For what you are looking at i wouldnt look at metal tooling at all. I use invar for aerospace tooling and it usually costs more than a house for even small tools.
As above a good grp/composite tool will suffice, you need to find a good tooling resin that will have a tg of 120+. Ive recently used a sicomin resin system that was infused at room temp and with a post cure was good up to 200 degrees.
I would make sure you have some stiffeners in both directions of the tool.

Thank you to everyone for their input.

For the glass fiber mould, I am presuming chopped fiber glass mat should be good enough? Cheap and relatively strong. I don’t think the mould needs strength in a particular direction thats why chopped sounds alright to me... please correct me im wrong

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.

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.

+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?

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.

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