Wacky idea...epoxy foam

[ShatnersBassoon]

As usual I have been mulling over some crazy, possibly too off the wall ideas. 

I love the thought of using alternative materials. Wanted to utilise epoxy in a guitar body for quite some time but firstly it’s kind of been done to death now and secondly it’s extremely weighty.

Anyway, some internet detective work brought up a material known as epoxy expanding foam. I wonder if this could be utilised? Anyone familiar with this stuff? Is it overly spongy or relatively rigid? 

Probably a bonkers idea, as is normally the case in my quest to become a mad scientist

[curtisa]

Might depend a bit on the particular epoxy expanding foam product you're looking at, but I'd be leery about how much strength is in such products. At the very least you're probably looking at some kind of neck-thru construction where the neck bears all the tension from the strings and mounting points for the bridge; the epoxy foam could only be used in non-load bearing structures of the guitar (the body wings).

The MSDS for the first product result I got in Google suggests that the most-dense foam variety (PD600) when set has a modulus of elasticity of 468N/mm2 and a shear strength of 13.4N/mm2, which works out to 0.468GPA and 0.0134GPA respectively. For comparison a piece of mahogany is around 8GPA MoE and 6GPA shear strength - about 20 times less bendy and 400 times less break-y. You couldn't embed a screw into the stuff and use it to mount a bridge, for example.

The other thing to consider is that the stuff gets HOT when curing. Epoxy is exothermic by nature, and the expanding stuff even more so. Datasheet shows temperatures of up to 180 degC depending on the casting thickness and expansion amount for several hours. That's a lot of heat to manage while it's curing. You have to work quickly too, as the datasheet suggests a working time of only 4 minutes once mixed up.

Not a completely mad idea, but one that would require pretty careful planning and thought to pull off successfully. 

[ShatnersBassoon]

8 hours ago, curtisa said:

Might depend a bit on the particular epoxy expanding foam product you're looking at, but I'd be leery about how much strength is in such products. At the very least you're probably looking at some kind of neck-thru construction where the neck bears all the tension from the strings and mounting points for the bridge; the epoxy foam could only be used in non-load bearing structures of the guitar (the body wings).

The MSDS for the first product result I got in Google suggests that the most-dense foam variety (PD600) when set has a modulus of elasticity of 468N/mm2 and a shear strength of 13.4N/mm2, which works out to 0.468GPA and 0.0134GPA respectively. For comparison a piece of mahogany is around 8GPA MoE and 6GPA shear strength - about 20 times less bendy and 400 times less break-y. You couldn't embed a screw into the stuff and use it to mount a bridge, for example.

The other thing to consider is that the stuff gets HOT when curing. Epoxy is exothermic by nature, and the expanding stuff even more so. Datasheet shows temperatures of up to 180 degC depending on the casting thickness and expansion amount for several hours. That's a lot of heat to manage while it's curing. You have to work quickly too, as the datasheet suggests a working time of only 4 minutes once mixed up.

Not a completely mad idea, but one that would require pretty careful planning and thought to pull off successfully. 

Thanks Curtisa. Looks like the guitar builder Teuffel has employed this material in one of his builds. So it must be doable. As you say, would have to be on a non load/strain bearing area...this was one of my initial concerns. 

[curtisa]

What you could do is make it a bit like a surfboard. Cast yourself an oversized block of the stuff and then cut/shape/machine/sand etc the body wings down to your desired profile. The foam section(s) could be skinned in fibreglass and then glued to either side of the neck-thru section to make an extremely light but rigid body.

Similar construction techniques are used by radio controlled aircraft hobbyists. You can buy precast sheets of the recommended foam from various outlets if you don't want the mess of handling the raw chemicals.