The really good pictorials were "disappeared".
I salvaged and somewhat simplified the explanations here...
http://www.queensown.org/photo-gallery/?g2_itemId=2224And Here A two sided mold.
http://www.queensown.org/photo-gallery/?g2_itemId=2223.
A slightly different two sided mold.
http://www.queensown.org/photo-gallery/?g2_itemId=2267I pour polyurethane plastics. It is far less brittle than Polyesters.
For the clear casts, I let the silicone molds cure for a week before use.
Otherwise the clear polyurethanes tend to remain sticky..
Several people I know use
"LEGOS" to build up their mold boxes for two sided molds.
The
LEGOS allow the plasticene clay to be built up and worked, and then pulled out
of the mold easily. They also make the mold box is rebuildable and reusable.
Avoid any sulfer in your clays.
The sulfer can inhibit the curing of mold material and leave you a gooey mess.
Also, use a good scale to measure portions "by weight".
This also prevents a a bad mix and a gooey mess.