Making a prepreg carbon front clip for my Lotus Elan. So far so good.



Mould with the reinforcement that caused the problem. I bagged the whole thing. This resulted in a differential pressure between the inner surface and the outer skin under the reinforcement which resulted in the mould sinking under the pressure!

As long as you didn't have bridging, it doesn't make a difference if you bagged the whole mould or just the inside. The air pushes against the outside mold with the mold with the same pressure, with or without the bag. As the pressure in your stringers should have been atmospheric pressure, I wonder why the bending didn't went outward, as the trapped air expanded with the temperature? The pressure inside the ribs should not be 0psi, but >atmospheric pressure?
Or did you drill venting holes from the outside? If so, the pressure inside the ribs would have been 0psi due to your bagging technique. The same would happen, if your upper two layers are not airtight.
The main mistake here was to use hollow profiles. You should always try to avoid air pockets in both moulds and parts, unless the air is securely trapped inside a foam or micro glass bubbles. Or at least honeycombs, but I saw heat problems with those, too.