Muses About A Low Mass Electric Furnace

Too funny. Just in case I'm gonna give it 8kw!

You don't know how true that analogy is. That pumpable ceramic fiber spreads like frosting. The moldable is more like putty or a loose cookie dough and dries much faster. I hope the stuff is all it's cracked up to be because the build is heavily invested in it.

Best,
Kelly

 

Still plugging away at it.

Built up the last 1 ½” of the basin diameter. I decided to use my electric furnace to dry it out. Just sat it on top, and inserted some ceramic fiber and thermocouple in the drain hole. Set to 275F. I think I’ll be able to cure the furnace body in the same manner.



The lid now has hot face on all surfaces. It probably didn’t need it on the top side but it makes it more structurally sound.



…and the furnace body flanges are over a ½” thick now….getting close.



…..and coming soon to a theater near you…....why should you guys get to have all the fun?





Best,
Kelly

 

It's what Campbell calls slimmed streamline for improved metal yield. Puhakka prefers reverse delta...which is very strange, and definitely not square, but for high aspect ratio sprues (which I don't have). Campbell, Marquardt, Puhakka.......I think all the subtle differences may be a case of picking the fly sh!t out f the pepper. It's an offset pouring cup with a volume that's proven to work on many, many, of my lost foam pours.

Best,
Kelly

 

Feels like we’re getting somewhere now boys. Refractory work on the lid is done.




Furnace basin refractory work is done too.



Waiting for the furnace body to dry and will put the finishing touches on it.

I was able to pop the foam plug out of the pouring basin intact with compressed air.



Planning on filling the loose sand mold to within ¼” of the top of the foam sprue and then just placing the pouring basin on the protruding sprue, then fill mold near the top of the pouring well.



My intent is to reuse the pouring basin, remove it from the loose sand and let the melt drain into the void it leaves. There is some draft on the basin wall too. We’ll see how that goes.

Best,
Kelly

 

I hear ya! me too.

Thanks OIF. Looking forward to using it.

True

It's that moldable ceramic fiber I used for hot face furnace on the furnace components in this thread. Non-wetting to aluminum, insulating.....specs here:

http://forums.thehomefoundry.org/in...w-mass-electric-furnace.607/page-2#post-12360

Best,
Kelly

 

Finally, time to demold the furnace body…..been a long time coming.



Removed the support structure so the only thing left attached to the hotface was the cardboard tube/plug.



The foam heating element profiles were hot melt glued to the plug. I didn’t want to risk damaging the grooves pulling them out with the cardboard tube so I placed it on top my electric furnace and set the temp to 170F for 30 minutes……..hot enough to reactivate the hot melt glue but not melt the foam or plastic barrier on the cardboard tube.



That worked like a champ. I can even reuse the cardboard tube someday if the deed ever needs to be done again.



Now, to get that foam profile out of there. I suppose I could have just set it back on top the furnace and turned up the wick, but it would melt, burn, stink, and make a mess. So, I just grabbed a carving gouge, drug it down the center of the foam profile, and pulled it out with needle nose pliers 2-3 inches at a time. Took about 10 minutes.



Then I dressed up the edges lightly with 220 grit. Sure looks the part…..better work!



So let’s see how we’re doing on weight. I'm pleased with 8.45lbs for the hot face structure. I figure by volume another 5.5lbs of ceramic fiber to complete it so let's call it 14lbs total for the furnace body.



The lid weighs in at 7.25 lbs….surprised by that one…..more than I had estimated.



…and the basin weighs in at 15.4lbs. The firebrick pedestal with the dense refractory cap added a few lbs.



So 14 + 7.25 + 15.4 + 2.4 (plinth) = 39lbs. That’s a lot more than my original estimate of 26lbs. I think the biggest difference is I used more moldable ceramic fiber than originally estimated for more structural integrity. There may also be some weight gain from the colloidal silica ridgidizer, and possibly some more water yet to be cooked off since it has not been fired. The materials are such good insulators, a good percentage of the total mass may never see appreciable temperature rise.

