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Old Sep 12, 2012, 09:26 PM
AvB
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I received the Foam-It 3 (FI3) yesterday did some comparisons with the GP330 I’ve previously used. But I’m not happy with it.

On the positive side, the FI3 does have a longer working time, which we want. These foams reach a point while they gel, when they break down completely when touched. Let’s call that the break time. I started the timing from when I’d mixed for 15 seconds and poured the liquid. The FI3 break time was a good 2 minutes, and even then it seemed to hold up against handling to some degree. The GP330 reached break time at 1:10 at which time it completely collapses to the touch.

The FI3 is slower to reach the “firm and dry to the touch” point … it took nearly 10 minutes, vs the GP330’s 6 minutes. But that isn’t a factor that affects us greatly. The break point is the critical limit to handling, when using it to cover a big surface area like wing moulds.

But the big disappointment was that the FI3 gets really big bubbles and is more inconsistent than the GP330. I took some video to show the differences and to explain how I compared them.

GP330 vs Foam-It 3 (4 min 45 sec)
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Old Sep 13, 2012, 01:00 AM
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Estonia, Harju, Tallinn
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There's a problem with using PU foam as core material. It is very hard to get consistent results. There are too many variables that affect the end result - temperature, humidity, how the PU is poured, etc. The industry has tried this method with inconsistent results. Because of this they have abandoned using 2 component PU foam as core material.
With model airplanes wings it might be ok that on core has less compressive strength than the other.
It's mentioned in the book "Composite Basics" by Andrew C. Marshall. Recommended reading to anyone who's working with composite materials.

If you mix PU glue with water you'll get a foam that is very inconsistent in density - some areas have very large bubbles and others are almost solid.
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Old Sep 13, 2012, 08:50 AM
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United States, NV, Fernley
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Av

I'm not getting what your showing. I wonder if it is the difference in locations. I'm at 4200 ft altitude with humidity between 10-20% with day temps around 90 deg. I do close my molds leaving just a couple of breathers so that I do get compression.
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Old Sep 13, 2012, 05:23 PM
AvB
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Ace, one guy I spoke to said that inadequate mixing caused big bubbles. So I made another batch yesterday and mixed it very very thoroughly, and it was quite significantly better than the first ones. (they were pretty well mixed I thought). But I do like the consistency of the Era foams and am ordering a couple of others to try, which have longer working times too.

The guy from ERA said this:
Hi Andrew
The meaning of the letters in the name of the foam is BS for BLOCK FOAM and MF for MOLDING FOAM. The MF foams are known to be the stronger foam with better compressive strength( crushing strength) and thicker skin than BS foams at the same foam density.

Ecofoam BS320 has the same density as ECOFOAM GP330. You would have more time to mix and spread Ecofoam BS320 into the mold as this foam has a longer cream time.

For the higher density foam the choice is between Ecofoam BS700 or Ecofoam MF710. I would try Ecofoam MF710 first as this foam has the longer cream.

Other foams you can consider that have a heavy density and long cream time would be ECOFOAM MF 62, ECOFOAM MF710 and ERATHANE MF95

regards Kim
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Old Sep 13, 2012, 06:25 PM
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All your research is great guys. I guess foams aint foams hey! Just from curiosity, what is the density of the ampreg, and would a PU foam of the same density do more or less the same thing?
Cheers
Josh
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Old Sep 13, 2012, 07:30 PM
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Hey guys, finally got a chance to pour some foam today. I did a free-rise with the TC-300 6 pound density. The first thing I did was apply our adhesion promoter TC-89 which is sprayed via airbrush or touch up gun. This stuff works great with a variety of polyurethanes to promote bonding on different substrates. I let it dry for 30-40 min to allow solvents to flash off before casting my material. I did a mix of 100g TC-300 for the first wing mold half. This covered a majority of the tool as I worked it around using a brush. I stopped messing with it as it began to thicken/gel so I had to go back with some more to get a little more coverage and thickness. On the second half I mixed up 150g and this gave much better coverage. I touched up a few spots with one more 50g mix to ensure I had certain spots above the split line. After about 20-30 min it was firmed up rather nicely. By the time I took it home for the day it was rock hard. I will use a sure-form rasp to knock it down close to level and then a sanding to fine-tune it.
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Old Sep 13, 2012, 08:52 PM
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Well done Troy...
That's the way to do it..

Josh.
Ampreg is 230kg/m2...
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Old Sep 13, 2012, 11:12 PM
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And here is the top half after sanding. The 6 pound is not delicate stuff to sand. I found a mix between 50 grit sand paper and a hand-planer would knock it down the fastest. I tried a large serrated kitchen knife but it didn't cut through as I hoped.
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Old Sep 14, 2012, 05:35 AM
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Looking great Troy!
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Old Sep 14, 2012, 05:53 PM
Arrarrar!
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+1!
Thanks Tim.
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Old Sep 15, 2012, 04:55 PM
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I finished sanding the second half of the wing today. Considering it was pushing 100F in my garage, I am a bit done with the heat.

The good news is that the mold is joined and curing as we speak. I opted to use Gorilla Glue to join and I pre-wet the foam to get the GG foaming up. I was originally going to use a fluffy mix of epoxy and micro balloons but wanted to see how GG would bond. Plus, I wanted the expansion of the GG to fill any gaps in the joint. Considering the heat and humidity today, I should be able to demold tonight.
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Old Sep 15, 2012, 08:24 PM
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Great looking wing and mould by the way Troy! Do you have a thread for that one?
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Old Sep 15, 2012, 10:54 PM
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Thanks Josh. The design for the wing is a long, interesting story but basically I got to borrow some male plugs we shipped from Germany. More info on the wing is in this thread and Biber's blog: http://www.rcgroups.com/forums/showthread.php?t=534401

The mo(u)ld is cast from TC-1651 aluminum filled epoxy. I don't have a thread dedicated to it but there are photos of the build in the link above.

So I demolded my wing tonight and am pleasantly surprised with the results. The weight is about 200g and is very stiff in both lateral and torsional. The dent resistance is great. The lay up is only a layer of .75oz and 3oz for the skin. The spar caps are pre-cured high modulus carbon. My plan for this thing is either a slope airframe or a modest electric pusher to test the funky control set up. This is not a conventional elevon wing (more in the thread above).
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Old Sep 16, 2012, 12:10 AM
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Wow , sweet work Troy .
200g is great.

Tim
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Old Sep 16, 2012, 12:49 AM
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Quote:
Originally Posted by Troy View Post
The 6 pound is not delicate stuff to sand. I found a mix between 50 grit sand paper and a hand-planer would knock it down the fastest. I tried a large serrated kitchen knife but it didn't cut through as I hoped.
What about this concept Troy: since you have a mold flange, clamp down a series of surfaced MDF planks just after the wet foam is applied. They are positioned with a small, consistent gap between their edges, acting kind of like vent holes. When foam is cured, pop off the boards & all you have to sand is a thin section that oozed in through the gap. Not sure what releases expanding foam though...always seems to stick to everything. Release treated mylar 3M'd to the MDF maybe?
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