As part of an assignment, I needed to propose an alternate manufacturing method for making surfboards. The current method is hand layup, the method I am proposing is prepreg. Now I have a few questions:

1) One of the reasons Im suggesting prepreg is to decrease the time taken to produce a surf board. For my suggestion to make sense, the mould used should be able to withstand thousands of cycles in an autocure upto 120C. My prepreg is glass fiber which has a thermal expansion of around 5, I am very confused as to what would be a good mould material to use? Glass fiber epoxy mould would be suitable in terms of CTE match but would not withstand the number of cycles I would like it to.

2) From what I have read the prepreg and the mould material should have similar CTE to avoid thermal stresses due to expansion. This would mean I could not use Glass fiber prepreg in an Invar tool as Invar has a CTE of zero whereas glass fiber has a CTE OF 5, which effectively means GF would expand upon heating whereas Invar would remain the same leading to stress buildup. Or have I understand this incorrectly? Im a little doubtful because I keep reading that Carbon fiber and Invar mould/tools are great cuz they have a CTE of zero, but surely that means they arent a great for materials that have higher CTE.

I would greatly appreciate any input on what mould material would be suitable for a glass fiber epoxy prepreg for a big number of cycles in an autoclave at 120C. 


TIA

You should definitely calculate the price of a steel mould of that size. Afterwards you will be cured of that thought. It's not only the material itself, you also need to have someone with a really, really expensive cnc mill, wich will still need a huge amount of time to make the mould. Expensive machine combined with a long process time won't end well for you wallet.
And then you could make a single size of surfboards. I have no experience in surfing, but I know, that the size have to fit you body weight.

Hanaldo is absolutely right. Same cte: always good, mould cte higher: usually good, mould cte: lower bad.