Sealing glass for molding?

starving4rtist

starving4rtist

Active Member
Hey everyone,

I'm trying to make a mold for my Paterson Trident slide viewer, which has a big glass lens. I spent the entire weekend making a mold....only to learn the hard way that silicone sticks to glass :unsure

What would be the best way to seal the lens before I attempt to cast it again? I'm looking for something that won't add any texture and will preserve the smooth / glossy surface.

Any suggestions are appreciated!
 
Spray on mould release should work. It won't do any damage to glass, however not all lenses are made out of glass.

TazMan2000
 
What type of silicone did you use? I've never heard of silicone sticking unless it is caulking silicone. Caulking silicone has an adhesive and why it is used for fish tanks.
 
cavx said:
What type of silicone did you use? I've never heard of silicone sticking unless it is caulking silicone. Caulking silicone has an adhesive and why it is used for fish tanks.
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I once had Mold Max 30 stick to glass. Never would've thought it would stick the way it did (it's like it was bonded). Still don't understand why it sticks to glass, but it does. Use ease release to prevent it.
 
GhostMinion said:
I once had Mold Max 30 stick to glass. Never would've thought it would stick the way it did (it's like it was bonded). Still don't understand why it sticks to glass, but it does. Use ease release to prevent it.
Click to expand...
I am going to have to test this when I get more silicone.

Sent from my CPH1701 using Tapatalk
 
couple layers spray of macwax (should be able to find it on ebay) will give a decent boundary for your silicone moulding
 
ChickenHaunt said:
I've seen it before. I think it's because silicone and glass are both derived from silica, so they actually can bond.
Click to expand...

They say that only silicone sticks to silicone. Yet I have made molds in layers where the layers separate later on. A very different story if i rough the surface.

I hope it doesn't stick, because I have some EXPENSIVE glass parts that I want to mold soon.
 
cavx said:
I hope it doesn't stick, because I have some EXPENSIVE glass parts that I want to mold soon.
Click to expand...


Well, I hate to burst your bubble, but it will stick. But again, using a release agent should protect your glass parts.

Also, although I doubt this will help, it didn't bond to the glass evenly. Some areas did peel off, while other areas were totally bonded. Again, I can't explain the science behind it, I just know a mold I attempted to make was ruined because of it. After it happened, I Googled it and didn't have any trouble finding others who've had the same problem.
 
GhostMinion said:
Well, I hate to burst your bubble, but it will stick. But again, using a release agent should protect your glass parts.

Also, although I doubt this will help, it didn't bond to the glass evenly. Some areas did peel off, while other areas were totally bonded. Again, I can't explain the science behind it, I just know a mold I attempted to make was ruined because of it. After it happened, I Googled it and didn't have any trouble finding others who've had the same problem.
Click to expand...

Yep, I hear you, but adding a release agent is not going to work for what I am doing as I need the optical glass surface to remain "optically clear".

So was the silicone that stuck to the glass, tin based or platinum based? These have a difference.

Example, in Australia, our main suppliers for the hobbyists is Barnes. Their most popular silicone is a 1:1 tin based product called Pinkysil. If you make a mold of a part using that, then cast the part in their quick set polyurethane (D65 resin), then attempt to make another mold from that cast part, it won't cure. Yet move up to their platinum rage like Vario, and there is no issues molding cast parts.
 
GhostMinion said:
When it happened, I was using Mold Max 30, which is tin based.
Click to expand...

OK now we are getting somewhere. There are so many variables to consider when molding and casting. I avoid tin based silicones unless I have no other choice. I've only been doing this since about 2012, but in the time I have been molding, I have discovered silly things like silicones reacting to different superglues when building mold walls. One discovery was anything air cured won't let the silicone cure. Yet,use a kicker on the same superglue (now it is additive cured, not air cured) and the silicone doesn't react.
 
GhostMinion said:
When it happened, I was using Mold Max 30, which is tin based.
Click to expand...

Mine reacted the exact same way: the silicone bonded to about 75% of the glass (had to be scraped off with a razor), the other 25% came off just fine. I was using Smooth On Rebound 25 (platinum cure).

FWIW, I found this page that also confirms that silicone sticks to glass: https://www.smooth-on.com/support/faq/101/

I'm currently testing out Krylon Crystal Clear as I had some on hand.
 
having made many moulds of scopes and various glass items, skip the mold release and opt for Vaseline. It may take a try or two to perfect the surface but just lighting brush some onto the glass surface and hit it lightly with a heat gun to smooth it out. Once you get the method down pact, youll not even notice the Vaseline once its done. The glass surface will be as smooth as glass and shiney. If you dont want the shine, then hit it with mold release or clear coat.
 
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