How I would do it (applicable to any core):
If thinking of pouring a PU, this is what you need to do first, full vacuum in a glass pot:
Why would I do a pre-cast rails:
http://www.smooth-on.com/tb/files/Smoot ... 1,_322.pdf
"For Best Results . . . Best results are obtained using a pressure casting technique.
After pouring the mixed compound, the entire casting assembly (mold, dam
structure, etc.) is placed in a pressure chamber and subjected to 60 PSI (4.2 kg/
cm2) air pressure for the full cure time of the material.
Post Cure - Castings will reach “full cure” faster and achieve maximum physical
properties if post cured. Allow material to cure for recommended cure time at
room temperature followed by 4 - 6 hours at 150°F/65°C. Allow casting to
come to room temperature before handling."
I've done lots of experimenting. A lot is described here, even some destructive PU testing :
http://boardbuilders-forum.1077691.n5.n ... 34i40.html
The idea of using a PU pre-cast rails is to make as many rails as possible in the shortest period of time. If not, the PU in a container will go off very soon hence waisted...
1.8kg (part A+B) of PU can make 16 boards (1/4 inches rails width).
I would never ever go back to poured rails which is a disaster for the most 1st time DIY builders (even with epoxy)...