K’s Shop Journal

a beginners’ notes on handicraft shoemaking

Cast Lasts in the Wild

dense polyurethane foam, but where to get it?

The best public information I’ve found on casting shoe lasts so far comes from a Swiss hobbyist shoemaker, Andi, who has posted two YouTube videos making shoes on custom lasts made of expanding polyurethane foam cast in plaster, build up from plaster casts over the feet: one making Hiking Boots, the other making ladies’ high-heel boots.

In a reply to a YouTube comment, Andi mentions he used a product called ASTI-OP from Beil, a German orthopedic shoe findings distributor. I dug into their catalog and found that they offer that foam, a polyurethane expanding foam, in several densities. They specifically mention ASTI-OP 700 kg/m³, which is roughly 44 lb/ft³, for shoe lasts.

I live in the United States, and haven’t been able to find a distributor of Beil ASTI-OP stateside. Nor have I been able to find a similarly dense alternative from resellers of polyurethane casting foams in consumer quantities, like Smooth-On, BJB, or Silpak. The closest is Smooth-On Foam-It 26. Smooth-On’s online store has 1.9-pound trial units for $25 and 15 pound kits for $127. Smooth-On’s casting estimator guesses that 1.9 pounds could expand to cast about 115 cubic inches, so roughly 2 quarts expanding by 2 to make a gallon.

However, the 26 in Foam-It 26 standards for 26 lb/ft³, which is about 415 kg/m³. That’s much less dense than ASTI-OP 700. Comparing to Beil products, it’s denser than ASTI-OP 300, described as a “light” foam suitable for prostheses coverings. But it’s still lighter than ASTI-OP 500, which is must lighter than the 700 recommended for lasts, but which Beil describes as similarly “nailable”.

I have zero prior experience in casting and am just reading up on all this. But my tentative conclusions so far:

  1. Resin casting probably costs too much. For $100, it might be cheaper to 3D scan with the Lidar camera on a smartphone and send to a last manufacturer with a CNC lathe.

  2. Polyurethane foam casting might be cost-effective, but I’m concerned the foam available to me on the US consumer market wouldn’t be dense enough to stand up to tack lasting and hammering.