[One of Many]

I used the urethane on vacuum fixtures periodically. It being flexible compared to tuf-fil(bondo), I prefered the bondo. If you know all the parts raw material ever used on the mold will shrink consistant, creating a plug from the mold would be adequate providing the shrinkage of the plug materials is relative. Creating a plug from the parts should cancel out one step in shrinkage since the parts will be the one to fit the plug anyways.

Thermal formed raw materials as in ABS, POLY, PVC's etc. have different shrink factors. These are extruded sheet, but when capped with color and/or texture, the rollers put some stretch to the material as it cools. That stretch or tension can vary from run to run(or during the run) and it shows up after parts are formed. For this reason we would have to relieve the fixture on one run, then use duct tape shimming layers on other runs to get tight or loose parts to register repeatably for trimmed in tolerance. Not much could be done on our end, but to futz with it to keep product flowing out the door.

I'm not sure how much of that would apply to composites, but there should still be ways to compensate for variables out of your immediate control.

One other thing to make note of. If the part were shaped like a bowl or symetrical such that the parts fit the fixture in more than one position with no indexing reference on the mold. If the mold were machined, the parts should have a consistant surface. If the mold surface is made from a casting or by hand, the parts will technically only fit the mold in one position. That goes double for the fixture the parts will be trimmed on, if any hope of repeatable precision is expected from the parts.

DC