Autoclave venting & compression moulding with condensation reaction polymers

You are asking many different questions,so I may not address them all. It is easier for all to deal with only one at a time.

Yes, a vacuum bag is not a vacuum chamber, and it is critical to understand the distinction. A chamber has inflexible, self-supporting walls so that there can be different pressures on each side of the walls. A bag is a thin, flexible film that cannot support a pressure difference from one side to the other. The purpose of the film is to be impermeable to gas, although everything is permeable to some degree, and sometimes this is important. Polymer films are surprisingly permeable to many gasses, but this is often overlooked with no problem.

Dry, woven or non-woven fabrics that are permeable to gas flow (breathers) are used under the vacuum-bag to maintain open channels for gas flow while supporting the compaction pressure of the bag. It is important to understand that compaction pressure is not a hydrostatic pressure. It is a force that has direction (perpendicular to the bagging film) that is applied to the surface area of a solid (the breather and the composite part under the bag. On the outside of the bag you have the hydrostatic pressure of atmosphere or autoclave. On the inside of the bag you have the hydrostatic pressure in the pores of the breather and part and the compaction pressure. The total pressure on either side of the bagging film are are always equal.

A major complication is that there are many different cure processes for composite parts that work, but the specific details that make each one work correctly are different for each one. Often some detail that may be essential for one process are inappropriately specified in a different process. For example, bleed-cures are different than no-bleed-cures. Compression molding is different than vacuum-bagging, and oven curing is different than autoclave curing.

Now, phenolic resins are condensation cure, meaning water is a product of the cure reaction. Phenolics are one of the first commercial plastics, and have traditionally been processed in compression molds under more than 1000 psi. At this pressure the water remains in the resin after cure, either dissolved or phase separated as microscopic droplets. This water diffuses out of the part during postcure, and care must be taken to heat parts slowly enough that they don't explode due to steam pressure. The loss of water does add to shrinkage during cure. Some water can be removed by bumping the mold open slightly during cure before the resin gels.

There is nothing special about 100C when it comes to composite processing. 100C is just the boiling point of pure water at 14.7 psia. In composite processing the pressure is rarely 14.7 psia, and the water is dissolved in resin. The major problem with moisture is the magnifying effects it has on air bubbles that may be present. But moisture or other volatile material can be very helpful in removing air when there is a path for gasses to move to the vacuum port.

Cold traps can be used to remove some water or volatiles from the vacuumm line before the vacuum pump, but that requires chilling to -100C or so to be effective. This is common practice in laboratories, but I have never seen it done in composites production.

The purpose of venting vacuum at 20 psig is to avoid excessive resin bleed during cure due to volatile pressure pushing resin toward the vacuum source as bubbles grow in the resin. This is a valid procedure when the resin is hot and fluid, but when the resin is still cool and semi solid and has not started to flow yet, all it does is allow air the flow back into the permeable pores in the composite part. Cold debulking can still be useful for removing large air pockets between plies, but warm debulking is usually far more effective (but time consuming).