing mould spreading it fully, I'm getting pretty good expansion but still, I'm using only 10% volume (expanding 10x) when the foam is capable of 33x! Which is good I think because it forces it to the edges and gives a more solid matrix and a better skin under the composite skin.

Best thing is test it in a mockup situation to get an idea of how much it's gonna expand in the situation, and go from there.
AvB is offline Find More Posts by AvB
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Dow 1/2" super rigid foam Polyisocyanurate Rigid Foam Insulated Sheathing inline300 Foamies (Scratchbuilt) 7 Aug 15, 2009 05:55 PM
Discussion Moulded Wing, or Foam Core...Which Do You Prefer? Kenny Sharp Hand Launch 10 Nov 13, 2006 04:38 AM
Discussion Rigid Polyurethane Foam wing cores? moreil Hand Launch 9 Jul 11, 2006 04:01 AM