Low melt metals query.

Faye · Post by **Faye** » Thu Jan 28, 2010 8:51 pm

Hi there, just a query on casting with low melt metals.

Ive read many a book claiming that theres a special type of silicone required for this. Thats all well and good but its pricey. So just wondering how well a plaster mould stands up to metal casts. Im looking at casting a basic knife shape and I only need round 2-3 casts, will the mould stand up to that treatment or will it only take one before it burns?

Thanks in advance.

Kylwell · Post by **Kylwell** » Fri Jan 29, 2010 12:24 am

IIRC, unless it's a really low melt metal, plaster is a bad idea more so if it's hasn't fully cured (which may take weeks).

I'll see what my friend down the row says, he's been casting metal miniatures for a while now.

Stu Pidasso · Post by **Stu Pidasso** » Fri Jan 29, 2010 1:22 am

Kylwell wrote:I'll see what my friend down the row says, he's been casting metal miniatures for a while now.

Give him my email address, I have work for him if he wants.

Lt. Z0mBe · Post by **Lt. Z0mBe** » Fri Jan 29, 2010 6:39 am

Kylwell wrote:IIRC, unless it's a really low melt metal, plaster is a bad idea more so if it's hasn't fully cured (which may take weeks).

I'll see what my friend down the row says, he's been casting metal miniatures for a while now.

I can speak for that from Boy Scout experience We made our own tiny sinkers for hand carved lures using some plaster molds. Molds weren't cured enough and they cracked violently. Why? I didn't allow them to cure long enough and the heat caused the trapped water to steam in a hurry.

Kenny

Faye · Post by **Faye** » Fri Jan 29, 2010 10:27 am

Thanks Klywell any advice on the matter is much appreciated.

Lt. Z0mBe, define "violently"... are we talking explosive...

Has anyone had experience with the special Silicone they recommend? Anything funky about it or is it like every other silicone?

Umi_Ryuzuki · Post by **Umi_Ryuzuki** » Fri Jan 29, 2010 3:03 pm

Faye wrote:Thanks Klywell any advice on the matter is much appreciated.

Lt. Z0mBe, define "violently"... are we talking explosive...

Has anyone had experience with the special Silicone they recommend? Anything funky about it or is it like every other silicone?

Yes, especially in larger molds. any moisture in the plaster
will immediately expand as steam when hot metal is poured
into the mold. It is an extreme hazard. Even metal molds should
be preheated to evaporate moisture.

In metal molds the steam has been know to splatter the
molten metal up and onto the person pouring the metal causing
third degree burns. A plaster mold could shatter and in the
worse case scenario act like a molten metal grenade...

Do be careful.

Many silicone molding materials such as GI-1000, can be use to
cast material up to 300° or 400°, which is fine for many bismuth and
other low melting point metals. Again, the mold should be dry, and
probably preheated to remove moisture.

Kylwell · Post by **Kylwell** » Fri Jan 29, 2010 3:15 pm

http://www.smooth-on.com/index.php?cPath=2_1113_1135

Smooth-on Mold Max® Performance Silicone Rubber looks to be the stuff.

Faye · Post by **Faye** » Fri Jan 29, 2010 7:33 pm

All safety warnings are duly noted, I had a feeling this would be that case but ive never had an accurate assessment of the damage... I was hoping to find a cheaper alternative to the silicone but I may have to bite the bullet, I can get some smaller quantity tubs here in the UK for about £30 ish.

Thanks for all the advice guys

Chacal · Post by **Chacal** » Sat Jan 30, 2010 9:51 am

Umi_Ryuzuki wrote:...In metal molds the steam has been know to splatter the molten metal up and onto the person pouring the metal causing
third degree burns...

Just like letting water drip into a heated deep-frier: water flashing into steam will splash hot, searing oil everywhere. Hot oil is still preferable to molten metal, IMO.

The mold has to withstand the metal temperature (otherwise it won't be any good), so it is always a good idea to pre-heat the mold, so the poured metal won't 'flash freeze' onto the surface of the mold when poured, which can cause surface defects, keep the metal from flowing into details, all sorts of problems. Also, slowly heating the mold up in an oven instead of having it be suddenly, unevenly heated by onrushing molten metal can only be good for it.

Faye · Post by **Faye** » Sat Jan 30, 2010 3:51 pm

Hmmm, sound advice, I would take oil over burning metal anyday

As far as details go, will I still get crisp lines? There isnt a large amount of this, just one simple raised logo but will the edges round themselves?

Thank you all for the advice so far.

Jagdson · Post by **Jagdson** » Sun Jan 31, 2010 1:59 am

Metal miniatures can be made with surprisingly fine details. The key seems to be in the mold temperature and simple repetition. There will be flaws in any production run.

Umi_Ryuzuki · Post by **Umi_Ryuzuki** » Sun Jan 31, 2010 2:39 am

Jagdson wrote: Metal miniatures can be made with surprisingly fine details.
The key seems to be in the mold temperature and
simple repetition. There will be flaws in any production run.

Aahh, but the flaws in metal casting can be thrown back
into the pot and melted down for another run.

seam-filler · Post by **seam-filler** » Mon Feb 01, 2010 5:23 am

Faye, as you're in the UK, your best source for metals, rubber and advice is Alec Tiranti. No, I'm not a shill for them - they've just given me sound advice and good service over the years. A 1kg pack of their RTV-101 is only about £20 and is intended for low-melt alloys. It's the same stuff that Hobbycraft sell for twice as much.

I back up what everyone says about water. Even a tiny amount of water can result in an explosion of molten metal. So heat the mould in the oven at about 150C for at least 1/2 hour immediately before you pour. A fan oven helps. Do not use a microwave.

Depending on the mould, temperature of the metal and the exact mix that makes up the alloy, incredible detail is possible. In general, though, the lower the melt temperature, the less crisp detail will be. Also, lower temperature alloys usually give softer results - more susceptible to damage.

If you're thinking of doing a lot of casting, think about getting a centrifuge.

Another safety issue is that lead is still used in some low-melt alloys in the UK - there are alternatives, but you should be aware that if it does contain lead, you should wear gloves, eye protection and a face mask when casting. Gloves are also a good idea when working on the part later, at least until it has been primed.

Faye · Post by **Faye** » Tue Feb 02, 2010 7:23 pm

Yeah I use Tiranti a lot, pretty reliable but they have a habit of 'overlooking' safety factors! Either way I think its my best bet, not many smooth-on suppliers here in the UK...

I think ill give plaster a miss in the end, besides, it gives me a chance to try some new silicone and I might be bale to piggy back the purchase on someone else's stuff.

Thanks for all the advice guys, really helps.

Darth Humorous · Post by **Darth Humorous** » Wed Feb 10, 2010 2:36 am

You might look into “investment powders” for casting metals. It is plaster-like, but made specifically for mold material to melt metal into. It is used in “investment casting”, which is also known as the “lost wax” process. Jewelers use it to make, well…jewelry. I don’t think it is very expensive, but it has been a very long time since I’ve used it.

Mark