Hi, so I have this mould that will be used just one time and theb scrapped.
It is made with epoxy gelcoat and a few layers of 200gsm cloth laminated with epoxy.
I want to infuse the part and I think the mould is too flexible for the job.
What do you suggest as the cheapest way to stiffen it?
I was thinking about breather cloth impregnated with cheap polyester resin but maybe you have better and cheaper ideas?

Thanks for any advice!

Breather cloth with polyester will not work at all: there are not fibres, that would make the mould any stiffer, the polyester resin will shrink a lot and warp the mould, and polyester resin does not stick to epoxy to begin with.

The cheapest way are some eps foam strips and 1-2 layers of any glass cloth you've got laminated with epoxy.

As oakmont says, eps or some foam or even wood strips used as stiffening ribs.  Couple of layers of glass over and it should help considerably. 

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

No, it just won't stick to it very well.

Depending on the size & shape of your mold, it may be more effective to cut some strips of MDF (or whatever other stiff material you have around) roughly to match the outside (or back) of the mold, attach them to the mold with resin & glass. Any leftover fiberglass strips you've got laying around should work fine. 

Really, pretty much anything rigid that you can attach to the mold should work. I've got parallel strips of 1/2" MDF on the back of a large hood mold (5' x 4'). It was a lot cheaper than adding many layers of fiberglass to an already-expensive mold, and I was able to cut some handles into the strips so it is easy to move and hang out of the way on a wall. MDF has the benefit of being cheap, easily available and very dimensionally-stable.

MDF is not dimensionally stable, it changes dimension quite dramatically with weather and temperature. Unlikely to be an issue with a one-off mould though.

It is also quite likely with rigid support structures that you end up with it printing through onto the surface of your mould, so its often a good idea to only put it on your flanges.

Hmmm. I should say, it's a lot more dimensionally stable than, say, typical dimensional lumber. That stuff

Yeh the forces involved in print through are very strong. It is typically more an issue for elevated temp tooling, you tend to get away with it more for ambient stuff, but it can still catch you out. I've had 1 or 2 bonnet moulds that were 10mm thick rapid tooling reinforcement and they still suffered print from the support structure. Will see if I can find the photos.