I want to make a 2-part mould roughly covering 3.5sqmtrs but the overall cost is quite large.

Would it be possible to use a cheaper alternative to the Uni-mould Tooling resin (which appears to be the most expensive part) such as a cheap Polyester resin to bond all the layers of chopped strand mat? Or is there another cheaper alternative to the uni-mould system that would give adequate results?

I don't intend on pulling hundreds of releases, at most I would say 5 in total?

I use regular polyester almost all the time.  You just need to watch it for shrinkage.  Build it up a layer at a time and let it cure before adding the next. I found this stops any shrink and removes the need for a filled resin.  If you're just after a few pulls you can use regular gel coat too. It's just softer than tooling gel so wears poorly.

Thanks for the reply, would you still require the barrier layer (coupling coat?) between the gelcoat and polyester resin?

No need for a specific coupling coat either; but it's good practice to do the same light-weight single layer of CSM behind the gelcoat to reduce print. Once that has cured, you can proceed with your bulk 450+g reinforcement, doing as FLD suggested and letting it cure layer by layer. If you use an unwaxed resin then you can proceed with the next layer as soon as the resin is hard to the touch, without the need for surface prep. 

Excellent stuff, that has reduced the overall cost massively! I think I would prefer to use a proper tooling gelcoat but skip the coupling coat and use polyester to bind the CSM.

What are the advantages of using the full system i.e. all three stages of the uni-mould? Seems like a much more expensive way of doing it!?

Are you going to use the mold to produce carbon parts? The i would highly recommend to make a test sample. Because epoxy sometimes releases quite poorly from polyester molds. Even with most release agents excluding pva.
Have you guys really measured the difference between doing the main reinforcement in one shot, or doing it layer by layer? Because if the sounds to me like a good way to get an even "warpier" laminate. In polyester resins, most shrinkage happens during the solid state (reference: prof. Schürmann from the university of darmstadt). So after your first layer you've got a slightly warped laminate, caused by the shrinkage of the resin. After that you apply the next layer. This cures until the resin becomes solid, without warping the pice further, because the resin will flow in a way wich compensates the shrinkage. But after that the resin of your second layer will shrink even more, now unable to flow any more, bending your mold in the same direction as the first layer did. Ever new layer will warp your mold even further. Compared to that, applying all layers at once will result in the same linear shrinkage, but will not warp the part that much. Never tried layer by layer, but in theory it doesn't sound like an good idea to me. At least with regular polyester.

Unsure, may do a fibreglass one first and a carbon one later on.

However, the polyester would only be used to bond the CSM to the back of the tooling gelcoat? The part would release from the same gelcoat whether I used polyester or the uni-mould system?

If the description is true (I guess it is):

A: the unimold tooling resin is vinylester based, wich hasn't got that epoxy problem.
B: the coupling coat seems to be a chemical bond between the surface vinylester resin and the tooling polyester resin. So using the coupling coat without using the whole system might not make that much sense.
C: the resin for the main reinforcement seems to be filled in a way (using some special thermoplastics) that there is (allmost) zero shrinkage, resulting in a accurate and straight mold.





 

A rapid tooling system like Unimould will produce a much better mould than regular GP laminating resins. The main advantage is time, it is very quick and easy to produce a tool because you can layup the bulk reinforcement in one go. You can do a complete mould lamination in a few hours and have a ready-to-use mould 24 hours later. This is more important for professionals than hobbiests, but still a great advantage to have. 

Thanks Hanaldo, yes this will just be a hobby project so time is not critical and I will not need the higher temperature mould as I will most likely be wet-laying and vacuum bagging.