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Information for Preparation

Urethane molds tend to be a cheaper alternative to silicones, but they do have some drawbacks, namely the tendency for plastics to adhere if inappropriate seperators are used. Another drawback is that seperators can degrade the resolution of the cast produced. Lastly, urethane molds are difficult to recycle into new molds, compared to silicones.

Here, when we use Urethane (or latex for that matter) a combination of a thin film of Johnson's Paste Wax and Parfilm 4 is used. When applied correctly, sticking is not a problem.

Molding the original is carried out the same way any silicone block mold is done. Personally, I like the interlocking ropes, clay bottom and clay walls method. The Lego method ends up wasting too much moldmaking material in anything but small molds. On very large molds, a plywood box may be made to retain the urethane. One must be careful, especially when working with real bones, to apply both a wax and parfilm seperator to the surface in order not to damage it.

When flipping the mold for pouring the second side, the same steps must be taken, however additionally the use of "rocket sauce" in between the pours where urethane touches urethane is reccomended. It's basically vaseline and mineral spirits that are heated and mixed to form a thick-ish (slimy) liquid. This brushes on well, and can also be used as a seperator between sides of a silicone mold.

Here's a pic of a Dolicorhynchops pubis in the process of being molded. The first side has been poured, and the seperator has been applied to the urethane in preperation for the second side.

And a photo of the cast on the mounted skeleton