3D printing foam

Hi all. New to the forums here but hoping someone can help me out. I'm wondering if anyone has built a printer that will print foam. I would like to make some very cheap foam parts but only need accuracy to a couple of mm. Printing layer thickness could be similar. It would be large scale prints compared to what most rep raps use so to keep print times within reasonable accuracy and layer thickness should increase accordingly. Ideally I'd like a closed cell foam which is already available at a reasonable price for fairly large quantities. Any ideas?
Thanks.

The Joris Laarman MX3D Resin is a robot arm that has a 2-part mixer and a pair of heat guns at the end. The mixture forms a heat-curing expanding foam that gets quickly cured by the heat guns as soon as it exits the nozzle.

In theory, this could be scaled down to the size of a regular hot end on a RepRap, but I've not seen it done yet.

That is exactly what I was hoping for! Very cool design but I'd only like to be able to print in layers and shapes that are easily 3d printable.
Could anyone forsee a problem with using regular polyurethane foam? After some research I found it takes a few minutes to expand and an hour to set. So ideally it would be mixed right before the nozzle and printed unexpanded while allowing appropriate distance between printed lines and layers to allow for expansion. Then on small prints it would need to wait a certain amount of time before starting the next layer to allow it to fully expand. Unfortunately there would be small gaps between the lines and layers as it expands in a fairly circular shape. I think it would be best to print the outside of the layers first with a small amount of extra material to be removed afterwards, then print the internal parts of the layer slightly closer than it would fully expand to. Since it does not need to cure as quick as the above design there would be no need for heat guns. I can't see anything overly difficult about doing that.
Would the RepRap control software be scalable to allow for relatively large layer sizes and print lines? I am thinking in the range of 5mm would be suitable for both.

I think the best way to go about it would be to have a canister for each liquid similar to a syringe controlled by a screw. These would push the liquid through a tube to a mixing nozzle which could have a small motor which agitates the two in the nozzle and the combined liquid is forced through the nozzle. The rest of the system would be exactly like any other 3D printer.

Well, unless you do some to very quickly cure the polyurethane as it gets deposited, it will continue to expand and then contract as it out-gasses, giving you a warped surface on the individual beads, which I assume would not do well for the larger structure. If you add a heat source to quickly cure a shell on the bead, the pressure for the gasses is contained.

basically, there's nothing stopping the intrepid RepRapper from basically copying the MX3D head and having it on a normal Cartesian-style printer.

Hmm I didn't think of that. The only thing I am wondering is if its fully cured when the next layer is deposited I don't know how well it would stick between layers? Though since it is expanding it might expand into the layer below it slightly.

Might be easier to mill from solid dry foam. The cutting forces are pretty low even with a big cutter so a reprap-style build is probably rigid enough.

Yeah but there would be a massive amount of waste material and no option for internal geometry.

How about pressurizing the liquid components of the foam and using two automotive fuel injectors to spray and air mix the two components in small enough quantities to allow rapid curing via a heat gun (fire hazards to be considered and addressed) attached to the print head. Then between each layer, slice the top with a nichrome wire (an extra axis to deal with - as well as removing the scrap) so each layer gets a fresh level start. On the plus side the more porous surface of the "cut" layer should allow for excellent adhesion of the next layer. On the other hand, a slower process allowing each layer to "skin up" as it cures would, I believe, create a stronger finished product as the skin of each layer would act as a more solid web at each layer within the printed structure, and the expansion of the new layer should back fill any cavities left in earlier layers.

Unless you air-mix the two parts, I can see a problem will arise with material starting to expand and cure inside the mixing head. The issue being that the extrusion rate is not constant during a print, so on slow moves and non-printing moves the mix will sit in the head for a longer time than when printing long straight thick lines. The expanding foam will extrude by itself during pauses of non-printing moves (lots of strings/dribbles). Even if nothing cures inside the head while printing, you will have the problem of cleaning the head after the print (though possibly there is a solvent that would do that).

Dave