rtv rubber mold making question

I have started to make a mold with Alumilite RTV Quick Set Rubber and I didn't have near enough. (Did some REAL bad math I guess.
So here's the question. Can I order more and just pour more in on top, even if the first part would be fully cured by then? It's a one part mold. Or I guess it will now be a two part mold.
Thanks Madelynne
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I'm not familiar with that particular product but the RTVs I have used will bond solidly with cured product. Keep the mold clean (say in a shoebox) until you make your next mix. (I think you can clean the mating surface with alcohol if it gets dusty.)
In fact, if you have molds that didn't turn out right you can chop them into little cubes with a razor and use them as filler on new molds. I make sure the pattern and mold box are completely wetted with pure RTV, then mix the cubes into the RTV and fill the rest of the mold with this "slurry".
KL
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snipped-for-privacy@gmail.com wrote:

I have used a couple of brands. The RTV MAY fuse with the old stuff but there is no guarantee. If you can make it a two part or multiple part mold, that might be a more reliable solution than expecting the stuff to bond well. I have found it sometimes bonds well, other times not.
How many casts do you hope to make in the mold? One piece molds sometimes are damaged taking the pattern and then the cast part out, so you may not get as many pours as a well-designed two or multiple part mold.
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snipped-for-privacy@gmail.com wrote:

The answer is yes. I have run out of RTV in the middle of pouring and added some at a leter date with no ill affects. One neat thing about RTV is the only thing that it will adhear to is itself. As someone has already mentioned, keep the mold clean until your new batch arrives.
Rusty White Flagship Models Inc. flagshipmodels.com
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Rusty White wrote:

Not true if you are referring to Silicone RTV. It will bond very well to glass and other materials containing silica.
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Billy Hiebert
HIEBERT SCULPTURE WORKS
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Not entirely true also. I use Essil 120 which is a polycondensation silicone.
I Use a glass base for my mould makeing and it peals right off after curing.
Cheers,
Dennis
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This is a new supplier to me((essil) axxon-na.com) - they really seem to have it together! Thanks for the tip.

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The Essil 120 is a real good stuff. Use their F 16 poy-urethane resin with it. Pot life of a few minutes and demouldable after about 30 mins.
Mould life is about 30-40 casts, but use talcum powder in the mould and youl prolongue its life by some 25%. (you will not notice this with the resin casting)
Stay away from F 31. altough it's thinner (more fluid) it will not cast nearly as crisp and eat yer mould.
An more q's just ask.
HTH,
Dennis
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Have you tried any of their clear casting systems?

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I've used one of their clear resins but that needed degassing BIG TIME. So I ended up with clear resin foam coming out of my moulds.
But that was only one of their clear resins, cant think of its name though.
I am certain though, that if you pop them an e-mail with your requirements, they will come up with a sulution.
HTH
Cheers,
Dennis
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Billy Hiebert wrote

I know Bob Wells at Perma-Flex, and he does know what he is talking about.
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Battersby.

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Could all very well be, what you say about the Essil 120. I really shouldn't know. I only know it works for me and that is what's important too me.
Only thing is now, that now I'm gonna test this on some very well cleaned and degreased piece of glass. Just to see how it works out.
Maybe I can use that technique in the future.
Cheers,
Dennis
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Mechanical Menace wrote:

Great! Let us know how it comes out. And as you said, it's what works for you that is important. I try to test materials and conditions before starting a project. Occasionally I don't and bad things happen. I emailed tech support about Essil 120 and the bonding to glass question. The reply: "I have never heard of our silicones bonging to glass." The results of your test will be interesting. You might try a piece of glass that has not been used. Wax can be difficult to remove. I've had students using large ceramic tiles for work surfaces. The tiles were new and they forgot to wax them. They poured silicone molds and got a good bond to the glazed(glass)tiles. The silicone ripped and had to be scraped off. Once the tiles were waxed, silicone didn't bond even when they were used several times without waxing. Let us know your results.
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Billy Hiebert
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snipped-for-privacy@hieberts.com says...

I don't think that the general rule. I've cast many sheets of silicone rubber sheets between glass plates, and never experienced bonding. I don't recall reading about it in manufacturer's brochures either.
Back to the original question: in my experience the bond between cured and freshly cast silicone rubber is excellent. Even with release agent applied to the cured rubber (to create a two part mold) it was often difficult to pry the two apart.
Rob
My models: www.xs4all.nl/~robdebie/models.htm Me 163B site: www.xs4all.nl/~robdebie/me163.htm AQM-34 site: www.xs4all.nl/~robdebie/aqm34.htm
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