Hey.

I read that you could not use polyester gelcoat into molds to make best use of epoxy, but that you should use vinylester gelcoat.

Is that correct?

Why? 

Magnus

Hi Magnus,

Although it is possible to use polyester moulds we have (and many of our customers have) found them to be problematic.

Newly made moulds are particularly troublesome, the reason for this is that the cross-linking of the polyester molecules is not fully completed even in a 'cured' laminate, in fact around 5% of the potential polyester remains reactive, this reactive surface limits the effectiveness of release agents leaving the epoxy able to bond to areas of the tool surface. lower quality polyesters are often even more prone to leaving unreacted linking sites making the problem worse.

Properly post-cured and 'seasoned' polyester moulds do sometimes work effectively for epoxy moulding, particular attention must be made to release agent application, PVA would provide the most reliable system as this will create a physical barrier between the tool and resin, however this is a the sacrifice of surface finish.

Epoxy resins will achieve a much better straight bond to polyester than they do to cured epoxy or vinylester meaning you will pay dearly if there are any insufficiently treated areas of a tool.

So in summary although it is defiantly possible to use polyester tooling the potential for problems generally makes it a poor choice, the only times that we have ever experience a true 'sticker' has been with polyester moulds, we've never used them since!

Paul Statham
Easy Composites / Carbon Mods - Technical

All right.
I am a boat builder and has many polyester molds .. Can I spray them with pu lacquer to solve the problem ...
Or is it just pva applies?
Magnus

Hi Magnus,

You could spray them with PU and then re-finish that surface (the paint will be orange-peel like and will need flatting and polishing) and that would make them much likely to release epoxy parts if using a chemical release agent but not only would the PU paint finish require quite a bit of work (all the flatting and polishing on the *inside* of a mould, which is always the more difficult face to work on) but PU paint is also no where near as tough and durable as a mould finish as a tooling gel coat and so if you're taking multiple releases out of these moulds then I'd certainly prefer to have a gel coat finish than a paint finish. Paint is fine for going from a pattern to a mould or from a mould for a very short production run but as your day-to-day mould finish I don't think it would be a good move.

My suggestion would be to try a small area of your mould first with epoxy. If the moulds are old and have already release many parts then it might not be such a problem to release epoxy out of them (there are members on this forum who manage to do just that). The important thing is to expect the worst when releasing epoxy out of polyester and so test thoroughly and prepare accordingly.

--Matt

Matt Statham
Easy Composites / Carbon Mods - Technical Sales