The purpose of core materials is to separate laminates and to prevent them buckling under load, I haven't explained that very well. To bend two thin laminates separate by a few mm, requires considerable compression on the inner one and extension in the outer one. The greater the distance between them the greater the compression and/or extension required. The surfaces must be supported to prevent them buckling; Aluminum or Nomax honeycomb can be used, or in certain applications foam does the job quite well. The properties of the Carbon are utilized in this sort of laminate, excellent tensile strength and good compressive strength.

The core I use most is blue Styrofoam, polystyrene foam used for household insulation. For large low stress pieces it appears to work well the epoxy bonds well to the sanded foam and it is great for putting webs into a laminate.

I had a lot of offcuts of Nomax honeycomb. Just to see what it was like, I made a mudguard with 1mm skin and several pieces of 6mm nomax honeycomb were bonded in at high load areas then covered with an inner laminate of 1 mm carbon fibre. It was quite amazing you could stand on it when it was bolted to the bike. I will try and find it, I recovered it after the bike had been crashed, most of it is intact it is ground through in one place and one of the sides was broken off.

Core materials are required in moulds to give them rigidity, foam is good for this. Pieces of foam are used to form webs, which are laminated over and the result is like a webbed casting, rigidity with low weight.