Pour-a-mold not hardening

[jafisto]

I am making a mold of a large head; about twice the size of a normal head. I am using Pour-a-mold with a plaster counter mold. I don't know if i just didn't mix it correctly or what, but the rubber pour-a-mold is not hardening sufficently, it is sort of patchy, the thinner parts seem good, though a bit tacky, but the thicker parts are squishy. I have only put it on the the face and have decided to seek more information before i began the back side of the mold. I am working an a very tight schedule, but i do have enough time and materials to scrap what i have done so far, but i dont know if i have enough time to resculpt the whole head. I am really just looking for a way to to save what already have done.
HELP! :eek:

[justme]

Hi,
That happened to me with fiber glass once, and again with a rubber mold. Fiberglass did not have enough hardener, rubber had been used over plastilene with sulphur in it, as i recall.
Just a warning...I never found a fix. Had to clean and resculpt, etc.
Hope someone out there has some helpful advice.
Good Luck
J

[Merlion]

I also had similar nasty experience with silicone rubber, twice. It is supposed to set in about 30 min, but it just would not set after 24 hours, even under the hot sun (which is supposed to speed up the setting). I rely on mental arithmetic for the amount of catalyst, and I did it wrong.

It was tough cleaning it off, as this has to be complete without leaving some behind. In one case, I abondoned the clay model.

With all these two-component hardening/setting chemicals, one other common problem is not mixing them thoroughly enough. This leaves some parts soft.

[bluewolfe]

Jafisto>
When you refer to "pour-a-mold", are you referring to a product name or a mold that is pourable?

I once used one that had that name that had the same problem. Turns out that it was a latex solution that needed to be brushed on a layer at a time (the whiff of ammonia should have alerted me but then I'm known not to have real sense of smell! :P)

It formed a "skin" on surface contact with air but the rest of the stuff under it never set. It did eventually harden ... a month later... by which i had generally abandoned the project.

[Daniel]

Hi Jafisto,

If I'm not mistaken, the "Pour-a-Mold" is a urethane rubber system. Urethanes are cheaper than silicone, but they can be a real pain in the neck. They are very sensitive to moisture contamination. Don't ever use them over a water-based clay unless it is completely sealed. Their shelf life usually isn't very long, especially if you have opened the container previously. The material will absorb moisture from the air and won't set up right. Also, you should seal your sculpture and be very generous with the mold release because these rubbers like to stick, not like silicone that usually won't stick to anything. I sculpted a custom-shaped sign for this hotel in Beverly Hills twice because the urethane rubber stuck to the first one and I could't get it off.
Be sure to measure the 'A' and 'B' ratio properly, too.

[Landseer]

Synair also makes a product they call "Por-a-mold" it's urethane too.

After years of use I had similar problems with Polytek not curing, it was not sticky but just tore like paper, after pulling some teeth they admitted that they found one of the two parts of the kit's retained batch they tested at their plant was not working right and they replaced the kit free- which promptly did the same thing on the replacement mold.

That's when I switched companies.

So your problem could be something you did, didn't do, or a defective product. Urethanes as I've read- some are so sensitive the opening of the cans and reclosing them a few times during use will absorb enough humidity from the -AIR- to affect the cure, so like how confident should I be then that a micron thickness of sprayed on lacquer or "release" spray is going to make an absolutely 100% impenetrable barrier over 100% of the entire surface of the model?

How likely is it spraying on a thin coating so as not to clog up fine details and affect the texture is going to absolutely get into every undercut, opening, line, carving etc? Ever spray paint on something and you know how many times you have to keep going over all those little areas that seem to repel the spray because they are below the surrounding surfaces? Would it be any different on the model using lacquer or relase spray you can't see like you can red primer or paint?



Urethanes are cheaper than silicone, but they can be a real pain in the neck. If you check prices NOW you might find urethanes are more expensive than silicones- polytek's 74-30 is about $11 more than the Quantum QM140 silicone.


They are very sensitive to moisture contamination. Don't ever use them over a water-based clay unless it is completely sealed. Frankly I'd never take the chance with urethanes on an important model, it takes too many hours to sculpt a nice model and have it ruined, miss just one tiny little spot and the mold is ruined- it won't cure in that area where the moisture eeks through.

The Black Tuffy polysulphide rubber CAN be applied directly over damp clay and damp plaster, but the only outlets I've found for this- Smooth-on is one, only sells it in 5 gallon pails for $350 or so. It also appears it is a pour only rubber not brushasble on vertical surfaces.




Their shelf life usually isn't very long, especially if you have opened the container previously. The material will absorb moisture from the air and won't set up right.
Six months might be stretching it for shelf life, that's why I always buy just what I need and use it ALL up within a couple of weeks.

ALWAYS mix part A and B and then mix together, transfer to another clean container and mix in that with clean tools so you have no residue of unmixed material left clinging to the container or tools. that could wind up on a brush on your model unmixed from scraping the sides of the container to get the last bit- it won't cure.

[Daniel]

Wow, I hadn't realized urethanes have become more expensive than silicone. I guess that makes sense with petroleum prices going through the roof. I haven't used urethane rubber since that last mold stuck to my piece almost nine years ago. I agree it's way too touchy to mess with after you've spent so much time and patience on your sculpture. I use Quantum silicone, too, and haven't had any problems with it.

[Landseer]

Wow, I hadn't realized urethanes have become more expensive than silicone. I guess that makes sense with petroleum prices going through the roof. I haven't used urethane rubber since that last mold stuck to my piece almost nine years ago.
Hmmm, the cynic side in me say's- more like they are recouping their costs for so many screwed up kits being replaced. I'll never use urethanes again.


I use Quantum silicone, too, and haven't had any problems with it.Same here, no problems of any kind, I kind of like the psychedellic purple color too ;)
I need to order two gallons about now too.