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

+x

 

oekmont - 12/22/2018 8:15:15 AM

for mass production I don't see prepreg to be the fastest method, except for sheet like products. I would go for RTM, which can reach cycle times under 10 min, with the right resin, and a preforming station.

for such a simple shape  a well made grp mould virtually lasts forever. Like a couple of thousand pulls. Additionally, a surfboard sized invar mould is so expensive, that you can build a small armada of grp moulds for the same or less money. Wich also gives the option to use them parallel.

the thermal expansion is not a problem, at least not in the direction you are suggesting. Uncured, the expanding prepregs can not build up stresses on the mould, and cured the fibreglass shrinks faster than the invar mould, which will simply demould the part. As long as there is no undercut, this doesn't result into great stresses for the mould or the part.

Thanks so much, your reply was very helpful. If u have a minute, I have some follow up questions, apologies if im repeating myself:

1) i can’t use rtm because i have a eps foam core which can’t be subjected to 120C (curing temp of my prepreg)

2) if GF has a CTE of 7 and Invars CTE is 0. Doesnt this mean that when heated the GF will expand in size whereas the Invar tooling will remain the same size, which essentially means that the GF will be pushing against the tool when it expands, will this not introduce stresses?
3) i quite like the idea of using multiple gfrp moulds for the price of one invar mould. Just out of curiosity, Steel has a CTE of 7 (same as gf), would this make steel a better choice than gfrp for the mould?
4) if my prepreg has a curing temp of 120C, the epoxy in my mould should have a higher tg, would 200C be enough or should it be higher?