Muses About A Low Mass Electric Furnace

Discussion in 'Furnaces and their construction' started by Al2O3, Jan 22, 2019.

  1. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Foam doesn't do well with radiative heat sources. It has so little mass and is such a poor conductor that once it gets a good look at a bright red heating element, it's all over. A similar thing happens when heat treating aluminum in a kiln or electric furnace. To combat this I shield the castings with an open top metal container lined with 1/4" of ceramic paper. Then, it works very well. Without, attempts at T6 will produce localized melting.

    As you say, with the wet white coating, there'd be no line of sight for radiative heat transfer to the foam, I doubt the problem would exist to that degree but still, no good way to control it and I couldn't get comfortable with the idea of leaving it unattended.

    I think the foam will withstand 140F but all the commercial lost foam drying room temps cite ~110F as the drying temp, apparently related to foam strength and preventing warpage from coating shrinkage.

    Best,
    Kelly
     
  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I added two more coats of moldable/pumpable fiber, let it dry for 4-5 days between coats, and now have about ½” of thickness built onto the entire plug. It feels very solid and should be a low mass, relatively strong hot face, and a good foundation from which to start building the outward diameter with 2 ½” of rigidized ceramic fiber. The moldable ceramic fiber carves and sands easily. Not much invested working time, but plenty of calendar time for drying. I started on some of the pieces to construct the basin and lid. I'll post those up when I get them a little farther along.

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    Best,
    Kelly
     
    _Jason, Tobho Mott and oldironfarmer like this.
  3. Looking good!
     
    Last edited: Feb 23, 2019
  4. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    You're right. I'll fix that. I use so much of it I've got foam on the brain.

    Best,
    K
     
  5. Melterskelter

    Melterskelter Gold Banner Member

    Very interesting tutorial on the use of a product new to me. Though it’s service temp is probably too low for iron furnaces even if protected with Satanite (or maybe not!), it opens up new worlds for hot face fabrication. I plan to use it in a planned electric iron heat-treat oven as that use will be well under its rated 2300F service range.

    How expensive is it?

    Great work,

    Denis
     
  6. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    While the layers of the furnace body were drying, I started building the rest of the pieces. What would a low mass furnace be without a low mass plinth? A figured a 6” diameter by 3” thick dense refractory plinth would be about 7.5lbs by itself. After all this effort to get mass out of the furnace, that didn’t make much sense, so I made a mold and cast this plinth base from Greenlite 45L insulating castable refractory which is about half the density of dense castable but still reasonably good strength. This part weighs 1.2lb. I added a ½” thick cap of dense castable for durability. It also weighs 1.2lbs. I fired them separately because I thought they may have slightly different shrink rates. Then glued them together with a thin layer of mortar and fired them again.

    10 Low Mass Plinth.jpg

    I also started building the furnace basin and lid. For the basin, I thought the strength of a thin shell of moldable ceramic fiber and rigidized ceramic fiber might be a little suspect for supporting a full A20 so I made a 6” diameter pedestal out of K23 IFB, coated it with Satanite mortar for a little extra strength, and capped it with a ½” disc of dense refractory for added durability. I was able to re-use the same foam mold for both the dense refractory cap on the pedestal and the plinth and just added the small foam disc in the center to form the drain hole in the pedestal cap. I did include a drain hole, but figure I’ll just stuff loose ceramic fiber in it. Like the plinth, I fired the pieces separately then mortared them together, and fired them again.

    11 Basin Pedestal.jpg

    Firing the pedestal and plinth pieces.

    12 Cure Pedestal Plinth.JPG

    For the basin, I decided to start with the pedestal and build outward. So I made a foam plug the shape of the basin and applied paste wax for release agent.

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    Then coated it. Afterward, for some reason I was craving Angel Food Cake.

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    Best,
    Kelly
     
  7. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    As a starting point for the lid, I made a 3.5” diameter by 4” tall foam plug and applied a ½” of moldable ceramic fiber for the hot face in the vent hole. Since this is a resistive electric furnace it’s really just an access hole but I have a number of different plugs that install in that opening size for various accessories, so I stayed with the same diameter.

