Silicone not setting?

[Stephen Duncan]

I've been doing a fair bit of practising building my models and testing different techniques for mould-making. But the couple moulds I made last night didn't set completely, despite doing nothing different and leaving it for 12 hours.

Could this be something to do with the high temperatures we're having at the moment (UK)? I've placed the moulds in the fridge for the last 4 hours to see, but worried I might be wasting my time.

 

[valiant]

Quite probably - they are very susceptible to humidity. Polyurethane resin is, too. The humidity seems to leech the oils out to the surface.

Steve

 

[Stephen Duncan]

Thanks for the reply, Steve. Do you think putting the silicone in the fridge will help it set?

 

[valiant]

Possibly, but I would prepare myself for redoing whatever it was!!

 

[Stephen Duncan]

I'm going to stay positive and leave the crying for another day!

 

[Stephen Duncan]

I've started cutting further into the silicone and I think (hope) that it's not what I originally thought it was. It looks like an area of the two silicone parts wasn't mixed properly, so it'll never set. Every other part is perfectly fine and the wet area doesn't actually come in contact with the sculpture/cast, so it might still be use-able.

 

[valiant]

Sounds positive - Ill keep my fingers crossed for you...!

 

[garyhiggins]

Stephen, before you start thinking there's anything mystical going on, check that you have the right catalyst and then check that you used the right amount, and then check that you mixed it properly. I really don't think that putting it in the fridge will help. As for the humidity, that's not likely to have any effect, although if you add a few drops of water when you're mixing the rubber it can massively accelerate the curing time of the rubber.
Best wishes, Gary.

 

[Michaelminiatures]

I cast all the time and have had this problem, the fix is weak. However, let the mold set sometimes takes up to 48 hours it will set up, be patient.

 

[garyhiggins]

Stephen, Michael is probably right about the mould setting eventually, but if it doesn't, scoop away the offendong area, and refill the gap with fresh silicone

 

[Stephen Duncan]

check that you have the right catalyst and then check that you used the right amount, and then check that you mixed it properly. Gary

The two parts came together in a set; I've been buying >this< which is a 10:1 ratio. I think where I might have gone wrong is the utensils I'm using to mix it up. I'm using (microwaved to remove moisture) lollipop sticks, which can be a bit too short and difficult to use.

EDIT: I hadn't thought of removing/refilling the wet area, though it's good advice. Thanks!

 

[garyhiggins]

The two parts came together in a set; I've been buying >this< which is a 10:1 ratio. I think where I might have gone wrong is the utensils I'm using to mix it up. I'm using (microwaved to remove moisture) lollipop sticks, which can be a bit too short and difficult to use.

EDIT: I hadn't thought of removing/refilling the wet area, though it's good advice. Thanks!

OK Stephen, personally I've never heard of Polycraft so I don't know what it's like. As my own rule of thumb I would say don't buy anything off E- bay because you don't know where it's been.
For pretty much anything you need, go either to Alec Tiranti or Sylmaster, both have websites, both are long standing suppliers and are reliable, and both have very good technical departments that are happy to discuss all your needs.
Regarding the mixing, yes lollypop sticks are way too small. You need to be able to really attack the rubber, not forgetting to change direction now and then, and don't forget to really scrape rount the bottom and edge of the pot.
Best wishes, Gary.

 

[valiant]

I always get my Pu resin and silicon rubber from Ebalta UK -they are based at Castle Donington close to East Midlands Airport. Considerably cheaper than Tiranti....

Steve

 

[Stephen Duncan]

Yes I think the length of lollipop stick was the issue, couldn't reach the bottom without getting stuff everywhere! I'll take up on your advice regarding E-bay too. Hopefully I'll have some good results to show from your help!

 

[Stephen Duncan]

Excellent, cheaper the better - and a big issue for me at them moment! Cheers Steve

 

[Michaelminiatures]

I use a thick wooden flat spatula wiped clean after every mix. Scraping the sides and bottom of the mixing pot (I use small plastic paint buckets that have a flat bottom,toss when done) gets the rubber and the catalyst mixed well. The rubber is very thick so it takes a strong mixing tool and arm. I also use foam board and hot met glue cheap and fast.mount the spur on a piece of foam board and cut foam board for the sides, make sure you make the sides tall enough to allow for the rubber expansion when de-gassing the mold. I de-gas twice once in the mixing bucket and once in the mold, you can top off the mold with rubber once you have de gassed. When the mold is set you just break off the foam board and toss it away. You can trim the mold edges with an x-acto knife. Hope this helps, I better go mix some rubber ,yuck!

 

[jasmils]

G'day Stephen,
http://www.ammsbrisbane.com/ajax/articles/aug_09_casting.html
This is a small casting workshop I did a few years ago.
The products used are not the same as what your using, but the principles are the same.
Cheers.

 

[Stephen Duncan]

I've checked out your workshop page, Jasmils, and it looks very useful. I'll certainly take some notes from it and give a it a few more read overs, thanks very much!

 

[Stephen Duncan]

I'm now having a similar problem with my resin not setting/curing. I've chucked the wooden sticks away and am now using plastic stirrers, and I'm meticulous with the 50:50 ratio as well as how long/thoroughly I'm stirring. Instead of turning white as usual, the resin remains a murky pale terracotta colour and is semi-transparent.

It seems instead of improving with practice, my results are deteriorating and my materials are being wasted.


UPDATE: I've just tried again and had perfect result. The only difference was the order in which I mixed the two parts of resin. Does it really make a difference whether you pour part A or B into your container first?

 

[Mookie]

When i cast with my mate he puts the b into the a then mix slowly.As for the Rtv please make sure you mix slowly thoroughly mate.I use a clear mixing bowl to see if all the catalyst is mixed and the mix is homogenised together.Made some moulds yesterday and my rtv set overnight and our flat is very warm.Tomps on e.bay do great rtv and service.
Good luck from Richard.