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I recall (from probably over ten years ago) someone demonstrating laminated sides that had a foam core. Purely for fun and experimentation, I was interested in giving that a try. Sonically, that seems like it would be the absolute opposite of the rigid live walls that, in theory, bounce back the vibration of the top. I'm curious what a guitar like that would sound like.

Problem is, I can't seem to find the thread now. What kind of foam were they using? How thick? Was the foam flexible, or did they have to score it? Did the foam compress under vacuum when laminating? I know the sides ended up quite thick, but thick enough not to need kerfed lining? Were these 6mm sides?

My memory is that they used some kind of closed cell rigid foam, that it was 6mm or 1/4" thick, that they didn't need any lining for glue up, and that at 6mm it was flexible enough to bend into a typical guitar shape. But I think I may be remembering wrong.

I'm happy to experiment. I get a little bored building the same thing over and over. But I don't want to reinvent the wheel if someone is already worked through some of these issues.

Anyone have any experience with this technique?

I was lucky enough to attend a seminar by Charles Fox about 15 years ago and he demonstrated his use of foam core laminated sides. He was using 1/4" thick closed cell foam. And he laminated wood veneer on both sides of the foam. Never tried it myself.

I think one challenge would be figuring out a way to install bindings and purflings into foam.

I never saw the old thread, but it sounds like a fun experiment

Dollar store foam board is a good source for 3/16" foam. I use it with carbon fiber (on non-guitar projects). The paper peels off easily, unlike more expensive foam boards. Though it does only come in 30x20" sheets, so the length is a little short. Maybe cutting diagonally could get a large enough single piece for a side. It's fairly brittle, so practice with smaller pieces to figure out how to get it into the waist without breaking.

Seems like binding would be a problem, routing through into the foam and then not having a solid surface to clamp against. Maybe plastic binding would be flexible enough to glue to the foam without crushing it.

Whether you need linings probably depends on how clean of a surface you can get on the foam rim (chunks tear out easily). And with wood veneer and 3/16" foam, I think you would need the added glue area of linings. But if the wood is 2mm, then maybe ok.

I'd probably use epoxy for everything that bonds to the foam. Though I'm not sure how well it could grip plastic binding, even with the surface roughed up. Would need testing. Regular plastic adhesive would probably dissolve the foam. I know CA glue dissolves it.

Anyone know what the theory is behind doing this? The only thing I can think of is that it might make the guitar light in weight. And of course now there's lots of talk about making sides heavy for producing better tone.

I’m interested for a super light weight archtop. But I admit I’m confused on the sonic qualities. My poor recollection is that the builder in question (a German chap, if I remember right) considered them the ultimate in rigid sides. Seems to me that it would be the opposite.


PS Carbon fiber is an excellent idea for sides. More impact resistant than a sheet of .5mm veneer. I’ve been wanting to make a classical guitar with carbon fiber sides just to see some people’s heads explode. Sacrilege!

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You might consider honeycomb. It'll bend along the ribbon direction so it shouldn't have any trouble conforming to the body shape.
Closed cell polyurethane foam (General Plastics), PVC Foam (Divinycell) and acrylic foam (Rohacell) are all used in structural panel applications. They come in a variety of densities but will more than likely have to be heat formed to the side shape.
Also, (people look at me cross eyed when I suggest it) end grain balsa is another choice. An end grain core makes a much stronger panel than a long grain core due to the fiber orientation in regard to the shear forces.
Lots of options!

_________________
Jim Watts
http://jameswattsguitars.com

Do you seal the balsa with shellac or something before using it that way? Otherwise it seems like it would either starve the joint or soak up so much resin that it becomes more like a solid block of plastic with a few wood fibers in it. Though shellac will also saturate it unless applied carefully with a relatively dry cloth to bridge the pores without penetrating.

You can let your epoxy advance and thicken prior to applying it, or a mix of hot glue might also work as a sealer.
I haven't made too many panels from balsa, but using a thick epoxy seams to work well.

_________________
Jim Watts
http://jameswattsguitars.com

Thanks Jim. When I look for "closed cell foam" on Amazon I get a lot of "neoprene". You don't suppose that is what they used? I was imagining something less spongy and more rigid.

I see Polyurethane as "upholstery" foam, so I guess also very spongy. PVC foam looks more like what I imagined. But what I see has slick skins on either side. I would worry that anything would bond to them. Hmmm....

You're right that is not what you want, you need rigid closed cell foam.
Here's a link to Fiberglast, they sell small quantities of various cores. Specifically Divinycell.
www.fibreglast.com/category/Vacuum_Bagg ... dwich_Core

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Jim Watts
http://jameswattsguitars.com

That reminds me, the foam board stuff I mentioned in my first post does need a light sanding to open up some pores.

Tom Sands has been doing a lot with what he calls “structured sides,” which sounds very similar in concept. I’m not sure what he uses as the inner layer, but we’re talking about two veneers separated by some kind of inner layer. You can find more info on his Instagram account or by messaging him.


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I have worked with honeycomb Nomex in building boat hulls a long time ago and just looked it up to see if it's still made. To my surprise it says that innovative luthiers are using it for double tops. Seems it would make a great lightweight stiffener for sides too

https://www.lmii.com/nomex/3593-nomex-18-honeycomb-060-sheet.html?gclid=CjwKCAjwy42FBhB2EiwAJY0yQhpoXy6_5OGTB2nOMWJNm_e8gWprmDysr3BcKl3AC629L5hiYBGiDhoC7dsQAvD_BwE

Ed M

Allan Beardsell has been doing this for many years. More than 15 that I know of. Might be longer. The sides 2mm and are bent normally. The inner layer is basswood, (kerfed on both sides.) The inside veneer is .020”

He used to have photos of this posted on his site. The Beardsell Platform rim.

For those who don’t know his work, check him out. https://www.beardsellguitars.com

Bob


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Bob

Pvc sheet foams like Divinycell or rohacell in 5lb density are widely used in making composite structures and readily available. These are Easily formed into guitar shaped pieces using a heat gun and much easier to laminate than balsa cores because they do not absorb the resin. End grain balsa is lighter and better shear strength, but it can be difficult achiece consistant lightweight structures with because it wicks up so much resin and each piece wicks differently.

Definitely the Divinycell in 5lbs looks to be the best bet. I'll order some for my little experiment.