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    Since the lid is shaped like stepped hat, the rigidize wool gets wound around the foam plug in two different heights.

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    I cut strips of ½” ceramic fiber and dry fitted them around the plug.

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    Then the strips were painted with rigidizer and wrapped around the plug. Wow, did the wool drink a lot of rigidizer. Not sure how much but probably 1/3 of the gallon. That’s going to be a challenge to get dry without heat. I used a piece of SonaTube, and for release agent, taped the inside with packing tape and wiped it down with paste wax. Then clamped to compress the layers and get good contact, and used filament tape to hold it together.

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    I removed the foam plug to expose both sides of the ceramic fiber, then suspended it over a furnace vent inside a cardboard box. I’ll just have to wait and see how the drying progresses. The outer fiber diameter and hot face surfaces still need to be applied. Many more steps to come. -Stay tuned.

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    Best,
    Kelly
     
  8. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Thanks Denis. It's new to me too. I paid $136 for 5 gallons of the pumpable version, $57 for 1 gallon of the moldable version, and $68 for a gallon of rigidizer. They only had 1 gallon units of the moldable. Not sure why the unit cost varied between the two. I would have bought a 5 gal unit of moldable too if it was the same price as the pumpable. The rigidizer seems pretty expensive for what it is. The pumpable it wetter than the moldable. Drying times are long but only because I can't apply heat. The directions say it can be rapidly cured with high heat and immediately placed into service.

    I posted the data sheet in post #22.

    I don't know how it would fair behind a layer of Satanite in your iron duty furnace......about the same as ceramic wool I would guess. Should be fine at resistive electric furnace temperatures.

    Best,
    Kelly
     
  9. Great job on the new furnace. You're serious about low mass.:D

    It will be real interesting to see how short you can get your melt times. Energy to actually melt the aluminum will soon be the driver.

    Kind of off topic, but aluminum seems to melt faster when adding it to a molten puddle. That's deceptive, because the colder the crucible is the more BTU's it is absorbing from the furnace, for the same furnace temperature. So when I'm adding charge I try to push it and put in as much as I can as soon as I can to minimize melt times. I can't remember, do you add material to your crucible or put it all in before you lower the furnace?
     
  10. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Thanks Andy, hopefully that will be the pay off. I'm expecting much improvement in time to first melt, but not so in subsequent melts. In fact, subsequent melts could even suffer slightly since this material is highly insulating and low mass, I doubt this hot face surface will store and radiate heat like dense refractory. We'll see on that one.

    I charge the crucible as full as possible, taking care to pack in a manner the charge remains loose and unable to expand and crack the crucible. My primary reason for that, being electric, I have no good way to preheat added charge stock, so since that goes into the crucible at/near room temp, it really drops the temp of the melt and could potentially even freeze it, so I keep the charge small compared to the melt it is being added to. In the end, I don't think it makes much difference in time to first melt. In subsequent melts, I do think molten metal contact accelerates things a bit.

    I also stopped returning my crucible to the furnace after the last melt and just place it on a fire brick to cool. When I place them in the furnace, the aluminum skin on the crucible wall oxidizes and sticks tenaciously to the crucible making it difficult to start the next melt with a clean crucible. When it cools more rapidly, the foil on the crucible wall and heal easily pull out as a single piece leaving the crucible clean as a whistle.

    Best,
    Kelly
     
  11. JCSalomon

    JCSalomon Copper

    I’ve seen this suggested for putting castable refractory inside kaowool, but it might work for you: How about covering the dried hot-face with a layer of Saran Wrap before putting the foam around it? From what I’ve read, that should be a sufficient water barrier, and should entirely vaporize when you fire the furnace.
     
