5 years ago #1

Polyester casting problem with small (say 2-10cm3) silicone molds of fossils. We use laminating resin. Most often it does not cure properly where resin contacts mold; surface ends up soft, tacky and with pull-marks where it has shrunk from contact. Surface in contact with air sets fine. Catalyst ratio 3%. Tried heating molds (warm, not hot), didn't help. Help!!

4 years ago #2
me again


you should first of all try to use a casting polyester resin(works way better than laminating resing),
besides that while casting polyester I allways use only up to 0,5%(small things maybe 0,7% or at the very most 1%) catalyst ratio, of course it depends a lot which exact type of catalyst you use, a slow one or a fast one, the one for laminating polyester most times is a fairly agressive and fast one.(try to find a slower one special for casting)

you should try with most of all the catalyst ratio but also with cooling or maybe even heating a little bit to make the hardning proces an equal process, so during 12 hours it constantly hardnens bit by bit in stead of a lot at once.

(watch out for fire an too much heat!!!!!!!!)(polyster can be very flammable and you wouldn´t be the first person who would burn down their workshop)be carefull of fire while using more than 1% catalyst and also with blowdryers etc.
also be carefull not to throw away lumps of paper with polyester all together while still hardning. (i seriously know 4 worksshops burning down from which two with interns who had cleaned up and put all the polyester leftovers in one bin)

dont catch me wrong, i think polyester is if you know what you´re doing the best casting resin for the fact how things come out of the mould in the end, though casting epoxy is much and much more easie and safe (more toxic though).

if i were you i´d buy a bit of epoxy for you casting projects. this really is the easiest and safest resin to cast.

and of course for both goes, where a gas mask they are highly toxic (also epoxy you can´t smell)

4 years ago #3
me again

two things i forgot mentioning

the pull-marks are from shrinking (hardning) too fast, so slow down your hardning process and probably it would work better to keep the temperature a bit lower in stead of warmer (you slow down the process with this)(sometimes small fairly cold polyester things can need 1 or two days hardning)

another thing is.
if you´ll use epoxy take notice of the fact it doesn´t really shrink. this will have big consequences for how easy your model will come out of your mould. think of that if you want to save your mould
(another thing, my english is not so well still, how do i spell mould? mould ? mold? is one of those oalso the fungi thing?)

4 years ago #4
Andrew Grebneff

Many thanks for your response!

Mold... The English and most of their colonies spell it "mould". To me that looks like it should be pronounced "moold. I'm not American, but I use their spelling of "mold", for both the fungus and manufacturing molds.

I'm aware that the shrinkage marks are caused when the body of the cast starts setting and shrinks before the surfaces set, causing still-wet strings of polyester to hang between the mold and the cast.

I have tried warmth for setting, which hasn't helped. However I'd been under the impression that 3% catalyst by mass was the norm for laminating resin. Maybe it is for laminating, where very thin layers are made. I'll try the 1% you suggest! If it doesn't work, than I guess I'll try the other polyesters.

I have tried colorless encapsulating polyester; this seems to set all by itself too quickly, that is, it has a very short shelf-life, whereas laminating resin can last for years and when it does go off you can still use the liquid portion.

I have used epoxy and prefer it, as it doesn't shrink and seems to be a little less brittle than polyester, BUT it often damages silicone molds (it's about the only thing other than silica which I have found will adhere firmly to silicone!) and it has problems with retaining bubbles, which polyester doesn't (yes, I've thinned epoxy by heating, but that doesn't leave time to degas it before it sets due to the heat). Admittedly this epoxy is 2-3 years old.

At 3% catalyst polyester, if allowed to remain in any significant volume, gets extremely hot and cracks while setting. I have had one lot start smoking... definitely a fire hazard. 1%, 1%, 1%...

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