Curtis is correct. Moisture, as we've mentioned quite a bit lately, causes havic with urethanes. Typically if you have moisture in the cactus and you pressure cast it thinking it was dry, the result will be air bubbles not throughout the piece but closely attached to the cactus itself. There may be a small number of sporadic tiny bubbles floating randomly off of the cactus but most will be sitting on or very close to the surface of the cactus itself.
If you have moisture in the resin, in a dye, in a filler, or in the mold box you will typically see more random air bubbles throughout the part. They are typically spread out very evenly throughout the pour and in worst cases it will foam out of the box completely as we saw in airrat's case with the pearl colorant he tried.
To the question of whether you can vacuum or degas Alumilite, the answer is yes. 95% of all Alumilite Clear users will simply mix the resin, vacuum it to remove all of the mixed in air and then pour a perfectly clear part. In a situation such as this how to ... http://www.alumilite.com/index.php?page=show_info&type=HOWS&id=47 ... we recommend also pressure casting the resin once it has been vacuumed and poured due to the resin more than likely creating air bubbles as it fills the detailed mold of the tail light lense. So in this example we not only recommend degassing the resin before pouring to remove all of the air we whipped in while mixing but also pressure cast it to eliminate any air bubbles in the mold that we may have created while pouring it. This process is still rare that you have to do both. Typically people either vacuum or pressure the Alumilite Clear with the large majority only vacuuming to effectively remove all the air out of the cast.
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Here is a picture of a piece I poured yesterday and the left over resin poured in a cup. All I did was mix, vacuum, and pour the resin and the parts came out perfect with absolutely no air (without using any pressure). So stating that pressure during curing is the only way to reduce/eliminate the bubbles of Alumilite resin is simply not accurate. Vacuuming Alumilite has been used for close to 20 years and is a perfectly suitable mean of removing air from the urethane resins whether they be clear, opaque, or flexible.
Mike
Alumilite
800 447-9344