Largely inspired by this video in which a Swiss maker named Andi casts lasts in expanding foam, I’ve been looking into materials and methods for casting shoe lasts.
The idea would be to sculpt lasts in some cheap or reusable medium—oil-based clay, oven-bake polymer clay, plaster with leather or foam build-ups, etc.—then wrap it in plaster and use the plaster as a mold to cast a copy in some harder material. That material needs to be tough enough for cutting off a scoop block, lasting with tacks, and hammering on, for at least one pair. And cheaper than 3D scanning and getting one of the last companies in Mexico to turn and ship copies back.
There have been many twists and turns so far—bad CNC lathe pun there—but I now feel I’ve got a few things worth sharing:
My top candidate is thermoplastic polyurethane.
These are polymers, sold in pellets, that are very hard at room temperature but liquefy around 300°F. So you melt them in a thrift store cookpot over a stove, like a witch from Macbeth, then pour them into a mold.
Atlas International, an orthopedic supplier in Rancho Cordova, California, stocks a product called THM 160 from Exact Plastics in Germany, noted specifically for lastmaking:
As of today, they offer at $200 for 5 kg, which they describe as enough for 2 pairs, or 1k for $45.
The kicker is it’s reusable. You can melt down a cast last and pour the same material again and again.
The manufacturer has a demo on YouTube, but it’s casting a footbed in a foam-box impression, rather than a full last:
I’ve followed up with Atlas to see if they have more information on full-last casting of this product. Maybe somebody using it that way I can talk to.
Maybe other German orthopedic finders have it, too.
@Customboots, do I remember correctly that you redistribute from Götz Service? Their catalog also has a reusable thermoplastic polyurethane, dubbed “Thermo-Blue”, listed for lastmaking, catalog numbers 748 012 00 for 1kg and 748 012 05 for 5 kg.
Any interest in seeing if you could bring that over for sale?
From the catalog:
Thermo-Blue
re-usable thermoplastic casting mass, i.e. for manufacturing of moulds, work models, orthotic lasts and so on. Can be used indefinite times, is usable as filler, tacker and nailable, sandable, minimal shrinking (approx. 0.8%), nonpoisonous, free of solvents, material melts at approx. 280 – 320°F.
This reads eerily familiar. 280–320°F is 140–160°C, the same melting range listed for THM 160. It could just be two different German orthopedic suppliers reselling the same stuff under different names.
Rigid polyurethane expanding foam probably costs too much.
PU expanding foam is the canned blow foam you see on home construction sites. That stuff’s way too light for lastmaking, but PU foams can also be far, far denser.
Digging through the comments to Andi’s video, I found a response from him to a question where he mentions he used a product called ASTI-OP from a German orthopedic supplier called Beil. Sure enough, per their catalog, it’s a two-part polyurethane casting foam, offered in three different densities. For making lasts, they specifically recommend the heaviest, at 700 kg/m³, which is roughly 44 lb/ft³.
Working backwards from there, I was able to find a similar product from Exact Plastics, the German finder making THM 160. The product is called HEKAPUR, and they recommend the “H400” version for lasts. H400 is even denser than ASTI-OP 700, at over 1,000 kg/m³, or roughly 68 lb/ft³. They have a video demo of that product, too, this time casting a full last in plaster:
Alas, I couldn’t find any US Beil redistributor. Atlas doesn’t list HEKAPUR for sale, though I’ve asked if they can get it.
Nor could I find I find any company selling similar polyurethane foams that dense in retail quantities. The closest I found was Smooth-On’s FOAM-iT! product, but the densest they sell is FOAM-iT! 26, which is 26 lb/ft³, much less than ASTI-OP 700 at 44 or HEKAPUR H400 at 68. FOAM-iT! 26 is closer to the foams Beil lists as “light”.
Even if the FOAM-iT! weren’t too light, I fear it might be too expensive. I think Smooth-On’s trial kits—2 pounds filling 2 quarts expanding to 1 gallon for $25—might just suffice for a pair of small lasts. But the next available size up is 15 pounds for $127. And these foams aren’t reusable.
Götz’ catalog also has a bunch of other PU foams and resins, but I can’t imagine it would be cost-effective to import them. That may not even be practically possible, looking at the material safety sheets on some of the hardeners.
Resin casting is probably too expensive.
My best candidate here was Smooth-On 380, but it costs a little over $100 for a gallon kit. Since resin does’t expand, that’s actually more expensive that the polyurethane foam per area cast. Resin also takes much longer than foam to cure—potentially much longer for a relatively thick, dense shape like a shoe last.
This was the approach I thought of first, as the closest to the high-density polyethylene factory lasts tend to be made of. Apart from a couple options in Götz’ catalog, I have’t seen any signs of resin casting lasts on a wide scale.
