[BurrMan]

Sketchup is a polygon modeler. Alibre is a NURBS modeler. The last time I looked, Alibre used the ACIS modeling kernel. Did they change something?

A polygon modeler only makes little flat planar surfaces that "represent", at some resolution, a curve. A NURBS curve or surface is a mathematically precise definition of the curve or surface. For machining, you will always want to choose the NURBS option. For 3d printing, you would still want to use the NURBS option to model, then Mesh your object to a polygon format, at the required resolution. If Alibre is saving to STL, then it is doing this conversion then. The Ideal would be if Alibre is allowing you control over this meshing to Polygons, or if it just doing something "automatically" and presenting something to you in the end.

FYI.

[BurrMan]

You can make the solids and do what you are asking to do in Alibre, with a few things added to qualify this. When you start talking "Molds," it can become very complicated and this is a very complex task. Though as a hobbiest, you can probably get some results with simple model splitting and making "air holes", true mold designing is a very complex operation (Especially injection) and the 6K software have built in tools for helping this stuff out. They will calculate flow+time and know that this little corner will not get filled before the plastic cools., kind of thing. Others here have mentioned drafting qualities that would be required for proper results, but the hobbiest can get by with prying stuff apart too.

[tobyaxis]

Sketchup is a polygon modeler. Alibre is a NURBS modeler. The last time I looked, Alibre used the ACIS modeling kernel. Did they change something?

Alibre uses Direct X not NURBS.