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I have been working on a project that is without doubt my best work thus far (no spoilers yet!) but ran into a snag with casting the fittings. I already carved the wax, made a silicon mold of it and a few copies. That all went fine, and actually came out better than expected. However, after setting it in investment, the bronze well, failed miserably. I had a small sprue vent to help the air escape, and that filled about half way before a void the size of the thing I was casting formed between it and the top of the flask. Is this because the bronze was too cold? Or the mold not preheated enough? Any advice would be greatly appreciated. Hopefully, I'll be able to give it another go this afternoon.

 

Thanks

 

John

 

(Large cone part was the top of the mold, thin wiry thing the vent, and the bit on the end of it being the bottom part of what I was trying to cast)

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Edited by John Page

Not all those who wander are lost. -J.R.R. Tolkien

-Shards of the Dark Age- my blog
-Nine Worlds Workshop-
-Last Apocalypse Forge-

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Yea, too cold on Both issues .

Robert D. Yates , 13 & On Forge

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Figures :(

Next question- would it be safe to keep a thermocouple probe in the bronze, or is that bad for it? The 1905 degree mark is hard to judge be eye not having seen it in person before. Is there a better way to approximate pouring temp?

Not all those who wander are lost. -J.R.R. Tolkien

-Shards of the Dark Age- my blog
-Nine Worlds Workshop-
-Last Apocalypse Forge-

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Hello Brother John ,

 

What type of thermocouple probe do you have ? also do you have a Laser type Temp reader ?

Robert D. Yates , 13 & On Forge

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I have a stainless K-type probe thermocouple rated upwards of 2100(F), but I am worried that the steel of the probe will adversely effect the bronze. Also, the way it sits in the forge is not the greatest, and I am a little worried that the plastic on the far end will melt. If only I had an IR thermometer!

 

Cheers!

Not all those who wander are lost. -J.R.R. Tolkien

-Shards of the Dark Age- my blog
-Nine Worlds Workshop-
-Last Apocalypse Forge-

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Don't take this the wrong way, because I may be reading it wrong, but you have a sprue and a vent, right? They are not one and the same. Your vent should only fill with metal after the part is full. That doesn't mean temper wasn't also a problem. Just from reading it wasn't clear (to me at lest) that your system was set up correctly. Also make sure that all of your wax is burned out completely, as it will really gas off and stop up the gating system if there is any residue left. We do lost wax at one of our foundries at work, and we take every mold up to 1700-2000F for at least an hour to burn out all the wax, then let the temp drop to the actual pre-heat temp desired; typically 1000-14000F. These are for 30+ pound pour weight molds though.

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The vent was sort of an improvised idea, more to help the air escape than anything. In the newest mold I made, I did not use one. The geometry of the piece is not conducive to having a conventional vent, at least in the way I configured it. The confusion probably comes from my poor choice of words. I used a length of wax that I made the sprues from to make the vent.

 

As far as burnout goes, I put it in an oven at 500 for a few hours, then into the forge as the bronze came up to temp. A minute or so before the pour, I took it out and let it cool a little. I think I need to be a little more patient and let everything soak longer. This time, I will use a different crucible (graphite) that is larger and will retain heat better. Unless of course graphite crucibles are no good for bronze!

 

Thanks for the input

 

John

Not all those who wander are lost. -J.R.R. Tolkien

-Shards of the Dark Age- my blog
-Nine Worlds Workshop-
-Last Apocalypse Forge-

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John, are you using a centrifugal, vacuum or steam caster for such a small casting? If not, then you probably have more than temperature problems. Molten bronze in small quantities has a high surface tension (notice it forms a high curved surface when you melt a small blob), so it will not flow well into tiny areas without some sort of pressure behind it. As Jerrod alludes to, a 30 pound pour creates its' own pressure thanks to gravity, but a few ounces won't without encouragement. Anything that slows down the flow will allow the metal to harden and choke off unfilled areas. So, really large spruces and ample venting if you are doing just gravity pours.

 

I've found I'm happier carving metal by hand than when I used to do centrifugal lost wax casting. By the time I created the wax, sprued and vented the wax, invested the wax, vibrated the mold (I didn't have vacuum), burned out the mold, melted the metal, cringed as I let the caster spin, quenched, cleaned off the investment, cut off the sprues, carved and chased away the problems, dealt with the inevitable shrinkage, and then reaccomplished the above steps for the occasional and inevitable failure, carving by hand is actually faster for a one-off piece.

