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When composites are laid up over honeycomb, what prevents the uncured composite from being pushed into the holes in the honeycomb structure when the vacuum bag / autoclave pressure is applied?

Thanks
Jon

for very thin skins the core material is definitely making visible impressions on the skin.
The skin is laid up against the mold to drag it of that surface would be quite something when the hexagonal core pushes it hard against the mold .
Plus the core is filled with air at ambient pressure so when you apply vaccum to the part for curing this will effectively push the skin material closer to the mold.you would not vaccum back the open core laid up on the skin.Usually the skinlayup facing the (female) mold is finished up applying a syntactic film adhesive on which the core is draped,followed up by another sntactic film adhesive and layup of the inner skin system.the whole thing is cured in One shot, after putting some venting material on the inside as a finish and bagging the whole thing .
plus Carbonfibres do not elongate much under tension .
Pick your choice.

A lot of that work these days, does not require an autoclave, but instead is laid up a lot like fiberglass.
I have a friend whose brother makes body parts for all kinds of classes and types of racing.
Oddly I was watching a TV show just last night on kit cars and kit jet airplanes.
The body of the jet was laid up just as you say, a skin of carbon over a honeycomb and they showed that you did that yourself in the shop.
Have to say the jet was pretty cool...
http://www.viper-aircraft.com/

out of autoclave curing is surely gaining momentum also for structural parts. but the technique to process is far from conventional fibreglass.
usually it is some resin infusion (dry cloth draped into the mold before),or a film based resin system ,or thermoplastic based resin systems.
You still need to have the vaccum (or press) + a controllable heating of the mold for curing.
You should not waste expensive carbonfibres by using wet layup techniques...but in bota building resin transfer technique is state of the art admittedly...no roll or brush on resin todays.. and no smelly workshops.

It is a good question. My best guess is that they are using a prepreg system with minimal crimp and low drapability in order to avoid the issue.

In order to produce carbon fiber parts with good mechanical properties outside an autoclave you need to use a RTM process of some kind. VARTM (Vacuum Assisted RTM) is easily managed even for complex structures, but obviously not honeycomb compatible. You could use a different sandwich material such as closed cell foam, but it does not give the same structual capabilities as honeycomb.

one way
http://youtu.be/IAdVO8Rkv6c
Then there is the method of using a vaccuum cleaner and a plastic bag...
http://youtu.be/BgD9oVw3GJw
More vids
http://youtu.be/f8cLE5xR-hg

The way they are handling the epoxy resin is NOT the way to do it. Avoid skin contact, use safety glasses and nitril gloves or even better the ones where the penetration time is higher.

It is not a vacuum cleaner, but a compressor which should be hooked up with a resin trap.