Molding and RTV help!

Cmast45 · 08-21-2013, 01:38 PM

So I am molding a sculpt and am using rtv silicone. I am using old pieces of cured silicone (same brand) as a filler so I don't have to use as much new material. This method is recommended in Bruckners Pop Sculpture book and it has worked for me in the past. But this go 'round the silicone will jot bond to the cured silicone. Is remains tacky around the contact points. Has anyone experienced this? My cured silicone is about a year old but I did not think that its age would matter. Any thoughts?

barneyjoseph · 08-23-2013, 08:55 AM

Dear Cmast45,

The message or problem isn't clear... I have understood so far that you have cut up and used 1 yr old silicone as filler. You then poured in the new silicone and now you feel that it is tacky. The silicone and the filler are from the same company. Are they the same product? Is there any other data that you can give?

As far as I know once silicone has been poured out and cured (or imperfectly cured - tacky) THERE IS NOTHING that can be done!

To prevent this from happening again here are some tips. There are several factors that can lead to tacky finish/imperfect cure:

1. Improper measurements of the silicone and hardener. Use a weighing scale (analog or digital) if it has to be a "parts by weight" mixture. For "parts by volume" mixtures, use a transparent measuring cup. The cups should be clean and have no left over uncured silicone from before (just to be on the safe side)

2. Improper mixing of the two parts.

3. The sculpture material and the silicone (or polyuerethane) might be incompatible with each other. Eg. Oil based clays with sulphur in them cannot be moulded with platinum cure silicones. Check the data sheet that came along with the silicone.

4. Silicones have an expiry date, using it after the expiration date reduces the capability of forming cross-linking polymer bonds between the silicone and the hardener.

5. If the silicone was manufactured locally then temperature shouldn't be a problem. For us in India the climate is different and we have to refer to the product's data sheet to see if it is compatible to our climates. Most international brands DO NOT have this problem. I buy the silicone from a local manufacturer in India. His silicones would not work well certain climates.

Hope this helps

Cheers!

Spazzy · 08-23-2013, 02:20 PM

I use to use 1:10 ratio and hated it was usually off, I now switched to smooth on 1:1 and it is much easier to use. I have yet to have any problem using older pieces though, unless mixed improperly

Cmast45 · 09-05-2013, 08:59 AM

Thanks for the thoughts!
Barneyjoseph, I think the issue was number 2... Improper mixing. I think what happened was that I has material in the bottom that was not mixed well. Since it was the last to be poured it ended up on the top of my mold. The inside of the mold was just fine.
Thanks again!