It seems to me that a vacuum chamber allows the bubbles (and dissolvables) to escape, by causing them to form and rise to the surface. A compression chamber drives them into solution, and, if you can cast rapidly enough, prevents them from coming out of solution. Just a thought.To get bubbles out of plastic resin that's to be used in casting, the uncured resin can be put in a vacuum chamber. However, I notice that a pressure chamber is also advocated for getting out bubbles (e.g. http://www.smooth-on.com/faq_display.php?faq_id=81 ). Are bubbles in a plastic resin in some sort of delicate equilibrium state so that both more pressure and less pressure can be used to eliminate them ?
My understanding is that the two methods are very different.
In both cases bubbles are typically formed when the resin is mixed.
Then in the case of pressure moulding the resin is poured into the mould and subjected to increased pressure. As your link says.. "The bubbles are collapsed into solution and the result is a bubble free casting". So the gas is still in the resin just not in the form of a visible bubble.
In the case of vacuum casting the resin is degassed before it's put into the mould. During degassing individual bubbles becomes larger which increases their buoyancy so they rise to the surface more easily. Gas dissolved in solution is also removed so there might be more bubbling than you expect..