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Old Feb 11, 2013, 05:48 PM
AvB
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Australia, QLD, Woody Point
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On TV the other day I heard a guy quoting

"If you're not prepared to have failures you're never going to come up with anything original".

Well, I have had a go at making a polyurethane filled wing using a mylar splitter plate. Unfortunately the wing is a failure! But there's hope! ... read on ...

As mentioned at the beginning of this thread I made a light wing using Erapol GP330 PU, expanded in a closed mould and it was an outstanding success with a perfectly strong and homogenous foam core. I am still using that wing – it’s a favourite! But the weak point of the technique was that the PU foam does not provide a durable bond of the wing skin seams so the edges are prone to damage and big crashes can tear the skins away from the foam.

So I planned to try using a piece of waxed mylar in between the clamped mould halves as the PU is expanding, so that they can be split apart. The advantage of this would be that areas of dry PU foam could be scraped out around the edges and these areas then filled with a thick bead of expanding epoxy, and the moulds closed again. It should give the best of both worlds … a very quick build method, a stiff and light wing, and good durability. It would also allow trenches to be cut for spars and aileron stiffeners for stronger faster wings, and wiring etc could be put in place too.

On Sunday I spray painted the mould and laid up the wing skins and vac bagged them. Yesterday I stripped the bagging stuff off, and got ready to do the splitter plate and PU process. A couple of friends and my wife Janet helped with the PU spreading ... there is no time to spare and we had 4 people - one per wing side half, mixing for about 10 seconds then pouring and spreading the liquid over the whole skin with gloved fingers. Then the mylar was laid over one half, and the mould closed and clamped.

Although this first attempt failed, I think the mylar split technique was a success, and if I'd used fresh GP330 PU, it would have been a good result. As it was using my old and nearly empty tins, I’m sure the PU foam has “gone off” and it didn't expand as much as previously, or as consistently. When I split the moulds apart and removed the mylar splitter plate, there were some significant deficits I attempted to fix it by “patching" the deficits with a bit more PU liquid just before closing the mould (after spreading the Sicomin expanding epoxy around the cleared edges) but I didn’t do it very well and it didn't work.

Removing the wing this morning, it’s easy to see that the TE is not bonded together, and the LE’s are good but not perfect. Worst of all, a good "flex" test and the wing creases quite easily. Very different to the first wing, which is very strong in flex.

So the first problem was the poor expansion of the PU and the second issue was that the Sicomin PB170 didn't give a good result on the LE's and TE's. These expanding epoxies seem to need a critical mass in one spot to expand well. Where I put a thin strip on the TE, it didn't expand and the TE's are hopeless - barely adhered. The tips where I put a bigger volume are really solid and the foam has pushed the PB170 out into the flanges. The PB170 behind the LE's is not bad, but more bubbly and fragile than PB250 or Ampreg F230 which I'd use next time.

A third issue is that the faces of the foam surfaces (the faces that were against the mylar) didn't bond well together. I smeared them with some PB170, thinking that would be enough, but it isn't. The lack of bonding here is the main factor in allowing the wing to crease under bending force. So next time I'll need to rough up the faces and smear some Gorilla type PU glue over them to give them a strong bond.

It's interesting how when you go and get dirty doing something it moves from ideas, to concrete thoughts about improving it.

One issue I've thought of with the mylar splitter plate is this. (if you can wrap your head around it ...) Using the simple method of putting PU on both skins and closing the mould, there's one big space into which the PU moves as it tries to grow to 20 - 30x its volume. If it moves up and pushes against PU growing out from the other side, it's going to bulge out sideways and into gaps. If the PU is thin in one area, there's a pretty good chance there's plenty more in the vicinity, about to push into the area. But the splitter divides the cavity so that the only "growth" of PU is from the skin on that side. I suspect that the PU I used is not working well, but I also wonder if the splitter, dividing the cavity, inhibits full expansion of the PU?

