Machining or casting mold cavity

[Thorpydo]

Hey Guys,

I'm working a bit over my head on a project and thought it'd be best to get some advice.

The part I need to make needs to be molded and will be caste with a flexible urethane in one area and hard urethane in another. (1)Will the two materials stick together if I do the hard first? Would they bond to a fiberglass part inserted inside?

The part is roughly 3.5' x 1.3' x .1' (42" x 15.6" x 1.2"). I have a benchtop X3 mill with about 14" in the X and 6 " in the Y to work with. I'm trying to decide if it is best to make a plug and pour urethane for both female sections of the mold or to machine the female cavities in acetal, 3D machining the mold in sections.

Economically and timewise, I think it makes more sense to pour the mold. The part, for the most part, has a constant cross section. I was thinking I could easily contour a bunch of small sections and glue them together, connecting and aligning them with two rods down the center. I wonder though, how well the final part will come out and if I'll see the seems.

The more I think about it, the more I'd like caste the female mold. In doing so I could make the mold clear, so that I can see the urethane inside and make sure there are no air pockets.

I'm looking at needing to buy either about 10 ft^2 (@.75" thickness) of acetal plate which will cost an arm and a leg. Or buy about 5 gallons of casting resin, which will still cost about an arm. . .

Though I plan to do a few smaller parts before tackling this big one, this is really the first time I will be making my own molds and casting with urethane.

Do you guys see other options? Advice appreciated!

Thanks,
Adam