 

And, I hate to say, 500 F burnout isn't even close to the temps you need.

 

Good luck,

 

Tom

Edited by tsterling
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Thanks Tom, I think you are right about the time in this case. I am just using gravity, but might be able to improvise some sort of centrifugal device if worst comes to it. The pieces are, with the exception of a few spacers, large enough that I (hope) will be able to overcome the surface tension problem. I noticed that when I was experimenting with green sand, and had similar issues. Also, the true burnout is happening around 1900, after the majority has melted out, but evidently not for long enough that all the remnants have been cleared out.

 

thanks again,

 

John

Not all those who wander are lost. -J.R.R. Tolkien

-Shards of the Dark Age- my blog
-Nine Worlds Workshop-
-Last Apocalypse Forge-

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Thanks for that Dan, I've never heard of steam casting before and looks like something that would not be too difficult to try. I'll try that out on the spacers and see how it turns out!

Not all those who wander are lost. -J.R.R. Tolkien

-Shards of the Dark Age- my blog
-Nine Worlds Workshop-
-Last Apocalypse Forge-

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Bronze melts @1750 or there about depending on the 'metal formula' of the bronze. If you are casting with a 'flask temperature' of 500 F, the metal will cool to quickly, resulting in an incomplete fill like your casting. Here is some information I copied fron another site:

 

TRUBRONZE - yellow base metal alloy

TruBronze is a non-precious alloy that was developed to make models, sample line jewelry or finished costume jewelry. TruBronze has the look of 14K yellow gold which will show your jewelry well without the cost of karat gold. This casting alloy can resist tarnishing for long periods of time in sample cases but may tarnish when worn against the skin. TruBronze does not need to be gold plated and can be directly rhodium plated without the need for pre-nickel plating.

Recommended Uses: Tru-Bronze is an excellent casting alloy for non-precious applications.

PRODUCT FORMS AVAILABLE: Casting grain Quick Order - allow 3 days for shipping PROPERTIES: Melting Range 1795° - 1845°F (980°- 1005°C) Specific Gravity 8.64 Density 4.55 Troy ozs/in³ Hardness 130HV annealed PROCESSING RECOMMENDATIONS: Investment Casting

TruBronze can be cast using standard gypsum bonded investments. Recommended casting temperature: 1895° - 1945°F (1035° - 1065°C). Recommended flask temperature: 1020° - 1200°F (550° - 650°C). Quench time: minimum 5 minutes. Temperatures may differ from these recommendations depending on processing considerations.

Castings can be pickled using sodium bisulfate solution. Never use ammonia based solutions with TruBronze. Hoover & Strong recommends using a minimum of 50% fresh metal with each cast to avoid brittle castings.

Soldering Standard soldering techniques may be used with Tru-Bronze. Results can only be guaranteed when using H&S recommended solders. Recommended Solders: First solder: N-14 solder (1490° - 1520°F), second solder: APY solder (1110° - 1290°F)

 

As is illustreated here, the flask temperature should be at least 1020/1200 F. Lower than that and there will be an incomplet fill, as you experienced.

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Thanks for the info, it seems that the brass I was using is similar to what you mentioned. The casting temp of the grain I used as 1905(F), but I think I missed it by 100 degrees or so. The flask was somewhere below that, but was in the furnace while the bronze melted. At the peak, I would estimate it reached somewhere around 1700 degrees, cooling in still air while I waited for the bronze to heat a little more.

 

John

Not all those who wander are lost. -J.R.R. Tolkien

-Shards of the Dark Age- my blog
-Nine Worlds Workshop-
-Last Apocalypse Forge-

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I was going to recommend the steam casting too. This is a really good first move if gravity doesn't work because it is so cheap and easy. Just make sure that the steam pushes the metal in and you have a vent that lets the air out. My first casting experience (first 4 actually) were failures due to poor venting. I thought my ceramic was porous enough, it wasn't. Also make sure your sprue is as big as possible. Think about drinking a milkshake through a straw, easier with a wider straw than smaller. You just want it to be a bit smaller in cross sectional area at the in-gate (where it touches the part) than the part so it solidifies there first or you will get a shrink from your sprue into your part.

 

Standard rule of thumb for vents in castings (all media - sand, investment, etc.): Vent, vent, and vent some more. If you think you have enough vents, double it and maybe you'll be OK.

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After an interesting afternoon, got half way there. Some of the old bronze became a spongy crust and clogged the opening. The good news is now I know what not to do next time, and with 2 more molds curing I'll have a good cast tomorrow thanks to all the advice.

