I am looking for a source of foam molding compound in the US.  I need
to make foam cast bike tires such as the ones described here
http://www.airfreetires.com/Information/Manufacturing.html.

I have used a 10lb density polyurethane flexible foam from Smooth-On
(www.smoothon.com) but this material is not dense enough or rigid
enough for a bike tire application.  I have also found other sources
such as PolyTec and Burman Foam.  But their foams also seem to be too
light and soft.

Where I can I find a source of resins for a more rubbery and rigid
foam to make solid bike tires out of?  A 20 lb density flexible foam
might do the trick, or possibly an additive to the foam I have right
now.

Thanks,

Chris

It depends on what product you have now. Alternatively you can contact
the PU trade association who can point you in the direction of the
relevant manufacturer who will pleased to help.

http://www.pmahome.org/

Alternatively you can use material consultants like www.bayworth.com

The density of PU foam is generally controlled by the water / polyol ratio 
(normally component B). Less water / polyol will reduce the density.
However the flexibility of the resin will also depend on the original
isocyanate structure (component A).