Once I've licked my wounds a bit I'll try again, but this time doing it as a more basic experiment. I was so optimistic about this experiment that I spent hours painting the mould and used carbon, but next time (with fresh PU) I'll just lay up glass skins with no paint. Do the mylar splitter again and see if we get proper PU expansion. If it's good I'll hack out narrower trenches at the edges, and fill them more solidly with with PB250.

We got some great video of the process but I think I'll wait till I have a success before posting it!
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Old Feb 12, 2013, 10:15 AM
built flown + crashed it
Pewsey, Great Britain (UK)
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Hi Andrew, really appreciate your efforts and willingness to share your results. Moulded wings have scared me off for several years now, the internal bits are the main reason, just looks so much work compared to bagging but am coming around to it, partly by being inspired by this thread and your alternative approaches. Really like the idea of solid core wings as that is what I am used to dsing and repairing, so am working towards getting a few wing moulds done and having a play, along with reconfiguring my fuselage moulds for inflatable bladders, keeps me busy and out of mischief anyway.
Curious about the sicomin foams, particularly the pb250 which is what I want to use, I understand its nowhere near as good as the ampreg F230 but can not get hold of it in small quantities in the uk. My thoughts are to start off using a similar method to how you began with blue foam vac bagged into the moulds, hot wire the excess and bond with pu glue and pb250 around the edges and the tips. Why are you using the pb250 in the last wing and not the ampreg if its better? Another thought is using a larger cavity around the le, more a d- box of pb250 to aid in the expansion and bonded in to the spar. One last thought is maybe on my first try to not use expanding foam at all on the leading and trailing edges but cut smaller cavities and fill with wetted carbon tows before clamping together, slight overfill should get a bit of compression going and form a solid lump on the front of the wing for a good bond and ding resistance for a lightish wing, which is similar to how I do my vac bagged wings currently any how.

Mark
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Old Feb 12, 2013, 04:10 PM
AvB
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Mark, more a subject for the Scratcho thread or another composites thread, digressing from the PU theme! But a couple of quick comments/ replies:

1) Ampreg F230 and Sicomin PB250 in my experience so far are exactly the same - one is no better than the other. Fresh product is best. At least with Sicomin you can get small packs.

2) yes I like the idea of the expanding epoxy filled D-Box. Wings fly better if you keep them light behind the CG, so a good method is vacuum bonding pre-cut blue foam cores into the rear 3/4 of the chord (from TE forward). It allows you to cut slots for a socked spar and aileron stiffeners etc, which you do need for serious stiffness because the expanding epoxy is a bit plasticky. Fill the front empty section and the edges with the ExpEpoxy. One big tip is to make sure you use a thick bead it in areas where it is a thin join... it doesn't like to expand if it starts from a small layer.
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Old Feb 12, 2013, 10:23 PM
built flown + crashed it
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Thanks Andrew, thats good to know, sorry for going off topic.
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Old Feb 13, 2013, 11:55 AM
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Istanbul, Turkey
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Hey Andrew!
That is a great building-technique. I especially like the "rocksolid-version" with the expanding epoxy foam.
We did quite some experiments with PU-foam as core material at some moulded parts (for example building moulded ailerons). I like it wherever weight is not a primary factor and "ease of building" is the dominant criterium. At parts with no interior structure like shearwebs or subspars it works incredibly well.
For us it took a little while until we figured out how to build good parts out of our moulds.
I also am yearning for some low density PU-foam with longer (lets say at least 5min.) potlife but found nothing up to now.
Therefore we needed to cope with this crazy fast stuff and found a - lets call it: "three steps closing procedure".
1st:
We apply thickened epoxy (I use 2 portions of erosil and 1 portion of flox) to the leading edge on both halves of the trimmed parts in the molds. It's about the same thing you'd do when closing a hollow moulded structure.
Then we wait for the epoxy to reach its gel-time. That's about 2 times its potlife but tests should give you the right moment. It's perfect when the "epoxy-worm" barely sticks to your glove anymore but still is soft enough to fully close the mould.
2nd.
We use a wide brush (1.5 - 2 inch) and apply fresh epoxy to the vacuumed and cured skins. This epoxy is mixed with about 3% of foaming agent.
http://www.shop.r-g.de/4DCGI/ezshop?...CKCOUNT=686312
It helps especially on the trailing edges but as well with bonding the PU-foam to the wingskins. I was researching about combining PU and EP systems and found nothing accurate, so I gave it a try ... and it worked.
3rd:
Pour and brush in the PU-foam like you used to do it, just be a little more careful at the leading edge.
With this technique the leading edges are solid. The expanding PU-foam has no real chance to push the epoxy onto the parting board because it is already to thick to move.
Give it a try! ...and keep up the good work!

