RTV Silicone problems

[vistaVision]

I've been making molds with RTV Silicone and lately been having some trouble. I think the problem may be isolated to an issue with the primer on my patterns. Patterns are hardened Sculpy, with a sandable grey auto primer finish. First attempt at mold was less than perfect, a thin layer of the RTV at the pattern surface did not set. Is this a problem with sulphur in the primer?

I tried top coating the primer with several sprayed acrylic clear coats, results not much better.

The Sculpy is not the problem, test shots of the RTV seem to be okay, and I've not had similar trouble with the clear acrylic, so now the primer is the suspect...

Anyone with experience with this? I'd like some ideas of what I can use over the primer to seal the pattern properly, or any other ideas.

Marcus

 

[replicaprops]

none of those situations would screw up the cure.
Sounds to me like the rubber was not mixed properly, or you did not pre mix parts A and B prior to mixing them together.

 

[vistaVision]

<div class='quotetop'>(replicaprops @ Sep 23 2006, 02:19 PM) [snapback]1325109[/snapback]</div>

none of those situations would screw up the cure.
Sounds to me like the rubber was not mixed properly, or you did not pre mix parts A and B prior to mixing them together.
[/b]

Thanks for the thoughts on mixing. However, I think I'm getting the stuff mixed well, and the entire mold is cureing properly, as is all of the "left-over" material in the mixing cups. The only area not cureing is a very thin layer of rubber just on the surface of the pattern, and, after de-molding the RTV in these areas sets up fine...??

Any other thoughts? All will be very welcome.

Marcus

 

[Blad]

Brush on a very thin coat of RTV on the areas where it apparently does not set. Use a fast catalyst. See what happens.

If it sets fine then you can mould as normal, the RTV will stick to the initial thin coat of RTV.

I can't see that auto primer could be a problem, unless you haven't let it dry. Even then it shouldn't inhibit the cure.

You could also try using a release agent on the master, wax-spray would give a barrier to anything in the paint or sculpy.

 

[vistaVision]

<div class='quotetop'>(Blad @ Sep 23 2006, 02:59 PM) [snapback]1325126[/snapback]</div>

Brush on a very thin coat of RTV on the areas where it apparently does not set. Use a fast catalyst. See what happens.

If it sets fine then you can mould as normal, the RTV will stick to the initial thin coat of RTV.

I can't see that auto primer could be a problem, unless you haven't let it dry. Even then it shouldn't inhibit the cure.

You could also try using a release agent on the master, wax-spray would give a barrier to anything in the paint or sculpy.
[/b]

Wax-spray release? Something specific to moulding/casting or...? I'd love to find something readily available that will create a barrier. Of course, if this works, still leaves me with a bit of a mystery.?

I'll play around with all these ideas. Any additional advice is appreciated.

Marcus

 

[nickytea]

Are you using a tin or platinum based silicone rtv? Platinum based rtv's are very finicky and have the problems you are describing. Tin will usually set on anything.

 

[vistaVision]

<div class='quotetop'>(nickthsaberman @ Sep 23 2006, 04:00 PM) [snapback]1325169[/snapback]</div>

Are you using a tin or platinum based silicone rtv? Platinum based rtv's are very finicky and have the problems you are describing. Tin will usually set on anything.
[/b]

I've been using a platinum material. Polytek 71-20. Nice stuff, haven't had these problems before. Maybe the stuff is just getting old? Too cold room temp? (mid 70's)... Any advice on getting the platinum cure RTV's to behave?

I might have to try the tin cure. I understand the alcohol that is released during the cure can lead to problems with urethane resin castings?

All of this advice is great. Thanks guys..

Marcus

 

[replicaprops]

71-20 is the pink super soft stuff. The pink part really needs mixing prior to mixing with the clear catalyst.
It is probably the worst mold rubber polytek makes.
I suggest going with a 1 to 1 mix ratio rubber like Dragon Skin from smooth-on.
Thats just me, but I really hate polytek rubbers.

 

[clonesix]

WARNING: HIJACK

Since we are on the subject of RTV problems, I tried to repair a GI-1000 mold with some silicone caulking and it isn't curing. I had some small tears in the RTV and tried to glue them back in place using GE silicone in a tube. Here it is 24 hrs later and the white caulk hasn't even surface cured.

thoughts? Is there a better repair method?




HIJACK OVER

 

[Blad]

<div class='quotetop'>(eightperf @ Sep 23 2006, 09:50 PM) [snapback]1325164[/snapback]</div>

<div class='quotetop'>(Blad @ Sep 23 2006, 02:59 PM) [snapback]1325126[/snapback]

Brush on a very thin coat of RTV on the areas where it apparently does not set. Use a fast catalyst. See what happens.

If it sets fine then you can mould as normal, the RTV will stick to the initial thin coat of RTV.

I can't see that auto primer could be a problem, unless you haven't let it dry. Even then it shouldn't inhibit the cure.

You could also try using a release agent on the master, wax-spray would give a barrier to anything in the paint or sculpy.
[/b]

Wax-spray release? Something specific to moulding/casting or...? I'd love to find something readily available that will create a barrier. Of course, if this works, still leaves me with a bit of a mystery.?

I'll play around with all these ideas. Any additional advice is appreciated.

Marcus
[/b][/quote]




May not be available in the US the release agent I use is:
Bonde
AFH Aerosol Spray Wax
Description:
A versatile spray wax release agent for most mould surfaces for moulding in polyester, epoxy and polyurethane resins. Also used for on silicone rubbers when separation is required between layers. Gives gloss, (or slight matt or satin finish on fibreglass moulds)

You can use it on the master to give a glossy - or matt finish, you can apply several coats.

The problem you have does seem odd in that it's just happening in a certain area.

<div class='quotetop'>(clonesix @ Sep 24 2006, 12:36 AM) [snapback]1325227[/snapback]</div>

WARNING: HIJACK

Since we are on the subject of RTV problems, I tried to repair a GI-1000 mold with some silicone caulking and it isn't curing. I had some small tears in the RTV and tried to glue them back in place using GE silicone in a tube. Here it is 24 hrs later and the white caulk hasn't even surface cured.

thoughts? Is there a better repair method?




HIJACK OVER
[/b]

For very small tears then what you've done usually works, it can take days to cure though.

Another method i've used is RTV mixed with thixiotropic Agent which makes the silicone look and feel like butter (depending upon colour of course), you can smear it onto a tear, break or thin area and it will stay there and bond with the existing RTV.

 

[clonesix]

OK, I will wait a few days for it to cure. If that doesn't work, I will be doing as you mentioned and using a little of the same GI-1000 to repair it. At least I know that will cure. Thanks.