Pressure / vac setups have various uses.
1. Some woods greatly benefit from being "stabilized" in which a stabilizing agent such as acrylic, polyurethane or other is forced into the pores of the softer wood, making it harder and more stabile in turning. After blanks are placed in the stabilizer material, a vac is pulled to get "pull" as much micro air bubbles out so that more stabilizing material can fill in the space when pressure is applied.
2. In casting, some casting material take hours to set, some take 5 minutes. For most of casting - pressure is used to "shrink" any bubbles in it, especially for 5 minute setting casts. There is no time for a vac to be pulled for this. 40 to 60 lbs of pressure will greatly reduce the sizes of bubbles and micro bubbles will become miniscule enough to be practically invisible. This is often the difference in a " very usable" cast and a "non usable" cast.
On the several hour set up casting agent, IIRC, a few have pulled a vac for a several minutes to get bubbles to float to the top and then pressure to shrink them. Also, IIRC, a vac on resin casting creates more bubbles than most people have been able to get rid of.
Even with this simple explanation, there are variables, ideas and experiences that are much more detailed. But it all boils down to: 1. getting rid of, or minimizing bubbles on casts and 2. impregnating wood as much as possible for stabilizing.
