Thinking ahead for some projects, I was wondering if there is a foam out there that has the same density/strength as balsa, but can be hot wired cut?

Originally Posted by Guz

Thinking ahead for some projects, I was wondering if there is a foam out there that has the same density/strength as balsa, but can be hot wired cut?

No.

Balsa all by itself is a complex, composite material with a huge range of weights and densities. In addition, it has huge differences of compressive and tensile strengths in different directions, and huge differences in stiffness (Youngs) depending on the direction of grain versus applied force.

Foam is a monolithic, non-direction material with the same properties from every direction; its advantage is its uniformity. It is also low weight, cheap, resilient at low loadings, and has the ability to be formed easily.

Balsa was dropped from assembly line production use from all but the most advanced, lightest, sailplanes.

Properly selected and graded balsa is far superior to foam in just about any/every application within an airframe other than cost and ease of use. Foam is fine for just adding bulk, but balsa becomes a load bearing portion of the composite structure as well as adding bulk.

OK, so out of all the foams out there, which would be the best for a load bearing core?

This isn't for a plane, btw.

I just don't want to shape a balsa core. I can get a more accurate shape with a hot wire, than with planer and sandpaper. It can be a 'heavy' foam for my use (just need to keep it at or under 'heavy' balsa).

I think we need a bit more info on what you are creating.

There are foams that have a compression strength that is over 200 psi. They can be toxic to wire cut though.

Dow makes a foam called High Load 100 that has a compression strength of 100 psi. It's extruded polystyrene. It can be hot wire cut. Owens Corning also makes Formular 1000 that is XPS and 100 psi.

3.0 pound density expanded polystyrene (can be wire cut) has a compression strength of 100 psi. 4 pound density has 175 psi. 4 pound density is pretty tough to come by though.

The longitudinal compression strength of balsa is around 700 psi. I can find any firm figures on the axial compression strength of balsa.

I see numbers for balsa of 3.7Mpa (530PSI) to 27MPa (3900PSI) for the compressive strength parallel to the grain.

Cross grain compressive strength of most woods is about 1/10th the parallel compressive strength. Balsa might be about 50 to 400PSI on that basis.

http://www.scribd.com/doc/37966050/Balsa-Propertie-5

Kevin

http://f1.grp.yahoofs.com/v1/QGmfTdS...m_modulus2.txt

Elastic Modulus Of Common Foams last revision: 5 May 03
================================================== ======================

foam rho E11 E22 E33 E33/rho data source
type (lb/ft^3) (psi) (psi) (psi) (in/10^6)
---------- ------- ------ ------ ------ -------- ------------
Spyder foam 2.30 4000 6900 5.18 Mark Drela
Spyder foam 2.36 1218 3358 7027 5.14 Phil Barnes

Hi-Load 60 2.25 1100 3400 6000 4.61 Mark Drela
Hi Load 60a 2.30 1229 2819 6212 4.67 Phil Barnes
Hi Load 60b 2.30 1412 2888 6412 4.82 Phil Barnes

Dow blue 1.75 2000 640 2500 2.47 Mark Drela

Foamular 250a 1.63 2317 1899 2372 2.51 Phil Barnes
Foamular 250b 1.63 2430 1972 2476 2.62 Phil Barnes

Foamular 400a 1.98 2788 2946 2941 2.54 Phil Barnes
Foamular 400b 1.98 2920 3088 3087 2.67 Phil Barnes

Foamular 600a 3.04 4927 4927 5108 2.90 Phil Barnes
Foamular 600b 3.04 4716 4807 5251 2.98 Phil Barnes

Grey Foam a 1.54 2894 1054 640 0.72 Phil Barnes
Grey Foam b 1.54 2845 1105 588 0.66 Phil Barnes

Stylite a 1.93 1877 2906 3489 3.12 Phil Barnes
Stylite b 1.93 1847 2854 3416 3.06 Phil Barnes

Rohacell 31 1.90 5120 5120 5120 4.66 CST
Rohacell 51 3.10 9950 9950 9950 5.55 CST

4.7lb balsa 4.70 160000 Mark Drela

---------------------------------------------------


Definitions
-----------

rho = density
E11 = modulus along sheet (8' dimension)
E22 = modulus across sheet (2' dimension)
E33 = modulus across thickness
E/rho = specific modulus


For foam core applications, the relevant modulus is E33.

For bare foam wings (slow fliers, etc), the relevant modulus
is either E11 (span is along foam sheet), or E22 (span is
across foam sheet).

Originally Posted by talonxracer

I think we need a bit more info on what you are creating.

Bagging more sailboat fin/keels. They need to be as stiff as possible.

Use Balsa ...Endgrain...Dont think you can get much stiffer than that

Originally Posted by Guz

Bagging more sailboat fin/keels. They need to be as stiff as possible.

Depending on the size required, a foam covered torsion box(similar to Burt Rutan's homebuilder wing construction) and then hotwired, should provide a very strong core for the keel.