Hey guys
I’ve thought about this for a while but never could find much information on it.
Would it be beneficial to use epoxy for a mould? Obviously better strength, less flex = less fatigue, and polyester/vinylester doesn’t like to stick to it very much.
Is there any cons to this? Obviously cost, but money aside, any downside?
Cheers
Mitchell
Most of the moulds I have made have been done using epoxy and prefer it to poly/vinyl. As most of the moulds were used in the autoclave so the higher Tg of the epoxy was very important.
We also found that the moulds didn’t shrink and warp over time. One customer of ours had a vinylester mould to make a 1.5m dia yacht steering wheel, over a few years it had shrunk 70mm in diameter.
How do you find the release in comparison to poly/vinyl moulds?
That’s quite interesting! 7cm shrink!
What system do you use? Do you use an “epoxy gelcoat”?
70mm is a bit of a surprise to me. over how many cooks/parts is this? I know cquip/advanced aero use a fair amount of low profile resin for their moulds, I figured this was a fairly stable option.
Ah, I guess I might as well fire off a question that relates to this, instead of starting a new thread:
Could I, in an epoxy mold, make my own DIY tooling gelcoat? I am thinking of adding graphite and then vac’ing the resin to get rid of air and then use it as a regular epoxy.
Furthermore, if I post cure the mold to 100-125C would it then be safe to post cure parts at about 80C?
I made only 2-3 molds out of polyester resin and the rest were made from an epoxy. I still have the molds that I made back in 2009 and they are still in a good shape. I use to heat them up to let the part cure and so far the molds for 1-2 hours. The temperatures use to get up to 140-165 degree (65-80C)
There’s a few downsides to epoxy moulds, aside from the cost.
Quite often, epoxy tooling gelcoat doesn’t polish up to a very high gloss. You can get some that do, but be prepared to sell your first born to get it.
You need to use woven fabric, or powder-bound CSM. Both are difficult to use to make moulds. With woven, you need consider load axis and fibre orientation, and it doesn’t conform to corners as well as CSM. Powder-bound CSM is an asshole. It just falls apart and doesn’t work very well at all. I refuse to use the stuff.
Epoxy isn’t UV stable. You need to keep them in the dark or covered with something, otherwise they will degrade over time.
You would be much better off using a vinyl ester tooling system, building monolithic moulds to reduce flex, and spending the money you save by not buying epoxy on a gallon of Frekote. Pol/vinyl/epoxy don’t like to stick to that either 
We sell really good quality epoxy at work, i’ve never buffed to a mirror finish but I don’t doubt that I could. I’m sure this would suffice as a “gelcoat”.
Yeh, I can imagine the issues of layup. Chopped strand is so easy to use in comparison, but I dare say in some of my finished products I’ll be using different materials besides choppy. Wouldn’t mind making a bonnet out of double bias 450, the stuff we sell is quite soft and pliable. Or even 268 which is a little looser.
Never used our powder bound as it’s only ever sold in rolls and I can’t commit to epoxy just yet.
Really interesting that you mention uv stability as an issue with epoxy. With two of our hardeners i notice yellowing, but I’ve never seen any brittleness or physical break down from uv. We’ve got some really old epoxy parts laying around, they seem fine. It’s not common that vinyl or polyesters have uv stabalizers or uv blockers, wouldn’t they be just as susceptable?
I doubt I’d save money on epoxy, I buy at cost 
haha The perks to my job 
But i definitely like the idea of this frekote. Will investigate, cheers Hanaldo!
I have been building carbon pre preg parts from fglass polyester and vinyl ester moulds for a long time and also tried epoxy moulds.
For me I prefer the Vinylester since there seemed to be no advantage in epoxy. Mostly I find that if you have any corners or fine detail that this can break and chip your gelcoat (due to expansion/contraction or other?) so I focus more on mould/part design and techniques to avoid the damage.
The epoxy moulds were more expensive resins, more expensive with the layers of cloth fabric to make any thickness and more expensive with all the labour in cloth layups yet in the end no benefit over polyester/vinylester.
When I can justify spending more money I go for metal moulds.
Only advantage in epoxy is less risk of alligatoring (seems to be non existent for me with epoxy moulding) and fumes.
VE is the way imo
From the usual fabricator it’s obvious that VE would be the way to go, and I do love it. But I was just curious in a “if money was no issue” way.
As pointed out, you’re steered towards less convenient reinforcements. You can use choppy, but I’ve always thought that using choppedstrand with epoxy wasn’t making the most of the resin… Anyone agree with that statement?
id love metal moulds. It combines two loves… CNC & Composites
Alligatoring is caused by styrene, so it won’t happen with epoxy.
Personally, I think there is a place for epoxy moulds, but I wouldn’t go exclusive. I used to prefer using epoxy, given the difficulties with paint compatibility, layup timing etc. But really, those issues come from a lack of experience or mistake, and not a material issue.
Just take it on a job by job basis, some parts might make sense to do in epoxy, others in vinyl, others in poly and others in metal.
What is the size of the mold? What is the processing method? Bagged? Bladder? Elevated temp curing? Are there other internal structures to support the mold? Is the part a precision part that must interface accurately with other structures?
The answer to these questions are what I use to determine the molding resin.