Cast-Iron Furnace Build Using Partially 3D-Printed Molds

Discussion in 'Furnaces and their construction' started by k.kuhn, Dec 21, 2020.

  1. k.kuhn

    k.kuhn Silver

    This is my first post on this forum, but I've gotten a lot of good advice and information from what others have posted. So, I've decided to post a build log of making my own furnace to add my share of experiences to the hive mind.

    This build is inspired by PatJ's thin hotface furnace thread found here: http://forums.thehomefoundry.org/in...er-keg-drum-size-furnace-for-melting-iron.26/

    The furnace will be run on a diesel burner using a Delevan style atomizing nozzle with a propane pilot flame for easy lighting and re-lighting if necessary. With this pilot flame it is possible to start the furnace with diesel and simply adjust the fuel and air as it heats up (eventually you can turn off the propane too once the interior is hot enough for spontaneous reignition). I've already been using this burner in my forge with great results. I decided to go with diesel because there is a gas station on the end of my street, which adds immense convenience. If I can find some pictures of building the nozzle I'll make another post with some more info, but the design is based on this very helpful Youtube video:


    I'm not innovating too much at all in terms of furnace design, but one thing I'm going to try that I haven't seen too many others doing is using a 3D printed mold for forming the tuyere. I'm using two sizes of cardboard concrete tubes for the main forms and a 3D printed form that is contoured perfectly to fit the outside of the tube for the tuyere and to provide a perfectly tangential entry for the burner. The furnace will be approximately sized for a #10 ProCast crucible, which I think will be large enough for anything I'll ever want to do. I'll attach some images to this post of my CAD models and will post updates as I proceed.

    The insulation will be at least 2" of ceramic wool, and then, since it is relatively cheap, some rockwool/mineral wool insulation around that. I've found that the mineral wool insulation (the high-temp green stuff for homes) works ok as extra layers of insulation on top of higher temperature insulation. The refractory I'm going to be using is Kast-O-Lite 30, which is a calculated choice for several reasons. First, it is slightly cheaper per volume than denser castables, but this comes at the cost of being less durable. An upside is that being less dense also sinks less heat. I am also planning to coat it with ITC-100. The combination of thin wall and low-density refractory will hopefully make for a fairly hot (and quick) furnace. In the end, I'm not going to be running this furnace too much so I think the tradeoff is worth it in my case.

    My thought is that it would be better to have the crucible not getting directly exposed to the diesel flame, so I'm planning to elevate it off the bottom of the furnace a bit more than I've seen in other designs to get it mostly above the burner.
    97ea72d53c5a1c75db06e6209c097152.png 7276a37dc82a544847a43ce0a7e48b27.png View attachment 14534
    Furnace floor is 1.5" thick so with 14" overall height that leaves 12.5" deep for the internal chamber. The ProCast #10 is 9" tall, burner tube is 2 3/8".

    Diameter of the inner chamber will be 9-13/16" and the crucible is 7-9/16" diameter. This leaves about 1-1/8" of space between the wall and the crucible at its widest point. From what I've read this might be pushing it a little, but I think it will work out.
    07844e8b041f954010bea28fff462337.png
    The 3D printable tuyere mold (translucent to make it easier to see how it is going to work):
    Screenshot 2020-12-21 001347.png
    The profile of the 3D printed part will be cut out of the concrete tube and then the mold will be glued, taped, or both in order to firmly attach it over the hole. This way I can cast the complex geometry of the tuyere with basically no effort if it works out. ;)
    Screenshot 2020-12-21 001836.png
    The way I'm planning to cast the main part will put the tuyere at the top, allowing me to hopefully pack it from the side before filling the forms all the way up to insure it fills properly.
    Screenshot 2020-12-21 001948.png
    Pretty awful surface finish and made the walls pretty thin to make it print faster. Had to print it in two parts because my printer only has ~5" of vertical travel.
    20201221_004859.jpg
    The aforementioned 3D printer; an ungodly conglomeration of leftover parts reconstituted into this semi-workable format.
    20201221_004934.jpg

    Edited to fix my stupid mistakes.
     
    Last edited: Dec 21, 2020
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  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Hi K and welcome to THF. It's all pretty standard practice accept you may find that gap to be on the small side for an oil burner. Probably ok for propane. If so, it can create difficulty tuning the burner. So you'd be well advised to increase the furnace bore or use a smaller crucible. I didn't watch the video but looks like a lot of burner for that size furnace. What's the size of the vent hole in your furnace lid?

