: Interesting observation. So, it would seem to be a highwire balancingact,
: between pulling too much vacuum and starving the cloth, or not enough and
: obviating the advantages of the process. Is the answer to gradually
: increase the vacuum over time? What with all the different resins and
: hardeners, and ambient temperature fluctuations, that sounds much too
: fussy for the garage builder. Maybe we should agree that hand-laid is good
: enough and move on...

Pete,

Right again. For single layers of thin woven materials, there is no reason to vacuum bag, unless you want to experiment with the process. In that case, go for it! Yes, the process is best suited to situations where you can do some prototyping (if only on test panels) so you can tailor the pressure cycles to the resin/fiber system.

In the situations where bagging would offer advantages (stitched goods, thick laminates, pre-pregs…), the best approach is not to rely on the vacuum to control the fiber content. The main purpose of the vacuum should be compaction. Bleeders can help remove excess resin that made the lay-up easier, but the initial resin content should not be too far above the target level of the finished part. That is why wet-out machines are used on big jobs.

To avoid excessive bleed out, a low vacuum is usually applied until the resin has started to gel, then higher vacuum is applied to ensure consolidation. If minimum resin bleed out is desired, one can use a release film (more poly sheeting) rather than a release cloth.

Vacuum bagging is fun. If you are interested, try it.

Sam