  12. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Good thought. So far, it looks like the concern is unwarranted and though not essential, I actually do want the wool to bond to the hot face. The wrapped rigidized wool layers did not seem to affect the cured moldable ceramic fiber hot face. However, I think getting the interior of the rigidized wool to fully dry is going to require heat. The exterior surfaces dried overnight enough to make the whole thing quite rigid, and when I removed the cardboard form it seemed quite structurally sound and further dried. Even so, I can tell by the weight that the interior still has water that needs to be driven off.

    This is quite the slow motion build because of required drying time between all the steps to build up the structures. It doesn't take long to do the work to complete a step but it takes days to dry. Almost feels like it needs a time elapsed video......and a more patient builder.

    If I apply heat, it would probably take no more than an hour to dry each build step. On the lid, the only thing stopping me from applying heat is the inner foam core. With the hot face and wool cured enough to be structurally sound, I'm going to remove the foam and fire it between steps. I have a furnace large enough to do the basin and lid, but not the entire furnace body......so I may slightly rethink the remaining build up process on the furnace body. Also, based upon the lid build, with the wool portion of the furnace body being 16" OD, 15" tall, and 2 1/2" wall, I think it would drink gallons of rigidizer. At $68/gal that is a bit of a deterrent, but if they layers were done all at once, that would be one heavy, very hard to dry mess of wet wool. I'm going to finish the lid and basin as planned, but think I have a better alternative dreamed up for the remainder of the furnace body.

    -Thanks for the post.

    Best,
    Kelly
     
  13. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    A little more progress tonight. Here’s the lid. Will build the step onto it next with rigidized wool.

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    ….and here’s the basin. Both the lid and the basin are built out to the diameter of the furnace bore now.

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    Into the drying box it goes. One step at a time.

    Best,
    Kelly
     
  14. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    More time elapse photography :)

    The step is on the lid.

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    Built out the diameter of the basin.

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    They're going to air dry for a few days in the drying box over the furnace vent and then get fired in my big furnace. The basin is now at 14" in outside diameter and needs to be built out to 16" but I can only get 14" in my furnace. I'll be thinking on what to do about that.

    Best,
    Kelly
     
  15. The last inch of wool with rigidizer really does not need to be fired, does it?
     
  16. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I'm saying fired but I'm not certain any of it except the maybe hot face really needs to be fired.......more like heated/cooked to dry it and achieve strength. The outer diameters will never reach any significant temperature in use so the only way that's going to happen is with heat/accelerated drying or (lots) of time. Good news is several hundred degrees F is probably good enough to drive off residual water and cure the rigidizer and moldable ceramic fiber so drying doesn't really need a furnace, just a warm box. The hot face will shrink a little more when fired and that could probably (and may) be done in use. I figure I should try to get it as dry as possible before applying hot face to the remaining surfaces.

    I'll have 2 gallons of rigidizer into the lid and basin when completed. I haven't calculate how much would be needed for the furnace body but I'm guessing at least 4 maybe 5 gallons....didn't plan on that. Drying that mass would be a challenge and at $68/gal, that isn't going to be practical so current plan is to build top and bottom flanges onto the furnace plug with the moldable ceramic fiber and make the hot face shell a self-supporting structure wrapped with dry ceramic fiber blanket, just like my present dense refractory furnace body. I already have enough material to do that from the original purchase. I think it will work fine. It'll just take more time.

    Best,
    Kelly
     
  17. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Applied more hot face to the lid. I had to control the step diameters to fit the furnace bore and the lid lifter ring on my furnace rig so I tacked together a troweling guide.

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    Started building the flanges onto the furnace body.

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    Best,
    Kelly
     
  18. I generally think of you as a heavyweight, Kelly, but you're going kind of light on this project.:D
     
  19. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I feel like I'm building this thing one atom at a time........45 minutes of work, 3 days of drying......then do it again.

    It's killing me, like watching refractory dry!

    K
     
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  20. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Got the basin dried out and pulled the foam plug. It still needs to increase 1 1/2” in diameter but this is the biggest I can get in my furnace.

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    With the plinth

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    Pulled the plug on the lid and applied some more hot face.

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    Into the furnace they went.

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    Also applied another layer to the furnace body flanges.

    Best,
    Kelly
     

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