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

I agree with Oekmont that pre-preg is not going to be the fastest method for a surfboard. Pre-preg isnt really a fast process, it just CAN be faster if you have really complex geometry. For geometry as simple as a surfboard, throwing all your dry reinforcement into a mould is going to be far less labour intensive than pre-preg.

The CTE of the materials is only going to cause stress when both are rigid - the uncured fiberglass is soft and wont cause any stress. Once it is cured, it will be fully expanded and not cause any stress. As it cools down, the fibreglass will shrink again and self-release. Hence why it is a good idea to have a mould with a low CTE. If your mould has a higher CTE than the material you are curing in it, like carbon pre-pregs in a fibreglass mould. In this case, the materials are fine as they heat up, but then the low CTE material (the carbon) stays the same size while the fibreglass shrinks back down, causing stress.

Theres no need to have your mould Tg so far above your pre-preg cure temperature. To maintain surface finish for as many cycles as possible, it is probably ideal to have a mould Tg of around 135-140. That said, I've got moulds with a Tg of 120 that I frequently use to cure pre-pregs at 120. I'm not aiming for thousands of cycles so it isnt as much of a concern for me, but after 50 or so cycles there is no sign of any mould degradation.