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Old May 17, 2003, 09:35 AM
Stuck on a rock
epilot's Avatar
Germany, NDS, WOB
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Comparison of various building materials

I made 15x15cm pieces of various materials and weighed them for comparison. Here are my findings:

3mm Depron - 3,3 grams
3mm Depron with skin sanded off (2,8mm) - 2,5 grams
2mm Selitac (lightly sanded) - 1,5 grams
4mm beaded styro (wallpaper backing) - 1,0 grams
2mm beaded styro (wallpaper backing) - 0,6 grams
1,7mm hot wire cut pink foam - 1,5 grams
1mm medium grade balsa - 3 grams

Yes, we mainland Europeans use commas instead of dots

Some thoughs on the results:

Sanding 3mm Depron reduces weight by app. 20% without sacrificing much strength. Quick to do with an orbital sander. It becomes more bendy. Great for fuselages but leave the skin on for wings on heavy models.
Don't despair if you can't get Depron - decent balsa is lighter and stiffer although not as forgiving in a crash. Hot wire cut blue/pink foam is another good alternative.
For light/small models like Bit conversions nothing beats the 4mm wall foam in terms of weigth/strength ratio and even wings work well in 4mm styro if a 1mm carbon spar is glued in. If you want a smooth surface Selitac is great but since it comes on a roll it needs to be carefully ironed to straighten it.

Depron that has not been sanded is best glued with UHU Por which is a solvent free contact cement. Unfortunately it remains rubbery and cannot be sanded. If you sand the skin off Depron or Selitac you can use aliphatic glue which sands better. Need to hold pieces together while the glue dries though.

Hope this is useful info. It has probably all been mentioned before.

Michael
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Old May 17, 2003, 01:58 PM
in persuit of low wing loading
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Michael, Thanks.
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Old May 18, 2003, 09:48 AM
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I have some sheets of Depron from Flitehook in various thicknesses down to 0.5mm. Unfortunately it does almost weigh the same *per area* as the 3mm stuff, so I did not use it for models yet because at the same weight the 3mm material is much stiffer. Up to now I have only used 3mm Depron and 2mm Selitac for micro models (besides 1mm balsa, of course).

In my opinion the crash resistance is best with Selitac, especially when glued with UHU Por, because an UHU Por joint will never break due to its flexibility. My Junkers F13 is carved from blue foam, and it is very brittle. Also these quick flying field repairs with styro-safe CA are brittle, very often the plane will break again near the repaired part.

I agree with Michael that sanding UHU Por joints is a pain, it simply does not work. This is probably the biggest disadvantage of UHU Por.

The biggest advantages of Depron and Selitac over balsa is that you are able to build models much quicker, and they are more crash resistant.

Regards, Jochen
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Last edited by jberg; May 18, 2003 at 09:52 AM.
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Old Sep 15, 2003, 09:52 PM
Who Says Penguins Can't Fly?
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Bump!
Thanks Michael!

Thanks!
-Michael
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Old Sep 16, 2003, 06:20 PM
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Michael,

That's very useful information - thanks for posting it. I build with both Depron and hot wire slit blue Styrofoam and your measurements confirm my own observations, that Depron is slightly heavier for a given thickness than raw blue foam.

The extra weight in Depron apparently comes from the hard skin, which adds considerable stiffness to a sheet compared to raw blue foam. For this reason, I prefer to use it for profile fuselages etc., whilst sticking to blue foam for wings which usually have carbon tubes or rods to stiffen them up anyway.

I also suspect that the skin on Depron makes it more resistant to fracturing in a crash, due to the denser skin being able to take more tensile stress than bare foam.

Graham.
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Old Sep 16, 2003, 08:00 PM
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I would like to bring one of my favorite materials to light.(no pun intended). I have used cardboard.The corregated type,about 2mm thick.I touch it with just enough cyano glue to lightly flow down the "flutes" This results in some of the toughest lightest materials I have ever worked with. I made a main frame for a Hornet helicopter...half the weight and stiffer than the origional..
Im making the bulkhead form my Ducted fans with it...there is no grain weakness to worry about.
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Old Jun 14, 2004, 05:52 AM
Stuck on a rock
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I decided to bump this thread because I know someone can use the info. I also have something to add.

I found that the best way to flatten Selitac is to bake it in the oven between two sheets of glass (or in my case between a glass cutting board and a sheet of 1.5mm brass because this was all I had). The temp should be 95-100C. Any more than that and the foam starts to shrink. Let the sandwich cool down a bit before removing the foam sheets.

As both Jochen and I have discovered, gluing foam with foam safe CA is not really a good idea. The glue joint is brittle and you can be sure that the foam will break around the joint in a crash. It then becomes very difficult to re-glue. Better use the contact adhesives like UHU-POR. This stays flexible and you can rub off old glue before re-glueing a part.

I use a cheap orbital sander from the DYI store for sanding Depron sheets. DO THIS OUTDOORS! Fine grade abrasive paper and light pressure makes it and easy and fast job.

An indispensable tool for foam sanding is the fine-grit Permagrit sanding block. It will no tear the foam easily and will likely last a Century or more when used on foam.

Michael
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Old Jun 14, 2004, 06:43 AM
Soaring freak
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Stockholm
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Flatten SELITAC

When I flatten selitac I use an iron, with a paper between. I have also experimented with foot copies (carbon print) to get a print on the selitac. Some times it works but mostly not, it seems to be very tricky with speed and pressure!
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Old Jun 14, 2004, 06:48 AM
Stuck on a rock
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I used to iron Selitac too but it is a bit off a hit and miss way to flatten it. Oven method is more consistent. I wonder if it would be possible to print on lightly sanded Selitac with an inkjet printer? Any tried this?

Michael
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Old Jun 14, 2004, 07:04 AM
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2mm Depron does not seem to have as much of a skin as the 3mm type BTW

Selitac has a definate chewyness to it which seems great for crashability, Depron too to some extent. Blue foam comes in many grades etc, it is more brittle but boy you can build light and the finish is supurb if done right. Some people seem only to be able to get the more coarse BF and the finish is not as nice

Graham
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Old Jun 14, 2004, 07:14 AM
Stuck on a rock
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Some of the tendency for blue foam to crack can be removed by "painting" the part with watered down PVA. It effectively creates a skin like you have on Depron. PVA works quite well for glueing wall foam.

My biggest wish: An UHU-POR type glue that is thin enough to be applied with a brush and has a slightly longer drying time. I wonder if UHU-POR can be thinned with something that won't attack foam?

Michael
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Old Jun 14, 2004, 07:26 AM
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michael.
you can try ethanol or methanol or maybe white spirit to thin uhu por.
havn't try yet myself!!
patrick.
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Old Jun 14, 2004, 07:32 AM
Stuck on a rock
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Patrick, it is worth a try. Some German "Korn" might do the trick
I have "tried" ethanol many times but it always leaves me with a headache the day after. Methanol? Yuck, I'd have to visit a slimer flying field to get some of that

Michael
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Old Jun 14, 2004, 07:33 AM
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or hexane!
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Old Jun 14, 2004, 07:38 AM
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methanol not good for your headhache !!and your eyes too!
patrick
that's why the new glues are water based!(some one not bad!)
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