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I began my foray into casting just recently. I am using the RTV Silicone mould kit from the Alumite mini casting kit. It's a 1:10 ratio silicone if that matters.

The problem is that I had a couple pieces of it break because the directions forgot to warn me that the resin expands and that I shouldn't fill the mould fully because of this.

Now, my question is this: Does anyone have any clever ways for fixing pieces of a mould? The part that broke off was not very complicated; I could probably actually replace it with greenstuff or a piece of sprue, I just don't know if that's a "good" idea. I'm casting my own design of turrets for some razorbacks I have, and I need the holes for the ammo belts to pass through, but one of the posts that would accommodate that on the mould was what broke. It's a one piece mould, not a two piece clam-shell, so technically the post is at the fill point, so there would be minimal flexing to remove the piece from any sort of greenstuff addition I were to add to it.


Thoughts?

What product are you using? Most casting resins don't expand any noticeable amount. Did part of the rubber mold rip away? There is no good way to fix that - you could try using more RTV rubber to glue the piece back on - but you'll probably lose the detail at the joint. Using green stuff/sprue is just going to stick to the resin the next time you cast the part.

This is the precise kit I have: http://www.alumilite.com/ProdDetail.cfm?Category=Starter%20Kits&Name=Mini%20Casting%20Kit

Yeah, the piece that was in the rubber tore up. As far as the expanding goes, I'm doing about a 1:1 mix for the resin per the directions. Not sure if humidity or temperature could cause expansion, but it definitely did. Also, I've had the kit for quite a while though, possibly a year or two; not sure if the resin degrades over time. One of the test castings I made had sort of a "bubbily" texture to the underside of it. I'm not sure if I'm introducing too much air while mixing or something, but that's a different issue I think I can work out through playing with it.

At the particular place I'm looking at, detail on the joint isn't a huge loss; it'll be covered up by the ammo belt anyway. Really, the thing is still quite salvageable, it's just now I have to drill out the hole on every single one of these I produce.

The bubbles are probably caused by moisture contamination. Do you use talc in the mold? Dusting the mold with talc does wonders for fighting moisture bubbles.

I've not tried the talc yet. Sounds like it would work pretty well. I assume you need a very light dusting to prevent it from interfering with the actual casting process, or does the resin mix well with the talc?

Hi daedalus. Unless you can somehow glue the other piece back on, which isn't likely, you're probably better off redoing your mold or just repairing the issue post-cast. Attaching anything onto the mold rubber is going to be a pain and is likely to continue breaking off at the attachment point every time you demold your casts. Instead of using the poured rubber, perhaps you can try one of the mold putties? They might be a bit stronger and stand up to what you're doing better, though I can't speak from experience.