Sandable foam . . . ?

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raymondbird

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Been studying the Mark Langford method of making wing skins(seems like the easiest method to me) but before taking the plunge, could anybody advise me if there is a better choice of foam?
I understand PVC has far superior properties but from my experience, would be very hard to sand down to profile like PU or XPS.
Any comments greatly appreciated!
 

raymondbird

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Superior properties to what?
Do you have a link to Mark's method?

Duncan
Just google "Mark langford KR2". Absolutely required viewing for anybody into homebuilt airplanes.
PVC foam has markedly superior qualities to PU or XPS foams, especially as a sandwich core. Really wanted to use Last-a-foam as it's easy to sand and sounds great but not after reading all the negative posts about it here on HBA.
 

wsimpso1

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Each type of foam has its use. Issues being balanced are weight, cost, man-hours, laminating methods, and sturdiness against handling, processing, and use:

Extruded polystyrene foam is safely hot wired, and so is great for massive core work, but is both softer and less strong, so requires a minimum of 3 UNI (21 oz of cloth per square yard) to be sturdy for build and use. It sculpts poorly, but can be done. And cost wise it is reasonable for sculpting;

Expanded polystyrene foam is safely hot wired, but is fragile and may be OK for tooling, but I would never use it for airplane components. It requires a careful hand to sculpt it, and is inexpensive;

Polyurethane foam gives off toxic gases when hot wired - just don't expose the stuff to hot surfaces. It sculpts nicely and its cost is OK even when you are throwing away much of it. Depending upon the density chosen can use 3 UNI or 2 BID glass, but I would never go down to single ply glass with it;

Last-a-Foam has a checkered history. Some have had awful experiences with it, others have found it to be OK. That some knowledgeable and capable folks have had a bad time with the densities we like to use for skins is enough for me to back away from it;

PVC foams are sturdy, tough to sand, and make substantial feeling parts that easily survive de-molding processes. BoKu reports that the sailplane community covers it with 1 ply of 6 oz carbon fiber fabric with excellent results, so it seems to backup the fiber-resin laminating products better than the above foams. It is also pricey stuff and sanding it to shapes can be a serious chore - the result being we homebuilders do not sculpt it much. We may heat bend it where we must to get it into the molds, but much of the time it is left flat until it is drawn down into the mold.

Now to your point - Seriously now - ALL materials are compromises made to reflect the needs of the processes and use of the products made from them...

Is there something in the airplane you are contemplating that would require a stronger and/or heavier foam? Have planes built this way had issues in the field or in-flight break-ups attributable to wing skin failures? Is your max q substantially higher than other planes built with those materials and processes? Without a serious reason to deviate, you are just adding cost and man-hours without benefit to the airplane.

Billski
 

vhhjr

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I have only made small parts and models from foam and always found the glue line difficult to sand flat to the foam. After much experimenting I developed a method that seems to work. Mixing foaming Gorilla glue with the right amount of lightweight spackle compound works quite well and leaves a glue line that sands flat. Because the mix starts foaming immediately it is tricky to glue large blocks of foam with this mix. The foaming rate can be slowed by keeping the water and Gorilla glue very cold before mixing. I have attached two detailed articles on this method that I wrote for Kitplanes magazine a few years ago.

Vince Homer
 

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vhhjr

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Slight correction: Should read, "keeping the spackle and Gorilla glue very cold....."

Vince Homer
 

raymondbird

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Each type of foam has its use. Issues being balanced are weight, cost, man-hours, laminating methods, and sturdiness against handling, processing, and use:

Extruded polystyrene foam is safely hot wired, and so is great for massive core work, but is both softer and less strong, so requires a minimum of 3 UNI (21 oz of cloth per square yard) to be sturdy for build and use. It sculpts poorly, but can be done. And cost wise it is reasonable for sculpting;

Expanded polystyrene foam is safely hot wired, but is fragile and may be OK for tooling, but I would never use it for airplane components. It requires a careful hand to sculpt it, and is inexpensive;

Polyurethane foam gives off toxic gases when hot wired - just don't expose the stuff to hot surfaces. It sculpts nicely and its cost is OK even when you are throwing away much of it. Depending upon the density chosen can use 3 UNI or 2 BID glass, but I would never go down to single ply glass with it;

Last-a-Foam has a checkered history. Some have had awful experiences with it, others have found it to be OK. That some knowledgeable and capable folks have had a bad time with the densities we like to use for skins is enough for me to back away from it;

PVC foams are sturdy, tough to sand, and make substantial feeling parts that easily survive de-molding processes. BoKu reports that the sailplane community covers it with 1 ply of 6 oz carbon fiber fabric with excellent results, so it seems to backup the fiber-resin laminating products better than the above foams. It is also pricey stuff and sanding it to shapes can be a serious chore - the result being we homebuilders do not sculpt it much. We may heat bend it where we must to get it into the molds, but much of the time it is left flat until it is drawn down into the mold.