    Best,
    Kelly
     
  3. Melterskelter

    Melterskelter Gold Banner Member

    Agree with Kelly’s comments. I’d suggest increasing the bore diameter 20% as a minimum. As presently drawn I don’t see how lifting tongs could be placed around the crucible for lift-out. I’d also suggest a taller plinth and bore by 20%. There is no advantage to minimizing furnace volume other than reducing raw material cost. On the other hand, I think the consensus is that increasing internal volume increases burn efficiency.

    If you are using a Delevan nozzle (a good idea I think), there is no need to increase complexity by adding propane for starting. Done correctly using 12 pounds compressed atomizing air, very low initial combustion air, and .15L/m fuel flow, the furnace will light easily with a burning fuel soaked single-sheet paper towel starter. Works first time every time without blowouts. Once the furnace is red inside, it will relight on its own by just turning on fuel flow and air. Use a nominal 1 L/m nozzle. Pressurize fuel to 10 pounds. FILTER your fuel as the last step in the fuel line using an unbroken connection to the nozzle. Don’t use excess combustion air.

    In practice, precise fabrication of furnace parts is far less important than proper scaling and material selection. Furnaces are brutish devices bent on self-destruction.

    Denis
     
    Last edited: Dec 21, 2020
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  4. DavidF

    DavidF Administrator Staff Member Banner Member

    As the others have said, the bore is too small for that big of a crucible...
     
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  5. Petee716

    Petee716 Gold Banner Member

    Congrats on a well planned furnace. The comments above are spot-on in my experience.
    Welcome to the forum.

    Pete
     
  6. Billy Elmore

    Billy Elmore Silver

    Its tight around the crucible but if you are building your own tongs you can make it work....just need a steady hand and good eye. I left just a little more than an inch and I can tell you it would have been easier to use a smaller cucible which I will most likely do.
     
  7. Jason

    Jason Gold

    Unless you are burning propane, that bore is way too tight. Oil burners like lots of breathing room. If you haven't bought your crucible yet, I highly suggest you buy the cart before the horse on this one. Having it on hand will really help when seeing the bigger picture. You'll need it anyways to build your lifting tools also.

    I'm not sure about not seeing people elevate the crucible that height out of the path of the flame. Maybe the asshats on youtube like to point flames at expensive crucibles. We here do not.
    That thing has a name. It's called a plinth. Either cast a plinth in the floor with the refractory on your build or do what most here do. Cast a few solid plinths of different heights out of refractory when you have the stuff mixed up. You'll find the one that works best at a later date.
     
  8. k.kuhn

    k.kuhn Silver

    Thanks for all the advice, I had a feeling that the crucible may be too big. I am planning to build my own crucible tongs and also eventually build a propane burner, just to experiment. I think I will proceed with building the furnace at this size, but per your recommendations I will opt for a smaller crucible. I've already basically finished the molds for this size, so most of the hard work is already done. I wouldn't complain about getting to build a bigger furnace in the future either. ;)

    I haven't designed the lid yet, but I was thinking that it would probably be a good idea to make a larger hole than I'd need and then covering it partly by preheating ingots or firebrick in operation. I was thinking about 5" in diameter.
    Additionally, I'd like some expert advice on what would be the best way to lift the lid. I really like the design where a lever is used to lift and swivel the lid out of the way, but it seems to me there really isn't a great way to attach the refractory to the lifting arm. Do most people imbed steel supports into the hotface for lifting points? This seems like it would cause cracking due to unequal thermal expansion. Or maybe a tension band around the the whole thing?

    Thanks,
    kk
     
  9. Jason

    Jason Gold

    5inches is PLENTY of exhaust hole size. 4 is more common.
     
  10. k.kuhn

    k.kuhn Silver

    Ok sounds good, thanks for the feedback.
     
  11. k.kuhn

    k.kuhn Silver

    I just realized I had made a mistake when designing the model: the diameter of the inner tube was actually 9-13/16 instead of 9-3/16. I knew it looked off somehow LOL. This means that the gap between the crucible and the wall is actually 1.125", which is a little better. Also the wall in actuality is a bit less thick 1 1/8" instead of ~1.5".

    51f5ec5175602e900f3e349f04daf3d5.png b6dd1871362e4c7d37aec1063bda36a7.png
     
    Last edited: Dec 21, 2020
  12. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    That's helpful.

