Casting tin in silicone, poor results? (1 Viewer)

[Gerrydelasel]

I joined this forum in the hope of finding someone who can help me with small detailed object casting (not necessarily soldiers).
I made a mould of a medallion using RTV high-temp silicone.
The mould came out beautifully, but try as I might I cannot get a good casting from it using lead, tin, or pewter! All the fine detail comes out subdued or kind flattened. I can get a perfect casting with low-temp bismuth alloy, so I know the mould is fine.
What is going on here? I'm guessing something to do with the surface tension of lead/tin doesn't agree with the silicone, but is there anything I can do about this? Should I be using a different mould material? Ideally I want to get a good casting using tin because I have ready access to tin.
Can anyone advise?

 

[king59]

I joined this forum in the hope of finding someone who can help me with small detailed object casting (not necessarily soldiers).
I made a mould of a medallion using RTV high-temp silicone.
The mould came out beautifully, but try as I might I cannot get a good casting from it using lead, tin, or pewter! All the fine detail comes out subdued or kind flattened. I can get a perfect casting with low-temp bismuth alloy, so I know the mould is fine.
What is going on here? I'm guessing something to do with the surface tension of lead/tin doesn't agree with the silicone, but is there anything I can do about this? Should I be using a different mould material? Ideally I want to get a good casting using tin because I have ready access to tin.
Can anyone advise?

One of the motive is the temperature of lead and the mould. More casting make and the mould is more warm and the lead pouring well. Two make more air vents must be made around the object. Third dust the walls of the mold with talc or graphite to allow the meta to slide better. Good luck!
[/FONT]
air vents must be made around the object.air vents must be made around the object.

 

[theBaron]

I joined this forum in the hope of finding someone who can help me with small detailed object casting (not necessarily soldiers).
I made a mould of a medallion using RTV high-temp silicone.
The mould came out beautifully, but try as I might I cannot get a good casting from it using lead, tin, or pewter! All the fine detail comes out subdued or kind flattened. I can get a perfect casting with low-temp bismuth alloy, so I know the mould is fine.
What is going on here? I'm guessing something to do with the surface tension of lead/tin doesn't agree with the silicone, but is there anything I can do about this? Should I be using a different mould material? Ideally I want to get a good casting using tin because I have ready access to tin.
Can anyone advise?

If you're trying to cast and get fine detail, then don't use unalloyed lead. Lead does not generally give you crisp detail, regardless of the quality of the mold. Commercial casting alloys that you buy will be alloys of tin and bismuth or antimony. Bismuth and antimony allow the alloy to fill the fine details and produce a good casting.

Unalloyed tin is too brittle and the casting will likely break or crack.

"Pewter" as a term covers alloys of tin and antimony or bismuth, and sometimes some copper, but also alloys of tin and lead. So unless you can confirm the makeup of the pewter you used, I'd avoid it. I would buy casting metal from a supplier, like Prins August, or I would look for old linotype metal and use that. That's what I use for my castings.

Beyond that, as king59 said, make sure your mold is vented, if it isn't, so the air in the mold can escape as the metal pours in.

Also, if you can show us pictures of your mold, that would help us help you, too.

Prost!
Brad

 

[Gerrydelasel]

Also, if you can show us pictures of your mold, that would help us help you, too.

Thanks everyone for indulging me, I'm a newb!
Here is the mold: https://i.ibb.co/zHBR6g6/IMG-20200408-175052768.jpg
There are two vents in it, coming up either side of the spout. You can see one in the tin example below.

And here are the results, from left to right: https://i.ibb.co/Dt23zxK/IMG-20200319-164942212.jpg

• Lead: very poor detail
• 'White metal' (unknown alloy) I bought from the estate of an old guy who used to cast soldiers: Pretty poor detail.
• Tin: About the same result as with white metal. It does not seem to be brittle, I can try to bend it hard and it barely flexes and doesn't snap.
• Bismuth alloy (unknown alloy from the 1980s, probably lead and bismuth, maybe some cadmium?): Near perfect detail
• 50/50 bismuth alloy with tin (just for fun): Good detail but very brittle, snaps with no effort, they didn't want to be mixed!
• Alloy from a toy gun, zinc maybe, it's higher temp than the others: Also poor detail

BTW they're supposed to have those holes in them, and the rough outline

 

[theBaron]

OK, with that additional info, I stand by what I said in my previous post. Don't use lead. If you want the best results you can get, I recommend buying casting metal from a commercial source, eg, Prins August, among other sources, where you can be assured of the composition of the alloy. But I think you've answered your own question, with that bismuth alloy you mentioned as giving you your best results. Every other alloy or metal you've listed has only given you less-than-satisfactory results. Consider those the errors in a process of trial and error.

Prost!
Brad

 

[Gerrydelasel]

But that doesn't explain why it's happening. I'm sure I've seen detailed tin castings before, what are they doing differently? I suppose they might be doing centrifugal or vaccum chamber stuff.

 

[Wolfgang_UK]

But that doesn't explain why it's happening. I'm sure I've seen detailed tin castings before, what are they doing differently? I suppose they might be doing centrifugal or vaccum chamber stuff.

The spout is much to small, the needed pressure - for a gravity casting like yours - can not be build up if the spout volume is of such small size.
Rule of the thumb goes like this " You need (at least) twice the volume of your casting in volume for the spout".

Since you only got very limited space to attach the spout to your casting you need to redesign the entire mold. Feed the mold from below (the coin) so that the incoming metal can push the air (through the top) out of the mold. Design the spout like an incomplete U-shape. Casting your metal from one side, release of trapped air on the other. That will vent the mold much better, create more pressure due to higher volume and fill the mold complete. The incomplete casting clearly shows lack of ventilation, trapped air inside the mold and to little pressure build up due to the small volume of the the spout.

Hope this will help

 

[Tinsoldier]

Wolfgang is right, the amount of metal above the cavity to be filled should be twice, or at least equal to the amount necessary to fill the cavity. I create a cavity from LEGO blocks with maybe several pour vents made from Q-Tip pieces. They are smooth and round, providing a nice flow to the cavity, especially when you are making a round piece. Similar to getting a spoked wheel.

While getting quality alloy metal is the best, a tin/lead mix will work but remember the baby powder (not the one with corn starch because the heat of the metal will burn the starch slowly destroying mould).

I even use the Q-Tip for creating air vents.

don

 

[theBaron]

But that doesn't explain why it's happening. I'm sure I've seen detailed tin castings before, what are they doing differently? I suppose they might be doing centrifugal or vaccum chamber stuff.

Yes, commercial castings are usually made using centrifugal casting machines, as well as using quality casting metal. There is only so much you can do with drop-casting.

Prost!
Brad

 

[Gerrydelasel]

Thanks I'll try making some kind of hopper above the mold, and if that fails I will try making a new mold with Wolfgang's U-bend method.

 

[Gerrydelasel]

Quick update:
After trying everything I could think of including graphite powder and a hopper (turns out a hopper adds almost no extra pressure, it's a *heat* reservoir, not a pressure thing) the solution turned out to be to add 20% bismuth to my tin, turn my pot up as hot as it will go and get everything to max temp, then pour it as fast as possible (talc powder, no hopper). Near perfect results every time now