 

John

image.jpg

Not all those who wander are lost. -J.R.R. Tolkien

-Shards of the Dark Age- my blog
-Nine Worlds Workshop-
-Last Apocalypse Forge-

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If you insist :)

 

The mold was prepared a little different, not having a vent go from the bottom around and back out to the top, and the wax was about a centimeter closer to the surface. Because of that, the channel to the void was considerably larger. However, part of the bridge where the tang hole would be broke off, and that was on the half that didn't cast. Collectively, I heated the bronze and mold about twice as long. Also, I used a graphite crucible rather than a shallow clay one. Last, I tried the steam casting, but it was inconclusive whether that helped because of the partial cast.

 

John

Not all those who wander are lost. -J.R.R. Tolkien

-Shards of the Dark Age- my blog
-Nine Worlds Workshop-
-Last Apocalypse Forge-

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Have you thought of adding a bit of flux to help? The break could be from impurities filling that part of the mould, I think flux would help especialy if you are reusing material...

As they say, 3rd time lucky!

To become old and wise... You first have to survive being young and foolish! ;) Ikisu.blogsot.com. Email; milesikisu@gmail.com mobile: +27784653651

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I did use a little flux, but I think I could use more B)

Yesterday I did the burnout but ran out of propane, so the pour got pushed back to today. More flux it is! (I was worried about the flux eating through the graphite crucible, but after looking at it, there was virtually no difference between the inside surface and outside, so it looks like I'm safe)

Not all those who wander are lost. -J.R.R. Tolkien

-Shards of the Dark Age- my blog
-Nine Worlds Workshop-
-Last Apocalypse Forge-

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I havn't done any casting for about 25 years, so I'm a touch rusty, if you post a pic of what it is you're casting, I may be able to help a bit, I used to cast carved chess pieces, so know a bit about handling undercuts, vents etc...hope you get it right! I'm planing a run of knives for a company as corporate gifts, so may get back to casting soon!

To become old and wise... You first have to survive being young and foolish! ;) Ikisu.blogsot.com. Email; milesikisu@gmail.com mobile: +27784653651

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Here's a picture of the original waxes (purple), the copies (pink on the blade) and the silicon molds I used to duplicate them. A chess set is something I want to try out, I'd love to see see some of yours!

image.jpg

Not all those who wander are lost. -J.R.R. Tolkien

-Shards of the Dark Age- my blog
-Nine Worlds Workshop-
-Last Apocalypse Forge-

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Ah! Every piece has a flat section, so instead of a sprue, you could make the mold with the 'flat' as the sprue...just make it +- 1/4 inch taller and file down to size afterwards, this will prevent trapped air problems...

I don't have any pics of the chess sets, but the theme was 'African' castles-thatched huts, knights- spear wielding Zulu wariors, bishops-sangomas (witch-doctors) etc...tourists loved them!

To become old and wise... You first have to survive being young and foolish! ;) Ikisu.blogsot.com. Email; milesikisu@gmail.com mobile: +27784653651

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That sounds awesome! Neat theme, wish I could have seen it!

That's a great idea for orientation, didn't think of that. For the next pieces I'll definitely give that a try!

Not all those who wander are lost. -J.R.R. Tolkien

-Shards of the Dark Age- my blog
-Nine Worlds Workshop-
-Last Apocalypse Forge-

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I used to put an orbital sander on the bench, no paper, and just let it vibrate to help prevent bubbles when pouring, seemed to shake the medium into all corners and such, this also helps whenn making the mould, shakes the material into holes and other detail, like the eyes on the figurines, and is easier than setting up a vacuum chamber!

To become old and wise... You first have to survive being young and foolish! ;) Ikisu.blogsot.com. Email; milesikisu@gmail.com mobile: +27784653651

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That's ingenious, way better than the bad vacuum chamber I built from recycling ;)

How long should I let the plaster dry before firing? This last round sat for a day and felt solid, but when I first heated it, water began to rise out. Not a lot, but visible amounts. I don't think it did that when I preheated to 500 for a few hours on the toaster oven but I want watching that closely. I would be inclined to think I did not mix it right, but it was thicker than the previous ones so I don't think that's the case.

 

John

Not all those who wander are lost. -J.R.R. Tolkien

-Shards of the Dark Age- my blog
-Nine Worlds Workshop-
-Last Apocalypse Forge-

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