Now to your point - Seriously now - ALL materials are compromises made to reflect the needs of the processes and use of the products made from them...

Is there something in the airplane you are contemplating that would require a stronger and/or heavier foam? Have planes built this way had issues in the field or in-flight break-ups attributable to wing skin failures? Is your max q substantially higher than other planes built with those materials and processes? Without a serious reason to deviate, you are just adding cost and man-hours without benefit to the airplane.

Billski
Ya, you make a lot of sense of course as usual and thanks very much! I know PU core surf boards survive better than you would think with that crumbly, friable stuff as a core. No airplane skin wing failures that I've ever heard about either. Say no more . . .
Ray
 

patrickrio

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Aug 15, 2020
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278
Each type of foam has its use. Issues being balanced are weight, cost, man-hours, laminating methods, and sturdiness against handling, processing, and use:

Extruded polystyrene foam is safely hot wired, and so is great for massive core work, but is both softer and less strong, so requires a minimum of 3 UNI (21 oz of cloth per square yard) to be sturdy for build and use. It sculpts poorly, but can be done. And cost wise it is reasonable for sculpting;

Expanded polystyrene foam is safely hot wired, but is fragile and may be OK for tooling, but I would never use it for airplane components. It requires a careful hand to sculpt it, and is inexpensive;

Polyurethane foam gives off toxic gases when hot wired - just don't expose the stuff to hot surfaces. It sculpts nicely and its cost is OK even when you are throwing away much of it. Depending upon the density chosen can use 3 UNI or 2 BID glass, but I would never go down to single ply glass with it;

Last-a-Foam has a checkered history. Some have had awful experiences with it, others have found it to be OK. That some knowledgeable and capable folks have had a bad time with the densities we like to use for skins is enough for me to back away from it;

PVC foams are sturdy, tough to sand, and make substantial feeling parts that easily survive de-molding processes. BoKu reports that the sailplane community covers it with 1 ply of 6 oz carbon fiber fabric with excellent results, so it seems to backup the fiber-resin laminating products better than the above foams. It is also pricey stuff and sanding it to shapes can be a serious chore - the result being we homebuilders do not sculpt it much. We may heat bend it where we must to get it into the molds, but much of the time it is left flat until it is drawn down into the mold.

Now to your point - Seriously now - ALL materials are compromises made to reflect the needs of the processes and use of the products made from them...

Is there something in the airplane you are contemplating that would require a stronger and/or heavier foam? Have planes built this way had issues in the field or in-flight break-ups attributable to wing skin failures? Is your max q substantially higher than other planes built with those materials and processes? Without a serious reason to deviate, you are just adding cost and man-hours without benefit to the airplane.

Billski
I have been saving pages with engineering information on structural foams for use in spar webs and ribs, specifically XPS, PU and PVC foams but I haven't found a good source for engineering qualities and variations for various densities, types (closed/open etc), manufacturers/sources etc. any recommendations? Has someone here already compiled an engineering qualities table or something like that?
 

TLAR

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Mark used sand paper that floor sanders use, glued to a piece of pvc pipe, probably sch80.
That method would be effective on any foam and would leave a nice flat surface.
Vacuum the foam a lot during the process.
The setup will take a while but the sanding time will be short
 

TLAR

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I don’t remember what foam he used, but my tests clearly showed to avoid lastafoam.
DOW billet is the only foam that I would use with confidence

One key to this is to avoid sanding epoxy glue lines, get a billet and Hotwire/sand
 

challenger_II

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Fisher County, Tx. USA
I use Minwax Polyurethane to laminate foam board (blue, pink purple'ish insulation board) for making larger sections. The hotwire cuts smoothly through the glue joints. Minwax, mixed with the Evil talc powder, makes a good material for coating the exterior surface, for sanding to a fine finish. Once surface is prepped, a couple of coats of Minwax has you ready for direct painting, or glassing.


I have only made small parts and models from foam and always found the glue line difficult to sand flat to the foam. After much experimenting I developed a method that seems to work. Mixing foaming Gorilla glue with the right amount of lightweight spackle compound works quite well and leaves a glue line that sands flat. Because the mix starts foaming immediately it is tricky to glue large blocks of foam with this mix. The foaming rate can be slowed by keeping the water and Gorilla glue very cold before mixing. I have attached two detailed articles on this method that I wrote for Kitplanes magazine a few years ago.

Vince Homer
 

TLAR

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Sep 29, 2020
Messages
290
How did you discover that Challenger2?
Good tip but I gotta run that through a delam test. I assume you have already done that
 

challenger_II

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Discovered the technique by fighting with foam cutting. :)
As for de-lam, the longer, and broader, the section laminated, the greater the resistance to de-lamination. Truthfully, this laminating method is used for non-structural applications, or where One will have structural reinforcement, such as wood stringers, or carbon fibre inlays.

How did you discover that Challenger2?
Good tip but I gotta run that through a delam test. I assume you have already done that
 
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