    Have you ever worked with castable refractory before? If not mix a small batch and cast a plinth first. Your first impression will be the instruction result the in an impossibly dry mixture. It is not. Buy a mixing paddle they're cheap at the big box stores. Not a fancy one, a simple one like this:

    Mixer 1.jpg

    ....use a stout 1/2" drill and mix 10-15lb premeasured batches in a 5 gal bucket held between your feet. Then use vibration to place it. With vibe, it will flow like a viscous liquid and yield a solid refractory casting. You'll be thankful you did. Cast a plinth and you'll see what I mean.

    Best,
    Kelly
     
  13. k.kuhn

    k.kuhn Silver

    I've worked with Mizzou before when I built my forge, but for that build I didn't use a mold and simply formed it by hand. I will definitely be taking your suggestion to get a mixer, that seems like a convenient way to do it. I'm currently working on making a ball bearing pneumatic vibrator for use as a match plate vibrator, so maybe that will work for vibrating the refractory mold.
     
  14. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    The first time I used castable, it was a small batch, like 5lbs and I mixed it by hand. The next time I cast two successive 25lb batches mixed by hand and wow, what a work out that was, but I think I worked that stuff for 20-25 minutes for each batch. I bought that paddle and a 10-15lb batch takes about 2 minutes......and no real effort, and is very well mixed. The other reason for small batches is so the drill can handle it. If you premeasure the castable and water the smaller batches are no problem to do in succession with the paddle.
    Usually, you will want a match plate vibrator to be lower energy than what you would use for a massive batch of castable. Almost all castable refractory manufacturers recommend vibratory placement (unless it's a big job, then it is usually gunned).

    Hobbyist use two approaches for vibratory placement. Ramming is common too but doesn't work as well IMO. Make a wand you can insert into your mold. This has the advantage of being small, verstile, requires less vibe energy, but only makes the castable flow locally in the vicinity of the wand, and you still have spend more time packing the mold. This is similar to what you see concrete workers doing when they pour home concrete foundations.

    I prefer full mold vibration because you can literally just shovel in the refractory flip the switch for a few seconds, and it will magically flow into place. Here's thread with a small foam mold and vibrator. There is a video of the castable flowing into pace.

    http://forums.thehomefoundry.org/index.php?threads/degassing-lance.204/page-2#post-3870

    But, you need a much higher energy vibrator for molds that are 10s of lbs of refractory. Mine were 50-100lbs. I made a spring table to mount the mold. That is a method used by old time brick makers, and those that make concrete figurines, and allows the mold to be agitated with lower energy. Here is an example of that and if you look on the second page of the thread, you can see the results. Makes casting complex shapes a breeze.

    http://forums.thehomefoundry.org/index.php?threads/furnace-fail-reconstruction.209/#post-3287

    If you're an automotive guy, a few old valve springs work well for platform springs. I remove the springs and stor the two flat pieces against the wall, but it's not much work to reconstitute the spring table from scratch.

    Best,
    Kelly
     
  15. Melterskelter

    Melterskelter Gold Banner Member

    One simple “high energy” vibrator is my Makita impact driver engaging a screw(s) on strategic points on the mold. Plenty of power. Good frequency. Simple.

    Denis
     
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  16. Mach

    Mach Silver

    I used a HF vibratory tumbler screwed to plywood with an inner tube sandwiched between another sheet of plywood with a hole cut in it. This allowed the tumbler to hang free with weight supported by the inner tube. It worked well for me. Here's a time lapse of 3d printed plinth mold using the vibrating table. I did use 3d printing for my furnace forms. I was surprised to see layer lines show up in my furnace face so print with finer resolution if that matters to you.

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  17. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    That's a good one Mach!

    Best,
    Kelly
     
  18. OMM

    OMM Silver

    Keath does not give a very good demonstration of torch run inside a furnace. I'm not sure why, He does not show his lightup, stable flame, or lid closing. A lot of editing… Boo.

    In my opinion he is, another king of random.
     
    Last edited: Dec 22, 2020
  19. k.kuhn

    k.kuhn Silver

    The one I built using his method is running great, so maybe I'll upload a video to fill that particular void! I started watching his channel for his machine tool restoration videos, he seems to really know his stuff when it comes to old iron.
     
  20. Petee716

    Petee716 Gold Banner Member

    Pretty unfair assessment.

    I run the same burner minus the propane feed and turbulator. I burn diesel utilizing the siphon ability of the nozzle from an unpressurized bucket to preheat and then switch fuel lines to my pressurized waste oil tank when the furnace is hot. Somethimes I preheat with propane utilizing a separate simple burner that just dumps propane and air into the burner tube and then switch out the burners, but the first method is easier for me.

    Pete
     
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