Actually it turned out amazingly well, so far. Pores of the foam near the tools are a bit large in places, but I wanted 10 cm of foam, so dumped a whole can in. Ok, details and procedure so far: Box 60 cm by 30 cm, about 10 to 15 cm of foam to be under the tools. Tools basically flat, some undercuts. The goal is a pleasing tool box insert, with the shapes of the tools in a soft padding foam. This is more for display than use, and to see how the whole idea works out. So didn't use much of the depth of the box.
I had no idea how closely the foam would conform to details, nor did it say on the can how many liters the foam would expand to. This was the cheapest can of one-component polyurethane foam with a one-off screw-on nozzle I could find, but less than four weeks old. (I have had problems with foam a couple on months old being reluctant to come out of the can and expand).
So:
First, I placed the tools in pleasing arrangement, turned so that the less undercut side is up. Photographed arrangement. Wrapped tools in clingfilm in case foam gets through to them. This also reduces the detail somewhat -- I didn't want foam to mold to every loop and crevice, just to the general outline. Placed tools on cardboard face down, mirroring arrangement, referring to photograph.
Taped tools in place on a piece of cardboard, using masking tape to reduce undercuts. (I decided to leave out some small boxy bits, and just cut holes for those later.) Covered tools with a thin bit of jersey fabric, to provide protection from foam, and a little overall clearance between foam and tools -- there will be fabric over the foam in the final inset. This also softens any sharp edges that may cut the plastic bin liner. Placed a bin liner over the lot: tools, cardboard, fabric. I used a polyethylene bin liner so that the foam will not stick to the liner -- PE is one of the things listed on the can that PU foam won't adhere to. Now, I sucked the air from bin liner. I used an injector pump and a bit garden hosepipe attached with a cable tie. Fiddled with the plastic to get it to conform well to the tools, and get the folds and wrinkles to lie flat. (This was the part that had me really worried: I wanted a vacuum to get the plastic foil good an close, but any tiny hole would result in foam being sucked all over all the tools.)
Built four cardboard sides, a bit smaller than the box it will fit in. Fold out bottom edge, and attach over the cardboard/tools/bin liner using binder clips. Taped gaps on the inside to keep foam inside. Dampened the foil and cardboard -- the foam wants a bit of moisture to cure.
Put on old clothes, rubber gloves, shook can and foamed away, squirting the stuff in the undercuts first. As the final foam block was to be about 30 liters, I ignored the "maximum thickness of 30 mm per layer" and emptied the can. (I have had half-used cans sitting in the corner harmlessly at first to find that they had oozed their guts out the moment I turned my back. So out with it.) This thick layer may have contributed to the large pores near some of the tools.
It looked as if it were far too little for a while, but then expanded slowly and continuously. From previous experience with the stuff, it always does, and still I put in too much. So I'd left the cardboard sides very high, so no worries. (No canoes here!)
I let the injector pump run for about two hours, and then turned it off. The lightly moistened top layer had started to cure and was no longer tacky or soft by then, so I assumed the bottom with the tools would also have started to cure. This foam block cured overnight.
The tools came out easily this morning: I cut open the bin liner from underneath, pulled the tools out, pulled the fabric off, and peeling off the plastic sheet from the foam without any trouble. No leaks. There are large pores, gaps, and holes under the tools, but overall, details molded well. The pores tend to be at flat surfaces. Some narrow gasp of 1.5 cm wide an 2 deep have filled in well, with a fine-grained foam. Perhaps the moisture keeps the pore size down? The bin liner rounded out the edges nicely.
The next step will be to cut the remaining holes with a cutter knife and a scalpel, reduce some of the undercutting, and maybe provide some clearance holes to make it easier to grip and remove tools.
Yellow foam with large pores is not pretty - this would do for packing, but not for use or display. I have bought some crushed velvet to cover the foam. This is a synthetic fabric and very elastic, so placing it over the foam and inserting the tools one by one, from the middle out, pulls it into the cutouts nicely. The crushed look makes the inevitable folds and wrinkles much less noticeable and unobjectionable. Hot-melt or double-sided carpet tape should hold the fabric down nicely, perhaps aided by the odd pin; I have yet try this.
Should you think crushed red velvet a bit too much -- it was cheaper than felt by about half, and felt would have required a lot of cutting, joining and gluing!
Thomas Prufer