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Attaching sprues for vulcanized rubber molds


I am making my first vulcanized rubber mold today and I ran into a slight problem, I was hoping someone may have an answer!

I am making a silicone rubber mold of a silver wishbone, so I can eventually cast it into gold. I want to attach a sprue and rod to it so it comes out in the mold. Are there any suggestions how I should attach it to the silver wishbone? I don’t want to solder a rod to it because the piece is not mine and I don’t want to damage the original at all. Do I use wax? If so, how do I properly attach the wax to the metal so it will stay?

Thanks! I can use all the help I can get :slight_smile:

Silicone or rubber mold? If silicone then you can use a wax sprue. If rubber and vulcanizing, you need a sprue that will withstand the heat of vulcanization, and that means soldering a brass or silver rod to your mode

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It is silicone. Thank you so much for taking the time and responding, you helped me a lot!
Soldering a rod wont ruin the original piece?

even with silicone, you can always put in a brass sprue and sprue
cone near the piece and if necessary, exacto knife a connection to
the model after the mold is made. With rubber, it is also possible
to burn a channel to the mold of the object… silicone you can’t


You might try a 2 part jb weld. I regularly fabricate a thick copper wire sprue and attach it to the prototype body with the jb weld. After cooking the mold, the compound sort of breaks down and can be chipped & filed away. It does take 24 hours to cure, though.


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If you’re pouring a liquid silicone mold you can attach the sprue with super glue. Afterwards a soak in acetone will break the join and you can peel off any residue. The only downside is that super glue will not fill gaps, so there won’t be a nice fillet at the join as there would be with soft solder. But as Jeanne said, you can cut away any silicone flash with a knife or small scissors.

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OR, just add a sprue to the wax(s) before investing. Not the difficult unless you are planning on casting 1000’s of unit. A bit of a mountain here when in fact it is an ant hill.


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You guys are the best! Thank you so much for taking the time to help me out!