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