Crushable foam for mold making

Would that be the Custom Replicas 66" fiberglass Nautilus? if so, the molds still exist. It might be cheaper/quicker just to contact Jim at Custom Replicas and have him cast up the parts you need.

http://www.customreplicas.com

 Is it possible to make a two-part mold using the Lego-modeling clay-Alumilite approach?  

I wouldn't.. I use RTV for that, since you MUST cut "aligment keys" into the first half of the mold before you pour the second half... If you don't, or try to short-cut it by not doing it, you'll be VERY disappointed with your new part's seams... Never try to short-cut two-part molds...

I just suspend the release-compound-coated part in the mold-box, pour the RTV in until it's half-way full, then let it cure.. Then I cut the keys, pour, and air-vent holes... Then pour the second half of the RTV...

Alumalite makes a "Starter Kit" that's perfect for a first-time parts-caster... It only runs about 25.00 (if you got a Hobby Lobby near, you can use the 40% off coupon for it), and has all you need to start mold-making...

Clay molds are only good for "one-sided" parts...  Like if you wanted to cast a bunch of Pro-masks, pistol holsters, rucksacks, and stuff like that...  Modeling clay molds aren't very good for casting a second part either... The generally deform a bit when you remove the part..

 You'll want to get it right the first time...

Once again I'm going to recommend the Smooth-On How-To site.  It will answer many of your questions including two-part molding. 

Simply, the process is imbed the master part to be copied half way in clay.   Mix and pour RTV rubber to cover.   When the rubber hardens,   flip the mold over and strip away the clay.  Leave the master part undisturbed in the RTV..   Cut in some sprue gates/pour channels.  Coat the first RTV with molds release,  and mix and pour a second lift of RTV.   Separate the halves and remove the master.      Reassemble the RTV halves and mix & pour the resin.

The Lego box is there to form and support the clay and RTV pour.   Once you have a RTV mold the Lego box is no longer required.   You don't need to make the mold box out of Legos.    You can make a simple open-faced box from some sheet plastic (try a no trspassing sign from the hardware stor) and some hot glue.

The clay isn't there to provide a mold.  Impressions made by the master in the clay will be distorted when the master part is removed - thus distorting the final cast part.

Sculpey isn't what you need,  you're not going to fire-harden the mold piece.   You need a good clean mineral clay.  Be sure as to what type you do get as high sulfur clays will hinder the RTV from hardening.

Check with the technical people at the rubber and resin supplier as to specific product recommendations.   

Thanks to everyone for the advice.  Does anyone have a suggestion as to the type of modeling clay to use?  Would I need to use Super Sculpey or would something else be better?  Is it possible to make a two-part mold using the Lego-modeling clay-Alumilite approach?  The parts I want to copy will probably need a two-part mold.  What can you tell me about two-part modeling clays that harden into flexible rubber molds?

Thanks, again

Mark

I've made my fair share of pieces using Legos, modeling clay and Alumilite. Make a box from Legos, fill it with modeling clay and press the part into the clay (lubricate the part to keep it from sticking to the clay) then fill the impression with Alumilite.

One substitute is modeling clay.  Real mineral clay will work with resins.  Many of the polymer clays will not, though I do remember seeing one advertised for mold making that will work with urethane resin.

I think what you're looking at doing is a sure recipe for disaster.    The orthotic foam will be too coarse and anything which you use to seal it will: 

1) cover up impression detail you're looking to copy, and/or

2)  melt the foam

I suggest you check the Technical Info page at Smooth-On  http://www.smooth-on.com/p100/Product-Technical-Information/pages.html and see how RTV molds are made.  

Then check the distributor list for a supplier near you.   Most will be able to provide you with specific technical help on what you intend to do.   

If there isn't a supplier near by, google resin suppliers to locate one.   Most will be able to get you right up and running.  

And, FWIW,  I've found that the resin parts produced using Alumilite are coarser  ( bubbles & texture) than parts produced using Smooth-On or other similar producer's materials