Forge Sintering Bronze Day 1:
I’m in the shop today working on some new ways to sinter parts that have already been debound. We call them ‘brown’ at this stage. I’m working with bronze today.
Our normal debind recipe for all of our materials is to:
- Ramp at 150f per hour to 900f
- Hold for 3 hours
- Shut off and let cool to ambient
We typically run this cycle over night, Debind one night, Sinter the next. This makes the full cycle take 48 hours. This is our published recipe and will give you a good Copper or Bronze part.
My problem with this recipe is that over 50% of this time is spent either waiting for the kiln to either warm up or cool down.
In the world of Precious Metal Clay there is a common technique where bronze (specifically) is sintered with a propane torch. This method will also work with Silver (per its nobility), but we don’t have that in our store yet.
So today I’m hacking my way through a way to dramatically abbreviate the Sintering cycle for Filamet™ Bronze. Please note, these parts were Debound in a kiln using our standard recipe. This is the second day so I have some Brown Bronze parts prepped for this experiment.
This is my initial test bench setup. I’ll add more to this post as I build out the test rig. The clock is ticking rapidly today, so this might be a multi-day test. But I’ll just keep adding this post as I go.
See Where I’m going?
This is the link where I bought the big-ass burners. They came with a kit called the Mr. Volcano. I wound up buying the whole kit mostly to get the burners, but at $135 there was no cheaper way to get even half these parts (and it’s made in U.S.A.). At some point I will go back and actually sinter in this nifty little forge. But part of my motivation today is to collect data and gain experience working with this equipment.
Any prior experience I may have with propane burners is fixing gas grills. And while this isn’t a lot different, I still like to go slow when starting out. The inside of the crucible will need to hit about 1650f in order to sinter Bronze.
The forge was in pristine shape when received. It’s been kicking around the shop for a few months.
Dry Fitting the Gas Lines:
A view from the top: Here you can see what I’m thinking. It’s an Alumina crucible sitting between 2 large propane burners. I can’t find data on how many btu’s these are. I’m going to put several thermocouples into the crucible from the top. Some near the edge and at least 1 in contact with the part.
When starting the Sinter cycle, it’s much less important to ramp up to temperature slowly. This slow ramp is imperative during the debind (too fast boils the binder), but for sintering it’s just a matter of not shocking the metal so hard that it cracks. I will use gas flow slow the ramp rate somewhat, but I’m not expecting this to be a big problem.
Basic Burner Setup/Layout
I’ve decided to split this into a 2 day project. The following is what I guess I’ll call ‘first heat’. There are still some bugs to tweak, but this is a solid days work.
Tomorrow I’ll add in the temperature sensing equipment and we’ll see how how we can get that crucible sitting in there. I’ve filled the crucible with ‘Pearlite”. Not for any particular reason other than that I’m always on the lookout for easily accessible refractory materials. Since you can buy this stuff just about everywhere, I hove it works out.
First Heat with both burners on: The first thing I noticed was that the crucible had to be in place in order to tune the burners. Getting them both running at a stochiometric balance was a minor challenge, probably took me a half hour or so. What they wanted to do was burn inside of the burner tube. There must be a word for this, but I think of it as “Pulse Jetting”. When running rich or lean it seamed equally likely to ignite inside the tube with a steady frequency pulse.
First Video with both burners running.
The ceramic fiber blanket around the rim is just a sort of gasket. I’m out of fire brick, so had to use this broken piece. I’ll scavenge up some more for tomorrows run at full-speed.
Busted out the Thermal Imager: It looks like most of the heat is staying where I want it. I think this is going to wind up being a battle over adding heat more quickly that it’s able to escape.
I better calibrate the Flir before my next run. Images aren’t lining up.
Does anyone want to guess how hot I can get the inside of that crucible? I’ve easily hit 1600f with one burner, so I’m certain I can go higher than that. I’ve seen other people melt iron in small crucible furnaces that burn used motor oil and things like that. I’m not sure if straight propane can get as hot.
To consider this experiment a success, I’m really only hoping to hit a bit over 2000f. If I can hit 2500f that will be a bonus, I’ll be able to sinter Inconel and most of the other Super Allows and Nickel Alloys.
So, check back tomorrow for the next phase of testing. This is gonna be cool.
Forge Sintering Bronze, Day 2.
Let’s get right to it!
Flew past 1700f in about 20 minutes.
Went past 2000f at about 30 minutes.
Cycle peaked out at just over 2200f:
I probably could have kept going but decided to shut down, check for damage and set up for an actual sintering run. For bronze I’ll need to keep the temperature at around 1550 to 1600f.
For anyone that’s curious. Pearlite from the garden store starts to melt at around 2050f and shrivels up by 2200f. This means it’s actually useful for sintering Bronze or Copper Filamet™. Will likely try this later.
So, it looks like all of this is pretty well documented by others that have done similar experiments. The temperatures, not the sintering part. Theoretical max is around 2400f, i’m close enough to that number that I’m happy with my test rig here. Link here.
Now, Let’s Sinter Something:
This is the coin sample i sintered. I’ll give it about a B-, which is a resounding success for a first attempt. Here are some photo’s. I broke and ground the piece so I could see the inside.
Bronze part in the Forge-Like setup.
Total Sinter time for this part was 15 minutes. Which is likely some sort of record for 3d printed metal.
This is about halfway through the 15 minute sinter cycle.
The piece is dense enough, roughly 95% solid (5% air)
The gnarly texture was there when I started.
It was in the debound brown part. This was defective, and why I set it aside to experiment on. I now regret this, I should have started with a good part.
Generally speaking. I’m calling this a success. The metal quality is good, density is good and I was able to create this more quickly than anything I’ve ever made using this method.
I would love to hear any feedback, comments, improvements etc.
Democratizing Metal 3D Printing has been my passion since I started this project about 7 years ago. With the help of our users we’ve made tremendous progress. The Virtual Foundry has found excellent traction in high-education, research and R&D labs all over the world.
More recently we’ve started gaining speed with people working in home shops, basements, garage labs, etc. This is a really cool and welcome bit of progress. In fact, some home users are taking this concept to the next level with things like Microwave Sintering. And I’m talking about a standard, common microwave. People are just buying them at Wal Mart, and sintering pure metal parts. That’s amazing!!! If curious, here’s Mr Highball. He’s been working on microwave sintering for a few months. There are others building on his research.
Here’s one more of the whole piece, not broken:
This is still rough. I’ll tweak the printing parameters so I can get the text to look nicer. So, perfect specimen to come, but this will prove the concept for now.