Basket,
They build the molds from carbon so the thermal expansion of the mold will exactly hatch the part (hull) being molded. This is not an issue at meduim temps but can become important for 350 degree cure prepreg. It is depends on mold shape - a complex shape that has reverse curves and acute angles can produce residual stresses when cooling down. These can damage the mold or part or cause the part to get stuck. shouldn't be a problem for a simply curved mold like a hull half.

Kevin

Kevin