Casting New Foam Orthotics From Old Ones

So my podiatrist retired and his craft of hand making shoe orthotics is lost with him. I really like the orthotics he made for me and I'm worried about the single pair I have now eventually wearing out (I hike a lot!). I want to be able to make more of the same!

This mold making stuff is kind of expensive. In total, I bought:

• 1 MoldStar 30 trial unit size (2lbs in two 1lb bottles) - $40 including shipping
• 1 Mold Max 30 gallon unit - $140 + $25 shipping
• 1 Foam-iT! 26 trial size - $35 + $25 shipping
• 1 FlexFoam-It! 25 trial size - $40 + $25 shipping
• 1 Minwax paste finishing wax - $20 including shipping
• Mann Ease Release 2831 - $20 + $20 shipping

$350 total. Well, I guess that's not too bad as it was $350 for the pair of orthotics in the first place.

This was enough for me to make two silicone molds (one left and one right orthotic). And several pairs of orthotics.

I was a little worried about how well the castable silicone would release from the foam of my orthotics. I made a little test patch by forming a circular well out of some 2 part silicone putty that I had around (Silicone Plastique from makeyourownmolds.com). Then I mixed a small batch of the liquid silicone and poured it in the well.

Releases fine!

I used an old plastic container as the mold form. It happened to be just the right size.

I used some water to imagine how the liquid silicone would set around the orthotic shape. The trickiest bit was deciding where the parting line would be. I decided to set it so that it intersected with the two distal vertices of the orthotic rim. I could trim the silicone first half later so that there wouldn't be an undercut or flashing in a weird place. There'll still be a slight undercut around the rim of the orthotic in the second (top) half of the mold, but the silicone material is plenty flexible to demold from that.

I used 3 push pins to set the orthotic at the right height and angle.

The orthotic positive is made of foam (probably an expanded polyethylene), so it floats in water, and also floats in the liquid silicone. So I had to weigh it down with this green coaster weight.

Used the full trial size of MoldStar 30 for the first half. Just enough. Since this was 1:1 by volume, I just poured the entirety of both containers into my mixing container after shaking and mixing them. Probably not a great shortcut though as there was some congealed material in part A that didn't pour out...I tried to match the volume from part B.

Mixed thoroughly by hand with a stick, but I probably could have done a better job. Some inhomogeneity.

It set fine in the end. Lucky? maybe.

I first cut out a tongue of material that had crept into the heel of the orthotic. Then I removed the orthotic.

Then I trimmed the silicone to remove the undercut and cut an angle to make a nice drafted surface. A super sharp scalpel made this easier.

I also trimmed around the perimeter to make a flat parting surface where there was a bit of a meniscus of silicone.

I clipped off flush the ends of the pushpins that had been holding the foam orthotic in the right place.

Time to think about pour direction, vents, and alignment!

The orthotic shape is simple, so there won't be places where there is trapped air (so no vents needed), but only if we pour with the orthotic oriented vertically, with the heel side up. Casting with the silicone negative horizontal might trap air bubbles around the rim of the negative space.

The mold form should mostly hold the two halves in place, but I cut in some alignment features in the first half just in case I wanted to try casting outside of the form.

I applied some "finishing wax" to the silicone first half as mold release. We don't want the two halves sticking together. Basically just picked some up from the can, kinda like shoe polish, and rubbed it onto the silicone.

I put the orthotic back into the mold, cuz now we're gonna cast around it. The foam orthotic will float in the liquid silicone, so I weighed it down with a couple sticks and weights.

Then I mixed a similar 910g (2lb) batch of MoldMax 30, and poured it in. It's pink compared to the MoldStar blue.

It cured fine overnight, but when I tried to take it apart, the two silicone halves had bonded in places! The finishing wax I applied as mold release didn't work so well, or I didn't coat it well enough.

I tried casting foam in this mold once, and it kinda worked but not very nice quality outcome.

Ok, I started again with a new bottom half mold. Then I applied Mann Ease Release 2831, which seems to be wax dissolved in some nasty solvents. Use gloves for sure for this one! Just dabbed some onto a folded paper towel and wiped it on. A brush would probably be better. I didn't hesitate to coat it several times.

Ok, take 2. Nothing new here.

Yes! it released great!

I trimmed off some silicone that had leaked around the sides, and a bit underneath the orthotic.

Then I cut a vent at the heel of the orthotic, where the excess foam will exit the mold. It's lined up with the hole in my plastic tupperware form.

The densities are good for both, but the rigid Foam-iT! 26 is too stiff and rigid, and the flexible FlexFoam-It! 25 is too floppy! I wondered if you could mix fractions of these two products to get something in between.

You can! I found that a mixture of about 20% by weight flex, and 80% by weight rigid matched pretty well the stiffness of my original orthotics. You may prefer something different, so try out different combinations!

You'll also have to do some trial and error to get the right amount of foam to fill the mold without too much excess. I liked about 100g total for my size.

I used the "waffle method". Basically, you mix up a batch and pour the "batter" into the bottom half, then close the top half on it, and clamp!

This stuff has a 90s potlife, so it's important to have everything ready. I mixed the two part Bs together first (polyols), and then the two part As together in another container (catalysts i think), then poured the As into Bs. 10-15s to pour the As into the Bs, 20-30s to mix them both together, a few seconds do to the pour, a few seconds to place the top half on, and a few seconds to clamp it.

I actually tilt the assembly vertical (so the foam fully coats the mold and to minimize trapped air bubbles) until I start to see the foam coming out the vent. Then I quickly put it horizontal and weigh it down. It needs a lot of weight! Even more than that full jug of Tide. You can see the foam pushed up and made a gap.

A big kettlebell is better!

After it's cured fully (I give it 2 hours), pull the mold apart, and trim the vent and flashing away to leave the finished orthotic!

looks pretty good and works pretty well!