This started coming up on Dave T’s build thread. I also know John C happens to know a thing or too about casting metals. I have played with it and know others have also. But I wanted to get some ideas and understanding on casting pewter mainly but also other metals such as brass/bronze. Dan and I have also discussed this a bit. Its an interesting topic.

Here is where I would like to start the discussion. When I have played with lower temp metals mainly lead and pewter pouring into RTV mold or even aluminum molds (fishing weights and jigs). I find it very frustrating trying to get the metal to flow nicely to all parts of the mold. I have pre-heated, I have used talc, I have flamed my aluminum molds and all have helped but getting a consistent pour each and every time to all parts of a detail oriented mold is eluding me. I don’t have the money to go buy a centrifugal metal slinger for the few parts at a time that i would make them. Lost wax casting with centrifugal force seems the best way to accomplish what I want to do. But the slinger is expensive. I have watched videos on hand held sling casting and while flinging hot metal around and about my head and face seems fun I am wonder if their are other ways. I do believe i will try the sling casting as I have a near endless supply of bronze at my disposal and I can easily make RTV molds for pouring wax to latter make the plaster molds for sling casting. This would give me a reproducible product.

One idea I have thought about trying and would love input on is the idea of using low melt temp pewter in a high temp RTV mold in an oven that is set above the melt point of the metal and then adding a vacuum much like vacuum casting resin. I think a very simple oven could be fashioned out of a pressure cooker that then could be sealed and a vacuum applied. Instead of the pressure being explosive in a pressure cooker you would be having implosive pressure and not a lot would be required as long as the temp stays above the melt point of the metal you are using. Any thoughts or concerns on this idea.

Devon, have you been making air escape passages in your rtv molds? these would be small passages cut into the rtv, from the bottom of the mold to allow air to escape. makes a huge difference.

AL P.

Al Pomeroy said:

Devon, have you been making air escape passages in your rtv molds? these would be small passages cut into the rtv, from the bottom of the mold to allow air to escape. makes a huge difference.

AL P.

Yes Al,

But apprently not good enough ones, or incorrectly made/positioned.

Bronze will boost your melt temps way up to 1900+ degrees. I have an electro-melt oven to safely melt for vacuum assist casting. I use lost wax techniques involving a burn out oven to fire a casting flask. The flask must be hot so it doesn’t spaul (flake) into the cast. My vacuum pump can boil water.

I started with a slinger, but like vac assist much better.

There’s a big difference between my safe method and your jerry rigged calamity.

Get 2 rectangles of 1/8" steel plates a 1/4" bigger than your molds. Use a C clamp to not too tightly hold them to your mold. Fashion a box to hold one side of the mold exposed for your pour. Attach the vacuum hose to box.

Turn on vac and pour.

You can experiment on the clamp pressure by dusting your mold with talc, clamp and blow into it. Some talc should puff out of the sides or it’s too tight. This is why I like C clamps vs spring loaded clamps.

Hey John thanks. I say bronze without having any idea what so ever about what I am talking about. That was just an idea as I have a source for it. I would be just as happy with low temp metals like pewter. I dont hink I care to try to come up with a plan to reach 1900 degrees.

But the vacuum assist sounds not only doable but safer than flinging molten metal around my head.

Edit for some clarification. John I understand the principle of vacuum idea but am not really fully understanding how I would build one. I might need a little more clarification on this one. Are we trying to hold the mold under a vacuum or are wen in essence putting a continuous vacuum on the bottom of the mold (from inside the box) and then as the metal is pour the vacuum is pulling it to the nether regions of the mold?

I think the vacuum is pulling the air out of the mold so it can completely fill.

I’m following along but I don’t know enough about this process to ask intelligent questions at this point. I am very interested and as Devon indicated he and I have been discussing this process behind the scenes. I’m going to sit over here in the corner and keep my mouth closed and ears open for a little while and see if I can learn enough to ask the appropriate questions. Thanks for all the help. Carry on.

Joe,

That’s the basics if it but not sure how its accomplished. In resin casting you pour your resin and put the mold in a vacuum pot and apply the vacuum and close it of holding a vacuum in the entire container which pulls the air out. Reintroducing the vacuum only if enough air has caused the vacuum pressure to drop to much. In parts casting I dont think this is ever an issue but in wood stabilization with resin it can and does happen. The point being the entire mold is in an air tight container.

In order to pour the metal while under vacuum the top of the mold is not in the vacuum chamber. So where I am confused is how are we applying the vacuum to the mold through the box. I am assuming this is why John says the mold cant be air tight.

If the box is air tight around the bottom half of the mold and the bottom “lid” on the mold is loose as John describes and the box is put under vacuum then I can see pouring metal in the top and having it pulled toward the bottom in the direction of the vacuum.

I suspect this is what we are doing.

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