Is it possible to use a spray-in product to make a foam core? We currently machine cores to shape but thought it would be a lot easier to simply inject some foam into the mold, let it cure, demold it and then use it as the core material.
What product to use? How do I treat the mold so the foam will release cleanly?
I have only had problems with spray ureathane foam. The problem is it keeps expanding slowly for ever. The 2 part is way better than the can spray foam. That’s my experience others could have different results.
Sicomin expanding/foaming epoxy available in various grades. Not a spray type product though, just pour mostly.
250kg density is popular.
This stuff expands slowly.
Should work with release systems used as with any other epoxy based resins.
Frekote, waxes etc.
http://www.sicomin.com/documents/Sicomin-Brochure-FoamSys-Eng-WEB-dl52.pdf
and make sure your mould is bullet proof … 
biggest problem is that most expanding foams are very dense (heavy)
I was thinking about doing this recently… Of course you’d need a mold to create the core but, BJB has some 2.5# foam: http://www.bjbenterprises.com/media/wysiwyg/pdfs/Foams/TC-300.pdf
looks like it’d work out. THe core we use is nomex honeycomb that ranges 2.5 to 4.5 PCF. So it should be similar in weight. With a mold you could make the cores the way surf boards were made with polyurethane foam. Seems like the compression strength is there to withstand the vacuum as well.
Looks interesting, but you better be ready! Those are some pretty short working tubes.
One of the biggest problems is lack of stability.
When urethane foam is produced it is made in large blocks. These blocks are monitored for internal temp until it reaches ambient. This can take up to 30 days. Then they are sliced on a large band saw.
If the foam is not allowed to completely react it is not stable enough to take temp changes.
We once sold a product that was foam filled honeycomb. On one batch we received it was not allowed to completely react. It was made and shipped in 1 week. It was used as a core material in some race car wings. When it was exposed to direct sun and was heated the foam expanded more resulting in very distorted wings.
I am not saying it cannot be done, (because it has) you need to be aware of the limitations of the process.
I use an 8 lb two part foam with good results, it’s not too heavy either. You can add pigment to color it if needed.
It is a urethane, so…your mold should be released with the best sealer/release possible. Urethane can stick to waxed tool surfaces, I have had it destroy a silicone rubber mold as well.
I have found that fluoro-carbon based release agents work the best with this pernicious bond-to-anything material.
Another moldable core material is microballon fillers/fairing compounds mixed with an epoxy resin (west system). How well it fills in the core mold is up to the technicians ability to apply it. But it is rigid, sand-able, stable, and compatible with most resin systems.