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Tooling polyurethane closed cell foam. Foam does not have to retain vacuum (the foam does not have to hold vacuum (envelope mold)). Strongly was not wanted ( suggested not bagging to the surface of the mold because it may not work and may deform). Suggested instead main part of the tool mold made of the tooling foam and then fiber glassing over the top but you may lose the structural integrity. Andy Phillips is their composites advisor (practical side of composites manufacturing/processes) industry advisor. Get into contact with Andy Phillips.
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