Philip
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Old Feb 13, 2013, 05:03 PM
AvB
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Thanks very much Philip - very impressed that you have been down this path!

So it's OK to apply the PU foam over the top of a wet layer of epoxy???

Do you have any problems with air being trapped in the mould preventing the PU from filling completely??
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Old Feb 14, 2013, 03:36 AM
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Istanbul, Turkey
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Hi Andrew,
"So it's OK to apply the PU foam over the top of a wet layer of epoxy??? "
It seems to be no problem at all to apply it on wet epoxy. It just doesn't work to apply the PU-foam over a wet layup of wet cloth because the pressure of the PU-foam will distort the layup.

"Do you have any problems with air being trapped in the mould preventing the PU from filling completely??"
In my case: No. As the ends of the aileron-moulds are open at their root and their tip, excess-PU-foam and possible airvoids will find their way out. For a totally closed one-piece-wingmould, if you really might encounter problems with enclosed air, it might help to drill a hole into the lower side of the mould (exactly where the later electric-connector will be placed) and/or maybe even two tiny little channels or wholes on the trailing edge of the wingtips.
Best of success,
Philip
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Old Feb 14, 2013, 04:07 AM
AvB
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That's great thanks Philip. I remember the other day when we closed the mould with the mylar inside, Alan clamped it up very hard so it's possible there was an air trap. If I do it again I'll put some strips of masking tape here and there to allow air to get out.
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Old Feb 14, 2013, 05:54 AM
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Something that intrigues me - when I tried PU cores some years ago, when the wing was de-moulded, the foam carried on expanding and distorted the part. Is this something that you've encountered? Or has PU foam chemistry moved on [significantly] in the 10ish years since I did this?
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Old Feb 14, 2013, 01:33 PM
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Quote:
Originally Posted by Shedofdread View Post
Something that intrigues me - when I tried PU cores some years ago, when the wing was de-moulded, the foam carried on expanding and distorted the part. Is this something that you've encountered? Or has PU foam chemistry moved on [significantly] in the 10ish years since I did this?
I've had warping happen , but I didn't do closed mold expansion. Just fill both halves , cut to parting plane and join.
I always put it down to moisture in the PU....maybe what you have said was the cause.
Got me thinking.

Tim
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Old Feb 15, 2013, 10:59 PM
AvB
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I have not experienced (as far as I can tell) expansion after the initial cure, using the GP330 light PU-foam.

But someone told me the other day how some time ago he bought a Destiny when he was learning to DS, and drilled big holes in the underside of the D-Box area, and filled it with light PU-foam till it came out the vent holes. It didn't expand the skins at the time but a week or 2 later he noticed that the skins had bulged out a bit. But in his words "But it still flew GREAT, and man that thing was TOUGH!!! And I flew the living daylights out of that thing for a long time too!" (it was Bruce Tebo and knowing Bruce I'd say that plane did a lot of flying!!!)
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Old Mar 03, 2013, 04:21 PM
AvB
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I am about to do another PU filled, expanded in mould wing - version 3 for me.

I'm not using the Splitter plate idea. I am going to glue a thin kite spar behind the LE of the top skin. Then I'm going to run a big bead of "Techniglue" around the leading edges, sitting a bit higher than the flange and let it gel for about an hour before getting all hands on deck to do the PU and close the mould. As Kevin suggested, I'm going to make up some notched spreaders from ice cream can lid, similar to tiling glue spreaders only smaller - to aid with quickly and evenly spreading the mixed PU.
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Old Mar 03, 2013, 04:44 PM
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I wish I could be there to help. I would love to be involved in this sort of stuff.

