It depends on how dimensionally critical the part is, the shrinkage on the mould materials, CTE and also temperature resistance.

If you are using polyester, ideally low shrink filled ones and one with the requisite temperature resistance. 
Allthough there is a difference in CTE, it is not massive so is unlikely to cause any major issues (taking into account the resin shrinkage in the first place).

In almost all circumstances, you will likely need to post cure the mould before use to achieve the temperature resistance for the post cure of the finished part.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Hi,

I'm planning on making a large mold for a carbon fiber part. I will probably have to post-cure this part somewhere between 60 and 80 degrees C. Could a glass-fiber mold be used for this, seeing as the CTE is quite different to carbon fiber. Or should I be making a carbon fiber mold as well.
Also, I will be stiffening this mold with a support structure or cradle, what materials should I be using for this. I've considered simple wood, but I don't know if this is suitable when working with the elevated temperatures needed for the post-cure.

Does somebody have experience or any advice about this?