Naphtha and RTV molds

[rasputen]

I tried searching for more information on using Naphtha in RTV molds but the search tool doesn't seem to be able to come up with anything. When I add high percentages of Naphtha, like 80% - 110%, I find that my RTV molds want to curl up a fair amount as they cure. It is almost as if the top of the pour is not shrinking as much as the bottom of the pour, which seems backwards from what I would have expected. I have noticed too that the curing time for the RTV can be more like three hours instead of the advertised 75 minutes - I'm not sure if this symptom is related or not. I am using OOMOO 25 tin-curing RTV from Smooth-On, which is the fastest curing time I could find here in the US. I think I can still make a casting from the molds by weighting down each corner of the mold when I pour the resin in, but I was hoping to be able to make several castings from my Naphtha/RTV mold which will not be easy. The molds have been sitting for about a week, and the dimensions of the vehicle seem to be correct. I also tried making the mold in a larger box, leaving more rubber around the sides of the pattern, but this didn't help. Is there any way to prevent the molds from curling up as they shrink?

[only87]

Like I said, the naphtha tends to sink down in the mix to the bottom of the pour. Therefore the bottom has a higher shrinking grade because there is more fluid that is able to evaporate from the mix.

The interesting data is the cure time not the demold time. After the silicone has cured the naphtha won't sink down any more. To be precise, the best silicone is the one that switches fast from liquid to solid regardless of cure time. But since the manufacturers don't show that info in their specs, cure time is the best indicator.

Leaving more rubber around the model should have the exact opposite effect than what you were aiming for. The curling of the mold comes from the inner tension of the material, the more material there is, the more tension you get.
Also with the naphta gone from the mix, a certain percentage of the volume is gone. The percentage is the same no matter how strong the "walls" of the mold are. But 1 percent of 2 inches is double the stretch than 1% percent of 1 inch. A bigger mold has a bigger (or better higher) "movement" in terms of measurement. Thankfully the silicone mold moves towards the center of the mold where the models sits so this is not as strong an argument as it seems first. Still, you have an open end of the mold, the opening to release the model. There, the mold will curl a bit since tension in the mold is slightly off direction. More tension with from stronger walls will enhance that effect.

One idea around these effects would be to use as little silicone as possible for good casting results. But you would still have the problem with the naphtha sinking down. You could try to turn the mold while the material sets. That would of course mean you would need to seal the mold off. Another idea would be to shake the mold, maybe using a vibrating plate or an ultrasound cleaner. But I don't know if that would work good enough.

The best solution would still be to get a faster RTV silicone. I did a quick search and as you said, it's hard to find fast materials in the States. The only thing I found is the QM 225 here:
www.quantumsilicones.com/pages/ProductListing.aspx?app=Moldmaking
Theoretically it is very close to the silicone I use, but I don't know if they supply small quantities.

Another idea would be to look out for a pourable body part casting silicone with the right specs. For casting body parts a fast curing time is essential. Don't know if the whole Naphtha procedury makes any sense with that material, but I would try if you don't find another solution.

[rasputen]

Thanks for the help only87, I guess I was thinking the Naphtha would rise to the top, but
your explanation helps me out a lot. I think I have seen additives advertised that can change the curing time of RTV too - I wonder if anyone has tried those with Naphtha added?
The other thing I don't like about the OOMOO 25 is that it is very soft when cured, so small cross sections will tear very easily. I'll look into the Quantum Silicones page and see if their products are any better.

Thanks for the helpful insight!