REALLY keen for you to find a good method to make this work! I have so many projects that would benefit
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Old Mar 04, 2013, 02:05 AM
AvB
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It went well Ö first just a bit of background on the layup. I painted in the mould, then 100gm tight weave glass against the paint and 120gm 1K carbon on the bias. Broad triangles of light unicarbon stitched mat, like spar caps. Innegra for the hingelines, and carbon patches with ply for the bolt holes. I put extra squares of carbon under the servo positions. This was vac bagged down last night and trimmed this morning.

I again laid very thin (1.3mm) f/glass kite rods full span. One across the usual spar line, and and one behind the servo areas. Top and bottom so there are 4 of them which I think adds about 10 grams including glue! This stuff is really cheap Ė you can buy it by the metre from kite shops. In a foam core, light skin/ no spar layup these glass rods under the skin provide enormous resistance against the dreaded kinking. Huge. Try it sometime. They are initially tacked down with tiny dobs of hot glue which works beautifully and is fast. Donít get the gun too hot. Then thin CA was drizzled over them and they locked rock solid to the skin.

I also similarly tacked and CAíed a thicker 2.4mm glass kite rod up tight behind the LE on the top half. This is really fast and easy stuff to do.

Last time we mixed PU it wasnít a good result so my very interested daughter and I did some test mixes, and it wasnít good. I started to worry and considered whether it was worth the risk. I had made up some notched spreaders and we spread it instantly it was poured, but it was to no avail - the stuff just went off much too fast. We couldnít spread it before it thickened and began to foam and we couldnít get a good spread. Once itís thickening, if you handle it any further it wontí expand properly.

I started to get nervous and phoned the PU supplier for some advice. He reckoned that it was the high humidity. I wondered if cooling the liquids would help but he was sure it wouldnít, while the air is so steamy. He also strongly recommended getting a drill mixer to speed up the mixing process. So I turned on the airconditioner in the office, in preparation to do the PU treatment later on in the cooler dryer air. But I also put the PU tins (GP330) in the fridge. Later after getting and modifying a small drill paint mixer I got the PU out of the fridge and did another test patch of 20 + 20mL. Wow!!! The cool mixture was thicker and harder to suck up in a syringe, but man did it spread better! I got heaps more working time (well, maybe 10 seconds more but thatís a lot in this game!) and was able to spread it thinly and evenly on the paper, and got a great result Ė a big thick homogenous patch. You can see in the photos the patch closest is the one mixed with cold liquids.

So I mixed up some Techniglue to use around the edges. Itís an excellent bonding epoxy with a jelly like consistency. You could make something similar with colloidal silica Iím sure but itís nice just to get the Techniglue out of the can and mix it at 2:1 and push it into a syringe. I added some glass microspheres too, and syringed a bead all around the tips and LE of both mould sides, with the bead sitting up proud of the flange. I left most of the trailing edge non- Techniglued because I want the excess PU to be able to bleed out. Itís easy to toughen up the TE later by wiping it with some CA.

After about 45 minutes the Techniglue bead was gelling nicely so we did the PU mix. This time I mixed 55 + 55mL with the drill for 7 seconds and poured it across one mould side, full span. My wife and daughter began spreading it FAST while I wiped out the rest from the container with my finger Ė this is so important! We got quite a good even spread and it hadnít gelled when we finished. Excellent! Then I quickly repeated the process for the other mould half, and we closed and clamped the mould. I wish I'd got some photos or video to give you guys an idea of how fast it is done, but with the stress of it I clean forgot!

I really hope this one works out. The PU application went more smoothly than ever before, so thereís a good chance. Iíll leave it for 24 hours to let the Techniglue set really hard.
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Old Mar 04, 2013, 04:53 AM
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Look cool Andrew , great to see the family joining in , now you just have to convince them to have chip night in the shed..
I love techniglue CA....so tough and lovely consistency , they use it to glue composite helicopter blades together. No need to add any fillers. But I guess you just wanted it a little lighter.

Tim
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