Furnace Fail & Reconstruction

This morning I put that 200w light bulb in there and it was a little too much. Within an hour it was at 270F and started to char the internal MDF support ring so I pulled it and put a 75w bulb in there and it settled in around 190F which is where I’ll leave it until tomorrow.



I decided a new furnace body deserved a new lid. The old one is IFB and probably just a matter of time before it follows the body into failure. My over arm router really is a great tool for doing things like this and knocked it out very quickly. It’s a completely expendable mold. I just hot melt glued all the pieces in in place. The filament tape on the segments are pull handles for extracting/demolding the foam.



Instead of spending all the time mummifying the foam with packing tape, I wiped the foam down with Carnuba wax. If it doesn’t release, I’ll burn it out. The opening is 3 ½” so I can get my degassing lance through the lid. I’ll just make IFB plugs for use with the pyrometer and lance.



Here it is mounted to the vibe table, just shovel in the refractory, hit vibe, and watch it flow and fill like magic….I should have video’d it. That’s the bottom of the lid showing in the opening. Positioning it that way made it very easy to feed and fill it with refractory. The walls are ½” thick. It is 12 lbs of dense refractory.



I’m looking forward to demolding them.

-Happy New Year to all.

Best,
Kelly

 

Demolded it this morning.



That excursion to 270F melted some of the foam in the element grooves. Fortunately, the refractory had cured enough to hold its shape. I was able to remove the foam in the groove by pulling a screw driver through the groove. While it was still somewhat green, I dressed the edges with abrasive cloth and used a masonry bit to drill the four holes through the wall at the end of each groove for the heating element connections.







It’s clean enough I think I can just bake it out inside with electric heat. It’s -15F here this morning and that isn’t wind chill boys, it’s ambient temp! I may demold the lid tonight.

Best,
Kelly

 

I was more nervous de-molding it. Always a relief to see a successful cast. Drilling wasn't too bad. Only .125" diameter and I held a board on the backside so it didn't break out.

Which model are you thinking? Make sure you have enough compressed air to run the one you pick. The GT25 can do a good job on 50-60lbs of mass. You get more action out of them if you have a spring mounted vibe table. A 5 gallon bucket full of sand is about that weight so that works in my lost foam rig too. I'm sure you could dial the air consumption back a bit with lower pressure at the expense of a lower frequency vibe.

Best,
Kelly

 

Here ya-go



I cobbled together a heating element from the old coils and made this improvised heating coil to cure the refractory with. Should be about 4kw and run on one of the two control circuits in my rig. Back to real life work tomorrow. Pace of progress will slow a bit.


Best,
Kelly

 

Mixed that batch of refractory about the same as previous....between 5&6% H2O. I tend to get those bug holes on the underside of lateral surfaces/overhangs. Blind pockets are worse. If you partially fill the mold and tip it while under vibration buoyancy tends to move the air bubbles and improve it but you still get some. This was a shallow mold so tougher to do that on this one. You could probably bend a wire and pull it around under the ledge but didn't want to snag the foam.

It's worse when the mold surface is foam because that stuff just doesn't wet as easily and seems to have a lot of surface tension. Venting and the equivalent of risering to get some flow through the area might help. It's cosmetic and compared to the ease of just shoveling in the refractory and vibing it into place in complex shapes....I'll take it. -The best molds are open face if you can design them that way. If you look at the web pockets the finish and detail are pretty much the quality of the mold surface finish.

Yes, to reduce weight and thermal mass. I'll stuff the lid cavities with wool. Wanted to keep the weight increase down so the counter weight in my lid lifting mechanism was still in range. It will take longer to come to temperature with the dense refractory compared to the IFB...how much longer....dunno.

Scroll down to the very bottom of the post on this link to my furnace build and have a look at the lid on my larger furnace. Now that's a shape.

http://www.alloyavenue.com/vb/showt...nace-Build-Log&p=187792&viewfull=1#post187792

Best,
Kelly

 

Thanks OCD. No needles but the refractory does have ceramic fibers. Just gonna have to see how it stands heat cycles. I'm gonna have to try needles some day. Seems like they may inhibit castable refractory flow a bit in a tight thin-walled mold like this one but believe their place is really in hearths and lower temperature sections of furnaces as opposed to hot face, and personally, I would opt for ceramic fiber reinforcement over stainless steel.

Best,
K

 

The refractory sat all week with the 200w light bulb in inside the furnace body. It wasn’t that I thought it needed it, more a matter of why not? Better some heat than none. I was pretty amazed that it managed an internal temperature of 280F and I suspect the wall close to the bulb was even hotter. It was hot enough to make the refractory sweat some water.



I made some plugs for the lid. One is for the lance, the second the pyrometer, and the third just for cooking.



I lined it with wool to get heat into it and put the sheet metal skin back on.





Back into my rig it went. I’m following the ramp schedule posted earlier in the thread, with a PiD controlled electric, why not?





It has that improvised coil posted earlier in the thread in there providing the heat. I’ll wind a couple new heating elements for it tomorrow for it tomorrow. Not sure I’ll be able to complete the cure cycle and get it to cool off in time to install them but we’ll see how it goes. There must have been a little polystyrene left in the grooves. The odor ran me out of my shop. Seems to have been just as it went through 500F. If not I may need to move it outside. I need to make a new bracket to attach the lid to the lifting mechanism. That’ll give me something productive to do while the refractory is cooking.

Best,
Kelly

 

Well it sat all night at 1150F. I had to fix the coil early this morning as the coil failed and also once yesterday so I patched it with a splice from the remnants. However, I was able to run it up to 1900F at 100F/hr today where it sat for a couple hours and then tripped the breaker. I left it shut down because I suspect the insulation on the MG appliance wire either failed in short or the resistance became so high it exceeded the breaker. I just had that IFB coil brick sitting on some wool in the bottom of the furnace and the wire ran out through the drain hole. It got the job done.

Here’s a shot I took of the controller before the breaker went. 242vac @ 18a = 4.7kw. The 44.5 kw/hr is power consumed during the cure cycle. Which was about 26hrs so the duty cycle was about 36%. With the IFB it was about 25%.



I won’t be able to get it cooled down enough to make and install the new heating coils so I think I’ll just let it cool off slowly and in the meantime, complete the new lid lift hardware, wind the new coils, and call it a day.



Best,
Kelly

 

Thanks Pat, not quite there yet.

Pretty much finished up adapting the new lid to the lift this afternoon though. I like this arrangement better than the old lid and interface with the lift. Everything is out in the open with no obstruction and I have an access hole to see what's going on inside, take metal temp, and degas, without lifting the lid. I need to make a sheet metal hat for it to cover the wool and paint it and it'll be done. In the evenings this week I’ll have to see if I can disassemble the furnace body and install the new heating elements. After that it should be ready to go and I’ll be back in business. At some point I’ll re-do the base and replace the IFB with castable but that may need to wait.




Best,
Kelly