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oekmont - 6/12/2020 9:21:50 AM

When you do infusion, full vacuum is important. Otherwise there would be trapped air in every corner, because the resin would overflow the corner, leaving behind a cavity of 50% pressurised air. During the process the cavity will shrink down to half it's size while it reaches atmospheric pressure. Accordingly a 100% vacuum cavity shrinks down to zero size.
And you should remember that rotary vane pumps were not invented for composite use only, especially not for vacuum bagging. Many applications require a high vacuum, vacuum bagging does not.

Prepreg is produced in a way that there is not much air inside the resin itself. The vacuum will not degas the resin, but it helps to get air out of the system. The pressure inside the autoclave is for the last percents of performance. Out of autoclave prepreg works too, just not quite as good as regular prepreg in an autoclave.