having just made my first epoxy mold ....

i was just thinking what are the differences in making the mold out of polyester to pull epoxy parts ?

pro's and cons ? obviously its cheaper hence why i am interested , but what else ?

It really depends on your expectations from the mold.

Polyester molds are less stable (more prone to warping), the edges are more brittle, and the higher temps generated during curing can cause issues.

Adam

ta adam

whats the shrinkage like using polyester ? on a plug 800-900mm long do you think it will warp ? hmmm i think i should just stick to epoxy as this mold worked so well

Originally Posted by Blackrat

ta adam

whats the shrinkage like using polyester ? on a plug 800-900mm long do you think it will warp ? hmmm i think i should just stick to epoxy as this mold worked so well

The shrinkage is typically what causes the warping.

Think about 1-2% linear shrinkage.

Internal shrinkage is more like 4%, so the structure is pre-stressed and will warp readily.

Wow... We (I) have been making molds using polyester resin before all these new "fancy" fabrics, resins and materials became available and we were getting pretty good results.

Yes, using polyester needs a bit of extra care if one want to keep a good geometrical definition and mold stability over time and with temperature changes. But it is doable.

I have made molds for a 5 meter 130+ kilos aircraft over 15 years ago and they are still in use after all this years. We are currently pulling out of them a new modified prototype after using them for the full production of the original aircraft. Granted I used vinyl-ester resin for these, but the difference in properties with polyester are not "that dramatic".

Geometrical stability is a direct result of the mold design/manufacturing. With a balanced layup, the proper structure reinforcements and the proper curing/post-curing the results should be of a pretty high standard.

The thermal expansion is a problem only if the parts pulled out of the tooling are of materials with drastic thermal characteristics differences and if high temperature in mold post-curing is required. A note on this: for the molds I spoke above, because of their mass and sizes they are mounted on a steel frame. The securing of the composite mold on the steel frame is done with a silicon adhesive/bedding: this is absolutely critical even if the thermal expansion of polyester/vinyl-ester structures is close to the one of steel.

Originally Posted by fnev

Wow.............................................

The securing of the composite mold on the steel frame is done with a silicon adhesive/bedding: this is absolutely critical even if the thermal expansion of polyester/vinyl-ester structures is close to the one of steel.

I am interested to know what this silicone is excatly. Is it 2 part casting silicone?

I have used vinylester tooling resin for my recent moulds. Only because my local supplier and advisor had lots in stock approaching its best before date and gave it to me very cheap. He suggested that I do the lay up in stages, with cool down time in between to avoid or minimise the thermal distortion problem.
As I am only at entry level with moulding, this seems to be a good compromise (if it is one!) for me to make, taking into account likely failures and the expected number of parts that will be pulled.

And one minor advantage of the vinylester/polyester resins is the option of using chopped strand mat, which is easy to use, cheap and efficient. And also available locally for me at the commercial laminators.

No doubt, if I was to advance to a commercial level, I would use epoxy.

Jim.

Jim's right. Vinylester (especially the low-shrink variety) can be quite good.

The benefit (as Jim mentioned) of using styrene-based resins is that you can use starch-bound materials like continuous strand, chopped strand, or veil matting. 450 Chopped Strand Mat builds up thickness and stiffness really fast!

Do beware exotherm. It's real and it's a PITA. I've "boiled off" a batch of polyester and it's really nasty. High exotherm can cause the wax on your plug to melt and thus you get a complete stick-up.

I am just starting to use epoxy for molds. I love it!

-Alex

I have found a very cool polyester resin call optimould tooling resin.

It is a good compromise price wise between epoxy and polyester price wise, and it is stable

5kg will do a 2m wing quite nicely.

lay up is specified as gell coat, the one layer of 300g with general purpose polyester resin. Let that set, key and hit it with 5mm to 6mm (About 6 layers of 300g) with the optimould. i really like it.

Up to know my moulds have been flat, but I am keen on laying up like this, with sides, then filling the back with poraver, and another 6mm on the back. Will be heavy but stable and strong.

http://www.grpms.com/files/documents...pniwupxppx.pdf
http://www.grpms.com/products/resins...ooling-system/
http://www.ecfibreglasssupplies.co.u...-catalyst.aspx

Optimold is a low-shrink Isophthalic polyester resin.

I believe it has lower shrinkage, less exotherm, and more resistance to heat than standard polyester resins.

There's a series of videos explaining the optimold process on Youtube.

Originally Posted by Blackrat

having just made my first epoxy mold ....

i was just thinking what are the differences in making the mold out of polyester to pull epoxy parts ?

pro's and cons ? obviously its cheaper hence why i am interested , but what else ?

Stick with the epoxy for your molds.
To justify the few extra dollars your parts will release better, your plugs will release better with less damage and your molds will last longer.

Polyester is a heartbreaker!

Regards Daryl (who made the move and won't go back)
PS no experience with vinylester though!

Vinylester is viable option but it has a very short shelf life. Most manufactures want you to use the resin within 3 months of the purchase date whether the containers have been opened or not. I've had vinylester go bad twice (always test it before use). The loss of material didn't justify the savings $ over epoxy.

Adam

Originally Posted by wyowindworks

Vinylester is viable option but it has a very short shelf life. Most manufactures want you to use the resin within 3 months of the purchase date whether the container have been opened or not. I've had vinylester go bad twice (always test it before use). The loss of material didn't justify the savings $ over epoxy.

Adam

Thanks Adam, the short shelf life is probably why I can't even buy it locally.

regards Daryl

Daryl

I've bought it at FMS (now Valspar) which have an outlet in Dandenong. FGI might have it also?

Since going to epoxy, I'd never go back - it's thinner, easier to use (though less tolerant of measuring errors) and doesn't stink like poly.

Steve

I only use epoxy for the moulds.
I did try with polyester a few months ago and it was ok I suppose but you really use a lot of resin.
Then half the resin had gone hard in the bottle so I had to ditch it. Not impressed.
Epoxy is used at 1 to 1 resin to the weight of the fabric, polyester is 3 resin to 1 fabric weight so you end up using 3 times the amount of polyester resin. This means it is not any cheaper really.
The polyester mould is heavier and more prone to warping.
I now use very lightweight epoxy moulds with no problems.
I do like the chopped strand mat that you use with polyester though. Pity there is nothing like it available for epoxy, or is there?.
Jim