There’s going to be some shrink when fully fired. Hopefully it won’t pull it out of shape. I’ll let it slow cook at 500F for a day or so.



Best,
Kelly

 

Thanks fellas. I was pleased with the low mass of the furnace body hot face especially since I used about twice as much material as originally intended to make it a self-supporting structure. The furnace body is about 75% of the internal surface area of the furnace. There's a fair percentage of the overall mass in the basin mass but very little surface area, so I suspect that works in my favor.

Already thinking about things I may do differently. I don't think I would rigidize the wool if I did it again. Very expensive, difficult to dry, probably (small) added mass, not needed structurally if a little more moldable fiber used in the hot face.

Thanks ESC, I'll fire the pouring basin prior to use. I may have to occasionally repeat that but sort of doubt it since its used in transfer troughs. I'll be interested in its hot strength and how "non-wetting" it actually is.

The other thing that is kind of neat about this moldable ceramic fiber material, it's such a good insulator I can handle parts soaked at 500F with bare hands. I should lose less heat compared to the pouring cup since I don't have to raise the temp of the steel cup and it will be strongly insulated from heat loss to the sand.

I was going to cast today. Woke up to 4" of wet snow. Bummer

Best,
Kelly

 

Cut and fit the wool for the first layer.



Then finished the job. It’s 2 ½” of fiber blanket. No rigidizer. It will get a light gauge steel skin to hold everything together.



I figure I’ll run it up to temperature before I fit the heating elements. I also want to see how much shrinkage occurs before I make the sheet metal skin. The spec sheet says virtually no shrink up to 1500F, 3% @2000F, and 6.7% at 2300F. I don’t think it will ever see 2300F except possibly in the immediate vicinity of the heating elements. It does produce a fairly strong odor, so I'll just be popping in and out of the shop every now and then to see how it’s doing.



Best,
Kelly

 

Too many projects started so I need to finish up a few, such as this one.

I wound up and stretched the resistive heating elements. I won’t repeat the coil winding description or specs but all that was previously posted here for any who may be interested.

http://forums.thehomefoundry.org/index.php?threads/furnace-fail-reconstruction.209/page-3#post-3660

This time I tripled up the extension leads. This is done to reduce the resistance in order to reduce the wire temperature from ~2300F to <500F in about 3” of length from the furnace interior to the outer wall. Here’s what they look like before they are stretched ~3x+ close wound length.



I also made these 90 degree lugs from 300 series stainless to make the transition to the high temp MG conductor wire (MG = nickel clad copper with fiberglass wrap over mica insulation, 850F continuous). They bind the resistive wire and MG conductor with set screws. A typical kiln would extend the resistive wires further outside the wall of the furnace to achieve more thermal isolation but since I package the refractory structures as slip-in furnace modules, I route the high temp wire around the very outer diameter of the insulation and then out the Tuyere hole to connect to the furnace controller via conventional conductor cable. I fished this thick wall stainless scrap out of the dumpster at work. Not sure of the alloy but man was it hard to work. The set screws are only 8-32. Even with lot’s of thread cutting fluid, it took me an hour to tap 8 holes, 1/8 turn in, 1 ½ out, in constant fear of snapping a low grade tap…...



Made the sheet metal skins for the furnace body. For the most part that was just rolling up some sheet metal hoops on my slip roll.



Here it is assembled. The furnace body skins get a mouse hole for the high temp conductor exit. The whole thing just swaps in as a module into my furnace lift.




As with my previous dense refractory lid, the low mass lid gets a sheet metal clamp around the perimeter with a couple weld nuts that are a direct swap to the existing lid lifter on my current rig.



I need to make a sheet metal skin for the base but other than that, the moment of truth for the low mass furnace is near.

Best,
Kelly

 

Looks great!
When your done with that can you come help finish up my 500 started projects too....

 

Thanks fellas.

I wouldn't do that to ya David. I know you become uncomfortable without a full backlog of projects.

I did but a tap drill did give it's life for the cause.

Best,
Kelly