Moldmaking - Can I Layer Silicone Caulk between RTV Silicone Layers?

[Jesuit24]

I've bought some RTV silicone for a few molding projects but I'm trying to make it last, so I have a helmet I'm planning on molding and slush casting and was trying to save silicone.

What I was thinking was painting on one or two layers of regular RTV silicone and then building up the middle layers with silicone caulk before finishing off with a final layer of RTV, effectively making a caulk sandwich.

Would this method be wrought with issues with the mold? Since it's a helmet, if I were to use RTV for the whole thing, I'd need to thicken it up whereas the caulk would be the right consistency right out of the tube. However, I've used caulk for molds before and while it works, the molds tend to tear after a few uses so I was wondering whether the RTV it's encased in would help the stability.

What do you think?

 

[Tarchinoko]

I dont have any experience with this personally but the issues you would likely face are:

the adhesion between the RTV silicone and the silicone caulk
the difference in properties between the silicones, for example shore hardness, elongation before break etc.

how did the caulk mold fail previously? also what was the surface quality of the mold when you used caulk?

 

[Jesuit24]

The caulk worked well as a two part mold for the quality and ability to pick up detail. Due to the design, casts were a bit awkward to remove from the mold so it eventually tore. Also, every time I wanted to make a cast, I would have to seal all the edges with silicone to prevent leakage, so there was an extra step of messing around along with more flashing than I was happy with. Going by feel, I think the caulk is a slightly softer shore than the RTV, but that may be because all my RTV molds are poured in thick blocks whereas the caulk is applied in layers and is thinner.

 

[detenten]

I dont have any experience with this personally but the issues you would likely face are:

the adhesion between the RTV silicone and the silicone caulk
the difference in properties between the silicones, for example shore hardness, elongation before break etc.

Seconding both of these issues
I'll add that your RTV silicone has a potential to not like some of the crap in the caulk and then you have cure inhibition problems for your outside layer.

I'm also not sure what value the extra caulk layer is going to give you, for how little RTV silicone it's going to save you.
Aren't you making mold mothers on top of your silicone layer anyway? I guess I'm not understanding why you want to do a silicone sandwich. How are you pouring RTV in blocks and then putting caulk layers...? I think I'm unclear on your entire idea so maybe my whole post is irrelevant!

I'd imagine a caulk sandwich mold will eventually give you similar issues as your other caulk molds, especially since you say it tore during demolding; it's not going to have much increased tear strength even with a thin layer of higher quality silicone surrounding it.

 

[8 perf]

If you are using RTV that is thick enough to be brushed on, as I think you are, then just use it all up and try to get about 1/4 coverage over the whole piece. As inferred by detenten above, back it up with a plaster mother mold made out of plaster and gauze bandages or even better still, burlap if you have it. Mother molds are used extensively in the industry for this very reason. Silicone costs a lot of $$; plaster is cheap.

 

[Jesuit24]

I was going to brush on the first thin layer although I'm a bit concerned the silicone will just drip off without thixotropy. Then I was going to add the thixotropic for the next layer to give it some thickness before switching to caulk to bulk out the mould and then giving it a final thixotropic added layer of RTV. But it sounds like mixing the pair is a bad idea.

< It's this helmet I'm molding, so at some point I'll slip aluminum mesh in the T-visor so when I split the mold down the visor area, the whole thing doesn't eventually tear apart.

I also have two two part molds I need to work on, so it looks like I'll have to buy more. The hope was I could get 2kg to last the three projects but it's not looking like it will.

And yes, I was planning on a fiberglass mother mold.

 

[Tarchinoko]

If i were you i would test this type of mold on something smaller and see what problems you run into. I usually use about 2kg of silicone on a full helmet silicone jacket so i find it very unlikely that you will manage to make 2kg last 3 helmet molds

 

[renaissance_man]

I would say, if you want a mold that is going to last and give you good clean resin casts from it, don't scrimp on the mold.
If you've taken the time to make a good quality master part and want to replicate it exactly you have to invest in a high quality RTV rubber.

An addition cure rubber is the best as they give little to no shrinkage and allow for multiple resin pulls to be made without the need for mold release.

Where you think you'll make a saving by trying to cut corners using caulk, ultimately the quality of your end product will suffer. I have experience making RTV molds and casting resins and I can tell you you can't cut corners.