Silicone rubber in polyester gel coat mold gives sticky surface

[mleenheer]

I’m trying to make some custom silicone rubber plugs using a female polyester gel coat split mold. When I release the mold, the silicone rubber is sticky / gummy on the area which have touched the mold.

Here’s what I’ve been doing:

1. Machine a female split mold from mdf on my CNC router
2. Apply polyester gel coat and let it cure
3. Run a final pass with the CNC router on the cured gel coat to get a smooth finish
4. Sand the gel coat up to 600
5. Apply Meguiars Mirror Glaze 87 release wax (containing carnauba)
6. Fit the two mold parts together
7. Mix shore 40 A silicone rubber according to specs
8. Pour rubber in mold

When I split the mold, the core of the plug is cured nicely. The surface however is very sticky. Also the surface of the gel coat mold has this sticky stuff on it.
The silicone rubber in the cup in which I mixed the rubber however cured nicely without sticky surface.
When I remove the sticky stuff from the gel coat mold, the gelcoat itself seems untouched.

Does anyone have any idea what goes wrong here?

Thanks a lot!

[Hanaldo]

I assume you’re using addition or platinum cured silicone? You can’t use this with polyester surfaces, the presence of styrene inhibits the cure of this type of silicone. You can use a condensation or tin cured silicone no problem, but if you need to use an addition cure silicone then you will need to change your mould surface. Epoxy for example would work perfectly.

[Roger]

Hanaldo is correct. Styrene inhibits the cure. That’s not to say you can’t use platinum cure silicone over polyester in every situation. I’ve made tons of silicone bags over polyester mold surfaces. The difference is that I was using a production mold built off a master. The molded surface is adequately cured/ sealed to eliminate styrene migration from the mold surface. Any time you cut the surface, i.e., sand, saw, CNC, etc., you are exposing fresh surface that will inhibit the cure.

You can also seal the surface with acrylic spray paint. I do this when I make a polyester offset to replicate part thickness. In your scenario, I would sand to 400 and then spray with acrylic.

[mleenheer]

Roger and Hanaldo, thanks for the excellent information / help!

I’ll go for the acrylic spray paint, and when my supply of rubber is empty I will make sure the next batch I order is condensation or tin cured rubber.

[Hanaldo]

Hanaldo is correct. Styrene inhibits the cure. That’s not to say you can’t use platinum cure silicone over polyester in every situation. I’ve made tons of silicone bags over polyester mold surfaces. The difference is that I was using a production mold built off a master. The molded surface is adequately cured/ sealed to eliminate styrene migration from the mold surface. Any time you cut the surface, i.e., sand, saw, CNC, etc., you are exposing fresh surface that will inhibit the cure.

You can also seal the surface with acrylic spray paint. I do this when I make a polyester offset to replicate part thickness. In your scenario, I would sand to 400 and then spray with acrylic.

Both good points, although you need to be careful even when using a well broken in tool. Not too long ago I decided to make a silicone bag for one of my production vinyl ester tools that had already done around 40 or so pre-preg cycles at 90 degrees, so I would have expected it to be sufficiently cured. But I still suffered inhibition.

Really I think the easiest thing to do (and what I did to solve my problem) is just to spray the entire mould with PVA before you pour the silicone. PVA prevents the migration of styrene to the silicone, and doesn’t sacrifice the mould by having to coat it with acrylic. Of course the surface won’t be quite as perfect as your mould surface, so if that is critical then it isn’t a great option, but in most cases it won’t matter so much.

[mleenheer]

Roger and Hanaldo, thanks for your quick and useful answers! I’ll give PVA and acrylic paint a try. And when my stock of silicone is empty I will switch to tin-cured silicone.

[Roger]

Both good points, although you need to be careful even when using a well broken in tool. Not too long ago I decided to make a silicone bag for one of my production vinyl ester tools that had already done around 40 or so pre-preg cycles at 90 degrees, so I would have expected it to be sufficiently cured. But I still suffered inhibition.

Really I think the easiest thing to do (and what I did to solve my problem) is just to spray the entire mould with PVA before you pour the silicone. PVA prevents the migration of styrene to the silicone, and doesn’t sacrifice the mould by having to coat it with acrylic. Of course the surface won’t be quite as perfect as your mould surface, so if that is critical then it isn’t a great option, but in most cases it won’t matter so much.

Yes, PVA or PV-6 would work better than a paint on the mold surface. I’ve never had an issue on a molded surface. I’ve pulled bags off brand new molds maybe 30-40 times and never had a problem. I use petroleum jelly on the mold surface as a release. Maybe that helps?