Matt's Home Foundry Page June/2002

Goal was to build an electrical furnace capable of heating a #16 crucible to a temperature of 1300° C (@ 2400F). The design should be suitable for melting of aluminum alloys, brass and silicon bronze as well as the firing of ceramic bisque and glazes (firing cone #10). This means that a fast heat up of short duration (for metal work) and otherwise a long lasting steady ramp up/down over a 12hour time frame for ceramic work are required. The wall materials should have a high refractoriness and low thermal capacitance paired with shock resistance and mechanical stability.

 Chamber size 12"x12"x12"

 two layer refractory

  stainless steel outer shell 1/32" thickness

  electrical data:

  max. core temperature 1300° C (@ 2400F),

 other shell steady state ~250F (worst case calculated), steady state ~180F (hotspot measured), ~140F typical measured

 heater elements resting on Alumina rods to prevent heat transfer to core wall (no element grooves)

Front view of the primary and secondary liner. The primary core consists of a 6 plate self supporting Ceramic board structure (1/2" thick). 2" thick fiber blanket builds the secondary liner. The Alumina rods on left and right side will hold the elements and are anchored to the front/back boards using self cast Alumina blocks (Rescorä 780) for higher mechanical and thermal stability.

Back side view of the core structure. The enlarged picture (click on thumbnail picture) shows the element rod anchors glued into the primary liner. Generally, all ceramic connections as well as chamber coverings where made using Rescorä 901 ceramic adhesive and protective coating (3000F stability).

The structural skeleton. Build of 1/4" angle iron to connect the refractory core to the outer cold shell. The core slides into this frame and will be held in place by the stainless steel shell. The bottom shows additional support irons which have a ceramic counterpart on the core to carry the load of a full #16 crucible at mean temperature. Measurements showed those parts as the coldest spots, so the concern about load is relative.

Chamber view. The enlargement shows two of the four heater elements of Thyssenä Elastrochrom wire coils resting on Alumina rods. Each element has two sections of coiling to fit the chamber dimensions. Each element consumes 120V/15A. Both elements of each side are connected as line to 240V whereas the two sides are connected in parallel. The front board is covered by UltraTempä 390 ceramic tape (3200F) to tighten the front refractory to the furnace door. The very back shows a partial covering of the ceramic board with Rescorä 901 coating. The furnace bottom has been covered already. On the left, the alu/steel rod hinges connect the furnace door stable to the steel frame and the stainless steel shell.

The (almost) finished furnace. The door consists of a steel angle iron frame, holding a 1/2" thick ceramic board backed up with 2" fiber blanket. The thermo element (type K) is fed through with a thin two hole alumina rod, approximately reaching 2" deep into the chamber. The wire ends are visible on top of the door (temporary fastened there). The stainless steel (304 alloy) shell is completely fastened to the skeleton.

The back side before wire up. Behind the back plate lays a 2" fiber blanket with the element connectors on top. The plate is electrically insulated using a four layer fiber glass mat. All connections are embedded between the two fiber blankets. A weak point and subject to future change is the heat transfer through the element wire to the (well insulated) connector which builds up hot spots (250F). The design change will feed the element wires through the back plate and supports and holds them with ceramic threaded studs outside. The side view shows the simple aluminum handle for easy transport.

The finished furnace. Until the digital controller is finished, the Pyrometer is a sufficient means to control the furnace for metal casting. Longer enduring and more elaborate firing schemes for ceramics would need a controller. The right side shows the locking knob to close and tighten the furnace door.

Back side view of the finished furnace. In the middle, right above the wire feed through, the solid state relay with heat sink (Omega 25A,400V with DC control). Left to it, the switch box, containing the main power switch (25A) and a small 12V DC power supply for the relay control and the digital controller. Both, relay and switch box stay on metal studs approx. 1/2" away from the back side plate for thermal insulation. The element connections behind the back plate utilizing copper clamps with screws and are insulated by fish spine ceramic beads. The wires to the outside switch box are fish spine insulated with additional woven fiber sleeves.

#8 silicon carbide crucible with simple tong. The crucible is lifted out by squeezing the left metal band to the handle (foundry gloves required!!)

Some of the outcome in trying to melt down the big 25pound foundry ingot into manageable portions. First, a lot of workout had to be done to hacksaw the ingot into furnace fitting portions. Metal casting is exciting and fun! Pouring mercury like liquid aluminum the first time is cool! Now I have to start with more demanding projects - I think a power Hacksaw would be a good thing. A couple of casting applications will be there, I'm sure.

(C) M.Knoth 06/2002
mknoth@earthlink.net
unauthorized copying and publishing prohibited
 

http://www.cotronics.com/ - high temperature refractories, coatings, glues, ceramics etc...

http://www.euclids.com/ - a good source to have your heating elements produced (much better than self made)

http://www.omega.com/ - solid state relay, everything about process measurement and control

http://www.metalsupermarkets.com/ - all metals for structural work

http://www.mcmaster.com/ - everything one can think of

http://www.sundanceglass.com/ - pyrometers and many, many other cool things

http://www.budgetcastingsupply.com/ - everything to do hobby metal casting

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