Silicone Curing Issues

[DerKraken]

Hey guys, hoping someone can lend a bit of advice on this.



I'm currently molding a couple of pieces for some more Portal stuff, and I'm running into a real pain in the ass problem. I have a puck that looks very similar to this:

This is a shot from my Daft Punk build, but the shape and size are very similar. The piece is made out of MDF, and there is an ABS disc in the center that adds some detail (it also covers the hole from my lathe that you see above.) On top of that are a few coats of Rustoleum gray primer which has been sanded smooth with 000 steel wool. After this I went over the piece with 50/50 water and windex and wet sanded with 1000 grit, then let dry overnight.



I've poured the silicone for this three times now, and twice I've ended up with silicone that has not cured in the center of the puck (recessed area above) This is where the ABS is on the actual piece, but I can't see how that would be an issue seeing as how it is coated in the same primer the rest of the piece is covered in. This isn't the thickest area of silicone either, but it is one of them.



On the first pass I let it dry for 18 hours. Pass 2 dried for 12 hours. Both did not cure fully in the middle and were still very gooey when I pulled the mold open - but again this was only in the center area where the ABS disc sits. I'm currently on mold #3, which has been curing since 8pm yesterday evening. I'll be opening that one at 8pm today to see if this is a time issue, but on the chance that it isn't, does anyone have any advice on this? I'd really rather not waste any more silicone trying to mold this thing any more if I can avoid it.



Thanks in advance!

 

[Yodajammies]

How old is your silicone? Are you mixing in the right proportions? Are you mixing long enough? What kind of silicone are you using?

 

[DerKraken]

How old is your silicone? 3 months at oldest



Are you mixing in the right proportions? 1:1 by volume, yes.



Are you mixing long enough? 3 minutes, more if the color is not consistent.



What kind of silicone are you using? Rebound 25



Further information: Curing is done in my garage, and it has been pretty humid out here in Georgia over the past few summer months. My old house (I recently moved) had a VERY damp garage, though this never proved to be a problem with any other molds I have done. Also, I have used this primer in combination with this silicone for other molds, with no issues (see pic):

Average temp inside my current garage is about 78 degrees. Its not air conditioned, but it is insulated and connected to the house, so it stays cool residually.



The mold jacket is a section of 4" PVC pipe with a single cut line on one side to release the mold when its dry.

 

[Yodajammies]

I recently talked to a service rep from Smooth-on, and he said the average life of rebound 25 is about 6 weeks. More if you use a gas blanket.

 

[rube]

Try stuffing it in the oven for 10 min. at 190 deg.

 

[Drgnfyre4]

Here is a thought, is that piece hollow? It is very possible that moisture and heat condensation is building up inside the piece. As your silicone is curing condensation my be forming on the ABS part because of trapped air heating up inside the master part, and any excess moisture will interfere with silicone curing evenly. The fact that this keeps occuring around the center ABS part tells me this could be what is happening.

 

[Ithica]

The issue is the paint. Do not use Rustoleum. Use Krylon Clear Coat and call it a day. The Rustoleum had a oil release and also has a small amount of sulfur. When you pull the mold off the piece it will still be uncure but it will cure once it is removed but the issue is losig detail and so on. You will need to remold and not use Rustoleum.



Ithica

 

[rube]

The issue is the paint. Do not use Rustoleum. Use Krylon Clear Coat and call it a day. The Rustoleum had a oil release and also has a small amount of sulfur. When you pull the mold off the piece it will still be uncure but it will cure once it is removed but the issue is losig detail and so on. You will need to remold and not use Rustoleum.



Ithica

That's probably it. The oil in Rustoleum is fish oil, for corrosion protection.

 

[DerKraken]

That's probably it. The oil in Rustoleum is fish oil, for corrosion protection.

Thanks guys. I keep telling myself not to use that crap (I HATE the way it sands) but when I run out of all my Krylon paints there's that one can of Rustoleum sitting on the back of the shelf, about half-full, and I always think "I really don't want to go to the store. It will be fine for now"



Mold #3 seems to have done the trick though. I let it sit for 24 hours, and prior to molding I went over the piece with mineral spirits to remove any oils. I also tried Rube's trick of tossing it in the oven for 10 minutes at low temp, then letting it sit for another hour before demolding.



Crappy cellphone pic, but you get the idea. Ignore the color issues, I was trying out some dyes (these will be painted anyways.) Top one is the master.

Thanks again!